ER stress: role in myocyte apoptosis and myocardial remodeling

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DESCRIPTION (provided by applicant): Heart failure represents a major cause of morbidity and mortality among veterans. Increased sympathetic activity is a central feature in patients with heart failure. Cardiac myocyte loss due to apoptosis plays an important role in the progression of heart failure. Stimulation of 2-adrenergic receptor (2-AR) increases apoptosis in cardiac myocytes in vitro and in vivo. Stimulation of 22-AR and inhibitory G-protein (Gi) protects against 2-AR-stimulated apoptosis. Recently, we provided evidence that activation of glycogen synthase kinase-32 (GSK-32) plays a pro-apoptotic role in 2-AR- stimulated apoptosis via the involvement of mitochondrial death pathway. However, molecular signals involved in the activation of GSK-32 and mitochondrial death pathway of apoptosis are largely unknown in cardiac myocytes. The endoplasmic reticulum (ER or sarcoplasmic reticulum in cardiac myocytes) is a principal site for protein folding, calcium storage and calcium signaling. ER stress induced by accumulation of misfolded proteins and alterations in calcium homeostasis can trigger apoptosis. Our recent preliminary data demonstrate that 2-AR stimulation induces ER-stress in vitro in adult cardiac myocytes and in vivo in the heart as evidenced by increased expression of GRP-78 (sensor of ER stress) and Gadd153 (a transcription factor induced by ER stress), and activation of caspase-12( a protease which plays a central role in ER stress-induced apoptosis). Increased expression of GRP-78 and Gadd153 and activation of caspase-12 was also observed in the heart following myocardial infarction (MI). Inhibition of GSK-32 inhibited 2-AR-stimulated increases in Gadd153 levels. Well-known pharmacological ER stressors (brefeldin A, thapsigargin and tunicamycin) activated GSK-32 and increased cardiac myocyte apoptosis. Salubrinal, known to protect cells from ER stress, inhibited 2-AR-stimulated increases in cardiac myocyte apoptosis in vitro. Infusion/treatment of mice with salubrinal reduced the extent of 2-AR-stimulated and MI-induced left ventricular dysfunction and cardiac myocyte apoptosis in the heart. These observations have led to our central hypothesis that 2-AR-stimulated induction of ER stress activates GSK-32, and activation of GSK-32 plays a pro-apoptotic role in 2-AR-stimulated apoptosis by augmenting ER stress and involving mitochondrial death pathway. Studies of Specific Aim 1 are focused on understanding the proximal signaling pathway leading to ER stress and cardiac myocyte apoptosis. Specific Aim 2 will use Gadd153 knockout mice to investigate the in vivo role of ER stress in apoptosis and myocardial remodeling following 2-AR stimulation and MI. Specific aim 3 will use pharmacologic, adenoviral and siRNA strategies to define the role of ER stress in the activation of GSK-32, and get an insight into the mechanism by which active GSK-32 augments ER stress and activates mitochondrial death pathway. Specific aim 4 will use transgenic mice with cardiac-specific overexpression of dominant negative GSK-32 to investigate the role of GSK-32 in cardiac myocyte apoptosis and myocardial remodeling following 2-AR stimulation and MI. The proposed studies investigating the role of ER stress in cardiac myocyte apoptosis and myocardial remodeling may uncover novel therapeutic strategies for the treatment of heart failure.