cGMP Signaling Increases Antioxidant Gene Expression by Activating Forkhead Box O3A in the Colon Epithelium

Abstract

Signaling through cGMP has therapeutic potential in the colon, where it has been implicated in the suppression of colitis and colon cancer. In this study, we tested the ability of cGMP and type 2 cGMPdependent protein kinase (PKG2) to activate forkhead box O (FoxO) in colon cancer cells and in the colon epithelium of mice. We show that activation of PKG2 in colon cancer cells inhibited cell proliferation, inhibited AKT, and activated FoxO. Treatment of colon explants with 8Br-cGMP also activated FoxO target gene expression at both RNA and protein levels, and reduced epithelial reduction-oxidation (redox) stress. FoxO3a was the most prominent isoform in the distal colon epithelium, with prominent luminal staining. FoxO3a levels were reduced in Prkg2 À/À animals, and FoxO target genes were unaffected by 8Br-cGMP challenge in vitro. Treatment of mice with the phosphodiesterase-5 inhibitor vardenafil (Levitra) mobilized FoxO3a to the nucleus of luminal epithelial cells, which corresponded to increased FoxO target gene expression, reduced redox stress, and increased epithelial barrier integrity. Treatment of human colonic biopsy specimens with 8Br-cGMP also activated catalase and manganese superoxide dismutase expression, indicating that this pathway is conserved in humans. Taken together, these results identify a novel signaling pathway in the colon epithelium, where FoxO tumor suppressors could provide protection from redox stress. Moreover, this pathway is regulated by endogenous cGMP/PKG2 signaling, and can be targeted using phosphodiesterase-5 inhibitors. (Am J Pathol 2017, 187: 377e389; http://dx

Sildenafil Suppresses Inflammation-Driven Colorectal Cancer in Mice

“…The mechanisms underlying the effects of cGMP on epithelial homeostasis and barrier protection in the normal colon epithelium are not fully understood. Suppression of Akt, β-catenin/TCF, and JNK signaling, with subsequent activation of FoxO have all been suggested to occur downstream of cGMP in either normal mouse tissue, in some human colon cancer cell lines, or both (18,20,28,29). In the present study, reduced levels of β-catenin were observed in the polyps from sildenafil treated animals, but this was more likely due to the lower abundance of proliferating cells and inflammation, because there was no evidence of PKG activity such as phosphorylation of vasodilator stimulated phosphoprotein (Supplementary Fig.…”

“…In both cell lines and in mice, cGMP has been shown to inhibit the AKT and JNK pathways (18,20), which regulate homeostasis in the intestinal epithelium. More recently it was reported that by suppressing AKT, cGMP/PKG2 activates Forkhead Box O (FoxO) transcription factors in both colon cancer cell lines and in vivo (29). As tumor suppressors, FoxO can inhibit proliferation in dividing cells and provide antioxidant protection to quiescent cells (30,31).…”

Protective role of endothelial progenitor cells stimulated by riociguat in chronic thromboembolic pulmonary hypertension

“…Previous reports showed that reactive oxygen species (ROS) negatively affect the number and function of EPCs. [23] Riociguat is one of the sGCs that increase the level of cGMP and smooth muscle relaxation; enhanced cGMP signaling increases antioxidant gene expression [24].…”

The Guanylate Cyclase C—cGMP Signaling Axis Opposes Intestinal Epithelial Injury and Neoplasia

“…A complementary mechanism was recently proposed, examining the role of reactive oxygen species in disrupting barrier integrity. Wang et al suggest that cGMP signaling enhances barrier integrity through activation of the transcription factor, FOXO3a and its downstream antioxidant transcriptional targets ( 125 ). FOXO3a is phosphorylated and inactivated by AKT.…”