The roles of different pathways in the release of cholesterol from macrophages

Maria Pia Adorni, Francesca Zimetti, Jeffrey T. Billheimer, Nan Wang, Daniel J. Rader, Michael C. Phillips, and George H. Rothblat

Children's Hospital of Philadelphia, Philadelphia, PA 19104

Corresponding Author: rothblat{at}email.chop.edu

Cholesterol efflux occurs by different pathways, including transport mediated by specific proteins. We determined the effect of enriching cells with free cholesterol (FC) on the release of FC to human serum. Loading Fu5AH cells with FC had no effect on fractional efflux whereas enriching mouse peritoneal macrophages (MPM) resulted in a doubling of fractional efflux. Efflux from cholesterol-normal MPM and Fu5AH cells to 15 human sera correlated well with HDL parameters. However, these relationships were reduced or lost with cholesterol-loaded MPM. By using macrophages from SR-BI-, ABCA1- and ABCG1-knockout mice together with inhibitors of SR-BI- and ABCA1-mediated efflux, we were able to quantitate efflux upon loading macrophages with excess cholesterol and establish the contributions of the various efflux pathways in cholesterol-normal and –enriched cells. The removal of ABCA1 had essentially no effect on the total efflux when cell cholesterol levels were normal. However in cholesterol-enriched cells the removal of ABCA1 reduced efflux by 50%. Approximately 20% of the efflux stimulated by FC-loading MPM is attributable to ABCG1. The SR-BI contribution to efflux is small. Another pathway that is in all cells is aqueous diffusion. Our studies demonstrate that this mechanism is one of the major contributors to efflux, particularly in cholesterol-normal cells.

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