PTGS2 and colorectal cancer: PGE2 derived from tumor or stroma cells induces high levels of COX-2 expression in MDSCs through prostaglandin E (EP) 2/EP4 receptors, and COX-2 consecutively initiates the autocrine production of endogenous PGE2 and stabilizes the suppressive functions of MDSCs.249 Recently, a novel signaling circuit has been demonstrated in colorectal cancer.