Mechanisms of tumor promotion: potential role of intercellular communication

Gap Junction Intercellular Communication in the Carcinogenesis Hallmarks: Is This a Phenomenon or Epiphenomenon?

If occupational tumors are excluded, cancer causes are largely unknown. Therefore, it appeared useful to work out a theory explaining the complexity of this disease. More than fifty years ago the first demonstration that cells communicate with each other by exchanging ions or small molecules through the participation of connexins (Cxs) forming Gap Junctions (GJs) occurred. Then the involvement of GJ Intercellular Communication (GJIC) in numerous physiological cellular functions, especially in proliferation control, was proven and accounts for the growing attention elicited in the field of carcinogenesis. The aim of the present paper is to verify and discuss the role of Cxs, GJs, and GJIC in cancer hallmarks, pointing on the different involved mechanisms in the context of the multi-step theory of carcinogenesis. Functional GJIC acts both as a tumor suppressor and as a tumor enhancer in the metastatic stage. On the contrary, lost or non-functional GJs allow the uncontrolled proliferation of stem/progenitor initiated cells. Thus, GJIC plays a key role in many biological phenomena or epiphenomena related to cancer. Depending on this complexity, GJIC can be considered a tumor suppressor in controlling cell proliferation or a cancer ally, with possible preventive or therapeutic implications in both cases.

Induction of Apoptosis by PQ1, a Gap Junction Enhancer that Upregulates Connexin 43 and Activates the MAPK Signaling Pathway in Mammary Carcinoma Cells

The mechanism of gap junction enhancer (PQ1) induced cytotoxicity is thought to be attributed to the change in connexin 43 (Cx43) expression; therefore, the effects of Cx43 modulation in cell survival were investigated in mammary carcinoma cells (FMC2u) derived from a malignant neoplasm of a female FVB/N-Tg(MMTV-PyVT)634Mul/J (PyVT) transgenic mouse. PQ1 was determined to have an IC₅₀ of 6.5 µM in FMC2u cells, while inducing an upregulation in Cx43 expression. The effects of Cx43 modulation in FMC2u cell survival was determined through transfection experiments with Cx43 cDNA, which induced an elevated level of protein expression similar to that seen with PQ1 exposure, or siRNA to silence Cx43 protein expression. Overexpression or silencing of Cx43 led to a reduction or an increase in cell viability, respectively. The mitogen-activated protein kinase (MAPK) family has been implicated in the regulation of cell survival and cell death; therefore, the gap junctional intercellular communication (GJIC)-independent function of PQ1 and Cx43 in the Raf/Mitogen-activated protein kinase/ERK kinase/extracellular-signal-regulated kinase (Raf-MEK-ERK) cascade of cellular survival and p38 MAPK-dependent pathway of apoptosis were explored. PQ1 treatment activated p44/42 MAPK, while the overexpression of Cx43 resulted in a reduced expression. This suggests that PQ1 affects the Raf-MEK-ERK cascade independent of Cx43 upregulation. Both overexpression of Cx43 and PQ1 treatment stimulated an increase in the phosphorylated form of p38-MAPK, reduced levels of the anti-apoptotic protein Bcl-2, and increased the cleavage of pro-caspase-3. Silencing of Cx43 protein expression led to a reduction in the phosphorylation of p38-MAPK and an increase in Bcl-2 expression. The mechanism behind PQ1-induced cytotoxicity in FMC2u mammary carcinoma cells is thought to be attributed to the change in Cx43 expression. Furthermore, PQ1-induced apoptosis through the upregulation of Cx43 may depend on p38 MAPK, highlighting that the effect of PQ1 on gap junctions as well as cellular survival via a MAPK-dependent pathway.

Possible role of hemichannels in cancer

In humans, connexins (Cxs) and pannexins (Panxs) are the building blocks of hemichannels. These proteins are frequently altered in neoplastic cells and have traditionally been considered as tumor suppressors. Alteration of Cxs and Panxs in cancer cells can be due to genetic, epigenetic and post-transcriptional/post-translational events. Activated hemichannels mediate the diffusional membrane transport of ions and small signaling molecules. In the last decade hemichannels have been shown to participate in diverse cell processes including the modulation of cell proliferation and survival. However, their possible role in tumor growth and expansion remains largely unexplored. Herein, we hypothesize about the possible role of hemichannels in carcinogenesis and tumor progression. To support this theory, we summarize the evidence regarding the involvement of hemichannels in cell proliferation and migration, as well as their possible role in the anti-tumor immune responses. In addition, we discuss the evidence linking hemichannels with cancer in diverse models and comment on the current technical limitations for their study.

Different mechanisms of modulation of gap junction communication by non-genotoxic carcinogens in rat liver in vivo

This is a comparative study of the mechanisms by which three different rodent non-genotoxic carcinogens modulate connexin-mediated gap junction intercellular communication in male rat liver in vivo. In the case of the peroxisome proliferating agent Wy-14,643, a non-hepatotoxic dose of 50mg/kg led to a marked loss of inter-hepatocyte dye transfer associated with a loss of both Cx32 and Cx26 protein expression. In contrast, p,p'-dichlorodiphenyltrichloroethane (DDT) at a non-hepatotoxic dose (25mg/kg) was not found to alter Cx32 or Cx26 expression or to produce a measurable Cx32 serine phosphorylation but did give a small, significant reduction of cell communication. Carbon tetrachloride (CCl(4)) did not affect cell communication (despite a small significant reduction of Cx32 content) at a non-hepatotoxic dose. Both loss of communication and Cx32 expression was observed only at a dose that caused hepatocyte toxicity as evidenced by increased serum alanine aminotransferase activity. Overall, the findings emphasise that loss of gap junctional communication in vivo can contribute to carcinogenesis by non-genotoxic carcinogens through different primary mechanism. In contrast to Wy-14,643 and DDT, the results with CCl(4) are consistent with a requirement for hepatotoxicity in its carcinogenic action.

The cancer stem cell: evidence for its origin as an injured autoreactive T cell

This review explores similarities between lymphocytes and cancer cells, and proposes a new model for the genesis of human cancer. We suggest that the development of cancer requires infection(s) during which antigenic determinants from pathogens mimicking self-antigens are co-presented to the immune system, leading to breaking T cell tolerance. Some level of autoimmunity is normal and necessary for effective pathogen eradication. However, autoreactive T cells must be eliminated by apoptosis when the immune response is terminated. Apoptosis can be deficient in the event of a weakened immune system, the causes of which are multifactorial. Some autoreactive T cells suffer genomic damage in this process, but manage to survive. The resulting cancer stem cell still retains some functions of an inflammatory T cell, so it seeks out sites of inflammation inside the body. Due to its defective constitutive production of inflammatory cytokines and other growth factors, a stroma is built at the site of inflammation similar to the temporary stroma built during wound healing. The cancer cells grow inside this stroma, forming a tumor that provides their vascular supply and protects them from cellular immune response.

As cancer stem cells have plasticity comparable to normal stem cells, interactions with surrounding normal tissues cause them to give rise to all the various types of cancers, resembling differentiated tissue types. Metastases form at an advanced stage of the disease, with the proliferation of sites of inflammation inside the body following a similar mechanism. Immunosuppressive cancer therapies inadvertently re-invigorate pathogenic microorganisms and parasitic infections common to cancer, leading to a vicious circle of infection, autoimmunity and malignancy that ultimately dooms cancer patients. Based on this new understanding, we recommend a systemic approach to the development of cancer therapies that supports rather than antagonizes the immune system.

MX, a by-product of water chlorination, lacks in vivo genotoxicity in gpt delta mice but inhibits gap junctional intercellular communication in rat WB cells

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a by-product of water chlorination, is a potent bacterial mutagen and rat carcinogen. In the present study, the in vivo mutagenicity, cell proliferative activity, and carcinogenicity of MX were investigated in gpt delta mice. Groups of 5 male and female 7-week-old gpt delta C57BL/6J transgenic mice were given MX at doses of 0, 10, 30, or 100 ppm in their drinking water for 12 weeks, and then killed to assess in vivo mutagenicity using 6-thioguanine and Spi- selection, and cell proliferative activity using immunohistochemistry for proliferating cell nuclear antigen (PCNA). Further groups of 10 male and female gpt delta mice were given 0 or 100 ppm MX for 78 weeks, and a full necropsy with histopathological examination of all organs was conducted to detect neoplastic lesions. The 12-week MX treatment did not result in mutagenicity in the livers or lungs or cell proliferative activity in several organs of the mice, and the 78-week treatment did not cause carcinogenicity. Additional investigations were conducted to evaluate the potential of MX to inhibit gap junctional intercellular communication (GJIC) in rat liver epithelial cells (WB cells) by the scrape loading/dye transfer method. Inhibition of GJIC was detected within 2 hr with a noncytotoxic dose of MX (4 microg/ml), followed by partial restoration after 5 hr. A second phase of inhibition occurred after 10 hr and then the lowered level persisted for the 24 hr-incubation period. Dose-dependent inhibition was evident at both 2 hr and 24 hr, with much stronger effects at the former time. These findings indicate that MX is not mutagenic, mitogenic or carcinogenic in mice, and suggest that the compound exerts epigenetic actions leading to GJIC inhibition.

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.

Inhibition of apoptosis by pentachlorophenol in v-myc-transfected rat liver epithelial cells: relation to down-regulation of gap junctional intercellular communication

Pentachlorophenol (PCP), a promoter of murine hepatocarcinogenesis, inhibits gap junctional intercellular communication (GJIC) in rat liver epithelial cells in vitro. To test the hypothesis that both inhibition of GJIC and apoptosis contribute to tumor promotion, we investigated the effect of PCP on both GJIC and serum deprivation-induced apoptosis in v-myc-transfected rat liver epithelial cells. The results showed that PCP inhibited apoptosis, as measured by the TUNEL assay and DNA ladder formation. Inhibition of apoptosis was associated with a decrease in GJIC. The study demonstrated that PCP has a potential for inhibiting apoptosis and GJIC, supporting the hypothesis.

Prevention of the down-regulation of gap junctional intercellular communication by green tea in the liver of mice fed pentachlorophenol

Much evidence has been documented supporting the hypothesis that the down-regulation of gap junctional intercellular communication (GJIC) is a cellular event underlying the tumor promotion process and that treatment to prevent the down-regulation or to up-regulate GJIC is important in preventing tumor promotion. We explored the potential preventive effects of green tea against the promoting action of pentachlorophenol (PCP) in mouse hepatocarcinogenesis, examining whether drinking green tea prevents the down-regulation of GJIC inhibition in the liver caused by tumorigenic doses of PCP. We used a modified in vivo GJIC assay, the incision loading/dye transfer method. Male B6C3F1 mice were given a green tea infusion for 1 week and then PCP was fed at a dose of 300 or 600 p.p.m. in the diet for the following 2 weeks, along with green tea treatment. A dose-related inhibition of GJIC in the hepatocytes was evident in the mice treated with PCP alone that was associated with a reduction in connexin32 (Cx32) plaques in the plasma membrane and an increase in the cell proliferation index. Drinking green tea significantly protected mice against GJIC inhibition, the reduction in Cx32 and the elevation of the labeling index. These findings suggest that green tea might act as an anti-promoter against PCP-induced mouse hepatocarcinogenesis via its ability to prevent down-regulation of GJIC.

Risk assessment for possible carcinogens: a critical look

Our overall understanding of mechanisms of toxicology in relation to human disease, with prevention of disease as a major objective, depends in part on the development of an adequate number of ways to assess risks, both short term and long term. Despite the cost, the long duration of the test, and some pitfalls, the long-term animal tests remain, to date, the only reliable assay for possible carcinogens. Recent work has concentrated increasingly on the development of short-term tests to replace the long-term tests. Such a development would be most welcome from several points of view. To date, a variety of approaches have been or are being used. These include (1) activation to an alkylating agent with DNA as the most important target, generating possible mutations in DNA and DNA damage with or without repair, (2) induction of cell proliferation, at least a few cycles, with DNA synthesis as the major target, again favoring mutations, and (3) decrease in cell-to-cell communication (gap-junctional intercellular communication) as a supposed test for promotion. None of these proposed assays are reliable indices for possible carcinogenic effects of chemicals or other agents; the scientific basis for this negative conclusion is discussed.

Preventive effect of epicatechin and ginsenoside Rb(2) on the inhibition of gap junctional intercellular communication by TPA and H(2)O(2)

The anticarcinogenic effects of epicatechin (EC) and ginsenoside Rb(2) (Rb(2)), which are major components of green tea and Korea ginseng, respectively, were investigated using a model system of gap junctional intercellular communication (GJIC) in WB-F344 rat liver epithelial cells. 12-O-tetradecanoylphorbol-13-acetate (TPA) and hydrogen peroxide, known as cancer promoters, inhibited GJIC in the epithelial cells as determined by the scrape loading/dye transfer assay, fluorescence redistribution assay after photobleaching, and immunofluorescent staining of connexin 43 using a laser confocal microscope. The inhibition of GJIC by TPA and H(2)O(2) was prevented with treatment of Rb(2) or EC. The effect of EC on GJIC was stronger in TPA-treated cells than in H(2)O(2)-treated cells, while the effect of Rb(2) was opposite to that of EC. EC, at the concentration of 27.8 microg/ml, prevented the TPA-induced GJIC inhibition by about 60%. Rb(2,) at the concentration of 277 microg/ml, recovered the H(2)O(2)-induced GJIC inhibition by about 60%. These results suggest that Rb(2) and EC may prevent human cancers by preventing the down-regulation of GJIC during the cancer promotion phase and that the anticancer effect of green tea and Korea ginseng may come from the major respective components, EC and Rb(2).

Cell population dynamics (apoptosis, mitosis, and cell-cell communication) during disruption of homeostasis

The sequence of events involved in maintenance of homeostasis must encompass mechanisms within single cells as well as interactions between cells within a population. To investigate the interaction among these inter- and intracellular mechanisms, disruption of homeostasis by serum deprivation was performed in WB-F344, a normal diploid epithelial cell line. Changes in cell-cell communication (gap junction function) at the population level and in individual cells were monitored using the scrape load/dye transfer and fluorescence redistribution after photobleaching assays. Apoptosis and mitosis were measured using internucleosomal DNA ladder assays and fluorescence-activated cell sorting. The results indicate that a common element in early apoptosis and early mitosis is sustained gap junction function. As cell life (mitosis) and cell death (apoptosis) progressed, a common process of change in gap junction function occurred. A transient stimulation of mitosis concomitant with increased apoptosis was also observed during serum deprivation. Gap junctions may play a regulatory role during initiation of these opposite yet equally important mechanisms of maintaining homeostasis. This model system is useful for further studies on the relationships among inter- and intracellular mechanisms of homeostasis.

Cell interactions control the fate of malignant keratinocytes in an organotypic model of early neoplasia

The role of cell interactions during early neoplastic progression in human skin is not well understood. We report that the fate and behavior of low-grade malignant cells in stratified epithelium is dependent on their interactions with neighboring cells and with extracellular matrix during the early events in neoplastic progression. We utilized an organotypic tissue model which mimics premalignancy to monitor malignant cells (II-4) genetically marked with beta-gal and grown in the context of either normal human keratinocytes or the immortalized cell line HaCaT. HaCaT cells were permissive for clonal expansion of II-4 cells at ratios of 4:1, 12:1, and 50:1 (HAC:II-4) when compared with coculture with normal human keratinocytes. This II-4 cell expansion was associated with the failure of neighboring HaCaT cells to induce differentiation and cell cycle withdrawal of II-4, as had been seen in the context of normal human keratinocytes. When 12:1 mixtures (NHK:II-4) were stripped of all suprabasal cells and regrown, all beta-gal cells were lost showing that these normal human keratinocyte-suppressed II-4 cells had been actively sorted to a suprabasal position where their clonal expansion was limited. These growth-suppressive effects of normal human keratinocytes were found to be conditional on direct cell-cell contact, as II-4 formed colonies when trypsinized from 12:1 (NHK:II-4) mixtures and grown at clonal density in submerged culture. The distribution and behavior of low-grade malignant cells was therefore dependent on the state of transformation of adjacent keratinocytes and on cell-matrix interactions. These results demonstrate that alterations in the cellular microenvironment are central to the induction of clonal expansion and early neoplastic progression in stratified epithelium.

Epigenetic toxicology as toxicant-induced changes in intracellular signalling leading to altered gap junctional intercellular communication

Communication mechanisms [extra-, intra-, and gap junctional inter-cellular communication (GJIC)] control, from the fertilized egg, through embryogenesis to maturity and aging, whether a cell proliferates, differentiates, dies by apoptosis, or if differentiated, adaptively responds to endogenous and exogenous signals. From the egg to the 100 trillion cells in the human body, health is maintained when these communication processes between stem, progenitor and terminally differentiated cells are integrated. Each cell choice involves 'epigenetic' mechanisms to alter the expression of genes at the transcriptional, translational or post-translational levels. Disruption of the communication mechanisms can be either adaptive or maladaptive. Modulation of extra-cellular communication, either by genetic imbalances of growth factors, hormones or neurotransmitters or by environmental, exogenous chemicals can trigger signal transducing intra-cellular mechanisms. These intra-cellular signals can modulate gene expression at the transcriptional, translational or post-translational levels while also modulating GJIC. Untimely or chronic disruption of GJIC during embryonic or fetal development could lead to embryonic lethality or teratogenesis. By modulation of GJIC, homeostatic control of cell growth, differentiation or apoptosis could lead to specific diseases, such as neurological, cardiovascular, reproductive or endocrinological dysfunction. Chemical modulation or oncogene down-regulation of GJIC in initiated tissues has been shown to lead to tumor promotion. Genetic syndromes carrying a mutated gap junction gene, together with some transgenic and knock-out gap junction gene mice, provide evidence for the importance of this organelle found only in metazoans. Implications for 'thresholds' to toxicants and for risk assessment are evident.

Inhibitory effect of pentachlorophenol on gap junctional intercellular communication in rat liver epithelial cells in vitro

To understand the initiating/promoting actions of pentachlorophenol (PCP), a non-mutagenic hepatocarcinogen, and its metabolite, tetrachlorohydroquinone (TCHQ), we investigated the effects of each chemical on gap junctional intercellular communication (GJIC) in rat liver epithelial cells (WB cells) by the scrape-loading and dye transfer method. After treatment with PCP, the GJIC was initially inhibited at 4 h but was restored in 6-8 h, followed by a second phase of inhibition between 16 and 24 h. Both the first and second inhibitions were concentration-dependent and were restored by 2-4 h after removal of PCP. The phosphorylation state of connexin 43 (CX43) and its localization on the plasma membrane were unchanged up to 24 h after treatment; however, this was accompanied by a decrease in the CX43 protein level. No inhibitory effect was apparent on the GJIC of cells treated with TCHQ. These results suggest that PCP may play a critical role of promoting activity via non-mutagenic mechanisms.

Involvement of oxidative stress in motorcycle exhaust particle-induced DNA damage and inhibition of intercellular communication

In this study, we investigated the involvement of reactive oxygen species (ROS) in the motorcycle exhaust particle (MEP)-induced genotoxic and non-genotoxic activity in mammalian cell systems. Initially, the capability of MEP to induce ROS was evaluated by using 2',7'-dichlorofluorescin diacetate (DCFH-DA) to detect hydrogen peroxide (H2O2). A five-fold increase in H2O2 was observed in Chinese hamster lung V79 and human lung carcinoma Calu-1 cells treated with 100 microg/ml MEP for 2 h. Under the same experimental conditions, only a two-fold elevation in H2O2 was detected in hepatic cell systems such as BNL.Cl.2, HepG2, and Hep3B. Treatment of the V79 cells with varying concentrations of MEP caused a dose-dependent increase in sister chromatid exchanges (SCEs), which are effectively inhibited by addition of antioxidants, N-acetyl-l-cysteine (NAC) and ascorbic acid. Furthermore, we determined the oxidized bases in the V79 cells after exposure to MEP. The result showed that 500 microg/ml MEP induced a 3.7-fold increase in thymine glycol (TG) and a seven-fold increase in 8-hydroxy-guanosine (8-OHGua) as compared to untreated cells. We subsequently examined whether MEP would affect gap junctional intercellular communication (GJIC), a tumor promotion process, in V79 cells. We found that MEP inhibited GJIC in a dose-response fashion. Maximal inhibition occurred at 500 microg/ml. The concentration that inhibited at 0.5 of the fraction of the control was 200 microg/ml. Interestingly, when cells were pretreated with NAC or ascorbic acid, they could abolish the MEP-mediated inhibition of GJIC. In addition, a moderate decrease of glutathione was observed in the V79 cells during exposure to MEP. Taken together, our findings suggest that MEP can induce oxidative stress in a broad range of cell lines, especially in lung cell systems. The MEP-induced oxidative stress was critically involved in both genotoxic and non-genotoxic activity.

2,3,7,8-Tetrachlorodibenzo-p-dioxin mechanistic data and risk assessment: gene regulation, cytotoxicity, enhanced cell proliferation, and tumor promotion

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been found to cause several tumor types in rodents, but TCDD has not been proven to cause cancer in humans, although there have been reported associations. TCDD does not bind to DNA, and indirect tests for DNA damage have been mostly negative. Tumorigenicity by TCDD in rodents has been linked to cellular necrosis, enhanced cell proliferation and tumor promotion. TCDD binds to the Ah receptor, which induces CYP1A1. This binding may be involved in tumorigenicity in rodents; however, additional TCDD-induced toxic changes appear to be required. Biopersistence and organ distribution may play an important role in TCDD dosage extrapolation to humans, but these have not been adequately determined.

Freeze-fracture and immunohistochemical studies of gap junctions in human gastric mucosa with special reference to their relationship to gastric ulcer and gastric carcinoma

Our aim was to determine whether the development of gap junctions in the human gastric mucosa has any relation to gastric ulcer and gastric carcinoma. Freeze-fracture replicas were prepared from the endoscopic biopsy specimens of 20 patients with gastric ulcer and 7 healthy volunteers. Large fractured areas of lateral cell membranes of surface mucous cells were examined randomly under an electron microscope. Small gap junctions were observed between gastric surface mucous cells in all healthy volunteers. Gap junctions in the patients with gastric ulcer were significantly fewer than in the healthy volunteers. In addition, gap junctions in patients with recurrent ulcer were significantly fewer than in those with first-onset ulcer. There was no obvious relationship between age and the development of gap junctions in patients with gastric ulcer or in healthy volunteers. In the areas of intestinal metaplasia, gap junctions were occasionally seen between absorptive cells of the villi, but not in the lateral membranes of goblet cells. Fresh frozen sections for indirect immunofluorescence were prepared from the endoscopic biopsy specimens of 19 patients with gastric ulcer and 5 patients with gastric cancer. Monoclonal antibody against liver gap junction protein (anti-connexin 32, 6-3G11) was used for the indirect immunofluorescence. On the border of gastric ulcer, fluorescent spots in the surface mucous cells were significantly fewer than in the surface mucous cells of the body and antrum which were distant from the ulcer area in the same patients. In gastric cancer tissue specimens, fluorescent spots were not observed at all.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of intra- and intercellular calcium signaling in a mouse malignant glioma cell line

Intra- and intercellular calcium signaling in glioma cells was examined by mechanical stimulation of a monolayer cell line of methylcholanthrene-induced mouse ependymoblastoma, 203-glioma, with a fine round-tip glass needle. A fura-2 fluorescence image of the glioma revealed a four- to eightfold increase in the cytosolic calcium ion concentration in directly stimulated signal cells. The increased calcium spread to surrounding cells at a speed of 20 microns/sec for a distance of up to 200 microns. Calcium was transmitted between adjacent cells and even in cells up to 200 microns distant from the initially stimulated cell. Microinjection of Lucifer yellow dye showed no gap junctional communication between cells. Depletion of extracellular calcium ion inhibited both cytosolic calcium elevation and propagation to neighboring cells by mechanical stimulus. An intracellular calcium blocker, TMB-8, eliminated the cytosolic calcium mobilization in a mechanically stimulated cell, but had no effect on calcium diffusion to surrounding cells. Nifedipine and verapamil, antagonists of voltage-dependent calcium channels, did not act on the mechanically induced calcium response. This suggests that some stimulating factor may trigger transmission of calcium, which may be ejected directly from single stimulated cells and mediated via a membrane receptor but not through a gap junction. The calcium signaling in a mechanically stimulated cell may be related to both an influx and a redistribution of intracellular calcium from internal stores, while calcium propagation to neighboring cells may involve calcium influx alone.

The role of modulated gap junctional intercellular communication in epigenetic toxicology

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

Effect of 1,1'-(2,2,2-trichloroethylidene)-bis(4-chlorobenzene) (DDT) on gap junctional intercellular communication and morphological transformation of Syrian hamster embryo cells

The organochlorine insecticide 1,1'-(2,2,2-trichloroethylidene)bis(4-chlorobenzene) (DDT) did not induce or promote induction of morphological transformation in Syrian hamster embryo (SHE) cells, but it was a potent inhibitor of gap junctional intercellular communication (GJIC). The kinase inhibitor staurosporine did not affect DDT induced inhibition of GJIC, although it has been shown to decrease the inhibitory effect of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on GJIC. In addition, pretreatment with TPA made the cells refractory to further TPA induced inhibition of GJIC, while they remained sensitive to DDT. Thus, DDT and TPA inhibit GJIC through different mechanisms. Elevation of cellular cyclic adenosine monophosphate (cAMP) level by exposure to forskolin counteracted the inhibitory effect of DDT similar to that observed for TPA. Continuous exposure to DDT at concentrations near the effective concentration (50%) value (EC50 value) resulted in a slight recovery of GJIC following the initial inhibition. This recovery was not accompanied by the cells becoming refractory to further DDT induced inhibition of GJIC. The recovery of GJIC after removal of the DDT containing medium seemed to be related to a reduction in the amount of cell-associated DDT.

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

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

Endogenous and exogenous modulation of gap junctional intercellular communication: toxicological and pharmacological implications

During the evolution of single-celled organisms to multicellular metazoans, a family of highly conserved genes coding for proteins (connexins), which as hexameric units (connexins), has evolved to form intercellular channels (gap junctions). These gap junctions allow ions and small molecular weight molecules to flow between coupled cells, thereby facilitating synchronization of electrotonic or metabolic cooperation. Control of cell proliferation, cell differentiation and adaptive responses of differentiated cells have been speculated to be biological roles of gap junctions. The regulation of these gap junctions can occur at the transcriptional, translational and posttranslational levels. Transient downregulation by endogenous or exogenous chemicals can bring about adaptive or maladaptive consequences depending on circumstances. Stable abnormal regulation of gap junction function has been associated with the activation of several oncogenes. Several tumor suppressor genes have also been associated with the up-regulation of gap junction function. Since gap junctions exist in all organs of the multi-cellular organisms, the dysfunction of these gap junctions by various toxic chemicals which have cell type/tissue/organ specificity could bring about very distinct clinical consequences, such as embryo lethality or teratogenesis, reproductive dysfunction in the gonads, neurotoxicity of the CNS system, hyperplasia of the skin, and tumor promotion of initiated tissue. Understanding how many non-mutagenic chemicals might alter normal gap junction function should form the basis of "epigenetic" toxicology. On the other hand, restoring normal gap junction function to cells which have dysfunctional intercellular communication could be the basis for a new approach for therapeutic pharmaceuticals.

Regulation of the 12-O-tetradecanoyl-phorbol-13-acetate-induced inhibition of intercellular communication

Effects of the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and protein kinase C (PKC) inhibitors on cell-cell communication were studied in a normal rat liver cell line, clone 9. Communication was observed and quantitated with microspectofluorometric and image analysis techniques following scrape-loading of the cells with lucifer yellow. Lucifer yellow migrated as far as ten cells away from the scraped edge in control populations. Two minute TPA (25-50 micrograms/ml) treatment inhibited dye movement such that the dye remained mainly in the cells at the cut edge. The TPA-induced inhibition of cell-cell communication could be partially blocked by 15 min treatment of the cell populations with the PKC inhibitors trifluoperazine (30 micrograms/ml), staurosporine (2 x 10(-8) or 2 x 10(-6) M), sangivamycin (15 or 200 microM), or a PKC inhibitor peptide (20 micrograms/ml) scraped in at the same time as lucifer yellow. Normal communication was observed in cultures treated only with PKC inhibitors. Lower concentrations of TPA (50 ng/ml-1 micrograms/ml) used for 2 min did not inhibit dye communication. Our results demonstrate the phorbol ester-induced interruption of cell-cell communication. The inhibition of PKC by inhibitors eliminates the effect of TPA on communication. Our data are consistent with a role of PKC in the control of junctional communication.

Oxidants are a major contributor to aging

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

Homologous and heterologous gap-junctional intercellular communication in v-raf-, v-myc-, and v-raf/v-myc-transduced rat liver epithelial cell lines

We examined gap-junctional intercellular communication (GJIC) in a series of normal and v-raf-, v-myc-, and v-raf/v-myc-transduced rat liver epithelial (RLE) cell lines using the scrape loading-dye transfer and fluorescence-recovery-after-photobleaching (FRAP) assays. Whereas the normal RLE cell line, the control helper virus-transduced cell line, and the v-myc-transduced cell line all showed excellent GJIC, the v-raf-transduced cell lines displayed decreasing levels of GJIC associated with their increasing tumorigenicity. The v-raf/v-myc-transformed cell lines showed the lowest levels of GJIC and were also the most tumorigenic. Heterologous GJIC of these oncogene-transduced cell lines was also compared with that in the normal RLE cells. A modified FRAP assay, using fluorescent-microbead labelling to identify the oncogene-transduced cell from surrounding normal cells, was used to quantify the heterologous GJIC. The v-raf/v-myc-transformed RLE cells had no heterologous communication with the normal RLE cells, whereas v-raf- and v-myc-transduced cell lines maintained heterologous GJIC. Northern analysis showed that connexin 43 was the only gap-junction protein message expressed in these cell lines; connexin 32 and connexin 26 were not expressed. The levels of connexin 43 mRNA expression were relatively unchanged in all cell lines, suggesting that the reduction in GJIC was primarily at the posttranslational level. These findings suggest that reduction of homologous GJIC in v-raf- and v-raf/v-myc-transformed RLE cells is linked to their tumorigenic potential. Furthermore, the loss of heterologous GJIC, which we observed only in the v-raf/v-myc-transformed cells, might release such cells from the growth-regulating effects of surrounding normal cells, possibly contributing to their enhanced tumorigenic potential.

Involvement of gap junctions in tumorigenesis: transfection of tumor cells with connexin 32 cDNA retards growth in vivo

Gap junction channels provide a pathway for exchange of ions and small molecules between coupled cells, and this exchange is believed to be critical for normal tissue growth and development. As a test for a role of gap junction-mediated intercellular communication in control of cell growth, we have compared growth rates of communication-deficient human tumor cells (SKHep1) with clones stably transfected with cDNA encoding the rat liver gap junction protein connexin 32. In culture, growth rates for parental and transfected clones were similar. However, when sizes of tumors were evaluated following injection of these clones into athymic nude mice, growth rates for two well-coupled clones were significantly lower than for communication-deficient or poorly coupled clones. This study demonstrates that growth rate of these tumor cells in situ is negatively correlated with strength of intercellular communication.

Endogenous mutagens and the causes of aging and cancer

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

Inhibition of metabolic coupling by metals

Several metals were evaluated in cell cultures for their ability to inhibit metabolic coupling, the intercellular transfer of low-molecular-weight metabolites by directly connecting gap junctions. To monitor inhibition of metabolic coupling, wild-type Chinese hamster V79 cells proficient in metabolism of 6-thioguanine (6-TG) were cocultured with mutant V79 cells unable to metabolize 6-TG to its toxic metabolite (6-TG-resistant cells). In the presence of 6-TG, inhibition of metabolic coupling by the metals was manifested as increased recovery of 6-TG-resistant cells compared to recovery in untreated cocultures. The toxic metal compounds, arsenic(V) acid, mercury(II) chloride, lead(II) acetate, and nickel(II) chloride, significantly (p less than .05) increased recovery of 6-TG-resistant cells at concentrations that did not alter cell survival. However, because the increased recovery observed with nickel showed no concentration dependence, its importance may be negligible. Cadmium chloride increased 6-TG-resistant cell recovery only at a toxic concentration, and zinc sulfate failed to increase recovery. These data demonstrate that some metal compounds inhibit metabolic coupling, and suggest that inhibition of junctional communication should be further evaluated as a potential mechanism of toxicity of some metals.

Two classes of continuous cell lines established from Syrian hamster 9 day gestation embryos: preneoplastic cells and progenitor cells

Primary cultures of 9-d-gestation Syrian hamster embryo (E9) cells are distinct from primary cultures of later gestational age in terms of their growth and differentiation. First, primary E9 cell cultures express multiple mesenchymal differentiation lineages (e.g., adipocyte, myoblast) only rarely seen in cultures of 13-d-gestation fetal (F13) cells. Second, although most primary E9 cultures have a limited in vitro proliferative life span and exhibit cellular senescence similar to primary cultures of F13 cells, E9 cultures seem to have higher frequency of escape from senescence and conversion to continuous cell lines compared to F13 cells. Moreover, this frequency can be further increased 4- to 5-fold by continuous exposure of the E9 cells to tumor promoters or epidermal growth factor. Eleven continuous cell lines have been isolated from untreated, promoter-treated, or epidermal growth factor-treated primary E9 cultures. Seven of these are neoplastic or preneoplastic. However, the remaining four do not show any evidence of being in neoplastic progression and three of these continue to express the same differentiated phenotype observed in ther parental primary cell cultures.

Inhibition of intercellular communication in smooth muscle cells of humans and rats by low density lipoprotein, cigarette smoke condensate and TPA

Inhibition by tumor promoting chemicals of intercellular communication via gap junctions may be important in carcinogenesis. In order to investigate the possible role of gap junctional intercellular communication in atherogenesis, we examined the effect of known inhibitors of intercellular communication, 12-O-tetradecanoylphorbol-13-acetate (TPA) and cigarette smoke condensate (CSC), and low density lipoproteins (LDL) and high density lipoproteins (HDL) on cellular communication in smooth muscle cells of human and rat by the microinjection-dye transfer technique. When lucifer yellow CH solution is injected into a cell, the average numbers of human and rat smooth muscle cells that become fluorescent is about 22 and 6, respectively. The tumor promoter (TPA) almost completely blocked gapjunctional communication between smooth muscle cells at 100 ng/ml after 4 h exposure. LDL and CSC were able to inhibit intercellular communication in human and rat cells in a dose-dependent manner up to 60%. LDL-pretreatment of human smooth muscle cells did not affect inhibition of intercellular communication, which suggests that this effect is mainly non-receptor mediated. HDL did not influence junctional communication. The results indicate that inhibition of intercellular communication may also contribute to the pathogenesis of atherosclerotic lesions, such as plaques.

TPA-induced inhibition of gap junctional intercellular communication is not mediated through free radicals

The present investigation was designed to determine whether the inhibition of gap junction-mediated intercellular communication (GJIC) induced by TPA (12-O-tetradecanoylphorbol-13-acetate) in rat liver epithelial (WB-F344) cells in vitro is mediated through free radical production. As assessed by fluorescence redistribution after photobleaching (FRAP) analysis, GJIC was significantly inhibited in cells treated for 1 hr with either 10 ng/ml TPA or 500 microM hydrogen peroxide (H2O2). Addition of 1000 U/ml catalase or 25 microM N',N'-diphenyl-p-phenylenediamine (DPPD) to TPA-treated cells did not alleviate the TPA-induced inhibition of GJIC. However, the concurrent addition of 1000 U/ml catalase to the culture medium prevented the H2O2 inhibition of GJIC. 2'-7'-dichlorofluorescein-mediated fluorescence, a measure of free radical production utilizing the Meridian ACAS 470 interactive laser cytometer, was not significantly increased in WB-F344 cells treated with 10 and 100 ng/ml TPA when compared to control cells. However, polymorphonuclear leukocytes (PMNs) treated for 10 min with 100 ng/ml TPA showed a substantial oxidative burst, as did WB-F344 cells treated for 1 hr with 500 microM H2O2. The concurrent addition of 1000 U/ml catalase to the culture medium attenuated H2O2-mediated free radical production in both PMNs and WB-F344 cells. Data from this study do not support a role for free radicals in the TPA-induced inhibition of GJIC in WB-F344 cells.

Tumor promoters retard the loss of a transient subpopulation of cells in low passage Syrian hamster cell cultures

Early passage normal diploid Syrian hamster (SH) fetal cell cultures contain a transient subpopulation of contact-insensitive (CS-) cells which lack density-dependent inhibition of cell division. The size of this CS- subpopulation decreases during in vitro passage by conversion of the CS- cells to contact-sensitive (CS+) cells. Approximately 10-15 population doublings after the frequency of the CS- cells has declined to below 0.001%, mass cultures cease proliferating and exhibit cellular senescence. Cultures with higher initial numbers of CS- cells exhibit longer in vitro proliferative life spans than cultures with smaller initial numbers of CS- cells. Active tumor promoting phorbol esters (12-O-tetra-decanoyl-phorbol-13-acetate [TPA] and phorbol-12,13-didecanoate [PDD]) retard the decline in the proportion of CS- cells during in vitro passage, while the inactive tumor promoting phorbol ester, 4 alpha-phorbol-12,13-didecanoate (4 alpha PDD) has no effect on the rate of loss of the CS- cells. In addition, continuous treatment from secondary culture with TPA or PDD extends by approximately twofold the in vitro proliferative life span of SH fetal cell cultures. Treatment must, however, begin at passage 1 or 2 when the CS- cells are still present. After the proportion of the CS- cells has decreased to less than 0.001% as in passage 6 cultures, promoters have no effect on the life span of the culture. This finding that exposure to promoters results in both a prolonged maintenance of the CS- cellular subpopulation, as well as an extension of in vitro proliferative life span, suggests that the conversion of CS- cells to CS+ cells is involved in the mechanism of in vitro senescence.

Infection of rat liver epithelial cells with v-Ha-ras: correlation between oncogene expression, gap junctional communication, and tumorigenicity

The role of v-Ha-ras oncogene in tumorigenesis in an in vitro/in vivo model system was studied by investigating the expression of the Ha-ras gene, gap junctional intercellular communication, and tumorigenicity as endpoints. Infection of a Fischer 344 rat liver epithelial cell line (WB 344) with a retrovirus containing the v-Ha-ras oncogene resulted in altered cell morphology and decreased contact sensitivity. Gap junctional intercellular communication in v-Ha-ras infected WB cells (WBHa-ras), assessed by fluorescence redistribution after photobleaching (FRAP), microinjection/dye transfer, and scrape-loading/dye transfer techniques, was markedly decreased compared with the level in control WB cells. Injection of 10(7) WBHa-ras cells into the portal vein of male F344 rats caused multiple focal hepatic lesions within 1 and 2 wk, merging to large invading tumors after 3 and 4 wk. Examination of the methylation pattern of the Ha-ras gene in WBHa-ras and control WB cells showed that the infected Ha-ras gene was relatively hypomethylated in comparison to the normal cellular Ha-ras gene, indicating a greater potential for expression. There was an increased level of Ha-ras mRNA in hepatomas as compared with both adjacent nontumor liver tissue and liver tissue obtained from normal animals. Three cell lines derived from three different primary hepatic tumors induced by an injection of WBHa-ras cells in a F344 rat displayed similar growth characteristics, levels of gap junctional communication, and methylation patterns as the original WBHa-ras cells. The results of these studies have established a strong positive correlation between expression of the Ha-ras oncogene, reduced gap junctional intercellular communication, decreased contact sensitivity, and tumorigenicity of the v-Ha-ras-infected rat liver epithelial cells.

Carcinogenicity of polyhalogenated biphenyls: PCBs and PBBs

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) are compounds whose physical/chemical properties led to their widespread commercial use. Although their production has been banned or severely limited in most countries since the 1970s, the persistence and stability of these compounds have resulted in a worldwide distribution, especially of PCBs. PBB contamination is limited principally to the state of Michigan, where a series of tragic errors eventually resulted in the accumulation of residues in livestock and the general human population. Long-term exposure to PCBs and PBBs in animals has been associated with the induction of neoplastic nodules in the liver and in some cases hepatocellular carcinoma. This review discusses the role of PCBs and PBBs in the process of carcinogenesis. The mutagenicity/genotoxicity of these compounds, as well as their initiation/promotion potential is discussed. The epidemiology of PCB and PBB exposure is reported along with an estimation of the risk of cancer to humans. Finally, possible molecular mechanisms of action are suggested for polyhalogenated biphenyls in cancer development.

Stem cell theory of carcinogenesis

Our present understanding of the carcinogenic process, involving complex interactions of genetic, developmental, sex, dietary and environmental factors during the multistage initiation/promotion/progression process of carcinogenesis, would lead us to reject simplistic non-biologically based risk assessment models. This understanding, plus recent results of the National Toxicology Bioassay program and of the studies of short-term tests for genotoxicity, has challenged the primary paradigm of 'carcinogens as mutagens' which governs our current risk assessment models. The concepts of the stem cell theory of cancer, of oncogenes/tumor suppressor genes, of gap junctional intercellular communication, and of mutagenic and epigenetic mechanisms must be integrated into a biologically-based model of the multistage nature of carcinogenesis. Current understanding of the complex interactions during this process prevents us from believing that a simple and accurate, biologically-based risk assessment model will be developed soon, if ever.

Proto-oncogene activation during chemically induced hepatocarcinogenesis in rodents

The liver is a frequent site for the development of chemically induced cancer in rodents. This is primarily owing to the capability of the liver to activate a large variety of exogenous chemicals metabolically to reactive electrophilic species that can covalently interact with cellular DNA and other macromolecules (Miller and Miller, 1966; Miller, 1978). It is the potential alteration of the hepatocellular genome by mutational events that forms the theoretical basis for the heritable nature of cancer as well as, at least in part, the altered phenotype of neoplastic cells; however, our understanding of the exact nature of these heritable genetic alterations remains fragmentary. Within the last decade the delineation of the molecular basis of viral oncogenesis, especially by retroviruses, has revealed potential targets in the cell genome for the reactive forms of chemical agents in relation to their carcinogenic action (Bishop, 1987). Primary among such potential targets are proto-oncogenes, homologous to the transforming genes of oncogenic retroviruses from which they have evolved (Temin, 1974). The objective of this brief review is to consider the evidence that induced alterations in the structure and/or regulation of expression of proto-oncogenes may play one or more roles in rodent hepatocarcinogenesis, especially in relation to the stages of initiation, promotion, and progression (Pitot et al., 1988).

Concentration/response effect of 2,2', 4,4', 5,5'-hexabromobiphenyl on cell-cell communication in vitro: assessment by fluorescence redistribution after photobleaching ("FRAP")

Inhibition of gap junction-mediated cell-cell communication might be a mechanism for several types of cellular dysfunctions, including tumor promotion. Although many different assays have been designed to measure gap junction-mediated intercellular communication, we applied a new technique, termed Fluorescence Redistribution After Photobleaching ("FRAP"), to assess the ability of a known tumor promoter, 2,2', 4,4', 5,5'-hexabromobiphenyl (245-HBB), to inhibit cell-cell communication in a concentration-dependent manner. WB-F344 (rat epithelial) cells were plated at low density, exposed to noncytotoxic concentrations of 1, 5, or 20 micrograms 245-HBB/ml medium, and stained with 6-carboxyfluorescein diacetate. Single cells in pairs or clusters of touching cells in each exposure group were examined with FRAP. The results revealed an inverse correlation between the degree of fluorescence redistribution in photobleached cells and the concentration of 245-HBB. Therefore, FRAP appears to be a sensitive and rapid technique for determining complete or partial inhibition of chemically induced intercellular communication in vitro. These results also provide further evidence for the ability of 245-HBB to inhibit gap junction-mediated cell-cell communication in a concentration-dependent manner.

Anchored cell analysis/sorting coupled with the scrape-loading/dye-transfer technique to quantify inhibition of gap-junctional intercellular communication in WB-F344 cells by 2,2',4,4',5,5'-hexabromobiphenyl

Inhibition of intercellular communication has been hypothesized to play a role in tumor promotion. The compound 2,2',4,4',5,5'-hexabromobiphenyl (245-HBB) is a tumor promoter in vivo and blocks intercellular communication in vitro. The scrape-loading/dye-transfer (SL/DT) assay was used to assess this in vitro effect at varying concentrations of 245-HBB. The SL/DT technique is based on the intracellular loading of a fluorescent dye, lucifer yellow (LY), and monitoring its transfer into adjacent cells via patent gap junctions. Confluent WB-F344 (rat epithelial) cells were exposed to various noncytolethal concentrations of 245-HBB. Transfer of LY was then quantified with anchored cell analysis/sorting (ACAS 470, Meridian Instruments, Okemos, Mich.). The results indicate an inverse correlation between the degree of fluorescence in secondary LY-recipient cells and the treatment concentration. The coupling of these two new methods of cellular biology provided rapid quantitative analysis of dye transfer in measuring the concentration/response of modulation of gap-junctional permeability in cultured cells.

Loss of intercellular junctional communication correlates with metastatic potential in mammary adenocarcinoma cells

A series of rat 13762NF mammary adenocarcinoma cell sublines and clones of various spontaneous pulmonary metastatic potentials from the mammary fat pads of syngeneic rats were examined for their intercellular junctional communication. Using the scrape-loading dye-transfer technique to introduce Lucifer yellow (Mr 457) into cells, we measured the abilities of 13762NF cells to transfer dye to adjacent cells. There was an excellent correlation between loss of Lucifer yellow dye transfer and spontaneous metastatic potential (average total volume of lung metastases inversely correlated to % cells coupled, r = 0.93; average total number of lung metastases inversely correlated to % cells coupled, r = 0.91). The data suggest that high metastatic potentials are closely correlated with loss of intercellular junctional communication in these malignant mammary tumor cells.