Angiogenesis gene therapy to rescue ischaemic tissues: achievements and future directions

Abstract

Ischaemic diseases are characterized by an impaired supply of blood resulting from narrowed or blocked arteries that starve tissues of needed nutrients and oxygen. Coronary-atherosclerosis induced myocardial infarction is one of the leading causes of mortality in developed countries. Ischaemic disease also affects the lower extremities. Considerable advances in both surgical bypassing and percutaneous revascularization techniques have been reached. However, many patients cannot benefit from these therapies because of the extension of arterial occlusion and/or microcirculation impairment. Consequently, the need for alternative therapeutic strategies is compelling. An innovative approach consists of stimulating collateral vessel growth, a natural host defence response that intervenes upon occurrence of critical reduction in tissue perfusion (Isner & Asahara, 1999). This review will debate the relevance of therapeutic angiogenesis for promotion of tissue repair. The following issues will receive attention: (a) vascular growth patterns, (b) delivery systems for angiogenesis gene transfer, (c) achievements of therapeutic angiogenesis in myocardial and peripheral ischaemia, and (d) future directions to improve effectiveness and safety of vascular gene therapy.

Figure 1

Immunohistochemical identification of vascular endothelial cells using antibodies against von Willebrand factor. Skeletal muscle sections were harvested from ischaemic hindlimbs of mice, 21 days after surgical removal of the femoral artery. Representative pictures show higher capillary density in adductor muscle injected with an adenoviral vector carrying the human tissue kallikrein gene (B) or reporter gene encoding for beta-galactosidase (A). (Emanueli et al., 2001a; reproduced with permission of publishers).

Figure 2

Representative sketch showing putative mechanisms (numbered in circles) of the angiogenic action exerted by human tissue kallikrein (HK). After delivery to targeted tissue by an adenoviral vector (Ad.CMV-HK, step 1), HK is released from infected skeletal muscle into the interstitial space and the blood stream (step 2). HK may contribute to digest vascular basal membrane (VBM), thus favouring vascular endothelial cell (EC) detachment and migration (step 3). Furthermore, as indicated in magnification of an EC, TK cleaves kininogen (KNG) to generate kinins which, via activation of inducible B₁ (B₁R, dashed squares on EC surface) and constitutive B₂ (B₂R, full squares on EC surface) receptors (step 4) and subsequent release of nitric oxide and cyclooxygenase 2 (COX2)-formed prostaglandins (step 5), stimulate vascular EC proliferation (step 6). Binding of kinin receptors may shift the receptor repertoire in favour of the inducible B₁R (inducibility is indicated by dashed line-squares reproducing the B₁R). Thus, the B₁R can act as magnets for TK-generated kinins. Other growth factors, including vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF-2), could be up-regulated by kinins. Furthermore, kinin-attracted leukocytes (PMNs) may contribute to stimulation of angiogenesis by providing an additional source of growth factors, kininogen, and kinin generating enzyme (step 7). VEGF released by leukocytes stimulates endothelial proliferation by interacting with its own receptors on EC.