The pathogenesis of foam cell formation - Modified LDL stimulates uptake of co-incubated LDL via macropinocytosis

Previously, modified LDLs were shown to stimulate macropinocytosis in pigeo n macrophages. Simultaneous intracellular trafficking of LDL and AcLDL, dif ferentially labeled with colloidal gold, was done to determine whether upta ke of LDL, which does not cause foam cell formation, was internalized via a separate route from AcLDL, which stimulates foam cell formation. AcLDL and LDL were followed at either low (12 mu g/mL) concentrations near the satur ation of high affinity binding sites or high (50 to 150 mu g/mL) lipoprotei n concentrations used to induce foam cell formation. The colloidal gold dis tribution and percentage of co-labeling as observed by transmission electro n microscopy were determined for organelles involved with coated-pit endocy tosis or macropinocytosis. LDL simultaneously incubated with AcLDL on macro phages at the low concentration was predominately internalized via coated-p it endocytosis. AcLDL was internalized via both coated-pit endocytosis and macropinocytosis at low concentration. At higher lipoprotein concentrations (50 to 150 mu g/mL), AcLDL continued to be internalized via macropinocytos is. Interestingly, a significant portion of the co-incubated LDL, at high c oncentrations, also trafficked via macropinocytosis. LDL internalized by ma cropinosomes at high lipoprotein concentrations suggests that AcLDL-stimula ted macropinocytosis might increase uptake of co-incubated lipoproteins. Wh en I-125-LDL was incubated with cold AcLDL, LDL degradation at 37 degrees C doubled, without a corresponding increase in cell association or total bin ding of LDL at 4 degrees C. These studies suggest that modified LDL-stimula ted macropinocytosis is a mechanism for increased degradation of co-incubat ed LDL potentially leading to foam cell formation.