The Differential Roles of pH sensing G-Protein Coupled Receptors in Inflammation and Cancer

Intestinal inflammation is a burdening disease that may occur due to an aberrative immune response to certain drugs, infections, genetics, or unknown environmental factors. Inflammatory bowel disease (IBD) is of special concern as long-standing chronic intestinal inflammation may increase the risk for colitis-associated colorectal cancer (CAC) development due to cellular transformation to neoplastic lesions. Both the inflamed and tumor microenvironments are complex in nature and are characterized by an acidic environment. Bacterial byproducts, leukocytes respiratory bursts, and tissue ischemia and glycolysis are the main sources of protons produced in the inflamed environment, which cause local tissue acidosis. Tissue acidosis may alter both the immune and vasculature responses and subsequent cytokines and chemokines production. The pH sensing G-protein coupled receptors (GPCRs), GPR4, GPR65 (TDAG8), GPR68 (OGR1), and GPR132 (G2A) have emerged as a new class of proton sensing receptors that are expressed by immune and non-immune cells. GPR65 is mainly expressed in immune cells and is functionally critical for intestinal homeostasis, as identified by Genome-Wide Association Study (GWAS), for being a genetic risk factor in patients with IBD. We and others observed an anti-inflammatory role for GPR65 in pre-clinical mouse models, possibly through modulating the innate immune response towards a less inflammatory phenotype. GPR4, on the other hand, is mainly expressed in endothelial cells and confers a proinflammatory role that our group had previously uncovered the mechanism for. Upon acidic activation of GPR4, a proinflammatory program in the endothelium is activated, upregulating cytokines, chemokines, adhesion molecules, and ER stress responses. This activation is particularly crucial in the process of immune cell extravasation to the site of inflammation which is a critical step in IBD pH homeostasis and chronic inflammation. We and others observed reduced inflammatory response in IBD preclinical mouse models using both genetic and pharmacological inhibition for GPR4. Additionally, a biological, as well as a pathological proangiogenic role for GPR4 has been previously described. Therefore, based on the differential roles observed for GPR65 and GPR4 in inhibiting and mediating intestinal inflammation, respectively, we sought to investigate their functional roles in CAC development. \nTo this end, we utilized the well-established azoxymethane/dextran sodium sulfate (AOM/DSS) CAC mouse model, using GPR65-null, and GPR4-null mice, to study their functional roles in CAC. Our observations indicate an anti-inflammatory role during chronic intestinal inflammation and an anti-tumoral role of GPR65, leading to less tumor development in CAC mice. Thus, we propose that the use of GPR65 agonist will be of therapeutic benefit in IBD treatment and CAC prophylaxis. Conversely, our results indicate a proinflammatory role for GPR4 during intestinal inflammation in addition to protumorigenic and proangiogenic roles, contributing to CAC development. Hence, we propose the use of GPR4 antagonism as a strategy for IBD and CAC treatment. Finally, we observed that abolishing GPR4 alleviates another type of colitis, immune checkpoint inhibitors-mediated colitis (IMC). This type of colitis occurs as a side effect for the use of immunotherapy treatment in cancer patients. Using an IMC mouse model, our results indicate that abolishing GPR4 reduces disease activity, macrophage clusters, and fibrosis, suggesting that inhibiting GPR4 may provide an effective treatment for IMC. \nCollectively, this dissertation work provides new insights into the roles of GPR4 and GPR65 in intestinal inflammation and cancer development