The orphan receptor GPRC5B modulates inflammatory and fibrotic pathways in cardiac fibroblasts and mice hearts.

Inflammation is a major driver of cardiac remodeling. Cardiac fibroblasts play an integral role in cardiac inflammation, fibrosis and remodeling. The orphan G-protein-coupled-receptor 5B of family C (GPRC5B) has recently been shown to have pro-inflammatory effects in adipocytes via the NFκB-signaling-pathway. Here, we investigated whether GPRC5B is involved in myocardial inflammation and fibrosis. Using neonatal rat cardiac fibroblasts (NRCF) we show that the transcription and the expression of endogenous GPRC5B is induced by stimulation with TNFα and LPS as well as through cyclic mechanical stretch, while the principle pro-fibrotic factor TGFβ has no effect on the GPRC5B expression. Furthermore, we demonstrate that adenoviral overexpression and siRNA-mediated knockdown of GPRC5B in NRCF significantly alters the transcription level of the pro-inflammatory and pro-fibrotic cytokines TNFα, IL-1β, IL-6 and MCP-1, and extracellular matrix-degrading MMP-9 in vitro. Additionally, in adult GPRC5B-transgenic mice the protein expression of collagen-1A1 is decreased and the production of MMP-9 is increased, indicating remodeling of the extracellular matrix in vivo. Our data show that GPRC5B is up-regulated by inflammatory signals and mechanical stress in NRCF, while GPRC5B modulates the inflammatory response of cardiac fibroblasts and the degradation of extracellular matrix-proteins in the mice heart. Thus, our findings are the first to report a novel role of the orphan receptor GPRC5B in fibroblast-driven myocardial inflammation and cardiac remodeling.