THE KINASE INHIBITOR ISO-H7 STIMULATES RAT SATELLITE CELL-DIFFERENTIATION THROUGH A NONPROTEIN KINASE-C PATHWAY BY INCREASING MYOGENIN EXPRESSION LEVEL

We analysed the signaling pathways involved in myogenic differentiatio n of primary cultures of rat satellite cells using substances targetin g the protein kinase C (PKC) and the cAMP protein kinase (PKA) pathway s. We have previously shown that iso-H7, which putatively inhibits bot h PKC and PKA, strongly stimulates satellite cell differentiation, as well as the PKA inhibitor HA1004. In the study reported here, the effe cts of iso-H7 on satellite cell differentation were compared to those observed in the presence of agents which reduce PKC activity. It was s hown that treatments with the highly specific PKC inhibitor GF109203X or with 12-O-tetradecanoylphorbol 13-acetate (TPA) which induced a par tial PKC downregulation, did not significantly alter myogenic differen tiation. Northern blot analyses showed that iso-H7 activated the expre ssion of myogenin but not that of MyoD mRNA. Concurrently, iso-H7 incr eased myosin light-chain mRNA expression. In contrast, TPA had no effe ct on these syntheses. Taken together, these results showed that iso-H 7 did not act intracellularly as a PKC inhibitor but rather as a PKA i nhibitor as previously suggested. Our results are compatible with the hypothesis that a reduction in-PKA activity controls satellite cell my ogenesis through an increased myogenin mRNA expression.