INHIBITION OF MYOGENIC DIFFERENTIATION IN MYOBLASTS EXPRESSING A TRUNCATED TYPE-II TGF-BETA RECEPTOR

Transforming growth factor-beta (TGF-beta) is thought to play a role i n mesenchymal cell development and, specifically, in muscle differenti ation, yet its precise role in the latter process remains unclear. TGF -beta has been shown to both inhibit and induce myoblast maturation in vitro, depending on the culture conditions. Whether the type I or typ e II TGF-beta receptor mediates the various TGF-beta effects on myogen esis is not known. In the present study, C2C12 myoblasts were transfec ted with an expression vector for a truncated type II TGF-beta recepto r, which has been shown to act as a dominant negative inhibitor of typ e II receptor signaling. In contrast to the parental cells, the transf ected clones did not efficiently form myotubes or induce expression of MyoD, myogenin and several other differentiation markers following in cubation in low serum media. However, some muscle differentiation mark ers continued to be expressed in the transfected cells suggesting that at least two pathways are involved in muscle cell differentiation. Th ese cells could still growth arrest in low serum media, showing that d ecreased proliferation can be dissociated from differentiation. Unlike several oncogenes known to block myogenic differentiation, expression of the truncated TGF-beta receptor did not result in myoblast transfo rmation. Injection of the parental or the transfected C2C12 cells into the limb muscle of nude mice revealed quantitative and qualitative di fferences in their behavior, and suggested that myoblasts expressing t he truncated TGF-beta receptor cannot fuse in vivo. Finally, retroviru s-mediated expression of MyoD in the transfected cells restored their ability to form myotubes in vitro, indicating that inhibition of myobl ast differentiation by the truncated TGF-beta receptor may depend on d ecreased MyoD expression. We propose that TGP-beta signaling through t he type II receptor is required for several distinct aspects of myogen ic differentiation and that TGF-beta acts as a competence factor in th is multistep process.