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 tissuederived 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/EBP␣, leptin, and aP2, but not C/EBP␤. Overexpression of PPAR␥, but not C/EBP␤, 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 ␤-catenin and formation of the Smad3-␤-catenin-TCF4 complex, together with the altered expression of a number of Wnt/␤-catenin pathway genes in hMSCs. The inhibitory effects of myostatin on adipogenesis were blocked by RNAi silencing of ␤-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 TGF␤/Smad signal to Wnt/␤-catenin/TCF4 pathway, leading to down-regulation of PPAR␥.Whereas the role of myostatin in the regulation of skeletal muscle mass in animals has been widely recognized (1-8), its effects on adipogenesis are poorly understood. Inactivating mutations of the myostatin gene in a number of mammalian species are associated with hypermuscularity and decreased fat mass (9 -13). Similarly, myostatin knockout mice are characterized by a lower fat mass than wild-type controls (14, 15). These in vivo observations have led to speculation that myostatin promotes adipogenesis. However, the data on the effects of myostatin on fat mass and metabolism are conflicting. Transgenic mice that hyperexpress myostatin protein either systemically or in the skeletal muscle have increased fat mass (5), whereas adipose-specific hyperexpression of myostatin leads to reduced fat mass and improved insulin sensitivity (16). Mice bearing tumor cells that hyperexpress myostatin experience loss of lean as well as fat mass (17); it is unclear whether the loss of fat mass is a consequence of myostatin hyperexpression or the tumor-associated cachexia.In vitro studies using various cell lines also have yielded inconsistent results. Some studies have reported that myostatin inhibits adipogenic differentiation of adipocyte precursor cell lines of murine, bovine, and human origins (16,18,19), whereas others have reported promotion of adipogenic differentiation by recombin...

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