Hostname: page-component-586b7cd67f-t8hqh Total loading time: 0 Render date: 2024-11-26T09:02:01.222Z Has data issue: false hasContentIssue false

Through fat and thin – a journey with the adipose tissues

Published online by Cambridge University Press:  12 February 2020

Paul Trayhurn*
Affiliation:
Obesity Biology Unit, Institute of Ageing & Chronic Disease, University of Liverpool, Liverpool, UK Clore Laboratory, University of Buckingham, BuckinghamMK18 1EG, UK
*
Corresponding author: Paul Trayhurn, email [email protected]
Rights & Permissions [Opens in a new window]

Abstract

The paper is based on the lecture that I gave on receiving the Nutrition Society's inaugural Gowland Hopkins Award for contributions to Cellular and Molecular Nutrition. It reviews studies on the adipose tissues, brown and white, conducted by the groups that I have led since entering nutrition research in 1975. The initial focus was on exploring metabolic factors that underpin the development of obesity using animal models. This resulted in an interest in non-shivering thermogenesis with brown adipose tissue being identified as the key effector of facultative heat production. Brown fat is less thermogenically active in various obese rodents, and major changes in activity are exhibited under physiological conditions such as lactation and fasting consistent with a general role for the tissue in nutritional energetics. My interests moved to white adipose tissue following the cloning of the Ob gene. Our initial contributions in this area included demonstrating nutritional regulation of Ob gene expression and circulating leptin levels, as well as a regulatory role for the sympathetic nervous system operating through β3-adrenoceptors. My interests subsequently evolved to a wider concern with the endocrine/signalling role of adipose tissue. Inflammation is a characteristic of white fat in obesity with the release of inflammation-related adipokines, and we proposed that hypoxia underlies this inflammatory state. O2-deprivation was shown to have substantial effects on gene expression and cellular function in white adipocytes. The hypoxia studies led to the proposition that O2 should be considered as a critical macronutrient.

Type
Gowland Hopkins Award Lecture
Copyright
Copyright © The Author 2020

Background

I am greatly honoured to receive the Gowland Hopkins Award from the Nutrition Society, and indeed to be the first recipient. I never met Sir Frederick Gowland Hopkins OM PRS, who received the Nobel Prize for Physiology or Medicine in 1929 for the discovery of vitamins, being born a year after he died. There is, however, a tangential link in that my entry into nutrition from a basic science background in physiology and biochemistry came through joining the MRC Dunn Nutritional Laboratory in Cambridge in 1975. The Dunn had been founded in 1927 when Gowland Hopkins was Professor of Biochemistry in the University, and the original intent was that he should be directly involved in its research. In practice, because of extensive other commitments, including as President of the Royal Society, his primary role with respect to the Dunn was as an advisor and member of the Management Committee. Gowland Hopkins has, of course, a close association with the Nutrition Society as one of the principal figures behind its foundation.

My research over the past 40+ years since entering nutrition has centred on the adipose tissues; first brown and then white. This began following my initial studies at the Dunn where I had been recruited by Philip James as a member of the then newly formed Energy Group (Fig. 1). The group had been established in recognition of obesity beginning to emerge as a public health problem. At the time, the incidence of obesity was considerably less than now; in 1980, for example, 6 % of adult males and 8 % of adult females in the UK were classified as obese(Reference Prentice and Jebb1) on the basis of a BMI ≥30, while by 2017 the figure was approximately 30 % of all adults (https://www.worldobesitydata.org/country-profiles/). Obesity is, of course, important primarily because of the increased risk of several associated diseases, particularly type-2 diabetes, hypertension, CHD and certain cancers(Reference Kopelman2,Reference Blüher3) .

Fig. 1. (Colour online) ‘Engaging’ with nutrition at the Dunn: Friday morning group ‘coffee and cake’. Eating and drinking in the laboratory is, of course, prohibited now, but was normal in my early years as a scientist.

The ethos prevailing when we began in the mid-1970s was that obesity is the product of ‘gluttony and sloth’, but our focus was on exploring whether there are important metabolic factors which underpin the development of the disorder. My remit was to investigate the fundamentals of the regulation of energy balance using animal models. The animal of choice was the genetically obese ob/ob (Lepob/Lepob) mouse, and a colony of the Aston strain of these mutants was set-up. The attraction of ob/ob mice, which at the time were the most widely used animal model in obesity research, was that not only is the obese state extreme with body weight being up to three times that of lean siblings but that it is reducible to a mutation in a single recessively inherited gene(Reference Trayhurn4). The link to a mutant gene meant that the obesity of ob/ob mice results from a change in just one protein, and that protein must play a critical role in the regulation of energy balance.

The ob/ob mouse is not, of course, the only rodent in which obesity is the result of a single gene mutation, and we subsequently established colonies of the other major obese mutants: the Zucker fa/fa (Leprfa/Leprfa) rat and the diabetic-obese db/db (Leprdb/Leprdb) mouse(Reference Trayhurn4). We were later able to house colonies of the adipose mouse (Ad), golden hamsters (Mesocricetus auratus) and Djungarian hamsters (Phodopus sungorus), each as a specific model within our energy regulation studies. This was based on the August Krogh Principle that ‘for a large number of problems there will be some animal of choice or a few such animals on which it can be most conveniently studied’(Reference Krogh5). As I have noted previously, the ability to maintain multiple colonies of experimental animals at the Dunn without direct cost to the investigator was remarkable(Reference Trayhurn6).

Energy balance and thermogenesis

Hyperphagia is part of the basis for the obesity of the ob/ob mouse, and indeed that of the other obese mutants, food intake being greater than in lean siblings(Reference Trayhurn4). However, studies where young ob/ob mice were either directly pair-fed to the ad libitum intake of their lean siblings, or otherwise given restricted amounts of food, indicated that obesity still develops without hyperphagia(Reference Thurlby and Trayhurn7,Reference Romsos8) . Our own work, in which full energy balance studies were performed, clearly illustrates the point; young ob/ob mice pair-fed to the ad libitum intake of lean siblings at room temperature (23°C) exhibited a rate of energy deposition 2⋅3 times that of the lean(Reference Thurlby and Trayhurn7). The study was conducted at four different environmental temperatures: 33 (thermoneutral for the mouse), 28, 23 and 17°C. At each temperature, the energy gain of the obese animals was greater than the lean, but the lower the temperature the higher the excess gain(Reference Thurlby and Trayhurn7).

The capacity for excess energy deposition in the absence of hyperphagia indicated that one or more components of energy expenditure is reduced in the ob/ob mutants. Of the main components of expenditure, facultative (or adaptive) non-shivering thermogenesis (NST) was particularly attractive as the key element. Not only is thermoregulatory thermogenesis a major part of total expenditure in small mammals in order to maintain body temperature, but reduced expenditure on thermogenesis was also being advocated by Miller and Stock as a causal factor in the development of obesity(Reference Miller and Mumford9,Reference Miller, Mumford and Stock10) . Furthermore, some 25 years earlier impaired homoeothermy had been noted in ob/ob mice(Reference Davis and Mayer11). This appeared counter-intuitive given the improved insulation provided by the additional body fat of the obese animals, and was suggestive of a reduced capacity to generate heat. In our own studies, core temperature fell rapidly, to as low as 15°C, just 3 h after exposure of ob/ob mice to 4°C, whereas lean siblings maintained their temperature above 35°C(Reference Trayhurn and James12).

Direct measurements of NST from the peak increase in the RMR at thermoneutrality following the administration of noradrenaline indicated that the capacity for this form of heat production was 2-fold lower in ob/ob mice than that in lean siblings(Reference Trayhurn and James12). Furthermore, RMR expressed ‘per animal’, as should be done in energetic studies, was reduced in the obese mice relative to the lean at every temperature examined below thermoneutrality, indicating a lower expenditure on NST(Reference Trayhurn and James12).

Brown adipose tissue

An immediate question raised by these physiological studies was the nature of the molecular and cellular mechanisms of NST. Several possibilities were under consideration at the time, including protein turnover, the α-glycerophosphate shuttle, Na+ transport across the plasma membrane mediated by Na+-K+ATPase, and futile/substrate cycles such as that between fructose-6-phosphate and fructose-1,6-bisphosphate(Reference Stirling and Stock13Reference Trayhurn, Goodbody and James19); in practice, most are in effect a form of energy-consuming substrate cycle. There were, however, substantial reservations with each of these mechanisms(Reference Trayhurn20) in that it seemed unlikely that they had the potential to generate sufficient quantities of heat and to do so acutely without disrupting normal metabolic control. In addition, there was a central issue of tissue localisation; several of the mechanisms, particularly protein turnover and Na+ transport, being essentially universal rather than restricted to a specific tissue site.

The question of the tissue basis for NST subsequently centred on brown adipose tissue (BAT, or brown fat), which had first been described by Conrad Gessner in 1551. Although different roles had been proposed for this tissue, including as an endocrine organ, the principal function was resolved in the early 1960s, as a thermogenic organ with heat as the primary product(Reference Smith and Horwitz21). The tissue is prominent in hibernating species, in the newborn of many mammals (including human subjects) and in rodents acclimated to the cold(Reference Nicholls and Locke22,Reference Cannon and Nedergaard23) . The quantitative importance of BAT in adaptive NST in rodents was demonstrated in influential studies by Foster and Frydman(Reference Foster and Frydman24Reference Foster and Frydman26). These authors mapped regional blood flow to different tissues using radioactively-labelled microspheres in rats in which NST was maximally stimulated following either cold-acclimation or the administration of noradrenaline. From the measurements of regional blood flow, together with the cardiac output and the oxygen extraction across the interscapular depot, BAT was estimated to account for 60 % of NST in cold-acclimated rats(Reference Foster and Frydman25). Our own studies on mice using the same approach suggested a broadly similar figure(Reference Thurlby and Trayhurn7).

In parallel with the identification of BAT as the principal locus for NST, the unique bioenergetic properties of the tissue's mitochondria were being elucidated. Heat was shown by Nicholls to be generated by a regulated uncoupling of oxidative phosphorylation, the energy inherent in the proton gradient across the inner mitochondrial membrane being dissipated as heat rather than coupled to ATP synthesis(Reference Nicholls and Locke22). This process is controlled by the 32 000-Mr mitochondrial uncoupling protein-1 (UCP1) discovered by Ricquier(Reference Nicholls and Locke22,Reference Ricquier27) . Acute stimulation of BAT thermogenesis leads to an activation of UCP1, while chronic stimulation results in an increase in the amount of the protein, through a combination of a higher concentration in the mitochondria and through mitochondriogenesis(Reference Cannon and Nedergaard23). These changes, both acute and chronic, are primarily driven by the release of noradrenaline from the extensive sympathetic innervation of BAT, acting mainly via β3-adrenoceptors(Reference Himms-Hagen28,Reference Arch29) .

My group at the Dunn, in parallel with several other groups, began to explore the potential role of BAT in energy balance and the development of obesity. Two seminal observations were pivotal; in the first, Himms-Hagen and Desaultels in Ottawa demonstrated reduced GDP binding to BAT mitochondria in ob/ob mice relative to lean siblings, this reflecting a reduction in the thermogenic proton conductance pathway of the tissue(Reference Himms-Hagen and Desautels30). Our own blood flow studies indicated that the reduced NST and consequent lower energy expenditure of the obese mutant is entirely due to decreased metabolic activity in BAT(Reference Thurlby and Trayhurn31). In the second key study, Rothwell and Stock in London proposed that BAT is the locus of the diet-induced thermogenesis that they were observing in rats overfed through the provision of a cafeteria diet(Reference Rothwell and Stock32). In follow-up studies with our group, key molecular indices of BAT thermogenic activity were demonstrated in the cafeteria-fed animals; increased mitochondrial mass and GDP binding, as well as GDP-sensitive respiration(Reference Brooks, Rothwell and Stock33).

These initial reports were followed by a series of studies in which the thermogenic activity of BAT was shown to be reduced in a variety of obese rodents. They included other single gene mutants, fa/fa rat and db/db mouse, and rodents with experimentally-induced obesity such as that following lesioning of the ventromedial hypothalamus, the administration of gold thioglucose, and treatment with corticosteroids(Reference Trayhurn20,Reference Himms-Hagen34,Reference Trayhurn, Trayhurn and Nicholls35) . Along with the studies on obese animals, the role of BAT in nutritional energetics was further explored in a range of physiological and pathophysiological situations in which body fat and energy flux change (Fig. 2). These included the reproductive cycle (pregnancy and lactation) hibernation, photoperiod, cancer cachexia and nutritional manipulations such as fasting/refeeding and the provision of a low protein diet(Reference Himms-Hagen34,Reference Trayhurn, Trayhurn and Nicholls35) .

Fig. 2. (Colour online) Schematic of different physiological and pathological conditions in experimental animals in which energy flux and/or balance are altered where increases, or decreases, in brown adipose tissue thermogenesis have been demonstrated. Examples of key situations in which brown fat thermogenesis changes are shown. DIT, diet-induced thermogenesis; VMH, ventromedial hypothalamus.

Lactation was a physiological stress of particular interest to us at the Dunn. The energy cost of lactation is high in small mammals, and energy intake is increased approximately 3-fold in lactating rats, for example, compared to virgin animals(Reference Fell, Smith and Campbell36,Reference Williamson37) . Our studies in mice showed that BAT thermogenesis is suppressed in lactation, the suppression being maximal in late lactation when milk production peaks(Reference Trayhurn, Douglas and McGuckin38Reference Krol and Speakman40). Mitochondrial mass and GDP binding are both markedly reduced in BAT of lactating mice, the latter to the same level as virgin animals at thermoneutrality(Reference Trayhurn, Douglas and McGuckin38,Reference Trayhurn and Jennings39) . The concentration of UCP1 in the mitochondria is also reduced relative to that of virgin mice, with the total UCP1 content of the interscapular pad at late lactation being <10 % of the virgin animals (Fig. 3)(Reference Trayhurn and Jennings39). These changes in BAT activity effectively lead to a substantial energy saving, helping to meet the high energy cost of milk production. However, this adaptation essentially reflects the limited scope for heat dissipation in the face of the high metabolic heat generation associated with milk synthesis rather than a specific energy saving mechanism as such, as convincingly argued in a recent review on BAT in lactation(Reference Krol and Speakman40).

Fig. 3. (Colour online) The thermogenic activity and capacity of brown adipose tissue (BAT) is decreased in lactation (Lact). The changes in mitochondrial GDP binding, the mitochondrial concentration of uncoupling protein-1 (UCP1; UCP1 conc) and the total UCP1 content of the interscapular BAT depot are shown for mice at late lactation (when milk production is close to maximal) relative to virgin mice (virgin = 1)(Reference Trayhurn and Jennings39).

Perhaps the strongest illustration of the link between energy expenditure and BAT thermogenesis comes from the changes in the tissue that occur when small rodents are acclimated to the cold. In mice, energy expenditure and food intake are increased 3-fold between thermoneutrality and 4°C, reflecting the energy cost of generating heat for homoeothermy(Reference Trayhurn and James12,Reference Trayhurn41) . In rats acclimated at 4°C, the mitochondrial content, mitochondrial GDP binding and UCP1 concentration were each substantially higher than in rats acclimated to thermoneutrality (29°C), while the total UCP1 content of the interscapular BAT depot was increased >100-fold(Reference Trayhurn, Ashwell and Jennings42).

Brown adipose tissue in human subjects

By the beginning of the 1990s, the importance of BAT in nutritional energetics had been firmly established across a range of obesity models and under other conditions in experimental animals in which energy flux is altered. In the case of human subjects, interest in the tissue had been driven to a considerable extent by the concept that reduced thermogenesis is a key factor in the development of obesity in human subjects and that BAT is a potential therapeutic target for the treatment of the disorder.

Although brown fat was widely recognised to be an important locus of heat production in the human neonate, the tissue appeared, on the basis of histological appearance, to be absent after the first few years of life. The presence of BAT in adult human subjects was confirmed, however, by immunological studies identifying UCP1 in fat depots, including in some elderly subjects(Reference Bouillaud, Combes and Ricquier43Reference Lean45). In addition, expression of the UCP1 gene was evident through detection of the encoded mRNA(Reference Bouillaud, Villarroya and Hentz46). Activation of the tissue in patients with phaeochromocytoma was also demonstrated(Reference Ricquier, Néchad and Mory47,Reference Lean, James and Jennings48) .

Despite the clear evidence for the presence of BAT in adults, with the capacity for adaptive changes, the prevailing view was that the tissue was of little, or no, significance in human energetics other than in neonates and during the first years of life. Interest in BAT then declined markedly, with the notable exception of those groups (particularly that of Cannon and Nedergaard in Stockholm(Reference Cannon and Nedergaard23)) whose principal focus was on understanding the fundamental biology of the tissue. Since 2009 there has, however, been renewed interest in BAT in human subjects following the application of fluorodeoxyglucose positron emission tomography(Reference Cypess, Lehman and Williams49Reference van Marken Lichtenbelt, Vanhommerig and Smulders51). This has firmly demonstrated active BAT in adults, activity being reduced in obesity and with ageing, for example, while being stimulated on cold exposure and by the administration of a selective β3-adrenoceptor agonist(Reference Cypess, Lehman and Williams49Reference Moonen, Nascimento and van Marken Lichtenbelt55).

White adipose tissue: the discovery of leptin

As interest in BAT declined, my own research focus changed and abruptly so following the cloning of the Ob (Lepob) gene and the identification of the encoded protein(Reference Zhang, Proenca and Maffei56). Within days of the report in Nature on 1 December 1994, my group at the Rowett in Aberdeen (where I had relocated in 1988) had designed and validated oligonucleotide probes to examine Ob gene expression. This move reflected the fact that some 2 years earlier a consortium of us in the UK, which included Michael Stock and John Stirling, had sought funding to identify the defective genes in the obese mouse mutants. We were, however, unsuccessful since it was argued (correctly) that at least one group in the United States was well-advanced in the goal and that it was unlikely that we could be competitive. My response was that once the Ob gene had been identified, our strategy would be to explore the physiology of the protein product.

The Ob gene was reported to be expressed in white adipose tissue (WAT) and the protein, initially termed ‘OB’, and then leptin, to act as a lipostatic signal(Reference Zhang, Proenca and Maffei56Reference Friedman58). Subsequently, the hormone was found to be produced by several tissues, including BAT(Reference Dessolin, Schalling and Champigny59) and the placenta(Reference Hoggard, Hunter and Duncan60), although WAT is the major source. Similarly, the functions attributed to leptin quickly expanded and it became regarded as a pleiotropic factor(Reference Trayhurn61). The early studies of my group at the Rowett demonstrated that expression of the Ob gene is nutritionally regulated, the mRNA level in WAT of lean rodents rapidly decreasing on fasting with a restoration on refeeding(Reference Trayhurn, Thomas and Duncan62). The circulating levels of the hormone change in parallel with the alterations in gene expression(Reference Hardie, Rayner and Holmes63).

We then showed that acute exposure of mice to cold led to a strong inhibition of Ob expression, and a fall in the circulating leptin level, both of which are rapidly reversed on return to a warm environment(Reference Trayhurn, Duncan and Rayner64,Reference Trayhurn, Duncan and Rayner65) . The cold-induced reduction in the Ob mRNA level was mimicked by the administration of noradrenaline and by the β-adrenoceptor agonist isoprenaline. From these observations we proposed that the sympathetic system plays a key role in the regulation of Ob gene expression(Reference Trayhurn, Duncan and Rayner64). Subsequent observations indicated that this operates primarily through β3-adrenoceptors(Reference Trayhurn, Duncan and Rayner65,Reference Mantzoros, Qu and Frederich66) . Further studies on leptin at the Rowett included the demonstration by in situ hybridisation that the receptor, and particularly the long form responsible for signalling, is strongly expressed in the regions of the hypothalamus, consistent with being an adipocyte-derived signal for appetite(Reference Mercer, Hoggard and Williams67).

Adipokines and the secretory function of white adipocytes

Leptin quickly became a major area in research on obesity and its associated disorders. One of the key outcomes of the discovery of the hormone was a radical change in perspective on the functions of white adipocytes and therefore of WAT itself. Adipocytes were recognised as endocrine cells with WAT as a major signalling organ(Reference Frühbeck, Gómez-Ambrosi and Muruzabal68Reference Trayhurn71). Although secreted protein factors had been identified previously, this had not led to the conceptualisation of white adipocytes as endocrine and signalling cells. The secreted proteins known prior to leptin were adipsin (complement factor D)(Reference Cook, Min and Johnson72), which is a serine protease, the cytokine TNFα(Reference Hotamisligil, Shargill and Spiegelman73), and lipoprotein lipase. Lipoprotein lipase is, of course, released from adipocytes to catalyse the breakdown of circulating TAG to enable the uptake of fatty acids into adipocytes; it was not, however, regarded as a fat cell secretory protein as such.

The secretome of adipocytes, and of WAT as a whole, is extensive (Fig. 4). Quantitatively, fatty acids are the largest secretory product, but there are several other lipid groups released from the cells. Some, such as specific prostaglandins and the endocannabinoid anandamide, are synthesised de novo within adipocytes, while others, including cholesterol and vitamin A, are taken up, stored and subsequently released(Reference Trayhurn61). A question raised by the discovery of leptin was whether there are a range of protein hormones and signals synthesised and secreted by fat cells. The answer is very much in the affirmative and one of the earliest of these adipokines, as they are termed, identified was another major adipocyte hormone, adiponectin, whose functions encompass insulin sensitising, angiogenic and anti-inflammatory actions (Fig. 3)(Reference Ouchi, Kihara and Arita74Reference Brakenhielm, Veitonmaki and Cao78).

Fig. 4. (Colour online) The secretome of white adipocytes. Fatty acids and other lipids are secreted, together with a multiplicity of adipokines (proteins); examples of some of the lipids and key adipokines are shown. The major adipocyte hormones, leptin and adiponectin, are highlighted. angptl4, angiopoietin-like protein-4; CETP, cholesteryl ester transfer protein; DPP4, dipeptidyl peptidase-4; IGF, insulin-like growth factor-1; LPL, lipoprotein lipase; MCP-1, monocyte chemoattractant protein-1; MIC-1, macrophage inhibitory cytokine-1; MIF, macrophage migration inhibitory factor; MMP, matrix metalloproteinase; NGF, nerve growth factor; PAI-1, plasminogen activator inhibitor-1; RBP4, retinol binding protein-4; TGFβ, transforming growth factor-β; VEGF, vascular endothelial growth factor; ZAG, zinc-α2-glycoprotein.

The search for novel adipokines became a core focus of my group, both at the Rowett and later at the University of Liverpool to where I moved in 2002. Among the several adipokines that we discovered were: the neurotrophic signal nerve growth factor(Reference Peeraully, Jenkins and Trayhurn79); specific metallothioneins(Reference Trayhurn, Duncan and Wood80), these having metal binding actions, and the lipolytic/cachectic factor zinc-α2-glycoprotein(Reference Bing, Bao and Jenkins81,Reference Bao, Bing and Hunter82) . Nerve growth factor was found to be linked to the inflammatory response in WAT, secretion of the protein being strongly stimulated by TNFα(Reference Peeraully, Jenkins and Trayhurn79). As in mice, zinc-α2-glycoprotein expression increased substantially in WAT of patients with cachexia associated with gastrointestinal cancer(Reference Mracek, Stephens and Gao83).

An extensive range of individual adipokines has now been identified, and proteomic studies and in silico analysis suggest that there are several hundred in total(Reference Lehr, Hartwig and Lamers84Reference Deshmukh, Peijs and Beaudry86). The wide-ranging secretory function of white adipocytes established over the past two decades has in part served as a model for other cell types which were not previously regarded as having a significant endocrine or signalling function. Myocytes, for example, are now known to release a range of protein signals, myokines(Reference Pedersen and Febbraio87,Reference Lee and Jun88) , while another example is hepatocytes which secrete multiple hepatokines(Reference Stefan and Haring89).

The identification of a multiplicity of protein signals and factors from adipocytes indicated that WAT is involved in a range of physiological and regulatory processes(Reference Trayhurn61,Reference Rajala and Scherer70,Reference Trayhurn and Wood90Reference Scherer92) . While some adipokines are endocrine in function, signalling to tissues and organs distant to the adipose depots, others have local paracrine and/or autocrine actions. The processes in which various adipokines play a role include appetite and energy balance, lipid metabolism, vascular haemostasis, blood pressure, angiogenesis and insulin sensitivity (see(Reference Trayhurn61,Reference Trayhurn91) ). A number of adipokines are linked to immunity and inflammation, these including classical cytokines and chemokines such as IL-1β, IL-6, IL-10 and monocyte chemoattractant protein-1; they also include inflammation-related factors, examples being vascular endothelial growth factor (VEGF), serum amyloid A and adiponectin (see(Reference Trayhurn61,Reference Rajala and Scherer70,Reference Trayhurn91 ).

In obesity, WAT exhibits chronic mild inflammation with increased production and release of inflammatory adipokines. There is a notable exception to this in that the synthesis and release of adiponectin, with its anti-inflammatory action, falls(Reference Arita, Kihara and Ouchi93,Reference Hotta, Funahashi and Arita94) . Inflammation in expanded WAT is augmented by the infiltration and activation of macrophages in particular, but also of other immune cells(Reference Scherer92,Reference Weisberg, McCann and Desai95Reference Mraz and Haluzik97) .

Hypoxia and the metabolic response to oxygen deprivation in adipocytes

Inflammation in WAT has been considered a key factor in the development of the major obesity-associated disorders, particularly insulin resistance and the other components of the metabolic syndrome(Reference Blüher3,Reference Rajala and Scherer70,Reference Yudkin98,Reference Hotamisligil99) . The question that intrigued me in the early 2000s was why does inflammation develop as adipose tissue mass expands? A ‘News’ article in Science on how cells endure low oxygen(Reference Marx100) encouraged me to consider the possibility that hypoxia might be a key. This was presented as a hypothesis in a Horizons article in the British Journal of Nutrition in 2004(Reference Trayhurn and Wood90). I am particularly proud of this paper: not only does it describe the hypoxia hypothesis, but it is my most highly cited publication (>1400 citations in the Web of Science; >2450 citations in Google Scholar) as well as being the fourth most highly cited article in the Nutrition Society's flagship journal (or indeed in all of its journals).

The hypothesis proposed that as adipose tissue mass expands with the development of obesity, areas within the tissue become relatively hypoxic as the enlarging adipocytes become more distant from the vasculature, this leading to major adaptive changes involving the hypoxia-inducible transcription factor-1 (HIF-1). The recruitment of HIF-1 was hypothesised to lead to increased expression of a series of hypoxia-sensitive genes linked to inflammation and the inflammatory response in WAT. The proposition was based on the following: (i) hypoxia occurs in situations such as ischaemic injury, wound healing and solid tumours leading to extensive metabolic changes(Reference Trayhurn and Wood90), (ii) blood flow to WAT is not increased in obese subjects, despite the higher mass of the tissue(Reference Blaak, van Baak and Kemerink101Reference Frayn and Karpe104), (iii) in contrast to lean subjects, blood flow to WAT does not increase post-prandially in the obese(Reference Frayn and Karpe104Reference Goossens, Bizzarri and Venteclef106), (iv) large adipocytes (which may be up to 200 μm diameter(Reference Skurk, Alberti-Huber and Herder107)) are further from the vasculature than the normal diffusion distance for O2 (100 μm)(Reference Brahimi-Horn and Pouysségur108). These observations refer to local hypoxia, but the provision of O2 on a whole-body level is reduced in specific environmental and pathological situations, such as high altitude, deep sea dives, lung diseases and obstructive sleep apnoea(Reference Trayhurn61,Reference Brahimi-Horn and Pouysségur108) .

In 2007, studies using two separate techniques reported that WAT depots in different types of obese mice are hypoxic, with the O2 tension being 2- to 3-fold lower than in lean mice(Reference Hosogai, Fukuhara and Oshima109,Reference Ye, Gao and Yin110) . Subsequent studies on mice were consistent with these observations(Reference Rausch, Weisberg and Vardhana111). In contrast, although some human studies have indicated that WAT depots are relatively hypoxic(Reference Kabon, Nagele and Reddy103,Reference Pasarica, Sereda and Redman112) others have reported either the same or an increase in pO2 (partial pressure of oxygen)(Reference Goossens, Bizzarri and Venteclef106,Reference Hodson, Humphreys and Karpe113) . The issue remains unresolved, but there is evidence that differences in the way in which O2 is delivered in terms of vascularisation and utilisation may occur(Reference Goossens, Bizzarri and Venteclef106,Reference Hodson, Humphreys and Karpe113,Reference Lempesis, van Meijel and Manolopoulos114) .

From 2004 the focus of my group in Liverpool was on examining the direct effects of hypoxia on gene expression and cellular function in adipocytes. Almost all of our studies were conducted on human adipocytes, differentiated in culture from fibroblastic preadipocytes. The initial priority was to examine whether incubation under a low O2 tension leads to increased expression and release of inflammation-related adipokines consistent with our initial hypothesis. A candidate gene approach was employed and increased production of several adipokines was observed, including IL-6, VEGF and leptin(Reference Wang, Wood and Trayhurn115). Raised production of VEGF and leptin, as well as of specific matrix metalloproteinases, had been reported earlier in 3T3-F442A adipocytes (a mouse cell line), reflecting a pro-angiogenic response to hypoxia(Reference Lolmède, Durand de Saint Front and Galitzky116).

The key cellular adaptation to O2 deficiency is a switch from aerobic to anaerobic metabolism. Mitochondrial oxidative phosphorylation cannot, of course, continue when O2 is severely limited, and there is instead increased anaerobic glycolysis. As expected, adipocytes exhibit greater glucose uptake under hypoxic conditions, as demonstrated by 2-deoxy-d-glucose uptake studies(Reference Wood, Wang and Lorente-Cebrián117) and by measurement of glucose in the culture medium(Reference Lolmède, Durand de Saint Front and Galitzky116). This is mediated through increased synthesis of the GLUT1 facilitative glucose transporter, driven by a marked stimulation of GLUT1 gene expression(Reference Wood, Wang and Lorente-Cebrián117). The expression of several genes encoding glycolytic enzymes is also raised, glucose-6-phosphate isomerase and phosphofructokinase, for example(Reference Geiger, Leiherer and Muendlein118,Reference Mazzatti, Lim and O'Hara119) . Lactate release is augmented in hypoxic adipocytes(Reference Lolmède, Durand de Saint Front and Galitzky116,Reference Pérez de Heredia and Wood120) , reflecting the increased glucose utilisation and glycolytic flux, this being mediated by increases in the synthesis of the monocarboxylate transporter, MCT1(Reference Pérez de Heredia and Wood120).

While our initial exploration of the effects of hypoxia on gene expression in human adipocytes probed selective candidate genes, in subsequent studies more comprehensive approaches were taken. In the first of these, PCR arrays for eighty-four genes linked to the hypoxia-signalling pathway were employed. The expression of a number of the genes changed, with one particular gene exhibiting dramatically increased expression(Reference Wang, Wood and Trayhurn121). The gene in question was MT3, which encodes a member of the metallothionein family, metallothionein-3 (also known as growth inhibitory factor). This protein binds zinc and copper, and linked to its marked induction by O2-deprivation has been implicated as an angiogenic factor and to protect against hypoxic damage(Reference Penkowa, Carrasco and Giralt122,Reference Zbinden, Wang and Adenika123) .

PCR arrays are themselves limited in terms of the number of genes whose expression can be screened and a specific pathway or metabolic system needs to be selected. DNA microarrays offer an unbiased approach in which all, or almost all, the genes expressed in a tissue or cell can be probed simultaneously. Our microarray studies at Liverpool, in collaboration with colleagues at Unilever, indicated that the expression of >1300 genes was altered in human adipocytes cultured under hypoxic conditions, stringent criteria being used to evaluate changes(Reference Mazzatti, Lim and O'Hara119). Of these genes, the expressions of approximately half were up-regulated and half down-regulated under low pO2. Bioinformatic analysis showed that a number of metabolic pathways and functions are altered in human adipocytes by hypoxia, these including lipolysis, lipid oxidation, glucose utilisation, cell to cell signalling and cell death(Reference Mazzatti, Lim and O'Hara119).

It is evident from these and other studies that hypoxia results in extensive changes in gene expression in adipocytes. Several important functional changes have been described, in addition to increased anaerobic glycolysis (Fig. 5). These include the rapid induction of insulin resistance through the direct inhibition of insulin signalling(Reference Regazzetti, Peraldi and Gremeaux124,Reference Yin, Gao and He125) , and the disruption of the extracellular matrix within WAT that characterises fibrosis(Reference Halberg, Khan and Trujillo126,Reference Sun, Tordjman and Clément127) . With respect to fibrosis, hypoxia leads to changes in the expression of collagens released as components of the extracellular matrix, as well as matrix metalloproteinases(Reference Lolmède, Durand de Saint Front and Galitzky116,Reference Halberg, Khan and Trujillo126) involved in tissue remodelling. The overall cellular response to low O2 is regulated by a series of hypoxia-responsive transcription factors of which HIF-1, consisting of two subunits (HIF-1α and HIF-1β), is the best characterised(Reference Brahimi-Horn and Pouysségur108,Reference Semenza128,Reference Coleman and Ratcliffe129) .

Fig. 5. Schematic representation of the central cellular responses to hypoxia in white adipocytes. The effect of low partial pressure of O2 on gene expression, glucose uptake and utilisation, and the production of selected key adipokines is shown. angptl4, angiopoietin-like protein-4; FA, fatty acid; GLUT1, facilitative glucose transporter 1; HIF-1, hypoxia-inducible factor-1; MCT1, monocarboxylate transporter-1; MIF, macrophage migration inhibitory factor; MMP, matrix metalloproteinase; MT-3, metallothionein-3; PAI-1, plasminogen activator inhibitor-1; TF, transcription factors (other than HIF-1); VEGF, vascular endothelial growth factor. Modified from(Reference Trayhurn131).

Our studies, like most that examine the response of cells to hypoxia, were undertaken by comparing 1 to 20 % O2 (95 % air/5 % CO2; ‘normoxia’). However, this is an extreme, effectively representing a comparison between ambient air (higher than arterial pO2) and marked hypoxia. A question that interested me was whether there is a critical point at which adaptation to reduced O2 is initiated in adipocytes, or if there is a gradual response to falling O2 tension. Experiments in which human adipocytes were incubated with a range of O2 levels between 20 and 1 % clearly demonstrated a dose-dependent response to lowering O2 tension with differences being observed between 21 and 15 % and between 15 and 10 % O2(Reference Wood, Stezhka and Trayhurn130). This was true for the expression and secretion of several adipokines, including leptin and VEGF, as well as 2-deoxy-d-glucose uptake and GLUT1 gene expression. Nevertheless, changes tended to be more marked between 10 and 5 % O2. Since the pO2 in WAT of lean mice is equivalent to about 7 % O2 while in obese mice it is about 2 %, it is evident that there are responses to O2-deprivation over physiologically relevant differences in tissue oxygenation between the phenotypes(Reference Trayhurn91).

These experiments on the effects of a range of O2 levels demonstrate that while the customarily employed protocols in hypoxia studies offer proof of principle, they lead to an exaggerated view of the scale of the cellular response to relative O2 lack under normal physiological conditions. This raises a question of the extent to which our understanding of cellular processes has been conditioned, or even distorted, by the routine use of 20 % O2 as the gas phase in cell culture and other in vitro experiments. It is intriguing that careful attention is paid to the pH (7⋅4), temperature (37°C) and the concentration of glucose and other nutrients in cell culture to ensure physiological conditions' (except for when they are the parameters under investigation), but the O2 tension employed is quite unphysiological and indeed reflects overt hyperoxia.

Oxygen: an overlooked macronutrient

A corollary of our studies on hypoxia is that they underscore that O2 is a key nutrient at the cellular level. Indeed, investigation of hypoxia is in effect exploration of the molecular and metabolic consequences of the deficiency of a nutrient. However, O2 is not considered as a nutrient as such in the context of nutritional science. Textbooks of nutrition do not contain sections on O2, and reference to it is generally restricted to discussion of metabolic rate and respiratory quotient. I have argued recently that O2 should be included alongside the other elements/molecules/macromolecules that are defined as nutrients(Reference Trayhurn131,Reference Trayhurn132) .

O2 undoubtedly meets dictionary definitions of a nutrient; for example, ‘as a substance that provides nourishment for the maintenance of life and for growth’ (Oxford English Dictionary). The central reason why O2 is not considered as part of nutritional science is because of the route of entry; the nose/lungs in higher terrestrial animals, rather than the mouth/gastrointestinal tract. However, I argue that the route of entry should not be the critical determinant of whether O2 is, or is not, considered a nutrient, but rather its function and essentiality(Reference Trayhurn132). O2 is, of course, critical to all aerobic species without which mitochondrial oxidative phosphorylation cannot take place.

Early organisms developed under anoxic conditions, the level of O2 in the atmosphere being just one part in a million soon after the Earth was formed some 4⋅54 billion years ago(Reference Kerr133,Reference Lyons, Reinhard and Planavsky134) . It was only after the initiation of the ‘Great Oxidation Event’ some 2⋅45 billion years ago that considerable amounts of O2 began to appear in the atmosphere(Reference Lyons, Reinhard and Planavsky134Reference Gumsley, Chamberlain and Bleeker136), the present level of 21 % being essentially stable over the past 600 million years(Reference Lyons, Reinhard and Planavsky134). The availability of O2 in abundance in the atmosphere was critical to the evolution of life as we know it.

Conclusions

An odyssey with the adipose tissues that began for me over 40 years ago has provided much by way of riches and changed perspectives. The unique bioenergetic properties of BAT mitochondria, through the presence of the cell-specific UCP1, were initially thought to generate heat only in relation to temperature regulation. Subsequently, the link to energy balance was established and the tissue has provided a theoretical target for the treatment of obesity. BAT is also implicated in metabolic regulation more broadly than was originally envisaged, through roles in glucose homoeostasis and TAG clearance(Reference Moonen, Nascimento and van Marken Lichtenbelt55,Reference Nedergaard and Cannon137Reference Stanford, Middelbeek and Townsend140) . Whether it is a realistic target for the treatment of obesity and the metabolic syndrome, as many propose(Reference Moonen, Nascimento and van Marken Lichtenbelt55,Reference Nedergaard and Cannon137Reference Stanford, Middelbeek and Townsend140) , remains a matter of continuing debate; my own view, as noted recently, is that there are formidable barriers to this concept(Reference Trayhurn141).

Perspectives on the physiological role of WAT have changed radically since the discovery of leptin. An organ that appeared confined to fuel storage, a view reinforced by the histological structure with a single lipid droplet taking up most of the volume of mature white adipocytes, has emerged as having major endocrine and signalling functions. For specific adipose tissue depots there is good evidence of local impact in relation to the organs and tissues with which they abut(Reference Zwick, Guerrero-Juarez and Horsley142); examples are the epicardial fat, postulated to play a role in CVD(Reference Ouwens, Sell and Greulich143,Reference Cherian, Lopaschuk and Carvalho144) , and dermal adipose tissue which is implicated in hair cycling and wound healing(Reference Kruglikov, Zhang and Scherer145,Reference Zhang, Shao and Hepler146) . A specific role in relation to cancer and tumour microenvironment is also evident for some depots(Reference Catalán, Gómez-Ambrosi and Rodríguez147,Reference Chkourko Gusky, Diedrich and MacDougald148) .

My research has centred throughout on what are traditionally considered to be BAT and WAT, both of which are defined by their respective signature cells. However, a third type of adipocyte is now recognised, namely the beige or brite cell(Reference Petrovic, Walden and Shabalina149,Reference Wu, Boström and Sparks Lauren150) . Beige adipocytes have some, though not all, the characteristics of brown fat cells, and in particular are thermogenic through the presence of UCP1. Beige adipocytes are found predominantly within what are regarded as WAT depots and a number of factors lead to their recruitment, particularly cold exposure and β-adrenergic stimulation(Reference Nedergaard and Cannon151,Reference Carobbio, Guénantin and Samuelson152) . The complexity and diversity of fat cells may be even greater with a recent study reporting four distinct human adipocyte subtypes(Reference Min, Desai and Yang153).

Although work on hypoxia has focused on WAT, with substantial changes in gene expression and function being demonstrated in white adipocytes, a deficiency in O2 availability can also occur with BAT. BAT has an exceptionally high O2 demand in order to fuel thermogenesis and hypoxia has been noted in the tissue of normal mice exposed to cold(Reference Xue, Petrovic and Cao154). Hypoxia is not evident, however, in mice acclimated to a warm environment (30°C), and studies on Ucp1 knockout animals indicate that it occurs only with thermogenesis(Reference Xue, Petrovic and Cao154). Obese mice exhibit vascular rarefaction and a substantial reduction in pO2 in BAT compared with lean mice, leading to a ‘whitening’ of the tissue together with mitochondrial dysfunction and loss(Reference Shimizu, Aprahamian and Kikuchi155).

From the effects on hypoxia on white adipocytes, it was stressed earlier that as cells are customarily incubated under hyperoxic conditions (20 % O2) we may have obtained a somewhat distorted view of cellular processes. This may be true for many types of cells, including brown adipocytes. Finally, one of the implications with the response of white adipocytes to graded levels of O2 is that cells carefully titrate small changes in the concentration of this critical nutrient and this results, as with other nutrients, in the continuous modulation of cellular function.

Acknowledgements

I am most grateful for the contributions of the many people with whom I have worked whilst based in Oxford, Strasbourg, Cambridge, Edmonton, Aberdeen, Oslo, Liverpool and Buckingham; regretfully, they are too many to be listed individually. I would, however, particularly like to acknowledge three scientists who in their different ways were pivotal in my early scientific development: Dr Ruth van Heyningen, my DPhil supervisor at Oxford (who died recently, just before her 102nd birthday) who inculcated high standards and gave me considerable scientific freedom; Professor Philip James who appointed me to the nascent Energy Group at the Dunn, despite my lack of training in nutrition, and who encouraged a sense that everything is possible; and Professor David Fraser, whose scholarly commitment and integrity remain a beacon. Finally, I wish to acknowledge the unfailing support of my wife of 50 years, Deborah, to whom I simply say ‘thank you for everything’.

Financial Support

Funding of my studies over the past 45 years has come from a number of sources. I wish to highlight the following: the Medical Research Council, the Alberta Heritage Foundation for Medical Research, the Scottish Office, the Biotechnology and Biological Sciences Research Council, the European Union, the Throne Holst Foundation and King Saud University.

Conflict of Interest

None.

Authorship

The author has sole responsibility for all aspects of the preparation of this article.

References

Prentice, AM & Jebb, SA (1995) Obesity in Britain: gluttony or sloth? Br Med J 311, 437439.CrossRefGoogle ScholarPubMed
Kopelman, PG (2000) Obesity as a medical problem. Nature 404, 635643.CrossRefGoogle ScholarPubMed
Blüher, M (2009) Adipose tissue dysfunction in obesity. Exp Clin Endocrinol Diabetes 117, 241250.CrossRefGoogle ScholarPubMed
Trayhurn, P (1984) The development of obesity in animals: the role of genetic susceptibility. Clin Endocrinol Metab 13, 451474.CrossRefGoogle ScholarPubMed
Krogh, A (1929) The progress of physiology. Science 70, 200204.CrossRefGoogle ScholarPubMed
Trayhurn, P (2018) A basic scientist's odyssey in nutrition. Eur J Clin Nutr 72, 923928.CrossRefGoogle ScholarPubMed
Thurlby, PL & Trayhurn, P (1979) The role of thermoregulatory thermogenesis in the development of obesity in genetically obese (ob/ob) mice pair-fed with lean siblings. Br J Nutr 42, 377385.CrossRefGoogle ScholarPubMed
Romsos, DR (1981) Efficiency of energy retention in genetically obese animals and in dietary-induced thermogenesis. Fed Proc 40, 25242529.Google ScholarPubMed
Miller, DS & Mumford, P (1967) Gluttony (1): an experimental study of overeating low- or high-protein diets. Am J Clin Nutr 20, 12121222.CrossRefGoogle ScholarPubMed
Miller, DS, Mumford, P & Stock, MJ (1967) Gluttony. 2. Thermogenesis in overeating man. Am J Clin Nutr 20, 12231229.CrossRefGoogle ScholarPubMed
Davis, T & Mayer, J (1954) Imperfect homeothermia in the hereditary obese-hyperglycemic syndrome of mice. Harvard School Publ Health 177, 222226.Google ScholarPubMed
Trayhurn, P & James, WP (1978) Thermoregulation and non-shivering thermogenesis in the genetically obese (ob/Ob) mouse. Pflügers Arch Eur J Physiol 373, 189193.CrossRefGoogle ScholarPubMed
Stirling, JL & Stock, MJ (1968) Metabolic origins of thermogenesis induced by diet. Nature 220, 801802.CrossRefGoogle Scholar
Newsholme, EA & Crabtree, B (1976) Substrate cycles in metabolic regulation and in heat generation. Biochem Soc Symp 41, 61109.Google Scholar
Miller, BG, Grimble, RF & Taylor, TG (1977) Liver protein metabolism response to cold in genetically obese (ob/ob) mice. Nature 266, 184186.CrossRefGoogle ScholarPubMed
Newsholme, EA (1978) Substrate cycles: their metabolic, energetic and thermic consequences in man. Biochem Soc Symp 43, 183205.Google Scholar
York, DA, Bray, GA & Yukimura, Y (1978) An enzymatic defect in the obese (ob/ob) mouse; loss of thyroid-induced sodium – and potassium-dependent adenosinetriphosphatase. Proc Natl Acad Sci USA 75, 477481.CrossRefGoogle ScholarPubMed
Lin, MH, Romsos, DR, Akera, T et al. (1978) Na+, K+-ATPase enzyme units in skeletal muscle from lean and obese mice. Biochem Biophys Res Commun 80, 398404.CrossRefGoogle ScholarPubMed
Trayhurn, P, Goodbody, AE & James, WPT (1982) A role for brown adipose tissue in the genesis of obesity? Studies on experimental animals. Proc Nutr Soc 41, 127131.CrossRefGoogle ScholarPubMed
Trayhurn, P (2017) Origins and early development of the concept that brown adipose tissue thermogenesis is linked to energy balance and obesity. Biochimie 134, 6270.CrossRefGoogle ScholarPubMed
Smith, RE & Horwitz, BA (1969) Brown fat and thermogenesis. Physiol Rev 49, 330425.CrossRefGoogle ScholarPubMed
Nicholls, DG & Locke, RM (1984) Thermogenic mechanisms in brown fat. Physiol Rev 64, 164.CrossRefGoogle ScholarPubMed
Cannon, B & Nedergaard, J (2004) Brown adipose tissue: function and physiological significance. Physiol Rev 84, 277359.CrossRefGoogle ScholarPubMed
Foster, DO & Frydman, ML (1977) Comparison of microspheres and 86Rb+ as tracers of the distribution of cardiac output in rats indicates invalidity of 86Rb+-based measurements. Can J Physiol Pharmacol 56, 97109.CrossRefGoogle Scholar
Foster, DO & Frydman, ML (1978) Nonshivering thermogenesis in the rat. II. Measurements of blood flow with microspheres point to brown adipose tissue as the dominant site of the calorigenesis induced by noradrenaline. Can J Physiol Pharmacol 56, 110122.CrossRefGoogle ScholarPubMed
Foster, DO & Frydman, ML (1979) Tissue distribution of cold-induced thermogenesis in conscious warm- or cold-acclimated rats re-evaluated from changes in tissue blood flow: the dominant role of brown adipose tissue in the replacement of shivering by non-shivering thermogenesis. Can J Physiol Pharmacol 57, 257270.CrossRefGoogle ScholarPubMed
Ricquier, D (1989) Molecular biology of brown adipose tissue. Proc Nutr Soc 48, 183187.CrossRefGoogle ScholarPubMed
Himms-Hagen, J (1991) Neural control of brown adipose tissue thermogenesis, hypertrophy, and atrophy. Front Neuroendocrinol 12, 3893.Google Scholar
Arch, JR (2002) β3-adrenoceptor agonists: potential, pitfalls and progress. Eur J Pharmacol 440, 99107.CrossRefGoogle Scholar
Himms-Hagen, J & Desautels, M (1978) A mitochondrial defect in brown adipose tissue of the obese (ob/ob) mouse: reduced binding of purine nucleotides and a failure to respond to cold by an increase in binding. Biochem Biophys Res Commun 83, 628634.CrossRefGoogle Scholar
Thurlby, PL & Trayhurn, P (1980) Regional blood flow in genetically obese (ob/ob) mice: the importance of brown adipose tissue to the reduced energy expenditure on non-shivering thermogenesis. Pflügers Archiv Eur J Physiol 385, 193201.CrossRefGoogle ScholarPubMed
Rothwell, NJ & Stock, MJ (1979) A role for brown adipose tissue in diet-induced thermogenesis. Nature 281, 3135.CrossRefGoogle ScholarPubMed
Brooks, SL, Rothwell, NJ, Stock, MJ et al. (1980) Increased proton conductance pathway in brown adipose tissue mitochondria of rats exhibiting diet-induced thermogenesis. Nature 286, 274276.CrossRefGoogle ScholarPubMed
Himms-Hagen, J (1989) Brown adipose tissue thermogenesis and obesity. Prog Lipid Res 28, 67115.CrossRefGoogle ScholarPubMed
Trayhurn, P (1986) Brown adipose tissue and energy balance. In Brown Adipose Tissue, pp. 299338 [Trayhurn, P and Nicholls, DG, editors]. London: Edward Arnold.Google ScholarPubMed
Fell, BF, Smith, KA & Campbell, RM (1963) Hypertrophic and hyperplastic changes in the alimentary canal of the lactating rat. J Pathol Bacteriol 85, 179188.CrossRefGoogle ScholarPubMed
Williamson, DH (1980) Integration of metabolism in tissues of the lactating rat. FEBS Lett 117, K93K104.CrossRefGoogle ScholarPubMed
Trayhurn, P, Douglas, JB & McGuckin, MM (1982) Brown adipose tissue thermogenesis is ‘suppressed’ during lactation in mice. Nature 298, 5960.CrossRefGoogle ScholarPubMed
Trayhurn, P & Jennings, G (1987) Functional atrophy of brown adipose tissue in mice: effects of lactation and weaning on mitochondrial GDP binding and uncoupling protein. Biochem J 248, 273276.CrossRefGoogle ScholarPubMed
Krol, E & Speakman, JR (2019) Switching off the furnace: brown adipose tissue and lactation. Mol Aspects Med 68, 1841.CrossRefGoogle ScholarPubMed
Trayhurn, P (1981) Fatty acid synthesis in mouse brown adipose tissue: the influence of environmental temperature on the proportion of whole-body synthesis in brown adipose tissue and the liver. Biochim Biophys Acta 664, 549560.CrossRefGoogle ScholarPubMed
Trayhurn, P, Ashwell, M, Jennings, G et al. (1987) Effect of warm or cold exposure on GDP binding and uncoupling protein in rat brown fat. Am J Physiol Endocrinol Metab 252, E237E243.CrossRefGoogle ScholarPubMed
Bouillaud, F, Combes, GM & Ricquier, D (1983) Mitochondria of adult human brown adipose tissue contain a 32,000-Mr uncoupling protein. Biosci Rep 3, 775780.CrossRefGoogle ScholarPubMed
Lean, MEJ, James, WPT, Jennings, G et al. (1986) Brown adipose tissue uncoupling protein content in human infants, children and adults. Clin Sci 7l, 29l297.Google Scholar
Lean, MEJ (1989) Brown adipose tissue in humans. Proc Nutr Soc 48, 243256.CrossRefGoogle ScholarPubMed
Bouillaud, F, Villarroya, F, Hentz, E et al. (1988) Detection of brown adipose tissue uncoupling protein mRNA in adult humans by a genomic probe. Clin Sci 75, 2127.CrossRefGoogle ScholarPubMed
Ricquier, D, Néchad, M & Mory, G (1982) Ultrastructural and biochemical characterization of human brown adipose tissue in pheochromocytoma. J Clin Endocrinol Metab 54, 803807.Google ScholarPubMed
Lean, MEJ, James, WPT, Jennings, G et al. (1986) Brown adipose tissue in patients with phaeochromocytoma. Int J Obes 10, 219227.Google ScholarPubMed
Cypess, AM, Lehman, S, Williams, G et al. (2009) Identification and importance of brown adipose tissue in adult humans. New Engl J Med 360, 15091517.CrossRefGoogle ScholarPubMed
Virtanen, KA, Lidell, ME, Orava, J et al. (2009) Functional brown adipose tissue in healthy adults. New Engl J Med 360, 15181525.CrossRefGoogle ScholarPubMed
van Marken Lichtenbelt, WD, Vanhommerig, JW, Smulders, NM et al. (2009) Cold-activated brown adipose tissue in healthy men. New Engl J Med 360, 15001508.CrossRefGoogle ScholarPubMed
Orava, J, Nuutila, P, Lidell Martin, E et al. (2011) Different metabolic responses of human brown adipose tissue to activation by cold and insulin. Cell Metab 14, 272279.CrossRefGoogle ScholarPubMed
Ouellet, V, Labbe, SM, Blondin, DP et al. (2012) Brown adipose tissue oxidative metabolism contributes to energy expenditure during acute cold exposure in humans. J Clin Invest 122, 545552.CrossRefGoogle ScholarPubMed
Cypess Aaron, M, Weiner Lauren, S, Roberts-Toler, C et al. (2015) Activation of human brown adipose tissue by a β3-adrenergic receptor agonist. Cell Metab 21, 3338.CrossRefGoogle Scholar
Moonen, MPB, Nascimento, EBM & van Marken Lichtenbelt, WD (2019) Human brown adipose tissue: underestimated target in metabolic disease? Biochim Biophys Acta – Mol Cell Biol Lipids 1864, 104112.CrossRefGoogle ScholarPubMed
Zhang, Y, Proenca, R, Maffei, M et al. (1994) Positional cloning of the mouse obese gene and its human homologue. Nature 372, 425432.CrossRefGoogle ScholarPubMed
Friedman, JM & Halaas, JL (1998) Leptin and the regulation of body weight in mammals. Nature 395, 763770.CrossRefGoogle ScholarPubMed
Friedman, JM (1998) Leptin, leptin receptors, and the control of body weight. Nutr Rev 56, S38S46.CrossRefGoogle ScholarPubMed
Dessolin, S, Schalling, M, Champigny, O et al. (1997) Leptin gene is expressed in rat brown adipose tissue at birth. FASEB J 11, 382387.CrossRefGoogle ScholarPubMed
Hoggard, N, Hunter, L, Duncan, JS et al. (1997) Leptin and leptin receptor mRNA and protein expression in the murine fetus and placenta. Proc Natl Acad Sci USA 94, 1107311078.CrossRefGoogle ScholarPubMed
Trayhurn, P (2014) Hypoxia and adipocyte physiology: implications for adipose tissue dysfunction in obesity. Annu Rev Nutr 34, 207236.CrossRefGoogle ScholarPubMed
Trayhurn, P, Thomas, ME, Duncan, JS et al. (1995) Effects of fasting and refeeding on ob gene expression in white adipose tissue of lean and obese (ob/ob) mice. FEBS Lett 368, 488490.Google Scholar
Hardie, LJ, Rayner, DV, Holmes, S et al. (1996) Circulating leptin levels are modulated by fasting, cold exposure and insulin administration in lean but not Zucker (fa/fa) rats as measured by ELISA. Biochem Biophys Res Commun 223, 660665.CrossRefGoogle Scholar
Trayhurn, P, Duncan, JS & Rayner, DV (1995) Acute cold-induced suppression of ob (obese) gene-expression in white adipose-tissue of mice – mediation by the sympathetic system. Biochem J 311, 729733.CrossRefGoogle Scholar
Trayhurn, P, Duncan, JS, Rayner, DV et al. (1996) Rapid inhibition of ob gene expression and circulating leptin levels in lean mice by the β3-adrenoceptor agonists BRL 35135A and ZD2079. Biochem Biophys Res Commun 228, 605610.CrossRefGoogle Scholar
Mantzoros, CS, Qu, DQ, Frederich, RC et al. (1996) Activation of β3 adrenergic receptors suppresses leptin expression and mediates a leptin-independent inhibition of food intake in mice. Diabetes 45, 909914.CrossRefGoogle ScholarPubMed
Mercer, JG, Hoggard, N, Williams, LM et al. (1996) Localization of leptin receptor mRNA and the long form splice variant (Ob-Rb) in mouse hypothalamus and adjacent brain regions by in situ hybridization. FEBS Lett 387, 113116.CrossRefGoogle Scholar
Frühbeck, G, Gómez-Ambrosi, J, Muruzabal, FJ et al. (2001) The adipocyte: a model for integration of endocrine and metabolic signaling in energy metabolism regulation. Am J Physiol Endocrinol Metab 280, E827E847.CrossRefGoogle Scholar
Trayhurn, P & Beattie, JH (2001) Physiological role of adipose tissue: white adipose tissue as an endocrine and secretory organ. Proc Nutr Soc 60, 329339.CrossRefGoogle ScholarPubMed
Rajala, MW & Scherer, PE (2003) The adipocyte – at the crossroads of energy homeostasis, inflammation, and atherosclerosis. Endocrinology 144, 37653773.CrossRefGoogle ScholarPubMed
Trayhurn, P (2005) Endocrine and signalling role of adipose tissue: new perspectives on fat. Acta Physiol Scand 184, 285293.CrossRefGoogle ScholarPubMed
Cook, KS, Min, HY, Johnson, D et al. (1987) Adipsin: a circulating serine protease homolog secreted by adipose tissue and sciatic nerve. Science 237, 402405.CrossRefGoogle ScholarPubMed
Hotamisligil, GS, Shargill, NS & Spiegelman, BM (1993) Adipose expression of tumor necrosis factor-α – direct role in obesity-linked insulin resistance. Science 259, 8791.CrossRefGoogle ScholarPubMed
Ouchi, N, Kihara, S, Arita, Y et al. (1999) Novel modulator for endothelial adhesion molecules – adipocyte-derived plasma protein adiponectin. Circulation 100, 24732476.CrossRefGoogle ScholarPubMed
Yokota, T, Oritani, K, Takahashi, I et al. (2000) Adiponectin, a new member of the family of soluble defense collagens, negatively regulates the growth of myelomonocytic progenitors and the functions of macrophages. Blood 96, 17231732.CrossRefGoogle ScholarPubMed
Berg, AH, Combs, TP, Du, X et al. (2001) The adipocyte-secreted protein Acrp30 enhances hepatic insulin action. Nat Med 7, 947953.CrossRefGoogle ScholarPubMed
Yamauchi, T, Kamon, J, Waki, H et al. (2001) The fat-derived hormone adiponectin reverses insulin resistance associated with both lipoatrophy and obesity. Nat Med 7, 941946.CrossRefGoogle ScholarPubMed
Brakenhielm, E, Veitonmaki, N, Cao, R et al. (2004) Adiponectin-induced antiangiogenesis and antitumor activity involve caspase-mediated endothelial cell apoptosis. Proc Natl Acad Sci USA 101, 24762481.CrossRefGoogle ScholarPubMed
Peeraully, MR, Jenkins, JR & Trayhurn, P (2004) NGF Gene expression and secretion in white adipose tissue: regulation in 3T3-L1 adipocytes by hormones and inflammatory cytokines. Am J Physiol Endocrinol Metab 287, E331E339.CrossRefGoogle ScholarPubMed
Trayhurn, P, Duncan, JS, Wood, AM et al. (2000) Metallothionein gene expression and secretion by white adipose tissue. Am J Physiol: Regul Integr Comp Physiol 279, R2329R2335.Google ScholarPubMed
Bing, C, Bao, Y, Jenkins, J et al. (2004) Zinc-α2-glycoprotein, a lipid mobilising factor, is expressed in adipocytes and upregulated in mice with cancer cachexia. Proc Natl Acad Sci USA 101, 25002505.CrossRefGoogle Scholar
Bao, Y, Bing, C, Hunter, L et al. (2005) Zinc-α2-glycoprotein, a lipid mobilizing factor, is expressed and secreted by human (SGBS) adipocytes. FEBS Lett 579, 4147.CrossRefGoogle ScholarPubMed
Mracek, T, Stephens, NA, Gao, D et al. (2011) Enhanced ZAG production by subcutaneous adipose tissue is linked to weight loss in gastrointestinal cancer patients. Br J Cancer 104, 441447.CrossRefGoogle ScholarPubMed
Lehr, S, Hartwig, S, Lamers, D et al. (2012) Identification and validation of novel adipokines released from primary human adipocytes. Mol Cell Proteomics 11, [Epublication 26 September 2011].CrossRefGoogle ScholarPubMed
Dahlman, I, Elsen, M, Tennagels, N et al. (2012) Functional annotation of the human fat cell secretome. Arch Physiol Biochem 118, 8491.CrossRefGoogle ScholarPubMed
Deshmukh, AS, Peijs, L, Beaudry, JL et al. (2019) Proteomics-based comparative mapping of the secretomes of human brown and white adipocytes reveals EPDR1 as a novel batokine. Cell Metab 30, 963975.CrossRefGoogle Scholar
Pedersen, B & Febbraio, M (2008) Muscle as an endocrine organ: focus on muscle-derived interleukin-6. Physiol Rev 88, 13791406.CrossRefGoogle ScholarPubMed
Lee, JH & Jun, H-S (2019) Role of myokines in regulating skeletal muscle mass and function. Front Physiol 10, 42. [Epublication 30 January 2019].CrossRefGoogle ScholarPubMed
Stefan, N & Haring, HU (2013) The role of hepatokines in metabolism. Nat Rev Endocrinol 9, 144152.CrossRefGoogle ScholarPubMed
Trayhurn, P & Wood, IS (2004) Adipokines: inflammation and the pleiotropic role of white adipose tissue. Br J Nutr 92, 347355.CrossRefGoogle ScholarPubMed
Trayhurn, P (2013) Hypoxia and adipose tissue function and dysfunction in obesity. Physiol Rev 93, 121.CrossRefGoogle ScholarPubMed
Scherer, PE (2016) The multifaceted roles of adipose tissue – therapeutic targets for diabetes and beyond: the 2015 Banting Lecture. Diabetes 65, 14521461.CrossRefGoogle ScholarPubMed
Arita, Y, Kihara, S, Ouchi, N et al. (1999) Paradoxical decrease of an adipose-specific protein, adiponectin, in obesity. Biochem Biophys Res Commun 257, 7983.CrossRefGoogle ScholarPubMed
Hotta, K, Funahashi, T, Arita, Y et al. (2000) Plasma concentrations of a novel, adipose-specific protein, adiponectin, in type 2 diabetic patients. Arterioscl Thromb Vasc Biol 20, 15951599.CrossRefGoogle ScholarPubMed
Weisberg, SP, McCann, D, Desai, M et al. (2003) Obesity is associated with macrophage accumulation in adipose tissue. J Clin Invest 112, 17961808.CrossRefGoogle ScholarPubMed
Xu, H, Barnes, GT, Yang, Q et al. (2003) Chronic inflammation in fat plays a crucial role in the development of obesity-related insulin resistance. J Clin Invest 112, 18211830.CrossRefGoogle Scholar
Mraz, M & Haluzik, M (2014) The role of adipose tissue immune cells in obesity and low-grade inflammation. J Endocrinol 222, R113R127.CrossRefGoogle ScholarPubMed
Yudkin, JS (2003) Adipose tissue, insulin action and vascular disease: inflammatory signals. Int J Obes 27, Suppl. 3, S25S28.CrossRefGoogle ScholarPubMed
Hotamisligil, GS (2006) Inflammation and metabolic disorders. Nature 444, 860867.CrossRefGoogle ScholarPubMed
Marx, J (2004) How cells endure low oxygen. Science 303, 14541456.CrossRefGoogle ScholarPubMed
Blaak, EE, van Baak, MA, Kemerink, GJ et al. (1995) β-adrenergic stimulation and abdominal subcutaneous fat blood flow in lean, obese, and reduced-obese subjects. Metabolism 44, 183187.CrossRefGoogle ScholarPubMed
Virtanen, KA, Lonnroth, P, Parkkola, R et al. (2002) Glucose uptake and perfusion in subcutaneous and visceral adipose tissue during insulin stimulation in nonobese and obese humans. J Clin Endocrinol Metab 87, 39023910.CrossRefGoogle ScholarPubMed
Kabon, B, Nagele, A, Reddy, D et al. (2004) Obesity decreases perioperative tissue oxygenation. Anesthesiology 100, 274280.CrossRefGoogle ScholarPubMed
Frayn, KN & Karpe, F (2014) Regulation of human subcutaneous adipose tissue blood flow. Int J Obes 38, 10191026.CrossRefGoogle ScholarPubMed
Karpe, F, Fielding, BA, Ilic, V et al. (2002) Impaired postprandial adipose tissue blood flow response is related to aspects of insulin sensitivity. Diabetes 51, 24672473.CrossRefGoogle ScholarPubMed
Goossens, GH, Bizzarri, A, Venteclef, N et al. (2011) Increased adipose tissue oxygen tension in obese compared with lean men is accompanied by insulin resistance, impaired adipose tissue capillarization, and inflammation. Circulation 124, 6776.CrossRefGoogle ScholarPubMed
Skurk, T, Alberti-Huber, C, Herder, C et al. (2007) Relationship between adipocyte size and adipokine expression and secretion. J Clin Endocrinol Metab 92, 10231033.CrossRefGoogle ScholarPubMed
Brahimi-Horn, MC & Pouysségur, J (2007) Oxygen, a source of life and stress. FEBS Lett 581, 35823591.CrossRefGoogle ScholarPubMed
Hosogai, N, Fukuhara, A, Oshima, K et al. (2007) Adipose tissue hypoxia in obesity and its impact on adipocytokine dysregulation. Diabetes 56, 901911.CrossRefGoogle ScholarPubMed
Ye, J, Gao, Z, Yin, J et al. (2007) Hypoxia is a potential risk factor for chronic inflammation and adiponectin reduction in adipose tissue of ob/ob and dietary obese mice. Am J Physiol Endocrinol Metab 293, E1118E1128.CrossRefGoogle Scholar
Rausch, ME, Weisberg, SP, Vardhana, P et al. (2008) Obesity in C57BL/6J mice is characterised by adipose tissue hypoxia and cytotoxic T-cell infiltration. Int J Obes 32, 451463.CrossRefGoogle Scholar
Pasarica, M, Sereda, OR, Redman, LM et al. (2009) Reduced adipose tissue oxygenation in human obesity: evidence for rarefaction, macrophage chemotaxis, and inflammation without an angiogenic response. Diabetes 58, 718725.CrossRefGoogle ScholarPubMed
Hodson, L, Humphreys, SM, Karpe, F et al. (2013) Metabolic signatures of human adipose tissue hypoxia in obesity. Diabetes 62, 14171425.CrossRefGoogle ScholarPubMed
Lempesis, IG, van Meijel, RLJ, Manolopoulos, KN et al. (2020) Oxygenation of adipose tissue: a human perspective. Acta Physiol 228, e13298.CrossRefGoogle ScholarPubMed
Wang, B, Wood, IS & Trayhurn, P (2007) Dysregulation of the expression and secretion of inflammation-related adipokines by hypoxia in human adipocytes. Pflügers Archiv Eur J Physiol 455, 479492.CrossRefGoogle ScholarPubMed
Lolmède, K, Durand de Saint Front, V, Galitzky, J et al. (2003) Effects of hypoxia on the expression of proangiogenic factors in differentiated 3T3-F442A adipocytes. Int J Obes 27, 11871195.CrossRefGoogle ScholarPubMed
Wood, IS, Wang, B, Lorente-Cebrián, S et al. (2007) Hypoxia increases expression of selective facilitative glucose transporters (GLUT) and 2-deoxy-D-glucose uptake in human adipocytes. Biochem Biophys Res Commun 361, 468473.CrossRefGoogle ScholarPubMed
Geiger, G, Leiherer, A, Muendlein, A et al. (2011) Identification of hypoxia-induced genes in human SGBS adipocytes by microarray analysis. PLoS ONE 6, e26465.CrossRefGoogle ScholarPubMed
Mazzatti, D, Lim, F-L, O'Hara, A et al. (2012) A microarray analysis of the hypoxia-induced modulation of gene expression in human adipocytes. Arch Physiol Biochem 118, 112120.CrossRefGoogle ScholarPubMed
Pérez de Heredia, F, Wood, IS et al. (2010) Hypoxia stimulates lactate release and modulates monocarboxylate transporter (MCT1, MCT2, and MCT4) expression in human adipocytes. Pflügers Arch Eur J Physiol 459, 509518.CrossRefGoogle ScholarPubMed
Wang, B, Wood, IS & Trayhurn, P (2008) PCR Arrays identify metallothionein-3 as a highly hypoxia-inducible gene in human adipocytes. Biochem Biophys Res Commun 368, 8893.CrossRefGoogle ScholarPubMed
Penkowa, M, Carrasco, J, Giralt, M et al. (2000) Altered central nervous system cytokine-growth factor expression profiles and angiogenesis in metallothionein-I + II deficient mice. J Cereb Blood Flow Metab 20, 11741189.CrossRefGoogle ScholarPubMed
Zbinden, S, Wang, J, Adenika, R et al. (2010) Metallothionein enhances angiogenesis and arteriogenesis by modulating smooth muscle cell and macrophage function. Arterioscler Thromb Vasc Biol 30, 477482.CrossRefGoogle ScholarPubMed
Regazzetti, C, Peraldi, P, Gremeaux, T et al. (2009) Hypoxia decreases insulin signaling pathways in adipocytes. Diabetes 58, 95103.CrossRefGoogle ScholarPubMed
Yin, J, Gao, Z, He, Q et al. (2009) Role of hypoxia in obesity-induced disorders of glucose and lipid metabolism in adipose tissue. Am J Physiol Endocrinol Metab 296, E333E342.CrossRefGoogle ScholarPubMed
Halberg, N, Khan, T, Trujillo, ME et al. (2009) Hypoxia-inducible factor 1α induces fibrosis and insulin resistance in white adipose tissue. Mol Cell Biol 29, 44674483.CrossRefGoogle ScholarPubMed
Sun, K, Tordjman, J, Clément, K et al. (2013) Fibrosis and adipose tissue dysfunction. Cell Metab 18, 470477.CrossRefGoogle ScholarPubMed
Semenza, GL (2001) HIF-1 and mechanisms of hypoxia sensing. Curr Opin Cell Biol 13, 167171.CrossRefGoogle ScholarPubMed
Coleman, ML & Ratcliffe, PJ (2007) Oxygen sensing and hypoxia-induced responses. Essays Biochem 43, 115.Google ScholarPubMed
Wood, I, Stezhka, T & Trayhurn, P (2011) Modulation of adipokine production, glucose uptake and lactate release in human adipocytes by small changes in oxygen tension. Pflügers Archiv Eur J Physiol 462, 469477.CrossRefGoogle ScholarPubMed
Trayhurn, P (2017) Oxygen – the forgotten nutrient. J Nutr Sci 6, 14, e47.CrossRefGoogle ScholarPubMed
Trayhurn, P (2019) Oxygen – a critical, but overlooked, nutrient. Front Nutr 6, article 10, 19.CrossRefGoogle ScholarPubMed
Kerr, RA (2005) The story of O2. Science 308, 17301732.CrossRefGoogle ScholarPubMed
Lyons, TW, Reinhard, CT & Planavsky, NJ (2014) The rise of oxygen in Earth's early ocean and atmosphere. Nature 506, 307315.CrossRefGoogle ScholarPubMed
Blaustein, R (2016) The great oxidation event. Evolving understandings of how oxygenic life on Earth began. Bioscience 66, 189195.CrossRefGoogle Scholar
Gumsley, AP, Chamberlain, KR, Bleeker, W et al. (2017) Timing and tempo of the great oxidation event. Proc Natl Acad Sci USA 114, 18111816.CrossRefGoogle ScholarPubMed
Nedergaard, J & Cannon, B (2010) The changed metabolic world with human brown adipose tissue: therapeutic visions. Cell Metab 11, 268272.CrossRefGoogle ScholarPubMed
Bartelt, A, Bruns, OT, Reimer, R et al. (2011) Brown adipose tissue activity controls triglyceride clearance. Nat Med 17, 200205.CrossRefGoogle ScholarPubMed
Bartelt, A & Heeren, J (2012) The holy grail of metabolic disease: brown adipose tissue. Curr Opin Lipidol 23, 190195.CrossRefGoogle ScholarPubMed
Stanford, KI, Middelbeek, RJW, Townsend, KL et al. (2013) Brown adipose tissue regulates glucose homeostasis and insulin sensitivity. J Clin Invest 123, 215223.CrossRefGoogle ScholarPubMed
Trayhurn, P (2018) Brown adipose tissue – a therapeutic target in obesity? Front Physiol 9, article 1672, 15.CrossRefGoogle ScholarPubMed
Zwick, RK, Guerrero-Juarez, CF, Horsley, V et al. (2018) Anatomical, physiological, and functional diversity of adipose tissue. Cell Metab 27, 6883.CrossRefGoogle ScholarPubMed
Ouwens, DM, Sell, H, Greulich, S et al. (2010) The role of epicardial and perivascular adipose tissue in the pathophysiology of cardiovascular disease. J Cell Mol Med 14, 22232234.CrossRefGoogle ScholarPubMed
Cherian, S, Lopaschuk, GD & Carvalho, E (2012) Cellular cross-talk between epicardial adipose tissue and myocardium in relation to the pathogenesis of cardiovascular disease. Am J Physiol Endocrinol Metab 303, E937E949.CrossRefGoogle ScholarPubMed
Kruglikov, IL, Zhang, Z & Scherer, PE (2019) The role of immature and mature adipocytes in hair cycling. Trends Endocrinol Metab 30, 93105.CrossRefGoogle ScholarPubMed
Zhang, Z, Shao, M, Hepler, C et al. (2019) Dermal adipose tissue has high plasticity and undergoes reversible dedifferentiation in mice. J Clin Invest 129, 53275342.CrossRefGoogle ScholarPubMed
Catalán, V, Gómez-Ambrosi, J, Rodríguez, A et al. (2013) Adipose tissue immunity and cancer. Front Physiol 4, Article 275, 113.CrossRefGoogle ScholarPubMed
Chkourko Gusky, H, Diedrich, J, MacDougald, OA et al. (2016) Omentum and bone marrow: how adipocyte-rich organs create tumour microenvironments conducive for metastatic progression. Obes Rev 17, 10151029.CrossRefGoogle ScholarPubMed
Petrovic, N, Walden, TB, Shabalina, IG et al. (2010) Chronic peroxisome proliferator-activated receptor γ (PPARγ) activation of epididymally derived white adipocyte cultures reveals a population of thermogenically competent, UCP1-containing adipocytes molecularly distinct from classic brown adipocytes. J Biol Chem 285, 71537164.CrossRefGoogle ScholarPubMed
Wu, J, Boström, P, Sparks Lauren, M et al. (2012) Beige adipocytes are a distinct type of thermogenic fat cell in mouse and human. Cell 150, 366376.CrossRefGoogle ScholarPubMed
Nedergaard, J & Cannon, B (2014) The browning of white adipose tissue: some burning issues. Cell Metab 20, 396407.CrossRefGoogle ScholarPubMed
Carobbio, S, Guénantin, A-C, Samuelson, I et al. (2019) Brown and beige fat: from molecules to physiology and pathophysiology. Biochim Biophys Acta – Mol Cell Biol Lipids 1864, 3750.CrossRefGoogle Scholar
Min, SY, Desai, A, Yang, Z et al. (2019) Diverse repertoire of human adipocyte subtypes develops from transcriptionally distinct mesenchymal progenitor cells. Proc Natl Acad Sci USA 116, 1797017979.CrossRefGoogle ScholarPubMed
Xue, Y, Petrovic, N, Cao, R et al. (2009) Hypoxia-independent angiogenesis in adipose tissues during cold acclimation. Cell Metab 9, 99109.CrossRefGoogle ScholarPubMed
Shimizu, I, Aprahamian, T, Kikuchi, R et al. (2014) Vascular rarefaction mediates whitening of brown fat in obesity. J Clin Invest 124, 20992112.CrossRefGoogle ScholarPubMed
Figure 0

Fig. 1. (Colour online) ‘Engaging’ with nutrition at the Dunn: Friday morning group ‘coffee and cake’. Eating and drinking in the laboratory is, of course, prohibited now, but was normal in my early years as a scientist.

Figure 1

Fig. 2. (Colour online) Schematic of different physiological and pathological conditions in experimental animals in which energy flux and/or balance are altered where increases, or decreases, in brown adipose tissue thermogenesis have been demonstrated. Examples of key situations in which brown fat thermogenesis changes are shown. DIT, diet-induced thermogenesis; VMH, ventromedial hypothalamus.

Figure 2

Fig. 3. (Colour online) The thermogenic activity and capacity of brown adipose tissue (BAT) is decreased in lactation (Lact). The changes in mitochondrial GDP binding, the mitochondrial concentration of uncoupling protein-1 (UCP1; UCP1 conc) and the total UCP1 content of the interscapular BAT depot are shown for mice at late lactation (when milk production is close to maximal) relative to virgin mice (virgin = 1)(39).

Figure 3

Fig. 4. (Colour online) The secretome of white adipocytes. Fatty acids and other lipids are secreted, together with a multiplicity of adipokines (proteins); examples of some of the lipids and key adipokines are shown. The major adipocyte hormones, leptin and adiponectin, are highlighted. angptl4, angiopoietin-like protein-4; CETP, cholesteryl ester transfer protein; DPP4, dipeptidyl peptidase-4; IGF, insulin-like growth factor-1; LPL, lipoprotein lipase; MCP-1, monocyte chemoattractant protein-1; MIC-1, macrophage inhibitory cytokine-1; MIF, macrophage migration inhibitory factor; MMP, matrix metalloproteinase; NGF, nerve growth factor; PAI-1, plasminogen activator inhibitor-1; RBP4, retinol binding protein-4; TGFβ, transforming growth factor-β; VEGF, vascular endothelial growth factor; ZAG, zinc-α2-glycoprotein.

Figure 4

Fig. 5. Schematic representation of the central cellular responses to hypoxia in white adipocytes. The effect of low partial pressure of O2 on gene expression, glucose uptake and utilisation, and the production of selected key adipokines is shown. angptl4, angiopoietin-like protein-4; FA, fatty acid; GLUT1, facilitative glucose transporter 1; HIF-1, hypoxia-inducible factor-1; MCT1, monocarboxylate transporter-1; MIF, macrophage migration inhibitory factor; MMP, matrix metalloproteinase; MT-3, metallothionein-3; PAI-1, plasminogen activator inhibitor-1; TF, transcription factors (other than HIF-1); VEGF, vascular endothelial growth factor. Modified from(131).