chylomicron remnant

**Formation and metabolism (exogenous/dietary pathway):** 1. Enterocytes package dietary triglycerides (TG), cholesterol, and fat-soluble vitamins into nascent chylomicrons. These contain apoB-48, made by intestinal mRNA editing of the apoB gene (APOBEC-1) that creates a premature stop codon. The particles are secreted into lymph (lacteals), then reach the blood through the thoracic duct. 2. In blood, nascent chylomicrons receive apoC-II and apoE from HDL. ApoC-II activates endothelial lipoprotein lipase (LPL), which hydrolyzes TG. This releases free fatty acids to adipose tissue and muscle. 3. As TG is removed, the particle shrinks and becomes a **chylomicron remnant**. It is relatively cholesterol ester-enriched and TG-depleted. It retains apoB-48 and apoE, and it loses most of its apoC-II. 4. The liver takes up remnants by receptor-mediated endocytosis. ApoE binds hepatic LRP1 (LDL receptor-related protein 1) and LDL receptors, with help from heparan sulfate proteoglycans. Remnant cholesterol and TG are then used in the liver, for example to make VLDL or bile acids.

**Key facts for boards:** - ApoB-48 is the marker of intestinally derived particles. It lacks the LDL receptor-binding domain of apoB-100, so remnant uptake depends on **apoE**, not apoB. - Chylomicrons are the largest and least dense lipoproteins. They cause the creamy supernatant layer in refrigerated plasma, and they are present postprandially. - Remnants are more atherogenic than intact chylomicrons. They are small enough to enter the arterial intima and become trapped, and they are taken up by macrophages to form foam cells.

**Clinical correlations:** - **Familial dysbetalipoproteinemia (Type III hyperlipoproteinemia):** - Cause: apoE2/E2 homozygosity, with defective binding to the LDL receptor. It is often unmasked by a second hit such as obesity, diabetes, or hypothyroidism. - Lab findings: accumulation of chylomicron and VLDL remnants (IDL), with elevated TG and cholesterol. A broad β-band appears on electrophoresis. - Clinical findings: palmar xanthomas (palmar crease), tuberoeruptive xanthomas, and premature coronary and peripheral vascular disease. - **Familial LPL deficiency (Type I) or apoC-II deficiency:** chylomicrons build up (not remnants), causing severe hypertriglyceridemia, pancreatitis, eruptive xanthomas, and hepatosplenomegaly. Lipemia retinalis can also occur. - **Abetalipoproteinemia (MTP deficiency):** no chylomicron formation, causing fat malabsorption, acanthocytes, and vitamin E deficiency with neurologic findings. - **Chronic kidney disease, diabetes, and metabolic syndrome:** these can impair remnant clearance and raise remnant cholesterol, which is an independent ASCVD risk factor.

**Mnemonic:** "Chylomicron remnant = Cholesterol-rich, TG-poor, apoE-ready for the liver."

**Therapeutics:** For Type III, treat secondary causes, use diet modification, and start fibrates and/or statins. Fibrates upregulate LPL and lower TG.