U01.01.131 Lipid transport

Written by

in

Learning Objectives

  • Trace the path of Exogenous (dietary) and Endogenous (hepatic) lipid transport.
  • Identify the specific roles of Apolipoproteins (B-48, B-100, CII, and E).
  • Explain the function of Lipoprotein Lipase (LPL) and Hepatic Lipase (HL).
  • Connect transport defects to Familial Dyslipidemias (Types I, II, and IV).

1. Exogenous Pathway (Dietary Lipids)

Dietary fats are processed in the intestine and transported to peripheral tissues and the liver.

  • Formation: Enterocytes package dietary TG and cholesterol into Chylomicrons using Apo B-48.
  • Lymphatic Entry: Chylomicrons enter the lacteals and travel through the thoracic duct to the subclavian vein, bypassing the portal circulation.
  • Maturation: In the blood, HDL “donates” Apo CII (to activate LPL) and Apo E (for liver reuptake).
  • Utilization: Apo CII activates Lipoprotein Lipase (LPL) on capillary walls to release FFA for energy or storage.
  • Remnant Clearance: The Chylomicron Remnant returns to the liver via the Apo E receptor.

2. Endogenous Pathway (Hepatic Lipids)

The liver synthesizes and exports lipids to satisfy the body’s energy and structural needs between meals.

  • VLDL: The liver secretes VLDL containing Apo B-100. Like chylomicrons, VLDL picks up Apo CII/E from HDL and delivers TG to tissues via LPL.
  • IDL: As VLDL loses TG, it becomes IDL (VLDL remnant). IDL can be taken up by the liver via Apo E or further processed.
  • LDL: Hepatic Lipase removes more TG from IDL to form LDL. LDL is rich in cholesterol and delivers it to peripheral cells via the LDL Receptor (recognizes Apo B-100).

        1. 1. Chylomicron enters lymphatics,  2. HDL transfers Apo CII and apoE, 3. Chylomicron apo CII activates LPL (impaired in type I familial dyslipidemia), 4. Liver releases VLDL (overproduction in type IV familial dyslipidemia), 5. VLDL apo CII activates LPL, 6. IDL delivers TGs and cholesterol to the liver via apoE, 7. Endocytosis of LDL (impaired in type II familial dyslipidemia)

       

       

 

 

Activity: Apolipoprotein Identification

3. Summary of Major Lipoproteins

Lipoprotein Main Apolipoprotein Primary Function
Chylomicron B-48 Transports dietary TGs to tissues.
VLDL B-100, CII Transports hepatic TGs to tissues.
LDL B-100 Delivers cholesterol to peripheral cells.
HDL A-I, CII, E Reverse cholesterol transport; Apo “reservoir.”

Clinical Notes & Corrections:

  • Type I Familial Dyslipidemia: Caused by a deficiency in LPL or Apo CII. Results in severe hypertriglyceridemia and pancreatitis.
  • Type II Familial Hypercholesterolemia: Caused by a defect in LDL receptors or Apo B-100. Leads to very high LDL and early atherosclerosis.
  • Type IV Familial Dyslipidemia: Characterized by hepatic overproduction of VLDL, often associated with insulin resistance.

Activity: Dyslipidemia Diagnostic Flow

Memory Hook:

B-48: Mediates intestinal absorption (from 48 states).

B-100: Mediates hepatic export (100% from liver).

CII: Cuts and Clears (activates LPL).

E: Enters the liver (remnant uptake).

Activity