Diminished GATA4 Protein Levels Contribute to Hyperglycemia-induced Cardiomyocyte Injury

Abstract

Hyperglycemia is an independent risk factor for diabetic heart failure. However, the mechanisms that mediate hyperglycemiainduced cardiac damage remain poorly understood. The transcription factor GATA4 is essential for cardiac homeostasis, and its protein levels are dramatically reduced in the heart in response to diverse pathologic stresses. In this study, we investigated if hyperglycemia affects GATA4 expression in cardiomyocytes and if enhancing GATA4 signaling could attenuate hyperglycemia-induced cardiomyocyte injury. In cultured rat cardiomyocytes, high glucose (HG, 25 or 40 mM) markedly reduced GATA4 protein levels as compared with normal glucose (NG, 5.5 mM). Equal amount of mannitol did not affect GATA4 protein expression (NG, 100 ؎ 12%; mannitol, 97 ؎ 8%, versus HG, 43 ؎ 16%, p < 0.05). The GATA4 mRNA content, either steady-state or polysome-associated, remained unchanged. HG-induced GATA4 reduction was reversed by MG262, a specific proteasome inhibitor. HG did not activate the ubiquitin proteasome system (UPS) in cardiomyocytes as indicated by a UPS reporter, nor did it increase the peptidase activities or protein expression of the proteasomal subunits. However, the mRNA levels of ubiquitin-protein isopeptide ligase (E3) carboxyl terminus of Hsp70-interacting protein (CHIP) were markedly increased in HG-treated cardiomyocytes. CHIP overexpression promoted GATA4 protein degradation, whereas small interfering RNA-mediated CHIP knockdown prevented HG-induced GATA4 depletion. Moreover, overexpression of GATA4 blocked HG-induced cardiomyocyte death. Also, GATA4 protein levels were diminished in the hearts of streptozotocin and db/db diabetic mice (44 ؎ 7% and 67 ؎ 13% of control, p < 0.05), which correlated with increased CHIP mRNA abundance. In summary, increased GATA4 protein degradation may be an important mechanism that contributes to hyperglycemic cardiotoxicity.Diabetes is a major risk factor for the development of various cardiovascular diseases, including atherosclerosis, hypertension, and diabetic cardiomyopathy, which collectively constitute the leading causes of mortality from diabetes. Diabetic cardiomyopathy, independent of vascular pathology, is now recognized as an important causative factor for the heightened risk of heart failure and mortality in diabetic patients (1, 2). The characteristic metabolic abnormalities of diabetes include hyperlipidemia, hyperinsulinemia (type 2 diabetes), and hyperglycemia. The development of diabetic cardiomyopathy correlates with the duration and severity of hyperglycemia. Hyperglycemia induces cardiac damage through a number of biochemical mechanisms, including the formation of advanced glycation end products (3), altered calcium homeostasis (4), enhanced renin-angiotensin system (5), and protein kinase C activation (6). Ultimately, it is thought that hyperglycemia increases oxidative stress inducing cardiomyocyte death leading to heart failure in animals (7, 8) and humans (9) with diabetes. In fact, high concentrations of glucose induce the generation of r...

Suppression of autophagy is protective in high glucose-induced cardiomyocyte injury

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“…Neonatal rat ventricular cardiomyocytes were isolated from 0-to 2-d old Harlan Sprague-Dawley rat neonates using a kit from Worthington Biochemical Corporation (LK003300) as described previously. 12 After tissue digestion, cells were preplated for 1 h to remove non-myocytes and then plated on gelatinized dishes and cultured overnight in Dulbecco's modified essential medium (DMEM; Cellgro, 10-017CV) with 15% bovine serum. The following day, the cells were washed in phosphate-buffered saline and cultured in serum-free DMEM containing 100 mM of 5-bromo 2'-deoxy-uridine (BrdU; Sigma, B5002).…”

“…[4][5][6] Indeed, the cardiotoxic effects of hyperglycemia have been corroborated in numerous in vitro and in vivo studies. High levels of glucose can directly induce cardiomyocyte death in culture, [7][8][9][10][11][12][13][14] which has been attributed to excessive production of reactive oxygen species (ROS) triggered by high glucose. The critical role of oxidative stress in mediating diabetic cardiac injury has been strongly supported by the ability of various antioxidants to prevent or slow the development of diabetic cardiomyopathy in animal studies.…”

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