Insulin Resistance Affects the Cytoprotective Effect of Insulin in Cardiomyocytes Through an Impairment of MAPK Phosphatase-1 Expression

Objective: Insulin protects cardiomyocytes from apoptosis. Insulin resistance usually refers to a defect in the ability of insulin to stimulate glucose uptake. It is unknown, however, whether or not insulin resistance compromises the cell-protective effect of the hormone. Caspases are a family of cysteine proteases that regulate apoptosis. We explored the effects of insulin resistance on hypoxia-induced caspase-3 activation in cardiomyocytes.

Methods: Experiments were performed in cultured neonatal rat cardiomyocytes. Insulin resistance was induced by treating cardiac myocytes with isoproterenol, a beta-adrenergic receptor agonist.

Results: Twelve hours of hypoxia-induced caspase-3 cleavage, which was inhibited by treatment with insulin, while pre-treatment with isoproterenol abolished the insulin effect. Hypoxia-induced cleavage of caspase-3 was mediated by p38 mitogen-activated protein kinase (MAPK). Insulin inhibited hypoxia-induced phosphorylation of p38 through MAPK phosphatase-1 (MKP-1). Insulin-induced MKP-1 expression was mediated by extracellular signal-regulated protein kinases (ERK) 1/2, c-Jun NH2-terminal kinases (JNK) MAPK, and phosphatidylinositol 3-kinase (PI3K)/Akt pathways. Isoproterenol stimulation failed to induce expression of MKP-1; moreover, insulin resistance induced by long-term beta-adrenergic stimulation inhibited insulin-evoked expression of MKP-1 by impairing insulin-induced phosphorylation of both ERK1/2 and JNK without affecting Akt kinase activity. Furthermore, concomitant activation of Akt, ERK 1/2, and JNK was required for insulin to exert its protective effect against the hypoxia-induced cleavage of caspase-3.

Conclusions: The results of this study lead to the conclusions that, in cardiac myocytes, antiapoptotic signals induced by insulin are mediated by more than one signaling pathway, and that long-term beta-adrenergic receptor stimulation impairing some of these pathways affects the cytoprotective action of insulin.