Connexins as targets for cancer chemoprevention and chemotherapy

Biophysical control of plasticity and patterning in regeneration and cancer

Cells and tissues display a remarkable range of plasticity and tissue-patterning activities that are emergent of complex signaling dynamics within their microenvironments. These properties, which when operating normally guide embryogenesis and regeneration, become highly disordered in diseases such as cancer. While morphogens and other molecular factors help determine the shapes of tissues and their patterned cellular organization, the parallel contributions of biophysical control mechanisms must be considered to accurately predict and model important processes such as growth, maturation, injury, repair, and senescence. We now know that mechanical, optical, electric, and electromagnetic signals are integral to cellular plasticity and tissue patterning. Because biophysical modalities underly interactions between cells and their extracellular matrices, including cell cycle, metabolism, migration, and differentiation, their applications as tuning dials for regenerative and anti-cancer therapies are being rapidly exploited. Despite this, the importance of cellular communication through biophysical signaling remains disproportionately underrepresented in the literature. Here, we provide a review of biophysical signaling modalities and known mechanisms that initiate, modulate, or inhibit plasticity and tissue patterning in models of regeneration and cancer. We also discuss current approaches in biomedical engineering that harness biophysical control mechanisms to model, characterize, diagnose, and treat disease states.

The Role of Connexin 43 in Lung Disease

The term lung disease describes a broad category of disorders that impair lung function. More than 35 million Americans have a preventable chronic lung disease with high mortality rates due to limited treatment efficacy. The recent increase in patients with lung disease highlights the need to increase our understanding of mechanisms driving lung inflammation. Connexins, gap junction proteins, and more specifically connexin 43 (Cx43), are abundantly expressed in the lung and are known to play a role in lung diseases. This review focuses on the role of Cx43 in pathology associated with acute respiratory distress syndrome (ARDS), chronic obstructive pulmonary disease (COPD) and asthma. Additionally, we discuss the role of Cx43 in preventing disease through the transfer of mitochondria between cells. We aim to highlight the need to better understand what cell types are expressing Cx43 and how this expression influences lung disease.

Connexins-Therapeutic Targets in Cancers

Connexins (Cx) are members of a protein family that forms intercellular channels localised in gap junction (GJ) plaques and single transmembrane channels called hemichannels. They participate in intercellular communication or communication between the intracellular and extracellular environments. Connexins affect cell homeostasis, growth and differentiation by enabling the exchange of metabolites or by interfering with various signalling pathways. Alterations in the functionality and the expression of connexins have been linked to the occurrence of many diseases. Connexins have been already linked to cancers, cardiac and brain disorders, chronic lung and kidney conditions and wound healing processes. Connexins have been shown either to suppress cancer tumour growth or to increase tumorigenicity by promoting cancer cell growth, migration and invasiveness. A better understanding of the complexity of cancer biology related to connexins and intercellular communication could result in the design of novel therapeutic strategies. The modulation of connexin expression may be an effective therapeutic approach in some types of cancers. Therefore, one important challenge is the search for mechanisms and new drugs, selectively modulating the expression of various connexin isoforms. We performed a systematic literature search up to February 2020 in the electronic databases PubMed and EMBASE. Our search terms were as follows: connexins, hemichannels, cancer and cancer treatment. This review aims to provide information about the role of connexins and gap junctions in cancer, as well as to discuss possible therapeutic options that are currently being studied.

Cx32 mediates norepinephrine-promoted EGFR-TKI resistance in a gap junction-independent manner in non-small-cell lung cancer

The second-generation EGFR-TKI Afatinib is an irreversible ErbB family blocker used to treat patients with non-small-cell lung cancer (NSCLC). Unfortunately, resistance to this drug develops over time, and patients are always under great psychological pressure. A previous study showed that chronic stress hormones participate in EGFR-TKI resistance via β2 -AR signaling via an IL-6 dependent mechanism. Our study further explores a novel potential underlying mechanism. In the present study, we show that the stress hormone norepinephrine (NE) promotes Afatinib resistance by upregulating Cx32 expression. Furthermore, we, for the first time, find that Cx32 is a target gene for transcription factor CREB and NE enhances Cx32 mRNA expression by activation of CREB. We also demonstrate that Cx32 promotes Afatinib resistance by decreasing the degradation of EGFR-TKI resistance-associated proteins (MET, IGF-1R) and by increasing their transcription levels. Together, these results reveal that the stress hormone NE accelerates Afatinib resistance by increasing the expression of Cx32, which augments MET and IGF-1R levels in cancer cells and provides a promising therapeutic strategy against EGFR-TKI Afatinib resistance in NSCLC.

Betacellulin enhances ovarian cancer cell migration by up-regulating Connexin43 via MEK-ERK signaling

Epithelial ovarian cancer is the fifth common cause of cancer death in women and the most lethal gynecological malignancies. Our previous studies have shown that up-regulation of Connexin43, a gap-junction subunit crucial for cell-cell communication, enhances ovarian cancer cell migration. Betacellulin is a member of the epidermal growth factor (EGF) family which can bind to multiple EGF family receptors. Overexpression of betacellulin is found in a variety of cancers and is associated with reduced survival. However, the specific roles and molecular mechanisms of betacellulin in ovarian cancer progression are poorly understood. In the current study, we tested the hypothesis that betacellulin induces ovarian cancer cell migration by up-regulating Connexin43. Our results showed that treatment with betacellulin significantly increased Connexin43 expression and cell migration in both OVCAR4 and SKOV3 ovarian cancer cell lines. Moreover, betacellulin induced the activation of MEK-ERK signaling, and its effects on Connexin43 were inhibited by pre-treatment with U0126. Pre-treatment with AG1478 totally blocked the activation of MEK-ERK signaling but only partially inhibited betacellulin-induced Connexin43 expression and cell migration. Most importantly, betacellulin-induced cell migration was attenuated by knockdown of Connexin43, and co-treatment with gap junction inhibitor carbenoxolone did not alter this effect. Our results suggest a bilateral role of Connexin43 in ovarian cancer migration, and also demonstrate a gap junction-independent mechanism of betacellulin.

Targeting different domains of gap junction protein to control malignant glioma

A rational treatment strategy for glioma, the most common primary central nervous system tumor, should focus on early invasive growth and resistance to current therapeutics. Connexin 43 (Cx43), a gap junction protein, plays important roles not only in the development of the central nervous system and but also in the progression of glioma. The different structural domains of Cx43, including extracellular loops, transmembrane domains, and an intracellular carboxyl terminal, have distinct functions in the invasion and proliferation of gliomas. Targeting these domains of Cx43, which is expressed in distinct patterns in the heterogeneous glioma cell population, can inhibit tumor cell invasion and new tumor formation. Thus, this review summarizes the structural characteristics of Cx43, the effects of regulating different Cx43 domains on the biological characteristics of glioma cells, intervention strategies targeting different domains of Cx43, and future research directions.

[Inhibitory effect of connexin43 protein on autophagy in cisplatin-resistant testicular cancer I-10 cells]

OBJECTIVE

To investigate the effect of connexin43 (Cx43) protein on autophagy in cisplatin (DDP)-resistant testicular cancer I-10 cells.

METHODS

The expression of Cx43 proteins in testicular cancer I-10 cells and I-10/DDP cells were detected with Western blotting. I-10/DDP cells were transfected with a full- length mouse Cx43 vector (mCx43) via Lipofectamine²⁰⁰⁰, the empty vector or Lipofectamine²⁰⁰⁰ (blank control group), and the changes in the expressions of LC3 and p62 proteins were determined with Western blotting. mCherry-GFP-LC3B transfection and transmission electron microscopy were used to analyze the changes in autophagy of the cells with Cx43 overexpression.

RESULTS

Cx43 was significantly decreased in I-10/DDP cells compared with I-10 cells (P < 0.01). Transfection of the I-10/DDP cells with mCx43 vector resulted in significantly increased Cx43 expression in the cells (P < 0.01) and caused significantly decreased expression of LC3-Ⅱ (P < 0.01) and increased expression of p62 (P < 0.05) as compared with the negative control cells. Both transmission electron microscopy and mCherry-GFP-LC3B transfection showed that the number of autophagosomes was obviously reduced in mCx43-transfected cells as compared with the negative control cells.

CONCLUSIONS

Cx43 inhibits autophagy in cisplatin-resistant testicular cancer I-10 /DDP cells.

Connexins in cancer: bridging the gap to the clinic

Gap junctions comprise arrays of intercellular channels formed by connexin proteins and provide for the direct communication between adjacent cells. This type of intercellular communication permits the coordination of cellular activities and plays key roles in the control of cell growth and differentiation and in the maintenance of tissue homoeostasis. After more than 50 years, deciphering the links among connexins, gap junctions and cancer, researchers are now beginning to translate this knowledge to the clinic. The emergence of new strategies for connexin targeting, combined with an improved understanding of the molecular bases underlying the dysregulation of connexins during cancer development, offers novel opportunities for clinical applications. However, different connexin isoforms have diverse channel-dependent and -independent functions that are tissue and stage specific. This can elicit both pro- and anti-tumorigenic effects that engender significant challenges in the path towards personalised medicine. Here, we review the current understanding of the role of connexins and gap junctions in cancer, with particular focus on the recent progress made in determining their prognostic and therapeutic potential.

Connexin-43 Enhances the Redesigned Cytosine Deaminase Activity for Suicide Gene Therapy in Human Breast Cancer Cells

Background:

Escherichia coli cytosine deaminase (CD) converts 5-fluorocytosine (5-FC), a prodrug, into 5-fluorouracil (5-FU), a chemotherapeutic drug. However, the poor binding affinity of CD towards 5-FC as compared to the natural substrate cytosine, limits its application towards a successful suicide gene therapy. Although F186W mutant was developed to enhance the effect of wild-type CD, still scope for its improvement remains to further minimize the dose-dependent cytotoxicity of the drugs. Hence, in this study, we employ the anti-tumour attribute of the gap junction forming protein connexin-43 (Cx43) in conjunction with CD or F186W mutant.

Methods:

Lipofectamine was used to co-transfect CD/F186W-pVITRO2 and Cx43-pEGFP-N1 plasmids construct into MCF-7 cells. Comparative analysis of cell viability was observed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-bromide (MTT) and trypan blue–based assays. To further confirm the mode of cell death was apoptosis, propidium iodide and annexin V/7-aminoactinomycin D (7-AAD)-based apoptosis assays were performed.

Results:

Semi-quantitative polymerase chain reaction (PCR) confirmed the expression of both Cx43 and CD/F186W genes after transfection. Furthermore, cell viability assays revealed the enhanced activity of F186W-Cx43 compared with CD-Cx43 and F186W alone. The trend of the reduction in cell viability was also reflected in the flow cytometry–based apoptosis analyses. Overall, F186W-Cx43 combination demonstrated its superiority over the CD-Cx43 and F186W mutant alone.

Conclusions:

The enhanced cytotoxic activity of F186W mutant was further amplified by gap junction protein Cx43.

Disassembling a cancer puzzle: Cell junctions and plasma membrane as targets for anticancer therapy

Despite an enhanced permeability and retention effect typical of many solid tumors, drug penetration is not always sufficient. Possible strategies for the drug delivery improvement are a modification of the tumor cell-to-cell junctions and usage of cell membrane permeabilization proteins. In this review we discuss epithelial cell junctions as targets for a combined anticancer therapy and propose new possible sources of such agents. We suggest considering viral and bacterial pathogens disrupting epithelial layers as plentiful sources of new therapeutic agents for increasing tumor permeability for other effector agents. We also observe the application of pore forming proteins and peptides of different origin for cytoplasmic delivery of anti-cancer agents and consider the main obstacles of their use in vivo.

Connexin 32 affects doxorubicin resistance in hepatocellular carcinoma cells mediated by Src/FAK signaling pathway

Doxorubicin (DOX) is first-line chemotherapy for hepatocellular carcinoma (HCC), but the effect is not satisfactory. The resistance of HCC cells to DOX is the main reason leading to treatment failure. Therefore, it is necessary to study the mechanism of DOX resistance in HCC. In this study, expression of connexin (Cx)32 was significantly decreased in HCC tissues compared with corresponding paracancerous tissues, and activity of the Src/focal adhesion kinase (FAK) signaling pathway was significantly enhanced. Expression of Cx32 was closely associated with activity of the Src/FAK signaling pathway, Cx32, and the Src/FAK signaling pathway was also correlated with degree of HCC differentiation. In DOX-resistant HepG2 cells, compared with DOX-sensitive HepG2 cells, expression of Cx32 was significantly reduced and activity of the Src/FAK pathway increased. After silencing Cx32 in HepG2 cells, activity of the Src/FAK pathway increased and sensitivity to DOX decreased. In contrast, overexpression of Cx32 in HepG2/DOX cells decreased activity of the Src/FAK pathway and increased sensitivity to DOX. Dasatinib and KX2-391, inhibitors of the Src/FAK pathway, significantly increased the sensitivity of HepG2/DOX cells to DOX. The results suggest that Src/FAK is a downstream regulator of Cx32 and Cx32 regulates the sensitivity of HCC cells to DOX via the Src/FAK signaling pathway. Our study demonstrates a potential mechanism of DOX resistance in HCC cells and supports that Cx32-Src/FAK is an important target for reversing drug resistance of HCC.

Application of a co‑expression network for the analysis of aggressive and non‑aggressive breast cancer cell lines to predict the clinical outcome of patients

Breast cancer metastasis is a demanding problem in clinical treatment of patients with breast cancer. It is necessary to examine the mechanisms of metastasis for developing therapies. Classification of the aggressiveness of breast cancer is an important issue in biological study and for clinical decisions. Although aggressive and non‑aggressive breast cancer cells can be easily distinguished among different cell lines, it is very difficult to distinguish in clinical practice. The aim of the current study was to use the gene expression analysis from breast cancer cell lines to predict clinical outcomes of patients with breast cancer. Weighted gene co‑expression network analysis (WGCNA) is a powerful method to account for correlations between genes and extract co‑expressed modules of genes from large expression datasets. Therefore, WGCNA was applied to explore the differences in sub‑networks between aggressive and non‑aggressive breast cancer cell lines. The greatest difference topological overlap networks in both groups include potential information to understand the mechanisms of aggressiveness. The results show that the blue and red modules were significantly associated with the biological processes of aggressiveness. The sub‑network, which consisted of TMEM47, GJC1, ANXA3, TWIST1 and C19orf33 in the blue module, was associated with an aggressive phenotype. The sub‑network of LOC100653217, CXCL12, SULF1, DOK5 and DKK3 in the red module was associated with a non‑aggressive phenotype. In order to validate the hazard ratio of these genes, the prognostic index was constructed to integrate them and examined using data from the Cancer Genomic Atlas (TCGA) and Gene Expression Omnibus (GEO) databases. Patients with breast cancer from TCGA in the high‑risk group had a significantly shorter overall survival time compared with patients in the low‑risk group (hazard ratio=1.231, 95% confidence interval=1.058‑1.433, P=0.0071, by the Wald test). A similar result was produced from the GEO database. The findings may provide a novel strategy for measuring cancer aggressiveness in patients with breast cancer.

Potassium Ascorbate with Ribose: Promising Therapeutic Approach for Melanoma Treatment

While surgery is the definitive treatment for early-stage melanoma, the current therapies against advanced melanoma do not yet provide an effective, long-lasting control of the lesions and a satisfactory impact on patient survival. Thus, research is also focused on novel treatments that could potentiate the current therapies. In the present study, we evaluated the effect of potassium ascorbate with ribose (PAR) treatment on the human melanoma cell line, A375, in 2D and 3D models. In the 2D model, in line with the current literature, the pharmacological treatment with PAR decreased cell proliferation and viability. In addition, an increase in Connexin 43 mRNA and protein was observed. This novel finding was confirmed in PAR-treated melanoma cells cultured in 3D, where an increase in functional gap junctions and a higher spheroid compactness were observed. Moreover, in the 3D model, a remarkable decrease in the size and volume of spheroids was observed, further supporting the treatment efficacy observed in the 2D model. In conclusion, our results suggest that PAR could be used as a safe adjuvant approach in support to conventional therapies for the treatment of melanoma.

Association between connexin 40 and potassium voltage-gated channel subfamily A member 5 expression in the atrial myocytes of patients with atrial fibrillation

Structural and electrical remodeling within the atrium mediate the pathogenesis of atrial fibrillation (AF). Two key genes that sever a role in this remodeling are connexin 40 (Cx40) and potassium voltage-gated channel subfamily A member 5 (KCNA5), respectively. Electrical remodeling is considered to induce structural remodeling during AF. In the present study, the left atrial appendage section and atrial myocytes of patients with AF were evaluated. It was observed that Cx40 and KCNA5 mRNA (P<0.05) and protein (P<0.01) expression was significantly downregulated in AF compared with rheumatic heart disease. In addition, a positive correlation between the mRNA expression Cx40 and KCNA5 was observed in the atrial myocytes of patients with AF (P<0.05; r=0.42). The association between Cx40 and KCNA5 expression was subsequently investigated in primary cultured atrial myocytes using siRNA transfection. In atrial myocytes, downregulation of Cx40 inhibited the expression of KCNA5. Similarly, silencing of KCNA5 suppressed the expression of Cx40. These results indicate that synergistic regulation may occur between Cx40 and KCNA5 expression. Furthermore, the combined effects of electrical and structural remodeling in the atrial myocytes of patients with AF may contribute to the pathogenesis of AF.

Implication of connexin30 on the stemness of glioma: connexin30 reverses the malignant phenotype of glioma by modulating IGF-1R, CD133 and cMyc

Gap-junctional intercellular communication (GJIC) plays a major role in the malignant growth of glioma. Although the mechanistic aspects of GJIC have been extensively studied, the role of connexins in the regulation of the malignant behavior of glioma stem cells (GSCs) remains unclear. In our previous studies, we have shown that connexin30 can interfere with the insulin-like growth factor 1 receptor (IGF-1R), which is known for self-renewal and pluripotency. Following our earlier in vitro observation, in this work, we aimed to study the consequence of this influence of Cx30 on IGF-1R by evaluating the marker of GSCs, CD133 and oncoprotein, cMyc. We strengthened our basis by examining human glioma samples of different grades as well as rat C6 xenografts (Cx30-transfected and -non-transfected C6 cells) along with the sphere formation assays in vitro. Investigation of stemness-related CD133 and cMyc in human samples and rat xenografts exhibited a reciprocal relationship between Cx30 and IGF-1R in the low and high grades (HG) of glioma. Cx30 was completely abolished in HG; levels of IGF-1R, CD133 and cMyc expression were positively correlated with HG. Cx30 transfection could attenuate the malignant burden of glioma in rat xenografts. Cx30 transfection also altered the tumor sphere formation of C6 glioma cells in vitro, an important property of GSCs, and there was a significant reduction of CD133 and cMyc expression by Cx30 both in vitro and in vivo. These factors indicate that dysfunction of Cx30 plays a crucial role in the prevention of the stemness of glioma, and the exploitation of this feature will help in the management of glioma.

Connexin 43 enhances paclitaxel cytotoxicity in colorectal cancer cell lines

Colorectal cancer has a relatively low sensitivity to paclitaxel. The purpose of this study was to investigate the role of connexin 43 (Cx43), which is a structural component of gap junctional communication (GJC), in paclitaxel cytotoxicity in colorectal cancer cells. Three colorectal cancer cell lines (HCT106, HCT116 and LoVo) were transfected with Cx43 and used to examine paclitaxel cytotoxicity. A western blot assay was used to confirm Cx43 expression in transfected cell lines as well as the expression of several proteins that are associated with paclitaxel cytotoxicity. A parachute dye-coupling assay was used to measure GJC function. An MTT assay was used to analyze the viability of paclitaxel-treated cells. Cx43 expression level and GJC function were significantly upregulated by the transfection (P<0.05). The viability of transfected cells was significantly inhibited compared with that of untransfected cells when treated with paclitaxel (20 or 80 nM) at high culture density but not at low culture density (P<0.05). Cx43 transfection significantly increased the mitotic arrest, tubulin polymerization and apoptosis effects of paclitaxel (P<0.05). It was also found that paclitaxel had an inhibitory effect on GJC function after 12 h of treatment in LoVo cells (P<0.05). These results indicate that Cx43 may serve as a target of paclitaxel chemotherapy for colorectal cancer.

Kanglaite sensitizes colorectal cancer cells to Taxol via NF-κΒ inhibition and connexin 43 upregulation

Taxol, a first-line anti-tumour drug, has low effectiveness against colorectal cancer. Combination with other agents is an effective strategy to enhance Taxol cytotoxicity. Kanglaite injection is an extract from Coix lacryma-jobi seed and is usually combined with other agents to treat cancer. The aim of this study was to investigate the treatment effect of Taxol combined with Kanglaite on colorectal cancer cell lines. Kanglaite pretreatment followed by Taxol treatment was found to show the best synergism among all combination strategies. This combination also resulted in the smallest tumour volume in a Balb/c mice model. Kanglaite inhibited the expression of nuclear factor (NF)-κΒ and upregulated that of connexin 43, both of which sensitized cancer cells to Taxol. Moreover, Kanglaite increased many cellular variations caused by Taxol, including tubulin polymerization, caspase-3 cleavage, and upregulated expression of survivin and cyclin B1. These results suggest that Kanglaite pretreatment may increase the effect of Taxol on colorectal cancer.

Inhibition of gap junction intercellular communication is involved in silica nanoparticles-induced H9c2 cardiomyocytes apoptosis via the mitochondrial pathway

Gap junction intercellular communication (GJIC) between cardiomyocytes is essential for synchronous heart contraction and relies on connexin-containing channels. Connexin 43 (Cx43) is a major component involved in GJIC in heart tissue, and its abnormal expression is closely associated with various cardiac diseases. Silica nanoparticles (SNPs) are known to induce cardiovascular toxicity. However, the mechanisms through which GJIC plays a role in cardiomyocytes apoptosis induced by SNPs remain unknown. The aim of the present study is to determine whether SNPs-decreased GJIC promotes apoptosis in rat cardiomyocytes cell line (H9c2 cells) via the mitochondrial pathway using CCK-8 Kit, scrape-loading dye transfer technique, Annexin V/PI double-staining assays, and Western blot analysis. The results showed that SNPs elicited cytotoxicity in H9c2 cells in a time- and concentration-dependent manner. SNPs also reduced GJIC in H9c2 cells in a concentration-dependent manner through downregulation of Cx43 and upregulation of P-Cx43. Inhibition of gap junctions by gap junction blocker carbenoxolone disodium resulted in decreased survival and increased apoptosis, whereas enhancement of the gap junctions by retinoic acid led to enhanced survival but decreased apoptosis. Furthermore, SNPs-induced apoptosis through the disrupted functional gap junction was correlated with abnormal expressions of the proteins involved in the mitochondrial pathway-related apoptosis such as Bcl-2/Bax, cytochrome C, Caspase-9, and Caspase-3. Taken together, our results provide the first evidence that SNPs-decreased GJIC promotes apoptosis in cardiomyocytes via the mitochondrial pathway. In addition, downregulation of GJIC by SNPs in cardiomyocytes is mediated through downregulation of Cx43 and upregulation of P-Cx43. These results suggest that in rat cardiomyocytes cell line, GJIC plays a protective role in SNPs-induced apoptosis and that GJIC may be one of the targets for SNPs-induced biological effects.

Improved Anticancer Photothermal Therapy Using the Bystander Effect Enhanced by Antiarrhythmic Peptide Conjugated Dopamine-Modified Reduced Graphene Oxide Nanocomposite

Despite tremendous efforts toward developing novel near-infrared (NIR)-absorbing nanomaterials, improvement in therapeutic efficiency remains a formidable challenge in photothermal cancer therapy. This study aims to synthesize a specific peptide conjugated polydopamine-modified reduced graphene oxide (pDA/rGO) nanocomposite that promotes the bystander effect to facilitate cancer treatment using NIR-activated photothermal therapy. To prepare a nanoplatform capable of promoting the bystander effect in cancer cells, we immobilized antiarrhythmic peptide 10 (AAP10) on the surface of dopamine-modified rGO (AAP10-pDA/rGO). Our AAP10-pDA/rGO could promote the bystander effect by increasing the expression of connexin 43 protein in MCF-7 breast-cancer cells. Because of its tremendous ability to absorb NIR absorption, AAP10-pDA/rGO offers a high photothermal effect under NIR irradiation. This leads to a massive death of MCF-7 cells via the bystander effect. Using tumor-bearing mice as the model, it is found that NIR radiation effectively ablates breast tumor in the presence of AAP10-pDA/rGO and inhibits tumor growth by ≈100%. Therefore, this research integrates the bystander and photothermal effects into a single nanoplatform in order to facilitate an efficient photothermal therapy. Furthermore, our AAP10-pDA/rGO, which exhibits both hyperthermia and the bystander effect, can prevent breast-cancer recurrence and, therefore, has great potential for future clinical and research applications.

Gap junctions and cancer: communicating for 50 years

Fifty years ago, tumour cells were found to lack electrical coupling, leading to the hypothesis that loss of direct intercellular communication is commonly associated with cancer onset and progression. Subsequent studies linked this phenomenon to gap junctions composed of connexin proteins. Although many studies support the notion that connexins are tumour suppressors, recent evidence suggests that, in some tumour types, they may facilitate specific stages of tumour progression through both junctional and non-junctional signalling pathways. This Timeline article highlights the milestones connecting gap junctions to cancer, and underscores important unanswered questions, controversies and therapeutic opportunities in the field.

The interplay between genetic and bioelectrical signaling permits a spatial regionalisation of membrane potentials in model multicellular ensembles

The single cell-centred approach emphasises ion channels as specific proteins that determine individual properties, disregarding their contribution to multicellular outcomes. We simulate the interplay between genetic and bioelectrical signals in non-excitable cells from the local single-cell level to the long range multicellular ensemble. The single-cell genetic regulation is based on mean-field kinetic equations involving the mRNA and protein concentrations. The transcription rate factor is assumed to depend on the absolute value of the cell potential, which is dictated by the voltage-gated cell ion channels and the intercellular gap junctions. The interplay between genetic and electrical signals may allow translating single-cell states into multicellular states which provide spatio-temporal information. The model results have clear implications for biological processes: (i) bioelectric signals can override slightly different genetic pre-patterns; (ii) ensembles of cells initially at the same potential can undergo an electrical regionalisation because of persistent genetic differences between adjacent spatial regions; and (iii) shifts in the normal cell electrical balance could trigger significant changes in the genetic regulation.

Connexin and pannexin channels in cancer

Communication among cells via direct cell-cell contact by connexin gap junctions, or between cell and extracellular environment via pannexin channels or connexin hemichannels, is a key factor in cell function and tissue homeostasis. Upon malignant transformation in different cancer types, the dysregulation of these connexin and pannexin channels and their effect in cellular communication, can either enhance or suppress tumorigenesis and metastasis. In this review, we will highlight the latest reports on the role of the well characterized connexin family and its ability to form gap junctions and hemichannels in cancer. We will also introduce the more recently discovered family of pannexin channels and our current knowledge about their involvement in cancer progression.

Connexins, Pannexins, and Their Channels in Fibroproliferative Diseases

Cellular and molecular mechanisms of wound healing, tissue repair, and fibrogenesis are established in different organs and are essential for the maintenance of function and tissue integrity after cell injury. These mechanisms are also involved in a plethora of fibroproliferative diseases or organ-specific fibrotic disorders, all of which are associated with the excessive deposition of extracellular matrix components. Fibroblasts, which are key cells in tissue repair and fibrogenesis, rely on communicative cellular networks to ensure efficient control of these processes and to prevent abnormal accumulation of extracellular matrix into the tissue. Despite the significant impact on human health, and thus the epidemiologic relevance, there is still no effective treatment for most fibrosis-related diseases. This paper provides an overview of current concepts and mechanisms involved in the participation of cellular communication via connexin-based pores as well as pannexin-based channels in the processes of tissue repair and fibrogenesis in chronic diseases. Understanding these mechanisms may contribute to the development of new therapeutic strategies to clinically manage fibroproliferative diseases and organ-specific fibrotic disorders.

Detection of Connexins in Liver Cells Using Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis and Immunoblot Analysis

Since connexin expression is partly regulated at the protein level, immunoblot analysis represents a frequently addressed technique in the connexin research field. The present chapter describes the setup of an immunoblot procedure, including protein extraction and quantification from biological samples, gel electrophoresis, protein transfer, and immunoblotting, which is optimized for analysis of connexins in liver tissue. In essence, proteins are separated on a polyacrylamide gel using sodium dodecyl sulfate followed by transfer of proteins on a nitrocellulose membrane. The latter allows specific detection of connexins with antibodies combined with revelation through enhanced chemiluminescence.

Investigation of an SPR biosensor for determining the influence of connexin 43 expression on the cytotoxicity of cisplatin

The real-time and label free detection abilities of surface plasmon resonance (SPR) biosensors provide a way of evaluating the influence of some genes’ expression on anti-tumor drug cytotoxicity.

Immunohisto- and Cytochemistry Analysis of Connexins

Immunohistochemistry (IHC) is a ubiquitous used technique to identify and analyze protein expression in the context of tissue and cell morphology. In the connexin research field, IHC is applied to identify the subcellular location of connexin proteins, as this can be directly linked to their functionality. The present chapter describes a protocol for fluorescent IHC to detect connexin proteins in tissues slices and cells, with slight modifications depending on the nature of biological sample, histological processing, and/or protein expression level. Basically, fluorescent IHC is a short, simple, and cost-effective technique, which allows the visualization of proteins based on fluorescent-labeled antibody-antigen recognition.

Transforming growth factor-β stimulates human ovarian cancer cell migration by up-regulating connexin43 expression via Smad2/3 signaling

Reduced connexin43 (Cx43) expression is frequently detected in different types of human cancer. Cx43 has been shown to regulate cancer cell migration in a cell-type dependent manner. In both primary and recurrent human ovarian cancer, overexpression of TGF-β ligand and its receptors have been detected. TGF-β can regulate Cx43 expression in other cell types and stimulate human ovarian cancer cell migration. However, whether Cx43 can be regulated by TGF-β and is involved in TGF-β-stimulated cell migration in human ovarian cancer cells remain unknown. In this study, we demonstrate that TGF-β up-regulates Cx43 in two human ovarian cancer cell lines, SKOV3 and OVCAR4. The stimulatory effect of TGF-β on Cx43 expression is blocked by inhibition of TGF-β receptor. Treatment with TGF-β activates Smad2 and Smad3 signaling pathways in both ovarian cancer cell lines. In addition, siRNA-mediated knockdown of Smad2 or Smd3 abolishes TGF-β-induced up-regulation of Cx43 expression. Moreover, knockdown of Cx43 attenuates TGF-β-stimulated cell migration. This study demonstrates an important role for Cx43 in mediating the effects of TGF-β on human ovarian cancer cell migration.

Internal ribosomal entry site (IRES) activity generates endogenous carboxyl-terminal domains of Cx43 and is responsive to hypoxic conditions

Connexin43 (Cx43) is the most abundant gap junction protein in higher vertebrate organisms and has been shown to be involved in junctional and non-junctional functions. In addition to the expression of full-length Cx43, endogenously produced carboxyl-terminal segments of Cx43 have been described and have been suggested to be involved in manifold biological functions, such as hypoxic preconditioning and neuronal migration. Molecular aspects, however, behind the separate generation of carboxyl-terminal segments of Cx43 have remained elusive. Here we report on a mechanism that may play a key role in the separate production of these domains. First, stringent evidence derived from siRNA treatment and specific knockouts revealed significant loss of the low molecular weight fragments of Cx43. By applying a dicistronic vector strategy on transfected cell lines, we were able to identify putative IRES activity (nucleotides 442–637) in the coding region of Cx43, which resides upstream from the nucleotide sequence encoding the carboxyl terminus (nucleotides 637–1149). Functional responsiveness of the endogenous expression of Cx43 fragments to hypoxic/ischemic treatment was evaluated in in vitro and in vivo models, which led to a significant increase of the fastest migrating form (20 kDa) under conditions of metabolic deprivation. By nano-MS spectrometry, we achieved stringent evidence of the identity of the 20-kDa segment as part of the carboxyl-terminal domain of full-length Cx43. Our data prove the existence of endogenously expressed carboxyl-terminal domains, which may serve as valuable tools for further translational application in ischemic disorders.

Long-range gap junctional signaling controls oncogene-mediated tumorigenesis in Xenopus laevis embryos

In addition to the immediate microenvironment, long-range signaling may be an important component of cancer. Molecular-genetic analyses have implicated gap junctions-key mediators of cell-cell communication-in carcinogenesis. We recently showed that the resting voltage potential of distant cell groups is a key determinant of metastatic transformation and tumor induction. Here, we show in the Xenopus laevis model that gap junctional communication (GJC) is a modulator of the long-range bioelectric signaling that regulates tumor formation. Genetic disruption of GJC taking place within tumors, within remote host tissues, or between the host and tumors significantly lowers the incidence of tumors induced by KRAS mutations. The most pronounced suppression of tumor incidence was observed upon GJC disruption taking place farther away from oncogene-expressing cells, revealing a role for GJC in distant cells in the control of tumor growth. In contrast, enhanced GJC communication through the overexpression of wild-type connexin Cx26 increased tumor incidence. Our data confirm a role for GJC in tumorigenesis, and reveal that this effect is non-local. Based on these results and on published data on movement of ions through GJs, we present a quantitative model linking the GJC coupling and bioelectrical state of cells to the ability of oncogenes to initiate tumorigenesis. When integrated with data on endogenous bioelectric signaling during left-right patterning, the model predicts differential tumor incidence outcomes depending on the spatial configurations of gap junction paths relative to tumor location and major anatomical body axes. Testing these predictions, we found that the strongest influence of GJ modulation on tumor suppression by hyperpolarization occurred along the embryonic left-right axis. Together, these data reveal new, long-range aspects of cancer control by the host's physiological parameters.

Regulation of FAK Activity by Tetraspan Proteins: Potential Clinical Implications in Cancer

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that regulates multiple cell signaling pathways in both physiological and pathological conditions. Overexpression and activation of FAK is associated with many advanced stage cancers through promoting cancer cell tumorigenicity and progression as well as by regulating the tumor microenvironment. FAK has multiple binding partners through which FAK exerts its functions including RhoGEF, Src family, talin, cortactin, and paxilin. Over the last few years, it has been proposed that a novel group of four transmembrane proteins can interact with FAK and regulate its activity. These include select tetraspanins such as CD151 and CD9 as well as the GAS3 family members epithelial membrane protein-2 (EMP2) and peripheral myelin protein-22 (PMP22). In this review, we discuss the current knowledge of the interaction between FAK and tetraspan proteins in physiological and pathological conditions, with an emphasis on the potential of tetraspan family members as therapeutic targets in cancer.

Impact of obesity on 7,12-dimethylbenz[a]anthracene-induced altered ovarian connexin gap junction proteins in female mice

The ovarian gap junction proteins alpha 4 (GJA4 or connexin 37; CX37), alpha 1 (GJA1 or connexin 43; CX43) and gamma 1 (GJC1 or connexin 45; CX45) are involved in cell communication and folliculogenesis. 7,12-dimethylbenz[a]anthracene (DMBA) alters Cx37 and Cx43 expression in cultured neonatal rat ovaries. Additionally, obesity has an additive effect on DMBA-induced ovarian cell death and follicle depletion, thus, we investigated in vivo impacts of obesity and DMBA on CX protein levels. Ovaries were collected from lean and obese mice aged 6, 12, 18, or 24 wks. A subset of 18 wk old mice (lean and obese) were dosed with sesame oil or DMBA (1mg/kg; ip) for 14days and ovaries collected 3days thereafter. Cx43 and Cx45 mRNA and protein levels decreased (P<0.05) after 18 wks while Cx37 mRNA and protein levels decreased (P<0.05) after 24 wks in obese ovaries. Cx37 mRNA and antral follicle protein staining intensity were reduced (P<0.05) by obesity while total CX37 protein was reduced (P<0.05) in DMBA exposed obese ovaries. Cx43 mRNA and total protein levels were decreased (P<0.05) by DMBA in both lean and obese ovaries while basal protein staining intensity was reduced (P<0.05) in obese controls. Cx45 mRNA, total protein and protein staining intensity level were decreased (P<0.05) by obesity. These data support that obesity temporally alters gap junction protein expression and that DMBA-induced ovotoxicity may involve reduced gap junction protein function.

Vitamin D3 regulates the formation and degradation of gap junctions in androgen-responsive human prostate cancer cells

1α-25(OH)2 vitamin D3 (1-25D), an active hormonal form of Vitamin D3, is a well-known chemopreventive and pro-differentiating agent. It has been shown to inhibit the growth of several prostate cancer cell lines. Gap junctions, formed of proteins called connexins (Cx), are ensembles of cell-cell channels, which permit the exchange of small growth regulatory molecules between adjoining cells. Cell-cell communication mediated by gap junctional channels is an important homeostatic control mechanism for regulating cell growth and differentiation. We have investigated the effect of 1-25D on the formation and degradation of gap junctions in an androgen-responsive prostate cancer cell line, LNCaP, which expresses retrovirally-introduced Cx32. Connexin32 is expressed by the luminal and well-differentiated cells of normal prostate and prostate tumors. Our results document that 1-25D enhances the expression of Cx32 and its subsequent assembly into gap junctions. Our results further show that 1-25D prevents androgen-regulated degradation of Cx32, post-translationally, independent of androgen receptor (AR)-mediated signaling. Finally, our findings document that formation of gap junctions sensitizes Cx32-expressing LNCaP cells to the growth inhibitory effects of 1-25D and alters their morphology. These findings suggest that the growth-inhibitory effects of 1-25D in LNCaP cells may be related to its ability to modulate the assembly of Cx32 into gap junctions.

Targeting gap junction intercellular communication as a potential therapy for HCV-related carcinogenesis

ABSTRACT: 

Worldwide, at least 170 million people are infected with hepatitis C virus (HCV), which is associated with hepatocellular carcinoma (HCC). With the recent success of Sofosbuvir (and other agents) antiviral therapy may be used as a future early-stage HCC treatment; however, in the short term, a cost-effective solution is needed to treat patients with viral-associated HCC. Here, we emphasize the potential of targeting gap junction intercellular communication (GJIC) as a therapeutic approach for HCC as HCV perturbs GJIC, which is linked to cellular transformation. We review the ROCK inhibitor Y-27632 and structurally related compounds that may inhibit the carcinogenic properties of HCV.

Connexins in colorectal cancer pathogenesis

The connexins constitute a family of integral membrane proteins that form channels between adjacent cells. These channels are assembled in plasma membrane domains known as gap junctions and enable cells to directly exchange ions and small molecules. Intercellular communication via gap junctions plays important roles in regulating cell growth and differentiation and in maintaining tissue homeostasis. This type of cell communication is often impaired during cancer development, and several members of the connexin protein family have been shown to act as tumor suppressors. Emerging evidence suggests that the connexin protein family has important roles in colorectal cancer development. In the normal colonic epithelial tissue, three connexin isoforms, connexin 26 (Cx26), Cx32 and Cx43, have been shown to be expressed at the protein level. Colorectal cancer development is associated with loss of connexin expression or relocalization of connexins from the plasma membrane to intracellular compartments. Downregulation of connexins in colorectal carcinomas at the transcriptional level involves cancer-specific promoter hypermethylation. Recent studies suggest that Cx43 may constrain growth of colon cancer cells by interfering with the Wnt/β-catenin pathway. There is also increasing evidence that the connexins may have potential as prognostic markers in colorectal cancer. This review discusses the role of connexins in colorectal cancer pathogenesis, as well as their potential as prognostic markers and targets in the prevention and treatment of the disease.

Baicalein increases the cytotoxicity of cisplatin by enhancing gap junction intercellular communication

Drug resistance limits the clinical application of cisplatin, a widely used chemotherapeutic agent. Gap junction (GJ) is a channel that enhances cytotoxicity of certain chemotherapeutic agents. Baicalein is well known for its antitumor activity. This study investigated the effect of baicalein on cisplatin cytotoxicity and the relationship between this effect and the modulation of the GJ function in connexin 26 (Cx26)‑transfected HeLa cells. The sulforhodamine B (SRB) assay was used to examine the effect of baicalein on cell viability. A 'parachute' assay was used to investigate the effect of baicalein on GJ function. The effects of baicalein on cisplatin cytotoxicity and GJ function were assayed by standard colony‑forming assays. The expression of Cx26 was monitored by western blotting. It was observed that exposure of Cx26‑transfected cells to cisplatin reduced the number of colonies formed in low‑density cultures (no GJ formation) and in high‑density cultures (GJ formation), but the toxic effect was greater when cells were seeded at a high density. In the absence of connexin expression or with blockage of connexin channels however, cell density had no effect on cisplatin toxicity. Baicalein significantly enhanced cisplatin cytotoxicity, but this effect required the presence of functional GJs between the cells. In conclusion, the dependence of cisplatin toxicity on cell density is mediated by GJs. Baicalein increases cisplatin cytotoxicity through enhancing GJ intercellular communication.

Benzo[a]pyrene-7,8-diol-9,10-epoxide inhibits gap junction intercellular communication via phosphorylation of tumor progression locus 2 in WB-F344 rat liver epithelial cells

Benzo[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE), a major metabolite of benzo[a]pyrene, has been reported to function as a human carcinogen. However, the molecular mechanism of how B[a]PDE regulates signaling pathways during tumor promotion remains unclear. In this study, we investigated the effects of B[a]PDE on the regulation of gap junction intercellular communication (GJIC), one of the major carcinogenic processes, and its main regulatory signaling pathways using WB-F344 rat liver epithelial (WB-F344 RLE) cells. Treatment of benzo[a]pyrene or B[a]PDE resulted in GJIC inhibition, and B[a]PDE was more active at lower concentrations than benzo[a]pyrene in the suppression of GJIC. This suggests that B[a]PDE is a stronger GJIC inhibitor. B[a]PDE at 1 µM reversibly inhibited GJIC in WB-F344 RLE cells, which was attributable to hyperphosphorylation of connexin43 (Cx43) via phosphorylation of mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK) and extracellular signal-regulated kinase (ERK). We found that B[a]PDE induced phosphorylation of tumor progression locus 2 (Tpl2), a direct upstream regulator of MEK. Tpl2 inhibitor recovered B[a]PDE-induced GJIC inhibition and attenuated B[a]PDE-induced MEK/ERK phosphorylation in WB-F344 RLE cells. Collectively, our results suggest that B[a]PDE suppresses GJIC by activating Tpl2 and subsequently the MEK/ERK pathway and Cx43 phosphorylation in WB-F344 RLE cells. These results outline the potential importance of Tpl2 as a novel therapeutic target for B[a]PDE-induced GJIC inhibition during cancer promotion.

Connexin 43 a check-point component of cell proliferation implicated in a wide range of human testis diseases

Gap junction channels link cytoplasms of adjacent cells. Connexins, their constitutive proteins, are essential in cell homeostasis and are implicated in numerous physiological processes. Spermatogenesis is a sophisticated model of germ cell proliferation, differentiation, survival, and apoptosis, in which a connexin isotype, connexin 43, plays a crucial role as evidenced by genomic approaches based on gene deletion. The balance between cell proliferation/differentiation/apoptosis is a prerequisite for maintaining levels of spermatozoa essential for fertility and for limiting anarchic cell proliferation, a major risk of testis tumor. The present review highlights the emerging role of connexins in testis pathogenesis, focusing specifically on two intimately interconnected human testicular diseases (azoospermia with impaired spermatogenesis and testicular germ cell tumors), whose incidence increased during the last decades. This work proposes connexin 43 as a potential cancer diagnostic and prognostic marker, as well as a promising therapeutic target for testicular diseases.

Gap junctions and non-neoplastic liver disease

Because of their critical role as goalkeepers of hepatic homeostasis, gap junctions are frequent targets in liver disease. This concept has been demonstrated on many occasions in the light of hepatocarcinogenesis. Relatively little focus has been put on the fate of gap junctions in other liver pathologies, including hepatitis, liver fibrosis and cirrhosis, cholestasis and hepatic ischemia and reperfusion injury. The present paper provides an in-depth description of the multiple changes in expression, localization and function of connexins, the molecular constituents of gap junctions. The use of connexins as biomarkers and therapeutic targets in liver disease is also illustrated.

Extracellular loop cysteine mutant of cx37 fails to suppress proliferation of rat insulinoma cells

Although a functional pore domain is required for connexin 37 (Cx37)-mediated suppression of rat insulinoma (Rin) cell proliferation, it is unknown whether functional hemichannels would be sufficient or if Cx37 gap junction channels are required for growth suppression. To test this possibility, we targeted extracellular loop cysteines for mutation, expecting that the mutated protein would retain hemichannel, but not gap junction channel, functionality. Cysteines at positions 61 and 65 in the first extracellular loop of Cx37 were mutated to alanine and the mutant protein (Cx37-C61,65A) expressed in Rin cells. Although the resulting iRin37-C61,65A cells expressed the mutant protein comparably to Cx37 wild-type (Cx37-WT)--expressing Rin cells (iRin37), Cx37-C61,65A expression did not suppress the proliferation of Rin cells. As expected, iRin37-C61,65A cells did not form functional gap junction channels. However, functional hemichannels also could not be detected in iRin37-C61,65A cells by either dye uptake or electrophysiological approaches. Thus, failure of Cx37-C61,65A to suppress the proliferation of Rin cells is consistent with previous data demonstrating the importance of channel functionality to Cx37's growth-suppressive function. Moreover, failure of the Cx37-C61,65A hemichannel to function, even in low external calcium, emphasizes the importance of extracellular loop cysteines not only in hemichannel docking but also in determining the ability of the hemichannel to adopt a closed configuration that can open in response to triggers, such as low external calcium, effective at opening Cx37-WT hemichannels.

Retinoids regulate the formation and degradation of gap junctions in androgen-responsive human prostate cancer cells

The retinoids, the natural or synthetic derivatives of Vitamin A (retinol), are essential for the normal development of prostate and have been shown to modulate prostate cancer progression in vivo as well as to modulate growth of several prostate cancer cell lines. 9-cis-retinoic acid and all-trans-retinoic acid are the two most important metabolites of retinol. Gap junctions, formed of proteins called connexins, are ensembles of intercellular channels that permit the exchange of small growth regulatory molecules between adjoining cells. Gap junctional communication is instrumental in the control of cell growth. We examined the effect of 9-cis-retinoic acid and all-trans retinoic acid on the formation and degradation of gap junctions as well as on junctional communication in an androgen-responsive prostate cancer cell line, LNCaP, which expressed retrovirally introduced connexin32, a connexin expressed by the luminal cells and well-differentiated cells of prostate tumors. Our results showed that 9-cis-retinoic acid and all-trans retinoic acid enhanced the assembly of connexin32 into gap junctions. Our results further showed that 9-cis-retinoic acid and all-trans-retinoic acid prevented androgen-regulated degradation of gap junctions, post-translationally, independent of androgen receptor mediated signaling. Finally, our findings showed that formation of gap junctions sensitized connexin32-expressing LNCaP cells to the growth modifying effects of 9-cis-retinoic acid, all-trans-retinoic acid and androgens. Thus, the effects of retinoids and androgens on growth and the formation and degradation of gap junctions and their function might be related to their ability to modulate prostate growth and cancer.