Studies On The Compositional And Metabolic Heterogeneity Of Very Low-density Lipoproteins In Hypertriglyceridemic Human Subjects

Experiments were designed to characterize more fully specific aspects of the compositional and metabolic heterogeneity of Sf 60-400 very low density lipoproteins (VLDL) known to be associated with Type IV hypertriglyceridemia in humans. Apolipoprotein E (apo E) is thought to be an important mediator of VLDL catabolism. Heparin-Sepharose chromatography was shown to be a reproducible method for separating Type IV VLDL into apo E-poor and apo E-rich subfractions. The apo E-poor fraction was found to contain more intestinally-derived lipoproteins than the apo E-rich fraction. VLDL from subjects with Types I, III and V hyperlipoproteinemia could not be subfractionated in an apo E-dependant manner by the heparin-Sepharose method. Compared to normal subjects, Type IV subjects accumulated apo E-poor VLDL with the amount this material being reduced on treatment for hypertriglyceridemia, suggesting that accumulation of the apo E-poor fraction results from VLDL overproduction. In vitro and in vivo studies showed that the apo E-poor fraction was also a poorer substrate for lipoprotein lipase (LPL) relative to the apo E-rich fraction. In contrast to normal subjects, upwards of 50% of the Sf 60-400 VLDL in Type IV subjects is removed from their plasma at an unknown tissue site. Widely-accepted macrophage (J774) and hepatocyte (Hep G2) cell lines were used to study the interaction and mechanism of uptake of Type IV VLDL by these cell types. Cultured J774 cells catabolized Type IV VLDL by a two-step uptake mechanism involving extracellular lipolysis of VLDL triglycerides, by macrophage-derived LPL, and apo E-mediated uptake of VLDL-borne cholesterol. Type IV VLDL was not taken up by Hep G2 cells unless an exogenous source of LPL was added to the culture media, suggesting that large VLDL particles must interact with LPL prior to uptake by hepatocytes. In addition, these experiments were unable to show that VLDL-associated apo E was the critical ligand for uptake of the VLDL-borne cholesterol by Hep G2 cells