Pathogenesis of Uric Acid Nephrolithiasis: The Multifaceted Role of Renal Lipids

描述（由申请人提供）：肥胖和代谢综合征已达到流行病的比例，尿酸结石是这些人常见的卫生保健问题。尿酸结石的发病率和患病率随着时间的推移而上升，对生活质量和医疗保健费用造成重大负担。虽然肥胖/代谢综合征和尿酸结石之间的流行病学联系是无可辩驳的，但病理生理学联系一直难以捉摸。导致尿酸沉淀的最重要因素是尿液pH值过低。我们已经证明，这源于利用氨缓冲质子的内在肾脏缺陷，未缓冲的质子自由地滴定成不溶性尿酸并启动级联反应。成石。我们建议使用细胞培养，动物和人体研究的组合，采用磁共振光谱和单光子发射计算机断层扫描的一些最新技术，结合经典的生理学，生物化学和分子生物学来测试四个相互关联的假设。1.由于较高的循环水平以及作为“调节”作用的一部分的近端小管的优先转运，肾脏对游离脂肪酸的摄取增加。2.游离脂肪酸的增加提供了ATP生成的代谢底物，因此减少了其他底物如谷氨酰胺的消耗，谷氨酰胺是近端小管产氨的主要来源。这种底物竞争或代谢转换可以降低主要尿缓冲氨的形成，即使在近端小管没有损伤的情况下。3.随着近端小管的持续脂质负荷超过其氧化能力，脂质储存首先被激活，但随着时间的推移，毒性脂质代谢物可能会积聚。我们有证据表明，过量的饱和脂肪，这是普遍存在于西方饮食，导致近端小管脂毒性表现为内质网（ER）泄漏/应力，我们提出，缺陷ammoniagenesis是一个更广泛的脂毒性表型的一部分。我们进一步提出，一个特定的脂质物质的积累可能是负责的毒性。4.为了测试人类近端小管脂肪变性和脂毒性是否具有功能性后果，我们将研究尿酸结石形成者。之前已经证明噻唑烷二酮（TZD）可以减少动物的肾脏脂肪变性和脂毒性并改善铵排泄，我们已经在人类尿酸结石形成者中启动了一项使用TZD或安慰剂的随机干预试验。中期分析显示，TZD治疗6个月后，结石形成者的尿生化参数改善，尿酸沉淀倾向降低。我们将继续这项试验，但增加了一种新的高灵敏度的方法来非侵入性地测量肾脂肪，测试尿生化的改善是否与肾脂肪的减少有关。该提案阐述了肾小管脂质生物学和脂毒性的基本概念，并在临床上将尿酸结石治疗的范式从经验性尿碱化转变为肾脂肪的特异性减少。我们还将介绍用于肾脏研究的尖端人体成像研究。

DESCRIPTION (provided by applicant): Obesity and the metabolic syndrome have reached epidemic proportions and uric acid stones are a common health care problem in these individuals. The incidence and prevalence of uric acid stones are escalating with time, imparting a significant burden on quality of life and health care cost. While the epidemiologic link between obesity / metabolic syndrome and uric acid stones is irrefutable, the pathophysiologic link has been elusive. The single most important factor driving uric acid precipitation is unduly low urine pH. We have shown that this stems from an intrinsic renal defect in the utilization of ammonia to buffer protons, with unbuffered protons free to titrate urate to insoluble uric acid and initiate te cascade of lithogenesis. We propose to use a combination of cell culture, animal, and human studies employing some of the latest technologies in magnetic resonance spectroscopy and single-photon emission computed tomography, combined with classical physiology, biochemistry, and molecular biology to test four interrelated hypotheses. 1. There is increased uptake of free fatty acids into the kidney as a result of higher circulating levels as well as preferential transport by the proximal tubule as part of a "conditioning" effect. 2. The increased provision of free fatty acid supplies metabolic substrate for ATP generation hence reducing the consumption of other substrates such as glutamine, which is the principal source of ammoniagenesis by the proximal tubule. This substrate competition, or metabolic switch, can lower the formation of the major urinary buffer ammonia, even in the absence of injury to the proximal tubule. 3. With sustained lipid loading of the proximal tubule that exceeds its oxidative capacity, lipid storage is first activated but with time, toxic lipid metabolites may build up. We have evidence that excess saturated fat, which is prevalent in the Western diet, leads to proximal tubule lipotoxicity manifested as endoplasmic reticulum (ER) leakage/stress, and we propose that defective ammoniagenesis is part of a broader lipotoxic phenotype. We further propose that accumulation of a specific lipid species may be responsible for the toxicity. 4. To test whether proximal tubule steatosis and lipotoxicity in humans have a functional consequence, we will study uric acid stone formers. Having previously shown that thiazolidinediones (TZD) reduce renal steatosis and lipotoxicity and improve ammonium excretion in animals, we have initiated a randomized intervention trial with TZD or placebo in human uric acid stone formers. The interim analysis showed that after 6 months of TZD therapy, stone formers had improved urinary biochemical parameters and reduced propensity for uric acid precipitation. We will continue this trial but add a novel highly sensitive method to non-invasively measure renal fat, testing whether improvement in urinary biochemistry associates with reduction of renal fat. This proposal addresses fundamental concepts of renal tubular lipid biology and lipotoxicity, and clinically will shift the paradigm of uric acid stone therapy from empiric urinary alkalinization to specific reduction in renal fat. We will also introduce cutting-edge human imaging studies for kidney research.