Dan Rader: The Confusion around HDL and its Link to Cardiovascular Disease

ApoA One and HDL Particles

• Several molecules of ApoA One on any given HDL particle
• ApoA One doesn’t stay with the particle; it can exchange onto other types of mostly HDL particles and sometimes onto apob containing lipoproteins
• ApoA One is put into the blood by either the intestinal enterocyte or the hepatocyte in the liver as a free protein
• HDL formation requires ApoA One to acquire lipid, particularly phospholipid and cholesterol, soon after it’s been secreted
• Humans that lack ABCA One have virtually undetectable HDL

Tangier Disease

• Disorder primarily associated with accumulation of cholesterol in macrophages throughout the body
• May or may not have increased risk of atherosclerotic cardiovascular disease

HDL Particle Size and Density

• HDL particles are much smaller than LDL particles (about 1/​​5 to 1/​​10 the size)
• HDL particles are higher density than LDL particles due to their lipid content

HDL Particle Formation and LCAT

• LCAT enzyme is responsible for attaching a fatty acid to the cholesterol molecule, creating a cholesterol ester
• Cholesterol ester forms the core of the mature HDL particle
• People who lack LCAT have extremely low levels of HDL cholesterol and low ApoA One levels

HDL in Other Mammals

• ApoA One is highly conserved among mammals
• In many lower-​​level mammals, the primary lipoprotein is HDL
• HDL is the primary lipoprotein in mice, dogs, and other mammalian species
• HDL may have evolved as a transport mechanism for lipids within circulatory systems in lower-​​level species

Evolutionary Questions

• Humans, as well as other species, have apob in both the gut and the liver
• Differences in steady-​​state plasma levels are due to the apob system being markedly revved up in other species
  Apolipoprotein B (ApoB) and Apolipoprotein A (ApoA)
• ApoB and ApoA are essential for transporting fats in the diet for energy purposes and parceling out available fat in adipose stores 
• ApoB system is mostly a problem in modern times due to lifestyle and aging
• ApoA lipoproteins: ApoA1, ApoA2, ApoA4, ApoA5 
  • ApoA5 has a role in stimulating lipoprotein lipase and metabolism of triglyceride-​​rich lipoproteins
• HDL is a platform for transporting proteins and lipids in the blood and transferring them to other lipoproteins

HDL Fractionation

• HDL can be fractionated into different sizes and densities: HDL1, HDL2, HDL3
• HDL2 is bigger, HDL3 is smaller
• HDL fractionation is not clinically relevant for predicting risk

HDL Metabolism

• HDL is acted upon by lipases (hepatic lipase and endothelial lipase) which modify the phospholipid composition of the HDL particle
• Cholesterol ester transfer protein (CETP) transfers cholesterol esters between ApoB-​​containing lipoproteins and HDL, modifying the HDL particle in terms of size and composition
• People who lack CETP have elevated HDL cholesterol levels
• Mice do not have CETP
  CTEP Inhibitors and HDL Cholesterol
• Mice have more HDL cholesterol relative to apob due to lack of CTP protein 
• High HDL cholesterol was considered “good cholesterol” due to early studies in the 70s and 80s
• CTEP inhibitors were developed to raise HDL cholesterol levels 
  • First CTEP inhibitor, torcetropib, by Pfizer showed adverse effects and was discarded
  • Three additional CTEP inhibitors were developed, but none showed significant benefits
• HDL cholesterol itself is not directly and causally protective against atherosclerotic cardiovascular disease

Importance of Hard Outcome Trials

• First lipid-​​lowering drug, triparanol, introduced in the 1950s 
  • Lowered total cholesterol but increased cardiovascular mortality
  • Drug was pulled off the market
• Importance of hard outcome trials in cardiovascular medicine to ensure safety and efficacy of drugs

Clomiphene (Clomid) and Testosterone Replacement

• Clomiphene used off-​​label for testosterone replacement in men 
  • Preserves testicular function compared to exogenous testosterone
• However, clomiphene can raise desmosterol levels, which may be harmful 
  • Takes a long time for desmosterol levels to return to normal after stopping clomiphene
  • Some doctors have stopped using clomiphene for testosterone replacement due to this concern
    Clomid and Hormone Use
• Clomid clinics opening up, increasing use of the hormone 
• Long-​​term use of Clomid could potentially be concerning
• No clear data on how high Desmo levels were in triparanol trials

CTP Inhibition

• One CTP inhibitor, Obacetropib, still in clinical development
• More effective at lowering LDL and apob
• Raising HDL cholesterol with CTP inhibition not believed to help or hurt

Sex Differences in HDL Cholesterol

• Significant sex differences in HDL cholesterol levels
• High HDL not uniformly associated with protection
• High HDL should not be a reason to avoid statin or other LDL-​​lowering treatments

Mendelian Randomization Studies

• Low HDL cholesterol not causally linked to atherosclerosis
• Genetically high HDL cholesterol not causally linked to protection from ASVD
• High HDL should not be used as a reason to not treat in the presence of other risk factors

Clinical Implications

• High HDL should not be used as a reason to avoid treatment
• Low HDL could contribute to the decision to treat someone on the fence about starting a statin
• Individuals of African ancestry may need to be more cautious about using HDL as a predictor
  Low HDL Cholesterol and Insulin Resistance
• Low HDL cholesterol can be used as an indicator of overall risk profile 
• HDL cholesterol is associated with insulin resistance
• Ratio of triglyceride to HDL cholesterol is associated with insulin resistance
• HDL cholesterol is an integrator of information related to insulin resistance, triglycerides, and inflammation

HDL Cholesterol and Triglyceride Metabolism

• HDL cholesterol reflects 24-​​hour triglyceride metabolism better than fasting triglyceride measurements
• Repeated high-​​fat meals can have a chronic effect on HDL cholesterol levels
• HDL cholesterol integrates information over time, reflecting both fasting and postprandial triglyceride levels

Insulin Resistance and Adiponectin

• Adiponectin is an adipokine secreted by fat that has an inverse relationship with insulin resistance
• Adiponectin may have a direct effect on HDL metabolism, possibly by affecting the liver
• Insulin resistance may impact HDL cholesterol through adiponectin secretion

Niacin and HDL Cholesterol

• Niacin raises HDL cholesterol and lowers Apob (Apolipoprotein B) levels
• The clinical utility of measuring adiponectin levels is still uncertain
  Niacin and HDL Cholesterol
• Niacin was prescribed to raise HDL cholesterol 
  • HDL is considered “good cholesterol”
  • Niacin also lowers triglycerides, ApoB, and LDL
  • Modestly lowers lipoprotein(a) by 15–20%
• Clinical trials (Aim High and Merck) showed minimal or disappointing effects on reducing cardiovascular events 
  • Led to a decrease in niacin prescriptions
• Mechanism of niacin raising HDL cholesterol not fully understood 
  • Triglyceride lowering may play a role
  • Increases in HDL disproportionate to triglyceride lowering

HDL, Lipidation, Delipidation, and Reverse Cholesterol Transport

• Foam cells (lipid-​​loaded macrophages) are core pathological features of atherosclerotic vascular disease 
  • Accumulate in subintimal and intimal spaces in large vessels
• Macrophages need effective ways to rid themselves of cholesterol 
  • ABCA1 transporter helps macrophages get rid of cholesterol
  • ApoA1 is the main acceptor of cholesterol via ABCA1
• Reverse cholesterol transport (RCT) is a process that helps protect against atherosclerosis 
  • Involves picking up cholesterol via efflux and returning it to the liver for excretion
  • Promoting the first step of RCT (efflux from macrophages) could potentially protect or regress atherosclerotic plaque
• Mature HDL particle is not the main driver of RCT 
  • Measuring HDL cholesterol only provides information on mature particles, not the entire process of RCT
    HDL Cholesterol and Functionality
• HDL cholesterol levels may not accurately represent functionality 
• Different individuals with the same HDL cholesterol levels may have varying abilities to extract cholesterol from cells
• Cholesterol reflux capacity measurement of HDL is a better predictor of risk than just measuring HDL cholesterol

Ex Vivo Cholesterol Reflux Assay

• Developed to measure HDL functionality in humans
• Involves isolating HDL from blood or plasma and measuring its effectiveness in removing cholesterol from cells
• Results show significant differences in individuals’ HDL abilities to extract cholesterol from cells, even with the same HDL cholesterol levels

Clinical Applicability

• Currently, assays using radioactivity are cumbersome for high throughput clinical use
• Efforts are being made to develop cleaner, simpler, faster, and cheaper assays
• A more widely available clinical assay could be available within the next 2–3 years

Correlation with Risk

• Studies have shown that eFlux capacity is predictive of incident cardiovascular events
• Statistical analyses have been conducted to correct for clinical risk factors, but more sophisticated markers of insulin resistance have not been included
• Further research is needed to determine if eFlux capacity is causal or associative

Potential for AI and Composite Metrics 

• AI could potentially be used to develop a composite metric of biomarkers that predict eFlux capacity
• This could provide a more accurate assessment of risk without having to directly measure eFlux capacity
• Further research is needed to explore this possibility
  HDL and Human Disease
• HDL can promote nitric oxide production 
• Beneficial for blood pressure lowering and protection against atherosclerosis
• SR-​​B1 receptor mediates part of HDL’s effect on endothelial cells
• HDL can interact with skeletal muscle to promote insulin sensitivity 
  • Solid mechanistic evidence, but unclear relevance to human disease and physiology

ASCVD, Cancer, and Neurodegenerative Diseases

• ASCVD, cancer, and neurodegenerative diseases are the big three killers in the modern world 
  • ASCVD has more known information and tools for treatment
  • Alzheimer’s disease has no tools for treatment
  • Cancer treatments mostly extend median survival but don’t cure people

HDL Function and Risk Prediction

• Need a reproducible, easy-​​to-​​run, automatable assay to test HDL function in large numbers of people 
  • Several assays are in development
  • Must prove that the measurement enhances risk prediction enough to be worth doing in clinical practice

Lipid Metabolism in the Brain

• Lipid metabolism in the brain is an under-​​investigated area with potential implications for therapeutic intervention to prevent neurodegenerative diseases 
  • The brain is the most lipid-​​rich organ in the body
  • Understanding lipid metabolism in the brain could lead to new discoveries and treatments for neurodegenerative diseases