Kobayashi Satoru, Mao Kai, Zheng Hanqiao, Wang Xuejun, Patterson Cam, O'Connell Timothy D, Liang Qiangrong
Cardiovascular Research Institute, Sanford Research, University of South Dakota Sanford School of Medicine, Sioux Falls, SD 57105, USA.
J Biol Chem. 2007 Jul 27;282(30):21945-52. doi: 10.1074/jbc.M703048200. Epub 2007 May 24.
Hyperglycemia is an independent risk factor for diabetic heart failure. However, the mechanisms that mediate hyperglycemia-induced 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.
高血糖是糖尿病性心力衰竭的一个独立危险因素。然而，介导高血糖诱导心脏损伤的机制仍知之甚少。转录因子GATA4对心脏内环境稳定至关重要，在心脏中，其蛋白质水平会因多种病理应激而显著降低。在本研究中，我们调查了高血糖是否会影响心肌细胞中GATA4的表达，以及增强GATA4信号传导是否能减轻高血糖诱导的心肌细胞损伤。在培养的大鼠心肌细胞中，与正常葡萄糖（NG，5.5 mmol/L）相比，高糖（HG，25或40 mmol/L）显著降低了GATA4蛋白水平。等量的甘露醇不影响GATA4蛋白表达（NG组为100±12%；甘露醇组为97±8%，而HG组为43±16%，p<0.05）。GATA4 mRNA含量，无论是稳态还是多核糖体相关的，均保持不变。HG诱导的GATA4减少可被特异性蛋白酶体抑制剂MG262逆转。如蛋白酶体报告基因所示，HG并未激活心肌细胞中的泛素蛋白酶体系统（UPS），它也未增加蛋白酶体亚基的肽酶活性或蛋白质表达。然而，在HG处理的心肌细胞中，泛素-蛋白质异肽连接酶（E3）热休克蛋白70相互作用蛋白（CHIP）的羧基末端mRNA水平显著增加。CHIP过表达促进GATA4蛋白降解，而小干扰RNA介导的CHIP敲低可防止HG诱导的GATA4耗竭。此外，GATA4过表达可阻止HG诱导的心肌细胞死亡。同样，在链脲佐菌素诱导的糖尿病小鼠和db/db糖尿病小鼠的心脏中，GATA4蛋白水平降低（分别为对照组的44±7%和67±13%，p<0.05），这与CHIP mRNA丰度增加相关。总之，GATA4蛋白降解增加可能是导致高血糖心脏毒性的一个重要机制。
