Background/Purpose: Osteoarthritis (OA) is the most prevalent joint disease, accounting for 50% of the musculoskeletal burden. Inflammation plays a crucial role in the development of OA and is often accompanied by synovial fluid acidosis and an acidic extracellular environment.As OA progresses, chondrocytes experience changes in osmolarity and acidity in their surrounding environment. However, the mechanism by which chondrocytes detect and respond to acidic stress is currently unknown. Recently, there has been growing interest in the role of proton-sensing GPCRs such as Ovarian Cancer G-Protein Coupled Receptor (OGR1 or GPR68) in musculoskeletal tissues. Nevertheless, the expression and function of OGR1 in cartilage during OA remain unexplored. This study aims to examine whether the OGR1 contributes to the regulation of inflammation during OA progression.

Methods: High throughput RNA-Seq data (GSE114007) in healthy donors and OA cartilage was analyzed to determine the role of proton-sensing orphan GPCRs during OA pathogenesis. Primary human OA chondrocytes were obtained from discarded cartilage of patients undergoing arthroplasty. siRNA mediated OGR1 knockdown was used to determine its role in inflammatory signaling in human chondrocytes. Activation of OGR1 was done by treating chondrocytes with Ogerin and activity was measured using Ca+ release and fluorescent based biosensor assay. Western immunoblotting was performed to analyze the expression at protein level. Statistical analyses were performed using one-way analysis of variation (ANOVA) with Tukey’s post-hoc tests.

Results: Differential gene expression analysis of RNA-seq data identified OGR1 was highly expressed in human OA cartilage. Immunohistochemistry and qPCR further confirmed the expression of OGR1 in human OA cartilage and in chondrocytes treated with IL1β. Low density array analysis of OGR1 knockdown showed that silencing of OGR1 significantly induced the expression of various inflammatory cytokines and chemokines including IL6, NOS2, CSF2, CXCL6, CCL3, CXCL2. Interestingly, Ogerin mediated OGR1 activation in chondrocytes repressed the expression of these inflammatory genes suggesting the role of GPR68 in the regulation of inflammatory gene expression. Furthermore, MAPK inhibitors studies showed that inhibition of ERK and P38 MAPK pathways significantly reverse the effect of OGR1 knockdown on the induction of IL6 gene expression in IL1β stimulated OA chondrocytes. These data suggest the involvement of ERK1/2 and P38 MAPK signaling in OGR1 mediated regulation of inflammatory genes in chondrocytes during OA pathogenic conditions.

Conclusion: Our data suggest that OGR1 is highly expressed in human OA cartilage and regulates inflammatory gene expression via MAPK activation. Our results showed the involvement of OGR1 in the inflammatory pathways and highlights its potential as a therapeutic target for OA treatment. Exploring the mechanisms through which OGR1 influences the development of OA could offer novel perspectives and lead to the development of novel therapies that can modify the course of the disease for individuals with OA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/activation-of-ovarian-cancer-g-protein-coupled-receptor-ogr1-attenuates-chondrocytes-inflammation-via-erk1-2-signaling-pathways-in-an-in-vitro-model-of-osteoarthritis/