Lecture 12: Lipoprotein Metabolism

Lipoprotein: noncovalent lipid-protein complexes that allow movt of apolar lipids through aqueous environments like blood and lymph 
- Various families exist and each plays defined role in lipid transport
- Classified by density as determined by centrifugation

Take blood coagulate, plasma on top

Take small portion into a tube

Take sucrose solution, spin it, it creates a gradient of sucrose from top to bottom

Top = high concentration

These vesicles according to their density will exist in diff layers

- Lipoproteins are heterogenous in buoyancy and size but also in lipid and protein composition
Lots of TAGs in chylomicrons, VLDL and IDL
LDL and HDL: Transport Cholesterol

Free cholesterol = within the lipoprotein membrane
Cholesterol ester = stored form of cholesterol; stable, inert

Lipoprotein Exogenous Pathway
How do we get it in our body?

Absorption and transport of fat (TAGs) and cholesterol from diet. (Also, reabsorption of recycled cholesterol; 80% of what's rejected by the liver is reabsorbed by the intestine

1) Bile salts synthesized by hepatocytes then exported out with cholesterol
2) Form micelles with lipids from hepatocyte plasma membrane through emulsification
- Micelle = phospholipid + bile salt + a bit of cholesterol
3) Micelles get stored in the gall bladder and will be released into small intestine after a meal

- Where? Synthesized in hepatocytes
- Them and cholesterol are then exported out.

@ Normal pH, cholic acid is all protonated; bile salts become more polar -> lead to better absorption of Fat
4) Dietary cholesterol + TAGs absorbed by bile salts-cholesterol micelles
5) Hydrophilic side of bile salts allow recruitment of water-soluble enzymes (attached to outside of micelle)
- ENZYMES: pancreatic lipase + colipase
6) Hydrolytic action from 5) releases FAs from triglycerides -> mix of glycerol, FFAs, MAGs and DAGs

Dynamic reaction: result = tiny vesicles
7) FFAs, MAGs, and cholesterol are absorbed by enterocytes lining small intestine lumen; FFAs + MAGs reassembled into TAGs
8) Portion of cholesterol converted into cholesterol ester (FFA + hydroxyl group of cholesterol)
- Cholesterol esters less polar, used for transport in plasma 
9) Within enterocytes (in the Golgi apparatus) TAGs and cholesterol and cholesterol ester are combined with APOLIPOPROTEIN to form chylomicrons
10) Chylomicrons released into lymphatic system and then into bloodstream

Aside: Apolipoproteins are associated with lipoproteins (always apo- something)
- Two subcategories: Exchangeable and non-exchangeable

Exchangeable Apolipoprotein:
- Tethered to lipoprotein surface, can disassociate from one lipoprotein and become associated with another lipoprotein
- Composition: multiple amphipathic alpha-helices 
- Involved in recognition

Non-exchangeable: tightly bound to a lipoprotein particle, do not disassociate from it
- ApoB proteins: insoluble in aqueous solutions
- Non-exchangeables are heavier

Distribution of TAGs
- After being secreted into bloodstream, chylomicron-carrier TAG are hydrolyzed by lipoprotein lipase; specific in capillaries the released FAs are delivered to adipose tissue and other cells
- Result chylomicron "remnants" taken up by liver in endocytosis; contain small amounts of TAGs and cholesterol
    - Mediator: ApoE receptor
- In adipocytes: reform TAG and store, otherwise used right away 

Apo CII will recognize lipoprotein lipase and increase lipase activity


Lipoprotein Endogenous Pathway for TAG-rich lipoprotein (VLDL)

1. VLDL synthesized in liver via assembly of synthesized lipids (mostly TAGs) and apolipoproteins
2. TAGs associated w/ VLDL are hydrolyzed by lipoprotein lipase and released FAs go to adipose tissue for storage/other places for energy

VLDL remnants (aka IDL) taken up by liver through endocytosis
- Mediator = ApoE to a receptor
- IDL may exchange lipoproteins+lipids w/ HDL to become LDL and are cleared from circulation by LDL-receptor-mediated process
LDL = bad b/c it circulates cholesterol to everyone 
HDL = good b/c it just takes it and puts it in the liver

Endogenous = VLDL

How IDL -> LDL?
- Loses Apo CI, II and III and they end up in HDL
- Loses a ton of TAGs

IDL: Loses Apo C1,2,3 to HDL; loses TAG to HDL
HDL: Gains Apos, Gains TAG, loses cholesterol esters to IDL
RECALL: Cholesterol important in:
1) Structure
2) Consituent of lipoprotein
3) precursor for bile acids + hormones

Cholesterol-rich lipoprotein = LDL
- Primary function = deliver cholesterol to extrahepatic tissues
- LDL-receptor-dependent endocytosis responsible for update of plasma LDL (integral membrane protein expressed in liver+peripheral tissues; part of regulatory mech for cholesterol homeostasis)

ACAT: enzyme to store cholesterol ester

HDL Cycle
Cholesterol-rich lipoprotein = HDL
- Opposite to LDL: removes cholesterol from tissues
- Often called HDL cycle/reverse transport of cholesterol

1) ApoA-1/phospholipid complexes (HDL precursors) secreted from liver
2) They interact w/ ABC-A1 transporter located @ cell surface of peripheral cells (acquire free cholesterol+phospholipid)
Product = disk-shaped nascent HDL (like a hockey puck)
3) Esterification of free cholesterol (via LCAT) -> transform puck into sphere (aka HDL3)

4) start = The discoid from 3)
transfer of cholesterol ester to ApoB-containing lipoproteins (VLDL, IDL, LDL)

5) HDL acquires new exchangeable apolipoprotein ApoC1,2,3 + TAGs 
ENZYME: Cholesteroyl ester transfer protein (CETP)

6) FATE OF HDL2? Selective uptake of Cholesterol esters 
- Liver + steroidogenic tissues uptake of CE from HDL: mediated by SR-B1; result = generate ApoA1/phospholipid complexes
- Another way? research proposed endocytosis