Novel mechanisms of niacin to improve HDL function: Role of myeloperoxidase

描述(由申请人提供)： 生理功能的高密度脂蛋白(HDL)对动脉粥样硬化性心血管疾病(CVD)的发展具有保护作用。高密度脂蛋白的抗动脉粥样硬化特性归因于载脂蛋白A-I(apoA-I，高密度脂蛋白的主要蛋白质)能够促进胆固醇外流，从而将多余的胆固醇从外周组织(如动脉)中清除出去，并具有抗炎特性。最近的研究表明，冠心病患者或相当于冠心病患者的高密度脂蛋白功能紊乱，缺乏动脉粥样硬化保护特性，这表明保留和/或改善高密度脂蛋白的功能特性在预防动脉粥样硬化中起着关键作用。关于了解高密度脂蛋白功能障碍的形成机制，有证据表明，中性粒细胞髓过氧化物酶(MPO)生成的氧化剂在导致高密度脂蛋白功能障碍的过程中发挥了作用。MPO生成的氧化剂催化apoA-I的修饰，包括其特定部位的氯化和硝化，导致生理功能障碍的高密度脂蛋白颗粒的形成，损害了动脉粥样硬化保护胆固醇的流出和抗炎特性。中性粒细胞和血液中的MPO水平与冠心病有关，并预测未来的心血管事件。烟酸是一种广泛用于治疗血脂异常的药物，临床试验表明，烟酸治疗可减少总死亡率，减少冠状动脉事件，延缓冠状动脉粥样硬化的进展和诱导其消退。虽然烟酸对血脂的有益作用有助于减少动脉粥样硬化性心血管疾病，但目前尚不清楚烟酸是否具有改善动脉粥样硬化的其他非脂类特性。为了填补这一空白，并促进我们在定义烟酸在动脉粥样硬化中作用的新的抗炎范式方面的知识，在这一应用中，我们提出了烟酸通过抑制循环白细胞分泌MPO，抑制MPO介导的血液中功能障碍的高密度脂蛋白的产生，从而改善胆固醇流出和高密度脂蛋白的抗炎特性的新概念。假设和特定目的：在体外初步研究的基础上，我们假设烟酸通过减少细胞内cAMP/蛋白激酶A(PKA)信号而激活循环中性粒细胞(HL-60细胞)中的Gpr109a，抑制葡萄糖-6-磷酸脱氢酶(G6PD)的磷酸化，从而导致G6PD活性增加，细胞氧化还原状态增加，NADPH和GSH水平增加。烟酸通过抑制NADPH氧化酶介导的GSH升高抑制中性粒细胞中ROS的产生。ROS的减少，通过蛋白酪氨酸磷酸酶(PTPs)介导的事件，抑制了Src和p38MAP激酶信号，导致中性粒细胞脱颗粒减少，MPO的释放及其活性减少。烟酸减少MPO的释放和活性，抑制MPO介导的功能紊乱的高密度脂蛋白的形成，从而改善高密度脂蛋白的功能。具体目标：1.明确烟酸增加细胞氧化还原状态和抑制中性粒细胞ROS产生的细胞机制：Gpr109a和G6PD的参与。2.阐明ROS介导的烟酸抑制中性粒细胞脱颗粒和MPO释放/激活的细胞机制：PTPs、Src和p38MAP激酶信号转导的作用。3.明确烟酸作用的细胞机制：MPO介导的高密度脂蛋白-载脂蛋白AI的氧化修饰，形成“功能失调的高密度脂蛋白”，改善高密度脂蛋白的抗动脉粥样硬化特性。

DESCRIPTION (provided by applicant): Physiologically functional high density lipoproteins (HDL) are protective against development of atherosclerotic cardiovascular disease (CVD). Anti-atherogenic properties of HDL have been attributed to the ability of apolipoprotein A-I (apoA-I, the major protein of HDL) to facilitate cholesterol efflux which removes excess cholesterol from peripheral tissues (such as arteries) and anti-inflammatory properties. Recent studies indicate that patients with coronary heart disease (CHD) or CHD equivalents had "dysfunctional" HDL lacking in atheroprotective properties, suggesting that retaining and/or improving functional properties of HDL play a pivotal role in protecting against atherosclerosis. In regards to understanding the mechanisms for formation of dysfunctional HDL, evidence points to a role for neutrophils myeloperoxidase (MPO)-generated oxidants as participants in rendering HDL dysfunctional. MPO-generated oxidants catalyze modification of apoA-I including its site-specific chlorination and nitration, resulting in the formation of physiologically dysfunctional HDL particles with impaired athero- protective cholesterol effluxing and anti-inflammatory properties. MPO levels in neutrophils and blood have been associated with CHD, and predict future cardiovascular events. Nicotinic acid (Niacin) is a widely used agent in the treatment of dyslipidemia, and clinical trials indicate that the treatment with niacin reduces total mortality, coronary events, and retards the progression and induces regression of coronary atherosclerosis. Although beneficial effects of niacin on lipids contribute in its action to reduce atherosclerotic CVD, it is not clear whether niacin has additional non-lipid properties that ameliorate atherosclerosis. To fill this gap and advance our knowledge in defining new anti-inflammatory paradigm for niacin action on atherogenesis, in this application we propose the novel concept that niacin, by inhibiting MPO secretion from circulating leukocytes, inhibits MPO-mediated generation of "dysfunctional HDL" in blood resulting in improvements in cholesterol effluxing and anti-inflammatory properties of HDL. Hypotheses and Specific Objectives: Based on in-vitro preliminary studies, we hypothesize that niacin- mediated activation of GPR109A in circulating neutrophils (HL-60 cells) through decreased cellular cAMP/Protein kinase A (PKA) signaling inhibits phosphorylation of glucose-6-phosphate dehydrogenase (G6PD) which in turn results in increased G6PD activity and increased cellular redox state with increased levels of NADPH and GSH. Niacin-mediated increased GSH by inhibiting NADPH oxidase inhibits reactive oxygen species (ROS) generation in neutrophils. The reduced ROS, through protein tyrosine phosphases (PTPs)-mediated events, inhibits Src and p38MAP kinase signaling resulting in reduced neutrophil degranulation and the release of MPO and its activity. Decreased MPO release and activity by niacin inhibits MPO-mediated formation of dysfunctional HDL and thus improves HDL function. Specific Objectives: 1. Identify cellular mechanisms by which niacin increases cellular redox state and inhibition of ROS production in neutrophils: Involvement of GPR109A and G6PD. 2. Elucidate ROS-mediated cellular mechanisms by which niacin inhibits neutrophil degranulation and MPO release/activation: Role of PTPs, Src, and p38MAP kinase signaling. 3. Define cellular mechanism of action of niacin on: MPO-mediated oxidative modification of HDL- apoAI, formation of "dysfunctional HDL", and improvement in HDL's anti-atherogenic properties.