Endocrine Signalling From Adipose Tissue (5/6)

Question 1

What is the Randle Hypothesis?

Answer 1

Mechanism of inhibition of glucose utilization by fatty acid oxidation

Question 2

How does endocrine signalling from adipose tissue change in obesity?

Answer 2

The release of FFA and pro-inflammatory cytokines is increased due to the increased size in adipose tissue
This causes an influx to other organs and fat accumulation there
Increased glucose output from the liver
Decreased glucose uptake by muscles

Question 3

How does fatty acid oxidation inhibit glucose utilisation?

Answer 3

Acetyl-CoA is produced from fatty acid oxidation and glycolysis
It is converted into citrate which moves into the cytoplasm
Citrate can be converted back into acetyl-CoA and then malonyl-CoA
However, it can also inhibit GLUT4 (glucose uptake) and PFK-1 (glycolysis enzyme)

Question 4

What does an overload of glucose and/or fatty acids cause?

Answer 4

Excess energy production
Overproduction of acetyl-CoA
This inhibits glucose uptake and causes an increased uptake of fat in muscles

Question 5

How is the action of malonyl-CoA implicated in obesity?

Answer 5

In healthy tissue malonyl-CoA inhibits CPT1
In obesity, too much fat is entering the cells - overwhelms malonyl-CoA

Question 6

What causes muscle insulin resitance?

Answer 6

Genes
Aging
Obesity

Question 7

What does muscle insulin resistance lead to?

Answer 7

β-cell compensation (hyperinsulinemia), then decompensation (some die)
Increased lipolysis in visceral fat (FA increase)
Increased gluconeogenesis in the liver (glucose output increase)

Together, these lead to impaired glucose tolerance -> decreased insulin secretion -> diabetes

Question 8

What does insulin binding to its receptor cause?

Answer 8

Insulin interacts with the α-subunits of its receptor
This increases the autophosphorylation and the tyrosine kinase activity of the β-subunits
It phosphorylates docking proteins:
-SHC
-IRS1
-APS (PH and SH2 domains)

Question 9

What does the phosphorylation of docking protein SHC activate in insulin signalling?

Answer 9

Recruitment of the Grb2–SOS
MAPK pathway activated
Ras -> Raf -> MEK -> MAPK -> cell growth/ gene expression regulation

Question 10

What does the phosphorylation of docking protein IRS1 activate in insulin signalling?

Answer 10

Association between PI3-kinase (PI3K) and IRS-1 increases the amount of PI3P
This activates PDK1 and its effectors: the Ser/Thr kinases PKB and atypical PKCζ
These kinases are involved in the stimulation of glucose uptake
PKB is also involved in insulin-induced glycogen synthesis- phosphorylatesm(inactivates) glycogen synthase kinase-3

Question 11

What does the phosphorylation of docking protein APS activate in insulin signalling?

Answer 11

APS and the CAP–Cbl complex interact, allowing the tyrosine phosphorylation of Cbl by the insulin receptor
This mediates glucose transport by a pathway dependent on the activation of the small GTPase TC10

Question 12

How does fatty acid accumulation affect insulin receptor substrate 1 (IRS1)?

Answer 12

Increased FFAs activate JNK and PKCθ
JNK phosphorylates Ser 307 (also activated by insulin but increased with FFA presence)
PKCθ alternatively phosphorylates at Ser612 and Ser632 (insulin phosphorylates Tyr608 and Tyr628) - less affinity for PI3K if alternatively phosphorylated- more insulin needed for same signal

Question 13

What is ceramide produced in response to?

Answer 13

Stress stimuli including in obesity (e.g., chemotherapy, inflammatory agonists, saturated fatty acids)

Question 14

How is ceramide synthesised?

Answer 14

By a de novo biosynthesis from precursor palmitate

Through the activation of inflammatory pathways triggered by TLR4 recognition of saturated fatty acids, which induce the upregulation of genes driving ceramide biosynthesis

Through the breakdown of more-complex sphingolipids as part of a “salvage pathway”

Through the disruption of endoplasmic reticulum homeostasis (ER stress)

Question 15

How is ceramide synthesis regulation implicated in diabetes?

Answer 15

The action of adiponectin at AdipoR1/AdipoR2 receptors activate their ceramidase activity which decreases ceramide production by converting it into sphingosine.
In diabetes, adiponectin levels are decreased.

Question 16

What does ceramide accumulation lead to?

Answer 16

Impaired cellular function, including insulin action

Ceramide directly activates PKCζ isoform which phosphorylates and inhibits the translocation of Akt/PKB
Stimulates the activity of a cytosolic protein phosphatase 2A (PP2A)- phosphatase that dephosphorylating Akt/PKB
Induce further ER stress and mitochondrial dysfunction

Question 17

What cells are common in lean adipose tissue?

Answer 17

M2 macrophages
Th2 cells
Tregs
iNKT cells
Eosinophils

Question 18

What cells are common in obese adipose tissue?

Answer 18

Increased influx of monocytes
Proinflammatory M1 macrophages
Th1 cells
Th17 cells
Neutrophils
B cells

Question 19

What happens to adipocytes in obesity?

Answer 19

They become overloaded with fat burst -> cell death

Question 20

What are the 2 types of macrophages?

Answer 20

M1: pro-inflammatory
M2: anti-inflammatory

Question 21

What changes occur in adipose tissue in obesity?

Answer 21

Lean adipose: type 2 environment - type 2 cytokines: IL-4, IL-5, and IL-13, cell surface molecule interactions

Obesity: type 1 inflammation due to activation of M1 macrophages etc.
Increase of saturated fatty acids, AT hypoxia, danger-associated molecular patterns (DAMPs), and metabolic endotoxemia with increased plasma levels of lipopolysaccharide (LPS)

Question 22

What is the difference between white and beige adipocytes?

Answer 22

White adipocytes: store fat
Beige adipocytes: generate heat in cold conditions

Question 23

What happens to the number of white and beige adipocytes in obesity?

Answer 23

Decrease in beige adipocytes due to change in cytokines released

Question 24

What is dysfunctional adipose remodelling?

Answer 24

When beige adipogenesis is suppressed and insulin resistance is induced

Question 25

Which receptors detect TNFα?

Answer 25

TNFR1 (mainly) and TNFR2

Question 25

Which adipokine levels change in obesity?

Answer 25

Decrease in adiponectin Increase in TNF-α and IL-6

Question 26

What is TNFα?

Answer 26

Pro-inflammatory cytokine synthesised in adipocytes 26kDa precursor - cleaved to 17kDa

Question 27

What does TNFα do in obesity?

Answer 27

Promotes insulin resistance

Question 28

What are the effects of elevated TNFα levels?

Answer 28

Increase FFA levels Increase hepatic triglyceride production Downregulate GLUT-4 Inhibit insulin receptor autophosphorylation and IRS-1 tyrosyl phosphorylation Increase IRS-1 serine/threonine phosphorylation- causes alternate phosphorylation of IRS through activation of mTOR: decreases signal transduction Inhibit PPARγ synthesis and/or function and impair adipocyte function via NFkB activation

Question 29

What happens in TNFα signalling?

Answer 29

Sphingomyelinase activation Increase fats (e.g., ceramide)

Question 30

What happens in insulin-mediated glucose uptake?

Answer 30

Insulin -> activates insulin receptor (autophosphorylates) -> PI3K -> PDK -> AKT -> GLUT4 translocates to membrane -> glucose uptake

Question 31

What is the role of IL-6?

Answer 31

Activates pro-inflammatory cytokines Also activates SOCS: silencer of cytokine signalling

Question 32

What is the role of SOCS3?

Answer 32

Inhibits insulin signalling: Binds to phosphorylated JAK and insulin receptor IR Can also bind to elongin which ubiquitinated IRS1 and 2: degradation Less insulin signal transduction

Question 33

What is adiponectin?

Answer 33

Anti-inflammatory action – blocks TNFα activation of NFκB by blocking IκB phosphorylation Reduces hepatic glucose and triglyceride output Increases muscle and liver fatty acid oxidation reducing insulin resistance

Question 34

What is the role of adiponectin in skeletal muscle?

Answer 34

AdipoR1/AdipoR2 -> APPL1/APPL2 -> enhance fatty acid oxidation through MAPK-PPARα-mediated expression and activation of ACOX1 and AMPK-NAD+/NADH-mediated response of ACC Activated AdipoR1/AdipoR2 also increase mitochondrial biogenesis through APPL1-SERCA-Ca2+-CaMKKβ or LKB1-AMPK-NAD+/NADH-SIRT1-mediated activation of PGC1α Activation of adiponectin receptors enhances glucose uptake via APPL1-Rab5 or APPL1-AMP-AMPK-mediated translocation of GLUT4 as well as attenuation of Rheb-mTOR-S6K1-induced inhibition of insulin signalling

Question 35

What is the role of adiponectin and AdipoR1 in the liver?

Answer 35

AdipoR1 activates AMPK via APPL1/LKB1 or APPL1/PLC/Ca2+/CaMKK mediated signalling pathways Activated AMPK: Inhibits the activities of PEPCK and G6Pase through the activation of EGR1 and DUSP4, which leads to decreased gluconeogenesis Also decreases lipid biosynthesis through inhibiting sterol regulatory element-binding transcription factor 1c (SREBP1c)-mediated transcription of acetyl-CoA carboxylase (ACC), FAS and stearoyl-CoA desaturase (SCD)1 Increases abnormality in mitochondrial morphologies and fatty acid oxidation via sequestering the inhibitory effect of ACC-1 on carnitine palmitoyltransferase (CPT)-1 activation

Question 36

What is the role of adiponectin and AdipoR2 in the liver?

Answer 36

AdipoR2 activates: Fatty acid oxidation through PPARα-induced activation of acyl-CoA oxidase (ACO) and uncoupling protein (UCP)2 Adiponectin also protects hepatocytes through the inhibition of ROS-mediated activation of proinflammatory molecules including TNF-α and MCP1

Question 37

How does adiponectin relate to insulin?

Answer 37

Adiponectin increases insulin sensitivity through the activation of AdipoR1/R2 following CDase-mediated degradation of ceramides