Abstract

Cyclin D1 Expression Mediated by Phosphatidylinositol 3-Kinase through mTOR-p70 S6K -Independent Signaling in Growth Factor-Stimulated NIH 3T3 Fibroblasts Noriko Takuwa 1 , * , Yasuhisa Fukui 2 , and Yoh Takuwa 1 , 3 Department of Molecular and Cellular Physiology, Graduate School of Medicine, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, 1 Laboratory of Biological Chemistry, Division of Applied Biological Chemistry, Graduate School of Agriculture and Life Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, 2 and The Foundation for Advancement of International Science, Ibaraki 305-0005, 3 Japan ABSTRACT Phosphatidylinositol (PI) 3-kinase is required for G 1 to S phase cell cycle progression stimulated by a variety of growth factors and is implicated in the activation of several downstream effectors, including p70 S6K . However, 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 the expression of a dominant negative (DN) form of either the p110α catalytic or the p85 regulatory subunit of heterodimeric PI 3-kinase strongly inhibited epidermal growth factor (EGF)-induced upregulation of cyclin D1 protein in NIH 3T3(M17) fibroblasts. The PI 3-kinase inhibitors LY294002 and wortmannin completely abrogated increases in both mRNA and protein levels of cyclin D1 and phosphorylation of pRb, inducing G 1 arrest in EGF-stimulated cells. By contrast, rapamycin, which potently suppressed p70 S6K activity throughout the G 1 phase, had little inhibitory effect, if any, on either of these events. PI 3-kinase, but not rapamycin-sensitive pathways, was also indispensable for upregulation of cyclin D1 mRNA and protein by other 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(M17) 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 LY294002, but was independent of the Ras-MEK-ERK pathway. Unlike mitogen stimulation, the p110 induction of cyclin D1 was sensitive to rapamycin. These results indicate that the catalytic activity of PI 3-kinase is necessary, and could also be sufficient, for upregulation of cyclin D1, with mTOR signaling being differentially required depending upon cellular conditions.