Recent advances in understanding endothelial dysfunction in atherosclerosis

Abstract

Over the last two decades, it has become evident that decreased bioavailability of endothelial nitric oxide (NO) produced from endothelial NO synthase (eNOS), referred to as endothelial dysfunction, plays a crucial role in the development and progression of atherosclerosis. Much progress has been made in understanding the mechanisms of decreased endothelial NO bioavailability at the levels of regulation of eNOS gene expression, eNOS enzymatic activity and NO inactivation. Initial studies suggest that increasing eNOS gene expression would improve endothelial NO release in the hope of inhibiting the progression of atherosclerosis. Recent experimental studies, however, do not always support this therapeutic concept and show some evidence that overexpression of eNOS in atherosclerosis may be even harmful for the disease progression.Thus, recent research to improve endothelial function in atherosclerosis has focused on regulation of eNOS enzymatic activity and prevention of NO inactivation by oxidative stress. Since the role of oxidative stress in endothelial NO bioavailability has been reviewed in a large number of comprehensive articles, this article focuses on the relevant regulatory mechanisms of eNOS enzymatic activity that are emerging to play a role in endothelial dysfunction in atherosclerosis.

Figures

Figure 1.

Regulatory mechanisms of endothelial NO production at three different levels. While shear stress, insulin, VEGF, estrogen, and statins increase eNOS gene expression, several factors that are involved in atherosclerotic disease, such as thrombin, TNF-α and Ox-LDL, suppress eNOS gene expression. Whether the −786T/C substitution in the promoter region reduces eNOS gene expression is not clear. Subcellular targeting of eNOS regulated by co- or post-translational modifications, such as N-myristoylation and cysteine, palmitoylation is critical for optimal NO production. The mechanism by which localization influences eNOS activation is not fully understood. Interaction of eNOS with caveolin-1 (Cav-1) and NOSIP proteins reduces eNOS enzymatic activity, and interaction with heat shock protein 90 (Hsp90) and dynamin-2 increases eNOS enzymatic activity. Increase in intracellular Ca2+ concentration forms the Ca2+-calmodulin (CaM) complex, which also interacts with eNOS and stimulates eNOS enzymatic activity. Whether the eNOS Glu298Asp variant (G894T polymorphism) affects eNOS enzymatic activity remains obscure. Phosphorylation of human eNOS at serine 1177 (Ser1177) by protein kinases Akt, PKA or AMPK enhances, whereas phosphorylation of the enzyme at threonine 495 (Thr495) by PKC inhibits eNOS enzymatic activity. The eNOS enzymatic activity is also dependent on the availability of co-factors, such as BH4, NADH, FAD, and substrate L-arginine. Increase in production of endogenous ADMA reduces NO production. Increase in oxidative stress inactivates NO resulting in decreased NO bioavailability.

Figure 2.

Mechanisms of eNOS uncoupling in atherosclerotic endothelial dysfunction. (A) Under physiological conditions, endothelial cells produce NO from L-arginine in the presence of optimal concentration of the co-factor tetrahydrobiopterin (BH4). (B) Under pathological conditions, such as atherosclerosis, a decrease in endothelial BH4 production, an increase in formation of endogenous eNOS inhibitor ADMA and an increase in arginase activity which metabolizes L-arginine into urea and ornithine, causes eNOS uncoupling, a condition that leads to superoxide anion (O2−) production and less NO release. O2− produced from eNOS uncoupling and other enzymes reacts with NO to generate a more potent oxidant peroxynitrite (ONOO−) which further inactivates BH4 and increases ADMA accumulation in endothelial cells, leading to endothelial dysfunction and may promote atherogenesis.