Contrary Effects of BMP-2 and ATRA on Adipogenesis in Mouse Mesenchymal Fibroblasts

This study evaluates the effects of bone morphogenetic protein 2 (BMP-2) and all trans retinoic acid (ATRA) on adipogenesis in primary mouse embryo fibroblasts (MEFs). In BMP-2-treated MEFs, lipid accumulation and substantial induction of the adipocyte specific marker 442-aP2 suggested the conversion of MEFs into adipocytes. Such adipogenesis was found to be mediated through sequential induction of C/EBPα, C/EBPβ, and PPARγ. Both the BMP/Smad and BMP/p38 pathways contributed to the adipocyte differentiation. Contrary to the effects of BMP-2, ATRA was demonstrated to inhibit adipocyte differentiation in MEFs. Semi-quantitative RT-PCR analysis revealed that ATRA caused a selective inhibition of both the basal and induction levels of C/EBPα and PPARγ, without altering the expression pattern of C/EBPβ. Taken together, these data suggest the roles of BMP-2 and ATRA in adipogenic differentiation of primary MEFs, and the possible molecular mechanism that involves the regulation of C/EBPα, C/EBPβ, and PPARγ.     

Deregulated MAPK activity prevents adipocyte differentiation of fibroblasts lacking the retinoblastoma protein

A functional retinoblastoma protein (pRB) is required for adipose conversion of preadipocyte cell lines and primary mouse embryo fibroblasts (MEFs) in response to treatment with standard adipogenic inducers. Interestingly, lack of functional pRB in MEFs was recently linked to elevated Ras activity. Ras-dependent signaling plays a significant, although incompletely understood, role in adipocyte differentiation, because activated Ras has been reported to either promote or inhibit adipogenesis depending on the cellular context. In various cell types activation of Ras leads to activation of the mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase 1/2 (ERK1/2), and protein kinase B (PKB)/Akt, which exert opposing effects on adipogenesis, with ERK1/2 inhibiting and PKB/Akt promoting terminal differentiation. Here we report that the levels of activated ERK1/2 and PKB/Akt are significantly increased in pRB-deficient MEFs both before and after the addition of adipogenic inducers. Consistently, we detected higher levels of activated Ras in MEFs lacking pRB. Suppression of ERK1/2 activation by the MEK inhibitor UO126 restored the ability of pRB-deficient MEFs to undergo adipocyte differentiation, as manifested by expression of adipocyte marker genes and lipid accumulation. Furthermore and reflecting the elevated levels of activated PKB/Akt in the pRB-deficient MEFs, differentiation proceeded in an insulin-independent manner. In conclusion, we suggest that pRB plays a pivotal role in adipogenesis by suppressing MAPK activity.     

Role of Wnt10b and C/EBPalpha in spontaneous adipogenesis of 243 cells

This report examines the balance of positive and negative adipogenic factors in a line of immortalized 243 embryonic fibroblasts that undergo spontaneous preadipocyte differentiation. Control of adipogenesis reflects the interplay of factors that promote or inhibit expression of C/EBPalpha and PPARgamma. The 243 cells express C/EBPalpha early and at elevated levels compared to 3T3-F442A preadipocytes or adipocytes. Cell clones were derived from the heterogeneous 243 population for ability or inability to differentiate into adipocytes. Wnt10b, a secreted protein that inhibits adipogenesis, is expressed at high levels in cells with low adipogenic potential and is undetectable in preadipocytes that spontaneously differentiate. In contrast, C/EBPalpha is expressed at reduced levels in cells with low adipogenic potential, and is expressed at high levels in preadipocytes that spontaneously differentiate. These data are consistent with a model in which decreased Wnt10b, coupled with increased C/EBPalpha, results in induction of PPARgamma and spontaneous adipogenesis of 243 cells.