RBP2 Biology and Pathobiology

视黄醇结合蛋白2(RBP2)早在30多年前就被鉴定为维甲酸(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，并且仅从其在饮食中对维甲酸摄取和代谢的作用进行研究。我们出人意料地发现了RBP2在维持体重、对葡萄糖挑战的正常反应以及正常的空腹肝脏甘油三酯水平方面的作用。6-7月龄雄性Rbp2基因缺陷(Rbp2-/-)小鼠仅以对照饲料喂养时，白色脂肪组织的体重显著增加，对葡萄糖挑战的反应明显较差，肝脏脂肪含量显著高于野生型对照小鼠。当两个月大的雄性Rbp2-/-小鼠被喂食高脂饮食6周或更长时间时，也观察到同样的代谢表型。尽管我们试图确定可能解释这些代谢表型的维甲酸依赖机制，但我们没有发现组织维甲酸水平或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与单酰甘油(MAG)结合，包括1-和2-花生四烯基甘油、2-油酰甘油和2-亚油酰甘油，亲和力非常高，与视黄醇与RBP2结合的亲和力相当。基于这些和其他数据，我们得出结论，RBP2是一个生理上相关的MAG结合蛋白。我们建议深入了解RBP2在调节肠上皮细胞MAG代谢中的作用(S)。我们认为这是朝着我们的长期目标迈出的第一步，该目标是确定我们在喂养食物和高脂饮食的雄性Rbp2-/-小鼠中观察到的代谢表型背后的分子过程。我们假设这些表型涉及RBP2作为MAG结合蛋白的作用。为此，我们提出了三个具体目标。在特定的目标1中，我们将鉴定能够与RBP2结合的非视黄酸类脂，并建立RBP2的结构属性，以促进或阻止非视黄酸配体与RBP2的结合。在特定的目标2中，我们将阐明RBP2介导的与肠道细胞内MAG摄取和代谢相关的生化途径。在具体目标3中，我们将在小鼠身上研究RBP2如何在空肠内影响代谢表型发育，以响应饲喂以饱和脂肪为主或以不饱和脂肪为主的不同高脂肪饮食，以及这些饮食如何影响MAG和类维A/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.