Objective: To characterize the associations and functions of apoE with the HepG2 cell surface through the study of the relationship between proteoglycans and apoE on HepG2 cell surface.
Methods: The specific binding of 7C9, a monoclonal antibody against the N-terminal domain of apoE was employed to demonstrate the effects of glycosaminoglycans, heparinase, chondroitinase, xyloside, and chlorate on the apoE of HepG2 cell surface.
Results: Growth of cells in beta-D-xyloside decreased cell surface apoE by 45% with a concomitant increase in apoE secretion (4.3-fold), underlining the importance of glycosaminoglycan association of apo E. Incubations with heparinase (3 U/ml) and heparin (1 mg/ml) decreased apoE by 25.0% and 30.5% respectively indicating association through cell surface haparin sulfate proteoglycans. Incubations with chondroitinase ABC (1.5 U/ml) reduced cell surface apoE by 40.0%. Chondroitin sulfate A and chondroitin sulfate B reduced cell surface apoE by 23.6% and 15.3% respectively while incubations with chondrointin sulfate C not effective. Decreasing the levels of cell surface apoE with haparin or chondrointin sulfates A and B increased the subsequent binding of LDL to the HepG2 cell surface.
Conclusions: ApoE associates with the cell surface mainly through chondrointin sulfate proteoglycans and to a lesser extent through heparan sulfate proteoglycans, decreased levels of cell surface apoE increase the binding of LDL to the cell surface.