Human GPR4 and the Notch signaling pathway in endothelial cell tube formation

G protein-coupled receptor 4 (GPR4) is hypothesized to function as a pH sensor and is important in the regulation of proliferation, migration and angiogenesis of vascular endothelial cells (ECs). Furthermore, the Notch signaling pathway is significant in the regulation of the angiogenic behavior of ECs. However, whether GPR4 regulates angiogenesis via the Notch signaling pathway remains unclear. The present study evaluated the effect of Notch signaling in human GPR4-induced angiogenesis in HMEC-1 cells. The results revealed that GPR4 increased Notch1 expression in a time-dependent manner. In addition, the inhibition of Notch1 expression using small interfering RNA or the Notch receptor inhibitor, γ-secretase inhibitor I, significantly blocked GPR4-induced HMEC-1 tube formation and lymphocyte transendothelial migration. Furthermore, the inhibition of Notch1 blocked GPR4-induced vascular endothelial growth factor and hypoxia-inducible factor 1α expression. Thus, it was demonstrated that GPR4 affects ECs by regulating Notch1, a function that may be important for physiological and pathological angiogenesis.