Endogenous apoE expression modulates HDL3 binding to macrophages.

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 HDL3, specifically examining a role for the cell surface pool. Enhanced binding of apoE-free human HDL3 to apoE-expressing macrophages, compared to non-expressing macrophages, was observed at 37 degrees C and 4 degrees 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 HDL3 binding produced by apoE expression. Cellular cholesterol mass measurements demonstrated that experimental conditions that reduced HDL3 binding to apoE-expressing macrophages, did not substantially reduce cholesterol efflux to HDL3. In summary, our results document a clear role for cell surface pool of apoE in modulating HDL3 interaction with macrophages. The enhanced binding, however, does not appear to be a major mechanism contributing to the increased cholesterol efflux to HDL3, which results from endogenous macrophage expression of apoE.