apolipoprotein A-I

**Structure/Synthesis:** ApoA-I is a ~28 kDa protein encoded on chromosome 11 (APOA1–APOC3–APOA4–APOA5 gene cluster). It is synthesized in the liver and small intestine (enterocytes) and secreted lipid-poor, then lipidated to form nascent (discoid) HDL.

**Key functions:** 1. **Structural protein of HDL** – the main apolipoprotein of HDL. 2. **ABCA1 interaction** – lipid-poor ApoA-I accepts free cholesterol and phospholipids from peripheral cells (e.g., macrophages) via the ABCA1 transporter, forming nascent HDL. 3. **LCAT activator** – lecithin-cholesterol acyltransferase esterifies free cholesterol on HDL, trapping cholesteryl esters in the core and generating mature spherical HDL (HDL3 → HDL2). 4. **Reverse cholesterol transport** – mature HDL delivers cholesterol to the liver via SR-B1 (selective uptake) or transfers cholesteryl esters to VLDL/IDL/LDL via CETP (in exchange for triglycerides). 5. Ancillary effects: anti-inflammatory, antioxidant, and endothelial-protective properties.

**Clinical correlations:** - **Tangier disease** (ABCA1 mutation, autosomal recessive): very low/absent HDL and ApoA-I (rapid catabolism of lipid-poor ApoA-I); orange/yellow tonsils, hepatosplenomegaly, peripheral neuropathy, corneal opacities, and cholesterol ester accumulation in reticuloendothelial cells. - **LCAT deficiency:** low HDL, corneal opacities, hemolytic anemia, and renal disease (proteinuria/renal failure). - **ApoA-I Milano variant:** very low HDL yet paradoxically low CV risk; investigated as a therapeutic (recombinant ApoA-I Milano infusion). - **ApoA-I amyloidosis:** rare hereditary amyloidosis (AApoAI) with variants causing renal, hepatic, and cardiac deposition. - **Cardiovascular risk:** low ApoA-I/HDL correlates with increased atherosclerotic risk; ApoA-I and the ApoB/ApoA-I ratio are used as risk markers (a high ratio indicates higher risk). However, raising HDL pharmacologically (e.g., CETP inhibitors, niacin) has not reliably reduced CV events, so HDL-C is a marker rather than a proven therapeutic target. - **Increased HDL/ApoA-I:** exercise, moderate alcohol, estrogen, and niacin/fibrates (modest). **Decreased:** smoking, obesity, insulin resistance, hypertriglyceridemia, and anabolic steroids. - **CETP deficiency:** markedly high HDL levels.

**Board pearls:** ApoA-I = HDL structural protein + LCAT activator. ApoB-100 = LDL/VLDL/IDL (ligand for LDL receptor); ApoB-48 = chylomicrons; ApoC-II = LPL activator; ApoE = remnant uptake; ApoA-V helps regulate triglyceride metabolism.