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 t hat endogenous apoE expression produces a cell surface pool of apoE as sociated with proteoglycans. In this series of experiments, we first d emonstrate the presence of a cell surface proteoglycan-associated apoE pool in human monocyte-derived macrophages. We then examine the hypot hesis that endogenous expression of apoE modulates HDL3 binding to mac rophages, thereby, accounting for enhanced cholesterol efflux to HDL3, specifically examining a role for the cell surface pool. Enhanced bin ding of apoE-free human HDL3 to apoE-expressing macrophages, compared to non-expressing macrophages, was observed at 37 degrees C and 4 degr ees C. The enhanced binding was not due to apoE secreted into the medi um, as determined by experiments utilizing conditioned medium from apo E-secreting cells. Removal of the cell surface pool of apoE, however, substantially reduced the incremental HDL3 binding produced by apoE ex pression. Cellular cholesterol mass measurements demonstrated that exp erimental conditions that reduced HDL3 binding to apoE-expressing macr ophages, did not substantially reduce cholesterol efflux to HDL3. In s ummary: our results document a clear role for cell surface pool of apo E in modulating HDL3 interaction with macrophages. The enhanced bindin g, however, does not appear to be a major mechanism contributing to th e increased cholesterol efflux to HDL3, which results from endogenous macrophage expression of apoE.