HDL-apoA-I Exchange: Rapid Detection and Association with Atherosclerosis

Abstract

High density lipoprotein (HDL) cholesterol levels are associated with decreased risk of cardiovascular disease, but not all HDL are functionally equivalent. A primary determinant of HDL functional status is the conformational adaptability of its main protein component, apoA-I, an exchangeable apolipoprotein. Chemical modification of apoA-I, as may occur under conditions of inflammation or diabetes, can severely impair HDL function and is associated with the presence of cardiovascular disease. Chemical modification of apoA-I also impairs its ability to exchange on and off HDL, a critical process in reverse cholesterol transport. In this study, we developed a method using electron paramagnetic resonance spectroscopy (EPR) to quantify HDL-apoA-I exchange. Using this approach, we measured the degree of HDL-apoA-I exchange for HDL isolated from rabbits fed a high fat, high cholesterol diet, as well as human subjects with acute coronary syndrome and metabolic syndrome. We observed that HDL-apoA-I exchange was markedly reduced when atherosclerosis was present, or when the subject carries at least one risk factor of cardiovascular disease. These results show that HDL-apoA-I exchange is a clinically relevant measure of HDL function pertinent to cardiovascular disease.

HDL-apolipoprotein A-I exchange is independently associated with cholesterol efflux capacity

“…However, factors controlling HDL sterol effl ux capacity are largely unknown. Our previous work indicated that the ability of HDL to remodel and/or exchange apoA-I is attenuated in CVD subjects with similar apoA-I and HDL-C levels, suggesting that direct measurement of HDL dynamics measures a HDL property distinct from apoA-I or cholesterol concentration ( 31 ). To establish whether HAE and cholesterol effl ux capacity assays measure independent HDL properties, we investigated the relationship between HDL dynamics, as measured by the HAE assay, and cholesterol effl ux capacity, as measured by the cell-based assays.…”

“…HAE assay methods were performed with apoB-depleted plasma as described in Borja, et al ( 31 ), except that the EPR spectrum collection at 6°C was omitted, as analysis of previous data revealed the assay gave the same results when this step was not performed. Spin labeling of apoA-I is described in Oda et al ( 32 ).…”

“…HAE provides a measure of the ability of HDL to remodel and release lipid-poor apoA-I ( 24,31 ). The assay capitalizes on the high specifi city of apoA-I for HDL and the exchangeable nature of HDL-associated apoA-I, which can be released upon the addition of exogenous lipid-free apoA-I ( 31 ).…”

“…The assay capitalizes on the high specifi city of apoA-I for HDL and the exchangeable nature of HDL-associated apoA-I, which can be released upon the addition of exogenous lipid-free apoA-I ( 31 ). As spin-labeled apoA-I associates with HDL, the EPR spectra's peak amplitude increases due to structural changes in apoA-I from a lipid-free to a lipid-bound conformation ( 31,32 ). The ratio of lipid-free to lipidbound apoA-I provides a measure of the relative exchangeability of endogenous apoA-I and the dynamic nature of HDL particles.…”

“…The ratio of lipid-free to lipidbound apoA-I provides a measure of the relative exchangeability of endogenous apoA-I and the dynamic nature of HDL particles. Our early investigation into the clinical relevance of HDL dynamics suggests that HAE is impaired in people with prevalent CVD, and as such it may be a CVDrelevant measure of HDL function ( 31 ).…”