Effect of transforming growth factor-β1 on embryonic and posthatch muscle growth and development in normal and low score normal chicken

Abstract

During skeletal muscle development, transforming growth factor-β1 (TGF-β1) is a potent inhibitor of muscle cell proliferation and differentiation. The TGF-β1 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-β1 inhibits muscle cell proliferation and differentiation has not been well documented in vivo. The present study investigated the effect of TGF-β1 on in vivo skeletal muscle growth and development. A chicken line, Low Score Normal (LSN) with reduced muscling and upregulated TGF-β1 expression, was used and compared to a normal chicken line. The injection of TGF-β1 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-β1 is likely due to different levels of endogenous TGF-β1 where the LSN birds have increased TGF-β1 expression in their p. major muscle at both 17 ED and 6 wk posthatch. Smad3 expression was reduced by TGF-β1 from 10 ED to 1 wk posthatch in normal p. major muscle. Unlike Smad3, Smad7 expression was not significantly affected by TGF-β1 until posthatch in both normal and LSN p. major muscle. Expression of MyoD was reduced 35% by TGF-β1 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-β1 treatment. Myogenin expression was reduced 29% by TGF-β1 after hatch in normal p. major muscle. In LSN p. major muscle, TGF-β1 treatment significantly decreased myogenin expression by 43% at 1 d posthatch and 32% at 1 wk posthatch. These data suggested that TGF-β1 reduced p. major muscle growth by inhibiting MyoD and myogenin expression during both embryonic and posthatch development. Furthermore, TGF-β1 also reduced the expression of the cell adhesion receptor β1 integrin subunit during embryonic and posthatch muscle growth in normal and LSN chickens. Therefore, the reduction of β1 integrin in response to TGF-β1 is also associated with decreased posthatch muscle growth. The results from this study indicate that TGF-β1 inhibits skeletal muscle growth by regulating MyoD and myogenin expression. These data also suggest that a β1 integrin-mediated alternative pathway is likely involved in the TGF-β1-induced reduction of muscle growth.