Cholesteryl ester-loaded macrophages (Mfs), or foam cells, which are prominent and important in atherosclerosis, form by the interaction of Mf with atherogenic lipoproteins (Lps), followed by stimulation of intracellular cholesterol esterification (CE'tion). That one type of atherogenic particle, b-VLDL, enters Mfs by unique cell-surface invaginations called STEMs (surface tubules for entry into Mfs) was recently discovered. The STEM pathway appears to be important in CE'tion and, remarkably, partial b-VLDL catabolism occurs in these cell-surface sites. Preliminary studies have revealed that another important class of atherogenic lipoproteins--retained & aggregated LDL--interacts with the Mf cell surface for an even longer time period than b-VLDL. This finding, together with the physiological importance and paucity of prior data regarding the interaction of retained & aggregated LDL with Mfs, provides a strong rationale for further study in this area. Thus, the overall objectives of this proposal are to first elucidate important cellular and biochemical events involved in the interaction of retained & aggregated LDL with Mfs and then to test specific hypotheses linking the cell-surface pathway involved with both b-VLDL (STEMs) and retained & aggregated LDL to the stimulation of intracellular CE'tion. To evaluate and refine these hypotheses, particularly as they relate to retained and aggregated LDL, fluorescence & electron microscopy and biochemical methods will be used in aim 1 to elucidate precisely how these lipoproteins interact with the Mf cell-surface and how they affect intracellular cholesterol metabolism. An important hypothesis to be tested is that CE'tion is stimulated during the period of prolonged cell-surface interaction.
In aim 2, the lipoprotein component(s) and the hydrolase(s) that are involved in the partial cell-surface catabolism known to occur with b-VLDL in STEMs and predicted to occur with retained & aggregated LDL will be identified.
Aim 3 will use the information obtained in the first two aims to address the roles of four processes that are hypothesized to link cell-surface events to intracellular CE'tion: (1) cell-surface catabolism as a facilitator of Lp-cholesterol transfer to cells; (2) direct transfer of the free cholesterol moiety of Lps to the Mf plasma membrane; (3) physical interactions between cholesterol and ACAT, including possible Lp-induced alterations in ACAT localization; and (4) signal transduction events involving changes in protein phosphorylation. The information gained from these studies may suggest new therapeutic strategies, directed at the level of the lesion foam cell, for further lowering the risk of atherosclerotic heart disease.
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