Current Pharmaceutical Design - Volume 9, Issue 7, 2003

Angiogenesis is a complex process, where several cell types and mediators interact to establish a specific microenvironment suitable for the formation of new capillaries from pre-existing vessels. Such biological processes occur in several physiological conditions, such as embryo development and wound healing, as well as in pathological conditions, including tumours and diabetic retinopathy. T lymphocytes, neutrophils and monocytes fully participate in the angiogenic process by secreting cytokines that may control endothelial cell (EC) proliferation, their survival and apoptosis, as well as their migration and activation. Angiogenesis is the result of a net balance between the activities exerted by positive and negative regulators. This balance is conceptually very similar to that of the Th1 / Th2 cells that modulate an appropriate and specific immune response. Th1 or Th2 cytokines may control angiogenesis directly, by acting on cell growth and differentiation, indirectly by inducing the release of other cytokines in the microenvironment, and by modulating the expression of specific receptors, involved in the control of angiogenic processes, such as EC proliferation and migration. In this review we will mainly discuss the role of Th1- and Th2-type cytokines in the angiogenic process, emphasizing the complexity of the cytokine and leukocyte / EC network, and highlighting the care that needs to be taken when designing new therapeutic interventions involving Th1 and Th2 cytokines.

Angiogenesis, the development of new capillaries form pre-existing vessels, requires the coordinate activation of endothelial cells, which migrate and proliferate in response to growth factors to form functional vessels. Therapeutic angiogenesis is proposed to restore tissue integrity and function following damage and ischemia, while strategies aimed to block or suppress the neovascular growth are designed as adjuvant therapies for cancer treatment. Different experimental and clinical observations support the existence of a molecular / biochemical link between vasodilation, nitric oxide (NO) production and angiogenesis. NO significantly contributes to the prosurvival / proangiogenic program of capillary endothelium by triggering cell growth and differentiation via endothelial-constitutive NO synthase (ecNOS) activation, and cyclic GMP (cGMP) dependent gene transcription. Re-establishment of a balanced NO production in the cardiovascular system results in a reduction of cell damage during inflammatory and vascular diseases. Elevation of NOS activity in correlation with angiogenesis and tumor growth and aggressiveness has been extensively reported in experimental and human tumors. On these bases, the nitric oxide pathway appears to be a promising target for the development of pro- and anti-angiogenic therapeutic strategies. In particular, the use of NOS inhibitors or NO scavengers seems appropriate to reduce edema, block angiogenesis and facilitate antitumor drug delivery.

In the last few decades it has become clear that detailed understanding of the mechanisms of angiogenesis, a process leading to growth of new blood vessels, should lead to improved treatment of diseases such as ischemic disorders and cancer where neovascularization is impaired or activated, respectively. In this review, we will outline some of our recent findings concerning the regulation of the vascular endothelial growth factor (VEGF), a key player in angiogenesis and one of its transcription factors, the hypoxia-inducible factor-1 (HIF-1) a master gene product driving adaptation to hypoxia. We will discuss the observation that growth factors and oncogenic transformation via the mitogen-activated protein kinases p42 / p44 MAPKs not only activate the VEGF promoter through the Sp1 / AP-2 transcriptional factor complex but also phosphorylate HIF-1α leading in turn to enhance HIF-1 dependent transcriptional activation of VEGF. The stress-activated protein kinases (SAPK) also contribute to angiogenesis by stabilizing VEGF mRNA. Finally, we will present recent advances into oxygen-sensing, in particular the HIF-hydroxylases that govern HIF-1α instability (PHD2) or inactivation (FIH-1). The revelation of these oxygen sensors has provided pharmacologists with new molecular targets for the development of novel therapies to control angiogenesis either positively or negatively.

The plethora of cellular pathways and events involved in angiogenesis are a prime example of the widespread role of calcium ion flux in biological functions. Indeed, calcium is a main point of intersection for many distinct molecular signaling pathways that promote and modulate angiogenesis. Here, we illustrate some of the important aspects of calcium induction, function, downstream effects, and resulting cellular changes that ensue. We describe some of the main mechanisms of calcium regulation in cells as well as intracellular and cross-membrane flux, highlighting key players that are known to facilitate these events. We review some of the major signaling pathways that tie into angiogenesis, and also describe how cellular phenotypic changes that occur during angiogenesis require processes rich in calcium ion stimulation of gradient shifts. Lastly, we hypothesize on current thinking of the role of calcium as a whole in angiogenic cellular function and propose new insight into calcium as a universal effector molecule and a prime target for therapeutic intervention.

Angiogenesis is the process of generating new capillary blood vessels. Uncontrolled endothelial cell proliferation is observed in tumor neovascularization and in angioproliferative diseases. Tumors cannot growth as a mass above few mm3 unless a new blood supply is induced. It derives that the control of the neovascularization process may affect tumor growth and may represent a novel approach to tumor therapy.Angiogenesis is controlled by a balance between proangiogenic and antiangiogenic factors. The angiogenic switch represents the net result of the activity of angiogenic stimulators and inhibitors, suggesting that counteracting even a single major angiogenic factor could shift the balance towards inhibition.Heparan sulfate proteoglycans are involved in the modulation of the neovascularization that takes place in different physiological and pathological conditions. This modulation occurs through the interaction with angiogenic growth factors or with negative regulators of angiogenesis. Thus, the study of the biochemical bases of this interaction may help to design glycosaminoglycan analogs endowed with angiostatic properties.The purpose of this review is to provide an overview of the structure / function of heparan sulfate proteoglycans in endothelial cells and to summarize the angiostatic properties of synthetic heparin-like compounds, chemically modified heparins, and biotechnological heparins.

Fibroblast growth factor receptors (FGFR) are members of a family of polypeptides synthesized by a variety of cell types during the processes of embryonic development and in adult tissues. FGFR have been detected in normal and malignant cells and are involved in biological events that include mitogenic and angiogenic activity with a consequent crucial role in cell differentiation and development. To activate signal transduction pathways, FGFR are coupled to fibroblast growth factors (FGF) and heparan sulfate (HS) proteoglycans to form a biologically fundamental ternary complex. Based on these considerations, a variety of inhibitors able to block the signaling cascade through a direct interaction with FGFR have been designed and investigated for their biological properties related to antiangiogenesis and antitumor activity. The purpose of this review is to focus on synthetic chemical approaches aimed at blocking tyrosine kinase (TK) receptors, members of the FGFR family. In particular, a literature survey aimed at summarizing on the structural properties that a compound should possess to show affinity toward FGFR is presented, and structure-activity relationships (SAR) on FGFR inhibitors are delined.

Thrombospondin-1 (TSP-1) was one of the first endogenous inhibitors of angiogenesis to be discovered. This large multimodular protein of around 600 kDa inhibits endothelial cell proliferation, migration and morphogenic organization into capillary tubes. TSP-2 shares homology with TSP-1 in primary sequence, structural organization and angiostatic properties. TSP-1-null and TSP-2-null mice display increased tissue vascularity and enhanced sensitivity to carcinogenesis. Conversely, overexpression of TSP-1 or TSP-2 in cancer cells results in reduced tumor vascularization and tumor growth. In this review, we focus on the preclinical data obtained in experimental anti-tumorigenic assays using either TSP-1, TSP-2 or shorter peptides derived from the type 1 repeats of these molecules. In contrast with the full length thrombospondin molecules, which present a poor bioavailability and are highly susceptible to proteolytic degradation, TSP-derived angiostatic peptides appear as potent and promising therapeutic agents in anti-angiogenic therapy.

Recently, therapeutic angiogenesis has been proposed as an alternative for the treatment of ischemic diseases unresponsive to conventional therapy. This strategy is based on the concept that a supply-side approach with growth factors would overcome the endogenous deficit and result in more robust collateralization. We have developed a strategy based on local delivery of human tissue kallikrein gene for potentiation of microcirculation and rescue of peripheral ischemia. Following successful application in otherwise healthy animals, the approach resulted to be of therapeutic value in rats with endothelial dysfunction caused by arterial hypertension. In addition, human tissue kallikrein prevents or rescues microvascular rarefaction caused by diabetes mellitus. In this model, human tissue kallikrein was able to stimulate vascular growth and contrast apoptosis. The strategy displays interesting pharmacological features because is devoid of obvious side effects and is effective even at low infecting doses. In addition, the neovascularization promoted by human tissue kallikrein is well organized and durable. It is reasonable to anticipate that the new approach will have a great impact in the treatment of cardiovascular ischemic complications.