Inhibition of intercellular communication between liver cells by the liver tumor promoter 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

Brain Disorders and Chemical Pollutants: A Gap Junction Link?

The incidence of brain pathologies has increased during last decades. Better diagnosis (autism spectrum disorders) and longer life expectancy (Parkinson's disease, Alzheimer's disease) partly explain this increase, while emerging data suggest pollutant exposures as a possible but still underestimated cause of major brain disorders. Taking into account that the brain parenchyma is rich in gap junctions and that most pollutants inhibit their function; brain disorders might be the consequence of gap-junctional alterations due to long-term exposures to pollutants. In this article, this hypothesis is addressed through three complementary aspects: (1) the gap-junctional organization and connexin expression in brain parenchyma and their function; (2) the effect of major pollutants (pesticides, bisphenol A, phthalates, heavy metals, airborne particles, etc.) on gap-junctional and connexin functions; (3) a description of the major brain disorders categorized as neurodevelopmental (autism spectrum disorders, attention deficit hyperactivity disorders, epilepsy), neurobehavioral (migraines, major depressive disorders), neurodegenerative (Parkinson's and Alzheimer's diseases) and cancers (glioma), in which both connexin dysfunction and pollutant involvement have been described. Based on these different aspects, the possible involvement of pollutant-inhibited gap junctions in brain disorders is discussed for prenatal and postnatal exposures.

Hexachlorocyclohexane-Induced Tumorigenicity in Mice under Different Experimental Conditions

Continuous administration of 500 ppm hexachlorocyclohexane (HCH) in the diet induced 100 % liver tumors in mice, but none were observed in rats and hamsters treated with the same dose of HCH. In Swiss male mice, tumor incidence at earlier ages was greater than that observed in corresponding age groups of Swiss female mice. It was further observed that in Swiss females, sex hormones retarded the tumorigenic effect of HCH; in BALB/c mice a similar phenomenon was not observed.

Potential Role of Intercellular Communication in the Rate-Limiting Step in Carcinogenesis

In order to ascertain whether there might be a scientific basis for determining practical “thresholds” for “carcinogens,” the concepts of thresholds and carcinogens were examined in the context of some current ideas on cardnogenesis. The observation that cardnogenesis seems to involve the donal expansion of a pre-malignant cell through a series of pheno-typic changes was explained by the initiation/promotion model of cardnogenesis. Unrepaired DNA lesions, acting as substrates for mutations in dividing cells, were speculated to play a role in the initiation phase of cardnogenesis (and indirectly to the promotion phase if the lesions lead to significant cell killing, forcing “compensatory hyperplasia”). Inhibition of intercellular communication, either by cell removal, cell death, growth factors or chemical promoters, was speculated to allow the donal expansion of initiated cells to reach a “critical mass.” During that donal expansion of initiated cells, additional phenotypic changes were speculated to occur during cell replication by mutational and/or epigenetic events. Therefore, it was concluded, on the basis of this model, that conditions which prevented the inhibition of intercellular communication between normal cells and the initiated cell(s) contributed to the rate limiting step of cardnogenesis.

Assuming the initiation and promotion model of cardnogenesis, the classical concepts of “thresholds” and “carcinogens” were viewed as grossly inadequate because they did not symbolically represent the known determinants of the complex carcinogenic process. Unless genetic, developmental stage, tissue, nutritional, stress, life style, as well as concurrent antagonists and/or synergists, factors are known, extrapolation about the potential carcinogenicity of a given chemical from molecular, in vitro or even in vivo experiments or epidemiological data would be extremely risky. It was concluded that, at this stage of our understanding of the mech-anism(s) of carcinogenesis, attempts to determine “thresholds” for “carcinogens” naively assume “carcinogens” are the single determinants for carcinogenesis, and that all chemicals which might influence the appearance of tumors act the same way.

Carcinogen induced unscheduled DNA synthesis in mouse hepatocytes

Mouse primary liver cell cultures were examined for evidence of unscheduled DNA synthesis (UDS) following treatment with the carcinogens; dimethylnitrosamine (DMNA), diethylnitrosamine (DENA), 2-acetylaminofluorene (2-AAF), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), benzo(a)pyrene (BP), dimethylbenzanthracene (DMBA), 1,1,-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), safrole, diethylstilbestrol (DES), aflatoxin B1 (AFB1), and dieldrin and the noncarcinogens; dimethylformamide (DMF), fluorene, and pyrene. Mouse hepatocyte cultures were simultaneously treated with three concentrations of each compound and 3H-thymidine. After 24 hrs, cells were fixed and processed for autoradiography. 3H-thymidine incorporation in both experimental and control cell nuclei, as evidenced by autoradiographic grains, was quantitated microscopically. DMNA, DENA, 2-AAF, MNNG, BP, AFB1 and DMBA significantly increased UDS over untreated cells at all concentrations studied. DDT, DMF, fluorene, pyrene, safrole, DES, and dieldrin were negative for UDS in all concentrations examined. DMNA, 2-AAF and MNNG were also studied for UDS induction in 2 hr old, 1 day old and 4 day old cultures. A progressive decrease in UDS with increased time after plating was found in DMNA and 2-AAF treated cultures. After 4 days DMNA and 2-AAF induced UDS only at the highest concentrations examined (10(-3) M and 10(-4) M respectively). MNNG induced UDS at all time periods and concentrations sampled. An attempt to enhance the sensitivity of the UDS assay by inducing the mixed function oxidative enzyme activity in the hepatocytes with phenobarbital administered in vivo resulted in no statistically significant increase in UDS with DMNA, 2-AAF, MNNG, DDT, and dieldrin when compared with cells from non-induced animals.

Absence of DNA adduct formation by phenobarbital, polychlorinated biphenyls, and chlordane in mouse liver using the 32P-postlabeling assay

Phenobarbital (PB), polychlorinated biphenyls (PCBs), and chlordane (CLD) increase liver tumor incidences in rodents, and all are tumor promoters. Most indirect tests for DNA reactivity, including mutagenicity and chromosomal damage, have been negative with these agents. Consequently, the modes of action for tumorigenesis by these compounds are not believed to involve direct DNA reactivity; however, only limited information from direct tests is available for the lack of DNA adduct formation. PB, PCBs, and CLD were tested for DNA adduct formation in the liver of male and female B6C3F1 mice after either single or 2-week dietary exposures. Single gavage dose levels were as follows: PB, 200 mg/kg; PCBs, 50 mg/kg; and CLD, 50 mg/kg. Dietary dose levels were as follows: PB, 1000 ppm; PCBs, 200 ppm and CLD, 200 ppm. Animals were killed 24 h following the end of test-substance administration. DNA was extracted from the liver, and DNA adduct concentrations were enriched using either 1-butanol extraction of adducted nucleotides or nuclease P1 digestion of unadducted nucleotides. Using this protocol, none of the three test compounds produced DNA adducts detected by 32P-postlabeling. Similar negative results were obtained for DNA from the livers of both male and female mice receiving either single or 2-week exposures. The two positive controls, benzidine for the 1-butanol extraction procedure and 2-acetylaminofluorene for the nuclease P1 procedure, showed the expected patterns of DNA adducts. These results support the conclusion that the carcinogenicity of PB, PCBs, and CLD in experimental animals is not the result of direct DNA reactivity, but involves epigenetic mechanisms.

Differential decrease in connexin 32 expression in ischemic and nonischemic regions of rat liver during ischemia/reperfusion

The effect of a localized hepatic injury, regional ischemia/reperfusion, on the expression of connexin 32 (Cx32) was studied. Cx32 is the component of the major hepatic gap junction. Two regions of the injured liver were analyzed: the area directly affected by the ischemic insult (ischemic liver), and the remainder of the organ (nonischemic liver). In the ischemic liver, there were simultaneous reductions in Cx32 mRNA steady-state levels and the encoding polypeptide from the plasma membrane within 1 h of reperfusion. In contrast, Cx32 mRNA steady-state levels were only reduced after 4 h of reperfusion in the nonischemic liver. This reduction of Cx32 mRNA levels was followed by the disappearance of Cx32 on the plasma membrane within 24 h of the insult. Administration of actinomycin D prior to the ischemic insult prevented the reduction in Cx32 mRNA in both ischemic and nonischemic liver regions. Protein synthesis was blocked during the first hour of reperfusion in the ischemic liver but not in the nonischemic liver. To mimic this effect, animals were treated with cycloheximide in absence of the ischemic insult. A reduction in Cx32 mRNA and polypeptide in the liver was observed in cycloheximide treated animals. This finding suggests that the decrease in Cx32 expression in the ischemic, but not in the nonischemic, liver may be due to the inhibition of protein synthesis during ischemia/reperfusion. These observations suggest that an ischemic insult produces a selective deteriorating effect on Cx32 expression in both ischemic and nonischemic liver regions probably through different mechanisms.

Triazine derivatives inhibit rat hepatocarcinogenesis but do not enhance gap junctional intercellular communication

We report here novel candidate chemopreventive agents active against experimental hepatocarcinogenesis. The triazine derivatives 6-(2-chlorophenyl)-2,4-diamino-1,3,5-triazine (2CPDAT), 6-(3-chlorophenyl)-2,4-diamino-1,3,5-triazine (3CPDAT), 6-(4-chlorophenyl)-2,4-diamino-1,3,5-triazine (4CPDAT), 6-(4-pyridyl)-2,4-diamino-1,3,5-triazine (PyDAT), and 6-(pyridine N-oxid-4-yl)-2,4-diamino-1,3,5-triazine (PyNODAT), synthesized in our laboratory, in addition to 6-(2,5-dichloro-phenyl)-2,4-diamino-1,3,5-triazine (DCPDAT), or irsogladine, which is a widely used anti-ulcer drug, were investigated for potential chemopreventive effects in a rat liver medium-term bioassay system. A significant inhibitory influence on enzyme-altered liver foci was found for 2CPDAT, 3CPDAT, 4CPDAT, and PyNODAT, but not for DCPDAT or PyDAT. The involvement of gap junctional intercellular communication in the inhibition was studied, but no change in gap junctional intercellular communication capacity in rat liver cells in vitro or in gap junction protein (connexin 32) expression in rat liver in vivo was noted. These results indicate that, although these irsogladine analogues exert inhibitory effects on rat liver carcinogenesis, their action is independent of modification of gap junctional intercellular communication.

Role of cytoplasmic membrane in the screening of potential tumour promoters

The functional changes of cytoplasmic membrane of mammalian cells as an alternative approach for evaluating the tumor promoter activity were investigated. The V79 metabolic cooperation assay, as the standard method for detection of tumor promoters in vitro was used. From three known tumor promoters only phenobarbital increased the efflux of [3H]2-deoxy-D-glucose-6-phosphate from the cells in the same concentration range where the inhibition of metabolic cooperation was estimated. Neither TPA, saccharin nor theophylline caused any functional changes of cytoplasmic membrane of V79 cells. On the basis of the results with known tumor promoters we suggest that following the functional changes of cytoplasmic membrane is not a convenient approach for screening potential tumor promoters.

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) induces gene mutations and inhibits metabolic cooperation in cultured Chinese hamster cells

Investigations were carried out on 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a potent direct-acting bacterial mutagen found in chlorinated tap water, for the potential to induce 6-thioguanine (6-TG) resistant mutations in cultured Chinese hamster V79 cells. The mutagenicity of MX was manifested when cells were treated with MX in Hanks balanced salt solution, but not in serum-free Eagle's minimal essential medium. A 12-fold higher mutation frequency over control was obtained at 12.5 micrograms/ml MX. In addition, the inhibitory effect of MX on metabolic cooperation between cocultivated 6-TG- sensitive (6-TGs) and -resistant (6-TGr) V79 cells was investigated. Recovery of 6-TGr cells significantly increased at 0.8-2.0 micrograms/ml MX due to impaired intercellular communication with 6-TGs cells. MX is believed to exert tumor-promoting and mutagenic activity in mammalian cells.

Effects of a synthetic polyprenoic acid (E-5166) on the gap junction of rat hepatocytes treated with liver tumor promoters, phenobarbital, and p,p'-dichlorodiphenyltrichloroethane

Effects of in vivo exposure to the synthetic polyprenoic acid, 3,7,11,15-tetramethyl-2,4,6,10,14-hexadecapentaenoic acid (E-5166) using 6 groups of rats treated simultaneously or individually with phenobarbital (PB) or with p,p'-dichlorodiphenyltrichloroethane (DDT) on the gap junctions of hepatocytes were examined by freeze-fracture analysis. Experimental groups were as follows: Group 1, basal diet alone; Group 2, E-5166 treatment; Group 3, 0.05% PB diet; Group 4, PB diet and E-5166 treatment; Group 5, 0.05% DDT diet; and Group 6, DDT diet and E-5166 treatment. E-5166 was given by gavage (40 mg/kg body weight, 3 times/wk). Experimental diets and E-5166 were started at 6 wk of age. All animals were sacrificed at 2 wk after the start of the experiments. The density of gap junctions in PB- or DDT-treated rats was higher and that in E-5166-treated rats was lower than that in controls. The density of gap junctions in rats given PB and E-5166-/or DDT and E-5166-treated rats was slightly increased when compared to controls. The average area of gap junctions in DDT- or PB and E-5166-treated rats was significantly smaller than in controls (p < 0.005), although no statistical differences were found among PB-, E-5166-, and DDT and E-5166-treated rats and controls. Unit area of gap junctions to membrane area in rats given E-5166, DDT, or PB and E-5166 was lower than in controls. However, the unit area of gap junctions in DDT and E-5166-treated rats did not differ from in controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell culture assays for chemicals with tumor-promoting or tumor-inhibiting activity based on the modulation of intercellular communication

The ability of chemicals with tumor-promoting or tumor-inhibiting activity to modulate gap junctional intercellular communication is reviewed. The two most extensively used types of assays for screening tests are (1) metabolic cooperation assays involving exchange between cells of precursors of nucleic acid synthesis and (2) dye-transfer assays that measure exchange of fluorescent dye from loaded cells to adjacent cells. About 300 substances of different biological activities have been studied using various assays. For tumor promoters/epigenetic carcinogens, metabolic cooperation assays have a sensitivity of 62% and dye-transfer assays 60%. Thirty percent of DNA-reactive carcinogens also possess the ability to uncouple cells. The complete estimation of the predictive power of these assays could not be made because the majority of the substances studied for intercellular communication effects in vitro have not yet been studied for promoting activity in vivo. Both metabolic cooperation assays and dye transfer assays respond well to the following classes of substances: phorbol esters, organochlorine pesticides, polybrominated biphenyls, promoters for urinary bladder, some biological toxins, peroxisome proliferators, and some complex mixtures. Results of in vitro assays for such tumor promoters/nongenotoxic carcinogens, such as some bile acids, some peroxides, alkanes, some hormones, mineral dusts, ascorbic acid, okadaic acid, and benz(e)pyrene, do not correlate with the data of in vivo two-stage or complete carcinogenesis. Enhancement of intercellular communication was found for 18 chemicals. Among these, cAMP, retinoids, and carotenoids have demonstrated inhibition of carcinogenesis. We examine a number of factors that are important for routine screening, including the requirement for biotransformation for some agents to exert effects on gap junctions. We also discuss the mechanisms of tumor promoter and tumor inhibitor effects on gap junctional permeability, including influences of protein kinase activation, changes in proton and Ca2+ intracellular concentrations, and effects of oxy radical production.

Effect of tumor promoting stimuli on gap junction permeability and connexin43 expression in ARL18 rat liver cell line

The ARL18 rat liver cell line has previously been used for screening tumor promoters in the metabolic cooperation assay (Williams 1980; Williams et al. 1981; Telang et al. 1982). These cells display high levels of gap junctional communication, as assessed functionally and immunologically. Intracellularly injected Lucifer Yellow diffused extensively and there was rapid fluorescent recovery after photobleaching. Moreover, expression of connexin43 (Cx43) was high as evaluated by immunocytochemistry of cell monolayers and Western blot analysis of total cell homogenates. Western blot analysis revealed multiple forms of Cx43, which presumably correspond to known dephosphorylated and phosphorylated states of this protein. Gap junction permeability and Cx43 expression in ARL18 cells were studied after exposure to the tumor promoters 12-0-tetradecanoyl-phorbol-13-acetate (TPA), and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)-ethane (DDT), and after wounding the cell monolayer. TPA and DDT strongly inhibited gap junction permeability; whereas monolayer wounding did not affect the degree of fluorescent recovery after injury, either in the cells on the edge of the wound or in distal regions. No changes in the cellular distribution of Cx43 were observed after any of these treatments, although Western blots revealed a decrease in total Cx43 after 24-h exposure to DDT (10 micrograms/ml) and a slight increase after TPA treatment (30 min, 0.1 microgram/ml). Relative abundance of different phosphorylated Cx43 forms was increased after 1 h exposure to DDT (10 micrograms) and 30 min exposure to TPA (0.1 microgram/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The effect of K+/H+ antiporter nigericin on gap junction permeability

The K+/H+ antiporter nigericin inhibits the intercellular exchange of the fluorescent dye Lucifer Yellow between DM15-transformed fibroblasts derived from the Djungarian hamster. The efficacy of nigericin action was related to its concentration and time of incubation. The nigericin-induced uncoupling effect on gap junctions was reversible and was shown to be based on its ability to cause cystolic acidification. The effect of nigericin on dye-coupling in intact and 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-pretreated cells did not differ, indicating that the uncoupling effect of H+ on gap junctions in DM15 cells was not mediated by the TPA-dependent isoform of protein kinase C.

Depression of the Ca2+-ATPase activity of the rat liver endoplasmic reticulum by the liver tumour promoters 1,1,1-trichloro-2,2-bis(p-chlorophenyl)- ethane and phenobarbital

The effects of a short-term in vivo administration of two liver tumour promoters (phenobarbital and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane on rat liver endoplasmic reticulum Ca(2+)-ATPase were investigated. The specific activity values of this membrane-bound enzyme significantly decreased (P less than 0.01) by 51% for phenobarbital-treated rats and by 48% for 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane-treated rats compared with control animals. The depression of liver endoplasmic reticulum Ca(2+)-ATPase appears to be a manifestation of the toxicological effect of tumour promoters.

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.

Toxicity studies of butylated hydroxyanisole and butylated hydroxytoluene. I. Genetic and cellular effects

The cellular effects of the antioxidants butylated hydroxyanisole and butylated hydroxytoluene were studied in a battery of in vitro tests. No evidence of genotoxicity was obtained for either compound in the hepatocyte primary culture/DNA repair test, the Salmonella/microsome mutagenesis test, the adult rat liver epithelial cell/hypoxanthine guanine phosphoribosyl transferase test, or for butylated hydroxyanisole in the Chinese hamster ovary cell/sister chromatid exchange test. Both compounds inhibited intercellular molecular exchange between cultured liver cells, an effect that has been observed for many agents with neoplasm-promoting activity.

The effect of complete carcinogens on intercellular transfer of lucifer yellow in fibroblast culture

The effect on permeability of gap junctions of complete powerful carcinogens, 3-methylcholanthrene (MC), 7,12-dimethylbenz(a)anthracene (DMBA), ethyl methanesulfonate (EMS), and weak carcinogens, benz(a)anthracene (BA), benzo(e)pyrene (B(e)P) as well as the aryl-hydroxylase inhibitor 7,8-benzoflavone (7,8-BF) has been studied with the use of a dye-coupling technique and transformed Djungarian hamster DM15 fibroblasts. MC, EMS and 7,8-BF were found to exert a strong inhibitory effect on cell-to-cell dye transfer. BA and DMBA had the uncoupling activity only in 2 out of 4 experiments. B(e)P was not shown to affect LY transfer between DM15 cells. The uncoupling effect of MC, 7,8-BF and EMS (only when EMS used at the concentration of 600 micrograms/ml but not 1000 micrograms/ml) appeared reversible. The causes of failure to detect DMBA and B(e)P effects on gap junctions are discussed.

Synergistic inhibition of metabolic cooperation by oleic acid or 12-0-tetradecanoylphorbol-13-acetate and dichlorodiphenyltrichlorethane (DDT) in Chinese hamster V79 cells: implication of a role for protein kinase C in the regulation of gap junctional intercellular communication

The effects of TPA and/or DDT and oleic acid and/or DDT on gap junction-mediated intercellular communication (i.e. metabolic cooperation) between Chinese hamster V79 cells was examined. Addition of TPA, DDT or oleic acid alone to cocultures of 6-thioguanine-resistant (6-TGR) and 6-thioguanine-sensitive (6-TGS) V79 cells significantly increased the recovery of 6-TGR cells indicating inhibition of metabolic cooperation. In the presence of TPA and DDT or oleic acid and DDT the observed recovery of 6-TGR cells was significantly greater than the expected (calculated) additive 6-TGR cell recovery. No synergistic increases in 6-TGR cell recovery were observed when co-cultures of V79 cells were exposed to dieldrin and DDT. These results indicate that TPA and DDT or oleic acid and DDT can act synergistically to inhibit metabolic cooperation. These data suggest a role for protein kinase C in the regulation of gap junction-mediated intercellular communication.

Effects of tetradecanoyl phorbol acetate, pyrethroids and DDT in the V79

The effects of the pyrethroids flucythrinate, cyfluthrin, bioallethrin and resmethrin on metabolic cooperation between V79 cells were investigated. Addition of flucythrinate to cocultures of 6-thioguanine-resistant and 6-thioguanine-sensitive V79 cells significantly increased the mutant cell recovery, indicating inhibition of intercellular communication. No such effect was observed by the other pyrethroids tested. To compare the modes of action of TPA-, DDT-, and pyrethroid-induced inhibition of intercellular communication, co-exposure experiments were undertaken. Addition of TPA, together with increasing doses of fenvalerate or flucythrinate, produced a synergistic response. Various combinations of fenvalerate-, flucythrinate- and DDT-exposure gave results in accordance with an additive response. The result suggest different pathways of action for TPA and the insecticides investigated in this study.

Identification of tumor promoters by their inhibitory effect on intercellular transfer of lucifer yellow

The effect of the tumor promoters 12-O-tetradecanoylphorbol-13-acetate (TPA), mezerein, teleocidin, anthralin, the Ca2+-ionophore A23187, butylated hydroxytoluene (BHT), dichlorodiphenyltrichloroethane (DDT) and phenobarbital (PB) on lucifer yellow transfer in cultures of SV-40-transformed Djungarian hamster fibroblasts was studied. TPA, mezerein, teleocidin, A23187, DDT and BHT exerted a strong inhibitory effect on cell-to-cell dye transfer. Anthralin uncoupled cells in 3 experiments out of 6. PB appeared to enhance lucifer yellow transfer. Sodium nitrite, a substance with unknown promoting activity, effectively uncoupled cells. All the promoters investigated had a reversible effect on the dye transfer. The value of the dye transfer method for promoter screening is discussed.

Tumour promotion related effects by the cyclodiene insecticide endosulfan studied in vitro and in vivo

The cyclodiene insecticide endosulfan is structurally related to the tumour promoting pesticides chlordane and heptachlor. Divergent conclusions have been reported regarding the carcinogenic activity of endosulfan. In this study we have investigated if endosulfan and four of its metabolites possess tumour promotion related effects. Two in vitro test systems detecting inhibition of intercellular communication were used; the Chinese hamster lung fibroblast (V79) metabolic cooperation assay and a scrape loading/dye transfer assay using rat liver WB epithelial cells. At non-cytotoxic concentrations, technical grade endosulfan, analytical grade endosulfan (alpha- and beta-isomers and an alpha beta-isomer mixture) and endosulfan-sulfate inhibited gap junctional communication in both assay systems. In addition, the metabolite endosulfan-ether was effective in the rat liver WB epithelial cells. Endosulfan was also studied for enhancement of gamma-glutamyl transpeptidase positive enzyme altered foci incidence in partially hepatectomized, nitrosodiethylamine-initiated male Sprague-Dawley rats. However, endosulfan administered orally (1 or 5 mg/kg/day) five days a week for ten weeks did not enhance enzyme altered foci incidence. These apparently contradictory results with regard to possible tumour promoting activity of endosulfan are discussed in relation to metabolism, systemic toxicity and tissue/species specificity in tumour promotion.

Multistage carcinogenesis: implications for risk estimation

In undertaking a quantitative estimation of carcinogenesis risk, it is essential to keep in mind that carcinogenesis is a multistage process, and that each stage can be affected by different classes of risk factors. Furthermore, different mechanisms are involved in the various stages of carcinogenesis. Thus, a dose-response analysis of one given factor cannot provide an accurate estimation of carcinogenic risk. Carcinogenic risk estimation is usually undertaken for a specific chemical or group of chemicals; however, the concept of multistage carcinogenesis is based on biological processes and not on the mechanisms of action of the agents involved. It is therefore important to consider three related, but different, factors involved in carcinogenesis: stage, agent, and activity of agent. This is especially important in developing a short-term test for stage-related risk factors, such as tumor-promoting agents. For this reason, carcinogens should not be classified according to only one chemical activity. This article briefly reviews the cellular and molecular mechanisms involved in multistage carcinogenesis, and discusses their implications for risk estimation. Special consideration is given to the effect of treatment frequency on the response of tumor-promoting agents, as seen in long-term tests in experimental animals. It is proposed that exposure frequency be taken into account together with exposure dose.

Dibutyryl cyclic AMP modulation of gap junctions in SW-13 human adrenal cortical tumor cells

Cultured human adrenal cortical adenocarcinoma cells (SW-13) form a confluent monolayer of epithelial-like cells when seeded into culture flasks. Following a 24-48 hr non-mitotic period, cells begin to divide and become confluent within a week after seeding at 5 X 10(4) cells/cm2. The SW-13 cells were exposed to dibutyryl cyclic AMP (DbcAMP), cyclic AMP (cAMP), sodium butyrate, and adrenocorticotropin (ACTH). The rate of SW-13 cell proliferation was measured with a DNA microfluorometric assay, as well as by procedures measuring the incorporation of 3H-thymidine. In addition, following administration of ACTH and DbcAMP, the fractional area of membrane covered by gap junctions was quantitated with freeze-fracture electron microscopic techniques. Dibutyryl cyclic AMP at a concentration of 1 X 10(-3) M decreased the growth rate of the cell population. There was a corresponding increase in the fractional area of gap junctions found on the cell membrane in 96-hr DbcAMP-treated cultures. ACTH (40 mU/ml) exposure failed to produce an increase in the fractional area of gap junctions or to alter the rate of cell proliferation. From these data it can be suggested that elevations in cAMP levels within the cell can be related to both the proliferation of gap junctions and the decrease in cell proliferation in the SW-13 tumor cell.

Inhibition of metabolic cooperation in vitro and enhancement of enzyme altered foci incidence in rat liver by the pyrethroid insecticide fenvalerate

The synthetic pyrethroids cypermethrin, delta-methrin, fenvalerate, permethrin, and the fenvalerate metabolite p-chlorophenylisovaleric acid were investigated for inhibition of gap-junctional intercellular communication in vitro in the Chinese hamster lung fibroblast (V79) metabolic cooperation assay. Fenvalerate was furthermore studied for enhancement of gamma-glutamyl transpeptidase-positive enzyme altered foci incidence in partially hepatectomized, nitrosodiethylamine-initiated male Sprague Dawley rats. The in vitro studies showed that fenvalerate and p-chlorophenylisovaleric acid were inhibitors of intercellular communication at non-cytotoxic concentrations while cypermethrin, deltamethrin, and permethrin were inactive. In the in vivo study in rat liver, fenvalerate administered p.o. (75 mg/kg/day) 5 days a week for 10 weeks induced significantly more foci per cm3 and a larger percentage of liver tissue occupied by foci tissue compared to a vehicle control group. Analysis of size distributions of foci in fenvalerate- and vehicle-treated rats showed elevated foci incidences in fenvalerate-treated rats at all foci sizes. Fenvalerate induced no hepatotoxic effects as judged by plasma transaminase activities and histopathology. The results of this study suggest fenvalerate to be a potential tumour promoter.

Calmodulin involvement in TPA and DDT induced inhibition of intercellular communication

The organochlorine pesticide DDT is a liver tumour promoter and a potent inhibitor of intercellular communication. Present knowledge of the mechanism by which DDT inhibits intercellular communication is limited but it has been suggested that increased intracellular free calcium induced by DDT could be of importance. As the effects of calcium are closely associated with the multifunctional protein calmodulin (CaM) in most cells the potential binding of DDT to CaM and subsequent effects on CaM-stimulated Ca2+/Mg2+-ATPase activity were studied. DDT inhibited CaM-stimulated Ca2+/Mg2+-ATPase activity and bound to CaM in a manner similar to established CaM-inhibitors. Subsequently an in vitro assay for measuring inhibition of metabolic cooperation between 6-thioguanine (TG)-sensitive and TG-resistant Chinese hamster (V79) cells was used to investigate the possible involvement of CaM in the regulation of intercellular communication. Calmidazolium (CzM), a potent CaM inhibitor, was tested alone or in combination with the tumour promoters 12-O-tetradecanoyl phorbol-13-acetate (TPA) or DDT known inhibitors of intercellular communication. The results showed that CzM alone was without effect with regard to inhibition of metabolic cooperation but potentiated the response induced by TPA, an effect not noticed with DDT. These results suggest different mechanisms of action of TPA and DDT on metabolic cooperation and support the hypothesis that with calcium CaM may be of importance for drug-induced inhibition of intercellular communication and tumour promotion.

Effects of fatty acids on gap junctional communication: possible role in tumor promotion by dietary fat

Dietary lipids, in particular unsaturated fat, promote the development of many experimental tumors. However, no mechanisms to fully explain these effects have been elucidated. Recent reports, which we summarize here, suggest a role for gap junction-mediated intercellular communication in the process of tumor promotion. We also review tumor-promoting effects of dietary fat on experimental, particularly mammary, carcinogenesis. Our main focus is to review recent data examining the inhibitory effects of unsaturated fatty acids on metabolic cooperation in Chinese hamster V79 cells. These data suggest that inhibition of junctional communication may be involved mechanistically in the promotion of tumors by high levels of dietary unsaturated fat. Finally, potential mechanisms by which unsaturated fatty acids inhibit metabolic cooperation are examined.

Carbon tetrachloride at hepatotoxic levels blocks reversibly gap junctions between rat hepatocytes

Electrical coupling and dye coupling between pairs of rat hepatocytes were reversibly reduced by brief exposure to halogenated methanes (CBrCl3, CCl4, and CHCl3). The potency of different halomethanes in uncoupling hepatocytes was comparable to their hepatotoxicity in vivo, and the rank order was the same as that of their tendency to form free radicals. The effect of carbon tetrachloride (CCl4) on hepatocytes was substantially reduced by prior treatment with SKF 525A, an inhibitor of cytochrome P-450, and by exposure to the reducing reagent beta-mercaptoethanol. Halomethane uncoupling occurred with or without extracellular calcium and did not change intracellular concentrations of calcium and hydrogen ions or the phosphorylation state of the main gap-junctional protein. Thus the uncoupling appears to depend on cytochrome P-450 oxidative metabolism in which free radicals are generated and may result from oxidation of the gap-junctional protein or of a regulatory molecule that leads to closure of gap-junctional channels. Decreases in junctional conductance may be a rapid cellular response to injury that protects healthy cells by uncoupling them from unhealthy ones.

Characterization of a rat liver epithelial cell line to detect inhibitors of metabolic cooperation

A normal rat liver epithelial cell line, with phenotype characteristics of "oval" cells (WB-F344), was examined for its ability to perform gap-junctional intercellular communication as measured by metabolic cooperation. To test for gap-junctional intercellular communication, 6-thioguanine-sensitive cells were cocultivated with 6-thioguanine-resistant cells. It was found that the recovery of 6-thioguanine-resistant cells depended on the densities of the 6-thioguanine-sensitive cells. Higher densities of 6-thioguanine-sensitive cells reduced the recovery of 6-thioguanine-resistant cells. These observations demonstrate that rat liver epithelial cells could metabolically cooperate, implying they could perform gap-junctional intercellular communication. Two tumor-promoting organochlorine pesticides, aldrin and dieldrin, were potent inhibitors of metabolic cooperation for these cells, but 12-0-tetradecanoyl-phorbol-13-acetate and teleocidin, known mouse skin tumor promoters, were not significantly effective in inhibiting metabolic cooperation. The results suggest that these cells might provide the basis for an in vitro assay specifically to study liver tumor promoters.

Effects of tumor promoters, genotoxic carcinogens and hepatocytotoxins on mouse hepatocyte intercellular communication

Intercellular communication via gap junctions may be an important mechanism of cellular growth control. Tumor promoters can inhibit intercellular communication between cultured cells, while genotoxic carcinogens apparently lack this capability. The inhibition of intercellular communication by tumor promoters may be an essential mechanism by which tumor promotion occurs in vivo. In this study, the liver tumor promoters phenobarbital, lindane (1,2,3,4,5,6-hexachlorocyclohexane, gamma-isomer), DDT (1,1-Bis[4-chlorophenyl]-2,2,2-trichloroethane), Aroclor 1254 (a polychlorinated biphenyl mixture) and dieldrin inhibited intercellular communication between male B6C3F1 mouse hepatocytes in primary culture. Intercellular communication was detected as the passage of [5-3H]uridine nucleotides from pre-labelled donor hepatocytes to non-labelled recipient hepatocytes. Mouse hepatocyte intercellular communication was also inhibited by the skin tumor promoter TPA (12-0-tetradecanoyl-phorbol-13-acetate), but not by the bladder tumor promoter saccharin. The genotoxic hepatocarcinogens dimethylnitrosamine, diethylnitrosamine, benzo[a]pyrene and 2-acetylaminofluorene, and the hepatocytotoxins bromobenzene, acetaminophen, carbon tetrachloride, chloroform and methotrexate had no effect on mouse hepatocyte intercellular communication at non-cytotoxic levels. These results suggest that the ability to inhibit mouse hepatocyte intercellular communication is an effect specific to tumor promoters.

Epigenetic promoting effects of butylated hydroxyanisole

Studies on the potential genotoxicity and epigenetic effects of BHA are reviewed. BHA was not DNA-reactive, but studies showed that it did have an effect on membrane systems, blocking exchange between hepatocytes and epithelial cells. The available data strongly suggest that BHA is an epigenetic carcinogen that produces forestomach neoplasms through a promoting effect. Standard food safety procedures are suggested as appropriate for setting safe levels of exposure.

Intercellular communication of transformed and non-transformed rat liver epithelial cells. Modulation by TPA

Gap-junctional intercellular communication of transformed and non-transformed rat liver epithelial cell lines was compared using a dye transfer method in the presence and absence of 12-O-tetradecanoylphorbol 13-acetate (TPA). Whereas non-transformed cells (IAR 20, non-tumorigenic in newborn rats and in nude mice) showed very high communication capacity throughout a culture period of 3 weeks, transformed cells (IAR 6-1, tumorigenic in newborn rats and in nude mice) were less able to communicate. Similar correlation between intercellular communication and expression of transformed phenotypes were also found in newly cloned epithelial cell lines, IAR 27E and IAR 27F. When TPA was added to culture medium at 100 ng/ml, intercellular communication in all lines tested was reduced within 60 min. However, communications recovered completely from the effect within 10 h after addition of TPA. Further addition of TPA to the cultures every 24 h for 3 weeks had no effect on intercellular communication (measured 30 min after each TPA addition), suggesting that a single application of TPA made these cells refractory to further doses. A known stimulator of gap-junctional communication, db-cAMP, also increased dye transfer in IAR 20 and IAR 6-1 cells. TPA added to db-cAMP-treated cultures of IAR 20 and IAR 6-1 cells inhibited intercellular communication, suggesting that cAMP is not an antagonist of the effect of TPA on intercellular communication in these cell lines. These results are in sharp contrast to those obtained with the fibroblast cell line BALB/c 3T3, in which db-cAMP antagonized TPA effect and inhibition by TPA of intercellular communication was transient only when administered during their growth phase, and was stable and continuous when TPA was applied at confluence, and suggest that TPA may not be an effective tumour promoter in rat liver.

Reversible inhibition of intercellular junctional communication by glycyrrhetinic acid

Intercellular gap-junctional communication was measured using metabolic co-operation in co-cultures of argininosuccinate synthetase-deficient and argininosuccinate lyase-deficient human fibroblasts. 18-alpha-glycyrrhetinic acid (AGA) was found to inhibit communication by more than 95% at concentrations as low as 2 microM. Concentrations up to 100 microM were not cytotoxic over a period of 2 hours. Communication inhibition was of rapid onset and was readily reversible. Communication remained continuously yet reversibly blocked in cells cultured in the presence of AGA for 20 days. The related compounds 18-beta-glycyrrhetinic acid and carbenoxolone also caused communication inhibition. The effect is probably not mediated via mineralocorticoid or glucocorticoid receptors since aldosterone and glucocorticoids had no effect on communication. AGA thus has properties of a useful inhibitor in the study of intercellular junctional communication.

Effects of a teratogen on [3H]uridine nucleotide transfer between human embryonal cells and on gap junctions

Intercellular communication (IC) mediated by gap junctions (gj) occurs during embryonal development and appears to be important for normal differentiation through the exchange of morphogenetic signalling substances. Disruption of IC by chemicals may induce abnormal development resulting from failed cell-cell interactions. It was established in the present study that genotypically normal human embryonal palate mesenchyme (HEPM) cells displayed IC in cell culture and that the transfer of [3H]uridine nucleotides was inhibited by the potent embryotoxin and teratogen 12-O-tetradecanoyl-phorbol-13-acetate (TPA). IC was mediated by gj which were revealed by freeze-fracture and electron microscopy. Quantitative morphometric analysis showed that inhibition of IC by TPA coincided with a significant reduction in the number of gj. The observations suggest that inhibition of IC by the teratogen TPA may be one among the many mechanisms believed to be responsible for the induction of abnormal development by chemical teratogens.

The effect of tumor promoters and antagonists on in vitro directional migration of 10T1/2 mouse embryo cells

In vitro directional migration of 10T1/2 fibroblasts is partially inhibited by TPA but not by its non-promoting analogues. Other tumor promoters, e.g., phenobarbital, saccharin, and benzoylperoxide had no measurable effect when added in concentrations known to affect in vitro two-step transformation or intercellular communication. Inhibitors of in vitro transformation do not affect migration, except for dexamethasone, which inhibited it. Hence, there is no evidence for a general correlation between tumor promoting potential and inhibition of in vitro directional migration.

Identification of genotoxic and epigenetic carcinogens in liver culture systems

Liver culture systems are available for identifying the DNA reactivity of carcinogens and a cell membrane effect, which appears to be associated with neoplasm-promoting ability. Using these and other approaches, carcinogens can be categorized as genotoxic or epigenetic. This distinction has implications for risk assessment.

Inhibited intercellular communication as a mechanistic link between teratogenesis and carcinogenesis

Teratogenesis and carcinogenesis share many characteristics, leading to the speculation that they may also share pathogenic mechanisms. Direct intercellular communication mediated by membrane junctions is known to occur between a variety of cells and may play an important role in the control of cell growth and differentiation. Inhibition of junctional communication may be a mechanism common to both teratogenesis and carcinogenesis whereby cells and tissues are diverted from their normal differentiation paths. The multistage model of carcinogenesis predicts that the irreversibly initiated cell is at least partially regulated by the surrounding cells of a tissue, and that the initiated cell remains inactive until stimulated to proliferate by a tumor promotor. Tumor promoters may release the initiated cell from control of the surrounding tissue by interrupting intercellular communication, since many tumor promoters have now been shown to interfere with junctional communication in cultured mammalian cells. Furthermore, many tumorigenic cells have compromised junctional communication abilities. Similarly, it has been reasoned that the cells of an embryo must be able to communicate with each other to define tissue specificity and pattern formation, and to coordinate morphogenetic events. Many studies have chronicled alterations in junctional communication that occur coincident with major developmental events and some studies suggest that junctional communication may be modified at boundaries of morphogenetic fields. A recent in vivo study has provided evidence that inhibition of junctional communication may interfere with embryonic development, and several teratogens are known to interrupt junctional communication in mammalian cells in culture. These observations suggest that inhibition of junctional intercellular communication may be a shared mechanism of carcinogenesis and teratogenesis.

The use of sister-chromatid exchange in Chinese hamster primary lung cell cultures to measure genotoxicity

Primary cell cultures derived from Chinese hamster lung (CHL) were established, and their response for the induction of sister-chromatid exchange (SCE) by direct- and indirect-acting mutagens was characterized. An increase in SCE frequency was induced in CHL cells by 3-methylcholanthrene (MCA), benzo[a]pyrene (BaP), and 2-aminoanthracene (2AA). The SCE frequency increased slightly after exposure to cyclophosphamide, but did not respond to the hepatocarcinogen dimethylnitrosamine (DMN). A slight increase in SCE frequency by DMN was observed in the CHL system with use of Aroclor-1254-induced rat liver homogenate fraction (S9). This response to DMN in CHL cells was lower than that seen when CHO cells were the target in the presence of S9. At low (1) and high (20) passages, the CHL cells responded with a similar dose-related increase in SCE frequency to direct- (ethyl methanesulfonate, EMS) and indirect-(MCA) acting mutagens. This response indicates that even after prolonged culturing in vitro, the cells retained the ability to metabolically activate xenobiotic promutagens. The induction of SCE by MCA occurred at concentrations that also induced macromolecular binding. SCE induction was also examined in primary lung cell cultures from animals exposed by nose-only inhalation to MCA aerosol. A significant increase in SCE frequency above controls was observed in cells from animals after a single exposure to MCA. No detectable increase in SCE frequency was observed after repeated inhalation exposures. Because CHL cells are of lung origin and showed metabolic activity, the CHL system appears to be appropriate for study of the genotoxic potential of inhaled compounds.

Inhibition of metabolic cooperation in Chinese hamster V79 cells by various organic solvents and simple compounds

Gap-junctional intercellular communication is a biological process implicated in the regulation of cell proliferation and differentiation. Metabolic cooperation between 6-thioguanine-sensitive and resistant Chinese hamster cells, in vitro, has been used as a means to detect chemicals which can inhibit this form of intercellular communication. To further characterize this in vitro system as a potential screening assay for potential teratogens, tumor promoters and reproductive toxicants, a series of common solvents as well as other chemicals representing eight different functional groups, i.e., alcohols with straight or side chains, glycols, ketones, esters, ethers, phenols, aldehydes, amines and amino compounds and oxygen-heterocyclic compounds, were tested for their ability to inhibit colony-formation and to inhibit metabolic cooperation. A wide range of effects were observed which suggested a structure/activity relationship between a chemical's ability to inhibit gap junction-mediated intercellular communication and the cytotoxicity of a chemical. Possible mechanisms affecting the modulation of gap junctional communication by these chemicals are discussed.

Polybrominated biphenyls are nongenotoxic and produce an epigenetic membrane effect in cultured liver cells

Polybrominated biphenyls (PBB) were studied for their genotoxic and epigenetic effects in cultured liver cells. PBB did not elicit DNA repair synthesis in rat, mouse, or hamster hepatocytes in primary cultures and did not cause mutations at the hypoxanthine-guanine phosphoribosyl transferase locus in a line of rat liver epithelial cells or in human fibroblasts cocultivated with rat hepatocytes as an activating system. In contrast, PBB inhibited intercellular molecular exchange between rat hepatocytes and liver epithelial cells indicating an epigenetic membrane effect. These data are consistent with the interpretation that PBB act as neoplasm promoters in the production of rodent liver neoplasms.

Hormone-induced intercellular signal transfer dissociates cyclic AMP-dependent protein kinase

We used co-cultures of porcine ovarian granulosa cells and mouse adrenocortical tumor cells (Y-1) to examine the kinetics of contact-dependent intercellular signal transfer and to assess the molecular mechanisms employed by this process. Exposure to follicle-stimulating hormone (FSH) caused cAMP-dependent protein kinase dissociation in granulosa cells and, with time, in Y-1 cells if, and only if, they contacted a responding granulosa cell. Y-1 cells close to a granulosa cell but not touching it failed to respond similarly. In reciprocal experiments, co-cultures were stimulated with adrenocorticotropic hormone (ACTH). Y-1 cells dissociated protein kinase as did granulosa cells in contact with Y-1 cells; however, granulosa cells that were not in contact with Y-1 cells failed to respond to the hormone. Fluorogenic steroids were secreted by Y-1 cells cultured alone and stimulated with ACTH, but were not secreted by cultures exposed to FSH. Neither hormone caused fluorogenic steroid production by granulosa cells. On the other hand these steroids were secreted in co-cultures stimulated with ACTH and to a lesser degree in co-cultures exposed to FSH. Autoradiography revealed that I125-FSH bound only to granulosa cells, never to Y-1 cells, even if they were in contact with an ovarian cell. The possibility of cell fusion was tested by experiments in which Y-1 cell membranes were labeled with cationized ferritin. These cells were then placed in co-culture with ovarian granulosa cells that had previously been allowed to ingest latex spheres. At regions of gap junctions between Y-1 and granulosa cells ferritin remained attached to the adrenal cell membrane and was never observed to migrate to the granulosa cell membrane. From these data, we conclude that hormone specific stimulation of one cell type leads to protein kinase dissociation in heterotypic partners only if they contact a hormone responsive cell. This signal transfer is bidirectional, exhibits temporal kinetics and occurs in the absence of apparent cell fusion. The only structural feature connecting Y-1 and granulosa cells were gap junctions implying they provided the communication channels; however, alternative mechanisms cannot be excluded. We have not established the identity of the signal being transferred although cAMP is a logical candidate.