MACROPHAGES STIMULATE CHOLESTERYL ESTER ACCUMULATION IN COCULTURED SMOOTH-MUSCLE CELLS INCUBATED WITH LIPOPROTEIN-PROTEOGLYCAN COMPLEX

Foam cells of atherosclerotic lesions originate from both macrophages and smooth muscle cells (SMCs). We explored the mechanism by which SMC s may become Lipid laden. Confluent bovine aortic SMCs were cocultured with P388D(1) macrophages, and the cocultures were incubated for vari ous times with low-density lipoprotein (LDL), acetyl-LDL, or lipoprote in-proteoglycan (PG) complex isolated from human atherosclerotic lesio ns. Macrophages were then removed from the SMCs and the cholesteryl es ter (CE) content of the SMCs was quantitated. Lipoprotein-PG complex b ut not LDL or acetyl-LDL produced a 6-fold to 9-fold stimulation of CE synthesis and a 4.4-fold increase in cellular CE mass in cocultured S MCs relative to control SMCs. In similar studies with human aortic SMC -macrophage cocultures, macrophages stimulated lipoprotein-PG complex- mediated CE synthesis 7-fold to 13-fold and CE mass 7.8-fold in cocult ured SMCs compared with SMCs cultured alone. CE synthesis that was med iated by lipoprotein-PG complex was dose dependent and increased linea rly with time. Incubation of lipoprotein-PG complex with SMC-macrophag e cocultures but not with SMCs or macrophages alone resulted in aggreg ation of the complex and stimulation of cholesterol esterification in SMCs by the conditioned media containing the aggregated complex. Cytoc halasin D, an inhibitor of phagocytosis, inhibited CE synthesis mediat ed by lipoprotein-PG complex by 73%, whereas polyinosinic acid, an inh ibitor of the scavenger receptor, had no effect. Upregulation or downr egulation of apolipoprotein B, E receptors did not affect the lipoprot ein-PG complex-mediated CE synthesis by cocultured SMCs, Lipoprotein-P G complex did not stimulate CE synthesis in SMCs cocultured with aorti c endothelial cells or macrophages cocultured with SMCs. These results indicate that macrophages can stimulate CE synthesis and accumulation in cocultured SMCs when incubated with lipoprotein-PG complexes isola ted from atherosclerotic lesions. This could be a potential mechanism for myocyte foam cell formation.