p70 S6 kinase is activated by sodium arsenite in adult rat cardiomyocytes: Roles for phosphatidylinositol 3-kinase and p38 MAP kinase

p70 S6 kinase (p70 S6k) is important in regulating a variety of cellular functions including mRNA translation and cell cycle progression and is activated by mitogens and hormones. Unexpectedly, we have found that, in adult rat cardiomyocytes, arsenite, which generally induces stress responses, markedly and rapidly activates p70 S6K. This activation of p70 S6k is completely blocked by rapamycin but only partially prevented by inhibitors of phosphatidylinositol 3-kinase. In trying to delineate the mechanism underlying this effect, we found that arsenite did not activate protein kinase B, JNK or MAP kinase, but did activate p38 MAP kinase in cardiac myocytes. A specific inhibitor of p38 MAP kinase (SB203580) partially attenuated the stimulation of p70 S6k by arsenite. These data indicate that the activation of p70 S6k by arsenite involves p38 MAP kinase and phosphatidylinositol 3-kinase but not PKB.