GPR4 and cancer: It was found that malignant tumors can affect the expression of GPR4 and vascular endothelial growth factor (VEGF) and its receptors directly or indirectly due to excessive proliferation, severe local tissue hypoxia, and the formation of an acidic environment [16–18], which affects the microvascular density of tumor tissues and thus the development, infiltration, and metastasis of tumors, and GPR4 plays an essential role in the vascular endothelial cells of angiogenesis.