Chronic β-adrenoreceptor stimulation in vivo decreased Bcl-2 and increased Bax expression but did not activate apoptotic pathways in mouse heart

Prolonged activation of the sympathetic nervous system is deleterious to heart function. In vitro β1-adrenergic activation promotes apoptosis, whereas β2-adrenergic activation reduces apoptosis in cultured adult cardiomyocytes. To determine the effect of chronic catecholamine infusion in vivo, we measured apoptosis marker expression in C57Bl/6 and catecholamine-sensitive Egr-1 deficient mice after treatment with the nonspecific β-adrenergic agonist, isoproterenol, the β1-specific agonist, dobutamine, or the β2-specific agonist, metaproterenol. Antiapoptotic and proapoptotic protein expression, cytochrome c release and caspases 3, 9, and 12 activation products were measured on immunoblots. Catecholamine-treated mice had decreased Bcl-2 and increased Bax and BNIP1 expression, suggesting mitochondria-dependent apoptosis pathway activation. However, cytosolic cytochrome c or caspase 3 or 9 activation products were not detected. In mice, increased molecular chaperone expression and caspase 12 activation characterize endoplasmic-reticulum-driven apoptosis. Clusterin expression was increased in catecholamine-treated mice, but GRP78 expression was not increased, and caspase 12 activation products were not detected. Thus, neither the mitochondrial nor the endoplasmic apoptotic pathway was fully activated. Further, Egr-1 deficiency did not increase cardiac apoptosis. We conclude that although chronic in vivo infusion of β1- or β2-adrenergic receptor agonists partially activates the apoptosis program, full activation of the caspase cascade requires more, or other, cardiac insults.Key words: apoptosis, catecholamine infusion, mice, Egr-1.