PTGER4 and cancer: Other pathways also induce an ‘exhausted’ phenotype, including cancer cell release of transforming growth factor (TGF)-β, the activation of the adenosine A2A receptor (A2Ar) and the induction of the cyclooxygenase (COX)2-prostaglandin (PG)E2 path leading to the activation of the PGE2 receptor, (EP)4.