* Veterinary Research Institute, Hudcova 70, CZ-62132 Brno, Czech Republic;
Research Center for Atmospheric and Environmental Chemistry and Ecotoxicology (RECETOX), Masaryk University, CZ-63700 Brno, Czech Republic;
Institute of Biophysics, Czech Academy of Sciences, CZ-61265 Brno, Czech Republic; and
Department of Pediatrics and Human Development and the National Food Safety and Toxicology Center, Michigan State University, East Lansing, Michigan 48824
Received July 14, 2001; accepted October 5, 2001
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key Words: gap-junctional intercellular communication (GJIC); polycyclic aromatic hydrocarbons (PAHs); nongenotoxic carcinogenicity; tumor promotion; in vitro.
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The downregulation of gap junctional intercellular communication (GJIC) by tumor promoting compounds is considered to be a critical step in the removal of a cell from growth suppression (Trosko and Ruch, 1998; Upham et al., 1998
; Yamasaki et al., 1995
). Gap junctions are channels formed by connexin proteins that permit small regulatory molecules and ions <1000 molecular weight (e.g., glutathione, cAMP, Ca2+, inositoltriphosphate, etc.) to pass directly between adjacent cells (Loewenstein, 1987
; Upham et al., 1997b
). GJIC has been linked to the regulation of development, cellular proliferation, differentiation, and apoptosis (Trosko and Goodman, 1994
; Trosko and Ruch, 1998
; Yamasaki et al., 1995
). Downregulation of GJIC by chronic exposure to toxicants has been suggested as playing an important role in the tumor-promoting steps of cancer. Strong correlations between the results of in vivo two-stage carcinogenesis tests and in vitro GJIC inhibition assays were found for a number of known tumor promoters, such as phorbol 12-myristate 13-acetate (PMA) (Fitzgerald and Yamasaki, 1990
) and several organochlorine compounds (Baker et al., 1995
; Flodström et al., 1988
; Ren et al., 1998
; Sai et al., 1998
; Warngard et al., 1985
, 1989
), using rat liver WB-F344 epithelial cells, hamster V79 fibroblasts, or primary rat hepatocytes as model systems. A structure activity-relationship model on GJIC inhibiting activity resulted in a very strong concordance between experimental and predicted results (Rosenkranz et al., 1997
) and indicated that inhibition of GJIC is linked to the carcinogenic process in rodents (Rosenkranz et al., 2000
). Therefore, a potency of a given chemical to inhibit GJIC in vitro can be assumed to be a representative marker of tumor-promoting properties for a majority of known classes of tumor promoters.
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental and food contaminants formed mainly during the incomplete combustion of organic materials. High concentrations of PAHs were found in various environmental samples and complex mixtures, such as air particulate matter, soil, river, and marine sediments, petroleum products, and tar or tobacco smoke (Marvin et al., 1999; WHO, 1998
). Their presence in the environment is of concern, since many of them are suspected of being strong mutagens and carcinogens (Delistraty, 1997
; Durant et al., 1996
, 1999
). In vivo tests have shown that many PAHs induce tumors in rodents (IARC, 1983
). Until recently, the studies on the carcinogenicity of PAHs focused almost exclusively on genotoxic events. However, PAHs have been shown to cause adverse nongenotoxic effects, such as aryl hydrocarbon receptor (AhR)-mediated activation of genes (Bols et al., 1999
; Clemons et al., 1998
; Machala et al.
, in press; Piskorska-Pliszczynska et al., 1986
; Willett et al., 1997
), perturbation of Ca2+ levels (Tannheimer et al., 1997
), activation of mitogen-activated protein kinase (MAPK)-mediated intracellular signaling (Rummel et al., 1999
), and inhibition of GJIC (Upham et al., 1994
, 1998
). PAHs form an extremely heterogeneous class of individual chemicals numbering in the hundreds, but it is practical to routinely monitor only a few selected PAHs in complex environmental matrices. The U.S. EPA requires monitoring of 16 so-called priority PAHs (Callahan, 1979
). However, the selection of monitored PAHs does not sufficiently reflect the real toxicity of complex mixtures (mutagenicity, AhR-mediated toxicity, or tumor-promoting activity). Therefore, in the present study, we compared the potencies of 35 PAHs to inhibit GJIC using an in vitro rat liver epithelial cell system, which included U.S. EPA priority PAHs as well as some high molecular weight PAHs, most of them detected in various environmental samples (Machala et al., 2001
; Marvin, 1999
). Based on the relative potencies (REPs) of individual PAHs expressed as a ratio of the IC50 of the reference compound (benzo[a]pyrene) and the IC50 of individual PAHs, arbitrary inhibition equivalency factors (GJIC-IEFs) were suggested in this study for the purposes of toxicity estimation of complex samples and environmental risk assessment. Such values allow us to characterize individual PAHs of ecotoxicological importance by multiplying the IEF value with the concentration of the respective compound. The concept of GJIC-IEFs is similar to a widely accepted approach using toxic equivalency factors (TEFs) for assessment of dioxin-like toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds (van den Berg et al., 1998
), and the TEF approach for risk assessment of carcinogenic PAHs (Delistraty, 1997
).
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The chemical structures of PAHs under study, with the exception of known U.S. EPA priority PAHs, are presented in Figure 1.
|
Data analyses.
The ratio of GJIC inhibition related to the negative control was evaluated and expressed in percentage (fraction of control, FOC). Nonparametric statistical methods were used for the data analysis. Kruskal-Wallis ANOVA followed by the Mann-Whitney test were used for the assessment of significance, and p values of less than 0.05 were considered statistically significant. The concentrations of reference toxicants (PMA and benzo[a]pyrene) and concentrations of the PAHs under study causing 50% inhibition of GJIC (IC50) were determined by the nonlinear logit regression, and 95% confidence intervals for IC50 were estimated; relative error did not exceed 15%. The values of relative inhibition potencies (REPs) were based on calculated ratios of reference IC50 (benzo[a]pyrene) vs. the IC50 of the respective PAH under study.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
Nine PAHs showing no significant inhibitory effects on GJIC included anthracene, benzo[k]fluoranthene, dibenzo[a,h]anthracene (U.S. EPA priority PAHs), as well as dibenzo[a,i]pyrene, dibenz[a,j]anthracene, benzo[j]fluoranthene, benzo[e]pyrene, perylene, and coronene. Due to the overlap of fluorescence spectra of the other 5 compounds (indeno[1,2,3-cd]pyrene, naphtho[2,3-a]pyrene, benzo[a]perylene, dibenzo[a,e]fluoranthene, dibenzo[a,k]fluoranthene) with the fluorescence of lucifer yellow, it was not possible to estimate the effect of these compounds at the highest tested concentration (100 µM). However, none of the compounds showed significant inhibitory activity at 50 µM concentration. Thus, these were considered as noninhibiting compounds of GJIC in the WB-F344 cells.
Based on our results, relative inhibition potencies were calculated for all the PAHs under study, as the ratio of the IC50 of the reference PAH benzo[a]pyrene and IC50 of each respective PAH. The values of IC50 and calculated ratios (REPs) are summarized in Table 1. Based on the REP values, the GJIC-IEFs for the purposes of risk assessment were arbitrarily chosen (Table 1
).
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Several nongenotoxic effects of PAHs, which could play a role in carcinogenesis, were reported. These include strong Ah receptor-mediated activities of PAHs (Bols et al., 1999; Clemons et al., 1998
; Machala, 1996
; Machala et al., 2001
; Piskorska-Pliszczynska et al., 1986
; Willett et al., 1997
), estrogenic receptor-mediated activities, i.e., agonistic activation of estrogenic receptor and/or antiestrogenic effects associated with AhR activation (Clemons et al., 1998
; Safe et al., 1998
), oxidative stress (Burczynski et al., 1999
), modulation of intracellular signal transduction pathways involving release of Ca2+ (Tannheimer et al., 1997
), and MAPK activation (Rummel et al., 1999
). The known carcinogenic, genotoxic, and AhR-mediated potencies of PAHs are presented in Table 1
, in order to compare them with the inhibitory effects of individual compounds on GJIC.
Of all the in vitro methods for detecting tumor-promoting activity, the assays for GJIC inhibition seem to have the best predictive power (Autrup and Dragsted, 1987; Rosenkranz et al., 2000
, 1997
). In previous reports (Ghoshal et al., 1999
; Upham et al., 1994
; Weis et al., 1998
) several low molecular weight PAHs (such as fluorene, phenanthrene, fluoranthene, and their methylated derivatives) have been shown to elicit a strong downregulation of GJIC in the WB-F344 cells, while pyrene, benzo[a]pyrene and benzo[e]pyrene induced only a partial inhibition within micromolar exposure concentrations. However, a number of PAHs that are prevalent in environmental samples have not been included in those studies. Therefore, in the present study, the inhibitory potencies of a broader set of PAHs were investigated.
Exposure of WB-F344 cells to relatively smaller molecules (such as fluorene, phenanthrene, fluoranthene, pyrene and others) resulted in higher than 70% inhibition of GJIC (up to 98% inhibition), while the other nine PAHs (mostly higher molecular weight compounds, such as benzo[a]pyrene, dibenzo[a,e]pyrene, dibenzo[a,h]pyrene and others) caused only 2550% inhibition of GJIC where a plateau in the response was reached (exposure to higher concentration did not further inhibit GJIC). Since pyrene and dibenz[a,c]anthracene caused higher than 50% inhibition and elicited relatively low IC50 values, these compounds seemed to belong to the group of strong inhibitors (Fig. 3, Table 1
). No inhibition up to 100 µM was observed for the remaining PAHs. Therefore, we classified these compounds as noninhibitors of GJIC in the cellular model used (Table 1
).
The results showed that several PAHs considered to possess a nonmutagenic or low mutagenic activity in human and bacterial in vitro models, such as phenanthrene, fluoranthene, pyrene, benzo[c]phenanthrene and picene (Delistraty, 1997; Durant et al., 1996
), belong among the most potent inhibitors of GJIC. Other compounds that are usually not analysed during routine monitoring procedures, such as cyclopenta[cd]pyrene and 5-methylchrysene (both known to be potent mutagens) were found to be also potent inhibitors of GJIC. On the other hand, some of the strong mutagens and carcinogens, such as dibenzopyrenes and dibenzofluoranthenes, showed very low or no inhibition of GJIC. Taken together, in agreement with the results of Rosenkranz et al. (2000), no apparent correlations were found between the reported genotoxic potencies of the PAHs and their potencies to inhibit GJIC in vitro (see Table 1
). Similarly, we have previously shown that the data on the Ah receptor-mediated potencies of PAHs also did not correlate with their mutagenic properties (Machala et al., 2001
). The results suggest an urgent need to study carcinogenicity of PAHs, not only by means of mutagenicity or AhR-mediated toxicity in a single assay, but also in using test systems on GJIC inhibition and/or other important nongenotoxic modes of toxic action.
Similar to time- and dose-dependent GJIC inhibition previously reported for several PAHs (Ghoshal et al., 1999; Weis et al., 1998
), responses to the exposures of 21 GJIC-inhibiting PAHs in our study were observed to have a transient character. Strong inhibitory effects were found within 30 min, while a recovery of GJIC was observed after prolonged exposure periods (14 h). Several model tumor promoters such as PMA also produce only a transient inhibition of GJIC (Kanemitsu et al., 1993
). Despite the transient character of this process, the inhibition of GJIC is currently considered to be a suitable in vitro biomarker of tumor promoting potency of tested compounds.
PMA and PAHs inhibited GJIC at significantly different concentrations (IC50 of PMA was 8 nM, while PAHs inhibited within micromolar concentrations after a 30-min exposure). Interestingly, estimated IC50 values of individual strongly inhibiting PAHs in this study did not differ more than one order of concentration range (1050 µM). Similar micromolar concentrations were previously reported for GJIC inhibiting methylated and chlorinated PAHs (Rummel et al., 1999; Weis et al., 1998
), and also for persistent chlorinated tumor promoters such as DDT (Fransson et al., 1990
) and lindane (Upham et al., 1997a
). However, the concentrations of PAHs required to inhibit GJIC could approach the levels known from human exposure data (WHO, 1998
).
With respect to rapid occurrence of GJIC inhibition, the mechanism of action of these compounds is probably at the posttranslational level, as suggested by Rummel et al. (1999). One of the possible mechanisms of GJIC control is phosphorylation of connexins. Several protein kinases, such as protein kinase C (PKC), mitogen-activated protein kinases (MAPK), or tyrosine kinases are known to be involved in this process (Lampe and Lau, 2000). Although PKC-mediated phosphorylation of connexin is supposed to be a major mechanism by which PMA disrupts GJIC (Lampe and Lau, 2000
; Madhukar et al., 1996
), the mode of action of PAHs may be different. It has been shown, that PAHs induce MAPK activation downstream of GJIC inhibition (Rummel et al., 1999
). Another mechanism may involve phospholipase-induced release of arachidonic acid metabolites after PAH exposure (Upham et al., 2000
). In conclusion, the molecular mechanisms involved in the inhibition of GJIC by PAHs are still poorly understood and additional studies are required to elucidate this process.
Several structure-activity relationships have been described, based on the previous studies on downregulation of GJIC by PAHs (Rummel et al., 1999; Upham et al., 1994
, 1998
; Weis et al., 1998
). The methylated and chlorinated PAH derivatives that had bay-like regions were more inhibitory than the PAH-counterparts that did not contain the angular pocket of the bay-like region, which had been formed by either a methyl or a chlorine group, and the 3-ringed PAHs appeared to possess the higher inhibition when compared to 2-, 4-, and 5-ringed PAHs. In the present study, the following structural dependencies were apparent and corresponded to previous findings discussed above: PAHs with higher molecular masses and higher lipophility (Kow values) elicited low or negligible inhibition activity; stronger GJIC inhibition potency of bay region-forming or bay-like PAHs (methylated derivatives) was confirmed. In contrast, some higher molecular mass PAHs with bay-like regions (picene and dibenz[a,c]anthracene) showed strong inhibitory effect. Thus, several physicochemical characteristics seem to affect inhibitory potencies of individual PAHs.
In this study, IC50 values were estimated for individual compounds, inhibitory potencies (REPs) related to a reference PAH such as benzo[a]pyrene were calculated, and arbitrary inhibitory equivalency factors (GJIC-IEFs) were suggested (Tab. 1). Arbitrary IEFs values 10.0 and 5.0 were chosen for strongest inhibitors, 1.0 and 0.5 were attributed to weak GJIC-inhibitors.
The similar arbitrary approach has been proposed formerly for the formulation of genotoxic equivalents of PAHs (Nisbet and LaGoy, 1992). This concept is based on the similar toxic-equivalency-factor (TEF) approach that is used in environmental and health risk assessment of various toxic pollutants. The approach is generally accepted for persistent dioxin-like chemicals (van den Berg et al., 1998
). For PAHs, the genotoxicity equivalency factors relative to benzo[a]pyrene (Delistraty, 1997
; Nisbet and LaGoy, 1992
) and AhR-mediated activity equivalency factors relative to TCDD or benzo[a]pyrene (Clemons et al., 1998
; Delistraty, 1997
; Jones and Anderson, 1999
; Machala et al., 2001
; Willett et al., 1997
) have been previously suggested. Specific TEFs (based on multiple in vivo and in vitro studies) or REPs/IEFs (derived from a single assay) are multiplied by concentrations measured in complex samples. Calculated values representing "benzo[a]pyrene equivalents" can be used for comparative studies of promotional and carcinogenic effects, as well as ecological risk assessment of PAHs and their mixtures. Such comparison of environmental significance of individual compounds or complex mixtures is generally accepted (WHO, 1998
).
In a previous study, high concentrations of some strong GJIC inhibitors, such as fluoranthene, pyrene and picene (up to 779, 1.033, and 382 ng/g dry weight, respectively), were found in river sediments contaminated by PAHs (Machala et al., 2001). Due to their high levels in the environment and high relative potencies to inhibit GJIC, these PAHs may contribute most significantly to the promotional potency of complex environmental mixtures.
In conclusion, our results indicate that many environmentally important PAHs are potent in vitro inhibitors of GJIC in rat liver epithelial cell line WB-F344. Inhibition of GJIC seems to be an important mode of action of a series of PAHs, especially for those with lower molecular mass. On the other hand, when considering complete carcinogenic effects of other PAHs, showing only weak or no GJIC inhibitory properties, such as dibenzopyrenes and dibenzofluoranthenes, then other mechanisms could be involved in the tumor promoting effects. Suggested arbitrary equivalency factors (GJIC-IEFs) should serve for evaluation of promotional activity of complex environmental samples contaminated with PAHs. Although the GJIC inhibition assay remains the most suitable in vitro system to detect potential tumor promoters, it would be beneficial to combine the GJIC results with assays detecting other important, nongenotoxic modes of action, such as modulation of intracellular signals leading to an increased cell proliferation and survival.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
NOTES |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Autrup, H., and Dragsted, L. (1987). Overview of tumour promoters and test systems to identify promoters, Vol. 2. Nordic Council of Ministers, Copenhagen, Denmark.
Baker, T. K., Kwiatkowski, A. P., Madhukar, B. V., and Klaunig, J. E. (1995). Inhibition of gap junctional intercellular communication by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rat hepatocytes. Carcinogenesis 16, 23212326.[Abstract]
Bols, N. C., Schirmer, K., Joyce, E. M., Dixon, D. G., Greenberg, B. M., and Whyte, J. J. (1999). Ability of polycyclic aromatic hydrocarbons to induce 7-ethoxyresorufin-O-deethylase activity in a trout liver cell line. Ecotoxical. Environ. Safety 44, 118128.
Burczynski, M. E., Lin, H. K., and Penning, T. M. (1999). Isoform-specific induction of a human aldo-keto reductase by polycyclic aromatic hydrocarbons (PAHs), electrophiles, and oxidative stress: Implications for the alternative pathway of PAH activation catalyzed by human dihydrodiol dehydrogenase. Cancer Res. 59, 607614.
Callahan, M. A., Slimak, M. W., Gabel, N. W., May, I. P., Flower, C. F., Freed, J. R., Jennings, P., DuPree, R. I., Whitmore, F. C., Maestri, B., Mabey, W. R., Holt, B. R., and Gould, C. (1979). Water-related environmental fate of 129 priority pollutants. EPA-440/4-79-029a and b, Vols. I and II. U.S. Environmental Protection Agency, Springfield, VA.
Clemons, J. H., Allan, L. M., Marvin, C. H., Wu, Z., McCarry, B. E., Bryant, D. W., and Zacharewski, T. R. (1998). Evidence of estrogen- and TCDD-like activities in crude and fractionated extracts of PM10 air particulate material, using in vitro gene expression assays. Environ. Sci. Technol. 32, 18531860.[ISI]
Delistraty, D. (1997). Toxic equivalency factor approach for risk assessment of polycyclic aromatic hydrocarbons. Toxicol. Environ. Chem. 64, 81108.
Durant, J. L., Busby, W. F., Jr., Lafleur, A. L., Penman, B. W., and Crespi, C. L. (1996). Human cell mutagenicity of oxygenated, nitrated, and unsubstituted polycyclic aromatic hydrocarbons associated with urban aerosols. Mutat Res. 371, 123157.[ISI][Medline]
Durant, J. L., Lafleur, A. L., Busby, W. F., Donhoffner, L. L., Penman, B. W., and Crespi, C. L. (1999). Mutagenicity of C24H14 PAH in human cells expressing cyp1A1. Mutat Res. 446, 114.[ISI][Medline]
El-Fouly, M. H., Trosko, J. E., and Chang, C. C. (1987). Scrape-loading and dye transfer. A rapid and simple technique to study gap-junctional intercellular communication. Exp. Cell Res. 168, 422430.[ISI][Medline]
Fitzgerald, D. J., and Yamasaki, H. (1990). Tumor promotion: Models and assay systems. Teratog. Carcinog. Mutagen. 10, 89102.[ISI][Medline]
Flodström, S., Warngard, L., Hemming, H., Fransson, R., and Ahlborg, U. G. (1988). Tumour promotion-related effects by the cyclodiene insecticide endosulfan: Studies in vitro and in vivo. Pharmacol. Toxicol. 62, 230235.[ISI][Medline]
Fransson, R., Nicotera, P., Warngard, L., and Ahlborg, U. G. (1990). Changes in cytosolic Ca2+ are not involved in DDT-induced loss of gap-junctional communication in WB-F344 cells. Cell Biol. Toxicol. 6, 235244.[ISI][Medline]
Ghoshal, S., Weber, W. J., Rummel, A. M., Trosko, J. E., and Upham, B. L. (1999). Epigenetic toxicity of a mixture of polycyclic aromatic hydrocarbons on gap junctional intercellular communication before and after biodegradation. Environ. Sci. Technol. 33, 10441050.[ISI]
IARC (1983). IARC Monographs on the evaluation of the carcinogenic risk of chemicals in humans: I. Chemical, environmental, and experimental data. International Agency for Research on Cancer, Lyon, France.
Jones, J. M., and Anderson, J. W. (1999). Relative potencies of PAHs and PCBs are based on the response of human cells. Environ. Toxicol. Pharmacol. 7, 1926.[ISI]
Kanemitsu, M. Y., and Lau, A. F. (1993). Epidermal growth factor stimulates the disruption of gap junctional communication and connexin43 phosphorylation independent of 12-O-tetradecanoylphorbol 13-acetate-sensitive protein kinase C: The possible involvement of mitogen-activated protein kinase. Mol. Biol. Cell 4, 837848.[Abstract]
Lampe, P. D., and Lau, A. F. (2000). Regulation of gap junctions by phosphorylation of connexins. Arch. Biochem. Biophys. 384, 205215.[ISI][Medline]
Loewenstein, W. R. (1987). The cell-cell channel of gap junctions. Cell 48, 725726.[ISI][Medline]
Madhukar, B. V., de Feijter-Rupp, H. L., and Trosko, J. E. (1996). Pulse treatment with the tumor promoter TPA delays the onset of desensitization response and prolongs the inhibitory effect on gap-junctional intercellular communication of a rat liver epithelial cell line WB F-344. Cancer Lett. 106, 117123.[ISI][Medline]
Machala, M., Mátlová, L., Svoboda, I., and Nezveda, K. (1996). Induction effects of polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and other widespread aromatic environmental pollutants on microsomal monooxygenase activities in chick embryo liver. Arch. Toxicol. 70, 362367.[ISI][Medline]
Machala, M., Vondráek, J., Bláha, L., Ciganek, M., and Ne
a, J. V. (2001). Aryl hydrocarbon receptor-mediated activity of mutagenic polycyclic aromatic hydrocarbons determined using in vitro reporter-gene assay. Mutat Res. 497, 4962.[ISI][Medline]
Marvin, C. H., McCarry, B. E., Lundrigan, J. A., Roberts, K., and Bryant, D. W. (1999). Bioassay-directed fractionation of PAH of molecular mass 302 in coal tar-contaminated sediment. Sci. Total Environ. 231, 135144.[ISI][Medline]
Mosmann, T. (1983). Rapid colorimetric assay for cellular growth and survival: Application to proliferation and cytotoxicity assays. J. Immunol. Methods 65, 5563.[ISI][Medline]
Nisbet, I. C. T., and LaGoy, P. K. (1992). Toxic equivalency factors (TEFs) for polycyclic aromatic hydrocarbons (PAHs). Regul. Toxicol. Pharmacol. 16, 290300.[ISI][Medline]
Piskorska-Pliszczynska, J. P., Keys, B., Safe, S., and Newman, M. S. (1986). The cytosolic receptor-binding affinities and AHH induction potencies of 29 polynuclear aromatic hydrocarbons. Toxicol. Lett. 34, 6774.[ISI][Medline]
Ren, P., Mehta, P. P., and Ruch, R. J. (1998). Inhibition of gap junctional intercellular communication by tumor promoters in connexin43- and connexin32-expressing liver cells: Cell specificity and role of protein kinase C. Carcinogenesis 19, 169175.[Abstract]
Rosenkranz, H. S., Pollack, N., and Cunningham, A. R. (2000). Exploring the relationship between the inhibition of gap junctional intercellular communication and other biological phenomena. Carcinogenesis 21, 10071011.
Rosenkranz, M., Rosenkranz, H. S., and Klopman, G. (1997). Intercellular communication, tumor promotion, and nongenotoxic carcinogenesis: Relationships based upon structural considerations. Mutat. Res. 381, 171188.[ISI][Medline]
Rummel, A. M., Trosko, J. E., Wilson, M. R., and Upham, B. L. (1999). Polycyclic aromatic hydrocarbons with bay-like regions inhibited gap-junctional intercellular communication and stimulated MAPK activity. Toxicol. Sci. 49, 232240.[Abstract]
Safe, S., Wang, F., Porter, W., Duan, R., and McDougal, A. (1998). Ah receptor agonists as endocrine disruptors: Antiestrogenic activity and mechanisms. Toxicol. Lett. 102103, 343347.
Sai, K., Upham, B. L., Kang, K.-S., Hasegawa, R., Inoue, T., and Trosko, J. E. (1998). Inhibitory effect of pentachlorophenol on gap junctional intercellular communication in liver epithelial cells in vitro. Cancer Lett. 130, 917.[ISI][Medline]
Stocker, K. J., Howard, W. R., Statham, J., and Proudlock, R. J. (1996). Assessment of the potential in vivo genotoxicity of fluoranthene. Mutagenesis 11, 493496.[Abstract]
Tannheimer, S. L., Barton, S. L., Ethier, S. P., and Burchiel, S. W. (1997). Carcinogenic polycyclic aromatic hydrocarbons increase intracellular Ca2+ and proliferation in primary human mammary epithelial cells. Carcinogenesis 18, 11771182.[Abstract]
Trosko, J. E. (1997). Challenge to the simple paradigm that "carcinogens" are "mutagens" and to the in vitro and in vivo assays used to test the paradigm. Mutat. Res. 373, 245249.[ISI][Medline]
Trosko, J. E., Chang, C. C., Upham, B. L., and Wilson, M. (1998). Epigenetic toxicology as toxicant-induced changes in intracellular signalling leading to altered gap junctional intercellular communication. Toxicol. Lett. 102103, 7178.
Trosko, J. E., and Goodman, J. I. (1994). Intercellular communication may facilitate apoptosis: Implications for tumor promotion. Mol. Carcinog. 11, 812.[ISI][Medline]
Trosko, J. E., and Ruch, R. J. (1998). Cell-cell communication in carcinogenesis. Front. Biosci. 3, D208D236.[Medline]
Tsao, M. S., Smith, J. D., Nelson, K. G., and Grisham, J. W. (1984). A diploid epithelial cell line from normal adult rat liver with phenotypic properties of "oval" cells. Exp. Cell Res. 154, 3852.[ISI][Medline]
Upham, B., Sai, K., Tithof, P. K., Chen, G., Wilson, M. R., and Trosko, J. E. (2000). Inhibition of gap junction communication, activation of MAPK, and release of arachidonic acid by specific isomers of methylated anthracenes. Toxicol. Sci. 54(Suppl.), 323 (Abstract).
Upham, B. L., Boddy, B., Xing, X., Trosko, J. E., and Masten, S. J. (1997a). Nongenotoxic effects of selected pesticides and their disinfection by-products on gap junctional intercellular communication. Ozone Sci. Engineer. 19, 351369.
Upham, B. L., Kang, K.-S., Cho, H.-Y., and Trosko, J. E. (1997b). Hydrogen peroxide inhibits gap junctional intercellular communication in glutathione-sufficient but not gluthathione-deficient cells. Carcinogenesis 18, 3742.[Abstract]
Upham, B. L., Masten, S. J., Lockwood, S. J., and Trosko, J. E. (1994). Nongenotoxic effects of polycyclic aromatic hydrocarbons and their oxygenation by-products on the intercellular communication of rat liver epithelial cells. Fundam. Appl. Toxicol. 23, 470475.[ISI][Medline]
Upham, B. L., Weis, L. M., and Trosko, J. E. (1998). Modulated gap-junctional intercellular communication as a biomarker of PAH epigenetic toxicity: Structure-function relationship. Environ. Health Perspect. 106(Suppl.), 975981.[ISI][Medline]
van den Berg, M., Birnbaum, L., Bosveld, A. T. C., Brunstrom, B., Cook, P., Feeley, M., Giesy, J. P., Hanberg, A., Hasegawa, R., Kennedy, S. W., Kubiak, T., Larsen, J. C., VanLeeuwen, F. X., Liem, A. K. D., Nolt, C., Peterson, R. E., Poellinger, L., Safe, S., Schrenk, D., Tillitt, D., Tysklind, M., Younes, M., Waern, F., and Zacharewski, T. (1998). Toxic equivalency factors (TEFs) for PCBs, PCDDs, and PCDFs for humans and wildlifea review. Environ. Health Perspect. 106, 775792.[ISI][Medline]
Wang, J.-S., and Busby, W. F., Jr. (1993). Induction of lung and liver tumors by fluoranthene in a preweaning CD-1 mouse bioassay. Carcinogenesis 14, 18711874.[Abstract]
Warngard, L., Flodström, S., Ljungquist, S., and Ahlborg, U. G. (1985). Inhibition of metabolic cooperation in Chinese hamster lung fibroblast cells (V79) in culture by various DDT-analogs. Arch. Environ. Contam. Toxicol. 14, 541546.[ISI][Medline]
Warngard, L., Hemming, H., Flodström, S., Duddy, S. K., and Kass, G. E. N. (1989). Mechanistic studies on the DDT-induced inhibition of intercellular communication. Carcinogenesis 10, 471476.[Abstract]
Weis, L. M., Rummel, A. M., Masten, S. J., Trosko, J. E., and Upham, B. L. (1998). Bay or bay-like regions of polycyclic aromatic hydrocarbons were potent inhibitors of gap junctional intercellular communication. Environ. Health Perspect. 106, 1722.[ISI][Medline]
Willett, K. L., Gardinali, P. R., Sericano, J. L., Wade, T. L., and Safe, S. H. (1997). Characterization of the H4IIE rat hepatoma-cell bioassay for evaluation of environmental samples containing polynuclear aromatic hydrocarbons (PAHs). Arch. Environ. Contam. Toxicol. 32, 442448.[ISI][Medline]
WHO (1998). Selected Nonheterocyclic Polycyclic Aromatic Hydrocarbons. World Health Organization, Geneva.
Yamasaki, H., Mesnil, M., Omori, Y., Mironov, N., and Krutovskikh, V. A. (1995). Intercellular communication and carcinogenesis. Mutat. Res. 333, 181188.[ISI][Medline]