Advisory Committee Chair
Gary A Piazza
Advisory Committee Members
Graeme B Bolger
William E Grizzle
Yonghe Li
Thomas M Ryan
David A Schneider
Document Type
Dissertation
Date of Award
2013
Degree Name by School
Doctor of Philosophy (PhD) Heersink School of Medicine
Abstract
Nonsteroidal anti-inflammatory drugs (NSAIDs) display promising antineoplastic activity for colorectal and other cancers, but toxicity from cyclooxygenase (COX) inhibition limits their long-term use for chemoprevention. However, many investigators have concluded that COX-independent mechanisms are responsible for their antineoplastic activity. We previously reported that cyclic guanosine monophosphate phosphodiesterase (cGMP PDE) is a non-COX target of sulindac sulfide (SS). However, the specific isozyme families involved have not been well studied. Here we demonstrate that SS inhibits the growth of colon tumor cells through a novel mechanism involving inhibition of two cGMP degrading isozymes, PDE5 and PDE10, to activate cGMP-dependent protein kinase (PKG). SS does not activate the cGMP/PKG pathway, nor affect proliferation or apoptosis in normal colonocytes, which suggest that cGMP PDEs are important targets for tumor selectivity. PDE5 and 10 were found to be overexpressed in human colon tumor cell lines and in clinical specimens from colorectal cancer patients compared with colonocytes and uninvolved mucosa. Inhibition of PDE5 and PDE10 by specific inhibitors or RNAi selectively suppressed colon tumor cell proliferation and induced apoptosis through activation of PKG, which mimicked the effects of SS. Knockdown of PDE10 also inhibited colony formation in soft agar. The mechanism by which cGMP/PKG pathway inhibits colon tumor cell growth appears to involve suppression of ß-catenin expression to inhibit TCF transcriptional activity.Conversely, ectopic expression of PDE10 in colonocytes and adenoma cells promoted cell growth involving up-regulation of ß-catenin signaling. Moreover, combined inhibition of PDE5 and 10 caused additive antitumor effect through greater inhibition of ß-catenin signaling compared with inhibiting either isozyme alone. By chemically modifying SS to block COX binding, we identified a novel sulindac derivative, MCI-1, that potently and selectively inhibited colon tumor cell growth and induced apoptosis by inhibiting both PDE5 and 10, activating cGMP/PKG pathway, and attenuating of ß-catenin/TCF mediated transcription. Overall, this dissertation demonstrates PDE5 and 10 are novel targets for prevention and treatment of colorectal cancer and likely other cancer types. It should therefore be feasible to develop safer and more efficacious chemopreventive drugs by targeting PDE5 and PDE10, while removing COX-inhibitory activity from sulindac and possibly other NSAIDs.
Recommended Citation
Li, Nan, "Cyclic GMP Phosphodiesterase as a Novel Target for Colorectal Cancer Chemoprevention" (2013). All ETDs from UAB. 2269.
https://digitalcommons.library.uab.edu/etd-collection/2269