Pharmacology · Cardiovascular

Lipids, Lipoproteins, and Cardiovascular Risk

Visual summary — lipoprotein classes, metabolic pathways, and drug targets

Three lipoprotein classes define the lipid transport system: chylomicrons carry dietary fat from intestine to peripheral tissues and their remnants are atherogenic; the endogenous VLDL-to-LDL pathway exports hepatic triglycerides and remodels them through lipoprotein lipase into cholesterol-enriched LDL, the primary atherogenic particle; and HDL mediates reverse cholesterol transport back to the liver, making it a risk marker but not a useful drug target.
LDL-C lowering is the principal pharmacological goal because LDL is causally related to atherosclerosis — established by genetic studies (familial hypercholesterolemia), Mendelian randomization, and every major outcomes trial; apolipoprotein B-100 is the structural protein on LDL and its precursors, and it is a more precise atherogenic particle count than LDL-C alone.
All three major drug classes work by upregulating hepatic LDL receptor expression through complementary mechanisms: statins deplete intrahepatic cholesterol by inhibiting HMG-CoA reductase, driving compensatory receptor upregulation; ezetimibe blocks NPC1L1 in the intestinal brush border, reducing cholesterol delivery to the liver and triggering the same compensatory response; PCSK9 inhibitors prevent receptor degradation, allowing maximal receptor recycling to the cell surface.
PCSK9 is the enzyme that tags LDL receptors for lysosomal destruction — blocking it with a monoclonal antibody allows each receptor to cycle back to the surface repeatedly instead of being degraded after a single internalization, producing LDL-C reductions of 50 to 60 percent on top of statin therapy.
Secondary causes of dyslipidemia — hypothyroidism, uncontrolled diabetes, nephrotic syndrome, and drugs including thiazides, beta-blockers, and glucocorticoids — must be identified and treated before escalating lipid-lowering pharmacotherapy, because treating the lipid without correcting the cause produces suboptimal results and may delay diagnosis of a treatable condition.

Abbreviations: VLDL = very low-density lipoprotein · LDL = low-density lipoprotein · HDL = high-density lipoprotein · IDL = intermediate-density lipoprotein · LDL-C = LDL cholesterol · apo = apolipoprotein · LPL = lipoprotein lipase · HMG-CoA = 3-hydroxy-3-methylglutaryl coenzyme A · NPC1L1 = Niemann-Pick C1-Like 1 · PCSK9 = proprotein convertase subtilisin/kexin type 9

Lipoprotein Classes — Structure and Role

Dietary pathway

Chylomicrons

Made in intestine after fat absorption
Carry apolipoprotein B-48
Largest, least dense lipoprotein
Remnants are atherogenic

Endogenous pathway

Very Low-Density Lipoprotein → Low-Density Lipoprotein

Very low-density lipoprotein: liver exports triglycerides
Carries apolipoprotein B-100, apolipoprotein C-II, apolipoprotein E
Remodeled by lipoprotein lipase to low-density lipoprotein
Low-density lipoprotein: primary atherogenic particle

Reverse transport

High-Density Lipoprotein

Carries apolipoprotein A-I
Collects cholesterol from peripheral tissues
Returns cholesterol to liver for excretion
Risk marker — not a drug target

The Endogenous Pathway — From Very Low-Density Lipoprotein to Low-Density Lipoprotein

Liver

Very Low-Density Lipoprotein

Secreted with triglycerides and apolipoprotein B-100

→

Lipoprotein lipase

Triglyceride hydrolysis

Activated by apolipoprotein C-II on particle surface

→

Intermediate form

Intermediate-Density Lipoprotein

Transient; further remodeled

→

Primary target

Low-Density Lipoprotein

Cholesterol-enriched; cleared by low-density lipoprotein receptor

Low-Density Lipoprotein Receptor — Regulation and Drug Targets

Statins

Block cholesterol synthesis

Inhibit 3-hydroxy-3-methylglutaryl coenzyme A reductase
Hepatic cholesterol falls
Low-density lipoprotein receptor expression increases
More low-density lipoprotein cleared from plasma

Ezetimibe

Block intestinal absorption

Inhibits Niemann-Pick C1-Like 1 transporter
Less cholesterol delivered to liver
Compensatory low-density lipoprotein receptor upregulation
Additive to statin effect

Proprotein convertase subtilisin/kexin type 9 inhibitors

Prevent receptor degradation

Block proprotein convertase subtilisin/kexin type 9 from destroying receptors
More receptors recycle to cell surface
Maximum low-density lipoprotein clearance
Additive to statin and ezetimibe

Secondary Causes of Dyslipidemia — Screen Before Prescribing

Cause	Effect on Low-Density Lipoprotein	Effect on Triglycerides	Effect on High-Density Lipoprotein	Key Screening Test
Hypothyroidism	Elevated	Variable	Normal or low	Thyroid-stimulating hormone
Type 2 diabetes / insulin resistance	Normal or mildly elevated	Elevated	Low	Fasting glucose, hemoglobin A1c
Nephrotic syndrome	Markedly elevated	Elevated	Low	Urine protein
Thiazides / beta-blockers	Variable	Elevated	Low	Medication review
Glucocorticoids	Elevated	Elevated	Variable	Medication review

Clinical Rule

Always exclude secondary causes — particularly hypothyroidism, uncontrolled diabetes, nephrotic syndrome, and causative drugs — before initiating or escalating lipid-lowering pharmacotherapy. Treating the lipid without correcting the cause produces suboptimal results and may delay diagnosis of a treatable condition.

Suggested References

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