Tissue metabolism and regulation

digestive gland (bile)

numerous metabolic functions (proteins, lipids, carbohydrates, vitamins, oligo-elements), antitoxic function

storage, oxidation & synthesis

nutrients from the intestine have to pass the liver through portal perfusion before entering central circulation

organ with a fibrous capsule = Glisson capsule

2 lobes in humans, more in rats and mice

most voluminous organ of the human body (1.8-1-9 kb with 800 g blood)

high capacity to regenerate (in rat:ablation of 70% and recuperation in 7 days)

—>double afferent vascularisation: hepatic portal vein (75% of blood), blood from the digestive tract and the spleen, hepatic artery (25%) bring oxygen for liver activities

—>efferent vascularisation = centrolobular veins —>sus-hepatic veins —>inferior vena cava ( one collects everything and sends it to the heart)

50 000 to 100 000 / liver —>functional unit

composition: hepatocytes (80%), endothelial cells, macrophages (Kuppfer cells), stellate or Ito cells

blood from exterior to interior

voluminous cell

numerous organelles (mitochondria)

glucagon sensitive

organized into plates separated by vascular channels

supported by reticulin (collagen type III) network

two functional poles: exchange surface biliary pole —> bile, vascular pole

bilateral transport of molecule: hepatocytes —>blood

peripheral zone (16 % O2): peripheral hepatocytes oxidative—>use lipids as energy source, release glycogen if needed

centrilobular vein (8% O2): perivenous hepatocytes glycolytic, use glucose as source of energy

storage in glycogenogenesis

mobilization in glycogenolysis

neoglucogenesis

regulated by insulin & highly sensitive to glucagon

GLUT2 release of glucose

GLUT1 &2

Glucokinase slow—>traps glucose as G6P inside the cell due to irreversible coupling —>ATP hydrolysis

specific hexokinase (HK IV) of the liver

with high Km —>speed of irreversible trapping of glucose inside the cell dependent on [Glc]

glucose sensor

stimulated in longterm with insulin

not inhibited by G6P

for energy production (glycolysis, Krebs cycle)

storage (via G1P & UDP-glucose added in glycogen)

lipid production: via acetyl-CoA & malonyl-CoA to fatty acid

glucose supply of the liver vital to maintain glycemia

sources of glucose: blood glucose in minutes

hepatic glycogen in 10-12 h

production of glucose in the liver

glycemia in the blood quite stable

after meal glycogen storage produced that is destroyed during the night to produce glucose

hepatic glycogen increases with glucose and insulin

glucose only energy supply of the brain and red blood cells

Glycogen synthesis (liver and muscles)

glucose —>G6P—>G1P—>UDP-glucose—>glycogen (n+1 glucose)

glycogen degradation until G6P in liver and muscles that can be used in glycolysis

to glucose in liver, glucose can enter the circulation

glycogen glucose monomer with n=5000-30000 (branched)

α-1,4/1,6-linkages with branching enzymes

100g in liver & 300 g in muscle

phosphoglucomutase for isomerization of G1P

UDP-glucose-phosphorylase for UDP-glucose

glucogen synthase adds UDP-glucose to glycogen

alternative substrates: galactose, G6P, fructose

direct pathway 40%

indirect pathway 60%: from G6P first to other neoglycogenic substrates

Glycogen phosphorylase major enzyme for degradation of glycogen removes 1 molecule of glucose

glycogen synthase active if not phosphorylated & inactive if phosphorylated

phosphorylation via cAMP—>PKA—>synthase kinase

activation via protein phosphatase (ISPK)—> insulin-stimulated PK —>insulin pushes storage

insulin also decreases the level of cAMP via IRS-1 which activates phosphodiesterase

glucose: increased glycogenogenesis by inhibited glycogen phosphorylase by allostery binding of glucose (no glucose released if high [Glc], actived glycogen phosphorylase if phosphorylated

increases cAMP —>protein kinase A activated (phosphorylation)

activates glycogen phosphorylase

inhibits glycolysis (PFK-1 & pyruvate kinase)

—>increase in release of glucose into the circulation with glucose-6-phosphatase—>increased glycogenolysis

inhibition glycogen synthase (inactivating phosphorylation)—>decreased glycogenogenesis

no glucagon receptor present in the muscle

decrease in cAMP

glycogen synthase active —>increased glycogenogenesis

via adrenergic recetpors

activation of PKC

Ca2+ calmodulin complex activates in the presence of Ca2+ phosphorylase kinase

activation of glycogen phosphorylase

liver: glycogen —>G6P—>glucose—>released in blood (G6Pase important to release glucose in blood for other organs)

muscle: glycogen—>G6P—>pyruvate (in situ utilization of glycogen via glycolysis)

glucose synthesis form non carbohydrate precursors in liver (and kidney)

specific enzyme in the liver = glucose-6-phosphatase

G6P can be built from glycerol, 2 pyruvate (can be formed from alanine or glutamine) or 2 lactate

no neoglucogenesis from FA

need high levels of ATP (energy cost of 6 ATP)

for glycogenolysis no ATP needed

via lactic acid cycle (cori cycle)

lactate produced in muscle under anaerobic conditions and transported to the liver via blood

6 ATP needed to be transformed into glucose

glucose-alanine cycle

transamination of pyruvate to alanine to go into circulation (muscle proteolysis)

alanine from muscle—>blood—>liver

3 irreversible reactions reversing glycolysis, need energy

1. 
   

   • Pyruvate carboxylase: in mitochondria of liver & kidney, transforms pyruvate into oxaloacetate (ATP hydrolysis) using acetyl-CoA

   stimulated by acetyl-CoA & high ATP/ADP

   inhibited by malonyl-CoA & oxaloacetate

   • transfer to oxaloacetate: into the cytosol via transformation to malate using NADH—>malate transporter and reverse in cytosol back to oxaloacetate

   • phosphoenol pyruvate carboxykinase PEPCK: in human 50% in cytosol and mitochondria, no regulator known

     —>control via gene expression (halflife short with 6 hours)

     transcription activated via fasting, cAMP (glucagon), PPARs

     inhibition via food intake, insulin, glucose, adiponectin

2. Fructose-1,6-diphosphatase: F1,6BP—>F6P (ATP hydrolysis), activity-dependent on F1,6BP (the more the higher activity)

3. Glucose6phosphatase: G6P—>glucose, in periportal liver (give it to periphery & uses lipids itself), transmembrane enzyme of the ER

activator: none known, long: fasting & cortisol

inhibitor: F1,6P, glucose, ATP, Pi, citrate, α-ketoglutarate, unsaturated FA, insulin (long)

and regulation also via quantity of enzymes (transcriptional regulation)

high ATP/ADP: when high, de novo synthesis of glucose is possible via stimulation of enzymes needing ATP & inhibition of major steps of glycolysis

high NADH/NAD+: high NADH inhibits glycolysis

high acetyl-CoA: decreases the use of glucose

high citrate: major product of Krebs inhibits glycolysis

decreased glycolysis (use of glucose) and oxidation of glucose = preservation of glycemia

increased glucose production (neoglucogenesis)

in the liver via GLUT2 released into circulation after G6P dephsophorylation

synthesis of lipoproteins (chylomicrons)

Ketogenesis: synthesis of Ketone bodies

synthesis of cholesterol & phospholipids

lipogenesis (conversion of carbohydrates and proteins into lipids)

no storage of triglycerides except in pathologies

classified according to their size, density & composition

HDL (high-density lipoprotein), LDL, IDL (intermediate), VLDL (very low)

consist of: apolipoproteins, triglycerides, phospholipids, free cholesterol

chylomicrons first lipo particle after intestine, during distribution to peripheral tissues decrease in size

de novo synthesis in the liver from FA to VLDL

synthesis of fatty acids

in the cytosol of liver and adipose tissue from acetyl-CoA

acetyl-CoA from mitochondria transferred to cytosol

acetyl-CoA —> malonyl-CoA via acetyl CoA carboxylase ACC

successive reactions to the synthesis of palmitic acid (fatty acid synthase FAS)

regulation through ACC

in humans mainly hepatic but also in adipose tissue & mammary gland

1. when FA synthesis no oxidation

2. FA synthesis can start only when glucose and ATP are high in the cell to ensure energy needs (fed state, insulin)

3. FA synthesis needs mitochondrial acetyl-CoA and NADPH

4. In humans, the liver is the major site of FA synthesis (other adipocytes)

hormonal and nutritional factors

• diet rich sugars

• low-fat diet

• insulin

• glucagon

• glucocorticoids

regulate glycolysis & ACC

1. transfer of acetyl-CoA to the cytosol via citrate-malate (pyruvate) shuttle (transfer as citrate)

2. addition of CO2 via Acetyl-CoaAcarboxylase ACC under ATP hydrolysis (active by dephosphorylation, major regulation), ACC inhibited by malonyl-CoA (product of ACC)

3. each circle of fatty acid synthase adds 2 carbons

total ATP for synthesis of palmitic acid: 7 ATP & 7 acetyl-CoA

fed state, glucose present —>hyperglycemia

increased de novo lipogenesis (increase in FA, triglycerides, decrease ß-oxidation)

activates ACC

inhibits the use of lipids as a source of energy

at low levels of energy

via AMP-dependent kinase AMPK ACC inhibition (phosphorylation ACC)

AMPK: if depletion in ATP decrease of lipogenesis & increase in ß-oxidation

allows indirect use of lipids by the brain

in peripheral tissues, KB used in ketolysis

produced if insulin low & glucagon high (starvation)—>for production increase in lipolysis & oxidation of FA

only in the brain & muscle

degradation of proteins, AA

(release of AA, can go into circulation after deamination)

oxidation of AA

neoglucogenesis

ketone bodies ketogenesis

1. transamination: oxidation of amino group to ketone allows circulation in the blood

1. desamination: amine group removal hydrolytic /release of ammonia) or oxidative (major in physiology)

2. transport of ammonia to liver: in the form of glutamine, formation of glutamine first line of defense against ammonia toxicity

   transport of alanine: degradation of proteins in the muscle —>ammonia production —>toxicity

   alanine in liver transformed into pyruvate

   most other tissues send glutamine to liver ->transformed into glutamate and release of ammonium

for elimination of ammonium and regulation of blood pH

high energy cost: 3 ATP

in periportal zone

precurosors for neoglucogenesis = glucognic

precursors of acetyl-CoA = ketogenic

alcohol metabolism: can be used as a source of energy (via alcohol dehydrogenase to acetaldehyde—>acetate—>Acetyl-CoA—>Krebs)

if high concentration or chronic: microsomal ethanol oxidizing system

xenobiotics metabolism: biliary or urinary elimination

steatosis: storage of triglycerides

can be reversed by changing the diet, linked with obesity

caused by chronic consumption of alcohol—>excess of acetyl-CoA stimulates storage of lipids

mainly lipid & glucose metabolism

storage of triglycerides for own needs (main storage in adipose tissue)

transport of fatty acids in mitochondria via CPT & used for production of energy

also storage of glycogen

at rest: muscle use FA as main fuel source

short-time fasting: muscle still use FA from adipose tissue and ketone bodies from the liver

long-term fasting: muscles exclusively use FA from adipose tissue and let ketone bodies for the brain—>proteolysis furnishes liver with AA for glucose production

post-prandial: insulin increases glucose uptake via GLUT4 and muscles use glucose as main fuel source—>storage

storage—>glycogenogenesis

glycogenolysis

hormone regulation: very sensitive to insulin

GLUT4

increase protein synthesis, glycogen synthesis & glucose transport (GLUT4 translocation)

signaling cascade via PI3-kinase

2 different GLUT4 pools for stimulation of insulin via PI3 kinase and AMPK signaling (under anaerobic conditions)

also GLUT1

hexokinase II transforms into G6P irreversible (ATP hydrolysis)

for energy production (glycolysis, krebs), anaerobic glycolysis (lactate)

storage as glycogen

activation via: cell work, low ATP/AMP, low NADH/NAD+ (low energy), after food intake (insulin)

inhibition: at rest, oxidation of FA, low cell work (ATP/AMP & NADH/NAD+ high)

small local storage of energy as triglycerides

ß-oxidation

lipogenesis (less active in the liver)

ketone bodies hydrolysis

depends on CPT1 activity (transfer or malonyl-CoA into mitochondria)

carnitine palmitoyltransferase I transports fatty acyl-CoA by exchange to acylcarnitine

also carnitine-acylcarnitine translocase (CAT)

activated via: AMPK, T3, fasting & PPARs

inhibited by: high malonyl-CoA & insulin

also controlled by redox: high NADH/NAD+ & FADH2/FAD decreases ß-oxidation

controlled by [acetyl-CoA]/[CoASH]

inhibition of glucose oxidation by fatty acids

use of one source of energy inhibits the degradation of another source of energy

if lipids high: ß-oxidation and Krebs —>production of citrate —>inhibits glycolysis at different steps (mostly at F6P but also entry and PDH)

if glucose is high (insulin): increase in malonyl-CoA inhibits CPT1 (pyruvate—acetyl-CoA excess transformed in cytoplasm to malonyl-CoA—>inhibits CPT-1 & blocks entry of FA

decreases AMPK activity

AMPK directly regulates ACC

increase in malonyl-CoA & inhibition CPT-1

pushes glycogen synthase to store glucose (active glycogen phosphorylase (phosphorylation))

hormonal changes: decrease in insulin & release of epinephrine

increased production of lactate in the muscle

in the liver increased glycogenolysis & gluconeogenesis to provide glucose (glucagon)

no glucagon receptor in the muscle

but activation of glycogenolysis by epinephrine (ß-receptor activates kinase phosphorylation—>breakdown glycogen)

signals need for energy: phosphorylation of ACC—>decrease malonyl-CoA, transport of FA & ß-oxidation

in skeletal muscle: fatty acid uptake & oxidation, glucose uptake & mitochondrial biogenesis

liver: fatty acid synthesis, cholesterol synthesis, gluconeogenesis inhibited

adipose tissue: inhibition FA synthesis & lypolysis

energy storage and thermoregulation

white and brown AT

20% BWT in human but can be >80%

endocrine function—>100 secreted factors (90% by associated cells)

fat subcutaneous and visceral

mostly lipid metabolism (storage of FA (TG), mobilisation (lipolysis))

regulated by: insulin (GLUT4)

glucose uptake via GLUT1 or 4

storage of lipids

mobilisation of lipids

secretion (endothelial cells, pre-adipocytes & macrophages)

lipids come from the intestine as Chylomicrons

synthesis of FA in WAT & release to reach the periphery

ACC (acetyl-CoA carboxylase)

FASN (fatty acid synthase)

ester of 3 FA with glycerol

=lipogenesis / FA esterification

storage form of FA

needs glycerol-3-phosphate

produced by the liver from glycerol (glycerol kinase)

no direct production in the adipose tissue (no glycerol kinase), but indirect via glycolysis (uses intermediate from degradation from glycolysis)

translocation of GLUT4 (in AT only one pool)

no pool of GLUT4 that is translocated upon AMPK signal

storage hormone

hormone sensitive lipase normally degrading TG —>release FA inhibited

strong anti-lipolytic action by decrease of cAMP & decrease of PKA

signal need of energy

activeates protein kinase A

phosphorylation of HSL (on serine residue) leads to increased lipolysis —> glycerol & FA released

more than energy storage: thermoregulation & endocrine function

white & brown adipose tissue

lot in rodents & babies

in adults: cervical, supraclavicular & paravertebral—>next to vertebrates

role not known

subcutaneous, intra-abdominal around organs

main function storage of lipids

big lipid vacuole

thin membrane —>few mitochondria

some lipid droplets but mostly mitochondria—>give color

responsible for ß-oxidation and thermogenesis

regulated as a function of stress

—>regulate heat production

essential role of UCP1—>increased leak

energy from ß-oxidation induced by cold/stress via cAMP

UCP1 responsible for hihg level of ß-oxidation

from precurosr to preadipocytes to brown adipocytes

preconditiong depends on adrenergic stimulation

GLUT4 translocation stimulated by insulin

use glucose as energy source post-prandial

also constitutive GLUT1 at cell curface

detects BAT with radiotracer 18F-fluoro-deoxy-D-glucose

normally used for cancer & metastasis but after exposure to cold also increased glucose uptake in BAT —>background signal

not visible under theromneutral conditions

positron emission tomography

radioactive ß+ emitter molecule used—>emission of 2 γ photons apart from 180°

—>rearrangement of nucleus

high sensitivity

ring detector for static acquistion

also dynamic imaging possible

glucose: transported by GLUT, substrate of hexokinase, metabolized

18FDG: transported by GLUT, substrate of hexokinase, not metabolized—>trapped inside the cell

from pre-adipocytes

maturation stimulated by Insulin, IGF-1, glucocorticoids, AG

due to exercise training derived from scWAT

in between brown & white: storage of lipids & many mitochondria

increased mitochondrial biogenesis & mitochondrial activity

no maturation from brown but developed from white with exercise or drugs

PPARs

large familiy inducing gene transcription

PPAR α-γ have polyinsaturated FA as ligand—>lipids sensor

thight regulation & association with nuclear factors

without ligand: corepressor complex

ligand-independent actiovation: co-activator complex & target gene transcription

ligand-dependent activation: different conformation due to bound ligand increases level of transcription

depending on tissue different isoforms, many adverse effects if stimulated with frugs —>activation of many gene clusters

in childhood/adolescence: hyperplasia (increase in adipose number)

in adluthood: hypertrophy (increase in adipocyte size)

leptin & adiponection important

appetite control through the CNS

encoded by ob gene

db gene: leptine receptor

fat mass informs the brain for the level of energy requirement (=lipostatic hypothesis)

—>proportional to body fat

Insulin sensitizer, anti-inflammatory

cross-circulation (shared bloodstream)

connect animals with different qualtities / conditions

investigate how blood factors influence helath

lesion in hypothalamus (ventromedial nucleus)

WT eats less & weight loss —> appetite suppressing factor & site of action in hypothalamus

obesity ob/ob mouse without parabiosis

ob/ob will eat less & weight loss —>appetite-suppressing factor not produced but acts on hypothalamus

db/db mouse has obesity & diabetes (w/o parabiosis)

no change in db/db —> site of action not present

normal mouse: weight loss because non-functional receptor leads to overproduction of leptine in db/db mouse

ob/ob mouse eats less, weight loss

no changes in db/db mouse—>no functional receptor and overproduction of leptine that works on functional receptors of ob/ob mouse

in the ventromedial nucleus of the hypothalamus

regulation of the appetite

acts through dimerization & autophosphorylation (tyrosine kinase)

if truncated or spontaneous mutation (in Jak-STAT box motif) no full signaling acitivity

STAT= signal transducer and activator of transcription

regulated by: PTP1B protein tyrosine phosphatase 1B & SOCS suppressor of cytokine signalling

activation of transcription inhibiting the appetite

if high: reduced food intake & increase energy expenditure

hypothalamus receives & integrates neural, metabolic & hormonal signals to regulate homeostasis

PYY (large intestine), ghrelin (stomach), NPY (neuropeptide Y), agouti related prot neurons lead to orexigenic signals

effect: increased appetite, decreased energy expenditure & thermogenesis

leptin (adipose tissue), insulin (ß-pancreatic vells), pro-opiomelanocrotin POMC, cocaine & amphetamine related transcript neurons CART & α-MSH (meloncyte stimulating hormone) lead to anorexigenic signals

effect: decreased appetite, increased energy expenditure & thermogenesis

mainly secreted by the adipose tissue

has collagen domains

most common: globular adiponectin

↑ glucose uptake (muscle) & fatty acid oxidation (liver&muscle)

↓ gluoneogenesis (liver)

AdipoR1 & AdipoR2

activates adaptor protein APPL1 (adaptor protein containing a pleckstrin homology domain)

phosphorylates other proteins: p38MAPK (increased GLUT4 translocation), ACC (FA oxidation), eNOS (vasodilation)