The role of low-density lipoprotein modified by myeloperoxidasederived oxidants on vascular dysfunction in atherosclerosis

Abstract

Atherosclerosis is characterised by inflammation, arterial stiffening and aberrant lipid transport by deposition of low-density lipoproteins (LDL) in the artery wall that is susceptible to oxidation. Oxidised LDL is taken up by endothelial cells, which can lead to impaired vasorelaxation. There is strong evidence for the role of myeloperoxidase (MPO), an inflammatory enzyme released by neutrophils, in oxidative damage to biomolecules such as LDL in atherosclerosis. MPO produces the oxidants HOCl and HOSCN. While it is well established that HOCl causes extensive LDL modification, less is known regarding HOSCN in atherosclerosis, which may be of relevance to smokers whom have significantly elevated plasma levels of the MPO substrate thiocyanate (SCN-), which can shift MPO to favour HOSCN production. Therefore, modifications of LDL by HOSCN may be a contributing factor to the deleterious effects of smoking in the development of atherosclerosis. This study examined modification of the LDL protein apoB-100 by HOSCN and found it selectively cleaved the N- and C-termini by free-radical mediated reaction. However, studies into the specific mechanism and determining the cleavage sites are still required. The role of HOSCN- or HOCl-modified LDL in inducing vascular cell dysfunction was also investigated. Exposing these modified LDL to endothelial cells did not induce inflammatory adhesion molecule or cytokine expression, unfolded protein response gene expression, or alter neutrophil adhesion. However, HOSCN- and HOCl-modified LDL did significantly impair nitric oxide (NO) production by uncoupling endothelial NO synthase (eNOS). This supported functional assessment of vasorelaxation by wire myography in rat aortic segments exposed to HOSCN- or HOCl-modified LDL ex vivo, which had impaired endothelium-dependent vasodilation. In cultured vascular smooth muscle cells (VSMC) exposed to these modified LDL, the gene expression of contractile markers α-smooth muscle actin and calponin were stunted, while the synthetic marker osteopontin was elevated. This is consistent with promotion of a synthetic VSMC phenotype. The data from this study support a role for HOSCN as well as HOCl in promoting vascular dysfunction by modification of LDL, which is an important mechanism in promoting atherosclerosis.