A novel model for the evaluation of human HDL function

Abstract

Background High-density lipoprotein (HDL) mediates macrophage reverse cholesterol transport (RCT). This process leads to decreased atherosclerotic plaque formation and progression. A causal relationship between HDL levels and CVD has been challenged, requiring investigation of HDL function independent of HDL levels. Whilst an ex vivo assay to measure the first step of RCT has been developed, a model to evaluate the capacity of human HDL to enhance in vivo macrophage RCT is lacking. Methods and Results This thesis investigated the application of a validated assay of macrophage RCT in the apolipoproteinA-I knockout (apoa1-/-) mouse, a model that has low background levels of HDL and RCT. These experiments show for the first time, that: 1) human HDL injection in apoa1-/- mice resulted in a dose-dependent appearance of human HDL in plasma and stimulation of macrophage RCT; 2) Human HDL modified with myeloperoxidase is impaired in enhancing RCT, validating the model for detection of HDL dysfunction; 3) HDL from acute coronary syndrome patients (ACS-HDL) demonstrated impaired RCT; 4) An ex vivo assay, using a cell line expressing the main receptor for HDL lipid uptake, scavenger receptor B type 1 (SR-B1), showed impaired SR-B1 receptor mediated uptake of lipid from ACS-HDL supporting defective hepatic SRBI-mediated lipid uptake as a mechanism for this dysfunction; 5) The ability of human HDL to promote ex vivo ABCA1-specific cellular cholesterol efflux, is not retained following mice injection; 6) serum CEC of mice injected HDL is not a good indicator of ex vivo HDL cholesterol efflux. Conclusion The thesis presents a novel model for the evaluation of human HDL function. HDL functionality does vary between patients, and simple evaluation of HDL concentration is limited in its ability to reflect HDL biology in vivo. These results also indicate that two simple, widely used assays of HDL function- HDL efflux and CEC- give incomplete insight into HDL function.