Cancer cachexia

Cachexia is derived from the Greek words ‘kakos’ and ‘hexis’, meaning ‘bad condition’. It is a complex metabolic syndrome that comprises weight loss with reductions in skeletal muscle and adipose tissue mass, anorexia and weakness⁽Reference Morley, Thomas and Wilson¹⁾. It usually occurs in chronic diseases such as cancer, chronic obstructive pulmonary disease, chronic heart failure and end-stage renal failure⁽Reference Morley, Thomas and Wilson¹⁾. Cachexia not only markedly impairs the quality of life but is associated with increased morbidity and mortality.

Most patients with cancer develop cachexia at some point during the course of their disease and approximately half all patients with cancer have weight loss at diagnosis⁽Reference Tisdale²⁾. Clinically, cachexia should be suspected if involuntary weight loss of >5% premorbid weight occurs within a 6-month period⁽Reference Morley, Thomas and Wilson¹⁾. The frequency of weight loss varies with the type of malignancy, being more common and severe in patients with cancers of the gastrointestinal tract, prostate and lung⁽Reference Tisdale³⁾. Cachexia has a detrimental effect on cancer treatment as a result of, for example, poor responses to chemotherapy⁽Reference Slaviero, Read and Clarke⁴⁾. Weight loss is also a prognostic indicator of decreased survival in patients with cancer⁽Reference Fearon, Voss and Hustead^5, Reference Deans, Wigmore and de Beaux⁶⁾. It is considered that ⩽20% of all cancer deaths are directly attributable to cachexia⁽Reference Tisdale^2, Reference Skipworth, Stewart and Dejong⁷⁾.

Evidence is accumulating that cancer cachexia arises from multiple metabolic alterations, such as reduced appetite, increased energy expenditure and tissue breakdown. The main tissues that are affected during the development of cachexia are skeletal muscle and white adipose tissue. The mechanisms of muscle wasting have been the focus of intensive research, with the demonstration of reduced protein synthesis and enhanced proteolysis in experimental models of cachexia and in patients with cancer cachexia⁽Reference Tisdale^3, Reference Argiles, Busquets and Lopez-Soriano⁸⁾. Although profound loss of adipose tissue is a hallmark of cancer cachexia, much less is known of the underlying cellular and molecular mechanisms. A better understanding of fat depletion in malignancy is crucial for the development of effective treatments for the syndrome. The present article reviews studies on the mechanisms and potential mediators of adipose atrophy in cancer cachexia.

Adipose tissue in metabolic health

It is well documented that adipose tissue plays an important metabolic role by storing TAG in periods when energy input exceeds expenditure and releasing NEFA during energy deprivation⁽Reference Spiegelman and Flier⁹⁾. As the largest energy reserve in the body, adipose tissue has a major impact on energy flux, plasma lipid levels and glucose uptake. There is compelling evidence that alterations in adipose tissue mass and metabolism have a major impact on whole-body energy homeostasis⁽Reference Bluher¹⁰⁾. It has been shown that too little fat in lipodystrophy, as with too much fat in obesity, is a major risk for insulin resistance, dyslipidaemia and vascular diseases⁽Reference Rosen and Spiegelman¹¹⁾.

In addition to its primary role as a fuel reservoir, white adipose tissue has been affirmed as a major endocrine organ, since the tissue synthesises and secretes an array of hormones and proteins, termed adipokines⁽Reference Trayhurn and Wood¹²⁾. Adipose tissue has extensive cross talk with other organs including the brain, liver and skeletal muscle through these adipokines. Over the last decade a growing number of adipokines have been identified, such as leptin, adiponectin, TNFα, visfatin and chemerin, which act locally in an autocrine and/or paracrine manner and/or as endocrine signals to modulate appetite, nutrient metabolism, insulin sensitivity, inflammation and adipose tissue development⁽Reference Mohamed-Ali, Pinkney and Coppack^13–Reference Trayhurn and Bing¹⁷⁾.

Adipose atrophy in cachexia

Extensive loss of adipose tissue is a prominent feature of cancer cachexia. Although it is not clear whether there are site differences in fat loss, a study using computed tomography scanning has revealed that patients with cancer cachexia who have gastrointestinal carcinoma have a smaller visceral adipose tissue area than control subjects⁽Reference Ogiwara, Takahashi and Kato¹⁸⁾. In a follow-up study of patients with cancer the analysis of body composition (by dual-energy X-ray absorptiometry) has shown that in progressive cancer cachexia the loss of body fat is more rapid than that of lean mass and occurs preferentially from the trunk followed by leg and arm adipose tissue⁽Reference Fouladiun, Korner and Bosaeus¹⁹⁾. A recent study using retrospective computed tomography scan images of patients with advanced colo-rectal cancer has shown that the most rapid loss of adipose tissues (⩽41%) occurs within 3 months of death⁽Reference Lieffers, Mourtzakis and Hall²⁰⁾. Marked falls of ⩽85% body fat have been observed in patients with lung cancer, which may lead to hyperlipidaemia and insulin resistance as well as complicating anti-tumour therapies⁽Reference Fearon and Preston^21, Reference Fearon²²⁾. With the current escalation in obesity, the paradox of higher BMI as a long-term risk factor but having better survival has been observed in several wasting diseases, including chronic obstructive pulmonary disease, chronic heart failure, end-stage renal failure and cancer⁽Reference Kalantar-Zadeh, Horwich and Oreopoulos²³⁾. A recent study has reported that obesity estimated by an elevated BMI appears to have a protective effect against prostate cancer-specific mortality⁽Reference Halabi, Ou and Vogelzang²⁴⁾.

Fat loss cannot be explained by reduced appetite alone, as it often precedes the onset of anorexia and is much more severe in animal models of cachexia than in food restriction⁽Reference Ishiko, Nishimura and Yasui²⁵⁾. This pattern is also observed at the tissue level, as morphological examination has shown marked changes in adipose tissue plasticity in mice with cancer cachexia compared with ad libitum-fed and pair-fed controls⁽Reference Bing, Russell and Becket²⁶⁾. Adipose tissue from tumour-bearing mice contains shrunken adipocytes of various sizes with a dilated interstitial space. Further morphometric analysis has revealed that the adipocyte size is dramatically reduced. In the tissue matrix increased fibrosis is evident with strong collagen-fibril staining (Fig. 1). Moreover, the ultrastructural features of ‘slimmed’ adipocytes are characterised by severe delipidation and alterations in cell membrane conformation, with irregular cytoplamic projections and increased mitochondria that are electron dense. Taken together, these changes illustrate adipose remodelling in cancer cachexia. A recent study of adipose tissue from patients with cancer has also shown tissue atrophy in subjects with cachexia but not in those without cachexia (C Bing and NA Stephens, unpublished results).

Fig. 1. Light microscopy of Sirius Red-stained sections of epididymal adipose tissue from control (A), pair-fed (B) and MAC16 tumour-bearing (C) mice at day 18 after tumour inoculation. (Adapted from Bing et al. 2006⁽Reference Bing, Russell and Becket²⁶⁾.)

Mechanisms of adipose atrophy

Although the molecular basis of adipose atrophy is poorly understood, evidence suggests that fat loss may arise from an enhanced catabolic response and disrupted anabolic processes. Increased lipolysis appears to be a key factor⁽Reference Zuijdgeest-van Leeuwen, van den Berg and Wattimena^27, Reference Agustsson, Ryden and Hoffstedt²⁸⁾. In patients with cancer cachexia there is an increase in glycerol and fatty acid turnover compared with patients with cancer without cachexia⁽Reference Shaw and Wolfe²⁹⁾. It has also been shown that whole-body lipolysis, measured by the rate of appearance of glycerol, is higher in patients with cancer who are losing weight than in healthy subjects⁽Reference Zuijdgeest-van Leeuwen, van den Berg and Wattimena²⁷⁾. Studies have demonstrated that lipolytic activity (fasting plasma glycerol or fatty acids) is increased in patients with cancer cachexia⁽Reference Agustsson, Ryden and Hoffstedt^28, Reference Drott, Persson and Lundholm³⁰⁾. Increased expression and activity of hormone-sensitive lipase, a rate-limiting enzyme of the lipolytic pathway, is thought to promote lipolysis. Hormone-sensitive lipase mRNA and protein levels have been shown to be increased in adipose tissue of patients with cancer cachexia⁽Reference Agustsson, Ryden and Hoffstedt^28, Reference Thompson, Cooper and Parry³¹⁾. It is therefore proposed that inhibition of hormone-sensitive lipase may prevent or reverse cachexia-associated fat loss. Furthermore, in mature adipocytes isolated from subcutaneous fat of patients with gastrointestinal adenocarcinoma the lipolytic effects of catecholamines and natriuretic peptide are increased by >2-fold in patients with cachexia, although the basal lipolysis is unchanged⁽Reference Agustsson, Ryden and Hoffstedt²⁸⁾. However, gene expression of adipose TAG lipase is not affected in patients with cachexia⁽Reference Agustsson, Ryden and Hoffstedt²⁸⁾. It is also postulated that the fatty acids liberated by lipolysis may serve as substrates for oxidation⁽Reference Laurencikiene, Stenson and Arvidsson Nordstrom³²⁾, which might be mediated by the adipocyte-specific gene cell death-inducing DNA fragmentation factor-α-like effector A. Cell death-inducing DNA fragmentation factor-α-like effector A mRNA levels are increased in patients with cancer cachexia and its overexpression in vitro stimulates adipocyte fatty acid oxidation while decreasing glucose oxidation through inactivation of the pyruvate dehydrogenase complex⁽Reference Laurencikiene, Stenson and Arvidsson Nordstrom³²⁾.

In addition to increased lipolysis, fat loss may be attributable to a decrease in lipid deposition. Circulating insulin, the hormone that promotes fat deposition and glucose transport in adipose tissue, is reduced in the tumour-bearing state⁽Reference Yoshikawa, Noguchi and Satoh^33–Reference Bing, Taylor and Tisdale³⁵⁾. A fall in lipoprotein lipase activity in white fat has been reported in tumour-bearing mice⁽Reference Thompson, Koons and Tan³⁶⁾. This outcome may lead to reduced cleavage of TAG from plasma lipoproteins into glycerol and NEFA for storage, resulting in an increased net flux of lipid into the circulation. There is also evidence that fat diminution in cachexia could be the result of impairment in the formation and development of adipose tissue. A recent study has shown that the expression of the genes encoding several key adipogenic transcription factors, including CCAAT/enhancer-binding protein-α and -β, PPARγ and sterol regulatory element-binding protein-1c, is markedly reduced in white fat of mice with cancer cachexia⁽Reference Bing, Russell and Becket²⁶⁾. mRNA levels of sterol regulatory element-binding protein-1c targets, genes encoding lipogenic enzymes fatty acid synthase, acetyl-CoA carboxylase, stearoyl-CoA desaturase 1 and glycerol-3-phosphate acyl transferase, also fall markedly⁽Reference Bing, Russell and Becket^26, Reference Hale, Price and Sanchez³⁷⁾. Finally, glucose, which serves as a substrate for lipid synthesis, is transported into the adipocyte via the insulin-responsive facilitative glucose transporter GLUT-4. However, there is a decrease in GLUT-4 mRNA in white fat of mice with cancer cachexia⁽Reference Bing, Russell and Becket²⁶⁾, which could be a downstream effect of inhibited CCAAT/enhancer binding protein α since its deficiency is associated with abnormal subcellular localisation of GLUT-4⁽Reference Rosen³⁸⁾.

Potential mediators of adipose atrophy

Several factors produced by tumours and host tissues in the presence of a tumour burden are suggested to be able to mediate fat loss in cachexia. These factors include pro-inflammatory cytokines such as TNFα, IL-1β and IL-6 and the lipid-mobilising factor zinc-α2-glycoprotein (ZAG; also known as AZGP1), each of which can be derived from the tumour and also from the host tissues. Their potential involvement in cachexia-associated fat depletion will be discussed.

Adipose-derived ZAG in cancer cachexia

The secretory function of adipose tissue and the potent fat-mobilising effect of ZAG have led to the postulation that this protein could also be produced by adipose tissue, thereby modulating adipocyte metabolism⁽Reference Bing, Bao and Jenkins⁷⁸⁾. Work from the authors' group has demonstrated that the ZAG gene and protein are expressed by the major white fat depots (epididymal, perirenal, subcutaneous, mammary gland) and the interscapular brown fat of mice⁽Reference Bing, Bao and Jenkins⁷⁸⁾. Further detection using immunocytochemistry has shown the presence of ZAG protein in the cytoplasm of adipocytes in adipose tissue⁽Reference Bing, Bao and Jenkins⁷⁸⁾. In human subjects ZAG mRNA and protein have been shown to be expressed in both visceral and subcutaneous fat depots⁽Reference Bing, Bao and Jenkins⁷⁸⁾. Futhermore, ZAG mRNA and protein are detected in differentiated human Simpson-Golabi-Behmel syndrome adipocytes. Most importantly, ZAG, which contains a secretory signal sequence⁽Reference Hale, Price and Sanchez³⁷⁾, has been shown to be secreted into the culture medium by differentiated Simpson-Golabi-Behmel syndrome adipocytes⁽Reference Bao, Bing and Hunter⁷⁹⁾. Subsequent quantification of ZAG secretion levels in the culture medium has revealed its concentration to be in the range of ng/ml per 24 h, close to that of adiponectin⁽Reference Mracek, Ding and Tzanavari⁸⁰⁾. Taken together, these findings indicate that ZAG is indeed a novel adipokine produced abundantly by adipocytes and the protein may have a major action locally in the regulation of fat mass.

Adipose-derived ZAG appears to be inversely associated with body fat mass. ZAG mRNA and protein levels are markedly increased in adipose tissue of mice with cancer cachexia and, furthermore, the increase in ZAG protein content is related to the extent of weight loss in these animals⁽Reference Bing, Bao and Jenkins⁷⁸⁾. In contrast, as a reference adipokine, leptin mRNA and circulating leptin levels are strikingly repressed in tumour-bearing mice⁽Reference Bing, Russell and Becket^26, Reference Bing, Taylor and Tisdale^35, Reference Bing, Bao and Jenkins⁷⁸⁾ (Fig. 2). Very recently, it has been shown that ZAG mRNA and protein expression are also up regulated in adipose tissue in patients with cancer cachexia (T Mracek, NA Stephens, X Xiao and C Bing, unpublished results). Contrarily, studies of obese subjects have shown that ZAG gene expression is down regulated in subcutaneous adipose tissue of obese women⁽Reference Dahlman, Kaaman and Olsson⁸¹⁾ and men⁽Reference Marrades, Martinez and Moreno-Aliaga⁸²⁾. Furthermore, recent work has demonstrated that ZAG mRNA levels are negatively correlated with total fat mass in human subjects with a wide range of BMI⁽Reference Mracek, Ding and Tzanavari⁸⁰⁾.

Fig. 2. mRNA levels of zinc-α2-glycoprotein (A), leptin (B), CCAAT/enhancer-binding protein α (C) and sterol regulatory element-binding protein-1c (D) in epididymal adipose tissue of control (□), pair-fed (PF; ) and MAC16 tumour-bearing (▪) mice, quantified by real-time PCR and normalised to β-actin. Values presented as fold changes relative to controls are means with their standard errors represented by vertical bars for eight mice per group. Mean values were significantly different from those for the control group: *P<0·05, **P<0·01. Mean values were significantly different from those for the PF group: †P<0·05, ††P<0·01. (Adapted from Bing et al. 2006⁽Reference Bing, Russell and Becket²⁶⁾.)

Although the role of ZAG in adipose tissue remains to be established, its effect on fat loss has been further supported by a recent study that shows that ZAG-knock-out mice are vulnerable to weight gain when fed a high-fat diet and this outcome appears to be the result of decreased lipolytic response to several stimuli, such as isoprenaline, CL316243, foskolin and isobutylmethylxanthine, in adipocytes⁽Reference Rolli, Radosavljevic and Astier⁸³⁾. Recent work has demonstrated that recombinant ZAG stimulates lipolysis in human adipocytes (T Mracek, P Trayhurn and C Bing, unpublished results). Further studies are required to unravel the nature of the action of ZAG in human cancer cachexia. Overall, current data point to ZAG, as well as TNFα, as a potential candidate for mediating lipid catabolism in cancer cachexia (Fig. 3). Other factors may also be involved in fat loss in malignancy, e.g. macrophage inhibitory cytokine-1, which causes cachexia and a reduction in fat mass via its central effects on appetite⁽Reference Johnen, Lin and Kuffner⁸⁴⁾. Interestingly, recent work has found that macrophage inhibitory cytokine-1 is also produced by adipocytes and this factor may have autocrine and/or paracrine effects in adipose tissue⁽Reference Ding, Mracek and Gonzalez-Muniesa⁸⁵⁾.

Fig. 3. A schematic diagram of lipid catabolism in cancer cachexia. Certain tumour and host tissue-produced factors, such as TNFα and zinc-α2-glycoprotein (ZAG), may act systemically and/or as autocrine and/or paracrine signals to stimulate adipocyte lipid metabolism. TNFα-induced lipolysis acts through a TNFα receptor 1 (TNFR-1) dependent pathway, which inhibits perilipin allowing hormone-sensitive lipase (HSL) to access the surface of lipid droplets. ZAG-stimulated lipolysis may be mediated by β3-adrenoceptors (β3AR) and the activation of the intracellular cAMP pathway. NEFA generated by enhanced lipolysis in cachexia may serve as substrates for lipid utilisation through uncoupling proteins (UCP) in brown adipose tissue (BAT) and skeletal muscle.↑, Increased; ↓, decreased.