Endogenous apoE expression modulates HDL3 binding to macrophages

Abstract

We have previously shown that expression of a human apoE cDNA in J774 macrophages enhances cholesterol efflux to HDL 3 . 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 HDL 3 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 Њ C and 4 Њ 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 HDL 3 binding to macrophages.

Interaction with proteoglycans enhances the sterol efflux produced by endogenous expression of macrophage apoE

“…In previously reported studies, we demonstrated the usefulness of the transfected J774 model to study the effect of apoE expression on cholesterol flux in macrophages (21,23,24). We showed that cholesterol efflux produced in this model is very similar to that observed in human monocyte macrophages, in terms of amount of cholesterol and phospholipid secreted, the density of the apoEcontaining lipid particle, and the response of cholesterol and phospholipid efflux to enriching cells with excess cholesterol.…”

“…Proteoglycans provide important binding sites for growth factors and cytokines, and the presence of apoE could potentially modulate retention of these factors on the macrophage cell surface. We previously showed that apoE on the macrophage cell surface modulates the interaction of cells with HDL (24). It was also recently shown that apoE in a pericellular proteoglycan matrix modulates the metabolism of apoE-containing triglyceride-rich lipoproteins by macrophages (40,41).…”

“…Acetylated LDL (ALDL) and apoE-free HDL 3 (HDL 3 ) were prepared as previously described (21,23). HDL 3 was radiolabeled as previously described (24). Purified apoE from human VLDL was purchased from Calbiochem (San Diego, CA).…”

LDL receptor binds newly synthesized apoE in macrophages: a precursor pool for apoE secretion

“…All tissue culture reagents were obtained from Gibco (Grand Island, NY). All other materials were from previously described sources (8)(9)(10)(11)(12)14).…”

“…Whether apoE has an intracellular function, or exists in a discrete intracellular pool separate from the secretory pathway, is not yet completely clear. However, there does appear to be a distinct cell surface pool of macrophage apoE which has been identified in human monocyte-derived macrophages and in J774 cells transfected to express human apoE (10,14,15). Involvement of cell surface proteoglycans in the formation of the macrophage cell surface pool of apoE has been clearly established (10,14,15).…”