The effect of hypoglucose on cardiac apoptosis: Insulin signaling pathway and c -Jun N -terminal kinase function

Cardiac myocytes cultured under conditions of hypoxia-reoxygenation were utilized as an in vitro model to study ischemia-reperfusion-induced cardiac apoptosis. JNK activation is strongly induced by reoxygenation in this model, but its function in hypoxia-reoxygenation-induced apoptosis is regulated by the glucose concentration in hypoxia medium. Through inhibition of JNK activation with dn-JNK or SP600125 (JNK inhibitor), I found that in hypoxic conditions JNK activity is anti-apoptotic when cells were incubated with medium containing a physiological concentration of glucose (5.5 mM), but pro-apoptotic when cells were incubated with low glucose medium (0.5 mM). The central questions were: (1) why does JNK switch from pro-apoptotic function under low glucose to anti-apoptotic function under normal glucose; (2) the molecular mechanism of the pro-apoptotic function of JNK in low glucose condition. My results indicate that, under the low glucose condition, JNK activation during reoxygenation phosphorylates IRS-1 on Ser307 and suppresses Akt activation that is essential for cell survival, thereby promoting cell death. Furthermore, we found that low glucose promoted AMPK activation and enhanced insulin sensitivity in cardiac myocytes under hypoxia. AICAR (AMPK activator) increased insulin sensitivity and diminished the anti-apoptotic function of JNK under normal glucose conditions, suggesting that the differential sensitivity of the PI3K-Akt signaling pathway under normal or low glucose conditions is involved in the 'glucose switch' of JNK function. The signaling pathways whereby hypoglycemia enhanced insulin activity were further studied. My results show that hypoglycemia promotes basal and insulin-stimulated Akt activity through a pathway that requires AMPK, PI3K and IRS-1. It was further demonstrated that hypoglycemia increased association of PI3K with IRS-1 as well as the phosphorylation of IRS-1 on Ser789 (AMPK phosphorylation site). Therefore, hypoglucose stimulates AMPK and leads to phosphorylation of IRS-1 on Ser789 by AMPK, and enhances the sensitivity of the protective IRS-I-PI3K-Akt signaling pathway. JNK inhibits this pathway through phosphorylatiog IRS-1 on Ser307 and promotes apoptosis. These results describe a novel pathway of signaling in response to glucose starvation in cardiac myocytes that is initiated by AMPK and involves signals associated with IRS-1, PI3K and Akt. The net effect is to increase Akt phosphorylation and promote cell survival.