Gap junctions in normal and neoplastic mammary gland

Inhibitory Effects of Alpha-Connexin Carboxyl-Terminal Peptide on Canine Mammary Epithelial Cells: A Study on Benign and Malignant Phenotypes

Mammary cancer is highly prevalent in non-castrated female dogs. Cell-to-cell communication is an important mechanism to maintain homeostasis, and connexins are proteins that assemble to form the communicating gap junctions. In many cancers, communication capacity is reduced; several approaches are being tested in order to increase the communication capacity in cancer cells and, therefore, alter their viability. This study analyzed the effects of the alpha-connexin carboxyl-terminal peptide (αCT1) on canine mammary non-neoplastic and neoplastic epithelial cells. Seven canine epithelial mammary cell lines were used. Among these, one was a normal canine epithelial mammary cell line (LOEC-NMG), two canine mammary adenomas (LOEC-MAd1 and LOEC-MAd2), and four canine mammary adenocarcinomas (LOEC-MCA1, LOEC-MCA2, LOEC-MCA3 and CF41). The αCT1 corresponds to a short Cx43 C-terminal sequence linked to an internalization sequence called the antennapedia. After 24 h of incubation, the medium containing different αCT1 peptide concentrations was added to the cells, and only the culture medium was used for control. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test was used to quantify cell viability before treatment and 48, 72, and 96 h after the treatment. Results showed that the normal mammary epithelial cell line (LOEC-NMG) was resistant to treatment with αCT1, which is consistent with a previous study on human mammary cell lines. One of the adenoma cell lines (LOEC-MAd2) was also resistant to treatment with αCT1, although the other (LOEC-MAd1) was susceptible to treatment, mostly at 72 h after treatment. Regarding the four canine adenocarcinoma cell lines, they differ regarding the susceptibility to the treatment with αCT1. Three cell lines, canine mixed adenocarcinoma (LOEC-MCA1), canine complex adenocarcinoma (LOEC-MCA2), and commercial canine mammary adenocarcinoma cell line CF41, were susceptible to treatment with αCT1, while one canine mammary adenocarcinoma cell line (LOEC-MCA3) was resistant to treatment. In most αCT1 treated cell lines, Cx43 was strongly detected in cell membranes by immunofluorescence. We propose that αCT1 restored the cell-to-cell communication capacity of neoplastic cells and induced inhibitory effects on cell viability.

Toward a Reasoned Classification of Diseases Using Physico-Chemical Based Phenotypes

Background: Diseases and health conditions have been classified according to anatomical site, etiological, and clinical criteria. Physico-chemical mechanisms underlying the biology of diseases, such as the flow of energy through cells and tissues, have been often overlooked in classification systems.

Objective: We propose a conceptual framework toward the development of an energy-oriented classification of diseases, based on the principles of physical chemistry.

Methods: A review of literature on the physical chemistry of biological interactions in a number of diseases is traced from the point of view of the fluid and solid mechanics, electricity, and chemistry.

Results: We found consistent evidence in literature of decreased and/or increased physical and chemical forces intertwined with biological processes of numerous diseases, which allowed the identification of mechanical, electric and chemical phenotypes of diseases.

Discussion: Biological mechanisms of diseases need to be evaluated and integrated into more comprehensive theories that should account with principles of physics and chemistry. A hypothetical model is proposed relating the natural history of diseases to mechanical stress, electric field, and chemical equilibria (ATP) changes. The present perspective toward an innovative disease classification may improve drug-repurposing strategies in the future.

Mechanical Stress as the Common Denominator between Chronic Inflammation, Cancer, and Alzheimer's Disease

The pathogenesis of common diseases, such as Alzheimer's disease (AD) and cancer, are currently poorly understood. Inflammation is a common risk factor for cancer and AD. Recent data, provided by our group and from others, demonstrate that increased pressure and inflammation are synonymous. There is a continuous increase in pressure from inflammation to fibrosis and then cancer. This is in line with the numerous papers reporting high interstitial pressure in cancer. But most authors focus on the role of pressure in the lack of delivery of chemotherapy in the center of the tumor. Pressure may also be a key factor in carcinogenesis. Increased pressure is responsible for oncogene activation and cytokine secretion. Accumulation of mechanical stress plays a key role in the development of diseases of old age, such as cardiomyopathy, atherosclerosis, and osteoarthritis. Growing evidence suggest also a possible link between mechanical stress in the pathogenesis of AD. The aim of this review is to describe environmental and endogenous mechanical factors possibly playing a pivotal role in the mechanism of chronic inflammation, AD, and cancer.

Targeting connexin 43 with α-connexin carboxyl-terminal (ACT1) peptide enhances the activity of the targeted inhibitors, tamoxifen and lapatinib, in breast cancer: clinical implication for ACT1

BACKGROUND

Treatment failure is a critical issue in breast cancer and identifying useful interventions that optimize current cancer therapies remains a critical unmet need. Expression and functional studies have identified connexins (Cxs), a family of gap junction proteins, as potential tumor suppressors. Studies suggest that Cx43 has a role in breast cancer cell proliferation, differentiation, and migration. Although pan-gap junction drugs are available, the lack of specificity of these agents increases the opportunity for off target effects. Consequently, a therapeutic agent that specifically modulates Cx43 would be beneficial and has not been tested in breast cancer. In this study, we now test an agent that specifically targets Cx43, called ACT1, in breast cancer.

METHODS

We evaluated whether direct modulation of Cx43 using a Cx43-directed therapeutic peptide, called ACT1, enhances Cx43 gap junctional activity in breast cancer cells, impairs breast cancer cell proliferation or survival, and enhances the activity of the targeted inhibitors tamoxifen and lapatinib.

RESULTS

Our results show that therapeutic modulation of Cx43 by ACT1 maintains Cx43 at gap junction sites between cell-cell membrane borders of breast cancer cells and augments gap junction activity in functional assays. The increase in Cx43 gap junctional activity achieved by ACT1 treatment impairs proliferation or survival of breast cancer cells but ACT1 has no effect on non-transformed MCF10A cells. Furthermore, treating ER+ breast cancer cells with a combination of ACT1 and tamoxifen or HER2+ breast cancer cells with ACT1 and lapatinib augments the activity of these targeted inhibitors.

CONCLUSIONS

Based on our findings, we conclude that modulation of Cx43 activity in breast cancer can be effectively achieved with the agent ACT1 to sustain Cx43-mediated gap junctional activity resulting in impaired malignant progression and enhanced activity of lapatinib and tamoxifen, implicating ACT1 as part of a combination regimen in breast cancer.

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.

The potential prognostic value of connexin 26 and 46 expression in neoadjuvant-treated breast cancer

BACKGROUND

Several classification systems are available to assess pathological response to neoadjuvant chemotherapy in breast cancer, but reliable biomarkers to predict the efficiency of primary systemic therapy (PST) are still missing. Deregulation of gap junction channel forming connexins (Cx) has been implicated in carcinogenesis and tumour progression through loss of cell cycle control. In this study we correlated Cx expression and cell proliferation with disease survival and pathological response to neoadjuvant chemotherapy in breast cancers using existing classification systems.

METHODS

The expression of Cx26, Cx32, Cx43, Cx46 and Ki67 was evaluated in 96 breast cancer patients prior to and after neoadjuvant chemotherapy using duplicate cores in tissue microarrays (TMA). Cx plaques of <1μm were detected with multilayer, multichannel fluorescence digital microscopy. Current classifications to assess residual tumour burden after primary systemic therapy included the EWGBSP, CPS-EG, Miller-Payne, Sataloff and NSABP systems.

RESULTS

In our cohort dominated by hormone receptor (ER/PR) positive and HER2 negative cases, only the CPS-EG classification showed prognostic relevance: cases with scores 1-2 had significantly better overall survival (p=0.015) than cases with scores 3-5. Pre-chemotherapy Cx43 expression correlated positively with hormone receptor status both before and after chemotherapy and had a negative correlation with HER2 expression pre-chemotherapy. There was a positive correlation between Cx32 and HER2 expression pre-chemotherapy and between Cx32 and Ki67 expression post-chemotherapy. A negative correlation was found between post-chemotherapy Cx46 and Ki67 expression. Decreased post-chemotherapy Cx26 expression (<5%) statistically correlated with better overall survival (p=0.011). Moderate or higher Cx46 expression (>20%) pre- and post-chemotherapy correlated with significantly better survival in the intermediate prognostic subgroups of EWGBSP TR2b (p(pre-chemo)=0.006; Sataloff TB (p(pre-chemo)=0.005; p(post-chemo)=0.029) and in Miller-Payne G3 (p(pre-chemo)=0.002; p(post-chemo)=0.012) classifications. Pre-chemotherapy, Cx46 expression was the only marker that correlated with overall survival within these subgroups.

CONCLUSION

Our results suggest that Cx46 and Cx26 expression in breast cancer may improve the assessment of pathological response and refine intermediate prognostic subgroups of residual tumour classifications used after neoadjuvant chemotherapy.

Directing the assembly of spatially organized multicomponent tissues from the bottom up

The complexity of the human body derives from numerous modular building blocks assembled hierarchically across multiple length scales. These building blocks, spanning sizes ranging from single cells to organs, interact to regulate development and normal organismal function but become disorganized during disease. Here, we review methods for the bottom-up and directed assembly of modular, multicellular, and tissue-like constructs in vitro. These engineered tissues will help refine our understanding of the relationship between form and function in the human body, provide new models for the breakdown in tissue architecture that accompanies disease, and serve as building blocks for the field of regenerative medicine.

Molecular connexin partner remodeling orchestrates connexin traffic: from physiology to pathophysiology

Connexins, through gap junctional intercellular communication, are known to regulate many physiological functions involved in developmental processes such as cell proliferation, differentiation, migration and apoptosis. Strikingly, alterations of connexin expression and trafficking are often, if not always, associated with human developmental diseases and carcinogenesis. In this respect, disrupted trafficking dynamics and aberrant intracytoplasmic localization of connexins are considered as typical features of functionality failure leading to the pathological state. Recent findings demonstrate that interactions of connexins with numerous protein partners, which take place throughout connexin trafficking, are essential for gap junction formation, membranous stabilization and degradation. In the present study, we give an overview of the physiological molecular machinery and of the specific interactions between connexins and their partners, which are involved in connexin trafficking, and we highlight their changes in pathological situations.

Heterocellular interaction enhances recruitment of alpha and beta-catenins and ZO-2 into functional gap-junction complexes and induces gap junction-dependant differentiation of mammary epithelial cells

Gap junctions (GJ) are required for mammary epithelial differentiation. Using epithelial (SCp2) and myoepithelial-like (SCg6) mouse-derived mammary cells, the role of heterocellular interaction in assembly of GJ complexes and functional differentiation (beta-casein expression) was evaluated. Heterocellular interaction is critical for beta-casein expression, independent of exogenous basement membrane or cell anchoring substrata. Functional differentiation of SCp2, co-cultured with SCg6, is more sensitive to GJ inhibition relative to homocellular SCp2 cultures differentiated by exogenous basement membrane. Connexin (Cx)32 and Cx43 levels were not regulated across culture conditions; however, GJ functionality was enhanced under differentiation-permissive conditions. Immunoprecipitation studies demonstrated association of junctional complex components (alpha-catenin, beta-catenin and ZO-2) with Cx32 and Cx43, in differentiation conditions, and additionally with Cx30 in heterocellular cultures. Although beta-catenin did not shuttle between cadherin and GJ complexes, increased association between connexins and beta-catenin in heterocellular cultures was observed. This was concomitant with reduced nuclear beta-catenin, suggesting that differentiation in heterocellular cultures involves sequestration of beta-catenin in GJ complexes.

Connexin channel permeability to cytoplasmic molecules

Connexin channels are known to be permeable to a variety of cytoplasmic molecules. The first observation of second messenger junctional permeability, made approximately 30 years ago, sparked broad interest in gap junction channels as mediators of intercellular molecular signaling. Since then, much has been learned about the diversity of connexin channels with regard to isoform diversity, tissue and developmental distribution, modes of channel regulation, assembly, expression, biochemical modification and permeability, all of which appear to be dynamically regulated. This information has expanded the potential roles of connexin channels in development, physiology and disease, and made their elucidation much more complex--30 years ago such an orchestra of junctional dynamics was unanticipated. Only recently, however, have investigators been able to directly address, in this more complex framework, the key issue: what specific biological molecules, second messengers and others, are able to permeate the various types of connexin channels, and how well? An important related issue, given the ever-growing list of connexin-related pathologies, is how these permeabilities are altered by disease-causing connexin mutations. Together, many studies show that a variety of cytoplasmic molecules can permeate the different types of connexin channels. A few studies reveal differences in permeation by different molecules through a particular type of connexin channel, and differences in permeation by a particular molecule through different types of connexin channels. This article describes and evaluates the various methods used to obtain these data, presents an annotated compilation of the results, and discusses the findings in the context of what can be inferred about mechanism of selectivity and potential relevance to signaling. The data strongly suggest that highly specific interactions take place between connexin pores and specific biological molecular permeants, and that those interactions determine which cytoplasmic molecules can permeate and how well. At this time, the nature of those interactions is unclear. One hopes that with more detailed permeability and structural information, the specific molecular mechanisms of the selectivity can be elucidated.

Breast cancer metastasis suppressor 1 functions as a corepressor by enhancing histone deacetylase 1-mediated deacetylation of RelA/p65 and promoting apoptosis

The antiapoptotic transcription factor NF-kappaB is constitutively activated in many cancers and is important for cytokine-mediated progression and metastatic movement of tumors. Breast cancer metastasis suppressor 1 (BRMS1) is a metastasis suppressor gene whose mechanisms of action are poorly understood. In this report, we demonstrate that BRMS1 decreases the transactivation potential of RelA/p65 and ameliorates the expression of NF-kappaB-regulated antiapoptotic gene products. BRMS1 immunoprecipitates with the RelA/p65 subunit of NF-kappaB with protein-protein interactions occurring at the C terminus region of the rel homology domain but not at its known transactivation domains. Moreover, BRMS1 functions as a corepressor by promoting binding of HDAC1 to RelA/p65, where it deacetylates lysine K310 on RelA/p65, which suppresses RelA/p65 transcriptional activity. Selective small interfering RNA knockdown of BRMS1 confirms that chromatin-bound BRMS1 is required for deacetylation of RelA/p65, while enhancing chromatin occupancy of HDAC1 onto the NF-kappaB-regulated promoters cIAP2 and Bfl-1/A1. We observed in cells lacking BRMS1 a dramatic increase in cell viability after the loss of attachment from the extracellular matrix. Collectively, these results suggest that BRMS1 suppresses metastasis through its ability to function as a transcriptional corepressor of antiapoptotic genes regulated by NF-kappaB.

Expression of MAP kinases and connexins in the differentiation of rat mammary epithelial cells

Gap junctional intercellular communication (GJIC) is involved in the regulation of many cellular processes. MAP kinases are known to affect GJIC and phosphorylation of connexin (Cx). MAP kinases can also be a regulator of cell proliferation and growth. This study was undertaken to show the relevance between expression patterns of Cxs and MAP kinases in rat mammary epithelial cells (RMECs). In order to characterize the RMECs, they were stained with Peanut lectin, which indicates most alveolar epithelial cells, and Thy-1.1 was used as a marker of luminal epithelial cells or myoepithelial cells, respectively. We studied the expression patterns of major gap junction proteins, Cx26, 32, and 43 in RMECs. Western blot analysis demonstrated that Cx26 gradually decreased from day 2, while Cx32 was expressed constantly from day 1 to 14. Cx43 dramatically increased on day 5 and decreased thereafter. The expression patterns and phosphorylation of ERK1/2 and JNK were similar to Cx43, but expression of p38 was like that of Cx32. These results showed that the MAP kinases that comprise ERK1/2, p38, and JNK were involved in regulation of Cxs. Our data suggests that GJIC plays an important role during rat mammary differentiation and that MAP kinases may be closely related functionally to regulate the gap junction.

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.

Reduced expression of the Connexin26 gene and its aberrant DNA methylation in rat lung adenocarcinomas induced by N-nitrosobis(2-Hydroxypropyl)amine

Gap junctions are mediated by intercellular channels that connect adjacent cells and are composed of Connexin (Cx) proteins. A member of the Cx family, Cx26 is considered a potential tumor suppressor in several cancers. The expression of Cx26 gene and its methylation status in rat lung adenocarcinomas induced by N-nitrosobis(2-hydroxypropyl)amine (BHP) were investigated. Six-wk-old male Wistar rats were given 2,000 ppm BHP in their drinking water for 12 wk and maintained without further treatment until they were sacrificed at 25 wk. A total of nine lung adenocarcinomas were obtained and total RNA was extracted to assess expression by real-time quantitative reverse transcription (RT)-polymerase chain reaction (PCR). Five out of nine adenocarcinomas showed reduced expression compared with normal lung tissue. We next performed a bisulfite sequence analysis to measure the methylation status of the 5' upstream region of the Cx26 gene in two normal lung tissues and five lung adenocarcinomas that showed reduced expression of Cx26. All five adenocarcinomas were highly methylated in the 5' upstream region, while the two normal lung tissues were unmethylated. This suggests that aberrant methylation of the Cx26 gene may be involved in the development of lung adenocarcinomas induced by BHP in rats.

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.

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.

Connexin 26 correlates with Bcl-xL and Bax proteins expression in colorectal cancer

AIM: To evaluate of Cx26 in correlation with Bcl-xL and Bax proteins in colorectal cancer.

METHODS: Immunohistochemical staining using specific antibodies was performed to evaluate the protein expression of Cx26, Bax and Bcl-xL in 152 colorectal cancer samples and the correlations among studied proteins as well as the relationships between the expression of Cx26, Bax, Bcl-xL and clinicopathological features were analyzed.

RESULTS: Both normal epithelial cells and carcinoma cells expressed Cx26, Bax and Bcl-xL, but Cx26 in cancer cells showed aberrant, mainly cytoplasmic staining. Expression of Cx26, Bax and Bcl-xL was observed in 55.9%, 55.5% and 72.4% of evaluated colorectal cancers respectively. We found the positive correlation between Cx26 and Bax expression (r = 0.561, P<0.0001), Cx26 and Bcl-xL (r = 0.409, P<0.0001) as well as between Bax and Bcl-xL (r = 0.486, P<0.0001). Association of Cx26, Bax and Bcl-xL expression with histological G2 grade of tumors was noted (P<0.005, P<0.001 and P<0.002 respectively).

CONCLUSION: Cytoplasmic presence of Cx26 and its association with apoptotic markers could indicate a distinct role from physiological functions of Cx26 in cancer cells and it could suggest that connexins might be a target point for modulations of apoptosis with therapeutic implications.

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.

Connexin expression and cell coupling fail to reverse the v-src transformed growth characteristics of a Cx43-/- cell line

Gap junctions, composed of connexins, have been shown to suppress transformation in a variety of malignancies and transformed cell types. In addition, transforming factors such as the src oncogene have been shown to directly phosphorylate some connexins (e.g., Cx43) and inhibit coupling. To investigate the role of gap junctions in cell transformsation by v-src, we utilized a clonal cell line derived from Cx43 knockout mice (KoA) that was immortalized, but not transformed. Transfection by v-src induced a marked transformed phenotype characterized by growth in low serum and anchorage-independent conditions. Subsequent transfections by Cx43, Cx32 or vector alone were then tested for their effects on growth. Activity of pp60v-src was confirmed in all transfectants as well as the ability of pp60v-src to phosphorylate Cx43 in several clones. Despite the documented effect of pp60v-src on Cx43 channel closure, modest coupling was still retained in many of the Cx43 and Cx32 transfectants. However, none of the four Cx43 transfected clones showed significant inhibitory effects on proliferation in either anchorage-independent or low serum growth conditions. Of the Cx32 clones, only one in five showed effects on growth in both assays, which was the same ratio observed for the control transfectants. Thus, based on the levels of expression achieved, which were comparable to endogenous levels in established cell lines, neither Cx43 nor Cx32 serve as effective suppressors of the transformed growth phenotype of this v-src expressing cell line.

Overexpression of estrogen receptor-alpha gene suppresses gap junctional intercellular communication in endometrial carcinoma cells

Stimulation of the endometrium by estrogens without the differentiating effect of progestins is the primary etiological factor associated with the development of endometrial hyperplasia and adenocarcinoma. However, the correlation between sex steroids and gap junctional intercellular communication (GJIC), which is considered to play an important role in the control of cell growth and differentiation, is not well known in endometrial carcinoma. In this study, we focused on the influence of estrogen and its receptor in connexin (Cx) expression and GJIC in endometrial carcinoma cells, established stable clone IK-ER1 overexpressing ER-alpha to transfect the expression vector and analysed them in various hormonal conditions. The growth of IK-ER1 was accelerated by 17beta-estradiol and the acceleration of the 5-bromo-25-deoxyuridine labeling index was observed. GJIC was assayed by scoring the number of dye-coupled cells after microinjection of single cells with Lucifer-Yellow, and subcellular localization of Cx26 and Cx32 was analysed by immunocytochemistry. In the presence of estradiol, dye-coupled cells of IK-ER1 were significantly reduced compared to those without estradiol and the reduction was completely inhibited by adding ICI182.780, a pure antiestrogen substrate. Cxs were detected as only small spots by immunocytochemistry, and Western blotting showed that the expression was decreased. These results suggest that activation of ER-alpha by estrogen results in tumor progression by stimulating cell growth and suppressing GJIC via suppression of the expression of Cxs in endometrial carcinogenesis.

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.

Breast cancer metastasis suppressor 1: update

This article reviews information related to the BRMS1 (BReast Metastasis Suppressor 1) metastasis suppressor gene. BRMS1 was identified by differential display comparing metastasis-suppressed chromosome 11 hybrids with metastatic, parental MDA-MB-435 human breast carcinoma cells. BRMS1 has subsequently been shown to suppress metastasis, but not tumorigenicity of human melanoma cells. The murine version, Brms1, also suppresses metastasis and exhibits a high level of homology to the human gene at the structure, nucleotide and amino acid levels. The mechanisms of action remain to be determined; however, BRMS1 transfectant cells have restored gap junctional intercellular communication. Recent data suggest that BRMS1 is part of the mSin3A histone deacetylase complex.

Cancer: the role of extracellular disease

Invasive carcinoma originates from the epithelial cells lining the lumen of an organ. It is often preceded by metaplasia, dysplasia or carcinoma in situ. The purpose of this review is to suggest that this disease of the epithelium may be, in part, the result of underlying tissue-based disorganization. Human cancer is frequently associated with pre-existing tissue disease. For example, hepatocellular carcinoma usually occurs in patients with a macronodular cirrhotic liver. Most lung cancers arise among patients with chronic lung disease (bronchitis, emphysema, and chronic infection). Mechanical forces appear to play a major role in regulating normal and cancer cell growth. The loss of cell polarity by neoplastic cells, coupled to an otherwise normal growth rate is enough to explain the cancer star-shaped pattern. By changing the plane of cell division, tumor cells may escape physical constraints from surrounding cells and divide. Loss of cell polarity and the resulting cell proliferation appears to be a consequence of either tissue-based disorganization (chronic inflammation, fibrosis) or of direct carcinogenic insult. The multiple mutations frequently described in cancer may be, in part, secondary to physical stress and not primary events. Several animal and clinical trials have shown that tissue disruption (i.e. radiation-induced fibrosis or liver cirrhosis) can be successfully treated. It is possible that treatment targeted at tissue disruption would delay or reduce cancer incidence regardless of the precise biological mechanism of carcinogenesis.

Variable promoter region CpG island methylation of the putative tumor suppressor gene Connexin 26 in breast cancer

Intercellular communication via gap junctions is a mechanism for tumor suppression. Connexin 26 (Cx26) is a structural component of gap junctions expressed by breast epithelial cells. Expression levels of Cx26 are reduced in many breast tumors. Methylation-sensitive single-stranded conformation analysis showed variable methylation in the promoter region CpG island in 11 out of 20 (55%) breast cancer patients. Heterogeneity in methylation patterns was observed both between and within tumors. The degree of methylation ranged from a few CpG dinucleotides to almost all the CpG dinucleotides in the analyzed region. The most frequently methylated CpG was in an Sp1 site known to be important for Cx26 gene expression. One of eight breast cancer cell lines (MD-MBA-453) was methylated in the promoter region and did not express Cx26. Treatment of MDA-MB-453 with 5-aza-2'-deoxycytidine resulted in the re-expression of Cx26 mRNA. Methylation of the promoter region is likely to be an important mechanism in modulating the expression of Cx26 in breast cancer.

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.

Putting tumours in context

The interactions between cancer cells and their micro- and macroenvironment create a context that promotes tumour growth and protects it from immune attack. The functional association of cancer cells with their surrounding tissues forms a new 'organ' that changes as malignancy progresses. Investigation of this process might provide new insights into the mechanisms of tumorigenesis and could also lead to new therapeutic targets.

Partial purification and characterization of proteins with growth promoting activities from ovine mammary gland secretions

Developmental regulation of growth promoting activities in mammary secretions of pregnant Awassi ewes was defined, and growth factors contained in these secretions were partially purified and characterised. Mammary secretions from pregnant ewes enhanced fibroblast cell (AKR-2B) and mammary cell (CID-9 cell strain) proliferation to levels comparable to that induced by 10% Foetal calf serum. Major milk proteins in mammary secretions collected from pregnant ewes one month prior to lambing up to one week after lambing, were resolved by SDS-PAGE, while gelatinases were resolved by zymography. Gelatinase activity was noted prior to P134 and decreased thereafter to reach a minimum during lactation. This decrease was concomitant with the onset of casein production. It is during this critical developmental period that highest growth promoting activity in mammary secretions was detected. Secretions with highest growth promoting activity were fractionated by ion exchange and gel filtration chromatography. Two heat-resistant, trypsin/chymotrypsin sensitive, growth-promoting activities were characterised. The first, designated ovine mammary derived growth factor-1 (oMDGF-1), had around a 30 kDa molecular weight and eluted at 0.65 M NaCl gradient on cation ion exchange chromatography. The second, oMDGF-2, eluted under gel filtration conditions at a molecular weight of 50 kDa and 150 kDa. oMDGF-1 induced changes in Connexin 43, but not in beta-casein mRNA expression by CID-9 mammary cells. In conclusion, growth factor activities in ewe mammary secretions peak during gestation at a period that overlaps maximal gelatinase expression and precedes milk protein synthesis. The factors modulate mammary cell function and may play a role in mammary gland development.

Suppressed gap junctional intercellular communication in carcinogenesis of endometrium

To examine whether and at which stage of endometrial carcinogenesis decreased connexin expression occurs, we investigated changes in the expression of the gap junction proteins, connexin 26 (Cx26), Cx32 and Cx43, in human endometrial hyperplasia and cancer samples. Forty-eight endometrial tissue samples (15 endometrial hyperplasias and 33 endometrial cancers) were subjected to immunofluorescence and RT-PCR analysis. In endometrial hyperplasia, Cx26 was aberrantly expressed in all samples as revealed immunohistochemically. There was weak or negative expression in 12 samples (80.0%) and diffuse expression in cytoplasm in 3 samples (20.0%). Cx32 expression in those samples was similar to that of Cx26; there was weak or negative expression in 11 samples (73.3%) and diffuse expression in 4 samples (26.7%). In endometrial cancer, Cx26 was expressed weakly or negatively in 25 samples (75.8%), diffusely in 6 samples (18.2%) and normally in 2 samples (6.1%), while Cx32 was expressed weakly or negatively in 26 samples (78.8%), diffusely in 5 samples (15.2%) and normally in 2 samples (6.1%). It was confirmed that weak staining of Cx26 and Cx32 was due to poor expression of their mRNA. All samples showed weak Cx43 protein expression as revealed by immunohistochemical analysis. In the majority of samples, concomitant expression levels of Cx26 and Cx32 protein were observed, confirming our long-term hypothesis that Cx26 and Cx32 are both abnormally regulated in a coordinated fashion in the endometrium. Our results indicate that during endometrial carcinogenesis, loss of gap junctional intercellular communication (GJIC) may occur due to the suppressed expression and the aberrant localization of connexin at relatively early stages.

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.