Abstract B59: Cyclic GMP phosphodiesterase: A molecular target for cancer chemoprevention

Nonsteroidal anti-inflammatory drugs (NSAIDs) can significantly reduce the incidence and risk of death from multiple cancers, but their long-term use for chemoprevention is not recommended because of potentially fatal toxicities resulting from cyclooxygenase (COX) inhibition. However, the mechanism responsible for their tumor cell growth inhibitory activity may not require COX inhibition, which suggests that NSAID derivatives lacking COX inhibitory activity have potential for improved safety and efficacy. Cyclic guanosine monophosphate phosphodiesterase (cGMP PDE) inhibition appears to be an important mechanism based on previous studies with the NSAID, sulindac. Here we characterize a novel sulindac derivative referred to as sulindac benzylamine (SBA) that does not inhibit COX-1 or COX-2, yet potently inhibits the growth and induces the apoptosis of human colon tumor cells. This activity is closely associated with cGMP PDE inhibition given that SBA can inhibit cGMP hydrolysis in colon tumor cell lysates, increase intracellular cGMP levels, and activate cGMP-dependent protein kinase G (PKG) at concentrations that suppress tumor cell growth and induce apoptosis. The cGMP-specific PDE isozyme, PDE5 is selectively inhibited by SBA and elevated in colon tumor cells compared with normal colonocytes that display reduced sensitivity to SBA. PKG activation by SBA is associated with the reduction of nuclear β-catenin levels and inhibition of Tcf transcriptional activity in which the anti-apoptosis gene, survivin is suppressed. These events preceded apoptosis induction by SBA and appear to result from the rapid elevation of intracellular cGMP levels following cGMP PDE inhibition. Transient transfection with PDE5 siRNA reduced PDE5 protein expression in both human normal (NCM460) colonocytes and colon tumor cell lines (HT29 and HCT116), but only suppressed cGMP PDE activity in the tumor cells, likely because of low PDE5 levels and/or the presence of other cGMP degrading isozymes in normal colonocytes. In accord, PDE5 siRNA selectively suppressed growth and induced apoptosis of colon tumor cells, but not normal colonocytes. Colon tumor cells transfected with PDE5 siRNA also displayed reduced β-catenin levels and Tcf transcriptional activity as well as increased sensitivity to SBA treatment. Finally, molecular modeling studies suggest that SBA directly interferes with cGMP binding within the catalytic domain of PDE5. However, additional cGMP degrading PDE isozymes may either be responsible for or contribute to the sensitivity of tumor cells to SBA given that PDE5 selective inhibitors (e.g. sildenafil) do not effectively suppress colon tumor cell growth. These results lead us to conclude that cGMP PDE inhibition is an important molecular target for cancer chemoprevention, although further studies are necessary to identify key cGMP degrading isozymes involved in tumorigenesis. This research was supported NIH grants, NCI 1R01CA131378 and 1R01CA148817-01A1. Citation Format: Gary A. Piazza, Nan Li, Gary D. Bernard, Xi Chen, Adam B. Keeton, Yaguang Xi, Bing Zhu, Wei Zhang, Alexandra Fajardo, Veronica Ramirez, Sara Sigler, Kevin Lee, Claire Cawthon, Evrim Gurpinar, Maya A. Brownby, Heather N. Tinsely, Ashraf Abadi, William Grizzle. Cyclic GMP phosphodiesterase: A molecular target for cancer chemoprevention. [abstract]. In: Proceedings of the Eleventh Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2012 Oct 16-19; Anaheim, CA. Philadelphia (PA): AACR; Cancer Prev Res 2012;5(11 Suppl):Abstract nr B59.