Low-density lipoprotein endocytosis. II. Influence of the multivalent ligand cationized ferritin on acetylated low-density lipoprotein endocytosis in cultured cells

Monocytes influence the fate of T cells challenged with oxidised low density lipoproteins towards apoptosis or MHC-restricted proliferation

Atherosclerosis has been implicated in myocardial infarction, stroke and a host of cardiovascular diseases. The presence of activated T lymphocytes and macrophages, and the increased expression of HLA-DR antigen are consistent with the notion of immune activity in the atherosclerotic plaque. The nature of the causative antigen has not been established although oxidised low density lipoproteins (oxLDL) that accumulate in atherosclerotic plaques could fulfil this role. Here, we report that monocytes play a key role in influencing the fate of purified peripheral human T lymphocytes from healthy donors when the cells are exposed to LDL oxidised under the controlled conditions of water radiolysis. Our data showed that oxLDL generated under these conditions were chemoattractants for T cells. However, they induced a state of apoptosis in T lymphocytes cultured in the absence of monocytes. The extent of apoptosis was related to the degree of oxidation of LDL and the time of T cell exposure to oxLDL. OxLDL-dependent apoptosis did not involve a scavenger-like receptor. CD4(+) cells were more sensitive to the apoptotic effect of oxLDL than CD8(+) cells. OxLDL-primed (12 h) autologous monocytes triggered a robust proliferation of T lymphocytes cultured in the absence of oxLDL. The strength of T cell stimulation was related to the degree of oxidation of the LDL used in priming. Heterologous monocytes exposed to oxLDL under similar conditions induced a response that was not different than monocytes exposed to untreated LDL (natLDL) which did not induce T cell proliferation. Fucoidan did not modify the oxLDL-, monocyte-dependent T cell response to proliferation, suggesting that a scavenger-like receptor was not involved. The expression of the HLA-DR marker and the B7.2 protein were up-regulated in monocytes exposed to oxLDL but not to natLDL. The levels of B7.1 were unchanged. Our data are consistent with the notion that monocytes are critical for T cell survival in the presence of oxLDL and MHC-restricted T cell proliferative response to oxLDL.

The pathogenesis of atherosclerosis: an overview

In this unifying hypothesis directed to the etiology and pathogenesis of atherosclerosis, the importance of focal arterial lesion-prone sites has been emphasized. Key initial participants in these sites include the focal intimal influx and accumulation of low-density lipoprotein (LDL) and a preferential recruitment of blood monocytes. Both are further enhanced in the presence of hyperlipidemia, when the quantity of intimal LDL and the oxidative potential of the intima exceed the capacity of macrophages to remove, via the non-down-regulating scavenger receptor, cytotoxic anionic (Ox-LDL) macromolecules. Foam cells, pathognomonic of the fatty streak, form during the receptor-mediated uptake of Ox-LDL by the macrophages. Interstitial free radicals and the excess of Ox-LDL particles injure and kill cells, including the foam cells, with the formation of the necrotic extracellular lipid core, a key transitional step in lesion progression. Monocyte-macrophage recruitment to the intima is likely to be regulated not only by a multiplicity of endothelial adhesive cytokines, integrins, and selectins, but also by the monocyte-specific chemoattractant, MCP-1, constitutively synthesized and secreted by intimal smooth muscle and endothelial cells. Its synthesis and secretion is augmented by mildly oxidized LDL. Free radicals, pivotal in the oxidation of LDL, and derived from activated macrophages, and also endothelial and smooth muscle cells. Smooth muscle cells migrate from the media through the intimal endothelial layer (IEL) and proliferate under the regulation of a number of mitogens, including platelet-derived growth factor (PDGF). Collagen synthesis by smooth muscle cells is substantial. Lymphocytes, as a source of interferons, invade the plaque and are present in the adventitia in substantial numbers, likely representing an autoimmune response in the later stages of plaque development. Platelets and mural thrombosis directly contribute to subsequent plaque growth, particularly after plaque rupture or fissure and disruption of the thromboresistant endothelial cells (EC). Plaque regression in all likelihood involves the conversion of the inert pool of extracellular lipid to a metabolically active intracellular pool and subsequent clearance by the high-density lipoprotein mediated reverse cholesterol transport system. The atherogenic cascades so described conceptually represent arterial inflammatory and healing processes occurring in a hyperlipidemic environment. Many components of pathogenesis are the targets for modulation by genetic, hemodynamic and selected risk factors. The prevention and treatment of the disease should logically target reduction in plasma LDL levels, the inhibition of the oxidative modification of lipoproteins, including LDL, by free radical scavengers, and augmentation of the reverse cholesterol transport system.

Low-density lipoprotein endocytosis. I. Influence of the multivalent ligand cationized ferritin on normal and receptor-negative human fibroblasts

Based upon the observation that the multivalent ligand cationized ferritin (CF) alters the cell surface distribution of anionic domains and significantly enhances the adsorptive endocytosis of 125I-labeled human serum albumin, these studies were undertaken to probe the influence of CF on receptor-mediated low-density lipoprotein (LDL) endocytosis and the nature of the mechanisms involved. A brief 1-min exposure of normal receptor upregulated fibroblasts to CF (0.2 mg/ml) resulted in a significant decrease (P less than 0.001) in the subsequent internalization and degradation of 125I-LDL. Studies with receptor downregulated normal fibroblasts indicated that CF pretreatment did not measurably influence 125I-LDL internalization and only slightly inhibited its degradation (P less than 0.05). In contrast, CF pretreatment of FH receptor-negative mutant skin fibroblasts resulted in a modest but significant increase in both 125I-LDL internalization and degradation (P less than 0.05). Scatchard analyses of binding data indicated that CF-pretreated upregulated normal fibroblasts exhibit a single class of LDL binding sites with an affinity, Kd = 24.7 +/- 4.1 nM, almost 10-fold lower than the affinity of binding sites in untreated controls, Kd = 3.2 +/- 0.06 nM. Increasing either the concentration or the duration of CF exposure resulted in additional inhibition of LDL internalization and degradation associated primarily with a decrease in the number of LDL binding sites without any further change in binding affinity. Total cellular LDL receptor-mediated binding, measured using an octylglucoside solubilization-filtration assay, confirmed the CF-induced decrease in high-affinity LDL binding. Pulse-chase experiments showed that CF had no direct influence on LDL degradation, nor did it influence targeting of the LDL-containing endosome toward exocytosis. Further, restoration of LDL receptor function to control values after CF pretreatment required de novo protein synthesis. The normal feedback inhibition of HMG-CoA reductase activity was nearly abolished by CF pretreatment. Additionally, CF pretreatment was found to induce not only a redistribution of surface anionic sites, but also a very rapid internalization of surface components labeled with 4,4'-[3H]diisothiocyano-1,2..diphenylethane-2,2'-disulfonic acid. It is concluded that the inhibitory influence of CF on LDL endocytosis is mediated via a decrease in the affinity and in the number of functional LDL receptors.