The proton-activated receptor GPR4 modulates glucose homeostasis by increasing insulin sensitivity. 
The proton-activated G protein-coupled receptor GPR4 is expressed in many tissues including white adipose tissue.  GPR4 is activated by extracellular protons in the physiological pH range (i.e. pH 7.7 - 6.8) and is coupled to the production of cAMP.  We examined mice lacking GPR4 and examined glucose tolerance and insulin sensitivity in young and aged mice as well as in mice fed with a high fat diet.  Expression profiles of pro- and anti-inflammatory cytokines in white adipose tissue, liver and skeletal muscle was assessed.  Here we show that mice lacking GPR4 have an improved intraperitoneal glucose tolerance test and increased insulin sensitivity.  Insulin levels were comparable but leptin levels were increased in GPR4 KO mice.  Gpr4-/- showed altered expression of PPARa, IL-6, IL-10, TNFa, and TGF-1b in skeletal muscle, white adipose tissue, and liver.  High fat diet abolished the differences in glucose tolerance and insulin sensitivity between Gpr4+/+ and Gpr4-/- mice.  In contrast, in aged mice (12 months old), the positive effect of GPR4 deficiency on glucose tolerance and insulin sensitivity was maintained.  Liver and adipose tissue showed no major differences in the mRNA expression of pro- and anti-inflammatory factors between aged mice of both genotypes.  Thus, GPR4 deficiency improves glucose tolerance and insulin sensitivity.  The effect may involve an altered balance between pro- and anti-inflammatory factors in insulin target tissues.