Targeting Truncated Retinoid X Receptor-Alpha By Cf31 Induces Tnf-Alpha-Dependent Apoptosis

A truncated version of retinoid X receptor-alpha, tRXR-alpha, promotes cancer cell survival by activating the phosphoinositide 3-kinase (PI3K)/AKT pathway. However, targeting the tRXR-alpha-mediated survival pathway for cancer treatment remains to be explored. We report here our identification of a new natural product molecule, CF31, a xanthone isolated from Cratoxylum formosum ssp. pruniflorum, and the biologic evaluation of its regulation of the tRXR-a-mediated PI3K/AKT pathway. CF31 binds RXR-alpha and its binding results in inhibition of RXR-a transactivation. Through RXR-a mutational analysis and computational studies, we show that Arg316 of RXR-a, known to form salt bridges with certain RXR-alpha ligands, such as 9-cis-retinoic acid (9-cis-RA), is not required for the antagonist effect of CF31, showing a distinct binding mode. Evaluation of several CF31 analogs suggests that the antagonist effect is mainly attributed to an interference with Leu451 of helix H12 in RXR-alpha. CF31 is a potent inhibitor of AKT activation in various cancer cell lines. When combined with TNF-alpha, it suppresses TNF-alpha activation of AKT by inhibiting TNF-alpha-induced tRXR-alpha interaction with the p85 alpha regulatory subunit of PI3K. CF31 inhibition of TNF-alpha activation of AKT also results in TNF-alpha-dependent activation of caspase-8 and apoptosis. Together, our results show that CF31 is an effective converter of TNF-alpha signaling from survival to death by targeting tRXR-alpha in a unique mode and suggest that identification of a natural product that targets an RXR-mediated cell survival pathway that regulates PI3K/AKT may offer a new therapeutic strategy to kill cancer cells. Cancer Res; 73(1); 307-18. (C)2012 AACR.