Bisphenol S enhances gap junction intercellular communication in ovarian theca cells

Gap junction intercellular communication (GJIC) is necessary for ovarian function, and it is temporospatially regulated during follicular development and ovulation. At outermost layer of the antral follicle, theca cells provide structural, steroidogenic, and vascular support. Inter- and extra-thecal GJIC is required for intrafollicular trafficking of signaling molecules. Because GJIC can be altered by hormones and endocrine disrupting chemicals (EDCs), we tested if any of five common EDCs (bisphenol A (BPA), bisphenol S (BPS), bisphenol F (BPF), perfluorooctanesulfonic acid (PFOS), and triphenyltin chloride (TPT)) can interfere with theca cell GJIC. Since most chemicals are reported to repress GJIC, we hypothesized that all chemicals tested, within environmentally relevant human exposure concentrations, will inhibit theca cell GJICs. To evaluate this hypothesis, we used a scrape loading/dye transfer assay. BPS, but no other chemical tested, enhanced GJIC in a dose- and time-dependent manner in ovine primary theca cells. A signal-protein inhibitor approach was used to explore the GJIC-modulatory pathways involved. Phospholipase C and mitogen-activated protein kinase (MAPK) inhibitors significantly attenuated BPS-induced enhanced GJIC. Human theca cells were used to evaluate translational relevance of these findings. Human primary theca cells had a ∼40% increase in GJIC in response to BPS, which was attenuated with a MAPK inhibitor, suggestive of a conserved mechanism. Upregulation of GJIC could result in hyperplasia of the theca cell layer or prevent ovulation by holding the oocyte in meiotic arrest. Further studies are necessary to understand in vitro to in vivo translatability of these findings on follicle development and fertility outcomes.

Early Mechanistic Events Induced by Low Molecular Weight Polycyclic Aromatic Hydrocarbons in Mouse Lung Epithelial Cells: A Role for Eicosanoid Signaling

Low molecular weight polycyclic aromatic hydrocarbons (LMW PAHs; < 206.3 g/mol) are under regulated environmental contaminants (eg, secondhand smoke) that lead to gap junction dysregulation, p38 MAPK activation, and increased mRNA production of inflammatory mediators, such as cytokines and cyclooxygenase (COX2), in lung epithelial cells. However, the early mechanisms involving lipid signaling through the arachidonic acid pathway and subsequent eicosanoid production leading to these downstream events are not known. Common human exposures are to mixtures of LMW PAHs, thus C10 cells (a mouse lung epithelial cell line) were exposed to a representative binary PAH mixture, 1-methylanthracene (1-MeA) and fluoranthene (Flthn), for 30 min-24 h with and without p38 and cytosolic phospholipase A2 (cPLA2) inhibitors. Cytosolic phospholipase A2 inhibition reversed PAH-induced phospho-p38 MAPK activation and gap junction dysregulation at 30 min. A significant biphasic increase in cPLA2 protein was observed at 30 min, 2, and 4 h, as well as COX2 protein at 2 and 8 h. Untargeted metabolomics demonstrated a similar trend with significantly changing metabolites at 30 min and 4 h of exposure relative to 1 h; a "cPLA2-like" subset of metabolites within the biphasic response were predominately phospholipids. Targeted metabolomics showed several eicosanoids (eg, prostaglandin D2 (PGD2), PGE2α) were significantly increased at 4, 8, and 12 h following exposure to the binary PAH mixture and this effect was p38-dependent. Finally, PAH metabolism was not observed until after 8 h. These results indicate an early lipid signaling mechanism of LMW PAH toxicity in lung epithelial cells due to parent PAH compounds.

Risk, Stigma, Trustworthiness, and Citizen Participation-A Multifaceted Analysis of Media Coverage of Dioxin Contamination in Midland, Michigan

In the United States, more than 200 communities are designated by the U.S. Environmental Protection Agency as areas of concern for dioxins. Informing the public about potential risks associated with dioxins and delivering information about how to avoid such risks are essential activities. News coverage of environmental and health problems affects how members of the public assess those problems in terms of both severity and how they are understood, as well as the extent of attention given to the problem by policy-makers. To contextualize public and institutional responses to dioxin contamination and remediation in a dioxin-affected community, we assessed 176 newspaper articles published over 30 years concerning dioxin contamination in Midland, Michigan, in terms of risk, trust in institutions, environmental stigma, and citizen participation. Articles about dioxin contamination and remediation in Midland appeared in both domestic and international newspapers. Domestically, both national and local newspapers covered this issue. The risks for human health and the environment caused by exposure to dioxins were widely covered, with much less media attention given to the trustworthiness of the organizations responsible for managing the risk, environmental stigma, and citizen participation. News coverage of these four themes also changed significantly overtime. Overall, our findings highlight the important role of local news media in communicating risk information, guiding safe behaviors, and facilitating community-level decision-making.

The Effect of Dioxin Contamination and Remediation on Property Values

Loss of property value is a major concern in communities faced with the toxic byproducts of industrial practices. Even after site remediation, stigma may persist and negatively affect market values of residential properties. To study the effects of contamination and of remediation on property values in Midland, Michigan, where dioxins have been released into the environment through the incineration of contaminated waste and the discharge of contaminated water for many years, records of assessed value were obtained for 229 homes within the same neighborhood for the previous 18 years. A multilevel, longitudinal analysis was conducted to determine if there was a relationship between level of dioxin and assessed value after controlling for housing characteristics. Remediated and un-remediated properties saw increases in value at a similar rate over time. However, a property's level of dioxin was found to have a small, significant, and negative relationship with assessed value, and this negative effect was present regardless if a home had been remediated or not. These results suggest that while environmental remediation may be effective at removing the contamination, its economic effects may persist for a longer period of time.

Isothermal assay targeting class 1 integrase gene for environmental surveillance of antibiotic resistance markers

Antimicrobial resistance genes (ARGs) present in the environment pose a risk to human health due to potential for transfer to human pathogens. Surveillance is an integral part of mitigating environmental dissemination. Quantification of the mobile genetic element class 1 integron-integrase gene (intI1) has been proposed as a surrogate to measuring multiple ARGs. Measurement of such indicator genes can be further simplified by adopting emerging nucleic acids methods such as loop mediated isothermal amplification (LAMP). In this study, LAMP assays were designed and tested for estimating relative abundance of the intI1 gene, which included design of a universal bacteria 16S rRNA gene assay. Following validation of sensitivity and specificity with known bacterial strains, the assays were tested using DNA extracted from river and lake samples. Results showed a significant Pearson correlation (R² = 0.8) between the intI1 gene LAMP assay and ARG relative abundance (measured via qPCR). To demonstrate the ruggedness of the LAMP assays, experiments were also run in the hands of relatively "untrained" personnel by volunteer undergraduate students at a local community college using a hand-held real-time DNA analysis device - Gene-Z. Overall, results support use of the intI1 gene as an indicator of ARGs and the LAMP assays exhibit the opportunity for volunteers to monitor environmental samples for anthropogenic pollution outside of a specialized laboratory.

Toll-like receptor expression in human non-small cell lung carcinoma: potential prognostic indicators of disease

Introduction

Lung cancer remains the highest cause of cancer mortality worldwide. Toll-like receptors (TLR) are innate immune receptors that have both pro- and anti-tumorigenic properties. Based on findings from epidemiological studies and in rodents, we hypothesized that elevated TLR expression would be a positive prognostic indicator of disease in non-small cell lung carcinoma patients.

Results

Higher mRNA expression of TLR1-3 and 5-8 were significantly associated with increased overall survival (OS) when analyzed individually or as a group in both non-small cell lung carcinoma (NSCLC) patients and in the adenocarcinoma (ADC) subtype. Significant co-expression of many TLR combinations in ADC patients were also observed via RNA sequencing. Immunostaining demonstrated TLR4 and 8 significantly correlated in tumor tissue, similar to RNA.

Methods

We used kmplot.com to perform a meta-analysis on mRNA expression of TLR1-10 to determine any significant associations with OS in NSCLC and the ADC subtype. cBioportal was also used simultaneously to assess co-expression in TLR1-10 in ADC patients via RNA sequencing and to identify any molecular alterations. Lastly, immunostaining for a subset of TLRs was conducted on ADC patients.

Conclusions

Expression of innate immune receptors TLR1-10 is associated with improved survival outcomes in NSCLC. Thus, further evaluation of their predictive capacity and therapeutic utility is warranted.

Secondhand Smoke-Prevalent Polycyclic Aromatic Hydrocarbon Binary Mixture-Induced Specific Mitogenic and Pro-inflammatory Cell Signaling Events in Lung Epithelial Cells

Low molecular weight polycyclic aromatic hydrocarbons (LMW PAHs; < 206.3 g/mol) are prevalent and ubiquitous environmental contaminants, presenting a human health concern, and have not been as thoroughly studied as the high MW PAHs. LMW PAHs exert their pulmonary effects, in part, through P38-dependent and -independent mechanisms involving cell-cell communication and the production of pro-inflammatory mediators known to contribute to lung disease. Specifically, we determined the effects of two representative LMW PAHs, 1-methylanthracene (1-MeA) and fluoranthene (Flthn), individually and as a binary PAH mixture on the dysregulation of gap junctional intercellular communication (GJIC) and connexin 43 (Cx43), activation of mitogen activated protein kinases (MAPK), and induction of inflammatory mediators in a mouse non-tumorigenic alveolar type II cell line (C10). Both 1-MeA, Flthn, and the binary PAH mixture of 1-MeA and Flthn dysregulated GJIC in a dose and time-dependent manner, reduced Cx43 protein, and activated the following MAPKs: P38, ERK1/2, and JNK. Inhibition of P38 MAPK prevented PAH-induced dysregulation of GJIC, whereas inhibiting ERK and JNK did not prevent these PAHs from dysregulating GJIC indicating a P38-dependent mechanism. A toxicogenomic approach revealed significant P38-dependent and -independent pathways involved in inflammation, steroid synthesis, metabolism, and oxidative responses. Genes in these pathways were significantly altered by the binary PAH mixture when compared with 1-MeA and Flthn alone suggesting interactive effects. Exposure to the binary PAH mixture induced the production and release of cytokines and metalloproteinases from the C10 cells. Our findings with a binary mixture of PAHs suggest that combinations of LMW PAHs may elicit synergistic or additive inflammatory responses which warrant further investigation and confirmation.

Gap Junctional Intercellular Communication: A Functional Biomarker to Assess Adverse Effects of Toxicants and Toxins, and Health Benefits of Natural Products

This protocol describes a scalpel loading-fluorescent dye transfer (SL-DT) technique that measures intercellular communication through gap junction channels, which is a major intercellular process by which tissue homeostasis is maintained. Interruption of gap junctional intercellular communication (GJIC) by toxicants, toxins, drugs, etc. has been linked to numerous adverse health effects. Many genetic-based human diseases have been linked to mutations in gap junction genes. The SL-DT technique is a simple functional assay for the simultaneous assessment of GJIC in a large population of cells. The assay involves pre-loading cells with a fluorescent dye by briefly perturbing the cell membrane with a scalpel blade through a population of cells. The fluorescent dye is then allowed to traverse through gap junction channels to neighboring cells for a designated time. The assay is then terminated by the addition of formalin to the cells. The spread of the fluorescent dye through a population of cells is assessed with an epifluorescence microscope and the images are analyzed with any number of morphometric software packages that are available, including free software packages found on the public domain. This assay has also been adapted for in vivo studies using tissue slices from various organs from treated animals. Overall, the SL-DT assay can serve a broad range of in vitro pharmacological and toxicological needs, and can be potentially adapted for high throughput set-up systems with automated fluorescence microscopy imaging and analysis to elucidate more samples in a shorter time.

Methoxychlor and Vinclozolin Induce Rapid Changes in Intercellular and Intracellular Signaling in Liver Progenitor Cells

Methoxychlor (MXC) and vinclozolin (VIN) are well-recognized endocrine disrupting chemicals known to alter epigenetic regulations and transgenerational inheritance; however, non-endocrine disruption endpoints are also important. Thus, we determined the effects of MXC and VIN on the dysregulation of gap junctional intercellular communication (GJIC) and activation of mitogen-activated protein kinases (MAPKs) in WB-F344 rat liver epithelial cells. Both chemicals induced a rapid dysregulation of GJIC at non-cytotoxic doses, with 30 min EC50 values for GJIC inhibition being 10 µM for MXC and 126 µM for VIN. MXC inhibited GJIC for at least 24 h, while VIN effects were transient and GJIC recovered after 4 h. VIN induced rapid hyperphosphorylation and internalization of gap junction protein connexin43, and both chemicals also activated MAPK ERK1/2 and p38. Effects on GJIC were not prevented by MEK1/2 inhibitor, but by an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), resveratrol, and in the case of VIN, also, by a p38 inhibitor. Estrogen (ER) and androgen receptor (AR) modulators (estradiol, ICI 182,780, HPTE, testosterone, flutamide, VIN M2) did not attenuate MXC or VIN effects on GJIC. Our data also indicate that the effects were elicited by the parental compounds of MXC and VIN. Our study provides new evidence that MXC and VIN dysregulate GJIC via mechanisms involving rapid activation of PC-PLC occurring independently of ER- or AR-dependent genomic signaling. Such alterations of rapid intercellular and intracellular signaling events involved in regulations of gene expression, tissue development, function and homeostasis, could also contribute to transgenerational epigenetic effects of endocrine disruptors.

Chemopreventive Agents Attenuate Rapid Inhibition of Gap Junctional Intercellular Communication Induced by Environmental Toxicants

Altered gap junctional intercellular communication (GJIC) has been associated with chemical carcinogenesis, where both chemical tumor promoters and chemopreventive agents (CPAs) are known to conversely modulate GJIC. The aim of this study was to investigate whether attenuation of chemically inhibited GJIC represents a common outcome induced by different CPAs, which could be effectively evaluated using in vitro methods. Rat liver epithelial cells WB-F344 were pretreated with a CPA for either 30 min or 24 h, and then exposed to GJIC-inhibiting concentration of a selected tumor promoter or environmental toxicant [12-O-tetradecanoylphorbol-13-acetate (TPA), lindane, fluoranthene, 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT), perfluorooctanoic acid (PFOA), or pentachlorophenol]. Out of nine CPAs tested, quercetin and silibinin elicited the most pronounced effects, preventing the dysregulation of GJIC by all the GJIC inhibitors, but DDT. Metformin and curcumin attenuated the effects of three GJIC inhibitors, whereas the other CPAs prevented the effects of two (diallyl sulfide, emodin) or one (indole-3-carbinol, thymoquinone) GJIC inhibitor. Significant attenuation of chemically induced inhibition of GJIC was observed in 27 (50%) out of 54 possible combinations of nine CPAs and six GJIC inhibitors. Our data demonstrate that in vitro evaluation of GJIC can be used as an effective screening tool for identification of chemicals with potential chemopreventive activity.

Scrape Loading/Dye Transfer Assay

The scrape loading/dye transfer (SL/DT) technique is a simple functional assay for the simultaneous assessment of gap junctional intercellular communication (GJIC) in a large population of cells. The equipment needs are minimal and are typically met in standard cell biology labs, and SL/DT is the simplest and quickest of all the assays that measure GJIC. This assay has also been adapted for in vivo studies. The SL/DT assay is also conducive to a high-throughput setup with automated fluorescence microscopy imaging and analysis to elucidate more samples in shorter time, and hence can serve a broad range of in vitro pharmacological and toxicological needs.

Phosphatidylcholine Specific PLC-Induced Dysregulation of Gap Junctions, a Robust Cellular Response to Environmental Toxicants, and Prevention by Resveratrol in a Rat Liver Cell Model

Dysregulation of gap junctional intercellular communication (GJIC) has been associated with different pathologies, including cancer; however, molecular mechanisms regulating GJIC are not fully understood. Mitogen Activated Protein Kinase (MAPK)-dependent mechanisms of GJIC-dysregulation have been well-established, however recent discoveries have implicated phosphatidylcholine-specific phospholipase C (PC-PLC) in the regulation of GJIC. What is not known is how prevalent these two signaling mechanisms are in toxicant/toxin-induced dysregulation of GJIC, and do toxicants/toxins work through either signaling mechanisms or both, or through alternative signaling mechanisms. Different chemical toxicants were used to assess whether they dysregulate GJIC via MEK or PC-PLC, or both Mek and PC-PLC, or through other signaling pathways, using a pluripotent rat liver epithelial oval-cell line, WB-F344. Epidermal growth factor, 12-O-tetradecanoylphorbol-13-acetate, thrombin receptor activating peptide-6 and lindane regulated GJIC through a MEK1/2-dependent mechanism that was independent of PC-PLC; whereas PAHs, DDT, PCB 153, dicumylperoxide and perfluorodecanoic acid inhibited GJIC through PC-PLC independent of Mek. Dysregulation of GJIC by perfluorooctanoic acid and R59022 required both MEK1/2 and PC-PLC; while benzoylperoxide, arachidonic acid, 18β-glycyrrhetinic acid, perfluorooctane sulfonic acid, 1-monolaurin, pentachlorophenol and alachlor required neither MEK1/2 nor PC-PLC. Resveratrol prevented dysregulation of GJIC by toxicants that acted either through MEK1/2 or PC-PLC. Except for alachlor, resveratrol did not prevent dysregulation of GJIC by toxicants that worked through PC-PLC-independent and MEK1/2-independent pathways, which indicated at least two other, yet unidentified, pathways that are involved in the regulation of GJIC. In conclusion: the dysregulation of GJIC is a contributing factor to the cancer process; however the underlying mechanisms by which gap junction channels are closed by toxicants vary. Thus, accurate assessments of risk posed by toxic agents, and the role of dietary phytochemicals play in preventing or reversing the effects of these agents must take into account the specific mechanisms involved in the cancer process.

Alcohol Disrupts Human Liver Stem/Progenitor Cell Proliferation and Differentiation

Objective

Excessive alcohol consumption injures the liver resulting in various liver diseases including liver cirrhosis. Advanced liver disease continues to be a major challenge to human health. Liver stem/progenitor cells (LSPCs) are tissue specific precursors with a distinct capacity of multi-lineage differentiation. These precursor cells may play an important role in the process of tissue injury repair and pathological transition of liver structures. At the present time, knowledge about the effect of alcohol on LSPC function during the development of alcoholic liver disease remains absent. This study was conducted to investigate changes in LSPC activity of proliferation and differentiation following alcohol exposure. The disruption of cell signaling mechanisms underlying alcohol-induced alteration of LSPC activities was also examined.

Methods

Primary and immortalized human liver stem cells (HL1-1 cells and HL1-hT1 cells, respectively) were cultured in media optimized for cell proliferation and hepatocyte differentiation in the absence and presence of ethanol. Changes in cell morphology, proliferation and differentiation were determined. Functional disruption of cell signaling components following alcohol exposure was examined.

Results

Ethanol exposure suppressed HL1-1 cell growth [as measured by cell 5-bromo-2-deoxyuridine (BrdU) incorporation] mediated by epidermal growth factor (EGF) or EGF plus interleukin-6 (IL-6) in an ethanol dose-dependent manner. Similarly, ethanol inhibited BrdU incorporation into HL1-hT1 cells. Cyclin D1 mRNA expression by HL1-hT1 cells was suppressed when cells were cultured with 50 and 100 mM ethanol. Ethanol exposure induced morphological change of HL1-1 cells toward a myofibroblast-like phenotype. Furthermore, ethanol down-regulated E-cadherin expression while increasing collagen I expression by HL1-1 cells. Ethanol also stimulated Snail transcriptional repressor (Snail) and α-smooth muscle actin (α-SMA) gene expression by HL1-1 cells.

Conclusion

These results demonstrate that the direct effect of alcohol on LSPCs is inhibiting their proliferation and promoting mesenchymal transition during their differentiation. Alcohol interrupts LSPC differentiation through interfering Snail signaling.

Abstract 3587: Polycyclic aromatic hydrocarbons exhibit differential tumor promoting properties through mitogen activated protein kinases and inhibition of gap junctions in murine lung cells

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants as well as major components of tobacco smoke. Exposure to PAHs represents a significant health concern due to their association with a variety of cancer endpoints. While a majority of research has focused on the ability of the high molecular weight PAHs to act as carcinogens through their ability to bind and adduct DNA, recent evidence suggests that the low molecular weight PAHs, two to four ring compounds, may act through distinct mechanisms to promote, rather than initiate, neoplasia. We evaluated five sidestream smoke PAHs, 1-Methylanthracene (1-MeA) and 2 Methlyanthracene (2-MeA), Phenanthrene (Phe) and Anthracene (A) and Fluoranthene (Flthn) for their ability to (1) inhibit gap junction intercellular communication (GJIC), (2) with a concomitant effect on the gap junction protein Connexin 43 (Cxn43) and (3) to activate mitogen activated protein kinases (MAPK) in murine lung cells. Since both dysregulation of GJIC and activation of MAPK have been previously reported to contribute to tumor promotion, these pathways represent potential mechanistic targets for PAH perturbation. Murine C10 cells (a non-tumorigenic type II alveolar pneumocyte, and progenitor cell type of lung adenocarcinoma) were used to assess the effects of these low molecular weight PAHs on GJIC and MAPKs over a time course of 15 minutes to 4 hours. Detection of MAPKs and Connexin 43 protein was determined by immunoblot, and GJIC was assessed by a scrap-load dye transfer assay. Pathway specific antagonists were used to further elucidate the mechanism by which 1-MeA 2-MeA, Phe, A, and Flthn inhibited GJIC in a dose and time dependent manner while the isomers, 2-MeA and A, did not. With 1-MeA, this inhibition was prevented by blocking either the phosphotidylcholine-phospholipase or p38 pathway, while inhibiting the MAP kinase kinase (MEK) pathway had no effect. Treatment with 1-MeA also activated ERK1/2 and p38 MAP kinases and also decreased the overall amount of Connexin 43, the major lung connexin, in a time-dependent manner. The ability of 1-MeA to both inhibit GJIC as well as activate MAPKs supports a role for low molecular weight tobacco-smoke PAHs to act as tumor promoters in lung cells, and is consistent with previous studies that suggest these compounds may act as tumor promoters in liver cells. The differences observed between 1-MeA and 2-MeA and Phe and A suggests that the ability of low molecular weight PAHs to inhibit GJIC and activate MAPKs is highly dependent on chemical structure. Such structure-activity relationships are often indicative of discrete toxicological mechanisms.

Citation Format: Ross S. Osgood, Thomas Hill, Katherine L. Helms, Pavel Babica, Brad L. Upham, Alison K. Bauer. Polycyclic aromatic hydrocarbons exhibit differential tumor promoting properties through mitogen activated protein kinases and inhibition of gap junctions in murine lung cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3587. doi:10.1158/1538-7445.AM2013-3587

Bronchoalveolar Lavage Fluid Utilized Ex Vivo to Validate In Vivo Findings: Inhibition of Gap Junction Activity in Lung Tumor Promotion is Toll-Like Receptor 4-Dependent

TLR4 protects against lung tumor promotion and pulmonary inflammation in mice. Connexin 43 (Cx43), a gap junction gene, was increased in Tlr4 wildtype compared to Tlr4-mutant mice in response to promotion, which suggests gap junctional intercellular communication (GJIC) may be compromised. We hypothesized that the early tumor microenvironment, represented by Bronchoalveolar Lavage Fluid (BALF) from Butylated hydroxytoluene (BHT; promoter)-treated mice, would produce TLR4-dependent changes in pulmonary epithelium, including dysregulation of GJIC in the Tlr4-mutant (BALB^Lps-d) compared to the Tlr4-sufficient (BALB; wildtype) mice. BHT (4 weekly doses) was injected ip followed by BALF collection at 24 h. BALF total protein and total macrophages were significantly elevated in BHT-treated BALB^Lps-d over BALB mice, similar to previous findings. BALF was then utilized in an ex vivo manner to treat C10 cells, a murine alveolar type II cell line, followed by the scrape-load dye transfer assay (GJIC), Cx43 immunostaining, and quantitative RT-PCR (Mcp-1, monocyte chemotactic protein 1). GJIC was markedly reduced in C10 cells treated with BHT-treated BALB^Lps-d BALF for 4 and 24 h compared to BALB and control BALF from the respective mice (p < 0.05). Mcp-1, a chemokine, was also significantly increased in the BHT-treated BALB^Lps-d BALF compared to the BALB mice, and Cx43 protein expression in the cell membrane altered. These novel findings suggest signaling from the BALF milieu is involved in GJIC dysregulation associated with promotion and links gap junctions to pulmonary TLR4 protection in a novel ex vivo model that could assist in future potential tumor promoter screening.

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.

A paradigm shift is required for the risk assessment of potential human health after exposure to low level chemical exposures: a response to the toxicity testing in the 21st century report

Chemicals are known to be associated with birth defects, cancer, cardiovascular diseases, immunological, reproductive, and neurological disorders. In response to recent reviews of limitations of current concepts and techniques for toxicity testing, this commentary challenges the paradigm that chemicals are directly responsible for DNA damage in the genomic-nuclear DNA in relevant cells of the human body. This challenge is not that mutations do not play roles in human-inherited or somatic diseases but that chemical exposures bring about disease end points by epigenetic mechanisms or by alterations in adult stem cell numbers in utero (ie, the Barker hypothesis) or postnatally, by selecting preexisting mutated cells. Classic concepts, that is, multistage, multimechanism process of carcinogenesis, stem cell theory of cancer, and newer and ignored concepts, such as cancer stem cells and cell-cell communication, will be used to support the view that the toxic effect of chemicals is mediated by nonmutagenic mechanisms at human relevant exposures.

Single-walled carbon nanotubes dispersed in aqueous media via non-covalent functionalization: effect of dispersant on the stability, cytotoxicity, and epigenetic toxicity of nanotube suspensions

As the range of applications for carbon nanotubes (CNTs) rapidly expands, understanding the effect of CNTs on prokaryotic and eukaryotic cell systems has become an important research priority, especially in light of recent reports of the facile dispersion of CNTs in a variety of aqueous systems including natural water. In this study, single-walled carbon nanotubes (SWCNTs) were dispersed in water using a range of natural (gum arabic, amylose, Suwannee River natural organic matter) and synthetic (polyvinyl pyrrolidone, Triton X-100) dispersing agents (dispersants) that attach to the CNT surface non-covalently via different physiosorption mechanisms. The charge and the average effective hydrodynamic diameter of suspended SWCNTs as well as the concentration of exfoliated SWCNTs in the dispersion were found to remain relatively stable over a period of 4 weeks. The cytotoxicity of suspended SWCNTs was assessed as a function of dispersant type and exposure time (up to 48 h) using general viability bioassay with Escherichia coli and using neutral red dye uptake (NDU) bioassay with WB-F344 rat liver epithelia cells. In the E. coli viability bioassays, three types of growth media with different organic loadings and salt contents were evaluated. When the dispersant itself was non-toxic, no losses of E. coli and WB-F344 viability were observed. The cell viability was affected only by SWCNTs dispersed using Triton X-100, which was cytotoxic in SWCNT-free (control) solution. The epigenetic toxicity of dispersed CNTs was evaluated using gap junction intercellular communication (GJIC) bioassay applied to WB-F344 rat liver epithelial cells. With all SWCNT suspensions except those where SWCNTs were dispersed using Triton X-100 (wherein GJIC could not be measured because the sample was cytotoxic), no inhibition of GJIC in the presence of SWCNTs was observed. These results suggest a strong dependence of the toxicity of SWCNT suspensions on the toxicity of the dispersant and point to the potential of non-covalent functionalization with non-toxic dispersants as a method for the preparation of stable aqueous suspensions of biocompatible CNTs.

Inhibition of gap-junctional intercellular communication and activation of mitogen-activated protein kinases by cyanobacterial extracts--indications of novel tumor-promoting cyanotoxins?

Toxicity and liver tumor promotion of cyanotoxins microcystins have been extensively studied. However, recent studies document that other metabolites present in the complex cyanobacterial water blooms may also have adverse health effects. In this study we used rat liver epithelial stem-like cells (WB-F344) to examine the effects of cyanobacterial extracts on two established markers of tumor promotion, inhibition of gap-junctional intercellular communication (GJIC) and activation of mitogen-activated protein kinases (MAPKs) - ERK1/2. Extracts of cyanobacteria (laboratory cultures of Microcystis aeruginosa and Aphanizomenon flos-aquae and water blooms dominated by these species) inhibited GJIC and activated MAPKs in a dose-dependent manner (effective concentrations ranging 0.5-5mgd.w./mL). Effects were independent of the microcystin content and the strongest responses were elicited by the extracts of Aphanizomenon sp. Neither pure microcystin-LR nor cylindrospermopsin inhibited GJIC or activated MAPKs. Modulations of GJIC and MAPKs appeared to be specific to cyanobacterial extracts since extracts from green alga Chlamydomonas reinhardtii, heterotrophic bacterium Klebsiella terrigena, and isolated bacterial lipopolysaccharides had no comparable effects. Our study provides the first evidence on the existence of unknown cyanobacterial toxic metabolites that affect in vitro biomarkers of tumor promotion, i.e. inhibition of GJIC and activation of MAPKs.

Actin and Vimentin proteins with N-terminal deletion detected in tumor-bearing rat livers induced by intraportal-vein injection of Ha-ras-transfected rat liver cells

The introduction of the tumorigenic v-Ha-ras oncogene-transformed rat liver epithelial cells (WBras), which is deficient in gap junctional intercellular communication (GJIC), into F344 rats, induces significant formation of hepatocellular tumors. GJIC plays a major role in maintaining tissue homeostasis. Using this in vivo tumor model system, we used 2-dimensional electrophoresis with isoelectric focusing in the first dimension and SDS-PAGE in the second dimension to globally identify proteins that are uniquely expressed in the livers of WBras-treated rats as compared to the sham control. Immunoblotting was used to identify Ras and Connexin43, which were the positive and negative marker proteins, respectively, of the introduced WBras cells. As predicted, immunoblotting indicated that the whole liver of tumor-bearing animals exhibited a decreased level of Connexin43 and an increased level of Ras. Connexin43 and GJIC were expressed and functional in normal liver, but not in the tumor. In addition to these 2 markers, an additional 4 proteins exhibited decreased levels and 2 proteins exhibited increased levels in the livers of tumor-bearing animals. N-Terminal sequencing analysis was used to identify these proteins, which were glucose-regulated protein 78, 2 isoforms of heat shock protein 60, and the beta-chain of ATP synthase for the down regulated proteins, and beta-Actin with a 46 amino acid deletion from its N-terminus and Vimentin with a 71 amino acid deletion from its N-terminus for the up regulated proteins. These data offer potentially new markers of liver tumorigenicity, particularly, Vimentin. (

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.

Oxidative-dependent integration of signal transduction with intercellular gap junctional communication in the control of gene expression

Research on oxidative stress focused primarily on determining how reactive oxygen species (ROS) damage cells by indiscriminate reactions with their macromolecular machinery, particularly lipids, proteins, and DNA. However, many chronic diseases are not always a consequence of tissue necrosis, DNA, or protein damage, but rather to altered gene expression. Gene expression is highly regulated by the coordination of cell signaling systems that maintain tissue homeostasis. Therefore, much research has shifted to the understanding of how ROS reversibly control gene expression through cell signaling mechanisms. However, most research has focused on redox regulation of signal transduction within a cell, but we introduce a more comprehensive-systems biology approach to understanding oxidative signaling that includes gap junctional intercellular communication, which plays a role in coordinating gene expression between cells of a tissue needed to maintain tissue homeostasis. We propose a hypothesis that gap junctions are critical in modulating the levels of second messengers, such as low molecular weight reactive oxygen, needed in the transduction of an external signal to the nucleus in the expression of genes. Thus, any comprehensive-systems biology approach to understanding oxidative signaling must also include gap junctions, in which aberrant gap junctions have been clearly implicated in many human diseases.

3-Methylthiopropionic acid ethyl ester, isolated from Katsura-uri (Japanese pickling melon, Cucumis melo var. conomon), enhanced differentiation in human colon cancer cells

The fully ripened fruit of Katsura-uri Japanese pickling melon ( Cucumis melo var. conomon) has rarely been used for food because the midripened fruit is utilized for making pickles, but the fully ripened fruit is no longer valuable for pickles due to the fruit body being too soft. We have considered the utilization of the fully ripened Katsura-uri fruit that may be used for nonpickling products, particularly if the fully ripened fruit demonstrated health benefits such as anticarcinogenic properties. The phytochemical extract from the fully ripened fruit of Katsura-uri Japanese pickling melon was purified via a bioassay-guided fractionation scheme, which was based on the induction of differentiation in a RCM-1 human colon cancer cell line. On the criteria of two differentiation markers (duct formation and alkaline phosphatase activity), the most potent fraction contained a compound identified as 3-methylthiopropionic acid ethyl ester, based on GC retention time, EI-MS, (1)H NMR, and (13)C NMR spectra. Previously, the role of 3-methylthiopropionic acid ethyl ester was considered as an odor producing compound in many fruits, but this study indicates potential medical benefits of this compound.

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.

Inhibition of Gap Junctional Intercellular Communication and Activation of Mitogen-Activated Protein Kinase by Tumor-Promoting Organic Peroxides and Protection by Resveratrol

Dicumyl peroxide (di-CuOOH) and benzoyl peroxide (BzOOH) act as tumor promoters in SENCAR mice, whereas di-tert-butylhydroperoxide does not. Tumor promotion requires the removal of growth suppression by inhibition of gap junctional intercellular communication (GJIC) and the induction of mitogenic intracellular pathways. We showed that di-CuOOH and BzOOH both reversibly inhibited GJIC and transiently activated mitogen-activated protein kinase, specifically, the extracellular receptor kinase at noncytotoxic conditions in WB-F344 rat liver epithelial cells, whereas the non-tumor-promoting di-tert-butylhydroperoxide did not inhibit GJIC or activate extracellular receptor kinase. di-CuOOH but not BzOOH inhibited GJIC through a phosphatidylcholine-specific phospholipase C-dependent mechanism. N-acetylcysteine (NAC) was needed to prevent a cytotoxic, glutathione-depleting effect of BzOOH, whereas di-CuOOH was noncytotoxic and did not alter glutathione levels at all doses and times tested. Pretreatment of WB-F344 cells with resveratrol, a polyphenolic antioxidant present in red wine, prevented at physiological doses the inhibition of GJIC by di-CuOOH but not from BzOOH and was effective in significantly preventing extracellular receptor kinase activation by both peroxides. NAC did not prevent any of the peroxide effects on either GJIC or extracellular receptor kinase, suggesting a specific antioxidant effect of resveratrol.

Cigarette smoke components inhibited intercellular communication and differentiation in human pancreatic ductal epithelial cells

Smoking is a well-documented risk factor for the development of pancreatic adenocarcinoma. Although the most abundant polycyclic aromatic hydrocarbons (PAHs) in cigarette smoke are methylated anthracenes and phenanthrenes, the epigenetic toxicity of these compounds has not been extensively studied. We previously showed that methylanthracenes, which possess a bay-like structure, affect epigenetic events such as an induced release of arachidonic acid, inhibition of gap junctional intercellular communication (GJIC) and induction of mitogen-activated protein kinases in a pluripotent rat liver epithelial stem cell line. Anthracenes with no bay-like structures were inactive. These biological effects are all molecular events associated with the promotional phase of cancer. A human immortalized, nontumorigenic pancreatic ductal epithelial cell line, H6c7, was examined to study the epigenetic toxicity of PAHs related to pancreatic cancer by using scrape-loading dye transfer, immunostaining, RT-PCR and telomerase assay methods. H6c7 cells were GJIC-incompetent and exhibited high telomerase activity when grown in growth factor and hormone-supplemented medium. In the presence of the cAMP elevating drugs (forskolin and IBMX) the cells became GJIC competent and expressed connexins. Telomerase activity was also decreased by cAMP elevating drug treatment. After induction of cAMP, 1-methylanthracene with bay-like structures inhibited GJIC, whereas the 2-methylanthracene lacking a bay-like structure had no effect on GJIC. Telomerase activity remained high in 1-methylanthracene treatment but not with 2-methylanthracene. These results indicate that a prominent component of cigarette smoke, namely methylanthracenes with distinct structural configurations, could be a potential etiological agent contributing to the epigenetic events of pancreatic cancer.

The role of human adult stem cells and cell-cell communication in cancer chemoprevention and chemotherapy strategies

Since carcinogenesis is a multi-stage, multi-mechanism process, involving mutagenic, cell death and epigenetic mechanisms, during the "initiation/promotion/and progression" phases, chemoprevention must be based on understanding the underlying mechanism(s) of each phase, In principle, prevention of each of these phases could reduce the risk to cancer. However, because reducing the mutagenic/initiation phase to a zero level is impossible, the most efficacious intervention would be at the promotion phase that requires a sustained exposure to promoting conditions/agents. In addition, assuming the "target" cells for carcinogenesis are the pluri-potent stem cells and their early progenitor or transit cells, chemoprevention strategies for inhibiting the promotion of these two types of pre-malignant "initiated" cells will require different kinds of agents. A hypothesis will be proposed that involves adult stem cells, which express Oct-4 gene and lack gap junctional intercellular communication (GJIC-) or the early progenitor cells which express GJIC+ and are partially-differentiated, if initiated, will be promoted by agents that either inhibit secreted negative growth regulators or by inhibitors of GJIC. Consequently, anti-tumor promoting chemopreventing agents to each of these two types of initiated cells must have different mechanisms of action and work on different target cells. Assuming stem cells are target cells for carcinogenesis, an alternative method of chemoprevention would be to reduce the stem cell pool. Many classes of anti-tumor promoter chemopreventive agents, such as green tea components, resveratrol, caffeic acid phenethylene ester, either up-regulate GJIC in stem cells or prevent the down regulation of GJIC by tumor promoters in early progenitor cells.

Ignored hallmarks of carcinogenesis: stem cells and cell-cell communication

Hanahan and Weinberg (2000, Cell 100: 57-70) listed "hallmarks" of cancer that must be considered in order to understand the underlying determinants of carcinogenesis: (a) self-sufficiency in growth signals; (b) insensitivity to growth-inhibitory (antigrowth) signals; (c) evasion of programmed cell death (apoptosis); (d) limitedless replicative potential; (e) sustained angiogenesis; and (f) tissue invasion and metastasis. While these are important phenotypic markers, important concepts--the role of pluripotent stem cells and gap junctional intercellular communication (GJIC)--must be brought into this analysis of carcinogenesis. Carcinogenesis is a multistage, multimechanism process consisting of a single cell that has been irreversibly blocked from terminal differentiation (the initiation stage). The promotion phase is a potentially reversible or interruptible clonal expansion of the initiated cell by a combination of growth stimulation and inhibition of apoptosis. When the expanded initiated cells accrue sufficient mutations and epigenetic alterations to become growth stimulus independent and resistant to growth inhibitors and apoptosis, to have unlimited replicative potential and invasive and metastatic phenotypes, then the progression phase has been achieved. The hypothesis that integrates these hallmarks is that the stem cell and its early progenitor cell are the target cells for the initiation event. These cells are naturally immortal and become mortal only when they are induced to terminally differentiate and lose their telomerase activity. These two types of initiated cells are suppressed by either secreted negative growth regulators (the stem cells) or GJIC (the early initiated progenitor cells). Promoters inhibit either the secreted growth inhibitor to initiated stem cells or GJIC between the initiated progenitor cells and the normal progenitor cells. When a stable resistance to the secreted negative growth regulator or permanent downregulation of GJIC has occurred, the cell has entered the progression phase. These two new concepts contradict the current paradigm that the first phase of carcinogenesis is the immortalization of a normal cell followed by its neoplastic transformation. Our hypothesis is that the first stage of carcinogenesis must prevent the "mortalization" or terminal differentiation of a naturally immortal cell. Chemoprevention and chemotherapeutic implications suggest that one must induce connexin genes in initiated stem cells and restore GJIC in initiated early progenitor cells.

Biphasic lindane-induced oxidation of glutathione and inhibition of gap junctions in myometrial cells

The insecticide lindane (gamma-hexachlorocyclohexane) inhibits gap junction intercellular communication in rat myometrial cells by a mechanism involving oxidative stress. We hypothesized that oxidation of reduced glutathione (GSH) to glutathione disulfide (GSSG) and subsequent S-glutathionylation provide a mechanistic link between lindane-induced oxidative stress and lindane's inhibition of myometrial gap junction communication. Gap junction communication between cultured rat myometrial myocytes was assessed by Lucifer yellow dye transfer after microinjection. A biphasic pattern was confirmed, with dye transfer nearly abolished after 1 h of exposure to 100 microM lindane followed initially by recovery after lindane removal, and then the development 4 h after termination of lindane exposure of a delayed-onset, sustained inhibition that continued for 96 h. As measured by HPLC, cellular GSH varied over a 24-h period in a biphasic fashion that paralleled lindane-induced inhibition of dye transfer, whereas GSSG levels increased in a manner inversely related to GSH. In accordance, GSH/GSSG ratios were depressed at times when GSH and dye transfer were low. Lindane substantially increased S-glutathionylation in a concentration-dependent manner, measured biochemically by GSSG reductase-stimulated release of GSH from precipitated proteins. Furthermore, treatments that promoted accumulation of GSSG (50 microM diamide and 25 microM 1,3-bis(2-chloroethyl)-1-nitrosourea [BCNU]) inhibited Lucifer yellow dye transfer between myometrial cells. Findings that lindane induced GSH oxidation to GSSG with increased S-glutathionylation, together with the diamide and BCNU results, suggest that oxidation of GSH to GSSG is a component of the mechanism by which lindane inhibits myometrial gap junctions.

Low-dose ionizing radiation: induction of differential intracellular signalling possibly affecting intercellular communication

Given the complexity of the carcinogenic process and the lack of any mechanistic understanding of how ionizing radiation at low-level exposures affects the multistage, multimechanism processes of carcinogenesis, it is imperative that concepts and paradigms be reexamined when extrapolating from high dose to low dose. Any health effect directly linked to low-dose radiation exposure must have molecular/biochemical and biological bases. On the other hand, demonstrating some molecular/biochemical or cellular effect, using surrogate systems for the human being, does not necessarily imply a corresponding health effect. Given the general acceptance of an extrapolated LNT model, our current understanding of carcinogenesis cries out for a resolution of a real problem. How can a low-level acute, or even a chronic, exposure of ionizing radiation bring about all the different mechanisms (mutagenic, cytotoxic, and epigenetic) and genotypic/phenotypic changes needed to convert normal cells to an invasive, malignant cell, given all the protective, repair, and suppressive systems known to exist in the human body? Until recently, the prevailing paradigm that ionizing radiation brings about cancer primarily by DNA damage and its conversion to gene and chromosomal mutations, drove our interpretation of radiation carcinogenesis. Today, our knowledge includes the facts both that epigenetic events play a major role in carcinogenesis and that low-dose radiation can also induce epigenetic events in and between cells in tissues. This challenges any simple extrapolation of the LNT model. Although a recent delineation of "hallmarks" of the cancer process has helped to focus on how ionizing radiation might contribute to the induction of cancers, several other hallmarks, previously ignored--namely, the stem cells in tissues as targets for carcinogenesis and the role of cell-cell communication processes in modulating the radiation effects on the target cell--must be considered, particularly for the adaptive response, bystander effects, and genomic instability phenomena.

The emperor wears no clothes in the field of carcinogen risk assessment: ignored concepts in cancer risk assessment

The following is a position paper challenging the paradigm that 'carcinogen = mutagen', and that the current rodent bioassay to predict risks to human cancers is relevant and useful. Specifically, we review current observations concerning carcinogenesis that might lead to another approach for assessing the identification of human carcinogenic hazards and the risk assessment that chemicals might pose. We give a brief review of the multistage and multimechanism process of cancer in a tissue that involves not only genotoxic but also epigenetic events, and the importance of stem and progenitor cells in the development of cancer. We focus on the often ignored 'epigenetic' effects of carcinogens and the role of cell communication systems in epigenetically altering gene expression that leads to an imbalance of cell proliferation, differentiation and apoptosis in a tissue that can contribute to the cancer process. To draw attention to the fact that the current paradigm and policy to test toxic chemicals is often misleading and incorrect, we discuss how oxidative stress, in spite of the DNA damaging data, most probably contributes to cancer at the epigenetic level. Additionally, we briefly review how this mutagenic concept has greatly diverted attention away from doing research on the lower molecular weight, non-genotoxic, polycyclic aromatic hydrocarbons (PAHs), and how these low molecular weight PAHs are etiologically more relevant to the disease potential of environmental mixtures such as cigarette smoke.

β-Sitosterol From Psyllium Seed Husk (Plantago ovata Forsk) Restores Gap Junctional Intercellular Communication in Ha-ras Transfected Rat Liver Cells

We purified compounds from the husks of psyllium seeds (Plantago ovata Forsk; desert Indian wheat), beginning with an ethanol extraction then followed by HP-20 and silica gel chromatography, which restored gap junctional intercellular communication (GJIC) in v-Ha-ras transfected rat liver epithelial WB-F344 cell line (WB-Ha-ras). GJIC was assessed by a scrape loading dye transfer assay. The active compound was identified as beta-sitosterol based on gas chromatography retention times and electron ionization mass spectroscopy (EI-MS) spectrum of authentic beta-sitosterol. Authentic beta-sitosterol restored GJIC in the tumorigenic WB-Ha-ras GJIC-deficient cells at a dose of 2.4 microM. In addition, a similar phytosterol, stigmasterol, also restored GJIC, albeit at a lower activity. beta-sitosterol and stigmasterol increased the level of connexin43 protein (Cx43) and restored phosphorylation of Cx43 to levels similar to the parental nontransfected cell line. We concluded that the restoration of intercellular communication in the GJIC-deficient, tumorigenic WB-Ha-ras cell line by the ethanol soluble fraction of psyllium seed husks is largely due to the presence of the phytosterol, beta-sitosterol. We discuss implications for dietary modulation of cancer by beta-sitosterol.

Divergent roles for glutathione in lindane-induced acute and delayed-onset inhibition of rat myometrial gap junctions

Previous studies have shown that the insecticide lindane (gamma-hexachlorocyclohexane) induces a biphasic inhibition of gap junction intercellular communication that is accompanied by oxidative stress. The present study investigates the hypothesis that depletion of cellular glutathione (GSH) is a mechanistic link between lindane-induced oxidative stress and inhibition of myometrial gap junctions. Exposure to 100 or 200 microM lindane rapidly (within 1 min) increased myometrial cell generation of superoxide, as measured by superoxide dismutase-inhibitable cytochrome c reduction, and superoxide production remained elevated for up to 60 min of exposure. To measure gap junction communication, Lucifer yellow dye was injected into myometrial cells, and dye transfer to adjoining cells was monitored. Cells were exposed to lindane with or without GSH modulators, and dye transfer was determined at the end of a 1-h exposure to 100 microM lindane (acute phase) and 24 h after termination of lindane exposure (secondary phase). The acute phase of lindane-induced inhibition of dye transfer was prevented by GSH depletion with L-buthionine-[S,R]-sulfoximine (BSO) and enhanced by GSH augmentation with GSH monoethyl ester or L-2-oxothiazolidine-4-carboxylate (OTC). In contrast, the secondary, delayed-onset phase of lindane-induced inhibition of dye transfer was enhanced by GSH depletion with BSO and prevented by GSH augmentation with GSH monoethyl ester or OTC. Changes in cellular GSH by the pharmacological modulators were confirmed by high performance liquid chromatography. These results suggest that GSH is required in the acute phase but protects against the secondary phase of lindane-induced inhibition of myometrial gap junctions.

Augmentation of differentiation and gap junction function by kaempferol in partially differentiated colon cancer cells

Kaempferol induces differentiation in partially differentiated colon cancer cells which express low levels of connexin43 protein and connexin43 mRNA (KNC cells). Differentiation was observed as changes in cell morphology and the activity of alkaline phosphatase. Increased differentiation in kaempferol-treated KNC cells correlated with restoration of gap junctional intercellular communication (GJIC), increased levels of connexin43 protein and its phosphorylation status. Phosphorylation (activation) of Stat3 and Erk was also reduced by kaempferol. An inhibitor of Stat3 phosphorylation also induced morphological changes in KNC cells similar to those in kaempferol-treated cells, suggesting that kaempferol-induced differentiation may be mediated by inhibition of Stat3 phosphorylation. These effects were not observed in HCT116 cells, a poorly differentiated colon cancer cell line deficient in expression of connexin43 mRNA and connexin43 protein. In conclusion, kaempferol might function as an anticancer agent by re-establishing GJIC through enhancement of the expression and phosphorylation of connexin43 protein in a tumorigenic colon cancer cell line that already expresses connexin43 mRNA via a Stat3-dependent mechanism. In contrast, kaempferol had no effect in a tumorigenic colon cancer cell line that did not express connexin43 mRNA and was deficient in GJIC.

Psyllium extracts decreased neoplastic phenotypes induced by the Ha-Ras oncogene transfected into a rat liver oval cell line

Inhibition of gap junctional intercellular communication (GJIC) by tumor promoters and oncogenes has been implicated in the removal of initiated cells from the suppression of growth by neighboring cells in the tumor promoting step of carcinogenesis. The GJIC of WB-Ha-ras cell line is GJIC-deficient and they are capable of anchorage independent growth (AIG). The ethanol extract of psyllium increased GJIC 1.65-times and decreased AIG in both number and size of colonies in WB-Ha-ras cells. Histochemical staining of the gap junction protein, connexin43, showed that psyllium restored gap junction plaques on the plasma membrane of the WB-Ha-ras cells. In conclusion, the ethanol extract of psyllium reversed two tumor cell phenotypes, namely reduced GJIC and AIG, induced by the Ha-ras oncogene.

Inhibition of gap junctional intercellular communication by noncoplanar polychlorinated biphenyls: inhibitory potencies and screening for potential mode(s) of action

Polychlorinated biphenyls (PCBs), a structurally diverse group of environmental pollutants, are effective promoters in two-stage cancer models, which implies that epigenetic mechanisms are involved. Inhibition of gap junctional intercellular communication (GJIC) belongs among critical epigenetic events of tumor promotion. We determined the relative potencies of a series of environmentally relevant PCB congeners to inhibit GJIC in vitro in a rat liver epithelial cell line with pluripotent oval cell characteristics. The nonplanar PCBs were potent inhibitors of GJIC, whereas the coplanar PCBs did not inhibit GJIC. We then compared the effects of the coplanar PCB 126 (3,3',4,4',5-pentachlorobiphenyl) and the noncoplanar PCB 153 (2,2',4,4',5,5'-hexachlorobiphenyl) with effects of two model GJIC inhibitors, a tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and epidermal growth factor (EGF). In contrast to TPA or EGF, PCB 153 elicited a long-term downregulation of GJIC (up to 48 h). Using Western blot analysis with phospho-specific antibodies, it was found that PCB 153, and not PCB 126, activated mitogen-activated protein kinases ERK1/2; however in contrast to TPA and EGF, this activation was observed at the time points subsequent to GJIC inhibition. Moreover, blocking of ERK1/2 activation did not prevent the GJIC inhibition induced by PCB 153. Therefore, additional intracellular signaling pathways potentially involved in the downregulation of GJIC by PCBs were screened by using specific chemical probes inhibiting serine/threonine kinases, tyrosine kinases, and phospholipases. The inhibition of diacylglycerol lipase partially blocked and the selective inhibition of Src kinases and phosphatidylcholine-specific phospholipase C (PC-PLC) completely blocked the inhibitory effects of the noncoplanar PCB on GJIC, indicating that PC-PLC or sphingomyelinase and Src might be upstream regulators of noncoplanar PCB-induced inhibition of GJIC.

Oxidative stress in laboratory-incubated double-crested cormorant eggs collected from the Great Lakes

Double-crested cormorant (Phalacrocorax auritus) eggs were collected in 1998 from three sites on Lakes Huron and Superior and either analyzed for 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD)-like residues or artifically incubated. Some of the incubated eggs were injected with vitamin E (antioxidant) or piperonyl butoxide (CYPIA blocker) to examine the role of CYPIA and oxidative stress in normal bird development. Embryos (day 23) were analyzed for hepatic ethoxyresorufin O-deethylase (EROD) activity and different measures of oxidative stress. Glutathione-related parameters were also measured in brain. In contrast to the historical data, there were no statistically significant differences in concentrations of chlorinated dioxins, furans, dioxin-like PCBs, or total TCDD-equivalents (TEQs) in eggs among sites. Survival and incidence of abnormalities were comparable at all study sites. Slight differences in liver, heart, and egg weight were observed among sites. A greater incidence of eye deformities was observed in embryos treated with vitamin E. Treatment with the CYPIA blocker, piperonyl butoxide, decreased the body weights of embryos. EROD activities were similar at all locations, but measures of oxidative stress varied among locations. There were greater levels of oxidized glutathione and oxidative DNA damage at Little Charity Island in Saginaw Bay. There was relatively great interindividual variation in biochemical responses and significant interrelation of the parameters of oxidative stress. While exposure to PCDD/DF and PCBs does not seem to explain the observed oxidative stress, the potential of these compounds to cause the observed effects can not be completely excluded.

Oxidative stress in liver and brain of the hatchling chicken (Gallus domesticus) following in ovo injection with TCDD

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was injected into chicken eggs prior to incubation to study possible mechanisms of toxicity and teratogenicity. One of the suggested mechanisms of teratogenicity is oxidative stress. Eggs were injected simultaneously with TCDD and cotreatment compounds in an attempt to prevent oxidative stress or to block cytochrome P450 activity. Indicators of oxidative stress were assessed in livers and brains of hatchling chicks. In ovo, exposure to TCDD caused significant effects on indicators of oxidative stress in liver, but not in the brain of the hatchling chicks. TCDD did not significantly affect superoxide production. In liver, TCDD treatment caused a decrease in glutathione content and glutathione peroxidase activity and an increase in the ratio of oxidized to reduced glutathione. TCDD increased the susceptibility to lipid peroxidation and oxidative DNA damage in liver. Administration of the antioxidants vitamin E and vitamin A provided partial protection against TCDD-induced oxidative stress in liver. The lack of effect of TCDD in chicken brain could be due to the low cytochrome P4501A activity in this tissue and little accumulation of TCDD in brain compared to liver. Phenytoin, a known inducer of oxidative stress, caused a decrease in glutathione content and an increase in susceptibility to lipid peroxidation in both liver and brain and increased oxidative DNA damage in brain. Responsiveness varied among individual animals, but measures of the oxidative stress were correlated.

Reduced gap junctional intercellular communication and altered biological effects in mouse osteoblast and rat liver oval cell lines transfected with dominant-negative connexin 43

Gap junctional intercellular communication (GJIC) maintains normal growth and differentiation of cells in a tissue. The intercellular molecules traversing gap junctions are largely unknown, but the molecular weight (MW) cutoff is normally 1200 Da. No differences in dye transfer were observed in normal or vector controls of WB-F344 rat liver epithelial or mouse osteoblastic MC3T3-E1 cells with either Lucifer Yellow (LY) with a MW of 457 Da (LY-457) or LY with a MW of 649 Da (LY-649). Transfection of a dominant negative-connexin 43 (Cx43) gene decreased GJIC (>50%) when LY-649 was used, however, normal GJIC was observed in both cell lines when LY-457 was used. Therefore, the MW cut off in these clones was considerably less than the wild type. The dominant negative clones of the MC3T3-E1 cells exhibited over 90% less alkaline phosphatase (ALPase) activity and calcium deposition after the induction of differentiation. Similarly, dominant negative Cx43 inhibited gene expression of ALPase and bone sialoprotein but not osteocalcin in MC3T3-E1. WB-F344 cells normally exhibit a biphasic response to 12-O-tetradecanoylphorbol-13-acetate (TPA) where inhibition of GJIC recovers after 2 h, but the dominant negative clones showed no recovery from inhibition of GJIC by TPA. Dominant negative Cx43 also inhibited the formation of network-like structures by WB-F344 cells on Matrigel. These results demonstrate that the dominant negative gene transfected into cell types containing the wild-type connexins result in diminished channel sizes, thus allowing the determination of whether distinct biological endpoints, i.e., differentiation, are dependent upon either small or high MW intercellular signals.

Cannabinoids inhibit gap junctional intercellular communication and activate ERK in a rat liver epithelial cell line

Many tumor promoters suppress the immune system; however, the direct effect of immunosuppressants on the tumorigenic pathways of nonimmune cells in solid tissue has not been well documented. Cannabinoids were chosen to explore this question further. Cannabinoids are immune modulators that affect specific intracellular signaling pathways in leukocytes. Since these compounds are nongenotoxic, any tumorigenic effect that might be associated with these compounds would need to occur through an epigenetic mechanism. Therefore, we determined the effect of Delta(9)-THC and CBN, 2 plant-derived cannabinoids, on 2 key epigenetic markers of tumor promotion: inhibition of GJIC, which is essential in removing a cell from growth suppression, and activation of the ERK-MAPK pathway, which is crucial in activating the appropriate genes for mitogenesis. Both Delta(9)-THC and CBN reversibly inhibited GJIC at noncytotoxic doses (15 microM) in a normal diploid WB rat liver epithelial oval cell line within 20 min and activated ERK1 and ERK2 within 5 min. Inhibition of MEK with PD98059 prevented the inhibition of GJIC by either cannabinoid, suggesting that inhibition of GJIC was MEK-dependent. Based on RT-PCR analysis and employment of an antagonist of CB1 and CB2, the effects on GJIC and MAPK were independent of both cannabinoid receptors. Cannabinoids affected crucial epigenetic pathways associated with cell proliferation in a rodent liver epithelial cell model system.

Differential roles of 2, 6, and 8 carbon ceramides on the modulation of gap junctional communication and apoptosis during carcinogenesis

The inhibition of apoptosis and gap junctional intercellular communication (GJIC) has been implicated in tumor promotion. Ionizing radiation and oxidative toxicants activate sphingomyelinases resulting in the release of ceramides that control cell proliferation and apoptosis. A rat liver epithelial cell line treated with ceramides containing a 6 (C6) or 8 (C8) carbon acyl-group were potent inhibitors of GJIC and apoptosis, whereas a C2-ceramide was only a weak inhibitor of GJIC and strong inducer of apoptosis. Apoptosis induced by either serum deprivation or C2-ceramide was inhibited by the GJIC inhibitory C8-ceramide. In conclusion, these results suggest that a chronic release of ceramides with acyl groups larger than C6 might act as tumor promoters.

Inhibition of gap junctional intercellular communication by perfluorinated compounds in rat liver and dolphin kidney epithelial cell lines in vitro and Sprague-Dawley rats in vivo

Gap junctional intercellular communication (GJIC) is the major pathway of intercellular signal transduction, and is thus important for normal cell growth and function. Recent studies have revealed a global distribution of some perfluorinated organic compounds, especially perfluorooctane sulfonic acid (PFOS) in the environment. Because other perfluoroalkanes had been shown to inhibit GJIC, the effects of PFOS and related sulfonated fluorochemicals on GJIC were studied using a rat liver epithelial cell line (WB-F344) and a dolphin kidney epithelial cell line (CDK). In vivo effects on GJIC were studied in Sprague-Dawley rats orally exposed to PFOS for 3 days or 3 weeks. Effects on GJIC were measured using the scrape loading dye technique. PFOS, perfluorooctane sulfonamide (PFOSA), and perfluorohexane sulfonic acid (PFHA) were found to inhibit GJIC in a dose-dependent fashion, and this inhibition occurred rapidly and was reversible. Perfluorobutane sulfonic acid (PFBS) showed no significant effects on GJIC within the concentration range tested. A structure activity relationship was established among all 4 tested compounds, indicating that the inhibitory effect was determined by the length of fluorinated tail and not by the nature of the functional group. The results of the studies of the 2 cell lines and the in vivo exposure were comparable, suggesting that the inhibitory effects of the selected perfluorinated compounds on GJIC were neither species- nor tissue-specific and can occur both in vitro and in vivo.

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.

Toxicant-induced oxidative stress in cancer

The article highlighted in this issue is "The Role of Oxidative Stress in Indium Phosphide-Induced Lung Carcinogenesis in Rats" by Barbara C. Gottschling, Robert R. Maronpot, James R. Hailey, Shyamal Peddada, Cindy R. Moomaw, James E. Klaunig, and Abraham Nyska (pp. 28-40). The article integrates a traditional pathologic study of toxicant-induced pulmonary carcinogenesis with an immunohistologic assessment of oxidative stress, thereby determining a potential mechanism of action of a toxicant, specifically indium phosphide.

Effect of selected pesticides and their ozonation by-products on gap junctional intercellular communication using rat liver epithelial cell lines

The non-genotoxic effects of two commonly used pesticides, 1,1-bis (p-chlorophenyl)-2,2,2-trichloroethane (DDT) and malathion, and one widely used commercial insect repellent N,N-diethy-m-toluamide (DEET) on gap junction intercellular communication (GJIC) were determined using a rat liver epithelial cell line. Malathion and DDT reversibly inhibited GJIC in a treatment time- and dose-dependent manner at non-cytotoxic doses, whereas, DEET did not inhibit GJIC. Malathion was very reactive with ozone, while DEET and DDT did not react to any appreciable extent with ozone. The mixtures of ozonation products from malathion and DEET did not inhibit GJIC. The mixtures of ozonation by-products formed from DDT inhibited GJIC, but to a lesser extent than did DDT, itself. These results suggest that ozone can effectively remove malathion from solution without forming GJIC-toxic products, but is less effective in eliminating DEET and DDT from solution.

Effect of electromagnetic field exposure on chemically induced differentiation of friend erythroleukemia cells

Whether exposure of humans to extremely low frequency electromagnetic fields (ELF-EMF) can cause cancer is controversial and therefore needs further research. We used a Friend erythroleukemia cell line that can be chemically induced to differentiate to determine whether ELF-EMF could alter proliferation and differentiation in these cells in a manner similar to that of a chemical tumor promoter. Exposure of this cell line to 60 Hz ELF-EMF resulted in a dose dependent inhibition of differentiation, with maximal inhibition peaking at 40% and 40 mG (4 microT). ELF-EMF at 10 mG (1.0 microT) and 25 mG (2.5 microT) inhibited differentiation at 0 and 20%, respectively. ELF-EMF at 1.0 (100) and 10.0 G (1,000 microT) stimulated cell proliferation 50% above the sham-treated cells. The activity of telomerase, a marker of undifferentiated cells, decreased 100[times] when the cells were induced to differentiate under sham conditions, but when the cells were exposed to 0.5 G (50 microT) there was only a 10[times] decrease. In summary, ELF-EMF can partially block the differentiation of Friend erythroleukemia cells, and this results in a larger population of cells remaining in the undifferentiated, proliferative state, which is similar to the published results of Friend erythroleukemia cells treated with chemical-tumor promoters.

Lambda-carrageenan-induced inhibition of gap-junctional intercellular communication in rat liver epithelial cells

lambda-Carrageenan, a food additive extracted from red seaweed, is widely used as an emulsifier, stabilizer, or thickener. Previously, it has been shown that carrageenan could play a role in carcinogenesis. However, the mechanism by which it might influence the multimechanism, multistep process of carcinogenesis is not known. Gap-junctional intercellular communication (GJIC) has been associated with maintaining homeostatic regulation of cell proliferation and differentiation. Most cancer cells have dysfunctional GJIC, and many tumor-promoting chemicals, growth factors, and oncogenes can downregulate GJIC. The experiments in this study were designed to test the hypothesis that carrageenan might function as a tumor-promoting chemical by inhibiting GJIC. To test this hypothesis, nontumorigenic rat liver epithelial cells were exposed to carrageenan, and GJIC was measured. Western blot analysis and immunofluorescent staining were used to monitor the phosphorylation and localization of connexin 43. The data revealed inhibition of GJIC by carrageenan similar to that by the well-documented tumor promoter phorbol ester. However, the phosphorylation and localization of connexin 43 were not altered. Although the mechanism by which carrageenan inhibits GJIC is unknown, carrageenan's influence on the carcinogenic process might be via its ability to be a tumor promoter.

Polycyclic aromatic hydrocarbons with bay-like regions inhibited gap junctional intercellular communication and stimulated MAPK activity

Many polycyclic aromatic hydrocarbons (PAHs) are known carcinogens. A considerable amount of research has been devoted to predicting the genotoxic, tumor-initiating potential of PAHs based on chemical structure. However, information on the correlation of structure with the non-genetoxic, epigenetic events of tumor promotion is sparse. PAHs containing a bay or bay-like region were shown to be potent inhibitors of gap-junctional intercellular communication (GJIC), an epigenetic event involved in the removal of an initiated cell from growth suppression. We tested the epigenetic toxicity of PAHs containing bay-like regions by comparing the effects of methylated vs. chlorinated isomers of anthracene on the temporal activation of mitogen-activated protein kinase (MAPK) and the regulation of GJIC. Specifically, we used anthracene, 1-methylanthracene, 2-methylanthracene, 9-methylanthracene, 9,10-dimethylanthracene, 1-chloroanthracene, 2-chloroanthracene, and 9-chloroanthracene. We determined the effect of these compounds on GJIC and on the activation of extracellular receptor kinase (ERK 1 and 2), a MAPK, in F344 rat liver epithelial cells. Results showed that bay or bay-like regions, formed by either chlorine or a methyl group, reversibly inhibited GJIC at the same doses, time, and time of recovery, whereas the linear-planar isomers had no effect on GJIC. Similarly, the GJIC-inhibitory isomers also induced the phosphorylation of ERK 1 and ERK 2, while the non-inhibitory isomers had no effect on the activation of these MAPKs. MAPK activation occurred 10-20 min after the inhibition of GJIC, which indicates that MAPK is not involved in the initial regulation of GJIC; instead altered GJIC may be affecting MAPK activation. The present study revealed that there are structural determinants of PAHs, which clearly affect epigenetic events known to be involved in the non-genetoxic steps of tumor promotion. These events are the release of a cell from growth suppression involving the reduction of GJIC, followed by the activation of intracellular mitogenic events.

Inhibition of gap junctional intercellular communication by perfluorinated fatty acids is dependent on the chain length of the fluorinated tail

Perfluorinated fatty acids (PFFAs), such as perfluorooctanoic acid (PFOA) and perfluorodecanoic acid (PFDA), are known peroxisome proliferators and hepatocarcinogens. A causal link between an increase in the oxidative stress by peroxisomes and tumor promotion has been proposed to explain the hepatocarcinogenicity of PFOA and PFDA. However, the down-regulation of gap junctional intercellular communication (GJIC) has also been linked to the tumor-promoting properties of many carcinogens. Therefore, the effect of PFFAs on GJIC in WB-rat liver epithelial cells was determined. The chain length of the PFFAs tested for an effect on GJIC ranged from 2 to 10, 16 and 18 carbons. Carbon lengths of 7 to 10 inhibited GJIC in a dose-response fashion, whereas carbon lengths of 2 to 5, 16 and 18 did not appreciably inhibit GJIC. Inhibition occurred within 15 min and was reversible, with total recovery from inhibition occurring within 30 min after the removal of the compound from the growth medium. This short time of inhibition suggests that GJIC was modified at the post-translational level. Also, this short time period was not long enough for peroxisome proliferation. The post-translational modification of the gap junction proteins was not a consequence of altered phosphorylation as determined by Western blot analysis. Perfluorooctanesulfonic acid also inhibited GJIC in a dose-response fashion similar to PFDA, indicating that the determining factor of inhibition was probably the fluorinated tail, which required 7-10 carbons. Our results suggest that PFFAs could potentially act as hepatocarcinogens at the level of gap junctions in addition to or instead of through peroxisome proliferation.