GR 63799X, an EP3 receptor agonist, induced S phase arrest and 3T6 fibroblast growth inhibition

The importance of arachidonic acid metabolites on the control of cell growth, particularly those derived from cyclooxygenase pathway has long been recognized. Recently, we observed that prostaglandin E2 (PGE2) interaction with EP1 and EP4 receptors is involved in serum-induced 3T6 fibroblast growth due to their effect at various levels of the cell cycle machinery. This study shows that prostanoid EP3 receptor was expressed in 3T6 fibroblast. We studied the role of EP3 receptor agonist GR 63799X in serum-induced 3T6 cell proliferation. This was concentration-dependent inhibit (IC50 ∼10 μM) to a complete inhibition without any cytotoxic or proapoptotic effect. The prostanoid EP3 receptor agonist treatment decreased the G0/G1 and G2/M populations whereas cells were accumulated in S phase. This arrest in S phase was associated with a decrease in cyclin B levels and the enhancement of p21 expression. Our data show that EP3 agonist decreases cAMP levels in our experimental conditions. Interestingly, the S arrest caused by prostanoid EP3 receptor agonist seems to be cAMP dependent, at least in part, because forskolin treatment allowed S-arrested cells to progress through cell cycle and consequently growth. Thus, our results suggest that PGE2 EP3 receptor interaction may be involved in serum-induced 3T6 fibroblast growth due to their effects on cAMP levels and on cell cycle machinery of the S phase.