β-Adrenergic receptor stimulation induces endoplasmic reticulum stress in adult cardiac myocytes: role in apoptosis

Accumulation of misfolded proteins and alterations in calcium homeostasis induces endoplasmic reticulum (ER) stress, leading to apoptosis. In this study, we tested the hypothesis that beta-AR stimulation induces ER stress, and induction of ER stress plays a pro-apoptotic role in cardiac myocytes. Using thapsigargin and brefeldin A, we demonstrate that ER stress induces apoptosis in adult rat ventricular myocytes (ARVMs). beta-AR-stimulation (isoproterenol; 3h) significantly increased expression of ER stress proteins, such as GRP-78, Gadd-153, and Gadd-34, while activating caspase-12 in ARVMs. In most parts, these effects were mimicked by thapsigargin. beta-AR stimulation for 15 min increased PERK and eIF-2 alpha phosphorylation. PERK phosphorylation remained higher, while eIF-2 alpha phosphorylation declined thereafter, reaching to similar to 50% below basal levels at 3 h after beta-AR stimulation. This decline in eIF-2 alpha phosphorylation was prevented by beta 1-AR, not by beta 2-AR antagonist. Forskolin, adenylyl cyclase activator, simulated the effects of ISO on eIF-2 alpha phosphorylation. Salubrinal (SAL), an ER stress inhibitor, maintained eIF-2 alpha phosphorylation and inhibited beta-AR-stimulated apoptosis. Furthermore, inhibition of caspase-12 using z-ATAD inhibited beta-AR-stimulated and thapsigargin-induced apoptosis. In vivo, beta-AR stimulation induced ER stress in the mouse heart as evidenced by increased expression of GRP-78 and Gadd-153, activation of caspase-12, and dephosphorylation of eIF-2 alpha. SAL maintained phosphorylation of eIF-2 alpha, inhibited activation of caspase-12, and decreased beta-AR-stimulated apoptosis in the heart. Thus, beta-AR stimulation induces ER stress in cardiac myocytes and in the heart, and induction of ER stress plays a pro-apoptotic role.