The effects of triacylglycerols and heparin binding on the structural stability and remodeling of very low- and low-density lipoproteins: implications for type-2 diabetes mellitus
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Plasma triacylglycerols (TG) are elevated in diabetes, metabolic syndrome, obesity, and dyslipidemia. Very-low density lipoprotein (VLDL) is the main plasma carrier of TG and the direct metabolic precursor of low-density lipoprotein (LDL), the main carrier of plasma cholesterol and the major causative risk factor for atherosclerosis. Binding of LDL to heparan sulfate on the arterial wall initiates retention and modifications of LDL in the arterial intima, triggering atherosclerosis. Studies presented in this dissertation show that variations in TG levels and lipoprotein binding to heparin, a model for heparan sulfate, alter the structural and biochemical stability of VLDL and LDL, and increase their atherogenic potential. The molecular consequences of variations in the lipoprotein TG content and LDL-heparin binding were determined by combining heparin affinity chromatography with biochemical, spectroscopic and electron microscopic techniques. Remodeling of human VLDL and LDL by thermal denaturation was used to mimic key aspects of lipoprotein remodeling in vivo. Our studies revealed that increasing the TG content in VLDL promotes changes in the lipoprotein size and release of the exchangeable apolipoproteins. Similarly, increased TG content in LDL promotes lipoprotein remodeling and fusion. Additionally, an increase in TG content increases lipoprotein susceptibility to oxidation and lipolysis, thereby promoting the generation of free fatty acids that augment fusion. Consequently, TG-induced destabilization may be a general property of plasma lipoproteins. Our studies showed that binding to heparin initiates irreversible pro-atherogenic remodeling of human LDL. As a result of heparin binding, LDL showed decreased structural stability and increased susceptibility to hydrolysis and fusion. Further, phospholipid hydrolysis and/or glycation of LDL (as occurs in diabetes) increased the proteolytic susceptibility of apolipoprotein (apo)B (the major apolipoprotein of VLDL and LDL) and its heparin binding affinity. LDL derived from hyperglycemic patients with type-2 diabetes, became particularly destabilized following heparin binding causing apoB fragmentation and LDL fusion. In summary, binding to heparin alters apoB conformation and triggers pro-atherogenic LDL modifications including proteolysis, lipolysis and structural destabilization. Furthermore, phospholipid lipolysis and glycation of LDL in vitro strengthen its binding to heparin. Together, these findings help establish a mechanistic link between diabetes and atherosclerosis.