Exchange Proteins directly Activated by cAMP (EPAC): Involvement in electrophysiological remodeling and underlying signaling in human atrial cardiomyocyte

EPAC proteins act as downstream effectors of cAMP pathway. EPAC activation lengthens Action Potential Duration (APD) and dysruptes Ca2+ homeostasis in ventricular myocytes. In atria , APD modulations contribute to pathogenesis of Atrial Fibrillation (AF). Recent studies revealed EPAC proteins as a putative AF promotor in mice by APD lengthening. However, the EPAC-induced variation of human atrial cardiomyocyte electrophysiology is not yet described. To evaluate the influence of EPAC proteins in human atrial cardiomyocytes electrophysiology . Right auricular appendages were collected from patients in Sinus Rhythm (SR) and AF undergoing open-heart surgery. Action Potentials and K+ currents (IK) were recorded in human isolated cardiomyocytes or in cultured HL-1 atrial myocytes with the patch-clamp technique. EPAC proteins were acutely activated or inhibited by pharmacological tools. EPAC proteins levels and phosphorylation states of NO-synthases were measured by Western-Blot. In human atrial cardiomyocytes, EPAC activation with 8-CPTAM (10 μM) lengthened APD by downregulation of both transient peak and sustained (IKsus) components of repolarizing K+ currents. Pre-treatment of myocytes with selective EPAC1 or EPAC2 pharmacological blockers (AM-001, 20 μM and ESI-05, 25 μM, respectively) revealed that both EPAC isoforms participate in this effect. Mechanistically, pharmacological inhibition of NO-synthases (NOS) and PKG significantly reduced the EPAC-induced decrease of IKsus. The inhibition of AMPK , a positive regulator of NOS activity, significantly reduced the EPAC-dependent IKsus alteration, whereas the blockade of Akt, a NOS activator, failed to influence EPAC effect. In HL-1 myocytes, a similar downregulation of IKsus occured after EPAC activation with 8-CPTAM. In addition, 8-CPTAM treatment significantly increased the phosphorylation level of the endothelial isoform of NOS in its activating site Ser1177, whereas AMPK inhibition prevented this increase. Of note, 8-CPTAM treatment did not modify Reactive Oxygen Species levels in HL-1 cells. Interestingly, we found that the EPAC1 isoform, but not EPAC2, was overexpressed in right auricular appendages from AF patients compared to SR patients. Consistent with this finding, the EPAC1 blocker AM-001 significantly rectified the IK alteration in cardiomyocytes from AF patients treated with 8-CPTAM, but not in cells from SR patients. EPAC stimulation lengthens APD by decreasing IK through an AMPK-eNOS-PKG pathway in human atrial cardiomyocytes. Finally, alteration of EPAC1 signaling could promote AF onset.