What is the difference between the exogenous and endogenous lipoprotein pathways?

The exogenous pathway handles dietary fat: intestinal cells package triglyceride and cholesterol into chylomicrons built on apoB-48, which reach the blood through the lymph, are unloaded by lipoprotein lipase and leave remnants that the liver clears through apoE. The endogenous pathway handles fat made or processed in the liver: hepatocytes secrete VLDL built on apoB-100, which lipoprotein lipase converts to IDL and hepatic lipase then converts to LDL. LDL delivers cholesterol to cells by binding the LDL receptor through apoB-100.

Which apolipoprotein activates lipoprotein lipase?

Apolipoprotein C-II activates lipoprotein lipase, the enzyme on the capillary endothelium of muscle and adipose tissue that hydrolyzes triglyceride in chylomicrons and VLDL. Chylomicrons and VLDL pick up apoC-II from HDL after they enter the blood. A deficiency of apoC-II or of lipoprotein lipase itself causes familial chylomicronemia, with very high triglycerides, pancreatitis and eruptive xanthomas. Apolipoprotein C-III has the opposite effect and inhibits the enzyme.

What does apolipoprotein E do?

ApoE is the receptor ligand that lets the liver take up chylomicron remnants and IDL, and it is carried on chylomicrons and their remnants, VLDL, IDL and HDL. People with two copies of the E2 variant clear remnants poorly and can develop familial dysbetalipoproteinemia (type III), with high cholesterol and triglycerides and palmar or tuberous xanthomas. The E4 variant increases the risk of late-onset Alzheimer disease.

How does HDL carry out reverse cholesterol transport?

Lipid-poor apoA-I made by the liver and intestine receives free cholesterol and phospholipid pumped out of peripheral cells by the ABCA1 transporter, forming nascent HDL. LCAT, which apoA-I activates, esterifies the cholesterol so it moves into the particle's core, creating mature spherical HDL. The cholesteryl ester reaches the liver directly through the SR-B1 receptor, or indirectly after CETP transfers it to VLDL and related particles that the liver takes up.

What is the difference between lipoprotein lipase and hormone-sensitive lipase?

Lipoprotein lipase sits on the capillary endothelium of muscle and adipose tissue and breaks down triglyceride carried in circulating chylomicrons and VLDL; apoC-II activates it and insulin increases its production after meals. Hormone-sensitive lipase works inside adipocytes and breaks down stored triglyceride during fasting, when glucagon and epinephrine activate it. The fatty acids it releases travel in the blood bound to albumin.

Why are all apoB lipoproteins missing in abetalipoproteinemia?

Abetalipoproteinemia is an autosomal recessive defect in microsomal triglyceride transfer protein (MTP), which loads lipid onto apoB-48 in the intestine and apoB-100 in the liver. Without it, chylomicrons and VLDL cannot be assembled, so LDL never forms and apoB is absent from plasma. Patients have fat malabsorption, acanthocytes and deficiencies of vitamins A, E and K; the vitamin E deficiency in particular causes ataxia, neuropathy and retinal degeneration. Treatment includes high-dose vitamin E with the other fat-soluble vitamins.

Lipoprotein Metabolism: Pathways, Apolipoproteins & Enzymes

Lipoprotein metabolism moves fat along three routes: the exogenous pathway (intestinal chylomicrons, built on apoB-48, carry dietary triglyceride), the endogenous pathway (hepatic VLDL → IDL → LDL, built on apoB-100, carry liver-made triglyceride and cholesterol) and reverse cholesterol transport (HDL, built on apoA-I, brings cholesterol back to the liver). Four handoffs explain most exam questions: apoC-II activates lipoprotein lipase (LPL), apoE lets the liver clear remnants, apoB-100 binds the LDL receptor, and LCAT, activated by apoA-I, esterifies cholesterol in HDL. When a clearance step fails, the particle just before it piles up; when assembly fails, the particles never form.

The particles at a glance

Particle	Made in	Main apolipoproteins	Main cargo	Where it goes
Chylomicron	Small intestine	B-48 (plus C-II and E borrowed from HDL)	Dietary triglyceride; largest, least dense	Muscle and adipose via LPL; the remnant goes to the liver
VLDL	Liver	B-100 (plus C-II and E from HDL)	Liver-made triglyceride	Muscle and adipose via LPL, leaving IDL
IDL	Blood, from VLDL	B-100, E	Triglyceride and cholesterol	Liver (via apoE), or hepatic lipase turns it into LDL
LDL	Blood, from IDL	B-100	Cholesteryl ester; the most cholesterol-rich particle	LDL receptors on liver and peripheral cells
HDL	Liver and intestine	A-I	Cholesteryl ester; smallest, densest	Collects cholesterol from tissues and returns it to the liver

Density rule: the more triglyceride, the lighter the particle: chylomicron < VLDL < IDL < LDL < HDL.

Apolipoproteins: the shipping labels

Apolipoprotein	Found on	Job	Exam hook
A-I	HDL	Main HDL protein; activates LCAT	Accepts cholesterol pumped out by ABCA1
B-48	Chylomicrons	Structural; needed to assemble and secrete chylomicrons	Made by intestinal mRNA editing that creates a stop codon, so it is the first 48% of apoB-100 and lacks the LDL receptor-binding domain
B-100	VLDL, IDL, LDL, Lp(a)	Structural; LDL receptor ligand	Defective apoB-100 causes familial hypercholesterolemia
C-II	Chylomicrons, VLDL, HDL	Activates LPL	Deficiency causes familial chylomicronemia (type I)
C-III	Chylomicrons, VLDL, HDL	Inhibits LPL	Fibrates reduce its production
E	Chylomicrons and remnants, VLDL, IDL, HDL	Receptor ligand for hepatic uptake of remnants	E2/E2 underlies most dysbetalipoproteinemia (type III); E4 raises late-onset Alzheimer risk
Apo(a)	Lp(a)	Plasminogen-like protein linked to an LDL-like particle	High Lp(a) raises cardiovascular risk and may impair fibrinolysis

Memory hook: B-48 is what you ate, B-100 is what the liver made; C-II cuts triglyceride by switching on LPL; E is the exit pass that gets remnants into the liver; A-I activates LCAT.

Enzymes and transfer proteins

Protein	Where	What it does	Defect
MTP (microsomal triglyceride transfer protein)	Endoplasmic reticulum of enterocytes and hepatocytes	Loads lipid onto apoB-48 and apoB-100 to form chylomicrons and VLDL	Abetalipoproteinemia
LPL	Capillary endothelium of muscle and adipose tissue, anchored by GPIHBP1	Hydrolyzes triglyceride in chylomicrons and VLDL to fatty acids and glycerol; needs apoC-II; insulin increases its synthesis	Familial chylomicronemia
Hepatic lipase	Liver	Converts IDL to LDL	Rare; raises cholesterol and triglyceride
LCAT	Plasma, mainly on HDL	Esterifies free cholesterol so it moves into the HDL core	Complete deficiency: corneal clouding, hemolytic anemia, kidney failure; partial (fish-eye disease): corneal clouding
CETP	Plasma	Moves cholesteryl ester from HDL to VLDL in exchange for triglyceride	No classic exam disease
ABCA1	Cell membranes, including macrophages	Pumps cholesterol and phospholipid onto lipid-poor apoA-I to form nascent HDL	Tangier disease
PCSK9	Made by the liver	Sends LDL receptors for degradation	Gain-of-function variants cause familial hypercholesterolemia

Exogenous pathway: dietary fat

Enterocytes rebuild triglyceride from absorbed fatty acids, and MTP loads it with cholesterol and fat-soluble vitamins onto apoB-48, forming the nascent chylomicron.
Chylomicrons enter the lacteals, travel in lymph and reach the blood through the thoracic duct, so dietary fat skips the portal circulation.
In the blood they pick up apoC-II and apoE from HDL.
ApoC-II activates LPL on muscle and adipose capillaries. Merck puts the share of chylomicron triglyceride converted to fatty acids and glycerol at about 90%; muscle burns the fatty acids by beta-oxidation and adipocytes re-store them.
The shrunken, cholesterol-rich chylomicron remnant keeps apoB-48 and apoE. Because apoB-48 can't bind the LDL receptor, hepatic uptake depends on apoE.

Endogenous pathway: liver fat

The liver packages triglyceride (from excess carbohydrate under insulin, or from incoming fatty acids) and cholesterol onto apoB-100 via MTP, forming VLDL. Obesity, uncontrolled diabetes and high-fat diets raise hepatic fatty acids and VLDL output.
VLDL borrows apoC-II and apoE from HDL, and LPL strips its triglyceride, leaving IDL.
The liver takes up some IDL through apoE; hepatic lipase converts the rest to LDL.
LDL binds LDL receptors through apoB-100, especially on hepatocytes, and enters by clathrin-coated pit endocytosis. In the lysosome, cholesteryl esters are hydrolyzed, and free cholesterol leaves with the help of NPC2 and NPC1, the proteins that fail in Niemann-Pick type C.
Rising cell cholesterol turns down cholesterol synthesis (HMG-CoA reductase) and LDL receptor production through SREBP, and ACAT re-esterifies the excess for storage. Statins exploit this loop: less synthesis means more LDL receptors (see the cholesterol synthesis pathway).
Macrophages take up oxidized LDL through scavenger receptors and become the foam cells of atherosclerotic plaque.

Reverse cholesterol transport: HDL

The liver and intestine secrete lipid-poor apoA-I.
ABCA1 on peripheral cells, including macrophages, loads free cholesterol and phospholipid onto it, forming nascent HDL.
LCAT esterifies that cholesterol; the cholesteryl ester moves into the core, producing mature, spherical HDL.
HDL delivers cholesteryl ester to the liver directly through SR-B1, or indirectly through CETP, which hands it to VLDL and its remnants for hepatic uptake.
HDL is also the apolipoprotein bank, lending apoC-II and apoE to new chylomicrons and VLDL.

When a handoff breaks

Broken step	Disease (inheritance)	Lipid picture	Classic clue
MTP	Abetalipoproteinemia (AR)	No chylomicrons, VLDL or LDL; apoB absent	Steatorrhea, acanthocytes, ataxia, retinitis pigmentosa from fat-soluble vitamin (mainly E) deficiency
LPL, apoC-II (or apoA-V, GPIHBP1, LMF1)	Familial chylomicronemia, type I (AR)	Chylomicrons and triglyceride very high	Pancreatitis, eruptive xanthomas, lipemia retinalis, hepatosplenomegaly, creamy plasma
LDL receptor, apoB-100 or PCSK9 gain of function	Familial hypercholesterolemia, type IIa (AD)	LDL high	Tendon xanthomas (Achilles), xanthelasma, corneal arcus, premature coronary disease
ApoE (usually E2/E2)	Dysbetalipoproteinemia, type III (usually recessive)	Chylomicron remnants and IDL high; cholesterol and triglyceride both up	Tuberous and palmar xanthomas, yellow palmar creases
ABCA1	Tangier disease (AR)	HDL very low	Enlarged orange tonsils, neuropathy, hepatosplenomegaly

Merck notes that triglycerides above 500 mg/dL can cause acute pancreatitis, and levels above 2,000 mg/dL can make retinal vessels look creamy white (lipemia retinalis). All five Fredrickson phenotypes are decoded in the familial dyslipidemias mnemonic.

Drugs mapped to the pathway

Drug	Step it hits	Main lipid effect
Statins	Inhibit HMG-CoA reductase, so hepatocytes make more LDL receptors	LDL down (up to about 60%)
Ezetimibe	Blocks the intestinal sterol transporter NPC1L1, so less intestinal cholesterol reaches the liver	LDL down
Bile acid sequestrants	Block bile acid reabsorption; the liver adds LDL receptors to make new bile acids	LDL down; triglycerides may rise
PCSK9 inhibitors (evolocumab, alirocumab; inclisiran is an siRNA)	Stop PCSK9 from degrading LDL receptors	LDL down
Fibrates	Activate PPAR-α: more LPL activity, less apoC-III, more apoA-I	Triglycerides down, HDL up
Niacin	Partly inhibits fatty acid release from adipose tissue and slows hepatic VLDL synthesis	HDL up, triglycerides and LDL down

Exam traps

LPL is not hormone-sensitive lipase. LPL sits on capillary endothelium and unloads circulating particles after meals; hormone-sensitive lipase works inside adipocytes and is switched on by glucagon and epinephrine in fasting.
Chylomicron remnants are cleared through apoE, because apoB-48 has no LDL receptor-binding domain.
Pancreatitis tracks triglycerides (chylomicrons), not LDL; atherosclerosis tracks LDL.
Calculated LDL fails when triglycerides are high. The Friedewald equation, $\text{LDL-C} = \text{TC} - \text{HDL-C} - \frac{\text{TG}}{5}$ , was validated only for fasting triglycerides below 400 mg/dL.
Absent apoB particles mean an assembly defect (MTP); a selective, very low HDL with orange tonsils is Tangier disease (ABCA1).

Practice question

Vignette. An 8-year-old boy has had greasy, foul-smelling stools and poor weight gain since infancy. Over the past year he has developed an unsteady gait with decreased vibration sense, and an eye examination shows retinitis pigmentosa. A blood smear shows acanthocytes. Fasting total cholesterol is 38 mg/dL, triglycerides are 12 mg/dL, LDL is undetectable and apoB is absent from plasma. A defect in which of the following is the most likely cause?

A. ABCA1
B. Apolipoprotein E
C. LDL receptor
D. Lipoprotein lipase
E. Microsomal triglyceride transfer protein

Answer: E. MTP loads lipid onto apoB in both enterocytes and hepatocytes. Without it, neither chylomicrons (apoB-48) nor VLDL (apoB-100) can be assembled, so LDL never forms and apoB is absent. Fat and fat-soluble vitamins aren't absorbed, and vitamin E deficiency drives the ataxia and retinal disease. This is abetalipoproteinemia, treated with high-dose vitamin E and the other fat-soluble vitamins.

Why the others are wrong:

A: ABCA1 defects (Tangier disease) lower HDL selectively and cause orange tonsils; LDL and triglycerides are present.
B: apoE defects block remnant clearance, so cholesterol and triglycerides rise.
C: LDL receptor defects raise LDL.
D: LPL deficiency makes triglyceride-rich chylomicrons accumulate, with pancreatitis and eruptive xanthomas.

Key takeaways

Exogenous: intestine → chylomicron (apoB-48) → LPL (activated by apoC-II) → remnant → liver (apoE).
Endogenous: liver → VLDL (apoB-100) → IDL → LDL → LDL receptor (apoB-100).
Reverse transport: ABCA1 loads apoA-I → LCAT matures HDL → liver via SR-B1, or via CETP and apoB particles.
HDL lends apoC-II and apoE to chylomicrons and VLDL.
Assembly failure (MTP) removes all apoB particles; clearance failure raises the upstream particle: LPL or apoC-II (chylomicrons), apoE2 (remnants), LDL receptor, apoB-100 or PCSK9 (LDL).
Very high triglycerides cause pancreatitis; high LDL causes atherosclerosis.