Abstracts


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Emerging Roles of Cysteine Cathepsins in Disease and their Potential as Drug Targets
Vasiljeva, Olga; Reinheckel, Thomas; Peters, Christoph; Turk, Dusan; Turk, Vito; Turk, Boris

The general view on cysteine cathepsins, which were long believed to be primarily involved in intracellular protein turnover, has dramatically changed in last 10 to 15 years. The discovery of new cathepsins, such as cathepsins K, V, X, F and O, and their tissue distribution suggested that at least some of them are involved in very specific cellular processes. Moreover, gene ablation experiments revealed that cathepsins play a vital role in numerous physiological processes, such as antigen processing and presentation, bone remodelling, prohormone processing and wound healing. Their involvement in several pathologies, including osteoporosis, rheumatoid arthritis, osteoarthritis, bronchial asthma and cancer have also been confirmed and today several of them have been validated as relevant targets for therapies. Compounds targeting cathepsins S and K are already in clinical evaluation, whereas others are in experimental phases. The cathepsin K inhibitor AAE-581 (balicatib) as the most advanced of them passed Phase II clinical trials in 2005. In this review, we discuss the current view on cathepsins as an emerging group of targets for several diseases and the development of cathepsin K and S inhibitors for treatment of osteoporosis and various immune disorders.


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Matrix Metalloproteinases as Valid Clinical Target
Fingleton, Barbara

The matrix metalloproteinase family of enzymes has been a pharmaceutical target for over 20 years. In that time, many drugs have been developed but none have successfully passed clinical trials. A significant problem has been development of dose-limiting side-effects that were revealed during long-term clinical trials in diseases such as arthritis and various cancers. There are, however, other clinical settings where evidence for MMP function contributing to the pathophysiology of disease is strong. A number of these settings will be discussed here together with evidence from animal models that MMP inhibition is a valid strategy to be considered. A major advantage with many of these settings is that drug exposure may not have to be long-term and/or systemic thus reducing the possibility that side-effects will stymie MMPI-based therapy.


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The Role of Insulin Receptor Isoforms and Hybrid Insulin/IGF-I Receptors in Human Cancer
Belfiore, Antonino

This review will focus on the emerging role of the insulin receptor (IR) in cancer. Several epidemiological studies have shown that insulin resistance states, characterized by hyperinsulinemia, are associated with an increased risk for a number of malignancies, including carcinomas of the breast, prostate, colon and kidney. Recent data have elucidated some molecular mechanisms by which IR is involved in cancer. First, IR is overexpressed in several human malignancies. Interestingly, one of the two IR isoform (IR-A) is especially overexpressed in cancer. IR-A is the IR fetal isoform and has the peculiar characteristic to bind not only insulin but also IGF-II. Second, IR forms hybrid receptors with the homologous IGF-IR, which is also commonly overexpressed in cancer. These hybrid receptors containing IR-A hemidimers have broad binding specificity as they bind IGF-I and also IGF-II and insulin. By binding to hybrid receptors, insulin may stimulate specific IGF-IR signaling pathways. Overexpression of IR-A is, therefore, a major mechanism of IGF system overactivation in cancer.

These findings may have important implications for both the prevention and treatment of common human malignancies. They underline the concept that hyperinsulinemia, associated with insulin resistance and obesity, should be treated by changes in life style and/or pharmachological approaches to avoid an increased risk for cancer. IR-A isoform and hybrid receptors should be regarded, therefore, as potential molecular targets for novel anti-cancer therapies.


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Adipose Tissue Macrophages, Low Grade Inflammation and Insulin Resistance in Human Obesity
Heilbronn, Leonie K; Campbell, Lesley V. Vol: 14-12

Obesity was first described as a low-grade inflammatory condition more than a decade ago. However, it is only relatively re-cently that obese individuals have been described with increased macrophage infiltration of adipose tissue, as well as an increase in the number of “M1” or “classically activated” macrophages. Furthermore, macrophages have been identified as the primary source of many of the circulating inflammatory molecules that are detected in the obese state and are postulated to be causal both in the development of insulin resistance and in the progression to type 2 diabetes. There is also novel evidence to suggest that macrophages inhibit adipocyte differentiation, potentially leading to adipocyte hypertrophy, altered secretion of adipokines and ectopic storage of lipid within liver, muscle and other non-adipose tissues. Currently, it is not clear what causes increased macrophage infiltration of adipose tissue in obese individuals. Theories include altered signalling by adipocytes, nutritional induction of metabolic endotoxemia or reduced angiogenesis and local adipose cell hypoxia. Importantly, PPAR-gamma agonists have been shown to alter macrophage phenotype to “M2” or an “alternatively activated” anti-inflammatory phenotype and may induce macrophage specific cell death. Consequently, excitement surrounds the potential for specific inhibition of macrophage infiltration of adipose tissue via pharmacotherapy for obese patients and more particularly as adjunct therapy to improve insulin sensitivity in obese individuals with insulin resistance and overt type 2 diabetes.


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Heparanase: Structure, Biological Functions, and Inhibition by Heparin-Derived Mimetics of Heparan Sulfate
Israel Vlodavsky; Neta Ilan; Annamaria Naggi; Benito Casu

Heparanase is an endoglycosidase which cleaves heparan sulfate (HS) and hence participates in degradation and remodeling of the extracellular matrix (ECM). Heparanase is preferentially expressed in human tumors and its over-expression in tumor cells confers an invasive phenotype in experimental animals. The enzyme also releases angiogenic factors from the ECM and thereby induces an angio-genic response in vivo. Heparanase upregulation correlates with increased tumor vascularity and poor postoperative survival of cancer patients. Heparanase is synthesized as a 65 kDa inactive precursor that undergoes proteolytic cleavage, yielding 8 kDa and 50 kDa protein subunits that heterodimerize to form an active enzyme. Heparanase exhibits also non-enzymatic activities, independent of its involvement in ECM degradation. Among these, are the enhancement of Akt signaling, stimulation of PI3K- and p38-dependent endothelial cell migration, and up regulation of VEGF, all contributing to its potent pro-angiogenic activity. Studies on relationships between structure and heparanase inhibition activity of nonanticogulant heparins systematically differing in their O-sulfation patterns, degrees of N-acetylation, and glycol-splitting of both pre-existing nonsulfated uronic acid residues (prevalently D-glucuronic) and/or those (L-iduronic acid/L-galacturonic acid) generated by graded 2-O-desulfation, have permitted to select effective inhibitors of the enzymatic activity of heparanase. N-acetylated, glycol-split heparins emerged as especially strong inhibitors of heparanase, exerting little or no release of growth factors from ECM. N-acetylated glycol-split species of heparin, as well as heparanase gene silencing inhibit tumor metastasis, angiogenesis and inflammation in experimental animal models. These observations and the unexpected identification of a single functional heparanase, suggest that the enzyme is a promising target for anti-cancer and anti-inflammatory drug development.


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The IGF-I Signaling Pathway
Laviola, Luigi; Natalicchio, Annalisa; Giorgino, Francesco

The insulin-like growth factor (IGF)-I is implicated in the regulation of protein turnover and exerts potent mitogenic and differentiating effects on most cell types. IGF-I biological actions are mediated by the IGF-I receptor, comprised of two extra-cellular α-subunits, containing hormone binding sites, and two membrane-spanning β-subunits, encoding an intracellular tyrosine kinase. Hormone binding activates the receptor kinase, leading to receptor autophosphorylation and tyrosine phosphorylation of multiple substrates, including the IRS and Shc proteins. Through these initial tyrosine phosphorylation reactions, IGF-I signals are transduced to a complex network of intracellular lipid and serine/threonine kinases that are ultimately responsible for cell proliferation, modulation of tissue differentiation, and protection from apoptosis. This review will focus on the IGF-I receptor structure and function, its intracellular signaling pathways, and some important implications of the activation of the IGF-I signal transduction system in specific tissues.


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JNK Signalling: A Possible Target to Prevent Neurodegeneration
Tiziana Borsello; Gianluigi Forloni

The c-Jun N-terminal kinases (JNK) belong to the subfamily of mitogen-activated protein kinase (MAPK). JNK is an important transducing enzyme that is involved in many facets of cellular regulation including gene expression, cell proliferation and programmed cell death. The activation of JNK pathways is critical for naturally occurring cell death during development as well as for pathological death associated with neurodegenerative diseases. Initial research concentrated on defining the components and organization of JNK signalling cascades, but more recent studies see JNK as a target to prevent cell death. Several in vitro and in vivo studies have reported alterations of JNK pathways potentially associated with neuronal death in Parkinson’s and Alzheimer’s disease. So efforts are now aimed at developing chemical inhibitors of this pathway. These have proved effective in vivo, reducing brain damage and some of the symptoms of arthritis in animal models. An alternative cell penetrating peptide approach is now available, with the identification of the JNK permeable peptide inhibitor, which modifies JNK action rather than activation, preventing neuronal death with unprecedented specificity and efficacy in several experimental conditions, including two animal models of ischemia. In this review we examine in detail the role of JNK in neurodegeneration, particularly in Alzheimer’s and Parkinson’s disease. The possibility of intervention on the JNK pathway as a therapeutic approach is also illustrated.


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Peripheral TRPV1 receptors as targets for drug development: new molecules and mechanisms
Gunthorpe MJ, Szallasi A.

Based on the painful effects of exposure to capsaicin, TRPV1 (transient receptor potential vanilloid subfamily member 1) localization is most readily associated with peripheral sensory neurons, however, TRPV1 is now known to be expressed, albeit at lower levels, in the spinal cord, brain and a wide-range of non-neuronal cells. The latter includes epithelial cells (e.g. keratinocytes, urothelium, gastric epithelial cells, enterocytes, and pneumocytes) through vascular endothelium and cells of the immune system (e.g. T-cells and mast cells) to smooth muscle, fibroblasts and hepatocytes. Despite extensive research, the physiological function of TRPV1 in the brain and in non-neuronal tissues remains elusive. The preliminary results are exciting, but many are unconfirmed and/or contradictory. As yet, studies with TRPV1 knock-out mice have proven unhelpful in clarifying such biological roles. Now that a range of potent and selective TRPV1 antagonists are available in this rapidly expanding research field, further understanding of the biological roles of TRPV1 throughout the body is within reach. In this article, we will summarize the known roles of peripheral TRPV1 receptors in physiology and disease and review the current perspectives for the therapeutic potential of TRPV1 agonists and antagonists in the treatment of a wide range of conditions such as pain, cancer, migraine, chronic cough, asthma, rectal hypersensitivity, inflammatory bowel disease, obesity, overactive bladder and diabetes. New applications of targeting central TRPV1 receptors are reviewed in the accompanying article by Starowicz et al. (in this issue).


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Copper Chelation Chemistry and its Role in Copper Radiopharmaceuticals
Wadas, TJ.; Wong, EH.; Weisman, GR.; Anderson, CJ.

Molecular imaging is an important scientific discipline that plays a major role in clinical medicine and pharmaceutical development. While several imaging modalities including X-ray computed tomography (CT) and magnetic resonance imaging (MRI) generate high-resolution anatomical images, positron emission tomography (PET) and single photon emission computed tomography (SPECT) offer insight into the physiological processes that occur within a living organism. Of these two nuclear medicine imaging techniques, PET has advantages with respect to sensitivity and resolution, and this has led to the production and development of many positron emitting radionuclides that include non-traditional radionuclides of the transition metals. Copper-64 (t_1/2 = 12.7 h, β+: 17.4%, E_β+max = 656 keV; β-: 39%, E_β-max = 573 keV) has emerged as an important positron emitting radionuclide that has the potential for use in diagnostic imaging and radio-therapy. However, ⁶⁴Cu must be delivered to the living system as a stable complex that is attached to a biological targeting molecule for effective imaging and therapy. Therefore, significant research has been devoted to the development of ligands that can stably chelate ⁶⁴Cu. This review discusses the necessary characteristics of an effective ⁶⁴Cu chelator, while highlighting the development and evaluation of ⁶⁴Cu-complexes attached to biologically-targeted ligands.


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Aspartic Proteases in Drug Discovery
Eder, Jorg; Hommel, Ulrich; Cumin, Frederic; Martoglio, Bruno; Gerhartz, Bernd

Aspartic proteases are the smallest class of human proteases with only 15 members. Over the past years, they have received considerable attention as potential targets for pharmaceutical intervention since many have been shown to play important roles in physiological and pathological processes. Despite numerous efforts, however, the only inhibitors for aspartic proteases currently on the market are directed against the HIV protease, an aspartic protease of viral origin. Nevertheless, several inhibitors including those targeting renin, BACE1 and γ-secretase are in clinical or preclinical development, and some other aspartic proteases are discussed as potential drug target. The crystal structures of seven human aspartic proteases have now been solved and, together with a detailed kinetic understanding of their catalytic mechanism, this has greatly contributed to the design and discovery of novel inhibitors for this protease class. This review describes current aspartic protease drug targets and summarizes the drug discovery efforts in this field. In addition, it highlights recent developments which may lead to a new generation of aspartic protease inhibitors.