Free Fatty Acids and Hepatic Insulin Resistance

描述（由申请人提供）：导致内源性葡萄糖生成（EGP）增加的肝脏胰岛素抵抗是2型糖尿病（T2 DM）发病机制的主要因素。肥胖个体中常见的游离脂肪酸（FFA）血浆水平升高已被证实可引起外周（肌肉）以及肝脏胰岛素抵抗。近年来，大多数研究工作都集中在外周胰岛素抵抗。主要是由于方法学问题，肝脏胰岛素抵抗尚未得到重视。然而，最近，几种方法已经变得可用，其允许非侵入性测量EGP的两个组分--糖原生成（GNG）和糖原分解（GL）的体内速率。使用2 H2O方法，我们最近在健康志愿者中发现，血浆FFA的急性升高通过抑制GL的胰岛素抑制作用引起肝脏胰岛素抵抗。在当前的应用程序中，我们计划扩展这些发现。在具体目标1中，我们建议在健康受试者和轻度和重度T2 DM患者中评估血浆FFA急性升高对GL/EGP胰岛素抑制作用的剂量依赖性、持续时间和可能的性别差异。这些研究将涉及在正常血糖-高胰岛素钳夹（正常对照）或等血糖-高胰岛素钳夹（T2 DM患者）期间测量GL、GNG和EGP的速率，同时输注和不输注肝素化脂质（以不同的输注速率）以将血浆FFA急性升高至不同水平。在具体目标2中，我们建议评估血浆FFA长期升高对肝脏胰岛素敏感性的影响。实验方法将是在轻度或重度T2 DM肥胖患者中使用Niaspan（烟酸类似物）降低血浆FFA过夜（12 h），并在第二天早晨（高胰岛素钳夹期间）测量GL/EGP的胰岛素抑制。在具体目标3中，我们将讨论FFA升高导致肝脏胰岛素抵抗的机制。具体而言，我们将测试FFA介导的肝胰岛素抵抗与肝内二酰基甘油（DAG）的积累，蛋白激酶C（PKC）的激活，IRS-1/2的丝氨酸磷酸化增加和酪氨酸磷酸化减少，以及PI 3激酶对胰岛素的反应减少相关的假设。实验方法将是在有和没有脂质/肝素输注的情况下进行的高胰岛素-正常血糖钳夹期间的不同时间间隔处死大鼠，并测定DAG、PKC活性和亚型、IRS-1/2、酪氨酸磷酸化和PI 3激酶活性的肝脏浓度。这些研究将有望为FFA诱导的肝脏胰岛素抵抗的重要细节和机制提供急需的信息。

DESCRIPTION (provided by applicant): Hepatic insulin resistance resuIting in increased endogenous glucose production (EGP) is a major factor in the pathogenesis of type 2 diabetes (T2DM) Increased plasma levels of free fatty acids (FFAs) which are characteristically seen in obese individuals, have been established to cause peripheral (muscle) as well as hepatic insulin resistance. Most of the research efforts in recent years have focused on peripheral insulin resistance. Mainly due to methodological problems, hepatic insulin resistance has not received much attention. Recently, however, several methods have become available which allow non-invasive measurement of in vivo rates of gluconeogenesis (GNG) and glycogenolysis (GL), the two components of EGP. Using the 2H2O method, we have recently shown in healthy volunteers that acute elevations of plasma FFAs cause hepatic insulin resistance through inhibition of insulin suppression of GL. In the current application, we plan to expand these findings. In Specific Aim 1, we propose to assess dose dependency, duration and possible gender differences of the effects of acute elevation of plasma FFAs on insulin suppression of GL/EGP in healthy subjects and in patients with mild and severe T2DM. These studies will involve measurements of rates of GL, GNG and EGP during euglycemic-hyperinsulinemic clamping (in normal controls) or during isoglycemic-hyperinsulinemic clamping (in patients with T2DM) with and without simultaneous infusion of heparinized lipid (at different rates of infusion) to acutely raise plasma FFAs to different levels. In Specific Aim 2, we propose to evaluate effects of prolonged elevation of plasma FFAs on hepatic insulin sensitivity. The experimental approach will be to lower plasma FFAs overnight (12 h) with Niaspan (a nicotinic acid analog) and to measure insulin suppression of GL/EGP the next morning (during hyperinsulinemic clamping) in obese patients with mild or severe T2DM. In Specific Aim 3, we will address the mechanism by which elevated FFAs cause hepatic insulin resistance. Specifically, we will test the hypothesis that FFA mediated hepatic insulin resistance is associated with intrahepatic accumulation of diacylglycerol (DAG), activation of protein kinase C (PKC), with increased serine and decreased tyrosine phosphorylation of IRS-1/2, and a decrease in PI3 kinase responses to insulin. The experimental approach will be to sacrifice rats at various time intervals during hyperinsulinemic-euglycemic clamping performed with and without lipid/heparin infusions and determine hepatic concentrations of DAG, PKC activity and isoforms, IRS-1/2, tyrosine phosphorylation and PI3 kinase activity. These studies will hopefully provide much needed information relative to important details and mechanisms of FFA induced hepatic insulin resistance.