Akt/PKB, AMPK, mTOR

dysfunctional metabolic sensing of blood sugar

insulin normally stimulates the uptake of glucose from the blood

incidence rapidly increasing

genetically

no insulin production in the pancreas

target tissue doesn’t respond to insulin

triggered with age & lifestyle (consume more processed food (lipids) than glucides

central obesity

high blood pressure

high triglycerides

low HDL-cholesterol

insulin resistance

10-30%

increases with age

more frequent in woman

depends on ethnic origin

live longer with caloric restrictions

obesity, diabetes type 2, cardiovascular diseases

on the cellular scale: apoptosis, energy stress

enzymatic phosphotransfer

metabolic feedback

cell signaling by protein kinases

transcriptional regulation

extracellular signal molecule sensed by receptor protein in the membrane

intracellular signaling protein cascade—>transmission of activation state

activation target protein (altered metabolism, altered gene expression, altered cell shape or movement)

have on and off-state

reversible addition of phosphate group to proteins altering the nucleotide state of a small G-protein

signaling by phosphorylation: reversible phosphorylation of the protein

signaling by GTP-binding protein: exchange between GDP & GTP (GTP bound active protein)

Insulin receptor IR phosphorylated upon binding of insulin

phosphorylation cascade via insulin receptor substrate IRS

activation of Akt (central kinase, triggers many cellular pathways)

inhibits TSC2 which normally dephosphorylates Rheb to inactive Rheb-GDP

due to inhibition active Rheb-GTP

phosphorylation of mTOR—>activated mTOR leads to protein synthesis (use of energy for proliferation)

amino acids also activate mTOR, but in the absence of glucose TSC2 active & inactive Rheb-GDP—>reduced protein synthesis

energy: AMPK sense rati between AMP/ATP, activated at low levels of energy —>activates TSC2—>inactivation Rheb & mTOR inactive—>downregulation of protein synthesis (high energy consumption

via Akt

1. insulin binding to IR leads to phosphorylation of IRS-1 recruiting GRB2 (activation Ras pathway)

2. IRS-1 activates PI-3 kinase: phosphorylation membrane lipid PIP2 to PIP3 (back via PTEN)

3. PIP3 binds protein kinase Akt which is activated by other protein kinases

4. Akt catalyzes the phosphorylation of key proteins leading to an increase in glycogen synthase activity & recruitment of glucose transporter GLUT4

1. enzymes/transporters regulated in activity—>direct response in cytosol

2. goes into the nucleus & changes gene expression—>slow response, longterm changes

via mTOR

mTORC1 involved in insulin pathway & AA sensing —>ribosome biogenesis & translation

mTORC2 involved in WNT pathway (different subunits in heterotrimer)—>works on the cytoskeleton

also stimulation of gene expression & protein synthesis

via AMPK

AMP

ATP stable for a long time as regenerated via adenylate kinase (2 ADP —> AMP+ATP)

AMP concentration changes over 4 orders of magnitude, ATP rather stable—>better to sense ADP & AMP

in catabolism normally rephosphorylation of ADP to ATP

if limited/too much ATP consumed accumulation ADP & activation adenylate kinase—>accumulation AMP

activates AMPK (disequilibrium ATP/AMP) —>accelerated ATP regeneration and slows down ATP consumption

inhibition catabolism: under metabolic stress (hypoxia, glucose deprivation, metformin)

increased ATP consumption under cell growth/division, activation of motor proteins

allosteric activation: allosteric binding of AMP, conformational change exposes phosphorylation site

LKB1/CamKKß: phosphorylates AMP bound AMPK to activate it, itself controlled by endocrine signals & calcium (extracellular signals)

PP2Cα: phosphatase inactivating AMPK, inhibited by ADP&AMP

activated aMPK: increases catabolic pathways & decreases anabolic pathways

Glucose metabolism

—>increase glucose uptake (GLUT4/1), glycolysis

Lipid metabolism

—>increased fatty acid uptake, oxidation & synthesis

protein metabolism

—>decreased rRNA & protein synthesis

mitochondrial biogenesis & autophagy

complex of 3 subunits

α chain with kinase domain, ß &γ regulatory

γ has a binding site for AMP & regulates kinase activity

—>upon binding big conformational change allows phosphorylation of threonine

interplay of the subunits allows sensing & regulation

• metformin/phenformin: slight increase in AMP leads to chronic basal activation of AMPK at chronic low level

• AICAR: metabolized to AMP analog ZMP activating AMPK but many side effects

• A-769662 (binds ß subunit AMPK)

• PT1 acts on α-subunit (with kinase domain)

• regulation of insulin-dependent glucose uptake and fat utilization - potential drug for diabetes type II (AMPK activators)

AMPK stimulated by adiponectin, metformin, TZD —>activation has same effect as exercise & low calorie intake—>benefical against metabolic syndrome

decrease: glucose, lipid synthesis

increase: insulin sensitivity, mitochondrial biogenesis, lipid oxidation

exercise mimetic

via acitvation of AMPK

sensitize tissue for insulin & accelerate glycolysis (activation of rate-limiting enzyme phosphofructokinase PFK1 via allosteric activation with fructose 2,6 bisphosphate generated by PFK2)

activate translocation of GLUT4 on the insulin-independent pathway

transport of activated fatty acids = acyl CoA with Carnitine Palmitoyltransferase CPTI as Acyl-Carnitine

uptake of lipids into mitochondria limiting step for ß-oxidation

AMPK inhibits Acetyl-CoA carboxylase ACC —>prevents fatty acid synthesis due to the reduction of malonyl-CoA

activation CPT1 & uptake of fatty acids in mitochondria and fatty acid oxidation

without AMPK fatty acid synthesis & inhibition of CPT1 when enough glucose present

inhibition by malonyl-CoA

has different effects on different organs

hypothalamus: reduced food intake by leptin mediated by decreased AMPK activity

increased food intake by ghrelin is mediated by stimulated AMPK activity

in heart FA oxidation, glucose uptake & glycolysis

cant be used to lower blood glucose in diabetes type II as too many side effects

used in AICAR doping

production of nucleotide analog ZMP and activation AMPK

used widely even if not tested in humans before, nowadays detectable

due to mutations in γ-subunit causing glycogen storage diseases

AMPK inhibition?

leads to reduced but constitutive AMPK activation

mutation close to the AMP binding side

altered glycogen storage and cell growth, mainly causing cardiac syndrome

AMPK upstream kinase LKB1 is a tumor suppressor and metformin reduces cancer incidence

slows down protein synthesis & cell cycle progression—>diminishes proliferation bad in the early stage of cancer

in the beginning inhibitory effect on cancer might be used as prevention against hereditary cancer but would need tight surveilence

metabolic adaptation and resistance against apoptosis mediated by AMPK (would need inhibitor)

anti-stress factor in solid tumors where cellular metabolism is not in equilibrium

release of ATP would allow cell division & protein synthesis

in later stages rather tumor progressor

used to shut down protein synthesis