Why are traditional HDL cholesterol measurements potentially misleading?

Traditional HDL cholesterol measurements are misleading because they only indicate the amount of cholesterol in HDL particles, not their functionality. High HDL cholesterol doesn't always correlate with protection from heart disease, as HDL particles can be dysfunctional and not effectively transport cholesterol back to the liver.

How do LDL particles acquire cholesterol, and is it always 'bad'?

LDL particles can acquire cholesterol through various pathways, including receiving it from HDL. While high LDL cholesterol is a risk factor, the 'good' or 'bad' designation is more about the lipoprotein's function and destination: if it successfully returns cholesterol to the liver for excretion, it's part of a beneficial pathway; if it deposits cholesterol in arterial walls, it's detrimental.

What is the significance of apolipoprotein C3 (ApoC3) in lipid metabolism?

ApoC3 plays a critical role by increasing the residence time of VLDL, IDL, and LDL particles in circulation, making them more prone to causing atherosclerosis. Higher ApoC3 levels are often associated with insulin resistance and increased cardiovascular risk.

How do VLDL and LDL particles transform and clear from the body?

VLDL particles deliver triglycerides to tissues, transforming into smaller remnants. These remnants, along with IDL, are generally cleared quickly by the liver, largely due to ApoE. VLDL can further transform into LDL, which primarily binds to ApoB and has a longer residence time, though ApoE can enhance its clearance.

Why did CETP inhibitors fail to show significant benefits in clinical trials?

CETP inhibitors were designed to raise HDL cholesterol by preventing the transfer of cholesterol from HDL to LDL. However, trials showed that simply raising HDL cholesterol didn't necessarily reduce cardiovascular events. Issues with drug toxicity and the complexity of HDL functionality likely contributed to their failure.

What is the difference between direct and indirect reverse cholesterol transport?

Direct RCT involves HDL particles directly delivering cholesterol to the liver or intestine. Indirect RCT occurs when HDL transfers its cholesterol to other lipoproteins, like LDL, which then transport it to the liver or intestine for clearance.

Can high HDL cholesterol be a sign of a problem, not a benefit?

Yes, in some rare genetic conditions, very high HDL cholesterol can indicate dysfunctional HDL particles that are rich in cholesterol but lack the necessary proteins for proper clearance, leading to accelerated atherosclerosis.

What are the limitations of measuring HDL particle size and number?

Measuring HDL particle size and number, while more informative than just HDL cholesterol, is still crude. These metrics don't fully capture the functional status of HDL particles, which is influenced by their phospholipid content and the diverse array of proteins they carry.

Why is a standard ApoB or LDL particle number assay considered valuable?

Measuring ApoB or LDL particle number is considered valuable because it directly quantifies the number of atherogenic particles in circulation, which is often a more accurate predictor of cardiovascular risk than LDL cholesterol concentration alone, especially in discordant cases (different LDL-P and LDL-C).

Key Moments

#22 – Tom Dayspring Part III of V: reverse cholesterol transport, CETP inhibitors, & apolipoproteins

Peter Attia MD

People & Blogs3 min read65 min video

Jan 1, 2020|9,446 views|180|7

HDL CETP CETP inhibitors apolipoproteins apoC-III lipoproteins

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Key Moments

TL;DR

Reverse cholesterol transport is complex; HDL function is nuanced and not solely about cholesterol levels. CETP inhibitors failed to show benefit.

Key Insights

Reverse cholesterol transport (RCT) is more complex than the simple HDL-to-liver pathway traditionally taught.

HDL cholesterol levels are a poor indicator of its functional role in reverse cholesterol transport.

Apolipoproteins, particularly ApoB, ApoE, and ApoC3, play critical roles in lipoprotein function and clearance.

CETP inhibitors, designed to raise HDL cholesterol, failed in clinical trials, suggesting HDL-C is not a reliable therapeutic target.

Lipid transport involves constant exchange and interaction between various lipoprotein particles.

Understanding lipoprotein function requires looking beyond simple cholesterol levels to particle numbers and protein composition.

DEBUNKING THE SIMPLISTIC MODEL OF REVERSE CHOLESTEROL TRANSPORT

The traditional understanding of reverse cholesterol transport (RCT) as a straightforward process where HDL particles ferry cholesterol back to the liver for excretion is overly simplistic. While the liver is a major cholesterol producer, peripheral cells also synthesize significant amounts. Cholesterol excess within these cells needs to be effluxed, and initially, it was believed only HDL particles, especially nascent ones, facilitated this. However, the reality involves complex interactions with other molecules and transport mechanisms beyond the textbook HDL-to-liver pathway.

THE MULTIFACETED ROLE OF HIGH-DENSITY LIPOPROTEINS (HDL)

HDL particles are far more complex than initially understood. While they can accept cholesterol from cells via transporters like ABCA1 and membrane diffusion, their journey doesn't solely end at the liver. HDL can transfer cholesterol to other lipoproteins, like LDL, a process mediated by CETP. Furthermore, HDL's functionality is dictated by its composition, including apolipoproteins and phospholipids, leading to numerous subpopulations, each with specific roles, akin to a fire department responding to different emergencies.

THE CRITICAL IMPORTANCE OF APOLIPOPROTEINS IN LIPID METABOLISM

Apolipoproteins, such as ApoB, ApoE, and ApoC3, are central to lipoprotein function. ApoB is crucial for LDL receptor binding, facilitating clearance. ApoE significantly amplifies clearance efficacy by acting as a ligand for LDL receptors. ApoC3, conversely, increases the plasma residence time of lipoproteins like VLDL and LDL, making them more atherogenic. Measuring ApoC3 is proposed as a valuable clinical assay for identifying high-risk remnant lipoproteins, particularly in insulin-resistant states.

LIPID TRANSPORT IS A DYNAMIC EXCHANGE SYSTEM

Lipoprotein particles are not static entities but are constantly interacting and exchanging core lipids (triglycerides and cholesterol esters) through processes like homotypic and heterotypic transfer. This exchange, often facilitated by proteins like ApoD (or CETP), means that cholesterol acquired by HDL from cells can be transferred to LDL particles. The 'goodness' or 'badness' of cholesterol is thus context-dependent, not inherent to the molecule itself, but rather dependent on the fate of the lipoprotein carrying it.

THE FAILURE OF CETP INHIBITORS AND THE LIMITATIONS OF HDL CHOLESTEROL AS A TARGET

The pursuit of CETP inhibitors, aimed at raising HDL cholesterol, has largely failed. While these drugs increase HDL-C levels, clinical trials like Pfizer's did not demonstrate cardiovascular benefit and some even showed adverse effects. This outcome strongly suggests that simply increasing HDL cholesterol is not a viable therapeutic strategy. The complexity of HDL function and the intricate lipid transport network indicate that focusing solely on HDL-C levels provides a misleading picture of cardiovascular risk.

RETHINKING LIPID ASSESSMENT: BEYOND CHOLESTEROL LEVELS

Traditional lipid panels, focusing on LDL-C and HDL-C, are insufficient for comprehensive cardiovascular risk assessment. Metrics like LDL particle number (ApoB) and non-HDL cholesterol offer a more accurate reflection of risk, particularly in insulin-resistant individuals. The intricate pathways of lipid transport, including direct and indirect RCT and the role of various apolipoproteins, highlight the need for more sophisticated biomarkers and a nuanced understanding of lipid metabolism beyond simplistic interpretations.

Mentioned in This Episode

●Supplements

●Software & Apps

●Companies

●Drugs & Medications

●Studies Cited

●Concepts

●People Referenced

Common Questions

Reverse cholesterol transport (RCT) is the process by which excess cholesterol is removed from cells and tissues and transported back to the liver for excretion. It's crucial for maintaining cholesterol homeostasis and preventing its buildup in artery walls, which can lead to atherosclerosis.

Topics

Health & Longevity Neuroscience & the Brain Science & Mathematics Lipid Metabolism HDL Function Cardiovascular Risk Factors Cholesterol Transport LDL Particles Lipid Biomarkers

Mentioned in this video

People

Tom Dayspring

Co-host of the podcast, providing expertise on lipidology and contributing to the nuanced understanding of cholesterol transport.

Brian Brewer

An expert in HDL, mentioned alongside other researchers contributing to the understanding of HDL biology.

John Chapman

An expert in HDL, mentioned for his contributions to understanding HDL's role in lipid transport.

Dan Rader

An expert in HDL biology, mentioned as one of the figures who helped elucidate the complexity of HDL in lipid transport.

Josh Knowles

An expert in familial hypercholesterolemia (FH), mentioned in the context of red blood cell cholesterol and FH patients.

Allan Sniderman

A researcher who has published extensively on the discordance between ApoB, LDL particle number, and non-HDL cholesterol.

Software & Apps

CETP

Cholesterol Ester Transfer Protein, a key protein in lipid transport; its inhibition was explored as a therapeutic strategy.

LCAT

Lecithin-cholesterol acyltransferase, an enzyme that esterifies cholesterol, enabling it to move to the core of HDL particles.

Scavenger Receptor B1

A receptor involved in delivering cholesterol esters from HDL to the liver.

Concepts

ApoB

Abbreviation for Apolipoprotein B, a critical marker in lipidology for assessing cardiovascular risk through particle numbers.

Apolipoprotein B

A key apolipoprotein found on VLDL, IDL, and LDL particles, used as a marker for particle number and cardiovascular risk.

ApoA1 Milano

A specific variant of apolipoprotein A1 discovered in Italy, associated with longevity despite low HDL cholesterol levels.

Apolipoprotein D

A protein identified on HDL particles, also known as Cholesterol Ester Transfer Protein (CETP).

Apolipoprotein C3

A protein that increases the plasma residence time of VLDL, IDL, and LDL particles, making them more atherogenic.

Trans Intestinal Cholesterol Efflux

A pathway where HDLs can eliminate cholesterol in the intestine, without needing to go to the liver first.

Non-HDL Cholesterol

A calculated lipid metric (Total Cholesterol - HDL Cholesterol) used as a marker for assessing potential atherogenic cholesterol.

Familial hypercholesterolemia

A genetic condition characterized by high LDL cholesterol due to LDL receptor deficiencies, relevant to discussions on cholesterol transport mechanisms.

Hemolytic Anemias

A condition where red blood cells are destroyed faster than they can be made, linked to phytosterol accumulation in red blood cell membranes.

APOE

Apolipoprotein E, an important ligand for LDL receptors, which can enhance particle clearance when present on LDL particles.

SR-B1

Abbreviation for Scavenger Receptor B1, a key receptor in cholesterol transport.

Drugs & Medications

Dalcetrapib

A weak CETP inhibitor that was tested in clinical trials for cardiovascular risk.

Celebrex

A selective COX-2 inhibitor, mentioned in comparison to Vioxx.

Vioxx

A selective COX-2 inhibitor that was withdrawn from the market due to cardiovascular risks, discussed as a case study in pharmaceutical risk assessment and regulatory decisions.

Evacetrapib

A potent CETP inhibitor developed by Merck, which showed efficacy in reducing cardiovascular events but was not commercialized due to safety concerns and lack of clear benefit over existing therapies.

Companies

Merck

A pharmaceutical company that developed and tested potent CETP inhibitors, including Evacetrapib.

Pfizer

A pharmaceutical company that developed an early CETP inhibitor trial which was terminated.

Supplements

Phytosterols

Plant sterols that can cause hemolytic anemias when incorporated into red blood cell membranes, as discussed in the context of red blood cell cholesterol.

ApoC3

Abbreviation for Apolipoprotein C3, a key player in lipid metabolism and cardiovascular risk.

Studies & Research

Framingham study

A long-term observational study that provided foundational data on cardiovascular risk factors, including lipid profiles, although its limitations are discussed regarding HDL and LDL metrics.

Garvey paper

A study referenced in the discussion on VLDL and LDL particle numbers, particularly in the context of insulin resistance.

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