Cholesterol induces apoptosis-associated loss of the activated leukocyte cell adhesion molecule (ALCAM) in human monocytes

Abstract

The activated leukocyte cell adhesion molecule (ALCAM/CD166) is associated with cell migration and leukocyte invasion into the vessel wall. This study investigates the impact of cholesterol loading on the expression of ALCAM, as compared with P-selectin glycoprotein ligand-1 (PSGL-1), vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) in monocytic U937 cells and human primary monocytes. Cells were enriched with cholesterol by incubation with a cyclodextrin–cholesterol complex. Expression of adhesion molecules and apoptosis was determined by flow cytometry. Migration was quantified by chemotaxis toward serum. Incubation with cholesterol (10–100 μg/ml) for 16 h caused a concentration-dependent increase in apoptosis. Enhanced apoptosis was associated with reduction of ALCAM by > 70%. While PSGL-1 was affected similarly, expression of VCAM-1 was markedly increased by cholesterol and ICAM-1 levels were not regulated. The nonselective caspase/apoptosis inhibitor Q-VD-OPh partially prevented cholesterol-modulated alteration of adhesion molecule expression. Migration of cholesterol-rich monocytic cells toward serum was greatly reduced. This effect was partially restored by Q-VD-OPh and was dependent on ALCAM as shown by ALCAM-neutralizing antibodies. In conclusion, cholesterol-induced apoptosis in monocytes is accompanied by reduced expression of ALCAM and attenuated monocyte migration. This may restrain monocytes at cholesterol-rich sites and thereby expedite vascular lesion formation.