Lipoproteins and Atherosclerosis

Overview:

• Lipoproteins play a critical role in the development of atherosclerosis, with elevated LDL, oxidized LDL, IDL, and triglyceride-rich lipoproteins contributing to plaque formation within the arteries.
• These plaques are primarily formed due to dyslipidemia, which affects over 70% of patients with premature coronary heart disease (CHD).
• In contrast, HDL has protective, antiatherogenic properties. The management of dyslipidemia,
• through lifestyle modifications, lipid-lowering drugs, and antioxidants, can significantly reduce the risk of atherosclerosis and its associated complications, including CHD and stroke.

Key Lipoproteins (Non-HDL cholesterol) Involved in Atherosclerosis:

1. Low-Density Lipoprotein (LDL):
   • Composition: LDL contains cholesterol, triglycerides, phospholipids, and apolipoproteins (Apo B-100 and C-III). LDL is crucial in cholesterol transport, but high plasma concentrations of LDL, especially small, dense LDL particles, promote atherogenesis (plaque formation).
   • Mechanism of Atherogenesis:
     • Subendothelial Retention: Apo B-100-containing LDL particles are retained in the subendothelial space of arteries through interactions with proteoglycans. Small, dense LDL particles penetrate the endothelial barrier more easily and bind to proteoglycans, staying in the vessel wall for longer periods.
     • Oxidative Modification: Once trapped, LDL is modified by reactive oxygen species (oxidation), making it more atherogenic. Oxidized LDL attracts monocytes, which transform into macrophages, ingest the oxidized LDL, and become foam cells—central to plaque formation.
     • Scavenger Receptors and Foam Cells: LDL not cleared by normal Apo B/E (LDL) receptors is taken up by macrophages via unregulated scavenger receptors like CD36, leading to foam cell formation. This process contributes to plaque buildup in the artery wall.
2. Intermediate-Density Lipoprotein (IDL):
   • Role in Atherogenesis: IDL is an intermediate between LDL and very low-density lipoprotein (VLDL). It is associated with an increased risk of CHD, especially in individuals with normal total cholesterol levels. IDL can also be taken up by macrophages and contribute to foam cell formation.
   • Impact: Studies have shown that elevated IDL levels predict CHD incidence and progression, independently of LDL cholesterol levels.
3. Very Low-Density Lipoprotein (VLDL):
   • Triglycerides and Atherosclerosis: Hypertriglyceridemia, often associated with elevated VLDL, contributes to atherosclerosis, particularly when accompanied by low HDL levels. VLDL particles enriched in apolipoprotein E (apo E) can bind to macrophages, promoting foam cell formation.
   • Small, Dense LDL: In hypertriglyceridemic states, smaller, denser LDL particles are formed. These particles have a prolonged circulation time and are more susceptible to oxidation, further increasing atherosclerotic risk.
4. Lipoprotein(a) [Lp(a)]:
   • Atherogenic Potential: Lp(a) is a variant of LDL that contains an additional protein, apolipoprotein(a). High levels of Lp(a) are an independent risk factor for atherosclerosis and cardiovascular events.
   • Mechanism: Lp(a) contributes to plaque formation by interfering with fibrinolysis (the breakdown of blood clots), increasing the risk of thrombosis (clot formation) within the plaques.
   • Target: <0.9 g/L in high-risk individuals.

Oxidized LDL and its Role in Atherosclerosis:

• Oxidation Process: LDL particles are oxidized in the arterial wall by reactive oxygen species. This oxidation modifies the surface phospholipids and cholesterol, leading to increased retention in the vessel wall and a stronger inflammatory response.
• Foam Cell Formation: Oxidized LDL is taken up by macrophages via scavenger receptors, leading to foam cell formation. Foam cells accumulate and eventually rupture, releasing harmful substances like oxidized LDL, enzymes, and reactive oxygen species, further damaging the arterial wall.
• Endothelial Dysfunction: Oxidized LDL impairs endothelial cell function, reducing the release of nitric oxide (NO), which is essential for vasodilation (relaxation of blood vessels). This contributes to vasoconstriction, increased blood pressure, and thrombosis (formation of blood clots).

High-Density Lipoprotein (HDL) and its Protective Role:

• Anti-Atherogenic Properties: HDL has several protective functions, including:
  • Cholesterol Efflux: HDL promotes the removal of cholesterol from macrophages (cholesterol efflux), helping to prevent foam cell formation.
  • Antioxidation: HDL has antioxidant properties, preventing the oxidation of LDL and reducing the formation of oxidized LDL.
  • Endothelial Function: HDL enhances endothelial function by promoting the production of nitric oxide (NO), which improves vasodilation and reduces blood pressure.
  • Anti-Thrombotic Effects: HDL also reduces platelet aggregation and thrombus formation, thereby reducing the risk of plaque rupture and heart attacks.

Role of Inflammatory Markers and Immune Response in Atherosclerosis:

• Monocyte Recruitment and Inflammation: Oxidized LDL acts as a chemoattractant for monocytes, increasing their binding to the endothelial cells. Once in the vessel wall, monocytes differentiate into macrophages, contributing to the inflammatory response that drives plaque formation.
• Endothelial Adhesion Molecules: In patients with elevated LDL, increased expression of adhesion molecules like VCAM-1 (vascular cell adhesion molecule-1) and ICAM-1 (intercellular adhesion molecule-1) facilitates the binding of inflammatory cells to the endothelium, promoting plaque development.

Impact of Antioxidants and Treatment:

• Antioxidants: Nutritional antioxidants, such as vitamin E, may reduce the uptake of oxidized LDL by decreasing the expression of scavenger receptors like CD36. This can potentially lower the risk of foam cell formation.
• Lipid-Lowering Drugs: Statins and other lipid-lowering drugs not only reduce LDL levels but also reduce the oxidative susceptibility of LDL, improving endothelial function and slowing the progression of atherosclerosis.

Triglyceride-Rich Lipoproteins and Apolipoprotein C-III (Apo C-III):

• Triglyceride-Rich Lipoproteins: High levels of triglyceride-rich lipoproteins, including VLDL, are associated with increased atherosclerosis risk. Apo C-III, found on these lipoproteins, may delay their clearance from the bloodstream, enhancing their atherogenic potential.
• Apo C-III’s Role: Apo C-III promotes inflammation in endothelial cells and interferes with nitric oxide production, leading to endothelial dysfunction and contributing to plaque formation.

Triglycerides

Triglycerides are primarily synthesized in the liver and adipose (fat) tissue. However, they can also be produced in the intestines after consuming food, especially high-fat meals. The process involves the breakdown of dietary fats by enzymes, known as lipases, into free fatty acids and monoglycerides in the intestines. Key pathways include:

• Synthesis in the Liver and Intestines
  • Liver: Triglycerides are synthesized from excess carbohydrates and proteins through the de novo lipogenesis pathway.
  • Intestines: Triglycerides are synthesized after food consumption, particularly from dietary fats, and are then repackaged for transport to other tissues.
• Packaging of Triglycerides into Lipoproteins
  • Once synthesized, triglycerides are packaged into lipoproteins for transport through the bloodstream.
  • Chylomicrons
    • Source: Intestines.
    • Function: Transport dietary triglycerides from the intestines to tissues after a meal.
  • Very Low-Density Lipoproteins (VLDL)
    • Source: Liver.
    • Function: Transport triglycerides produced by the liver to peripheral tissues.
• Circulation of Triglycerides in Lipoproteins
  • Triglycerides circulate in the bloodstream as part of lipoproteins. The two main types of lipoproteins involved in this transport are:
  • Chylomicrons: Responsible for transporting dietary triglycerides after meals.
  • VLDL (Very Low-Density Lipoprotein): Transports triglycerides synthesized in the liver.

Factors Associated with Hypertriglyceridemia

• Elevated triglyceride levels, known as hypertriglyceridemia, are linked to several cardiovascular risks and metabolic disturbances. These include:
• Low HDL-C (High-Density Lipoprotein Cholesterol)
  • Hypertriglyceridemia often coexists with low HDL cholesterol, which further elevates the risk of cardiovascular disease. The combination of low HDL-C and high triglycerides enhances the likelihood of atherosclerosis.
• Small, Dense LDL (Low-Density Lipoprotein) Particles
  • Elevated triglycerides are associated with the formation of small, dense LDL particles, which are highly atherogenic and more likely to contribute to arterial plaque formation.
• Insulin Resistance
  • Insulin resistance, a key feature of metabolic syndrome and type 2 diabetes, frequently coexists with elevated triglycerides and low HDL-C. This increases the overall cardiovascular risk.
• Other Contributing Factors
  • Increased coagulability and blood viscosity: These factors contribute to endothelial dysfunction and tissue ischemia.
  • Atherogenic triglyceride-rich lipoproteins: Lipoprotein remnants such as intermediate-density lipoproteins (IDL) also contribute to cardiovascular risk.

Triglycerides and Atherosclerosis

UptoDate: Approach to the patient with hypertriglyceridemia – Robert S Rosenson, MD

• Role in Atherosclerosis
  • Elevated triglycerides have been linked to cardiovascular diseases, particularly coronary heart disease (CHD). Though triglycerides are less likely to accumulate in atherosclerotic plaques compared to cholesterol, high levels still pose significant cardiovascular risks.
• Uncertainty in Causality
  • It is still debated whether elevated triglycerides directly cause atherosclerosis or if other lipid abnormalities contribute to this risk. Research in this area is ongoing.
• Mendelian Randomization Studies
  • Genetic studies suggest that triglyceride-related pathways are causally linked to CHD, although the precise mechanisms remain unclear.
• Coronary Heart Disease (CHD) and Hypertriglyceridemia
  • Association with CHD Risk
    • Studies have shown a consistent association between elevated triglycerides and increased CHD risk. Notable findings include:
    • 2007 Meta-Analysis: A review of 27 studies involving over 10,000 CHD cases found that individuals in the top third of serum triglyceride levels had a 1.7x increased risk of CHD compared to those in the bottom third.
    • Study in Younger Males (2007): Men aged 26–45 with high triglyceride levels were found to have a 4.1x higher risk of CHD. This risk further increased with rising triglyceride levels over time.
  • Increased Mortality
    • Patients with elevated triglycerides and existing CHD have higher mortality rates and reduced event-free survival after procedures such as coronary artery bypass grafting (CABG).
• Non-Fasting Triglycerides
  • Research has shown that non-fasting triglyceride levels are also predictive of cardiovascular events, further supporting the link between triglycerides and CHD risk.
• Triglycerides and Cerebrovascular Disease
  • Elevated triglycerides are associated with an increased risk of ischemic stroke. Two large studies demonstrated a significant correlation between non-fasting triglyceride levels and stroke risk, highlighting the broader impact of triglycerides on cerebrovascular health.

Summary of Mechanisms Contributing to Atherosclerosis:

1. LDL Oxidation: LDL oxidation promotes foam cell formation and triggers a pro-inflammatory response within the arterial wall.
2. Foam Cell Formation: Macrophages take up oxidized LDL, forming foam cells, which are a hallmark of early plaque formation.
3. Endothelial Dysfunction: Oxidized LDL impairs nitric oxide production, leading to reduced vasodilation and increased blood pressure.
4. Thrombosis: Oxidized LDL promotes platelet aggregation and clot formation, increasing the risk of plaque rupture and heart attack.
5. Inflammation: Monocytes are recruited to the site of oxidized LDL accumulation, where they differentiate into macrophages and contribute to inflammation.

HDL to Total Cholesterol Ratio

The HDL to total cholesterol ratio is used in the Australian Cardiovascular Disease Risk Calculator because it provides a simple and effective way to assess the balance between “good” cholesterol (HDL) and the total cholesterol load, which includes both “bad” cholesterol (LDL, VLDL) and HDL. Here’s why this ratio is important:

1. Better Predictor of CVD Risk:
   • Research has shown that the HDL/total cholesterol ratio is a stronger predictor of cardiovascular risk compared to using total cholesterol or LDL cholesterol alone . The ratio provides insight into both the protective effect of HDL and the overall atherogenic (plaque-forming) potential of the lipid profile.
   • Studies have found that individuals with a low HDL to total cholesterol ratio are at higher risk for CVD, even if their total cholesterol levels are not dramatically elevated.
2. Simplifies Risk Stratification:
   • In population-level assessments, calculating the HDL/total cholesterol ratio is simpler and more cost-effective than calculating individual lipoproteins like LDL cholesterol. It allows clinicians to quickly stratify patients into different risk categories without the need for more advanced lipid tests.
   • Since HDL levels often decline as LDL and total cholesterol levels rise, this ratio offers a consolidated measure of both atherogenic and anti-atherogenic forces within a patient’s lipid profile.
3. Reflects Overall Lipoprotein Health:
   • The total cholesterol number includes all forms of cholesterol, but not all cholesterol is equally harmful. HDL cholesterol has a protective effect, so by comparing HDL against total cholesterol, the ratio reflects the overall balance between harmful and protective lipoproteins.
   • For example, two individuals may have the same total cholesterol, but if one has a higher proportion of HDL, their cardiovascular risk will be lower. The ratio adjusts for this variability.
4. Evidence from Large Studies:
   • Data from large epidemiological studies, such as the Framingham Heart Study and the INTERHEART Study, have shown that the HDL to total cholesterol ratio is an independent predictor of cardiovascular events. In the INTERHEART study, which examined risk factors for myocardial infarction across multiple countries, the ApoB/ApoA1 ratio (similar to LDL/HDL ratio) was found to be one of the strongest predictors of heart attack risk .

Australian guidelines for cholesterol targets

Prevention Type	LDL Cholesterol	HDL Cholesterol	Triglycerides	Total Cholesterol	HDL : Cholesterol Ratio
Primary Prevention	<2.0 mmol/L	≥1.0 mmol/L (M)	<2.0 mmol/L	<4.0 mmol/L	≥0.4
		≥1.2 mmol/L (F)
Secondary Prevention	<1.8 mmol/L	≥1.0 mmol/L (M)	<2.0 mmol/L	<4.0 mmol/L	≥0.4
		≥1.2 mmol/L (F)

Non-fasting lipids: A change in practice

https://www1.racgp.org.au/ajgp/2022/may/non-fasting-lipids

• Global guidelines shift: Many cardiovascular societies now recommend non-fasting lipid testing over fasting lipid testing for assessing cardiovascular risk.
• Australia’s outdated guidelines: Australia’s latest cardiovascular guidelines (2012) still recommend fasting lipids, but GPs may not be aware of the international shift to non-fasting testing.
• Realistic representation of risk: Non-fasting lipids are thought to reflect a more accurate cardiovascular risk, as people spend most of their time in a non-fasting state.
• Patient and system benefits:
  • Reduces inconvenience for patients (no need to return after fasting).
  • Eases strain on pathology centres (reduces morning surge).
  • Reduces risk of hypoglycemia in diabetic patients.
  • Improves compliance with testing (tests more likely done on the same day).
• Triglycerides and LDL-C estimation:
  • Argument against non-fasting lipids: higher triglycerides post-meal may lead to underestimation of LDL-C (calculated via the Friedewald equation).
  • However, normal food intake causes minor increases in triglycerides and is clinically insignificant.
  • In cases of elevated triglycerides with non-fasting testing, a fasting sample may be required.
• Management concerns:
  • Some argue fasting lipids are necessary for management decisions because older studies used fasting lipids.
  • Recent meta-analysis and statin trials show no significant difference between fasting and non-fasting lipids in cardiovascular risk assessment.
• Current global guidelines:
  • Canada, Europe, and the U.S. recommend non-fasting lipids, with fasting retested only if triglycerides are ≥4.5 mmol/L or higher.