Cholesterol and Lipoprotein Metabolism

Cholesterol Synthesis and Regulation

• De novo synthesis occurs primarily in the liver and intestines via the HMG-CoA reductase pathway; acetyl-CoA is converted to malonyl-CoA, then to HMG-CoA, then to mevalonate, ultimately producing cholesterol and steroid hormones
• Rate-limiting enzyme: HMG-CoA reductase is inhibited by high intracellular cholesterol (negative feedback) and by statins (competitive inhibitors); enzyme expression is upregulated by low cholesterol via SREBP-2 (sterol regulatory element-binding protein-2)
• LDL receptor regulation: High intracellular cholesterol downregulates LDL receptor expression, reducing uptake of exogenous cholesterol; low cholesterol upregulates receptors via SREBP-2

Lipoprotein Structure and Classification

• Chylomicrons (intestinal origin): Transport dietary triglycerides and fat-soluble vitamins; contain apolipoprotein B-48; metabolized by lipoprotein lipase (LPL) in capillaries to chylomicron remnants, which are taken up by liver via apoE and apoB-48 receptors
• VLDL (very low-density lipoprotein, hepatic origin): Transport endogenous triglycerides; contain apoB-100; metabolized by LPL to IDL then LDL
• IDL (intermediate-density lipoprotein): Transition particle between VLDL and LDL; can be taken up by liver or further metabolized
• LDL (low-density lipoprotein): Primary carrier of cholesterol; taken up by hepatic and peripheral tissues via LDL receptors (apoB-100/E receptor); major atherogenic particle; concentration correlates with cardiovascular risk
• HDL (high-density lipoprotein): Produced by liver and intestines; facilitates reverse cholesterol transport (removal of cholesterol from peripheral tissues and atherosclerotic plaques); protective against ASCVD; contains apoA-I as major protein

Reverse Cholesterol Transport

• HDL-mediated removal: Nascent HDL (discoidal) acquires cholesterol from tissues via ABCA1 transporter and from apoB-containing particles via CETP (cholesteryl ester transfer protein)
• Maturation: Cholesterol is esterified by LCAT (lecithin-cholesterol acyltransferase), forming spherical HDL particles with hydrophobic cholesteryl ester core
• Hepatic uptake: Mature HDL delivers cholesteryl esters to liver via SR-BI receptor; cholesterol is either stored, converted to bile acids, or secreted into VLDL
• Bile acid synthesis and fecal excretion: Final step of cholesterol catabolism; 24-hydroxylase (CYP7A1) catalyzes initial step; loss of bile acids in stool is main excretory route for cholesterol

Apolipoprotein Functions

• ApoB-100: Structural protein of VLDL, IDL, and LDL; recognized by LDL receptors; one molecule per particle; mutations cause familial defective apoB-100
• ApoB-48: Structural protein of chylomicrons; produced by intestinal RNA editing of apoB-100 gene (48% of full-length protein)
• ApoE: Ligand for hepatic uptake of remnant particles and HDL; three polymorphic isoforms (ε2, ε3, ε4) affecting cholesterol metabolism and Alzheimer's risk
• ApoA-I: Major protein of HDL; activates LCAT; promotes reverse cholesterol transport

Asymptomatic Dyslipidemia (Majority of Cases)

• Most patients with elevated LDL or low HDL are completely asymptomatic and discovered on screening lipid panels
• No physical findings in early disease; dyslipidemia is a "silent" risk factor until complications develop
• Screening recommended for all adults ≥20 years old (universal screening in US per ATP III and ACC/AHA guidelines)

Physical Examination Findings (Advanced or Genetic Dyslipidemia)

• Xanthomas: Lipid deposits in skin/tendons; firm, yellowish nodules typically on Achilles tendon, dorsal hands, or elbows; pathognomonic for severe hypercholesterolemia (total cholesterol often >300 mg/dL); seen in homozygous familial hypercholesterolemia
• Corneal arcus: White/gray opacification of corneal periphery; indicates markedly elevated cholesterol; may be age-appropriate finding in older patients
• Eruptive xanthomas: Small, pruritic, yellow papules in clusters on buttocks/extensor surfaces; associated with severe hypertriglyceridemia (triglycerides >1000 mg/dL); may indicate familial combined hyperlipidemia or familial chylomicronemia
• Palmar xanthomas: Linear or crease xanthomas in palms; classic for Type III hyperlipoproteinemia (dysbetalipoproteinemia)
• Lipemia retinalis: "Milky" appearance of retinal vessels; seen with severe hypertriglyceridemia (triglycerides >1000-1200 mg/dL); reversible with treatment

Manifestations of Atherosclerotic Complications

• Premature coronary artery disease: MI in men <55 years or women <65 years; may be initial presentation in familial hypercholesterolemia
• Cerebrovascular disease: Stroke or TIA from carotid atherosclerosis
• Peripheral arterial disease: Claudication, ischemic rest pain
• Sudden cardiac death: May be first presentation in young patients with homozygous familial hypercholesterolemia

Important Clinical Pearls about Presentation

• Absence of symptoms does not indicate absence of disease; lipid abnormalities promote atherosclerosis asymptomatically over years to decades
• Physical findings suggest advanced/severe disease and warrant aggressive treatment
• Patient ethnicity, family history, and presence of comorbidities (diabetes, hypertension, smoking) dramatically increase pretest probability of significant dyslipidemia

Lipid Panel Components and Interpretation

• Total cholesterol: Sum of all cholesterol forms; generally <200 mg/dL is desirable; highly influenced by LDL and HDL levels
• LDL-cholesterol: "Bad cholesterol"; primary target of therapy; calculated using Friedewald equation (TC - HDL - TG/5) when triglycerides <400 mg/dL; optimal <100 mg/dL; <70 mg/dL recommended for very high-risk patients (prior ASCVD, diabetes with additional risk factors)
• HDL-cholesterol: "Good cholesterol"; inverse correlation with cardiovascular risk; optimal ≥40 mg/dL in men, ≥50 mg/dL in women; protective; each 1 mg/dL increase decreases CAD risk ~2%
• Triglycerides: Transport form of dietary and endogenous fat; fasting level <150 mg/dL is normal; 150-199 borderline; 200-499 high; ≥500 very high (associated with pancreatitis risk); measured fasting (6-12 hours)

Diagnostic Criteria for Dyslipidemia Subtypes

• Hypercholesterolemia: Total cholesterol >200 mg/dL with elevated LD

The enzymes examiners actually test

• HMG-CoA reductase: rate-limiting step of cholesterol synthesis (HMG-CoA → mevalonate); the mevalonate pathway also yields isoprenoids and ubiquinone, the proposed basis for statin-associated myalgias.
• Lipoprotein lipase (LPL): clears triglyceride from chylomicrons and VLDL; requires apoC-II as cofactor. Deficiency of either → type I hyperlipoproteinemia with milky serum, eruptive xanthomas, and pancreatitis, but no increase in ASCVD risk — a favorite distractor.
• LCAT esterifies cholesterol on HDL; CETP swaps cholesteryl ester for triglyceride between HDL and apoB particles; ABCA1 loads free cholesterol onto nascent HDL (defective in Tangier disease).

One-line disease associations

• Familial hypercholesterolemia: autosomal dominant LDL-receptor (or apoB-100, or gain-of-function PCSK9) defect → tendon xanthomas, corneal arcus, premature MI. Homozygotes present in childhood.
• Type III dysbetalipoproteinemia: apoE2/E2 → remnant accumulation, palmar crease xanthomas.
• Abetalipoproteinemia: MTP mutation → no apoB-containing particles, fat malabsorption, acanthocytes, ataxia from vitamin E deficiency.

Pharmacology traps

• Statins: lower LDL most of any oral agent by depleting hepatic cholesterol → SREBP-2-mediated upregulation of LDL receptors. Watch for myopathy/rhabdomyolysis (worse with fibrates) and transaminase elevation.
• PCSK9 inhibitors (e.g., evolocumab): PCSK9 normally targets the LDL receptor for lysosomal degradation, so inhibition increases receptor recycling; loss-of-function variants cause lifelong low LDL.
• Ezetimibe blocks intestinal NPC1L1; bile acid sequestrants (cholestyramine) increase LDL-receptor expression but can raise triglycerides and impair fat-soluble vitamin absorption; fibrates are PPAR-α agonists (LPL up) and the drug of choice for very high triglycerides; niacin inhibits adipose lipolysis, causes prostaglandin-mediated flushing blunted by aspirin, plus hyperuricemia and hyperglycemia.

Single best next step

• Statin therapy is the intervention that changes outcomes: per the 2018 AHA/ACC multisociety cholesterol guideline, clinical ASCVD or untreated LDL ≥190 mg/dL warrants high-intensity statin, and risk-stratified statins are recommended for diabetes (also per the ADA Standards of Care). Raising HDL pharmacologically has not reduced events — do not pick a "HDL-raising drug" as the answer.
• Friedewald-calculated LDL is invalid when triglycerides exceed ~400 mg/dL; obtain a direct LDL.