ADORA2A and head and neck squamous cell carcinoma: The A2AR is overexpressed in some cancers, such as head and neck squamous cell carcinoma (HNSCC) [28,116], and in several breast and melanoma cell lines, [76,82,117] and has been shown to induce PLC, PKC, AKT, MAPK/ERK, and JNK signaling pathways to promote cell proliferation in vitro [118].