Role of Epac1 in mediating anti-proliferative effects of prostanoid EP2 receptors and cAMP in human lung fibroblasts

In lung fibroblasts, proliferation is inhibited by activation of EP2 prostanoid receptors which are known to couple to adenylyl cyclase. Beside the classic target of cAMP, protein kinase A (PKA), alternative cAMP effectors have been identified, among them Epac (exchange protein activated by cAMP). The present study aimed to illuminate transduction pathways mediating the anti-proliferative effects of EP2 receptors in lung fibroblasts. Proliferative activity of human lung fibroblasts was determined by measuring [3H]-thymidine incorporation. The selective EP2 receptor agonist butaprost inhibited 3H]-thymidine incorporation by 75%, an effect mimicked by forskolin, the phosphodiesterase inhibitor IBMX, the stable cAMP analogues dibutyryl-cAMP and bromo-cAMP, as well as by the Epac selective cAMP analogues 8-pCPT-2'-O-Me-cAMP and Sp-8-pCPT-2'-O-Me-cAMPS, whereas the PKA selective agonist 6-Bnz-cAMP was inactive. The PKA inhibitor Rp-8-Br-cAMPS inhibited butaprost-induced phosphorylation of CREB (cAMP response element-binding protein), but did not affect butaprost-induced inhibition of [3H]-thymidine incorporation. Partial knockdown of Epac1 by specific siRNA transfection resulted in a marked attenuation of the inhibitory potency of butaprost, whereas transfection of Epac2 siRNA or non-silencing siRNA did not affect the effectiveness of butaprost to inhibit [3H]-thymidine incorporation. In conclusion, Epac1 rather than the classic cAMP effector PKA is a crucial element in the signal transduction pathway mediating anti-proliferative effects of EP2 receptor activation.