Retinoids and cell differentiation

Cell differentiation is involved in the immune response through both innate and cell-mediated responses. Retinoids are responsible for cell differentiation through gene transcription and are mediated by the activation of the interaction of their nuclear retinoic acid receptor (RAR) and retinoid X receptor (RXR)⁽Reference Giguere, Ong and Segui^30–Reference Mangelsdorf, Thummel and Beato³²⁾. These receptors belong to the nuclear hormone receptors superfamily, like PPARγ⁽Reference Mangelsdorf, Thummel and Beato^32–Reference Robinson-Rechavi, Escriva and Laudet³⁴⁾. The RAR receptors respond to both all-trans-RA and 9-cis-RA, while RXR respond specifically to 9-cis-RA⁽Reference Aranda and Pascual³³⁾.

Both RAR and RXR regulate the expression of hundreds of genes such as skin keratins, retinol-binding protein, leptin and IFN-γ⁽Reference Blomhoff and Blomhoff^35–Reference Ziuzenkova and Plutzky³⁹⁾. The regulation of gene expression by retinoids has been recently reviewed by Amann et al.⁽Reference Amann, Eichmüller and Schmidt⁵⁾. Transcription regulation by the retinoid receptors can be through two different pathways. The first direct pathway involves binding of RAR:RXR or RXR:RXR to specific response elements located in the promoter or enhancer region of a specific target gene⁽Reference Mangelsdorf, Thummel and Beato^32–Reference Robinson-Rechavi, Escriva and Laudet^34, Reference Mongan and Gudas⁴⁰⁾. Also, the transcription of genes can be regulated indirectly by interfering with other transcription factors⁽Reference Amann, Eichmüller and Schmidt^5, Reference Aranda and Pascual³³⁾.

Retinoic acid and immunity

The role of RA on inflammation and immunity has been reviewed recently⁽Reference Kim⁴¹⁾. Briefly, RA production is induced in tissue cells through retinol metabolism when an inflammation process begins. RA induces and regulates the expression of immune cells such as neutrophils, macrophages, dendritic cells and lymphoid cells, which include T-cells, T-regulatory cells and B-cells⁽Reference Kim⁴¹⁾. B-cells are part of the immune regulatory response and are responsible for the production of antibodies. RA is necessary for the development and maturity of B-cells, B-cell proliferation, B-cell differentiation and antibody-secreting plasma cells⁽Reference Kim^41–Reference Allan, Stax and Zheng⁴³⁾. The effect that RA has on cell differentiation involves the RAR and RXR pathways. RA induces cell differentiation by binding to RAR, and thus induces gene transcription of transcription factors, signalling proteins, and interacts with RXR to activate the transcription of primary target genes⁽Reference Gudas and Wagner^44, Reference Du, Tabeta and Mann⁴⁵⁾. The RXR pathway seems to be the one involved in maintaining a Th2-type response⁽Reference Du, Tabeta and Mann^45, Reference Stephensen, Jiang and Freytag⁴⁶⁾.

RA has been shown to inhibit NF-κB activity in human cell lines and also repress the transcription of NF-κB genes such as IL-6, monocyte chemoattractant protein-1 and cyclooxygenase-2, all involved in inflammation processes⁽Reference Austenaa, Carlsen and Hollung^47–Reference Delage, Bairras and Buaud⁵¹⁾. Also, it is now recognised that RA is responsible for the down-regulation of the gene expression of IFN-γ and TNFα, contributing to the Th2 anti-inflammatory response⁽Reference Rocha and Folco^20, Reference Delage, Bairras and Buaud^51, Reference Cantorna, Nashold and Hayes^21, Reference Luo and Ross^29, Reference Garbe, Jochen and Hämmerling⁵²⁾.

RA is also produced at high concentrations in the intestine and is a key factor in mucosal immunity⁽Reference Kim^41, Reference Hammerschmidt, Friedrichsen and Boelter⁵³⁾. It has been shown that RA promotes epithelial integrity to produce mucosal secretions, and induces immunity to protect the intestine after immunisation in animal models⁽Reference Hammerschmidt, Friedrichsen and Boelter^53–Reference Maggini, Wintergerst and Beveridge⁵⁴⁾. In addition, RA is known to modulate transforming growth factor β in the epithelial intestine cells and this contributes to an anti-inflammatory response in the gut⁽Reference Kim^28, Reference Kang, Wang and Matsumoto⁵⁵⁾.

Vitamin A and the adipose tissue

Vitamin A actively participates in the adipocyte metabolism. It is estimated that 15–20% of total retinoids are located in the adipose tissue⁽Reference Zovich, Orolonga and Okuno⁵⁶⁾. The role of vitamin A in adipose tissue development and metabolism has been previously reviewed⁽Reference Villarroya, Giralt and Iglesis^57–Reference Yasmeen, Jeyakumar and Reichert⁵⁹⁾.

During obesity, the adipose tissue undergoes an important expansion and cell differentiation as well as adipocyte maturation. PPARγ is the major regulator of adipogenesis and its expression is mediated through several pathways, one of which involves retinoids. Vitamin A, retinaldehyde and RA are all present in the adipocyte and are actively involved in cell differentiation, as well as transcription of PPARγ⁽Reference Yasmeen, Jeyakumar and Reichert^59, Reference Xue, Shwarz and Chawla⁶⁰⁾. Also, vitamin A is involved in the production of anti-inflammatory cytokines in the adipocyte promoting a Th2-type response⁽Reference Kumar, Sunvold and Scarpace^27, Reference Luo and Ross²⁹⁾.

White adipose tissue (WAT) and brown adipose tissue (BAT) regulate retinoid metabolism, and differentiation between WAT and BAT is associated with the amount of intracellular retinoids⁽Reference Villarroya, Giralt and Iglesis^57, Reference Tsutsumi, Okuno and Tannous⁶¹⁾. Both RAR and RXR are expressed differently in WAT and BAT, and thus, there are different gene expressions depending on the type of adipose tissue⁽Reference Villarroya⁶²⁾. In BAT, for example, RA induces transcription of the uncoupling protein (UCP)-1 gene mediated through RAR and RXR and the response element located at the enhancer; UCP-1 is the protein responsible for the thermogenic function attributed to BAT⁽Reference Alvarez, De Andrés and Yubero^63–Reference Alvarez, Checa and Brun⁶⁷⁾. In vitro, RA increased 1·6 times UCP-1 mRNA expression in the BAT and slightly decreased its expression in WAT adipocytes⁽Reference Lehr, Canola and Léger⁶⁸⁾.

In addition to the UCP-1 gene transcription, RA has been found in vitro to decrease preadipocyte survival time and to inhibit or promote adipose cell differentiation depending on the dose, probably regulated by RA intracellular lipid-binding protein-II⁽Reference Yasmeen, Jeyakumar and Reichert^59, Reference Yasmeen, Jeyakumar and Reichert^69–Reference Berry, Soltanian and Noy⁷²⁾.

In addition to RA, retinaldehyde can also play an important role in gene expression through nuclear receptors, similar to RA⁽Reference Ziouzenkova, Orasanu and Sharlack⁸⁾. Both in vivo and in vitro adipogenesis have been shown to regulate retinaldehyde metabolism. In addition, Ziouzenkova et al. ⁽Reference Ziouzenkova, Orasanu and Sharlack⁸⁾ demonstrated in rodents that retinaldehyde inhibits adipogenesis and suppresses PPAR and RXR response.

Retinoids and leptin

Leptin is an important adipocytokine that actively participates in the regulation of energy homoeostasis and its importance in body weight control has been demonstrated in human subjects and animals⁽Reference Friedman^73–Reference La Cava and Matarese⁷⁵⁾. High leptin concentrations are associated with an increase in adipose tissue observed in obesity⁽Reference Friedman^73–Reference La Cava and Matarese⁷⁵⁾. Also, leptin is induced as part of the acute phase response in response to inflammation, but it is also part of the chronic inflammation response associated with obesity⁽Reference Sarraf, Frederich and Turner^76, Reference Conde, Scotece and Gómez⁷⁷⁾. It is now recognised that leptin regulates both the innate and adaptive immune response; low leptin concentration is associated with a higher risk of infections and high leptin concentration is responsible for increasing pro-inflammatory cytokine levels⁽Reference Conde, Scotece and Gómez^77, Reference Stofkova⁷⁸⁾. Thus, leptin is a key modulator of inflammation and obesity.

Leptin is involved in the activation and infiltration of macrophages into WAT, the activation and development of natural killer cells, induces chemotaxis of neutrophils, suppresses the proliferation of T-regulatory cells and regulates the Th1 and Th2 response⁽Reference La Cava and Matarese^75, Reference Lord, Matarese and Howard^79–Reference Caldefie-Chezet, Poulin and Vasson⁸³⁾. Leptin has been shown to induce a Th1 response with an increased production of IFN-γ and TNFα⁽Reference Zarkesh-Esfahani, Pockley and Wu^82, Reference Lord, Matarese and Howard⁸⁴⁾. In ob/ob mice, decreased secretion of pro-inflammatory cytokines, such as TNFα, IL-6, IFN-γ and IL-1β, was observed, which is reversed when leptin is administered⁽Reference Siegmund, Lehr and Fantuzzi⁸⁵⁾. The interactions between leptin and other pro-inflammatory cytokines is complex, and leptin can help modulate the inflammation response⁽Reference Xiao, Xia-Zhang and Vulliémoz⁸⁶⁾. Therefore, leptin plays an important role in the pathways leading to chronic inflammation.

Of the different active metabolites of vitamin A, RA has been shown to participate in leptin's metabolism in both mice and human tissue⁽Reference Felipe, Mercader and Ribot^87–Reference Menendez, Lage and Peino⁹⁰⁾. Different isomers of RA seem to have different effects on leptin expression and adipogenesis. In vitro, leptin expression is inhibited by both 9-cis- and all-trans-RA ⁽Reference Hong, Ahn and Jung^89, Reference Menendez, Lage and Peino⁹⁰⁾. However, Menendez et al.⁽Reference Menendez, Lage and Peino⁹⁰⁾ found that all-trans-RA has a more potent inhibitor of leptin secretion compared with 9-cis-RA. In addition, all-trans-RA increases adipogenesis in a dose-dependent way through the activation of RXR and PPARγ, while 9-cis-RA has no effect⁽Reference Hong, Ahn and Jung⁸⁹⁾.

Vitamin A deficiency

Vitamin A deficiency is a major public health concern worldwide. Clinical signs of vitamin A deficiency include xerophthalmia, and also an increase of infectious diseases⁽Reference Cunningham-Rundles, McNeeley and Moon^1, Reference Stephensen^3, Reference Sommer, Katz and Tarwotjo^91, Reference Barreto, Santos and Assis⁹²⁾. According to the World Health Organisation, the global prevalence of vitamin A deficiency, defined as serum retinol concentrations <0·7 μmol/l (<20 μg/dl), is 33·3 (95% CI 31·1, 35·4)% for preschool children and 15·3 (95% CI 7·4, 23·2)% for pregnant women⁽⁹³⁾. These numbers represent 190 million children and 19·1 million pregnant women worldwide.

The clinical manifestations of vitamin A deficiency are due to its effect on the immune system, mainly lymphopenia, impairment of the mucosal barriers and a decrease of the T-cell response⁽Reference Stephensen^3, Reference Viroonudomphol, Pongpaew and Tungtrongchitr^19, Reference Maggini, Stephen and Marcos⁹⁴⁾. Since the immune response is compromised, vitamin A deficiency increases the risk of parasites, bacterial and viral infections. In vitamin A deficiency, normal regeneration of the mucosal epithelial barriers is impaired and the resistance to infections is diminished. In animal models, vitamin A deficiency impaired both the humoral and cellular immunity of the intestinal mucosa probably through the modulation of the dendritic cells⁽Reference Yang, Yuan and Tao⁹⁵⁾.

Vitamin A deficiency and T-helper types 1 and 2 response

Vitamin A deficiency impairs both Th1 and Th2 response⁽Reference Stephensen^3, Reference Stephensen, Rasooly and Jiang⁹⁶⁾. However, Th2 immune response is affected the most. Vitamin A deficiency down-regulates the Th2 response and up-regulates Th1 response⁽Reference Cantorna, Nashold and Hayes^21, Reference Long and Nanthakumar^23, Reference Garbe, Jochen and Hämmerling^52, Reference Stephensen, Rasooly and Jiang⁹⁶⁾. In vitamin A deficiency, the antibody-mediated immunity is impaired and there is an increase of pro-inflammatory cytokines that promotes a Th1 type inflammatory response, while the production of anti-inflammatory cytokines, such as IL-4 and IL-10, is reduced⁽Reference Cantorna, Nashold and Hayes^21, Reference Garbe, Jochen and Hämmerling^52, Reference Long and Santos^97, Reference Aukrust, Mueller and Ueland⁹⁸⁾. It has been observed that vitamin A-deficient children had a markedly Th1 response with low concentrations of IL-10 compared with non-deficient children⁽Reference Jason, Archibald and Nwanyanwu²⁵⁾. In Indonesian children, for example, vitamin A deficiency was associated with a Th1 response⁽Reference Wieringa, Dijkhuizen and West⁹⁹⁾. A population of children in Venezuela with a high prevalence of vitamin A subclinical deficiency had lower IL-10 concentrations, an anti-inflammatory cytokine, compared with a group with adequate vitamin A status⁽Reference Leal, Castejón and Romero¹⁰⁰⁾. Thus, even in subclinical vitamin A deficiency, Th2 response is compromised. The elevated Th1:Th2 ratios observed in vitamin A deficiency lead to inflammation and low capacity of the body to fight infections. In animal models, vitamin A deficiency shifts from a Th2 response to a Th1 response, decreasing antibody production and Ig maturation⁽Reference Cantorna, Nashold and Hayes²¹⁾. This increase in the inflammatory response is due to a higher production of IFN-γ by T lymphocytes⁽Reference Wiedermann, Chen and Enerback^101, Reference Cantorna and Hayes¹⁰²⁾.

In addition to its effect on the T-cells response, vitamin A deficiency also diminishes the activity of neutrophils, macrophages and natural killer cells⁽Reference Cunningham-Rundles, McNeeley and Moon^1, Reference Stephensen^3, Reference Dawson, Li and DeCicco¹⁰³⁾. Animal models have shown that chronic marginal vitamin A status decreases number of natural killer cells and lymphocytes, affects both T-cells distribution and function, and thus increases the risk of infections with aging⁽Reference Dawson, Li and DeCicco^103–Reference Nauss, Mark and Suskind¹⁰⁵⁾.

Vitamin A deficiency and the adipose tissue

The increase of the adipose tissue observed in obesity produces an increased secretion of adipocytokines, such as leptin, that, as mentioned earlier, induces an inflammatory response. Vitamin A deficiency is highly prevalent in obesity and this may produce inadequate Th1:Th2 ratio, increased leptin concentrations, and elevate pro-inflammatory cytokines levels⁽Reference Villaça Chaves, Pereira and Saboya^18, Reference Maffei, Funicello and Vottari^106, Reference Harford, Reynolds and McGillicuddy¹⁰⁷⁾. Vitamin A deficiency reduces not only RA levels but also retinaldehyde, and therefore may be contributing to fat deposition.

In a mouse model, vitamin A deficiency increases the expression of UCP-1 mRNA and UCP-2 mRNA expressions, thus promoting reduced thermogenesis in BAT, increased body weight and increased leptin concentrations⁽Reference Bonet, Oliver and Picó¹⁰⁸⁾. In addition, vitamin A deficiency might promote adipogenesis, adipocyte differentiation through PPAR and increased preadipocyte survival time.

Vitamin A deficiency might increase leptin expression and thus, increase the production of cytokines, contributing with the low-grade systemic inflammation observed in obesity. For example, in mice, vitamin A deficiency increased body weight, BAT, adiposity and leptin mRNA expression⁽Reference Bonet, Oliver and Picó¹⁰⁸⁾. Low RA concentrations increase leptin and resistin expressions in brown and white adipocytes⁽Reference Bonet, Oliver and Picó^108, Reference Felipe, Bonet and Ribot¹⁰⁹⁾. Resistin is an adipocytokine that inhibits adipocyte differentiation and an insulin resistance factor in both cell and animal models. In vitro, RA inhibits resistin expression in both brown and white adipocytes through RAR and RXR pathways⁽Reference Felipe, Bonet and Ribot¹⁰⁹⁾.

Thus, vitamin A deficiency in obesity increases Th1 type response, increases the expression of leptin, resistin and UCP, and promotes adipogenesis. Vitamin A deficiency in obesity might be increasing the risk of fat deposition and also the risk of chronic inflammation associated with obesity.

The main activities of vitamin A, the role of vitamin A in the immune function and the effect of vitamin A deficiency on the immune function in obesity are summarised in Table 1.

Table 1. Vitamin A functions, major roles in the immune system and effects of vitamin A deficiency in undernutrition and obesity (adapted from Maggini and Stephen⁽Reference Maggini, Stephen and Marcos⁹⁴⁾ and Maggini and co-workers⁽Reference Maggini, Wintergerst and Beveridge⁵⁴⁾)

Th1, T-helper type 1 response; Th2, T-helper type 2 response; UCP, uncoupling protein; BAT, brown adipose tissue.