Abnormal composition of hypertriglyceridemic very low density lipoprotein determines abnormal cell metabolism

The metabolism of very low density lipoprotein (VLDL) from normolipemic (NTG) subjects, hypertriglyceridemic (HTG) subjects, and hypertriglyceridemic subjects treated with bezafibrate (BZ) was studied in cultured human skin fibroblasts. The binding, cell association, and proteolytic degradation of 125I-labeled lipoproteins and the capacity to regulate cellular sterol synthesis was determined with and without maximal stimulation of the lipoprotein by exogenous recombinant or plasmatic apolipoprotein (apo) E-3. The VLDL was separated into three density subfractions: I, II, and III. Multiple differences between HTG and NTG lipoproteins were found, which all reverted toward normal with therapy. Even in the presence of an optimal concentration of apo E-3, HTG-VLDL demonstrated 100% to 200% higher metabolic activities, indicating a better association or a better biological expression of apo E-3 at the surface of the lipoprotein. There was a strong and linear relationship between the cholesterol ester/protein ratios of the different VLDLs and their proteolytic degradations by the cells (r = 0.95). Thus, the composition/structure alterations of VLDL appear to determine their apo E-3-dependent cellular catabolism. In addition, HTG-VLDLs not enriched with apo E-3 exhibited a capacity to down-regulate cellular sterol synthesis independently of their uptake and degradation by the cells. This abnormality appeared to reflect the ability of the VLDL to donate cholesterol to the cells and was not observed in receptor-negative cells. Thus, HTG-VLDL is much more capable than NTG-VLDL of introducing cholesterol to cells by at least two mechanisms: 1) accelerated uptake and degradation and 2) direct transfer of cholesterol to the cells. Both processes are potentially atherogenic and are reversible when triglyceride-lowering therapy is instituted.