AbstractActivated phosphoinositide 3‐kinase (PI3K) and its downstream target Akt/PKB are important signaling molecules and key survival factors involved in the control of cell proliferation, apoptosis and oncogenesis. We investigated the role of the PI3K‐Akt signaling pathway in the invasion of prostate cancer cell lines and activation of this pathway in primary human prostate tumors. Treatment of human prostate cancer cells viz. LNCaP, PC‐3 and DU145 with PI3K pharmacological inhibitor, LY294002, potentially suppressed the invasive properties in each of these cell lines. Restoration of the PTEN gene to highly invasive prostate cancer PC‐3 cells or expression of a dominant negative version of the PI3K target, Akt also significantly inhibited invasion and downregulated protein expression of urokinase‐type plasminogen activator (uPA) and matrix metalloproteinase (MMP)‐9, markers for cell invasion, indicating a central role of the PI3K‐Akt pathway in this process. Immunoblot analysis of PI3K and total/activated levels of Akt showed increased protein levels of catalytic (p110α/β) and regulatory (p85) subunits of PI3K and constitutive Akt activation in high‐grade tumors compared to low‐grade tumor and benign tissue. Immunohistochemical analyses further confirmed a progressive increase in p‐Akt (p‐Ser473) levels but not of total‐Akt (Akt1/2) in cancer tissues compared to benign specimens. A successive increase in p‐Akt expression was further noted in specimens serially obtained from individuals with time‐course disease progression. Taken together, these results suggest that aberrant activation of PI3K‐Akt pathway may contribute to increased cell invasiveness and facilitate prostate cancer progression. © 2007 Wiley‐Liss, Inc.