CEACAM AND INSULIN ACTION

我们将继续研究CEACAM1在胰岛素代谢中的作用。CEACAM1是肝脏中胰岛素受体激酶的底物。在最初四年的资助中，我们已经证明了肝脏CEACAM1在胰岛素受体内吞复合体的形成中起着关键作用，以促进胰岛素的摄取和降解，这一事件构成了肝脏中胰岛素清除的基本机制。利用表达显性-负性突变体CEACAM1的转基因小鼠，我们发现CEACAM1在肝脏中的功能失活损害了胰岛素的清除，并导致高胰岛素血症、脂代谢改变和内脏脂肪增加。我们建议使用Ceacam1缺陷(Cc1-/-)小鼠来研究CEACAM1功能改变引起的代谢综合征的发病机制。与人类基因组只有一个CEACAM基因不同，小鼠基因组含有两个基因(Cc1和CC2)，它们的蛋白产物在组织中的分布不同，CEACAM1是肝脏的主要亚型，CEACAM2是肾脏和胰腺b细胞的主要亚型。我们提供的初步数据显示，单个Cc1/-和CC2-/-基因敲除以明显不同的机制发展代谢异常。CC1/-缺失的小鼠出现胰岛素清除受损和胰岛素抵抗而不患糖尿病，而CC2-/-小鼠则发展为糖尿病并伴有胰岛素分泌受损。我们还发现，在其他肥胖和糖尿病的啮齿动物模型和高脂饮食的小鼠中，肝脏CEACAM1的表达减少。在这些观察的基础上，在目标1中，我们将研究Cc1-/-小鼠胰岛素抵抗的机制是否源于胰岛素清除受损。在目标2中，我们将确定恢复CEACAM1表达是否逆转肥胖啮齿动物的代谢异常。在目标3中，我们将探讨CC2-/-基因敲除中b细胞功能障碍的机制，以及恢复胰腺b细胞中CEACAM2的表达是否能拯救CC2-/-小鼠的糖尿病表型。在目标4中，我们将通过产生缺乏这两种异构体(Cc1-/-/CC2-/-)的小鼠来研究这两个基因是否发挥重叠或不同的功能。拟议的研究应该描绘出一种新的胰岛素抵抗糖尿病的机制，这种机制突出了胰岛素代谢在调节胰岛素敏感性方面的作用，与胰岛素信号截然不同。

We will continue studies of the role of CEACAM1 in insulin metabolism. CEACAM1 is a substrate of the insulin receptor kinase in the liver. In the first four years of funding, we have demonstrated that hepatic CEACAM1 plays a pivotal role in the formation of the insulin-receptor endocytosis complex to promote insulin uptake and degradation, an event that constitutes the basic mechanism of insulin clearance in liver. Using transgenic mice expressing a dominant-negative mutant CEACAM1, we have shown that functional inactivation of CEACAM1 in liver impairs insulin clearance and results in hyperinsulinemia, altered lipid metabolism and increased visceral adiposity. We propose to investigate the pathogenesis of the metabolic syndrome caused by altered CEACAM1 function using Ceacam1-deficient (Cc1-/-) mice. In contrast to the human genome, which harbors a single Ceacam gene, the mouse genome harbors two genes (Cc1 and Cc2), differing in the tissue distribution of their protein products, with CEACAM1 being the predominant liver isoform and CEACAM2 being the predominant isoform in kidney and pancreatic b-cells. We present preliminary data showing that individual Cc1/- and Cc2-/- knockouts develop metabolic abnormalities with apparently different mechanisms. Cc1-/- null mice develop impaired insulin clearance and insulin resistance without diabetes, while Cc2-/- develop diabetes with impaired insulin secretion. We also show decreased hepatic CEACAM1 expression in other models of obesity and diabetes in rodents and in mice on a high-fat diet. Based on these observations, in Aim 1 we will investigate whether the mechanism of insulin resistance in Cc1-/- mice is due to impaired insulin clearance. In Aim 2, we will determine whether restoring CEACAM1 expression reverses the metabolic abnormalities of obese rodents. In Aim 3, we will investigate the mechanism of b-cells dysfunction in Cc2-/- knockouts, and whether restoring CEACAM2 expression in pancreatic b-cells rescues the diabetic phenotype of Cc2-/- mice. In Aim 4, we will study whether the two genes play overlapping or distinct functions by generating mice lacking both isoforms (Cc1-/-/Cc2-/-). The proposed studies should delineate a novel mechanism of insulin-resistant diabetes, one that highlights the role of insulin metabolism in regulating insulin sensitivity distinctly from insulin signaling.