The G protein-coupled pH-sensing receptor OGR1 is a regulator of intestinal inflammation

Background: A novel family of proton sensing G-protein coupled receptors (GPCRs), including ovarian cancer G-protein coupled receptor 1 (OGR1, GPR68), GPR4, and T-cell death associated gene 8 (TDAG8, GPR65), was previously identified to play an important role in physiological pH homeostasis. Luminal and tissue pH values in patients with active inflammatory bowel disease (IBD) are known to be significantly lower than values in the normal subject. We investigated whether OGR1 may play a role for the pathophysiology of IBD. Methods: OGR1 expression in colonic tissues was investigated in controls and IBD patients. Expression of OGR1 upon cell activation was studied in the Mono Mac 6 (MM6) cell line and primary human and murine monocytes by real-time PCR. Ogr1 knockout mice were crossbred with Il-10 deficient mice, compared to Il-10 -/- from the same litters and studied over 200 days. Microarray profiling (Metacore GeneGo and ranked fold change analysis) was performed using Ogr1-/- and Ogr1+/+ (WT) residential peritoneal macrophages. Results: IBD patients expressed higher levels of OGR1 in the mucosa as compared to controls. Treatment of MM6 cells with TNF, but not other cytokines (IFN-γ, IL-1β, IL-6, TGF-β) tested, led to significant up-regulation of OGR1 mRNA expression. Induction of OGR1 expression by TNF was dose-dependent. Macrophage differentiation of MM6 cells with PMA also led to a significant increase in OGR1 expression. Dose-dependence of TNF and PMA mediated induction of OGR1 expression was confirmed in primary human monocytes. TNF-mediated induction of OGR1 mRNA expression was reversed by simultaneous treatment of cells with the NF-κB inhibitors MG132, AICAR, BAY-11-7082, CAY10512, and SC-514. Kaplan-Meier survival analysis showed a significantly delayed onset and progression of rectal prolapse in female Ogr1-/- // Il-10-/- mice (p=0.005). Female Ogr1 -/- // Il-10 -/- mice had significantly less rectal prolapses (21%, n = 19 versus 75%, n = 8; p = 0.025; Odd ratio = 0.089). Up-regulation of gene expression, mediated by OGR1, in response to extracellular acidification in mouse macrophages was enriched for inflammation and immune response, actin cytoskeleton, and cell adhesion gene pathways. Conclusion: OGR1 expression is induced in cells of human macrophage lineage and primary human monocytes by TNF and PMA. NF-κB inhibition reverses the induction of OGR1 mRNA expression by TNF. OGR1 deficiency protects from spontaneous inflammation in the Il-10 KO model. Our data indicate an important pathophysiological role for the pH sensing G-protein coupled receptor OGR1 during the pathogenesis of mucosal inflammation