Role of SGLT3 in diabetes-mediated increased renal sodium reabsorption

描述(申请人提供)：糖尿病肾病是美国终末期肾病的主要原因。高血糖已被认为是糖尿病肾病发生发展的主要危险因素。在糖尿病患者和糖尿病动物模型中，高血糖诱导的近端小管(PT)钠滞留增加已被证明。这种增强的钠重吸收可能在高血压的发展中起作用，高血压是糖尿病肾病和终末期肾脏疾病发展的另一个促成因素。PT钠-葡萄糖共转运体(SGLT)在高血糖诱导的钠滞留增加中的作用已被提出，但该转运体尚未被确定。肾脏在PT细胞顶侧表达SGLT1和SGLT2。这些蛋白在钠依赖的肾小球滤液葡萄糖摄取中起着关键作用。已在人肾癌细胞和猪肾中发现了SGLT3mRNA。当人SGLT3在非洲爪哇卵母细胞中过表达时，它不转运葡萄糖，但对葡萄糖的反应，它介导了钠的内流。小鼠有两个编码SGLT3a和3b的基因，我们已经证明它们的mRNAs在小鼠肾脏和培养的小鼠肾脏细胞中都有表达。我们还发现，虽然镉暴露培养的小鼠原代肾细胞SGLT3s的mRNA水平比未处理的细胞高出数倍，但镉处理细胞对葡萄糖的钠依赖摄取减少了，这支持了SGLT3不是葡萄糖转运体。基于上述研究和对SGLT3激动剂脱氧诺吉霉素(DNJ)的初步研究，我们推测肾脏SGLT3可能是一种新的葡萄糖刺激的PT中的钠转运体，可能在高血糖诱导的糖尿病钠滞留中发挥作用。为了验证我们的假设，我们建议：(1)定位SGLT3蛋白在人和小鼠肾脏中的位置；(2)确定SGLT3在体外葡萄糖介导的PT细胞钠摄取中的作用；(3)确定SGLT3在体内高血糖介导的PT Na重吸收中的作用。这项体外和体内研究旨在探讨SGLT3作为一种新的葡萄糖刺激的钠转运蛋白的作用，它可能在糖尿病患者增强钠重吸收中发挥作用。

DESCRIPTION (provided by applicant): Diabetic nephropathy is the leading cause of end-stage renal disease in the United States. Hyperglycemia has been recognized as a major risk factor in the development and progression of diabetic nephropathy. Hyperglycemia-induced increase in proximal tubule (PT) Na+ retention has been shown in diabetic patients and in animal models of diabetes. This enhanced Na+ reabsorption may play a role in the development of hypertension, which is an additional contributing factor in the development of diabetic nephropathy and the end stage renal disease. A role for a PT sodium-glucose cotransporter (SGLT) in hyperglycemia-induced increased sodium retention has been suggested but this transporter has not yet been identified. Kidney expresses SGLT1 and SGLT2 on the apical side of the PT cells. These proteins play crucial roles in sodium-dependent uptake of glucose from the glomerular filtrate. SGLT3 mRNA has been found in the human kidney carcinoma cells and in the pig kidney. When over-expressed in Xenopus oocytes, human SGLT3 did not transport glucose but in response to glucose, it mediated inward flux of sodium. Mouse has two genes encoding SGLT3a and 3b and we have shown that their mRNAs were expressed in the kidney and in the cultured kidney cells from mouse. We also showed while mRNA levels of SGLT3s in cultured mouse primary kidney cells exposed to cadmium were several folds higher than in their levels in untreated cells, the sodium-dependent uptake of glucose in Cd-treated cells had decreased supporting that SGLT3 is not a glucose transporter. Based on the above and additional preliminary studies with the potent agonist of SGLT3, deoxynojirimycin (DNJ), we hypothesize that kidney SGLT3 serves as a novel glucose-stimulated Na+ transporter in the PT that may play role in hyperglycemia-induced Na+ retention in diabetes. To test our hypothesis, we propose: (1) To localize the SGLT3 protein in human and mouse kidneys, (2) To determine the role of SGLT3 in glucose-mediated Na+ uptake in PT cells in vitro, (3) To determine the role of SGLT3 in hyperglycemia-mediated PT Na+ reabsorption in vivo. The proposed in vitro and in vivo studies are designed to investigate the role of SGLT3 as a novel glucose-stimulated Na+ transporter that may play role in the enhanced Na+ reabsorption in diabetes.