THROMBIN CAUSES A MARKED DELAY IN SKELETAL MYOGENESIS THAT CORRELATESWITH THE DELAYED EXPRESSION OF MYOGENIN AND P21(CIP1 WAF1)/

Thrombin is a multifunctional serine protease whose activity is regula ted in the extravasculature by an extracellular inhibitor, protease ne xin-l. Because protease nexin-l expression has been shown to be regula ted during skeletal muscle cell differentiation, we reasoned that thro mbin inactivation may be an important requirement for this development al process, To test this hypothesis, we examined the effects of thromb in on differentiating C2C12 myoblasts. We report here that myogenesis, as scored by myotube formation, is considerably delayed by thrombin, This regulation correlated with delayed expression of myogenin and p21 (CIP1/WAF1), both considered critical components of the skeletal muscl e cell differentiation program, Regulation occurred at the RNA level, indicating that the effect of thrombin is either transcriptional or po st-transcriptional. Further more, we present evidence suggesting that this regulation is mediated by the thrombin receptor. Although thrombi n is mitogenic for certain cell types, we found that delay of myogenes is in C2C12 cells did not involve a mitogenic signal, Taken together, these results imply that inhibition of the serine protease thrombin ma y be required for proper progression through the myogenic differentiat ion program, The data point to potentially important roles that thromb in and protease nexin-l may play during skeletal muscle development.