Changes in gap junction protein (connexin 32) gene expression during rat liver carcinogenesis

Diversity of Intercellular Communication Modes: A Cancer Biology Perspective

From the moment a cell is on the path to malignant transformation, its interaction with other cells from the microenvironment becomes altered. The flow of molecular information is at the heart of the cellular and systemic fate in tumors, and various processes participate in conveying key molecular information from or to certain cancer cells. For instance, the loss of tight junction molecules is part of the signal sent to cancer cells so that they are no longer bound to the primary tumors and are thus free to travel and metastasize. Upon the targeting of a single cell by a therapeutic drug, gap junctions are able to communicate death information to by-standing cells. The discovery of the importance of novel modes of cell-cell communication such as different types of extracellular vesicles or tunneling nanotubes is changing the way scientists look at these processes. However, are they all actively involved in different contexts at the same time or are they recruited to fulfill specific tasks? What does the multiplicity of modes mean for the overall progression of the disease? Here, we extend an open invitation to think about the overall significance of these questions, rather than engage in an elusive attempt at a systematic repertory of the mechanisms at play.

The role of caveolin-1 in tumors of the brain - functional and clinical implications

BACKGROUND

Caveolin-1 (cav-1) is the major structural protein of caveolae, the flask-shaped invaginations of the plasma membrane mainly involved in cell signaling. Today, cav-1 is believed to play a role in a variety of disease processes including cancer, owing to the variations of its expression in association with tumor progression, invasive behavior, metastasis and therapy resistance. Since first detected in the brain, a number of studies has particularly focused on the role of cav-1 in the various steps of brain tumorigenesis. In this review, we discuss the different roles of cav-1 and its contributions to the molecular mechanisms underlying the pathobiology and natural behavior of brain tumors including glial, non-glial and metastatic subtypes. These contributions could be attributed to its co-localization with important players in tumorigenesis within the lipid-enriched domains of the plasma membrane. In that regard, the ability of cav-1 to interact with various cell signaling molecules as well as the impact of caveolae depletion on important pathways acting in brain tumor pathogenesis are noteworthy. We also discuss conversant causes hampering the treatment of malignant glial tumors such as limited transport of chemotherapeutics across the blood tumor barrier and resistance to chemoradiotherapy, by focusing on the molecular fundamentals involving cav-1 participation.

CONCLUSIONS

Cav-1 has the potential to pivot the molecular basis underlying the pathobiology of brain tumors, particularly the malignant glial subtype. In addition, the regulatory effect of cav-1-dependent and caveola-mediated transcellular transport on the permeability of the blood tumor barrier could be of benefit to overcome the restricted transport across brain barriers when applying chemotherapeutics. The association of cav-1 with tumors of the brain other than malignant gliomas deserves to be underlined, as well given the evidence suggesting its potential in predicting tumor grade and recurrence rates together with determining patient prognosis in oligodendrogliomas, ependymomas, meningiomas, vestibular schwannomas and brain metastases.

Connexins and Pannexins: Important Players in Tumorigenesis, Metastasis and Potential Therapeutics

Since their characterization more than five decades ago, gap junctions and their structural proteins-the connexins-have been associated with cancer cell growth. During that period, the accumulation of data and molecular knowledge about this association revealed an apparent contradictory relationship between them and cancer. It appeared that if gap junctions or connexins can down regulate cancer cell growth they can be also implied in the migration, invasion and metastatic dissemination of cancer cells. Interestingly, in all these situations, connexins seem to be involved through various mechanisms in which they can act either as gap-junctional intercellular communication mediators, modulators of signalling pathways through their interactome, or as hemichannels, which mediate autocrine/paracrine communication. This complex involvement of connexins in cancer progression is even more complicated by the fact that their hemichannel function may overlap with other gap junction-related proteins, the pannexins. Despite this complexity, the possible involvements of connexins and pannexins in cancer progression and the elucidation of the mechanisms they control may lead to use them as new targets to control cancer progression. In this review, the involvements of connexins and pannexins in these different topics (cancer cell growth, invasion/metastasis process, possible cancer therapeutic targets) are discussed.

An update on minding the gap in cancer

This article is a report of the "International Colloquium on Gap junctions: 50Years of Impact on Cancer" that was held 8-9 September 2016, at the Amphitheater "Pôle Biologie Santé" of the University of Poitiers (Poitiers, France). The colloquium was organized by M Mesnil (Université de Poitiers, Poitiers, France) and C Naus (University of British Columbia, Vancouver, Canada) to celebrate the 50th anniversary of the seminal work published in 1966 by Loewenstein and Kanno [Intercellular communication and the control of tissue growth: lack of communication between cancer cells, Nature, 116 (1966) 1248-1249] which initiated studies on the involvement of gap junctions in carcinogenesis. During the colloquium, 15 participants presented reviews or research updates in the field which are summarized below.

Changes in expression of connexin 32, bile canaliculus-like structures, and localization of alkaline phosphatase in primary cultures of fetal rat hepatocytes

We devised an experimental design in primary cultures of fetal rat hepatocytes for studying hepatocyte differentiation over a short period. In the present study, hepatocytes were first cultured for 3 days in dexamethasone-supplemented medium and then for an additional 3 days in dexamethasone- or epidermal growth factor-supplemented medium. In hepatocytes cultured continuously in dexamethasone-supplemented medium, the expression of connexin 32 increased and bile canaliculus-like structures and localization of alkaline phosphatase in the plasma membrane around bile canaliculus-like structures were maintained. Few cells incorporated bromodeoxyuridine. On the other hand, in most of the hepatocytes cultured in epidermal growth factor-supplemented medium, the expression of connexin 32 was minimally recognized, bile canaliculus-like structures were shortened or eliminated, and alkaline phosphatase was localized as numerous fine spots throughout the cytoplasm. More than 20% of all hepatocytes incorporated bromodeoxyuridine.

The present study suggests that in hepatocytes, there is a close relationship among connexin 32 expression, the maintenance of bile canaliculus-like structures, and the localization of alkaline phosphatase to the plasma membrane around the bile canaliculus-like structures, and this indicates that the present experimental model is useful for studying hepatocyte differentiation over a short period.

Combinational treatment of gap junctional activator and tamoxifen in breast cancer cells

Tamoxifen is a drug of choice for endocrine-responsive breast tumor patients. However, tamoxifen resistance has become a major concern for the treatment of breast cancer. Combinational therapies of tamoxifen and different drugs are being frequently studied. In this study, we tested the efficacy of substituted quinolines (code name=PQ1; a gap junctional activator) in combination with tamoxifen in T47D cells. Colony growth assay was performed using soft agar to measure the colony growth, whereas cell proliferation was measured by the MTT assay in T47D cells. The level of Ki67, survivin, and BAX was measured using confocal microscopy along with western blot analysis. Apoptosis-bromodeoxyuridine triphosphate labeling was also examined in the induced treatment of T47D cells. We observed a 55% decrease in the colony growth in the presence of combination of PQ1 and tamoxifen, whereas tamoxifen alone had little effect. A combination of 10 micromol/l tamoxifen and 200 or 500 nmol/l PQ1 resulted in only 16% cell viability compared with controls at 48 h in T47D cells by the MTT assay. We found a significant increase in BAX protein at 1 h in the presence of 500 nmol/l PQ1 alone, 10 micromol/l tamoxifen alone, and the combination of PQ1 and tamoxifen. A two-fold increase was observed in active caspase 3 in the presence of combinational treatment of 10 micromol/l tamoxifen and 200 or 500 nmol/l PQ1. In addition, flow cytometric analysis showed a 50% increase in the number of apoptotic cells in the presence of the combination of tamoxifen and PQ1 compared with the control. Furthermore, the results show that combinational treatment of tamoxifen and PQ1 significantly reduces the expression of survivin in T47D cells. Gap junction inhibitor studies with carbenexolone were also performed confirming the role of gap junctions in cell proliferation and cell death. The combinational treatment of PQ1 and tamoxifen has a significant increase in BAX expression, caspase 3 activation, and DNA fragmentation. Tamoxifen alone and in combination with PQ1 showed a decrease in the expression of survivin, whereas PQ1 alone was shown to be independent of the survivin-mediated pathway. This suggests that an increase in gap junction activity can potentiate the effect of tamoxifen. The combinational treatment of tamoxifen and PQ1 also showed a significant decrease in cell viability compared with tamoxifen treatment alone. The gap junction inhibitor carbenexolone was shown to increase cell proliferation by increased cyclin D1 expression, MTT assay, and Ki67 expression. It further decreased cell death. This study shows for the first time that combinational treatment of tamoxifen and PQ1 (a gap junctional activator) can be used to potentiate apoptosis of T47D human breast cancer cells. Thus, a gap junctional activator, PQ1, could potentially alter either the length or dose of tamoxifen clinically used for breast cancer patients.

Development of a reference dose for the persistent congeners of weathered toxaphene based on in vivo and in vitro effects related to tumor promotion

Toxaphene is a mixture of chlorinated camphenes and bornanes that was produced and used in the United States until 1982. 1.3 million tons of toxaphene have been released worldwide. "Technical" toxaphene (TT) consists of a mixture of up to 800 different chemicals, known as congeners. TT weathers in the environment by both biotic and abiotic processes. The human body burden of toxaphene consists of only five persistent congeners that are not metabolized; three of these occur in considerably greater amounts than the other two. Because of the rapid metabolism and excretion of the non-persistent congeners, the persistent congeners that make up the human body burden most likely play a role in eliciting any potential adverse effects. EPA's toxicity assessment for TT is based on the occurrence of liver cancer in rodents, and considerable doubt exists whether this assessment is applicable to weathered toxaphene (WT). Using experimental results from European Union scientists, a reference dose (RfD) was developed for WT based on the three most persistent congeners that comprise the human body burden. The critical effect chosen was tumor promotion and this endpoint is considered protective for other endpoints as well. Although RfDs are typically derived for non-carcinogenic effects, the endpoint of tumor promotion is appropriate for RfD development because the experimental data suggest a dose threshold. The RfD for weathered toxaphene represented by the sum of the three major persistent congeners ( summation 3PC) is 2E-05 mg/kg-day. To apply this reference dose to a particular WT mixture, information is needed regarding the percentage of summation 3PC in the mixture.

Defective gap junctional intercellular communication in the carcinogenic process

Gap junctions are membrane structures made of intercellular channels which permit the diffusion from cytoplasm to cytoplasm of small hydrophilic molecules. Nearly 40 years ago, the loss of functional gap junctions has been described in cancer cells and led to the hypothesis that such type of intercellular communication is involved in the carcinogenesis process. From this time, a lot of data has been accumulated confirming that gap junctions are frequently decreased or absent in cancer cells whatever their tissue and species origins. Here, we review such data by insisting on the possible links existing between altered gap-junctional intercellular communication capacity (or the altered expression of their constitutive proteins, the connexins) and the stages of cancer progression in various cancer models. Then, we analyse particular aspects of the disturbance of connexin-mediated communication in cancer such as the cytoplasmic localization of connexins, the lack of heterologous communication between cancer cells and normal cells, the role of connexin gene mutations in cancer. In a separate part of the review, we also analyse the disturbance of gap-junctional intercellular communication during the late stages of cancer (invasion and metastasis processes).

Endotoxin unmasks the role of gap junctions in the liver

Gap junctions are thought to be necessary for proper tissue function. However, no clear hepatic phenotype has been described in patients lacking connexin 32 (Cx32), the principal gap junction in liver. To determine the physiological role of Cx32 in liver, we compared the response of wild type and Cx32-deficient mice to endotoxin, since this stress increases serum levels of hormones that bind to receptors that are asymmetrically distributed across the hepatic lobule. In hepatocyte couplets isolated from wild type mice, most hepatocytes could transfer microinjected dye to their neighbor even after treatment with endotoxin. Dye transfer was not observed in Cx32-deficient couplets. Treatment of hepatocyte couplets from wild type mice with vasopressin induced calcium (Ca(2+)) waves that crossed the couplets in a concentration-dependent fashion, but the delay in transmission was markedly prolonged at all concentrations in Cx32-deficient couplets. Expression of the vasopressin receptor and the inositol 1,4,5-trisphosphate receptor was not decreased by endotoxin or in Cx32-deficient couplets. Finally, endotoxin caused transient hypoglycemia and cholestasis in wild type animals, but hypoglycemia was slightly prolonged and cholestasis was much worse in Cx32-deficient mice treated with endotoxin. The hepatic response to endotoxin is markedly impaired in the absence of Cx32. Thus, an important role of gap junctions in the liver is to assure integrated and uniform tissue response in times of stress.

Disruption of beta-catenin pathway or genomic instability define two distinct categories of liver cancer in transgenic mice

BACKGROUND & AIMS

Human liver cancer can be divided into 2 categories that are characterized by activation of beta-catenin and genomic instability. Here we investigate whether similar categories exist among 5 transgenic models of liver cancer, including c-myc, transforming growth factor-alpha, E2F-1, c-myc/transforming growth factor-alpha, and c-myc/E2F-1 mice.

METHODS

The random amplified polymorphic DNA method was used to assess the overall genomic instability, and chromosomal loci affected by genomic alterations were determined by microsatellite analysis. beta-Catenin mutations and deletions were analyzed by polymerase chain reaction and sequencing screening. Cellular localization of beta-catenin and expression of alpha-fetoprotein, a prognostic marker of hepatocellular carcinoma, were investigated by immunohistochemistry.

RESULTS

Liver tumors from the transgenic mice could be divided into 2 broad categories characterized by extensive genomic instability (exemplified by the c-myc/transforming growth factor-alpha mouse) and activation of beta-catenin (exemplified by the c-myc/E2F-1 mouse). The c-myc/transforming growth factor-alpha tumors displayed extensive genomic instability with recurrent loss of heterozygosity at chromosomes 1, 2, 4, 6, 7, 9, 12, 14, and X and a low rate of beta-catenin activation. The genomic instability was evident from the early dysplastic stage and occurred concomitantly with increased expression of alpha-fetoprotein. The c-myc/E2F-1 tumors were characterized by a high frequency of beta-catenin activation in the presence of a relatively stable genome and low alpha-fetoprotein levels.

CONCLUSIONS

We have identified 2 prototype experimental models, i.e., c-myc/transforming growth factor-alpha and c-myc/E2F-1 mice, for the 2 categories of human hepatocellular carcinoma characterized by genomic instability and beta-catenin activation, respectively. These mouse models will assist in the elucidation of the molecular basis of human hepatocellular carcinoma.

Characterisation of the connexin32 promoter and changes in response element complexes in rat liver and hepatocytes during culture associated with oxidative stress

Hepatic gap junctional intercellular communication (GJIC), mediated principally by connexin 32, provides a mechanism for regulating multicellular activities between neighbouring cells. The control of Cx32 gene expression at the transcriptional level has been investigated in rat liver tissue and in primary rat hepatocytes during culture. Several response elements have been identified and characterised using the electrophoretic mobility shift assay. Nuclear protein extract prepared from rat primary hepatocytes cultured for 2 h gave a larger number of DNA-protein complexes than observed with extracts from liver in vivo, including complexes containing Sp1. In contrast, nuclear extracts prepared from primary rat hepatocytes cultured for 96 h, and subject to oxidative stress, gave altered DNA-protein complexes when compared to those from hepatocytes cultured for 2 h. These results indicate that culture conditions, known to cause a loss of connexin expression, can modulate the transcription of Cx32 in hepatocytes by affecting the regulatory trans/cis-interactions of redox-sensitive zinc finger proteins within the promoter.

Irsogladine malate up-regulates gap junctional intercellular communication between pancreatic cancer cells via PKA pathway

INTRODUCTION

Gap junctions (GJs) are intercellular channels that aid communication between coupling cells and may play a critical role in cell differentiation and growth. Connexins (Cxs) are structural proteins of GJs. Though several reports have demonstrated that Cx expression decreases in various malignant tumors, a pancreatic cancer cell line, PANC-1, was reported to express Cx43 mRNA. It is known that irsogladine malate (IM) can up-regulate gap junctional intercellular communication (GJIC). We examined the effects of IM on GJ between pancreatic cancer cells (PC cells) and the mechanism of GJ up-regulation.

METHODOLOGY

GJIC between PC cells (PANC-1) was evaluated by dye transfer methods. The expression of Cx43 was estimated by Western blot analysis with immunoprecipitation sample and immunohistochemical analysis. Intracellular cAMP level was estimated by enzyme-linked immunoassay.

RESULTS

IM increased cell coupling in a dose-dependent manner (0M-10 ). Western blot analysis of Cx43 revealed that PANC-1 cells expressed Cx43 protein. Treatment with IM was found to move localization of Cx43 immunoreactive spots from the cytoplasm to boundary lesions with neighboring cells, but no major change was seen in the phosphorylation state of Cx43. Intracellular cAMP level was increased by IM. The PKA inhibitor H-89 and adenylyl cyclase inhibitor SQ22536 inhibited the effects of IM.

CONCLUSION

These results suggest that IM up-regulates GJIC between PC cells via regulation of the PKA pathway. It also suggests a useful adjuvant of IM to pancreatic cancer therapy.

Intercellular communication and cell proliferation in precision-cut rat liver slices: effect of medium composition and DDT

Gap junctional intercellular communication (GJIC) and cell proliferation were studied in control and 1,1'-bis(p-chlorophenyl)-2, 2,2,-trichloroethane (DDT) treated precision-cut liver slices of rat by evaluating connexin 32 (Cx32) expression and 5-bromo-2'-deoxyuridine (BrdU) incorporation. In addition, the effect of different culture media (RPMI and WME) on control and DDT influenced Cx32 expression and cell proliferation was determined. Cx32 expression in control precision-cut liver slices was maintained during 8 h of culturing, but decreased after prolonged culturing. Control cell proliferation was significantly higher when WME was used as culture medium than when RPMI was used. In slices treated with DDT Cx32 expression was decreased. In slices cultured in RPMI medium, this decrease preceded a dose-dependent increase in cell proliferation. These results show the usefulness of precision-cut liver slices in studying cellular proliferation and intercellular communication.

Regulation of connexin32 and connexin43 gene expression by DNA methylation in rat liver cells

Gap junction proteins (connexins) are expressed in a cell-specific manner and expression is often reduced in neoplastic cells. We investigated the mechanisms of connexin32 (Cx32) and connexin43 (Cx43) expression in hepatic cells using MH1C1 rat hepatoma cells and freshly isolated, adult rat hepatocytes that express Cx32 but not Cx43 and WB-F344 rat liver epithelial cells that express Cx43 but not Cx32. Southern blotting after DNA restriction with MspI and HpaII indicated that two MspI/HpaII restriction sites in the Cx32 promoter (positions -147 and -847) were methylated in WB-F344 cells, but not in MH1C1 cells or hepatocytes. In contrast, an MspI/HpaII restriction site in the Cx43 promoter (position -38) was methylated in MH1C1 cells, but not in WB-F344 cells or hepatocytes. Transient transfection of the cell lines with connexin promoter-luciferase constructs indicated that the Cx32 promoter was 7-fold more active in MH1C1 cells and the Cx43 promoter was 5-fold more active in WB-F344 cells. These results suggest that transcription of Cx32 and Cx43 in hepatic cells is controlled by promoter methylation and by cell-specific transcription factors. Similar mechanisms may be involved in the reduced expression of these genes frequently observed in neoplastic cells.

Role of connexin (gap junction) genes in cell growth control and carcinogenesis

Gap junctional intercellular communication (GJIC) is considered to play a key role in the maintenance of tissue independence and homeostasis in multicellular organisms by controlling the growth of GJIC-connected cells. Gap junction channels are composed of connexin molecules and, so far, more than a dozen different connexin genes have been shown to be expressed in mammals. Reflecting the importance of GJIC in various physiological functions, deletion of different connexin genes from mice results in various disorders, including cancers, heart malformation or conduction abnormality, cataract, etc. The possible involvement of aberrant GJIC in abnormal cell growth and carcinogenesis has long been postulated and recent studies in our own and other laboratories have confirmed that expression and function of connexin genes play an important role in cell growth control. Thus, almost all malignant cells show altered homologous and/or heterologous GJIC and are often associated with aberrant expression or localization of connexins. Aberrant localization of connexins in some tumour cells is associated with lack of function of cell adhesion molecules, suggesting the importance of cell-cell recognition for GJIC. Transfection of connexin genes into tumorigenic cells restores normal cell growth, supporting the idea that connexins form a family of tumour-suppressor genes. Some studies also show that specific connexins may be necessary to control growth of specific cell types. We have produced various dominant-negative mutants of Cx26, Cx32 and Cx43 and showed that some of them prevent the growth control exerted by the corresponding wild-type genes. However, we have found that connexins 32, 37 and 43 genes are rarely mutated in tumours. In some of these studies, we noted that connexin expression per se, rather than GJIC level, is more closely related to growth control, suggesting that connexins may have a GJIC-independent function. We have recently created a transgenic mouse strain in which a mutant Cx32 is specifically overexpressed in the liver. Studies with such mice indicate that Cx32 plays a key role in liver regeneration after partial hepatectomy. A decade ago, we proposed a method to enhance killing of cancer cells by diffusion of therapeutic agents through GJIC. Recently, we and others have shown that GJIC is responsible for the bystander effect seen in HSV-tk/ganciclovir gene therapy. Thus, connexin genes can exert dual effects in tumour control: tumour suppression and a bystander effect for cancer therapy.

Increased P-glycoprotein messenger RNA stability in rat liver tumors in vivo

P-glycoproteins (Pgp) are comprised of a small family of plasma membrane proteins whose abundance in cultured cells is often associated with the multidrug resistance phenotype. Overexpression of Pgp has been observed in many types of human cancers, but the molecular basis for this overexpression has not been established. We have used primary monolayer cultures of adult rat hepatocytes and a stepwise model of rat liver carcinogenesis to study the regulation of Pgp gene expression. We observed a marked overexpression of Pgp, specifically the class II Pgp, in both systems. In addition, we observed that a number of unrelated genes including alpha-tubulin, beta-actin, gamma-actin, cytokeratin 8, cytokeratin 18, and c-myc are overexpressed in cultured hepatocytes, and they are also overexpressed during liver carcinogenesis and in transplantable tumors. Nuclear run-on assays showed no increase in the transcriptional activity of Pgp genes in transplantable liver tumors compared to normal liver. Studies of in vivo mRNA stability, however, revealed that all three Pgp mRNAs were relatively stable in transplantable liver tumors (t(1/2) > 12 h), in contrast to what was found in normal liver (t(1/2) < 2 h). In addition, mRNA for several other genes, including alpha-tubulin, c-myc, and cyclin D1, all appear to be stabilized in the tumors. These findings suggest that the overexpression of Pgp genes in rat liver tumors may be the result of a mechanism involving stabilization of a diverse group of mRNAs.

The ability to alter the gap junction protein expression outside GST-P positive foci in liver of rats was associated to the tumour promotion potency of different polychlorinated biphenyls

The results demonstrate different modes of action by a dioxin-like polychlorinated biphenyl (PCB 126) and a non dioxin-like PCB (PCB 153) in the alteration of connexin (cx) 26 and cx 32 expression outside GST-P positive foci in liver of female Sprague-Dawley rats, treated according to an initiation-promotion protocol. A decreased relative amount of immunopositive cx 26 and cx 32 spots in the parenchymal cell plasma membranes was observed after treatment with the potent tumour promoters PCB 126 or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). No reduction of cx 26 or cx 32 was noted after administration with the weaker tumour promoters PCB 153 or PCB 118 (PCB 118; both dioxin- and non dioxin-like). Additionally, we found that the down-regulation of connexins also occurred in rats treated with PCB 126 or TCDD without partial hepatectomy and initiation with nitrosodiethylamine. In summary, the results indicate that the ability to reduce the gap junction protein level in liver of rats can be associated to the tumour promotive potency of the different PCB-congeners and TCDD.

Prolonged decrease in hepatic connexin32 in chronic liver injury induced by carbon tetrachloride in rats

BACKGROUND/AIMS

Our previous study indicated that the amount of connexin32, the major gap-junctional protein of rat liver, is transiently reduced in acute liver injury after single administration of hepatotoxic chemicals. This study was designed to examine alteration in the expression of connexin32 in chronic liver injury, unassociated with hepatocyte proliferation.

METHODS

Rats were injected with carbon tetrachloride (CCI4, 0.5 ml/kg) twice a week for 12 weeks. After cessation of CCI4 injection, hepatic contents of connexin32 and its mRNA levels were measured by immunoblotting as well as immunohistochemical examination and by Northern-blot analysis.

RESULTS

The plasma alanine-aminotransferase activity was increased from 30 U/I to about 1000 U/l after 12 weeks of CCI4 injections, but recovered nearly to normal level in 7 days after cessation of the injection. Liver specimens 12 days after the last CCI4 injection appeared cirrhotic with a marked increase in fibrosis. Connexin32 contents in these livers decreased to about 37% of controls. The significant decrease in connexin32 content was sustained for at least 30 days and recovered to the control level by 60 days. The alteration of connexin32 content in chronically injured liver was confirmed immunohistochemically. The level of connexin32-mRNA, however, was not reduced, but rather increased by chronic injection of CCI4.

CONCLUSION

The results suggest that intercellular communication is disturbed in chronic liver injury, lasting even after recovery from the acute phase of injury. Since the mRNA levels of connexin32 were sustained, the prolonged decrease in connexin32 contents in these livers might be due to a post-transcriptional change that causes decrease in protein synthesis or a derangement of post-translational controls.

Perturbation in connexin 43 and connexin 26 gap-junction expression in mouse skin hyperplasia and neoplasia

To examine the possible role of gap junctions in mouse skin tumor progression, we generated a panel of mouse skin tissue samples exhibiting normal, hyperplastic, or neoplastic changes and characterized the expression of the gap-junction genes connexin 43 (Cx43) and connexin 26 (Cx26) by in situ hybridization and immunohistochemical analyses. In normal skin, these two gap junction genes were differentially expressed; Cx43 was found predominantly in the less differentiated lower spinous layers, whereas Cx26 was found in terminally differentiating upper spinous and granular layers. In hyperplastic epidermis exhibiting an expansion of the differentiated upper layer, i.e., epidermis with a thickened granular layer or in which the granular layer was replaced with keratinocytes exhibiting tricholemmal differentiation, expression of Cx43 and Cx26 remained segregated in the lower and upper spinous layers, respectively. However, in papillomas, Cx26 was localized in the lower but not upper spinous layer, an expression pattern identical to that of Cx43. In addition, the overall expression levels of both Cx43 and Cx26 appeared to be greatly elevated in the papillomas. It is interesting that such marked alteration in the pattern of Cx26 expression occurred within the context of hyperplastic changes histologically identical to those seen in the nonpapillomous hyperplasias. Interestingly, in neoplastic skin lesions containing a squamous cell carcinoma, Cx43 and Cx26 expression was extinguished. Moreover, expression of Cx43 was also significantly reduced in adjacent apparently nonneoplastic tissues. Overall, these observations show that perturbations in gap-junction gene expression are associated with skin hyperplasia and neoplasia. Such findings suggest a possible role for gap junctions in the malignant conversion of mouse epidermal cells.

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.

Connections with connexins: the molecular basis of direct intercellular signaling

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Lack of tumor-promoting effects of flavonoids: studies on rat liver preneoplastic foci and on in vivo and in vitro gap junctional intercellular communication

Possible tumor-promoting activity of four flavonoids, quercetin (QC), tangeretin (TG), flavone (FO), and flavanone (FN), was examined in a rat liver short-term carcinogenesis assay as well as with in vivo and in vitro assays of inhibition of gap junctional intercellular communication (GJIC). Rat hepatocarcinogenesis was induced by aflatoxin B1 treatment followed by a selection phase (2-acetylaminofluorene treatment and partial hepatectomy), then treatment with or without test chemicals (in vivo studies of antipromotion were not performed). Using glutathione S-transferase placental form (GST-P)-positive foci, we compared the effects of flavonoids (at 1,000 ppm in the diet) with the effects of phenobarbital (PB) on the occurrence of liver preneoplastic lesions. In addition, we studied the effects of flavonoids on GJIC in the livers derived from these experiments and in two types of cultured cells. No significant difference in the number and area of GST-P-positive foci was found after one or three months of treatment between any flavonoid group and control group. In the positive control group, PB markedly increased the numbers and areas of preneoplastic lesions at three months. Whereas PB also decreased by 60% the average size of lucifer yellow dye spread in slices of liver parenchyma free of preneoplastic lesions among the different flavonoids, only TG decreased the dye transfer in vivo: by 30% at one month and 50% at three months. With the dye transfer assay applied to a rat liver epithelial cell line (REL) and the Chinese hamster V79 metabolic cooperation assay, none of the tested flavonoids (< or = 25 microM) inhibited GJIC. Conversely, protective properties were seen for some of the compounds in antipromotion in vitro studies, because TG and FN enhanced the dye transfer in REL cells and FO, TG, and QC partly prevented the inhibition of metabolic cooperation by 12-O-tetradecanoylphorbol-13-acetate. Thus, taken together, our results suggest that QC, FO, and FN do not show tumor-promoting activity. Concerning TG, some discrepancies in the in vivo data are observed. Some of them (GJIC inhibition in liver slices) are probably more relevant to promotion of hepatocarcinogenesis.

Role of blocked gap junctional intercellular communication in non-genotoxic carcinogenesis

Gap junctional intercellular communication mediates the transfer of small molecules from the cytoplasm of one cell to that of neighbouring cells. Connexins are the proteins that form the channels responsible for this type of communication. Aberrant expression and function of connexins are often found in cells exposed to tumor-promoting agents and during carcinogenesis, both in cell culture systems and in tissues freshly removed directly from patients and exposed animals. Transfection of connexin genes into tumorigenic cells often exerts negative growth control, suggesting that connexins act as a family of tumor-suppressor genes. Connexin gene mutations appear to be the cause of two human diseases, i.e. X-linked Charcot-Marie-Tooth syndrome and visceroatrial heterotaxia. Connexin genes are therefore important for the maintenance of homeostasis and thus their dysfunction could lead to various forms of disease.

Clofibrate-induced neoplastic development in the rat liver is associated with decreased connexin 32 expression but not with a co-ordinated shift in expression of marker enzymes

Altered enzyme phenotype and expression of connexin 32 (Cx32), a gap junction protein were studied during the development of rat liver tumors induced by the non-genotoxic carcinogen, clofibrate. (1) In contrast to previous findings for nitrosamine-induced lesions, preneoplastic enzyme-altered foci (EAF) and neoplastic nodules (NN) lacked any clear association with degree of altered enzyme expression because of an almost complete negativity for GST-P and GGT. (2) Immunohistochemically demonstrated Cx32 spots on the hepatocyte membranes showed a clear decrease in clofibrate-induced lesions. (3) Naturally occurring EAF demonstrating GST-P and/or GGT positivity did not show a significant decrease of Cx32 counts suggesting a reversible nature. Therefore, the Cx32 decrease appears closely linked to progression of hepatocarcinogenesis irrespective of the enzyme phenotype of neoplastic focal lesions and the carcinogens used for their induction.

Expression of gap junction protein connexin 32 and E-cadherin in human hepatocellular carcinoma

The expression of connexin 32, a major gap junction protein, and E-cadherin, an intercellular adhesion molecule that is supposed to be involved in the regulation of gap junctional intercellular communications, was examined immunohistochemically in seven specimens of human hepatocellular carcinoma and surrounding non-carcinomatous tissues. We found that the number of connexin 32-positive spots per mm2 was significantly less in hepatocellular carcinoma tissues than in the surrounding non-carcinomatous cirrhotic tissues (4360 +/- 3390/mm2 vs 10,030 +/- 3690/mm2; p < 0.01). The number in the latter was also significantly less than that in normal controls (23,560 +/- 4170/mm2). E-cadherin was expressed in all non-carcinomatous hepatocytes as well as carcinomatous cells, except for one case of Edmondson's grade III hepatocellular carcinoma. These results suggest an impairment of cell-to-cell communications in human hepatocellular carcinomas.

The gap junctional intercellular communication is no prerequisite for the stabilization of xenobiotic metabolizing enzyme activities in primary rat liver parenchymal cells in vitro

In primary monocultures of adult rat liver parenchymal cells (PC), the activities of the xenobiotic metabolizing enzymes microsomal epoxide hydrolase (mEHb), soluble epoxide hydrolase (sEH), glutathione S-transferases (GST), and phenolsulfotransferase (ST) were reduced after 7 d to values below 33% of the initial activities. Furthermore, the gap junctional intercellular communication (GJIC), measured after microinjection by dye transfer, decreased from 90% on Day 1 to undetectable values after 5 d in monoculture. Co-culture of PC with nonparenchymal rat liver epithelial cells (NEC) increased (98% on Day 1) and stabilized (82% on Day 7) the homotypic GJIC of PC. Additionally, most of the measured xenobiotic metabolizing enzyme activities were well stabilized over 1 wk in co-culture. Because GJIC is one of several mechanisms playing an important role in cell differentiation, the importance of GJIC for the stabilization of xenobiotic metabolizing enzymes in PC was investigated. PC in monoculture were, therefore, treated with 2% dimethyl sulfoxide (DMSO), a differentiation promoting factor, and 1,1,1-trichloro-2,2,-bis (p-chlorophenyl) ethane (DDT) (10 micrograms/ml), a liver tumor promotor and inhibitor of GJIC, was given to co-cultures of PC with NEC. DMSO significantly stabilized (68% on Day 7), while DDT significantly inhibited (8% on Day 7) homotypic GJIC of PC in the respective culture systems. In contrast, the activities of mEHb, sEH, GST, and ST were not affected in the presence of DMSO or DDT. These results lead to the assumption that the differentiation parameters measured in this study (i.e., homotypic GJIC and the activities of xenobiotic metabolizing enzymes) are independently regulated in adult rat liver PC.

Basal promoter of the rat connexin 32 gene: identification and characterization of an essential element and its DNA-binding protein

The connexin 32 (Cx32) gene, a member of a multigene family, is expressed preferentially in the liver. The basal promoter complex of the rat Cx32 gene was previously localized to a 146-bp region (map positions [mp] -179 to -34) immediately upstream of the first exon. To investigate the biochemical factors contributing to the basal promoter activity, nuclear protein-DNA complexes within this region (mp -177 to -106) were investigated by using a DNA mobility shift assay. Three DNA-protein binding activities, termed Cx32-B1, Cx32-B2, and Cx32-B3, were identified with nuclear protein extracts from hepatoma cell lines, HuH7 and FAO-1. However, only Cx32-B2 binding activity was detected in nuclear protein extract from normal rat liver tissue. This activity was significantly more abundant in rat liver tissue than in hepatoma cell lines and tissues from various other organs. By using methylation interference footprinting, the Cx32-B2 complex was localized to the region between mp -152 and -127 and a DNA probe containing this region bound to a 60-kDa protein in rat liver nuclear extracts. Mutation of two nucleotides in the Cx32-B2 binding site abrogated the formation of the Cx32-B2 protein-DNA complex and significantly reduced the transcriptional activity of the Cx32 promoter. These results indicate that the Cx32-B2 complex is an essential component of the rat Cx32 basal promoter and is likely a major factor in the preferential expression of this gene in the liver.

Morphological alterations of gap junctions in phalloidin-treated rat livers

Morphological alterations in the pattern of liver cell gap junctions were examined in phalloidin-treated rats to assess the role of gap junctions in experimental intrahepatic cholestasis. Double-labelled fluorescent staining of gap junctions and F-actin were performed using a monoclonal antibody against rat hepatocyte connexin 32 and rhodamine-phalloidin. Immunoelectron microscopy, using the anti-connexin 32 antibody, freeze-fracture replica electron microscopy, and conventional electron microscopy were also performed. In phalloidin-treated rat livers, the specific immunofluorescent staining of connexin 32 was markedly decreased in the pericentral area after 1 day of phalloidin treatment and, after 5 days of phalloidin treatment, there was a decrease in connexin 32 staining in the entire hepatic lobule. On the other hand, F-actin staining at the cell periphery and at the bile canaliculi was markedly increased in the pericentral area of the hepatic lobule after 1 day of phalloidin treatment and in the entire lobule after 5 days of treatment. Immunoelectron microscopy showed that both sides of the cytoplasmic domains of gap junctions were stained with anti-connexin 32 antibody in controls, whereas, in cholestatic rats, only one side of the cytoplasmic domain of some gap junctions was stained with anti-connexin 32 antibody after 1 or 3 days of phalloidin treatment. No gap junctions were observed after 5 days of phalloidin treatment either by freeze-fracture replica electron microscopy or by conventional electron microscopy. These results indicate that with phalloidin treatment, hepatocyte gap junctions decrease, first in the pericentral area, and finally throughout the entire lobule.

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.

Altered homologous and heterologous gap-junctional intercellular communication in primary human liver tumors associated with aberrant protein localization but not gene mutation of connexin 32

Gap-junctional intercellular communication (GJIC) in 20 primary human liver tumors with different degrees of malignancy has been studied at the functional and molecular levels. When GJIC capacity was determined by dye-transfer assay performed directly with freshly removed tumor tissue, significant reduction was found in all samples, regardless of their morphology. In addition, a selective lack of GJIC between tumor and surrounding non-tumorous cells was observed in some cases, probably due to the physical separation between them resulting from encapsulation of tumors. There was, however, no essential change in the level of expression of the major liver gap-junction protein, connexin (cx) 32, in liver tumors as measured by Northern and Western blot analyses. Immunohistochemical study revealed aberrant localization of cx 32 in the majority of malignant liver tumors. Instead of cytoplasmic membrane localization at intercellular contacts, cx 32 was detected mainly either intracytoplasmically or in plasma membrane free from contact with other cells. We did not detect any mutation in the coding sequence of the cx 32 gene from any of the human liver tumors we tested. Thus it is likely that the aberrant localization of cx 32 in tumor cells is due to disruption of the mechanisms for establishment of this protein into gap-junction plaques, rather than to structural abnormality of the cx 32 protein itself. Another member of the connexin family, cx 43, not detectable in non-tumorigenic hepatocytes, was expressed in several tumors, especially in invasive areas, but was detected in only a few tumor cells and was localized intracytoplasmically, suggesting that cx 43 protein is not involved in GJIC in the tumors.

Multiple mechanisms are responsible for altered expression of gap junction genes during oncogenesis in rat liver

Although several abnormalities in gap junction (GJ) structure and/or function have been described in neoplasms, the molecular mechanisms responsible for many of the alterations remain unknown. The identification of a family of GJ proteins, termed connexins, prompted this study of connexin32 (Cx32), connexin26 (Cx26) and connexin43 (Cx43) expression during rat hepatocarcinogenesis. Using antibody, cDNA and cRNA probes, we investigated connexin mRNA and protein expression in preneoplastic and neoplastic rat livers. In normal liver, Cx32 is expressed in hepatocytes throughout the hepatic acinus, Cx26 is restricted to periportal hepatocytes, and Cx43 is expressed by mesothelial cells forming Glisson's capsule. Most preneoplastic altered hepatic foci generated by diethylnitrosamine (DEN) initiation and either phenobarbital (PB) or 2,3,7,8-dichlorodibenzo-p-dioxin (TCDD) promotion exhibited decreased Cx32 or increased Cx26 staining. Foci from either protocol failed to display Cx43 immunoreactivity. In the majority of PB-promoted foci, Cx32 immunoreactivity decreased independently of changes in mRNA abundance. Continuous thymidine labeling, following cessation of PB promotion, showed that downregulation of Cx32 staining is reversible in foci that are promoter-dependent for growth, but irreversible in lesions that are promoter-independent for growth. Hepatic neoplasms from rats initiated with DEN and promoted with PB or TCDD also displayed modified connexin expression. While all 24 neoplasms studied were deficient in normal punctate Cx32 and Cx26 staining, altered cellular localization of these proteins was apparent in some tumors. Immunoblotting of crude tissue extracts revealed that neoplasms with disordered Cx32 staining showed immunoreactive bands with altered electrophoretic mobility. These observations show that hepatomas may downregulate Cx32 expression through changes in the primary structure of Cx32 or by post-translational modifications. Northern blotting of total tumor mRNAs failed to demonstrate consistent changes in the abundance of Cx32, Cx26 or Cx43 transcripts. Some tumors expressed steady-state transcripts without observable immunoreactivity, indicating that some hepatomas downregulate connexin immunoreactivity independently of mRNA abundance. Increased levels of Cx43 mRNA and protein were found in several neoplasms, but immunostaining was always localized to nonparenchymal cells. Areas of bile duct proliferation and cholangiomas displayed Cx43 staining, whereas, cholangiocarcinomas were deficient in immunoreactivity. These findings show that alterations in the expression of connexins, by either downregulation or differential induction, represent common modifications during hepatocarcinogenesis. Although our results imply that connexins represent useful markers for the boundary between tumor promotion and progression, preneoplastic and neoplastic rat hepatocytes fail to use a common mechanism to modify connexin expression.

Identification of proximal and distal regulatory elements of the rat connexin32 gene

We have investigated the genetic basis of the transcriptional regulation of the rat connexin32 gene which encodes the major gap junction protein in rat liver. Primer extension analysis of RNA isolated from adult rat liver identified multiple initiation sites clustered between -110 bp and -50 bp upstream from the translation start codon. An approx. 760 bp genomic DNA fragment upstream of the first exon which included the mRNA start sites was cloned 5' to the luciferase reporter cassette in p19LUC to yield pCx32-800/-33-LUC. Transfection of pCx32-800/-33-LUC resulted in a 200-fold increase in luciferase activity above p19LUC in the human hepatoma cell line HuH-7. Using a series of vectors containing 5' deletions of the 760 bp fragment, a basal promoter was localized between -179 bp and -134 bp. Three DNA:protein complexes were identified with the basal promoter fragment by DNA mobility shift assay using nuclear extracts from HuH-7 cells. Two of the DNA-binding complexes appeared to be related to the transcription factor Sp1. In addition, three DNase hypersensitive (HS) sites were identified within the genomic locus of connexin32 in adult rat liver. Two of the DNase HS regions behaved as silencer elements with both the native promoter and a heterologous promoter in HuH-7 cells. These data demonstrate that (1) the active promoter responsible for rat connexin32 mRNA transcription is located upstream of the first exon, (2) a basal promoter region was localized to a 50 bp region which formed multiple DNA:protein complexes, and (3) multiple proximal and distant regulatory elements are involved in the expression of connexin32.

Immunohistochemical analysis of rat liver using a monoclonal antibody (HAM8) against gap junction

Four monoclonal antibodies were raised against crude gap junction fractions of rat liver to clarify the distribution of gap junctions during animal development and to analyze gap junction expression in vivo and the polarity of hepatocytes in vitro. Among the monoclonal antibodies obtained, HAM8 antibody recognized the 27-kDa rat liver gap junction protein connexin 32. This antibody recognized gap junctions at the contiguous faces of hepatocytes, and the antigen was also observed in exocrine pancreas and salivary gland but not in kidney, heart, esophagus, or thymus. HAM8 did not react with amphibian or fish liver, heart, esophagus, stomach, or intestine as assessed via the immunofluorescence method on frozen sections. A few hepatocytes and many hemopoietic cells were seen in rat fetal liver at 15 days of gestation. HAM8 antigen was expressed on some hepatocytes but not on any hemopoietic cells. As the fetus grew, the number of hepatocytes in the liver increased gradually, together with the amount of HAM8 antigen. The distribution of HAM8 antigen at 25 days after birth was similar to that in adult liver. When the expression of HAM8 antigen was examined in primary cultured hepatocytes using the immunofluorescence method, the antigen was observed clearly between the hepatocytes. However, most of the HAM8 antigen on the free surface of hepatocytes disappeared within 4 hr. HAM8 antigen was not expressed on AH-7974 rat hepatoma cells when they formed small islets in the rat peritoneal cavity or within the liver. When HAM8 IgG antibody was injected intravenously, the HAM8 signal was expressed in the liver.

Differential changes in expression of gap junction proteins connexin 26 and 32 during hepatocarcinogenesis in rats

We examined expressions of the gap junction proteins, connexin 26 (Cx26) and 32 (Cx32), in preneoplastic and neoplastic lesions during rat hepatocarcinogenesis. A marked reduction in the number of Cx32-positive gap junctions was observed in 17% of the glutathione S-transferase placental form-positive foci, whereas 44% of the foci showed increased expression of Cx26. Most hyperplastic nodules exhibited decreased expression of Cx32, whereas 16% of the nodules showed increased expression of Cx26. In hepatocellular carcinomas, expressions of both Cx32 and Cx26 were significantly reduced. These results suggest that the expressions of Cx32 and 26 are differentially regulated during hepatocarcinogenesis, and that the decrease in Cx32 expression occurs earlier, whereas reduction in Cx26 expression occurs later in association with promotion and progression of carcinogenesis.

Expression of Cx26, Cx32 and Cx43 gap junction proteins in normal and neoplastic human tissues

This report concerns the expression of the gap-junction proteins Connexin (Cx)26, 32 and 43 in different malignant and non-malignant human tissues. Affinity-purified polyclonal antibodies against Cx26, 32 and 43 were used for immunohistochemical as well as immunoblot analysis. Cx32, the major gap-junction protein in rat and mouse liver, was detected in human liver and kidney. By contrast, Cx43 was expressed in epithelial and mesenchymal tissues and Cx26 was detected in different epithelia. Whereas all of the benign tumors studied, and some malignant ones, showed stable expression of gap-junction proteins, breast cancer, renal-cell cancer and sarcomas showed a significant decrease in gap-junction proteins as opposed to normal tissue. Cx43, not detected in human normal liver, was found in human hepatocellular carcinoma and Cx26, not detected in human adult skin, was observed in tissue samples of basal-cell carcinoma. In immunoblot analysis, Cx32 antibodies recognized a 27-kDa protein in human liver and hepatocellular carcinoma. A 43-kDa polypeptide was detected in human kidney, renal-cell carcinoma, normal breast, connective tissue of invasive-duct carcinoma of the breast and hepatocellular carcinoma.

Molecular mechanisms of TPA-mediated inhibition of gap-junctional intercellular communication: evidence for action on the assembly or function but not the expression of connexin 43 in rat liver epithelial cells

We found that a rat liver epithelial cell line (IAR 20) expresses connexin 43, the major cardiac gap-junction protein, but not connexin 26 or connexin 32, major liver gap-junction proteins. The effects of TPA on connexin 43 expression in IAR 20 were investigated using northern blot analysis, western blot analysis, and an immunofluorescence technique. Gap-junctional intercellular communication (GJIC) in this cell line decreased within 60 min of 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment and recovered after 24 h. The number of immunofluorescence spots of connexin 43 on IAR 20 was closely related to the change in GJIC induced by TPA. However, TPA did not change the level of mRNA measured by northern blot analysis. Moreover, connexin 43 protein expression analyzed by western blotting suggests that connexin 43 proteins were still present in TPA-treated cells at a similar level. These results suggest that GJIC of these rat liver epithelial cells was mediated by connexin 43 protein and that TPA inhibited GJIC by inhibiting posttranslational processing of connexin 43 proteins, e.g., localization or assembly.