LDL protein nitration: Implication for LDL protein unfolding

Abstract

Oxidatively- or enzymatically-modified low-density lipoprotein (LDL) is intimately involved in the initiation and progression of atherosclerosis. The in vivo modified LDL is electro-negative (LDL −) and consists of peroxidized lipid and unfolded apoB-100 protein. This study was aimed at establishing specific protein modifications and conformational changes in LDL − assessed by liquid chromatography/tandem mass spectrometry (LC/MS/MS) and circular dichroism analyses, respectively. The functional significance of these chemical modifications and structural changes were validated with binding and uptake experiments to- and by bovine aortic endothelial cells (BAEC). The plasma LDL − fraction showed increased nitrotyrosine and lipid peroxide content as well as a greater cysteine oxidation as compared with native- and total-LDL. LC/MS/MS analyses of LDL − revealed specific modifications in the apoB-100 moiety, largely involving nitration of tyrosines in the α-helical structures and β 2 sheet as well as cysteine oxidation to cysteic acid in β 1 sheet. Circular dichroism analyses showed that the α-helical content of LDL − was substantially lower (∼25%) than that of native LDL (∼90%); conversely, LDL − showed greater content of β-sheet and random coil structure, in agreement with unfolding of the protein. These results were mimicked by treatment of LDL subfractions with peroxynitrite (ONOO −) or SIN-1: similar amino acid modifications as well as conformational changes (loss of α-helical structure and gain in β-sheet structure) were observed. Both LDL − and ONOO −-treated LDL showed a statistically significant increase in binding and uptake to- and by BAEC compared to native LDL. We further found that most binding and uptake in control-LDL was through LDL-R with minimal oxLDL-R-dependent uptake. ONOO −-treated LDL was significantly bound and endocytosed by LOX-1, CD36, and SR-A with minimal contribution from LDL-R. It is suggested that lipid peroxidation and protein nitration may account for the mechanisms leading to apoB-100 protein unfolding and consequential increase in modified LDL binding and uptake to and by endothelial cells that is dependent on oxLDL scavenger receptors.