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.

Objective The molecular mechanisms underlying sex differences in dyslipidemia are poorly understood. We aimed to distinguish genetic and hormonal regulators of sex differences in plasma lipid levels. Approach and Results We assessed the role of gonadal hormones and sex chromosome complement on lipid levels using the Four Core Genotypes mouse model (XX females, XX males, XY females, and XY males). In gonadally intact mice fed a chow diet, lipid levels were influenced by both male–female gonadal sex and XX–XY chromosome complement. Gonadectomy of adult mice revealed that the male–female differences are dependent on acute effects of gonadal hormones. In both intact and gonadectomized animals, XX mice had higher HDL cholesterol (HDL-C) levels than XY mice, regardless of male–female sex. Feeding a cholesterol-enriched diet produced distinct patterns of sex differences in lipid levels compared to a chow diet, revealing the interaction of gonadal and chromosomal sex with diet. Notably, under all dietary and gonadal conditions, HDL-C levels were higher in mice with two X chromosomes compared to mice with an X and Y chromosome. By generating mice with XX, XY and XXY chromosome complements, we determined that the presence of two X chromosomes, and not the absence of the Y chromosome, influences HDL-C concentration. Conclusions We demonstrate that having two X chromosomes versus an X and Y chromosome complement drives sex differences in HDL-C. It is conceivable that increased expression of genes escaping X-inactivation in XX mice regulates downstream processes to establish sexual dimorphism in plasma lipid levels.

ObjectiveWe tested the hypothesis that HDL-apolipoprotein A-I exchange (HAE), a measure of high-density lipoprotein (HDL) function and a key step in reverse cholesterol transport (RCT), is impaired in metabolic syndrome (MetSyn) patients who are asymptomatic for diabetes and cardiovascular disease. We also compared HAE with cell-based cholesterol efflux capacity (CEC) to address previous reports that CEC is enhanced in MetSyn populations.MethodsHAE and ABCA1-specific CEC were measured as tests of HDL function in 60 MetSyn patients and 14 normolipidemic control subjects. Predictors of HAE and CEC were evaluated with multiple linear regression modeling using clinical markers of MetSyn and CVD risk.ResultsHAE was significantly reduced in MetSyn patients (49.0 ± 10.9% vs. 61.2 ± 6.1%, P < 0.0001), as was ABCA1-specific CEC (10.1 ± 1.6% vs. 12.3 ± 2.0%, P < 0.002). Multiple linear regression analysis identified apoA-I concentration as a significant positive predictor of HAE, and MetSyn patients had significantly lower HAE per mg/dL of apoA-I (P = 0.004). MetSyn status was a negative predictor of CEC, but triglyceride (TG) was a positive predictor of CEC, with MetSyn patients having higher CEC per mg/dL of TG, but lower overall CEC compared to controls.ConclusionsMetSyn patients have impaired HAE that contributes to reduced capacity for ABCA1-mediated CEC. MetSyn status is inversely correlated with CEC but positively correlated with TG, which explains the contradictory results from earlier MetSyn studies focused on CEC. HAE and CEC are inhibited in MetSyn patients over a broad range of absolute apoA-I and HDL particle levels, supporting the observation that this patient population bears significant residual cardiovascular disease risk.

Objective: Metabolic syndrome (MetSyn) is associated with a host of cardiovascular risk factors including increased waist circumference, triglycerides, blood pressure, fasting glucose, and reduced high density lipoprotein cholesterol (HDL-C). Changes in HDL function, especially the ability to participate in reverse cholesterol transport, are more indicative of the atheroprotective properties of HDL compared to HDL-C levels alone. In this study, we determine the association of HDL-apolipoproteinA-I (apoA-I) exchange, a measure of HDL remodeling/apoA-I exchange, with MetSyn and its individual components. Methods and Results: We analyzed HDL-apoA-I exchange (HAE) in the plasma of 60 well-characterized subjects with MetSyn and 14 healthy control subjects. HAE measurements were obtained using electron paramagnetic resonance (EPR) following incubation of plasma with lipid-freeapoA-I containing a methanethiosulfonate spin label. HAE positively correlated with plasma HDL-C and apoA-I levels, and inversely correlated with fasting blood glucose levels, blood pressure, BMI, and triglycerides. Multiple linear regression showed that HAE is significantly correlated with MetSyn (HAE: r2 = 0.57, P<0.0001), though MetSyn subjects on statins exhibited significantly higher HAE compared to subjects not on statins. Conclusions: MetSyn has a significant negative impact on HDL remodelin/apoA-I exchange, as measured by HAE. HAE is a strong identifier of MetSyn status even after adjusting for individual components of MetSyn.

Objective: Niacin and omega-3 fatty acids are two therapeutic agents that have been extensively studied for their ability to reduce cardiovascular disease risk, but their effectiveness has more recently been called into question. In this study, we investigate whether these agents alone and in combination alter HDL function, in particular, HDL-apolipoprotein A-I exchange (HAE), a measure of HDL dynamics, and serum cholesterol efflux capacity (CEC). Approach: Fifty-six subjects with metabolic syndrome (MetSyn) were recruited to a double-blind trial and randomized to 16 weeks of treatment with dual placebo, extended release niacin (ERN, 2g/day), prescription omega-3 ethyl esters (P-OM3, 4g/day), or combination. HDL function was assessed at baseline and following 16 weeks of treatment by measuring HAE, macrophage CEC, and ABCA1-specific CEC. Results: Compared to placebo, ERN and P-OM3 alone significantly increased HAE by 15.1 [8.2, 22.0] (p<0.0001) and 12.4% [5.9, 18.9] ( P <0.0001) respectively while the combination therapy increased HAE by 10.0% [3.2, 16.8] ( P =0.002). When evaluated by HAE:apoA-I ratio (a measure of apoA-I specific activity), ERN increased apoA-I specific activity by 20.1% [4.8, 36.9] ( P =0.008), P-OM3 by 30.1% [14.4, 45.9] ( P< 0.0001), however with combination there was no increase, 9% [-6.6, 26.6] ( P =0.34). Triglyceride-adjusted macrophage CEC showed marginally significant increases with P-OM3 therapy ( P =0.05). No therapy significantly improved ABCA1-specific CEC. Conclusions: Much of the effect of ERN on HDL function can be attributed to this therapy raising apoA-I levels, but P-OM3 raises HDL function by independent means, increasing apoA-I specific activity. Future investigation is needed to determine whether interaction between ERN and P-OM3 therapies in combination reduces their overall effectiveness.