The contribution of vitamin A to autocrine regulation of fat depots

Abstract

Morbidity and mortality associated with increased white fat accumulation in visceral fat depots have focused attention on the pathways regulating the development of this tissue during embryogenesis, in adulthood, and while under the influence of obesogenic diets. Adipocytes undergo clonal expansion, differentiation (adipogenesis) and maturation through a complex network of transcriptional factors, most of which are expressed at similar levels in visceral and subcutaneous fat. Rigorous research attempts to unfold the pathways regulating expression and activity of adipogenic transcription factors that act in a fat-depot-specific manner. Peroxisome proliferator-activated receptor-γ (PPARγ) is the master regulator of adipogenesis, and is expressed at higher levels in subcutaneous than in visceral depots. PPARγ expression in adipogenesis is mediated by CCAAT/enhancer binding proteins (C/EBPs) and several transcription factors acting in conjunction with C/EBPs, although alternative pathways through zinc-finger protein-423 (ZFP423) transcription factor are sufficient to induce PPARγ expression and adipogenesis. Vitamin A and its metabolites, retinaldehyde and retinoic acid, are transcriptionally-active molecules. Retinoic acid is generated from retinaldehyde in adipose tissue by the aldehyde dehydrogenase-1 family of enzymes (Aldh1). In this review, we discuss the role of Aldh1 enzymes in the generation of retinoic acid during adipogenesis, in the regulation of the transcriptional network of PPARγ in a fat-depot-specific manner, and the important contribution of this autocrine pathway in the development of visceral obesity. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Figure 1. Schematic presentation of selected genes that are expressed in a fat-depot-specific manner

Red box, small font – repressed genes; Green box, large font – induced genes. Group 1. Developmental genes. Specific developmental genes differ in their expression pattern in subcutaneous and visceral fat depots. Among genes involved in organogenesis, homeobox genes HoxA5, HoxA9, homeobox transcription factor Engrailed 1 (En1), and nuclear receptor COUP-TF1/NR2F1 are expressed at higher levels in the subcutaneous fat depot, whereas in visceral fat, these genes are downregulated. Vice-versa, HoxC9 and homeobox transcription factor Engrailed 1 (En1) have nominal expression in subcutaneous fat, while visceral fat shows their higher expression [27,28]. Group 2. Fundamental adipogenic transcription factors. Among genes regulating adipogenesis, subcutaneous fat expresses higher levels of PPARγ, C/EBPα, and CREBP1 compared to visceral fat [29,30] Group 3. Proteins involved in vitamin A metabolism. Retinol binding protein RBP4 is preferentially expressed in the visceral fat depot. The expression of the Aldh1 family of enzymes regulating endogenous RA production is also dissimilar in the two fat depots but it differs between humans and mice (indicated by H and M respectively) [65,25].

Figure 2. Transcriptional network regulating PPARγ expression

Preadipocyte differentiation requires the coordinated action of multiple transcription factors induced by hormones or their synthetic analogs. Pathways that are dispensable in vivo are represented by dashed lines. IBMX activates the cAMP signaling cascade resulting in induction of C/EBPβ expression in vitro. C/EBPδ expression is induced by dexamethasone in vitro and mediated through glucocorticoid receptor (GR). C/EBPβ forms homo/heterodimers with C/EBPδ and binds to the C/EBP regulatory region of C/EBPα and PPARγ, activating both transcription factors. C/EBPα, in a positive feedback loop, can further induce PPARγ. Furthermore, C/EBPβ activates the KLF5 promoter, which in turn activates the PPARγ promoter, in conjunction with C/EBPδ. SREBP1c, a transcription factor acting downstream of C/EBPβ, activates PPARγ upon insulin stimulation, possibly through endogenous PPARγ ligand production. Some evidence suggests that fetal bovine serum (FBS) stimulates the STAT5/GR pathway, which induces PPARγ expression, although its role in vivo is yet to be explored. Early B cell factor (EBF1) regulates C/EBPβ expression. Recently, ZFP423 was identified as a potent inducer of PPARγ2 and adipogenesis.

Figure 3. Schematic mechanisms by which autocrine production of retinoic acid (RA) by the aldehyde dehydrogenase-1 family of enzymes regulates depot-specific fat formation and PPARγ expression

The MRI images of C57/Bl6 (WT) and Aldh1a1^−/− female mice on a regular diet showed marked loss of visceral (V) and, to lesser extent, subcutaneous (S) fat. Aldh1 is a family of three cytosolic enzymes (Aldh1a1, a2, a3) that catalyze retinaldehyde oxidation to RA. Our in vitro studies show that Aldh1a1 is responsible for 70% of endogenous RA production (gene names in italics represent mRNA levels). Subcutaneous fat of WT female mice expresses higher levels of Aldh1 enzymes compared to visceral fat (red arrow lengths show relative expression pattern). In both fat depots, Aldh1a1 is the predominantly expressed enzyme, followed by Aldh1a3. Consequently, disruption of RA production was observed in Aldh1a1^−/− visceral fat depleted of any Aldh1 enzymes, while subcutaneous fat had marginal Aldh1a3 expression. Such drastic changes in Aldh1 expression in the knockouts result in a 70% decrease in PPARγ expression in visceral fat and a 40% decrease in PPARγ expression in subcutaneous fat, as compared to WT, which corresponds to fat accumulation in visceral and subcutaneous depots [25].

Figure 4. Potential RA effects on the transcriptional network participating in PPARγ regulation

RA, acting through RAR and Smad3, blocks C/EBPβ and C/EBPα activity, which subsequently inhibits PPARγ expression in vitro. It is suggestive that RA also regulates other transcription factors, i.e. SREBP1c, STAT5, KLF5 and EBF1 in various tissues. Endogenous RA is produced from retinaldehyde by the cytosolic Aldh1 family of enzymes. Our studies showed that autocrine production of RA controls 70% of ZFP423 expression in visceral fat, which in turn induces PPARγ expression.