Insulin stimulates p70 S6 kinase in the nucleus of cells

Insulin action on both cytoplasmic and nuclear processes is dependent on activation of p70 S6 kinase (p70S6K). In CHO cells expressing human insulin receptors, Western blotting revealed the presence of p70S6K in the cell nucleus at a level of about 32% of that in the cytoplasm. Following insulin treatment, there was a retardation in mobility nuclear p70S6K in SDS-PAGE indicative of a change in phosphorylation of the enzyme, but no change in the amount of enzyme. Stimulation was maximal after 10 min of insulin treatment and decreased gradually at 30 min. There was also a rapid doubling of nuclear p70S6K activity in immunocomplex assays followed by a return to baseline by 30 min. Simultaneously, insulin stimulated cytoplasmic p70S6K by almost 10-fold at 10 min, and activity remained high up to 30 min. Tetradecanoylphorbol acetate (TPA) and fetal calf serum also stimulated nuclear p70S6K as judged by gel mobility shift. TPA also promoted a decrease in cytosolic p70S6K and an increase in nuclear enzyme suggestive of translocation of the enzyme. Rapamycin, a selective inhibitor of p70S6K, and the casein kinase II inhibitor DRB blocked insulin-stimulated nuclear and cytosolic p70S6K. Thus, nuclear p70S6K is regulated by insulin, serum and TPA. The insulin effect is downstream of rapamycin and DRB-sensitive targets and occurs without translocation of the enzyme.