PROTEIN PHOSPHATASE-1 ISOFORMS IN DIFFERENTIATING C2C12 MYOCYTES

In muscle protein phosphatase 1 (PP1) is involved in growth factor sig nal transduction and metabolic regulations. Three isoforms of the cata lytic subunit are found in mammalian cells (PP1 alpha, PP1 gamma 1 and PP1 delta), with potentially different functions. We investigated the changes in the PP1 isoforms in differentiating C2C12 myoblasts. Few h ours after differentiation induction the soluble PP1 activity was reve rsibly increased, displaying a peak at 6h. This was due to activation mainly of PP1 alpha, with no change in the immunodetected protein. A f urther indication of PP1 alpha involvement came from the observation t hat electroporation of inactive PP1 alpha into myoblasts induced a dif ferentiation delay of at least 24 h. Subsequently, starting from 9-12 h, the activities and protein levels of all the three soluble PP1 isof orms decreased, reaching a minimum around 48 h. By this time the cells had undergone morphological changes and myosin became immunodetectabl e. We conclude that PP1 may be involved in myoblast differentiation, b ased on: 1) its higher activity in myoblasts than in myocytes, 2) the reversible activation of soluble PP1 alpha during the first 6h from di fferentiation induction, 3) the delay in differentiation onset followi ng electroporation of inactive PP1 alpha into myoblasts.