Prostaglandin E₂ (PGE₂) suppresses natural killer cell function primarily through the PGE₂ receptor EP4

The COX-2 product prostaglandin E₂ (PGE₂) contributes to the high metastatic capacity of breast tumors. Our published data indicate that inhibiting either PGE₂ production or PGE₂-mediated signaling through the PGE₂ receptor EP4 reduces metastasis by a mechanism that requires natural killer (NK) cells. It is known that NK cell function is compromised by PGE₂, but very little is known about the mechanism by which PGE₂ affects NK effector activity. We now report the direct effects of PGE₂ on the NK cell. Endogenous murine splenic NK cells express all four PGE₂ receptors (EP1-4). We examined the role of EP receptors in three NK cell functions: migration, cytotoxicity, and cytokine release. Like PGE₂, the EP4 agonist PGE₁-OH blocked NK cell migration to FBS and to four chemokines (ITAC, MIP-1α, SDF-1α, and CCL21). The EP2 agonist, Butaprost, inhibited migration to specific chemokines but not in response to FBS. In contrast to the inhibitory actions of PGE₂, the EP1/EP3 agonist Sulprostone increased migration. Unlike the opposing effects of EP4 vs. EP1/EP3 on migration, agonists of each EP receptor were uniformly inhibiting to NK-mediated cytotoxicity. The EP4 agonist, PGE₁-OH, inhibited IFNγ production from NK cells. Agonists for EP1, EP2, and EP3 were not as effective at inhibiting IFNγ. Agonists of EP1, EP2, and EP4 all inhibited TNFα; EP4 agonists were the most potent. Thus, the EP4 receptor consistently contributed to loss of function. These results, taken together, support a mechanism whereby inhibiting PGE₂ production or preventing signaling through the EP4 receptor may prevent suppression of NK functions that are critical to the control of breast cancer metastasis.