Gap junction distribution and connexin expression in human breast

Insights into the role of connexins and specialized intercellular communication pathways in breast cancer: Mechanisms and applications

Gap junctions, membrane-based channels comprised of connexin proteins (Cxs), facilitate direct communication among neighbouring cells and between cells and the extracellular space through their hemichannels. The normal human breast expresses various Cxs family proteins, such as Cx43, Cx30, Cx32, Cx46, and Cx26, crucial for proper tissue development and function. These proteins play a significant role in breast cancer development, progression, and therapy response. In primary tumours, there is often a reduction and cytoplasmic mislocalization of Cx43 and Cx26, while metastatic lesions show an upregulation of these and other Cxs. Although existing research predominantly supports the tumour-suppressing role of Cxs in primary carcinomas through channel-dependent and independent functions, controversies persist regarding their involvement in the metastatic process. This review aims to provide an updated perspective on Cxs in human breast cancer, with a specific focus on intrinsic subtypes due to the heterogeneous nature of this disease. Additionally, the manuscript will explore the role of Cxs in immune interactions and novel forms of intercellular communication, such as tunneling nanotubes and extracellular vesicles, within the breast tumour context and tumour microenvironment. Recent findings suggest that Cxs hold potential as therapeutic targets for mitigating metastasis and drug resistance. Furthermore, they may serve as novel biomarkers for cancer prognosis, offering promising avenues for future research and clinical applications.

Mapping the Anti-Cancer Activity of α-Connexin Carboxyl-Terminal (aCT1) Peptide in Resistant HER2+ Breast Cancer

Connexin 43 (Cx43) is a protein encoded by the GJA1 gene and is a component of cell membrane structures called gap junctions, which facilitate intercellular communication. Prior evidence indicates that elevated GJA1 expression in the HER2-positive (HER2+) subtype of breast cancer is associated with poor prognosis. Prior evidence also suggests that HER2+ breast cancers that have become refractory to HER2-targeted agents have a loss of Cx43 gap junction intercellular communication (GJIC). In this study, a Cx43-targeted agent called alpha-connexin carboxyl-terminal peptide (aCT1) is examined to determine whether GJIC can be rescued in refractory HER2+ breast cancer cells. A proposed mechanism of action for aCT1 is binding to the tight junction protein Zonal Occludens-1 (ZO-1). However, the true scope of activity for aCT1 has not been explored. In this study, mass spectrometry proteomic analysis is used to determine the breadth of aCT1-interacting proteins. The NanoString nCounter Breast Cancer 360 panel is also used to examine the effect of aCT1 on cancer signaling in HER2+ breast cancer cells. Findings from this study show a dynamic range of binding partners for aCT1, many of which regulate gene expression and RNA biology. nCounter analysis shows that a number of pathways are significantly impacted by aCT1, including upregulation of apoptotic factors, leading to the prediction and demonstration that aCT1 can boost the cell death effects of cisplatin and lapatinib in HER2+ breast cancer cells that have become resistant to HER2-targeted agents.

The direct transfer approach for transcellular drug delivery

A promising paradigm for drug administration that has garnered increasing attention in recent years is the direct transfer (DT) of nanoparticles for transcellular drug delivery. DT requires direct cell-cell contact and facilitates unidirectional and bidirectional matter exchange between neighboring cells. Consequently, DT enables fast and deep penetration of drugs into the targeted tissues. This comprehensive review discusses the direct transfer concept, which can be delineated into the following three distinct modalities: membrane contact-direct transfer, gap junction-mediated direct transfer (GJ-DT), and tunneling nanotubes-mediated direct transfer (TNTs-DT). Further, the intercellular structures for each modality of direct transfer and their respective merits and demerits are summarized. The review also discusses the recent progress on the drugs or drug delivery systems that could activate DT.

Connexin 32 overexpression increases proliferation, reduces gap junctional intercellular communication, motility and epithelial-to-mesenchymal transition in Hs578T breast cancer cells

Connexins (Cx) are primary components of gap junctions that selectively allow molecules to be exchanged between adjacent cells, regulating multiple cellular functions. Along with their channel forming functions, connexins play a variety of roles in different stages of tumorigenesis and their roles in tumor initiation and progression is isoform- and tissue-specific. While Cx26 and Cx43 were downregulated during breast tumorigenesis, Cx32 was accumulated in the cytoplasm of the cells in lymph node metastasis of breast cancers and Cx32 was further upregulated in metastasis. Cx32's effect on cell proliferation, gap junctional communication, hemichannel activity, cellular motility and epithelial-to-mesenchymal transition (EMT) were investigated by overexpressing Cx32 in Hs578T and MCF7 breast cancer cells. Additionally, the expression and localization of Cx26 and Cx43 upon Cx32 overexpression were examined by Western blot and immunostaining experiments, respectively. We observed that MCF7 cells had endogenous Cx32 while Hs578T cells did not and when Cx32 was overexpressed in these cells, it caused a significant increase in the percentages of Hs578T cells at the S phase in addition to increasing their proliferation. Further, while Cx32 overexpression did not induce hemichannel activity in either cell, it decreased gap junctional communication between Hs578T cells. Additionally, Cx32 was mainly observed in the cytoplasm in both cells, where it did not form gap junction plaques but Cx32 overexpression reduced Cx43 levels without affecting Cx26. Moreover, migration and invasion potentials of Hs578T and migration in MCF7 were reduced upon Cx32 overexpression. Finally, the protein level of mesenchymal marker N-cadherin decreased while epithelial marker ZO-1 and E-cadherin increased in Hs578T cells. We observed that Cx32 overexpression altered cell proliferation, communication, migration and EMT in Hs578T, suggesting a tumor suppressor role in these cells while it had minor effects on MCF7 cells.

Ouabain Promotes Gap Junctional Intercellular Communication in Cancer Cells

Gap junctions are molecular structures that allow communication between neighboring cells. It has been shown that gap junctional intercellular communication (GJIC) is notoriously reduced in cancer cells compared to their normal counterparts. Ouabain, a plant derived substance, widely known for its therapeutic properties on the heart, has been shown to play a role in several types of cancer, although its mechanism of action is not yet fully understood. Since we have previously shown that ouabain enhances GJIC in epithelial cells (MDCK), here we probed whether ouabain affects GJIC in a variety of cancer cell lines, including cervico-uterine (CasKi, SiHa and Hela), breast (MDA-MB-321 and MCF7), lung (A549), colon (SW480) and pancreas (HPAF-II). For this purpose, we conducted dye transfer assays to measure and compare GJIC in monolayers of cells with and without treatment with ouabain (0.1, 1, 10, 50 and 500 nM). We found that ouabain induces a statistically significant enhancement of GJIC in all of these cancer cell lines, albeit with distinct sensitivity. Additionally, we show that synthesis of new nucleotides or protein subunits is not required, and that Csrc, ErK1/2 and ROCK-Rho mediate the signaling mechanisms. These results may contribute to explaining how ouabain influences cancer.

The Use of Biomarkers as Alternatives to Current Animal Tests on Food Chemicals

Recent developments in biomarkers relating to the interrelationship of diet, disease and health were surveyed. Most emphasis was placed on biomarkers of deleterious effects, since these are of greatest relevance to the subject of this review. The area of greatest activity was found to be that relating to biomarkers of mutagenic, genotoxic and carcinogenic effects. This is also one of the major areas of concern in considerations of the beneficial and deleterious effects of dietary components, and also the area in which regulatory testing requires studies of the longest duration. A degree of progress has also been made in the identification and development of biomarkers relating to certain classes of target organ toxicity. Biomarkers for other types of toxicity, such as immunotoxicity, neurotoxicity, reproductive toxicity and developmental toxicity, are less developed, and further investigation in these areas is required before a comprehensive biomarker strategy can be established. A criticism that recurs constantly in the biomarker literature is the lack of standardisation in the methods used, and the lack of reference standards for the purposes of validation and quality control. It is encouraging to note the growing acknowledgement of the need for validation of biomarkers and biomarker assays. Some validation studies have already been initiated. This review puts forward proposals for criteria to be used in biomarker validation. More discussion on this subject is required. It is concluded that the use of biomarkers can, in some cases, facilitate the implementation of the Three Rs with respect to the testing of food chemicals and studies on the effects of diet on health. The greatest potential is seen to be in the refinement of animal testing, in which biomarkers could serve as early and sensitive endpoints, in order to reduce the duration of the studies and also reduce the number of animals required. Biomarkers could also contribute to establishing a mechanistic basis for in vitro test systems and to facilitating their validation and acceptance. Finally, the increased information that could result from the incorporation of biomarker determinations into population studies could reduce the need for supplementary animal studies. This review makes a number of recommendations concerning the prioritisation of future activities on dietary biomarkers in relation to the Three Rs. It is emphasised, however, that further discussions will be required among toxicologists, epidemiologists and others researching the relationship between diet and health.

Novel Techniques to Study the Bone-Tumor Microenvironment

Many cancers commonly metastasize to bone. After entering the bone, cancer cells can interact with surrounding stromal cells, which ultimately influences metastasis progression. Extracellular vesicles, direct cell contact and gap junctions, and cytokines are all mechanisms of intercellular communication that have been observed to occur in the bone microenvironment. These methods of cellular crosstalk can occur between cancer cells and a variety of stromal cells, with each interaction having a different impact on cancer progression. Communication between cancer cells and bone-resident cells has previously been implicated in processes such as cancer cell trafficking and arrest in bone, cancer cell dormancy, cancer cell reactivation, and proliferation. In this chapter we review innovative techniques and model systems that can be used to study bidirectional crosstalk between cancer cells and stromal cells in the bone, with an emphasis specifically on bone-metastatic breast cancer. Investigating how metastatic cancer cells interact with, and are influenced by, the bone microenvironment is crucial to better understanding of the progression of bone metastasis.

Connexin 43 maintains tissue polarity and regulates mitotic spindle orientation in the breast epithelium

Cell-cell communication is essential for tissue homeostasis, but its contribution to disease prevention remains to be understood. We demonstrate the involvement of connexin 43 (Cx43, also known as GJA1) and related gap junction in epithelial homeostasis, illustrated by polarity-mediated cell cycle entry and mitotic spindle orientation (MSO). Cx43 localization is restricted to the apicolateral membrane of phenotypically normal breast luminal epithelial cells in 3D culture and in vivo Chemically induced blockade of gap junction intercellular communication (GJIC), as well as the absence of Cx43, disrupt the apicolateral distribution of polarity determinant tight junction marker ZO-1 (also known as TJP1) and lead to random MSO and cell multilayering. Induced expression of Cx43 in cells that normally lack this protein reestablishes polarity and proper MSO in 3D culture. Cx43-directed MSO implicates PI3K-aPKC signaling, and Cx43 co-precipitates with signaling node proteins β-catenin (CTNNB1) and ZO-2 (also known as TJP2) in the polarized epithelium. The distribution of Cx43 is altered by pro-inflammatory breast cancer risk factors such as leptin and high-fat diet, as shown in cell culture and on tissue biopsy sections. The control of polarity-mediated quiescence and MSO may contribute to the tumor-suppressive role of Cx43.

Connexin 43 Loss Triggers Cell Cycle Entry and Invasion in Non-Neoplastic Breast Epithelium: A Role for Noncanonical Wnt Signaling

(1) Background: The expression of connexin 43 (Cx43) is disrupted in breast cancer, and re-expression of this protein in human breast cancer cell lines leads to decreased proliferation and invasiveness, suggesting a tumor suppressive role. This study aims to investigate the role of Cx43 in proliferation and invasion starting from non-neoplastic breast epithelium. (2) Methods: Nontumorigenic human mammary epithelial HMT-3522 S1 cells and Cx43 shRNA-transfected counterparts were cultured under 2-dimensional (2-D) and 3-D conditions. (3) Results: Silencing Cx43 induced mislocalization of β-catenin and Scrib from apicolateral membrane domains in glandular structures or acini formed in 3-D culture, suggesting the loss of apical polarity. Cell cycle entry and proliferation were enhanced, concomitantly with c-Myc and cyclin D1 upregulation, while no detectable activation of Wnt/β-catenin signaling was observed. Motility and invasion were also triggered and were associated with altered acinar morphology and activation of ERK1/2 and Rho GTPase signaling, which acts downstream of the noncanonical Wnt pathway. The invasion of Cx43-shRNA S1 cells was observed only under permissive stiffness of the extracellular matrix (ECM). (4) Conclusion: Our results suggest that Cx43 controls proliferation and invasion in the normal mammary epithelium in part by regulating noncanonical Wnt signaling.

Gap Junctions and Wnt Signaling in the Mammary Gland: a Cross-Talk?

Connexins (Cxs), the building blocks of gap junctions (GJs), exhibit spatiotemporal patterns of expression and regulate the development and differentiation of the mammary gland, acting via channel-dependent and channel-independent mechanisms. Impaired Cx expression and localization are reported in breast cancer, suggesting a tumor suppressive role for Cxs. The signaling events that mediate the role of GJs in the development and tumorigenesis of the mammary gland remain poorly identified. The Wnt pathways, encompassing the canonical or the Wnt/β-catenin pathway and the noncanonical β-catenin-independent pathway, also play important roles in those processes. Indeed, aberrant Wnt signaling is associated with breast cancer. Despite the coincident roles of Cxs and Wnt pathways, the cross-talk in the breast tissue is poorly defined, although this is reported in a number of other tissues. Our previous studies revealed a channel-independent role for Cx43 in inducing differentiation or suppressing tumorigenesis of mammary epithelial cells by acting as a negative regulator of the Wnt/β-catenin pathway. Here, we provide a brief overview of mammary gland development, with emphasis on the role of Cxs in development and tumorigenesis of this tissue. We also discuss the role of Wnt signaling in similar contexts, and review the literature illustrating interplay between Cxs and Wnt pathways.

The Complex Subtype-Dependent Role of Connexin 43 (GJA1) in Breast Cancer

Gap junction transmembrane channels allow the transfer of small molecules between the cytoplasm of adjacent cells. They are formed by proteins named connexins (Cxs) that have long been considered as a tumor suppressor. This widespread view has been challenged by recent studies suggesting that the role of Connexin 43 (Cx43) in cancer is tissue- and stage-specific and can even promote tumor progression. High throughput profiling of invasive breast cancer has allowed for the construction of subtyping schemes that partition patients into at least four distinct intrinsic subtypes. This study characterizes Cx43 expression during cancer progression with each of the tumor subtypes using a compendium of publicly available gene expression data. In particular, we show that Cx43 expression depends greatly on intrinsic subtype. Tumor grade also co-varies with patient subtype, resulting in Cx43 co-expression with grade in a subtype-dependent manner. Better survival was associated with a high expression of Cx43 in unstratified and luminal tumors but with a low expression in Her2e subtype. A better understanding of Cx43 regulation in a subtype-dependent manner is needed to clarify the context in which Cx43 is associated with tumor suppression or cancer progression.

Connexin 43 in the development and progression of breast cancer: What's the connection? (Review)

Connexin 43 is a prominent gap junction protein within normal human breast tissue. Thus far, there have been a number of research studies performed to determine the function of connexin 43 in breast tumor formation and progression. Within primary tumors, research suggests that the level of connexin 43 expression in breast tumors is altered when compared to normal human breast tissue. While some reports indicate that connexin 43 levels decrease, other evidence suggests that connexin 43 levels are increased and protein localization shifts from the plasma membrane to the cytoplasm. In either case, the prevailing theory is that breast tumor cells have reduced gap junction intercellular communication within primary tumors. The current consensus appears to be that the loss of connexin 43 gap junction intercellular communication is an early event in malignancy, with the possibility of gap junction restoration in the event of metastasis. However, additional evidence is needed to support the latter claim. The purpose of this report is to review the connexin 43 literature that describes studies using human tissue samples, in order to evaluate the function of connexin 43 protein in normal human breast tissue as well as the role of connexin 43 in human breast tumor formation and metastatic progression.

Non-coding RNA as mediators in microenvironment–breast cancer cell communication

The tumor microenvironment has a critical role in the survival and decision of the cancer cells. These include support by enhanced angiogenesis, and metastasis or adaptation of dormancy. This article discusses methods by which the microenvironment sustains the tumor. This process is important as it will identify avenues of drug targets. Non-coding RNAs (ncRNAs) are evolving as key mediators in the interaction between the cancer cells and the microenvironment. Thus, the question is how to develop methods to effectively block the effects of the ncRNA and/or to introduce them to prevent metastasis, dormancy or to reverse dormancy. We focused on the advantages of using mesenchymal stem cells (MSCs) for RNA delivery. MSCs can be available as "off-the-shelf" cells. Thus far, MSCs are shown to be safe when transplanted across allogeneic barriers. We discussed the various methods by which MSCs can interact with cancer cells to deliver ncRNA or antagomirs. We also include the advances and possible confounds of using these methods. Overall, this review article provides a potential method by which MSCs can be used for effective delivery of nucleic acid to treat cancer.

Insights into the role of connexins in mammary gland morphogenesis and function

Gap junctions formed of connexin subunits link adjacent cells by direct intercellular communication that is essential for normal tissue homeostasis in the mammary gland. The mammary gland undergoes immense remodeling and requires exquisite regulation to control the proliferative, differentiating, and cell death mechanisms regulating gland development and function. The generation of novel genetically modified mice with reduced or ablated connexin function within the mammary gland has advanced our understanding of the role of gap junctions during the complex and dynamic process of mammary gland development. These studies have revealed an important stage-specific role for Cx26 (GJA1) and Cx43 (GJB2), while Cx30 (GJB6) and Cx32 (Gjb1) can be eliminated without compromising the gland. Yet, there remain gaps in our understanding of the role of mammary gland gap junctions.

Correlations of differentially expressed gap junction connexins Cx26, Cx30, Cx32, Cx43 and Cx46 with breast cancer progression and prognosis

BACKGROUND AND AIMS

Connexins and their cell membrane channels contribute to the control of cell proliferation and compartmental functions in breast glands and their deregulation is linked to breast carcinogenesis. Our aim was to correlate connexin expression with tumor progression and prognosis in primary breast cancers.

MATERIALS AND METHODS

Meta-analysis of connexin isotype expression data of 1809 and 1899 breast cancers from the Affymetrix and Illumina array platforms, respectively, was performed. Expressed connexins were also monitored at the protein level in tissue microarrays of 127 patients equally representing all tumor grades, using immunofluorescence and multilayer, multichannel digital microscopy. Prognostic correlations were plotted in Kaplan-Meier curves and tested using the log-rank test and cox-regression analysis in univariate and multivariate models.

RESULTS

The expression of GJA1/Cx43, GJA3/Cx46 and GJB2/Cx26 and, for the first time, GJA6/Cx30 and GJB1/Cx32 was revealed both in normal human mammary glands and breast carcinomas. Within their subfamilies these connexins can form homo- and heterocellular epithelial channels. In cancer, the array datasets cross-validated each other's prognostic results. In line with the significant correlations found at mRNA level, elevated Cx43 protein levels were linked with significantly improved breast cancer outcome, offering Cx43 protein detection as an independent prognostic marker stronger than vascular invasion or necrosis. As a contrary, elevated Cx30 mRNA and protein levels were associated with a reduced disease outcome offering Cx30 protein detection as an independent prognostic marker outperforming mitotic index and necrosis. Elevated versus low Cx43 protein levels allowed the stratification of grade 2 tumors into good and poor relapse free survival subgroups, respectively. Also, elevated versus low Cx30 levels stratified grade 3 patients into poor and good overall survival subgroups, respectively.

CONCLUSION

Differential expression of Cx43 and Cx30 may serve as potential positive and negative prognostic markers, respectively, for a clinically relevant stratification of breast cancers.

Mammary gland specific knockdown of the physiological surge in Cx26 during lactation retains normal mammary gland development and function

Connexin26 (Cx26) is the major Cx protein expressed in the human mammary gland and is up-regulated during pregnancy while remaining elevated throughout lactation. It is currently unknown if patients with loss-of-function Cx26 mutations that result in hearing loss and skin diseases have a greater susceptibility to impaired breast development. To investigate if Cx26 plays a critical role in mammary gland development and differentiation, a novel Cx26 conditional knockout mouse model was generated by crossing Cx26^fl/fl mice with mice expressing Cre under the β-Lactoglobulin promoter. Conditional knockdown of Cx26 from the mammary gland resulted in a dramatic reduction in detectable gap junction plaques confirmed by a significant ∼65-70% reduction in Cx26 mRNA and protein throughout parturition and lactation. Interestingly, this reduction was accompanied by a decrease in mammary gland Cx30 gap junction plaques at parturition, while no change was observed for Cx32 or Cx43. Whole mount, histological and immunofluorescent assessment of breast tissue revealed comparatively normal lobuloalveolar development following pregnancy in the conditionally knockdown mice compared to control mice. In addition, glands from genetically-modified mice were capable of producing milk proteins that were evident in the lumen of alveoli and ducts at similar levels as controls, suggesting normal gland function. Together, our results suggest that low levels of Cx26 expression throughout pregnancy and lactation, and not the physiological surge in Cx26, is sufficient for normal gland development and function.

The severity of mammary gland developmental defects is linked to the overall functional status of Cx43 as revealed by genetically modified mice

Genetically modified mice mimicking ODDD (oculodentodigital dysplasia), a disease characterized by reduced Cx43 (connexin 43)-mediated gap junctional intercellular communication, represent an in vivo model to assess the role of Cx43 in mammary gland development and function. We previously reported that severely compromised Cx43 function delayed mammary gland development and impaired milk ejection in mice that harboured a G60S Cx43 mutant, yet there are no reports of lactation defects in ODDD patients. To address this further, we obtained a second mouse model of ODDD expressing an I130T Cx43 mutant to assess whether a mutant with partial gap junction channel activity would be sufficient to retain mammary gland development and function. The results of the present study show that virgin Cx43^I130T/+ mice exhibited a temporary delay in ductal elongation at 4 weeks. In addition, Cx43^I130T/+ mice develop smaller mammary glands at parturition due to reduced cell proliferation despite similar overall gland architecture. Distinct from Cx43^G60S/+ mice, Cx43^I130T/+ mice adequately produce and deliver milk to pups, suggesting that milk ejection is unaffected. Thus the present study suggests that a loss-of-function mutant of Cx43 with partial gap junction channel coupling conductance results in a less severe mammary gland phenotype, which may partially explain the lack of reported lactation defects associated with ODDD patients.

Biological role of connexin intercellular channels and hemichannels

Gap junctions (GJ) and hemichannels (HC) formed from the protein subunits called connexins are transmembrane conduits for the exchange of small molecules and ions. Connexins and another group of HC-forming proteins, pannexins comprise the two families of transmembrane proteins ubiquitously distributed in vertebrates. Most cell types express more than one connexin or pannexin. While connexin expression and channel activity may vary as a function of physiological and pathological states of the cell and tissue, only a few studies suggest the involvement of pannexin HC in acquired pathological conditions. Importantly, genetic mutations in connexin appear to interfere with GJ and HC function which results in several diseases. Thus connexins could serve as potential drug target for therapeutic intervention. Growing evidence suggests that diseases resulting from HC dysfunction might open a new direction for development of specific HC reagents. This review provides a comprehensive overview of the current studies of GJ and HC formed by connexins and pannexins in various tissue and organ systems including heart, central nervous system, kidney, mammary glands, ovary, testis, lens, retina, inner ear, bone, cartilage, lung and liver. In addition, present knowledge of the role of GJ and HC in cell cycle progression, carcinogenesis and stem cell development is also discussed.

Partial mesenchymal to epithelial reverting transition in breast and prostate cancer metastases

Epithelial to mesenchymal transition (EMT) is an oft-studied mechanism for the initiation of metastasis. We have recently shown that once cancer cells disseminate to a secondary organ, a mesenchymal to epithelial reverting transition (MErT) may occur, which we postulate is to enable metastatic colonization. Despite a wealth of in vitro and in vivo studies, evidence supportive of MErT in human specimens is rare and difficult to document because clinically detectable metastases are typically past the micrometastatic stage at which this transition is most likely evident. We obtained paired primary and metastatic tumors from breast and prostate cancer patients and evaluated expression of various epithelial and mesenchymal markers by immunohistochemistry. The metastases exhibited increased expression of membranous E-cadherin compared to primary tumors, consistent with EMT at the primary site and MErT at the metastatic site. However, the re-emergence of the epithelial phenotype was only partial or incomplete. Expression of epithelial markers connexins 26 and/or 43 was also increased on the majority of metastases, particularly those to the brain. Despite the upregulation of epithelial markers in metastases, expression of mesenchymal markers vimentin and FSP1 was mostly unchanged. We also examined prostate carcinoma metastases of varied sizes and found that while E-cadherin expression was increased compared to the primary lesion, the expression inversely correlated with size of the metastasis. This not only suggests that a second EMT may occur in the ectopic site for tumor growth or to seed further metastases, but also provides a basis for the failure to discern epithelial phenotypes in clinically examined macrometastases. In summary, we report increased expression of epithelial markers and persistence of mesenchymal markers consistent with a partial MErT that readily allows for a second EMT at the metastatic site. Our results suggest that cancer cells continue to display phenotypic plasticity beyond the EMT that initiates metastasis.

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.

Cx43 suppresses mammary tumor metastasis to the lung in a Cx43 mutant mouse model of human disease

Gap junctions, the channels formed by the connexin (Cx) family of proteins, are responsible for direct intercellular communication. Although connexins are considered as tumor suppressors, their overall role in cancer onset, progression and metastasis is somewhat controversial. This study uses a novel Cx43 mutant mouse model (G60S mice) and cross-breeding strategies to determine the role of Cx43 in all stages of breast tumorigenesis. G60S mice were cross-bred with ErbB2 overexpressing mice, and spontaneous and 7,12-dimethylbenz[α]anthracene (DMBA)-induced tumor development was evaluated. Mice were killed when tumors reached ∼1 cm(3) or when mice showed signs of critical illness. In both spontaneous and DMBA studies, onset of palpable tumors was delayed in G60S mice compared with mice in control groups. Moreover, while tumors from control mice reached the size threshold, most DMBA-exposed Cx43 mutant mice were killed prematurely because of labored breathing, independent of the presence of a palpable tumor. Reduced Cx43 levels in Cx43 mutant mice were accompanied by extensive mammary gland hyperplasia. Lung histology revealed that all Cx43 mutant mice exhibited mammaglobin-positive mammary gland metastases to the lung, and the number of metastases was increased by threefold in Cx43 mutant mice on treatment with DMBA. Thus, while reduced levels of Cx43 delayed the onset of palpable tumors, normal Cx43 levels inhibited mammary gland tumor metastasis to the lungs. Understanding the mechanisms of how Cx43, which is expressed primarily in myoepithelial cells, inhibits mammary gland tumor metastasis is critical as Cx43 is assessed as a candidate for therapeutic intervention.

Attenuation of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced gap junctional intercellular communication (GJIC) inhibition in MCF-10A cells by c9,t11-conjugated linoleic acid

The protective effect of c9,t11-conjugated linoleic acid (CLA) on the inhibition of gap junctional intercellular communication (GJIC) was examined in a human mammary epithelial cell line (MCF-10A) treated with 12-O-tetradecanoylphorbol-13-acetate (TPA), relative to t10,c12-CLA isomer. TPA inhibited GJIC in a dose-dependent and reversible manner and was associated with connexin 43 phosphorylation. Pretreatment of 20 μM c9,t11-CLA for 24 h prior to 60 nM TPA for 1 h prevented the inhibition of GJIC by reducing the phosphorylation of connexin 43 via suppressing extracellular signal-regulated kinases (ERK1/2) activation. Reactive oxygen species (ROS) accumulation by TPA was attenuated by c9,t11-CLA. The efficacy of c9,t11-CLA in protecting inhibition of GJIC, connexin 43 phosphorylation, and ROS production was superior to that of t10,c12-CLA. These results suggest that c9,t11-CLA, including t10,c12-CLA, prevents the carcinogenesis of MCF-10A cells by protecting down-regulation of GJIC during the cancer promotion stage, and lack of their toxicities could be an excellent indicator for the chemoprevention of breast cancer.

Connexins and gap junctions in mammary gland development and breast cancer progression

The development and function of the mammary gland require precise control of gap junctional intercellular communication (GJIC). Here, we review the expression and function of gap junction proteins, connexins, in the normal mouse and human mammary gland. We then discuss the possible tumor-suppressive role of Cx26 and Cx43 in primary breast tumors and through the various stages of breast cancer metastasis and consider whether connexins or GJIC may actually promote tumorigenesis at some stages. Finally, we present in vitro data on the impact of connexin expression on breast cancer cell metastasis to the bone. We observed that Cx43 expression inhibited the invasive and migratory potentials of MDA-MB-231 breast cancer cells in a bone microenvironment, provided by the MC3T3-E1 mouse osteoblastic cell line. Expression of either Cx26 or Cx43 had no effect on MDA-MB-231 growth and adhesion under the influence of osteoblasts and did not result in regulation of osteogenic gene expression in these breast cancer cells. Furthermore, connexin-expressing MDA-MB-231 cells did not have an effect on the growth or differentiation of MC3T3-E1 cells. In summary, we conclude that connexin expression and GJIC are integral to the development and differentiation of the mammary gland. In breast cancer, connexins generally act as tumor suppressors in the primary tumor; however, in advanced breast tumors, connexins appear to act as both context-dependent tumor suppressors and facilitators of disease progression.

Connexins act as tumor suppressors in three-dimensional mammary cell organoids by regulating differentiation and angiogenesis

Connexins are tumor suppressors, and human breast connexin 26 (Cx26) and connexin 43 (Cx43) gap junctions are often down-regulated in breast cancer. We previously showed that Cx26 and Cx43 overexpressed in MDA-MB-231 breast cancer cells inhibited tumor growth in vivo but not in two-dimensional cultures. In the current study, we show that overexpression of Cx26 or Cx43 has tumor-suppressive properties in a three-dimensional environment such that they reduced anchorage-independent cell growth and induced partial redifferentiation of three-dimensional organoids of MDA-MB-231 cells. Importantly, the majority of exogenous connexins did not localize to the cell-cell interface or rescue gap junctional intercellular communication (GJIC) as assessed by dye transfer, providing evidence of a GJIC-independent mechanism of mammary tumor suppression. To further elucidate the mechanisms involved in connexin-induced three-dimensional redifferentiation of tumor cells, we examined whether connexin expression has a role in epithelial to mesenchymal transition (EMT). Cx26 and Cx43 reduced cell migration, increased cytokeratin 18 expression, and decreased vimentin levels, indicating a shift from a mesenchymal towards an epithelial phenotype. In addition, we examined the role of connexins in angiogenesis by probing an angiogenesis antibody array with conditioned media from three-dimensional MDA-MB-231 cultures. This revealed that connexin overexpression regulated various angiogenesis-linked proteins. Furthermore, secreted factors from connexin overexpressing cells inhibited endothelial cell tubulogenesis and migration, and xenografts of Cx43 overexpressing MDA-MB-231 cells showed reduced tumor angiogenesis. In summary, Cx26 and Cx43 inhibit the malignant properties of MDA-MB-231 cells via GJIC-independent mechanisms, including regulation of EMT and angiogenesis.

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.

Increased expression of connexins 26 and 43 in lymph node metastases of breast cancer

BACKGROUND

Gap junctions are intercellular channels composed of connexins, which mediate the direct passage of small molecules between neighbouring cells. They are involved in regulation of cell cycle, cell signalling, and differentiation, and probably invasion and metastasis. The role of connexins in the metastatic process is controversial, because some studies indicate that connexin expression is inversely correlated with metastatic capacity. In contrast, others demonstrate that connexins may be involved in metastasis. In addition, connexin status in breast cancer metastasis has not been widely studied.

METHODS

We evaluated by immunohistochemistry the expression of connexin 26 (Cx26) and connexin 43 (Cx43) in primary breast tumours (PTs) and matched paired metastases to lymph nodes (MLNs).

RESULTS

In PTs, we observed predominantly cytoplasmic localisation of evaluated connexins, indicating alterations in connexin expression in breast cancer cells. We demonstrated that expression of Cx26 and Cx43 was increased in MLNs compared with PTs (p<0.00001 and p<0.001, for CX26 and Cx43, respectively). In addition, Cx26 and Cx43 negative PTs developed Cx26 and Cx43 positive MLNs. Furthermore, besides increased cytoplasmic staining, enhanced membranous localisation of Cx43, typical of normal cells, was found in MLNs. Additionally, membranous Cx26 expression appeared only in metastatic breast cancer cells.

CONCLUSIONS

These findings suggest that connexins may contribute to the efficient metastasising of breast cancer to the lymph nodes.

Expression of connexins 26 and 43 in canine hyperplastic and neoplastic mammary glands

Gap junctions are the only communicating junctions found in animal tissues and are composed of proteins known as connexins. Alterations in connexin expression have been associated with oncogenesis; reported studies in rodent and human mammary glands, which normally express connexins 26 and 43, confirm these alterations in malignancies. Mammary neoplasms represent the second most frequent neoplasm in dogs, and since there are no reports on the study of connexins in canine mammary glands, the present study investigated the expression of connexins 26 and 43 in normal, hyperplastic, and neoplastic mammary glands of this species, to verify if altered patterns of connexin staining are related to higher cell proliferation and malignant phenotypes. A total of 4 normal, 8 hyperplastic mammary glands, 9 benign, and 51 malignant mammary gland neoplasms were submitted for the immunostaining of connexins 26 and 43, E-cadherin, and proliferating cell nuclear antigen (PCNA). Normal, hyperplastic, and benign neoplastic mammary glands showed a punctate pattern for connexin 26 and 43 staining and an intercellular E-cadherin staining. Malignant neoplasms, especially the most aggressive cases with high cell proliferation rates, presented either fewer gap junction spots on the cell membranes or increased cytoplasmic immunostaining. Malignant tumors also expressed a less intense immunostaining of E-cadherin; the expression of this adhesion molecule is important for the transportation of connexins to cell membranes and in forming communicating gap junctions. Deficient expression of E-cadherin could be related to the aberrant connexin localization and may contribute to the malignant phenotype. In conclusion, the expression and distribution of connexins and E-cadherin are inversely correlated to cell proliferation in malignant mammary neoplasms of dogs and may well be related to their more aggressive histologic type and biologic behavior.

The phosphorylated form of connexin43 is up-regulated in breast hyperplasias and carcinomas and in their neoformed capillaries

We applied an antiserum (SA226P) specifically recognizing the phosphorylated form of connexin43 (P-Cx43) to human breast samples including normal breast samples, with fibrocystic disease (FCD), fibroadenomas (FA), in situ and infiltrating carcinomas of all major types, and miscellaneous extramammary tumors. The findings were compared with those obtained with commercial antisera recognizing all Cx43 forms (pan-Cx43). A subset of samples was stained for Her2-neu and p44/42 to mitogen-activated protein kinase. Paraffin step sections were used. Immunoblots were performed on frozen samples of a representative subset of cases. In the normal breast, FCD, and FA, SA226P stained strongly and extensively most myoepithelial cells (MECs); luminal cells remained unstained. In proliferative FCD and some cellular FA, SA226P stained MEC and the capillary endothelium (CE). In ductal and lobular in situ carcinomas, SA226P reacted strongly and diffusely with the remaining MEC, the CE, and the transformed luminal cells. SA226P stained all infiltrating carcinomas except the tubular variant. In all breast carcinomas, the CE within and adjacent to tumors and some myofibroblasts stained with SA226P. By contrast, pan-Cx43 stained weakly and sporadically the MEC and rare samples of invasive carcinomas. Notably, Mab p44/42 reacted in parallel with the samples stained with SA226P, whereas reactions with Her2 were negative. Immunoblot findings paralleled those obtained immunohistochemically. We conclude that P-Cx43, restricted to MEC in the normal breast, is up-regulated in the same cells in hyperplasias and dysplasias and FA and is strongly up-regulated in invasive carcinomas. Notably, in some proliferative FCD and in most in situ and infiltrating carcinomas, P-Cx43 is strongly expressed in CE within and adjacent to the lesions but not away from them. These findings were paralleled by the strong nuclear reactions noted with Mab p44/42. These phenomena, although not exclusive to malignancy, are particularly conspicuous in breast carcinomas and seemingly reflect active proliferation associated with abnormal gap junctional intercellular communication.

Myoepithelial cells: good fences make good neighbors

The mammary gland consists of an extensively branched ductal network contained within a distinctive basement membrane and encompassed by a stromal compartment. During lactation, production of milk depends on the action of the two epithelial cell types that make up the ductal network: luminal cells, which secrete the milk components into the ductal lumen; and myoepithelial cells, which contract to aid in the ejection of milk. There is increasing evidence that the myoepithelial cells also play a key role in the organizational development of the mammary gland, and that the loss and/or change of myoepithelial cell function is a key step in the development of breast cancer. In this review we briefly address the characteristics of breast myoepithelial cells from human breast and mouse mammary gland, how they function in normal mammary gland development, and their recently appreciated role in tumor suppression.

Connexin 43 mediated gap junctional communication enhances breast tumor cell diapedesis in culture

Introduction

Metastasis involves the emigration of tumor cells through the vascular endothelium, a process also known as diapedesis. The molecular mechanisms regulating tumor cell diapedesis are poorly understood, but may involve heterocellular gap junctional intercellular communication (GJIC) between tumor cells and endothelial cells.

Method

To test this hypothesis we expressed connexin 43 (Cx43) in GJIC-deficient mammary epithelial tumor cells (HBL100) and examined their ability to form gap junctions, establish heterocellular GJIC and migrate through monolayers of human microvascular endothelial cells (HMVEC) grown on matrigel-coated coverslips.

Results

HBL100 cells expressing Cx43 formed functional heterocellular gap junctions with HMVEC monolayers within 30 minutes. In addition, immunocytochemistry revealed Cx43 localized to contact sites between Cx43 expressing tumor cells and endothelial cells. Quantitative analysis of diapedesis revealed a two-fold increase in diapedesis of Cx43 expressing cells compared to empty vector control cells. The expression of a functionally inactive Cx43 chimeric protein in HBL100 cells failed to increase migration efficiency, suggesting that the observed up-regulation of diapedesis in Cx43 expressing cells required heterocellular GJIC. This finding is further supported by the observation that blocking homocellular and heterocellular GJIC with carbenoxolone in co-cultures also reduced diapedesis of Cx43 expressing HBL100 tumor cells.

Conclusion

Collectively, our results suggest that heterocellular GJIC between breast tumor cells and endothelial cells may be an important regulatory step during metastasis.

Down-regulation of Cx43 by Retroviral Delivery of Small Interfering RNA Promotes an Aggressive Breast Cancer Cell Phenotype

Connexins are gap junction proteins that assemble into channels that mediate direct intercellular communication. Connexins are well-documented tumor suppressors and are thought to regulate both cell growth and differentiation. As previously reported, most human breast tumors and cell lines down-regulate gap junctions or have defective gap junctional intercellular communication. Furthermore, overexpression of connexins in breast cancer cells inhibits tumor growth in vivo. In this study, we hypothesize that controlled Cx43 down-regulation would induce breast tumor cells to acquire a more aggressive phenotype. Here we report that Cx43 was down-regulated in both normal rat kidney (NRK) cells and human breast cancer cell lines (MDA-MB-231 and Hs578T) by transfection with chemically synthesized small interfering RNA (siRNA) or short hairpin RNA generated from a retroviral infection. Furthermore, we show that retroviral delivery and expression of siRNA directed to different coding regions of Cx43 resulted in differential levels of Cx43 silencing and impaired gap junctional intercellular communication. Cx43-silenced Hs578T cells grew faster and were more migratory. Finally, Western blot analysis revealed that down-regulation of Cx43 resulted in decreased expression of thrombospondin-1, an antiangiogenesis molecule, and increased expression of vascular endothelial growth factor. Taken together, these results suggest that Cx43 is required for maintaining cell differentiation and the regulation of molecules important in angiogenesis.

Developmental expression patterns and regulation of connexins in the mouse mammary gland: expression of connexin30 in lactogenesis

The mammary gland reaches a fully differentiated phenotype at lactation, a stage characterized by the abundant expression of beta-casein. We have investigated the expression and regulation of gap junction proteins (connexins, Cx) during the various developmental stages of mouse mammary gland. Immunohistochemical analysis, with specific antibodies, reveals that Cx26 and Cx32 are expressed and confined to the cell borders of luminal epithelial cells in all developmental stages of the gland. Cx26 and Cx32 expression, at the mRNA and protein levels, increases in pregnancy and peaks in lactation. Whereas Cx43 mRNA decreases in pregnancy and lactation, the functional activity of Cx43 protein, which has been localized to myoepithelial cells, is regulated (through phosphorylation) during pregnancy and peaks during lactation. Cx30 mRNA and proteins have, for the first time, been detected in mammary gland epithelia. Using reverse transcription/polymerase chain reaction and sequencing techniques, we show that Cx30 is abundant in pregnant and lactating mammary gland. Cx30 protein levels have not been detected in the mammary gland prior to day 15 of pregnancy, whereas maximum expression occurs at the onset of lactation. In mouse mammary cells in culture, Cx30 is epithelial-cell-specific and is induced by lactogenic hormones. These data identify a novel player in mammary differentiation and suggest a potential role for Cx30 in the fully differentiated gland.

Altered permeability and modulatory character of connexin channels during mammary gland development

Abrupt developmental changes occur in structural form and function of connexin (Cx) channels in the mouse mammary gland. Microarray study shows that the principal connexin isoform in epithelial cells during pregnancy is Cx26, up-regulated and persisting from the virgin. After parturition, there is rapid induction of Cx32. In epithelial plasma membranes, size exclusion chromatography reveals that Cx32 organizes initially with Cx26 as heteromeric (Cx26-Cx32) hemichannels and later in heteromeric and homomeric Cx32 channels. Dramatic alterations of connexin channel function following these developmental changes in channel composition are characterized using native channels reconstituted into liposomes. Changes to channel stoichiometry increase the allowable physical size limits of permeant after parturition; the new Cx32 channels are wider than channels containing Cx26. Most remarkably, heteromeric Cx26-Cx32 channels are selectively permeability to adenosine 3',5' cyclic phosphate (cAMP), guanosine 3',5' cyclic phosphate (cGMP), and inositol 1,4,5-triphosphate (IP(3)), whereas homomeric channels are not. Homomeric Cx26 and heteromeric channels with high Cx26/Cx32 stoichiometry are also inhibited by taurine, an osmolyte playing a key role in milk protein synthesis. Taurine effect is reduced where heteromeric channels contain Cx32 > Cx26 and eliminated when channels contain only Cx32. Connexin channel stoichiometry, permeability, and chemical gating character change in precisely the desired fashion after parturition to maximize molecular and electrical coupling to support coordinated milk secretion.

Loss of connexin 26 in mammary epithelium during early but not during late pregnancy results in unscheduled apoptosis and impaired development

Gap junctions are intercellular channels that are formed by the protein family of connexins (Cxs). In mammary tissue, Cx26 and Cx32 are present in the secretory epithelium and Cx43 is localized in the myoepithelium. The expression of Cx26 and Cx32 is induced during pregnancy and lactation, respectively, thus suggesting unique roles for them in the functional development of the gland. The requirement for these connexins was explored using several strains of genetically altered mice: mice with an inactivated Cx32 gene, mice in which the Cx43 gene had been replaced with the Cx32 gene (Cx43KI32 mice) and mice in which the Cx26 gene was specifically ablated in mammary epithelium at different stages of development using Cre-loxP-based recombination. Normal mammary development was obtained in Cx32-null mice and in Cx43KI32 mammary tissue. In contrast, loss of Cx26 in mammary epithelium before puberty resulted in abrogated lobulo-alveolar development and increased cell death during pregnancy, which was accompanied by impaired lactation. Loss of Cx26 in mammary epithelium during the later part of pregnancy did not adversely interfere with functional mammary development. These results demonstrate that the presence of Cx26 is critical during early stages but not during the end of pregnancy when the tissue has completed functional differentiation. Cx26 is considered a tumor suppressor gene and Cx26-null mammary tissue was evaluated after five pregnancies. No hyperproliferation or hyperplasia was observed, suggesting that Cx26 does not function as a tumor suppressor.

ECM-induced gap junctional communication enhances mammary epithelial cell differentiation

The relationship between gap junctional intercellular communication (GJIC) and mammary cell (CID-9) differentiation in vitro was explored. CID-9 cells differentiate and express beta-casein in an extracellular matrix (ECM)- and hormone-dependent manner. In response to interaction with the ECM, cells in culture modulated the expression of their gap junction proteins at the transcriptional and post-translational levels. In the presence of EHS-matrix, connexins (Cx)26, 32 and 43 localized predominantly to the plasma membrane, and enhanced GJIC [as measured by Lucifer Yellow (LY) dye transfer assays] was noted. Inhibition of GJIC of cells on EHS-matrix with 18 alpha glycyrrhetinic acid (GA) resulted in reversible downregulation of beta-casein expression. In the presence of cAMP, cells cultured on plastic expressed beta-casein, upregulated Cx43 and Cx26 protein levels and enhanced GJIC. This was reversed in the presence of 18 alpha GA. cAMP-treated cells plated either on a non-adhesive PolyHEMA substratum or on plastic supplemented with function-blocking anti-beta 1 integrin antibodies, maintained beta-casein expression. These studies suggest that cell-ECM interaction alone may induce differentiation through changes in cAMP levels and formation of functional gap junctions. That these events are downstream of ECM signalling was underscored by the fact that enhanced GJIC induced partial differentiation in mammary epithelial cells in the absence of an exogenously provided basement membrane and in a beta 1-integrin- and adhesion-independent manner.

Connexin expression patterns in the rat cornea: molecular evidence for communication compartments

PURPOSE

To identify and localize candidate connexin family members in adult rat cornea that may be important in coordinating corneal cell biology.

METHODS

To identify candidate connexin family members in adult rat cornea, a RT-PCR-based screening approach was initially adopted. Fourteen pairs of connexin isoform-specific primers were used to amplify connexin transcripts from two populations of RNA isolated from either the central cornea or the whole cornea. Immunohistochemistry and confocal microscopy were then used to confirm the presence and localization of connexins.

RESULTS

Eight connexin transcripts (Cxs 26, 30.3, 31, 31.1, 33, 37, 43, 50) are present in central cornea, and the peripheral cornea additionally expresses Cxs 30, 40, 45, and 46. No Cx32 or Cx36 transcripts were amplified. Immunohistochemistry revealed that Cxs 26, 30, 31.1, 37, and 43 are expressed in spatially distinct patterns within the cornea. Cx26 and Cx43 occur in basal cells of the whole corneal epithelium and between endothelial cells. Cx26 also immunolocalizes to the first layer of intermediate epithelial cells, and Cx43 antibody labels stromal keratocytes. Cx30 is expressed in the peripheral corneal epithelium and disappears toward the central cornea. Cx31.1 expression is restricted to superficial corneal epithelial cells, and Cx37 spans the intermediate corneal epithelium.

CONCLUSION

The spatially distinct cellular expression patterns of Cxs 26, 30, 31.1, 37, and 43 in the corneal epithelium imply that gap junctions play important roles in controlling corneal epithelial proliferation and differentiation and overall corneal maintenance.

Connexin26-mediated gap junctional communication reverses the malignant phenotype of MCF-7 breast cancer cells

A growing body of evidence indicates that the gap junction (GJ) plays a pivotal role in tumor suppression by exerting cell-cell communication. It has, however, been reported that expression of connexin26 (Cx26) protein is induced in human ductal carcinomas of the breast and that its amount increases in proportion to the grade of malignancy. We thus examined the effects of overexpressed Cx26 on growth characteristics in GJ-deficient human MCF-7 breast cancer cells that maintain the phenotype of early-stage cancers. MCF-7 cells were transfected with Cx26 cDNA, and several clones of stable transformants exhibiting a high level of cell-cell communication were established. When they were examined in terms of various growth characteristics in vitro, the proliferation rate and the saturation density were drastically reduced in Cx26-transfected clones compared with the mock-transfectant. The anchorage-independent growth capacity was also decreased by 50-75% after transfection of Cx26. Furthermore, the cell migration toward growth factors and cell invasion into Matrigel in a Boyden chamber were suppressed to 5-10% and 20-60%, respectively, of the control in Cx26-transfected clones. When implanted into the mammary fat pads of nude mice in the presence of an excess of 17beta-estradiol, Cx26-transfected clones tended to show slower tumor growth than the mock-transfectant, although the difference was not statistically significant. Our results strongly suggest that the induction of Cx26 protein observed in human breast cancers, reported previously, may not be very relevant to the development of breast cancers, and that Cx26 can function as a tumor suppressor in breast cancer cells.

Retroviral delivery of connexin genes to human breast tumor cells inhibits in vivo tumor growth by a mechanism that is independent of significant gap junctional intercellular communication

The mechanism by which gap junction proteins, connexins, act as potent tumor suppressors remains poorly understood. In this study human breast tumor cells were found to exhibit diverse gap junction phenotypes including (a) undetectable Cx43 and no intercellular communication (HBL100); (b) low levels of Cx43 and sparse intercellular communication (MDA-MB-231); and (c) significant levels of Cx43 and moderate intercellular communication (Hs578T). Although retroviral delivery of Cx43 and Cx26 cDNAs to MDA-MB-231 cells did not achieve an expected substantial rescue of intercellular communication, overexpression of connexin genes did result in a dramatic suppression of tumor growth when connexin-expressing MDA-MB-231 cells were implanted into the mammary fat pad of nude mice. Subsequent immunolocalization studies on xenograph sections revealed only cytoplasmic stores of Cx43 and no detectable gap junctions. Moreover, DNA array and Western blot analysis demonstrated that overexpression of Cx43 or Cx26 in MDA-MB-231 cells down-regulated fibroblast growth factor receptor-3. Surprisingly, these results suggest that Cx43 and Cx26 induce their tumor-suppressing properties by a mechanism that is independent of significant gap junctional intercellular communication and possibly through the down-regulation of key genes involved in tumor growth. Moreover, our studies show that retroviruses are effective vehicles for delivering connexins to human breast tumor cells, facilitating potential gene therapy applications.

Molecular profiling and cellular localization of connexin isoforms in the rat ciliary epithelium

The functionally distinct epithelial layers of the ciliary body act as a syncitium to produce the aqueous humour. Ultrastructural studies have shown that the pigmented (PE) and non-pigmented (NPE) cell layers of the ciliary epithelium are connected by gap junctions. However the molecular composition of gap junctions both between and within the two cell layers has not been comprehensively studied. To address this issue the authors have performed an extensive molecular screening of connexin (Cx) expression patterns in ciliary epithelium of the rat. Initially, mRNA was extracted from rat ciliary bodies, reverse-transcribed, and subjected to two rounds of PCR using primer sets designed against each of the 14 Cx isoforms known to be expressed in the rat. This initial screening protocol amplified eight candidate Cx isoforms (Cxs 26, 31, 33, 37, 40, 43, 45 and 46). The Cx isoforms identified in this initial screen were then first assigned to the ciliary epithelium itself (Cxs 26, 31, 40 and 43) or structures outside the epithelium (Cxs 37, 40, and 45) using immunohistochemistry performed on ciliary body whole mounts. No convincing evidence for either Cx 33 or 46 labelling was found in the ciliary body. Then the four Cx isoforms localized to the epithelium were further localized to specific membrane domains within the epithelial cell layers by performing high resolution imaging of the antibody labeling patterns obtained in cryosections. This enabled Cx26 and 31 to be specifically localized to spatially different gap junctions between NPE cells. Cx31 labeled gap junctions associated with an extensive network of membrane interdigitations found between NPE cells at their basal surfaces. In contrast Cx26 labeling in NPE cells was restricted to the basolateral membranes of adjacent NPE cells. Cx40 and Cx43 were both localized to the PE-NPE interface where they formed discrete homomeric/homotypic gap junction plaques. No convincing evidence was found for antibody labeling between PE cells. Thus it appears that intercellular communication, both within the NPE layer and between the PE and NPE cell layers, is mediated by gap junction channels that have distinctive permeability properties. In particular the results raise the possibility that the permeability of PE-NPE gap junctions can be modulated by changing the Cx43 : Cx40 expression ratio. Whether such a change in Cx expression ratios occurs and what effect it has on aqueous humour production and composition remains to be determined.

The role of connexin-mediated cell-cell communication in breast cancer metastasis

Gap junctional intercellular communication (GJIC) is a form of cell-cell communication mediating the exchange of small molecules between neighboring cells. Gap junctions (GJs) are formed by connexins (Cxs), and are subject to tight and dynamic regulation. They are involved in the cell cycle, differentiation, and cell signaling. The loss of Cxs and GJs is a hallmark of carcinogenesis, while their induction in cancer cells leads to a reversal of the cancer phenotype, induction of differentiation, and regulation of cell growth. On the basis of the observations about Cx loss in breast cancer, this review examines Cxs' involvement in breast cancer metastasis. Previous work indicates that Cx expression is inversely correlated to metastatic potential. This is probably because of the loss of cooperation between neighboring cells, leading to cell heterogeneity and cell dissociation in the tumor. The possible involvement of Cx activity during metastasis will be discussed.

Gap-junctional communication between feeder cells and recipient normal epithelial cells correlates with growth stimulation

LA7 rat mammary tumor cells stimulate the proliferation, in culture, of three normal epithelial cell types, namely mouse mammary, rat mammary, and mouse thymic cells. Gap-junctional communication between LA7 feeders and mouse mammary cells was demonstrated by microinjection of lucifer yellow, which traveled from LA7 to the surrounding mouse mammary cells. The amount of 3H-uridine exchange between feeder and recipient mouse mammary, rat mammary, and mouse thymus cells correlated with the growth rate induced by the feeders. Cells of the Madin Darby canine kidney (MDCK) line, which do not appreciably stimulate mouse mammary cell growth when used as feeder cells, also exchange little 3H-uridine with them. Expression of connexins Cx43, 32, and 26 was studied in all these cell lines and strains by immunocytochemistry. Mouse mammary cells expressed Cx26, and a few mouse thymic cells expressed Cx32. LA7, mouse mammary, mouse thymic, and rat mammary cells all expressed easily detectable amounts of the gap-junction protein Cx43, in contrast to MDCK cells, which expressed only a hint of the protein. These results suggest that gap junctions composed of Cx43 are those by which the normal epithelial cells communicate with the LA feeders. Thus, the ability of feeder cells to stimulate proliferation in recipients correlates with the expression of Cx43 in both members of the feeder/recipient pair and the capacity to form functional gap junctions between these cells.

Quiescent mammary epithelial cells have reduced connexin43 but maintain a high level of gap junction intercellular communication

Gap junctions have been implicated in growth control, but it remains unclear whether cells that enter a quiescent state continue to express connexins and maintain a high level of gap junction intercellular communication (GJIC). To this end, MAC-T cells, a bovine mammary epithelial cell line, were serum starved for 48 h to induce a quiescent (G0) state. In quiescent cells, [3H]thymidine incorporation was reduced by 97.3% from serum-fed controls. Western blotting in conjunction with Phosphorlmager analysis revealed up to a 20-fold decrease in the expression of the gap junction protein connexin43 (Cx43 or alpha 1) and a shift toward the unphosphorylated form in quiescent cells. However, cell-to-cell transfer of the gap junction-permeable fluorescent tracer, Lucifer yellow, was only moderately reduced in quiescent cells. In control cells, Cx43 was predominantly perinuclear, although it was also present at sites of cell-cell apposition. In quiescent cells, intracellular labeling for Cx43 decreased without a corresponding reduction at areas of cell-cell contact. Recovery from serum deprivation resulted in increased thymidine incorporation that corresponded with an elevation in Cx43 protein expression and phosphorylation. In parallel studies, MAC-T cells were also induced to enter a quiescent state through contact inhibition. Despite a 20-fold reduction in 5-bromo-2'-deoxyuridine and a substantial reduction in intracellular Cx43, contact inhibited MAC-T cells also maintained gap junctions and GJIC. These experiments demonstrate that the maintenance of dye coupling in quiescent mammary cells is correlated with a redistribution of intracellular stores of Cx43.

Connexins in tumour suppression and cancer therapy

Malignant cells usually show altered gap junctional intercellular communication and are often associated with aberrant expression or localization of connexins. Transfection of connexin genes into tumorigenic cells restores normal cell growth, suggesting that connexins form a family of tumour suppressor genes. Some studies have also shown that specific connexins may be necessary to control growth of specific cell types. Although we have found that genes encoding connexin32 (Cx32; beta 1), Cx37 (alpha 4) and Cx43 (alpha 1) are rarely mutated in tumours, our recent studies suggest that methylation of the connexin gene promoter may be a mechanism by which connexin gene expression is down-regulated in certain tumors. We have produced various dominant negative mutants of the genes encoding Cx26 (beta 2), Cx32 and Cx43, some of which prevent the growth control exerted by the corresponding wild-type genes. A decade ago, we proposed a method to enhance killing of cancer cells by diffusion of therapeutic agents through gap junctions. Recently, we and others have shown that gap junctional intercellular communication is responsible for the bystander effect seen in herpes simplex virus thymidine kinase/ganciclovir gene therapy. Thus, connexin genes can exert dual effects in tumour control: tumour suppression and a bystander effect for cancer therapy.

Gap junctions in normal and neoplastic mammary gland

Progress in the characterization of gap junctions and their constituent connexin sub-units is leading to a greater understanding of the structure, function, and regulation of this cell-cell communication channel. Although much of the experimental evidence generated to date is correlative, recent work utilizing reverse genetic approaches to manipulate connexin gene function has provided direct evidence that intercellular communication via gap junctions plays key roles in development, cellular differentiation, and organogenesis. Pathogenic mutations in human connexin genes have now been identified. Furthermore, a considerable body of experimental evidence correlates a loss of junctional communication with progression to a malignant phenotype. Although the cell biology of the mammary gland has been extensively studied, the role(s) of gap junctions in the development, differentiation, and maintenance of this tissue are unknown. Gap junctions were first reported in the mammary gland following freeze-fracture and electron microscopic analyses. The development of anti-connexin antibodies and the cloning of individual connexin isoforms have enabled this work to be extended, but there are contradictory reports in the temporal expression patterns of these proteins within mammary epithelium. In addition, a recent report in this Journal has implied by immunocytochemistry that there is up-regulation of connexin protein in some human breast tumours, a novel observation which may be inconsistent with the proposed tumour suppressor role for gap junctions.

Mapping and characterization of the basal promoter of the human connexin26 gene

Connexin26 (Cx26) is a major gap junction protein expressed in mammary and endometrial epithelial cells. Previously, we have cloned the genomic upstream sequence of the human connexin26 gene. In this paper, we studied the structure and function of its basal promoter. Various 5'-flanking regions of the human Cx26 gene were inserted upstream of the bacterial chloramphenicol acetyltransferase (CAT) reporter gene and transfected into human immortalized mammary MCF-10A and MCF-12A cell lines and endometrial RL95-2 cancer cell line. Through CAT reporter gene analysis, we identified the basal promoter of human Cx26 gene in the proximal 5'-flanking region from -128 to +2 (relative to the transcription initiation site). Further deletion analyses suggested that the critical regulatory area was located within a 29 bp region (from -97 to -69), where two GC consensus boxes (CCGCCC) resided, one at -93 and the other at -81. Labeled oligonucleotides encompassing these two GC box DNA sequences could bind the nuclear extracts from MCF-12A and RL95-2 cells in the electrophoretic mobility shift assay. These binding complexes could be competitively reduced by non-labeled self or Sp1 consensus oligonucleotide, and supershifted by antibodies against either Sp1 or Sp3. Mutations in the core sequence of these two GC boxes from CCGCCC to CCGAAC caused a loss of competitive ability and also produced a drastic reduction of basal promoter activity when integrated into promoter/reporter constructs. Furthermore, co-transfection of Sp1 and/or Sp3 expressing plasmids could trans-activate the expression of human Cx26 promoter/reporter constructs in Drosophila Schneider line 2 (SL2) cells. Taken together, these data indicated that the two GC boxes in the proximal promoter region play an important role in the control of human Cx26 gene expression.

Basic FGF increases communication between cells of the developing neocortex

We have found that basic fibroblast growth factor (bFGF), applied to cortical progenitor cells in vitro, produces an increase in the expression of the gap junction protein connexin (Cx) 43 and in the mRNA encoding Cx 43. This effect was evident in both proliferating and nonproliferating cells. The elevated levels of mRNA suggest that bFGF is likely to exert its effect by upregulating the rate of transcription of the Cx 43 gene. We have further shown that the increase in Cx 43 expression is mediated through the receptor tyrosine kinase pathway and is associated with enhanced intercellular dye-coupling mediated by gap junctions. These results suggest that gap junction channels provide a direct conduit for mitogens released in response to bFGF to effectively regulate proliferation during corticogenesis.

Sequence-specific antibodies to connexins block intercellular calcium signaling through gap junctions

Mechanical stimulation of a single cell in primary airway epithelial cell cultures induces an intercellular Ca2+ wave that has been proposed to be mediated via gap junctions. To investigate directly the role of gap junctions in this multicellular response, the effects of intracellularly-loaded sequence-specific connexin (gap junction) antibodies on the propagation of intercellular Ca2+ waves were evaluated. Electroporation of antibodies to the cytosolic loop (Des 1, generated to amino acids 102-112 + 116-124; and Des 5, amino acids 108-119), or to the carboxyl tail (Gap 9, amino acids 264-283) of connexin 32 inhibited the propagation of intercellular Ca2+ waves. The inhibitory effect of Des 1 antibody was competitively reversed by the co-loading of a peptide derived from a similar cytosolic loop sequence (Des 5 peptide). Conversely, the inhibitory effects on intercellular Ca2+ wave propagation of Gap 9 antibody was not altered by co-loading with the Des 5 peptide. Antibodies raised to peptide sequences within the extracellular loop (Gap 11, amino acids 151-187), or the cytoplasmically located amino terminus (Gap 10, amino acids 1-21) of connexin 32 did not inhibit mechanically-induced intercellular communication. Also ineffective in perturbing intercellular communication were antibodies raised to peptide sequences of the cytosolic loops of connexin 43 (Gap 15, amino acids 131-142) or connexin 26 (Des 3, amino acids 106-119). These data suggest that mechanically-induced Ca2+ waves in airway cell cultures are propagated through gap junctions made up of connexin 32 proteins.

Expression of gap junction proteins connexin 26 and connexin 43 in normal human breast and in breast tumours

Gap junctional intercellular communication (GJIC) has been proposed as a cellular mechanism for tumour suppression and there is experimental evidence in support of this. If aberrant GJIC contributes to the formation of human breast tumours, one might expect that the connexins (gap junction proteins) expressed by epithelial cells in normal human breast would be down-regulated in tumour epithelial cells, or that tumour cells might show aberrant expression of other connexin family members. This study examines the immunocytochemical expression of connexins 26 (Cx26) and 43 (Cx43) in normal human breast, 11 benign breast lesions, two special-type carcinomas, and 27 invasive carcinomas of no special histological type (NST). Cx26 generally was not expressed at detectable level in normal human breast, but punctate Cx43 immunostaining of the myoepithelial cells was found. Cx43 staining of the myoepithelium was also a feature of the benign lesions and ductal carcinoma in situ (DCIS). In general, the epithelial cells of benign lesions failed to stain for either connexin. Similarly, a lobular carcinoma did not express Cx26 or Cx43, but there was punctate Cx43 in the epithelial cells of a mucoid carcinoma. Cx26 was up-regulated in the carcinoma cells of 15 of the 27 invasive NST carcinomas, although the staining was usually cytoplasmic and heterogeneous. Cx43 was expressed by stromal cells, possibly myofibroblasts, in all NST carcinomas. Furthermore, there was heterogeneous Cx43 expression in the carcinoma cells of 14 of the 27 NST carcinomas and the staining was often intercellular and punctate, characteristic of functional connexins. Up-regulated of Cx26 and/or Cx43 in the carcinoma cells of over two-thirds of invasive lesions of NST is not necessarily inconsistent with a tumour suppressor role for GJIC. However, the role of gap junctions in the formation and progression of solid human tumours is likely to be more complex than indicated from experimental systems.