Current Pharmaceutical Design - Volume 11, Issue 7, 2005

Snake venom disintegrins are the natural products that have been investigated as potent inhibitors of various integrins. Functionally, disintegrins can be divided into three groups according to their integrin selectivity and presence of specific, active motifs. This classification includes RGD-disintegrins, MLD-disintegrins, and KTS-disintegrins. RGD-disintegrins have been the most intensively investigated, and their research resulted in design and synthesis of new pharmaceutical compounds, eptifibatide and tirofiban that are currently being evaluated for the therapy of acute coronary ischaemic syndrome. MLD- and KTS-disintegrins are specific for leukocyte integrins and collagen receptors, respectively, and are being investigated in the new fields of pharmaceutical research. This review summarizes the biological activities of snake venom disintegrins, as well as discusses their potential in the study of the pharmacology of various human diseases.

Disintegrins represent a family of polypeptides released in the venoms of Viperidae and Crotalidae snakes (vipers and rattlesnakes) by the proteolytic processing of multidomain metalloproteinases, which selectively block the function of β1 and β3 integrin receptors. Research on disintegrins is relevant for understanding the biology of viper venom toxins, but also provides information of new structural determinants involved in integrin recognition that may be useful in both, basic and clinical research. The role of the composition, conformation and concerted dynamics of the integrin inhibitory loop and the C-terminal tail in determining the selective inhibition of integrin receptors is discussed.

ADAMs (a disintegrin and metalloprotease) are a family of cell surface proteins related to the Class III snake venom metalloproteases (SVMP). ADAMs are members of the Metazincin family which includes the matrix matalloproteases and the ADAMTS proteins. Unlike their snake venom relatives, ADAMs are expressed as transmembrane cell surface proteins. The domain structure of ADAMs suggests that these proteins posses both proteolytic and adhesive functions. Several members of the ADAM protein family have been shown to be involved in ectodomain shedding of many important cell surface proteins resulting in the release of biologically active soluble factors. The carboxyl-terminal domains, especially the disintegrin-like domain of ADAMs, have been demonstrated to support cell adhesion. The disintegrin-like domains of many ADAMs are capable of acting as integrin ligands. Integrins known to interact with ADAM disintegrin-like domains include α4β1, α4β7, α5β1, α6β1, α9β1, αvβ3, and αvβ5. This integrin mediated interaction of the disintegrin-like domains with the cell surface suggests that ADAMs may function as cellular counter receptors. In this review we discuss the individual functions ascribed to members of the ADAM family especially those related to integrin interactions and the potential for integrin mediated regulation of ectodomain shedding.

Specific antagonists have been successfully developed for several different integrins. Clinical trials have been initiated to study therapeutic uses of these inhibitors in cancer, thrombosis, and inflammatory diseases. Most efforts to date have focused on the platelet integrin αIIbβ3, endothelial αvβ3, and the leukocyte integrin α4β1. However, the integrin family contains additional members with interesting tissue specificities and functional properties that could also be useful molecular targets for disease intervention. In many cases, specific recognition motifs for these integrins have not been identified, which has precluded development of specific antagonists. Our recent studies of thrombospondin-1 and thrombospondin-2 recognition by integrins have revealed novel motifs for a3b1 and a6b1 integrins as well as new motifs recognized by the well studied α4β1 integrin. These three integrins play distinct roles in angiogenesis and its modulation by thrombospondins. This review will discuss recent insights into the specificities of α3β1 and α6β1 integrins, their functions in angiogenesis, and potential applications for antagonists of these integrins and of α4β1 to control pathological angiogenesis and other diseases.

In recent years, our understanding of the molecular interaction of collagens with their cognate integrin receptors has remarkably improved. Structural elucidations of both the integrin and the collagenous triple helix have contributed to this achievement. The structures of an entire integrin ectodomain and of an A-domain, which is unique to the integrin α subunits of collagen-binding and leukocyte integrins, have been resolved crystallographically. Furthermore, a complex of such an integrin α subunit A-domain with its collagenous binding partner has revealed their interaction on the molecular level and gave first evidence in the conformational alterations which may convey the signal of ligand occupancy through the integrin into the cells. In parallel, the tissue distribution and biological functions of collagen-binding integrins have been characterised. Nowadays, the contribution of distinct integrins to different physiological and pathological processes is known. Among the best studied examples is the collagen-induced platelet activation and aggregation, in which α2β1 integrin is involved. Together with α1β1 integrin, it also plays a role in inflammatory processes. To manipulate processes which are mediated by collagen-binding integrins, compounds are developed which mimic the collagen ligand. Not only the structural information of the integrin:collagen-interaction but also improvements in the chemical synthesis of a collagenous triple helix facilitate the development of agonists and antagonists of collagen-binding integrins. Furthermore, another impact in this search comes from the discovery of high-affinity inhibitors from venoms, which lack a collagenous triple-helix.

Integrins are adhesion receptors that connect cells to components of the extracellular matrix or to counter receptors on other cells. Besides mediating stable adhesion, these receptors are implicated in the deposition of extracellular matrices and they are crucial for cell migration. Integrin-mediated adhesion also modulates signal transduction cascades downstream of other receptors and thereby regulates cell survival, proliferation, and the expression of differentiation-related genes. In this review, an overview of the evidence for roles of integrins in tissue development and function is given and the contribution of changes in integrin expression to cancer is discussed.

Integrins are a family of heterodimeric transmembrane glycoproteins that mediate cell-cell and cell-matrix interactions. They participate in inflammatory reactions mainly by regulation of leukocyte migration, activation and survival. Elevated expression of the cell adhesion molecules, such as VCAM, ICAM and MAdCAM on the lumenal surface of vascular endothelial cells is a critical early event in organ inflammatory processes - including the lung. Adhesive interactions with their counter-receptors on leukocytes, selectins and integrins, result in migration of the leukocytes to the inflammed tissues. Integrins also participate in physiological and pathological reorganization of the lung structure during e.g. pneumonia healing, airway remodeling, angiogenesis, emphysema and pulmonary fibrosis. Agents that could inhibit the function of one or more of these integrins could provide a novel therapeutic strategy targeted to inhibit inflammatory and immune phenomena in the lung.

The incidence of cardiovascular disease is increasing with the aging population. This has stimulated a need for innovative means to evaluate and develop therapeutic strategies intended to improve patient care. Positron emission tomography (PET) imaging is an advanced nuclear imaging technology. The advantage of PET over other non-invasive imaging modalities is its ability to accurately measure tissue concentrations of specific radiolabeled compounds. These radioligands can be used as molecular probes to quantify physiological processes and the effects of therapy. Molecular imaging with PET has been applied to evaluate new and established drugs and therapies, as well as their effects on physiological parameters. New radiolabeled receptor ligands will also allow in vivo pharmacokinetic studies following drug treatment, yielding insights into drug delivery, optimal drug occupancy, and mechanism of action at the receptor level. These exciting tissue pharmacokinetic data could revolutionize evaluation of drug therapies in cardiovascular diseases. In addition, serial evaluations of these processes are now possible in both animals and humans permitting sensitive means to evaluate disease progression and therapies. New tools for imaging such as PET/CT and small animal PET broaden the potential of PET in drug evaluation. This review will describe the accuracy of PET as a non-invasive modality to quantify various parameters, and the application of PET in evaluating new and established therapies. This paper will also review the application of receptor ligand imaging and the principles of using surrogate physiological end-points in early drug development and evaluation.

The powerful inducer of apoptosis Apo2L/TNF-related apoptosis-inducing ligand (TRAIL) has generated exciting promise as a potential tumour specific cancer therapeutic agent, since it selectively induces apoptosis in transformed versus normal cells. Interferons (IFNs) are important modulators of TRAIL expression, thus the ligand appears to play an important role in surveillance against viral infection and malignant transformation. In the light of the emerging importance of TRAIL in cancer therapy, we will discuss the molecular basis of the cooperation of TRAIL and IFNs or chemotherapeutic drugs. In particular, we will focus on the data known to date concerning the biochemical pathways leading to TRAIL-induced apoptosis in specific cancer cells and warranting further work to enable the investigation in cancer patients.