Functions of the Human OST-alpha and OST-beta proteins

描述(由申请人提供)：本申请的目的是促进对两类主要胆固醇代谢物的处置机制的了解，即胆汁酸和类固醇结合物，这两种化合物在许多生物过程和人类疾病中发挥重要作用。这一提议的背景是我们证明了有机溶质转运体Ost？Ost？在调节这些化合物的处置中发挥着核心作用。Ost？-Ost？是一种不同寻常的异构体转运蛋白，几乎在所有组织中都有表达，但在小肠、肾脏、肝脏、睾丸、肾上腺以及其他类固醇合成组织中含量最高。到目前为止，我们对转运蛋白的底物特异性、转运机制、组织分布、亚细胞定位、转录调控以及我们最近发现的Ost？缺陷小鼠的表型的研究表明，Ost？-Ost？是胆汁酸和结构相关分子的主要基侧膜输出物。特别是，对Ost？缺陷小鼠的研究表明，这些动物的胆汁酸池明显缩小，肠道胆汁酸吸收缺陷，肠道肥大，生长迟缓，血清胆固醇和甘油三酯水平下降。由于胆汁酸的肝-肠循环对肠道吸收膳食脂肪和维生素、胆固醇稳态、胆汁流动和胆汁脂肪分泌等过程是必不可少的，这些数据表明Ost？-Ost？也间接调节脂质的动态平衡。这项提议的目标是检验两个假设：第一，Ost？功能OST既是伴侣又是结构成分？-OST？第二，Ost？-Ost？介导的胆汁酸和相关类固醇的转运调节脂质稳态。其具体目的是：(1)鉴定Ost？对异二聚化、运输和/或功能活动至关重要；以及(2)检查OST？-/-小鼠是否对年龄相关、饮食或遗传诱导的肥胖具有抵抗力。总之，这些研究将为OST？-OST？的机制提供重要信息。调节胆汁酸和类固醇的处置，关于转运蛋白对脂质稳态的潜在贡献，并将提供结构-功能信息，应促进针对该转运蛋白的治疗药物的开发。 公共卫生相关性：我们最近的研究提供了强有力的证据表明，有机溶质转运体Ost？-Ost？在调节胆汁酸和类固醇结合物的处置方面发挥着核心作用，这些化合物在许多生物学过程和人类疾病中发挥着重要作用，包括胆汁淤积性和脂肪性肝病、吸收不良综合征和胆石症。这项提议的目标是检验假设，即Ost？功能OST既是伴侣又是结构成分？-OST？Ost？-Ost？介导的胆汁酸及其相关分子的转运调节脂质稳态。总体而言，拟议的研究将提供有关OST？-OST？调节胆汁酸和类固醇的处置，关于转运蛋白对脂质稳态的潜在贡献，并将提供结构-功能信息，应促进针对该转运蛋白的治疗药物的开发。这种疗法可能对许多与胆汁酸或脂质稳态失衡有关的人体状况有益。

DESCRIPTION (provided by applicant): The objectives of this application are to advance understanding of mechanisms of disposition of the two major classes of cholesterol metabolites, namely bile acids and steroid conjugates, compounds that play important roles in a number of biological processes and human diseases. The background to this proposal is our demonstration that the organic solute transporter, Ost?-Ost?, plays a central role in mediating the disposition of these compounds. Ost?-Ost? is an unusual heteromeric transporter that is expressed in nearly all tissues, but is most abundant in the small intestine, kidney, liver, testis, adrenal gland, as well as other steroidogenic tissues. Our studies to date of the transporter's substrate specificity, transport mechanism, tissue distribution, subcellular localization, transcriptional regulation, as well as the phenotype of our recently characterized Ost?- deficient mice suggest that Ost?-Ost? is the major basolateral membrane exporter of bile acids and structurally related molecules. In particular, studies with Ost?-deficient mice revealed that these animals have a markedly diminished bile acid pool size, a defect in intestinal bile acid absorption, intestinal hypertrophy, growth retardation, and a decrease in serum cholesterol and triglyceride levels. Because the enterohepatic circulation of bile acids is essential for processes such as intestinal absorption of dietary fats and vitamins, cholesterol homeostasis, bile flow, and biliary lipid secretion, these data suggest that Ost?-Ost? also indirectly regulates lipid homeostasis. The goals of this proposal are to test two hypotheses: first, that Ost? is both a chaperone and a structural component of the functional Ost?-Ost? transporter; and second, that the Ost?-Ost?-mediated transport of bile acids and related steroids modulates lipid homeostasis. The specific aims are: (1) Identify specific amino acid residues of Ost? that are critical for heterodimerization, trafficking, and/or functional activity; and (2) Examine whether Ost?-/- mice are resistant to age related, dietary, or genetically induced obesity. Overall, these studies will provide important information on the mechanism by which Ost?-Ost? mediates bile acid and steroid disposition, on the potential contribution of the transporter to lipid homeostasis, and will provide structure-function information that should facilitate the development of therapeutics targeting this transporter. PUBLIC HEALTH RELEVANCE: Our recent studies have provided strong evidence that the organic solute transporter, Ost?-Ost?, plays a central role in mediating the disposition of bile acids and steroid conjugates, compounds that play important roles in a number of biological processes and human diseases, including cholestatic and fatty liver diseases, malabsorption syndrome, and cholelithiasis. The goals of this proposal are to test the hypotheses that Ost? is both a chaperone and a structural component of the functional Ost?-Ost? transporter, and that the Ost?-Ost?- mediated transport of bile acids and related molecules modulates lipid homeostasis. Overall, the proposed studies will provide important information on the mechanism by which Ost?-Ost? mediates bile acid and steroid disposition, on the potential contribution of the transporter to lipid homeostasis, and will provide structure- function information that should facilitate the development of therapeutics targeting this transporter. Such therapeutics could be of benefit in many human conditions related to imbalances in bile acid or lipid homeostasis.