RBP2 Biology and Pathobiology

视黄醇结合蛋白2 （RBP2）最初是在30多年前被发现的，是类维甲酸（维生素A及其代谢物）的细胞内结合蛋白。RBP2仅在成人小肠中表达，主要在空肠中表达，并且仅从其在膳食类视黄醇摄取和代谢中的作用的背景下进行研究。我们意外地发现了RBP2在维持体重、对葡萄糖挑战的正常反应和正常空腹肝甘油三酯水平中的作用。当仅饲喂对照鼠粮时，6-7月龄雄性Rbp2缺陷（Rbp2-/-）小鼠的白色脂肪组织明显增加，对葡萄糖挑战的反应明显较差，肝脏脂肪明显多于匹配的野生型同窝鼠。当2个月大的雄性Rbp2-/-小鼠被喂食高脂肪饮食6周或更长时间时，观察到这些相同的代谢表型。尽管我们试图确定可能解释这些代谢表型的类维甲酸依赖机制，但我们没有检测到组织维甲酸水平或类维甲酸调节基因表达的任何差异，这些差异可以解释Rbp2-/-小鼠的代谢表型。通过生化研究，我们确定RBP2结合单酰基甘油（MAGs），包括1-和2-花生四烯醇甘油，2-油基甘油和2-亚酰基甘油具有非常高的亲和力，与视黄醇结合RBP2相当。基于这些和其他数据，我们得出结论，RBP2是生理上相关的mag结合蛋白。我们建议深入了解RBP2在介导肠细胞MAG代谢中的作用。我们认为这是实现我们长期目标的第一步，即确定我们在鼠粮喂养和高脂肪饮食喂养的雄性Rbp2-/-小鼠中观察到的代谢表型的分子过程。我们假设这些表型涉及RBP2作为mag结合蛋白的作用。为此，我们提出3个具体目标。在Specific Aim 1中，我们将鉴定能够结合RBP2的非视黄醇类脂质，并建立RBP2促进或阻止非视黄醇类配体与RBP2结合的结构特性。在Specific Aim 2中，我们将阐明rbp2介导的与肠道肠细胞内MAG摄取和代谢相关的生化途径。在具体目标3中，我们将在小鼠中研究RBP2如何在空肠中作用，影响以饱和脂肪或不饱和脂肪为主的不同高脂肪饮食的代谢表型发育，以及这些饮食如何影响MAG和类视黄醇/类胡萝卜素的吸收和代谢。

Retinol-binding protein 2 (RBP2) was originally identified more than 30 years ago as an intracellular binding protein for retinoids (vitamin A and its metabolites). RBP2 is expressed solely in the adult small intestine, primarily in the jejunum, and has been studied only from the context of its actions in dietary retinoid uptake and metabolism. We unexpectedly have identified a role for RBP2 in the maintenance of body weight, normal responses to a glucose challenge, and normal fasting hepatic triglyceride levels. When maintained solely on a control chow diet, 6–7 month-old male Rbp2-deficient (Rbp2-/-) mice accrue significantly more body weight as white adipose tissue, respond significantly less well to a glucose challenge, and possess significantly more hepatic fat than matched wild type littermate controls. These same metabolic phenotypes are observed when 2 month-old male Rbp2-/- mice are fed a high fat diet for 6 weeks or longer. Although we attempted to identify retinoid-dependent mechanisms that might account for these metabolic phenotypes, we did not detect any differences in tissue retinoic acid levels or in retinoid-regulated gene expression that could explain the metabolic phenotypes of the Rbp2-/- mice. Through biochemical studies, we established that RBP2 binds monoacylglycerols (MAGs), including both 1- and 2-arachidonoylglycerol, 2-oleoylglycerol, and 2-linoyleoylglycerol with very high affinities, comparable to that of retinol binding to RBP2. Based on these and other data, we concluded that RBP2 is a physiologically relevant MAG-binding protein. We are proposing to obtain in depth understanding of role(s) that RBP2 has in mediating enterocyte MAG metabolism. We view this as a first step towards our long-term goal of identifying the molecular processes that underlie the metabolic phenotypes we have observed in both chow fed and high fat diet fed male Rbp2-/- mice. We hypothesize that these phenotypes involve the actions of RBP2 as a MAG-binding protein. To this end, we propose 3 Specific Aims. In Specific Aim 1, we will identify non-retinoid lipids that are able to bind RBP2 and establish the structural properties of RBP2 that facilitate or prevent non-retinoid ligand binding to RBP2. In Specific Aim 2, we will elucidate RBP2-mediated biochemical pathways related to MAG uptake and metabolism within intestinal enterocytes. In Specific Aim 3, we will investigate in mice how RBP2 acts within the jejunum to affect metabolic phenotype development in response to feeding of different high fat diets consisting of either predominantly saturated or predominantly unsaturated fat and how these diets affect MAG and retinoid/carotenoid absorption and metabolism.