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经达到流行病的程度，尿酸结石是这些人常见的健康问题。尿酸结石的发病率和流行率随着时间的推移而上升，给生活质量和医疗费用带来了重大负担。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，但病理生理联系一直难以捉摸。驱动尿酸沉淀的最重要因素是尿液ph值过低。我们已经证明，这源于肾脏在利用氨来缓冲质子方面的内在缺陷，未缓冲的质子自由地将尿酸滴定为不溶性尿酸，并启动了级联的产石作用。我们建议结合细胞培养，动物和人类研究，采用磁共振波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近端小管作为“调节”效应的一部分的优先运输，游离脂肪酸进入肾脏的吸收增加。2. 游离脂肪酸的增加为ATP的生成提供了代谢底物，从而减少了其他底物的消耗，如谷氨酰胺，这是近端小管氨生成的主要来源。这种底物竞争或代谢开关可以降低主要尿缓冲氨的形成，即使在没有损伤近端小管的情况下也是如此。3. 随着近端小管持续的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，有毒的脂质代谢物可能会积累。我们有证据表明，在西方饮食中普遍存在的过量饱和脂肪会导致近端小管脂肪毒性，表现为内质网（ER）渗漏/应激，我们提出氨生成缺陷是更广泛的脂肪毒性表型的一部分。我们进一步提出，一种特定脂质物种的积累可能是毒性的原因。4. 为了测试人类近端小管脂肪变性和脂肪毒性是否具有功能后果，我们将研究尿酸结石患者。在先前的研究中，噻唑烷二酮类药物（TZD）可以减少动物的肾脂肪变性和脂毒性，并改善氨的排泄，我们在人类尿酸结石患者中启动了TZD或安慰剂的随机干预试验。中期分析显示，在TZD治疗6个月后，结石患者的尿液生化参数得到改善，尿酸沉淀倾向降低。我们将继续这项试验，但增加一种新的高灵敏度的方法来无创测量肾脏脂肪，测试尿生化的改善是否与肾脏脂肪的减少有关。本研究提出了肾小管脂质生物学和脂质毒性的基本概念，并将在临床上将尿酸结石的治疗模式从经经验的尿碱化转变为特异性的肾脂肪减少。我们还将介绍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.