Acidosis activates GPR4 and CHOP expression, and increases cell apoptosis and cAMP levels in HR-treated HK-2 cells. The pH of culture media was adjusted to 6.4 using 0.1 M HCl immediately following HR treatment. (A) The protein expression levels of GPR4, CHOP and cleaved caspase-3 were determined by western blotting. (B) Quantitative analyses of the protein expression levels of GPR4, CHOP and cleaved caspase-3, relative to control cells. (C) The intracellular cAMP levels were measured by ELISA. *P<0.05 vs. control group without pH adjustment. #P<0.05 vs. HR group without pH adjustment. Cells in normal oxygen conditions were used as a control group. GPR4, G protein-coupled receptor 4; CHOP, CCAAT/enhancer-binding protein homologous protein; cAMP, cyclic adenosine monophosphate; HR, hypoxia/reoxygenation.