G protein-coupled receptor GPR68 inhibits lymphocyte infiltration and contributes to gender-dependent melanoma growth

Targeting G Protein-Coupled Receptors in Immuno-Oncological Therapies

The advent of cancer immunotherapy based on PD-1 and CTLA-4 immune checkpoint blockade (ICB) has revolutionized cancer treatment. However, many cancers do not respond to ICB, highlighting the urgent need for additional approaches to achieve durable cancer remission. The large family of G protein-coupled receptors (GPCRs) is the target of more than 30% of all approved drugs, but GPCRs have been underexploited in cancer immunotherapy. In this review, we discuss the central role of GPCRs in immune cell migration and function and describe how single-cell transcriptomic studies are illuminating the complexity of the human tumor immune GPCRome. These receptors include multiple GPCRs expressed in CD8 T cells that are activated by inflammatory mediators, protons, neurotransmitters, and metabolites that accumulate in the tumor microenvironment, thereby promoting T cell dysfunction. We also discuss new opportunities to target GPCRs as a multimodal approach to enhance the response to ICB for a myriad of human malignancies.

The Roles of Proton-Sensing G-Protein-Coupled Receptors in Inflammation and Cancer

The precise regulation of pH homeostasis is crucial for normal physiology. However, in tissue microenvironments, it can be impacted by pathological conditions such as inflammation and cancer. Due to the overproduction and accumulation of acids (protons), the extracellular pH is characteristically more acidic in inflamed tissues and tumors in comparison to normal tissues. A family of proton-sensing G-protein-coupled receptors (GPCRs) has been identified as molecular sensors for cells responding to acidic tissue microenvironments. Herein, we review the current research progress pertaining to these proton-sensing GPCRs, including GPR4, GPR65 (TDAG8), and GPR68 (OGR1), in inflammation and cancer. Growing evidence suggests that GPR4 and GPR68 are mainly pro-inflammatory, whereas GPR65 is primarily anti-inflammatory, in various inflammatory disorders. Both anti- and pro-tumorigenic effects have been reported for this family of receptors. Moreover, antagonists and agonists targeting proton-sensing GPCRs have been developed and evaluated in preclinical models. Further research is warranted to better understand the roles of these proton-sensing GPCRs in pathophysiology and is required in order to exploit them as potential therapeutic targets for disease treatment.

Elucidating the Activation Mechanism of the Proton-sensing GPR68 Receptor

GPR68 is a proton-sensing G-protein Coupled Receptor (GPCR) involved in a variety of physiological processes and disorders including neoplastic pathologies. While GPR68 and few other GPCRs have been shown to be activated by a decrease in the extracellular pH, the molecular mechanism of their activation remains largely unknown. In this work, we used a combined computational and in vitro approach to provide new insight into the activation mechanism of the receptor. Molecular Dynamics simulations of GPR68 were used to model the changes in residue interactions and motions triggered by pH. Global and local rearrangements consistent with partial activation were observed upon protonation of the inactive state. Selected extracellular histidine and transmembrane acidic residues were found to have significantly upshifted pKa values during the simulations, consistently with their previously hypothesised role in activation through changes in protonation state. Moreover, a novel pairing between histidine and acidic residues in the extracellular region was highlighted by both sequence analyses and simulation data and tested through site-directed mutagenesis. At last, we identified a previously unknown hydrophobic lock in the extracellular region that might stabilise the inactive conformation and regulate the transition to the active state.

Recent advances in acid sensing by G protein coupled receptors

Changes in extracellular proton concentrations occur in a variety of tissues over a range of timescales under physiological conditions and also accompany virtually all pathologies, notably cancers, stroke, inflammation and trauma. Proton-activated, G protein coupled receptors are already partially active at physiological extracellular proton concentrations and their activity increases with rising proton concentrations. Their ability to monitor and report changes in extracellular proton concentrations and hence extracellular pH appears to be involved in a variety of processes, and it is likely to mirror and in some cases promote disease progression. Unsurprisingly, therefore, these pH-sensing receptors (pHR) receive increasing attention from researchers working in an expanding range of research areas, from cellular neurophysiology to systemic inflammatory processes. This review is looking at progress made in the field of pHRs over the past few years and also highlights outstanding issues.