PDK1 regulates cell proliferation and cell cycle progression through control of cyclin D1 and p27Kip1 expression

Abstract

PDK1 (3-phosphoinositide-dependent protein kinase 1) is a key mediator of signaling by phosphoinositide 3-kinase. To gain insight into the physiological importance of PDK1 in cell proliferation and cell cycle control, we established immortalized mouse embryonic fibroblasts (MEFs) from mice homozygous for a "floxed" allele of Pdk1 and from wild-type mice. Introduction of Cre recombinase by retrovirus-mediated gene transfer resulted in the depletion of PDK1 in Pdk1lox/lox MEFs but not in Pdk1+/+ MEFs. The insulin-like growth factor-1-induced phosphorylation of various downstream effectors of PDK1, including Akt, glycogen synthase kinase 3, ribosomal protein S6, and p70 S6 kinase, was markedly inhibited in the PDK1-depleted (Pdk1-KO) MEFs. The rate of serum-induced cell proliferation was reduced; progression of the cell cycle from the G0-G1 phase to the S phase was delayed, and cell cycle progression at G2-M phase was impaired in Pdk1-KO MEFs. These cells also manifested an increased level of p27 Kip1 expression and a reduced level of cyclin D1 expression during cell cycle progression. The defect in cell cycle progression from the G 0-G1 to the S phase in Pdk1-KO MEFs was rescued by forced expression of cyclin D1, whereas rescue of the defect in G2-M progression in these cells required both overexpression of cyclin D1 and depletion of p27Kip1 by RNA interference. These data indicate that PDK1 plays an important role in cell proliferation and cell cycle progression by controlling the expression of both cyclin D1 and p27Kip1. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.