The cell-cell channel in the control of growth

A Lack of GD3 Synthase Leads to Impaired Renal Expression of Connexins and Pannexin1 in St8sia1 Knockout Mice

The aim of this study was to determine the effects of altered ganglioside composition on the expression of Cx37, Cx40, Cx43, Cx45, and Panx1 in different kidney regions of St8sia1 gene knockout mice (St8sia1 KO) lacking the GD3 synthase enzyme. Experiments were performed in twelve male 6-month-old mice: four wild-type (C57BL/6-type, WT) and eight St8sia1 KO mice. After euthanasia, kidney tissue was harvested, embedded in paraffin wax, and processed for immunohistochemistry. The expression of connexins and Panx1 was determined in different regions of the kidney: cortex (CTX.), outer stripe of outer medulla (O.S.), inner stripe of outer medulla (IN.S.), and inner medulla (IN.MED.). We determined significantly lower expression of Cx37, Cx40, Cx45, and Panx1 in different parts of the kidneys of St8sia1 KO mice compared with WT. The most consistent decrease was found in the O.S. where all markers (Cx 37, 40, 45 and Panx1) were disrupted in St8si1 KO mice. In the CTX. region, we observed decrease in the expression of Cx37, Cx45, and Panx1, while reduced expression of Cx37 and Panx1 was more specific to IN.S. The results of the present study suggest that deficiency of GD3 synthase in St8sia1 KO mice leads to disruption of renal Cx expression, which is probably related to alteration of ganglioside composition.

Wilms' tumour 1 (WT1) negatively regulates the expression of connexin 43 via a non-canonical Wnt signalling pathway in cultured bovine Sertoli cells

The gap junction protein connexin (Cx) 43 between adjacent Sertoli cells (SCs) is the main testicular factor regulating the growth and development of SCs, and plays a vital role in controlling cell differentiation and maturation. However, the endogenous testicular factors that regulate Cx43 and the downstream signalling pathways that mediate Cx43-dependent SC differentiation are unclear. In this study, high-purity SCs were isolated from newborn calves' testes by differential adherence. The SCs were then cultured invitro and treated with short interference RNA to knockdown endogenous Wilms' tumour 1 (WT1). In WT1-knockdown SCs, Cx43 expression was upregulated. To elucidate the intracellular signalling mechanism of Cx43 in the differentiation and maturation of immature SCs, SCs were treated simultaneously with non-canonical Wnt signalling inhibitors CCG-1423 and GO-6983; in these SCs, Cx43 expression was upregulated. Together, these data indicate that WT1 negatively regulates the expression of Cx43 produced from SCs via a non-canonical Wnt signalling pathway in cultured bovine SCs.

Connexins and Gap Junctions in Cancer of the Urinary Tract

This review focuses on connexins and nexus or gap junctions in the genesis, progression, and therapy of carcinomas of the human urinary tract. Some decades ago, the idea was born that gap junctional intercellular communication might prevent both the onset and the progression of cancer. Later evidence indicated that, on the contrary, synthesis and the presence of connexins as a prerequisite for gap junctional intercellular communication might promote the occurrence of cancer and metastases. The research history of urinary bladder cancer is a good example of the development of scientific perception. So far, the role of gap junctional intercellular communication in carcinogenesis and cancer progression, as well as in therapeutical approaches, remains unclear.

Gap junctions in liver disease: Implications for pathogenesis and therapy

In the normal liver, cells interact closely through gap junctions. By providing a pathway for the trafficking of low molecular mass molecules, these channels contribute to tissue homeostasis and maintenance of hepatic function. Thus, dysfunction of gap junctions affects a wide variety of liver processes, such as differentiation, cell death, inflammation and fibrosis. In fact, dysfunctional gap junctions have been implicated, for more than a decade, in cholestatic disease, hepatic cancer and cirrhosis. Additionally, in recent years there is an increasing body of evidence that these channels are also involved in other relevant and prevalent liver pathological processes, such as non-alcoholic fatty liver disease, acute liver injury and portal hypertension. In parallel to these new clinical implications the available data include controversial observations. Thus, a comprehensive overview is required to better understand the functional complexity of these pores. This paper will review the most recent knowledge concerning gap junction dysfunction, with a special focus on the role of these channels in the pathogenesis of relevant clinical entities and on potential therapeutic targets that are amenable to modification by drugs.

Connexin43 Suppresses Lung Cancer Stem Cells

Alterations in gap junctions and their protein components, connexins, have been associated with neoplastic transformation and drug resistance, and more recently have been shown to play important roles in cancer stem cells (CSCs). However, there is less knowledge of connexins and gap junctions in lung CSCs. To address this, Connexin43 (Cx43), the major human lung epithelial gap junction protein, was expressed ectopically in poorly expressing National Cancer Institute-125 (NCI-H125) metastatic human lung adenocarcinoma cells, and phenotypic characteristics of malignant cells and abundance of CSCs were evaluated. The ectopic expression of Cx43 resulted in the formation of functional gap junctions; a more epithelial morphology; reduced proliferation, invasion, colony formation, tumorsphere formation, pluripotency marker expression, and percentage of aldehyde dehydrogenase (ALDH)-positive cells; and increased cisplatin sensitivity. Similarly, in NCI-H522 (human lung adenocarcinoma) and NCI-H661 (human lung large cell carcinoma) cell lines, which express Cx43 and functional gap junctions endogenously, the Cx43 content was lower in tumorspheres and ALDH-positive cells than in bulk cells. These results demonstrate that Cx43 can reverse several neoplastic characteristics and reduce the abundance of human lung CSCs.

Visualization of Annular Gap Junction Vesicle Processing: The Interplay Between Annular Gap Junctions and Mitochondria

It is becoming clear that in addition to gap junctions playing a role in cell⁻cell communication, gap junction proteins (connexins) located in cytoplasmic compartments may have other important functions. Mitochondrial connexin 43 (Cx43) is increased after ischemic preconditioning and has been suggested to play a protective role in the heart. How Cx43 traffics to the mitochondria and the interactions of mitochondria with other Cx43-containing structures are unclear. In this study, immunocytochemical, super-resolution, and transmission electron microscopy were used to detect cytoplasmic Cx43-containing structures and to demonstrate their interactions with other cytoplasmic organelles. The most prominent cytoplasmic Cx43-containing structures-annular gap junctions-were demonstrated to form intimate associations with lysosomes as well as with mitochondria. Surprisingly, the frequency of associations between mitochondria and annular gap junctions was greater than that between lysosomes and annular gap junctions. The benefits of annular gap junction/mitochondrial associations are not known. However, it is tempting to suggest, among other possibilities, that the contact between annular gap junction vesicles and mitochondria facilitates Cx43 delivery to the mitochondria. Furthermore, it points to the need for investigating annular gap junctions as more than only vesicles destined for degradation.

Coupling in Glial Cells: Who Is Coupled, and Why

One of the features that distinguishes glial cells from most other brain cells is their ability to communicate with each other through cytoplasmic connections established by gap- junctions. Gap-junctions are cell-cell channels composed of two connexons, one con tributed by each communicating cell. These connexons are formed by six connexin mol ecules that assemble in a hexagonal array to form a central pore. As is common for channel proteins, connexins comprise a superfamily of at least 12 members, with molecular weights ranging from 26 to 50 kDa and overall sequence identities of 50%. In the brain, only two connexins are expressed to a significant degree. These are connexin 43 (Cx43), which is the predominant molecular unit of astrocytic gap-junctions, and connexin 32 (Cx32), which forms the gap-junctions in oligodendrocytes and some neurons. The pore formed by these connexins is permeant to ions and small hydrophilic molecules with molecular weights <1 kDa. Molecules believed to be trafficking through gap-junctions include metabolites and second messengers, but hydrophobic molecules and most pro teins are excluded. It is believed that large numbers of glial cells, and particularly astro cytes, participate in syncytia in which cells share a common cytoplasm. Gap-junctions are thought to integrate regional variances between cells and provide a diffusional pathway to facilitate the sequestration and redistribution of ions and neurotransmitters. Recent evidence suggests that astrocytic gap-junction may also allow the passage of cell-cell signals, permitting information to spread in the form of regenerative Ca2+ changes through a glial network. The Neuroscientist 1:188-191, 1995

Molecular mechanisms regulating formation, trafficking and processing of annular gap junctions

Internalization of gap junction plaques results in the formation of annular gap junction vesicles. The factors that regulate the coordinated internalization of the gap junction plaques to form annular gap junction vesicles, and the subsequent events involved in annular gap junction processing have only relatively recently been investigated in detail. However it is becoming clear that while annular gap junction vesicles have been demonstrated to be degraded by autophagosomal and endo-lysosomal pathways, they undergo a number of additional processing events. Here, we characterize the morphology of the annular gap junction vesicle and review the current knowledge of the processes involved in their formation, fission, fusion, and degradation. In addition, we address the possibility for connexin protein recycling back to the plasma membrane to contribute to gap junction formation and intercellular communication. Information on gap junction plaque removal from the plasma membrane and the subsequent processing of annular gap junction vesicles is critical to our understanding of cell-cell communication as it relates to events regulating development, cell homeostasis, unstable proliferation of cancer cells, wound healing, changes in the ischemic heart, and many other physiological and pathological cellular phenomena.

Connexin 30 downregulates Insulin-like growth factor receptor-1, abolishes Erk and potentiates effects of an IGF-R inhibitor in a glioma cell line

Connexins (Cx) play a crucial role in cell communication though regulation of cell growth and proliferation. In recent decades, both suppressive and enhancing roles of gap junction proteins in malignancy have been proposed, though mechanisms remain unclear. We intend to evaluate the impact of Cx30 on dysregulated growth of glioma owing to an aberrant expression of Insulin-like growth factor-1 receptor (IGF-1R). The study also examined whether Cx30 expression influenced sensitivity of glioma cells to Picropodophyllin (PPP), the potent inhibitor of IGF-1R. C6 cells transfected with full length Cx30 resulted in complete abolition of colony-forming efficiency. Interestingly, PPP-supplemented cells behaved differently with and without exogenous Cx as confirmed by wound closure assay. The expressions of phosphorylated and unphosphorylated IGF-1R along with its key signaling enzymes, pAkt/pErk, were also varied significantly in transfected and non-transfected C6 cells. pIGF-1R and IGF-1R were significantly reduced on Cx30 transfection when compared with that of non-transfected cells. pErk expression was abolished in transfected C6 with no significant difference in the expression of pAkt. The potency of PPP against C6 was more pronounced in the presence of Cx30. We demonstrate that Cx30 has the potential to alter the IGF-1R mediated pathway thereby influencing the growth, proliferation and migration of glioma cells which could further enhance the effect of therapeutic intervention. Though it could not be corroborated that the observations made are due to Cx30-mediated channel-dependent and/or independent impact, we stress the impact of significance of Cx30 on IGF-1R in glioma and also in therapeutic aspects.

Adrenocortical Gap Junctions and Their Functions

Adrenal cortical steroidogenesis and proliferation are thought to be modulated by gap junction-mediated direct cell-cell communication of regulatory molecules between cells. Such communication is regulated by the number of gap junction channels between contacting cells, the rate at which information flows between these channels, and the rate of channel turnover. Knowledge of the factors regulating gap junction-mediated communication and the turnover process are critical to an understanding of adrenal cortical cell functions, including development, hormonal response to adrenocorticotropin, and neoplastic dedifferentiation. Here, we review what is known about gap junctions in the adrenal gland, with particular attention to their role in adrenocortical cell steroidogenesis and proliferation. Information and insight gained from electrophysiological, molecular biological, and imaging (immunocytochemical, freeze fracture, transmission electron microscopic, and live cell) techniques will be provided.

Interaction of Cx43 with Hsc70 regulates G1/S transition through CDK inhibitor p27

Connexin 43 (Cx43) functions as a cell growth suppressor. We have demonstrated that Cx43 interacts with heat shock cognate protein 70 (Hsc70) for regulating cell proliferation. Hsc70 interacts with CDK inhibitor p27, which regulates the assembly and subcellular localization of cyclin D1-CDK4-p27 complex. However, the involvement of p27 with Cx43-mediated cell cycle suppression is still poorly understood. Here, we report that nuclear accumulation of p27 is reduced by overexpression of Cx43, and that this reduction is restored by co-overexpression with Hsc70. We found that Cx43 competes with p27 for binding to Hsc70, and as a result, decreases the level of Hsc70 in cyclin D1-CDK4-p27 complex, leading to prevention of the nuclear translocation of the complex and the G1/S transition. Collectively, our findings suggest that, in Cx43 up-regulation, which is most likely an emergency measure, Cx43-Hsc70 interaction regulates cell cycle G1/S progression through a novel mechanism by which Cx43-Hsc70 interaction prevents the nuclear accumulation of p27 through controlling the nuclear translocation of cyclin D1-CDK4-p27 complex.

Connexin43 phosphorylation by PKC and MAPK signals VEGF-mediated gap junction internalization

Phosphorylation on well-recognized, regulatory connexin43 amino acid residues by a series of kinases serves as an early molecular signal that triggers not only inhibition of gap junction (GJ)–mediated cell-to-cell communication but also GJ internalization. The findings contribute to the newly evolving dynamic picture of GJs.

Gap junctions (GJs) exhibit a complex modus of assembly and degradation to maintain balanced intercellular communication (GJIC). Several growth factors, including vascular endothelial growth factor (VEGF), have been reported to disrupt cell–cell junctions and abolish GJIC. VEGF directly stimulates VEGF-receptor tyrosine kinases on endothelial cell surfaces. Exposing primary porcine pulmonary artery endothelial cells (PAECs) to VEGF for 15 min resulted in a rapid and almost complete loss of connexin43 (Cx43) GJs at cell–cell appositions and a concomitant increase in cytoplasmic, vesicular Cx43. After prolonged incubation periods (60 min), Cx43 GJs reformed and intracellular Cx43 were restored to levels observed before treatment. GJ internalization correlated with efficient inhibition of GJIC, up to 2.8-fold increased phosphorylation of Cx43 serine residues 255, 262, 279/282, and 368, and appeared to be clathrin driven. Phosphorylation of serines 255, 262, and 279/282 was mediated by MAPK, whereas serine 368 phosphorylation was mediated by PKC. Pharmacological inhibition of both signaling pathways significantly reduced Cx43 phosphorylation and GJ internalization. Together, our results indicate that growth factors such as VEGF activate a hierarchical kinase program—including PKC and MAPK—that induces GJ internalization via phosphorylation of well-known regulatory amino acid residues located in the Cx43 C-terminal tail.

Connexin 43 communication channels in follicular dendritic cell development and in follicular lymphomas

Follicular dendritic cells (FDC) show homo- and heterocellular metabolic coupling through connexin 43 (Cx43) gap junctions and support B cell selection and maturation in germinal centers. In follicular lymphomas B cells escape apoptosis while FDC develop abnormally. Here we tested Cx43 channels in reactive FDC development and follicular lymphomas. In culture, the treatment of FDC-B cell clusters (resembling to “ex vivo” germinal centers) with Gap27 peptide, mimicking the 2nd extracellular loop of Cx43 protein, significantly impaired FDC-B cell cluster formation and cell survival. In untreated cultures of intact clusters, cell proliferation showed a moderate reduction. In tissues, Cx43 protein levels run parallel with the density of FDC both in reactive germinal centers and in malformed follicles of follicular lymphomas and showed strong upregulation in newly generated and/or degrading bi-/multinuclear FDC of rudimentary processes. However, the inverse correlation between Cx43 expression and B cell proliferation seen in reactive germinal centers was not detected in follicular lymphomas. Furthermore, Cx43 levels were not associated with either lymphoma grade or bone marrow involvement. Our results suggest that Cx43 channels are critical in FDC and “ex vivo” germinal center development and in the persistence of FDC in follicular lymphomas but do not affect tumor progression.

Melanoma's connections to the tumour microenvironment

SUMMARY

Melanoma cells interact with and depend on seemingly normal cells in their tumour microenvironment to allow the acquisition of the hallmarks of solid cancer. In general, there are three types of interaction of melanoma cells with their microenvironment. First, there is bilateral communication between melanoma cells and the stroma, which includes fibroblasts, endothelial cells, immune cells, soluble molecules, and the extracellular matrix. Second, while under normal conditions keratinocytes control localisation and proliferative behaviour of melanocytes in the epidermis, once this balance is disturbed and a melanoma has developed, melanoma cells may take over the control of their epidermal tumour microenvironment. Finally, there are subcompartments within tumours with different microenvironmental milieu defined by their access to oxygen and nutrients. Therefore, different melanoma cells within a tumour face different microenvironments. Interactions between melanoma cells among each other and with the cell types in their microenvironment happen through endocrine and paracrine communication and/or through direct contact via cell-cell and cell-matrix adhesion, and gap junctional intercellular communication (GJIC). Connexins have been identified as key molecules for direct cell-cell communication and are also thought to be important for the release of signalling molecules from cells to the microenvironment. In this review we provide an update of the alterations in cell-cell communication in melanoma and the tumour microenvironment associated with melanoma development and progression.

A Cell-Cell Communication Marker for Identifying Targeted Tumor Therapies

Cell-cell communication through gap junctions is aberrant or absent in a majority of human cancer cells, compared to cells in corresponding normal tissues. This and other evidence has led to the hypothesis that gap junction channels, comprised of connexin proteins, are important in growth control and cancer progression. The major goal of this ongoing study was to identify bioactive compounds that specifically upregulate gap junction channel-mediated cell-cell communication as potential anti-tumor therapies. Control of cell-cell communication is linked to growth regulatory intracellular signaling pathways; we therefore further aimed to identify signaling pathways modulated by these compounds in order to assess their potential as targeted anti-tumor therapies. Compounds were screened for their ability to upregulate gap junction-mediated cell-cell communication by using a fluorescent dye transfer assay to measure cell-cell communication between tumor promoter-treated astroglial cells or ras-transformed epithelial cells. Western blotting using connexin-specific and phosphorylation site-specific antibodies was used to monitor phosphorylation changes in signaling pathway proteins. Our results identified three compounds that upregulate gap junction-mediated cell-cell communication in our screening assays, chaetoglobosin K(ChK), 4-phenyl-3-butenoic acid (PBA) and the methyl ester of PBA (PBA-Me). Further analyses demonstrated that in tumorigenic cells, ChK downregulates phosphorylation of Akt kinase, an enzyme in the PI3-kinase signaling pathway that is found to be upregulated in a number of human cancers, on a key activation site. However, ChK did not inhibit PI-3 kinase in vitro as did the classic PI-3 kinase inhibitor, Wortmannin. PBA and PBA-Me were found to upregulate phosphorylation of p38 MAPK on a key activation site in tumorigenic cells, which is downregulated in several human cancer cell types. ChK and PBA also decreased activation of SAPK/JNK, another kinase found to be upregulated in a number of human cancers. These studies highlight the potential of monitoring gap junction intercellular communication for identifying experimental anti-tumor compounds.

Gap junction and hemichannel-independent actions of connexins on cell and tissue functions--an update

Connexins, a family of transmembrane proteins, are components of both gap junction channels and hemichannels, which mediate the exchange of ions and small molecules between adjacent cells, and between the inside and outside of the cell, respectively. Substantial advancements have been made in the comprehension of the role of gap junctions and hemichannels in coordinating cellular events. In recent years, a plethora of studies demonstrate a role of connexin proteins in the regulation of tissue homeostasis that occurs independently of their channel activities. This is shown in the context of cell growth, adhesion, migration, apoptosis, and signaling. The major mechanisms of these channel-independent activities still remain to be discovered. In this review, we provide an updated overview on the current knowledge of gap junction- and hemichannel-independent functions of connexins, in particular, their effects on tumorigenesis, neurogenesis and disease development.

Connexin43 reduces melanoma growth within a keratinocyte microenvironment and during tumorigenesis in vivo

Connexins (Cx) have been identified as tumor suppressors or enhancers, a distinction that appears to be dependent on the type and stage of disease. However, the role of connexins in melanoma tumorigenesis and their status during cancer onset and progression remain controversial and unclear. Here, we show that the aggressive B16-BL6 mouse melanoma cell line expresses low basal levels of Cx26 and Cx43, rendering them gap junctional intercellular communication-deficient as elucidated by immunofluorescence, Western blotting, and dye transfer studies. Following ectopic expression of green fluorescent protein-tagged Cx26 and Cx43 in these connexin-deficient melanomas, punctate gap junction-like plaques were evident at sites of cell-cell apposition, and the incidence of dye transfer was significantly increased similar to connexin-rich keratinocytes. We found that the expression of Cx43, but not Cx26, significantly reduced cellular proliferation and anchorage-independent growth from control melanomas, whereas migration was unaffected. Additionally, melanomas expressing Cx43 displayed significantly reduced growth within the in situ-like microenvironment of keratinocytes, despite a lack of heterocellular gap junctional intercellular communication between the two cell types. Furthermore, when grown in vivo in the chicken chorioallantoic membrane, primary tumors derived from Cx43-expressing melanomas were significantly smaller than controls, whereas Cx26-expressing melanomas produced tumors similar to controls. Collectively, these results suggest that Cx43, and not Cx26, can act as a tumor suppressor during melanoma tumorigenesis.

The effect of the PQ1 anti-breast cancer agent on normal tissues

Gap junctions are intercellular channels connecting adjacent cells, allowing cells to transport small molecules. The loss of gap junctional intercellular communication (GJIC) is one of the important hallmarks of cancer. Restoration of GJIC is related to the reduction of tumorigenesis and increase in drug sensitivity. Previous reports have shown that PQ1, a quinoline derivative, increases GJIC in T47D breast cancer cells, and subsequently attenuates xenograft breast tumor growth. Combinational treatment of PQ1 and tamoxifen can lower the effective dose of tamoxifen in cancer cells. In this study, the effects of PQ1 were examined in normal C57BL/6J mice, evaluating the distribution, toxicity, and adverse effects. The distribution of PQ1 was quantified by high-performance liquid chromatography and mass spectrometry. The expressions of survivin, caspase-8, cleaved caspase-3, aryl hydrocarbon receptor (AhR), and gap junction protein, connexin 43 (Cx43), were assessed using western blot analysis. Our results showed that PQ1 was absorbed and distributed to vital organs within 1 h and the level of PQ1 decreased after 24 h. Furthermore, PQ1 increased the expression of survivin, but decreased the expression of caspase-8 and caspase-3 activity. Interestingly, the expression of AhR increased in the presence of PQ1, suggesting that PQ1 may be involved in the AhR-mediated response. Previously, PQ1 caused an increase in Cx43 expression in breast cancer cells; however, PQ1 induced a decrease in Cx43 in normal tissues. Hemotoxylin and eosin staining of the tissues showed no histological change between the treated and the untreated organs. Our studies indicate that the administration of PQ1 by an oral gavage can be achieved with low toxicity to normal vital organs.

Cx31.1 acts as a tumour suppressor in non-small cell lung cancer (NSCLC) cell lines through inhibition of cell proliferation and metastasis

Reduced connexin expression and loss of gap junction function is a characteristic of many cancers, including lung cancer. However, there are little reports about the relation between Cx31.1 and lung cancer. This study was conducted to investigate the effect of Cx31.1 on non-small cell lung cancer (NSCLC). We found that the Cx31.1 was down-regulated in NSCLC cell lines, and the expression levels were reversely related with their metastatic potential. We ectopically expressed Cx31.1 in H1299 NSCLC cell line to examine the influence of Cx31.1 overexpression. The results showed that overexpression of Cx31.1 in H1299 cells reduced cell proliferation, induced a delay in the G₁ phase, inhibited anchorage-independent growth and suppressed cell migration and invasion. The cell cycle delay and cell migration and invasion suppressive effects of Cx31.1 were partially reversed by siRNA targeting mRNA of Cx31.1. Moreover, xenografts of Cx31.1 overexpressing H1299 cells showed reduced tumourigenicity. These results suggested that Cx31.1 has tumour-suppressive properties. Further investigation indicated that cyclin D3 may be responsible for Cx31.1-induced G₁ phase delay. Importantly, Cx31.1 increased the expression of epithelial markers, such as cytokeratin 18, and decreased expression of mesenchymal markers, such as vimentin, indicating a Cx31.1-mediated partial shift from a mesenchymal towards an epithelial phenotype. We concluded that Cx31.1 inhibit the malignant properties of NSCLC cell lines, the mechanisms under this may include regulation of EMT.

Suppression of CX43 expression by miR-20a in the progression of human prostate cancer

The aberrant expression of microRNAs (miRNAs) has been found in various types of cancer. The present study found miR-20a to be significantly upregulated in prostate cancer compared with normal prostate tissues. The proliferation and colony formation assays revealed that the downregulation of miR-20a by miR-20a inhibitor suppresses the proliferation of MDA-PCa-2b cells in vitro and also inhibits tumor growth in vivo. Furthermore, a gap junction protein, α 1 (CX43), was identified as a direct target gene of miR-20a. The upregulation of CX43 was detected in MDA-PCa-2b cells after treatment with miR-20a inhibitor both in vitro and in vivo. In conclusion, the findings show that miR-20a significantly contributes to the progression of prostate cancer by targeting CX43.

Biology and biotechnology of follicle development

Growth and development of ovarian follicles require a series of coordinated events that induce morphological and functional changes within the follicle, leading to cell differentiation and oocyte development. The preantral early antral follicle transition is the stage of follicular development during which gonadotropin dependence is obtained and the progression into growing or atresia of the follicle is made. Follicular growth during this period is tightly regulated by oocyte-granulosatheca cell interactions. A cluster of early expressed genes is required for normal folliculogenesis. Granulosa cell factors stimulate the recruitment of theca cells from cortical stromal cells. Thecal factors promote granulosa cell proliferation and suppress granulosa cell apoptosis. Cell-cell and cell-extracellular matrix interactions influence the production of growth factors in the different follicular compartments (oocyte, granulosa, and theca cells). Several autocrine and paracrine factors are involved in follicular growth and differentiation; their activity is present even at the time of ovulation, decreasing the gap junction communication, and stimulating the theca cell proliferation. In addition, the identification of the factors that promote follicular growth from the preantral stage to the small antral stage may provide important information for the identification for assisted reproduction techniques.

Testicular connexin 43, a precocious molecular target for the effect of environmental toxicants on male fertility

Many recent epidemiological, clinical and experimental findings support the hypothesis that environmental toxicants are responsible for the increasing male reproductive disorders (congenital malformations, declining sperm counts and testicular cancer) over the past 20 years. It has also been reported that exposure to these toxicants, during critical periods of development (fetal and neonatal), represents a more considerable risk for animals and humans than exposure during adulthood. However, the molecular targets for these chemicals have not been clearly identified. Recent studies showed that a family of transmembranous proteins, named connexins, regulates numerous physiological processes involved in testicular development and function, such as Sertoli and germ cell proliferation, differentiation, germ cell migration and apoptosis. In the testis, knockout strategy revealed that connexin 43, the predominant connexin in this organ, is essential for spermatogenesis. In addition, there is evidence that many environmental toxicants could alter testicular connexin 43 by dysregulation of numerous mechanisms controlling its function. In the present work, we propose first to give an overview of connexin expression and intercellular gap junction coupling in the developing fetal and neonatal testes. Second, we underline the impact of maternally chemical exposure on connexin 43 expression in the perinatal developing testis. Lastly, we attempt to link this precocious effect to male offspring fertility.

The use of in vitro systems to assess cancer mechanisms and the carcinogenic potential of chemicals

Carcinogenesis is a highly complex, multi-stage process that can occur over a relatively long period before its clinical manifestation. While the sequence in which a cancer cell acquires the necessary traits for tumour formation can vary, there are a number of mechanisms that are common to most, if not all, cancers across the spectrum of possible causes. Many aspects of carcinogenesis can be modelled in vitro. This has led to the development of a number of mechanistically driven, cell-based assays to assess the pro-carcinogenic and anti-carcinogenic potential of chemicals. A review is presented of the current in vitro models that can be used to study carcinogenesis, with examples of cigarette smoke testing in some of these models, in order to illustrate their potential applications. We present an overview of the assays used in regulatory genotoxicity testing, as well as those designed to model other aspects that are considered to be hallmarks of cancer. The latter assays are described with a view to demonstrating the recent advances in these areas, to a point where they should now be considered for inclusion in an overall testing strategy for chemical carcinogens.

Altered expression and localization of connexin32 in human and murine gastric carcinogenesis

BACKGROUND

Intercellular communication via gap junctions, composed of protein subunits called connexins (Cxs), plays a key role in controlling cell growth, differentiation and carcinogenesis. Impaired gap junctional intercellular communication has been reported in various cancers and diseases.

AIMS

We investigated Cx32 expression patterns and semiquantitatively assessed Cx32 expression in cancers and preneoplastic lesions. To determine if cell proliferation is correlated with Cx32 expression, we evaluated Ki67 expression in a gastric cancer mouse model.

METHODS

In human and mouse, normal stomach and gastric adenocarcinoma tissues were used for immunohistochemical analyses.

RESULTS

Cx32 was detected at cell-cell (intercellular) contact points in normal cells and exhibited punctate intercellular and intracytoplasmic staining in cancer cells. The frequency of Cx32 loss of expression was significantly higher in human adenocarcinomas than in normal stomach. As tumor cells were less differentiated, Cx32 expression levels and intercellular and intracytoplasmic staining were also significantly lower. The Cx32 expression pattern in the mouse gastric cancer model was similar in several important respects to that of human. In mucous metaplasia of the mouse stomach, Cx32 was mainly expressed in the cytoplasm of epithelial cells. There was also an inverse correlation between Cx32 expression and cell proliferation in mouse tumors. However, there was no difference in the levels of Cx32 mRNA between normal and cancerous tissues.

CONCLUSIONS

These findings suggest that altered Cx32 expression, a loss of intercellular Cx32 and a gain of intracytoplasmic Cx32 in the form of punctate "dot", plays an important role in the formation of gastric adenocarcinomas.

All-trans retinoic Acid regulates cx43 expression, gap junction communication and differentiation in primary lens epithelial cells

PURPOSE

To examine the effect of all-trans retinoic acid (ATRA) treatment on connexin 43 (Cx43) expression, gap junction intercellular communication (GJIC), and cellular differentiation in primary canine lens epithelial cells (LEC).

METHODS AND MATERIALS

Dose and time-dependent effects of ATRA on Cx43 protein, mRNA and GJIC, were assessed by immunoblotting, quantitative reverse transcription polymerase chain reaction (qRT-PCR), and scrape loading/dye transfer assays, respectively. Expression of beta crystallin was evaluated by immunoblotting.

RESULTS

Treatment with ATRA at non-cytotoxic concentrations significantly increased Cx43 protein, mRNA and GJIC in primary canine LEC. Treatment with ATRA for five and seven days increased levels of beta crystallin, a protein marker of LEC differentiation. Inhibition of GJIC via pre-treatment with a synthetic inhibitor, 18-alpha glycyrrethinic acid (AGA), reduced ATRA-induced increases in Cx43 and GJIC and partially blocked ATRA-induced beta crystallin protein.

CONCLUSIONS

Treatment with ATRA significantly increased Cx43 expression and GJIC in canine LEC, and these effects were associated with increased LEC differentiation. Results from this study suggest that functional gap junctions may play a role in the modulation of cellular differentiation in primary canine LEC.

Implications and challenges of connexin connections to cancer

The idea that the gap junction family of proteins, connexins, are tumour suppressors has been widely supported through numerous cancer models. However, the paradigm that connexins and enhanced gap junctional intercellular communication is of universal benefit by restricting tumour growth has been challenged by more recent evidence that suggests a role for connexins in facilitating tumour progression and metastasis. Therefore, connexins might be better classified as conditional tumour suppressors that modulate cell proliferation, as well as adhesion and migration.

Regulation of renin release by connexin 43 in As 4.1 cell line

Gap junction channels facilitate chemical and electrical communication between adjacent cells. Gap junction protein, connexin (Cx), is expressed in the endothelial cells of vessels, glomerulus, and renin-secreting cells of the kidney. The purpose of this study was to investigate the role of Cx in renin release using Cx-overexpressing As 4.1 cells. The adenovirus-induced Cx overexpression was conducted by using recombinant adenovirus containing the cDNA encoding Cx37, Cx40, Cx43 (Ad-Cx), and beta-galactosidase (Ad-beta-gal). In 40-overexpressing cells, basal renin release increased in a time-dependent manner but it was significantly lower than that in Ad-beta-gal-treated cells. In Cx37- and Cx43-overexpressing cells, basal renin release was increased in a time-dependent manner, which was not different from control cells. 18-beta glycyrrhetinic acid (GA), a gap junction blocker, stimulated renin release dose-dependently and increased intracellular Ca(2+) in both Cx43-overexpressing cells and control cells. However, no significant differences were observed. An increase in renin release by 3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester, a putative antagonist of Ca(2+) release from intracellular sequestration sites, was also similar between two groups. These results suggest that Cx43 may unlikely alter the regulation of renin release and intracellular Ca(2+) by gap junction blocker in As 4.1 cells.

Cisplatin and oxaliplatin inhibit gap junctional communication by direct action and by reduction of connexin expression, thereby counteracting cytotoxic efficacy

Cisplatin [cis-diamminedichloroplatinum(II)]/oxaliplatin [1,2-diamminocyclohexane(trans-1)oxolatoplatinum(II)] toxicity is enhanced by functional gap junctions between treated cells, implying that inhibition of gap junctions may decrease cytotoxic activity of these platinum-based agents. This study investigates the effect of gap junction modulation by cisplatin/oxaliplatin on cytotoxicity in a transformed cell line. The effects were explored using junctional channels expressed in transfected HeLa cells and purified hemichannels. Junctional channels showed a rapid, dose-dependent decrease in dye coupling with exposure to cisplatin/oxaliplatin. With longer exposure, both compounds also decreased connexin expression. Both compounds inhibit the activity of purified connexin hemichannels, over the same concentration range that they inhibit junctional dye permeability, demonstrating that inhibition occurs by direct interaction of the drugs with connexin protein. Cisplatin/oxaliplatin reduced the clonogenic survival of HeLa cells at low density and high density in a dose-dependent manner, but to a greater degree at high density, consistent with a positive effect of gap junctional intercellular communication (GJIC) on cytotoxicity. Reduction of GJIC by genetic or pharmacological means decreased cisplatin/oxaliplatin toxicity. At low cisplatin/oxaliplatin concentrations, where effects on connexin channels are minimal, the toxicity increased with increased cell density. However, higher concentrations strongly inhibited GJIC, and this counteracted the enhancing effect of greater cell density on toxicity. The present results indicate that inhibition of GJIC by cisplatin/oxaliplatin decreases their cytotoxicity. Direct inhibition of GJIC and reduction of connexin expression by cisplatin/oxaliplatin may thereby compromise the effectiveness of these compounds and be a factor in the development of resistance to this class of chemotherapeutic agents.

Connexins and the kidney

Connexins (Cxs) are widely-expressed proteins that form gap junctions in most organs, including the kidney. In the renal vasculature, Cx37, Cx40, Cx43, and Cx45 are expressed, with predominant expression of Cx40 in the endothelial cells and Cx45 in the vascular smooth muscle cells. In the tubules, there is morphological evidence for the presence of gap junction plaques only in the proximal tubules. In the distal nephron, Cx30, Cx30.3, and Cx37 are expressed, but it is not known whether they form gap junctions connecting neighboring cells or whether they primarily act as hemichannels. As in other systems, the major function of Cxs in the kidney appears to be intercellular communication, although they may also form hemichannels that allow cellular secretion of large signaling molecules. Renal Cxs facilitate vascular conduction, juxtaglomerular apparatus calcium signaling, and tubular purinergic signaling. Accordingly, current evidence points to roles for these Cxs in several important regulatory mechanisms in the kidney, including the renin angiotensin system, tubuloglomerular feedback, and salt and water reabsorption. At the systemic level, renal Cxs may help regulate blood pressure and may be involved in hypertension and diabetes.

Maintaining Connexin43 Gap Junctional Communication in v-Src Cells Does Not Alter Growth Properties Associated with the Transformed Phenotype

Loss of connexin expression and/or gap junctional communication (GJC) has been correlated with increased rates of cell growth in tumor cells compared to their normal communication-competent counterparts. Conversely, reduced rates of cell growth have been observed in tumor cells that are induced to express exogenous connexins and re-establish GJC. It is not clear how this putative growth-suppressive effect of the connexin proteins is mediated and some data has suggested that this function may be independent of GJC. In mammalian cells that express v-Src, connexin43 (Cx43) is phosphorylated on Tyr247 and Tyr265 and this results in a dramatic disruption of GJC. Cells that express a Cx43 mutant with phenylalanine mutations at these tyrosine sites form functional gap junctions that, unlike junctions formed by wild type Cx43, remain functional in cells that co-express v-Src. These cells still appear transformed; however, it is not known whether their ability to maintain GJC prevents the loss of growth restraints that confine "normal" cells, such as the inability to grow in an anchorage-independent manner or to form foci. In these studies, we have examined some of the growth properties of cells with Cx43 gap junctions that remain communication-competent in the presence of the co-expressed v-Src oncoprotein.

Gap junctions as modulators of adrenal cortical cell proliferation and steroidogenesis

Gap junctions are membrane specializations that are composed of connexin protein. The assembly of these proteins into channels between cells allows for the intercellular transfer of regulatory molecules. In the adrenal gland, as in most other tissues, intercellular communication provides the potential for regulation of a number of complex interactive cell processes including differentiation, steroidogenesis, migration, and proliferation. This review is concerned with the regulation of gap junctions and cell function in cortical cells of the adrenal gland and in pathological disorders such as adrenal cancer.

Role of gap junctions in embryonic and somatic stem cells

Stem cells provide an invaluable tool to develop cell replacement therapies for a range of serious disorders caused by cell damage or degeneration. Much research in the field is focused on the identification of signals that either maintain stem cell pluripotency or direct their differentiation. Understanding how stem cells communicate within their microenvironment is essential to achieve their therapeutic potentials. Gap junctional intercellular communication (GJIC) has been described in embryonic stem cells (ES cells) and various somatic stem cells. GJIC has been implicated in regulating different biological events in many stem cells, including cell proliferation, differentiation and apoptosis. This review summarizes the current understanding of gap junctions in both embryonic and somatic stem cells, as well as their potential role in growth control and cellular differentiation.

The regulation of the gap junction of human mesenchymal stem cells through the internalization of quantum dots

The delivery mechanism of CdSe/ZnS quantum dots (QDs) into cells was previously found to critically determine the biocompatibility of QDs to human adult mesenchymal stem cells, but the associated mechanism remained unknown. The present study tried to establish a link between the above phenomenon and the change in gap junction upon QD internalization. By comparing Pep-1- and PolyFect-mediated QD internalizations, the connexin 43 (Cx43)-mediated gap junction intercellular communication (GJIC) of human adipose-derived adult stem cells was investigated in monolayer and in three-dimensional (3D) culture (alginate hollow spheres). The latter system offered cells more mobility, which was more similar as in vivo. The results showed that Pep-1-coated QDs, which escaped from the endo-/lysosome degradation, could activate the F-actin assembly and the ERK-dependent phosphorylation of Cx43. The consequence was a reduction in Cx43-mediated GJIC. When the cells were grown in high density 3D alginate hollow spheres instead of in monolayer, the decrease of GJIC caused by the QD internalization was restored. These results indicated that the adaptability in QDs-mediated regulation of GJIC with different delivery coatings depended on the culture systems. The study also suggested that the regulation of gap junction may play a key role in QD cytotoxicity.

The role of thyroid hormone in testicular development and function

Thyroid hormone is a critical regulator of growth, development, and metabolism in virtually all tissues, and altered thyroid status affects many organs and systems. Although for many years testis has been regarded as a thyroid hormone unresponsive organ, it is now evident that thyroid hormone plays an important role in testicular development and function. A considerable amount of data show that thyroid hormone influences steroidogenesis as well as spermatogenesis. The involvement of tri-iodothyronine (T(3)) in the control of Sertoli cell proliferation and functional maturation is widely accepted, as well as its role in postnatal Leydig cell differentiation and steroidogenesis. The presence of thyroid hormone receptors in testicular cells throughout development and in adulthood implies that T(3) may act directly on these cells to bring about its effects. Several recent studies have employed different methodologies and techniques in an attempt to understand the mechanisms underlying thyroid hormone effects on testicular cells. The current review aims at presenting an updated picture of the recent advances made regarding the role of thyroid hormones in male gonadal function.

Acute internalization of gap junctions in vascular endothelial cells in response to inflammatory mediator-induced G-protein coupled receptor activation

During the inflammatory response, activation of G-protein coupled receptors (GPCRs) by inflammatory mediators rapidly leads to inhibition of gap junction intercellular communication (GJIC); however, the steps that lead to this inhibition are not known. Combining high-resolution fluorescence microscopy and functional assays, we found that activation of the GPCRs PAR-1 and ET(A/B) by their natural inflammatory mediator agonists, thrombin and endothelin-1, resulted in rapid and acute internalization of gap junctions (GJs) that coincided with the inhibition of GJIC followed by increased vascular permeability. The endocytosis protein clathrin and the scaffold protein ZO-1 appeared to be involved in GJ internalization, and ZO-1 was partially displaced from GJs during the internalization process. These findings demonstrate that GJ internalization is an efficient mechanism for modulating GJIC in inflammatory response.

A potential role of connexin 43 in epidermal growth factor-induced proliferation of mouse embryonic stem cells: involvement of Ca2+/PKC, p44/42 and p38 MAPKs pathways

OBJECTIVES

The gap junction protein, connexin (Cx), plays an important role in maintaining cellular homeostasis and cell proliferation by allowing communication between adjacent cells. Therefore, this study has examined the effect of epidermal growth factor (EGF) on Cx43 and its relationship to proliferation of mouse embryonic stem cells.

MATERIALS AND METHODS

Expressions of Cx43, mitogen-activated protein kinases (MAPKs) and cell cycle regulatory proteins were assessed by Western blot analysis. Cell proliferation was assayed with [(3)H]thymidine incorporation. Intercellular communication level was measured by a scrape loading/dye transfer method.

RESULTS

The results showed that EGF increased the level of Cx43 phosphorylation in a time- (> or =5 min) and dose- (> or =10 ng/mL) dependent manner. Indeed, EGF-induced increase in phospho-Cx43 level was significantly blocked by either AG 1478 or herbimycin A (tyrosine kinase inhibitors). EGF increased Ca(2+) influx and protein kinase C (PKC) translocation from the cytosolic compartment to the membrane compartment. Moreover, pre-treatment with BAPTA-AM (an intracellular Ca(2+) chelator), EGTA (an extracellular Ca(2+) chelator), bisindolylmaleimide I or staurosporine (PKC inhibitors) inhibited the EGF-induced phosphorylation of Cx43. EGF induced phosphorylation of p38 and p44/42 MAPKs, and this was blocked by SB 203580 (a p38 MAPK inhibitor) and PD 98059 (a p44/42 MAPK inhibitor), respectively. EGF or 18alpha-glycyrrhetinic acid (GA; a gap junction inhibitor) increased expression levels of the protooncogenes (c-fos, c-jun and c-myc), cell cycle regulatory proteins [cyclin D1, cyclin E, cyclin-dependent kinase 2 (CDK2), CDK4 and p-Rb], [(3)H]thymidine incorporation and cell number, but decreased expression levels of the p21(WAF1/Cip1) and p27(Kip1), CDK inhibitory proteins. Transfection of Cx43 siRNA also increased the level of [(3)H]thymidine incorporation and cell number. EGF, 18alpha-GA or transfection of Cx43 siRNA increased 2-DG uptake and GLUT-1 protein expression.

CONCLUSIONS

EGF-induced phosphorylation of Cx43, which was mediated by the Ca(2+)/PKC, p44/42 and p38 MAPKs pathways, partially contributed to regulation of mouse embryonic stem cell proliferation.

Function of connexins in the renal circulation

Connexins form intercellular channels that span two plasma membranes and directly couple the cytoplasm of adjacent cells. This morphological contact enables the exchange of ions, second messengers, and metabolites, which act to regulate several biological functions. This review focuses on the significance of connexins in the renal circulation. Cells of the renal vasculature are coupled and express connexins in a vessel and cell-specific pattern. This finding indicates that renal connexins likely play an important role in renal autoregulatory mechanisms (Bayliss effect, tubuloglomerular feedback) and in the control of vasomotor responses. The described coupling of endothelial and vascular smooth muscle cells in the afferent arterioles may also contribute to the communication of neighboring nephrons, called 'nephron coupling.' Furthermore, deletion of the Cx40 and Cx43 genes results in an altered functional behavior of the renin-producing cells, suggesting involvement of these connexin isoforms in the regulation of renin secretion and synthesis. In addition, this review discusses the role of renal connexin expression in the pathogenesis of hypertension or diabetes.

The stage-specific function of gap junctions during tumourigenesis

Tumour development is a process resulting from the disturbance of various cellular functions including cell proliferation, adhesion and motility. While the role of these cell parameters in tumour promotion and progression has been widely recognized, the mechanisms that influence gap junctional coupling during tumorigenesis remain elusive. Neoplastic cells usually display decreased levels of connexin expression and/or gap junctional coupling. Thus, impaired intercellular communication via gap junctions may facilitate the release of a potentially neoplastic cell from the controlling regime of the surrounding tissue, leading to tumour promotion. However, recent data indicates that metastatic tumour cell lines are often characterized by relatively high levels of connexin expression and gap junctional coupling. This review outlines current knowledge on the role of connexins in tumorigenesis and the possible mechanisms of the interference of gap junctional coupling with the processes of tumour invasion and metastasis.

Accelerated internalization of junctional membrane proteins (connexin 43, N-cadherin and ZO-1) within endocytic vacuoles: an early event of DDT carcinogenicity

Stability of cell-to-cell interactions and integrity of junctional membrane proteins are essential for biological processes including cancer prevention. The present study shows that DDT, a non-genomic carcinogen used at a non-cytotoxic dose (1 microM), rapidly disrupted the cell-cell contacts and concomitantly induced the formation of cytoplasmic vacuoles close to the plasma membrane in the SerW3 Sertoli cell line. High-resolution deconvolution microscopy reveals that this vacuolization process was clathrin-dependent since a hyperosmotic media (0.2 M sucrose) blocked rhodamine-dextran endocytosis. In response to DDT, junctional proteins such as Cx43, N-Cadherin and ZO-1 were internalized and present in vacuoles. In Cx43-GFP transfected cells, time lapse videomicroscopy demonstrates that DDT rapidly enhanced fragmentation of the gap junction plaques and abolished the gap junction coupling without major modification of Cx43 phosphorylation status. Repeated exposure to DDT resulted in chronic gap junction coupling injury. The present results demonstrate that one of the early effect of DDT is to interfere with the plasma membrane and to perturb its function, specifically its ability to establish cell-cell junctions that are essential for tissue homeostasis and control of cell proliferation and differentiation. Such an alteration may play a specific role during carcinogenesis.

Organizational principles of the connexin-related brain transcriptome

We have found that deletion of genes encoding the gap junction proteins Cx43, Cx32 and Cx36 alter the expression levels of large numbers of genes in mouse brain located on all chromosomes and encoding proteins from all major functional categories. Gene regulation in Cx32 and Cx43 null brains was more similar than that in the Cx36 null brain, suggesting the possibility of transcriptomic controls exerted by both genes on both astrocytes and oligodendrocytes. In order to explore the nature of expression linkage among the genes, we examined coordinated expression patterns in wild-type and connexin null brains. Coordination with Cx43 in wild-type brain predicted regulation in Cx43 nulls with considerable accuracy. Moreover, interlinkage within gene networks was greatly perturbed in the Cx43 null brain. These findings suggest several principles regarding regulatory transcriptomic networks involving gap junction genes and raise the issue of the underlying cause of connexin null phenotypes as well as mechanisms of regulation.

Immunohistochemical analysis of connexin43 expression in infertile human testes

Connexin43 (Cx43) is abundantly expressed in mammalian testes and implicated in the regulation of cell-to-cell interaction between germ cells and Sertoli cells, which is essential to the normal process of spermatogenesis. In the present study, we investigated the relation between Cx43 expression and the degree of spermatogenesis in infertile human testes. Immunohistochemical analysis of Cx43 was performed on testicular biopsies from 29 patients with azoospermia (n=23) and severe oligospermia (n=6), who gave informed consent to this experiment. The degree of testicular spermatogenesis was evaluated by Johnsen score. In the interstitium, immunostaining for Cx43 was localized to some focal parts of plasma membrane between neighboring Leydig cells. In seminiferous tubules with normal spermatogenesis, Cx43 expression was found between Sertoli cells and germ cells. However, Cx43 expression in maturation arrest was decreased and located mainly in the basal compartment of seminiferous tubules. Finally, there was a significant positive correlation between histological score of spermatogenesis and intensity of Cx43 (p=0.0294). These data suggest that the alteration of Cx43 expression may be involved in spermatogenic impairment, and that the communication between Sertoli cells and germ cells through Cx43 may be important for maturation of spermatogenesis.

Regulation of gap junction intercellular communication in primary canine lens epithelial cells: role of protein kinase C

PURPOSE

Gap junction intercellular communication (GJIC) is important in maintaining lens epithelial cell homeostasis and reductions in GJIC may be associated with the development of cataract. Protein kinase C (PKC) activation can disrupt gap junction communication via phosphorylation of connexin 43 (C x 43) proteins that compose gap junction channels. This study examined the role of PKC activation in modulating GJIC in a primary canine lens epithelial cell (LEC) line.

METHODS

TPA (12-O-tetradecanoyl-phorbol-acetate), a potent PKC activator and inhibitor of GJIC, was utilized in the present study. Primary cultures of canine LEC were treated with TPA (0-1000 ng/ml) for 0.5 hr and GJIC was assessed by scrape loading/dye transfer (SL/DT), and immunoblotting to detect phosphorylation of C x 43 protein. Inhibition of general and calcium-dependent PKC activity was achieved by pretreatment of cells with GF109203X and Gö6976, respectively.

RESULTS

Treatment with TPA (1-1000 ng/ml) significantly decreased GJIC in canine LEC as assessed by SL/DT. Pretreatment with 10 and 100 ng/ml TPA decreased GJIC by 80% as compared to controls and increased Cx43 phosphorylation as assessed by immunoblotting. Pretreatment of cells with GF109203X and Gö6976, partially restored TPA-inhibited GJIC by 40% and 60%, respectively, and reduced C x 43 phosphorylation. Expression of calcium dependent PKC isoforms was detected in canine whole lens and LEC.

CONCLUSIONS

Treatment with TPA significantly reduces GJIC in canine LEC. These effects are mediated, in part, by activation of calcium-dependent PKC isoforms. Primary canine LEC are a useful model in the study of the molecular mechanisms involved in GJIC and cataractogenesis.

Cigarette smoke components inhibited intercellular communication and differentiation in human pancreatic ductal epithelial cells

Smoking is a well-documented risk factor for the development of pancreatic adenocarcinoma. Although the most abundant polycyclic aromatic hydrocarbons (PAHs) in cigarette smoke are methylated anthracenes and phenanthrenes, the epigenetic toxicity of these compounds has not been extensively studied. We previously showed that methylanthracenes, which possess a bay-like structure, affect epigenetic events such as an induced release of arachidonic acid, inhibition of gap junctional intercellular communication (GJIC) and induction of mitogen-activated protein kinases in a pluripotent rat liver epithelial stem cell line. Anthracenes with no bay-like structures were inactive. These biological effects are all molecular events associated with the promotional phase of cancer. A human immortalized, nontumorigenic pancreatic ductal epithelial cell line, H6c7, was examined to study the epigenetic toxicity of PAHs related to pancreatic cancer by using scrape-loading dye transfer, immunostaining, RT-PCR and telomerase assay methods. H6c7 cells were GJIC-incompetent and exhibited high telomerase activity when grown in growth factor and hormone-supplemented medium. In the presence of the cAMP elevating drugs (forskolin and IBMX) the cells became GJIC competent and expressed connexins. Telomerase activity was also decreased by cAMP elevating drug treatment. After induction of cAMP, 1-methylanthracene with bay-like structures inhibited GJIC, whereas the 2-methylanthracene lacking a bay-like structure had no effect on GJIC. Telomerase activity remained high in 1-methylanthracene treatment but not with 2-methylanthracene. These results indicate that a prominent component of cigarette smoke, namely methylanthracenes with distinct structural configurations, could be a potential etiological agent contributing to the epigenetic events of pancreatic cancer.

Myoblast proliferation and syncytial fusion both depend on connexin43 function in transfected skeletal muscle primary cultures

Muscles are formed by fusion of individual postmitotic myoblasts to form multinucleated syncytial myotubes. The process requires a well-coordinated transition from proliferation, through migratory alignment and cycle exit, to breakdown of apposed membranes. Connexin43 protein and cell-cycle inhibitor levels are correlated, and gap junction blockers can delay muscle regeneration, so a coordinating role for gap junctions has been proposed. Here, wild-type and dominant-negative connexin43 variants (wtCx43, dnCx43) were introduced into rat myoblasts in primary culture through pIRES-eGFP constructs that made transfected cells fluoresce. GFP-positive cells and vitally-stained nuclei were counted on successive days to reveal differences in proliferation, and myotubes were counted to reveal differences in fusion. Individual transfected cells were injected with Cascade Blue, which permeates gap junctions, mixed with FITC-dextran, which requires cytoplasmic continuity to enter neighbouring cells. Myoblasts transfected with wtCx43 showed more gap-junctional coupling than GFP-only controls, began fusion sooner as judged by the incidence of cytoplasmic coupling, and formed more myotubes. Myoblasts transfected with dnCx43 remained proliferative for longer than either GFP-only or wtCx43 myoblasts, showed less coupling, and underwent little fusion into myotubes. These results highlight the critical role of gap-junctional coupling in myotube formation.

Gap junctional communication in morphogenesis

Gap junctions permit the direct passage of small molecules from the cytosol of one cell to that of its neighbor, and thus form a system of cell-cell communication that exists alongside familiar secretion/receptor signaling. Because of the rich potential for regulation of junctional conductance, and directional and molecular gating (specificity), gap junctional communication (GJC) plays a crucial role in many aspects of normal tissue physiology. However, the most exciting role for GJC is in the regulation of information flow that takes place during embryonic development, regeneration, and tumor progression. The molecular mechanisms by which GJC establishes local and long-range instructive morphogenetic cues are just beginning to be understood. This review summarizes the current knowledge of the involvement of GJC in the patterning of both vertebrate and invertebrate systems and discusses in detail several morphogenetic systems in which the properties of this signaling have been molecularly characterized. One model consistent with existing data in the fields of vertebrate left-right patterning and anterior-posterior polarity in flatworm regeneration postulates electrophoretically guided movement of small molecule morphogens through long-range GJC paths. The discovery of mechanisms controlling embryonic and regenerative GJC-mediated signaling, and identification of the downstream targets of GJC-permeable molecules, represent exciting next areas of research in this fascinating field.

The role of connexins in controlling cell growth and gene expression

The purpose of this paper is to provide a brief overview of current thinking on the role of connexins, in particular Cx43, in growth regulation, and a more detailed discussion as to potential mechanisms involved with an emphasis on gene expression. While the precise molecular mechanism by which connexins can affect the growth of normal or tumor cells remains elusive, a number of exciting reports have expanded our understanding and are presented in some detail. Thus, we will discuss (Section 2): the role of protein-protein interactions in integrating connexins into multiple signal transduction pathways; phosphorylation at specific sites and reversal of growth inhibition; the role of the carboxy-terminal regulatory domain as a signaling molecule. Some of our latest work on the potential functions of endogenously produced carboxy-terminal fragments of Cx43 are also presented (Section 3). Finally, Section 4 will pay tribute to the rapidly emerging realization that connexins such as Cx43 and Cx32 exert important and extensive effects on gene expression, particularly those genes linked to growth regulation.

Adenosine stimulates connexin 43 expression and gap junctional communication in pituitary folliculostellate cells

Adenosine is known to stimulate interleukin (IL)-6 and vascular endothelial growth factor (VEGF) secretion from pituitary TtT/GF folliculostellate [corrected] (FS) cells indicating that it is an important paracrine regulator of anterior pituitary function. This study demonstrates that rodent anterior pituitary cell lines produce extracellular adenosine that is able to increase intercellular gap junction communication in FS cells. Ecto-5'-nucleotidase (CD73), the enzyme that generates adenosine from AMP, was demonstrated by immunocytochemistry in approximately 20% of anterior pituitary cells, and some of these cells colocalized with prolactin and growth hormone. CD73 mRNA and protein were detected in GH3 and MMQ (somatotroph-lactotroph lineages) and TtT/GF cells, and enzyme activity was demonstrated by the conversion of exogenously added fluorescent ethenoAMP to ethenoadenosine. Adenosine production, as measured by HPLC, was detected in GH3 (1 microM/h) and MMQ (3 microM/h) but not in TtT/GF cells. Adenosine (EC50: 0.5 microM) and NECA (universal adenosine receptor agonist; EC50 0.1 microM) stimulated connexin 43 (Cx43) mRNA and protein expression within 1-2 h in TtT/GF cells. Adenosine and NECA also stimulated gap junctional intercellular communication (as assessed by transmission of Alexa Fluor 488) by 6- to 8-fold in comparison with untreated TtT/GF cells. In cocultures of MMQ and TtT/GF cells, Cx43 expression in TtT/GF cells increased in proportion to the number of MMQ cells plated out. These data suggest that adenosine, formed locally in the anterior pituitary gland can stimulate gap junction communication in FS cells.