Effect of Smad3-mediated transforming growth factor-β1 signaling on satellite cell proliferation and differentiation in chickens

Transforming growth factor-beta 1 (TGF-beta 1) is a potent inhibitor of muscle cell proliferation and differentiation. The TGF-beta 1 signal is carried by Smad proteins into the cell nucleus, resulting in the regulation of the expression of key myogenic regulatory factors including MyoD and myogenin during myogenesis. However, to date, the molecular mechanism of the inhibition by Smad-mediated TGF-beta 1 signaling on the function of the myogenic regulatory factors has not been well understood. The present study was designed to investigate the effect of TGF-beta 1 on satellite cell proliferation and differentiation by a Smad3-dependent signaling pathway. A chicken line, low score normal (LSN) with reduced muscling and upregulated TGF-beta 1 expression, was used and compared with a normal chicken line. In LSN satellite cell cultures, both MyoD and myogenin expression was significantly decreased compared with the normal cells. Furthermore, in response to exogenous TGF-beta 1, the normal satellite cells had a significant decrease in both MyoD and myogenin expression, which suggests that TGF-beta 1 inhibited MyoD and myogenin expression, resulting in decreased satellite cell proliferation and differentiation. The expression of Smad3 and Smad7, key proteins of the Smad family, was greater in the LSN cultures than that measured in the normal culture. The addition of TGF-beta 1 reduced Smad3 expression, but did not affect the expression of Smad7. The reduction of Smad3 in response to TGF-beta 1 suggests that a negative regulatory feedback is likely involved in LSN satellite cell proliferation and differentiation. The overexpression of Smad3 inhibited both MyoD and myogenin expression in normal and LSN satellite cells. In contrast, the underexpression of Smad3 increased the expression of MyoD and myogenin in the LSN cells. However, in the normal cells, only myogenin expression was increased by Smad3 overexpression, but not MyoD. These data together suggest that LSN satellite cells are more responsive to a Smad3-dependent TGF-beta 1 signaling pathway than normal satellite cells, and a Smad3-independent pathway is also likely involved in the regulation of satellite cell proliferation and differentiation.