Connexin32 as a tumor suppressor gene in a metastatic renal cell carcinoma cell line

Connexin43 is associated with the progression of clear cell renal carcinoma and is regulated by tangeretin to sygergize with tyrosine kinase inhibitors

BACKGROUND

The roles of Connexin43 (Cx43) in clear cell renal cell carcinoma (ccRCC) microenviroment remains to be poorly defined.

METHODS

The expression profile, prognosis and immune analysis of Cx43 in various cancers, particularly in ccRCC were performed using TCGA database, and various biological function assays were applied to explore the physiological role of Cx43 and tangeretin in ccRCC. Western blot were applied to examine the protein expression and Kunming mice were used to evaluate preliminary safety or anti-tumor activity of tangeretin and sunitinib.

RESULTS

Compared with the normal group, higher expression levels of Cx43 in ccRCC, and distinct associations between Cx43 expression and ccRCC prognosis or immune infiltration, were found. Notably, the expression of Cx43 was found to be highly correlated with that of receptor tyrosine kinases (RTKs), particularly with VEGFR1, VEGFR2 and VEGFR3. The expression of Cx43 and EGFR was also found to be higher in ccRCC than that in the para-cancerous specimens. Knocking down Cx43 expression decreased RCC cell viability, cell migration, p-EGFR, MMP-9 and survivin expression. Using 14 Chinese medicine monomers, tangeretin was screened and found to inhibit tumor cell viability and Cx43 expression. Tangeretin also enhanced the sensitivity of RCC cells to tyrosine kinase inhibitors (TKIs) sunitinib and sorafenib. However, the same concentration of tangeretin exerted a less prominent effect on normal renal cell viability.

CONCLUSIONS

Cx43 is strongly associated with RTK expression and ccRCC progression, while tangeretin can inhibit RCC cell malignancy by inhibiting Cx43 expression and enhance the sensitivity of RCC cells to TKIs.

Connexins in Cancer: Jekyll or Hyde?

The expression, localization, and function of connexins, the protein subunits that comprise gap junctions, are often altered in cancer. In addition to cell-cell coupling through gap junction channels, connexins also form hemichannels that allow communication between the cell and the extracellular space and perform non-junctional intracellular activities. Historically, connexins have been considered tumor suppressors; however, they can also serve tumor-promoting functions in some contexts. Here, we review the literature surrounding connexins in cancer cells in terms of specific connexin functions and propose that connexins function upstream of most, if not all, of the hallmarks of cancer. The development of advanced connexin targeting approaches remains an opportunity for the field to further interrogate the role of connexins in cancer phenotypes, particularly through the use of in vivo models. More specific modulators of connexin function will both help elucidate the functions of connexins in cancer and advance connexin-specific therapies in the clinic.

Effects of Connexin 32-Mediated Lung Inflammation Resolution During Liver Ischemia Reperfusion

BACKGROUND

Hepatic ischemia reperfusion (HIR) leads to a lung inflammatory response and subsequent pulmonary barrier dysfunction. The gap junction communication protein connexin 32 (Cx32), which is widely expressed in the lungs, participates in intercellular signaling. This study determined whether the communication protein Cx32 could affect pulmonary inflammation caused by HIR.

METHODS

Mice were randomly allocated into four groups (n = 8/group): (i) Cx32+/+ sham group; (ii) Cx32+/+ HIR model group; (iii) Cx32-/- sham group; and (iv) Cx32-/- HIR model group. Twenty-four hours after surgery, lung tissues were collected for bright field microscopy, western blot (Cx32, JAK2, p-JAK2, STAT3, p-STAT3), and immunofluorescence (ZO-1, 8-OHDG) analyses. The collected bronchoalveolar fluid was tested for levels of interleukin-6 (IL-6), matrix metalloproteinase 12 (MMP-12), and antitrypsin (α1-AT). Lung mmu-miR-26a/b expression was detected using a PCR assay.

RESULTS

Increased expression of Cx32 mRNA and protein was noted in the lungs after HIR. Cx32 deletion significantly aggravated pulmonary function from acute lung injury induced by HIR. In addition, Cx32 deletion decreased the protein level of ZO-1 (pulmonary function) and increased the level of the oxidative stress marker 8-OHDG in the lungs. Moreover, in the Cx32-/- HIR model group, the levels of IL-6 and MMP-12 in bronchoalveolar lavage fluid were significantly increased leading to activation of the JAK2/STAT3 pathway, and decreased α1-AT levels. Furthermore, we found mmu-miR-26a/b was significantly downregulated in the Cx32-/- HIR model group.

CONCLUSION

HIR leads to acute lung inflammatory injury. Cx32 deletion aggravates hepatic-derived lung inflammation, partly through blocking the transferring of mmu-miR-26a/b and leading to IL-6-related JAK2/STAT3 pathway activation.

Redox-Mediated Mechanism of Chemoresistance in Cancer Cells

Cellular reactive oxygen species (ROS) status is stabilized by a balance of ROS generation and elimination called redox homeostasis. ROS is increased by activation of endoplasmic reticulum stress, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase family members and adenosine triphosphate (ATP) synthesis of mitochondria. Increased ROS is detoxified by superoxide dismutase, catalase, and peroxiredoxins. ROS has a role as a secondary messenger in signal transduction. Cancer cells induce fluctuations of redox homeostasis by variation of ROS regulated machinery, leading to increased tumorigenesis and chemoresistance. Redox-mediated mechanisms of chemoresistance include endoplasmic reticulum stress-mediated autophagy, increased cell cycle progression, and increased conversion to metastasis or cancer stem-like cells. This review discusses changes of the redox state in tumorigenesis and redox-mediated mechanisms involved in tolerance to chemotherapeutic drugs in cancer.

Connexin 32 downregulation is critical for chemoresistance in oxaliplatin-resistant HCC cells associated with EMT

Background: Oxaliplatin (OXA)-based chemotherapy is critical in the management of advanced hepatocellular carcinoma (HCC); however, acquired drug resistance has largely restricted its clinical efficacy. This study aims to explore the key mechanisms and regulatory factors determining chemosensitivity in HCC.

Methods: We developed OXA-resistant (OR) HCC cells and used multiple methods, including real-time RT-PCR, Western blot, immunofluorescence, transwell invasion assay, wound-healing assay, MTT assay, gene transfection, and immunohistochemistry to achieve our goals.

Results: We found that OR HCC cells showed a typical epithelial–mesenchymal transition (EMT) phenotype. Meanwhile, the expression of Cx32, a major member of the liver connexin (Cx) family, was lowly expressed in OR HCC cells. Downregulation of Cx32 in parental HCC cells led to EMT induction and thereby reduced OXA cytotoxicity, while Cx32 upregulation in OR HCC cells could reverse the EMT phenotype and partially restore chemosensitivity to OXA. Finally, in human HCC tissue samples, Cx32 was positively correlated with the expression of the EMT marker E-cadherin and negatively correlated with the expression of Vimentin.

Conclusion: Our findings demonstrated that downregulation of Cx32 may be an important determinant for HCC cells to acquire EMT-related acquired drug resistance to OXA, and targeting Cx32 could be a novel strategy to overcome OXA resistance in HCC.

Emerging roles of gap junction proteins connexins in cancer metastasis, chemoresistance and clinical application

Connexin, a four-pass transmembrane protein, contributes to assembly of gap junctions among neighboring cells and thus facilitates gap junctional intercellular communication (GJIC). Traditionally, the roles of connexins were thought to mediate formation of hemichannels and GJIC assembly for transportation of ions and small molecules. Many studies have observed loss of GJIC, due to reduced expression or altered cytoplasmic localization of connexins, in primary tumor cells. Connexins are generally considered tumor-suppressive. However, recent studies of clinical samples suggested a different role of connexins in that expression levels and membrane localization of connexins, including Connexin 43 (Cx43, GJA1) and Connexin 26 (Cx26, GJB2), were found to be enhanced in metastatic lesions of cancer patients. Cx43- and Cx26-mediated GJIC was found to promote cancer cell migration and adhesion to the pulmonary endothelium. Regulatory circuits involved in the induction of connexins and their functional effects have also been reported in various types of cancer. Connexins expressed in stromal cells were correlated with metastasis and were implicated in regulating metastatic behaviors of cancer cells. Recent studies have revealed that connexins can contribute to cellular phenotypes via multiple ways, namely 1) GJIC, 2) C-terminal tail-mediated signaling, and 3) cell-cell adhesion during gap junction formation. Both expression levels and the subcellular localization could participate determining the functional roles of connexins in cancer. Compounds targeting connexins were thus tested as potential therapeutics intervening metastasis or chemoresistance. This review focuses on the recent findings in the correlation between the expression of connexins and patients’ prognosis, their roles in metastasis and chemoresistance, as well as the implications and concerns of using connexin-targeting drugs as anti-metastatic therapeutics. Overall, connexins may serve as biomarkers for cancer prognosis and as therapeutic targets for intervening metastasis and chemoresistance.

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.

PBX1 attributes as a determinant of connexin 32 downregulation in Helicobacter pylori-related gastric carcinogenesis

AIM

To clarify the mechanisms of connexin 32 (Cx32) downregulation by potential transcriptional factors (TFs) in Helicobacter pylori (H. pylori)-associated gastric carcinogenesis.

METHODS

Approximately 25 specimens at each developmental stage of gastric carcinogenesis [non-atrophic gastritis, chronic atrophic gastritis, intestinal metaplasia, dysplasia and gastric carcinoma (GC)] with H. pylori infection [H. pylori (+)] and 25 normal gastric mucosa (NGM) without H. pylori infection [H. pylori (-)] were collected. After transcriptional factor array analysis, the Cx32 and PBX1 expression levels of H. pylori-infected tissues from the developmental stages of GC and NGM with no H. pylori infection were measured by real-time polymerase chain reaction (RT-PCR) and Western blot analysis. Regarding H. pylori-infected animal models, the Cx32 and PBX1 mRNA expression levels and correlation between the gastric mucosa from 10 Mongolian gerbils with long-term H. pylori colonization and 10 controls were analyzed. PBX1 and Cx32 mRNA and protein levels were further studied under the H. pylori-infected condition as well as PBX1 overexpression and knockdown conditions in vitro.

RESULTS

Incremental PBX1 was first detected by TF microarray in H. pylori-related gastric carcinogenesis. The identical trend of PBX1 and Cx32 expression was confirmed in the developmental stages of H. pylori-related clinical specimens. The negative correlation of PBX1 and Cx32 was confirmed in H. pylori-infected Mongolian gerbils. Furthermore, decreased PBX1 expression was detected in the normal gastric epithelial cell line GES-1 with H. pylori infection. Enforced overexpression or RNAi-mediated knockdown of PBX1 contributed to the diminished or restored Cx32 expression in GES-1 and the gastric carcinoma cell line BGC823, respectively. Finally, dual-luciferase reporter assay in HEK293T cells showed that Cx32 promoter activity decreased by 30% after PBX1 vector co-transfection, indicating PBX1 as a transcriptional downregulator of Cx32 by directly binding to its promoters.

CONCLUSION

PBX1 is one of the determinants in the Cx32 promoter targeting site, preventing further damage of gap junction protein in H. pylori-associated gastric carcinogenesis.

Downregulated connexin32 promotes EMT through the Wnt/β-catenin pathway by targeting Snail expression in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the common malignances in the world and is associated with high mortality and poor prognosis, partly due to early invasion and metastasis. Cx32 has been indicated to be involved in the progression of many cancers including HCC, but its relationship with tumor invasion and metastasis is still controversial. In the present study, the downregulated Cx32 in HCC tissue was found negatively correlated with histological grade and lymph node metastasis. Cx32 regulated HCC migration and invasion in vitro and inhibited tumor metastasis in xenograft models in vivo. We subsequently identified that Cx32 mediated epithelial-mesenchymal transition (EMT) by regulating Snail expression, and the enhanced Snail was due to activation of Wnt/β-catenin signaling in response to Cx32 inhibition. Finally, decreased expression of Cx32 showed strong correlation with loss/reduction of E-cadherin, higher expression of Snail, and nuclear accumulation of β-catenin in HCC tissues. Taken together, our results suggest that Cx32 inhibits HCC invasion and metastasis through Snail-mediated EMT, Cx32 and this signaling pathway molecules may offer potential targets for HCC cancer therapy.

Non-junctional Cx32 mediates anti-apoptotic and pro-tumor effects via epidermal growth factor receptor in human cervical cancer cells

The role of connexin proteins (Cx), which form gap junctions (GJ), in progression and chemotherapeutic sensitivity of cervical cancer (CaCx), is unclear. Using cervix specimens (313 CaCx, 78 controls) and CaCx cell lines, we explored relationships among Cx expression, prognostic variables and mechanisms that may link them. In CaCx specimens, Cx32 was upregulated and cytoplasmically localized, and three other Cx downregulated, relative to controls. Cx32 expression correlated with advanced FIGO staging, differentiation and increased tumor size. In CaCx cell lines, Cx32 expression suppressed streptonigrin/cisplatin-induced apoptosis in the absence of functional GJ. In CaCx specimens and cell lines, expression of Cx32 upregulated epidermal growth factor receptor (EGFR) expression. Inhibition of EGFR signaling abrogated the anti-apoptotic effect of Cx32 expression. In conclusion, upregulated Cx32 in CaCx cells produces anti-apoptotic, pro-tumorigenic effects in vivo and vitro. Abnormal Cx32 expression/localization in CaCx appears to be both a mechanism and biomarker of chemotherapeutic resistance.

Gene set enrichment analysis and ingenuity pathway analysis of metastatic clear cell renal cell carcinoma cell line

In recent years, genome-wide RNA expression analysis has become a routine tool that offers a great opportunity to study and understand the key role of genes that contribute to carcinogenesis. Various microarray platforms and statistical approaches can be used to identify genes that might serve as prognostic biomarkers and be developed as antitumor therapies in the future. Metastatic renal cell carcinoma (mRCC) is a serious, life-threatening disease, and there are few treatment options for patients. In this study, we performed one-color microarray gene expression (4×44K) analysis of the mRCC cell line Caki-1 and the healthy kidney cell line ASE-5063. A total of 1,921 genes were differentially expressed in the Caki-1 cell line (1,023 upregulated and 898 downregulated). Gene Set Enrichment Analysis (GSEA) and Ingenuity Pathway Analysis (IPA) approaches were used to analyze the differential-expression data. The objective of this research was to identify complex biological changes that occur during metastatic development using Caki-1 as a model mRCC cell line. Our data suggest that there are multiple deregulated pathways associated with metastatic clear cell renal cell carcinoma (mccRCC), including integrin-linked kinase (ILK) signaling, leukocyte extravasation signaling, IGF-I signaling, CXCR4 signaling, and phosphoinositol 3-kinase/AKT/mammalian target of rapamycin signaling. The IPA upstream analysis predicted top transcriptional regulators that are either activated or inhibited, such as estrogen receptors, TP53, KDM5B, SPDEF, and CDKN1A. The GSEA approach was used to further confirm enriched pathway data following IPA.

Sensitization of Ion Channels Contributes to Central and Peripheral Dysfunction in Neurofibromatosis Type 1

Neurofibromatosis type 1 (Nf1) is a progressive, autosomal disorder with a large degree of variability and severity of manifestations including neurological, cutaneous, ocular/orbital, orthopedic, and vascular abnormalities. Nearly half of Nf1 patients presents with cognitive impairment, specifically spatial learning deficits. These clinical manifestations suggest a global impairment of both central and peripheral nervous system functions in neurofibromatosis. Nf1 encodes for neurofibromin, a Ras GTPase-activating protein (Ras GAP) that has been implicated in the regulation of long-term potentiation (LTP), Ras/ERK (extracellular signal-regulated kinase) signaling, and learning in mice. Over the last decades, mice with a targeted mutation in the Nf1 gene, Nf1 ^-/- chimeric mice, Nf1 exon-specific knockout mice, and mice with tissue-specific inactivation of Nf1 have been generated to model the human Nf1 disease. These studies have implicated neurofibromin in regulation of the release of the inhibitory neurotransmitter γ-amino butyric acid (GABA) in the hippocampus and frontal lobe, which can regulate memory. Mutations in neurofibromin thus lead to perturbed ERK signaling, which alters GABA release, LTP, and subsequently leads to learning deficits. In addition to these cognitive deficits, Nf1 patients also have defects in fine and gross motor coordination as well as decreased muscle strength. Although the mechanisms underlying these motor deficits are unknown, deficits in GABAergic neurotransmission in both the motor cortex and cerebellum have been suggested. In this review, we present evidence to support the hypothesis that alterations of ion channel activity in Nf1 underscore the dysregulated neuronal communication in non-neuronal and neuronal cells that likely contributes to the clinical cornucopia of Nf1.

Association of STAT3 with Cx26 and Cx43 in human uterine endometrioid adenocarcinoma

Signal transducer and activator of transcription-3 (STAT3) drives endometrial carcinogenesis, while signaling via gap junctions gets weakened during cancer progression. Connexin 26 (Cx26), Cx43 and STAT3 were immunohistochemically evaluated in 78 endometrioid adenocarcinomas: Nuclear expression of STAT3 positively correlated with cytoplasmic immunoreactivity to Cx43 (P=0.004, r=0.318) and Cx26 (P=0.006, r=0.309). STAT3 correlated with Cx43 (P=0.022, r=0.411) and Cx26 (P=0.008 r=0.466) in G1 tumors. A statistically significant linkage remained in G2 cancers between STAT3 and Cx43 (P=0.061, r=0.262) and Cx26 (P=0.016, r=0.331); however, no correlations were observed in G3 tumors. STAT3 was significantly associated with Cx 43 (p=0.003, r=0.684) and Cx26 (p=0.049, r=0.500) in estrogen receptor (ER) negative adenocarcinomas. STAT3 did not correlate with Cx43 in ER positive adenocarcinomas; however, STAT3 expression remained correlated with Cx26 expression (P=0.035, r=0.268). In progesterone receptor negative tumors STAT3 was significantly associated with Cx43 (P=0.035, r=0.451) and Cx26 (P<0.0001, r=0.707). However, in PgR positive adenocarcinomas STAT3 correlated with Cx43 (P=0.03, r=0.290) but not with Cx26. Thus, it appears that hormone dependent acceleration of cancer growth breaks the association between STAT3 and Cx expression. These associations become weaker as the tumors dedifferentiate from G1 to G3 endometrioid adenocarcinomas. The present study provides evidence that the loss of correlation between STAT3 and selected Cx proteins occurs in tumors with more aggressive behavior.

Antitumor activity of phenanthroindolizidine alkaloids is associated with negative regulation of Met endosomal signaling in renal cancer cells

Met is a receptor tyrosine kinase for hepatocyte growth factor. Met mutations have been considered as a major cause of primary resistance to Met tyrosine kinase inhibitors (TKIs). Mutated Met enhances its endosomal signaling, which includes internalization, signaling within endosomes, recycling to membrane, and sorting for degradation. These sequential events lead to a plausible mechanism for resistance. (-)-Antofine, a phenanthroindolizidine alkaloid, has exhibited potent antitumor activity but the precise underlying mechanism has been poorly understood. We found that (-)-antofine effectively inhibited the proliferation of Met-mutated Caki-1 cells, which were resistant to well-known Met TKIs. (-)-Antofine negatively regulated Met endosomal signaling and consequently inhibited the nuclear translocation of STAT3 both in vitro and in vivo. These findings emphasize the potential of Met endosomal signaling as a novel target for Met TKI-resistant cancers and (-)-antofine as a novel lead compound associated with the suppression of Met endosomal signaling.

Downregulation of connexin 32 attenuates hypoxia/reoxygenation injury in liver cells

Gap junction intercellular communication is involved in ischemia-reperfusion (IR) injury of organs. Connexins are proteins that are critical to the function of gap junctions. To clarify the role of gap junctions in IR injury in liver cells, the function of gap junctions was modulated in an in vitro hypoxia/reoxygenation (H/R) model. BRL-3A rat liver cells, endogenously expressing connexins Cx32 and Cx43, were used to model the process of hepatic IR injury. Suppression of gap junction activity was achieved genetically, using Cx32-specific small interfering RNA (siRNA), or chemically, with pharmacological inhibitors, oleamide, and 18-α-GA. BRL-3A cells subjected to H/R exhibited reduced cell survival and pathologies indicative of IR injury. Cx32-specific siRNA, oleamide, and 18-α-GA, respectively, decreased gap junction permeability, as assessed by the parachute assay. Pretreatment with Cx32-specific siRNA increased cell survival. Pretreatment with oleamide or 18-α-GA did not improve cell survival. Modulating gap junction by Cx32 gene silencing protected BRL-3A liver cells from H/R.

Atomic force microscopy-based microrheology reveals significant differences in the viscoelastic response between malign and benign cell lines

Mechanical phenotyping of cells by atomic force microscopy (AFM) was proposed as a novel tool in cancer cell research as cancer cells undergo massive structural changes, comprising remodelling of the cytoskeleton and changes of their adhesive properties. In this work, we focused on the mechanical properties of human breast cell lines with different metastatic potential by AFM-based microrheology experiments. Using this technique, we are not only able to quantify the mechanical properties of living cells in the context of malignancy, but we also obtain a descriptor, namely the loss tangent, which provides model-independent information about the metastatic potential of the cell line. Including also other cell lines from different organs shows that the loss tangent (G″/G′) increases generally with the metastatic potential from MCF-10A representing benign cells to highly malignant MDA-MB-231 cells.

Connexins, gap junctions and tissue invasion

Formation of metastases negatively impacts the survival prognosis of cancer patients. Globally, if the various steps involved in their formation are relatively well identified, the molecular mechanisms responsible for the emergence of invasive cancer cells are still incompletely resolved. Elucidating what are the mechanisms that allow cancer cells to evade from the tumor is a crucial point since it is the first step of the metastatic potential of a solid tumor. In order to be invasive, cancer cells have to undergo transformations such as down-regulation of cell-cell adhesions, modification of cell-matrix adhesions and acquisition of proteolytic properties. These transformations are accompanied by the capacity to "activate" stromal cells, which may favor the motility of the invasive cells through the extracellular matrix. Since modulation of gap junctional intercellular communication is known to be involved in cancer, we were interested to consider whether these different transformations necessary for the acquisition of invasive phenotype are related with gap junctions and their structural proteins, the connexins. In this review, emerging roles of connexins and gap junctions in the process of tissue invasion are proposed.

Induction of the connexin 32 gene by epigallocatechin-3-gallate potentiates vinblastine-induced cytotoxicity in human renal carcinoma cells

BACKGROUND/AIM

Enforced expression of the connexin (Cx) 32 gene, a member of the gap junction gene family and a tumor suppressor gene in human renal cell carcinoma (RCC), enhanced vinblastine (VBL)-induced cytotoxicity in RCC cells due to suppression of multidrug resistance 1 (MDR1) expression. Furthermore, in RCC the Cx32 gene is silenced by hypermethylation of CpG islands in a promoter region of the Cx gene. In this study, we investigated if the green tea polyphenol epigallocatechin-3-gallate (EGCG) could enhance susceptibility of RCC cells (Caki-1, a human metastatic RCC cell) to VBL.

METHODS

The effects of EGCG on Caki-1 cells were estimated by WST-1 (cell viability), real-time RT-PCR (mRNA level) and immunoblotting (protein level). We estimated the methylation status in the promoter region of the Cx32 gene in RCC cells by methylation-specific PCR. Each protein function was inhibited by small interfering RNA (siRNA) and specific inhibitors.

RESULTS

The EGCG treatment elicited significant upregulation of Cx32 in Caki-1 cells, and the induction of the Cx led to the suppression of MDR1 mRNA expression through inactivation of Src and subsequent activation of c-Jun NH2-terminal kinase (JNK). Chemical sensitivity to VBL in Caki-1 cells was increased by EGCG pretreatment, and this effect was abrogated by siRNA-mediated knockdown of Cx32.

CONCLUSION

This study suggests that the restoration of Cx32 by EGCG pretreatment improves chemical tolerance on VBL in Caki-1 cells via the inactivation of Src and the activation of JNK.

Human Connexin Channel Specificity of Classical and New Gap Junction Inhibitors

Connexins are transmembrane proteins involved in gap junction intercellular communication. They present cell- and tissue-specific expression, with own electric and metabolic coupling specificities. These proteins are involved in numerous physiological processes in the brain and among them neuronal synchronization and trafficking of glucose. Such proteins are also described as being misregulated in various pathologies in the central nervous system. Thus, connexin blockers have been proposed as pharmacological tools to dissect these implications. However, such approaches lack accurate characterization of known inhibitors toward gap junction isoform specificity. In addition, those compounds are limited to few chemical classes and exhibit other activities, for example, an anti-inflammatory effect. The aims of this study were to evaluate the selectivity of described inhibitors and to enrich this pharmacopeia by new chemical classes. In this study, we present the specificity of published inhibitors toward several connexin isoforms expressed in the brain. Furthermore, after a screening of compounds using cellular models, we identified seven new inhibitors, with high functional reversibility and different relative selectivity toward isoforms. They constitute new chemical classes of connexin modulators completing those previously described. These new inhibitors might also provide new insights in understanding numerous pathophysiological processes involving gap junctions.

High-dose siRNAs upregulate mouse Eri-1 at both transcription and posttranscription levels

The eri-1 gene encodes a 3′ exonuclease that can negatively regulate RNA interference via siRNase activity. High-dose siRNAs (hd-siRNAs) can enhance Eri-1 expression, which in return degrade siRNAs and greatly reduces RNAi efficiency. Here we report that hd-siRNAs induce mouse Eri-1 (meri-1) expression through the recruitment of Sp1, Ets-1, and STAT3 to the meri-1 promoter and the formation of an Sp1-Ets-1-STAT3 complex. In addition, hd-siRNAs also abolish the 3′ untranslated region (UTR) mediated posttranscriptional repression of meri-1. Our findings demonstrate the molecular mechanism underlying the upregulation of meri-1 by hd-siRNA.

Bowman-Birk protease inhibitor from soybeans enhances cisplatin-induced cytotoxicity in human mesothelioma cells

Malignant mesothelioma (MM) is an aggressive cancer with no effective treatment options. Enforced expression of the gap junction (GJ) component connexin 43 (Cx43) increases the sensitivity of MM cells to cisplatin. Bowman-Birk protease inhibitor (BBI) induces the restoration of Cx43 in several types of tumor cells. In this study, we examined the capability of BBI to enhance the cytotoxic effect of cisplatin in MM cells via the induction of Cx43. Human MM H28 cells were used. Cell viability was evaluated by a WST-1 assay and proteasomal activity was determined by fluorometric analysis. Protein and mRNA levels were determined by immunoblot analysis and real-time RT-PCR, respectively. GJ function mediated by Cx43 was evaluated using the scrape-loading method. BBI effectively inhibited H28 cell growth in a dose-dependent manner (200-400 μg/ml). In parallel with the growth inhibition, Cx43 levels (mRNA and protein) and GJ function were elevated by BBI treatment. Knockdown of BBI-induced Cx43 by an antisense nucleotide treatment almost cancelled the growth inhibition. BBI enhanced cisplatin-induced cytotoxicity in H28 cells, and down-regulation of Cx43 by the antisense nucleotide treatment abrogated the enhancing effect of BBI. The induction of Cx43 by BBI contributed to Src inactivation and subsequent induction of Bax. Furthermore, an Src inhibitor (SU6656) also enhanced cisplatin-induced cytotoxicity in H28 cells. These results suggest that BBI improves the cytotoxic efficacy of cisplatin in H28 cells via the inhibition of Src signaling.

Propofol depresses the cytotoxicity of X-ray irradiation through inhibition of gap junctions

BACKGROUND

General anesthetics (e.g., propofol) influence the therapeutic activity of intraoperative radiotherapy but the mechanism of the effects is largely unknown. It has been reported that propofol inhibits gap junction (GJ) function briefly, and a functional GJ enhances the efficacy of radiotherapy in some cancer cells. Yet the mechanisms underlying the inhibition of GJ function by propofol and the influence of propofol on therapeutic activity of intraoperative radiotherapy are unknown.

METHODS

The role of propofol at clinically relevant concentrations in the modulation of radiograph-induced cytotoxicity in HeLa cells transfected with connexin 32 (Cx32) plasmid was explored by manipulation of connexin expression, GJ presence, and function. GJ function, Cx32 protein level, and Cx32 mRNA expression were determined by "Parachute" dye-coupling assay, Western blotting, and reverse transcriptase-polymerase chain reaction, respectively.

RESULTS

Propofol significantly reduced radiograph-induced cytotoxicity only in the presence of functional GJ. Four-hour propofol exposure inhibited GJ function mainly by diminution of Cx32 protein levels but without influence on Cx32 mRNA expression.

CONCLUSIONS

These results suggest that propofol inhibits the function of the GJ through the reduction of Cx32 protein levels by a transcription-independent mechanism. They further indicate that propofol depresses the cytotoxicity of radiograph irradiation through inhibition of GJ activity.

Up-regulation of connexin 32 gene by 5-aza-2'-deoxycytidine enhances vinblastine-induced cytotoxicity in human renal carcinoma cells via the activation of JNK signalling

Enforced expression of connexin (Cx) 32 gene, a member of gap junction gene family and a tumor suppressor gene in human renal cell carcinoma (RCC), enhanced vinblastine (VBL)-induced cytotoxicity on RCC cells, due to the suppression of multidrug resistance 1 (MDR1) gene product, P-glycoprotein (P-gp). Also, Cx32 gene in RCC is silenced by hypermethylation of CpG islands in a promoter region of the Cx gene. In this study, we investigated if a DNA demethylating agent, 5-aza-2'-deoxycytidine (5-Aza) could enhance susceptibility of RCC cells (Caki-1) to VBL. We found that 5-Aza treatment up-regulated Cx32 in Caki-1 cells, and the induction of the Cx led to the suppression of P-gp through inhibition of Src and subsequent activation of c-Jun NH(2)-terminal kinase (JNK). Moreover, increased transcription activity of c-Jun by the JNK activation contributed to the down-regulation of MDR1, thus indicating a central role of JNK signalling to suppress P-gp level in 5-Aza-treated Caki-1 cells. Chemical sensitivity to VBL in Caki-1 cells was increased by 5-Aza pre-treatment, and this effect was abrogated by short interfering RNA (siRNA)-mediated knockdown of Cx32. Furthermore, co-treatment of 5-Aza or a P-gp inhibitor with VBL drastically enhanced JNK activation comparing to only VBL treatment in Caki-1 cells. These results suggest that the restoration of Cx32 by 5-Aza pre-treatment improves chemical tolerance on VBL in Caki-1 cells and that the JNK activation is a key factor to induce the effect.

STAT3 activation induced by Epstein-Barr virus latent membrane protein1 causes vascular endothelial growth factor expression and cellular invasiveness via JAK3 And ERK signaling

The principal Epstein-Barr virus (EBV) oncoprotein, latent membrane protein 1 (LMP1), has been suggested to contribute to the highly invasive nature of nasopharyngeal carcinoma (NPC). Signal transducer and activator of transcription 3 (STAT3) is a master transcriptional regulator in proliferation and apoptosis and is newly implicated in angiogenesis and invasiveness, which, in turn, are likely to contribute to the highly invasive character of NPC. The fundamental molecular mechanisms of LMP1-regulated STAT3 activation in NPC cell invasion have not been completely explored. Here, we showed that LMP1 signals the Janus kinase 3 (JAK3) and extracellular signal-regulated kinase 1/2 (ERK1/2) pathways upon the activation of STAT3 as well as STAT transactivation activity. LMP1 induces vascular endothelial growth factor (VEGF) expression via the JAK/STAT and mitogen-activated protein kinase (MAPK)/ERK signalling pathways. Induction of STAT3 by the human viral oncoprotein LMP1 may contribute to the invasion of NPC.

Implications and challenges of connexin connections to cancer

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

Epithelial-to-mesenchymal transition and resistance to ingenol 3-angelate, a novel protein kinase C modulator, in colon cancer cells

Acquired resistance to protein kinase C (PKC) modulators may explain the failure of clinical trials in patients with cancer. Herein, we established a human colon cancer cell line resistant to PEP005, a drug that inhibits PKCalpha and activates PKCdelta. Colo205-R cells, selected by stepwise exposure to PEP005, were >300-fold more resistant to PEP005 than parental Colo205-S cells and were cross-resistant to phorbol 12-myristate 13-acetate, bryostatin, bistratene A, and staurosporine. No PKCalpha or PKCdelta mutation was detected in Colo205-S and Colo205-R cells. Changes in Colo205-R cells were reminiscent of the epithelial-to-mesenchymal transition (EMT) phenotype. Accordingly, Colo205-R cells were more invasive than Colo205-S in Matrigel assays and in mouse xenografts. We also found an increased mRNA expression of several EMT genes, such as those encoding for transforming growth factor-beta and vimentin, along with a decreased mRNA expression of genes involved in epithelial differentiation, such as CDH1 (E-cadherin), CLDN4 (claudin 4), S100A4, and MUC1, in Colo205-R compared with Colo205-S cells in vitro and in vivo. Interestingly, high expression of ET-1 was shown in Colo205-R cells and correlated with low sensitivity to PEP005 and staurosporine in a panel of 10 human cancer cell lines. Inhibition of the ET-1 receptor ETR-A with bosentan restored the antiproliferative effects of PEP005 in Colo205-R cells and decreased the invasive properties of this cell line. Exogenous exposure to ET-1 and silencing ET-1 expression using small interfering RNA modulated cell signaling in Colo205-S and Colo205-R. In summary, acquired resistance to PEP005 was associated with expression of EMT markers and activates the ET-1/ETR-A cell signaling.

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.

The inhibitory effect of connexin 32 gene on metastasis in renal cell carcinoma

We have previously reported that connexin (Cx) 32 gene, a member of gap junctions, was specifically downregulated in human renal cell carcinoma (RCC) and it acts as a tumor suppressor against RCC. Because there is no standard therapy for advanced RCC, we investigated the anti-metastatic effect of Cx32 to seek a possibility of new RCC therapy. In this study, we used human metastatic RCC cell (Caki-1), and established Cx32-expressed cell clone (Caki-1T) or only mock-transfected cell clone (Caki-1W). For experimental production of metastases, the cells were injected into the lateral tail vein of SCID mice. Seventy days after inoculation, metastatic colonies were observed in Caki-1W inoculated group, though none of them were in Caki-1T inoculated group. The plasma VEGF concentration was significantly lower in Caki-1T group compared to Caki-1W group. To investigate where Cx32 effects on, we also tried in vitro analysis and found that the malignant phenotypes involving metastasis steps were significantly decreased in Caki-1T under hypoxia, a mimic condition of internal tumor environment. After hypoxia treatment, the protein level of HIF-2alpha, which plays main role for hypoxia adaptation, was observed to increase in Caki-1W, whereas no expression was observed in Caki-1T. We investigated the activation of Src, which is required for stabilization of HIF-2alpha, is suppressed in Caki-1T compared to Caki-1W. These results suggest that Cx32 inhibits hypoxia adaptation governed by HIF-2alpha, and this may help to reduce the metastasis of RCC cells.

5-Aza-2'-deoxycytidine suppresses human renal carcinoma cell growth in a xenograft model via up-regulation of the connexin 32 gene

BACKGROUND AND PURPOSE

The connexin (Cx) 32 gene, a member of the gap junction gene family, acts as a tumour suppressor gene in human renal cell carcinoma (RCC) and is down-regulated by the hypermethylation of CpG islands in a promoter region of the Cx gene. The current study investigated whether the restoration of Cx32 silenced by hypermethylation in RCC by a DNA demethylating agent could be an effective treatment against RCC.

EXPERIMENTAL APPROACH

Using nude mice bearing Caki-1 cells (a human metastatic RCC cell line), the effects of 5-aza-2'-deoxycytidine (5-aza-CdR), a DNA demethylase inhibitor, on Cx32 mRNA expression and tumour growth were examined by RT-PCR, and by measuring tumour weight and volume. Cx32 expression in Caki-1 tumours was inhibited by Cx32 short interfering (si) RNA, and the effect of siRNA on 5-aza-CdR-dependent suppression of tumour growth in nude mice was evaluated.

KEY RESULTS

5-aza-CdR treatment inhibited the growth of Caki-1 cells in nude mice by 70% and increased 7-fold the level of Cx32 mRNA. The intratumour injection of Cx32 siRNA almost totally inhibited the expression of Cx32 mRNA and significantly reduced the suppression of tumour growth in 5-aza-CdR-treated nude mice.

CONCLUSIONS AND IMPLICATIONS

5-aza-CdR suppressed the growth of Caki-1 tumours in a xenograft model, by restoring Cx32 expression. This finding suggests that treatment with 5-aza-CdR could be a new effective therapy against human metastatic RCC and that Cx32 could be a potential target for the treatment of RCC.

Cytoplasmic accumulation of connexin32 protein enhances motility and metastatic ability of human hepatoma cells in vitro and in vivo

Connexins have long been believed to suppress tumour development during carcinogenesis by exerting gap junctional intercellular communication (GJIC). Although GJIC is abrogated in hepatocellular carcinoma (HCC), connexin32 (Cx32) protein tends to remain expressed in cytoplasm, but not in cell-cell contact areas; thus, it is incapable of forming gap junctions. Hypothesising that cytoplasmic Cx32 protein that has accumulated in HCC should have its proper functions, which are independent of GJIC, we established an inducible expression system of Cx32 in human HuH7 HCC cells, which were unable to support the formation of Cx32-mediated gap junctions, so that Cx32 protein could be overexpressed by doxycycline (Dox) withdrawal. Although the established clone HuH7 Tet-off Cx32 cells exhibited a 4-fold increase in Cx32 expression after Dox withdrawal, none of them were dye-coupled, and Cx32 protein was retained in the Golgi apparatus. However, the proliferation rate of the HuH7 Tet-off Cx32 cells was significantly higher in the Dox-free medium than in the Dox-supplemented one. Transwell assays also revealed that Dox withdrawal enhanced serum-stimulated motility and invasiveness into Matrigel of the HuH7 Tet-off Cx32 cells. Furthermore, when HuH7 Tet-off Cx32 cells were xenografted into the liver of SCID mice, only the mice to which no Dox was administered developed metastatic lesions, indicating that overexpression of cytoplasmic Cx32 protein induced metastasis of HuH7 cells. Our results suggest that, while Cx32-mediated GJIC suppresses the development of HCCs, cytoplasmic Cx32 protein exerts effects favourable for HCC progression, such as invasion and metastasis, once the cells have acquired a malignant phenotype.

Enhancing effect of connexin 32 gene on vinorelbine-induced cytotoxicity in A549 lung adenocarcinoma cells

PURPOSE

Connexin (Cx) genes exert negative growth effects on tumor cells with certain cell specificity, and tumor-suppressive effects of the Cx genes contribute to enhancement of chemotherapeutical agents-induced cytotoxicity in some cancer cells. Since we and others have been reported that Cx32 acts as a tumor suppressor gene in lung adenocarcinomas, this study was undertaken to estimate if the combination of Cx32-dependent tumor-suppressive effect and vinorelbine (VBN), a chemotherapeutic agent which has been utilized for clinical lung adenocarcinoma treatment, could be effective in enhancing the sensitivity of the lung cancer to VBN treatment.

METHODS

We established the A549 cells (a human lung adenocarcinoma cell line) which had stable expression of Cx32 and estimated effect of Cx32 on VBN-induced cytotoxicity in the established cells.

RESULTS

Cx32 expression in A549 cells significantly potentiated VBN-induced cytotoxicity on the cells due to enhancement of apoptosis induction. The enhancing cytotoxicity in A549 cells by Cx32 mainly depended on a decrease in expression of multi-drug resistance-1 (MDR-1) gene responsible for reduction of VBN accumulation into the cells. We also observed that silencing of Cx32 by siRNA treatment elevated the expression level of MDR-1 mRNA in A549 cells and that inhibition of MDR-1 gene product-dependent function enhanced VBN-induced cytotoxicity in the cells.

CONCLUSION

These results suggest that Cx32 contributes to the enhancement of VBN-induced cytotoxicity in A549 cells via the reduction of MDR-1 expression.

Regulation of renal cell carcinoma cell proliferation, invasion and metastasis by connexin 32 gene

Gap junctions composed of connexin (Cx), a large protein family with a number of subtypes, are a main apparatus to maintain cellular homeostasis in many organs. Gap junctional intercellular communication (GJIC) is actively involved in all aspects of the cellular life cycle, ranging from cell growth to cell death. It is also known that the Cx gene acts as a tumor-suppressor due to the maintenance of cellular homeostasis via GJIC. In addition to this function, recent data show that the GJIC-independent function of Cx gene contributes to the tumor-suppressive effect of the gene with specificity to certain cells. With respect to the tumor-suppressive effects, Cx genes acts as tumor-suppressors in primary cancers, but the effects are still conflicting in invasive and metastatic cancers. We have previously reported that Cx32 is specifically downregulated in human renal cell carcinoma (RCC) cell lines as well as cancerous regions when compared to normal regions in kidneys. In recent studies, we have also reported that Cx32 suppresses growth, invasion and metastasis of RCC cells. In this minireview, we refer to a new aspect of Cx32-dependent functions against cell proliferation, invasion and metastasis in RCC cells, especially in a GJIC-independent manner.

Connexin 32 potentiates vinblastine-induced cytotoxicity in renal cell carcinoma cells

We have reported that connexin (Cx) 32 gene, a member of gap junction protein family, acts as a tumor suppressor gene in human renal cell carcinoma (RCC). Of solid tumors, RCC is one of the most chemoresistant cancers, and there is no effective cancer chemotherapy against RCC at present. In this study, we examined if the combination of Cx32-dependent tumor-suppressive effect and vinblastine (VBL), a chemotherapeutic agent which has been utilized for clinical RCC treatment, could be effective in enhancing the sensitivity of RCC to VBL treatment. Cx32 expression in a human metastatic RCC cell (Caki-1 cell) significantly enhanced in vitro and in vivo VBL-induced cytotoxicity on the cell. Cx32 expression in the RCC cells potentiated VBL-induced apoptosis compared to the Cx32-negative RCC cells in vitro as well as in vivo. The enhancing apoptosis in the RCC cells by Cx32 mainly depended on the decrease of P-glycoprotein (P-gp), a multidrug resistance gene-1 (MDR-1) product responsible for reduction of VBL accumulation into the cells. We also observed that silencing of Cx32 by short interfering RNA (siRNA) treatment elevated the level of P-gp in Caki-1 cells and that inhibition of P-gp function enhanced VBL-induced apoptosis in the RCC cells. These results suggest that Cx32 is effective to enhance VBL-induced cytotoxicity in Caki-1 cells via the reduction of P-gp. Overall, it seems that the combination of Cx32-dependent tumor-suppressive effect and VBL is promising as a new cancer therapy against RCC.

What is hidden in the pannexin treasure trove: the sneak peek and the guesswork

Connexins had been considered to be the only class of the vertebrate proteins capable of gap junction formation; however, new candidates for this function with no homology to connexins, termed pannexins were discovered. So far three pannexins were described in rodent and human genomes: Panx1, Panx2 and Panx3. Expressions of pannexins can be detected in numerous brain structures, and now found both in neuronal and glial cells. Hypothetical roles of pannexins in the nervous system include participating in sensory processing, hippocampal plasticity, synchronization between hippocampus and cortex, and propagation of the calcium waves supported by glial cells, which help maintain and modulate neuronal metabolism. Pannexin also may participate in pathological reactions of the neural cells, including their damage after ischemia and subsequent cell death. Recent study revealed non-gap junction function of Panx1 hemichannels in erythrocytes, where they serve as the conduits for the ATP release in response to the osmotic stress. High-throughput studies produced some evidences of the pannexin involvement in the process of tumorigenesis. According to brain cancer gene expression database REMBRANDT, PANX2 expression levels can predict post diagnosis survival for patients with glial tumors. Further investigations are needed to verify or reject hypotheses listed.

Connexin 32 as an anti-invasive and anti-metastatic gene in renal cell carcinoma

Cellular homeostasis in many organs is maintained via gap junctions composed of connexin (Cx), a large protein family with a number of isoforms. In fact, gap junctional intercellular communication (GJIC) is actively involved in all aspects of the cellular life cycle, ranging from cell growth to cell death. It has been well known that Cx gene acts as a tumor suppressor gene due to the maintenance of cellular homeostasis via GJIC. On the other hand, recent data show that GJIC-independent function for Cx gene contributes to tumor-suppressive effect of the gene with cell certain specificity. However, the mechanistic aspect of the GJIC-independent function remains largely unknown. In this review, we briefly summarize the tumor-suppressive effects of Cx genes, refer to a new aspect of Cx32 as an anti-invasive and anti-metastatic gene against renal cell carcinoma in a GJIC-independent function and establishment of a new cancer therapy based on the new function of Cx32.

Connexin 32 down-regulates the fibrinolytic factors in metastatic renal cell carcinoma cells

Fibrinolytic factors have an important role in tumor progression through the degradation of extracellular matrix. The increased levels of urokinase-type plasminogen activator (uPA), uPA-receptor (uPAR) and type-1 PA inhibitor (PAI-1) are reported in human renal cell carcinoma (RCC). Connexin (Cx) gene, a member of gap junction, is known to act as a tumor suppressor gene. We have reported that Cx32 improves malignant phenotypes of metastatic RCC cells via the inhibition of Src-dependent signaling. In this study, we examined the effect of expression of Cx32 gene on the production of uPA, uPAR and PAI-1, and on the induction of PAI-1 stimulated by hypoxia in a human metastatic RCC cell line, Caki-1 cells. Cx32 expression decreased both mRNA level and production of PAI-1, uPA and uPAR in Caki-1 cells. Cx32 also decreased hypoxia-inducible factor (HIF)-1alpha and HIF-2alpha mRNA level. PP1, a Src inhibitor, significantly decreased PAI-1, uPA, uPAR and HIF-alpha mRNA levels in Caki-1 cells. Furthermore, Cx32 suppressed the induction of HIF-2alpha protein in Caki-1 cells under hypoxia. PAI-1 mRNA level in Cx32-transfected Caki-1 cells was lower than that of mock transfectant under hypoxic conditions. These results suggest that Cx32 might reduce PAI-1, uPA and uPAR production in metastatic RCC cells via the inhibition of Src-dependent induction of HIF-1alpha and HIF-2alpha gene expression and that Cx32 might suppress hypoxia-inducible gene expression under hypoxic conditions.