Human biochemistry, Medical Biotechnology, inglese

31.1.17! !

What!are!the!major!determinants!for!lipoprotein!density?!

Lipoproteins ! ; Amount/of/hydrophobic/lipids!→!in!particular!of!triacylglycerols.!87%!is!the!less!value!

Lipid!metabolism!occurs!in!four!major!areas!of!the!human!body:! in!density,!than!you!have!a!strong!decrease/in/triglycerides/and/an/increase/in/density/

; Intestine!→!site!of!absorption!of!lipids!! ; Ratio/between/hydrophobic/lipids/phospholipids/and/hydrophobic/lipids/proteins.!→!the!

; Liver!→!main!biosynthetic!processes!for!lipids! protein/ content/ increases/ along/ the/ increase/ in/ density,! and! the! same! is! for!

; Adipose!tissue/and/muscle!→!main!lipid!users!! phospholipids.!!

; Steroidogenic/tissues!→!need!for!cholesterol! Even! the! dimension/is/related/to/the/density! of! the! lipoprotein:! chylomicrons! are! the! biggest!

! structures,!while!HDL!are!the!most!little!structures.!!

How! can! lipids,! and! in! particular! cholesterol! and! triglycerides! that! are! absolutely! !

hydrophobic,!be!transported!in!the!plasma?! Phospholipid/composition/of/lipoproteins/

The! structures! that! are! engaged! for! lipid! transport! are! lipoproteins:! which! have! a! general! The! envelope! that! wraps! the! hydrophobic! can! be! a! little! bit! different! between! the! different!

structure:! lipoproteins:!PC!is!the!mainly!represented!in!all!lipoproteins,!but!there!are!some!differences!

Hydrophobic1core!→!we!find!triglycerols!and!cholesterol!esters!

• between!the!different!lipoproteins:!

Phospholipid! monolayer! it! surrounds! the! core,! and! ensure! the! water! solubility!

• →! ; LDL!are!enriched!in!SM,!!

because! phosholipids! can! interact! with! water! through! the! polar! head,! and! with! the! ; VLDL!are!enriched!in!PE!

hydrophobic!core!through!the!acid!chains! PHOSPHOLIPIDS IN LIPOPROTEINS

Ceramide!has!a!different!distribution!among!the!different!structures.!!

Apoproteins1

• →!they!are!amphipatic!proteins!associated!to!lipoproteins!that!have!an! !

aminoacid! composition! that! ensure! the! presence! of! an! hydrophobic! portion! and! an!

hydrophilic!portion.!

/

The/difference/between/lipoproteins/classes/is/the/composition/and/the/dimension./

! 161 1231

593

The!density!of!the!lipoprotein!that!is!the!feature!of!the!structure!that!gives!the!name!to!the! 30 209 191

lipoprotein:!

; Chylomicrons!→!it!is!the!less!dense!class,!the!density/is/<1,!than!density!increase!up!to! 6.6 7.6 21.4

values!>!1!(1!is!the!reference!number!for!water!density)! 3.7 9.12 20

; VLDL// 1.3 4.9 1.9 !

; IDL/ !

; LDL/ Protein/composition/

; HDL // The!proteins!associated!to!lipoproteins!differ!for!each!lipoprotein:!

2

; HDL !→! ApoA6I1 associated! to! HDL:! it! interacts! with! ABC! transporters! and! activates/ LCAT!

3 • →!

! (which!catalyses!the!esterification!of!cholesterol!using!PC!as!donor)!

ApoA6II!→!associated!to!HDL,!it!is!an!inhibitor/of/LCAT/

• ApoA6IV!→!associated!to!chylomicrons!

• ApoB648!→!characteristic!of!chylomicrons!

• ApoB6100!→!binds!to!LDL,VLDL!and!all!the!particles!derived!from!it!

• ApoC6I/II/III!→!associated!to!LP!and!most!exchangeable!between!the!different!classes!

• of! LP.! This! introduces! the! characteristic! that! LP! metabolism! is! highly! dynamic! and!

operates! in! blood! stream.! There’s! an! exchange! of! protein! and! lipids! between! the!

different!LPs.!These!3!proteins!are!an!example!of!this!event.!!

ApoD1

• ApoE!

•

! !

! /

Electrophoretic!mobility!(in!the!4 !column)!of!lipoproteins!is!important!for!one!of!the!clinical!

th

aspects! related! to! this! proteins:! you! can! identify! alterations! in! LP! based! on! electrophoretic!

analysis!of!the!plasma!proteins.!The!column!tells!the!region!in!which!you!find!the!different!LP!

in!the!electrophoretic!tract.!!

! 1! ! 2!

The!manipulation!of!LDL!and!chylomicrons!leads!to!the!formation!of!remnants!structure!that!

can!have!two!faiths:!

; Return!to!liver!

; Be!transformed!in!LDL!and!partially!to!be!addressed!once!again!to!liver!or!the!major!

amount!are!converted!in!LDL!that!can!transfer!lipids!to!extra;hepatic!tissues!and!are!

the!main!transporters!of!cholesterol!esters.!

1

HDL! that! is! synthesized! from! different! tissues! and! it! is! involved! in! the! reverse/ cholesterol/

transport/from/extraOhepatic/tissues/to/the/liver.!

!

To!summarize:!

; Chylomicrons,!VLDL,!HDL!are!lipoproteins!that!can!be!synthetized!by!tissues!

; IDL,! LDL,! VLDL! remnants! are! the! evolution! of! these! lipoproteins! synthesized! in!

! different! tissues! (there! is! no! the! synthesis! of! LDL,! it! is! the! evolution! of! another!

! structure).!

/ The!lifetime!of!chylomicrons!is!very!short:!they!are!produced!rightly!after!a!meal,!while!they!

Enzymes/and/transport/proteins/involved/in/lipoprotein/metabolism/ are!absent!in!fasting!condition.!

In!addition!to!the!apoproteins!associated!to!the!lipoprotein!structure!there!are!many!enzymes! !

and!one!transport!protein!that!are!involved!in!lipid!protein!metabolism:!! Chylomicrons!are!synthesized!in!intestinal!cells!and!from!the!lipids!from!the!diet,!which!are!

; LPL/(Lipoprotein/Lipase)!→!substrate!are!chylomicrones!and!LDL! the! major! source:! in! intestinal! cells! chylomicrons! are! assembled! and! don’t! have! these! any!

; HTGL/(Hepatic/Triglyceride/Lipase)→!has!substrate!rVLDL!and!IDL! apoproteins,!because!the!apoproteins!are!associated!to!chylomicrons!only!when!they’re!in!the!

; LCAT!(Lecithin–Cholesterol/Acyltransferase)→!the!substrate!is!HDL! blood! stream.! In! extra;hepatic! cells,! chylomicrons! are! metabolized! by! LPL! (lipoprotein;

; CETP! (Cholesterylester/ Transfer/ Protein)! catalyses! the! movement! of! cholesterol!

→! lipase):!the!products!of!the!reaction!are!chylomicron/remnants!that!are!taken!up!by!the!liver.!

esters!from!HDL!to!triglycerides!rich!particles.! The! VLDL! are! synthesized! in! the! liver:! the! synthesis! is! limited! to! the! assembly! only! of!

! ApoB100! and! not! of! ApoE/C! (which! are! then! taken! in! the! bloodstream):! the! LPL! can!

Lipoprotein/receptors/ metabolise! VLDL! and! produce! LDL! that! are! the! major/ transporters/ of/ cholesterol/ to/ extraO

We!have!also!lipoprotein!receptors,!that!are:! hepatic/tissues.!

; ApoB/E!and!LDL;R!→!they!interact!with!chylomicrons!and!VLDL!remnants,!and!LDL! The/common/feature/of/these/two/lipoproteins/is/to/release/fatty/acids/from/triglycerides./

; CD36/and/SROA!→!they!are!scavenger!receptors!that!interact!with!the!modified!LDL! !

; SROB1!→!it!is!a!scavenger!receptor!which!interacts!with!mature!HDL! Chylomicrons/ are! the! transfer! structures/ for/ fatty/ acids/ from/ the/ diet,! while! VLDL! are! the!

; ABCA1!→!able!to!associate!HDL!precursors!to!generate!major!HDL!by!the!transfer!of! transfer!structure!for!the!fatty/acids/synthesized/in/the/liver/from/glucose/and/aminoacids/when/

cholesterol!! they/overcome/the/energy/required.!

! The!first!step!of!the!assembly!is!different!for!these!two!classes!of!lipoproteins:!

The/metabolism/of/lipoproteins/need/all/these/components./ Chylomicrons! the! assembly! happens! in! enterocytes! and! is! associated! to! the!

• →!

! reassembly! of! triacylglycerol! (from! dietary! fats)! and! then! they! are! associated! to! the!

Lipoproteins1and1lipid1transport1 apoprotein!ApoB48.!!

Lipoproteins and lipid transport TG are carried by Chylomicrons and VLDL

! The! assembled! chylomicrons! are:! ApoB48/ +/ triglycerides/ +/ phospholipids.! They! are!

Chylomicrons! are! synthesized! in! the! intestine,! released!in!the!lymphatic!duct!and!then!can!reach!the!blood!stream!

while!VLDLs!in!the!liver.!

VLDL! and! chylomicrons! can! be! metabolized! in!

mammary!glands,!muscle!or!adipose!tissue!and!

they! act! as! releaser! of! FFAs,! thus! giving! an!

energy!contribution!in!term!of!energy!source!to! HTGL

these! tissues! and! a! storage! source! for! adipose!

tissue.!

!

!

! Extrahepatic cells

! !

Extrahepatic cells

!

! 3! ! 4!

LPL/is/synthesized/in/adipocytes/and/muscle/cells,!so!in!those!tissues!that!need!fatty!acids,!then!

VLDL!→!the!assembly!happens!in!enterocytes!where!the!main!source!of!triglycerides!

• it!is!transported!from!the!adipocyte/muscle!cell!to!the!epithelial!cell.!!

in!liver!is!glucose!than!can!be!converted!to!glycerol;3P!which!can!drive!the!synthesis!of!

TG are carried by Chylomicrons and VLDL After! LPL! activity,! the! structure! of! lipoproteins! is! smaller! than! before,! and! the! proteins!

triglycerides;!VLDL!are!then!associated!to!the!ApoB100.!! associated! to! chylomicrons! are! ApoB48/ApoCII/ApoE:! this! structures! can! be! removed! from!

Remnants!VLDL!(or!IDL)!can!then!return!to!the!liver,!so!a!little!triglyceride!component! the!blood!stream!because!they!can!interact!with!a!receptor!for!ApoE/B!in!the!liver;!this!is!the!

that!can!derive!from!these!structures!or!we!can!also!have!the!uptake!of!FFA!(free!fatty! end!point!metabolism!for!chylomicron.!

acids)!and!use!them!for!the!synthesis!of!triglycerides!(these!FFA!are!albumin!bound!to! !

FA!deriving!from!the!adipose!tissue).! While! VLDL! can! associate! to! the! liver! by! the! binding! to! the! ApoE/B! receptor! through! ApoE!

(that!it!is!more!represented);!there!is!than!the!loss!of!ApoA!and!ApoC!and!the!generation/of/

LDL/from/VLD.!!

HTGL So!after!the!removal!of!triglycerides!and!the!loss!of!apoprotein!VLDL!are!converted!in!LDL.!

LDL! is! the! particle! with! only! one! protein:! ApoB100! is! assembled! at! the! beginning;! this!

apoprotein!is!not!important!for!the!metabolism!of!VLDL!but!it/is/important/for/the/metabolism/

of/LDL.!!

/

Structure/of/LDL/

!

Extrahepatic cells

Extrahepatic cells LDL! is! a! particle! with! only! one! protein! associated! that! is! ApoB100.! This! protein! isn’t!

! important! for! metabolism! of! VLDL! but! it! is! important! for! the! metabolism! of! LDL.! It’s!

The!differences!in!assembly!step!between!CM!and!VLDL!are!due!to!the!different!origin!of!FFAs! LDL

synthesized!in!liver!and!associated!to!VLDL!but!it!is!operating!on!LDL!metabolism.!

and!to!the!Apo!proteins!associated!with!them.! !

ApoB48! and! ApoB100! are! codified! by! the! same! gene:! they! differ! in! length! because! in! In!LDL!we!have!mainly!cholesterol/esters!because!all!the!triglycerides!have!been!removed!and!

enterocytes/there/is/a/stop/in/the/transcription/of/the/protein/that/leads/to/a/shorter/protein.! also!because!of!the!action!of!CETP!that!happens!in!the!blood!stream.!

! The! amount! of! cholesterol! esters! depends! on! the! initial! amount! of! it! but! also! from! the!

In!the!bloodstream!phase!the!apoprotein!can!be!exchanged:!both!VLDL!and!chylomicrons!pick! exchange!of!cholesterol!esters!that!derives!from!HDL.!

up!ApoE!(specific!for!chylomicrons),!ApoA!and!ApoC.!! Also!HDL!can!directly!interact!with!a!membrane!receptor!that!can!internalize!the!lipoprotein!

CD36

SR-A

More! specifically! VLDL,/ that/ are/ TGOrich/ particles,/ can/ exchange/ cholesterol/ esters/ from/ HDL! (like!for!LDL)!because!the!dimension!of!lipoprotein!is!small!enough!to!be!assumed!by!the!cell.!!!

and!increase/their/content/in/cholesterol/esters./ !

Then!they!can!interact!with!lipoprotein;lipase.! There!are!two!different!receptors!involved!in!the!uptake!of!LDL:!

! LDL6R!→!it!has!a!ligand!binding!domain!not!directly!associated!to!the!membrane,!it!is!

! • far!from!the!membrane!in!the!extracellular!space.!It!is!specific!for!LDL.!!

Lipoprotein6lipase1(LPL)1 The! structure! is! composed! by:! cytoplasmic! domain,! small! transmembrane! domain,!

LPL! is! an! enzyme! that! works! outside/of/the/cell.! It! is! associated! to! endothelial! cells! and! it! is! glycosylation! site! and! a! Cys;rich! domain;! there! are! also! 3/ EGFOlike/ domains! that! are!

anchored/to/the/membrane/by/an!eparanOsolfate/structure!to!put!the!catalytic!site!far!from!the! interrupted!by!a!β;propeller!that!is!functional!to!the!conformational!changes!and!can!

membrane,!limiting!the!steric!interaction!between!chylomicrones/VLDL!and!the!membrane.! determine! the! release! of! LDL! from! their! receptor! when! they! are! in! the! endoplasmic!

! compartment!of!the!cell.!!

The!enzymatic!activity!is!a!lipase!activity!that!works!

on!triacylgycerol/associated/to/the/lipoproteins.!!

In! order! to! work! on! VLDL! and! chylomicron! LDL-R β O-glycosylation site

associated! trigacylglycerol,! the! enzyme! needs! a! coO Cys rich domain Prop

EGF-like domain eller

activator!that!is!ApoC6II,!so!the!lipoprotein!needs!to! cytoplasmic domain

TM domain !

exchange!the!apoproteins!and!pick;up!ApoC;II.!! ! LDL

LPL;ApoC;II!catalyses!the!removal/of/fatty/acids/from/ SR6A1 /1 CD36!→!they!are!scavenger/receptors!that!can!bind!

•

tri/diOacylglycerol! that! are! then! transported! to! LDL,!but!they!have!a!difference!in!the!expression!and!in!the!

extrahepatic!cells.! affinity! for! the! lipoproteins! and! in! the! possibility! to!

This! protein! is! expressed! specifically! in! endothelial! regulate! the! receptor! levels:! they! cannot! be! regulated! and! CD36

SR-A

cells!near!to!adipose!tissue!and!muscle!cells.! they!are!not!specific!for!LDL.!

! !

LPL!and!ApoCII!are!under!the!control!of:! !

; Insulin!→!increase!the!level!of!LPL!and!the!amount!of!ApoC;II! !

; PPAR!α!→!increase!the!level!of!LPL!and!decrease!the!amount!of!ApoC;II! !

So!both!insulin!and!PPARα!are!able!to!increase!the!uptake!of!fatty!acids.! !

LDL-R β

! 5! ! 6! O-glycosylation site

Cys rich domain Prop

EGF-like domain eller

cytoplasmic domain

TM domain

CD36 receptor

Mechanism/of/action// CD36/receptor/

LDL;R! is! localized! in! a! specialized! region! of! the! membrane! that! is! a! clathrin;associated! In! the! structure! of! the! CD36! receptor! we!

invagination;!after!the!binding!of!LDL!we!have!the!internalization!of!the!LDL;LDLR!structure.! can! see! all! the! particles! that! can! be!

Upon!the!change!in!the!pH!of!the!environment!(in!endosomes),!the!increase!in!H+!ions!causes! associated! to! the! receptor! such! as! OLDL,!

the!change/of/the/chemical/feature/of/amminoacids!(like!Asp)!that!become!dissociated!and!so! AGE! and! other! elements,! including!

more!hydrophobic.!! apoptotic!neutrophils./

In! this! condition! the! Asp! region! can! collapse! with! the! binding! domain! of! the! receptor:! the! This!receptor!is!able!to!interact!and!assume!

collapse!is!associated!to!the!release!of!the!particle.!! LDL!that!are!partially!modified!as!oxidised1

Then! the/ receptor/ can/ be/ recycled! and! the! LDL! reaches! the! lysosomal! district! where! it! is! LDL:! they! are! lipoproteins! in! which! the!

degraded! in! its! lipid! and! protein! components;! there! is! no/ recycling/ of/ ApoB100/ but/ it/ is/ oxidative!stress!can!generate!oxidised/lipids/

LDL Receptor regulation

degraded/as!well./ and/aminoacids/in/the/ApoB100/protein! and!

! the! oxidation! products! of! both! lipids! and!

Regulation/of/LDLOR/levels/ aminoacids! can! generate! even! more!

The!synthesis!of!this!protein!can!be!regulated!(different!from!scavenger!receptors)!through!a! complex! structures! that! can! form! imminoO

SREBP/SCAP

mechanism! that! shares! the! regulation! of! HMG;CoA! reductas,! another! important! protein! in! derivatives! between! the! products! of! lipid!

cholesterol!metabolism.! oxidation!and!protein!oxidation.!!

/ This!modification!changes!the!surface!of!the!

lipoproteins! that! consequently! changes! the! affinity! of! the! lipoprotein! for! the! receptor:! the!

oxLDL!is!recognized!by!a!scavenger!receptors,!that!is!a!low!specific!receptor!able!to!remove!

something!that!is!toxic!and!has!to!removed!from!the!blood!stream.!!

CD36!is!usually!expressed!by!macrophages,!so!modified!lipoprotein!can!be!assumed!by!these!

Increased cells,!but!since!CD36!cannot!be!regulated,!there/is/no/limit/in/the/accumulation/of/cholesterol/in/

LDLR macrophages!/

synthesis ! 1.2.17!

CD36!is!the!receptor!involved!in!the!uptake!of!oxidized!LDL!as!well!as!other!proteins,!but!also!

particles,!and!the!difference!between!the!LDLR!is!the!possibility!to!regulate!its!expression!in!

/ the!membrane.!CD36!plays!a!role!in!the!definition!of!the!atherosclerotic!lesion.!

1 !

SREBP! (Sterol/ Responsive/ Element/ Binding/ Protein)! is! a! protein! that! can! be! associated! to! a! LDL/oxidation/and/foam/cells/

sterol!responsive!element!at!the!level!of!DNA!and!generate!the!transcription!of!target!genes,! The!atherosclerotic!lesion!can!derive!from!an!increase!in!cholesterol!in!the!blood!stream!but!

for!example!the!gene!of!LDL;R.!! there! are! other! risk! factors! that! can! affect! the! pathogenic! mechanism! associated! to!

SREBP!is!synthesized!in!the!ER!where!it!is!associated!to!SCAP!that!is!an!adaptor!protein:!the! lipoproteins.!The!general!mechanism!is!the/wrong/uptake/of/lipoproteins.!

complex! SREBP/SCAP/ moves/ from/ the/ ER/ to/ the/ Golgi! where! there! is! the! first/ cleavage/ that/ Cholesterol!accumulated!in!foam!cells!doesn’t!derive!from!circulating!lipoproteins.!

removes/ SCAP! than! there! is! a! second/ cleavage! that! generates! the! active/ fragment! from! the! !

product!of!the!protease!activity.! The! main! source! of! cholesterol! are! the! native! LDL,! but! when! we! incubate! native! LDL! with!

This/fragment/reaches/the/nucleus/and/works/on/DNA./ macrophages/muscle! cells/endothelial! cells…! we! don’t! obtain! any! accumulation! of!

! cholesterol!inside!the!cell,!even!with!a!very!high!concentration!of!native!LDL.!!

When! the! level! of! cholesterol! and! cholesterol! derivatives! increase! in! cell,! they! can! bind! to! Why!can!we!accumulate!cholesterol!esters!in!macrophages!starting!from!native!LDL?!!

SCAP:! the! binding! prevents/ the/ movement/ of/ the/ SREPB/SCAP/ to/ the/ Golgi,! blocking! the! Even! in! macrophage! the! mechanism! of! LDL! uptake! is! under! control! of! LDLR! (that! can! be!

generation!of!the!active!form!of!SREBP.!! controlled),!and!native!LDL!that!can!be!internalized!only!through!LDLR!are!not!accumulated!

So!when!the!cholesterol!level!in!the!cell!increase!there!is!a!stop!mechanism!that!prevent/the/ into!macrophages;!so!you!need!a!modification!in!the!LDL!in!order!to!have!the!accumulation.!

assumption/of/cholesterol/from/the/outside/and/inhibits/the/synthesis/of/cholesterol/to/maintain/ We!have!two!models!of!modification:!

the/levels/of/cholesterol/based/on/the/need/of/the/cells./ ; Acetylation/at/the/level/of/ApoB100/aminoacids/

! ; Oxidized/LDL! it! occurs! both! at! the! level! of! the! ApoB100! and! phospholipids,! which!

This!mec