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Insulin protects isolated hearts from ischemia/reperfusion injury: cross-talk between PI3-K/Akt and JNKs.

Our previous results have demonstrated that insulin reduces myocardial ischemia/reperfusion (MI/R) injury and increases the postischemic myocardial functions via activating the cellular survival signaling, i.e., phosphatidylinositol 3-kinase (PI3-K)-Akt-endothelial nitric oxide synthase (eNOS)-nitric oxide (NO) cascade. However, it remains largely controversial whether c-Jun NH2-terminal kinase (JNK) is involved in the effects of insulin on MI/R injury. Therefore, the aims of the present study were to investigate the role of JNK, especially the cross-talk between JNK and previously expatiated Akt signaling, in the protective effect of insulin on I/R myocardium. Isolated hearts from adult Sprague-Dawley rats were subjected to 30 min of regional ischemia and followed by 2 or 4 h of reperfusion (n=6). The hearts were pretreated with PI3-K inhibitor LY294002, or phosphorylated-JNK inhibitor SP600125, respectively, then perfused retrogradely with insulin, and the mechanical functions of hearts, including the heart rate (HR), left ventricular developed pressure (LVDP) and instantaneous first derivation of left ventricular pressure (+/-LVdp/dt(max)) were measured. At the end of reperfusion, the infarct size (IS) and apoptotic index (AI) were examined. MI/R caused significant cardiac dysfunction and myocardial apoptosis (strong TUNEL-positive staining). Compared with the control group, insulin treatment in MI/R rats exerted protective effects as evidenced by reduced myocardial IS [(28.9 +/- 2.0)% vs (45.0 +/- 4.0) %, n=6, P<0.01], inhibited cardiomyocyte apoptosis [decreased AI: (16.0 +/- 0.7) % vs (27.6 +/- 1.3) %, n=6, P<0.01] and improved recovery of cardiac systolic/diastolic function (including LVDP and +/-LVdp/dt(max)) at the end of reperfusion. Moreover, insulin resulted in 1.7-fold and 1.5-fold increases in Akt and JNK phosphorylation in I/R myocardium, respectively (n=6, P<0.05). Inhibition of Akt activation with LY294002 abolished, and inhibition of JNK activation with SP600125 enhanced the cardioprotection by insulin, respectively. And the abolishment by LY294002 could be partly converted by SP600125 pretreatment. In addition, SP600125 also decreased the Akt phosphorylation (n=6, P<0.05). These results demonstrate that insulin simultaneously activates both Akt and JNK, and the latter further increases the phosphorylation of Akt which attenuates MI/R injury and improves heart function; this cross-talk between Akt and JNK in the insulin signaling is involved in insulin-induced cardioprotective effect.