| These experiments were designed to characterize the mechanism(s) responsible for the atherogenic nature of hypertriglyceridemic very low density lipoproteins (HTG-VLDL, Sf60-400) from subjects with type III or IV hyperlipoproteinemia (HLP), and VLDL from apoE-knockout mice (EKO-VLDL), an animal model that parallels human type III HL9. Cholesterol-loaded macrophages, morphologically recognized as foam cells, are characteristic of atherosclerotic plaques at all stages of lesion development. In contrast to type IV HTG-VLDL, type III HTG-VLDL and EKO-VLDL do not induce appreciable cholesteryl ester (CE) accumulation in cultured macrophages (J774A.1 cells). The process of copper-mediated oxidative-modification (CuOx-) of type III HTG-VLDL and EKO-VLDL significantly enhanced the ability of these lipoproteins to induce cellular CE accumulation in J774A.1 cells. In contrast, CuOx-type IV HTG-VLDL induced significantly less cellular CE accumulation compared to its native counterpart. In vitro incubation of HTG-VLDL with lipoprotein lipase (LPL) generated VLDL remnants (VLDL-REM), but did not enhance the ability of HTG-VLDL to induce CE accumulation. However, CuOx-type III VLDL-REM and CuOx-type IV VLDL-REM induced the greatest increase in cellular CE content, when compared to all other lipoprotein preparations tested. In contrast to their native counterparts, cellular uptake of CuOx-HTG-VLDL did not require cell secreted LPL activity. In addition, uptake of CuOx-HTG-VLDL, CuOx-VLDL-REM and CuOx-EKO-VLDL was found to be mediated entirely by the scavenger type A receptor (SR-A). Besides CuOx-modification, VLDL modification by p-hydroxyphenylacetaldehyde (pHA) was also examined. pHA is the major product derived from the oxidation of L-tyrosine by myeloperoxidase and is a component of human atherosclerotic lesions. Like CuOx-modification, pHA-modification created a lipoprotein that induced marked increases in cellular CE content. In contrast to CuOx-HTG-VLDL, cellular uptake of pHA-HTG-VLDL was found to require catalytically active LPL. Furthermore, uptake of pHA-HTG-VLDL, pHA-VLDL-REM and pHA-EKO-VLDL was only partially mediated by the SR-A. In addition to CuOx- and pHA-modification, uptake of native type IV HTG-VLDL, but not type III HTG-VLDL or EKO-VLDL was found to be enhanced by pre-treatment of J774A.1 cells with the cytokine interferon-... |
