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

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 beta (C/EBP beta) expression. C/EBP beta then acts to up-regulate the expression of adipogenic factors such as C/EBP alpha, which control the late stage of adipogenesis. Retinoic acid (RA) is a potent inhibitor of adipogenesis, and its action appears to block C/EBP beta transcriptional potential early during differentiation. Using preadipocytes and mesenchymal stem cell models, we show that RA specifically blocks the occupancy of C/EBP beta of the Cebpa promoter, thereby abrogating the differentiation process. RA does not act directly on C/EBP beta but rather stimulates the expression of the transforming growth factor beta-effector protein Smad3, which can interact with C/EBP beta via its Mad homology 1 domain and can interfere with C/EBP beta 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 beta 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 beta DNA occupancy suggesting that Smad3 is necessary to convey the inhibitory effects of retinoic acid during adipogenesis.