The Farnesoid X Receptor Promotes Adipocyte Differentiation and Regulates Adipose Cell Function in Vivo
The differentiation of a preadipocyte into a mature adipocyte is a highly regulated process that requires a scripted program of transcriptional events leading to changes in gene expression. Several genes are associated with adipogenesis, including the CAAT/enhancer-binding protein (C/EBPs) and peroxisome proliferator-activated receptor (PPAR) families of transcription factors. In this study, we have investigated the role of the farnesoid X receptor (FXR), a bile acid-activated nuclear receptor, in regulating adipogenesis in a preadipocyte cell line (3T3-L1 cells). Our results show that FXR is expressed in the white adipose tissue of adult mice and in differentiated 3T3-L1 cells but not in undifferentiated preadipocytes. Exposure of 3T3-L1 cells to INT-747 (6-ethyl cheno-deoxycholic acid), a potent and selective FXR ligand, increases preadipocyte differentiation induced by a differentiating mixture containing insulin. Augmentation of differentiating mixture-induced differentiation of 3T3-L1 cells by INT-747 associated with induction of aP2, C/EBPalpha, and PPARgamma2 mRNAs along with other adipocyte-related genes. This effect was reversed by guggulsterone, an FXR antagonist, and partially reverted by GW9662 (2-chloro-5-nitro-N-phenylbenzamide), a selective PPARgamma antagonist, indicating that FXR modulates adipocyte-related genes by PPARgamma-dependent and -independent pathways. Regulation of adipocyte-related genes by INT-747 was lost in FXR-/- mice, indicating that modulation of these genes by INT-747 requires an intact FXR. In addition, INT-747 enhances both insulin-induced serine phosphorylation of Akt and glucose uptake by 3T3-L1 cells. Taken together, these results suggest that activation of FXR plays a critical role in regulating adipogenesis and insulin signaling.