Plant flavonoid apigenin inactivates Akt to trigger apoptosis in human prostate cancer: an in vitro and in vivo study

Inappropriate activation of phosphatidylinositol 3-kinase-Akt signaling contributes to the development of several human malignancies. Modulation of Akt activity is a strategy that may be valuable in chemopreventive and chemotherapeutic regimens. We have previously demonstrated that apigenin, a plant flavone, causes decreased survival in human prostate cancer cells. However, the molecular mechanism underlying this observation remains elusive. In the present study, we investigated the mechanisms of apigenin action on human prostate cancer PC-3 cells, which possess constitutively active Akt. Treatment of PC-3 cells with apigenin (5-40 mu M) resulted in significant dose- and time-dependent decrease in Akt phosphorylation at Serine473. Apigenin-mediated dephosphorylation of Akt resulted in inhibition of its kinase activity, which was confirmed by reduced phosphorylation of proapoptotic proteins BAD and glycogen synthase kinase-3, essential downstream targets of Akt. Hypophosphorylation of BAD resulted in reduced interaction with 14-3-3 beta protein after 20 mu M apigenin exposure to PC-3 cells for 24 h. Inactivation of Akt seems to be associated with downregulation of insulin-like growth factor receptor 1 protein level and inhibition of its autophosphorylation upon apigenin treatment. Exposure to apigenin significantly induced caspase-9 activity and decreased the survival of PC-3 cells in a dose-dependent manner. Furthermore, Serine473 phosphorylation of ectopically expressed Akt in DU145 cells was significantly reduced upon 20 mu M apigenin treatment. In vivo, apigenin intake through gavage resulted in inactivation of Akt and induction of apoptosis in PC-3 tumors. These results suggest that Akt inactivation and dephosphorylation of BAD is a critical event, at least in part, in apigenin-induced decreased cell survival and apoptosis.