Background

Inflammation plays a key role in atherosclerosis. Sirt1 regulates transcription factors involved in inflammatory processes and blunts atherosclerosis in mice. However, its role in humans remains to be defined. This study was therefore designed to investigate the role of Sirt1 in the development of atherosclerosis.

Methods and Results

48 male subjects admitted for cardiac catheterization were subdivided into healthy subjects, patients with stable coronary artery disease (CAD), and with acute coronary syndromes (ACS). Monocytes were isolated and Sirt1 mRNA levels were determined. Sirt1 gene expression was higher in healthy subjects as compared to patients with CAD or ACS (P<0.05), respectively. Interestingly, HDL levels correlated positively with Sirt1 expression. Thus, HDL from the three groups was isolated and incubated with THP-1 monocytes to determine the effects of HDL on Sirt1 protein in controlled experimental conditions. HDL from healthy subjects stimulated Sirt1 expression in THP-1 monocytes to a higher degree than HDL from CAD and ACS patients (P<0.05). Paraoxonase-1 (PON-1), a HDL-associated enzyme, showed a reduced activity in HDL isolated from CAD and ACS patients as compared to the controls (P<0.001).

Conclusions

Monocytic Sirt1 expression is reduced in patients with stable CAD and ACS. Experiments on THP-1 monocytes suggest that this effect is HDL-dependent and is mediated by a reduced activity of HDL-associated enzyme PON1.

Incomplete combustion produces a pollutant mixture that includes polycyclic aromatic hydrocarbons (PAHs). Previous work by the Columbia Center for Children's Environmental Health (CCCEH) and others linked exposure to PAH with symptoms of asthma and other adverse health effects in young children. Inhaled β2-adrenergic agonists are mainstays in the treatment of reactive airway diseases. These exogenous catecholamines engage membrane-bound β2-adrenergic receptors (β2AR) on airway epithelial and smooth muscle cells to cause airway dilation. We hypothesized that exposure to PAH might similarly interfere with the function of β2AR in airway epithelial or smooth muscle cells, reducing the efficacy of a medication important for the treatment of asthma symptoms. A PAH mixture was devised, based on ambient levels measured prenatally among a cohort of pregnant women participating at the CCCEH. Primary airway epithelial and smooth muscle cells were exposed to varying concentrations of the PAH mixture, and expression, function, and signaling of β2AR were assessed. Murine tracheal epithelial cells and human airway smooth muscle cells, after exposure to a PAH mixture, exhibited reduced expression and function of β2AR. These findings support our hypothesis that environmentally relevant PAHs can impede β2AR-mediated airway relaxation, and suggest a new paradigm where air pollutants not only contribute to the pathogenesis of childhood asthma, but also diminish responsiveness to standard therapy.

Therapies that raise levels of HDL, which is thought to exert atheroprotective effects via effects on endothelium, are being examined for the treatment or prevention of coronary artery disease (CAD). However, the endothelial effects of HDL are highly heterogeneous, and the impact of HDL of patients with CAD on the activation of endothelial eNOS and eNOS-dependent pathways is unknown. Here we have demonstrated that, in contrast to HDL from healthy subjects, HDL from patients with stable CAD or an acute coronary syndrome (HDLCAD) does not have endothelial antiinflammatory effects and does not stimulate endothelial repair because it fails to induce endothelial NO production. Mechanistically, this was because HDLCAD activated endothelial lectin-like oxidized LDL receptor 1 (LOX-1), triggering endothelial PKCβII activation, which in turn inhibited eNOS-activating pathways and eNOS-dependent NO production. We then identified reduced HDL-associated paraoxonase 1 (PON1) activity as one molecular mechanism leading to the generation of HDL with endothelial PKCβII-activating properties, at least in part due to increased formation of malondialdehyde in HDL. Taken together, our data indicate that in patients with CAD, HDL gains endothelial LOX-1– and thereby PKCβII-activating properties due to reduced HDL-associated PON1 activity, and that this leads to inhibition of eNOS-activation and the subsequent loss of the endothelial antiinflammatory and endothelial repair–stimulating effects of HDL.