Activation of PKB/AKT promotes cell survival and invasion in choriocarcinoma

Abstract

Background: Activated phosphatidylinositol 3-kinase (PI3K) and its downstream serine/threonine protein kinase B (PKB)/AKT are important regulators of cell proliferation, apoptosis and invasion. The activation at Ser473 residue of PKB by phosphorylation (p-PKB Ser473) is required for its maximal activity. Dysregulation of PI3K/PKB signaling pathway has been found to be an important feature of many human malignancies. However, the role of activated PI3K/PKB in choriocarcinoma is not reported yet. In this study, we investigated the expression of p-PKB (Ser473) in gestational trophoblastic disease and the effects of inhibiting PI3K/PKB signaling pathway in choriocarcinoma. Methods: Choriocarcinoma cell lines JEG-3 and JAR were treated with PI3K inhibitor LY294002 and possible change in p-PKB (Ser473) expression was detected by Western Blot. The effects of activated PI3K/PKB on invasive activity, cell proliferation, apoptosis and cell cycle regulation in choriocarcinoma cells were determined by Matrigel invasion assay, MTT assay, TUNEL and flow cytometry, respectively. The expression of p-PKB (Ser473) in 96 clinical samples of normal placentas (n=30), hydatidiform moles (n=60) and choriocarcinomas (n=6) was also evaluated immunohistochemically. Results: After treating JEG-3 and JAR with PI3K inhibitor LY294002, significantly lower p-PKB (Ser473) but unchanged total PKB expressions was found compared with non-treated control. The invasive activity and viability of choriocarcinoma cells were significantly decreased. Cell cycle analysis by flow cytometry showed significant arrests in both G1/S and G2/M checkpoints with increased cell population in sub-G1 phase. The latter finding concurred with increased apoptosis in LY294002-treated cells by TUNEL assay. Immunohistochemical analysis in choriocarcinoma tissue samples confirmed significantly higher p-PKB (Ser473) expression comparing with that in placental and molar trophoblastic tissues (P<0.05). Furthermore, the immunohistochemical index of p-PKB (Ser473) inversely correlated with apoptotic index as assessed by TUNEL assay (P=0.001). Conclusion: We thus conclude that activation of PI3K/PKB signaling pathway is involved in the pathogenesis of choriocarcinoma. Activated PI3K/PKB signaling pathway promotes cell proliferation and invasion as well as reduces apoptosis by overcoming G1/S and G2/M checkpoints of the cell cycle. PI3K inhibitor may be a potential novel therapeutic target in choriocarcinomas resistant to conventional treatment.