In vitro analysis of modulators of intercellular communication: Implications for biologically, based risk assessment models for chemical exposure

The Use of Biomarkers as Alternatives to Current Animal Tests on Food Chemicals

Recent developments in biomarkers relating to the interrelationship of diet, disease and health were surveyed. Most emphasis was placed on biomarkers of deleterious effects, since these are of greatest relevance to the subject of this review. The area of greatest activity was found to be that relating to biomarkers of mutagenic, genotoxic and carcinogenic effects. This is also one of the major areas of concern in considerations of the beneficial and deleterious effects of dietary components, and also the area in which regulatory testing requires studies of the longest duration. A degree of progress has also been made in the identification and development of biomarkers relating to certain classes of target organ toxicity. Biomarkers for other types of toxicity, such as immunotoxicity, neurotoxicity, reproductive toxicity and developmental toxicity, are less developed, and further investigation in these areas is required before a comprehensive biomarker strategy can be established. A criticism that recurs constantly in the biomarker literature is the lack of standardisation in the methods used, and the lack of reference standards for the purposes of validation and quality control. It is encouraging to note the growing acknowledgement of the need for validation of biomarkers and biomarker assays. Some validation studies have already been initiated. This review puts forward proposals for criteria to be used in biomarker validation. More discussion on this subject is required. It is concluded that the use of biomarkers can, in some cases, facilitate the implementation of the Three Rs with respect to the testing of food chemicals and studies on the effects of diet on health. The greatest potential is seen to be in the refinement of animal testing, in which biomarkers could serve as early and sensitive endpoints, in order to reduce the duration of the studies and also reduce the number of animals required. Biomarkers could also contribute to establishing a mechanistic basis for in vitro test systems and to facilitating their validation and acceptance. Finally, the increased information that could result from the incorporation of biomarker determinations into population studies could reduce the need for supplementary animal studies. This review makes a number of recommendations concerning the prioritisation of future activities on dietary biomarkers in relation to the Three Rs. It is emphasised, however, that further discussions will be required among toxicologists, epidemiologists and others researching the relationship between diet and health.

PPARα Agonist-Induced Rodent Tumors: Modes of Action and Human Relevance

Widely varied chemicals--including certain herbicides, plasticizers, drugs, and natural products--induce peroxisome proliferation in rodent liver and other tissues. This phenomenon is characterized by increases in the volume density and fatty acid oxidation of these organelles, which contain hydrogen peroxide and fatty acid oxidation systems important in lipid metabolism. Research showing that some peroxisome proliferating chemicals are nongenotoxic animal carcinogens stimulated interest in developing mode of action (MOA) information to understand and explain the human relevance of animal tumors associated with these chemicals. Studies have demonstrated that a nuclear hormone receptor implicated in energy homeostasis, designated peroxisome proliferator-activated receptor alpha (PPARalpha), is an obligatory factor in peroxisome proliferation in rodent hepatocytes. This report provides an in-depth analysis of the state of the science on several topics critical to evaluating the relationship between the MOA for PPARalpha agonists and the human relevance of related animal tumors. Topics include a review of existing tumor bioassay data, data from animal and human sources relating to the MOA for PPARalpha agonists in several different tissues, and case studies on the potential human relevance of the animal MOA data. The summary of existing bioassay data discloses substantial species differences in response to peroxisome proliferators in vivo, with rodents more responsive than primates. Among the rat and mouse strains tested, both males and females develop tumors in response to exposure to a wide range of chemicals including DEHP and other phthalates, chlorinated paraffins, chlorinated solvents such as trichloroethylene and perchloroethylene, and certain pesticides and hypolipidemic pharmaceuticals. MOA data from three different rodent tissues--rat and mouse liver, rat pancreas, and rat testis--lead to several different postulated MOAs, some beginning with PPARalpha activation as a causal first step. For example, studies in rodent liver identified seven "key events," including three "causal events"--activation of PPARalpha, perturbation of cell proliferation and apoptosis, and selective clonal expansion--and a series of associative events involving peroxisome proliferation, hepatocyte oxidative stress, and Kupffer-cell-mediated events. Similar in-depth analysis for rat Leydig-cell tumors (LCTs) posits one MOA that begins with PPARalpha activation in the liver, but two possible pathways, one secondary to liver induction and the other direct inhibition of testicular testosterone biosynthesis. For this tumor, both proposed pathways involve changes in the metabolism and quantity of related hormones and hormone precursors. Key events in the postulated MOA for the third tumor type, pancreatic acinar-cell tumors (PACTs) in rats, also begin with PPARalpha activation in the liver, followed by changes in bile synthesis and composition. Using the new human relevance framework (HRF) (see companion article), case studies involving PPARalpha-related tumors in each of these three tissues produced a range of outcomes, depending partly on the quality and quantity of MOA data available from laboratory animals and related information from human data sources.

Effect of thioridazine on gap junction intercellular communication in connexin 43-expressing cells

Propagation of electrical activity between myocytes in the heart requires gap junction channels, which contribute to coordinated conduction of the heartbeat. Some antipsychotic drugs, such as thioridazine and its active metabolite, mesoridazine, have known cardiac conduction side-effects, which have resulted in fatal or nearly fatal clinical consequences in patients. The physiological mechanisms responsible for these cardiac side-effects are unknown. We tested the effect of thioridazine and mesoridazine on gap junction-mediated intercellular communication between cells that express the major cardiac gap junction subtype connexin 43. Micromolar concentrations of thioridazine and mesoridazine inhibited gap junction-mediated intercellular communication between WB-F344 epithelial cells in a dose-dependent manner, as measured by fluorescent dye transfer. Kinetic analyses demonstrated that inhibition by 10 micromol/L thioridazine occurred within 5 min, achieved its maximal effect within 1 h, and was maintained for at least 24 h. Inhibition was reversible within 1 h upon removal of the drug. Western blot analysis of connexin 43 in a membrane-enriched fraction of WB-F344 cells treated with thioridazine revealed decreased amounts of unphosphorylated connexin 43, and appearance of a phosphorylated connexin 43 band that co-migrated with a "hyperphosphorylated" connexin 43 band present in TPA-inhibited cells. When tested for its effects on cardiomyocytes isolated from neonatal rats, thioridazine decreased fluorescent dye transfer between colonies of beating myocytes. Microinjection of individual cells with fluorescent dye also showed inhibition of dye transfer in thioridazine-treated cells compared to vehicle-treated cells. In addition, thioridazine, like TPA, inhibited rhythmic beating of myocytes within 15 min of application. In light of the fact that the thioridazine and mesoridazine concentrations used in these experiments are in the range of those used clinically in patients, our results suggest that inhibition of gap junction intercellular communication may be one factor contributing to the cardiac side-effects observed in some patients taking these medications.

Effects of cadmium on gap junctional intercellular communication in WB-F344 rat liver epithelial cells

Cadmium has been associated with a number of tumors but its role in tumor promotion has not been elucidated clearly or the results obtained from various studies have been conflicting. This study was designed to investigate the effects of cadmium on the gap junctional intercellular communication (GJIC), number of gap junctions per cell, and cell proliferation in WB-F344 rat liver epithelial cells from the viewpoint of tumor promotion. GJIC was monitored by counting the cells stained with Lucifer yellow CH dye, using the scrape-loading and dye-transfer method. The numbers of gap junctions per cell were visually quantitated after an indirect immunostaining for gap junction protein using an antibody to connexin 43. Cell proliferation was assayed by direct counting of the living cells using the trypan blue dye exclusion method. In the time course study, cells treated with 200 microM CdCl2 showed rapid and nearly complete inhibition of GJIC (approximately 14% of the control) and a decrease in the number of gap junctions per cell (approximately 21% of the control) at 30 min, and the decrease continued up to 4 h without any changes in the cell viability. Treatment with CdCl2 (7.4-200 microM) for 4 h resulted in the decrease of GJIC and gap junction numbers per cell in a dose-response pattern without changes in the cell viability. In the long-term (14 days) exposure studies at doses of 0.01-7.4 microM CdCl2, an increase in cell proliferation was observed at low doses of 0.03-2.5 microM CdCl2, with GJIC also decreasing. These data demonstrate that cadmium inhibits GJIC, reduces the number of gap junctions per cell, and induces cell proliferation while decreasing the function of the gap junction.

The role of inhibition of gap junctional intercellular communication in rodent liver tumor induction by phthalates: review of data on selected phthalates and the potential relevance to man

Inhibition of gap junctional intercellular communication (GJIC) has been postulated as a nongenotoxic carcinogenic mechanism, probably related to tumor promotion. Recent studies assessed the role of GJIC in the induction of rodent liver tumors by high levels of phthalate esters. Studies with di(2-ethylhexyl) (DEHP) and diisononyl (DINP) phthalates demonstrated that inhibition of GJIC in rats and mice was well correlated with induction of both liver tumors and markers for peroxisomal proliferation. However, GJIC was unaffected in hamsters and primates, species in which phthalate treatment does not induce peroxisomal proliferation. In vitro studies which extended the database to include human liver cells mirrored the in vivo situation; GJIC was inhibited in rat and mouse cells but not in cells from unresponsive species including humans. Peroxisomal proliferation has been characterized as a species-specific process essential for phthalate-induced rodent liver tumor induction. That GJIC was not inhibited in primate liver or human liver cells provides evidence for a second species-specific carcinogenic process. Thus the GJIC data along with those from studies of peroxisomal proliferation support the view that the carcinogenic effects of DEHP and DINP in rodents are not relevant to humans.

Morphological transformation and effect on gap junction intercellular communication in Syrian hamster embryo cells as screening tests for carcinogens devoid of mutagenic activity

A large fraction of chemicals observed to cause cancer in experimental animals is devoid of mutagenic activity. It is therefore of importance to develop methods that can be used to detect and study environmental carcinogenic agents that do not interact directly with DNA. Previous studies have indicated that induction of in vitro cell transformation and inhibition of gap junction intercellular communication are endpoints that could be useful for the detection of non-genotoxic carcinogens. In the present work, 13 compounds [chlordane, Arochlor 1260, di(2-ethylhexyl)phthalate, 1,1,1-trichloro-2, 2-bis(4-chlorophenyl)ethane, limonene, sodium fluoride, ethionine, o-anisidine, benzoyl peroxide, o-vanadate, phenobarbital, 12-O-tetradecanoylphorbol 13-acetate and clofibrate] have been tested for their ability to induce morphological transformation and affect intercellular communication in Syrian hamster embryo cells. The substances were selected on the basis of being proven or suspected non-genotoxic carcinogens, and thus difficult to detect in short-term tests. The data show that nine of the 13 compounds induced morphological transformation, and seven of the 13 inhibited intercellular communication in hamster embryo cells. Taken together, 12 of the 13 substances either induced transformation or caused inhibition of communication. The data suggest that the combined use of morphological transformation and gap junction intercellular communication in Syrian hamster embryo cells may be beneficial when screening for non-genotoxic carcinogens.

Changes in gap-junction permeability, phosphorylation, and number mediated by phorbol ester and non-phorbol-ester tumor promoters in rat liver epithelial cells

The effects of three tumor promoters on gap-junction permeability; connexin 43 and 26 mRNA levels, protein levels, and phosphorylation; and the numbers of gap-junctional membrane plaques were studied in the rat liver epithelial cell line WB-F344 to determine whether changes in these parameters correlated with the inhibition of gap-junction function. 12-O-tetradecanoylphorbol-13-acetate (TPA; 10 ng/mL), dieldrin (10 micrograms/mL), and heptachlor epoxide (10 micrograms/mL) inhibited gap-junctional intercellular communication (GJIC) assayed by fluorescent dye transfer by 80-90% after a 5-min exposure and by more than 90% within 1 h. Decreases in steady-state connexin 43 mRNA levels were detected by northern blot analysis within 1 h and paralleled changes in steady-state beta-actin mRNA, but these changes did not occur rapidly enough to account for the rapid loss of gap-junction function. A substantial loss in the number of connexin 43 immunostained gap-junctional membrane plaques was detected after a 15-min exposure to all three promoters, but little change had occurred at 5 min. Western blot analyses using connexin 43-specific antibodies showed changes in the degree of connexin 43 phosphorylation for all three tumor promoters. TPA induced the appearance of a fourth connexin 43-immunoreactive band (P3) and a concomitant decrease in the relative intensity of the unphosphorylated (P0) band within 5 min of treatment. P3, in addition to bands P1 and P2, disappeared after treatment with alkaline phosphatase. In contrast, dieldrin and heptachlor expoxide induced loss of P2 with a concomitant increase in the relative staining intensity of P0 within 1 h of exposure, but no changes were seen after 5 min. Connexin 43 phosphorylation levels recovered in parallel with the recovery of GJIC for all three tumor promoters. Connexin 26 mRNA levels showed little change after a 1-h exposure to three promoters, but reductions in connexin 26 immunofluorescent staining were observed. These results suggest that (i) TPA-induced hyperphosphorylation of connexin 43 occurred fast enough to account for inhibition of GJIC, (ii) dieldrin and heptachlor expoxide modulated connexin phosphorylation in a manner different from TPA by promoting hypophosphorylation of connexin 43, (iii) redistribution of plasma membrane gap-junctional plaques after treatment with phorbol ester and non-phorbol-ester tumor promoters occurred subsequent to changes in gap-junction permeability, and (iv) changes in connexin mRNA levels could not account for the losses in fluorescent dye coupling induced by these promoters.

The role of modulated gap junctional intercellular communication in epigenetic toxicology

The normal development and health of all multicellular organisms, including the human being, depend on the adaptive maintenance of the integrity of the genetic information (e.g., DNA protective and repair mechanisms), as well as of the homeostatic and cybernetic regulatory systems within and between tissues. The primary focus of the past and current toxicological studies and risk assessment practices has been to ascertain and predict the "genotoxicity" of various physical and chemical agents. The paradigm of "carcinogen as mutagen," while valuable for stimulating studies of the detection of mutagens and of their potential role in "causing" somatic and germ line diseases, has tended to blunt research on the role of nongenotoxic mechanisms in disease causation. This brief analysis will emphasize the need to consider the role of modulated gap junctional intercellular communication (GJIC) in any biological risk assessment model. It is based on the following assumptions and facts. Because gap junctions exist in all metazoans, they have been associated with the regulation of cell proliferation, development, differentiation, and the adaptive function of both excitable and nonexcitable coupled cells. A highly evolutionarily conserved family of genes codes for proteins (connexins), which, as hexameric units (connexons), form membrane-associated channels of gap junctions. Cells coupled by gap junctions will have their ions and small regulatory molecules equilibrated. Regulation of GJIC can be at the transcriptional, translational, or posttranslational levels. Transient down or up regulation of GJIC can be induced by endogenous or exogenous chemicals via many mechanisms at any of these three levels. Stable abnormal regulation has been associated with activated oncogenes, and normal regulation has been associated with several tumor suppressor genes. The dysfunction of these gap junctions might play a role in the actions of various toxic chemicals that have cell type/tissue/organ specificity. This could bring about distinct clinical consequences, such as embryo lethality or teratogenesis, reproductive dysfunction in the gonads, neurotoxicity of the central nervous system, hyperplasia of the skin, and tumor promotion of initiated tissue. Modulation of GJIC should be viewed as a scientific basis of "epigenetic toxicology" because the alteration of intercellular communication would alter the internal physiological state of the cell. The inhibition of GJIC is a necessary component of mitogenesis (a necessary component of the multistage carcinogenic process). The modulation of GJIC can have both toxicological, as well as therapeutic potential.

Effect of glucocorticoids on TPA-induced inhibition of gap-junctional communication and morphological transformation in Syrian hamster embryo cells

The effect of glucocorticoids on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inhibition of gap-junctional intercellular communication (GJIC) and morphological transformation in Syrian hamster embryo (SHE) cells was examined. Fluocinolone acetonide (FA) and dexamethasone (DEX) almost completely suppressed the effect of TPA on induction of transformed morphology. On the other hand, up to 1000 times higher FA and DEX concentrations did not influence the inhibitory effect of TPA on GJIC. Neither treatment with these glucocorticoids for 4, 24 or 48 h before TPA exposure nor 24 h co-exposure with TPA altered the effect of TPA on GJIC. Thus the potent effect of glucocorticoids as inhibitors of the promotional effect of TPA on morphological transformation in SHE cells does not result in alterations of TPA-induced inhibition of GJIC.

Oxidants and mitogenesis as causes of mutation and cancer: the influence of diet

A very high level oxidative damage to DNA occurs during normal metabolism. In each rat cell, the steady-state level of this damage is estimated to be about 10(6) oxidative adducts, and about 10(5) new adducts are formed daily. This endogenous DNA damage appears to be a major contributor to cancer and aging. The oxidative damage rate in mammalian species with a high metabolic rate, short life span, and high age-specific cancer rate such as in rats is much higher than the rate in humans, long-lived mammals with a lower metabolic rate, and a lower age-specific cancer rate. It is argued that deficiency of micronutrients, that protect against oxidative DNA damage, is a major contributor to human cancer. Epidemiological studies, a large body of experimental evidence, and theoretical work on the mechanisms of carcinogenesis point to mitogenesis as a major contributor to cancer. Dividing cells, compared to nondividing cells, are at an increased risk for mutations due to: 1.) conversion of DNA adducts to mutations; 2.) chance of mitotic recombination, gene conversion, and nondisjunction; and, 3.) increased exposure of DNA to mutagens. Mitogenesis also increases the probability of gene amplification and loss of 5-methylcytosine. Dietary interventions that lower mitogenesis, such as calorie restriction, decrease cancer incidence.

Oxidants are a major contributor to aging

Very high level oxidative damage to DNA occurs during normal metabolism. In each rat cell the steady-state level of this damage is estimated to be about 10(6) oxidative adducts, and about 10(5) new adducts are formed daily. This endogenous DNA damage appears to be a major contributor to aging and to the degenerative diseases associated with aging such as cancer. The oxidative damage rate in mammalian species with a high metabolic rate, short life span, and high age-specific cancer rate such as rats is much higher than the rate in humans, long-lived mammals with a lower metabolic rate and a lower age-specific cancer rate. It is argued that deficiency of micronutrients that protect against oxidative DNA damage is a major contributor to human cancer. Epidemiological studies, a large body of experimental evidence, and theoretical work on the mechanisms of carcinogenesis point to mitogenesis as a major contributor to cancer. Dividing cells compared to nondividing cells are at an enormously increased risk for mutations in part due to the conversion of DNA adducts to mutations. Mitogenesis also increases the probability of gene amplification and loss of 5-methylcytosine. Dietary interventions that lower mitogenesis, such as calorie restriction, decrease the incidence of cancer.

Effects of hydrocarbons on transformation and intercellular communication in Syrian hamster embryo cells

The ability of 18 different hydrocarbons to induce and promote morphological transformation and to inhibit intercellular communication in primary Syrian hamster embryo cells in culture have been studied. The compounds were: the n-alkanes octane, nonane, decane, undecane, dodecane and tridecane; the iso-alkanes 2-methylheptane, 2-methyloctane and 2-methylnonane; the naphthenes 1,2-dimethylcyclohexane, 1,2,4-trimethylcyclohexane and tert-butylcyclohexane; the aromates 1,2-dimethylbenzene, 1,2,4-trimetylbenzene and tert-butylbenzene; and the alkenes 1-octene, 1-nonene and 1-decene. None of the hydrocarbons induced morphological transformation of Syrian hamster embryo cells. When the hydrocarbons were incubated together with benzo(a)pyrene, enhancement of the transformation frequency was observed for the naphthene 1,2-dimethylcyclohexane and the iso-alkanes 2-methylheptane and 2-methyloctane. None of the n-alkanes, alkenes or aromates enhanced the transformation frequency induced by benzo(a)pyrene. The alkane tridecane and the iso-alkanes 2-methyloctane and 2-methylnonane reduced intercellular communication in the primary Syrian hamster embryo cells.

Simulation modeling of carcinogenesis

A discrete-time simulation model of carcinogenesis is described mathematically using recursive relationships between time-varying model variables. The dynamics of cellular behavior is represented within a biological framework that encompasses two irreversible and heritable genetic changes. Empirical data and biological supposition dealing with both control and experimental animal groups are used together to establish values for model input variables. The estimation of these variables is integral to the simulation process as described in step-by-step detail. Hepatocarcinogenesis in male F344 rats provides the basis for seven modeling scenarios which illustrate the complexity of relationships among cell proliferation, genotoxicity, and tumor risk.

Endogenous mutagens and the causes of aging and cancer

A very large oxidative damage rate to DNA occurs as part of normal metabolism. In each rat cell the steady-state level is estimated to be about 10(6) oxidative adducts and about 10(5) new adducts are formed daily. It is argued that this endogenous DNA damage is a major contributor to aging and the degenerative diseases of aging, such as cancer. The oxidative damage rate in mammalian species with a high metabolic rate, short life span, and high age-specific cancer rate is much higher than the rate in humans, a long-lived creature with a lower metabolic rate and a lower age-specific cancer rate. It is argued that deficiency of micronutrients, such as dietary antioxidants or folate, is a major contributor to human cancer and degenerative diseases. Understanding the role of mitogenesis in mutagenesis is critical for clarifying the mechanisms of carcinogenesis and interpreting high-dose animal cancer tests. High-dose animal cancer tests have been done mainly on synthetic industrial chemicals, yet almost all of the chemicals humans are exposed to are natural. About half of natural chemicals tested in high-dose animal cancer tests are rodent carcinogens, a finding that is consistent with the view that high-dose tests frequently increase mitogenesis rates. Animals have numerous defenses against toxins that make them very well buffered against low doses of almost all toxins, whether synthetic or natural.