Week 7D: Liver Metabolism Flashcards
HC 46, 47
HC46: Lymph from capillaries
Capillary bed > blood plasma from arteriole to interstitial space and re-uptake in venule
> excess fluid and macromolecules drain into permeable lymphatic capillaries
Where do the lymph circulation flow to?
The subclavian vein to flow into blood
Lymphatic capillaries are derived from … cells
Venous endothelial cells
Lacteal
Blunt ended lymphatic vessel in villus of intestine
> drainage dietary lipids in intestine
> villi: large surface, quick and much uptake
> excess liquid collection
> excess liquid pushes it down into the lymph
Oil red O staining
After olive oil diet to mice
> little lipid accumulation in lamina propria: efficient transport through the lymph
Button junctions
Specialized, discontinuous junction between lacteal endothelial cells with open and closed regions
> allows chylomicron uptake (made in enterocytes)
> lipid through lymph, it cannot enter the blood stream directly
Junctions in endothelial cells and collecting lymphatics
Zipper junctions > tightly seal the ECs > no passage chylomicrons
Villi lengths in intestines
Duodenum > jejunum > ileum
Advantage chylomicrons to lymph
Gets to heart as first organ
> needs fats for energy (much energy needed): beta oxidation
> uses more fat than glucose
> high energy macronutrients
Reaction lymph capillaries to Vascular Endothelial Growth Factor (VEGF)
Lymph angiogenic signal transduction
> to express proteins to make zipper junctions
> stepwise proteolytic activation VEGF-A and binding VEGFR-2: pathway to zipper junctions
VEGF signalling in lacteals
VEGF binds to decoy (NRP1/FLP1) RTK on blood EC > limit VEGF binding to VEGFR-2 > resulting in discontinuous button junctions
-Waste of signal: no formation zipper junctions
Transition of button-to-zipper junctions
Inducible genetic deletion of decoy Nrp1/Flt1 increase bioavailability of VEGF and signalling through VEGFR-2.
> zippering up the lacteal junctions: prevent chylomicron uptake
Lipid droplet organelle: protein function
Regulate size and fusion etc
> on the outside layer
Lipid droplets in muscle
-Intramyocellular lipid storage
-Dynamic organelles
-Coated with proteins for regulation
-Independent or bound to mitochondria (couple to beta oxidation)
Core content of lipid droplets
TAGs and cholesterol ester (CE) > neutral lipids: hydrophobic
Outer layer lipid droplet
Monolayer phospholipids
Proteins on membrane lipid droplets from lipid metabolism
Lypolysis enzymes
> ATGL: adipose triglyceride lipase (TAG>DAG)
> HS lipase: hormone sensitive lipase
> Monoglyceride lipase
-Activated in glucagon/adrenalin signalling
Lipid synthesis and storage in liver
Temporary storage in liver lipid droplets and then to make VLDL or degrade in beta-oxidation
LC3 and lipid droplet
Receptor on phagosome > can bind lipases for complete degradation of lipid droplets through autophagy
After activation of lipids when entering the cell (Acyl-CoA), the only fate is not beta oxidation (committed step is transport in mitochondrion), other fates?
Storage in lipid droplet
> secretion as VLDL (liver)
> signalling: via PPARs
> make complex lipids in ER
Biogenesis lipid droplets > where?
Organized by proteins in ER > between two monolayers of ER
> cholesterol synthesis also in ER
What is done with DAGs and cholesterol in ER membrane to store them between the monolayers of the bilayer?
Esterification to TAGs or CEs > hydrophilic environment
> also a lot of proteins make it into monolayer of lipid droplet
Major steps lipid droplet biogenesis
-Nucleation
-Growth
-Budding
Membrane proteins from lipid droplets derived from…
ER membrane
> or made in cytosol and adhesion later
Lipid droplet nucleation
Synthesis lipids in ER > aggregate to form lens-like structure between ER membrane leaflets
SEIPIN function
Protein that forms ring in ER cytosolic leaflet
> keep lipids together > allows monolayer to bud, otherwise spontaneous reformation bilayer
Similarities and differences lipoproteins and lipid droplets
-Both consist of lipophilic core with TAGs and CEs surrounded by phospholipid monolayer
-Lipoproteins decorated by defined set of apolipoproteins that bind to the surface via amphipathic alpha-helices and beta-strands
-Lipid droplet proteome is highly diverse and dynamic for fusion, growth, shrinkage and fission: more organization
Budding lipid droplet
Proteins in ER membrane involved
> Sterol esterified to esterified sterol (CE) and Glycerol-3-P to TAG
> SEIPIN oligomer associates and forms pore around the neck of the buds with the help of other proteins
Class I and Class II proteins of lipid droplet
Class I proteins: inserted into ER and trafficked from ER to nascent lipid droplets
Class II proteins: inserted into lipid droplets directly from cytosol
What happens in lipid droplet biogenesis when Seipin KO
You can make lens-like structure, but budding cannot happen
Seipin disrupts which forces?
Hydrophobic forces in lipid droplet budding are blocked. These forces interfere with formation monolayer
Multiple functions Seipin, also in growth
Seipin
> stabilizes structure by surrounding neck of forming lipid droplet
> neutral lipids are channeled into nascent lipid droplet core: growth
> phospholipids are supplied from cytosolic leaflet of ER membrane
Dynamic size lipid droplet
-Shrinkage when TAGs or CEs are used > increased protein density > the weakest bound proteins will dissociate first to compensate
-At cytosolic side ER: proteins for lipid droplets are synthesized
With which organelles does the lipid droplet interact?
Mitochondria, peroxisomes, lysosomes, ER
Types of milk secretion in mammary glands
Lipid droplets in mammary gland
> pathway milk secretion involves merocrine and apocrine secretion
Merocrine secretion
Exocytosis via vesicles
Apocrine secretion
Secretion via lipid droplets: apical cytosol buds of surrounded by plasma membrane
> Total of monolayer (lipid droplet) + Bilayer (PM) = three membranes
What give milk white appearance
Light reflected back because three layers around lipid droplets: a lot of membranes with proteins
Holocrine secretion (does not occur in mammary gland!!)
Entire cell disintegrates > contents released
Pathway milk secretion during lactation
1: merocrine secretion: exocytosis milk proteins, lactose, calcium and other soluble components
2: apocrine secretion of milk fat globules with formation lipid droplets that move to apical side of cytoplasm for budding of surrounded by PM
3: vesicular transcytosis of proteins such as immunoglobin from intersitial space to lumen
4: transporters for direct transfer of monovalent ions, water and glucose
Difference lipid droplets and lipid globules
Globules are very large and droplets smaller
HC47: Where placement lipid droplets in muscle fibers?
At specific locations: in mitochondrial network: efficient shuttling of FAs to mitochondria
> first lipolysis to FAs
Muscle fuels
Glucose, lipids, ketone bodies
Muscle fibers and organization lipid droplets
Type I: slow twitch, mitochondria dependent, high use oxygen and myoglobin, lots of lipid droplets
Type II: fast twitch, less mitochondria, anaerobic glycolysis, less lipid droplets
Lipid droplets in muscle in trained state
Fat stored in more and smaller lipid droplets connected to (more) mitochondria
Lipid droplets in muscle of T2DM patient
Change organization
> less association with mitochondria
Lipid droplet organization in liver
Multiple contact points to orchestrate lipid metabolism
Lipid droplets to feed VLDL
Start with ApoB100: synthesis on RER
> protein for secretion of VLDL enters lumen ER
Droplets via secretory pathway: vesicles to Golgi to exocytosis of the vesicle with the VLDL particle inside it
> lipid droplets need to associate with ER to feed VLDL (in hepatocytes with lots of TAGs)
Other association lipid droplets in liver beside ER for VLDL feeding
Mitochondria or autophagosome
Which transporters transport FFAs into hepatocytes
CD36 and FATP
> FFAs in blood bound to serum albumin
High uptake FFAs by liver when…
Fasting
> from adipocytes
Obesitas glucose and insulin levels and result for beta cells
Same patterns as healthy
> during night, higher blood glucose
> Insulin levels (C-peptide) way higher in obese than healthy, to keep blood glucose levels at lower levels
> overactive beta cells
-Make sulfide bonds to cleave C-peptide, make H2O2 (hydrogen peroxide)
-Beta cells are compensating > insulin resistance.
Insulin receptor activation
Conformational change after binding insulin: from inverted-V conformation to T-conformation
> insulin receptor synthesized as one large protein: cleaved to alpha and beta parts
> receptor: heterotetramer: two alpha and beta from two insulin receptor primary products
> In inverted-V conformation: no trans-autophosphorylation, tyrosine kinase domains (beta) lay far apart
> T-conformation, tyrosine kinase domains in close proximity: trans-autophosphorylation and fully active tyrosine kinases: signal transduction
PKB activation
Insulin receptor binds IRS which binds PI3K which makes PIP3 from PIP2
PDK1 and PKB bind PIP3 in membrane
> PDK1 active: phosphorylation and activation PKB by PDK1
> dissociation active PKB/Akt
Insulin resistance in obesity
TAGs stored in lipid droplets (used for VLDL for example)
> TAGs made from DAG from Glycerol-3-P and 2 FAs
> accumulation TAGs: accumulation second messenger DAG
> DAG activates PKC theta (muscle) and epsilon (liver) isotypes
> PKC (serine/threonine kinase) phosphorlates threonine in activation loop of insulin receptor (where normally trans-autophosphorylation)
> phosphate group with negative charge and water shell prevents normal phsophorylation (not removed)
> some (20%) of insulin receptors not activated upon binding insulin
Result insulin resistance
-Hyperinsulinemia, hyperglycemia, hypertriglyceridemia
-Liver
> Higher gluconeogenesis (more signals glucagon)
> higher FA and fat syntesis (fatty liver)
-Adipose tissue
> Lower glucose uptake (no GLUT4)
> Higher lipolysis: more effect glucagon
-Skeletal muscle
> lower glucose uptake
> more FA uptake and fat storage
Obesitas to T2DM
Obesitas
> insulin resistance and hyperinsulinemia
> exhaustion pancreatic beta cells
> T2DM
Exhaustion beta cells
Insulin release after increased glucose production, but eventually beta cells die due to oxidative stress because overhours to make insulin in obesity (then hyperglycemia chronically).
> body cannot make insulin anymore
Reversible part in onset T2DM
Before the beta cell dysfunction
> reversible with life style changes
> genes in beta cells determine ‘survival’ after irreversible state: how long can beta cells make excessive insulin > after that: T2DM
Liver functional unit
Lobule
Lobule structural organisation
Portal triad: portal vein, hepatic artery and bile duct
> Between hepatocytes (at basal membrane) there are little vessels with loose endothelial cells and Kupffer cells lining within for blood flow towards central vein: sinusoids
> at apical membranes: bile canaliculli for bile flow towards bile duct
Gradient oxygen and metabolites in lobule
Portal triad to central vein
> high oxygen and nutrients to lower
Metabolic zoning of lobule
From high oxygen zone towards portal triad: Zone 1, and gradient to central vein to Zone 2 and Zone 3
Function space between endothelial cells lining sinusoids: liver sinusoidal endothelial cells (LSECs)
A lot of molecules, like chylomicrons and LDL can pass
Resident macrophages in liver
Kupffer cells
Quiescent stellate cells (qHSCs)
Produce collagen
> wrong collagen in injured liver > activation stellate cells: fibrosis: collagen on wrong positions, cells die and spaces filled in with scar tissue
Basal membrane of hepatocytes (face sinusoids) contain
Microvilli
Injured liver structure
Hepatocytes lose microvilli and finally their function, associated with ECM deposition from activated hepatic stellate cells (aHSCs) favoring progression to cirrhosis and finally hepatocellular carcinoma
Hepatic stellate cells are located at …
Space of Disse: between hepatocytes and liver sinusoidal endothelial cells (LSECs)
Collagen fibers in healthy vs injured liver
Healthy: collagen IV and VI > provide perfect scaffold for architecture and function of space of Disse
Injured: collagen is replaced with colalgen I and III fibers, fibrotic fibers
Bile salt function
Emulsifier of fats in intestinge
> surface active molecules
> amphipathic: hydrophobic and hydrophilic site
> like cholate
> solubilize large fat globules to small fat droplets with hydrophilic outside made up by the bile salts
Pancreatic lipase activation
Activated by co-lipase
> Co-lipase binds only to lipid droplets when there are bile salts there
> Co-lipase connects pancreatic lipase with lipid droplets
> breakdown TAGs to MAG and 2 FAs
> uptake by enterocytes
Bile acid formation, structure substrate and product
From cholesterol (C27) to bile acids (C24)
> remove three carbons to make for example cholate
> cholesterol is very hydrophobic but weakly amphipathic
> cholate is very amphipathic
Committed step bile acid formation
Hydroxylation cholesterol at 3, 7, and 12 alpha carbon atoms
> by CYP enzyme: make hydroxyl 7a carbon (committed step)
Second part bile acid formation in the ….
Peroxisomes
Peroxisomal steps bile acid formation
Oxidation > hydration > oxidation > thiolysis (release propionyl-CoA)
(like the beta-oxidation cycle!)
Glycocholate and taurocholate are conjugated bile salts of cholate, name their characteristics
Unconjugated cholate: pKa around 6, more in protonated form in duodenum (pH around 6) as bile acid > pKa is larger than pH
-Glycocholate: pKa: 4
-Taurocholate: pKa: 2 > deprotonated form mostly, negative charge, micelles easier formed in emulsifying
Bile salt is the acid/alkaline form
alkaline
Which form, bile salts or bile acids can solubilize the cells (dangerous)?
Bile acids > mostly protonated
> neutral, no charge and amphipathic > can enter cell > deprotonated in cell (pH 7.2 in cell) > detergent: solubilize the cell
Bile acid form of cholate (bile salt)
Cholic acid
Biliary bicarbonate umbrella theory
Secretion of bicarbonate (HCO3-) to protect hepatocytes and cholangiocytes from cytotoxic effects of cholic acid (unconjugated bile acid)
> deprotonating the bile acids (cholic acid) and keep them in bile salt form (cholate)
Bile salts recycling
Enterohepatic circulation
> bile salts synthesized in liver, stored in gallbladder, secreted in duodenum, reabsorbed in terminal ileum and returned to liver by portal blood
