Signals of oxidant-induced cardiomyocyte hypertrophy: Key activation of p70 S6 kinase-1 and phosphoinositide 3-kinase

Cardiomyocytes in culture can survive low or mild doses of oxidants but later increase cell volume and protein content. To understand the mechanism, we determined the early signaling events of oxidative stress. With 200 μM H₂O₂, the activity of p70 S6 kinase-1 (p70S6K1) increased at 30 min and reached a plateau at 90 min. Dose-response studies at the 60 min time point show that p70S6K1 activity reached its highest level with 150 μM H₂O₂. Increased p70S6K1 activity correlated with phosphorylation of Thr389 and Thr421/Ser424 residues, suggesting the involvement of an upstream kinase. Phosphoinositide 3-kinase (PI3K) activity was elevated by 5 min, reached a plateau at 10 min, and remained more than 6-fold induced for at least 60 min after 200 μM H₂O₂ exposure. The dose-response studies at 10 min found that 150 μM H₂O₂ induced the highest PI3K activity. Increased PI3K activity correlated with tyrosine phosphorylation of the 85-kDa regulatory subunit. Inactivating PI3K with wortmannin prevented H₂O₂ from inducing Thr389 phosphorylation and p70S6K1 activation. Wortmannin and rapamycin prevented H₂O₂ from inducing increases in cell volume and protein content. The antineoplastic drugs doxorubicin and daunorubicin also induced significant enlargement of cardiomyocytes at 10 to 100 nM dose range. Although the glutathione synthesis inhibitor buthionine sulfoximine potentiated the effect of doxorubicin and H₂O₂, the antioxidant N-acetylcysteine prevented induction of cell enlargement. Our data suggest that oxidative stress induces activation of PI3K, which leads to p70S6K1 activation and enlargement of cell size.