An a-Acetoxy-Tirucallic Acid Isomer Inhibits Akt/mTOR Signaling and Induces Oxidative Stress in Prostate Cancer Cells

Here we provide evidence that aATA(8,24) (3a-acetyloxy-tir-8,24dien-21-oic acid) inhibits Akt/mammalian target of rapamycin (mTOR) signaling. aATA(8,24) and other tirucallic acids were isolated from the acetylated extract of the oleo gum resin ofBoswellia serrata to chemical homogeneity. Compared with related tirucallic acids, aATA(8,24) was the most potent inhibitor of the proliferation of androgen-insensitive prostate cancer cells in vitro and in vivo, in prostate cancer xenografted onto chick chorioallantoic membranes. aATA(8,24) induced loss of cell membrane asymmetry, caspase-3 activation, and DNA fragmentation in vitro and in vivo. These effects were selective for cancer cells, because aATA(8,24) exerted no overt toxic effects on peripheral blood mononuclear cells or the chick embryo. At the molecular level, aATA(8,24) inhibited the Akt1 kinase activity. Prior to all biochemical signs of cellular dysfunction, aATA(8,24) induced inhibition of the Akt downstream targetmTORas indicatedby dephosphorylation of S6K1. This event was followed by decreased expression of cell cycle regulators, such as cyclin D1, cyclin E, and cyclin B1, as well as cyclindependent kinases CDK4 and CDK2 and phosphoretinoblastoma protein, which led to inhibition of the cell-cycle progression. In agreement with the mTOR inhibition, aATA(8,24) and rapamycin increased the volume of acidic vesicular organelles. In contrast to rapamycin, aATA(8,24) destabilized lysosomal and mitochondrial membranes and induced reactive oxygen species production in cancer cells. The ability of aATA(8,24) to inhibit Akt/ mTOR signaling and to induce simultaneously oxidative stress could be exploited for the development of novel antitumor therapeutics with a lower profile of toxic side effects.