Inflammation, oxidative stress, and repair capacity of the vascular endothelium in obstructive sleep apnea

Abstract

Background: Indirect evidence implicates endothelial dysfunction in the pathogenesis of vascular diseases associated with obstructive sleep apnea (OSA). We investigated directly whether dysfunction and inflammation occur in vivo in the vascular endothelium of patients with OSA. The effects of continuous positive airway pressure (CPAP) therapy on endothelial function and repair capacity were assessed.

Methods and results: Thirty-two patients with newly diagnosed OSA and 15 control subjects were studied. Proteins that regulate basal endothelial nitric oxide (NO) production (endothelial NO synthase [eNOS] and phosphorylated eNOS) and inflammation (cyclooxygenase-2 and inducible NOS) and markers of oxidative stress (nitrotyrosine) were quantified by immunofluorescence in freshly harvested venous endothelial cells before and after 4 weeks of CPAP therapy. Vascular reactivity was measured by flow-mediated dilation. Circulating endothelial progenitor cell levels were quantified to assess endothelial repair capacity. Baseline endothelial expression of eNOS and phosphorylated eNOS was reduced by 59% and 94%, respectively, in patients with OSA compared with control subjects. Expression of both nitrotyrosine and cyclooxygenase-2 was 5-fold greater in patients with OSA than in control subjects, whereas inducible NOS expression was 56% greater. Expression of eNOS and phosphorylated eNOS significantly increased, whereas expression of nitrotyrosine, cyclooxygenase-2, and inducible NOS significantly decreased in patients who adhered to CPAP > or = 4 hours daily. Baseline flow-mediated dilation and endothelial progenitor cell levels were lower in patients than in control subjects, and both significantly increased in patients who adhered to CPAP > or = 4 hours daily.

Conclusions: OSA directly affects the vascular endothelium by promoting inflammation and oxidative stress while decreasing NO availability and repair capacity. Effective CPAP therapy is associated with the reversal of these alterations.

Figure 1

Quantitative immunofluorescence analysis of eNOS protein expression in representative venous endothelial cells harvested from a control subject. Nuclear (blue) (A) and von Willebrand factor (green) (B) fluorescent staining identified cells and determined their endothelial origin. Cy-3 (red) fluorescent image of eNOS (C) was digitized and processed (D).

Figure 2

Representative immunofluorescent images from a control subject and untreated patient with OSA. Expression of eNOS and P-eNOS in harvested venous endothelial cells was lower in the OSA patient than in the control subject, whereas expression of iNOS, nitrotyrosine, and COX-2 was greater.

Figure 3

Expression of eNOS, P-eNOS, iNOS, nitrotyrosine, and COX-2 in harvested venous endothelial cells. Baseline expression of eNOS and P-eNOS in venous endothelial cells was lower in patients with OSA (n=30) than in control subjects (n=15), whereas expression of nitrotyrosine, COX-2, and iNOS was greater.

Figure 4

Effects of adherence with CPAP therapy on protein expression in venous endothelial cells in patients with OSA. A, Expression of eNOS and P-eNOS was increased significantly, whereas expression of nitrotyrosine, COX-2, and iNOS decreased significantly when patients adhered to CPAP ≥4 hours daily (n=14). B, Protein expression did not change when patients used CPAP <4 hours daily or declined CPAP therapy (n=16).