Persistently hyperphosphorylated Akt contributes to human
oncogenesis and resistance to
therapy.
Triciribine (TCN)
phosphate (
TCN-P), the active metabolite of the Akt phosphorylation inhibitor TCN, is in clinical trials, but the mechanism by which
TCN-P inhibits Akt phosphorylation is unknown. Here we show that in vitro,
TCN-P inhibits neither Akt activity nor the phosphorylation of Akt S473 and T308 by
mammalian target of rapamycin or
phosphoinositide-dependent
kinase 1. However, in intact cells, TCN inhibits
EGF-stimulated Akt recruitment to the plasma membrane and phosphorylation of Akt. Surface plasmon resonance shows that TCN, but not TCN, binds Akt-derived
pleckstrin homology (PH) domain (K(D): 690 nM). Furthermore, nuclear magnetic resonance spectroscopy shows that
TCN-P, but not TCN, binds to the PH domain in the vicinity of the PIP3-binding pocket. Finally, constitutively active Akt mutants, Akt1-T308D/S473D and myr-Akt1, but not the transforming mutant Akt1-E17K, are resistant to TCN and rescue from its inhibition of proliferation and induction of apoptosis. Thus, the results of our studies indicate that
TCN-P binds to the PH domain of Akt and blocks its recruitment to the membrane, and that the subsequent inhibition of Akt phosphorylation contributes to
TCN-P antiproliferative and proapoptotic activities, suggesting that this
drug may be beneficial to patients whose
tumors express persistently phosphorylated Akt.