Changes in gap-junction permeability, phosphorylation, and number mediated by phorbol ester and non-phorbol-ester tumor promoters in rat liver epithelial cells

Acute inhibition of spontaneous uterine contractions by an estrogenic polychlorinated biphenyl is associated with disruption of gap junctional communication

An estrogenic polychlorinated biphenyl, 4-hydroxy-2',4', 6'-trichlorobiphenyl (4-OH-TCB), inhibits oscillatory uterine contractions immediately. Because increased gap junction formation is associated with the development of synchronized uterine contractions at term, we examined whether the inhibitory effect of 4-OH-TCB on spontaneous oscillatory contractions was due to the disruption of gap junctional communication. The effect of 4-OH-TCB on gap junctional communication was determined by intercellular Lucifer yellow dye transfer in primary cultures of myometrial myocytes isolated from midgestation rats. Intercellular dye transfer was inhibited by 4-OH-TCB or 17beta-estradiol in a concentration-dependent manner. The inhibitory effect of 4-OH-TCB on intercellular dye transfer was reversed by tetraethylammonium (TEA). To examine effects on uterine contraction, longitudinal uterine strips were excised from midgestation rats and placed in muscle baths for isometric force measurement. Spontaneous uterine oscillation was suppressed by 4-OH-TCB or 17beta-estradiol. The inhibitory effects of 4-OH-TCB and 17beta-estradiol on spontaneous oscillations were counteracted by TEA but were not affected by a calcium ionophore (A23187) or a calcium-dependent potassium channel blocker (apamin). These results suggest that the acute inhibition of spontaneous oscillatory contractions by an estrogenic polychlorinated biphenyl may result from the disruption of intercellular communication.

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.

Platelet-derived growth factor-induced disruption of gap junctional communication and phosphorylation of connexin43 involves protein kinase C and mitogen-activated protein kinase

Previously we showed a rapid and transient inhibition of gap junctional communication (GJC) by platelet-derived growth factor (PDGF) in T51B rat liver epithelial cells expressing wild-type platelet-derived growth factor beta receptors (PDGFrbeta). This action of PDGF correlated with the hyperphosphorylation of the gap junction protein connexin43 (Cx43) and required PDGFrbeta tyrosine kinase activity, suggesting the participation of protein kinases and phosphatases many of which are activated by PDGF treatment. In the present study, two such kinases, namely protein kinase C (PKC) and mitogen-activated protein kinase (MAPK), are investigated for their possible involvement in PDGF-induced closure of junctional channels and Cx43-phosphorylation. Down-regulation of PKC-isoforms by 12-O-tetradecanoylphorbol-13-acetate or pretreatment with the PKC inhibitor calphostin C, completely blocked PDGF action on GJC and Cx43. Activation of MAPK correlated with PDGF-induced Cx43 phosphorylation, and prevention of MAPK activation by PD98059 eliminated the PDGF effects. Interestingly, elimination of GJC recovery by cycloheximide was associated with a sustained activated-MAPK level. Based on these results we postulate that the activation of PKC and MAPK are required in PDGF-mediated Cx43 phosphorylation and junctional closure.

Cooperative effects of v-myc and c-Ha-ras oncogenes on gap junctional intercellular communication and tumorigenicity in rat liver epithelial cells

The objective of this study was to isolate and partially characterize several rat liver epithelial cell clones containing myc, ras and myc/ras oncogenes in order to study their roles in apoptosis and to test the hypothesis that gap junctional intercellular communication is necessary for apoptosis in solid tissues and that the loss of junctional communication leads to tumorigenesis. The co-transfection of the myc and ras oncogenes in the normal rat liver epithelial cell line (WB-F344) resulted in a loss of functional channels and normal growth regulation; cell-cell communication was significantly decreased and tumorigenicity determined in adult male F344 rats was induced. We examined cell growth properties, gap junctional intercellular communication (GJIC), using the scrape-loading-dye transfer and fluorescence-redistribution-after-photobleaching assays, and tumorigenicity in a series of normal and v-myc-, c-Ha-ras- and v-myc/c-Ha-ras-transfected WB-F344 cell lines. The c-Ha-ras- and the v-myc/c-Ha-ras-transduced cell lines appeared distinctly different from the other lines, having spindle-shaped morphology, shorter generation time and contact insensitivity. On the other hand, the normal WB-F344 cell line and the v-myc-transduced cell line showed excellent GJIC. Moreover, the c-Ha-ras-transduced cell lines displayed decreasing levels of GJIC associated with their increasing tumorigenicity. The v-myc/c-Ha-ras-transformed cell lines showed the lowest levels of GJIC and were also the most tumorigenic. These findings suggest that the reduction of GJIC in c-Ha-ras- and v-myc/c-Ha-ras-transformed WB-F344 cells is linked to their tumorigenic potential. These cell lines should provide valuable tools to study the role of GJIC in apoptosis during tumorigenesis.

Neoplastic phenotype of gap-junctional intercellular communication-deficient WB rat liver epithelial cells and its reversal by forced expression of connexin 32

Gap-junctional intercellular communication (GJIC) is involved in cellular growth control and is often reduced in neoplastic cells. In this study, four GJIC-deficient rat liver epithelial cell lines (WB-aB1, WB-bA2, WB-cD6, and WB-dA2) were examined for altered growth and tumorigenicity in comparison with their GJIC-competent parental cell line, WB-F344. WB-aB1 cells were also forced to express connexin 32 (Cx32) by transduction with a Cx32 cDNA retroviral expression vector to help determine whether the restoration of GJIC could reverse their neoplastic phenotype. WB-aB1 and WB-bA2 cells had faster population doubling times (PDTs) and higher saturation densities (SDs) than did WB-F344 cells. In contrast, the growth of WB-cD6 and WB-dA2 cells was not significantly different from that of WB-F344 cells. WB-aB1 and WB-bA2 cells formed tumors in male F344 rats, but WB-cD6 and WB-dA2 cells did not. After transduction of WB-aB1 cells with Cx32, four stable clones (WB-a/32-3, -8, -9, and -10) were isolated that had GJIC levels of 5.2%, 44.5%, 69.8%, and 90.5%, respectively. The growth of poorly coupled clones 3 and 8 was similar to that of parental WB-aB1 cells, but the growth of well-coupled clones 9 and 10 was similar to that of WB-F344 cells. The tumorigenicity of WB-a/32-9 and WB-a/32-10 cells was also significantly lower than that of WB-aB1 cells. Our results suggest that reduced GJIC contributes to neoplastic transformation of WB cells, that additional changes are necessary, and that restoration of GJIC by forced Cx32 protein expression can suppress the neoplastic phenotype of these cells.

Expression of connexin32 and connexin43 gap junction proteins and E-cadherin in human lung cancer

We used immunohistochemical staining to examine the expression of the gap junction proteins connexin32 and connexin43 and of the intercellular adhesion molecule, E-cadherin, that is thought to be a prerequisite for gap junctional intercellular communication (GJIC), in 24 specimens of human lung cancer. Connexin32 was not found in cancer tissue and there were significantly fewer spots of connexin43 in the poorly differentiated versus the well differentiated (P = 0.0005) and moderately differentiated (P = 0.0002) adenocarcinomas and in the poorly differentiated versus the well differentiated (P = 0.0182) and moderately differentiated (P = 0.004) squamous cell carcinomas of the lung. E-Cadherin was expressed in all but three cases of poorly differentiated non-small cell lung cancer that showed a heterogeneously decreased expression of E-cadherin. These findings suggest that GJIC is decreased in poorly differentiated non-small cell lung cancer.

Regulation of gap junctions by protein phosphorylation

Gap junctions are constituted by intercellular channels and provide a pathway for transfer of ions and small molecules between adjacent cells of most tissues. The degree of intercellular coupling mediated by gap junctions depends on the number of gap junction channels and their activity may be a function of the state of phosphorylation of connexins, the structural subunit of gap junction channels. Protein phosphorylation has been proposed to control intercellular gap junctional communication at several steps from gene expression to protein degradation, including translational and post-translational modification of connexins (i.e., phosphorylation of the assembled channel acting as a gating mechanism) and assembly into and removal from the plasma membrane. Several connexins contain sites for phosphorylation for more than one protein kinase. These consensus sites vary between connexins and have been preferentially identified in the C-terminus. Changes in intercellular communication mediated by protein phosphorylation are believed to control various physiological tissue and cell functions as well as to be altered under pathological conditions.

Changes in the gap junctional intercellular communication in renal tubular epithelial cells in vitro treated with renal carcinogens

Gap junctional intercellular communications (GJIC) are known as the channels for the direct transfer of cytoplasmic molecules between neighboring cells and are lost during transformation of normal cells. To study the function and the molecular mechanism for the loss of GJIC, the effects of dimethylnitrosamine, KBrO3 and FeSO4 x 7H2O, which are known as chemical tumor promoters of the kidney on the GJIC function and the expression of connexin 43 of Madin-Darby canine kidney (MDCK) epithelial cells, were examined. These tumor promoters inhibited the GJIC in MDCK cells. The expression of connexin 43 mRNA and connexin 43 protein was not altered by these treatments, whereas immunocytochemical study revealed that the distribution of connexin 43 protein was changed from the cell surface to the cytoplasma. These data suggest that blockage of GJIC in MDCK cells treated with renal carcinogens support the hypothesis that loss of GJIC might be important in renal carcinogenesis.

Rapid disruption of gap junctional communication and phosphorylation of connexin43 by platelet-derived growth factor in T51B rat liver epithelial cells expressing platelet-derived growth factor receptor

Gap junctional communication (GJC) between contacting cells has been postulated to be involved in the regulation of cell proliferation. This suggestion stems from numerous studies showing modulation of GJC by agents that influence cellular proliferation. Platelet-derived growth factor (PDGF), a strong mitogen, inhibits GJC in many cell types. To understand the molecular nature of the signal transduction pathway responsible for the GJC blockade, T51B rat liver epithelial cells, which lack endogenous PDGF receptor (PDGFr), were infected with a retrovirus containing either wild-type full-length cDNA of human PDGFr beta (Kin+) or a mutant PDGFr beta lacking receptor tyrosine kinase activity (Kin-). PDGF caused a complete but transient interruption of cell communication in Kin+ cells within 15-20 min of addition. This interruption of GJC was not associated with a gross destabilization of gap junction plaques but with the phosphorylation of connexin43 (Cx43), the only known gap junction protein expressed in these cells. These effects were exhibited in either control T51B cells or in Kin- cells, indicating a requirement of the receptor tyrosine kinase activity. Further examination revealed that the newly phosphorylated Cx43 then undergoes a rapid degradation utilizing the lysosomal pathway resulting in a decreased total Cx43 protein level. The re-establishment of GJC following PDGF treatment was dependent on protein synthesis. This report describes a suitable cell system which is currently being utilized for the characterization of the PDGF signaling pathway responsible for the inhibition of GJC.

Altered function, localization and phosphorylation of gap junctions in rat liver epithelial, IAR 20, cells after treatment with PCBs or TCDD

Three different PCB-congeners 3,4,5,3',4'-pentachlorobiphenyl (IUPAC no. 126), 2,4,5,2',4',5'-hexachlorobiphenyl (IUPAC no. 153) and 2,4,5,3',4'-pentachlorobiphenyl (IUPAC no. 118) were investigated for possible structure-activity relationships in altering gap junction intercellular proteins. All tested PCB-congeners and TCDD decreased the gap junctional intercellular communication in IAR 20 cells, but at different treatment periods, suggesting different modes of action. The presence of the Cx43-P(2) band, a phosphorylated isoform of Cx43, was associated with a functional communication. A reduced Cx43 mRNA level was noted after 48 h of exposure with PCB 126, PCB 118 and TCDD. In summary, the non dioxin-like PCB 153 can decrease gap junctional intercellular communication rapidly by reducing the phosphorylated isoform of Cx43, whereas the dioxin-like PCB 126 and TCDD reduce the communication slowly by decreasing the mRNA level of Cx43, resulting in a reduced Cx43 protein level (which includes the P(2)-band). The mixed inducing PCB-congener, PCB 118, can act both as the dioxin-like and the non dioxin-like PCBs in gap junction regulation.

Stimulation of cell proliferation and inhibition of gap junctional intercellular communication by linoleic acid

The effect of linoleic acid (LA) on gap-junction permeability, connexin 43 mRNA level, protein level, and phosphorylation, and the numbers of gap-junctional membrane plaques were studied in the rat liver epithelial cell line WB-F344 to determine whether changes in these parameters correlated with the enhanced cell growth and the inhibition of gap-junction function. When cultured in a medium with low serum (1%), these cells exhibited a slower growth rate than in the high serum medium (7%). Addition of linoleic acid (0.01-3 mg/ml) to the low serum medium increased the growth rate and inhibited gap junctional intercellular communication (GJIC) in a dose-dependent manner. In a comparison of short-term and long-term treatments with LA, GJIC in short-term treated (1 h) WB cells was inhibited at 3 mg/ml LA but readily recovered by washing and removing LA from cells, whereas GJIC in long-term treated (6 days) WB cells did not recover by washing and removing LA from WB cells. Western blot analysis of connexin 43 showed that a short-term incubation with linoleic acid increased the relative amount of unphosphorylated connexin 43 protein, but a long-term incubation with linoleic acid decreased the amount of unphosphorylated connexin 43 protein and increased the relative amount of hyperphosphorylated connexin 43 protein. Connexin 43 and p53 mRNA levels decreased in a time- and dose-dependent manner in linoleic acid-treated cells. These results suggest that growth stimulation and gap junctional intercellular communication inhibition of rat liver epithelial cells by linoleic acid may be mediated in part through modulation of p53 expression and function.

Role of disrupted gap junctional intercellular communication in detection and characterization of carcinogens

Results from short-term tests for carcinogens and our advanced knowledge on cellular and molecular mechanisms of carcinogenesis strongly suggest that carcinogens do not induce genetic changes necessarily by directly interacting with DNA. Therefore, it is not surprising to see that many carcinogens are not detectable by available genetic toxicology tests. Thus, it has become necessary to study nongenotoxic mechanisms of carcinogenesis and to provide methods to predict those carcinogens which escape from conventional mutation tests. One possible nongenotoxic mechanism of carcinogenesis which is supported by abundant experimental evidence is inhibition of gap junctional intercellular communication. Many, but not all, tumor-promoting agents have been shown to inhibit the communication of cultured cells as well as in vivo. Molecular mechanisms of gap junctional intercellular communication control revealed that connexin (gap junction) genes form a family of tumor suppressor genes. Control mechanisms of expression as well as function of connexins are vulnerable to various carcinogenic insults, notably to nongenetoxic carcinogens. Thus, studies on the role of connexins in cell growth and carcinogenesis may prove to be useful for establishing a mechanism-based test to detect certain types of nongenotoxic carcinogens.

Localization and function of the connexin 43 gap-junction protein in normal and various oncogene-expressing rat liver epithelial cells

Clones of rat liver epithelial cells genotypically altered by mutation or by a variety of oncogenes were analyzed by microinjection-dye transfer, immunofluorescence confocal microscopy, and western blotting to determine at what level and to what degree these transformations disrupted gap-junctional intercellular communication (GJIC) mediated by connexin 43 (Cx43). Compared with normal rat liver epithelial cells, cells neoplastically transformed by src, neu, ras, and myc/ras all displayed reduced degrees of GJIC, reduced levels of membrane-associated Cx43 plaques, and hypophosphorylation of Cx43. Confocal analysis further demonstrated that the Cx43 protein was localized, at least in part, to the nucleus rather than to the plasma membrane in the src- and neu-transformed cells, but not in the ras- and myc/ras-transformed cells. Nuclei isolated from WB-neu cells showed substantially higher levels of Cx43 on western blotting than did nuclei from WB-neo control cells, supporting the idea that the nuclear-localized immunopositive material detected by confocal microscopy was Cx43 protein. In a GJIC-deficient mutant rat liver epithelial cell line containing normal numbers of plasma membrane-localized Cx43 plaques that appeared to be reduced in size, the Cx43 protein was also found to be hypophosphorylated. Cells overexpressing myc, on the other hand, displayed a normal degree of GJIC, increased levels of plasma membrane-localized Cx43 plaques, and hyperphosphorylation of the Cx43 protein. Cells expressing raf, previously shown to be GJIC competent, showed Cx43 immunostaining patterns similar to those in normal cells, whereas a cell line established from a tumor induced by injection of these raf-expressing cells into a mouse showed a marked reduction in GJIC and plasma membrane-associated Cx43 immunostaining. These data suggest that altered localization of the gap-junction protein Cx43, mediated in part by changes in the phosphorylation of this protein, contributes to the disruption of GJIC in neoplastically transformed rat liver epithelial cells.

Gap-junction disassembly and connexin 43 dephosphorylation induced by 18 beta-glycyrrhetinic acid

Gap-junction channels connect the interiors of adjacent cells and can be arranged into aggregates or plaques consisting of hundreds to thousands of channel particles. The mechanism of channel aggregation into plaques and whether plaques can disaggregate are not known. Many carcinogenic and tumor-promoting chemicals have been identified that inhibit cell-cell gap-junctional coupling. Here, we provide morphological evidence that 18 beta-glycyrrhetinic acid (18 beta-GA), a saponin isolated from licorice root that is an inhibitor of gap-junctional communication, caused the disassembly of gap-junction plaques in WB-F344 rat liver epithelial cells. This effect was dose (5-40 microM) and time dependent (1-4 h treatment). Gap-junction channels in WB-F344 cells are comprised of connexin 43 (Cx43), and the protein is phosphorylated to a species known as Cx43-P2 coincident with the assembly of channels into plaques. Consistent with this, the disassembly of plaques induced by 18 beta-GA was correlated with decreases in Cx43-P2 levels and increases in nonphosphorylated Cx43. Biochemical evidence indicated that these changes in the P2 and NP forms of Cx43 represented 18 beta-GA-induced dephosphorylation of Cx43-P2 and not its degradation or the inhibition of Cx43-NP phosphorylation. Okadaic acid and calyculin A, which are inhibitors of type 1 and type 2A protein phosphatases, prevented the dephosphorylation of Cx43, suggesting that one or both of these phosphatases were involved in Cx43 dephosphorylation. These data indicate that 18 beta-GA causes type 1 or type 2A protein phosphatase-mediated Cx43 dephosphorylation coincident with the disassembly of gap-junction plaques.

Tumor promoters induce inhibition of gap junctional intercellular communication in mouse epidermal cells by affecting the localization of connexin43 and E-cadherin

The molecular and histological effects of tumor promoters on gap junctional intercellular communication (GJIC) were studied in three mouse epidermal cell types, representing different stages of tumor formation. GJIC was inhibited by most of the studied compounds (l-ethionine, d-limonene, o-anisidine, clofibrate, Aroclor 1260 and 1,1'-(2,2,2-trichloroethylidene)bis(4-chlorobenzene) (DDT)) except NaF and phenobarbital (PB). Whatever their effect on GJIC, most of the studied compounds increased the phosphorylation state of the gap junction protein expressed in these cells, connexin43 (Cx43), as shown by Western analysis. All agents with GJIC inhibiting capacity changed the intensity of the immunofluorescent staining of Cx43 on the membrane of the cells, whereas NaF and PB had no effect on Cx43 immunostaining. No association could be found between the type of change in Cx43 localization (changed membrane and/or cytosolic staining) and Cx43 phosphorylation or GJIC inhibition. Because the cell adhesion molecule E-cadherin also regulates GJIC, the effects of tumor promoters on E-cadherin protein and localization were studied. No quantitative change could be observed in E-cadherin protein content of cells treated with any of the selected agents. However, all agents which decreased GJIC, affected E-cadherin immunostaining of the membrane, while PB and NaF had no effect. These results show that an association exists between inhibition of GJIC and localization of both connexin43 and E-cadherin protein, but not with Cx43 phosphorylation.

1995 STP Young Investigator Award recipient. Increased rate of apoptosis correlates with hepatocellular proliferation in Fischer-344 rats following long-term exposure to a mixture of groundwater contaminants

Apoptosis was evaluated in the livers of Fischer-344 rats following observations of increased hepatocellular proliferation from exposures, at low parts per million (ppm) levels, to a drinking water mixture of 7 groundwater contaminants during a 6-mo time-course study. The 7 chemicals used are among the most frequently detected contaminants associated with hazardous waste sites: arsenic, benzene, chloroform, chromium, lead, phenol, and trichloroethylene. Significant increases in 5-bromo-2'-deoxyuridine hepatocellular labeling were present in a unique pattern surrounding large hepatic veins (0.5-2.0 mm). This did not appear to be a regenerative response due to cytotoxicity, as assessed by the absence of increased plasma enzyme activity and the absence of hepatocellular lesions. Immunohistochemical staining for apoptosis, using the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) method showed patterns of labeling in treated animals that directly correlated to areas of increased hepatocyte proliferation. Apoptotic activity was maximum at the 1-mo exposure time point, whereas proliferating hepatocytes reached a maximum rate at the 10-day time point. This may have been triggered as a compensatory response to the increased cell proliferation or as a protective response to remove cells with altered DNA due to chemical mixture exposure. The principal findings of this paper are that (a) apoptosis directly correlated with changes in cell proliferation: (b) observed effects were produced by repeated exposures to a relatively low-level chemical mixture; and (c) the TUNEL method detected apoptotic cells at very early and late stages, potentially increasing the observable time period for apoptosis.

Inhibition of gap junctional intercellular communication in heptachlor- and heptachlor epoxide-treated normal human breast epithelial cells

Based on the concern of organochlorides in the environment and in human tissue, this study was designed to determine whether various noncytotoxic levels of heptachlor and heptachlor epoxide could inhibit, reversibly, gap junctional intercellular communication in human breast epithelial cells (HBEC). Cytotoxicity and gap junctional intercellular communication (GJIC) were evaluated by lactate dehydrogenase assay and fluorescence redistribution after photobleaching analysis, respectively. Both heptachlor and heptachlor epoxide were noncytotoxic up to 10 microg/ml. At this concentration, heptachlor and heptachlor epoxide inhibited GJIC of normal human breast epithelial cells after 1 h treatment. Within a 24 h treatment with heptachlor and heptachlor epoxide at 10 microg/ml, recovery of GJIC had not returned. GJIC completely recovered after a 12 h treatment of 1 microg/ml heptachlor epoxide, but it did not recover after a 24 h treatment of 1 microg/ml heptachlor. RT-PCR and Western blots were analyzed to determine whether the heptachlor or heptachlor epoxide might have altered the steady-state levels of gap junction mRNA and/or connexin protein levels or phosphorylation state. No significant difference in the level of connexin 43 (Cx43) message between control and heptachlor-treated cells was observed. Western blot analyses showed hypophosphorylation patterns in cells treated with 10 microg/ml heptachlor and heptachlor epoxide for 1 h with no recovery within 24 h. Immunostaining of Cx43 protein in normal HBEC indicated that heptachlor and heptachlor epoxide caused a loss of Cx43 from the cell membranes at noncytotoxic dose levels. Taken together, these results suggest that heptachlor and heptachlor epoxide can alter GJIC at the post-translational level, and that, under the conditions of exceeding a threshold concentration in the breast tissue containing 'initiated' cells for a long time and not being counteracted by anti-tumor-promoting chemicals, they could act as breast tumor promoters.

The inhibitory effects of boldine, glaucine, and probucol on TPA-induced down regulation of gap junction function. Relationships to intracellular peroxides, protein kinase C translocation, and connexin 43 phosphorylation

The naturally occurring antioxidant boldine and its di-methoxy analogue glucine, as well as the drug antioxidant probucol, all inhibit TPA-induced downregulation of gap junctional intercellular communication in WB-F344 rat liver epithelial cells in dose-dependent manners. The compounds were essentially 100% inhibitory to the effect of TPA (10 nM) at 50 microM each. Analysis of the mechanism of the antitumor promotive action of these agents in vitro revealed that boldine and probucol (both at 10 microM) totally inhibited the TPA-induced accumulation of intracellular oxidants. Additionally, boldine, glaucine, and probucol, each at 50 microM, inhibited TPA-induced translocation of protein kinase C (PKC) to the particulate fraction of the cells, with concomitant inhibition of TPA-induced hyperphosphorylation of gap junctional connexin 43 (cx43) and TPA-induced internalisation of cx43 protein from the plasma membrane of the cells. None of the compounds inhibited the binding of (3H)-PDBu to TPA-specific binding sites in the cells. The results indicate that antioxidant molecules, irrespective of structure, possess common antitumor promotive potential in this model of gap junctional intercellular communication. The data also indicate that the compounds may interfere with the promotive function of TPA, at least in part, by the destruction of oxidants within the cells. Xanthine oxidase was excluded as a major source of such intracellular oxidants because allopurinol (50 microM) did not significantly affect either the accumulation of oxidants in the cells or the downregulation of gap junctional communication in response to TPA. Taken together, these data also suggest that TPA-induced oxidants play a role in the translocation of PKC to cellular membranes and it is at this level where the antioxidants may interfere in TPA-induced downregulation of gap junctional function.

Human papillomavirus type 16 E5 protein affects cell-cell communication in an epithelial cell line

The human papillomavirus type 16 (HPV16) E5 protein is considered to have weak oncogenic properties, and its function in infected human keratinocytes is unknown. HPV16 E5 protein has been found to localize to the Golgi apparatus and the plasma membrane. To analyze the effect of E5 on plasma membrane properties, cells from the human keratinocyte cell line HaCaT were transfected with the HPV16 E5 open reading frame under the control of an inducible promoter. The gap junction-mediated cell-cell communication of E5- and vector-transfected cells was analyzed by microinjection of Lucifer yellow to measure dye coupling of the cells. A strong impairment of dye transfer in E5-transfected cells but not in vector-transfected cells was observed, with more than 80% dye transfer inhibition 40 min after injection. This impairment correlated with dephosphorylation of connexin 43, the major gap junctional protein in HaCaT cells. Furthermore, the dye coupling inhibition was not the result of differentiation of the E5-expressing cells, since no overexpression of cytokeratin 1 or filaggrin, markers of HaCaT cell differentiation, could be observed. These results therefore strongly suggest a correlation between expression of the HPV16 E5 open reading frame, impairment of gap junction-mediated dye coupling, and dephosphorylation of connexin 43.