Effects of oxidation on structural stability and remodeling of human very low density lipoprotein

Effects of triacylglycerol on the structural remodeling of human plasma very low- and low-density lipoproteins

Very low-density lipoprotein (VLDL) is the main plasma carrier of triacylglycerol that is elevated in pathological conditions such as diabetes, metabolic syndrome, obesity and dyslipidemia. How variations in triacylglycerol levels influence structural stability and remodeling of VLDL and its metabolic product, low-density lipoproteins (LDL), is unknown. We applied a biochemical and biophysical approach using lipoprotein remodeling by lipoprotein lipase and cholesterol ester transfer protein, along with thermal denaturation that mimics key aspects of lipoprotein remodeling in vivo. The results revealed that increasing the triacylglycerol content in VLDL promotes changes in the lipoprotein size and release of the exchangeable apolipoproteins. Similarly, increased triacylglycerol content in LDL promotes lipoprotein remodeling and fusion. These effects were observed in single-donor lipoproteins from healthy subjects enriched in exogenous triolein, in single-donor lipoproteins from healthy subjects with naturally occurring differences in endogenous triacylglycerol, and in LDL and VLDL from pooled plasma of diabetic and normolipidemic patients. Consequently, triacylglycerol-induced destabilization is a general property of plasma lipoproteins. This destabilization reflects a direct effect of triacylglycerol on lipoproteins. Moreover, we show that TG can act indirectly by increasing lipoprotein susceptibility to oxidation and lipolysis and thereby promoting the generation of free fatty acids that augment fusion. These in vitro findings are relevant to lipoprotein remodeling and fusion in vivo. In fact, fusion of LDL and VLDL enhances their retention in the arterial wall and, according to the response-to-retention hypothesis, triggers atherosclerosis. Therefore, enhanced fusion of triacylglycerol-rich lipoproteins suggests a new causative link between elevated plasma triacylglycerol and atherosclerosis.

VLDL Induced Modulation of Nitric Oxide Signalling and Cell Redox Homeostasis in HUVEC

High levels of circulating lipoprotein constitute a risk factor for cardiovascular diseases, and in this context, the specific role of the very-low-density lipoproteins (VLDL) is poorly understood. The response of human umbilical vein endothelial cells (HUVEC) to VLDL exposure was studied, especially focusing on the pathways involved in alteration of redox homeostasis and nitric oxide (NO) bioavailability. The results obtained by the analysis of the expression level of genes implicated in the NO metabolism and oxidative stress response indicated a strong activation of inducible NO synthase (iNOS) upon 24 h exposure to VLDL, particularly if these have been preventively oxidised. Simultaneously, both mRNA and protein expression of endothelial NO synthase (eNOS) were decreased and its phosphorylation pattern, at the key residues Tyr495 and Ser1177, strongly suggested the occurrence of the eNOS uncoupling. The results are consistent with the observed increased production of nitrites and nitrates (NOx), reactive oxygen species (ROS), 3-nitrotyrosine (3-NT), and, at mitochondrial level, a deficit in mitochondrial O₂ consumption. Altogether, these data suggest that the VLDL, particularly if oxidised, when allowed to persist in contact with endothelial cells, strongly alter NO bioavailability, affecting redox homeostasis and mitochondrial function.

Structural stability and functional remodeling of high-density lipoproteins

Lipoproteins are protein-lipid nanoparticles that transport lipids in circulation and are central in atherosclerosis and other disorders of lipid metabolism. Apolipoproteins form flexible structural scaffolds and important functional ligands on the particle surface and direct lipoprotein metabolism. Lipoproteins undergo multiple rounds of metabolic remodeling that is crucial to lipid transport. Important aspects of this remodeling, including apolipoprotein dissociation and particle fusion, are mimicked in thermal or chemical denaturation and are modulated by free energy barriers. Here we review the biophysical studies that revealed the kinetic mechanism of lipoprotein stabilization and unraveled its structural basis. The main focus is on high-density lipoprotein (HDL). An inverse correlation between stability and functions of various HDLs in cholesterol transport suggests the functional role of structural disorder. A mechanism for the conformational adaptation of the major HDL proteins, apoA-I and apoA-II, to the increasing lipid load is proposed. Together, these studies help understand why HDL forms discrete subclasses separated by kinetic barriers, which have distinct composition, conformation and functional properties. Understanding these properties may help improve HDL quality and develop novel therapies for cardiovascular disease.

Oxidative modification of lipoproteins

Lipoproteins consist of lipids and apolipoproteins that have functional roles in lipid metabolism. It has been suggested that oxidation of lipoproteins by reactive oxygen species (ROS) may be involved in the inception of various diseases. In particular, the relationship between low-density lipoprotein (LDL) oxidation and atherosclerosis has been studied in great detail. The main target molecules of lipoprotein oxidation are polyunsaturated fatty acid residues of lipids and apolipoproteins. Extensive investigations have characterized oxidative modifications of apolipoprotein B100 (apo B100) in LDL. Furthermore, modifications of apo B100 by oxidized lipids have been confirmed in oxidized LDL and atherosclerotic lesions using immunological techniques. In this chapter, characteristics and oxidation mechanisms of lipoproteins by ROS are described from in vitro and in vivo studies. Oxidative modifications of apo B100 by lipid hydroperoxides, major products of lipid peroxidation at the early stage, are principally reported.

Effects of phospholipase A(2) and its products on structural stability of human LDL: relevance to formation of LDL-derived lipid droplets

Hydrolysis and oxidation of LDL stimulate LDL entrapment in the arterial wall and promote inflammation and atherosclerosis via various mechanisms including lipoprotein fusion and lipid droplet formation. To determine the effects of FFA on these transitions, we hydrolyzed LDL by phospholipase A(2) (PLA(2)), removed FFA by albumin, and analyzed structural stability of the modified lipoproteins. Earlier, we showed that heating induces LDL remodeling, rupture, and coalescence into lipid droplets resembling those found in atherosclerotic lesions. Here, we report how FFA affect these transitions. Circular dichroism showed that mild LDL lipolysis induces partial β-sheet unfolding in apolipoprotein B. Electron microscopy, turbidity, and differential scanning calorimetry showed that mild lipolysis promotes LDL coalescence into lipid droplets. FFA removal by albumin restores LDL stability but not the protein conformation. Consequently, FFA enhance LDL coalescence into lipid droplets. Similar effects of FFA were observed in minimally oxidized LDL, in LDL enriched with exogenous FFA, and in HDL and VLDL. Our results imply that FFA promote lipoprotein coalescence into lipid droplets and explain why LDL oxidation enhances such coalescence in vivo but hampers it in vitro. Such lipid droplet formation potentially contributes to the pro-atherogenic effects of FFA.