Inhibition Akt by tetracyclic triterpenoids induces cell cycle arrest and apoptosis in prostate cancer cells.

Abstract

AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA 628 Akt are serine/threonine kinases controling cell metabolism, proliferation, and apoptosis. Akt activation is associated with aggressive behaviour of prostate cancer. Akt1 and Akt2 are the major isoforms expressed in androgen-dependent LNCaP and androgen-independent PC-3 and DU 145 prostate cancer cells as measured by quantitative real-time PCR. Three structurally different Akt inhibitors were cytotoxic for prostate cancer cells indicating that the Akt pathway is indispensable for their viability. Oleogum resins from Boswellia species contain a complex mixture of triterpenoids with antitumor properties. In search for well-tolerated stable Akt inhibitors, we isolated several tetracyclic triterpenoids and purified them to chemical homogeneity. 3-Oxo-tirucallic acid, 3-α-acetoxy-tirucallic acid and 3-β-acetoxy-tirucallic acid potently inhibited the activities of human recombinant Akt1 and Akt2 in in vitro kinase assays, and Akt immunoprecipitated from PC-3 cells, but not the activity of IκB kinase. Interaction of Akt with the triterpenoids was quantified using surface plasmon resonance. The tetracyclic triterpenoids inhibited the phosphorylation of cellular Akt and glycogen synthase kinase-3β, whereas extracellular signal-regulated kinase 1/2 phosphorylation was increased. Further, the compounds inhibited nuclear accumulation of p65/relA, the androgen receptor, β-catenin, c-myc and the expression of cyclin D1, followed by hypophosphorylation of retinoblastoma protein. These events culminated in cell cycle arrest and induction of apoptosis. Selective down-regulation of Akt1, but not of Akt2 by siRNA similarly reduced the cell viability. In addition, the tetracyclic triterpenoids inhibited proliferation and induced apoptosis in tumors xenografted onto chick chorioallantoic membranes. These results suggest that inhibition of Akt is sufficient to trigger apoptosis in prostate cancer cells. The isolated tetracyclic triterpenoids represent a new class of Akt inhibitors with anticancer properties in human prostate cancer cells. This work was supported by the Deutsche Krebshilfe.