Intercellular communication, tumor promotion and non-genotoxic carcinogenesis: relationships based upon structural considerations

Non-Genotoxic and Environmentally Relevant Lower Molecular Weight Polycyclic Aromatic Hydrocarbons Significantly Increase Tumorigenicity of Benzo[a]pyrene in a Lung Two-Stage Mouse Model

The World Health Organization has classified air pollution as a carcinogen, and polycyclic aromatic hydrocarbons (PAHs) are major components of air particulates of carcinogenic concern. Thus far, most studies focused on genotoxic high molecular weight PAHs; however, recent studies indicate potential carcinogenicity of the non-genotoxic lower molecular weight PAHs (LMW PAHs) that are found in indoor and outdoor air pollution as well as secondhand cigarette smoke. We hypothesize that LMW PAHs contribute to the promotion stage of cancer when combined with benzo[a]pyrene (B[a]P), a legacy PAH. We specifically determined the effects of an LMW PAH mixture containing 1-methylanthracene (1MeA), fluoranthene (Flthn), and phenanthrene (Phe) combined with B[a]P on lung tumor promotion. To test this hypothesis, we used a two-stage, initiation/promotion BALB/ByJ female lung tumor mouse model. The mice were initiated with 3-methylcholanthrene followed by exposures to B[a]P, the LMW PAH mixture, and the combination of the LMW PAH mixture plus B[a]P, all at 10 mg/kg. The LMW PAHs combined with B[a]P significantly increased the promotion and incidence of lung tumors over that of B[a]P alone. The LMW PAHs in the absence of B[a]P did not significantly promote tumors, indicating strong co-promotional activities. We further assessed the effects of these PAHs on other hallmarks of cancer, namely, bronchoalveolar lavage fluid inflammatory infiltrates, pro-inflammatory transcripts, KC protein content, and mRNA expression of the gap junction (Gja1) and epiregulin (Ereg) genes. The LMW PAHs increased the biomarkers of inflammation, decreased Gja1 expression, and increased Ereg expression, all consistent with tumor promotion. This study indicates that non-genotoxic LMW PAHs can contribute to the cancer process and warrants further studies to assess the carcinogenic risks of other LMW PAHs.

The Carcinogenic Properties of Overlooked yet Prevalent Polycyclic Aromatic Hydrocarbons in Human Lung Epithelial Cells

The WHO classified air pollution as a human lung carcinogen and polycyclic aromatic hydrocarbons (PAHs) are components of both indoor (e.g., tobacco smoke and cookstoves) and outdoor (e.g., wildfires and industrial and vehicle emissions) air pollution, thus a human health concern. However, few studies have evaluated the adverse effects of low molecular weight (LMW) PAHs, the most abundant PAHs in the environment. We hypothesized that LMW PAHs combined with the carcinogenic PAH benzo[a]pyrene (B[a]P) act as co-carcinogens in human lung epithelial cell lines (BEAS-2B and A549). Therefore, in this paper, we evaluate several endpoints, such as micronuclei, gap junctional intercellular communication (GJIC) activity, cell cycle analysis, anti-BPDE-DNA adduct formation, and cytotoxicity after mixed exposures of LMW PAHs with B[a]P. The individual PAH doses used for each endpoint did not elicit cytotoxicity nor cell death and were relevant to human exposures. The addition of a binary mixture of LMW PAHs (fluoranthene and 1-methylanthracene) to B[a]P treated cells resulted in significant increases in micronuclei formation, dysregulation of GJIC, and changes in cell cycle as compared to cells treated with either B[a]P or the binary mixture alone. In addition, anti-BPDE-DNA adducts were significantly increased in human lung cells treated with B[a]P combined with the binary mixture of LMW PAHs as compared to cells treated with B[a]P alone, further supporting the increased co-carcinogenic potential by LMW PAHs. Collectively, these novel studies using LMW PAHs provide evidence of adverse pulmonary effects that should warrant further investigation.

Applicability of Scrape Loading-Dye Transfer Assay for Non-Genotoxic Carcinogen Testing

Dysregulation of gap junction intercellular communication (GJIC) is recognized as one of the key hallmarks for identifying non-genotoxic carcinogens (NGTxC). Currently, there is a demand for in vitro assays addressing the gap junction hallmark, which would have the potential to eventually become an integral part of an integrated approach to the testing and assessment (IATA) of NGTxC. The scrape loading-dye transfer (SL-DT) technique is a simple assay for the functional evaluation of GJIC in various in vitro cultured mammalian cells and represents an interesting candidate assay. Out of the various techniques for evaluating GJIC, the SL-DT assay has been used frequently to assess the effects of various chemicals on GJIC in toxicological and tumor promotion research. In this review, we systematically searched the existing literature to gather papers assessing GJIC using the SL-DT assay in a rat liver epithelial cell line, WB-F344, after treating with chemicals, especially environmental and food toxicants, drugs, reproductive-, cardio- and neuro-toxicants and chemical tumor promoters. We discuss findings derived from the SL-DT assay with the known knowledge about the tumor-promoting activity and carcinogenicity of the assessed chemicals to evaluate the predictive capacity of the SL-DT assay in terms of its sensitivity, specificity and accuracy for identifying carcinogens. These data represent important information with respect to the applicability of the SL-DT assay for the testing of NGTxC within the IATA framework.

Polycyclic aromatic hydrocarbon-induced signaling events relevant to inflammation and tumorigenesis in lung cells are dependent on molecular structure

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental and occupational toxicants, which are a major human health concern in the U.S. and abroad. Previous research has focused on the genotoxic events caused by high molecular weight PAHs, but not on non-genotoxic events elicited by low molecular weight PAHs. We used an isomeric pair of low molecular weight PAHs, namely 1-Methylanthracene (1-MeA) and 2-Methylanthracene (2-MeA), in which only 1-MeA possessed a bay-like region, and hypothesized that 1-MeA, but not 2-MeA, would affect non-genotoxic endpoints relevant to tumor promotion in murine C10 lung cells, a non-tumorigenic type II alveolar pneumocyte and progenitor cell type of lung adenocarcinoma. The non-genotoxic endpoints assessed were dysregulation of gap junction intercellular communication function and changes in the major pulmonary connexin protein, connexin 43, using fluorescent redistribution and immunoblots, activation of mitogen activated protein kinases (MAPK) using phosphospecific MAPK antibodies for immunoblots, and induction of inflammatory genes using quantitative RT-PCR. 2-MeA had no effect on any of the endpoints, but 1-MeA dysregulated gap junctional communication in a dose and time dependent manner, reduced connexin 43 protein expression, and altered membrane localization. 1-MeA also activated ERK1/2 and p38 MAP kinases. Inflammatory genes, such as cyclooxygenase 2, and chemokine ligand 2 (macrophage chemoattractant 2), were also upregulated in response to 1-MeA only. These results indicate a possible structure-activity relationship of these low molecular weight PAHs relevant to non-genotoxic endpoints of the promoting aspects of cancer. Therefore, our novel findings may improve the ability to predict outcomes for future studies with additional toxicants and mixtures, identify novel targets for biomarkers and chemotherapeutics, and have possible implications for future risk assessment for these PAHs.

Role of integrative signaling through gap junctions in toxicology

Gap junctional intercellular communication (GJIC) plays a central role in coordinating signal-transduction pathways that control gene expression inside of cells with those of neighboring cells in maintaining the homeostasis of a tissue. The normal homeostatic set point of gap junctions within tissues is in an open state, and although transient closure of gap junctions in response to mitogenic effectors is normal, chronic closure of channels by continuous exposure to environmental and food-borne contaminants can result in adverse health effects such as cancer, teratogenesis, reproductive dysfunction, neuropathies, and cardiac arrhythmias. GJIC is the primary means of integrating signal transduction pathways controlling gene expression between contiguous cells. Thus, bioassay systems that can measure GJIC offer a central, more biosystems approach to assessing the potential for toxicants to epigenetically alter gene expression.

Supercritical CO(2)-extracted tomato Oleoresins enhance gap junction intercellular communications and recover from mercury chloride inhibition in keratinocytes

A nutritionally relevant phytochemical such as lycopene, found in tomatoes and other fruits, has been proposed to have health-promoting effects by modulating hormonal and immune systems, metabolic pathways, and gap junction intercellular communication (GJIC). This work analyzes lycopene extracts, obtained from tomato and tomato added with grape seeds by using a safe and environmentally friendly extraction process, based on supercritical carbon dioxide technology (S-CO(2)). Analysis of the innovative S-CO(2)-extracted oleoresins showed peculiar chemical composition with high lycopene concentration and the presence of other carotenoids, lipids, and phenol compounds. The oleoresins showed a higher in vitro antioxidant activity compared with pure lycopene and beta-carotene and the remarkable ability to enhance the GJIC and to increase cx43 expression in keratinocytes. The oleoresins, (0.9 microM lycopene), were also able to overcome, completely, the GJIC inhibition induced by 10 nM HgCl(2), mercury(II) chloride, suggesting a possible action mechanism.

Structure-activity-dependent regulation of cell communication by perfluorinated fatty acids using in vivo and in vitro model systems

BACKGROUND

Perfluoroalkanoates, [e.g., perfluorooctanoate (PFOA)], are known peroxisome proliferators that induce hepatomegaly and hepatocarcinogenesis in rodents, and are classic non-genotoxic carcinogens that inhibit in vitro gap-junctional intercellular communication (GJIC). This inhibition of GJIC is known to be a function of perfluorinated carbon lengths ranging from 7 to 10.

OBJECTIVES

The aim of this study was to determine if the inhibition of GJIC by PFOA but not perfluoropentanoate (PFPeA) observed in F344 rat liver cells in vitro also occurs in F344 rats in vivo and to determine mechanisms of PFOA dysregulation of GJIC using in vitro assay systems.

METHODS

We used an incision load/dye transfer technique to assess GJIC in livers of rats exposed to PFOA and PFPeA. We used in vitro assays with inhibitors of cell signaling enzymes and antioxidants known to regulate GJIC to identify which enzymes regulated PFOA-induced inhibition of GJIC.

RESULTS

PFOA inhibited GJIC and induced hepatomegaly in rat livers, whereas PFPeA had no effect on either end point. Serum biochemistry of liver enzymes indicated no cytotoxic response to these compounds. In vitro analysis of mitogen-activated protein kinase (MAPK) indicated that PFOA, but not PFPeA, can activate the extracellular receptor kinase (ERK). Inhibition of GJIC, in vitro, by PFOA depended on the activation of both ERK and phosphatidylcholine-specific phospholipase C (PC-PLC) in the dysregulation of GJIC in an oxidative-dependent mechanism.

CONCLUSIONS

The in vitro analysis of GJIC, an epigenetic marker of tumor promoters, can also predict the in vivo activity of PFOA, which dysregulated GJIC via ERK and PC-PLC.

Tumor promoting properties of a cigarette smoke prevalent polycyclic aromatic hydrocarbon as indicated by the inhibition of gap junctional intercellular communication via phosphatidylcholine-specific phospholipase C

Inhibition of gap junctional intercellular communication (GJIC) and the activation of intracellular mitogenic pathways are common hallmarks of epithelial derived cancer cells. We previously determined that the 1-methyl and not the 2-methyl isomer of anthracene, which are prominent cigarette smoke components, activated extracellular receptor kinase, and inhibited GJIC in WB-F344 rat liver epithelial cells. Using these same cells, we show that an immediate upstream response to 1-methylanthracene was a rapid (<1 min) release of arachidonic acid. Inhibition of phosphatidylcholine-specific phospholipase C prevented the inhibition of GJIC by 1-methylanthracene. In contrast, inhibition of phosphatidylinositol specific phospholipase C, phospholipase A(2), diacylglycerol lipase, phospholipase D, protein kinase C, and tyrosine protein kinases had no effect on 1-methylanthracene-induced inhibition of GJIC. Inhibition of protein kinase A also prevented inhibition of GJIC by 1-methylanthracene. Direct measurement of phosphatidylcholine-specific phospholipase C and sphingomyelinase indicated that only phosphatidylcholine-specific phospholipase C was activated in response to 1-methylanthracene, while 2-methylanthracene had no effect. 1-methylanthracene also activated p38-mitogen activated protein kinase; however, like extracellular kinase, its activation was not involved in 1-methylanthracene-induced regulation of GJIC, and this activation was independent of phosphatidylcholine-specific phospholipase C. Although mitogen activated protein kinases were activated, Western blot analyzes indicated no change in connexin43 phosphorylation status. Our results indicate that phosphatidylcholine-specific phospholipase C is an important enzyme in the induction of a tumorigenic phenotype, namely the inhibition of GJIC; whereas mitogen activated protein kinases triggered in response to 1-methylanthracene, were not involved in the deregulation of GJIC.

Lack of predictivity of the rat lethality (LD50) test for ecological and human health effects

The relationship between acute toxicity in rats (LD50 values) and indicators of potential health hazards in humans was investigated, based on a chemical population-based paradigm (i.e. the "chemical diversity approach"). These structure-activity relationship-based analyses indicate that high toxicity in rats (i.e. a low LD50 value) is not a good predictor of health effects in humans. In fact, it was found that high acute toxicity to minnows, as well as toxicity to cultured cells, showed significantly greater associations with the potential for health effects than rat LD50 values.

Metabolites of benzene are potent inhibitors of gap-junction intercellular communication

Chronic exposure to benzene has been shown to lead to bone marrow depression and the development of leukemia. The mechanism underlying the carcinogenicity of benzene is unknown, although a number of genetic changes including chromosomal aberrations have been associated with benzene toxicity. Metabolism of benzene is required for the induced toxicological effects. We have investigated the effect of trans,trans-muconaldehyde (MUC), hydroquinone (HQ), and four MUC metabolites on gap-junction intercellular communication (GJIC). Inhibition of GJIC has been considered a possible predictor of tumor promoters and non-genotoxic carcinogens, and shown to result in perturbation of hematopoiesis. MUC was found to be a strong inhibitor of GJIC (EC50=12 micromol L(-1)) in rat liver epithelial cells IAR20, with potency similar to that of chlordane (EC50=7 micromol L(-1)). HQ inhibited GJIC with an EC50 of 25 micromolmol L(-1), and the metabolite OH/CHO with an EC50 of 58 micromol L(-1). The other MUC metabolites tested, CHO/COOH and OH/COOH were weak inhibitors of GJIC whereas COOH/COOH had no effect. Benzene itself had no effect on GJIC when tested in concentrations up to 20 micromol L(-1). The relative potency observed for the metabolites on GJIC is similar to their hematotoxic effects. The effect of MUC on GJIC was observed to take place concordant with a dramatic loss of connexin 43 (Cx43) from the cells as visualized by Western blotting. Substances with the ability to inhibit Cx43-dependent GJIC have previously been observed to interfere with normal hematopoietic development. The ability of benzene metabolites to interfere with gap-junction functionality, and especially the dramatic loss of Cx43 induced by MUC, should therefore be considered as a possible mechanism for benzene-induced hematotoxicity and development of leukemia.

Connexin43 synthesis, phosphorylation, and degradation in regulation of transient inhibition of gap junction intercellular communication by the phorbol ester TPA in rat liver epithelial cells

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induces transient inhibition of gap junction intercellular communication (GJIC) in several cell types. The initial block in GJIC has been attributed to protein kinase C (PKC) mediated phosphorylation of connexin gap junction proteins, including connexin43 (Cx43). Restoration of GJIC, associated with normalization of the Cx43 phosphorylation status, has been ascribed to different events, including dephosphorylation of Cx43 and de novo synthesis of Cx43 or other, non-gap junctional, proteins. The data presented suggest that restoration of GJIC during continuous TPA exposure in normal and transformed rat liver epithelial cells is dependent on synthesis of Cx43 protein, as well as the transport of already synthesized Cx43 from intracellular pools to the plasma membrane. Reactivation of inactivated Cx43 by dephosphorylation does not appear to be involved in the recovery of GJIC. Both PKC and MAP kinase is involved in TPA-induced degradation of Cx43 and inhibition of GJIC. We show that coincubation of TPA with the protein synthesis inhibitor cycloheximide or the transcription inhibitor actinomycin D results in synergistic enhancement of the level of activated ERK1/2. Together, the present data highlight Cx43 degradation and synthesis as critical determinants in TPA-induced modifications of cell-cell communication via gap junctions.

Environmental odors and health hazards

Using the recently developed and validated 'chemical diversity approach', the potential of chemicals, to be detected by the human olfactory system and to cause adverse health effects, was investigated. The analyses found no significant association between odor perceptibility and potential for inducing health effects.

Synergy between systemic toxicity and genotoxicity: relevance to human cancer risk

The health risk manager and policy analyst must frequently make recommendations based upon incomplete toxicity data. This is a situation which is encountered in the evaluation of human carcinogenic risks as animal cancer bioassay results are often not available. In this study, in order to assess the relevance of other possible indicators of carcinogenic risks, we used the "chemical diversity approach" to estimate the magnitude of the human carcinogenic risk based upon Salmonella mutagenicity and systemic toxicity data of the "universe of chemicals" to which humans have the potential to be exposed. Analyses of the properties of 10,000 agents representative of the "universe of chemicals" suggest that chemicals that have genotoxic potentials as well as exhibiting greater systemic toxicity are more likely to be carcinogens than non-genotoxicants or agents that exhibit lesser toxicity. Since "genotoxic" carcinogenicity is a hallmark of recognized human carcinogens, these findings are relevant to human cancer risk assessment.

Recovery of gap junctional intercellular communication after phorbol ester treatment requires proteasomal degradation of protein kinase C

Reversible down-regulation of gap junctional intercellular communication (GJIC) is proposed to be an important cellular mechanism in tumor promotion. Gap junction function is modified by a variety of tumor promoters, including the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Treatment of cells with TPA results in the activation and subsequent depletion of the TPA-responsive protein kinase C (PKC) isoforms. TPA-induced degradation of the PKC isoforms alpha, delta and epsilon was recently shown to occur via the ubiquitin-proteasome pathway. In the present study we investigated the role of the proteasome in the TPA-induced modification of GJIC in IAR20 rat liver epithelial cells. TPA exposure of IAR20 cells induced hyperphosphorylation of gap junction protein connexin43 and inhibition of GJIC. Prolonged TPA treatment induced down-regulation of PKCalpha, delta and epsilon and a reduction in the total PKC activity, which was associated with recovery of GJIC. Co-treatment of IAR20 cells with TPA and the proteasomal inhibitor MG132 suppressed down-regulation of PKCalpha, delta and epsilon and caused prolonged PKC activity. Under these conditions, the recovery of GJIC was blocked. The general PKC inhibitor GF109203X reversed the effect of MG132, indicating that the prolonged TPA-induced inhibition of GJIC caused by MG132 was due to the prolonged PKC activity. These results indicate that proteasomal degradation of PKC is one mechanism by which the recovery of GJIC after TPA treatment is regulated.

A substructure-based SAR model for odor perception in humans relevant to health risk assessments

The ability of human to perceive odors is a very complex phenomenon involving the selective binding of molecules to approximately 1000 olfactory receptors. Accordingly, the derivation of a substructure-based SAR model can be expected to be problematic. Yet, based upon published data on odor thresholds of volatile organic chemicals, we were able to derive such an SAR model. An examination of the structural determinants and related modulators indicates that lipophilicity is a major contributor to olfactory perception. The availability of a substructure-based SAR model permits an examination of the relationship between the presence in the environment of odorous chemicals and public health risks.

A data mining approach for the elucidation of the action of putative etiological agents: application to the non-genotoxic carcinogenicity of genistein

A procedure designated "the virtual similarity index" (VSI) is described to determine the probability that two or more toxicants are related mechanistically. The approach is structure-activity relationship (SAR) based and generates the virtual toxicological profiles of the chemicals under investigation. It also determines the similarities between them. That commonality is compared to the frequency with which it is found among a population of 10,000 chemicals representing the "universe of chemicals". The similarities between the candidate chemicals and chemicals known to act by other recognized mechanisms are also determined. If the similarities between the candidate chemicals are significantly greater than for the non-related ones, the chemicals are assumed to act by a common mechanism. In that context, the putative non-genotoxic mechanism responsible for the carcinogenicity of genistein (GEN) and its relationship to the action of diethylstilbestrol is examined.

Relative molecular similarity in selected chemical carcinogens and the nucleoside triphosphate chain

Several markers of cell toxicity are useful as screening tests for epigenetic carcinogens. The direct effects of chemicals on ATPase and GTPase function are pertinent to the early stages of carcinogenesis. Interference with triphosphate-diphosphate exchange mechanisms may result from the interaction of carcinogens with the substrate triphosphate chain. To investigate this hypothesis, a computational chemistry programme is used in this study to investigate molecular similarity in ATPase inhibitors, carcinogens and tumour promoters, in relation to the nucleoside triphosphate chain. The results show that atoms in the investigated molecular structures superimpose on sets of oxygen atoms in the triphosphate chain with interatomic distances < 0.3A. Relative molecular similarity to the substrate triphosphate chain is discussed in terms of the established inhibitory properties of carcinogens/tumour promoters on ATPase function, the carcinogen/ tumour promoting properties of ATPase inhibitors and the prediction of carcinogenic activity from chemical structure.

Determination of the epigenetic effects of ochratoxin in a human kidney and a rat liver epithelial cell line

Epidemiological studies have implicated ochratoxin A (OTA), a fungal metabolic-contaminant of animal and human food sources, in Balkan Endemic Nephropathy and renal tumors. Many environmental toxicants operate through nongenotoxic mechanisms that epigenetically control gene expression leading to a diseased state. Gap junctional intercellular communication (GJIC) plays a central role in the epigenetic control of genes in which alteration of normal GJIC has been implicated in many human pathologies, including cancer, teratogenesis, reproductive dysfunction and peripheral neuropathies. The cell proliferative stages of human diseases, such as cancer, also involves the induction of signal transduction pathways controlling the mitogenic steps, in which the mitogen activated protein kinases (MAPK), such as extracellular receptor kinase (ERK) and p38, are central to mitogenesis. We therefore determined the effects of OTA on GJIC and MAPK in a human kidney and rat liver epithelial cell line. OTA reversibly inhibited GJIC at noncytotoxic doses in the rat liver but not the human kidney cell line. Similarly, OTA was also a strong activator of MAPK, ERK and p38, in the rat liver cells but only weakly activated ERK and had no affect on p38 in the human kidney cell line. Another hallmark of human diseases is an abnormal alteration of apoptosis, also known as programmed cell death. We used our myc-transfected cell line, which exhibits higher levels of apoptosis, to test the effects of OTA on apoptosis. OTA greatly induced apoptosis in this cell line, which is contrary to the effects of most tumor promoters. In summary, OTA exhibits tumor promoting properties in the liver, but the effects of OTA on the human kidney epithelial cells suggested a lack of tumorigenic activity assuming that these epithelial cells, like the rat liver epithelial cells, are a primary target for carcinogens. These results also indicate that the nephrotoxicity of OTA either does not involve GJIC, assuming these epithelial cells play a vital role in kidney physiology, or that a more differentiated kidney cell type is the target for OTA toxicity, of which the role of GJIC remains unknown.

Allergic contact dermatitis and its relationship to carcinogenicity

The relationship between allergic contact dermatitis (ACD) and carcinogenicity was investigated using a recently developed and validated simulation approach. The analyses indicated that while there are electrophilic and non-electrophilic components to ACD, these were not identical to those operating in carcinogenicity. Accordingly, with respect to carcinogenicity prediction, the results of ACD do not improve the results based upon mutagenicity testing alone, the latter being a surrogate for potential electrophilicity.

The role of connexin-mediated cell-cell communication in breast cancer metastasis

Gap junctional intercellular communication (GJIC) is a form of cell-cell communication mediating the exchange of small molecules between neighboring cells. Gap junctions (GJs) are formed by connexins (Cxs), and are subject to tight and dynamic regulation. They are involved in the cell cycle, differentiation, and cell signaling. The loss of Cxs and GJs is a hallmark of carcinogenesis, while their induction in cancer cells leads to a reversal of the cancer phenotype, induction of differentiation, and regulation of cell growth. On the basis of the observations about Cx loss in breast cancer, this review examines Cxs' involvement in breast cancer metastasis. Previous work indicates that Cx expression is inversely correlated to metastatic potential. This is probably because of the loss of cooperation between neighboring cells, leading to cell heterogeneity and cell dissociation in the tumor. The possible involvement of Cx activity during metastasis will be discussed.

The use of structure-activity relationships and markers of cell toxicity to detect non-genotoxic carcinogens

In contrast to the situation for genotoxic carcinogens, few in vitro tests exist that can detect early markers of the events thought to be associated with non-genotoxic carcinogenesis. Also, comparatively little is known about the quantitative structure-activity relationships (Q)SARs of these agents. This review discusses published SAR studies conducted on non-genotoxic carcinogens, in relation to the use of several markers of in vitro cell toxicity (inhibition of gap-junctional intercellular communication, inhibition of tubulin polymerization, modulation of apoptosis and induction of cell proliferation), which are used as endpoints for screening this class of carcinogen. Much of the work has involved the identification of new biophores (substructural features of molecules associated with toxicity), as well as other structural features, which are thought to predispose the chemicals to ligand binding with specific target molecules acting as possible receptors (e.g. protein kinase C, the oestrogen, peroxisome-proliferator and tubulin protein receptors), implicated in the mechanism of toxicity involved. It is concluded that (a) there is an urgent need for more information on (Q)SARs for non-genotoxic carcinogens; (b) this information should be acquired by using several different approaches in a variety of laboratories; and (c) such research should proceed together with more studies on the mechanisms of cell toxicity caused by these chemicals, including the identification and characterisation of further specific receptors involved in mediating the various types of cell toxicity associated with this type of carcinogenesis.

Phosphorylation of connexin43 on serine368 by protein kinase C regulates gap junctional communication

Phorbol esters (e.g., TPA) activate protein kinase C (PKC), increase connexin43 (Cx43) phosphorylation, and decrease cell-cell communication via gap junctions in many cell types. We asked whether PKC directly phosphorylates and regulates Cx43. Rat epithelial T51B cells metabolically labeled with (32)P(i) yielded two-dimensional phosphotryptic maps of Cx43 with several phosphopeptides that increased in intensity upon TPA treatment. One of these peptides comigrated with the major phosphopeptide observed after PKC phosphorylation of immunoaffinity-purified Cx43. Purification of this comigrating peptide and subsequent sequencing indicated that the phosphorylated serine was residue 368. To pursue the functional importance of phosphorylation at this site, fibroblasts from Cx43(-/-) mice were transfected with either wild-type (Cx43wt) or mutant Cx43 (Cx43-S368A). Intercellular dye transfer studies revealed different responses to TPA and were followed by single channel analyses. TPA stimulation of T51B cells or Cx43wt-transfected fibroblasts caused a large increase in the relative frequency of approximately 50-pS channel events and a concomitant loss of approximately 100-pS channel events. This change to approximately 50-pS events was absent when cells transfected with Cx43-S368A were treated with TPA. These data strongly suggest that PKC directly phosphorylates Cx43 on S368 in vivo, which results in a change in single channel behavior that contributes to a decrease in intercellular communication.

The high production volume chemical challenge program: the relevance of the in vivo micronucleus assay

The in vivo rodent bone marrow micronucleus assay (Mnt) has assumed a pivotal role in screening strategies for the identification of substances potentially carcinogenic to humans. The analysis of the results of the current international 5-year effort to provide toxicological data for high production volume chemicals will play a crucial role in developing future strategies for identifying health hazards. As part of that program, consideration is being given to accepting either in vitro genotoxicity data or results of the Mnt. The present analyses indicate that for hazard identification purposes that, in fact, in vitro genotoxicity test results, such as those derived from the Salmonella mutagenicity assay, may be an acceptable alternative.

Chemical diversity approach for evaluating mechanistic relatedness among toxicological phenomena

The CASE/MULTICASE structure-activity relationship (SAR) system was used to assess a new procedure to investigate the mechanistic relatedness of various toxicological endpoints. The method consisted of predicting the activity of 10,000 randomly selected chemicals using validated and characterized SAR models from a variety of biological and toxicological endpoints. The prevalence of chemicals predicted to possess the ability to induce two or more toxicological effects simultaneously should provide a measure of the mechanistic relatedness of these phenomena. Eight toxicological endpoints were predicted and the results were compared to predictions based on an eye irritation SAR model. Allergic contact dermatitis demonstrated a 29.6% greater than expected overlap between expected and observed results (p < 0.001). Similar results were seen for respiratory hypersensitivity (33.1%), sensory irritation (28.9%), cell toxicity (25.9%), and Ah receptor binding (19.8%). A lesser degree of overlap was seen between eye irritation and Salmonella mutagenicity (11.5%) and the inhibition of gap junction intercellular communication (6.7%). Moreover, a negative overlap, suggesting possibly an antagonistic phenomena, was observed between eye irritation and alpha 2 mu-induced nephropathy. These results indicate that this method can provide a useful tool to investigate mechanistic relatedness between diverse toxicological endpoints.

Structural determinants of developmental toxicity in hamsters

A CASE/MULTICASE structure activity relationship (SAR) model of developmental toxicity of chemicals in hamsters (HaDT) was developed. The model exhibited a predictive performance of 74%. The model's overall predictivity and informational content were similar to those of an SAR model of mutagenicity in Salmonella. However, unlike the Salmonella mutagenicity model, the HaDT model did not identify overtly chemically reactive moieties as associated with activity. Moreover, examination of the number and nature of significant structural determinants suggested that developmental toxicity in hamsters was not the result of a unique mechanism or attack on a specific molecular target. The analysis also indicated that the availability of experimental data on additional chemicals would improve the performance of the SAR model.

Chlorothalonil: lack of genotoxic potential

Based upon analyses using a number of validated structure-activity relationship models, it is concluded that the carcinogenicity in rodents of chlorothalonil is not due to a genotoxic mechanism.

Applications of the case/multicase SAR method to environmental and public health situations

The availability of validated and characterized SAR models of toxicological phenomena provides a method to apply SAR technology to a variety of environmental, public health and industrial situations. These include (i) the prioritization of environmental pollutants for control and/or regulation, (ii) the design of multi-action optimized therapeutics from which the potential for unwanted side-effects have been engineered out, (iii) the development of SAR-based computer-driven screening procedure to identify candidate therapeutics based upon combinatorial chemistry or compilations of molecular structures, (iv) the generation of toxicological profiles to be used in the selection of benign chemicals in the early stages of product development.

Human developmental toxicity and mutagenesis

A previously described SAR model of human developmental toxicity was analyzed further. The model shows a number of mechanistic similarities with SAR models of other toxicological phenomena (systemic toxicity, chromosomal and genomic effects). This implies that there are many targets associated with developmental effects. Surprisingly the analyses revealed no significant mechanistic overlap between developmental toxicity in humans and mutagenicity in Salmonella, a surrogate for the occurrence of point mutations. Our study indicates that this lack of similarity is likely the result of the pre-screening strategies which largely eliminate Salmonella mutagens from among the therapeutics introduced into human medicine.