A novel approach to measuring macrophage-specific reverse cholesterol transport in vivo in humans

The cholesterol contained within HDL is inversely associated with risk of coronary heart disease and is a key component of predicting cardiovascular risk. However, despite its properties consistent with atheroprotection, the causal relation between HDL and atherosclerosis is uncertain. Human genetics and failed clinical trials have created scepticism about the HDL hypothesis. Nevertheless, drugs that raise HDL-C concentrations, cholesteryl ester transfer protein inhibitors, are in late-stage clinical development, and other approaches that promote HDL function, including reverse cholesterol transport, are in early-stage clinical development. The final chapters regarding the effect of HDL-targeted therapeutic interventions on coronary heart disease events remain to be written. Copyright © 2014 Elsevier Ltd. All rights reserved.

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 (MPMs) 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 MPMs. Using macrophages from scavenger receptor class B type I (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 to 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 was small. Another pathway that is present 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.