Mechanisms of regulation of cell adhesion and motility by insulin receptorsubstrate-1 in prostate cancer cells

LNCaP cells are human prostatic cancer cells that have a frame-shift mutati on of the tumor suppressor gene PTEN and do not express the insulin recepto r substrate-1 (IRS-I), a major substrate of the type 1 insulin-like growth factor receptor (IGF-IR). Ectopic expression of IRS-1 in LNCaP cells increa ses cell adhesion and decreases cell motility by an IGF-I-independent mecha nism. We show nom that these effects of IRS-1 are accompanied by serine pho sphorylation of IRS-1 and are inhibited by inhibitors of phosphatidylinosit ol 3-kinase (PI3K), We have confirmed the requirement for PI3K activity and serine phosphorylation by the use of IRS-1 mutants, expressed in LNCaP cel ls. Serine phosphorylation inhibits IGF-I-induced tyrosyl phosphorylation o f IRS-1, which is restored by the expression of wild-type PTEN or by inhibi tion of PI3K activity. Finally, IRS-1 in LNCaP cells co-immunoprecipitates with integrin alpha 5 beta 1, and the association is again IGF-I-independen t. We conclude that in LNCaP cells, IRS-1 is serine phosphorylated by PI3K, generating effects that are different, and even opposite, from those gener ated by IGF-I.