Our previous studies have shown that methyl-2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oate (
CDDO-Me), a
oleanane synthetic
triterpenoid induces apoptosis in
prostate cancer cells by inhibiting the Akt/NF-κB/mTOR signaling cascade; however, the mechanism by which
CDDO-Me inhibits Akt/NF-κB/mTOR signaling has remained undetermined. Present studies show that Akt plays a critical role in the response of
prostate cancer cells to
CDDO-Me. Silencing of Akt sensitized PC-3 cells to
CDDO-Me, whereas its overexpression rendered them resistant to
CDDO-Me. Evaluation of the effect of
CDDO-Me on Akt which lies upstream of NF-κB and mTOR showed that
CDDO-Me directly inhibits the Akt
kinase activity in cell-free
kinase activity assay and in vivo without modulating the activity of PDK1, the upstream
kinase that phosphorylates and activates Akt. The inhibition of Akt activity resulted in inhibition of phosphorylation/inactivation of proapoptotic
procaspase-9, Bad and Foxo3a. Further, inhibition of p-Akt by
CDDO-Me was not attributable to an increase in the activity of
protein phosphatase 2A (PP2A) or PH domain/
leucine-rich repeat protein phosphatase1 (PHLPP1) both of which dephosphorylate p-Akt. These findings show that Akt is a direct target of
CDDO-Me in the Akt/NF-κB/mTOR prosurvival signaling axis.