Transcription Factor Smad3 Is Required for the Inhibition of Adipogenesis by Retinoic Acid

Abstract

The process of adipocyte differentiation is driven by a highly coordinated cascade of transcriptional events that results in the development of the mature adipocyte and in lipid accumulation. One of the early events of differentiation is the up-regulation of CCAAT/enhancer-binding protein ␤ (C/EBP␤) expression. C/EBP␤ then acts to up-regulate the expression of adipogenic factors such as C/EBP␣, which control the late stage of adipogenesis. Retinoic acid (RA) is a potent inhibitor of adipogenesis, and its action appears to block C/EBP␤ transcriptional potential early during differentiation. Using preadipocytes and mesenchymal stem cell models, we show that RA specifically blocks the occupancy of C/EBP␤ of the Cebpa promoter, thereby abrogating the differentiation process. RA does not act directly on C/EBP␤ but rather stimulates the expression of the transforming growth factor ␤-effector protein Smad3, which can interact with C/EBP␤ via its Mad homology 1 domain and can interfere with C/EBP␤ DNA binding. The RA-induced increase in Smad3 expression results in increased cytoplasmic and nuclear Smad3, an important event as ectopic expression of Smad3 in preadipocytes in the absence of RA treatment only modestly inhibits adipogenesis and C/EBP␤ DNA binding, suggesting that Smad3 alone is not sufficient to completely recapitulate the effects of retinoic acid treatment during differentiation. However, in the absence of Smad3, RA is not able to inhibit adipocyte differentiation or to elicit a decrease in C/EBP␤ DNA occupancy suggesting that Smad3 is necessary to convey the inhibitory effects of retinoic acid during adipogenesis.

β-Cryptoxanthin Suppresses the Adipogenesis of 3T3-L1 Cells via RAR Activation

“…Other reports have demonstrated that Smad3 (which binds to c/EBP␤ and inhibits its transcriptional activation) is upregulated by atRA (22), whilst the down-regulation of PPAR␥, a key regulator of adipocyte differentiation, following atRA treatment has been reported (19). Taken together, these studies indicate that RAR, when activated by ␤-cryptoxanthin, most likely binds to the c/EBP responsive element in the PPAR␥ promoter or inhibits PPAR␥ transcription by an indirect mechanism.…”

Noggin Is Novel Inducer of Mesenchymal Stem Cell Adipogenesis

“…Overexpression of Smad6, a natural antagonist of Smad1, blocks Smad1 activation and thus inhibits adipogenesis (35). Smad3, which is also a receptor-regulated Smad, inhibits adipogenic differentiation of MSC (36,37). Besides roles played by Smad proteins in adipogenesis, activation of p38 kinase also leads to adipocytes formation (33,34).…”

Adipogenesis at a glance

“…These pathways and regulators include glutathione (Vigilanza et al, 2010), the Janus kinase-signal transducer and activator of transcription 3 (JAK-STAT3) pathway (Zhang et al, 2011), SMAD signalling (Marchildon et al, 2010;Tan et al, 2011), ribosomal protein S6 kinase 1 (S6K1) (Carnevalli et al, 2010) and components of the insulin signalling cascade, such as AKT ) and a newly discovered regulator of this pathway, inositol pyrophosphate (Chakraborty et al, 2010). New transcriptional regulators of stem cell fate that are controlled by these pathways also continue to be identified.…”