15-Keto-PGE2 acts as a biased/partial agonist to terminate PGE2-evoked signaling.

Prostaglandin E2 (PGE2) is well-known as an endogenous proinflammatory prostanoid synthesized from arachidonic acid by the activation of cyclooxygenase-2. E type prostanoid (EP) receptors are cognates for PGE2 that have four main subtypes: EP1 to EP4. Of these, the EP2 and EP4 prostanoid receptors have been shown to couple to Gαs-protein and can activate adenylyl cyclase to form cAMP. Studies suggest that EP4 receptors are involved in colorectal homeostasis and cancer development, but further work is needed to identify the roles of EP2 receptors in these functions. After sufficient inflammation has been evoked by PGE2, it is metabolized to 15-keto-PGE2 Thus, 15-keto-PGE2 has long been considered an inactive metabolite of PGE2 However, it may have an additional role as a biased and/or partial agonist capable of taking over the actions of PGE2 to gradually terminate reactions. Here, using cell-based experiments and in silico simulations, we show that PGE2-activated EP4 receptor-mediated signaling may evoke the primary initiating reaction of the cells, which would take over the 15-keto-PGE2-activated EP2 receptor-mediated signaling after PGE2 is metabolized to 15-keto-PGE2 The present results shed light on new aspects of 15-keto-PGE2, which may have important roles in passing on activities to EP2 receptors from PGE2-stimulated EP4 receptors as a "switched agonist." This novel mechanism may be significant for gradually terminating PGE2-evoked inflammation and/or maintaining homeostasis of colorectal tissues/cells functions.

PGE1 and E3 show lower efficacies than E2 to ß-catenin-mediated activity as biased ligands of EP4 prostanoid receptors.

The 2-series of prostaglandin E (PGE2 ) is regarded as a pro-cancer prostanoid, whereas the 1-series (PGE1 ) and the 3-series (PGE3 ) are considered to act as anti-cancer prostanoids. In the present study, we provide possible reasons why PGE1 and PGE3 , but not PGE2 , exert anti-cancer effects by focusing on each diverged E-type prostanoid (EP)4 receptor-mediated signaling pathway. PGE1 , PGE2 and PGE3 function as full agonists in terms of Gαs - and Gαi -protein-mediated signaling. However, PGE1 and PGE3 function as partial agonists of T-cell factor (TCF)/ß-catenin (ß-cat)-mediated activity, the well-known cancer-related signaling pathway. Furthermore, pretreatment with PGE1 or PGE3 almost completely reduces PGE2 -induced TCF/ß-cat activity. These results provide a plausible reason why PGE1 and PGE3 function as anti-cancer prostanoids as a result of novel biased activity for EP4 receptors.