Signaling and metabolic functions of nSMase-2 in hepatic steatosis and onset of insulin resistance

修订摘要： 胰岛素抵抗的出现是代谢综合征及其并发症的一个标志。肝脏和其他组织(脂肪和肌肉)也会产生胰岛素抵抗，从而导致高血糖和II型糖尿病的发生。肝脏胰岛素抵抗的机制尚不清楚，但神经酰胺被认为是主要的罪魁祸首之一。神经酰胺是一种具有生物活性的脂肪代谢产物。针对神经酰胺从头合成途径的药物已显示出在改善小鼠代谢综合征方面的有效性。然而，这些方法通常对所有神经鞘脂脂产生非常广泛的影响，包括结构和生物活性，而不是仅对神经酰胺产生影响。由于神经酰胺在肝细胞的代谢“健康”中扮演着不同的角色，它所在的亚细胞位置的范围，以及它产生的代谢途径的多样性，需要针对不同神经酰胺效应的特定方法才能成功地转移到临床，但目前还不能获得。这项提议中要检验的假设是，一种名为中性鞘磷脂酶-2(nSMase-2，基因名称smpd3)的酶在调节肝细胞的胰岛素反应中发挥着独特的作用。中性鞘磷脂酶-2是四种不同的酶之一，可以通过鞘磷脂的周转来产生神经酰胺。这一假设得到了有力的初步数据的支持，这些数据记录了这一假说的关键方面。拟议的计划将确定这一新的信号轴在细胞对胰岛素反应中的潜在机制。目的1在体外破译质膜上nSMase-2对神经酰胺稳态和胰岛素信号转导的影响。目的2在体内检测nSMase-2与胰岛素抵抗的因果关系，以及质膜上产生的神经酰胺对体内新生脂肪生成、脂肪分泌、脂毒性和糖异生的影响。最近开发的肝脏特异性基因敲除小鼠将用于这些研究。

Revised ABSTRACT: The onset of insulin resistance is a hallmark of metabolic syndrome and its complications. Together with other tissues (the adipose and the muscle) liver also becomes insulin resistant, thus contributing to the onset of hyperglycemia and type II diabetes. The mechanisms of hepatic insulin resistance are not well understood; but ceramide, a bioactive lipid metabolite is considered one of the main culprits. Drugs targeting pathways for ceramide de novo synthesis have shown effectiveness in ameliorating aspects of metabolic syndrome in the mouse. These approaches however typically exert a very broad effect on all sphingolipids, both structural and bioactive, instead on ceramide alone. Due to the diverse roles that ceramide play in metabolic “health” of hepatocytes, the range of subcellular locations where it resides, and the variety of metabolic pathways for its generation, specific approaches to target distinct ceramide effects are needed for successful translation to the clinic, but are not available at the moment. The hypothesis to be tested in this proposal is that an enzyme called Neutral Sphingomyelinase-2 (nSMase-2, gene name smpd3), one of four distinct enzymes that can generates ceramide by the turnover of sphingomyelin, plays an unique role in the regulation of the insulin response of hepatocytes. This hypothesis is supported by strong preliminary data documenting its key aspects. The proposed plan will identify the underlying mechanisms of this novel signaling axis in cellular response to insulin. Aim 1 will decipher the impact of nSMase-2 at the plasma membrane on ceramide homeostasis and insulin signaling in vitro. Aim 2 is to test the cause-effect relationship between nSMase-2 and insulin resistance in vivo, and examine the impact of ceramide generated at the plasma membrane on de novo lipogenesis, lipid secretion, lipotoxicity and gluconeogenesis in vivo. A recently developed liver specific knockout mouse will be used in these studies.