L14 - Lipids (Lecture #1 Part A)

What is atherosclerosis?

A chronic disease of large and medium-sized arteries characterized by lipid accumulation, inflammation, smooth muscle proliferation, fibrosis, and possible calcification within the arterial intima, producing atherosclerotic plaques (atheromas).

What does the word “atherosclerosis” mean?

Greek athera = gruel; sclera = rigid; osis = condition. It describes the lipid-rich, hardened lesions that develop within arterial walls.

What are the three major theories underlying atherosclerosis?

1) Cholesterol/lipid hypothesis: explains atheroma formation. 2) Inflammatory hypothesis: explains atheroma instability/rupture. 3) Senescence hypothesis: explains the long lifetime and progression of atheromas with aging. Together they form the “Grand Unifying Theory” of atherosclerosis.

What is the cholesterol/lipid hypothesis of atherosclerosis?

Proposed by Nikolai Anitschkov before WWI after rabbits fed a high-cholesterol diet developed vascular lesions. It proposes that cholesterol/lipoprotein accumulation, especially LDL within the arterial intima, drives atheroma formation.

What is the inflammatory hypothesis of atherosclerosis?

Proposed by Rudolph Virchow in the mid-1800s after identifying cholesterol crystals and inflammatory cells in lesions. Inflammation contributes to endothelial activation, plaque progression, plaque instability, and rupture.

What is the senescence hypothesis of atherosclerosis?

Atherosclerosis develops and progresses partly because of natural aging (senescence), helping explain the long lifetime and gradual evolution of atherosclerotic lesions.

What is the “Grand Unifying Theory” of atherosclerosis?

The lipid hypothesis explains atheroma formation, the inflammation hypothesis explains atheroma rupture, and the senescence hypothesis explains the atheroma’s long lifetime. All three processes contribute to atherosclerosis.

What did the Framingham Heart Study demonstrate about atherosclerosis?

It showed a clear correlation between blood cholesterol levels and coronary artery disease (CAD) risk. LDL has an established role in atherosclerosis, and lowering LDL—particularly with statins—significantly lowers CAD risk.

What did the CANTOS clinical trial demonstrate?

Targeting inflammation can reduce cardiovascular events independently of lipid lowering. Canakinumab 150 mg every 3 months, targeting the IL-1β innate immunity pathway, significantly reduced recurrent cardiovascular events versus placebo without requiring lipid-level lowering.

Why are the Framingham Heart Study and CANTOS trial important together?

Framingham supports the lipid/cholesterol basis of atherosclerosis, while CANTOS supports the inflammatory basis. Together they demonstrate that both lipid accumulation and inflammation are important contributors to atherosclerotic cardiovascular disease.

How does inflammation affect endothelial cells during atherosclerosis?

Inflammation activates the endothelium → cytokines and adhesion/chemoattractant molecules are expressed → leukocytes adhere to and enter the intima → macrophage activity and lipid uptake increase → plaque development and inflammation continue.

What predisposes arteries to endothelial dysfunction and plaque formation?

Hemodynamic stress contributes to endothelial dysfunction. Plaques preferentially develop in certain arterial regions, particularly branch points where blood flow and shear stress are disturbed.

What is the step-by-step evolution of an atherosclerotic plaque?

1) Lipoproteins accumulate in the intima → 2) oxidative stress causes cytokine production → 3) adhesion molecules and chemoattractants recruit leukocytes → 4) blood monocytes/macrophages increase scavenger receptor expression → 5) scavenger receptors take up modified LDL (mLDL), producing foam cells → 6) smooth muscle cells migrate into the intima, increasing intimal thickness → 7) smooth muscle cells divide/proliferate, increasing plaque formation → 8) cell death and calcification contribute to formation of an acellular fibrous component/capsule surrounding a lipid-rich core.

How are foam cells formed?

Macrophages express scavenger receptors that take up modified LDL (mLDL). Because this uptake lacks normal negative-feedback regulation, lipid continues accumulating within macrophages until they become lipid-filled foam cells.

What happens when foam cells rupture?

They release cholesterol, which can form crystalline deposits and stimulate additional inflammatory responses. This recruits more macrophages and promotes further lesion progression.

What role do macrophages play in progression of atherosclerosis?

Macrophages take up modified LDL to become foam cells, contribute to inflammation, and secrete growth factors that stimulate smooth muscle cell migration and proliferation, increasing intimal thickness and plaque formation.

What role do vascular smooth muscle cells play in atherosclerosis?

Smooth muscle cells migrate into the intima, divide, and proliferate. This increases intimal thickness and contributes to growth and fibrous structure of the atherosclerotic lesion.

What occurs during the later stages of atherosclerotic plaque development?

Continued lipid accumulation, inflammation, smooth muscle proliferation, cell death, fibrosis, and calcification produce a plaque containing a lipid-rich core and fibrous components/cap. Plaque disruption can expose thrombogenic material and trigger thrombosis.

What causes most acute coronary syndromes (ACS) associated with atherosclerosis (6 steps that explain how atherosclerosis can cause ACS)?

1. Atherosclerotic plaque (atheroma) exists ➡ Thrombosis of a disrupted atheroma
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2. Plaque ruptures/disrupts 💥

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3. Thrombogenic material is exposed

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4. Platelets (Thrombocytes) activate + aggregate

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5. Coagulation activates → thrombus forms and fibrin produced🩸

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6. Thrombus obstructs/occludes coronary artery (obstruct = reduces flow, occludes = blocks flow)

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❤ ACUTE CORONARY SYNDROME

Does a myocardial infarction usually require severe pre-existing coronary stenosis?

No. Among MI patients in the lecture: 68% had

What is the relationship between plaque disruption, thrombosis, and coronary occlusion?

An atherosclerotic plaque develops → plaque becomes vulnerable/disrupted → thrombogenic material is exposed → platelets and coagulation factors are activated → thrombus forms → blood flow becomes severely restricted or completely occluded → acute coronary syndrome/MI may occur.

How does lipoprotein(a) [Lp(a)] contribute to atherosclerotic complications?

Lp(a) interferes with the conversion of plasminogen → plasmin, impairing fibrinolysis and favoring persistence of thrombi.

What are the major risk factors for atherosclerosis?

Gender, genetics/family history, obesity, smoking, increased LDL or triglycerides, decreased HDL, elevated homocysteine, high saturated-fat/trans-fat intake, lack of exercise, diabetes, and hypertension.

Which major atherosclerosis risk factors are potentially modifiable?

Smoking, obesity, elevated LDL/triglycerides, low HDL-related lifestyle factors, saturated/trans-fat intake, physical inactivity, hypertension, and diabetes can potentially be improved or controlled. Genetics/family history, aging, and some sex/gender-related risk are nonmodifiable.

How can atherosclerosis risk potentially be reduced?

Stop smoking; lower LDL and triglycerides; improve diet by reducing saturated and trans fats; exercise regularly; maintain a healthy body weight; and appropriately control associated diseases such as hypertension and diabetes. LDL-lowering therapy such as statins can significantly reduce CAD risk.

What is familial hypercholesterolemia (FH)?

An inherited disorder associated with markedly elevated cholesterol/LDL and accelerated atherosclerotic cardiovascular disease. The lecture notes >600,000 people in the U.S. have FH and that affected individuals have increased risk of coronary heart disease and MI at younger ages.

What physical findings may suggest familial hypercholesterolemia?

Xanthomas and xanthelasma can be associated with severe/inherited hypercholesterolemia and may suggest FH.

What is the overall sequence of the major stages of an atherosclerotic lesion?

Endothelial damage/dysfunction → LDL enters the intima → LDL becomes modified → macrophages take up modified LDL without normal negative feedback → foam cells form and rupture → cholesterol/crystals and inflammation accumulate → additional macrophages are recruited → macrophage growth factors stimulate smooth muscle migration/proliferation → intimal thickness and plaque increase → later calcification occurs → Lp(a) can inhibit plasmin formation → platelets and coagulation factors can activate → thrombosis and possible vascular occlusion.

What are the key components driving atherosclerotic lesion formation?

Endothelial dysfunction + LDL entry/modification + macrophage recruitment and foam-cell formation + chronic inflammation + smooth muscle migration/proliferation + fibrosis/calcification = progressive atherosclerotic plaque.

What are the major potential clinical consequences of atherosclerosis?

Progressive stenosis can reduce blood flow, while plaque disruption can cause acute thrombosis and partial or complete arterial occlusion. In coronary arteries, this can produce acute coronary syndromes such as myocardial infarction.