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The Journal of Lipid Research, Vol. 39, 293-301, February 1998
Copyright © 1998 by Lipid Research, Inc.

Original Article

Endogenous apoE expression modulates HDL₃ binding to macrophages

Correspondence to: Theodore Mazzone.

We have previously shown that expression of a human apoE cDNA in J774 macrophages enhances cholesterol efflux to HDL₃. We have also shown that endogenous apoE expression produces a cell surface pool of apoE associated with proteoglycans. In this series of experiments, we first demonstrate the presence of a cell surface proteoglycan-associated apoE pool in human monocyte-derived macrophages. We then examine the hypothesis that endogenous expression of apoE modulates HDL₃ binding to macrophages, thereby, accounting for enhanced cholesterol efflux to HDL ₃, specifically examining a role for the cell surface pool. Enhanced binding of apoE-free human HDL ₃ to apoE-expressing macrophages, compared to non-expressing macrophages, was observed at 37 °C and 4°C. The enhanced binding was not due to apoE secreted into the medium, as determined by experiments utilizing conditioned medium from apoE-secreting cells. Removal of the cell surface pool of apoE, however, substantially reduced the incremental HDL ₃ binding produced by apoE expression. Cellular cholesterol mass measurements demonstrated that experimental conditions that reduced HDL ₃ binding to apoE-expressing macrophages, did not substantially reduce cholesterol efflux to HDL ₃.

In summary, our results document a clear role for cell surface pool of apoE in modulating HDL ₃ interaction with macrophages. The enhanced binding, however, does not appear to be a major mechanism contributing to the increased cholesterol efflux to HDL ₃, which results from endogenous macrophage expression of apoE.—Lin, C-Y., M. Lucas, and T. Mazzone. Endogenous apoE expression modulates HDL₃ binding to macrophages. J. Lipid Res. 1998. 39: 293–301.

Supplementary key words: macrophages, apolipoprotein E, HDL ₃, cell cholesterol homeostasis

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