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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2000;20:773.)
© 2000 American Heart Association, Inc.

Atherosclerosis and Lipoproteins

The Pathogenesis of Foam Cell Formation

Modified LDL Stimulates Uptake of Co-Incubated LDL Via Macropinocytosis

Nancy L. Jones; Jerry W. Reagan; Mark C. Willingham

From the Department of Pathology, Wake Forest University School of Medicine of Wake Forest University, Medical Center Boulevard, Winston-Salem, NC.

Correspondence to Nancy L. Jones, PhD, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157-1092. E-mail njones{at}wfubmc.edu

Abstract—Previously, modified LDLs were shown to stimulate macropinocytosis in pigeon macrophages. Simultaneous intracellular trafficking of LDL and AcLDL, differentially labeled with colloidal gold, was done to determine whether uptake 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 µg/mL) concentrations near the saturation of high affinity binding sites or high (50 to 150 µg/mL) lipoprotein concentrations used to induce foam cell formation. The colloidal gold distribution and percentage of co-labeling as observed by transmission electron microscopy were determined for organelles involved with coated-pit endocytosis or macropinocytosis. LDL simultaneously incubated with AcLDL on macrophages at the low concentration was predominately internalized via coated-pit endocytosis. AcLDL was internalized via both coated-pit endocytosis and macropinocytosis at low concentration. At higher lipoprotein concentrations (50 to 150 µg/mL), AcLDL continued to be internalized via macropinocytosis. Interestingly, a significant portion of the co-incubated LDL, at high concentrations, also trafficked via macropinocytosis. LDL internalized by macropinosomes at high lipoprotein concentrations suggests that AcLDL-stimulated macropinocytosis might increase uptake of co-incubated lipoproteins. When ¹²⁵I-LDL was incubated with cold AcLDL, LDL degradation at 37°C doubled, without a corresponding increase in cell association or total binding of LDL at 4°C. These studies suggest that modified LDL-stimulated macropinocytosis is a mechanism for increased degradation of co-incubated LDL potentially leading to foam cell formation.

Key Words: lipoproteins • endocytosis • macropinocytosis • foam cell formation

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