Role of PI3K/AKT/mTOR signaling in the cell cycle progression of human prostate cancer

Prostate cancer is one of the most common cancers among men. Recent studies demonstrated that PI3K signaling is an important intracellular mediator which is involved in multiple cellular functions including proliferation, differentiation, anti-apoptosis, tumorigenesis, and angiogenesis. In the present study, we demonstrate that the inhibition of PI3K activity by LY294002, inhibited prostate cancer cell proliferation and induced the G₁ cell cycle arrest. This effect was accompanied by the decreased expression of G ₁-associated proteins including cyclin D1, CDK4, and Rb phosphorylation at Ser780, Ser795, and Ser807/811, whereas expression of CDK6 and β-actin was not affected by LY294002. The expression of cyclin kinase inhibitor, p21^CIP1/WAF1, was induced by LY294002, while levels of p16^INK4 were decreased in the same experiment. The inhibition of PI3K activity also inhibited the phosphorylation and p70^S6K, but not MAPK. PI3K regulates cell cycle through AKT, mTOR to p70^S6K. The mTOR inhibitor rapamycin has similar inhibitory effects on G₁ cell cycle progression and expression of cyclin D1, CDK4, and Rb phosphorylation. These results suggest that PI3K mediates G₁ cell cycle progression and cyclin expression through the activation of AKT/mTOR/p70^S6K signaling pathway in the prostate cancer cells.