A Phosphatidylinositol 3-Kinase/p70 Ribosomal S6 Protein Kinase Pathway Is Required for the Regulation by Insulin of the p85α Regulatory Subunit of Phosphatidylinositol 3-Kinase Gene Expression in Human Muscle Cells

Insulin acutely up-regulates p85 alpha phosphatidylinositol 3-kinase (p85 alpha PI 3-K) mRNA levels in human skeletal muscle (Laville, M., Auboeuf, D., Khalfallah, Y., Vega, N., Riou, J. P., and Vidal, H. (1996) J. Clin. Invest. 98, 43-49). In the present work, we attempted to elucidate the mechanism of action of insulin in primary cultures of human muscle cells. Insulin (10 super(-7) M, 6 h of incubation) induced a 2-fold increase in p85 alpha PI 3-K mRNA abundances (118 plus or minus 12 versus 233 plus or minus 35 amol/ mu g total RNA, n = 5, p < 0.01) without changing the expression levels of insulin receptor, IRS-1, glycogen synthase, and Glut 4 mRNAs in differentiated myotubes from healthy subjects. The effect is most probably due to a transcriptional activation of the p85 alpha PI 3-K gene because the half-life of the mRNA was not affected by insulin treatment (4.0 plus or minus 0.8 versus 3.1 plus or minus 0.4 h). PD98059 (50 mu M) did not modify the insulin response but increased p85 alpha PI 3-K mRNA levels in the absence of insulin, suggesting that the mitogen-activated protein kinase pathway exerts a negative effect on p85 alpha PI 3-K mRNA expression in the absence of the hormone. On the other hand, the insulin effect was totally abolished by LY294002 (10 mu M) and rapamycin (50 nM). In addition, overexpression of a constitutively active protein kinase B increased p85 alpha PI 3-K mRNA levels. These results indicate that the phosphatidylinositol 3-kinase/PKB/p70S6 kinase pathway is required for the stimulation by insulin of p85 alpha PI 3-K gene expression in human muscle cells.