Exendin-4 induces myocardial protection through MKK3 and Akt-1 in infarcted hearts

We have demonstrated that glucagon like peptide-1 (GLP-1) protects the heart against ischemic injury. However, the physiological mechanism by which GLP-1 receptor (GLP-1R) initiates cardioprotection remains to be determined. The objective of this study is to elucidate the functional roles of MAPK kinase 3 (MKK3) and Akt-1 in mediating exendin-4-elicited protection in the infarcted hearts. Adult mouse myocardial infarction (MI) was created by ligation of the left descending artery. Wild-type, MKK3^-/-, Akt-1^-/-, and Akt-1^-/-; MKK3^-/- mice were divided into one of several groups: 1) sham: animals underwent thoracotomy without ligation; 2) MI: animals underwent MI and received a daily dose of intraperitoneal injection of vehicle (saline); 3) MI + exendin-4: infarcted mice received daily injections of exendin-4, a GLP-1R agonist (0.1 mg/kg, ip). Echocardiographic measurements indicate that exendin-4 treatment resulted in the preservation of ventricular function and increases in the survival rate, but these effects were diminished in MKK3^-/-, Akt-1^-/-, and Akt-1^-/-;MKK3^-/- mice. Exendin-4 treatments suppressed cardiac hypotrophy and reduced scar size and cardiac interstitial fibrosis, respectively, but these beneficial effects were lost in genetic elimination of MKK3, Akt-1, or Akt-1^-/-;MKK3^-/- mice. GLP-1R stimulation stimulated angiogenic responses, which were also mitigated by deletion of MKK3 and Akt-1. Exendin-4 treatment increased phosphorylation of MKK3, p38, and Akt-1 at Ser129 but decreased levels of active caspase-3 and cleaved poly (ADP-ribose) polymerase; these proteins were diminished in MKK3^-/-, Akt-1^-/-, and Akt-1^-/-; MKK3^-/- mice. These results reveal that exendin-4 treatment improves cardiac function, attenuates cardiac remodeling, and promotes angiogenesis in the infarcted myocardium through MKK3 and Akt-1 pathway.