Xiaodan Lu, Ph.DPostdoctoral Fellow

Metabolic Impact of Adipose Tissue

The Simplified Model

Obesity Trends Among U.S. Adults between 1985 and 2002

Obesity Trends Among U.S. Adults between 1985 and 2002

Definitions:Obesity: having a very high amount of

body fat in relation to lean body mass, or Body Mass Index (BMI) of 30 or higher

Body Mass Index (BMI): a measure of an adult’s weight in relation to his or her height, specifically the adult’s weight in kilograms divided by the square of his or her height in meters

Definitions:Obesity: having a very high amount of

body fat in relation to lean body mass, or Body Mass Index (BMI) of 30 or higher

Body Mass Index (BMI): a measure of an adult’s weight in relation to his or her height, specifically the adult’s weight in kilograms divided by the square of his or her height in meters

Diseases Associated with Obesity

Diabetes: 80% related to obesity Hypertension: prevalence is >40% in

obesity Heart disease: 70% related to obesity Cancer: Obesity accounts for 15-20% of

cancer-related deaths Death: Obese individuals have a 50-

100% increased risk of death from all causes compared to lean individuals (most of this risk is due to cardiovascular disease)

Central obesityGlucose intolerance

HypertensionInsulin resistance

High TGLow HDL-C

Small, dense LDL particles

The Metabolic Syndrome

0

5

10

15

20

25

30

CHD Previous MI Previous Stroke

WITH Metabolic Syndrome All Subjects

NGT

IFG/IGT

Type 2 Diabetes

(n=3,928)(n=1,808)

(n= 685)(n=1,430)

Without Metabolic Syndrome

0

5

10

15

20

25

30

CHD Previous MI Previous Stroke

Prevalence of CHD, MI, and Stroke in Relation to the Presence of Metabolic Syndrome

Pre

vale

nce

Traditional CHD Risk Factors

Elevated Plasma CholesterolReduced Plasma HDL LevelsDiabetesHypertensionSmokingAge

Lipid Lowering only results in a relative risk reduction of 30%

New MarkersInflammatoryAdipokines

New Inflammatory Markers to assess Risk

CRP IL-6 TNF Serum Amyloid ANon-esterified fatty acids

Adipose tissue is an important source of inflammatory cytokine production

White Adipose Tissue (WAT)

Many different adipose tissue beds throughout the whole organism

Many distinct cell types: adipocytes, pre-adipocytes, fibroblasts, macrophages, vascular cells

Heterogeneous metabolic capabilities, depending on visceral or subcutaneous location of fat depot

Secrete adipokines with systemic effects

J. Magn. Reson. Imaging Vol.21, 4 Pages: 455-462

Gender Differences in Adipose Tissue Distribution

Female Male

Original Image

SCATSubcutaneous Adipose

TissueHIGHLIGHTED WHITE

VATVisceral Adipose

TissueHIGHLIGHTED WHITE

White Adipose Tissue

WAT is comprised of 2 Fractions:

1) adipocytes2) SVF which consists of: preadipocytes endothelial cells macrophages

WAT is central to energy storage in the body and the mobilization of this energy store is highly regulated.

Preadipocyte Mature adipocyte

http://www.hsph.harvard.edu/GSHLAB/adipos.html

Basic Adipose Tissue Expansion

Adipocyte Growth:Hypertrophy vs. Hyperplasia

Hypertrophy (increase in size) “Lipogenesis” Result of excess triglyceride accumulation in

existing adipocytes

Hyperplasia (increase in number) ‘adipogenesis’ results from the recruitment of

new adipocytes from precursor cells in adipose tissue and involves proliferation and differentiation

Hausman et al. Obesity Reviews (2001) 2, 239-254

Hormones, Adipokines, enzymes, molecules and other factors reportedly associated with Adipose Tissue

-1 acid glycoprotein (AGP) 3-hydroxysteroid dehydrogenase (3HSD) 3-HSD 5 reductase 7 hydroxylase 11HSD 17HSD Acylation-stimulatin protein (ASP or

C3adesArg) Adenosine Adipocyte differentiation factor (ADF) Adipogenin Adiponectin Adiponutrin Adipohilin (adipose differentiation-related

protein [ADRP]) Adipose protein 2 (aP2 or adipocyte-specific

fatty acid-binding protein, otherwise known as 422 protein)

Adipose triglyceride lipase (ATGL) Adipsin (ADN; complement factor D, C3

convertase activator and properdin factor D)

Agouti protein Androgens Angiotensin I and II Angiotensin-converting enzyme (ACE) Angiotensinogen (AGT) Annexin (lipocortin) Apelin Aoplipoproteins E, C1 and D Aquaporin 7 (AQP7) Autotaxin (lyosphospholipase D) Bone morphogenic protein (BMP) C-reactive protein (CRP) Calumenin Calvasculin Cathepsin D and G Ceramide Cholesteryl ester transfer protein (CETP) Chymase Collagen Type VI3 Complement factor C3 and B

• Cytochrome p450-dependent aromatase (P450arom)

• E2F proteins• Ecto-nucleotide

pyrophosphatase/phosphodiesterase 1• Eicosanoids• Endothelin• Entactin/nidogen• Eotaxin• Epidermal growth factor• Estrogen• Fasting-induced adipose factor• Fatty acid translocase• Fatty acid transport protein• Fibroblast growth factor• Free Fatty Acids• Galectin-12• Gelsolin• Glucose transporter 4• Glutamine• Glycerol• Haptoglobin• Hippocampal cholinergic neurostimulating

peptide• Hormone-sensitive lipase• Insulin-like growth factor• Insulin-like growth factor binding protein• Interleukin-1, -6, -8, and -10• Interleukin-1 receptor antagonist• Lactate• Leptin• Lipin• Lipocalins• Lipoprotein lipase• Lysophospholipid• Macrophage migration inhibitory factor• Metalloproteases• Metallothionein• Monobutyrin• Monocyte chemoattractant protein-1• Necdin• Nerve growth factor• Neuronatin

• Nitric oxide synthase• Nuclear factors• Omentin• Osteonectin (secreted protein, acidic and rich

in cystein/SPARC)• Pentraxin-3• p85phosphatidylinositol 3-kinase• Perilipins• Phosphoenolpyruvate carboxykinase• Phospholipid transfer protein• Pigment epithelium-derived factor• Plasminogen activator inhibitor• Pre-adipocyte factor-1• Prolactin• Protein kinases• Protease inhibitors (e.g., cystatin C and

Colligin-1)• Prostaglandin E, I2 and F2 prostacyclins

(PGI2/PGF2)• Regulated on activation, normal T-cell

expressed and secreted (RANTES)• Renin• Resistin (FIZZ3 or serine/cystein-rich

adipocyte-specific secretory factor [ADSF])• Resistin-like molecules• Retinol• Retinol-binding protein• Sergin protease inhibitors• Serum amyloid A• Stearoyl-CoA desaturase• Stromal cell-derived factors (e.g., stromal

cell-derived factor 1 precursor or pre-B growth stimulating factor)

• Tissue factor• Tonin• Transforming growth factor-• Tumor necrosis factor-• UDP-glucuronosyltransferase A242B15• Uncoupling proteins• Vascular endothelial growth• Visceral adipose tissue-derived serpin• Visfatin (pre- cell colony-enhancing factor

[PBEF])Bays H & Ballantyne C; Future Lipidol 2006; 1(4): 389-420

Free Fatty Acids (FFA)

Adipose tissue serves as a buffer for FFA levels following feeding (similar to the way liver buffers blood glucose levels)

FFA are released from WAT when systemic energy needs are not being met

FFA levels are a predictor of future development of type 2 diabetes

Effects of elevated plasma FFA levels

Insulin resistance in muscle Inhibition of normal function of insulin to

suppress hepatic glucose production Impair insulin-stimulated glucose uptake FFA are substrate for hepatic triglyceride

synthesis leading to increased plasma VLDL and triglyceride levels

Impair endothelial function

Control of Release of FFA From White Adipose Tissue

Hypertriglyceridemia Atherosclerosis

WATDietLipid storage

Lipolysis

Starvation

Insulin

Free Fatty Acid

Insulin Resistance

Liver

Triglycerides

PhospholipidsKetone bodies

Secretory Products of Adipose Tissue

AdiposeTissue

IGF-1IGFBP

TNF-αInterleukinsTGF βFGFEGF

Fatty AcidsLactateAdenosineProstaglandinsGlutamine

Unknown Factors

PAI-1Angiotensinogen

Ang II

Estrogen

Adiponectin

Resistin

BoneMorphogenic

Protein

Adapted from: Roth, J, et al, Obesity Research, Vol 12, supplement Nov 2004:88S-101S

Leptin

Adipokines

Vascular Disease Related Angiotensinogen PAI-1

Insulin Resistance Related ASP (Acylation-stimulating protein) TNF IL-6 Resistin Leptin Adiponectin

Angiotensinogen (AGE)

Links obesity with hypertensionPositive correlation of blood pressure with

AGE levelsPrimarily produced in liver, but also by WATDeficiency partially protects against diet-

induced obesity

Plasminogen Activator Inhibitor 1 (PAI-1)

Impairment of fibrinolytic system contributes to cardiovascular complications of obesity

WAT is main tissue source of PAI-1Produced by pre-adipocytes, primarily in

visceral WATActs to inhibit fibrinolysis (is pro-thrombotic)Also influences cell migration and angiogenesisCould impair pre-adipocyte migration leading

to WAT growth

TNF

Proinflammatory cytokineProduced by adipocytes and

macrophages in WATOver-expressed in obesityLink between TNF and insulin

resistanceAlters insulin signaling and MAPK

pathways in vitro and in vivo

IL-6

10-30% of circulating IL-6 is from WATHighly correlated with body mass and

inversely related to insulin sensitivityAlters insulin signaling in the liverIL-6 KO mice develop mature-onset obesity

and glucose intolerance

Resistin

Discovered in 2001Expressed in adipocytes in mice and in

macrophages in humansIncreased in obesityRecombinant resistin

Promotes insulin resistance in mice Decreased insulin stimulated glucose uptake in

WAT

Visfatin

Discovered in 2004Specifically expressed in visceral fat as

opposed to subcutaneous fatBinds to insulin receptor on an allosteric

site to insulinCan mimic insulin in down-stream insulin

signaling pathwaysPlasma levels are 1/100th level of insulin

and not controlled by nutritional statusThese original findings were not

reproducible, and the paper was later recalled!

Leptin

Secreted by adipocytesRegulates food intake – satiety factorLeptin receptor is expressed in hypothalamus

ob/ob and db/db mouse models

Spontaneous mutations first noted in 1950morbid obesity (3x normal weight)hyperinsulinemia (50-fold increase)hyperglycemiaInfertileslightly increased cholesterol

Leptin and Leptin Receptor

Both ob/ob and db/db mice originally developed from spontaneous mutations

The genes mutated in these mice were later identified as leptin and its receptor

ob/ob and db/db mice are obese, hyperphagic, hyperglycemic, and hyperinsulinemic and are commonly used models for studies of diabetes ob/ob littermate

1994: Cloning of Leptin

167 aa protein belonging to cytokine family

Circulates free or bound soluble receptor

Expressed in WAT, stomach and placenta84% homology between mouse and

human leptinLeptin administration to ob/ob mice

alleviates all aspect associated with the deficiency in mice

Possible Physiologic Roles of Leptin

ObesityAnorexiaDiabetesReproductionBone Mass Immune System

GlomerulosclerosisHematopoeisis Agingsweet-sensing

modulatorAngiogenic activityHypertension

Adiponectin (Acrp 30, AdipoQ)

Discovered in mid-1990’sProtein highly expressed in adipocytes

and circulates at high concentrationsCollagenous tail and globular headPlasma concentrations are reduced in

obesity and insulin resistanceTNF and IL-6 inhibit adiponectin

expressionAdministration of recombinant

adiponectin ameliorates IR in obese and lipodystrophic mice

Adiponectin is anti-atherogenic

Mobilization of Triglycerides Stored in Adipose Tissue

Low levels of glucose in the blood trigger the mobilization of triglycerides through the action of epinephrine and glucagon.

cAMP pathway activate hormone sensitive lipase to cause hydrolysis of triglycerides into glycerol and FFA

Modulators of Triglyceride Synthesis and Storage

Insulin – promotes conversion of carbohydrates into triglycerides

Epinephrine: activates release of fatty acids from adipose tissue

Glucagon: stimulates release of fatty acids from adipose tissue

Pituitary Growth Hormone Adrenal Cortical Hormones

Lipoprotein Lipase

LPL: lipolytic activity in the capillary bed; hydrolyzes NEFA from lipoproteins (chylomicrons and VLDL)

Secreted by adipocytes, macrophages, muscle but active in capillary endothelial beds

Has catalytic activity, but also functions as ligand for anchoring of lipoproteins to cell surface

Hormone Sensitive Lipase

lipolytic activity inside adipocytes releases NEFA from storage when energy is

needed Hydrolyses TG but also cholesteryl esters,

retinyl esters and steroid esters

Triglyceride

Glycerol backbone with 3 fatty acid chains

Fatty acids can be similar or varied

Stored in cytosol of adipocytes

Carbon atoms are more reduced than sugars – thus provide more energy

Together, ATGL and HSL are responsible for >95% of TG hydrolysis from WAT

Lipolysis of TG in WAT

Perilipins

Found within phospholipid monolayer surrounding neutral lipid containing lipid vacuole

Phosphorylated by PKA on multiple residuesPrevent lipolysis under basal conditions

Fatty Acid Binding Proteins(FABP: ap2 and mal-1)

Expressed in WAT, liver, intestine, heart, and brain

Sequester fatty acids in cytosol to protect cell from harmful effects of intracellular-FFA

Interact with HSL to favor translocation of lipid from cytosol to lipid droplet

Ap2-/- mice do develop diet-induced obesity, but do not develop IR or diabetes

Hormone Stimulated Lipolysis Paradigm

Lipolytic stimulation leads to phosphorylation of HSL and perilipin by PKA

Phosphorylation of perilipin causes them to lose their lipolysis-blocking capability

Phosphorylation of HSL promotes its translocation from the cytosol to the lipid droplet

Required for maximal TG storage in WAT

Yeaman, Biochem. J 379:11

Osuga. 2000. PNAS 97:787

Hormone Sensitive Lipase deficient mice

Mice do NOT become obese

Mice have increased Diacylglycerol in their WAT and muscle

Adipocytes in BAT were enlarged (5-fold)

BAT mass was enlargedMales were sterileWAT retained 40% of

TG lipase activity

Perilipin Knockout mice

Decreased adipose tissue mass

High levels of basal lipolysis

Resistant to diet-induced obesity

Javier Martinez-Botas Nature Genetics  26, 474 - 479 (2000)

Mechanism for Hormone-mediated lipolysis

Hormones include catecholamines, adrenaline, and noradrenaline

Receptors are called G-protein coupled receptors called β-adrenergic receptors

When activated, they transmit signals to adenylyl cyclase leading to cAMP production

cAMP activates PKAPKA phosphorylates HSL and perilipin

Gs/cAMP/PKA/HSL Pathway to Lipolysis