Effect of transforming growth factor-beta1 on embryonic and posthatch muscle growth and development in normal and low score normal chicken.

During skeletal muscle development, transforming growth factor-beta1 (TGF-beta1) is a potent inhibitor of muscle cell proliferation and differentiation. The TGF-beta1 signal is carried by Smad proteins into the cell nucleus, inhibiting the expression of key myogenic regulatory factors including MyoD and myogenin. However, the molecular mechanism by which TGF-beta1 inhibits muscle cell proliferation and differentiation has not been well documented in vivo. The present study investigated the effect of TGF-beta1 on in vivo skeletal muscle growth and development. A chicken line, Low Score Normal (LSN) with reduced muscling and upregulated TGF-beta1 expression, was used and compared to a normal chicken line. The injection of TGF-beta1 at embryonic day (ED) 3 significantly reduced the pectoralis major (p. major) muscle weight in the normal birds at 1 wk posthatch, whereas no significant difference was observed in the LSN birds. The difference between normal and LSN birds in response to TGF-beta1 is likely due to different levels of endogenous TGF-beta1 where the LSN birds have increased TGF-beta1 expression in their p. major muscle at both 17 ED and 6 wk posthatch. Smad3 expression was reduced by TGF-beta1 from 10 ED to 1 wk posthatch in normal p. major muscle. Unlike Smad3, Smad7 expression was not significantly affected by TGF-beta1 until posthatch in both normal and LSN p. major muscle. Expression of MyoD was reduced 35% by TGF-beta1 during embryonic development in normal p. major muscle, whereas LSN p. major muscle showed a delayed decrease at 1 d posthatch in MyoD expression in response to the TGF-beta1 treatment. Myogenin expression was reduced 29% by TGF-beta1 after hatch in normal p. major muscle. In LSN p. major muscle, TGF-beta1 treatment significantly decreased myogenin expression by 43% at 1 d posthatch and 32% at 1 wk posthatch. These data suggested that TGF-beta1 reduced p. major muscle growth by inhibiting MyoD and myogenin expression during both embryonic and posthatch development. Furthermore, TGF-beta1 also reduced the expression of the cell adhesion receptor beta1 integrin subunit during embryonic and posthatch muscle growth in normal and LSN chickens. Therefore, the reduction of beta1 integrin in response to TGF-beta1 is also associated with decreased posthatch muscle growth. The results from this study indicate that TGF-beta1 inhibits skeletal muscle growth by regulating MyoD and myogenin expression. These data also suggest that a beta1 integrin-mediated alternative pathway is likely involved in the TGF-beta1-induced reduction of muscle growth.