In this communication, we report the impact of chronic feeding of vitamin A-enriched diet on 11β-HSD1 activity in liver and visceral fat of WNIN/Ob obese rats, a new genetic rat model of obesity. Here, we demonstrate that feeding of vitamin A-enriched diet to WNIN/Ob obese rats decreases body weight, fat mass and 11β-HSD1 activity in liver and visceral fat. To our knowledge, this is the first study to link vitamin A and pre-receptor metabolism of glucocorticoids in normal and obese conditions.
Retinoic acid, one of the metabolically-active forms of vitamin A, regulates cellular gene expression through retinoic acid receptors (RARs) and retinoid × receptors (RXRs). Adipose tissue is a good target organ for vitamin A action, as it stores significant amount of vitamin A and expresses RAR and RXR transcription factors. Retinoic acid is known to inhibit preadipocyte differentiation [18
] and remodel white adipose tissue to brown adipose tissue [19
]. Retinaldehyde, another functional form of vitamin A is reported to decrease adipogenesis and ameliorate diet-induced obesity in rodent models [20
]. In line with our previous observation, high (but not toxic) dose of vitamin A decreased visceral fat mass and bodyweight in obese rats [10
]. Previous studies have reported that cortisol (corticosterone in rodents) is essential for preadipocyte differentiation [21
] and adipose-specific overexpression of 11β-HSD1 in mice, results in increased preadiocyte differentiation [3
]. As reported previously, WNIN/Ob obese rats have exhibited higher 11β-HSD1 activity in visceral fat than their lean counter parts [22
]. In the present study, vitamin A significantly decreased 11β-HSD1 activity in visceral fat, which is associated with significant decrease in visceral fat mass. Vitamin A mediated visceral fat loss in obese rats could be due to decreased preadipocyte differentiation, as reduced 11β-HSD1 activity results in decreased tissue corticosterone levels. In contrast to the observation in obese rats, vitamin A-challenged lean rats had increased 11β-HSD1 activity in visceral fat, however, increased enzyme activity had no impact on visceral fat mass. The differential effects of vitamin A on 11β-HSD1 activity and visceral fat mass in lean and obese rats could be due to other physiological factors that are altered in obese condition.
Vitamin A may regulate the 11β-HSD1 activity, by altering its gene expression through direct and indirect mechanisms. Presently, retinoic acid-response elements are not reported on 11β-HSD1 promoter to support the direct effect of vitamin A on 11β-HSD1 gene expression. C/EBPα has been shown to be obligatory for the expression of 11β-HSD1 [4
]. Previous studies have reported that retinoic acid downregulates the expression of genes like resistin that are regulated by C/EBPα in adipose tissue [13
]. Vitamin A-mediated regulation of 11β-HSD1 gene expression in adipose tissue may be mediated by indirect mechanism through C/EBPα.
11β-HSD1 plays an important role in the regulation of carbohydrate and lipid metabolism in liver. 11β-HSD1-KO mice have improved insulin sensitivity [4
], where as transgenic overexpression of 11β-HSD1 in liver results in the development of impaired glucose tolerance [23
]. WNIN/Ob obese rats have lower hepatic 11β-HSD1 expression and activity as observed in other obese rodent models [22
]. In the present study, vitamin A supplementation at high doses decreased hepatic 11β-HSD1 activity and expression in both lean and obese phenotypes. Supporting to our hypothesis, activity of GPDH, a glucocorticoid inducible enzyme, was also decreased in vitamin A supplemented lean and obese rats. Based on these observations, it is possible that vitamin A may regulate the expression of hepatic glucocorticoid target genes by regulating the expression of 11β-HSD1.
To understand the possible mechanisms involved in the downregulation of 11β-HSD1 in liver, we studied the expression of C/EBPα at mRNA and protein levels. In our study, vitamin A supplementation increased hepatic C/EBPα gene expression in vitamin A-treated lean and obese rats. In contrast to increased mRNA levels, C/EBPα protein levels decreased in vitamin A-supplemented obese rats, suggesting the role of post-transcriptional regulatory mechanisms. Possibly, vitamin A-mediated downregulation of hepatic 11β-HSD1 is mediated through different mechanisms in lean and obese rats, in other words, vitamin A-mediated down regulation of hepatic 11β-HSD1 may be C/EBPα-dependent in obese rats, while it is independent of C/EBPα in lean rats.
Previous studies have reported that LXRα ligands down regulate 11β-HSD1 through indirect mechanisms [7
]. In this study, vitamin A supplementation increased hepatic LXRα mRNA and protein levels in WNIN/Ob lean and obese rats. In support to the elevated LXRα gene expression, ABCA1 (ATP-binding cassette transporter protein 1), a classical target gene of LXRα is elevated in lean and obese rats (unpublished data). Vitamin A-mediated downregulation of hepatic 11β-HSD1 gene expression is may be mediated through LXRα. Another mechanism, through which dietary vitamin A may down regulate 11β-HSD1, is by providing high concentration of RXR ligand, 9-cis-retinoic acid. LXRα forms heterodimer with RXR and this step is essential for LXRα-mediated gene transcription. Higher concentration of RXR ligand may result in the increased recruitment of LXRα:RXR heterodimers to 11β-HSD1 gene promoter resulting in decreased 11β-HSD1 gene expression.
11β-HSD1 gene expression in liver and adipose tissue is also regulated by various hormones and cytokines present in the plasma [24
]. It is possible that Vitamin A-mediated alterations in the levels of these signaling molecules may also affect the 11β-HSD1 activity in liver and adipose tissue of WNIN/Ob lean and obese rats.