Importance of Evaluating Cell Cholesterol Influx With Efflux in Determining the Impact of Human Serum on Cholesterol Metabolism and Atherosclerosis

Objective— Cholesterol efflux relates to cardiovascular disease but cannot predict cellular cholesterol mass changes. We asked whether influx and net flux assays provide additional insights. Approach and Results— Adapt a bidirectional flux assay to cells where efflux has clinical correlates and examine the association of influx, efflux, and net flux to serum triglycerides (TGs). Apolipoprotein B–depleted (high-density lipoprotein-fraction) serum from individuals with unfavorable lipids (median [interquartile range]; high-density lipoprotein-cholesterol=39 [32–42], low-density lipoprotein-cholesterol=109 [97–137], TGs=258 [184–335] mg/dL; n=13) promoted greater ATP-binding cassette transporter A1–mediated [1,2- 3H ] cholesterol efflux (3.8±0.3%/4 hour versus 1.2±0.4%/4 hour; P P =0.04). However, whole sera from individuals with unfavorable lipids promoted more influx (5.14±0.65 versus 2.48±0.85; P =0.02) and lower net release of cholesterol mass (1.93±0.46 versus 3.36±0.47; P =0.04). The pattern differed when mass flux was measured using apolipoprotein B–depleted serum rather than serum. Although individuals with favorable lipids tended to have greater influx than those with unfavorable lipids, efflux to apolipoprotein B–depleted serum was markedly higher (6.81±0.04 versus 2.62±0.14; P Conclusions— When considering the efficiency of serum specimens to modulate cell cholesterol content, both influx and efflux need to be measured.