Lipid Homeostasis and Transport

CH353 February 12, 2008

Summary

• Major transported forms of lipids– Cholesterol esters – Triacylglycerols (triglycerides)

• Lipid transport particles (solubilize lipids & target cells)

– Chylomicrons (dietary triglyceride transport)– VLDL (hepatic lipid transport)– LDL (cholesterol transport and regulation)– HDL (reverse cholesterol transport)

• Cholesterol Homeostasis– Intracellular transport– Regulation of enzyme levels – Regulation of enzyme activity

Cholesteryl Ester Synthesis

• in liver• acyl-CoA-cholesterol acyl

transferase (ACAT)

• outside cells (on HDLs)• lecithin-cholesterol acyl

transferase (LCAT)

+

Triacylglycerol Biosynthesis

• Synthesis of triacylglycerol from phosphatidic acid

• Common precursor with membrane glycerolipid biosynthesis

• Mainly in liver and intestine (for transport) and adipose tissue (for storage)

Lipoproteins

a) Structure of Low-Density Lipoprotein (LDL)

b) Electron microscope images of lipoproteins

Chylomicrons

• Density: < 1.006 g/ml• Mass: 50–1000 x 103 kDa• Diameter: 75–1200 nm• Composition:

– 2% protein (apoB-48, apoA, apoC, apoE)– 9% phospholipids, 85% triglycerides, 4% cholesterol

• Origin: intestinal enterocytes (apoproteins and lipid)• Function: Transports dietary triglycerides from intestine

to tissues; returns remnants to liver

Very Low Density Lipoprotein (VLDL)

• Density: < 0.95–1.006 g/ml• Mass: 10–80 x 103 kDa• Diameter: 30–80 nm• Composition:

– 10% protein (apoB-100, apoC, apoE)– 20% phospholipids, 50% triglycerides, 20% cholesterol

• Origin: hepatocytes (apolipoproteins and lipid)• Function: Transports hepatic triglycerides to tissues; is

converted into LDL

Low Density Lipoprotein (LDL)

• Density: 1.006–1.063 g/ml• Mass: 2.3 x 103 kDa• Diameter: 18–25 nm• Composition:

– 25% protein (apoB-100)– 20% phospholipids, 10% triglycerides, 45% cholesterol

• Origin: derived from VLDL (by loss of triglycerides)• Function: major carrier of cholesterol to liver and other

tissues; regulates cholesterol biosynthesis by LDL receptor-mediated endocytosis

High Density Lipoprotein (HDL)

• Density: 1.063–1.210 g/ml• Mass: 0.175–0.360 x 103 kDa• Diameter: 5–12 nm• Composition:

– 55% protein (apoA)– 25% phospholipids, 5% triglycerides, 15% cholesterol

• Origin: pre-HDL secreted by hepatocytes• Function: Loads cholesterol from tissues and delivers it

to liver, steroidogenic tissues and other lipoproteins; apoA-1 binds to SR-B1; can release cholesterol without endocytosis

Systemic Transport of Lipid and Lipoprotein

1. Chylomicrons transport dietary lipid to tissues; endocytosis of remnants (apoB-48 or apoE)

2. VLDLs transport lipids from liver to tissues; remnants become LDLs or endocytosed by liver (apoB-100 or apoE)

3. LDLs transport cholesterol to liver and other tissues by endocytosis (apoB-100)

4. HDL precursors remove cholesterol from tissues; HDLs deliver cholesterol to the liver and other tissues (apoA) lipoprotein lipase regulated

by apoC on lipoproteins

Receptor-Mediated Cholesterol Endocytosis

• Allows sensing of plasma cholesterol by producing cells

• LDL endocytosis requires LDL receptor (binds apoB-100)

• genetic deficiency of functional LDL receptors causes familial

hypercholesterolemia • LRP (lipoprotein receptor-related

protein) binds apoE in remnants of chylomicrons and VLDLs

• Internalized cholesterol interacts with sensors on endoplasmic reticulum, regulating biosynthesis

Cell Membrane Transport Proteins

• ABC (ATP binding cassette) proteins transport cholesterol across cell membranes

• ABCA1 ubiquitous transport of cholesterol and phospholipid into HDLs – deficiency causes Tangier’s disease

• ABCG1 macrophage-specific transporter• Hepatic and intestinal ABCG5/8 export cholesterol and

other sterols into bile and lumen, respectively – deficiency causes β-sitosterolemia

Reverse Cholesterol Transport

• Secreted ApoA forms pre-HDL by acquiring transported lipids

• Cholesterol and phospholipid is transported across cell membrane by ABCA1 (and ABCG1)

• Cholesterol is esterified by plasma lecithin-cholesterol acyl transferase (LCAT) using lecithin from the cell

• Mature HDL binds to its receptor SRB1 on liver and steroidogenic cells, and delivers its cholesterol

• Cholesterol can be transferred to other lipoproteins by cholesteryl ester-transfer protein

Regulation of HMG-CoA Reductase

Regulating amount of enzyme (200x)• Transcription: 8x ↑ mRNA levels• Translation: 5x ↑ protein synthesis• Protein turnover: 5x ↓ protein degradation

Regulating activity of enzyme• by AMP-activated protein kinase

– high [AMP] inhibits (phosphorylates) HMG-CoA reductase

• by hormone-dependent kinases / phophatases– glucagon inhibits (phosphorylates) HMG-CoA reductase– insulin activates (dephosphorylates) HMG-CoA reductase

Regulation of Sterol Biosynthetic Genes

• Steroid Regulatory Element Binding Protein (SREBP) is retained in ER by SCAP (SREBP cleavage-activating protein)

• Low cholesterol allows migration of SREBP from ER to golgi where it can be cleaved by 2 proteases

• Cleavage of SREBP allows N-terminal fragment to enter nucleus and activate steroid biosynthetic genes

Cholesterol Regulation of Transcription and Protein Turnover of HMG-CoA

AbbreviationsSRE: sterol response element

(DNA sequence)

SREBP: SRE binding protein (transcription factor)

SCAP: SREBP cleavage-activating protein (SREBP chaperone)

INSIG: Insulin-induced gene 1 protein (ER retention)

SSD: sterol-sensing domain (5 membrane spanning helices)

S1P: Site 1 protease

S2P: Site 2 protease

HMG-CoAR: HMG-CoA reductase

LDLR: LDL receptor

INSIGINSIG

from Ikonen (2008) Nature Rev. Mol. Cell Biol. 9:125

Transcription of Lipid Biosynthetic Enzymes Regulated by SREBP

from Horton, Goldstein, Brown (2002) J. Clin. Invest. 109:1125

Regulation of Cholesterol Biosynthesis

• Hormones regulate the phosphorylation of HMG-CoA reductase:– Phosphorylated – Inactive– Dephosphorylated – Active

• High intracellular cholesterol:– stimulates ACAT for

cholesterol storage– lowers expression of LDL

receptor gene