cAMP analog mapping of Epac1 and cAMP kinase - Discriminating analogs demonstrate that Epac and cAMP kinase act synergistically to promote PC-12 cell neurite extension

Little is known about the relative role of cAMP-dependent protein kinase (cAPK) and guanine exchange factor directly activated by cAMP ( Epac) as mediators of cAMP action. We tested cAMP analogs for ability to selectively activate Epac1 or cAPK and discriminate between the binding sites of Epac and of cAPKI and cAPKII. We found that commonly used cAMP analogs, like 8-Br-cAMP and 8-pCPT-cAMP, activate Epac and cAPK equally as well as cAMP, i.e. were full agonists. In contrast, 6-modified cAMP analogs, like N-6-benzoyl-cAMP, were inefficient Epac activators and full cAPK activators. Analogs modified in the 2'-position of the ribose induced stronger Epac1 activation than cAMP but were only partial agonists for cAPK. 2'-O-Alkyl substitution of cAMP improved Epac/cAPK binding selectivity 10-100-fold. Phenylthio substituents in position 8, particularly with MeO- or Cl- in p-position, enhanced the Epac/cAPK selectivity even more. The combination of 8-pCPT- and 2'-O-methyl substitutions improved the Epac/cAPK binding selectivity about three orders of magnitude. The cAPK selectivity of 6-substituted cAMP analogs, the preferential inhibition of cAPK by moderate concentrations of Rp-cAMPS analogs, and the Epac selectivity of 8-pCPT-2'-O-methyl-cAMP was also demonstrated in intact cells. Using these compounds to selectively modulate Epac and cAPK in PC-12 cells, we observed that analogs selectively activating Epac synergized strongly with cAPK specific analogs to induce neurite outgrowth. We therefore conclude that cAMP-induced neurite outgrowth is mediated by both Epac and cAPK.