Arteriogenesis - Prognosis and Therapeutical Potential

Table of Content

1 Introduction

2 General Concepts of Arteriogenesis

3 Those Who Play the Game
3.1 The Endothelium
3.2 Arteriogenic Stimuli
3.2.1 MCP-1
3.2.1 VEGF
3.2.3 PlGF
3.2.4 FGF

4 How can it go wrong? Impairment of Arterio- genesis

5 To the Clinic: Prognosis and Future Therapy
5.1 Concrete Role of Monocytes
5.2 The Way to Predict Collateral Growth
5.3 Future Therapy

6 Discussion and Conclusion

7 References

Abstract

Due to the fast growing field of molecular cardiology, mechanisms responsible for collateral artery growth (arteriogenesis) are widely unravelled. After coronary 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 risk factors for atherosclerosis, as diabetes or hyperlipedemia. This can be attributed to the impaired migration capacity of monocytes towards pro-arteriogenic stimuli, like MCP-1 and VEGF-A. In these patients “at risk”, blood derived monocytes offer a possibility to predict the individual ability to form functional collaterals by an in vitro performed migration assay. Furthermore, growth factor therapy can be used to enhance and restore collateral growth in patients with impaired arteriogenesis. Here, the migration assay could perfectly be used to for individual adjustment of therapeutical interventions.

Abbreviations

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1 Introduction

Cardiovascular disease (CVD) is the major cause of death in the United States. 80,7 million American adults (one out of three) suffer from one or more types of CVD of which are nearly 50% (38,2 million) is 60 years of older. It is estimated that in 2008 about 770 000 Americans will experience a new coronary attack and additional 175 000 will have a first silent myocardial infarctions (MI). [2] However, a natural occurring escape mechanism in atherosclerosis saves cardiac tissue from ischemia, the growth of a collateral circulation, [3] already observed decades ago. [1] In patients with advanced coronary artery disease (CAD), anastomoses can transform to functional collaterals and contribute to tissue perfusion, by the process called arteriogenesis. [4] Moreover, collateral formation can nearly totally restore cardiac tissue perfusion in slowly progressive stenosis. In this way patients with occluded coronary arteries can stay asymptomatic. [5] Unfortunately, the process of arteriogenesis is negatively influenced by risk factors, like hyperlipedemia and diabetes, which also contribute to the development of CAD. [6] The growth of coronary collateral vessel is of functional importance in patients with regional cardiac ischemia after severe coronary stenosis or occlusion. Coronary collaterals can limit infarction size after MI and it is important to exactly understand the process of collateral formation, to predict collateral formations in patients at risk and for possible therapeutic interventions. [4] In this paper a broad overview of the mechanism of arteriogenesis is given and it is demonstrated how to achieve an individual prognosis of collateral growth. Furthermore possible future therapies are presented and discussed.

2 General Concepts of Arteriogenesis

Physiological blood vessels growth does not happen in an adult organism, with the exception of the female reproductive system. However, under pathophysiological circumstances, like tissue repair, the de novo growth and sprouting of new capillaries (angiogenesis) and the growth and remodelling of pre-existing collateral anastomoses (arteriogenesis), can be observed. [7,8,9] The main stimulus for angiogenesis is ischemia, with an up regulation of hypoxic factors like hypoxia-inducible factor (HIF) 1-alpha and vascular endothelial growth factor (VEGF), [5,7,10] whereas biomechanical forces, mainly fluid shear stress (FSS), induce the process of arteriogenesis. [7,11] A third form of vessel growth is vasculogenesis. In this process of de novo development of blood vessels, restricted to the embryonic phase, mesodermal cells give rise to angioblasts. [5,9] It is expected that pre­existing interconnection that can grow out to functioning collaterals are already created from the vascular plexus by vasculogenesis during the embryological phase. [3] Angiogenesis alone cannot sufficiently restore adequate tissue perfusion after the event of cardiac arterial occlusion. In fact, large diameter vessels are needed instead of small capillaries because only muscular arteries can assure the transport of blood over longer distances. The process of arteriogenesis takes place by mitosis of endothelial cells (ECs) and smooth muscle cells (SMCs) of pre-existing anastomoses under the influence of several growth signals. [12] Following maturation, collaterals only differ minor in histological aspects from other arteries, with a muscular wall and more collagen. From more importance is their 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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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 circumferential wall stress. [12] This strong enhancement of the pressure gradient between parts, before and proximal to the occlusion favours blood flow through pre-existing collaterals. [11] After a prior pressure peak, an increasing collateral vessel diameter diminishes shear stress. Not totally clear is, which affect of pressure, like longitudinal or radial pressure, finally induces and supports collateral growth. [7] When a major artery is occluded, many small diameter anastomoses are formed rather big ones, leading to a higher resistance and energy loss, according to Poiseuille's Law. Corresponding to this law, flow is directly dependent on the circular cross-section of the vessel. Finally, conductance that is achieved by newly formed 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 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]

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Figure 3: Mechanical forces in a blood vessel, created by blood-flow. [13]

The influence of FSS is more difficult to prove than that of vessel wall distension. [11] The tangential force of blood flow ranges between 10 and 70 dye cm2 and acts mainly on ECs [5], whereas the circumferential wall stress activates smooth muscle cells (SMCs) in the vessel wall by distension. It could be shown that the endothelial cytoskeleton is coupled to shear stress receptors, responsible for the up regulation of shear stress related genes after exposure to FSS changes, as shown below. Also structures like integrins and tyrosine kinases are able to pass information indirectly to shear stress receptors and finally induce to the alteration of gene expression. [11,12]

3 Those Who Play the Game

3.1 The Endothelium

FSS induces cell swelling in ECs that is 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 endothelial cell adhesion molecule (PECAM)-1. With its cytoplasmic domain this homophilic adhesion receptor binds to ß- and 7-catenins and contains an immunoreceptor tyrosine-based inhibitory motif (ITIM), which binds after phosphorylation to SH2-domain-containing protein tyrosine phosphatase (SHP)-2. After the onset of flow, mechano-transduction is triggered and phosphorylation of PECAM-1 ITIM tyrosines takes place with extracellular-signal regulated kinasen (ERK) activation as result. The event is down regulated after adaptation of ECs to laminar flow. In the case of shear stress, flow direction and magnitude change frequently, followed by a continual activation of intracellular pathways. This results in activation of NF-id3 and NF-id3 dependent genes. [14] Consequently, proteins as intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, E-selectin and platelet-derived growth factor (PDGF) are up regulated. [10,14] Mainly ICAM-1/2 and VCAM-1 are responsible for the adhesion of lymphocytes, further supported by stimulation of integrin expression on monocytes by the secretion of VEGF and monocyte chemoattractant protein-1 (MCP-1) by ECs, as discussed later. [11] Circulating monocytes can adhere to the activated endothelium and migrate into the vessel wall. After accumulation, these monocytes mature to macrophages and begin to produces a broad number of growth factors, cytokines and proteases. These are amongst others, basic fibroblast growth factor (bFGF), tumor necrosis factor-alpha (TNFa), MCP-1 and matrix metalloproteinases (MMPs). Latter are important because of their protease effect on the endothelium. [10]

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Figure 4: The central role of ECs in arteriogenesis. [3]

3.2 Arteriogenic Stimuli

In arteriogenesis complex interactions between numerous cells take place. One important step is the recruitment of inflammatory cells to the side of tissue hypo-perfusion, with a key role for chemokines. [15] Multiple agents play an important role in supporting the process of arteriogenesis, of which some essential are discussed here.

3.2.1 MCP-1

FSS leads to the expression and enhanced release of endothelial adhesion molecules and MCP-1. This molecule attracts inflammatory cells, especially monocytes/macrophages, which again stimulate the proliferation of endothelial and smooth muscle cells. [15] MCP-1 is seen as the most important chemo-attractant factor for monocytes and mediates its function via CC-chemokine receptor 2 (CCR2) binding, a member of the G-protein coupled receptors. [16]

In animal studies with mice that lack CCR2, arteriogenesis was drastically reduced as well as structure and function lost. The factor is up regulated during a strong angiogenic response and also during early phase of arteriogenesis. These findings are confirmed by many studies, often by local infusion of MCP-1 into the proximal stump in an occluded femoral artery model. [7] Arteriogenesis seems to takes place in two phases. In the early phase already existing arterioles are recruited, leading to an enhanced conductance in the first days after occlusion. In the sub-acute phase some vessels selectively grow out and increase 10 fold in diameter, accompanied by the regression of some smaller vessels (pruning process). This happens in a time course over weeks after occlusion. [17] Animal studies confirm that e.g. mice, deficient for either the MCP-1 receptor on monocytes (CCR2), ICAM-1 or CD44 show a strong impairment for arteriogenesis. [12]

3.2.2 VEGF

One of the strongest mitogens on endothelial and smooth muscle cells is VEGF, which function for ECs is known for long. [19,20] Members of the VEGF family are VEGF-A, VEGF-B, placental growth factor (PlGF), VEGF-C and VEGF-D and bind to the receptors VEGFR-1, 2 and 3, primary on the endothelium. Both, VEGF and PlGF are members of the tyrosine kinase family. [16] VEGF-A is important in the process of angiogenesis, whereas PlGF mainly mediates arteriogenesis. VEGF-A binds amongst other to two different receptors, VEGFR-1 and VEGFR-2, and is strongly induced under hypoxic condition by HIF and HIF regulated elements. [19] VEGF-2 is more prominent in ECs signalling and binds VEGF-A and E, whereas VEGFR-1, as the exclusive VEGF receptor on monocytes binds also PlGF. [21,22,23] In the process of arteriogenesis, a role of SMCs is also expected because an up regulation of VEGFR-1 after injury of the vessel wall can be observed. [23]

ECs up regulate VEGF-A mainly due to hypoxic stimuli but also after shear stress. VEGF-A induces chemotaxis of monocytes and also the release of MCP-1, as an even stronger stimulus for monocyte migration. The important role of VEGF-A in collateral formation was proven in several animal studies using ischemic cardiac and hind limp models. [24] It is expected that VEGF-A is able to form a heterodimer of VEGFR-1 and 2. In a study, neither VEGFR-2 stimulation via VEGF-E, nor VEGFR-1 stimulation via PlGF-2 alone could increase perfusion to the ischemic limp as strong as VEGF-A as a ligant for both receptor types. [23]

Besides many definitive results, concerning the role of VEGF in the process of arteriogenesis, there is also some controversy between several research groups described in literature. [12,20] One group, to cite as an example, could only find minor effects of VEGF in collateral diameter and conductance, also when given in concentration more than 30 times higher than the active concentration of MCP-1. This was further confirmed in a model, where VEGF was trapped, without negatively influencing arteriogenesis. [12]

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Figure 5: Different actions of VEGF-A. [4]

3.2.3 PlGF

An infusion of PlGF, with its high bindings-affinity for VEGFR-1, can significantly enhance visible collateral numbers and conductance.

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