Post on 12-Jan-2016
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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