Investigation on Molecular Link Between the Phosphatidylinositol 3-Kinase Signaling Pathway and the Cell Cycle Machinery

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Phosphatidylinositol (PI) 3-kinase is required for G1 to S phase cell cycle progression stimulated by a variety of growth factors, and is implicated as a regulator for activation of several downstream targets, including p70^<S6K>. However, molecular mechanisms by which PI 3-kinase is engaged in the activation of the cell cycle machinery is not fully understood. Here we report that a transient expression of wild type p110α catalytic subunit of PI 3-kinase was capable of inducing cyclin D1 protein in quiescent NIH3T3 (M17) fibroblasts. This effect of p110 was strongly attenuated by either the PI 3-kinase inhibitor LY294002 or rapamycin, but not by an induced expression of a dominant negative (DN-) Ras, Ras(Asn17). The expression of wild type p110 also greatly potentiated epidermal growth factor (EGF)-stimulated cyclin D1 protein expression. Conversely, the expression of a DN-form of either p110 or p85 regulatory subunit of PI 3-kinase strongly inhibited EGF-induced up-regulation of cyclin D1 protein. LY294002 and another PI 3-kinase inhibitor wortmannin completely abrogated EGF-stimulated increases in both mRNA and protein levels of cyclin D1, pRb phosphorylation and S phase entry. However, rapamycin had little inhibitory effect, if any, on either of these events despite potent p70^<S6K> inhibition throughout the G1 phase. These results indicate that PI 3-kinase is both necessary and sufficient for up-regulation of cyclin D1, with the downstream mTOR -p70^<S6K> signaling pathway differentially required depending on cellular conditions.