Biochemistry of Endothelial Biochemistry of Endothelial CellsCells

Doc. Dr. Mine KUCUR

ENDOTHELIUM

• Provides a cellular lining to all blood vessels in the circulatory system and forms a structural barrier between the vascular space and the tissue.

• In adults, the endothelium weights approximately 1kg, comprises 1.6x1013 cells and has a surface area between 1-7 m2.

• Each EC comes into contact with numerous smooth cells.

LUMEN

Tunica adventitia

Tunica media

Tunica intima

Note the individual Endothelial Cells

Vasoconstriction and dilatationVasoconstriction and dilatation

Normal Vasoconstriction Vasodilatation

VasoconstrictionVasodilatation

Vasoconstriction and dilatationVasoconstriction and dilatation

↓ Resistance to flow ↑ Resistance to flow

Endothelial ApoptosisEndothelial Apoptosis

Normal Apoptosed

The The EndotheliumEndothelium

As an Endocrine As an Endocrine OrganOrgan

• The inner lining of our bloods vessels is the Endothelium

• It plays a central role in regulating the vasomotor tone (vasoregulation)

• Local homeostasis & control of the coagulation process (provision of anti-thrombotic surface and selective permeability to haematopoietic cells and nutrients)

• Endothelial cells have ‘Sensors’ and release ‘Mediators’

• ‘Mediators’ are the functional molecules on the cell surface

The Vascular EndotheliumThe Vascular Endothelium

Vascular Endothelial MediatorsVascular Endothelial Mediators

Include the following

• Nitric oxide (NO)

• Cycloxygenase (CxO)

• Endothelin-1 (ET-1)

• Endothelium Depolarisation Factor (EDF)

• And many others - thus

• It is the largest endocrine gland

• NO is generated in ECs by the oxidation of L-arginine to L-citrulline by a family of enzumes, NO synthases (NOS)

• Half-life of NO, is affected by its chemical reaction and inactivation by superoxide anion

• NO is the most abundant free-radical in the body

• It is the only biological molecule in high concentrations to out-compete superoxide dismutase for superoxide

• NO has an anti-thrombogenic & anti-atherogenic role

Nitric Oxide (NO)Nitric Oxide (NO)

Endothelial NO has the following actions• Smooth muscle relaxation and vasodilatation-

maintenance of basal vasomotor tone• Essential for regulation of blood pressure• Reduces proliferation of vascular smooth muscle• Protects blood vessel intima from injurious

consequences of platelet aggregation-inhibition of thrombosis by inhibiting platelet adhesion, activation and agonist-induced secretion.

Protective actions of NOProtective actions of NO

(L-NMMA) = N(G)-mono-methyl-L-arginine

NO deficiency in the vessel wall promotes

• Inflammation

• Oxidation of lipoproteins

• Smooth muscle proliferation

• Accumulation of lipid rich material

• Platelet activation and thrombus formation

Finally results in atherosclerosis.

ED and NO ED and NO ↓↓

EndothelinsEndothelins• A family of 21 amino acid peptides.• Three members: ET-1,ET-2 and ET-3.• Regulate vasomotor tone, cellular proliferation and

hormone production.• ECs produce only ET-1, also syntesized by vascular

smooth cells.• Production is induced by hypoxia, ischemia.• ET-1 binds to specific receptors on smooth cells:

vasoconstriction.

Endothelins Endothelins Disease Disease

• Elevated plasma concentrations of ETs are found in congestive cardiac failure.

• Plasma ET-1 levels are normal in essential hypertension.

• Endothelins have been implicated in vascular diseases of the kidney and cyclosporin induced nephrotoxicity.

• Plasma ET levels are elevated after ischaemic cerebral infarction.

• Eicosanoid.

• Synthesis is induced by disturbances in endothelial function or vascular haemodynamics.

• Released from ECs and acts in a paracrine manner.

• Binds to a specific receptor on platelets and vascular smooth muscle cells to limit vasoconstriction and influence platelet deposition.

Prostacyclin(PGIProstacyclin(PGI22))

• Phospholipid, remains bound to the EC surface.

• Most important effect is recruiting leucocytes to the EC surface, and its effects on vascular tone are indirect and exerted through the generation of other eicosanoids and leukotrienes.

Platelet Activating Factor (PAF)

• Leucocyte transmigration to extravascular sites of inflammation.

• Initial step is the arrest of leucocytes and random contact with the ECs.

• Mediated by the selectins.

• Increasing adhesion occurs with activation of the leucocyte integrins.

• Leucocytes flatten and migrate along the endothelium: diapedesis.

ENDOTHELIUM IN INFLAMMATIONENDOTHELIUM IN INFLAMMATION

• Activated endothelium bind leucocytes.

• Is activated in response to cytokines including IL-1, TNF- and lipopolysaccharide which expresses adhesion molecules and bind leucocytes.

ENDOTHELIUM IN INFLAMMATIONENDOTHELIUM IN INFLAMMATION

• Three members: E-Selectin, L-Selectin and P-Selectin.

• Characterized by a C-terminal lectin like domain that binds complex carbohydrates.

• Involved in leucocyte recruitment to sites of inflammation.

Selectins

• Integrins relevant to leucocyte recruitment are 1 integrins and the 2 integrins.

1 integrins mediate binding to the ECM (fibronectin and laminin). Binds cell surface VCAM-1.

2 integrins are present only on leucocytes and their activity depends on conformational changes that occuron leucocyte activation.

• Integrin-immunglobulin superfamily interaction is essential for extravasation to occur.

Integrins

ECs maintain anticogulant activity:

• Prevent activation of thrombin- if activated, stimulates coagulation by causing platelet activation and the activation of several coagulation factors.

• Express heparan suphate- stimulates antithrombin III.

• Express tissue factor pathway inhibitor (TFPI)- prevents thrombin formation.

COAGULATIONCOAGULATION

• Express thrombomodulin.

• Thrombomodulin-thrombin interaction activates protein C- strong anticoagulant activity.

• Synthesize protein S, a cofactor for activated protein C.

THE BALANCE IS TOWARDS ANTICOAGULANT FACTORS IN HEALTHY ENDOTHELIUM !!!

COAGULATIONCOAGULATION

The Endothelium inThe Endothelium inHealth and DiseaseHealth and Disease

The Universal Damage

Coronary Coronary Heart Heart

DiseaseDisease

Endothelial Endothelial DysfunctionDysfunction

NO NO ↑ ↑ Inflammation Inflammation ↑ Thrombosis↑ Thrombosis

GenesGenes

Coronary Risk Factors

The Essential ComponentsThe Essential Components

• endothelial NOS (eNOS) is induced by receptor dependent agonists such as thrombin, bradykinin and substance P.

• NO causes vascular smooth muscle relaxation by binding to guanyl cyclase.

• plays a critical role in the inhibition of thrombosis by inhibiting platelet adhesion, activation and agonist-induced secretion.

• NO promotes platelet desegregation.

Regulatory Functions of the EndotheliumRegulatory Functions of the EndotheliumNormalNormal DysfunctionDysfunction

Vasodilation VasoconstrictionNO, PGI2, EDHF,

BK, C-NPROS, ET-1, TxA2,

A-II, PGH2

Thrombolysis Thrombosis

Platelet Disaggregation

NO, PGI2

Adhesion Molecules

CAMs, P,E Selectins

Antiproliferation

NO, PGI2, TGF-, Hep

Growth Factors

ET-1, A-II, PDGF, ILGF, ILs

Lipolysis Inflammation

ROS, NF-B

PAI-1, TF-α, Tx-A2tPA, Protein C, TF-I, vWF

LPLVogel R

Clinical Sequelae

Oxidative stress and Oxidative stress and Endothelial dysfunctionEndothelial dysfunction

• Oxidative Stress leads to ED

• Endothelial dysfunction is mainly due to reduced bioavailability and bioactivity of Nitric Oxide (NO)

• It is also a physiological process

• Takes place gradually by age and menopause.

• Oxidant stress and Endothelial dysfunction are major factors for atherosclerosis – the common pathway –

• for most of the cardiovascular risk factors including Hypertension, DM, Dyslipidemia and Smoking.

• Both endothelial dysfunction and oxidant stress result in clinical conditions - Heart failure, IHD and MI

The Effects of EDThe Effects of ED

• Advances in the understanding of endothelial function have been the basis for many therapeutic strategies.

• Expanding the understanding of endothelial function will lead to targeted therapies to a myriad of diseases such as cancer, cardiovascular disease and inflammatory conditions.

CONCLUSIONSCONCLUSIONS