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.
The proposal describes novel cellular mechanisms by which niacin inhibits MPO-mediated oxidative modification of HDL-apoAI and preserves functional properties of HDL, a primary lipoprotein involved in preventing atherosclerotic CHD. In view of the role of MPO in predicting future risk of coronary artery disease, the outlined studies in this application would also be very significant in defining the beneficial role of the cost- effective pharmacologic agent niacin in protecting against MPO-mediated atherosclerotic events and future coronary artery disease, and provide strong rationale for its continued use in reducing atherosclerotic coronary artery disease. Atherosclerotic cardiovascular disease is the prime cause of morbidity and death in veterans. Thus, findings from these studies will be directly relevant to the VA mission in developing long-term treatment strategies to prevent Veterans'morbidity and mortality.
