Role of SGLT3 in diabetes-mediated increased renal sodium reabsorption

描述（由申请人提供）：糖尿病肾病是美国终末期肾脏疾病的主要原因。高血糖已被认为是糖尿病肾病发生和发展的主要危险因素。在糖尿病患者和糖尿病动物模型中显示，高血糖诱导的近端小管（PT） Na+潴留增加。这种增强的Na+重吸收可能在高血压的发展中起作用，这是糖尿病肾病和终末期肾病发展的另一个促进因素。PT钠-葡萄糖共转运体（SGLT）在高血糖诱导的钠潴留增加中的作用已被提出，但这种转运体尚未被确定。肾脏在PT细胞的顶侧表达SGLT1和SGLT2。这些蛋白在从肾小球滤液中摄取钠依赖的葡萄糖中起关键作用。SGLT3 mRNA在人肾癌细胞和猪肾中均有表达。当在爪蟾卵母细胞中过表达时，人SGLT3不运输葡萄糖，但作为对葡萄糖的响应，它介导了钠的向内流动。小鼠有两个编码SGLT3a和3b的基因，我们已经证明它们的mrna在肾脏和培养的小鼠肾细胞中表达。我们还发现，虽然暴露于镉的培养小鼠原代肾细胞中SGLT3的mRNA水平比未处理的细胞高几倍，但cd处理的细胞对葡萄糖的钠依赖性摄取减少，支持SGLT3不是葡萄糖转运蛋白。基于上述以及对SGLT3强效激动剂脱氧诺吉霉素（DNJ）的进一步初步研究，我们假设肾脏SGLT3在PT中作为一种新的葡萄糖刺激的Na+转运体，可能在糖尿病高血糖诱导的Na+潴留中发挥作用。为了验证我们的假设，我们提出：(1)在人和小鼠肾脏中定位SGLT3蛋白，(2)在体外确定SGLT3在葡萄糖介导的PT细胞Na+摄取中的作用，(3)在体内确定SGLT3在高血糖介导的PT Na+重吸收中的作用。本研究旨在探讨SGLT3作为一种新的葡萄糖刺激Na+转运体的作用，该转运体可能在糖尿病患者Na+重吸收增强中发挥作用。

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. PUBLIC HEALTH RELEVANCE: One of the major adverse health effects of diabetes is damage to the kidneys. Increased kidney salt reabsorption in diabetes may play a role in the development of high blood pressure, which in turn can cause damage to the kidneys. The mechanism of diabetes-induced increased salt reabsorption is not known. We have evidence that a novel sodium transporter may be a mediator. The goal of this study is to show that human kidneys express this protein and also to show that glucose stimulates sodium uptake by this transporter. This study is designed to identify the mechanism for the enhanced salt retention in diabetes.