TY - JOUR T1 - A Novel Semisynthetic Inhibitor of the FRB Domain of Mammalian Target of Rapamycin Blocks Proliferation and Triggers Apoptosis in Chemoresistant Prostate Cancer Cells JF - Molecular Pharmacology JO - Mol Pharmacol SP - 531 LP - 541 DO - 10.1124/mol.112.081349 VL - 83 IS - 2 AU - Samy A. F. Morad AU - Maximilian Schmid AU - Berthold Büchele AU - Hans-Ullrich Siehl AU - Menna El Gaafary AU - Oleg Lunov AU - Tatiana Syrovets AU - Thomas Simmet Y1 - 2013/02/01 UR - http://molpharm.aspetjournals.org/content/83/2/531.abstract N2 - The mammalian target of rapamycin (mTOR) is a key regulator of cell growth and its uncontrolled activation is a hallmark of cancer. Moreover, mTOR activation has been implicated in the resistance of cancer cells to many anticancer drugs, rendering this pathway a promising pharmacotherapeutic target. Here we explored the capability of a semisynthetic compound to intercept mTOR signaling. We synthesized and chemically characterized a novel, semisynthetic triterpenoid derivative, 3-cinnamoyl-11-keto-β-boswellic acid (C-KβBA). Its pharmacodynamic effects on mTOR and several other signaling pathways were assessed in a number of prostate and breast cancer cell lines as well as in normal prostate epithelial cells. C-KβBA exhibits specific antiproliferative and proapoptotic effects in cancer cell lines in vitro as well as in PC-3 prostate cancer xenografts in vivo. Mechanistically, the compound significantly inhibits the cap-dependent transition machinery, decreases expression of eukaryotic translation initiation factor 4E and cyclin D1, and induces G1 cell-cycle arrest. In contrast to conventional mTOR inhibitors, C-KβBA downregulates the phosphorylation of p70 ribosomal S6 kinase, the major downstream target of mTOR complex 1, without concomitant activation of mTOR complex 2/Akt and extracellular signal–regulated kinase pathways, and independently of protein phosphatase 2A, liver kinase B1/AMP-activated protein kinase/tuberous sclerosis complex, and F12-protein binding. At the molecular level, the compound binds to the FKBP12-rapamycin-binding domain of mTOR with high affinity, thereby competing with the endogenous mTOR activator phosphatidic acid. C-KβBA represents a new type of proapoptotic mTOR inhibitor that, due to its special mechanistic profile, might overcome the therapeutic drawbacks of conventional mTOR inhibitors. ER -