Cell-cell communication and growth control of normal and cancer cells: evidence and hypothesis

Selectively-Packaged Proteins in Breast Cancer Extracellular Vesicles Involved in Metastasis

Cancer-derived extracellular vesicles are known to play a role in the progression of the disease. In this rapidly-growing field, there are many reports of phenotypic changes in cells following exposure to cancer-derived extracellular vesicles. This study examines the protein contents of vesicles derived from three well-known breast cancer cell lines, MCF-7, MDA-MB-231 and T47D, using peptide-centric LC-MS/MS and cytokine multiplex immunoassay analysis to understand the molecular basis of these changes. Through these techniques a large number of proteins within these vesicles were identified. A large proportion of these proteins are known to be important in cancer formation and progression and associated with cancer signaling, angiogenesis, metastasis and invasion and immune regulation. This highlights the importance of extracellular vesicles (EVs) in cancer communications and shows some of the mechanisms the vesicles use to assist in cancer progression.

Characterization of Human NK Cell-Derived Exosomes: Role of DNAM1 Receptor In Exosome-Mediated Cytotoxicity Against Tumor

Despite the pivotal role of natural killer (NK) cells in defenses against tumors, their exploitation in cancer treatment is still limited due to their reduced ability to reaching tumor sites and the inhibitory effects of tumor microenvironment (TME) on their function. In this study, we have characterized the exosomes from IL2- or IL15-cultured human NK cells. Both cytokines induced comparable amounts of exosomes with similar cargo composition. Analysis of molecules contained within or exposed at the exosome surface, allowed the identification of molecules playing important roles in the NK cell function including IFN-γ, Lymphocyte Function-Associated Antigen (LFA-1), DNAX Accessory Molecule-1 (DNAM1) and Programmed Cell Death Protein (PD-1). Importantly, we show that DNAM1 is involved in exosome-mediated cytotoxicity as revealed by experiments using blocking antibodies to DNAM1 or DNAM1 ligands. In addition, antibody-mediated inhibition of exosome cytotoxicity results in a delay in target cell apoptosis. We also provide evidence that NK-exosomes may exert their cytolytic activity after short time interval and even at low concentrations. Regarding their possible use in immunotherapy, NK exosomes, detectable in peripheral blood, can diffuse into tissues and exert their cytolytic effect at tumor sites. This property offers a clue to integrate cancer treatments with NK exosomes.

CEA: Combination-based gene set functional enrichment analysis

Functional enrichment analysis is a fundamental and challenging task in bioinformatics. Most of the current enrichment analysis approaches individually evaluate functional terms and often output a list of enriched terms with high similarity and redundancy, which makes it difficult for downstream studies to extract the underlying biological interpretation. In this paper, we proposed a novel framework to assess the performance of combination-based enrichment analysis. Using this framework, we formulated the enrichment analysis as a multi-objective combinatorial optimization problem and developed the CEA (Combination-based Enrichment Analysis) method. CEA provides the whole landscape of term combinations; therefore, it is a good benchmark for evaluating the current state-of-the-art combination-based functional enrichment methods in a comprehensive manner. We tested the effectiveness of CEA on four published microarray datasets. Enriched functional terms identified by CEA not only involve crucial biological processes of related diseases, but also have much less redundancy and can serve as a preferable representation for the enriched terms found by traditional single-term-based methods. CEA has been implemented in the R package CopTea and is available at http://github.com/wulingyun/CopTea/.

Tumor-stromal cross talk: direct cell-to-cell transfer of oncogenic microRNAs via tunneling nanotubes

Tunneling nanotubes (TnTs) represent a novel mechanism by which intercellular components such as proteins, Golgi vesicles, and mitochondria can be transferred from cell to cell in the complex tumor microenvironment. Here, we report data showing that microRNAs (miRNAs) are transferred through TnTs in osteosarcoma (OS) and ovarian cancer as in vitro model systems. miRNA array analysis demonstrated significant upregulation of miR-19a in OS tumors resected from human patients, and differential expression of miR-199a in ovarian cancer cell lines resistant or sensitive to platinum chemotherapy. K7M2 murine OS cells were transfected with miR-19a and cultured with nontransfected K7M2 cells in low-serum, hyperglycemic medium for up to 72 hours to induce TnT formation. miRNA transfer via TnTs was detected by time-lapse microscopic imaging. miR-19a was also transported via TnTs connecting transfected K7M2 cells and nontransfected stromal MC3T3 murine osteoblast cells. Similar findings were observed in studies of TnT-mediated transport of miR-199a among SKOV3 ovarian cancer cells and nonmalignant immortalized ovarian epithelial cells. To quantify TnT-mediated transport of miRNAs, we used modified Boyden chambers to separate miR-19a-transfected K7M2 cells (top chamber) and DiI-labeled MC3T3 cells (bottom chamber) compared with open culture of these cells. Fluorescence-activated cell sorting analysis of cells collected after 48 hours of culture indicated that miR-19a-positive MC3T3 cells were 3-fold higher in open culture; this finding suggests that miR-19a transfer occurred via TnTs, exclusive of other forms of cell-cell communication. These studies demonstrate that TnTs mediate direct transfer of genetic material between tumor and stromal cells.

Possible role of hemichannels in cancer

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

Modulation of cytotoxic and genotoxic effects of nanoparticles in cancer cells by external magnetic field

Magnetic nanoparticles are well known for anticancer activity by deregulating cellular functions. In the present study, cellular effects of low strength static magnetic field (SMF) were explored. How nanoparticles affect the cellular response in presence and absence of static magnetic field was also studied. Peripheral blood mononuclear cells (PBMC) and human lymphoma monocytic cell line U937 were chosen as representative normal and cancer cells models. The two effects we would like to report in this paper are, DNA damage induced by SMF of the order of 70 mT, and alteration in membrane potential. The other notable aspect was the changes were diametrically opposite in normal and cancer cell types. DNA damage was observed only in cancer cells whereas membrane depolarization was observed in normal cells. Iron oxide nanoparticles (IONP) and gold nanoparticles (AuNP) were also used for cellular response studies in presence and absence of SMF. The effects of the magnetic nanoparticle IONP and also of AuNP were sensitive to presence of SMF. Unlike cancer cells, normal cells showed a transient membrane depolarization sensitive to static magnetic field. This depolarization effect exclusive for normal cells was suggested to have correlations with their higher repair capacity and lesser propensity for DNA damage. The work shows cancer cells and normal cells respond to nanoparticle and static magnetic field in different ways. The static magnetic induced DNA damage observed exclusively in cancer cells may have therapeutic implications. From the conclusions of the present investigation we may infer that static magnetic field enhances the therapeutic potentials of nanoparticles. Such low strength magnetic field seems to be a promising external manoeuvring agent in designing theranostics.

Induction of the connexin 32 gene by epigallocatechin-3-gallate potentiates vinblastine-induced cytotoxicity in human renal carcinoma cells

BACKGROUND/AIM

Enforced expression of the connexin (Cx) 32 gene, a member of the gap junction gene family and a tumor suppressor gene in human renal cell carcinoma (RCC), enhanced vinblastine (VBL)-induced cytotoxicity in RCC cells due to suppression of multidrug resistance 1 (MDR1) expression. Furthermore, in RCC the Cx32 gene is silenced by hypermethylation of CpG islands in a promoter region of the Cx gene. In this study, we investigated if the green tea polyphenol epigallocatechin-3-gallate (EGCG) could enhance susceptibility of RCC cells (Caki-1, a human metastatic RCC cell) to VBL.

METHODS

The effects of EGCG on Caki-1 cells were estimated by WST-1 (cell viability), real-time RT-PCR (mRNA level) and immunoblotting (protein level). We estimated the methylation status in the promoter region of the Cx32 gene in RCC cells by methylation-specific PCR. Each protein function was inhibited by small interfering RNA (siRNA) and specific inhibitors.

RESULTS

The EGCG treatment elicited significant upregulation of Cx32 in Caki-1 cells, and the induction of the Cx led to the suppression of MDR1 mRNA expression through inactivation of Src and subsequent activation of c-Jun NH2-terminal kinase (JNK). Chemical sensitivity to VBL in Caki-1 cells was increased by EGCG pretreatment, and this effect was abrogated by siRNA-mediated knockdown of Cx32.

CONCLUSION

This study suggests that the restoration of Cx32 by EGCG pretreatment improves chemical tolerance on VBL in Caki-1 cells via the inactivation of Src and the activation of JNK.

Expression of connexin 26 and bovine papillomavirus E5 in cutaneous fibropapillomas of cattle

Bovine papillomaviruses (BPVs) can infect epithelial cells and fibroblasts, inducing fibropapillomas in cattle. Gap junctions are communication channels between cells composed of connexins (Cxs). This study evaluated expression of Cx26 and the major BPV oncoprotein E5 in bovine cutaneous fibropapillomas. BPV DNA was amplified from 20/20 fibropapillomas and 3/3 samples of normal skin. All fibropapillomas (20/20) were positive by immunostaining for E5, whereas the three normal skin samples were negative. Cx26 was expressed faintly in the normal skin epithelium. Positive cytoplasmic and juxtanuclear immunoreactivity for Cx26 was evident in 18/20 (90%) fibropapillomas. Western blot analysis demonstrated higher expression of Cx26 in 6/6 fibropapillomas compared to normal skin samples.

Up-regulation of connexin 32 gene by 5-aza-2'-deoxycytidine enhances vinblastine-induced cytotoxicity in human renal carcinoma cells via the activation of JNK signalling

Enforced expression of connexin (Cx) 32 gene, a member of gap junction gene family and a tumor suppressor gene in human renal cell carcinoma (RCC), enhanced vinblastine (VBL)-induced cytotoxicity on RCC cells, due to the suppression of multidrug resistance 1 (MDR1) gene product, P-glycoprotein (P-gp). Also, Cx32 gene in RCC is silenced by hypermethylation of CpG islands in a promoter region of the Cx gene. In this study, we investigated if a DNA demethylating agent, 5-aza-2'-deoxycytidine (5-Aza) could enhance susceptibility of RCC cells (Caki-1) to VBL. We found that 5-Aza treatment up-regulated Cx32 in Caki-1 cells, and the induction of the Cx led to the suppression of P-gp through inhibition of Src and subsequent activation of c-Jun NH(2)-terminal kinase (JNK). Moreover, increased transcription activity of c-Jun by the JNK activation contributed to the down-regulation of MDR1, thus indicating a central role of JNK signalling to suppress P-gp level in 5-Aza-treated Caki-1 cells. Chemical sensitivity to VBL in Caki-1 cells was increased by 5-Aza pre-treatment, and this effect was abrogated by short interfering RNA (siRNA)-mediated knockdown of Cx32. Furthermore, co-treatment of 5-Aza or a P-gp inhibitor with VBL drastically enhanced JNK activation comparing to only VBL treatment in Caki-1 cells. These results suggest that the restoration of Cx32 by 5-Aza pre-treatment improves chemical tolerance on VBL in Caki-1 cells and that the JNK activation is a key factor to induce the effect.

Enhancing effect of connexin 32 gene on vinorelbine-induced cytotoxicity in A549 lung adenocarcinoma cells

PURPOSE

Connexin (Cx) genes exert negative growth effects on tumor cells with certain cell specificity, and tumor-suppressive effects of the Cx genes contribute to enhancement of chemotherapeutical agents-induced cytotoxicity in some cancer cells. Since we and others have been reported that Cx32 acts as a tumor suppressor gene in lung adenocarcinomas, this study was undertaken to estimate if the combination of Cx32-dependent tumor-suppressive effect and vinorelbine (VBN), a chemotherapeutic agent which has been utilized for clinical lung adenocarcinoma treatment, could be effective in enhancing the sensitivity of the lung cancer to VBN treatment.

METHODS

We established the A549 cells (a human lung adenocarcinoma cell line) which had stable expression of Cx32 and estimated effect of Cx32 on VBN-induced cytotoxicity in the established cells.

RESULTS

Cx32 expression in A549 cells significantly potentiated VBN-induced cytotoxicity on the cells due to enhancement of apoptosis induction. The enhancing cytotoxicity in A549 cells by Cx32 mainly depended on a decrease in expression of multi-drug resistance-1 (MDR-1) gene responsible for reduction of VBN accumulation into the cells. We also observed that silencing of Cx32 by siRNA treatment elevated the expression level of MDR-1 mRNA in A549 cells and that inhibition of MDR-1 gene product-dependent function enhanced VBN-induced cytotoxicity in the cells.

CONCLUSION

These results suggest that Cx32 contributes to the enhancement of VBN-induced cytotoxicity in A549 cells via the reduction of MDR-1 expression.

Negative growth control of osteosarcoma cell by Bowman-Birk protease inhibitor from soybean; involvement of connexin 43

Bowman-Birk protease inhibitor (BBI) from soybean acts as a potential chemopreventive agent in several types of tumors. However, the mechanism is still unclear. The present study was undertaken to estimate a mechanism of BBI-dependent negative growth control of human osteosarcoma cell (U2OS cell). BBI had negative growth control of the cells via induction of connexin (Cx) 43, a tumor suppressor gene in U2OS cells. This negative growth control by BBI was abrogated under down-regulation of Cx43 induced by a Cx43 antisense nucleotide treatment. It was also found that the BBI-dependent induction of Cx43 was due to elevation of Cx43 mRNA and stabilization of Cx43 protein. Especially, BBI-dependent inhibition of chymotrypsin-like activity in proteasome contributed to stabilization of Cx43 protein. These results suggest that a major negative growth effect of BBI is based on the restoration of Cx43 expression in U2OS cells.

Regulation of renal cell carcinoma cell proliferation, invasion and metastasis by connexin 32 gene

Gap junctions composed of connexin (Cx), a large protein family with a number of subtypes, are a main apparatus to maintain cellular homeostasis in many organs. Gap junctional intercellular communication (GJIC) is actively involved in all aspects of the cellular life cycle, ranging from cell growth to cell death. It is also known that the Cx gene acts as a tumor-suppressor due to the maintenance of cellular homeostasis via GJIC. In addition to this function, recent data show that the GJIC-independent function of Cx gene contributes to the tumor-suppressive effect of the gene with specificity to certain cells. With respect to the tumor-suppressive effects, Cx genes acts as tumor-suppressors in primary cancers, but the effects are still conflicting in invasive and metastatic cancers. We have previously reported that Cx32 is specifically downregulated in human renal cell carcinoma (RCC) cell lines as well as cancerous regions when compared to normal regions in kidneys. In recent studies, we have also reported that Cx32 suppresses growth, invasion and metastasis of RCC cells. In this minireview, we refer to a new aspect of Cx32-dependent functions against cell proliferation, invasion and metastasis in RCC cells, especially in a GJIC-independent manner.

Connexin 32 potentiates vinblastine-induced cytotoxicity in renal cell carcinoma cells

We have reported that connexin (Cx) 32 gene, a member of gap junction protein family, acts as a tumor suppressor gene in human renal cell carcinoma (RCC). Of solid tumors, RCC is one of the most chemoresistant cancers, and there is no effective cancer chemotherapy against RCC at present. In this study, we examined if the combination of Cx32-dependent tumor-suppressive effect and vinblastine (VBL), a chemotherapeutic agent which has been utilized for clinical RCC treatment, could be effective in enhancing the sensitivity of RCC to VBL treatment. Cx32 expression in a human metastatic RCC cell (Caki-1 cell) significantly enhanced in vitro and in vivo VBL-induced cytotoxicity on the cell. Cx32 expression in the RCC cells potentiated VBL-induced apoptosis compared to the Cx32-negative RCC cells in vitro as well as in vivo. The enhancing apoptosis in the RCC cells by Cx32 mainly depended on the decrease of P-glycoprotein (P-gp), a multidrug resistance gene-1 (MDR-1) product responsible for reduction of VBL accumulation into the cells. We also observed that silencing of Cx32 by short interfering RNA (siRNA) treatment elevated the level of P-gp in Caki-1 cells and that inhibition of P-gp function enhanced VBL-induced apoptosis in the RCC cells. These results suggest that Cx32 is effective to enhance VBL-induced cytotoxicity in Caki-1 cells via the reduction of P-gp. Overall, it seems that the combination of Cx32-dependent tumor-suppressive effect and VBL is promising as a new cancer therapy against RCC.

Connexin 32 as an anti-invasive and anti-metastatic gene in renal cell carcinoma

Cellular homeostasis in many organs is maintained via gap junctions composed of connexin (Cx), a large protein family with a number of isoforms. In fact, gap junctional intercellular communication (GJIC) is actively involved in all aspects of the cellular life cycle, ranging from cell growth to cell death. It has been well known that Cx gene acts as a tumor suppressor gene due to the maintenance of cellular homeostasis via GJIC. On the other hand, recent data show that GJIC-independent function for Cx gene contributes to tumor-suppressive effect of the gene with cell certain specificity. However, the mechanistic aspect of the GJIC-independent function remains largely unknown. In this review, we briefly summarize the tumor-suppressive effects of Cx genes, refer to a new aspect of Cx32 as an anti-invasive and anti-metastatic gene against renal cell carcinoma in a GJIC-independent function and establishment of a new cancer therapy based on the new function of Cx32.

Expression patterns of connexin 26 and connexin 43 mRNA in canine benign and malignant mammary tumours

The expression patterns of connexin (Cx) genes, encoding gap junctional proteins, are tissue- and cell-specific and, as their expression is mostly suppressed during carcinogenic processes, they are appropriate for monitoring tumour development. In this study, using reverse transcriptase-coupled polymerase chain reaction (RT-PCR), the expression of Cx mRNAs was examined in seven normal canine mammary glands and in 31 mammary gland tumour samples. Cx26 and Cx43 gene expression was studied in all normal tissues using specific Cx26 and Cx43 primers. When the expression patterns of Cx26 and Cx43 genes were analyzed in several types of canine mammary gland tumours, it was noted that it was the loss of Cx26 expression rather than the occurrence of Cx43 expression that was associated with malignancy. These results suggest that Cx26 plays an important role in tumourigenesis of canine mammary gland.

A mouse strain difference in tumorigenesis induced by biodegradable polymers

The use of poly-L-lactic acid (PLLA) surgical implants for repair of bone fractures has gained popularity in the past decade. The aim of this study was to evaluate the in vivo effect of PLLA plates on subcutaneous tissue in two mouse strains, BALB/cJ and SJL/J, which have higher and lower tumorigenicity, respectively. Gap-junctional intercellular communication and protein expression of connexin 43 were significantly suppressed, whereas secretion of transforming growth factor-beta1 and expression of extracellular matrix, insulin-like growth factor binding protein 3, and cysteine-rich intestinal protein 2 were significantly increased in PLLA-implanted BALB/cJ mice when compared with BALB/cJ controls. Finally, tumors were formed after implantation of cultured cells from the more-tumorigenic BALB/cJ, but not SJL/J, mice into nude mice.

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).

A Src family inhibitor (PP1) potentiates tumor-suppressive effect of connexin 32 gene in renal cancer cells

Connexin (Cx) genes exert negative growth effects on tumor cells with certain cell specificity. We have recently reported that Cx32 acts as a tumor suppressor gene in renal cancer cells due to the inhibition of Src-dependent signaling. In line with the previous study, here we examined if a Src family inhibitor (PP1) could potentiate tumor-suppressive effect of Cx32 in Caki-2 cell from human renal cell carcinoma. In order to clarify the potentialization of PP1, using Cx32-transfected Caki-2 cells and mock-transfected Caki-2 cells, we estimated difference in cytotoxic effect of PP1 on the two cell clones in vitro as well as in vivo. PP1 showed more cytotoxic effect on Caki-2 cells having Cx32 positive expression than that of Cx32 negative expression at lower doses. This potentialization was also observed in xenograft model of nude mice. The potentialization of the effect mainly depended on the induction of apoptosis but not the control of cell growth. In conjugation with this event, the reduction of anti-apoptotic molecules (Bcl-2 and Bcl-xL) was caused by the combination of Cx32 expression and PP1 treatment in Caki-2 cells. These results suggest that PP1 potentiates tumor-suppressive effect of connexin 32 gene in renal cancer cells through the reduction of anti-apoptotic molecules.

Cytotoxic effect of the Her-2/Her-1 inhibitor PKI-166 on renal cancer cells expressing the connexin 32 gene

We have reported that connexin (Cx) 32 acts as a tumor suppressor gene in renal cancer cells partly due to Her-2 inactivation. Here, we determined if a Her-2/Her-1 inhibitor (PKI-166) can enhance the tumor-suppressive effect of Cx32 in Caki-2 cells from human renal cell carcinoma. The expression of Cx32 in Caki-2 cells was required for PKI-166-induced cytotoxic effect at lower doses. The cyctotoxicity was dependent on the occurrence of apoptosis and partly mediated by Cx32-driven gap junction intercellular communications. These results suggest that PKI-166 further supports the tumor-suppressive effect of the Cx32 gene in renal cancer cells through the induction of apoptosis.

Negative growth control of renal cell carcinoma cell by connexin 32: possible involvement of Her-2

Connexin (Cx) genes have negative growth effects on tumor cells with certain cell specificity. We have previously reported that Cx32 is specifically downregulated in human renal cell carcinoma cell (RCC) lines as well as cancerous regions of kidneys and that the Cx is expressed in the progenitor cells of the carcinoma. However, the precise role of Cx32 in growth control of RCC cells remains unknown. In this study, we examined whether Cx32 could act in growth control against a human RCC cell, Caki-2 cell. In order to estimate the cell growth control, we established Caki-2 cells that have stable expression of Cx32 genes. Cx32 expression in Caki-2 cells induced contact inhibition of growth and reduced anchorage-independent growth ability, but did not significantly affect lag phase growth rates. This growth control by Cx32 was dependent on the inhibition of the cell-cycle transition from G1 to S phase at high cell density, and the inhibition of the cell-cycle transition related to the suppression of Her-2 activation. Furthermore, the suppression of Cx32 expression in Caki-2 cells by short interfering RNA induced the activation of Her-2. These data suggest that Cx32 has negative growth control of Caki-2 cells, partly due to the inhibition of the Her-2 activation.

Hypermethylation of the CpG island of connexin 32, a candiate tumor suppressor gene in renal cell carcinomas from hemodialysis patients

Long duration of patients on hemodialysis is a large risk for the development of renal cell carcinoma (RCC) compared to general patients. However, the carcinogenic process is still unclear. On the other hand, we have reported that connexin (Cx) 32, a molecule of gap junction, is a new tumor suppressor gene in human RCC. In this study, we investigated the clinical significance of methylation-dependent silencing of Cx32 gene in the development of the RCC from the hemodialysis patients. As the result, we found that the inactivation of Cx32 through hypermethylation of the promoter regions frequently occurred in non-cancerous regions as well as cancerous regions of kidneys from hemodialysis patients. However, the hypermethylation of Cx32 occurred only in cancerous regions but not non-cancerous regions of kidneys from the general patients without hemodialysis. Furthermore, the hypermethylation of RASSAF1A, a representative tumor suppressor gene in human RCC, occurred in cancerous regions but not non-cancerous regions of kidneys from the hemodialysis and general patients. These results suggest that Cx32 is a promising tumor suppressor gene relating to the early stage of renal carcinogenesis in the hemodialysis patients.

Novel mechanism of tumorigenesis: Increased transforming growth factor-?1 suppresses the expression of connexin 43 in BALB/cJ mice after implantation of poly-L-lactic acid

Poly-L-lactic acid (PLLA) is a widely used promising material for surgical implants such as tissue-engineered scaffolds. In this study, we aimed to determine the in vivo effect of PLLA plates on the cellular function of subcutaneous tissue in the two mouse strains, BALB/cJ and SJL/J, higher and lower tumorigenic strains, respectively. Gap-junctional intercellular communication (GJIC) and the expression of connexin 43 (Cx43) protein were significantly suppressed, whereas the secretion of transforming growth factor-beta 1 (TGF-beta 1) level was significantly increased in PLLA-implanted BALB/cJ mice compared with BALB/cJ controls. However, no significant difference in TGF-beta 1 secretion was observed between the SJL/J-implanted and SJL/J control mice. We found for the first time that a significant difference was observed between the two strains; thus, the PLLA increased the secretion of TGF-beta 1 and suppressed the mRNA expression of Cx43 at the earlier stage after implantation into the higher-tumorigenic strain, BALB/cJ mice. This novel mechanism might have a vital role in the inhibition of GJIC and promote the tumorigenesis in BALB/cJ mice.

Gap junction-mediated intercellular communication in a long-term primary mouse hepatocyte culture system

Gap junction-mediated intercellular communication (GJIC) is critical for maintaining integral cellular processes including differentiation and growth control. The disruption of GJIC has been correlated with aberrant function in many cell types, including hepatocytes in vivo; therefore it is imperative that cellular model systems support intercellular communication to simulate normal cellular functions. Functional GJIC has been shown in long-term primary rat hepatocyte cultures, which have been implemented widely to study various aspects of hepatocellular function; however, the onset of transgenic technology in murine species has necessitated the development of a primary mouse hepatocyte system. In this report, we analyze GJIC in a dimethylsulfoxide (DMSO)-containing long-term primary mouse hepatocyte culture system. The cells retain morphologic and biochemical characteristics of differentiated hepatocytes through day 30 post plating, including liver-specific gene expression. We further show that connexin32 and connexin26 expression and gap junction plaque formation increase over time in culture concomitant with an increase in GJIC between adjoining primary mouse hepatocytes. In conclusion, the findings described in this study make it possible to maintain differentiated primary mouse hepatocytes that also show GJIC in long-term culture for 30 days. In addition, this system has the potential to be extended to study primary mouse hepatocytes isolated from genetically engineered mice.

Pharmacological modification of gap junction coupling by an antiarrhythmic peptide via protein kinase C activation

Antiarrhythmic peptides enhance gap junction current in pairs of cardiomyocytes and coupling in cardiac tissue. To elucidate the underlying mechanisms, we investigated the effects of the antiarrhythmic peptide AAP10 (GAG-4Hyp-PY-CONH2) on pairs of adult guinea pig ventricular cardiomyocytes and pairs of HeLa cells transfected with rat cardiac connexin 43 (Cx43). By using a double-cell voltage-clamp technique in pairs of cardiomyocytes, we found that under control conditions the gap junction conductance (gj) steadily decreased with time (by -0.292 +/- 0.130 nS/min). Use of 50 nmol/L AAP10 reversed this rundown and increased gj (by +0.290 +/- 0.231 nS/min, Pa). In HeLa-Cx43 cells, AAP10 exerted the same electrophysiological effect. In these cells, AAP10 activated PKC (determined by using ELISA) in CGP54345-sensitive manner and significantly enhanced incorporation of 32P into Cx43 with dependence on PKC. If G-protein coupling was inhibited with 1 mM GDP-BS, we found the effects of AAP10 on 32P incorporation were also completely abolished. Next, we performed a radioligand binding study with 14C-AAP10 as radioligand and AAPnat as competitor. We found saturable binding of 14C-AAP10 to cardiac membrane preparations, which could be displaced with AAPnat. The Kd of AAP10 was 0.88 nmol/L. We conclude that 1) AAP10 increases gj both in adult cardiomyocytes and in transfected HeLa-Cx43 cells, 2) AAP10 exerts its effect via enhanced PKC-dependent phosphorylation of Cx43, 3) AAP10 activates PKCa, and 4) a membrane receptor exists for antiarrhythmic peptides in cardiomyocytes.

A strategy for the suppression of tumorigenesis induced by biomaterials: Restoration of transformed phenotype of polyetherurethane-induced tumor cells by Cx43 transfection

Biomaterials such as polyetherurethans (PEUs) are the scaffolding, which is indispensable for the development of the bio-artificial organs. However, PEUs can induce tumors in subcutaneous implantation sites in rat. We have shown that the different inhibitory potential of gap junctional intercellular communication (GJIC) on the surface of the biomaterials, including PEUs, is a key step in determining the tumorigenic potential. Here we show that suppression of a gap junctional protein connexin 43 (Cx43) plays an important role in in vivo tumorigenesis induced by PEUs for the first time and that Cx43 transfection may be an effective strategy for preventing tumorigenesis induced by biomaterials. Rat tumor cell line U41 is derived from tumors in the subcutaneous implantation of PEU films. The GJIC and the expression of Cx43 were suppressed in U41. The restoration of normal phenotype, such as reduction of growth rate, recovery of contact inhibition and loss of colony formation ability in soft agar, was achieved by Cx43 transfection. These results strongly suggest that suppression of Cx43 expression plays an important role in the development of rat malignant fibrous histiocytoma (MFHC) caused by PEUs and that Cx43 transfection is effective for prevention of tumorigenesis induced by PEUs.

Influence of tris(4-chlorophenyl)methanol (TCPM) on gap junction-mediated intercellular communication of cultured bovine granulosa cells

Tris(4-chlorophenyl)methanol (TCPM) is a by-product in the manufacture of technical grade DDT, which is known to alter properties and functions of the female reproductive system. We investigated whether in vitro TCPM has an influence on the function of gap junction-mediated intercellular communication (GJIC) and gap junction protein expression of connexin 43 (Cx43) in cultured bovine granulosa cells. GJIC was assessed by fluorescent dye microinjection (dye-coupling). After a 1-h exposure to TCPM at a concentration of 32 microM, a significant (P<0.05) reduction in dye coupling occurred. The same result was obtained with o,p'-DDT. At a concentration of 32 microM both pesticides were cytotoxic as indicated by significant (P<0.05) increased propidium iodide staining of the cell nuclei. Little or no effect on the stainable pattern of connexons occurred after 1 h incubation time, while after 3 h treatment from 16 to 64 microM TCPM, a significant inhibition in the immunostaining resulted and the concentrations of 32 and 64 microM TCPM were cytotoxic for the granulosa cells. The freeze-fracture electron microscopy resulted in small differences in the morphology of gap junction plaques of cell cultures treated for 3 h with 8 or 16 microM TCPM in comparison to untreated cells. After treatment with 32 microM TCPM, gap junction plaques were very rarely detected and the lateral intramembraneous particles (IMP) distribution of many plasma membranes was strongly altered. Estimation of the cellular parameters may lead to an enhanced understanding of the mechanism of chemically induced toxicity by TCPM, that causes a general toxic effect on granulosa cells. We can conclude that TCPM is a toxic risk in the same manner as DDT.

Control of intracellular movement of connexins by E-cadherin in murine skin papilloma cells

The gap junctional intercellular communication-deficient mouse skin papilloma cell line P3/22 expresses Cx43 but not E-cadherin. The E-cadherin gene-transfected cells (P3E1) communicate in a calcium-dependent manner and they were used to study how E-cadherin restores the function of connexins. At low calcium, Cx43 molecules remain in the cytoplasm of P3E1 cells and appear at cell-cell contact areas only in high-calcium medium. While Cx43 is unphosphorylated in P3E1 cells in low-calcium medium, two phosphorylated bands appeared at high calcium. However, when Cx26, which has no C-terminal tail that can undergo phosphorylation, was expressed in P3E1 cells, this connexin also moved to the plasma membrane after the calcium shift and partly colocalized with Cx43, suggesting that C-terminal phosphorylation is not essential for E-cadherin-mediated intracellular transport of connexins. In low calcium, both Cx26 and Cx43 remained and colocalized in the endoplasmic reticulum. As early as 30 min after the shift to high-calcium medium, both Cx43 and Cx26 began to accumulate in the Golgi apparatus. Intracellular movement of connexins to the cytoplasmic membrane at high calcium was effectively blocked by cytochalasin D and brefeldin A. These results suggest that E-cadherin junction formation at high calcium leads to formation of actin cables, which directly or indirectly transport connexins from the cytoplasm to the cell-cell contact membranes via the Golgi apparatus.

Maintenance of connexin 32 and 26 expression in primary cultured rat hepatocytes treated with 3-acetylpyridine

BACKGROUND AND AIM

We recently reported that primary rat hepatocytes treated with 3-acetylpyridine (3-AP), an analog of nicotinic acid, could maintain hepatic differentiated functions such as albumin, tryptophan 2,3-dioxygenase, and connexin 32 (Cx32) mRNA expressions for more than a week. In the present experiment, we investigated the expression of not only Cx32, but also Cx26 in cells treated with 10 mmol/L 3-AP in detail.

METHODS

We examined the expression of Cx32 and Cx26 in primary rat hepatocytes by using the methods of immunocytochemistry, immunoelectron microscopy, northern blotting, and dye-transfer.

RESULTS

The hepatocytes treated with 3-AP were polygonal with a large cytoplasm from day 3, and were maintained for approximately 2 weeks, whereas the cells without 3-AP began to die from day 4. Immunocytochemically in the cells with 3-AP, many Cx32- and Cx26-positive spots were observed between most adjacent cells, and the intensity of positive spots increased with time in culture, whereas in the cells without 3-AP, Cx32- and Cx26-positive spots disappeared at day 4. Furthermore, most Cx26-positive spots were colocalized with Cx32-positive ones. The amounts of Cx32 and Cx26 mRNA transcripts in the cells with 3-AP at day 14 were more than 80% and approximately 30% of those of Cx32 and Cx26 mRNA transcripts in the cells at day 1, respectively. Gap junctional intercellular communication was maintained in the cells treated with 3-AP at day 8, although it was lost in the cells without 3-AP.

CONCLUSION

Thus, the addition of 10 mmol/L 3-AP to the medium enhanced the maintenance of Cx32 and Cx26 expression, which is one of the hepatic differentiated functions, in primary rat hepatocytes for a long time.

Regulation of cellular invasion and matrix metalloproteinase activity in HepG2 cell by connexin 26 transfection

We previously reported that connexin (Cx) 26 expression is involved in negative growth control of HepG2 cells established from a human hepatoma. We also found that induction of E-cadherin and subsequent formation of a cell adhesion complex were induced in HepG2 cells by Cx 26 expression. To examine the exact role of Cx 26-induced E-cadherin junctions in regulating appearance of malignant phenotypes of HepG2 cells, we expressed a Cx 26 antisense oligodeoxynucleotide (AS-ODN) in an established HepG2 cell clone that has stable expression of Cx 26 genes. We investigated changes in the expression of E-cadherin, the localization of beta-catenin, and some malignant phenotypes of HepG2 clone after the suppression of Cx 26 expression by AS-ODN treatment. The AS-ODN treatment prevented the expression of Cx 26 and E-cadherin, and the localization of beta-catenin was changed from cytoplasmic membrane to the cytoplasm. In parallel, a morphological change from a monolayer of polygonal cells to multilayered colonies was induced by the treatment, indicating a change of a malignant phenotype of HepG2 cells. The activity of matrix metalloproteinase 9 (MMP-9) was elevated by the AS-ODN treatment. A concomitant increase in invasiveness of the Cx 26-expressing cells by the treatment was also observed in an in vitro assay with Matrigel matrix. These results suggest that the induction of E-cadherin and formation of the cell adhesion complex by Cx 26 expression contribute to the reversal of some malignant phenotypes of HepG2 cells. Furthermore, the Cx 26-dependent expression of E-cadherin leads to reduction of the invasiveness of the cells through suppression of MMP-9 activity.

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

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

Neuronal differentiation and growth control of neuro-2a cells after retroviral gene delivery of connexin43

Given the roles proposed for gap junctional intercellular communication in neuronal differentiation and growth control, we examined the effects of connexin43 (Cx43) expression in a neuroblastoma cell line. A vesicular stomatitis virus G protein (VSVG)-pseudotyped retrovector was engineered to co-express the green fluorescent protein (GFP) and Cx43 in the communication-deficient neuro-2a (N2a) cell line. The 293 GPG packaging cell line was used to produce VSVG-pseudotyped retrovectors coding for GFP, Cx43, or chimeric Cx43.GFP fusion protein. The titer of viral supernatant, as measured by flow cytometry for GFP fluorescence, was approximately 2.0 x 10(7) colony form units (CFU)/ml and was free of replication-competent retroviruses. After a 7-day treatment with retinoic acid (20 microm), N2a transformants (N2a-Cx43 and N2a-Cx43.GFP) maintained the expression of Cx43 and Cx43.GFP. Expression of both constructs resulted in functional coupling, as evidenced by electrophysiological and dye-injection analysis. Suppression of cell growth correlated with expression of both Cx43 or Cx43.GFP and retinoic acid treatment. Based on morphology and immunocytochemistry for neurofilament, no difference was observed in the differentiation of N2a cells compared with cells expressing Cx43 constructs. In conclusion, constitutive expression of Cx43 in N2a cells does not alter retinoic acid-induced neuronal differentiation but does enhance growth inhibition.

Epigenetic mechanisms of chemical carcinogenesis

Chemically induced cancer is a multi-step process involving damage to the genome initially followed by clonal expansion of the DNA damaged cell eventually leading to a neoplasm. Chemical carcinogens have been shown to impact at all of the stages of the tumorigenesis process. It has become apparent that chemical and physical agents that induce cancer may do so through several different cellular and molecular mechanisms. Epigenetic (nongenotoxic) chemical carcinogens are those agents that function to induce tumor formation by mechanisms exclusive of direct modification or damage to DNA. These agents appear to modulate cell growth and cell death and exhibit dose response relationships between exposure and tumor formation. The exact and/or exclusive mechanisms by which these agents function have not been established, however, changes in cell growth regulation and gene expression are important to tumor formation. This review focuses on several potential mechanisms and cellular processes that may be involved in nongenotoxic chemical carcinogenesis.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Altered gap and tight junctions in human thyroid oncocytic tumors: a study of 8 cases by freeze-fracture

Human oncocytic tumors of the thyroid gland may be either adenomas or carcinomas. The morphology and the ultrastructure of these oncocytes are well-known. Numerous studies have demonstrated the role of gap and tight junctions in experimental and human carcinogenesis; however, the junctional complexes of the oncocytic tumors have never been studied. The aim of this study is to analyze gap and tight junctions in the oncocytic tumors of the thyroid. Because they are morphologically similar, whether benign or malignant, they offer an attractive model for studying the junctional complexes in both benign and malignant lesions. Eight oncocytic human thyroid tumors were collected and studied by freeze-fracture. Four of these cases were benign and four were malignant. Four cases of normal gland were also studied to represent the control group. Normal tight and gap junctions were described for the control group. No gap junctions could be found for the oncocytic tumors. Furthermore, alterations of the tight junctions were described; especially focal tights in the oncocytic adenomas and well organized and labyrinthic tight junctions in the oncocytic carcinomas. The lack of gap junction in the benign as well as in the malignant oncocytomas may suggest that the absence of gap junction is not sufficient for malignancy. The alterations of the tight junctions found in the oncocytic tumors of the thyroid are similar to those observed in poorly differentiated tissues or tumors, and may suggest a cellular regression rather than a tumorogenic factor.

Regulation of connexin-43 gap junctional intercellular communication by mitogen-activated protein kinase

Activation of the Ras/Raf/mitogen-activated protein kinase kinase/mitogen-activated protein (MAP) kinase signaling cascade is initiated by activation of growth factor receptors and regulates many cellular events, including cell cycle control. Our previous studies suggested that the connexin-43 gap junction protein may be a target of activated MAP kinase and that MAP kinase may regulate connexin-43 function. We identified the sites of MAP kinase phosphorylation in in vitro studies as the consensus MAP kinase recognition sites in the cytoplasmic carboxyl tail of connexin-43, Ser255, Ser279, and Ser282. In this study, we demonstrate that activation of MAP kinase by ligand-induced activation of the epidermal growth factor (EGF) or lysophosphatidic acid receptors or by pervanadate-induced inhibition of tyrosine phosphatases results in increased phosphorylation on connexin-43. EGF and lysophosphatidic acid-induced phosphorylation on connexin-43 and the down-regulation of gap junctional communication in EGF-treated cells were blocked by a specific mitogen-activated protein kinase kinase inhibitor (PD98059) that prevented activation of MAP kinase. These studies confirm that connexin-43 is a MAP kinase substrate in vivo and that phosphorylation on Ser255, Ser279, and/or Ser282 initiates the down-regulation of gap junctional communication. Studies with connexin-43 mutants suggest that MAP kinase phosphorylation at one or more of the tandem Ser279/Ser282 sites is sufficient to disrupt gap junctional intercellular communication.

Differential dose-dependent effects of alpha-, beta-carotenes and lycopene on gap-junctional intercellular communication in rat liver in vivo

In order to examine the relevance of alteration of gap-junctional intercellular communication (GJIC) to chemopreventive activity against carcinogenesis, the effects of alpha- and beta-carotene as well as lycopene, typical chemopreventive carotenoids, on cell coupling via gap junctions in rat liver in vivo were studied using a direct functional dye-transfer technique. We found that all three test compounds given at a dose of 50 mg/kg-body weight (b.w.) daily, 5 times by gavage, inhibited GJIC, while similar treatment with 5 mg/kg b.w. caused enhancement, especially in the beta-carotene- and lycopene-treated groups. At the dose level of 0.5 mg/kg b.w., the three compounds had no effect. The findings show that all three agents differentially modulate GJIC depending on the dose, with beneficial effects on cell communication only detected at the one dose. The result suggests that determination of the dose of chemicals to be used is crucial for human intervention studies.

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.

Gap junctions and growth control in liver regeneration and in isolated rat hepatocytes

The hepatocytes in the mature normal liver are tightly coupled through gap junctions, except during compensatory hyperplasia (regeneration) after partial hepatectomy when the gap junctions become down-regulated. The significance of this down-regulation has been a long-standing enigma. The present study of hepatocytes in primary culture and in the regenerating liver aimed at defining the relationship, if any, between hepatocyte gap junctional communication and proliferation. Gap junctional down-regulation in the regenerating liver appeared to be a specific phenomenon because desmosomes and the surface contact area between neighboring hepatocytes remained constant. All agents and conditions (dexamethasone in vivo; dexamethasone, cyclic adenosine monophosphate, serum, and high cell density in vitro) delaying gap junctional down-regulation also increased the lag before the cells reached competence to enter S phase. This raised the possibility that hepatocyte DNA replication was inhibited through preservation of gap junctions. However, we disproved this assumption by showing that the DNA replication (more specifically the G1/S transition rate constant) was inhibited even in hepatocytes completely devoid of gap junctional communication. The teleological advantage of linking gap junctional down-regulation to hepatocyte G1 progression therefore may not be to trigger DNA replication but to ensure that proliferating hepatocytes and hepatocytes responsible for liver-specific metabolic functions maintain separate pools of metabolites and signaling molecules.

The bovine papillomavirus type 4 E8 protein binds to ductin and causes loss of gap junctional intercellular communication in primary fibroblasts

The E8 open reading frame of bovine papillomavirus type 4 encodes a small hydrophobic polypeptide which contributes to cell transformation by conferring anchorage-independent growth. Using an in vitro translation system, we show that the E8 polypeptide binds to ductin, the 16-kDa proteolipid that forms transmembrane channels in both gap junctions and vacuolar H+-ATPase. This association is not due to nonspecific hydrophobic interactions. PPA1, a Saccharomyces cerevisiae polypeptide homologous (with 25% identity) to ductin, does not complex with E8. Furthermore, E5B, structurally similar to E8 but with no transforming activity, does not form a complex with ductin. Primary bovine fibroblasts expressing E8 show a loss of gap junctional intercellular communication, and it is suggested that this results from the interaction between E8 and ductin.

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.

Computer simulation modelling and visualization of 3D architecture of biological tissues. Simulation of the evolution of normal, metaplastic and dysplastic states of the nasal epithelium

Recent technical improvements, such as 3D microscopy imaging, have shown the necessity of studying 3D biological tissue architecture during carcinogenesis. In the present paper a computer simulation model is developed allowing the visualization of the microscopic biological tissue architecture during the development of metaplastic and dysplastic lesions. The static part of the model allows the simulation of the normal, metaplastic and dysplastic architecture of an external epithelium. This model is associated to a knowledge base which contains only data on the nasal epithelium. The latter has been well studied by numerous authors and its lesional states are well known. An inference engine allows the initialization of the static model parameters. A statistical comparison between simulated epithelia and real epithelia is achieved by adjusting the parameter values during the simulation. The dynamic part of the model allows the simulation of a growth process on a 3D representation based on the static model. The main hypothesis is that nasal epithelium is submitted to a continuous transformation from normal to cancer through metaplasia and dysplasia. The evolution of each cell (represented by its nucleus) depends on its local environment and also on its heritage from its mother-cell. Simulation of tissue renewal of the nasal pseudostratified epithelium has been achieved. The evolution from normal to hyperplasia has been simulated. After modification of the cell cycle modelling, the simulation of the development of metaplastic foci has been obtained.

Induction and regulation of connexin26 by glucagon in primary cultures of adult rat hepatocytes

In the adult rat hepatocyte, the gap junction proteins consist of a major component, connexin32 (Cx32) and a minor component, connexin26 (Cx26). Although we recently reported our success in inducing and maintaining Cx32 in adult rat hepatocytes cultured in serum-free L-15 medium supplemented with epidermal growth factor and 2% dimethyl sulfoxide, it was very difficult to induce Cx26 in the primary hepatocytes. In the present study, we found that the addition of 10(−7) M glucagon into the culture medium could dramatically induce Cx26 mRNA and protein. Although the expression of Cx32 mRNA was also influenced by glucagon, the increase of the expression was small. Immunocytochemically, Cx26-positive spots were observed between most adjacent cells and were co-localized with the Cx32-positive spots. We also examined whether 0.5 mM dibutyl cyclic AMP could induce expression of Cx26 in the cells. The effect of dexamethasone on the expression of Cx26 mRNA compared to that of Cx32 mRNA was examined. For the induction and maintenance of Cx26 mRNA, more than 10(−7) M dexamethasone was necessary in this culture. These results suggest that expression of Cx26 in hepatocytes may be regulated by the concentrations of glucagon and glucocorticoid hormones.

Non-genotoxic mechanisms of carcinogenesis: studies of cell transformation and gap junctional intercellular communication

It is widely accepted that a series of genetic changes accumulate during carcinogenesis. In addition, it is likely that various non-genotoxic mechanisms also operate at different stages of carcinogenesis. It is even possible that non-genotoxic mechanisms indirectly generate genetic changes, e.g., through induction of cell proliferation, active oxygen species or cytosine methylation. This may partially explain why many carcinogens are devoid of activity when tested in the usual genetic toxicology assays. In vitro cell transformation mimics certain stages of in vivo carcinogenesis. It has therefore been proposed that both genotoxic and non-genotoxic aspects of carcinogenesis can be studied in cell transformation systems, with tumor formation by transformed cells in syngenic animals or nude mice as the endpoint. Many genotoxic as well as non-genotoxic carcinogens induce transformation of Syrian hamster embryo, murine Balb/c 3T3 and murine C3H10T1/2 cells; interaction of genotoxic and non-genotoxic mechanisms can be clearly seen in 2-stage cell transformation studies in which a genotoxic initiating agent and a non-genotoxic promoting agent act synergistically to induce transformation of rodent cells. Aberrant control of gap junctional intercellular communication (GJIC) in cell transformation and carcinogenesis is well documented. Possible genotoxic as well as non-genotoxic mechanisms involved in abnormal gap junction communication control in multistage carcinogenesis are discussed.

Reappearance and long-term maintenance of connexin32 in proliferated adult rat hepatocytes: use of serum-free L-15 medium supplemented with EGF and DMSO

Intercellular communication, especially gap junctional communication, is thought to be one of the highly differentiated functions of hepatocytes. In primary cultures of rat hepatocytes, it has been considered that the maintenance and the reinduction of differentiated functions is very difficult. In the present study, we succeeded in inducing the gap junctional protein connexin32 (Cx32) in adult rat hepatocytes cultured in serum-free L-15 medium supplemented with epidermal growth factor (EGF) and dimethylsulfoxide (DMSO). When the hepatocytes were cultured in L-15 medium supplemented with 20 mM NaHCO3 and 10 ng/ml EGF in a 5% CO2:95% air incubator, the cells proliferated. Fluorescence immunocytochemistry showed spots immunoreactive to Cx32 on the cell membranes between adjacent cells until day 3, but only a few Cx32-positive spots were found after day 4. Western and northern blot analyses also showed that the amounts of both the protein and mRNA of Cx32 in the cells decreased with time in culture. However, when the cells were treated with 2% DMSO from day 4, the immunoreactive spots reappeared on the cell membranes from day 6 and both their number and intensity gradually increased. The reappearance of Cx32 was accompanied by increases in both the protein and mRNA of Cx32. Furthermore, the expression of Cx32 was well maintained, together with extensive gap junctional intercellular communication, for more than 4 weeks. In addition, ultrastructurally, many gap junctional structures were observed between the hepatocytes, and the antibodies to Cx32 were shown to bind to those structures. This culture system may be useful for studies of the reconstruction of the gap junctional structure, the intracellular pathways of the proteins, and the regulation of synthesis and processing in differentiated hepatocytes.

Retinoic acid enhances connexin43 expression at the post-transcriptional level in rat liver epithelial cells

The mechanism by which all-trans retinoic acid (RA) stimulates gap junctional intercellular communication (GJIC) in the rat liver epithelial cell line. IAR203, was investigated. When RA, at 0.1 microM for 24-48 h, enhanced the dye transfer in IAR203 cells (x 1.4), it increased the amount of connexin43 (Cx43) in the cell-cell contact regions of the plasma membrane, as evidenced by analysis by Western blot and by immunofluorescence. It had no effect on the level of Cx43 mRNA. Freeze-fracture analysis of the size of gap junctions revealed an increase of the proportion of small gap junctions in RA-treated cells. We conclude that, in IAR203 cells, RA stimulates GJIC by acting at the post-transcriptional level of Cx43 regulation. The possibility that RA acts indirectly on the regulation of Cx43 expression, and increases the half-life of Cx43 by inducing adhesion molecules is discussed.