CXCL8 and head and neck squamous cell carcinoma: In Ad-GPR4 cells, the expression of IL6, IL8, and VEGFA increased significantly compared with control at pH 5.9 but not at pH 6.8 and pH 7.4, which may indicate that GPR4 is fully activated at pH 5.9 in SCCHN and this is consistent with our previous study [12].