September 2008

Arteriogenesis:

Figure 1: Collateral formation after

right coronary artery occlusion. [1]

Prognosis and Therapeutical Potential

Darius Henatsch

Faculty of Health, Medicine and Science

Abstract

Maastricht University

Due to the fast growing field of molecular cardiology, mechanisms responsible

for collateral artery growth (arteriogenesis) are widely unravelled. After coronary

Table of Content

artery occlusion, flow shear stress induces endothelial cell expression and release

of adhesion molecules and chemokines. Monocytes are attracted and

transmigrate into the vessel wall to release pro-arteriogenic molecules. The

process of collateral vessel growth is severely impaired in patients with different

1 Introduction 2

risk factors for atherosclerosis, as diabetes or hyperlipedemia. This can be

attributed to the impaired migration capacity of monocytes towards pro-

2 General Concepts of

arteriogenic stimuli, like MCP-1 and VEGF-A. In these patients "at risk", blood

Arteriogenesis 2

derived monocytes offer a possibility to predict the individual ability to form

functional collaterals by an in vitro performed migration assay. Furthermore,

3 Those Who Play the Game

growth factor therapy can be used to enhance and restore collateral growth in

3.1 The Endothelium 4

patients with impaired arteriogenesis. Here, the migration assay could perfectly

3.2 Arteriogenic Stimuli

be used to for individual adjustment of therapeutical interventions.

3.2.1 MCP-1 4
3.2.1 VEGF 5
3.2.3 PlGF 5
3.2.4 FGF 6

Abbreviations

AAV

Adeno-associated virus

MCP-1 Monocyte chemoattractant

4 How can it go wrong?

bFGF Basic fibroblast growth factor

protein-1

Impairment of Arterio-

CAD

Coronary artery disease

MI

Myocardial Infarction

genesis 7

CCR2 CC-chemokine receptor 2

MMPs Matrix metalloproteinases

CTO

Total coronary occlusion

PDGF Platelet-derived growth factor

5 To the Clinic: Prognosis and

CVD Cardiovascular disease

PlGF

Placental growth factor

Future Therapy

DM

Diabetes mellitus

PECAM Platelet endothelial cell

5.1 Concrete Role of

ETT

Exercise tolerance test

adhesion molecule

Monocytes 8

ECs

Endothelial cells

SHP

SH2-domain-containing

5.2 The Way to Predict

ERK

Extracellular-signal regulated

protein tyrosine phosphatase

Collateral Growth 8

kinasen

SMCs Smooth muscle cells

5.3 Future Therapy 10

FSS

Fluid shear stress

TGF- Transforming growth factor

HC

Hypercholesterolemia

beta

6 Discussion and Conclusion...12

HIF

Hypoxia-inducible factor

TNFa

Tumor necrosis factor-alpha

ICAM Intercellular adhesion molecule

VCAM Vascular cell adhesion

ITIM

Immunoreceptor tyrosine-

molecule

7 References 14

based inhibitory motif

VEGF Vascular endothelial growth

LAD

Left anterior descending

factor

LDL

Low density lipo-protein

VRACs Volume regulated endothelial

chloride channels

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ARTERIOGENESIS

1 Introduction

2 General Concepts of
Arteriogenesis

Cardiovascular disease (CVD) is the major

cause of death in the United States. 80,7

Physiological blood vessels growth does not

million American adults (one out of three)

happen in an adult organism, with the

suffer from one or more types of CVD of

exception of the female reproductive system.

which are nearly 50% (38,2 million) is 60

H o w e v e r , u n d e r p a t h o p h y s i o l o g i c a l

years of older. It is estimated that in 2008 about

circumstances, like tissue repair, the de novo

770 000 Americans will experience a new

growth and sprouting of new capillaries

coronary attack and additional 175 000 will

(angiogenesis) and the growth and remodelling

have a first silent myocardial infarctions (MI).

of pre-existing collateral anastomoses

[2] However, a natural occurring escape

(arteriogenesis), can be observed. [7,8,9] The

mechanism in atherosclerosis saves cardiac

main stimulus for angiogenesis is ischemia,

tissue from ischemia, the growth of a collateral

with an up regulation of hypoxic factors like

circulation, [3] already observed decades ago.

hypoxia-inducible factor (HIF) 1-alpha and

[1] In patients with advanced coronary artery

vascular endothelial growth factor (VEGF),

disease (CAD), anastomoses can transform to

[5,7,10] whereas biomechanical forces, mainly

functional collaterals and contribute to tissue

fluid shear stress (FSS), induce the process of

perfusion, by the process called arteriogenesis.

arteriogenesis. [7,11] A third form of vessel

[4] Moreover, collateral formation can nearly

growth is vasculogenesis. In this process of de

totally restore cardiac tissue perfusion in

novo development of blood vessels, restricted

slowly progressive stenosis. In this way

to the embryonic phase, mesodermal cells give

patients with occluded coronary arteries can

rise to angioblasts. [5,9] It is expected that pre-

stay asymptomatic. [5] Unfortunately, the

existing interconnection that can grow out to

process of arteriogenesis is negatively

functioning collaterals are already created from

influenced by risk factors, like hyperlipedemia

the vascular plexus by vasculogenesis during

and diabetes, which also contribute to the

the embryological phase. [3] Angiogenesis

development of CAD. [6] The growth of

alone cannot sufficiently restore adequate

coronary collateral vessel is of functional

tissue perfusion after the event of cardiac

importance in patients with regional cardiac

arterial occlusion. In fact, large diameter

ischemia after severe coronary stenosis or

vessels are needed instead of small capillaries

occlusion. Coronary collaterals can limit

because only muscular arteries can assure the

infarction size after MI and it is important to

transport of blood over longer distances. The

exactly understand the process of collateral

process of arteriogenesis takes place by mitosis

formation, to predict collateral formations in

of endothelial cells (ECs) and smooth muscle

patients at risk and for possible therapeutic

cells (SMCs) of pre-existing anastomoses

interventions. [4] In this paper a broad

under the influence of several growth signals.

overview of the mechanism of arteriogenesis is

[12] Following maturation, collaterals only

given and it is demonstrated how to achieve an

differ minor in histological aspects from other

individual prognosis of collateral growth.

arteries, with a muscular wall and more

Furthermore possible future therapies are

collagen. From more importance is their

presented and discussed.

difference in anatomical appearance. They are

more tortuous and re-entry in the distal part of

the occluded artery with a non-physiological

angle (figure 2).

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ARTERIOGENESIS

force tangential to the tube is called shear

stress, whereas the force perpendicular to the

wall is called circumferential stretch (figure 3).

Shear stress causes morphological and

cytoskeletal changes, as well as a change in the

gene expression pattern. When shear stress is

elevated chronically, a dilatation in vessel

diameter can be observed, whereas a chronic

pressure elevation decreases vessel diameter.

[13]

Figure 2: Development of collaterals after arterial
occlusion is induced by a pressure gradient. [11]

The collateral growth process is dependent on

proliferation of SMCs, adventitial fibroblasts

and ECs. Proliferation starts about 25 hours

after occlusion with a peak after 3-7 days.

Mitotic activity is still higher than normal 3

weeks after the occlusion. [11]

In contrast to the growth of other vessel forms,

FSS is the primary inducer of arteriogenesis.

After the event of arterial stenosis and

occlusion a sudden evolving pressure change

leads to a pressure gradient along the collateral

network with a dramatically increasing FSS in

the anastomoses, [7] accompanied by

Figure 3: Mechanical forces in a blood vessel, created

circumferential wall stress. [12] This strong

by blood-flow. [13]

enhancement of the pressure gradient between

parts, before and proximal to the occlusion

The influence of FSS is more difficult to prove

favours blood flow through pre-existing

than that of vessel wall distension. [11] The

collaterals. [11] After a prior pressure peak, an

tangential force of blood flow ranges between

i n c r e a s i n g c o l l a t e r a l v e s s e l d i a m e t e r

10 and 70 dye cm2 and acts mainly on ECs [5],

diminishes shear stress. Not totally clear is,

whereas the circumferential wall stress

which affect of pressure, like longitudinal or

activates smooth muscle cells (SMCs) in the

radial pressure, finally induces and supports

vessel wall by distension. It could be shown

collateral growth. [7] When a major artery is

that the endothelial cytoskeleton is coupled to

occluded, many small diameter anastomoses

shear stress receptors, responsible for the up

are formed rather big ones, leading to a higher

regulation of shear stress related genes after

resistance and energy loss, according to

exposure to FSS changes, as shown below.

Poiseuille′s Law. Corresponding to this law,

Also structures like integrins and tyrosine

flow is directly dependent on the circular

kinases are able to pass information indirectly

c r o s s - s e c t i o n o f t h e v e s s e l . F i n a l l y,

to shear stress receptors and finally induce to

conductance that is achieved by newly formed

the alteration of gene expression. [11,12]

collaterals is not more than 40% of the

previous value (by normal blood pressure and

vasodilatation). Two types of force are exerted

by blood when flowing through a vessel. The

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ARTERIOGENESIS

macrophages and begin to produces a broad

number of growth factors, cytokines and

3 Those Who Play the Game

proteases. These are amongst others, basic

fibroblast growth factor (bFGF), tumor

necrosis factor-alpha (TNFa), MCP-1 and

3.1 The Endothelium

matrix metalloproteinases (MMPs). Latter are

important because of their protease effect on

FSS induces cell swelling in ECs that is

the endothelium. [10]

antagonized by the efflux of osmolytes by

volume regulated endothelial chloride channels

(VRACs). This in turn changes pH and

enhances Ca2+ influx and induces the

expression of multiple genes, responsible for

the attraction and adhesion of circulating blood

cells. [11,12] ECs express amongst others the

immunoglobulin family receptor platelet

e n d o t h e l i a l c e l l a d h e s i o n m o l e c u l e

(PECAM)-1. With its cytoplasmic domain this

homophilic adhesion receptor binds to ß- and

-catenins and contains an immunoreceptor

tyrosine-based inhibitory motif (ITIM), which

Figure 4: The central role of ECs in arteriogenesis.

binds after phosphorylation to SH2-domain-

[3]

containing protein tyrosine phosphatase

(SHP)-2. After the onset of flow, mechano-

3.2 Arteriogenic Stimuli

transduction is triggered and phosphorylation

of PECAM-1 ITIM tyrosines takes place with

In arteriogenesis complex interactions between

extracellular-signal regulated kinasen (ERK)

numerous cells take place. One important step

activation as result. The event is down

is the recruitment of inflammatory cells to the

regulated after adaptation of ECs to laminar

side of tissue hypo-perfusion, with a key role

flow. In the case of shear stress, flow direction

for chemokines. [15] Multiple agents play an

and magnitude change frequently, followed by

important role in supporting the process of

a continual activation of intracellular pathways.

arteriogenesis, of which some essential are

This results in activation of NF- B and NF- B

discussed here.

dependent genes. [14] Consequently, proteins

as intercellular adhesion molecule (ICAM)-1,

3.2.1 MCP-1

vascular cell adhesion molecule (VCAM)-1, E-

FSS leads to the expression and enhanced

selectin and platelet-derived growth factor

release of endothelial adhesion molecules and

(PDGF) are up regulated. [10,14] Mainly

MCP-1. This molecule attracts inflammatory

ICAM-1/2 and VCAM-1 are responsible for

cells, especially monocytes/macrophages,

the adhesion of lymphocytes, further supported

which again stimulate the proliferation of

by stimulation of integrin expression on

endothelial and smooth muscle cells. [15]

monocytes by the secretion of VEGF and

MCP-1 is seen as the most important chemo-

monocyte chemoattractant protein-1 (MCP-1)

attractant factor for monocytes and mediates its

by ECs, as discussed later. [11] Circulating

function via CC-chemokine receptor 2 (CCR2)

monocytes can adhere to the activated

binding, a member of the G-protein coupled

endothelium and migrate into the vessel wall.

receptors. [16]

After accumulation, these monocytes mature to

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