WELCOME

Fat and Beyond: The Diverse Biology of

PPARSPEAKER

P.RAMESH

PH.D

(ABC)

Credit Seminaron

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PPARs (peroxisome proliferator-activated

receptors ) are nuclear receptors & function as

“Transcription factor”

Essential roles in regulation of cellular

differentiation, atherosclerosis & macrophage

function

3 types have been identified: Alpha α, Gamma

, and Delta (Beta β)

α (alpha) - expressed in liver, kidney, heart,

muscle, adipose tissue, and others

INTRODUCTION

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β/δ (beta/delta) - expressed in many tissues but

markedly in brain, adipose tissue, and skin

γ (gamma) - although transcribed by the same

gene, this PPAR through alternative splicing is

expressed in 3 isoforms:

γ1 - expressed in virtually all tissues

γ2 - expressed mainly in adipose tissue (30

A.As longer)

γ3 - expressed in macrophages, large

intestine, white adipose tissue

Cont…

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STRUCTURE OF PPAR

PPAR is a ligand-activated transcriptional factor have

a dominant role in development of adipose cells

It is subfamily of structurally similar to nuclear

receptor

PPAR functions as an obligate heterodimer with RXRs

High affinity binding to DNA by PPAR requires

absolute dimerization with RXR

Domains of PPAR proteins present nearly all nuclear

hormone receptor Retinoid X Receptor

(Tontonoz P, et al., 1994)

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Cont…

PPAR has two N-terminal variants formed by

alternative splicing

PPAR 2 is expressed in more adipose selective

manner

N-terminal region influences the response to

ligand binding of LBD by phosphorylation at

ser112

N C

(Adams M, et al., 1997)

(Hu E, et al., 1996)

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DNA-binding domain (DBD): Highly conserved

domain containing two zinc fingers which binds

to specific sequences of DNA called hormone

response elements (HRE)

C-terminal region is responsible for dimerization

with RXR & contains Transcriptional activation

domain (AF2)

N C

(Ren D, et al., 2002)

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C-terminal region also

form ligand binding

pocket, with many

hydrophobic residuces

occuring inside the

pocket

Crystal structure

of PPAR LBD

Cont…

(Gampe RT, et al., 2000)

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PPAR activation induced by ligand dependent

and independent mechanisms

Absence of ligand, corepressors bind to

heterodimers & recruit Histone deacetylases to

repress transcription

After ligand binding, increase PPAR affinity for

number of co-activators

N-terminal regulatory domain: Contains the

activation function 1 (AF-1) whose action is

independent of the presence of ligand

Cont…

(Tontonoz P, et al., 1994)

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Co-activators: These are not themselves regulated

at expression level

CBP/p300

SRC family

TRAP220

PGC-1α

Co-repressors:

SMART

NCoR

RIP140

Co-activators,Co-repressors & Ligands

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Biological Ligands:

Polyunsaturated fatty acids

Prostanoids (15-deoxy-12,14 prostaglandin-

J2)

Leukotriene LTB4

Oxidized fattyacids (9-HODE & 13-HODE)

Lysophosphatidic acid

Synthetic Ligands:

Thiazolidinedion (TZD) Anti-Diabetic Drug

Fibrates (Hyperlipidemia)

Cont…

(Debevec D, et al., 2007)

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Adipocyte is central player in control of energy balance

& whole body lipid homeostasis

PPAR is dominant or “master” regulator of

Adipogenesis

It induces differentiation of pre-adipocytes into

adipocytes & expressed in BAT and WAT

C/EBP-β/ bind to PPAR promoter & activates PPAR

Upon ligand activation, PPAR induces many target

genes involved in Lipogenesis & Adipogenesis

PPAR & Pathway of Adipogenesis

(Sears I, et al., 1996)

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KLF15

PPAR

KLF2

KLF5

SREBP1C

KROX20 C/EBP

C/EBP C/EBP

C/EBP

CHOP

GATA2/3

Anti-adipogenic factors

Genes of Adipocytedifferentiation

Activation

Inhibition (Evan D, et al., 2006)

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PTC SMO

INSULINIGF-1

WNT10b SHH TGFFGF

BMPs

IRS

P13K

AKT/PKB

CREB

OTHERS

Testosterone

FOXO1/A2 TCF/LEF GATA2/3

-Catenin

? ?

?

C/EBP

PPAR

SMAD3 SMAD3ARP

+

SHN2SMAD1

(Evan D, et al., 2006)

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Anti-Diabetic drugsDietary fattyacids

Intracellular fattyacidsProstaglandins PGJ2

PPAR

RXR

p300SRC

CBP

RNA pol-II

Ap2

LPL

CD36

PEPCK

Aquaporin7

GLUT4

Perilipin

PGAR

GlyK

Fig: Role of PPAR pathway for Adipogenesis

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17Adipocyte

Glucose(Liver)

Triacylglycerols (Chylomicrons)

Glucose Fattyacids

Glycerol-3-P FattyacylcoA

Fattyacids

Triacylglycerols

Glycerol

HSL

Liver

LPLCD36

PerilipinPEPCK

GLUT4

Aquaporin7

18Adipocyte

Glucose(Liver)

Triacylglycerols (Chylomicrons)

Glucose Fattyacids

Glycerol-3-P FattyacylcoA

Fattyacids

Triacylglycerols

Glycerol

HSL

Liver

LPLCD36

PerilipinPEPCK

GLUT4

Aquaporin7

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PPAR extensively studied in WAT differentiation &

same receptor is also important in BAT development

& function

Thermogenic effect of PPAR in BAT is mediated by

PGC-1α, induced by cold exposure of animals

PGC-1α regulates activation of PPAR on

thermogenesis & fattyacid oxidation by interacting

with PPAR/RXR

Stimulation of uncoupling protein (UCP-1),

responsible for uncoupling β-oxidation

Role of PPAR in Brown Adipose Tissue

PPAR Coactivator-1

(Sears I, et al., 1996)

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Phosphorylated PGC-1α is recruited PPAR

binding site on UCP-1 promoter

PGC-1α is stabilized & activated by p38 MAP

kinases

PGC-1α binds to PPAR by its LBD in a ligand

independent manner

LXXLL motif containing co-activator binds to

PPAR

Insulin/akt pathway shutdown hepatic

gluconeogenesis, phosphorylation of PGC-1

Cont…

(McInerney E, et al., 1998)

cAMP

G protei

n

P

UCP-1

MAP kinase

PPAR

PGC-1

ATP

(Sears I, et al., 1996)

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Cont…

Fig: Thermogenesis by UCP-1 in BAT

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Insulin Resistance: is a condition in which body

cells become less sensitive to the glucose-

lowering effects of the hormone insulin

Type2 Diabetes mellitus: Pancreas secrete normal

or even greater than normal amount of insulin

Hallmark of type2 Diabetes is Insulin Resistance

In type2 Diabetes, plasma levels of FFAs &

Glucose are increased

PPAR and Insulin Resistance

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In T2DM inappropriate deposition of lipids in

liver & skeletal muscle

Thiazolidinedione (TZD), Rosiglitazone &

Pioglitazone are used for treatment of type2

Diabetes

Activation of PPAR target gene expression

enhance to store dietary fattyacids

Target genes contributing to this include AP2,

LPL, CD36, PEPCK & Aquaporin 7

(Haris P, et al., 1994)

(Kishida K, et al., 2001)

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Adipose tissue is primary target for effects of

TZD

It also promotes many signaling molecules

called Adipokines from adipocytes

Adiponectin

Resistin

TNFα

MCP-1

IL-6, IL-1β

Cont…

INFLAMMATOR

Y RESPONSE

(Bouskila M, et al., 2005)

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Ligand activation (TZD) of

PPAR in adipocytes is

associated with decreased

production of TNFα, Resistin

& MCP-1

Increase of Adiponectin gene

Decreased Insulin Resistance

Suppression of Hepatic

glucose uptake & stimulates

muscle glucose uptake

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ADIPONECTINS:

It is a protein hormone abundantly expressed in

adipocytes

Adiponectin affects:

Decreased gluconeogenesis

Increased glucose uptake

β-oxidation

Triglyceride clearance

Protection from endothelial dysfunction

insulin sensitivity

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TNF-α:

It is an inflammatory cytokine released in

obese & insulin resistance

Also propagates atherosclerotic lesion

formation

It also promotes apoptosis in endothelial

cells by dephosphorylating protein kinase

B or Akt & contribute endothelial injury

Cont…

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RESISTIN:

It is recently discovered fat-specific hormone,

directly induces insulin resistance in muscle &

liver

Neutralization of resistin by specific antibodies

results in decreased blood glucose level

It also express cell adhesion molecule (VCAM-

1), chemokine MCP-1(Atherosclerotic lesion

formation) with insulin resistance patients

Cont…

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Diet induced Obesity

Adipocyte

Adipocyte Hypertrophy

RESISTIN

?

MuscleLiverInsulin

(Olavi U, et al., 2002)

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Adipokines & Inflammation:

Increased in adipose tissue (WAT) with obesity

during inflammation (TNF-α, IL-6,IL-18, MIF, CRP &

PAI-1)

Circulatory inflammatory markers are raised in

obese

IL-6 released from adipose tissue stimulates hepatic

synthesis of CRP in obese

Similary IL-18 is acivated by TNF-α

Infiltration of Macrophages in adipose tissue(Trayhurn P, et al., 2005)

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Why Obese should be accompanied by

Inflammation???

It is mainly due to HYPOXIA

Adipose tissue (WAT) mass is very large in

Obese & Type-II diabetes are linked to

inflammation

Cont…

(Sears I, et al., 1996)

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TZD effects on Skeletal Muscle and Liver:

Skeletal Muscle is largest glucose utilizing organ,

ability of TZD is to improve whole body insulin

resistance

PPARγ expression is very low in Skeletal muscle

compare to fat

Effect of PPARγ ligands on skeletal muscle for glucose

uptake are likely to be indirect

Direct of effect of PPARγ activation in muscle is still

unclear(Castrillo A, et al., 2004)

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In liver also expression of PPARγ is very low,

activation of PPARγ signaling promotes lipid

accumulation (Hepatic Steatosis) in rodents

But does not appear in human hepatic

steatosis

TZD is beneficial in treating nonalcoholic fatty

liver disease in humans

Cont…

(Castrillo A, et al., 2004)

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PPARγ is induced differentiation of monocytes

into macrophages & highly expressed in

Macrophages

It also differentiates of monocytes into

dendritic cells

Oxidized ligands (9-HODE & 13-HODE) &

lipoproteins responsible for PPARγ signaling in

myeloid cells

Upon ligand activation, it promotes

expression of genes CD36, LXR, Arg-1& IL-10

PPAR in Inflammation

(Sears I, et al., 1996)

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PPARγ is a lipid-activated member of the nuclear

receptor superfamily of transcription factors

Biological receptor for the TZD class of

antidiabetic drugs

Master transcriptional regulator of adipocyte

differentiation & controls expression of a genes

involved in lipid metabolism

PPARγ action in myeloid cells, such as

macrophages and DCs, has been linked to the

modulation of immune and inflammatory

responses

Summary

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THANKS