Previous studies from this laboratory have shown that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (HCRI) causes apoptotic cell death of a muscle cell-derived cell line, L6 myoblasts, by involving the phosphatidylinositol-3 (PI-3) kinase pathway and tyrosine phosphorylation of several cellular proteins, although the relationship between PI-3 kinase pathway and tyrosine phosphorylation responses remained to be elucidated. Here, we show that HCRI induces tyrosine phosphorylation of catalytic subunit p110 of PI-3 kinase as early as 5 min after addition of HCRI into culture medium. We could not detect the tyrosine phosphorylation of the regulatory subunit p85 of PI-3 kinase under the present experimental conditions. Concomitantly, the kinase activity toward PI in p110 immunoprecipitates was decreased with a similar time course. Furthermore, both herbimycin A and genistein, potent inhibitors of tyrosine kinase activity, inhibited HCRI-induced inhibition of PI-3 kinase activity as well as HCRI-induced apoptotic cell death. Once the catalytic subunit p110 becomes tyrosine-phosphorylated, the regulatory subunit p85 appears to be dissociated from the catalytic subunit, because we observed a decreasing amount of p85 regulatory subunits in p110 immunoprecipitates in response to HCRI treatment. These results strongly suggest the novel function of tyrosine phosphorylation of catalytic subunit p110 of PI-3 kinase in the regulation of its kinase activity. The tyrosine phosphorylation of these catalytic subunits may play an important role in the intracellular signal transduction of apoptotic cell death. To our knowledge, this is the first report that tyrosine phosphorylation of p110 catalytic subunit acts as a negative regulator of its kinase activity.
