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 microM) 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
14-3-3beta protein after 20 microM
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 microM
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.