Lipoprotein Lipase (LPL) & Fatty Acid Delivery to Tissues

Lipoprotein Lipase (LPL)

• Is anchored to the cell surfaces of several tissues that are exposed to the circulation:

  • Anchored to heparan sulfate proteoglycans

• The highest levels of expression are within the heart and adipose tissue

• ApoC-II activates LPL, while ApoC-III inhibits LPL

• Responsible for the transition from triglyceride → diglyceride → monoglyceride

• Hydrolyzes the triglycerides that are carried through the circulation by lipoproteins containing ApoB-48 (chylomicrons) and ApoB-100 (VLDL)

The hydrolysis of triglycerides on VLDL by LPL:

Leads to the formation of a smaller more dense lipoprotein within the circulation – LDL

Lipoprotein Lipase (LPL) Deficiency

• A genetic disorder

• Causes very high triglycerides levels in circulation, and several health complications

• Fat intake has to be restricted to less than 20 g/day (=5 teaspoons of fat per day)

The effects of a chronic high fat diet or obesity on chylomicron metabolism:

1. Increased fat absorption + an altered gut

2. Altered chylomicrons

3. Increase in TG after meals

4. Fat storage + inflammation

5. Insulin resistance

Endotoxemia

Bacterial toxins (LPS) entering the blood as a result of a high-fat diet or obesity

• Causes adipose tissue inflammation

Endogenous Lipid Transport

1. Nascent VLDL are made in the Golgi apparatus of the liver

2. Additional apolipoproteins C and E are transferred from HDL

3. The FA from TAG are hydrolyzed by LPL found mainly in muscle and adipose tissue

4. As TAG is removed from the VLDL, the particle becomes smaller and becomes an IDL

5. Further loss of TAG and it becomes a LDL

6. LDL are taken up by LDL receptors found in the liver and non-hepatic tissue

ApoB-48

• Associated with chylomicrons

• 48% of the ApoB-100 protein

• Is generated in the intestine through the enzymatic removal of an amine group (‘deamination’) from a specific cytidine on the mRNA sequence for ApoB-100 to generate uracil

  • This introduces a stop codon ~48% into the ApoB-100 mRNA sequence

ApoB-100

• Associated with VLDL

Familial Hypercholesterolemia (LDL–Receptor Defect)

• 1 in 1 million are homozygotes

• 1 in 500 are heterozygotes

• Severe accumulation of cholesterol (primarily from LDL) in the circulation

Steps in endocytosis of LDL leading to synthesis and storage of cholesterol ester:

1. LDL particle with ApoB attaches to the LDL receptor

2. Endocytosis of LDL particle and receptor

3. LDL particle fuses with lysosome

4. LDL receptor returns to the membrane surface

5. Proteins of LDL particle hydrolyzed into amino acids

6. Free cholesterol released from LDL particle

7. HMG-CoA reductase is involved in cholesterol synthesis

• When excess cholesterol is present, synthesis of cholesterol and LDL receptors are inhibited

8. Cholesterol transferred to Golgi, esterified with ACAT, and stored in the cell

Hypercholesterolemia without detectable genetic defect:

Prolonged high saturated fat intake (and high cholesterol) down-regulate LDL-R

• High LDL-cholesterol levels are associated with an increased risk of cardiovascular disease

Reverse Cholesterol Transport

• The process of removing non-hepatic (extra-hepatic) cholesterol for delivery to the liver, and ultimately for excretion from the body (e.g. cholesterol is used for bile acids/salts synthesis; bile is excreted)

• Requires the transfer of cholesterol and phospholipids from cells to nascent HDL (almost exclusively proteins; very little lipid) in the circulation – a process called cholesterol efflux

• Nascent HDL is then converted to mature HDL in the circulation

• Mature HDL is taken up by the liver (only the lipids and not apoA-I) – a process called selective uptake

• ApoA-I can be reused to form more HDL

Cholesterol Efflux

The transfer of cholesterol and phospholipids from cells to nascent HDL (almost exclusively proteins; very little lipid) in the circulation

Selective Uptake

Mature HDL being taken up by the liver (only the lipids and not apoA-I)

ApoA-I

• The major protein component of HDL

• Has several functions, including maintaining the structure of HDL, and interacting with lipid transporters (ABCA1; ATP binding cassette-1)

• Can be reused to form more HDL

Steps of Reverse Cholesterol Transport

1. Lipid-free ApoA-1 is secreted by the liver and intestine

• It is also released from chylomicrons and VLDL during TAG hydrolysis

2. ApoA-1 acquires phospholipid and cholesterol from interaction with liver ABCA1, resulting in nascent HDL particles

3. Nascent HDL acquire additional phospholipids and cholesterol via ABCA1 and additional cholesterol via SR-B1 in peripheral tissues

4. The enzyme LCAT, carried on HDL particles, esterifies cholesterol to cholesteryl ester that migrate to the particle core

5. The now spherical mature HDL continue to acquire phospholipids and cholesterol via ABCG1 and cholesterol via SR-B1 in peripheral tissues

6. LCAT continues to esterify cholesterol to cholesteryl ester, forming larger HDL

7. Some cholesteryl ester are transferred to VLDL and LDL, mediated by CETP

8. Liver SR-B1 bind HDL

• Cholesteryl ester may be selectively removed, or the HDL particle may be internalized and degraded

Cholesteryl Esters (CE)

Storage form of cholesterol in macrophages

CE Hydrolase

• Provides cholesterol for RCT

• Cholesteryl ester + water → cholesterol + FA

Formation of mature HDL within the circulation:

The conversion of nascent HDL to mature HDL in the circulation requires one key enzyme, and a “transfer protein”:

• Lecithin cholesterol acyl transferase (LCAT)

• Phospholipid transfer protein (PLTP)

LCAT Activity

Lecithin + cholesterol → Lysolecithin + cholesterol ester

• ApoA-1 is a cofactor

PLTP (Phospholipid Transfer Protein)

Transfers excess phospholipid from ApoB-100-containing lipoproteins (VLDL, LDL) to HDL.

• Provides phospholipid to expand the HDL surface area.

• Provides lecithin (or phosphatidylcholine) for LCAT activity.

• Contributes to the metabolism of ApoB1-100-containing lipoproteins (VLDL) into LDL

High Dietary n-3 PUFA:

Increased Cholesterol Efflux in C57BL/6 Mice

Anti-Atherogenic Lipoproteins

HDL₃ and HDL₂

Pro-Atherogenic Lipoproteins

Small LDL, LDL, Large LDL, IDL, VLDL Remnants, VLDL, and Chylomicron remnants

Biological Roles of Lipoproteins in Trafficking Cholesterol and TAGs

Lipoproteins are transport particles that carry hydrophobic lipids through the bloodstream.

• Chylomicrons: Transport dietary TAGs and cholesterol from the intestine to muscle and adipose tissue.

• VLDL: Transport liver-made TAGs to peripheral tissues.

• LDL: Delivers cholesterol from the liver to cells for membranes and steroid synthesis.

• HDL: Carries excess cholesterol from tissues back to the liver for disposal (reverse cholesterol transport).

Chylomicrons

Transport dietary TAGs and cholesterol from the intestine to muscle and adipose tissue.

VLDL

Transport liver-made TAGs to peripheral tissues.

LDL

Delivers cholesterol from the liver to cells for membranes and steroid synthesis.

HDL

Carries excess cholesterol from tissues back to the liver for disposal (reverse cholesterol transport).

Lipid metabolism in the adipose cell following a meal:

1. Glucose is metabolized to make acetyl-CoA, which can be converted to FA

2. LPL acts on TAG in chylomicrons causing FFA and MAG to enter the adipocyte

3. LPL acts on VLDL so FFA and MAG enter the cell

4. The pathways favour energy storage as TAG

• Insulin stimulates lipogenesis by promoting entry of glucose into the cell by inhibiting the hormone-sensitive lipase that hydrolyzes the stored TAG to FFA and glycerol

What does insulin normally do to chylomicron secretion

Inhibits it

What happens to chylomicron secretion in insulin resistance?

It increases (hypersecretion)

What happens to de novo lipogenesis on insulin resistance?

It increases in enterocytes

What does increased de novo lipogenesis cause?

More lipid available for chylomicron production

What happens to circulating FFA in insulin resistance?

FFA levels increase

Where do increased FFA come from in insulin resistance?

Adipocytes

What do enterocytes do with circulating FFA?

Take them up

What happens to MTP activity in insulin resistance?

MTP activity increases

What does increased MTP help with?

Triglyceride re-synthesis and ApoB-48 lipidation

What else can increase chylomicron production

Increased glucose and fructose consumption

Blood Lipid Levels (mmol/L)

• Total cholesterol < 4.0

• Triglycerides < 1.8

• LDL-C < 2.0

• Non-HDL-C < 2.6

• HDL-C > 1.0 (male) and > 1.3 (female)

• routine screening for men > 40 yrs and women > 50 yrs

• earlier if any risk factors are present (high blood pressure, smoking, family history of early CVD, abdominal obesity, diabetes, chronic inflammatory conditions)

• treatment plan will consider the degree of hyperlipidemia and other risk factors (hypertension; high BP; smoking history etc.)