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BackgroundThe functional status of lipoprotein particles contributes to atherogenesis. The tendency of plasma LDL particles to aggregate and the ability of HDL particles to induce and mediate reverse cholesterol transport associate with high and low risk for cardiovascular disease in adult patients, respectively. However, it is unknown whether children with familial hypercholesterolemia (FH) display lipoprotein function alterations.HypothesisWe hypothesized that FH children had disrupted lipoprotein function.MethodsWe analyzed LDL aggregation susceptibility and HDL-apoA-I exchange to apoA-I ratio (HAE/apoA-I ratio), and activity of four proteins that regulate lipoprotein metabolism (CETP, LCAT, PLTP and PON1) in plasma samples derived from children with FH (n = 47) and from healthy children (n = 56). Potential biological mechanisms behind any variation in lipoprotein functionalities were explored using an NMR-based metabolomics profiling approach.ResultsLDL aggregation was higher and HAE/apoA-I ratio was lower in FH children than in healthy children. LDL aggregation associated positively with LDL-C and negatively with triglycerides, and HAE/apoA-I ratio associated negatively with LDL-C. Generally, the metabolomic profile for LDL aggregation was a mirror image of that for HAE/apoA-I ratio.ConclusionsFH children displayed increased atherogenicity of LDL and disrupted HDL function. These newly observed functional alterations in LDL and HDL may increase the risk for atherosclerotic cardiovascular disease in FH children.

Background and aims Measures of HDL function are emerging tools for assessing cardiovascular disease (CVD) event risk. HDL-apoA-I exchange (HAE) reflects HDL capacity for reverse cholesterol transport. Methods HAE was measured in 93 participants with type 2 diabetes (T2D) and at least one additional CVD risk factor in the Asker and Baerum Cardiovascular Diabetes study. At baseline and after seven years, the atherosclerotic burden was assessed by invasive coronary angiography. Major CVD events were registered throughout the study. Results Linear regression analysis demonstrated a significant inverse association between HAE and atherosclerotic burden. Cox proportional hazard regression analysis showed a significant association between HAE and a composite of major CVD events when controlling for waist-hip ratio, HR = 0.89, 95% CI = 0.80–1.00 and p=0.040. Conclusions Despite the relatively small size of the study population and the limited number of CVD events, these findings suggest that HAE provides valuable information in determining CVD risk.

The HDL (high-density lipoprotein) Workshop was established in 2009 as a forum for candid discussions among academic basic scientists, clinical investigators, and industry researchers about the role of HDL in cardiovascular disease. This ninth HDL Workshop was held on May 16 to 17, 2019 in Boston, MA, and included outstanding oral presentations from established and emerging investigators. The Workshop featured 5 sessions with topics that tackled the role of HDL in the vasculature, its structural complexity, its role in health and disease states, and its interaction with the intestinal microbiome. The highlight of the program was awarding the Jack Oram Award to the distinguished professor emeritus G.S. Getz from the University of Chicago. The tenth HDL Workshop will be held on May 2020 in Chicago and will continue the focus on intellectually stimulating presentations by established and emerging investigators on novel roles of HDL in cardiovascular and noncardiovascular health and disease states.

Cardiovascular disease risk depends on high-density lipoprotein (HDL) function, not HDL-cholesterol. Isolevuglandins (IsoLGs) are lipid dicarbonyls that react with lysine residues of proteins and phosphatidylethanolamine. IsoLG adducts are elevated in atherosclerosis. The consequences of IsoLG modification of HDL have not been studied. We hypothesized that IsoLG modification of apoA-I deleteriously alters HDL function. We determined the effect of IsoLG on HDL structure–function and whether pentylpyridoxamine (PPM), a dicarbonyl scavenger, can preserve HDL function. IsoLG adducts in HDL derived from patients with familial hypercholesterolemia (n = 10, 233.4 ± 158.3 ng/mg) were found to be significantly higher than in healthy controls (n = 7, 90.1 ± 33.4 pg/mg protein). Further, HDL exposed to myeloperoxidase had elevated IsoLG-lysine adducts (5.7 ng/mg protein) compared with unexposed HDL (0.5 ng/mg protein). Preincubation with PPM reduced IsoLG-lysine adducts by 67%, whereas its inactive analogue pentylpyridoxine did not. The addition of IsoLG produced apoA-I and apoA-II cross-links beginning at 0.3 molar eq of IsoLG/mol of apoA-I (0.3 eq), whereas succinylaldehyde and 4-hydroxynonenal required 10 and 30 eq. IsoLG increased HDL size, generating a subpopulation of 16–23 nm. 1 eq of IsoLG decreased HDL-mediated [(3)H]cholesterol efflux from macrophages via ABCA1, which corresponded to a decrease in HDL–apoA-I exchange from 47.4% to only 24.8%. This suggests that IsoLG inhibits apoA-I from disassociating from HDL to interact with ABCA1. The addition of 0.3 eq of IsoLG ablated HDL's ability to inhibit LPS-stimulated cytokine expression by macrophages and increased IL-1β expression by 3.5-fold. The structural–functional effects were partially rescued with PPM scavenging.

Objective— Measures of HDL (high-density lipoprotein) function are associated with cardiovascular disease. However, the effects of regular exercise on these measures is largely unknown. Thus, we examined the effects of different doses of exercise on 3 measures of HDL function in 2 randomized clinical exercise trials. Approach and Results— Radiolabeled and boron dipyrromethene difluoride–labeled cholesterol efflux capacity and HDL-apoA-I (apolipoprotein A-I) exchange were assessed before and after 6 months of exercise training in 2 cohorts: STRRIDE-PD (Studies of Targeted Risk Reduction Interventions through Defined Exercise, in individuals with Pre-Diabetes; n=106) and E-MECHANIC (Examination of Mechanisms of exercise-induced weight compensation; n=90). STRRIDE-PD participants completed 1 of 4 exercise interventions differing in amount and intensity. E-MECHANIC participants were randomized into 1 of 2 exercise groups (8 or 20 kcal/kg per week) or a control group. HDL-C significantly increased in the high-amount/vigorous-intensity group (3±5 mg/dL; P =0.02) of STRRIDE-PD, whereas no changes in HDL-C were observed in E-MECHANIC. In STRRIDE-PD, global radiolabeled efflux capacity significantly increased 6.2% (SEM, 0.06) in the high-amount/vigorous-intensity group compared with all other STRRIDE-PD groups (range, −2.4 to −8.4%; SEM, 0.06). In E-MECHANIC, non-ABCA1 (ATP-binding cassette transporter A1) radiolabeled efflux significantly increased 5.7% (95% CI, 1.2–10.2%) in the 20 kcal/kg per week group compared with the control group, with no change in the 8 kcal/kg per week group (2.6%; 95% CI, −1.4 to 6.7%). This association was attenuated when adjusting for change in HDL-C. Exercise training did not affect BODIPY-labeled cholesterol efflux capacity or HDL-apoA-I exchange in either study. Conclusions— Regular prolonged vigorous exercise improves some but not all measures of HDL function. Future studies are warranted to investigate whether the effects of exercise on cardiovascular disease are mediated in part by improving HDL function. Clinical Trial Registration— URL: https://www.clinicaltrials.gov . Unique identifiers: NCT00962962 and NCT01264406.

ObjectiveThe role of high-density lipoprotein (HDL) function in HIV-related atherosclerotic cardiovascular disease (CVD) is unclear. HDLs isolated from HIV [HIV(+)HDL] and HIV-uninfected individuals [HIV(-)HDL] were assessed for HDL function and ability to promote monocyte-derived foam cell formation (MDFCF; a key event in HIV-related CVD) ex vivo.Design/methodsUsing an established in-vitro model of atherogenesis and plasma samples from an established cross-sectional study of virologically suppressed HIV men on stable effective antiretroviral therapy and with low CVD risk (median age: 42 years; n = 10), we explored the impact of native HDL [HIV(+)HDL] on MDFCF. In this exploratory study, we selected HIV(+)HDL known to be dysfunctional based on two independent measures of impaired HDL function: antioxidant (high HDLox) ability of HDL to release apolipoprotein A-I (ApoA-I) (low HDL-ApoA-I exchange). Five healthy men matched by age and race to the HIV group were included. Given that oxidation of HDL leads to abnormal HDL function, we also compared proatherogenic effects of HIV(+)HDL vs. chemically derived HDLox. The ex-vivo atherogenesis assay was performed using lipoproteins (purchased or isolated from plasma using ultracentrifugation) and monocytes purified via negative selection from healthy donors.ResultsHIV(+)HDL known to have reduced antioxidant function and rate of HDL/ApoAI exchange promoted MDFCF to a greater extent than HDL (33.0 vs. 26.2% foam cells; P = 0.015). HDL oxidized in vitro also enhanced foam cell formation as compared with nonoxidized HDL (P

Apolipoprotein (apo)A-I is an organizing scaffold protein that is critical to high-density lipoprotein (HDL) structure and metabolism, probably mediating many of its cardioprotective properties. However, HDL biogenesis is poorly understood, as lipid-free apoA-I has been notoriously resistant to high-resolution structural study. Published models from low-resolution techniques share certain features but vary considerably in shape and secondary structure. To tackle this central issue in lipoprotein biology, we assembled a team of structural biologists specializing in apolipoproteins and set out to build a consensus model of monomeric lipid-free human apoA-I. Combining novel and published cross-link constraints, small-angle X-ray scattering (SAXS), hydrogen-deuterium exchange (HDX) and crystallography data, we propose a time-averaged model consistent with much of the experimental data published over the last 40 years. The model provides a long-sought platform for understanding and testing details of HDL biogenesis, structure and function.

Aims The biogenesis of high-density lipoprotein (HDL) particles by cholesterol-laden foam cells in atherosclerotic lesions is crucial for the removal of excess cholesterol from the lesions. Impairment in the HDL biogenic process contributes to the progression of atherosclerosis. The aim of this study is to identify novel cellular factors regulating HDL biogenesis. Methods and results HDL biogenesis is a process of apolipoprotein (apo)-mediated solubilization of specific plasma membrane (PM) microdomains generated in cholesterol-accumulated cells. We established a new method to isolate PM microdomains interacting with the major HDL protein constituent, apoA-I. Lipidomic and proteomic analyses of an isolated PM microdomain revealed that apoA-I binds to cholesterol-rich and desmocollin 1 (DSC1)-containing microdomains. In this novel apoA-I binding microdomain, DSC1 binds and prevents apoA-I from interacting with another PM microdomain created by adenosine triphosphate-binding cassette transporter A1 (ABCA1) for the formation of HDL. Inhibition of apoA-I-DSC1 binding by silencing DSC1 expression or using DSC1 blocking antibodies increases apoA-I accessibility to ABCA1-created microdomains and thus enhances HDL biogenesis. Importantly, DSC1 is abundantly expressed in macrophages and human atherosclerotic lesions, suggesting that DSC1 may contribute to cholesterol accumulation in atherosclerotic lesions by sequestering apoA-I and impairing HDL biogenesis. Conclusions The binding of apoA-I to two functionally opposing PM microdomains, ABCA1 and DSC1 domains, suggests that HDL biogenesis and PM cholesterol levels may be regulated by the relative abundance of the two domains and that novel HDL biogenic therapies may be developed by targeting DSC1.

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] (pP 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.

Background/synopsisLow-dose aspirin is an effective drug for the prevention of cardiovascular disease (CVD) events but individuals with diabetes mellitus can be subject to 'aspirin resistance'. Thus, aspirin's effect in these individuals is controversial. Higher blood levels of seafood-derived omega-3 polyunsaturated fatty acids (ω3) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) also have beneficial effects in reducing risk of CVD events but few studies have examined the interaction of plasma EPA and DHA with aspirin ingestion.Objective/purposeOur study examined the combinatory effects of EPA, DHA, and aspirin ingestion on HDL-cholesterol (HDL-C) and apoA-I exchange (shown to be associated with CVD event risk).Methods30 adults with Type 2 diabetes mellitus ingested aspirin (81mg/day) for 7 consecutive days, EPA+DHA (2.6g/day) for 28 days, then both for 7 days. Plasma was collected at baseline and at 5 subsequent visits including 4h after each aspirin ingestion. Mixed model methods were used to determine HDL-C-concentrations and apoA-I exchange compared to the baseline visit values. LOWESS curves were used for non-linear analyses of outcomes to help discern change patterns, which was followed by piecewise linear functions for formal testing of curvilinear relationships.ResultsSignificant changes (p < 0.05) compared to baseline in both HDL-C-concentrations and apoA-I exchange were present at different times. After 7 days of aspirin-only ingestion, apoA-I exchange was significantly modified by increasing levels of DHA concentration, with increased apoA-I exchange observed up until log(DHA) of 4.6 and decreased exchange thereafter (p = 0.03). These LOWESS curve effects were not observed for EPA or HDL-C (p > 0.05). Aspirin's effects on apoA-I exchange were the greatest when EPA or DHA concentrations were moderate compared to high or low. Comparison of EPA, DHA, and EPA+DHA LOWESS curves, demonstrated that the majority of the effect is due to DHA.ConclusionOur results strongly suggest that plasma concentrations of EPA and DHA influence aspirin effects on lipid mediators of CVD event risk where their concentrations are most beneficial when moderate, not high or low. These effects on HDL-C cholesterol and apoA-I exchange are novel. Personalized dosing of DHA in those who take aspirin may be a beneficial option for patients with type 2 diabetes mellitus.

We provide a historical perspective of how high-density lipoprotein (HDL) cholesterol became a clinical standard, the evidence in favor of HDL function as a more appropriate indication of HDL's antiatherogenic nature, and the options ahead.Recent studies have demonstrated a strong relationship between the cholesterol efflux capacity of plasma and prevalent cardiovascular disease (CVD) and CVD event risk, indicating the utility of HDL function as a diagnostic/prognostic of CVD.We will present how HDL cholesterol came to be the standard proxy of HDL function, the key observations that drew its clinical relevance into question, and the pros and cons of commercially available approaches to measuring HDL function.

High-density lipoprotein (HDL), a lipid nanoparticle containing many different low abundance proteins, is an attractive target for clinical proteomics because its compositional heterogeneity is linked to its cardioprotective effects. Selected reaction monitoring (SRM) is currently the method of choice for targeted quantification of proteins in such a complex biological matrix. However, model system studies suggest that parallel reaction monitoring (PRM) is more specific than SRM because many product ions can be used to confirm the identity of a peptide. We therefore compared PRM and SRM for their abilities to quantify proteins in HDL, using 15N-labeled apolipoprotein A-I (HDL's most abundant protein) as the internal standard. PRM and SRM exhibited comparable linearity, dynamic range, precision, and repeatability for protein quantification of HDL. Moreover, the single internal standard protein performed as well as protein-specific peptide internal standards when quantifying 3 different proteins. Importantly, PRM and SRM yielded virtually identical quantitative results for 26 proteins in HDL isolated from 44 subjects. Because PRM requires less method development than SRM and is potentially more specific, our observations indicate that PRM in concert with a single isotope-labeled protein is a promising new strategy for quantifying HDL proteins in translational studies. Biological significance HDL, a complex matrix composed of lipids and proteins, is implicated in cardioprotection. Its cholesterol content correlates inversely with cardiovascular disease and it is the current metric to assess cardiovascular risk. However, the cholesterol content does not capture HDL's complexity and heterogeneity. Devising metrics that better capture HDL's cardioprotective effects, we developed an optimized method for quantification of HDL proteome, using PRM in concert with a single labeled protein as internal standard. The availability of a method that increases sample throughput without compromising the reproducibility, sensitivity, and accuracy could therefore point to better risk assessment for CVD or other diseases.

Author(s): Kelesidis, Theodoros | Advisor(s): LLOYD-SMITH, JAMES O | Abstract: Objective: HDL function rather than absolute level may be a more accurate indicator for cardiovascular disease (CVD) but it is unclear what drives HDL dysfunction in HIV-1 infection. The objective of this study is to identify factors that may contribute to HDL dysfunction in chronic HIV-1 infection. Design: Retrospective study of HIV-1 infected males with low overall CVD risk and healthy males with no known CVD risk matched by race to the HIV-1 infected participants. Methods: We related parameters previously reported to be associated with HDL function to two different measures of HDL function: reduced antioxidant function (oxidized HDL, HDLox) and HDL-apoA-I exchange (HAE). Multivariable-adjusted linear regression analyses were employed adjusting for false discovery rate (FDR), age, race, body mass index (BMI), CD4 count, viremia, CVD risk, smoking, lipids, apoA-I, albumin. Results: In multivariate analysis among HIV-1 infected males (n=166) (median age 45 years, median CD4 T cell count 505 copies/ml, 30.1% were viremic), higher BMI, lower apoA-I and lower albumin were among the most notable correlates of higher HDLox and lower HAE (impaired HDL function)(pl0.05). In HIV-1 uninfected participants lower albumin and higher BMI were associated with lower HAE and higher HDLox, respectively (p≤0.05). HDLox was inversely related to HAE in HIV-1 infected (but not uninfected) individuals (pl0.001). Conclusion: Increased HDLox is associated with reduced HDL remodeling in chronic HIV-1 infection. Higher BMI, lower apoA-I and lower albumin were identified as factors associated with abnormal HDL function in chronic HIV-1 infection using two independent methods.

Objective: Apolipoprotein A-I (apoA-I) spontaneously exchanges between high-density lipoprotein (HDL)-bound and lipid-free states. This is a key event in reverse cholesterol transport, necessary for de novo HDL biogenesis. However, the mechanism of HDL-apoA-I exchange (HAE) is poorly understood. In this study, we test whether HDL remodeling is obligatory to HAE or an independent process. Method and Results: Recombinant human apoA-I modified with single cysteine mutations were expressed in bacteria and labeled with Alexa 488 and Alexa 647 fluorophores. ApoA-I Alexa647 was used to synthesize rHDL particles of defined sizes with phosphatidyl choline (POPC) and cholesterol. Reconstituted HDL of 17.0, 12.2, 9.6, 8.4 and 7.8 nm were obtained. These particles were incubated with excess lipid-free apoA-I Alexa488 to initiate HAE, which was quantified using non-denaturing gradient gel electrophoresis to separate HDL particles from lipid-free apoA-I. Gels were imaged using FITC and Cy5 filters. The extent of apoA-I Alexa488 binding and apoA-I Alexa647 released from HDL was determined by densitometry. HAE occurred without significant changes in rHDL particle size for all subclasses of rHDL. HAE in the forward (lipid-free to bound) direction did not exhibit size dependence, but HAE in the reverse (lipid-bound to free) direction appeared to be size-dependent, with the 8.4 nm rHDL displaying 2-fold higher apoA-I Alexa647 dissociation compared to the 17.0 nm particle. Conclusions: The bidirectional rate of HAE can be measured simultaneously using fluorescent apoA-I probes. Reconstituted HDL particles predominantly retained their size, indicating that apoA-I exchange occurs without remodeling of HDL particles. This result has significant implications as to the possible molecular processes driving apoA-I exchange with HDL.

Background Patients with type 1 diabetes (T1D) are at increased risk of cardiovascular disease (CVD). Measures of high-density lipoprotein (HDL) function provide a better risk estimate for future CVD events than serum levels of HDL cholesterol. The objective of this study was to evaluate HDL function in T1D patients shortly after disease onset compared with healthy control subjects. Methods Participants in the atherosclerosis and childhood diabetes study were examined at baseline and after 5 years. At baseline, the cohort included 293 T1D patients with a mean age of 13.7 years and mean HbA1c of 8.4%, along with 111 healthy control subjects. Their HDL function, quantified by HDL-apoA-I exchange (HAE), was assessed at both time points. HAE is a measure of HDL’s dynamic property, specifically its ability to release lipid-poor apolipoprotein A-I (apoA-I), an essential step in reverse cholesterol transport. Results The HAE-apoA-I ratio, reflecting the HDL function per concentration unit apoA-I, was significantly lower in the diabetes group both at baseline, 0.33 (SD = 0.06) versus 0.36 (SD = 0.06) %HAE/mg/dL, p

Apolipoprotein A-I (ApoA-I) is the major protein component of high-density lipoprotein (HDL), and is critical for maintenance of cholesterol homeostasis. During reverse cholesterol transport, HDL transitions between an array of subclasses, differing in size and composition. This process requires ApoA-I to adapt to changes in the shape of the HDL particle, transiting from an apolipoprotein to a myriad of HDL subclass-specific conformations. Changes in ApoA-I structure cause alterations in HDL-specific enzyme and receptor-binding properties, and thereby direct the HDL particle through the reverse cholesterol transport pathway. In this study, we used site-directed spin label spectroscopy to examine the conformational details of the ApoA-I central domain on HDL. The motional dynamics and accessibility to hydrophobic/hydrophilic relaxation agents of ApoA-I residues 99–163 on 9.6-nm reconstituted HDL was analyzed by EPR. In previous analyses, we examined residues 6–98 and 164–238 (of ApoA-I's 243 residues), and combining these findings with the current results, we have generated a full-length map of the backbone structure of reconstituted HDL-associated ApoA-I. Remarkably, given that the majority of ApoA-I's length is composed of amphipathic helices, we have identified nonhelical residues, specifically the presence of a β-strand (residues 149–157). The significance of these nonhelical residues is discussed, along with the other features, in the context of ApoA-I function in contrast to recent models derived by other methods.

Objective: The ability of high density lipoprotein (HDL) to promote cholesterol efflux from ATP binding cassette transporter A1 (ABCA1) is inversely associated with HDL particle size. The exchangeability of HDL’s primary protein constituent, apolipoprotein A-I (apoA-I), is positively correlated with cholesterol efflux capacity. However, it is unknown whether HDL particle size similarly affects the exchangeability of apoA-I. In this study, we investigated the effect of HDL particle size on the exchangeability of apoA-I on reconstituted (rHDL) and human-derived plasma HDL (pHDL) particles. Method and Results: Discoidal rHDL particles of well-defined size (17.0, 12.2, 9.6, 8.4 and 7.8 nm) were prepared from POPC and apoA-I by cholate dialysis and purified by size exclusion chromatography. Human HDL was isolated by subclass (HDL2b, 2a, 3a, 3b, 3c) from the plasma of healthy volunteers by sequential density gradient ultracentrifugation. HDL-apoA-I exchange (HAE) was measured by electron paramagnetic resonance (EPR) and non-denaturing gradient gel electrophoresis (NDGGE) following incubation with either spin-labeled or fluorescently-labeled, lipid-free apoA-I. Size-dependent effects were observed with rHDL particles, with the largest rHDL particles exhibiting 3-fold higher HAE compared to the smallest particles. In contrast, the HAE of human-derived pHDL particles was relatively constant across all subclasses at constant apoA-I concentration. Conclusions: HDL-apoA-I exchange is size-dependent in rHDL, with HAE positively associated with rHDL particle size, but it is size-independent in human pHDL. Our findings suggest that additional factors such as remodeling enzymes may facilitate apoA-I exchange from human pHDL.

Apolipoprotein A-I (apoA-I) is a prominent member of the exchangeable apolipoprotein class of proteins, capable of transitioning between lipid-bound and lipid-free states. It is the primary structural and functional protein of high density lipoprotein (HDL). Lipid-free apoA-I is critical to de novo HDL formation as it is the preferred substrate of the lipid transporter, ATP Binding Cassette Transporter A1 (ABCA1) Remaley et al. (2001) [1]. Lipid-free apoA-I is an important element in reverse cholesterol transport and comprehension of its structure is a core issue in our understanding of cholesterol metabolism. However, lipid-free apoA-I is highly conformationally dynamic making it a challenging subject for structural analysis. Over the past 20years there have been significant advances in overcoming the dynamic nature of lipid-free apoA-I, which have resulted in a multitude of proposed conformational models.

Low levels of high-density lipoprotein cholesterol (HDL-C) and high triglyceride levels contribute to the excess rate of cardiovascular events seen in subjects with type 2 diabetes. Fenofibrate treatment partially reverses dyslipidemia in these subjects. However, a paradoxical marked reduction in HDL-C and HDL's major protein, apolipoprotein A-I, is a complication of fenofibrate in combination with rosiglitazone, an insulin-sensitizing agent. Risk factors for this condition, termed hypoalphalipoproteinemia, have yet to be identified. Using a case-control study design with subjects enrolled in the Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial, we tested the hypothesis that alterations in HDL's protein cargo predispose diabetic subjects to fenofibrate/rosiglitazone-induced hypoalphalipoproteinemia. HDL was isolated from blood obtained from controls (no decreases or increase in HDL-C while receiving fenofibrate/rosiglitazone therapy) and cases (developed hypoalphalipoproteinemia after fenofibrate/rosiglitazone treatment) participating in the ACCORD study before they began fenofibrate/rosiglitazone treatment. HDL proteins were quantified by targeted parallel reaction monitoring (PRM) and selected reaction monitoring (SRM) with isotope dilution. This approach demonstrated marked increases in the relative concentrations of paraoxonase/arylesterase 1 (PON1), apolipoprotein C-II (APOC2), apolipoprotein C-I, and apolipoprotein H in the HDL of subjects who developed hypoalphalipoproteinemia. The case and control subjects did not differ significantly in baseline HDL-C levels or other traditional lipid risk factors. We used orthogonal biochemical techniques to confirm increased levels of PON1 and APOC2. Our observations suggest that an imbalance in HDL proteins predisposes diabetic subjects to develop hypoalphalipoproteinemia on fenofibrate/rosiglitazone therapy.

Human cholesteryl ester transfer protein (CETP) mediates the net transfer of cholesteryl ester mass from atheroprotective high-density lipoproteins to atherogenic low-density lipoproteins by an unknown mechanism. Delineating this mechanism would be an important step toward the rational design of new CETP inhibitors for treating cardiovascular diseases. Using EM, single-particle image processing and molecular dynamics simulation, we discovered that CETP bridges a ternary complex with its N-terminal β-barrel domain penetrating into high-density lipoproteins and its C-terminal domain interacting with low-density lipoprotein or very-low-density lipoprotein. In our mechanistic model, the CETP lipoprotein-interacting regions, which are highly mobile, form pores that connect to a hydrophobic central cavity, thereby forming a tunnel for transfer of neutral lipids from donor to acceptor lipoproteins. These new insights into CETP transfer provide a molecular basis for analyzing mechanisms for CETP inhibition.

Lipid transport in the brain is coordinated by glial-derived lipoproteins that contain apolipoprotein E (apoE) as their primary protein. Here we show that apoE is secreted from wild-type (WT) primary murine mixed glia as nascent lipoprotein subspecies ranging from 7.5 to 17 nm in diameter. Negative-staining electron microscropy (EM) revealed rouleaux, suggesting a discoidal structure. Potassium bromide (KBr) density gradient ultracentrifugation showed that all subspecies, except an 8.1 nm particle, were lipidated. Glia lacking the cholesterol transporter ABCA1 secreted only 8.1 nm particles, which were poorly lipidated and nondiscoidal but could accept lipids to form the full repertoire of WT apoE particles. Receptor-associated-protein (RAP)-mediated inhibition of apoE receptor function blocked appearance of the 8.1 nm species, suggesting that this particle may arise through apoE recycling. Selective deletion of the LDL receptor (LDLR) reduced the level of 8.1 nm particle production by approximately 90%, suggesting that apoE is preferentially recycled through the LDLR. Finally, apoA-I stimulated secretion of 8.1 nm particles in a dose-dependent manner. These results suggest that nascent glial apoE lipoproteins are secreted through multiple pathways and that a greater understanding of these mechanisms may be relevant to several neurological disorders.

Coronary disease risk increases inversely with high-density lipoprotein (HDL) level. The measurement of the biodistribution and clearance of HDL in vivo, however, has posed a technical challenge. This study presents an approach to the development of a lipoprotein MRI agent by linking gadolinium methanethiosulfonate (Gd[MTS-ADO3A]) to a selective cysteine mutation in position 55 of apo AI, the major protein of HDL. The contrast agent targets both liver and kidney, the sites of HDL catabolism, whereas the standard MRI contrast agent, gadolinium-diethylenetriaminepentaacetic acid-bismethylamide (GdDTPA-BMA, gadodiamide), enhances only the kidney image. Using a modified apolipoprotein AI to create an HDL contrast agent provides a new approach to investigate HDL biodistribution, metabolism and regulation in vivo.

For several decades, the standard model for high density lipoprotein (HDL) particles reconstituted from apolipoprotein A-I (apoA-I) and phospholipid (apoA-I/HDL) has been a discoidal particle ∼100 A in diameter and the thickness of a phospholipid bilayer. Recently, Wu et al. (Wu, Z., Gogonea, V., Lee, X., Wagner, M. A., Li, X. M., Huang, Y., Undurti, A., May, R. P., Haertlein, M., Moulin, M., Gutsche, I., Zaccai, G., Didonato, J. A., and Hazen, S. L. (2009) J. Biol. Chem. 284, 36605-36619) used small angle neutron scattering to develop a new model they termed double superhelix (DSH) apoA-I that is dramatically different from the standard model. Their model possesses an open helical shape that wraps around a prolate ellipsoidal type I hexagonal lyotropic liquid crystalline phase. Here, we used three independent approaches, molecular dynamics, EM tomography, and fluorescence resonance energy transfer spectroscopy (FRET) to assess the validity of the DSH model. (i) By using molecular dynamics, two different approaches, all-atom simulated annealing and coarse-grained simulation, show that initial ellipsoidal DSH particles rapidly collapse to discoidal bilayer structures. These results suggest that, compatible with current knowledge of lipid phase diagrams, apoA-I cannot stabilize hexagonal I phase particles of phospholipid. (ii) By using EM, two different approaches, negative stain and cryo-EM tomography, show that reconstituted apoA-I/HDL particles are discoidal in shape. (iii) By using FRET, reconstituted apoA-I/HDL particles show a 28-34-A intermolecular separation between terminal domain residues 40 and 240, a distance that is incompatible with the dimensions of the DSH model. Therefore, we suggest that, although novel, the DSH model is energetically unfavorable and not likely to be correct. Rather, we conclude that all evidence supports the likelihood that reconstituted apoA-I/HDL particles, in general, are discoidal in shape.

Amphotericin B (AMB), a potent antifungal agent, has been employed as an inhalable therapy for pulmonary fungal infections. We recently described a novel nano-sized delivery vehicle composed of phospholipid (PL) and apolipoprotein A-I, NanoDisk (ND), to which we added AMB as a payload (ND-AMB). The goal of the present study was to evaluate whether ND-AMB, compared to other formulations, preserves lung cell integrity in vitro, as AMB can be toxic to mammalian cells and reduce lung function when inhaled. Epithelial integrity was assessed by measuring K(+) ion flux across a model airway epithelium, Calu-3 cells. In this assay ND-AMB was at least 8-fold less disruptive than AMB/deoxycholate (DOC). Cell viability studies confirmed this observation. Unexpectedly, the ND vehicle restored the integrity of a membrane compromised by prior exposure to AMB. An alternative formulation of ND-AMB containing a high load of AMB per ND was not protective, suggesting that ND with a low ratio of AMB to PL can sequester additional AMB from membranes. ND-AMB also protected HepG2 cells from the cytotoxicity of AMB, as determined by cellular viability and lactate dehydrogenase (LDH) levels. This study suggests that ND-AMB may be safe for administration via inhalation and reveals a unique activity whereby ND-AMB protects lung epithelial membranes from AMB toxicity.

An important event in cholesterol metabolism is the efflux of cellular cholesterol by apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins (HDL). Lipid-free apoA-I is the preferred substrate for ATP-binding cassette A1, which promotes cholesterol efflux from macrophage foam cells in the arterial wall. However, the vast majority of apoA-I in plasma is associated with HDL, and the mechanisms for the generation of lipid-free apoA-I remain poorly understood. In the current study, we used fluorescently labeled apoA-I that exhibits a distinct fluorescence emission spectrum when in different states of lipid association to establish the kinetics of apoA-I transition between the lipid-associated and lipid-free states. This approach characterized the spontaneous and rapid exchange of apoA-I between the lipid-associated and lipid-free states. In contrast, the kinetics of apoA-I exchange were significantly reduced when apoA-I on HDL was cross-linked with a bi-functional reagent or oxidized by myeloperoxidase. Our observations support the hypothesis that oxidative damage to apoA-I by myeloperoxidase limits the ability of apoA-I to be liberated in a lipid-free form from HDL. This impairment of apoA-I exchange reaction may be a trait of dysfunctional HDL contributing to reduced ATP-binding cassette A1-mediated cholesterol efflux and atherosclerosis.

Dysfunctional high density lipoprotein (HDL) is implicated in the pathogenesis of cardiovascular disease, but the underlying pathways remain poorly understood. One potential mechanism involves covalent modification by reactive carbonyls of apolipoprotein A-I (apoA-I), the major HDL protein. We therefore determined whether carbonyls resulting from lipid peroxidation (malondialdehyde (MDA) and hydroxynonenal) or carbohydrate oxidation (glycolaldehyde, glyoxal, and methylglyoxal) covalently modify lipid-free apoA-I and inhibit its ability to promote cellular cholesterol efflux by the ABCA1 pathway. MDA markedly impaired the ABCA1 activity of apoA-I. In striking contrast, none of the other four carbonyls were effective. Liquid chromatography-electrospray ionization-tandem mass spectrometry of MDA-modified apoA-I revealed that Lys residues at specific sites had been modified. The chief adducts were MDA-Lys and a Lys-MDA-Lys cross-link. Lys residues in the C terminus of apoA-I were targeted for cross-linking in high yield, and this process may hinder the interaction of apoA-I with lipids and ABCA1, two key steps in reverse cholesterol transport. Moreover, levels of MDA-protein adducts were elevated in HDL isolated from human atherosclerotic lesions, suggesting that lipid peroxidation might render HDL dysfunctional in vivo. Taken together, our observations indicate that MDA damages apoA-I by a pathway that generates lysine adducts at specific sites on the protein. Such damage may facilitate the formation of macrophage foam cells by impairing cholesterol efflux by the ABCA1 pathway.

Accumulation of low-density lipoprotein (LDL)-derived cholesterol by artery wall macrophages triggers atherosclerosis, the leading cause of cardiovascular disease. Conversely, high-density lipoprotein (HDL) retards atherosclerosis by promoting cholesterol efflux from macrophages by the membrane-associated ATP-binding cassette transporter A1 (ABCA1) pathway. HDL has been proposed to lose its cardioprotective effects in subjects with atherosclerosis, but the underlying mechanisms are poorly understood. One potential pathway involves oxidative damage by myeloperoxidase (MPO), a heme enzyme secreted by human artery wall macrophages. We used mass spectrometry to demonstrate that HDL isolated from patients with established cardiovascular disease contains elevated levels of 3-chlorotyrosine and 3-nitrotyrosine, two characteristic products of MPO. When apolipoprotein A-I (apoA-I), the major HDL protein, was oxidized by MPO, its ability to promote cellular cholesterol efflux by ABCA1 was impaired. Moreover, oxidized apoA-I was unable to activate lecithin:cholesterol acyltransferase (LCAT), which rapidly converts free cholesterol to cholesteryl ester, a critical step in HDL maturation. Biochemical studies implicated tyrosine chlorination and methionine oxygenation in the loss of ABCA1 and LCAT activity by oxidized apoA-I. Oxidation of specific residues in apoA-I inhibited two key steps in cholesterol efflux from macrophages, raising the possibility that MPO initiates a pathway for generating dysfunctional HDL in humans.

Oxidized HDL has been proposed to play a key role in atherogenesis. A wide range of reactive intermediates oxidizes methionine residues to methionine sulfoxide (MetO) in apolipoprotein A-I (apoA-I), the major HDL protein. These reactive species include those produced by myeloperoxidase, an enzyme implicated in atherogenesis. The aim of the present study was to develop a sensitive and specific ELISA for detecting MetO residues in HDL. We therefore immunized mice with HPLC-purified human apoA-I containing MetO(86) and MetO(112) (termed apoA-I(+32)) to generate a monoclonal antibody termed MOA-I. An ELISA using MOA-I detected lipid-free apoA-I(+32), apoA-I modified by 2e-oxidants (hydrogen peroxide, hypochlorous acid, peroxynitrite), and HDL oxidized by 1e- or 2e-oxidants and present in buffer or human plasma. Detection was concentration dependent, reproducible, and exhibited a linear response over a physiologically plausible range of concentrations of oxidized HDL. In contrast, MOA-I failed to recognize native apoA-I, native apoA-II, apoA-I modified by hydroxyl radical or metal ions, or LDL and methionine-containing proteins other than apoA-I modified by 2e-oxidants. Because the ELISA we have developed specifically detects apoA-I containing MetO in HDL and plasma, it should provide a useful tool for investigating the relationship between oxidized HDL and coronary artery disease.

HDL protects against vascular disease by accepting free cholesterol from macrophage foam cells in the artery wall. This pathway is critically dependent on lecithin:cholesterol acyltransferase (LCAT), which rapidly converts cholesterol to cholesteryl ester. The physiological activator of LCAT is apolipoprotein A-I (apoA-I), the major HDL protein. However, cholesterol removal is compromised if apoA-I is exposed to reactive intermediates. In humans with established cardiovascular disease, myeloperoxidase (MPO) oxidizes HDL, and oxidation by MPO impairs apoA-I's ability to activate LCAT in vitro . Because a single methionine residue in apoA-I, Met-148, resides near the center of the protein's LCAT activation domain, we determined whether its oxidation by MPO could account for the loss of LCAT activity. Mass spectrometric analysis demonstrated that oxidation of Met-148 to methionine sulfoxide associated quantitatively with loss of LCAT activity in both discoidal HDL and HDL 3 , the enzyme's physiological substrates. Reversing oxidation with methionine sulfoxide reductase restored HDL's ability to activate LCAT. Discoidal HDL prepared with apoA-I containing a Met-148→Leu mutation was significantly resistant to inactivation by MPO. Based on structural data in the literature, we propose that oxidation of Met-148 disrupts apoA-I's central loop, which overlaps the LCAT activation domain. These observations implicate oxidation of a single Met in apoA-I in impaired LCAT activation, a critical early step in reverse cholesterol transport.

High-density lipoprotein (HDL) inhibits atherosclerosis by removing cholesterol from artery wall macrophages. Additionally, HDL is anti-inflammatory in animal studies, suggesting that this property might also be important for its cardioprotective effects. Recent studies in subjects with established cardiovascular disease (CVD) demonstrate that myeloperoxidase targets HDL for oxidation and blocks the lipoprotein's ability to remove excess cholesterol from cells, raising the possibility that the enzyme provides a specific mechanism for generating dysfunctional HDL in humans. Shotgun proteomic analysis of HDL identified multiple complement regulatory proteins, protease inhibitors, and acute-phase response proteins, supporting a central role for HDL in inflammation. Mass spectrometry and biochemical analyses demonstrated that HDL(3) from CVD subjects was selectively enriched in apolipoprotein E, suggesting that it carries a unique cargo of proteins in humans with clinically significant CVD. Thus, oxidative modifications to HDL and changes in its protein composition might be useful biomarkers-and perhaps mediators-of CVD.

Apolipoprotein A-I (apoA-I) is the major protein constituent of high density lipoprotein (HDL) and plays a central role in phospholipid and cholesterol metabolism. This 243-residue long protein is remarkably flexible and assumes numerous lipid-dependent conformations. Consequently, definitive structural determination of lipid-free apoA-I in solution has been difficult. Using electron paramagnetic spectroscopy of site-directed spin labels in the N-terminal domain of apoA-I (residues 1-98) we have mapped a mixture of secondary structural elements, the composition of which is consistent with findings from other in-solution methods. Based on side chain mobility and their accessibility to polar and non-polar spin relaxers, the precise location of secondary elements for amino acids 14-98 was determined for both lipid-free and lipid-bound apoA-I. Based on intermolecular dipolar coupling at positions 26, 44, and 64, these secondary structural elements were arranged into a tertiary fold to generate a structural model for lipid-free apoA-I in solution.

HDL is the primary mediator of cholesterol mobilization from the periphery to the liver via reverse cholesterol transport (RCT). A critical first step in this process is the uptake of cholesterol from lipid-loaded macrophages by HDL, a function of HDL inversely associated with prevalent and incident cardiovascular disease. We hypothesized that the dynamic ability of HDL to undergo remodeling and exchange of apoA-I is an important and potentially rate-limiting aspect of RCT. In this study, we investigated the relationship between HDL-apoA-I exchange (HAE) and serum HDL cholesterol (HDL-C) efflux capacity. We compared HAE to the total and ABCA1-specific cholesterol efflux capacity of 77 subjects. We found that HAE was highly correlated with both total (r = 0.69, P < 0.0001) and ABCA1-specific (r = 0.47, P < 0.0001) efflux, and this relationship remained significant after adjustment for HDL-C or apoA-I. Multivariate models of sterol efflux capacity indicated that HAE accounted for approximately 25% of the model variance for both total and ABCA1-specific efflux. We conclude that the ability of HDL to exchange apoA-I and remodel, as measured by HAE, is a significant contributor to serum HDL efflux capacity, independent of HDL-C and apoA-I, indicating that HDL dynamics are an important factor in cholesterol efflux capacity and likely RCT.

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 with 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 2 X chromosomes compared with mice with an X and Y chromosome. By generating mice with XX, XY, and XXY chromosome complements, we determined that the presence of 2 X chromosomes, and not the absence of the Y chromosome, influences HDL-C concentration. Conclusions— We demonstrate that having 2 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.

Evidence indicates that high density lipoprotein (HDL) is cardioprotective and that several mechanisms are involved. One important pathway is a membrane-associated ATP-binding cassette transporter, ABCA1, that clears cholesterol from macrophage foam cells. Anti-inflammatory and antioxidant properties also might contribute to HDL's ability to inhibit atherosclerosis.Myeloperoxidase targets HDL for oxidation, raising the possibility that the enzyme provides a specific mechanism for generating dysfunctional HDL in humans. Myeloperoxidase-dependent oxidation of apolipoprotein A-I, the major protein in HDL, blocks HDL's ability to remove excess cholesterol from cells by the ABCA1 pathway. Analysis of mutated forms of apoA-I and oxidized apoA-I treated with methionine sulfoxide reductase implicate oxidation of specific tyrosine and methionine residues in impairing the ABCA1 transport activity of apoA-I. The crystal structure of lipid-free apoA-I suggests that such oxidative damage might disrupt negatively charged regions on the protein's surface or alter its remodeling, resulting in conformations that fail to interact with ABCA1.Oxidation of HDL by myeloperoxidase may represent a specific molecular mechanism for converting the cardioprotective lipoprotein into a dysfunctional form, raising the possibility that the enzyme represents a potential therapeutic target for preventing vascular disease in humans. Moreover, oxidized HDL might prove useful as a blood marker for clinically significant cardiovascular disease in humans.

High density lipoprotein (HDL) isolated from human atherosclerotic lesions and the blood of patients with established coronary artery disease contains elevated levels of 3-chlorotyrosine. Myeloperoxidase (MPO) is the only known source of 3-chlorotyrosine in vivo, indicating that MPO oxidizes HDL in humans. We previously reported that Tyr-192 is the major site that is chlorinated in apolipoprotein A-I (apoA-I), the chief protein in HDL, and that chlorinated apoA-I loses its ability to promote cholesterol efflux from cells by the ATP-binding cassette transporter A1 (ABCA1) pathway. However, the pathways that promote the chlorination of specific Tyr residues in apoA-I are controversial, and the mechanism for MPO-mediated loss of ABCA1-dependent cholesterol efflux of apoA-I is unclear. Using site-directed mutagenesis, we now demonstrate that lysine residues direct tyrosine chlorination in apoA-I. Importantly, methionine residues inhibit chlorination, indicating that they can act as local, protein-bound antioxidants. Moreover, we observed near normal cholesterol efflux activity when Tyr-192 of apoA-I was mutated to Phe and the oxidized protein was incubated with methionine sulfoxide reductase. Thus, a combination of Tyr-192 chlorination and methionine oxidation is necessary for depriving apoA-I of its ABCA1-dependent cholesterol transport activity. Our observations suggest that biologically significant oxidative damage of apoA-I involves modification of a limited number of specific amino acids, raising the feasibility of producing oxidation-resistant forms of apoA-I that have enhanced anti-atherogenic activity in vivo.

Background Brain lipoprotein metabolism is dependent on lipoprotein particles that resemble plasma high‐density lipoproteins but that contain apolipoprotein (apo) E rather than apoA‐I as their primary protein component. Astrocytes and microglia secrete apoE but not apoA‐I; however, apoA‐I is detectable in both cerebrospinal fluid and brain tissue lysates. The route by which plasma apoA‐I enters the central nervous system is unknown. Methods and Results Steady‐state levels of murine apoA‐I in cerebrospinal fluid and interstitial fluid are 0.664 and 0.120 μg/ mL , respectively, whereas brain tissue apoA‐I is ≈10% to 15% of its levels in liver. Recombinant, fluorescently tagged human apoA‐I injected intravenously into mice localizes to the choroid plexus within 30 minutes and accumulates in a saturable, dose‐dependent manner in the brain. Recombinant, fluorescently tagged human apoA‐I accumulates in the brain for 2 hours, after which it is eliminated with a half‐life of 10.3 hours. In vitro, human apoA‐I is specifically bound, internalized, and transported across confluent monolayers of primary human choroid plexus epithelial cells and brain microvascular endothelial cells. Conclusions Following intravenous injection, recombinant human apoA‐I rapidly localizes predominantly to the choroid plexus. Because apoA‐I mRNA is undetectable in murine brain, our results suggest that plasma apoA‐I, which is secreted from the liver and intestine, gains access to the central nervous system primarily by crossing the blood–cerebrospinal fluid barrier via specific cellular mediated transport, although transport across the blood–brain barrier may also contribute to a lesser extent.