Cyclin D1 expression mediated by phosphatidylinositol 3-kinase through mTOR-p70(S6K)-independent signaling in growth factor-stimulated NIH 3T3 fibroblasts

Phosphatidylinositol (PT) 3-kinase is required for G(1) to S phase cell cyc le progression stimulated by a variety of growth factors and is implicated in the activation of several downstream effectors, including p70(S6K). Howe ver, the molecular mechanisms by which PI 3-kinase is engaged in activation of the cell cycle machinery are not well understood. Here we report that t he expression of a dominant negative (DN) form of either the p110 alpha cat alytic or the p85 regulatory subunit of heterodimeric PI 3-kinase strongly inhibited epidermal growth factor (EGF)-induced upregulation of cyclin D1 p rotein in NIH 3T3(M17) fibroblasts. The PI 3-kinase inhibitors LY29-4002 an d wortmannin completely abrogated increases in both mRNA and protein levels of cyclin D1 and phosphorylation of pRb, inducing G(1) arrest in EGF-stimu lated cells. By contrast, rapamycin, which potently suppressed p70(S6K) act ivity throughout the G(1) phase, had little inhibitory effect, if any, on e ither of these events. PI 3-kinase, but not rapamycin-sensitive pathways, w as also indispensable for upregulation of cyclin D1 mRNA and protein by oth er mitogens in NIH 3T3 (M17) cells and in wild-type NIH 3T3 cells as well. We also found that an enforced expression of wild-type p110 was sufficient to induce cyclin D1 protein expression in growth factor-deprived NIH 3T3 (M 17) cells. The p110 induction of cyclin D1 in quiescent cells was strongly inhibited by coexpression of either of the PI 3-kinase DN forms, and by LY2 94002, but was independent of the Ras-MEK-ERK pathway. Unlike mitogen stimu lation, the p110 induction of cyclin D1 was sensitive to rapamycin. These r esults indicate that the catalytic activity of PI 3-kinase is necessary, an d could also be sufficient, for upregulation of cyclin D1, with mTOR signal ing being differentially required depending upon cellular conditions.