Showing papers in "Annual Review of Pharmacology and Toxicology in 1990"

Muscarinic Receptor Subtypes

Abstract: In 1914, Sir Henry Dale provided the basis for the classical and comfortable definition of muscarinic and nicotinic acetylcholine receptors: Muscarinic receptors are selectively activated by muscarine and blocked by atropine; nicotinic receptors are activated by nicotine and blocked by curare. This definition lasted over 60 years, despite isolated reports that the picture might not be so simple. We now know, as the result of molecular biological studies, that there are multiple variants of both muscarinic and nicotinic receptors . These receptors are members of two quite separate gene superfamilies and only share the property of being activated by the same ligand, acetylcholine. The aim of this review is to discuss the structure, function, and binding properties of the different muscarinic receptor species, attempting where possible to coordinate the diverse experimental data into a uniform picture. For a more comprehensive review on muscarinic receptors, readers are di­ rected to a recent book by Brown (l).

Glutathione: Toxicological Implications

Abstract: Glutathione as reduced glutathione (GSH) and as the disulfide (GSSG) is a unique peptide that continues to interest toxicologists-as reflected in the numerous symposia and reviews ( 1-6). This article undertakes to describe the role of mitochondrial glutathione in cellular response to toxic exposures that include loss of the homeostases of Ca2+ and protein thiols. Because of the numerous reviews on the glutathione S-transferases (7-10), only the per­ oxidase activity of membrane-associated glutathione S-transferases is dis­ cussed. Mammalian cells have evolved protective mechanisms to minimize in­ jurious events that result from toxic chemicals and normal oxidative products of cellular metabolism. A major endogenous protective system is the glu­ tathione redox cycle (2). Glutathione is present in high concentrations as GSH in most mammalian cells (generally in the millimolar range), with minor fractions being GSSG, mixed disulfides of GSH and other cellular thiols, and minor amounts of thioethers ( 1) . GSH acts both as a nucleophilic "scavenger" of numerous compounds and their metabolites, via enzymatic and chemical mechanisms, converting electrophilic centers to thioether bonds, and as a substrate in the GSH peroxidase-mediated destruction of hydroperoxides. GSH depletion to about 20-30% of total glutathione levels can impair the cell's defense against the toxic actions of such compounds and may lead to cell injury and death (3, 1 1) .

G Proteins in Signal Transduction

Abstract: The last five years have seen major advances in our understanding of the central and generalized role of G-proteins as transducers of receptor signals into effector responses as outlined in Scheme 1. Signal-transducing G pro­ teins, in contrast to other GTP-binding proteins, are heterotrimers formed of one GTP-binding (and hydrolyzing) a subunit, one {3 subunit, and one 'Y subunit. I This increase in knowledge is perhaps best illustrated by the fact that

Genetic and molecular aspects of 2,3,7,8-tetrachlorodibenzo-p-dioxin action.

Abstract: Some halogenated aromatic hydrocarbons· (HAHs) such as the polychlori­ nated biphenyls have been used extensively for commercial purposes Related compounds, such as the halogenated dibenzo-p-dioxins and dibenzofurans, have no commercial value themselves but are generated during the man­ ufacture or combustion of other HAHs In general, HAHs are lipophilic and stable; halogenation renders them relatively resistant to enzymatic conversion to water-soluble derivatives Therefore, HAHs tend to persist in the environ­ ment and to accumulate in the food chain, posing a potential risk to human health (l, 2) Toxicologic evaluations reveal that the various HAHs evoke similar responses, although they differ by orders of magnitude in potency; therefore, they are believed to share a common mechanism of action

Excitotoxic amino acids and neuropsychiatric disorders

Abstract: A wealth of new information pertaining to excitatory amino acids (EAA) has been generated over the past two decades. The prototypic EAA, glutamate (Glu) and aspartate (Asp), which are abundantly present in the mammalian central nervous system, have become recognized as Jekyll and Hyde mole­ cules that serve vitally important metabolic and neurotransmitter functions while simultaneously harboring treacherous neurotoxic potential. The neurotoxicity of Glu and Asp, although first described 30 years ago, was relatively ignored until studies in the early 1970s linked the phenomenon to an excitatory mechanism. In subsequent years, mechanisms underlying ex­ citotoxic phenomena have been further elucidated, several EAA receptor subtypes have been delineated, drugs with anti-excitotoxic actions have been identified, and evidence for the potential complicity of excitotoxins in neurodegenerative disorders has begun to unfold. A specific subtype of EAA receptor, the N-methyl-D-aspartate (NMDA) receptor, has become a primary focus of attention because of evidence implicating it in a wide range of both neurophysiological and pathological processes. Evidence for the involvement of other EAA receptor subtypes in human neuropathological syndromes has also begun to appear and there is basis for believing that central neurons may be particularly vulnerable to excitotoxic degeneration during certain periods of development and in old age. Here I review highlights of research de-

Interactions between the brain and the immune system

Abstract: The observations and research described in this communication derive from a nontraditional view of the immune system. It has become abundantly clear that there are probably no organ systems or homeostatic defense mechanisms that are not, in vivo, subject to the influence of interactions between behavioral and physiological events. The complex mechanisms underlying these interactions and their relationship to health and illness, however, are imperfectly understood. The most imperfectly understood, perhaps, are the interrelationships among brain, behavior, and immune processes. Without attempting to cover all the literature, we have used stress effects and conditioning phenomena as illustrations to point out that behavior can influence immune function. We have also described data indicating that the immune system can receive and respond to neural and endocrine signals. Conversely, behavioral, neural, and endocrine responses seem to be influenced by an activated immune system. Thus, a traditional view of immune function that is confined to cellular interactions occurring within lymphoid tissues is insufficient to account for changes in immunity observed in subhuman animals and man under real world conditions. These data question seriously the notion of an autonomous immune system. Most of the research on the regulation of immune responses has been predicated on the assumption that such regulation is accomplished by the interacting components of the immune system itself, e.g. interactions among helper and suppressor T-lymphocytes, B-cells, and accessory cells that can result in the production of antibody and effector T cells. The immune system is, indeed, capable of considerable self-regulation, and immune responses can be made to take place in vitro. The functions of that component of adaptive processes known as the immune system that are of ultimate concern, however, are those that take place in vivo. There are now compelling reasons to believe that in vivo immunoregularity processes influence and are influenced by the neuroendocrine environment in which such processes actually take place--an environment that, on the one hand, can generate signals that resting and/or activated leukocytes can receive, and, on the other hand, is exquisitely sensitive to the individual's perception of and capacity to adapt to the demands of the environment. The immune system appears to be modulated, not only by feedback mechanisms mediated through neural and endocrine processes, but by feedforward mechanisms as well. The immunologic effects of learning, an essential feedforward mechanism, suggest that, like direct neural and endocrine processes, behavior can, under appropriate circumstances, serve an immunoregulatory function in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Prostaglandin H synthase and xenobiotic oxidation.

Abstract: We have attempted in this article to summarize and review cooxidation reactions that occur during the metabolism of AA and potential roles that these reactions can play in the activation and detoxification of chemicals. This review summarizes approximately 15 years of intensive investigation by a number of laboratories, and as such not all studies are cited, and in some cases data are not discussed with the emphasis that the original investigators may have intended. The major focus of many of these studies has been toward carcinogenesis. In the future, emphasis may shift to the formation of metabolites that will lead to other toxic effects. The cooxidation reactions that occur during AA metabolism are dependent upon the peroxidase activity of PHS. For some chemicals that are not cosubstrates, the epoxidation reactions that occur are dependent upon the subsequent formation of peroxyl radicals. A large and diverse number of chemicals are metabolized by an equally large and diverse number of chemical reactions. The unifying theme is the free radical nature of these oxidations. The subsequent reactions that these chemicals undergo is dictated by the nature of the free radical and the environment in which it is generated. Ample evidence now exists for the contribution of these free radical-mediated reactions not only in the formation of toxic metabolites, but also in some cases in the detoxification of chemicals. The overriding factor for this type of metabolism to occur is the relative concentrations in the specific tissue of PHS and peroxyl radicals with respect to other activating systems, particularly the monooxygenase system. In vivo investigations support the importance of the peroxidase and peroxyl radical systems in both activation and detoxification of chemicals in extrahepatic tissues.

Mechanisms of organophosphorus ester-induced delayed neurotoxicity: type I and type II.

Abstract: Some organophosphorus compounds produce neurologic dysfunctions, known as OPIDN, after a delay period that is accompanied by neuropathic damage in the central and peripheral nervous systems. This group of chemicals may be divided into two classes, Type I and II, based on chemical structure, species selectivity, age sensitivity, the length of latent period, clinical signs, morphology and distribution of neuropathologic lesions, protection with phenylmethyl sulfonyl fluoride, inhibition of neurotoxic esterase, and effect on catecholamine secretion from bovine adrenome-dullary chromaffin cells. The importance of this effect is underlined by the fact that incidents involving more than 40,000 cases of OPIDN in humans have been documented from 1899 to 1989. Most of these compounds are direct or indirect inhibitors of AChE, and produce acute cholinergic effects. Neurologic deficits are characterized by three phases: progressive, stationary, and improvement. Prognosis of OPIDN depends on the extent of damage of the nervous system. Improvement or even recovery of functions may follow mild cases, whereas severe toxicity results in long-lasting neurologic dysfunctions reflecting spinal cord damage. Recent studies have shown that delayed neurotoxic organophosphorus compounds interact with Ca2+/calmodulin kinase II (CaM kinase II), an enzyme responsible for the endogenous phosphorylation of cytoskeletal proteins, i.e. microtubules, neurofilaments, and MAP-2. This leads to an increased activity of CaM kinase II and enhanced phosphorylation of cytoskeletal elements, and eventually in the disassembly of cytoskeletal proteins. The dissociation of cytoskeletal proteins causes increased fast axonal transport in the treated animals resulting in the accumulation of altered cytoskeletal elements in the distal portions of the axon. Abnormal tubulin and neurofilaments are transformed into filamentous polymers and undergo condensation and dissolution. Concomitantly, proliferated endoplasmic reticulum and accumulated mitochondria degenerate and release Ca2+ ions. This leads to Ca2(+)-activated proteolysis of the cytoskeleton and interruption of ionic balance across the axonal membrane resulting in the uptake of water and axonal swelling, which subsequently degenerates. A similar mechanism may cause secondary myelin degeneration.

Molecular characterization of opioid receptors.

Abstract: Though opioid receptors are more difficult to purify and characterize than other cell surface receptors, significant progress has been made in the past several years. At least a dozen groups have now reported purification of opioid-binding proteins, either in a form that retains ligand-binding properties, or in a covalently bound form. Although there are some discrepancies in the molecular weights of these proteins, it is significant that many investigators have reported a molecular weight of about 60 kd for the receptor, regardless of whether it is of the mu, delta, or kappa type. This finding, together with immunological evidence, suggests that different opioid receptor types may be highly similar, and could conceivably even share a common ligand-binding subunit. Several groups have prepared monoclonal or polyclonal antibodies to purified opioid-binding proteins, which should be useful in mapping the brain regional distribution of the opioid receptors, determining the regions in the peptide involved in ligand binding and association with second messengers, and in determining the relationships among different opioid receptor types. One group has in fact already established an antigenic similarity between a mu-selective opioid-binding protein in mammalian brain, and the delta opioid receptor in NG108-15 neuroblastoma-glioma hybrid cells. One group has reported cloning of the cDNA for a purified opioid-binding protein. Somewhat surprisingly, its predicted amino acid sequence places it in the immunoglobulin superfamily, with strongest homologies to cell-adhesion molecules such as N-CAM. MAG, amalgam and fasciclin II, as well as receptors for peptides such as PDGF and interleukin-6. However, this is consistent with evidence that opioids can modulate cell-cell interactions of monocytes, and provides further support for links between opioids and the immune system. The second messengers mediating opioid actions are still unknown. Opioid agonists affect the activity of adenylate cyclase and ion channels in some tissues, but neither has been shown to mediate opioid analgesia. The sequence homologies of the purified opioid-binding protein OBCAM with tyrosine kinase growth factor receptors suggest additional possibilities for second messengers.

Subtypes of receptors for serotonin.

Abstract: Interest in the physiological functions of 5-hydroxytryptamine (5-HT; seroto­ nin) has increased steadily since its discovery in the intestine (1) and in serum (2). Its identification in brain in the early 1950s (3), followed a decade later by the initial studies of its distribution in brain by histofluorescence (4), stimulated investigations of the possible behavioral functions of this in­ dolealkylamine. In addition to studies of its function in brain, a steady stream of investigations have described its possible functions in the periphery, particularly in the gastrointestinal tract (see 5) and, more recently, in the cardiovascular system (see 6). Indeed, the initial suggestion that there might be more than one type of receptor for 5-HT came from experiments on the isolated guinea pig ileum. Gaddum & Picarelli (7) demonstrated that only a portion of its contractile response to 5-HT could be blocked by high con­ centrations of morphine but that the remainder of the response could be

Altered hepatic foci: their role in murine hepatocarcinogenesis.

Abstract: The recognition that the appearance of malignant disease in an organism is a reflection of a number of convergent processes, usually described as stages, has become one of the foremost areas of knowledge in the field of oncology. Notwithstanding the substantial epidemiologic and experimental evidence of extended latent periods between exposure to carcinogenic agents and the appearance of cancer, it was from the early studies of Rous and Kidd ( 1), Mottram (2), and Berenblum & Shubik (3) on the development of the two-stage process of epidermal carcinogenesis in the mouse that such descrip­ tive information was placed on a firm experimental basis. However, because epidermal neoplasms were seldom induced by dietary or parenteral adminis­ tration of putative carcinogenic agents, it was not until the multistage nature of carcinogenesis in other tissues was described that the impact of the natural history of carcinogenesis became a significant factor in both basic and applied cancer research. Of the extra-epidermal tissues studied, multistage carci­ nogenesis of the liver in both the mouse and the rat has become the best known. Prominent among early investigations of experimental carcinogenesis of �" the liver were those of Farber (4), Goldfarb & Zak (5) , and Gossner &

meso-2,3-Dimercaptosuccinic acid: chemical, pharmacological and toxicological properties of an orally effective metal chelating agent.

Abstract: The primary purpose of this article is to summarize the recent investigations dealing with the pharmacology and toxicology of meso-2,3-dimercaptosuccinic acid, an orally effective chelating agent. The need for a better chelating agent for treating young children and pregnant women with lead intoxication has been apparent for some time. Preclinical and clinical evidence now indicate that meso-2,3-dimercaptosuccinic acid, an Orphan Drug, shows the most promise for being effective in this regard. It has an extracellular distribution that may be responsible for its low toxicity compared to other dithiols. No attempt has been made to compare it in a rigorous and thorough manner with other chelating agents. That has not been the purpose of this review. The advantages of meso-DMSA, however, compared to CaNa2EDTA for the treatment of lead intoxication, have been outlined. Significant advances have been made recently in elucidating the structures of the metal chelates of DMSA. There is a striking difference between the structures of the lead chelate of meso-DMSA and those of racemic-DMSA. The former chelates by coordination of one sulfur and one oxygen atom with Pb. The latter can form chelates via the two sulfur atoms or via one oxygen and one sulfur atom. Solubility of the lead chelates depends on the ionization of the noncoordinated thiol and carboxylic acid groups. Bimane derivatization, HPLC, and fluorescence, as well as gas chromatography can be used for analysis of DMSA in biological fluids. The acid dissociation constants for meso- and racemic-DMSA have been summarized from the literature as have the formation constants of some of the DMSA chelates. DMSA is biotransformed to a mixed disulfide in humans. By 14 hr after DMSA administration (10 mg/kg), only 2.5% of the administered DMSA is excreted in the urine as unaltered DMSA and 18.1% of the dose is found in the urine as altered forms of DMSA. Most altered DMSA in the urine is in the form of a mixed disulfide. It consists of DMSA in disulfide linkages with two molecules of L-cysteine. One molecule of cysteine is attached to each of the sulfur atoms of DMSA. The remaining 10% of the altered DMSA was in the form of cyclic disulfides of DMSA. So far, the mixed disulfide has been found in human but not in rabbit, mouse, or rat urine. Apparently there are species differences in how organisms metabolize meso-DMSA.(ABSTRACT TRUNCATED AT 400 WORDS)

Covalent and noncovalent interactions in acute lethal cell injury caused by chemicals

Abstract: Several chemicals, including some drugs, cause cell injury by mechanisms that are poorly characterized. Although a few drugs are toxins that act directly, most require metabolic activation to form the ultimate toxic species. The concept of metabolic activation was first described by the Millers ( 1) in a review of their pioneering work on aminoazo dyes and 2-acetylaminofluorene as procarcinogens. Brodie et al (2) expanded on this concept to include a role for metabolic activation in acute cell injury. Since that time several reviews (3-12) and books ( 13-15) have discussed the role of metabolic activation in the pathogenesis of a variety of toxicities. Although a substantial body of information is available on the metabolic reactions that produce toxic metabolites and on the chemical nature of these metabolites, considerably less is known about how ultimate toxic species interact with cellular constituents and how the interactions cause cell injury. Reactions of toxic metabolites may result in covalent bond formation between the metabolite and a target molecule, or they may alter (usually by oxidation or reduction) the target molecule without formation of a covalent bond. Some of the more likely targets of these interactions are listed in Table 1.

Biochemical Mechanisms and Pathobiology of α2u-Globulin Nephropathy

Abstract: alpha 2u-N is a syndrome that has been characterized in male rats exposed to a number of environmental chemicals and pharmacological agents. The chemicals or their metabolites bind to alpha 2u, which is believed to lead to a less digestible chemical-protein complex. Because of the decreased hydrolysis of the chemical-protein complex in the lysosome, alpha 2u accumulates in the form of protein droplets. In extensive nephropathy, the accumulation of alpha 2u in the lysosome results in polyangular crystalloid droplets that lead to lysosomal overload and eventually cell death. This cell death stimulates restorative cell replication which promotes renal carcinogenesis in male rats. As such, it is imperative that extrapolation of risk to humans of chemicals causing this syndrome be performed. Because the nongenotoxic mechanism for carcinogenesis in the male rat involves a unique protein, such extrapolations can only be done incorporating species differences in the critical factors that result in alpha 2u-N in rats. Presently, these data suggest a markedly reduced risk for humans compared to male rats.

The Significance of Induced Forestomach Tumors

Abstract: Scientific interest in forestomach carcinogenesis has been greatly heightened by the observation that butylated hydroxyanisole (BHA) is carcinogenic to rats at this site (1). BHA is one of only three synthetic phenolic antioxidants that are permitted to be added to food in Canada. These chemicals inhibit oxidative spoilage of food (rancidity) during transportation and storage and are thus of importance to the functioning of the modern centralized food industry. The demonstration that high levels of BHA incorporated into the food of male and female rats resulted in papillomas and carcinomas of the forestomach led to an international research effort focused on the forestomach and designed to outline the way in which these tumors arose and determine whether they are relevant to humans (2). This report addresses the development of cancerous lesions in the rodent forestomach, the nature of the chemicals that induce these lesions, and their interactions with the animal body. Presently available evidence indicates there are at least two different types of chemical carcinogen that act on the forestomach and that these may have very different degrees of relevance to the human population.

Tissue Plasminogen Activator: The Biochemistry and Pharmacology of Variants Produced by Mutagenesis

Abstract: Thrombi, whatever their locations or etiologies, are to some extent composed of and held together by fibrin. Fibrin is formed by the action of thrombin on soluble fibrinogen and is the end result of the complex series of reactions in the coagulation cascade. Under normal circumstances the deposition of fibrin occurs at sites of blood vessel injury to stem the loss of blood from the vessel, and it is essential that the fibrin deposit be maintained until tissue repair has been initiated. Fibrinolysis comes into play as the mechanism by which fibrin is removed from the system. Regulation of the initiation of thrombolysis following wound repair is poorly understood, but it is clear that plasminogen activators are capable of converting plasminogen to plasmin, which degrades the fibrin matrix to soluble, readily cleared products . Under unusual circum­ stances, such as in myocardial infarction , a blood clot is deposited in such a way that blood flow through the vessel is significantly reduced. The damage initiating this misplaced coagulation event is the subject of considerable research; however, in the absence of ways to prevent such occurrences, it is essential that the thrombi be removed promptly. During myocardial infarc-

Mutational spectrometry in animal toxicity testing

Abstract: The microbial geneticists who pioneered the study of mutational spectra have shown us how rich a source of mutagen-specific information it can be for the pharmacologist and toxicologist. Contributions by physical chemists and molecular biologists have led to a practical means to observe mutation spectra directly from the DNA of cultured human cells and this review suggests that the same methodology may be applied successfully to the study of mutations and mutational spectra in tissues of experimental animals and humans. In toxicological testing, the new field of mutational spectrometry offers a previously unattainable level of sensitivity and specificity as well as new tools for dissecting mechanisms of pharmacologic and toxic action.

CHLOROETHYLENES: A Mechanistic Approach to Human Risk Evaluation

Abstract: The four chlorinated ethylenes, vinyl chloride (chloroethylene), vinylidene chloride (1, I-dichloroethylene), 1,1 ,2-trichloroethylene and 1,1,2,2tetrachloroethylene fonn a group of structurally related chemicals with prop­ erties invaluable to modem industrial society. Polymers and copolymers of vinyl chloride and vinylidene chloride have become the most important synthetic resins in use today, with worldwide production of PVC exceeding 18 million tons in 1988. The solvent properties of trichloroethylene and tetrachloroethylene have resulted in their widespread use in metal degreasing, dry cleaning, and a wide variety of industrial applications. These chemicals have now been in common use for more than 50 years. During this time workers have been exposed to a wide range of concentrations, in some cases for periods of 20 years or more, which has allowed a significant data base to be compiled about the shortand long-tenn effects of these chemicals on human health. In the last 20 years this infonnation has been supplemented by numerous studies in laboratory animals. The chloroethylenes are either gases or volatile liquids and the principal route of exposure is by inhalation. Exposure to high concentrations results in eNS depression and narcosis (1--4); trichloroethylene , for example, has been used as an anesthetic for many short-duration surgical procedures (5). Other effects have been reported, ranging from acro-osteolysis, part of a phenom­ enon known as vinyl chloride disease (6), to liver, kidney, and lung changes

Ability of short-term tests to predict carcinogenesis in rodents.

Abstract: Carcinogenesis is a complex, multistage process. Although an initial step in the process is believed to be the result of a genotoxic or mutagenic event, later stages are poorly understood. One of the major impediments to the study of carcinogenesis, and our ability to identify agents with tumorigenic potential, is the fact that the process is often protracted; the development of a chemically induced tumor may encompass the majority of the expected lifetime of the animal. This has given impetus to the development of short-term tests (STT) that can predict the carcinogenic potential of a chemical or serve as a model for one of the steps in the process. Numerous STT for genotoxicity have been proposed and developed, but attempts to evaluate these assays have suffered from the inadequate number of known noncarcinogens tested (l, 2). By having results on comparable num­ bers of carcinogens and noncarcinogens, not only can the sensitivity (i.e. the proportion of carcinogens correctly identified) of each assay, or combination of assays be estimated, but also the specificity (i.e. the proportion of noncar­ cinogens correctly identified) and the concordance (i.e. the proportion of