The triterpenoid acetyl-lupeolic acid (ac-LA) isolated from the oleogum resin of Boswellia carterii reduced the viability of a panel of cancer cell lines more efficiently than lupeol. There was no detectable intracellular conversion of ac-LA to lupeol and vice versa. In contrast to docetaxel, ac-LA did not induce selection of treatment-resistant cancer cells. By various parameters including DNA fragmentation, ac-LA was shown to induce apoptosis in androgen-independent PC-3 cells, whereas in MDA-MB-231 breast cancer cells, ac-LA led to cell accumulation in the G2/M phase of the cell cycle, but not to apoptosis. In silico docking combined with in vitro kinase assays implied that ac LA potently inhibits Akt mainly by direct binding to the pleckstrin homology domain. Consistently, an Akt1 mutant deficient of the PH domain afforded partial resistance to ac-LA and complete resistance to lupeol and the Akt inhibitor III. Ac-LA inhibited phosphorylation of downstream targets of the Akt signaling pathway, which was followed by inhibition of the mTOR target p70 ribosomal six protein kinase and the nuclear accumulation of p65/NF-κB, β-catenin, and c-myc, as well as loss of the mitochondrial membrane potential. Ac-LA exhibited antiproliferative, proapoptotic, and antitumorigenic effects on PC-3-tumors xenografted either on chick chorioallantoic membranes or in nude mice. Ac-LA exhibited a clearly better safety profile than docetaxel or lupeol during chronic administration in vivo. In contrast to lupeol, ac-LA also inhibited release of vascular endothelial growth factor in vitro and accordingly angiogenesis in vivo. Thus, ac-LA deserves further exploration as a potential new antitumor compound.
Keywords: allosteric Akt inhibitor; apoptosis; prostate cancer; triterpenoids.