3Obesity - Leptin Flashcards
How is STAT3 linked to POMC and AgRP expression?
Without leptin:
In a POMC neuron = closed chromatin conformation = no transcription
NPY/AgRP/GABA neuron = open chromatin conformation = transcription
Increased feeding
With leptin:
STAT3 signal transduction cascade = STAT3 enters nucleus = binds to POMC neuron in promoter region = opens up chromatin = switches on expression = acts directly as a transcription factor for POMC = elevated levels of POMC = subsequent downstream effects on hypothalamus
(Possibly) STAT3 binding to NPY/AgRP/GABA inhibits transcription
Reduced feeding
Balance between two different types of neurons is essential
Slightly controversial about STAT3 acting as a negative signal (on NPY/AgRP/GABA)
What is the concerted action of insulin and leptin on FOXO1 and STAT3?
Insulin helps leptin to cross the blood-brain barrier
Insulin and leptin signalling work in parallel with each other in the brain
Insulin linked to food consumption, high adiposity = high leptin
Relatively high levels of leptin and insulin suggest that you are well fed
Leptin and insulin have a concerted action on FOXO1
Without leptin and insulin:
POMC neuron with FOXO1 bound to it = switches off transcription = negative transcription factor
In NPY/AgRP/GABA neurons, FOXO1 bound = switches on transcription = positive transcription factor
With leptin and insulin:
STAT3 switches on POMC
STAT3 switches off NPY/AgRP/GABA
Insulin activates PI3 kinase and PKB pathway which causes FOXO1 to leave the nucleus
Leptin activates JAK2 causing STAT3 to enter nucleus
How does leptin mobilise the PI3 kinase/PKB pathway through JAK2 mediated phosphorylation of IRS2?
Leptin binds to LepRb receptor = phosphorylation of LepRb and JAK2 = recruits STAT3 = STAT3 phosphorylated = homodimerises = goes to the nucleus = switches on POMC = switches off NPY/AgRP/GABA
Leptin signals through PI3 kinase/PKB pathway as well
Insulin also activates this pathway
Activation of PKB phosphorylates the transcription factor FOXO1
FOXO1 phosphorylated can’t go to the nucleus = anorectic signal = chronically regulates energy intake = over time, gently reduces energy intake = lipostatic theory for body weight control
How is mTOR activated?
Second regulatory system = mTOR
mTOR = mammalian Target Of Rapamycin
PKB phosphorylated = phosphorylates TSC 1+2 complex on residues S939 and T1462 on TSC2 = GTPase activity inhibited = produce more RhebGTP
When FKBP38 is bound to mTOR = mTOR inactive
RhebGTP interacts with FKBP38 and removes it from mTOR = active mTOR = anorectic signal
mTOR regulates translation = RNA levels do not always represent protein levels = when RNA present but little or no protein = need translational activator
How does 5’AMP activate the rescue pathway?
In the fed state = insulin levels high = anorectic signal BUT need to eat if energy levels fall ie breakfast
Leptin levels do not fall overnight as adiposity stays the same
If cells have low energy, can generate ATP from 2 molecules of ADP via myokinase producing ATP and 5’-AMP
5’-AMP is an indicator of low energy in the cell = need energy
5’-AMP activates AMP kinase = LKB1 phosphorylates AMP kinase = AMP kinase highly active
AMP kinase phosphorylates TSC2 on residues T1227 and S1345 = alternate phosphorylation to PKB
Leads to GTPase activation = RhebGTP is converted to RhebGDP
RhebGDP means that FKBP38 remains firmly bound to mTOR = leading to an orectic signal
Phosphorylation of FKBP38 by AMP kinase may stabilise its interaction with mTOR
Leptin can control body weight over weeks and months but also allows body to deal with the effect of short term fasting on the cell’s energy levels
What are the downstream effects of mTORC signalling?
Not entirely sure of all the downstream effects of mTORC
Know that it phosphorylates proteins
Can phosphorylate 4EBP1 = decrease translation
How it influences energy balance is unknown
How does the leptin-regulated melanocortin circuit influence energy homeostasis and body weight (simplified)?
Adipocyte producing leptin and beta cells producing insulin
Both cross blood-brain barrier and hit the arcuate nucleus of the hypothalamus = first order targets
Leptin switches off NPY and AgRP expression and switches on POMC and CART expression
POMC is converted to alpha-MSH
Alpha-MSH hits MC4R
Alpha-MSH activates MC4R whereas AgRP inactivates MC4R
MC4R hits TRH, MCH, GABA etc. = have effects on endocrine, behavioural, autonomic systems
Autonomic = thermogenesis, insulin and glucose control
How does the leptin-regulated melanocortin circuit influence energy homeostasis and body weight (complex)?
The adipocyte hormone leptin crosses the blood brain barrier (BBB) and acts directly on two populations of neurons within the arcuate nucleus that express NPY and AgRP or POMC and CART.
Leptin stimulates production of a-MSH, an agonist for the MC4 receptor (as well as CART),and inhibits production of AgRP, an antagonist for this receptor (as well as NPY).
MC4 receptor–expressing neurons receive these leptin regulated signals, as well as others, such as NPY. Such MC4R neurons are just now being chemically and functionally identified, and include:
TRH neurons in the paraventricular nucleus (PVH) that regulate the thyroid,
MCH neurons in the lateral hypothalamus that regulate feeding,
GABAergic neurons in the PVH that modify other as yet unidentified neurons tied into energy balance, and others.
Several outputs of the MC4R-expressing neurons include:
Endocrine outputs such as thyroid, growth and reproduction, through control of pituitary function;
Behavioral outputs, including feeding;
Autonomic output, regulating energy expenditure; insulin secretion; and glucose homeostasis.
Sites in the pathway at which spontaneous loss of function mutations have caused obesity in rodents and humans are indicated in yellow, as are sites at which induced mutations have caused obesity in rodents (in blue). Not shown here are potential direct actions of leptin on peripheral tissues.
How is the leptin pathway downregulated by PTP1B?
Protein tyrosine phosphatase 1B is increased in leptin-resistant individuals
PTP1B dephosphorylates JAK = signal transduction events are decreased/dampened down
How does is the leptin pathway downregulated by SOCS3?
SOCS3 = silencer of cytokine signalling
SOCS3 = target of STAT3
A lot of leptin continually hitting the LepRb = a lot of STAT3 continually going to the nucleus = SOCS3 feeds back and inhibits binding of STAT3 to LepRb = blocks action of leptin on its receptor = signal dampened down
SOCS3 exists to decrease leptin signal once leptin is no longer present
How is leptin synthesis regulated?
Control of leptin levels crucial for maintenance of normal body weight
Fasting acutely decreases plasma leptin levels
Insulin increases leptin production by adipose tissue
PPAR gamma agonists increase leptin production
Regulatory signals all involved in pathogenesis of Type II diabetes
What is the action of leptin on peripheral tissue?
Leptin directly acts to prevent ectopic lipid overaccumulation during the development of obesity
Spares liver, muscle and other tissues including beta cells from metabolic trauma of lipid overload = prevents insulin resistance
Leptin deficiency leads to lipotoxicity caused by fatty acid derivatives eg. ceramide
Eventually leads to lipoapoptosis
How does leptin act on peripheral tissues?
How does leptin lead to a decrease of fatty acid synthesis and an increase in beta-oxidation of fatty acids?
Leptin acts on muscle via ObRb and via alpha-adrenergic receptor (activated by the SNS)
AMPK kinase/LKB1 phosphorylates AMP kinase
AMP kinase will phosphorylate Acetyl CoA carboxylase
Acetyl CoA Carboxylase = first enzyme in fatty acid synthesis
Phosphorylation of ACC inhibits it = does not form malonyl CoA
Malonyl CoA = inhibitor of Carnitine Palmitoyltransferase (CPT1)
CPT1 = transport mechanism which allows fatty acids to enter mitochondria = to allow beta-oxidation of fatty acids
Leptin decreases synthesis of fat and increases its beta-oxidation in the mitochondria = improves insulin sensitivity by decreasing fat stored in muscle
What is the direct effect of leptin on beta-cells?
Direct effect on beta-cells
Normally: Glucose = metabolised = ATP generation = K+ ATP channel = depolarisation = VDCC opening = Ca2+ influx = exocytosis of insulin
Leptin has a very gentle negative effect on most of this pathway
Doesn’t block insulin secretion but dampens down levels of insulin released in response to glucose
Think of insulin as a storage hormone:
When insulin is released it promotes glucose going into cells = being stored as glycogen = once glycogen stores are full = insulin promotes conversion of glucose into fat
Leptin gently nudges down insulin levels because don’t need to store any more fat
What evidence is there for PTP1B playing a role in leptin resistance?
Leptin resistance = might occur in peripheral tissues = However, high amounts of leptin in obese patients but still have high amount of fat in peripheral tissues
PTP1B knockout mice:
Decrease in ACC
Decrease in malonyl CoA
Decrease in FA-CoA
Decrease in TG
BUT despite this evidence that it could happen in transgenic mice, there is no evidence that it does happen in humans
