HDL’s Protein Cargo

Understanding HDL Metabolism and Biology Through In Vivo Tracer Kinetics

HDL (high-density lipoprotein), owing to its high protein content and small size, is the densest circulating lipoprotein. In contrast to lipid-laden VLDL (very-low-density lipoprotein) and LDL (low-density lipoprotein) that promote atherosclerosis, HDL is hypothesized to mitigate atherosclerosis via reverse cholesterol transport, a process that entails the uptake and clearance of excess cholesterol from peripheral tissues. This process is mediated by APOA1 (apolipoprotein A-I), the primary structural protein of HDL, as well as by the activities of additional HDL proteins. Tracer-dependent kinetic studies are an invaluable tool to study HDL-mediated reverse cholesterol transport and overall HDL metabolism in humans when a cardiovascular disease therapy is investigated. Unfortunately, HDL cholesterol-raising therapies have not been successful at reducing cardiovascular events suggesting an incomplete picture of HDL biology. However, as HDL tracer studies have evolved from radioactive isotope- to stable isotope-based strategies that in turn are reliant on mass spectrometry technologies, the complexity of the HDL proteome and its metabolism can be more readily addressed. In this review, we outline the motivations, timelines, advantages, and disadvantages of the various tracer kinetics strategies. We also feature the metabolic properties of select HDL proteins known to regulate reverse cholesterol transport, which in turn underscore that HDL lipoproteins comprise a heterogeneous particle population whose distinct protein constituents and kinetics likely determine its function and potential contribution to cholesterol clearance.

Good Cholesterol Gone Bad? HDL and COVID-19

The transmissible respiratory disease COVID-19, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has affected millions of people worldwide since its first reported outbreak in December of 2019 in Wuhan, China. Since then, multiple studies have shown an inverse correlation between the levels of high-density lipoprotein (HDL) particles and the severity of COVID-19, with low HDL levels being associated with an increased risk of severe outcomes. Some studies revealed that HDL binds to SARS-CoV-2 particles via the virus's spike protein and, under certain conditions, such as low HDL particle concentrations, it facilitates SARS-CoV-2 binding to angiotensin-converting enzyme 2 (ACE2) and infection of host cells. Other studies, however, reported that HDL suppressed SARS-CoV-2 infection. In both cases, the ability of HDL to enhance or suppress virus infection appears to be dependent on the expression of the HDL receptor, namely, the Scavenger Receptor Class B type 1 (SR-B1), in the target cells. SR-B1 and HDL represent crucial mediators of cholesterol metabolism. Herein, we review the complex role of HDL and SR-B1 in SARS-CoV-2-induced disease. We also review recent advances in our understanding of HDL structure, properties, and function during SARS-CoV-2 infection and the resulting COVID-19 disease.

Safety of regadenoson positron emission tomography stress testing in orthotopic heart transplant patients

OBJECTIVES

We sought to determine the safety of regadenoson (REG) stress testing in patients who have undergone orthotopic heart transplantation (OHT).

BACKGROUND

Routine screening for cardiac allograft vasculopathy (CAV) is necessary after OHT. Adenosine stress is contraindicated after heart transplantation due to supersensitivity in denervated hearts. Safety of regadenoson stress following OHT has not been well studied.

METHODS

We retrospectively reviewed data from OHT patients (N = 123) who were referred to REG stress testing. Medical records were reviewed to determine hemodynamic and ECG response to regadenoson and to identify adverse reactions.

RESULTS

No serious adverse events occurred. No life-threatening arrhythmias or hemodynamic changes occurred. Common side-effects related to regadenoson were observed, dyspnea being the most frequent (66.7%). On average the heart rate rose from 82.8 ± 12 to 95.7 ± 13.4 bpm (P < 0.001), systolic blood pressure decreased from 138.7 ± 20.9 to 115.9 ± 23.9 mmHg (P < 0.001) and mean arterial pressure decreased from 103.5 ± 14.1 to 84.72 ± 15.90 mmHg (P < 0.001) during stress protocol. There was no sustained ventricular tachycardia, ventricular fibrillation, or second-or third-degree atrioventricular block.

CONCLUSION

Regadenoson stress testing appears to be well tolerated and safe in OHT patients.

Remodeling of the HDL proteome with treatment response to abatacept or adalimumab in the AMPLE trial of patients with rheumatoid arthritis

BACKGROUND AND AIMS

To evaluate changes in the high-density lipoprotein (HDL) proteome and HDL function in active rheumatoid arthritis (RA) patients initiating therapy with abatacept or adalimumab in the Abatacept Versus Adalimumab Comparison in Biologic-Naïve RA Subjects with Background Methotrexate (AMPLE) study.

METHODS

Ultra high-pressure liquid chromatography (UHPLC) coupled with ion mobility mass spectrometry (LC-IM-MS) was used to analyze proteins associated with immunoaffinity-captured HDL from plasma of 30 patients with RA randomized to either abatacept (n = 15) or adalimumab (n = 15) therapy. Paraoxonase 1 (PON1) activity, HDL anti-oxidant capacity, cholesterol profiles, and homocysteine levels were also measured at baseline and following treatment. Repeated-measures analyses were performed using mixed-effect linear models to model the within-subject covariance over time.

RESULTS

In models controlling for age, sex and treatment group, improvement in inflammation measured by decreases in CRP was associated with improvement in HDL function and changes in several HDL-associated proteins including significant decreases in lipopolysaccharide-binding protein, serum amyloid A-I (SAA-I) and inter-alpha-trypsin inhibitor heavy chain H4 (p values < 0.05). Improvement in disease activity was also associated with changes in multiple HDL-associated proteins. Adalimumab was associated with higher PON1 activity, HDL-associated serotransferrin, and HDL-associated immunoglobulin J chain, and lower HDL-associated SAA-I over time compared with abatacept.

CONCLUSIONS

Improvement in inflammation associated with treatment of RA, using either abatacept or adalimumab in the AMPLE study, was associated with improvement in HDL function and significant alterations in the HDL proteome, including proteins involved in the immune response, proteinase inhibition, and lipid metabolism.

Multiphoton-Polymerized 3D Protein Assay

Multiphoton polymerization (MPP) enables 3D fabrication of micro- and nanoscale devices with complex geometries. Using MPP, we create a 3D platform for protein assays. Elevating the protein-binding sites above the substrate surface allows an optically sectioned readout, minimizing the inevitable background signal from nonspecific protein adsorption at the substrate surface. Two fluorescence-linked immunosorbent assays are demonstrated, the first one relying on streptavidin-biotin recognition and the second one on antibody recognition of apolipoprotein A1, a major constituent of high-density lipoprotein particles. Signal-to-noise ratios exceeding 1000 were achieved. The platform has high potential for 3D multiplexed recognition assays with an increased binding surface for on-chip flow cells.

Harm reduction-a systematic review on effects of alcohol reduction on physical and mental symptoms

Based on the knowledge that alcohol misuse causes a multitude of diseases and increased mortality, this systematic review examines whether a reduction of the individual alcohol consumption can contribute to a minimization of health risks within a harm reduction approach. In fact, the reviewed 63 studies indicate that interventions aiming at alcohol reduction (including total abstinence as one possible therapeutic aim) indeed resulted in or were associated with positive effects in harmful, hazardous or alcohol-dependent drinkers. Major benefits were observed for reducing alcohol-associated injuries, recovery of ventricular heart function in alcoholic cardiomyopathy, blood pressure lowering, normalization of biochemical parameter, body weight reduction, histological improvement in pre-cirrhotic alcohol-related liver disease and slowed progression of an already existing alcohol-attributable liver fibrosis. Furthermore, reduced withdrawal symptoms, prevalence of psychiatric episodes and duration of in-patient hospital days, improvement of anxiety and depression symptoms, self-confidence, physical and mental quality of life, fewer alcohol-related adverse consequences as well as lower psychosocial stress levels and better social functioning can result from reduced alcohol intake. The reviewed literature demonstrated remarkable socioeconomic cost benefits in areas such as the medical health-care system or workforce productivity. Individuals with heightened vulnerability further benefit significantly from alcohol reduction (e.g. hypertension, hepatitis C, psychiatric co-morbidities, pregnancy, but also among adolescents and young adults). Concluding, the reviewed studies strongly support and emphasize the importance and benefits of early initial screening for problematic alcohol use followed by brief and other interventions in first contact medical health-care facilities to reduce alcohol intake.

Inflammation, oxidative stress, and cardiovascular disease risk factors in adults with cystic fibrosis

Cystic fibrosis (CF) represents one of a number of localized lung and non-lung diseases with an intense chronic inflammatory component associated with evidence of systemic oxidative stress. Many of these chronic inflammatory diseases are accompanied by an array of atherosclerotic processes and cardiovascular disease (CVD), another condition strongly related to inflammation and oxidative stress. As a consequence of a dramatic increase in long-lived patients with CF in recent decades, the specter of CVD must be considered in these patients who are now reaching middle age and beyond. Buttressed by recent data documenting that CF patients exhibit evidence of endothelial dysfunction, a recognized precursor of atherosclerosis and CVD, the spectrum of risk factors for CVD in CF is reviewed here. Epidemiological data further characterizing the presence and extent of atherogenic processes in CF patients would seem important to obtain. Such studies should further inform and offer mechanistic insights into how other chronic inflammatory diseases potentiate the processes leading to CVDs.

Paraoxonase 1 and HDL maturation

Understanding the kinetics and function of paraoxonase 1 (PON1) is becoming an important issue in atherosclerosis. Low PON1 activity has been consistently linked with an increased risk of major cardiovascular events in the setting of secondary prevention of coronary artery disease. Recent studies have shown that there is a specific interaction of myeloperoxidase (MPO)-apoAI-PON1 on HDL surface that seems to be germane to atherogenesis. MPO specifically inhibits PON1 and PON1 mitigates MPO effects. Surprisingly, very little is known about the routes by which PON1 gets integrated into HDL or its fate during HDL remodeling in the intravascular space. We have developed a method that assesses PON1 activity in the individual HDL subclasses with the aid of which we have shown that PON1 is present across the HDL particle range and preferentially in HDL3, confirming data from ultracentrifugation (UC) studies. Upon HDL maturation ex vivo PON1 is activated and it shows a flux to both smaller and larger HDL particles as well as to VLDL and sdLDL. At the same time apoE, AI and AII are shifted across particle sizes. PON1 activation and flux across HDL particles are blocked by CETP and LCAT inhibitors. In a group of particles with such a complex biology as HDL, knowledge of the interaction between apo-lipoproteins, lipids and enzymes is key for an increased understanding of the yet multiple unknown features of its function. Solving the HDL paradox will necessitate the development of techniques to explore HDL function that are practical and well adapted to clinical studies and eventually become useful in patient monitoring. The confluence of proteomic, functional studies, HDL subclasses, PON1 assays and zymogram will yield data to draw a more elaborate and comprehensive picture of the function of HDL. It must be noted that all these studies are static and conducted in the fasting state. The crucial phase will be achieved when human kinetic studies (both in the fasting and post-prandial states) on HDL-PON1, apoA-I and lipid fate in the circulation are carried out.

Why targeting HDL should work as a therapeutic tool, but has not

Atherosclerosis is one of the most common causes of death and disability in the United States today despite the availability of statins, which reduce hyperlipidemia, a risk factor that predisposes individuals to this disease. Epidemiology of human populations has overwhelmingly demonstrated an inverse correlation between the concentration of plasma high-density lipoprotein (HDL) cholesterol (HDL-C) and the likelihood of developing cardiovascular disease (CVD). Decades of observations and mechanistic studies suggest that one protective function of HDL is its central role in reverse cholesterol transport. In this pathway, the ATP-binding cassette transporter A1 releases intracellular cholesterol, which is packaged with apolipoprotein A-I (apoA-I) into nascent HDL particles and released from the plasma membrane. Further lipidation and maturation of HDL occur in plasma with the eventual uptake by the liver where cholesterol is removed. It is generally accepted that CVD risk can be reduced if plasma HDL-C levels are elevated. Several different pharmacological approaches have been tried; the most popular approach targets the movement of cholesteryl ester from HDL to triglyceride-rich particles by cholesteryl ester transfer protein. Inhibition of cholesteryl ester transfer protein increases plasma HDL-C concentration; however, beneficial effects have yet to be demonstrated, likely the result of off-target effects. These revelations have led to a reevaluation of how elevating HDL concentration could decrease risk. A recent, landmark study showed that the inherent cholesterol efflux capacity of an individual's plasma was a better predictor of CVD status than overall HDL-C concentration. Even more provocative are recent studies showing that apoA-I, the principle protein component of HDL modulates cellular inflammation and oxidation. The following will review all these potential routes explaining how HDL apoA-I can reduce the risk of CVD.