Connexin 26 gene therapy of human bladder cancer: induction of growth suppression, apoptosis, and synergy with Cisplatin

Integrative analyses identified gap junction beta-2 as a prognostic biomarker and therapeutic target for breast cancer

Background

Increasing evidence has shown that connexins are involved in the regulation of tumor development, immune escape, and drug resistance. This study investigated the gene expression patterns, prognostic values, and potential mechanisms of connexins in breast cancer.

Methods

We conducted a comprehensive analysis of connexins using public gene and protein expression databases and clinical samples from our institution. Connexin mRNA expressions in breast cancer and matched normal tissues were compared, and multiomics studies were performed.

Results

Gap junction beta-2 mRNA was overexpressed in breast cancers of different pathological types and molecular subtypes, and its high expression was associated with poor prognosis. The tumor membrane of the gap junction beta-2 mutated group was positive, and the corresponding protein was expressed. Somatic mutation and copy number variation of gap junction beta-2 are rare in breast cancer. The gap junction beta-2 transcription level in the p110α subunit of the phosphoinositide 3-kinase mutant subgroup was higher than that in the wild-type subgroup. Gap junction beta-2 was associated with the phosphoinositide 3-kinase-Akt signaling pathway, extracellular matrix-receptor interaction, focal adhesion, and proteoglycans in cancer. Furthermore, gap junction beta-2 overexpression may be associated with phosphoinositide 3-kinase and histone deacetylase inhibitor resistance, and its expression level correlated with infiltrating CD8+ T cells, macrophages, neutrophils, and dendritic cells.

Conclusions

Gap junction beta-2 may be a promising therapeutic target for targeted therapy and immunotherapy and may be used to predict breast cancer prognosis.

The roles of connexins and gap junctions in the progression of cancer

Gap junctions (GJs), which are composed of connexins (Cxs), provide channels for direct information exchange between cells. Cx expression has a strong spatial specificity; however, its influence on cell behavior and information exchange between cells cannot be ignored. A variety of factors in organisms can modulate Cxs and subsequently trigger a series of responses that have important effects on cellular behavior. The expression and function of Cxs and the number and function of GJs are in dynamic change. Cxs have been characterized as tumor suppressors in the past, but recent studies have highlighted the critical roles of Cxs and GJs in cancer pathogenesis. The complex mechanism underlying Cx and GJ involvement in cancer development is a major obstacle to the evolution of therapy targeting Cxs. In this paper, we review the post-translational modifications of Cxs, the interactions of Cxs with several chaperone proteins, and the effects of Cxs and GJs on cancer. Video Abstract.

Combinational treatment of gap junction enhancers and paclitaxel attenuates mammary tumor growth

A class of substituted quinolines (PQs) acts as a gap junction enhancer with the ability to increase the gap junctional intercellular communication in breast cancer cells. This study examined the effect of a combinational treatment of PQs and the antineoplastic drug paclitaxel in a xenograft animal model. Mice were implanted with estradiol-17ß (1.7 mg/pellet) before the injection of 1 × 10 T47D breast cancer cells subcutaneously into the inguinal region of mammary fat pad. Animals were treated intraperitoneally with DMSO (control), paclitaxel (10 mg/kg), PQ (25 mg/kg), or a combinational treatment of paclitaxel and PQ. There was no significant difference between the paclitaxel-treated animals and the control group after seven injections of treatment for 2 weeks. All mice treated with PQ had a significant decrease in mammary tumor growth. The combinational treatment of paclitaxel and PQ1 or PQ7 showed a reduction in tumor growth by 2.3- and 2.2-fold, respectively, compared to paclitaxel alone after seven treatments every 2 days. Molecular analysis indicated a significant increase of gap junction proteins and caspase signaling in PQ and paclitaxel-treated tissues compared to control. Furthermore, there is evidence of an upregulation of Cyclin D1 expression in paclitaxel-treated tumors compared to control, suggesting that the neoplastic cells were highly proliferative and nonresponsive to the paclitaxel alone. We have showed for the first time an increase in the efficacy of antineoplastic drugs via the enhancement of gap junctions with PQs, a specific class of gap junction enhancers.

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.

Connexins and Gap Junctions in Cancer of the Urinary Tract

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

Synergistic effect of bladder cancer-specific oncolytic adenovirus in combination with chemotherapy

Gene therapy with adenoviral early region gene (E1A) may enhance the susceptibility of neoplastic cells to chemotherapy-induced cell death. Our previous study developed a urothelium-specific oncolytic serotype 5 adenovirus (Ad5) with the uroplakin II (UPII) promoter controlling E1A expression. The present study investigated whether this urothelium-specific recombinant adenovirus (Ad5-UPII-E1A) enhanced mitomycin (MMC) and hydroxycamptothecin (HCPT) sensitization and drug-induced apoptosis in bladder cancer cells. The results of the MTT assay revealed that combination therapy, using Ad5-UPII-E1A and MMC or HCPT, synergistically inhibited the viability of bladder cancer cells in a dose- and time-dependent manner when compared with either agent alone. When cells were treated with Ad5-UPII-E1A alone they arrested in the G1 phase, but cell cycle analysis by flow cytometry revealed S phase arrest when treated with combined therapy. Treatment with MMC or HCPT enhanced Ad5-UPII-E1A-induced apoptosis in 5,637 cells, observed by transmission electron microscopy. Western blot analysis revealed that MMC and HCPT enhanced the E1A expression of the Ad5-UPII-E1A vectorin a dose-dependent manner. The present study demonstrated that Ad5-UPII-E1A combined with MMC or HCPT resulted in synergistic cytotoxicity in a process which involved the promotion of apoptosis in bladder cancer cell lines. MMC and HCPT also promoted the oncolytic effect of Ad5-UPII-E1A. Thus, treatment using Ad5-UPII-E1A combined with MMC or HCPT may be an attractive strategy for the sensitization of bladder cancer to chemotherapy.

Impaired gap junctions in human hepatocellular carcinoma limit intrinsic oxaliplatin chemosensitivity: A key role of connexin 26

Hepatocellular carcinoma (HCC) is generally believed to have low sensitivity to chemotherapeutic agents including oxaliplatin (OXA). Studies have demonstrated that gap junctions (GJs) composed of connexin (Cx) proteins have the potential to modulate drug chemosensitivity in multiple tumor cells. In the present study, we investigated the characteristics of Cx and GJs in HCC at both histologic and cytologic levels, and the effects of GJ and its effective components on OXA cytotoxicity in HCC cells in vitro. Immunohistochemistry was performed in 76 HCCs and 20 normal liver tissues to detect and locate the expression of Cx26, Cx32 and Cx43. At cytologic levels, the expression and localization of Cxs were evaluated by RT-PCR, western blot and immunofluorescence assay, respectively. The GJ function between adjacent cells was detected using dye transfer assay. The role of GJs in the modulation of OXA toxicity in HCC cells was explored using pharmacologic and molecular biologic methods. We found that Cx expression in HCC tissues was significantly lower than in normal liver tissues, and the 'internalization' from cell membrane to cytoplasm was remarkable. In vitro experiments revealed the presence of functional GJs in the SMMC-7721 HCC cells due to a small amount of Cx protein along the plasma membrane at cell-cell contacts. Regulation of this part of GJs positively influenced OXA cytotoxicity. Using RNA interference, only specific inhibition of Cx26 but not Cx32 or Cx43 reduced OXA cytotoxicity. Conversely, Cx26 overexpression by transfection of Cx26 plasmid DNA enhanced OXA cytotoxicity. This study demonstrated that during hepatocarcinogenesis, the reduced expression and internalization of Cx proteins impaired the GJ function, which further attenuated OXA cytotoxicity. Impaired GJ function may contribute to low intrinsic chemosensitivity of HCC cells to OXA, mediated by Cx26.

Connexins: junctional and non-junctional modulators of proliferation

Mounting evidence indicates that dysregulation of gap junctions and their structural subunits-connexins-often occurs in, and sometimes causes, a variety of proliferative disorders, including cancer. Connexin-mediated regulation of cell proliferation is complex and may involve modulation of gap junction intercellular communication (GJIC), hemichannel signalling, or gap junction-independent paths. However, the exact mechanisms linking connexins to proliferation remain poorly defined and a number of contradictory studies report both pro- and anti-proliferative effects, effects that often depend on the cell or tissue type or the microenvironment. The present review covers junctional and non-junctional regulation of proliferation by connexins, with a particular emphasis on their association with cancer.

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.

Components of Panax notoginseng saponins enhance the cytotoxicity of cisplatin via their effects on gap junctions

Previously, Panax notoginseng saponin (PNS)-induced enhancement of gap junction (GJ) formation or function was observed to be responsible for the increased cytotoxic action of cisplatin. PNS has three constituents, ginsenoside Rg1 and Rb1, and notoginsenoside R1. The active compounds in PNS responsible for enhancing the cytotoxicity of cisplatin remain unknown. Thus, the effects of the main components of PNS on the cytotoxicity of cisplatin were investigated, as well as the correlation with the modulation of GJ function in transfected HeLa cells. The cytotoxicity of cisplatin (0.25-1 µg/ml) was increased in the presence of GJs. By contrast, the cytotoxicity of cisplatin was decreased when GJs were inhibited by a GJ blocker or by the inhibition of connexin expression. Ginsenoside Rg1 (100 µM) and notoginsenoside R1 (100 µM) were observed to significantly enhance cisplatin cytotoxicity in cells with functional GJs. Ginsenoside Rb1 had no effect on the cytotoxicity of cisplatin in the presence or absence of functional GJs. Cell exposure to ginsenoside Rg1 and notoginsenoside R1 for 4 h led to significant enhancement of a dye-coupled GJ in a dose-dependent manner; however, no effect was observed in cells exposed to ginsenoside Rb1. The present results indicate that ginsenoside Rg1 and notoginsenoside R1 are the active compounds responsible for enhancing the cytotoxic action of cisplatin induced by PNS in the presence of functional GJs.

Bioavailability and efficacy of a gap junction enhancer (PQ7) in a mouse mammary tumor model

The loss of gap junctional intercellular communication is characteristic of neoplastic cells, suggesting that the restoration with a gap junction enhancer may be a new therapeutic treatment option with less detrimental effects than traditional antineoplastic drugs. A gap junction enhancer, 6-methoxy-8-[(2-furanylmethyl) amino]-4-methyl-5-(3-trifluoromethylphenyloxy) quinoline (PQ7), on the normal tissue was evaluated in healthy C57BL/6J mice in a systemic drug distribution study. Immunoblot analysis of the vital organs indicates a reduction in Cx43 expression in PQ7-treated animals with no observable change in morphology. Next the transgenic strain FVB/N-Tg(MMTV-PyVT) 634Mul/J (also known as PyVT) was used as a spontaneous mammary tumor mouse model to determine the biological and histological effects of PQ7 on tumorigenesis and metastasis at three stages of development: Pre tumor, Early tumor, and Late tumor formation. PQ7 was assessed to have a low toxicity through intraperitoneal administration, with the majority of the compound being detected in the heart, liver, and lungs six hours post injection. The treatment of tumor bearing animals with PQ7 had a 98% reduction in tumor growth, while also decreasing the total tumor burden compared to control mice during the Pre stage of development. PQ7 treatment increased Cx43 expression in the neoplastic tissue during Pre-tumor formation; however, this effect was not observed in Late stage tumor formation. This study shows that the gap junction enhancer, PQ7, has low toxicity to normal tissue in healthy C57BL/6J mice, while having clinical efficacy in the treatment of spontaneous mammary tumors of PyVT mice. Additionally, gap junctional intercellular communication and neoplastic cellular growth are shown to be inversely related, while treatment with PQ7 inhibits tumor growth through targeting gap junction expression.

Nanoparticles for urothelium penetration and delivery of the histone deacetylase inhibitor belinostat for treatment of bladder cancer

Nearly 40% of patients with non-invasive bladder cancer will progress to invasive disease despite locally-directed therapy. Overcoming the bladder permeability barrier (BPB) is a challenge for intravesical drug delivery. Using the fluorophore coumarin (C6), we synthesized C6-loaded poly(lactide-co-glycolide) (PLGA) nanoparticles (NPs), which were surface modified with a novel cell penetrating polymer, poly(guanidinium oxanorbornene) (PGON). Addition of PGON to the NP surface improved tissue penetration by 10-fold in intravesically-treated mouse bladder and ex vivo human ureter. In addition, NP-C6-PGON significantly enhanced intracellular uptake of NPs compared to NPs without PGON. To examine biological activity, we synthesized NPs that were loaded with the histone deacetylase (HDAC) inhibitor belinostat (NP-Bel-PGON). NP-Bel-PGON exhibited a significantly lower IC50 in cultured bladder cancer cells, and sustained hyperacetylation, when compared to unencapsulated belinostat. Xenograft tumors treated with NP-Bel-PGON showed a 70% reduction in volume, and a 2.5-fold higher intratumoral acetyl-H4, when compared to tumors treated with unloaded NP-PGON.

FROM THE CLINICAL EDITOR

These authors demonstrate that PLGA nanoparticles with PGON surface functionalization result in greatly enhanced cell penetrating capabilities, and present convincing data from a mouse model of bladder cancer for increased chemotherapy efficacy.

Connexin 26 is down-regulated by KDM5B in the progression of bladder cancer

Connexin 26 (Cx26) expression is down-regulated and KDM5B (H3K4 demethylase) is up-regulated in the progression of bladder cancer, suggesting that Cx26 expression may be down-regulated by KDM5B in bladder cancer. To test the hypothesis, the HT1376 and T24 human bladder carcinoma cells were transfected with the plasmids pcDNA3.1-KDM5B, and caused the down-regulation of Cx26 expression. In contrast, the HT1376 and T24 cells transfected with the plasmids pTZU6+1-shRNA-KDM5B1 and pTZU6+1-shRNA-KDM5B2 caused the up-regulation of Cx26 expression. Immunohistochemistry and Spearman’s rank correlation analysis showed that the immunohistochemical expression of KDM5B and Cx26 was inversely related in bladder carcinoma tissues but no relationship in benign tissues. Taken together, these results indicate that KDM5B represses Cx26 expression in the bladder cancer development. Thus, a negative value to Cx26 immunohistochemical expression and a positive value to KDM5B immunohistochemical expression could be an ancillary diagnosis of primary bladder malignancy.

Lipopolysaccharide effects on the proliferation of NRK52E cells via alternations in gap-junction function

BACKGROUND

Gap junctions regulate proper kidney function by facilitating intercellular communication, vascular conduction, and tubular purinergic signaling. However, no clear relationship has been described between gap-junction function and acute kidney injury induced by the endotoxin lipopolysaccharide (LPS).

METHODS

Normal rat kidney epithelial cells (NRK52E cells) were seeded at high and low densities to promote or impede gap-junction formation, respectively, and establish distinctive levels of intercellular communication in culture. Cells were then challenged with LPS at various concentrations (10-1,000 ng/mL). LPS-induced formation and function of gap junctions were assessed by measuring changes in cell proliferation and colony-forming rates, fluorescent dye transmission to adjacent cells, expression levels of connexin43, and repositioning of confluent cells in response to the gap junction inhibitor oleamide or agonist retinoic acid.

RESULTS

The cell proliferation rate and colony-forming rate of high- and low-density NRK52E cells were decreased upon LPS challenge, in a dose-dependent manner. The colony-forming rate of confluent high-density cells was significantly lower than that of low-density cells. Oleamide treatment raised the LPS-induced colony-forming rate of high-density cells, whereas retinoic acid decreased the rate. Neither oleamide nor retinoic acid significantly affected the LPS-induced colony-forming rate of low-density cells. Fluorescence transmission of high-density cells was reduced by LPS challenge, in a dose-dependent manner, but inclusion of retinoic acid increased the LPS-induced transmission of fluorescence. LPS challenge of either high- or low-density NRK52E cells resulted in down-regulated connexin43 expression.

CONCLUSION

Gap-junction function plays an important role in concentration-dependent cytotoxic effect of LPS on normal rat kidney cells in vitro.

Gap junction enhancer increases efficacy of cisplatin to attenuate mammary tumor growth

Cisplatin treatment has an overall 19% response rate in animal models with malignant tumors. Increasing gap junction activity in tumor cells provides the targets to enhance antineoplastic therapies. Previously, a new class of substituted quinolines (PQs) acts as gap junction enhancer, ability to increase the gap junctional intercellular communication, in breast cancer cells. We examined the effect of combinational treatment of PQs and antineoplastic drugs in an animal model, showing an increase in efficacy of antineoplastic drugs via the enhancement of gap junctions. Mice were implanted with estradiol-17ß (1.7 mg/pellet) before the injection of 1×10⁷ T47D breast cancer cells subcutaneously into the inguinal region of mammary fat pad. Animals were treated intraperitoneally with DMSO (control), cisplatin (3.5 mg/kg), PQ (25 mg/kg), or a combining treatment of cisplatin and PQ. Cisplatin alone decreased mammary tumor growth by 85% while combinational treatment of cisplatin and PQ1 or PQ7 showed an additional reduction of 77% and 22% of tumor growth after 7 treatments at every 2 days, respectively. Histological results showed a significant increase of gap junction proteins, Cx43 and Cx26, in PQ-treated tissues compared to control or cisplatin. Furthermore, evidence of highly stained caspase 3 in tumors of combinational treatment (PQ and cisplatin) was seen compared to cisplatin alone. We have showed for the first time an increase in the efficacy of antineoplastic drugs through a combinational treatment with PQs, a specific class of gap junction enhancers.

Paris polyphylla Smith extract induces apoptosis and activates cancer suppressor gene connexin26 expression

BACKGROUND

The inhibition of tumor cell growth without toxicity to normal cells is an important target in cancer therapy. One possible way to increase the efficacy of anticancer drugs and to decrease toxicity or side effects is to develop traditional natural products, especially from medicinal plants. Paris polyphylla Smith has shown anti-tumour effects by inhibition of tumor promotion and inducement of tumor cell apoptosis, but mechanisms are still not well understood. The present study was to explore the effect of Paris polyphylla Smith extract (PPSE) on connexin26 and growth control in human esophageal cancer ECA109 cells.

METHODS

The effects of PPSE on Connexin26 were examined by RT-PCR, western blot and immunofluorescence; cell growth and proliferation were examined by the 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl-tetrazolium bromide (MTT) assay.

RESULTS

PPSE inhibited the growth and proliferation on esophageal cancer ECA109 cells, while increasing the expression of connexin26 mRNA and protein; conversely, PPSE decreased Bcl-2 and increased Bad.

CONCLUSION

This study firstly shows that PPSE can increase connexin26 expression at mRNA and protein level, exerting anti-tumour effects on esophageal cacner ECA109 cells via inhibiting cell proliferation and inducing cell apoptosis.

Non-channel functions of connexins in cell growth and cell death

Cellular communication mediated by gap junction channels and hemichannels, both composed of connexin proteins, constitutes two acknowledged regulatory platforms in the accomplishment of tissue homeostasis. In recent years, an abundance of reports has been published indicating functions for connexin proteins in the control of the cellular life cycle that occur independently of their channel activities. This has yet been most exemplified in the context of cell growth and cell death, and is therefore as such addressed in the current paper. Specific attention is hereby paid to the molecular mechanisms that underpin the cellular non-channel roles of connexin proteins, namely the alteration of the expression of tissue homeostasis determinants and the physical interaction with cell growth and cell death regulators. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Gap junctions and connexins as therapeutic targets in cancer

IMPORTANCE OF THE FIELD

Connexins (Cxs) and gap junctional intercellular communications (GJICs) play roles in cancer development, growth and metastasis. Experimental studies suggest that targeting Cxs may be a novel technique, either to inhibit tumor cell growth directly or to sensitize to various therapies.

AREAS COVERED IN THIS REVIEW

A brief introduction to the role of Cxs in cancer. The focus is mainly on data available in the literature regarding therapeutic aspects.

WHAT THE READER WILL GAIN

This article reviews the various strategies that take advantage of gap junctions and connexins to eliminate cancer cells, including use of the bystander effect (BE) in gene therapy, the effect of connexins on chemosensitization, the role of apoptotic processes and interactions with the microenvironment. Attempts to restore connexin expression at the transcriptional and post-transcriptional levels are described, as well as promising strategies recently explored. The potential and limitations of the approaches are discussed.

TAKE HOME MESSAGE

Connexins have multiple facets, singly, in hemichannel complexes, in gap junctions or interacting with different proteins. The regulation of their expression is not fully resolved and selective manipulation of Cxs expression is therefore a challenge. Although the therapeutic potential of connexins is undeniable, more effort is needed to study the regulation and functions of these proteins.

Connexins: sensors and regulators of cell cycling

It is nowadays well established that gap junctions are critical gatekeepers of cell proliferation, by controlling the intercellular exchange of essential growth regulators. In recent years, however, it has become clear that the picture is not as simple as originally anticipated, as structural precursors of gap junctions can affect cell cycling by performing actions not related to gap junctional intercellular communication. Indeed, connexin hemichannels also foresee a pathway for cell growth communication, albeit between the intracellular compartment and the extracellular environment, while connexin proteins as such can directly or indirectly influence the production of cell cycle regulators independently of their channel activities. Furthermore, a novel set of connexin-like proteins, the pannexins, have lately joined in as regulators of the cell proliferation process, which they can affect as either single units or as channel entities. In the current paper, these multifaceted aspects of connexin-related signalling in cell cycling are reviewed.

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

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

Tramadol and flurbiprofen depress the cytotoxicity of cisplatin via their effects on gap junctions

PURPOSE

Cancer patients are often concurrently treated with analgesics and antineoplastic drugs, yet the influence of analgesic agents on therapeutic activity of antineoplastic drugs is largely unexplored. This study investigates the effects of three commonly used analgesics, which produce analgesia by different mechanisms, on cytotoxicity induced by cisplatin, a widely used antitumor agent, and the relation between those effects and modulation of gap junction function by the analgesics.

EXPERIMENTAL DESIGN

The role of gap junctions in the modulation of cisplatin toxicity is explored by manipulation of connexin expression, and gap junction presence and function, using clinically relevant concentrations of the analgesics and cisplatin.

RESULTS

Short-term exposure of transformed cells to cisplatin reduced the clonogenic survival in low-density cultures (without gap junction formation) and in high density (with gap junction formation), but the toxic effect was greater at high density. In the absence of connexin expression or with block of connexin channels, cell density had no effect on cisplatin toxicity. Tramadol and flurbiprofen, but not morphine, significantly reduced cisplatin cytotoxicity, but this effect required functional gap junctions between the cells. Tramadol and flurbiprofen inhibited dye-coupling through gap junctions, but morphine did not.

CONCLUSIONS

The results suggest that the density dependence of cisplatin toxicity is mediated by gap junctions. They further indicate that tramadol and flurbiprofen depress cisplatin cytotoxicity through inhibition of gap junction activity, and more generally, that agents that depress junctional communication can counteract the effects of antitumor agents.

Correlation between connexin 26 expression and poor prognosis of esophageal squamous cell carcinoma

BACKGROUND

Failure of gap junction formation affects the development of various types of cancer. We aimed to clarify the clinicopathologic outcome and prognostic significance of connexin (Cx) 26 in human esophageal squamous cell carcinoma (ESCC).

METHODS

Immunohistochemical staining for Cx26 was performed on surgical specimens obtained from 123 patients with ESCC.

RESULTS

There was no positive staining for Cx26-specific expression in normal esophageal squamous cells. Primary ESCC with Cx26-positive expression was detected in the cytoplasm of cancer cell nests in 60 cases. Cx26 expression was correlated with N (lymph node metastasis, P = 0.014) and the number of metastatic lymph nodes (P = 0.047). The 5-year survival rates of ESCC patients with Cx26-positive expression were significantly lower than those with Cx26-negative expression (positive, 39.7%; negative, 65.7%; P = 0.007). By multivariate analysis, tumor-node-metastasis (TNM) clinical classification (T, P < 0.001; N, P = 0.002; M, P = 0.046) and Cx26 (P = 0.024) were independent prognosis predictors of ESCC.

CONCLUSIONS

These results suggest that abnormal expression of Cx26 participates in the progress of ESCC.

Connexin-related signaling in cell death: to live or let die?

Evidence is accumulating that some forms of cell death, like apoptosis, are not only governed by the complex interplay between extracellular and intracellular signals but are also strongly influenced by intercellular communicative networks. The latter is provided by arrays of channels consisting of connexin proteins, with gap junctions directly connecting the cytoplasm of neighboring cells and hemichannels positioned as pores that link the cytoplasm to the extracellular environment. The role of gap junctions in cell death communication has received considerable interest and recently hemichannels have joined in as potentially toxic pores adding their part to the cell death process. However, despite a large body of existing evidence, especially for gap junctions, the exact contribution of the connexin channel family still remains controversial, as both gap junctions and hemichannels may furnish cell death as well as cell survival signals. An additional layer of complexity is formed by the fact that connexin proteins as such, beyond their channel function, may influence the cell death process. We here review the current knowledge on connexins and their channels in cell death and specifically address the molecular mechanisms that underlie connexin-related signaling. We also briefly focus on pannexins, a novel set of connexin-like proteins that have been implicated in cellular responses to pathological insults.

Analysis of the expression of biomarkers in urinary bladder cancer using a tissue microarray

Dysregulation of Akt, PTEN, Drg-1, Cx-26, and L-plastin expression appear to be important in the progression of various cancers. Their expression in bladder cancer has not been well characterized. To assess the expression of these genes and their relationship to the outcome of bladder cancer, we used a bladder cancer tissue microarray (TMA) of 251 transitional cell carcinomas. We quantitated immunohistochemical staining of each protein using both automated and manual methods and correlated the expression levels with the clinicopathologic characteristics of the tumor and patient survival. Overall, the results from both automated and manual analyses were similar. We found a significant correlation between the expression of PTEN, Cx-26 and L-plastin with known clinically important pathologic features of bladder cancer (tumor grade, stage, and growth pattern). Aberrant localization patterns of Cx-26 and Drg-1 were observed in bladder tumors. There was also a significant correlation in expression among pAkt, PTEN, and L-plastin. Although the expression of these genes correlated with factors known to be associated with patient outcome, none of them was an independent predictor of progression-free or overall survival.

Aberrant expression of connexin 26 is associated with lung metastasis of colorectal cancer

PURPOSE

Connexin 26 (Cx26) is one of the gap junction-forming family members classically considered to be tumor suppressors. However, recent studies show association of elevated expression of Cx26 with poor prognosis in several human malignancies. Furthermore, Cx26 has been observed to be indispensable to spontaneous metastasis of melanoma cells. Here, we assessed Cx26 expression in primary colorectal cancer (CRC) and the metastatic lesions to elucidate its role in metastasis.

EXPERIMENTAL DESIGN

Cx26 expression was assessed in 25 adenomas, 167 CRCs, and normal mucosa, together with the metastatic lesions.

RESULTS

Normal mucosa and adenomatous tissue expressed Cx26 mainly in the plasma membrane, whereas cancer cells mostly contained Cx26 in the cytoplasm. The incidence of aberrant Cx26 expression varied widely in CRC (mean, 49.5 +/- 35.5%), and the expression levels were confirmed by Western blot and quantitative reverse transcription-PCR. Clinicopathologic survey revealed association of high expression with less differentiated histology and venous invasion (P = 0.0053 and P = 0.0084, respectively). Notably, high Cx26 expression was associated with shorter disease-free survival and shorter lung metastasis-free survival in 154 curatively resected CRC sets (P = 0.041 and P = 0.028, respectively). Survey of metastatic lesions revealed that lung metastasis, but not liver and lymph nodes metastases, expressed higher Cx26 than the CRC series or corresponding primary CRCs (P < 0.0001 and P = 0.0001, respectively).

CONCLUSIONS

These findings suggest that aberrant expression of Cx26 plays an essential role in lung metastasis. Thus, Cx26 is a promising therapeutic target, particularly for CRC patients who develop lung metastasis.

Gap junctional communication in morphogenesis

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

Connexin43 increases the sensitivity of prostate cancer cells to TNFalpha-induced apoptosis

To examine the effects of increased expression of connexin43 (Cx43) upon cell viability and response to cytotoxic agents, we expressed Cx43 in LNCaP and PC3 prostate cancer cells by infection with a recombinant adenovirus (Ad-Cx43). Infection with Ad-Cx43 led to the formation of Cx43-containing gap junction plaques at appositional membranes and increased Lucifer Yellow transfer in LNCaP cells, but not in PC3 cells. The increased intercellular communication was blocked by co-infection with an adenovirus containing a dominant-negative Cx43 (Ad-Cx43DN). Infection of LNCaP (but not PC3) cells with Ad-Cx43 greatly increased their sensitivity to killing by tumor necrosis factor alpha (TNFalpha), anti-Fas antibodies, and TRAIL as quantified using an MTS assay. The TNFalpha-induced cell death was dependent on cell density, and it was associated with increased annexin V staining, an increased proportion of sub-G1 cells, and activation of caspase 8. The TNFalpha-induced effects on Ad-Cx43-infected LNCaP cells were blocked by co-infection with Ad-Cx43DN or by pre-incubation with neutralizing antibodies directed against TNFalpha receptor 1. These results demonstrate that TNFalpha induces apoptosis in LNCaP cells by signaling through TNFalpha receptor 1 and that expression of functional Cx43 gap junction channels increases their sensitivity to TNFalpha.

Restoration of functional gap junctions through internal ribosome entry site-dependent synthesis of endogenous connexins in density-inhibited cancer cells

Gap junctions are composed of connexins and are critical for the maintenance of the differentiated state. Consistently, connexin expression is impaired in most cancer cells, and forced expression of connexins following cDNA transfection reverses the tumor phenotype. We have found that the restoration of density inhibition of human pancreatic cancer cells by the antiproliferative somatostatin receptor 2 (sst2) is due to overexpression of endogenous connexins Cx26 and Cx43 and consequent formation of functional gap junctions. Immunoblotting along with protein metabolic labeling and mRNA monitoring revealed that connexin expression is enhanced at the level of translation but is not sensitive to the inhibition of cap-dependent translation initiation. Furthermore, we identified a new internal ribosome entry site (IRES) in the Cx26 mRNA. The activity of Cx26 IRES and that of the previously described Cx43 IRES are enhanced in density-inhibited cells. These data indicate that the restoration of functional gap junctions is likely a critical event in the antiproliferative action of the sst2 receptor. We further suggest that the existence of IRESes in connexin mRNAs permits connexin expression in density-inhibited or differentiated cells, where cap-dependent translation is generally reduced.

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.

5-fluorouracil and hydroxyurea enhance adenovirus-mediated transgene expression in colon and hepatocellular carcinoma cells

PURPOSE

To investigate the efficient transduction of tumor cells which remains a major limitation of cancer gene therapy.

METHODS

In this study, we tested whether treatment with antimetabolic drugs 5-FU and hydroxyurea (HU) could improve adenovirus-mediated gene expression in tumor cells.

RESULTS

We found that 5-FU and HU treatment significantly increased beta-gal activity in adenovirus (Ad.CMVBG)-infected human colon carcinoma (LoVo) and hepatocellular carcinoma (SMMC7721) cells in a dose- and time-dependent manner. These increases were maximized at 5.01+/-0.42-fold and 3.32+/-0.32-fold for 5-FU (50 microM), and at 6.60+/-0.50-fold and 4.82+/-0.43-fold for HU (5 mM) treatment, respectively, after 48 h infection. Transient increases in viral uptake, determined by real-time PCR for viral DNA content and by confocal microscopy for viral particles, were observed in 5-FU or HU-treated cells that partially contribute to the overall increases of beta-gal expression. Moreover, mRNA levels for the beta-gal gene in infected cells were significantly increased in both LoVo and SMMC7721 cells by 5-FU and HU treatment in contrast to the inhibition of viral DNA replication and the unchanged mRNA levels for alpha-actin gene. The induction appeared to be the result of enhanced transcription since beta-gal mRNA half-life was not affected by drug treatment. However, similar induction was not detected in CMV-beta-gal-expressing stable cells, suggesting that an adenovirus-associated mechanism might be involved in this induction.

CONCLUSIONS

Our findings suggest that it may be possible to improve tumor cell transduction by adenovirus using chemotherapy.

Cell-interdependent cisplatin killing by Ku/DNA-dependent protein kinase signaling transduced through gap junctions

Cisplatin is one of the most widely used cancer chemotherapy agents, but its mechanism of action is not fully understood. Current models suggest that cell killing by cisplatin occurs in a cell-autonomous manner by means of formation of platinum-DNA adducts that, if not removed by DNA repair, block transcription and replication. Here, we show that there is a separate cell-interdependent pathway of cisplatin killing in which damaged cells can transmit a death signal to neighboring cells. This signal is produced within the damaged cell by the kinase function of the Ku70, Ku80, and DNA-dependent protein kinase complex and is conveyed to the recipient cell by direct cell-to-cell communication through gap junctions. These findings suggest that DNA-dependent protein kinase activity and gap junction expression in human cancers may influence the clinical response to cisplatin. In addition, strategies to manipulate these cellular components in conjunction with cisplatin treatment may provide new approaches to cancer therapy.

Connexins and apoptotic transformation

We examined the influence of connexin (Cx) expression on the development of apoptosis in HeLa parental cells (coupling deficient cell line) and HeLa cells expressing wild-type Cx43 and Cxs fused with enhanced green fluorescent protein (EGFP). EGFP was attached to the C-terminus of Cx32 and Cx43, Cx32-EGFP and Cx43-EGFP, respectively, and to the N-terminus of Cx32, EGFP-Cx32. All fusion proteins assembled into junctional plaques (JPs) at areas of cell-cell contact, but only the C-terminal fusion proteins formed functional gap junction (GJ) channels as well as hemichannels. In each cell line, apoptosis was induced by treatment with various agents including anisomycin, camptothecin, cis-platinum, colchicine, cycloheximide, etoposide, staurosporin and taxol. Using fluorescence microscopy, time-lapse imaging and dual whole-cell voltage clamp techniques, we correlated the changes in functional properties of GJ channels and Cx distribution with the progression of apoptosis based on cells' labeling with acridine orange and ethidium bromide (EB). The early phase of apoptosis (a viable apoptotic (VA) state) was characterized by shrinkage of the cells and by increased internalization of JPs accompanied by decreased cell-cell coupling. The apoptotic reagents had no direct effect on electrical cell-cell coupling. Transformation from a VA to a nonviable apoptotic (NVA) state was faster in HeLa cells expressing Cx43 or Cx43-EGFP than in HeLa parental cells. The potent GJ uncoupler, octanol, slowed the transition of HelaCx43-EGFP cells into a NVA state. In the absence of apoptotic reagents, the rate of EB uptake was higher in HeLaCx43-EGFP than in HeLa parental cells consistent with the presence of open Cx43-EGFP hemichannels. However, in both cell lines the rate of EB uptake decreased proportionally during the development of apoptosis suggesting that membrane permeability ascribed to Cx hemichannels is reduced. Cells expressing Cx32-EGFP and EGFP-Cx32 demonstrate the same apoptotic patterns as HeLaCx43-EGFP and HeLa parental cells, respectively. Intracellular levels of ATP in HeLaCx43-EGFP cells were substantially lower than in HeLa parental cells, and ATP added to the medium abolished the accelerated transition from a VA to a NVA state in HeLaCx43-EGFP cells. In summary, Cx32 or Cx43 accelerates transformation of cells into a NVA state or secondary necrosis and this depends on the ability of Cxs to form functional GJ channels and hemichannels.

In vivo gene therapy of human bladder cancer with PTEN suppresses tumor growth, downregulates phosphorylated Akt, and increases sensitivity to doxorubicin

The PTEN gene, located on chromosome 10, is a phosphatase in the phosphatidylinositol 3'-kinase (PI3'K)-mediated signal transduction pathway. PTEN inhibits the activation of Akt, a serine-threonine kinase involved in proliferative metabolic and antiapoptotic pathways, and has tumor suppressive properties. We created a PTEN adenoviral vector, Ad-MMAC, to assess the role of PTEN in the treatment of bladder cancer. Direct injection of Ad-MMAC into established subcutaneous UM-UC-3 (PTEN deleted, upregulation of phosphorylated Akt) and UM-UC-6dox (wild-type PTEN, upregulation of phosphorylated Akt) tumors in nude mice resulted in PTEN expression, apoptosis, and significantly decreased growth compared to Ad-CTR- or Phosphate-buffered saline (PBS)-treated tumors. UM-UC-3 tumors completely disappeared in all of mice treated with Ad-MMAC, but PBS- and Ad-CTR-treated UM-UC-3 tumors continued to grow rapidly. UM-UC-14 tumors (wild-type PTEN) were transiently suppressed by Ad-MMAC. Downregulation of vascular endothelial growth factor and decreased microvessel density were seen in tumors treated with Ad-MMAC in vivo. Combination therapy with Ad-MMAC and doxorubicin improved the in vivo efficacy of PTEN gene therapy in the doxorubicin-resistant cell line UM-UC-6dox. Treatment with Ad-MMAC and doxorubicin completely eradicated established UM-UC-6dox tumors in three of 10 mice. UM-UC-14 tumors were transiently suppressed by this combined treatment. These data demonstrate that PTEN gene therapy can effectively treat bladder cancers that have genomic alterations in PTEN. Furthermore, tumors that exhibit drug resistance associated with expression of phosphorylated Akt can be effectively treated with PTEN gene therapy and chemotherapy.

Therapeutic approaches to bladder cancer: identifying targets and mechanisms

Transitional cell carcinoma is the second most common genitourinary malignancy in US and third most common cause of death among genitourinary tumors. Treatment options for bladder cancer include surgery, often combined with chemotherapy, radiation, and/or immunotherapy. The MVAC adjuvant chemotherapy regimen has been most widely used in locally invasive as well as metastatic disease. Only a proportion of patients at risk will respond to therapy. There is thus need to identify good responder patients for adjuvant therapy and to identify new targets to treat a greater range of patients. Based upon patient-specific aberrations in pathways or known markers, both existing and new therapies can be tailored to benefit patients based on the risk of progression and molecular alterations specific to a patient's tumor. Targeted therapy, therefore, is defined as therapy that targets mechanism and risk. Utilizing the available knowledge of the molecular biology of cell-cycle regulation, signal transduction, apoptosis, and angiogenesis in bladder cancer, we review the potential therapeutic targets for rational drug development. Finally, using bladder cancer as a model for translational research, requirements for a desired clinical trial are presented.

Connexin 26 is abnormally expressed in bladder cancer

PURPOSE

Connexin 26 is the major gap junction protein in urothelial and mammary epithelial cells, and a putative tumor suppressor gene. We evaluated connexin 26 expression in normal urothelium and in bladder cancer.

MATERIALS AND METHODS

A total of 40 formalin fixed, paraffin embedded bladder tumors and 5 normal urothelial specimens were analyzed by immunohistochemistry. Two observers visually scored connexin 26 expression in these specimens.

RESULTS

Normal urothelium expressed connexin 26 in a punctate staining pattern with limited expression in the basal layer. Decreased connexin 26 expression was observed in 28 of 40 tumors (70%). Connexin 26 was diffusely expressed in 5 of 18 low grade, noninvasive tumors (28%), whereas loss of expression was observed in heterogeneous (30% to 70% positive staining) or extensive (less than 30% positive staining) fashion in 8 (44%) and 5 (28%), respectively. Seven of 22 high grade or invasive tumors (32%) showed diffuse connexin 26 expression, whereas expression was decreased in a heterogeneous or extensive pattern in 9 (41%) and 6 (27%), respectively. Intracytoplasmic localization of connexin 26 was also observed.

CONCLUSIONS

Expression of connexin 26 is altered in bladder cancer. These aberrant patterns of connexin 26 expression may contribute to the malignant phenotype of this disease.