Current Medicinal Chemistry - Volume 11, Issue 12, 2004

Retinoids exert pleiotropic effects in various biological processes by binding to their nuclear receptors, the retinoic acid receptors (RARs) and retinoid X receptors (RXRs), to regulate gene transcription. Apo-RARs and RXRs repress target gene expression by recruiting corepressors to the target DNA, triggering chromatin condensation by the action of histone deacetylases present in the corepressor complexes. In contrast, holo-RARs and RXRs recruit coactivators, some known to encode histone acetyl transferases, which trigger histone hyperacetylation, chromatin decondensation, and ultimately gene activation. Receptor interacting protein 140 (RIP140) represents a novel RAR / RXR coregulator that suppresses vitamin A-regulated gene expression in a retinoid- dependent manner. This review addresses the action of different retinoid ligands on gene expression, the molecular mechanisms underlying RAR/RXR-mediated gene regulation, and the unique properties of RIP140 as a novel retinoid hormone-dependent negative coregulator for RAR- and RXR-mediated gene regulation.

Ca2+ signalling is involved in virtually all cellular processes: among the others, it controls cell survival, proliferation and death regulating a plethora of intracellular enzymes located in the cytoplasm, nucleus and organelles. Changes in the cytosolic free Ca2+ concentration may be due either to release from the intracellular Ca2+ stores or to influx from the extracellular medium, through the opening of plasma membrane calcium-permeable channels. In particular, Ca2+ entry from the extracellular space is a mechanism able to sustain long lasting intracellular Ca2+ elevations: this signal, activated by many growth factors and mitogens in normal and tumoral tissues, is linked to DNA transcription and duplication, finally leading to cell proliferation. In the last years many informations have been provided about the transduction mechanisms related to Ca2+ entry induced by mitogenic factors, mostly binding to tyrosine kinase receptors, but also to G-protein coupled ones. Nevertheless, some key points remain to be fully clarified: among them, the molecular structure of the Ca2+ channels involved, their regulation by intracellular messengers, and the modes through which specificity is achieved. The increasing knowledge on Ca2+ entry-dependent control of proliferation may provide a more satisfactory understanding of pathological alterations, including cancer progression and angiogenesis. A detailed description of the mechanisms that trigger Ca2+ entry, and in particular the definition of calcium-permeable channels and their modulators at the molecular levels, will greatly improve our possibility to take advantage of Ca2+ entry regulation as a therapeutic approach for the control of cell proliferation, designing antibodies or molecules with low side effects and specific channel blocker functions. The review will focus on this topic.

Oxidative stresses such as oxidant stimuli, inflammation, exposure to xenobiotics, or ionizing irradiation provoke cellular protective responses, principally involving transcriptional activation of genes encoding proteins which participate in the defense against oxidative tissue injuries. Excess of free heme, which is released from hemeproteins under such conditions, may constitute a major threat because it can catalyze the formation of reactive oxygen species (ROS). Exposure of mammalian cells to oxidative stimuli induces heme oxygenase-1 (HO-1), the rate-limiting enzyme in heme degradation, as well as a 33-kDa heat shock protein. In various model systems, HO-1 induction confers protection on tissues from further injuries, while the abrogation of its induction accelerates cellular injuries. In this article, we review recent advances in the regulatory mechanism of ho-1 gene expression and the role of HO-1 in various models of experimental oxidative tissue injuries, and its potential therapeutic implications.

The human genome encompasses some 2,000 proteins that utilize adenosine 5'-triphosphate (ATP) in one way or another and some 500 of these are protein-tyrosine and protein-serine / threonine kinases (PTKs & PSTKs). Substrate phosphorylation by these enzymes is nature's predominant molecular way of organizing cellular signal transduction and regulating biochemical processes in general. It is not surprising, therefore, that abnormal phosphorylation of cellular proteins is a hallmark of disease and that there has been a growing interest in the use of kinase inhibitors as drugs. In fact the search for such agents has recently culminated in the approval of the first kinase inhibitor drugs for medical use. Although it has been demonstrated exhaustively that potent and structurally diverse ATP-antagonistic small molecule kinase inhibitors can be found through mass screening and structure-guided design, the question of biochemical, cellular, and in vivo selectivity of such inhibitors remains much less clear. Here the medicinal chemistry of kinase inhibitors is reviewed critically with particular emphasis on target selectivity and specificity. Approaches based on chemical genomics, combinatorial target-guided ligand assembly, computational chemistry, and structural biology techniques, which aim at classifying both inhibitors and kinase targets, are given special emphasis. The various strategies in which differences in biochemical mechanism of kinase function can be exploited in order to attain selective inhibition are also discussed. Furthermore, recent developments in the design of inhibitors to selected individual validated therapeutic kinase targets, including cell cycle kinases and receptor PTKs, etc. are summarised.

This review focuses on the advances in electron capture mass spectrometry. Electron-capture (EC) is a sensitive ionisation technique for mass spectrometry providing selectivity towards electrophoric compounds. Recent advances in instrumentation have led to a more widespread application of this method in biomedical and pharmaceutical analysis. After a brief introduction to EC-mass spectrometry (MS), potential targets for EC-MS analysis are defined and enhancement of sensitivity by electrophoric derivatisation is discussed. A wide range of applications is selected, including prostanoid analysis in biomedical systems (with the oxidative stress indicators isoprostanes) and the trace level analysis of endogenous low-molecular weight compounds. Application to the trace level gas chromatography-negative ion chemical ionization MS (GC-NICI-MS) analysis of complex glucuronides is also demonstrated, as well as a wide range of drugs analysed in human blood. The review should point out the versatility and unique sensitivity of the technique, making it useful for basic research in medicinal chemistry, as well as clinical diagnosis, pharmaceutical and toxicological applications.

The pattern of insulin release is crucial for regulation of glucose and lipid haemostasis. Deficient insulin release causes hyperglycemia and diabetes, whereas excessive insulin release can give rise to serious metabolic disorders, such as nesidioblastosis (Persistent Hyperinsulinemic Hypoglycemia of Infancy, PHHI) and might also be closely associated with development of type 2 diabetes and obesity. Type 2 diabetes is characterized by fasting hyperinsulinemia, insulin resistance and impaired insulin release, i.e. reduced first phase insulin release and decreased insulin pulse mass. The beta cell function of patients with type 2 diabetes slowly declines and will ultimately result in beta cell failureand increasing degrees of hyperglycemia. Type 2 diabetes, in combination with obesity and cardiovascular disorders, forms the metabolic syndrome. It has been possible to improve beta cell function and viability in preclinical models of type 1 and type 2 diabetes by reducing insulin secretion to induce beta cell rest. Clinical studies have furthermore indicated that inhibitors of insulin release will be of benefit in treatment or prevention of diabetes and obesity. Pancreatic beta cells secrete insulin in response to increased metabolism and by stimulation of different receptors. The energy status of the beta cell controls insulin release via regulation of open probability of the ATP sensitive potassium (KATP) channels to affect membrane potential and the intracellular calcium concentration [Ca2+]i. Other membrane bound receptors and ion channels and intracellular targets that modulate [Ca2+]i will affect insulin release. Thus, insulin release is regulated by e.g. somatostatin receptors, GLP-1 receptors, muscarinic receptors, cholecystokinin receptors and adrenergic receptors. Although the relationship between hyperinsulinemia and certain metabolic diseases has been known for decades, only a few inhibitors of insulin release have been characterized in vitro and in vivo. These include the KATP channel openers diazoxide and NN414 and the somatostatin receptor agonist octreotide.

The ability to permeate across the blood brain barrier (BBB) is essential for drugs acting on the central nervous system (CNS). Thus, for speeding up the drug discovery process in the CNS-area, it is of great importance to develop systems that allow rapid and inexpensive screening of the BBB-permeability properties of novel lead compounds or at least small subsets of combinatorial CNS-libraries. In this field, in silico prediction methods gain increasing importance. Starting with simple regression models based on calculation of lipophilicity and polar surface area, the field developed via PLS methods to grid based approaches (e.g. VolSurf). Additionally, the use of artificial neural networks gain increasing importance. However, permeation through the BBB is also influenced by active transport systems. For nutrients and endogenous compounds, such as amino acids, monocarboxylic acids, amines, hexoses, thyroid hormones, purine bases and nucleosides, several transport systems regulating the entry of the respective compound classes into the brain have been identified. The other way round there is striking evidence that expression of active efflux pumps like the multidrug transporter P-glycoprotein (P-gp) on the luminal membrane of the brain capillary endothelial cells accounts for poor BBB permeability of certain drugs. Undoubtedly, P-gp is an important impediment for the entry of hydrophobic drugs into the brain. Thus, proper prediction models should also take into account the active transport phenomena.

Modulation of chromatin structure through histone acetylation / deacetylation is known to be one of the major mechanisms involved in the regulation of gene expression. Two opposing enzyme activities determine the acetylation state of histones: histone acetyltransferases (HATs) and histone deacetylases (HDACs), respectively acetylating or deacetylating the ε-amino groups of lysine residues located in the aminoterminal tails of the histones. In general, transcriptionally active chromatin is associated with hyperacetylated histones, whilst silenced chromatin is linked to hypoacetylated histones. A number of structurally divergent classes of HDAC inhibitors have been identified. They have been shown to induce cell cycle arrest, terminal differentiation and / or apoptosis in various cancer cell lines and inhibit tumor growth in animals. In particular, the reversible HDAC inhibitor Trichostatin A (TSA) and its hydroxamate analogues can effectively and selectively induce tumor growth arrest at very low concentrations (nano- to micromolar range). They form a group of so-called promising antitumor agents of which some are currently under clinical trial. Since the selection of a molecule for further drug development requires a balance of biological potency, safety and pharmacokinetics, it is of paramount importance to elucidate the pharmacokinetic and toxicological properties of these HDAC inhibitors before they can be considered as potential new drugs. Primary hepatocytes and their cultures are well-differentiated in vitro models and can be used to study simultaneously the biological effects of HDAC inhibitors and their biotransformation. The present review provides a state-of-the-art of our current knowledge of the pharmacological and toxicological effects on proliferating cells of TSA and its hydroxamatebased structural analogues. Besides a theoretical basis, an overview of the experimental results, obtained by the authors using primary rat hepatocytes as an in vitro model, is given.

Polyenes constitute a large class of natural metabolites produced by giant multifunctional enzymes in a process resembling fatty acid biosynthesis. Like fatty acids, polyene macrolides and other polyketides are assembled by decarboxylative condensations of simple carboxylic acids. But while fatty acid intermediates are fully reduced, polyene macrolide intermediates suffer the suppression of reduction or dehydration reactions at given biosynthetic steps. In the last years, much progress has been made in our understanding of the linear and modular organization of the gene clusters, and the enzymes encoded by them, responsible for the biosynthesis of these macrocyclic metabolites. This know-how about the rules that govern polyene chain growth has provided the basis for the first rational manipulations of these fascinating systems for the production of engineered derivatives and promises a new era of novel polyene development, which will hopefully yield new molecules with improved pharmacological properties.

Selenium is an essential trace element. It is, however toxic at concentration little above which is required for health. Selenium is incorporated into proteins as selenocysteine, the 21st amino acid. Selenoproteins are found in bacteria, archaea and eukaryotes. Biochemical and physicochemical properties of selenium result in the unique redox characteristics of selenocysteine and its use in antioxidant enzymes. In this context of a redox reaction is the reduction of reactive oxygen metabolites by glutathione peroxidases, helping to maintain membrane integrity, reduces the oxidative damage to lipids, lipoproteins, and DNA. Selenium has structural and enzymatic roles. Selenium influences a number of endocrine processes, most notably, those involved in thyroid hormone synthesis and metabolism. Se is needed for the proper functioning of the immune system, a role in viral suppression, AIDS, and also is implicated in delaying the aging process. Its deficiency has been linked to a number of disorders such as heart disease, diabetes, and diseases of the liver, and it is required for sperm motility and may reduce the risk of miscarriage. Se supplementation has recently moved from the realm of correcting nutritional deficiencies to one of pharmacological intervention, especially in the clinical domain of cancer chemoprevention. During the last few years, a tremendous effort has been directed toward the synthesis of stable organoselenium compounds that could be used as antioxidants, enzyme modulators, antitumor, antimicrobials, antihypertensive agents, antivirals and cytokine inducers. The biochemistry and pharmacology of selenium-based compounds are subjects of intense current interest, especially from the point of view of public heath. The purpose of this review is to discuss the recent pharmacological applications of organoselenium compounds as therapeutic agents in the treatment of several diseases.