Abstract MP30: Excess HDL Free Cholesterol Bioavailability Drives Free Cholesterol Accretion Into Macrophages And Erythrocytes In Scarb1 ^-/- Mice

Aim: In humans, very high plasma HDL-cholesterol concentrations are associated with increased all cause- and atherosclerotic cardiovascular disease (ASCVD)-mortality. The HDL receptor-deficient mouse (Scarb1 -/- ), a robust model of this phenotype, is characterized by high free cholesterol (FC) bioavailability due to too many HDL particles that are FC-rich. Clinically, plasma LDL and HDL are quantified according to total cholesterol content, the sum of FC and esterified cholesterol, which likely contribute to ASCVD pathophysiology differently. A Western diet induces ASCVD in Scarb1 -/- mice, despite an attendant increase in HDL. We tested the hypothesis that high HDL-FC bioavailability contributes to ASCVD in Scarb1 -/- mice by increasing FC flux into macrophage cells, erythrocytes and other major tissues. Methods: Influx of HDL-FC and efflux of macrophage FC were determined between WT and Scarb1 -/- HDL and J774 macrophage cells. HDL of both genotypes were radiolabelled with [ 3 H]FC, injected into autologous mice, and the rates of plasma clearance and erythrocyte uptake were determined. Results: The magnitude of FC transfer from Scarb1 -/- HDL to LDL is greater than that from WT HDL; APOB-containing lipoproteins from Scarb1 -/- vs. WT mice are FC-enriched due likely to greater HDL-FC transfer. While macrophage efflux to HDL of Scarb1 -/- vs. WT HDL was not different, FC influx from Scarb1 -/- vs. WT HDL to macrophages was three-fold greater, a net effect that increased the FC burden of macrophages. In vivo studies showed that compared to WT mice, in Scarb1 -/- mice, autologous HDL-FC cleared more slowly and more FC transferred to erythrocytes. We compared the FC, CE, PL, and TG contents of all major tissues and determined that FC accretion by some tissues is higher among Scarb1 -/- vs. WT mice whereas in other tissues FC homeostasis is maintained. Lastly, we determined that the tissue compositions and plasma FC clearance kinetics varied according to sex, particularly among Scarb1 -/- mice. Conclusions: These findings are relevant to pathologies specific to Scarb1 -/- mice and to the evolving model of the role of HDL-FC in RCT. They provide a rationale for human studies to determine the utility of HDL-FC bioavailability as a risk factor for ASCVD and other pathologies.