The effects of myostatin on adipogenic differentiation are poorly understood, and the underlying mechanisms are unknown. We determined the effects of human recombinant myostatin protein on adipogenesis of bone marrow-derived human mesenchymal stem cells (hMSCs) and adipose tissue-derived preadipocytes. For both progenitor cell types, differentiation in the presence of myostatin caused a dose-dependent reduction of lipid accumulation and diminished incorporation of exogenous fatty acid into cellular lipids. Myostatin significantly down-regulated the expression of adipocyte markers PPAR¿, C/EBPa, leptin, and aP2, but not C/EBPß. Overexpression of PPAR-gamma, but not C/EBPß, blocked the inhibitory effects of myostatin on adipogenesis. Myostatin induced phosphorylation of Smad3 in hMSCs; knockdown of Smad3 by RNAi or inhibition of its upstream kinase by an Alk5 inhibitor blocked the inhibitory effect of myostatin on adipogenesis in hMSCs, implying an important role of Smad3 activation in this event. Furthermore, myostatin enhanced nuclear translocation of ß-catenin and formation of the Smad3-ß-catenin-TCF4 complex, together with the altered expression of a number of Wnt/ß-catenin pathway genes in hMSCs. The inhibitory effects of myostatin on adipogenesis were blocked by RNAi silencing of ß-catenin and diminished by overexpression of dominant-negative TCF4. The conclusion is that myostatin inhibited adipogenesis in human bone marrow-derived mesenchymal stem cells and preadipocytes. These effects were mediated, in part, by activation of Smad3 and cross-communication of the TGFß/Smad signal to Wnt/ß-catenin/TCF4 pathway, leading to down-regulation of PPAR¿.

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