G protein-coupled receptors (GPRs), including pro-inflammatory GPR4 and ovarian cancer GPR1 (OGR1/GPR68), are involved in the pH sensing of the extracellular space and have been implicated in inflammatory bowel disease (IBD). Previous data show that a loss of GPR4 or OGR1 independently is associated with reduced intestinal inflammation in mouse models of experimental colitis. In the present manuscript, we investigated the impact of the simultaneous loss of GPR4 and OGR1 in animal models of IBD. To study the effects of combined loss of Gpr4 Ogr1 in IBD we used the well-established acute dextran sodium sulfate (DSS) and spontaneous Il10^−/− murine colitis models. Disease severity was assessed using multiple clinical scores (e.g., body weight loss, disease activity score, murine endoscopic index of colitis severity (MEICS) and histological analyses). Real-time quantitative polymerase chain reaction (qPCR), Western blot, and flow cytometry were used to investigate changes in pro-inflammatory cytokines expression and immune cells infiltration. We found that a combined loss of GPR4 and OGR1 significantly reduces colon inflammation in IBD relative to single deficiencies as evidenced by reduced body weight loss, disease score, CD4/CD8 ratio, and Il1β, Il6, and Tnf in the colon. Similarly, in the II10 deficiency model, the inflammation was significantly ameliorated upon the simultaneous deletion of GPR4 and OGR1, evidenced by a reduction in the MEICS score, colon length, Tnf and Il1β measurements, and a decrease in the number of macrophages in the colon, as compared to single deletions. Importantly, hydroxyproline levels were decreased close to baseline in Il10^−/− × Gpr4^−/− × Ogr1^−/− mice. Our findings demonstrate that the simultaneous loss of GRP4 and OGR1 functions exerts an additive effect on multiple parameters associated with colonic inflammation. These results further reinforce the hypothesis that chronic inflammatory acidosis is a driver of fibrosis and is dependent on GPR4 and OGR1 signaling. The inhibition of both GPR4 and OGR1 by pH-sensing receptor modulators may constitute as a potential therapeutic option for IBD, as both pH-sensing receptors appear to sustain inflammation by acting on complementary pro-inflammatory pathways.

Fever reflects a physiological rise in body temperature accompanied by elevated production of adrenaline. The increased body temperature in fever is caused by shivering thermogenesis in skeletal muscle and non-shivering thermogenesis in brown adipose tissue (BAT), the latter being mediated by uncoupled oxidation of free fatty acids (FFAs). We hypothesized that an acute temperature rise to 40°C increases adrenalin-induced lipolysis in white adipocytes, thereby potentially providing FFAs as an energy substrate to sustain fever-induced thermogenesis in skeletal muscle and BAT. In 3T3-L1 and primary murine white adipocytes, isoproterenol-induced extracellular FFA accumulation was significantly increased at 40°C compared to 37°C. In contrast, isoproterenol-induced increase in extracellular glycerol concentrations and the protein levels of phosphorylated hormone sensitive lipase were comparable at both temperatures, suggesting a similar degree of lipolysis. Moreover, incubation at 40°C did neither increase isoproterenol-induced oxygen consumption nor intracellular FFA concentrations, indicating that the elevated extracellular FFA accumulation was not due to reduced intracellular consumption. Conversely, isoproterenol blunted FFA uptake into adipocytes to a significantly higher extent at 40°C compared to 37°C. Hence, an acute temperature rise to 40°C reduces FFA uptake into white adipocytes, thereby increasing extracellular FFA availability.