The B[a]P-increased intercellular communication via translocation of connexin-43 into gap junctions reduces apoptosis.

Gap junctions are channels in plasma membrane composed of proteins called connexins. These channels are organized in special domains between cells, and provide for direct gap junctional intercellular communication (GJIC), allowing diffusion of signalling molecules <1 kD. GJIC regulates cell homeostasis and notably the balance between proliferation, cell cycle arrest, cell survival and apoptosis. Here, we have investigated benzo[a]pyrene (B[a]P) effects on GJIC and on the subcellular localization of the major protein of gap junction: connexin-43 (Cx43). Our results showed that B[a]P increased GJIC between mouse hepatoma Hepa1c1c7 cells via translocation of Cx43 from Golgi apparatus and lipid rafts into gap junction plaques. Interestingly, inhibition of GJIC by chlordane or small interference RNA directed against Cx43 enhanced B[a]P-induced apoptosis in Hepa1c1c7 cells. The increased apoptosis caused by inhibition of GJIC appeared to be mediated by ERK/MAPK pathway. It is suggested that B[a]P could induce transfer of cell survival signal or dilute cell death signal via regulation of ERK/MAPK through GJIC.

Role of PKC and MAP kinase in EGF- and TPA-induced connexin43 phosphorylation and inhibition of gap junction intercellular communication in rat liver epithelial cells.

Gap junction intercellular communication (GJIC) is involved in the regulation of many cellular processes. The gap junction channels are made up of connexins and the flow of polar low molecular weight molecules through these channels is inhibited by several groups of substances, such as tumour promoters and growth factors. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), chlordane and the growth factor epidermal growth factor (EGF) are potent inhibitors of GJIC in several cell types, including the rat liver epithelial cell line IAR6.1. The induced inhibition of communication by TPA and EGF in IAR6.1 cells is associated with hyperphosphorylation of connexin43, the connexin responsible for GJIC. Two enzyme inhibitors, PD98059, a specific inhibitor of MEK kinase, and GF109203X, a selective inhibitor of protein kinase C (PKC), were used to study the signalling pathways involved in the effect of EGF and TPA on GJIC, with the following conclusions. The inhibition of cell communication in IAR6.1 cells by EGF is likely to be mediated by direct phosphorylation of connexin43 by MAP kinase. TPA blocks GJIC mainly by the direct action of PKC, but also partly through cross-talk with the MAP kinase pathway. Connexin43 hyperphosphorylation induced by TPA is, as for EGF, mediated through MAP kinase, while PKC seems to block GJIC either through other substrates or induces a type of connexin43 phosphorylation that causes no significant electrophoresis mobility shift.

Quantitative determination of gap junction intercellular communication using flow cytometric measurement of fluorescent dye transfer.

Gap junction intercellular communication (GJIC) is involved in several aspects of normal cell behaviour, and disturbances in this type of communication have been associated with many pathological conditions. Reliable and accurate methods for the determination of GJIC are therefore important in studies of cell biology. (Tomasetto, C., Neveu, M.J., Daley, J., Horan, P.K. and Sager, R. (1993) Journal of Cell Biology, 122, 157-167) reported some years ago the use of flow cytometer to determine transfer between cells of a mobile dye, calcein, as a measure of cell communication through gap junctions. In spite of this being a method with potential for quantitative and reliable determination of GJIC, it has been modestly used, possibly due to technical difficulties. In the present work we have illustrated several ways to use flow cytometric data to express cell communication through gap junctions. The recipient cells were pre-stained with the permanent lipophilic dye PKH26, and the donor cell population were loaded with the gap junction permeable dye, calcein. We show that the method may be used to measure the effect of chemicals on GJIC, and that the information is reliable, objective and reproducible due to the large number of cells studied. The data may give additional information to that obtained with other methods, since the effect observed will be on the establishment of cell communication as compared to what is observed for microinjection or scrape loading, where the effect is on already established communication. This is probably the reason for the more potent effects of DMSO on GJIC measured by the present method than on already existing GJIC measured by microinjection or quantitative scrape loading. We also show that the problem related to the mobile dye calcein not being fixable with aldehydes will not affect the results as long as the cells are kept on ice in the dark and analysed by flow cytometer within the first hours after formalin cell fixation.

Gap junctional intercellular communication is not a major mediator in the bystander effect in photodynamic treatment of MDCK II cells.

Photodynamic treatment (PDT) of confluent MDCK II cells resulted in a noticeable clustering of dead cells, consistent with a significant bystander effect. Likewise, PDT of cells in microcolonies resulted in an overabundance of microcolonies that had responded to the treatment as a single unit, that is, in which either all or no cells were dead. Confluent MDCK II cells appeared to communicate via gap junction channels, while cells in microcolonies did not. Monte Carlo simulation models were fitted to the distributions of dead cells in confluent monolayers and in microcolonies. The simulations showed that the degree of the bystander effect was higher in microcolonies than in confluent cells, suggesting that gap junction communication may be involved in the bystander effect. However, when the gap junction hypothesis was tested by treatment of microcolonies with 30 microM dieldrin, an inhibitor of gap junctional intercellular communication, there was no reduction of the bystander effect, indicating that this effect was not mediated by gap junctional intercellular communication. PDT influenced phosphorylation of tyrosine residues in several proteins in the cells. Protein phosphorylation is important in cellular signaling pathways and may be involved in the bystander effect, for example by influencing the mode of cell death.

Quantitative determination of gap junction intercellular communication by scrape loading and image analysis.

Gap junction intercellular communication (GJIC) consists of intercellular exchange of low molecular weight molecules. Chemically induced alterations of this communication have been suggested to result in abnormal cell growth and tumour promotion. Several in vitro assays have been developed to determine the effect of chemicals on gap junction communication in cultured cells. The scrape loading dye transfer technique is based on studying the transfer of the fluorescent dye Lucifer Yellow in cells where the dye is loaded through a cut in the cell monolayer. This technique is rapid and relatively uncomplicated, but has only been used to qualitatively demonstrate communication, due to lack of an appropriate method for quantification of the dye spreading. We show here that analysis of digital fluorescence images of cells scrape loaded with Lucifer Yellow can be used for quantitative determination of GJIC. We have analysed the images both by means of distance of diffusion of the dye in the cell monolayer, as well as by area of dye-coupled cells. The results are consistent with that obtained using microinjection of Lucifer Yellow and the method offers a simple way for quantitative determination of GJIC.

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

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

Role of Cx43 phosphorylation and MAP kinase activation in EGF induced enhancement of cell communication in human kidney epithelial cells.

Epidermal growth factor (EGF) has been found to induce enhanced gap junctional intercellular communication (GJIC) in the human kidney epithelial cell line K7. This is in contrast to what is reported for other cell types, which all show decreased GJIC in response to EGF. In the present study it is shown that 12-O-tetradecanoylphorbol-13-acetate (TPA) and EGF induce similar phosphorylation pattern of the gap junction protein connexin43 (Cx43) in K7 cells, although their effects on GJIC are opposite. Tyrosine phosphorylation of a 42 kD protein was observed to be induced concomitantly with phosphorylation of Cx43. EGF was however found to induce only serine phosphorylation of Cx43, indicating that the tyrosine kinase activity of the EGF receptor was not directly affecting the gap junction protein. The 42 kD protein phosphorylated on tyrosine was identified to be a mitogen activated protein (MAP) kinase. Both EGF and TPA was found to activate MAP kinase in these cells. Phosphorylation of Cx43 and enhancement of GJIC in response to EGF occurred with difference in time course. Phosphorylation of Cx43 was completed within 15 min, while the enhanced GJIC appeared 2-3 h later. It is therefore possible that regulation of synthesis or transport of Cx43 is responsible for the increase in GJIC, rather than direct involvement of Cx43 phosphorylation. This is in support of our previous finding that protein synthesis is necessary for EGF induced upregulation of GJIC in K7 cells.

Modulation of gap junctional intercellular communication by EGF in human kidney epithelial cells.

Modulation of gap junctional intercellular communication (GJIC) was studied in a multistep model of human renal epithelial carcinogenesis. We report that the majority of primary human kidney epithelial cells (NHKE) grown from fetal kidney explants did not communicate through gap junctions. Communication could, however, be observed within a subpopulation of the cells. Ni(II)-immortalized cells (IHKE) showed GJIC at a level of 10-20 communicating cells, but with heterogeneous regions on the dish, with regard to both communication and distribution of connexin43. The heterogeneity was less pronounced in a ras-transfected tumourigenic cell line (THKE), which also showed communication of approximately 10-20 dye-coupled cells. Communication within the IHKE sub-clone K7 decreased from 55 dye-coupled cells communicating on day 1 after seeding to approximately 13 in cells grown for 4 days. Daily change of growth medium reduced the decrease in GJIC. EGF enhanced communication following a lag period which depended on days in culture. The largest increase in GJIC was observed in 2-day-old cultures, where the number of communicating cells in some experiments increased from approximately 45 to 130 dye-coupled cells 4 h following change to medium with EGF. The induction was concentration dependent and communication was enhanced gradually between 2 and 7 h after exposure to EGF. A 15 min pulse of EGF was sufficient to induce the GJIC increase if the total incubation period was unchanged. Cycloheximide completely blocked the EGF-induced enhancement of communication, while actinomycin D had no effect. EGF exposure resulted in an increase in the cellular level of connexin43 protein in parallel with the enhancement in communication. Together, these results indicate that the EGF-induced enhancement of GJIC in human kidney epithelial cells was mediated through translational control of connexin43 expression.

Nongenotoxic carcinogens: development of detection methods based on mechanisms: a European project.

While the accumulation of genetic changes in a somatic cell is considered essential for the genesis of a cancer, it has become clear that not all carcinogens are genotoxic, suggesting that some carcinogens indirectly participate in the generation of genetic changes during carcinogenesis. A European project funded by the European Community was thus conceived to study mechanisms of nongenotoxic aspects of carcinogenesis. Two main strategical approaches were adapted: (i) to study whether and how Syrian hamster embryo (SHE), Syrian hamster dermal (SHD) and BALB/c 3T3 cell transformation systems simulate in vivo carcinogenesis, and to examine whether they can detect nongenotoxic carcinogens; (ii) to study, refine and validate mechanisms-based end-points for detection of nongenotoxic carcinogens. For mechanisms-based research, the proposed end-points included gap junctional intercellular communication (GJIC) inhibition, altered expression of critical genes, immortalization and aberrant cell proliferation. We also selected model compounds commonly usable for various endpoints. Our major results can be summarized as follows: (1) SHE and BALB/c 3T3 transformation systems reflect both genotoxic and nongenotoxic carcinogenic events; they detect not only genotoxic but also many although not all, nongenotoxic carcinogens. This is further supported by the fact that both genotoxic and nongenotoxic carcinogens were able to immortalize SHD cells. (2) Many nongenotoxic carcinogens, although not all, inhibit GJIC in vitro as well as in vivo. Mechanistic studies suggest an important role of blocked GJIC in carcinogenesis and that different mechanisms are involved in inhibition of the communication by different agents used. However, inhibition of GJIC is not a prerequisite for the enhancement (or induction) of transformation of SHE or BALB/c 3T3 cells. (3) Among compounds examined, there was a good correlation between induction of micronuclei and cell transformation in SHE cells while no such correlation was found between the induction of cell transformation and ornithine decarboxylase activity. (4) Two transgenic mouse mutation assays (lacI and lacZ) were established and validated. The genotoxin dimethylnitrosamine was shown to be mutagenic to the liver in both assays. Ortho-anisidine, a bladder-specific carcinogen that was inactive in standard rodent genetic toxicity assays was uniquely mutagenic to the bladder of the transgenic mice. The peroxisome proliferator methyl clofenipate was established as nonmutagenic to the liver of both transgenic mice. That eliminated DNA damage as a cause of the liver tumours produced by this chemical and weakened the idea that induced cell division leads to mutation induction. (5) With an in vitro DNA replication model, it was found that DNA damage induced by genotoxic agents can be responsible for inhibition of DNA replication, while certain nongenotoxic agents such as phorbol esters increase DNA replication. (6) An attempt to use structure-activity relationship for subfamilies of nongenotoxic carcinogens, e.g., receptor-mediated carcinogens, has been initiated with some promising results. Our results support the idea that there are multiple nongenotoxic mechanisms in carcinogenesis, and that working hypothesis-oriented approaches are encouraged rather than simple screening of chemicals in developing test systems for the detection of nongenotoxic carcinogens.

Nickel(II) induces alterations in EGF- and TGF-beta 1-mediated growth control during malignant transformation of human kidney epithelial cells.

We have previously described immortalization of normal human kidney epithelial cells by nickel(II) and the subsequent tumorigenic conversion by v-Ha-ras transfection. We report here that nickel(II) induces alterations in growth regulatory control. Normal human kidney epithelial cells (NHKE) were growth inhibited by transforming growth factor beta 1 (TGF-beta 1). This effect was abrogated in both the immortalized (IHKE) and transformed (THKE) cells. NHKE expressed approximately 4700 high-affinity binding sites/cell for TGF-beta 1. IHKE and THKE showed reduced binding of 47% and 44% relative to NHKE respectively. On the other hand, expression of epidermal growth factor (EGF) receptors was elevated in IHKE (260%) and THKE (236%) relative to NHKE, which expressed 1.5 x 10(5) receptors/cell. Preincubation of IHKE and THKE with TGF-beta 1 resulted in reduced EGF binding, whereas this binding was unaltered in NHKE. Exposure of human kidney epithelial cells to EGF led to tyrosine phosphorylation of the EGF receptor and other cellular proteins in the mol. wt range from 42 to > 300 kDa. The level of receptor phosphorylation induced by EGF reflected receptor expression. Tyrosine phosphorylated proteins appear to be identical in all three cell lines, and reach phosphorylation maxima independently of EGF receptor expression. These studies indicate that nickel carcinogenesis may involve changes in sets of genes important in normal growth regulation.

Role of serum in the morphological transformation of Syrian hamster embryo cells: Characterization and partial purification of protein factors in foetal bovine serum.

The expression of transformed colony morphology in Syrian hamster embryo (SHE) cells, and thus the results obtained in the SHE cell transformation assay, is dependent on the source of the foetal bovine serum (FBS) used. The purpose of this study was to characterize the factors in FBS that are necessary for the expression of transformed morphology. The factors were of protein nature (precipitated by ammonium sulfate and non-dialysable), sensitive to heating and thiol reagents, but resistant to acid and solvent treatment. The active factor(s) were found to bind to a number of protein purification media for ion exchange or affinity purification, and it was initially difficult to reconstitute the biological activity from eluted fractions. This loss of activity was not caused by the separation of more than one necessary factor, but by the factors being highly hydrophobic and negatively charged, and therefore strongly bound to the column material. The active factors could be eluted from Affigel Blue in 50% ethylene glycol, but not in 4m NaCl. The bioactive protein fraction could be further fractionated by gel permeation chromatography on Biogel P-60 in 1 m acetic acid, and cation exchange chromatography on MonoS with 20% acetonitrile added to the buffers. Isoelectric focusing on a Rotofor cell indicated two peaks of transforming activity, one with isoelectric point at about pH 8.5, and one at pH 9.5. The finding of two peaks of biological activity is supported by reversed phase chromatography studies. Bioactivity of two fractions from isoelectric focusing with pI around 8.5 and 9.5 were eluted at propanol concentrations of 20 and 27%, respectively. In the present studies, we were unable to identify the factors with transformation supporting activity, probably because of the high content of protein/peptides with similar biochemical properties in FBS. In further studies we will seek to demonstrate whether previously isolated growth factors, or signalling substances, with similar biochemical properties support the expression of the morphologically transformed phenotype in SHE cells.

Nickel-induced alterations in human renal epithelial cells.

Cellular progression to malignancy appears to require a number of distinct steps in which genetic damage in key regulatory genes accumulates. Immortalization, or escape from senescence, is considered to be one of the first phenotypic changes. Ni2+ treatment of normal human kidney epithelial (NHKE) cells in vitro resulted in immortalization of the cells IHKE cells). The combined action of Ni2+ and v-Ha-ras oncogene fully transformed the cells to tumorigenicity in athymic nude mice. Sequence analysis of DNA from IHKE cells revealed point mutation in the p53 gene at codon 238 with T-->C transition. These findings suggest that Ni-induced mutation in the p53 gene can be involved in the immortalization of the NHKE cells. The results also show that changes in the responses to EGF and TGF beta and in the expression of their receptors occur during malignant progression in vitro.

Inhibition of gap junctional intercellular communication in Syrian hamster embryo cells by TPA, retinoic acid and DDT.

12-O-Tetradecanoylphorbol-13-acetate (TPA), trans-retinoic acid (RA) and DDT inhibit gap junctional intercellular communication in Syrian hamster embryo (SHE) cells. The inhibition is rapid and takes place within minutes. Northern blot analysis shows that SHE cells express connexin 43 and that exposure to these compounds for up to 20 h has no effect on connexin 43 mRNA level. Immune cytochemistry shows that the connexon structures in SHE cells are scattered over the cell, and not confined to the cell-cell boundaries as is the case in the rat liver epithelial cell line IAR20. RA and TPA induce the disappearance of the connexon structures in parallel to the induced inhibition of communication in SHE cells. The disappearance of the connexon spots takes place with no apparent effect on the cellular content of connexin protein measured by immunoblotting, and is probably caused by disaggregation of the connexon structures rather than disappearance or degradation of the connexin protein. DDT shows little or no apparent effect on connexin immunostaining in SHE cells, indicating a different mechanism of action. In the IAR20 cells, exposure to TPA and RA also results in loss of immunostainable connexon structures while exposure to DDT results in relocalization of the connexons away from the cell-cell borders. Immunoblotting of connexin 43 in SHE cells results in three major bands with apparent mol. wts of 40-50 kDa where the two higher mol. wt bands represent phosphorylated connexin 43 protein. Exposure of the cells to the communication inhibiting compounds results in reduction or loss of the highest mol. wt phosphorylated band, indicating a relation between a specific connexin phosphorylation and aggregation of connexin 43 protein to functional communicating gap junctions. The results suggest the presence of various post-transcriptional control mechanisms in the regulation of connexin function which are vulnerable to exogenous stimuli.

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.

12-O-tetradecanoylphorbol-13-acetate-induced inhibition of gap junctional communication is differentially regulated in a transformation-sensitive Syrian hamster embryo cell line compared to early passage SHE cells.

The transformation-sensitive cell-line BPNi was more susceptible to 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inhibition of gap junctional intercellular communication (GJIC) than early passage Syrian hamster embryo (SHE) cells, while the potency of TPA to down-regulate EGF-binding was similar in the two cell types. The kinetics of TPA-induced inhibition of GJIC suggested that different mechanisms may operate at high and low TPA concentrations. The initial inhibition after exposure to high TPA concentrations was followed by a recovery of GJIC. The recovery was much more pronounced in SHE than in BPNi cells. This effect could not be explained by differences in down-regulation of protein kinase C. Removal of high TPA concentrations also resulted in a faster recovery of GJIC in SHE than in BPNi cells. In addition, although forskolin induced a similar protection against the inhibitory effect of TPA on GJIC, forskolin restored GJIC blocked by TPA much faster in SHE than in BPNi cells. Thus, BPNi cells are more sensitive to TPA induced inhibition of GJIC than SHE cells, and have reduced capability to recover from down-regulated GJIC as compared to SHE cells.

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

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

TPA inhibition of gap junctional intercellular communication in Syrian hamster embryo cells through two different pathways.

Exposure of early passage Syrian hamster embryo cells to high TPA concentrations (8 or 16 mum) decreased gap junctional intercellular communication (GJIC) to about 20% of the control during the first hour. Subsequently, the GJIC capacity recovered to 70-80% after 4-10 hr. Exposure to lower TPA concentrations also reduced GJIC, but did not result in the subsequent rapid enhancement of communication. Treatment of the cells to different TPA concentrations down-regulated protein kinase C (PKC), but this down-regulation did not seem to explain fully the induced recurrence of GJIC following high TPA concentrations. The maximal down-regulation of PKC took place at TPA concentrations above 0.16 mum. Addition of 8 mum TPA to cells pretreated with 0.016 and 8 mum for 10 hr reduced GJIC from 80 to 40%, whereas pretreatment with 0.16 or 1.6 mum TPA made the cells refractory to further TPA-induced inhibition. The PKC inhibitor staurosporine suppressed the effect of TPA on GJIC in cells exposed to 0.016 mum TPA, whereas no suppression was observed in cells depleted of PKC. The results indicate that TPA is capable of inhibiting GJIC through two different mechanisms, a staurosporine-sensitive mechanism at low TPA concentrations (EC(50) = 0.18 mum) and a staurosporine-insensitive mechanism at higher concentrations (EC(50) = 7 mum).

Effect of protein kinase C inhibitors on TPA-induced inhibition of gap junctional intercellular communication and induction of transformed morphology in Syrian hamster embryo cells.

Staurosporine and H-7, but not polymyxin B, were found partly to suppress the effect of TPA on both inhibition of gap junctional intercellular communication (GJIC), down-regulation of EGF-binding and induction of transformed morphology in Syrian hamster embryo (SHE) cells. The parallel effect of these kinase inhibitors suggests that TPA induces the effects through activation of protein kinase C (PKC). This is consistent with the view that PKC is involved in both the induction of transformed morphology and the inhibition of GJIC. All the inhibitors studied reduced PKC activity in SHE cell extracts. IC(50) values were determined to be 0.008 mum for staurosporine, 55 mum for H-7 and 16 mum for polymyxin B. The values for staurosporine and H-7 corresponded to those concentrations that suppressed TPA-induced cellular responses. The lack of effect of polymyxin B on whole SHE cells indicates that this compound does not enter the cells.

Effect of cAMP elevating compounds on inhibition of gap junctional communication and induction of morphological transformation in Syrian hamster embryo cells.

An enhancement of the cellular cAMP level has been shown to protect against phorbol ester-induced inhibition of gap junctional intercellular communication (GJIC) and induction of morphological transformation in Syrian hamster embryo (SHE) cells. Cholera toxin, forskolin, 3-isobutyl-1-methylxanthine (IBMX) and theophylline counteracted the 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inhibition of GJIC. The enhancement of communication by these compounds was independent of the TPA concentration, as well as whether the cells were treated with TPA prior to or after the cAMP elevating agents. The induced increase in the cAMP level by cholera toxin occurred in the same concentration range as the enhancement of GJIC. With forskolin the effect on GJIC was apparent at concentrations 10 times lower than needed to enhance the cAMP level. IBMX and theophylline were found to enhance GJIC with only a 20% elevation of cAMP. The cAMP elevating compounds also suppressed the response of TPA on induction of transformed morphology in SHE cells.