Jeremy

D. Pearson,

PhD

Endothelial

Cell

Biology’

The endothelium is not a passive blood-compatible lining for the containment of blood cells and plasma, but rather it is a metabolically active tissue that subserves a wide range of functions relating to vascular homeostasis. This article reviews the current understanding of endothelial cell biology in terms of the molecules and biochemical pathways involved. These regulate coagulant and thrombotic properties of the vessel wall, vascular tone, and hence blood flow and pressure; changes leukocyte

in solute traffic

permeability during the

tion

of inflammatory responses; and finally of vessel growth and

Index

media,

terms:

State-of-art

generaand immune the processes

Radiology

or angioplasty.

Blood

vessels,

9*91,2

Vessel tone

9*92

#{149}

179:9-14

Vessel

‘1

growth

+

Endothelium the regulation vascular

I

control of vascular of each of the processes

homeostasis. noted here,

and

homeostasis. molecules,

This

is generally

as outlined

achieved

in more

detail

the past 2 decades, primarily as a result of the ability to culture vascular endothelial cells from an increasing number of vessels and species, there has been a dramatic change

in

of this

cell

our perception type

of the

role

(1). Originally

as a metabolically

uninterest-

ing cell the function of which was serve as a passive, albeit selective, permeability

the Section of Vascular Biology, MRC Research Centre, Watford Rd, Harrow HAl 3UJ, England. Received November 27, 1991; accepted December 5. Address reprint requests to the author. 2 9* indicates generalized vein and artery involvement. I

From

c RSNA,

1991

Endothelial cells participate actively which contribute to the maintenance by the secretion or surface expression in the Table.

There

N

viewed

Clinical

Leucocyte traffic

4

:;::i

bioactive

reviews 1991;

agg

and

angiogenesis. The review concludes with a consideration of how these functional properties can be disturbed, and their possible consequences, in response to irradiation, intravascular

contrast

+

Platelet

barrier

between

to

evidence

that the important homeostatic functions of endothelium can be selectively or generally perturbed, either temporarily-for example, in response to inflammatory mediators and

cytokines-or

ly-in

the

litic ease

more

course

permanent-

of infection,

vascu-

diseases, or atherosclerotic or as a result of therapeutic

sures

blood

is accumulating

in of of

including

dismea-

angiography

and

and the tissue spaces, endothelium can now be regarded as an organ distributed throughout the body, which carries out a wide variety of functions relating to vascular homeostasis

angioplasty. The Table indicates the range of endothelial cell functional properties now characterized at the molecular level. Pathophysiologic stimuli evoke temporal changes in

(Figure).

endothelial

These

functions

range

relatively long-lived properties can be phenotypically distinct ious parts of the vascular tree, the ability to transport solutes macromolecules, to minute-by-mm-

ute

responses

synthesis mediators.

to stimuli, and

secretion

such

from

cell

functions

by altering

that in varsuch

as

or

as the

of vasoactive

Abbreviations:

relaxing inhibitor,

tor, tPA

EDRF

factor, =

PCI2 tissue

PAl-i =

endothelium-derived

plasminogen

prostacyclin,

plasminogen

TF

activator fac-

tissue

activator.

the

secretion

one

or more

cules. major

surface

This

or expression

of these

review

properties in this context ble consequences to vessel wall

of

effector

mole-

summarizes

the

Endothelial

Prostacyclin;

Tissue

AND

Growth

Villa

thelial

cells

and

contribute

further

process by synthesizing and ing protein 5, a cofactor that ates the reactions of activated

factors,

tissue

plasminogen

released

into

are

the

source

activator

the

of

(tPA),

circulation

in re-

sponse to a variety of stimuli in vivo including catecholamines, vasopressin, and thrombin (5). tPA acts at the surface of polymerized fibrin to cleave the zymogen plasminogen to the active enzyme plasmin, thus mitiating dissolution of clotted blood. Endothelium also produces plasminogen activator inhibitor (PAl-i), the circulating physiologic inhibitor of

tPA,

though

the

details

of how

the

secretion ed are The

of tPA and PAl-i are relatnot well understood (6). antithrombotic properties of endothelial cells are due primarily to the

bile,

stimulated

secretion

low-molecular-mass

prostacyclin derived

(PCI2) relaxing

of two

endothelium(EDRF). These

two compounds are discussed more detail below in relation effects on vascular tone, but separately a potent inhibitor

10

#{149} Radiology

la-

mediators: and factor

degrading

molecules

in to their each is of plate-

enzymes,

(ELAM-l,

complex

Factor

Binding

sites

for Factors

Vessel

ICAM-l,

class 11

converting

let aggregation, their actions are synergistic, and EDRF is additionally an effective inhibitor of platelet adhesion to collagen or extracellular ma-

(7,8). AND

PROTHROMBOTIC PROPERTIES highly

polymerized

glycopro-

ting factor VIII, is secreted tively by endothelial cells.

tion,

von

Willebrand

portant cofactor to subendothelium,

which in for clotconstituIn addi-

factor

is an im-

platelet adhesion particularly in of higher shear stress. It is directly into the subendo-

conditions secreted

for

thelial matrix and is also stored in endothelial granules, the exocytotic release

of which

by agonists

such

is rapidly

triggered

as thrombin

(9).

When the intrinsic pathway of blood coagulation has been initiated, endothelial cells, like platelets, may provide a favorable surface to coordinate the cascade of reactions, since they bind factors IXa, Xa, and possibly VIlla; synthesize factor V; and present Va at their surface; and express suitable phospholipids to form the ternary complexes that activate factor Xa and prothrombin (10). Endothelial cells, unlike extravascular cell types, do not normally express tissue factor (thromboplastmn;

TF) and extrinsic

therefore pathway

do not

trigger

the

of coagulation.

In

vitro studies have shown, however, that after a lag period requiring protein synthesis, endothelial cells will express thrombin

also

function growth

Leukocyte

and

angiogenesis

TF activity in response to (11). Furthermore, TF is increased by exposing endotheli-

adhesion

and

emigration

Immune system. lymphocyte activation Coagulation Coagulation Coagulation Vessel tone, platelet function

Va, Xa, XIa

angiotensin

tein von Willebrand factor, the circulation is the carrier

The endothelial glycosammnoglycan surface is rich in heparan sulfates. These bind antithrombin and markedly increase its affinity for thrombin, thus providing the most important physiologic mechanism for the inactivation of thrombin (4).

cells

matrix

PROCOAGULANT

C(3).

coagulation

molecules

adhesion

Tissue

The

adhesion,

vessel tone activation

proteoglycans

Ectonucleotidases,

to this

Platelet

Leukocyte

Thrombomodulin

trices

function and and leukocyte

Fibninolysis

colony

ICAM-2, VCAM-1) Major histocompatibility

Endosecretaccelerprotein

plasminogen

interleukin-6; factors

;

expressed

Leukocyte

enhances activity.

Endothelial

Surface

activator,

Platelet Platelet

inhibitor-l

stimulating

activator

Function

oxide

plasminogen

matrix

Va and

Homeostasis

labile

nitric

activator Interleukin-l

circumstances, endothelial cells present a noncoagulant and nonthrombotic surface to flowing blood. Several surface properties contribute to this, including the expression of thrombomodulin (2). This glycoprotein binds thrombin and alters its substrate-binding properties, simultaneously lowering its affinity for fibrinogen and increasing its affinity for protein C, a circulating component of the hemostatic system. Protein C, which is activated by cleavage by thrombin, is anticoagulant by virtue of at least three reactions: It inactivates clotting plasminogen

Vascular

Platelet activating factor High molecular weight von Willebrand factor

normal

factors

and

Secreted molecules Low molecular weight

ANTITHROMBOTIC PROPERTIES Under

Products Product

of endothelial cells and outlines the possiof their disturbance behavior.

ANTICOAGULANT

Cell

enzyme

al cells to the cytokines interleukin-1 or tumor necrosis factor or to bacterial lmpopolysaccharides (12). This response proves to be one example of a variety of overlapping but distinct phenotypic alterations in endothelial behavior elicited by cytokine treatment (13). In terms of hemostasis and thrombosis, these agents provoke several changes that alter the balance of endothelial cell behavior in the procoagulant direction. Thus, in addition to TF expression, they enhance the ability of agonists to induce Secretion of von Willebrand factor from its granular stores (14), decrease the secretion of tPA, and increase the

release

of PAl-i

(15).

CONTROL

OF VASCULAR TONE

PCI2, first described as an inhibitor of platelet aggregation, was subsequently demonstrated to be a powerful dilator in a variety of blood yessels, acting (as in platelets) by stimulation of adenylate cyclase. Its systemic circulating level is too low for biologic action, and PCI2 is therefore believed to act locally and transiently when its synthesis and release from endothelium are induced by specific stimuli, notably thrombin, bradykinin, or adenine nucleotides (released from aggregating platelets) (16). The mechanisms leading to PCI2 synthesis are now well characterized and depend on the ability of agonists to couple receptors to signals leading to elevation of intracellular ionized calcium [Ca2] above a threshold

value

(17,18).

This

rived predominantly stores, and hence directly dependent

Ca2

can

be de-

from internal PCI2 release is not on extracellular

April

1991

Ca2+.

Elevated

[Ca2+]j

pholipase

A2, leading

precursor

arachidonate

pholipids

and

thesis, idly

lower

back

resting

A similar

but

raplevels

there

levels.

not

identical

range

of

agonists induces the release of EDRF from endothelial cells. This highly labile dilator was first characterized by Furchgott and colleagues and definitively identified as nitric oxide by Palmer

et al (19,20).

Unlike

PGI,

EDRF activates soluble guanylate cyclase in smooth muscle cells and platelets (21). NO is formed from the guanidino nitrogen atom of arginine, and NO synthase in endothelium has been shown to be a calmodulin/Ca2+ dependent enzyme (22,23). EDRF release is often prolonged by comparison with PCI2 release in response to the same agonist and, unlike PCI2 release,

does

not

on repeated fist ence

show

tachyphylaxis

challenge

but is very of external

with

an ago-

sensitive to the presCa2 (24,25). This

may be related to a lower [Ca2+]1 threshold to activate NO synthase than phospholipase A,., and to the maintenance of a prolonged, but small

elevation

of

[Ca2j

EDRF

synthesis

of Ca2+

inof

has

demon-

strated that both venous and arterial smooth muscle respond to EDRF (21) and that EDRF also contributes to endothelium-dependent vasodilatation-and hence control of blood flow and pressure-in microvascular beds (26). NO release can be detected continuously in isolated perfused organs (27), suggesting detectable basal levels of production, which have been

that

confirmed

in vivo

inhibition

yates

of NO

systemic

and

pressure (28). NO a moment-by-moment role in the control

by

showing

synthesis

regional

eleblood

may

therefore play physiologic of blood pressure,

its release perhaps being regulated by shearor stretch-sensitive receptors on the endothelium. A relative lack of receptors, or their

failure to couple to intracellular signalling mechanisms, has been suggested to be responsible for the reduced ability of diabetic, hypertensive, or atherosclerotic vascular tissues

to exhibit

endothelium-de-

pendent vasodilatation more, several agonists lease are vasoconstrictors ing directly on vascular

(29). Furtherof EDRF rewhen actsmooth

Volume

1

179

#{149} Number

thrombox-

is failure

of EDRF

production

in response to some (but not fists (30,31). More recently, the synthesis potent dothelin,

vasoconstrictor by endothelium

described not

(32).

all)

agoof a

peptide,

in all

en-

has been endothelin

In vivo,

a constrictor

vascular

is

beds,

and at low doses it can cause release of EDRF and hence endothelium-dependent vasodilatation (33). The regulation of its synthesis and release is not fully understood, but in vitro studies suggest that agents such as angiotensin and thrombin or altered shear stress enhance endothelin production by endothelial cells (32,34, 35). The possible physiologic or

pathophysiologic roles for endothelin are thus currently under investigation;

there

have

already

been

re-

ports, not all confirmed, that plasma endothelin levels are raised in hypertensive patients or following myocardial ischemia (36,37). control

requires

inhibitors

or action

adenosine

serotonin,

Temporal

in response

to agonist by activation flux mechanisms. The use of selective

acetylcholine,

ane A2), and this mechanism has been implicated in the generation of vessel spasm in atherosclerotic sections of coronary vessels, where

syn-

short-lived [Ca2ij

(eg,

triphosphate,

phos-

mechanisms

elevated

toward

of

PG!2

is usually

regulatory

muscle

phos-

from

consequent

which

because

activates

to release

that

agents are (like PCI2

of vascular

constrictor

either

tone

dilator

chemically

NO) or that mechaspecifically to inactivate

exist

them. ognized

Endothelial to play

an

cells are important

tiated (44).

by

arteriolar

Neutrophil quence tractant

now role

recin

vasodilatation

emigration

on

endothelium

teins,

(45).

including

the

to the

endothelial such

glycopro-

characligand

translocated in response

surface as thrombin

or hista-

mine (46,47), but other adhesion molecules are expressed after a time delay requiring protein synthesis. ELAM-1, a ligand for neutrophils, is induced in response to interleukin-1, tumor necrosis factor, or lipopolysaccharides and greatly enhances neutrophil adhesion (48).

and

ny

Another

Some

recently

terized neutrophil-adhesive GMP-140, can be rapidly

Monocyte adhesion cells is also enhanced hum is activated by

(39,40).

of chemoatinflamma-

tory site and/or the increase of endothelial surface leukocyte adhesion molecules. The latter process is the subject of considerable recent research, forming one part of the array of changes in cell function (activation) that occurs when cytokines act

this process. Thus it has been known for many years that vasoactive amines are efficiently removed from the circulation by endothelial cell uptake in microvascular beds and subsequent metabolism (38). Endothelial cell uptake and metabolism also account for the rapid clearance of several vasoactive prostaglandins adenosine

is a conse-

of the generation molecules at the

to agents unstable

and

nisms

culature, during the acute inflammatory response, leading to edema formation. One mechanism by which this takes place, still poorly understood, is as a consequence of the action of mediators such as histamine and bradykinin on endothelial receptors, leading to increased intercellular permeability (42). Another mechanism is dependent on the margination and emigration of neutrophil leukocytes, though again it is not clear exactly how neutrophil diapedesis causes protein leakage (43). In both cases, edema formation is poten-

to endothelial when endothecytokine pre-

treatment (45) and in response to monocyte treatment with interleukin-3 or granulocyte/monocyte cobstimulating

factor

(49).

With

the

mechanism for inactivation is the action of surface proteins that either bind agonists, such as antithrombin and thrombomodulin, or are catabolic enzymes, such as angiotensin-con-

current emphasis on monocyte emigration as an early event in the pathogenesis of atherosclerotic besions, it is also of interest that it has

verting

modified hances thelium

enzyme

(which

inactivates

bradykinin) or the ectonucleotidases responsible for breakdown of adenosine triphosphate and adenosine diphosphate (40,41). INFLAMMATION

AND

IMMUNITY Endothelial a selective passage

the This

cells normally provide permeability barrier to the

of plasma

lumen barrier

on the

constituents

to extravascular

venular

is disrupted,

side

from

tissue. particularly

of the

microvas-

recently

been

shown

that

minimally

low-density lipoprotein enmonocyte adhesion to endo(50).

Endothelial phocyte traffic

cells

also

regulate

lym-

at sites of chronic inflammation. Normal lymphocyte recirculation is programmed by selective homing receptors on lymphocyte subpopulations, which recognize sins)

complementary on the endotheliab

endothelial sue (51,52). plicated tenance

ligands cells

(addresof high

venules in lymphoid Interferon-’y has been

in the of this

generation selective

and

tisim-

main-

interaction,

Radiology

#{149} 11

and

similar

alterations

enzymes

in lymphocyte

lymphocyte-derived

cytokine,

to degrade

trix components,

traffic and endothelial cell morphology occur within chronic inflammatory lesions (53). Exposure of endothelium to interferon-’y or another

toward

extraceblubar

ma-

directed migration angiogenic stimulus, and

the

subsequent

cell

perimentab

evidence

replication

(62).

also

Ex-

strongly

implicates vascular pericytes in normal microvessebs as inhibitors of endothelial cell growth, by a mecha-

inter-

leukin-4, enhances lymphocyte adhesion: In this case increase of adhesion molecules including ICAM-1 and VCAM-1 is involved (54,55).

nism

that

growth

may

involve

factor

transforming

(63).

targeting therapy may be causing

of the effects of radiation to tumor vasculature, which selectively radiosensitive, thrombosis and vessel occlusion, has long been proposed as an attractive therapeutic strategy. Intravascular contrast media,

whether

ionic

markably

safe,

documented

or nonionic, but

incidence

complications

VESSEL GROWTH ANGIOGENESIS It seems likely nance of an intact um

in barge

that the maintelayer of endotheli-

vessels

is important

in

several ubation

ways for the appropriate regof intimab thickness and cebbularity. In experimental models invobving physical damage to endothehal cells (see also next section), lipidrich atherosclerotic lesions can

of atherosclerosis

One mechanism disruption of an

ability

barrier

possible

adhesion platelets

to lipids,

component

and

to damaged and

a second of secre-

that

endotheliab

can

cells

normally contribute signals to the tima and media that help maintain smooth muscle cells in a quiescent state, such as by secreting antiproliferative heparin-related molecules (58,59).

Recently,

it has

been

in-

shown

experimentally that nitric oxide is also an inhibitor of smooth muscle cell proliferation (60). Except at sites of damage, endothehal cell turnover is normally extremely slow, but capillary endothehal

cells

must

retain

the

capacity

for

rapid migration and proliferation in response to angiogenic stimuli. Angiogenesis is thus tightly controlled in normal

tissue,

ing embryogenesis, generation, and however,

initiated

occurring

only

endometrial wound healing. in several

dur-

reIt is, disease

states, notably in response to tumors, but also contributes to the pathologic increase of small vessels found in diabetic retinopathy, psoriasis, and chronic inflammatory diseases (61). The process of angiogenesis is obviously mubtifactoriab, but key elements of the endothelial cell response to angiogenic molecules include the stimulated secretion of 12

#{149} Radiology

of normal

involve

tion of inflammatory and mitogenic products. There is also evidence, however,

concerned

with

of vascular homeostasis the many potential distur-

has provided how this may

enhanced with

reasonable fects are function

of endotheli-

vessel

biology

that

or and

some illustrations of happen, for example,

in response to cytokines. Investigative radiology, angiography, and angioplasty by their nature

endothebium

leukocytes,

functions

review

could occur as a result of selective general endothebiab dysfunction,

is the perme-

is the

al cell

bances

in humans

involved appropriate

foregoing

maintenance highlights

develop in normobipemic animals (56). Similarly, homocystinemia, which causes endothebial cell injury, is associated with premature deveb-

opment (57).

The

the

exposure

re-

is a well-

of thrombotic

attributed

to the

use

of

these agents (70,71). Since contrast media inhibit rather than stimulate platelet function ex vivo (72), it is

EFFECTS OF IRRADIATION, CONTRAST MEDIA, AND ANGIOPLASTY

AND

are

there

of endothebial

vessel

to presume that due to endothebial leading to altered

wall

vascular

which tile

interactions.

actions

The

of contrast

include force

these efcell dysplatelet-

depression

and

cardioagents,

of contrac-

dilatation,

seem

to be

direct effects on heart and vascular muscle cells (73,74). Again, there few experimental data on studies

are to

investigate selective inhibition of endothelial cell function, but it is clear that contrast media, arguably at doses and exposure times relevant to clini-

cells to potentially injurious agents: radiation, contrast media, and physicab trauma. It is therefore pertinent to conclude with a brief survey of the documented effects on endothebium of these interventions. In vitro studies with cultured endothelial cells have indicated that they are at beast as sensitive as other cell types to radiation-induced growth inhibition (64). However, since (as noted above) endothelial cell turnover in vivo is normally very low, these assays may not be relevant. Nonetheless, reports of animal studies have regularly noted progressive injury to the microvasculature following exposure to moderate radiation doses (65). Whether this is a direct consequence of altered endothehal cell functions is not clear, but

cab practice, do have cytotoxic effects on cultured endothebial cells; the extent of these effects is dependent on the osmolality and ionic nature of the agent in question (75,76).

there

dilator mechanisms. Nonanticoagulant heparin has been used successfully in animal experiments to re-

is evidence

of early

alterations

to endothelial morphology accompanied by intravascular thrombosis and

cal

Finally, problems

pbasty:

and

release

of von

Wil-

and

cliniangio-

diffuse

therapy, vasodibators

perhaps to com-

bat the vasoconstriction evoked the loss of endothelium-dependent

there

transport

restenosis

and anticoagulant combined with

doses of a few hundred to a few thousand rads beads to demonstrable alterations in endothelial cell functions in vitro without cell death, including depression of PCI2 production, ectoenzyme activity, and amino lebrand factor (67-69). These data are, however, far from providing evidence that radiation doses used to demonstrate contrast agents, rather than those used for radiation therapy, have significant clinical effects on vessel function. In contrast, the

major with

caused to endothelial cells (77,78). Rapid thrombotic occlusion may be ameliorated by the use of antiplatelet

duce

acid

acute

are two associated

intimab thickening. The former is the result of thrombosis, and the latter is a consequence of smooth muscle cell proliferation, but in both cases there is good reason to suspect that the primary event is the extensive damage

increases in vascular per(66). Exposure to radiation

substantial meabibity

there

tion

smooth and

cell

thickening

is growing

evidence

in vessels

generated

muscle

intimal where

prolifera(58),

that

endothelium

following

by

but

even has

balloon

re-

injury,

this renewed endotheliab cell layer does not behave in the same manner as the original cells, particularly with respect to bipoprotein transport and its ability to induce endothelium-de-

pendent

vasodilatation

(79,80).

present, it seems that the side effects of angioplastic dures may be an inevitable

quence caused

of the severe to endothelium,

At

unwanted proceconse-

disruption the active April

1991

metabolic

functions

of which

a series

of integral

components

quired

for

stasis.

successful

vascular

provide

rehomeo-

U. 19.

References 1.

2. 3.

4.

5.

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