Disruption of gap junctional intercellular communication in human renal cancer cell lines

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.

Up-regulation of connexin-43 expression in bone marrow mesenchymal stem cells plays a crucial role in adhesion and migration of multiple myeloma cells

Multiple myeloma (MM) cells accumulate mainly in the bone marrow (BM) and are closely related to the BM microenvironment. Connexin-43 (Cx43) is the main connexin present in bone marrow mesenchymal stem cells (BMSCs). We found that normal BMSCs expressed a lower level of Cx43 than those derived from patients with MM. Cx43 expression in BMSCs was up-regulated at both the gene and protein level, and gap junction-mediated intercellular communication was improved in BMSCs after co-culture with RPMI 8226 cells. The gap junction blocker 18α-glycyrrhetinic acid (18α-GA) inhibited the adherence of MM cells to BMSCs. Treatment with 18α-GA decreased stromal cell-derived factor-1α (SDF-1α) secretion by BMSCs and inhibited the migration of MM cells induced by BMSCs. These results suggest that alteration of Cx43 expression in BMSCs is involved in the interactions of MM cells with the BM environment and plays a crucial role in the adhesion and migration of MM cells.

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

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

Evidence for a role of oxidative stress in the carcinogenicity of ochratoxin a

The in vitro and in vivo evidence compatible with a role for oxidative stress in OTA carcinogenicity has been collected and described. Several potential oxido-reduction mechanisms have been identified in the past. More recently, the possibility of a reduction of cellular antioxidant defense has been raised as an indirect source of oxidative stress. Consequences resulting from the production of oxidative stress are observed at different levels. First, OTA exposure has been associated with increased levels of oxidative DNA, lipid, and protein damage. Second, various biological processes known to be mobilized under oxidative stress were shown to be altered by OTA. These effects have been observed in both in vitro and in vivo test systems. In vivo, active doses were often within doses documented to induce renal tumors in rats. In conclusion, the evidence for the induction of an oxidative stress response resulting from OTA exposure can be considered strong. Because the contribution of the oxidative stress response in the development of cancers is well established, a role in OTA carcinogenicity is plausible. Altogether, the data reviewed above support the application of a threshold-based approach to establish safe level of dietary human exposure to OTA.

Altered expression of connexin-43 and impaired capacity of gap junctional intercellular communication in prostate cancer cells

Connexin-43 (Cx43) expression in prostate cancer (PCa) cells and the potency of gap junctional intercellular communication (GJIC) in the cells were investigated, with an attempt to elucidate the reason why the so-called "bystander effect" mediated by thymidine kinase (TK) suicide gene therapy on PCa cells is not of significance and to explore the role of GJIC in PCa carcinogenesis. mRNA and protein expression of Cx43 in a PCa cell line PC-3m was detected by reverse-transcription polymerase chain reaction (RT-PCR) and strapt-avidin-biotin-enzyme complex (SABC) immunohistochemical staining, and inherent GJIC of PC-3m cells was assayed by scrape-loading and dye transfer (SLDT) assay. The expression of Cx43 in human normal and malignant prostate tissues was determined by SABC immunohistochemistry as well. It was found that Cx43 mRNA and protein expression in PC-3m cells was slightly reduced as compared with positive controls and the location of Cx43 protein was aberrant in cytoplasm rather than on membrane. Assessment of paraffin sections demonstrated that the expression of Cx43 protein in PCa cells was abnormally located and markedly diminished as compared with normal prostatic epithelial ones, displaying a negative correlation to the pathological grade (chi2=4.025, P<0.05). Additionally, capacity of inherent GJIC in PC-3m cells was disrupted, which was semi-quantified as (+) or (-). It was indicated that both down-regulated expression of Cx43 mRNA and aberrant location of Cx43 protein participated in the mechanisms leading to deficient GJIC in PC-3m cells. Lack of efficient GJIC is a molecular event, which may contribute not only to limited extent of "bystander effect", but also to initiation and progression of prostatic neoplasm.

Expression of the gap junction proteins connexin 26 and connexin 43 in human middle ear cholesteatoma

CONCLUSION

The results of this study showed upregulated expression and a change in localization of both connexin 43 (Cx43) and Cx26 in human middle ear cholesteatoma compared to those in normal retroauricular skins (RASs) and ear canal skins (ECSs). This suggests that perturbations of intercellular communication through gap junctions may be associated with the pathology of human cholesteatomas.

OBJECTIVE

Cholesteatomas in the middle ear require intercellular signal exchange through gap junctions as well as intracellular signal pathways for the hyperproliferation and differentiation of epithelial cells. Cx is a gap junction protein involved in intercellular communication. The objective of this study was to analyze the expression and possible roles of Cx43 and Cx26 in human cholesteatoma compared to normal epithelium.

MATERIAL AND METHODS

Ten RASs, 10 ECSs and 10 cholesteatomas were obtained during middle ear operations. Immunohistochemical staining, Western blotting and reverse transcriptase polymerase chain reaction (RT-PCR) were used to detect Cx43 and Cx26. The expression patterns of Cx43 and Cx26 were also compared with that of the proliferation marker Ki67.

RESULTS

In human cholesteatomas, Cx43 was expressed in whole suprabasal layers, except in the basal layer, and Cx26 was usually expressed in the suprabasal and basal layers. However, normal RASs showed weak expression of Cx43 in the upper spinosal and granular layers (with no expression in the basal layers) and restricted localization of Cx26 in the basal layer. The expression of Cx43 and Cx26 in ECSs was weak but showed similar patterns to that of cholesteatoma. RT-PCR and Western blotting showed that the expression of Cx43 and Cx26 was higher in cholesteatoma than in RASs. Epithelial cells expressing Cx43 and Cx26 in cholesteatoma were not exactly identical to Ki67-expressing cells on immunohistochemical staining.

Oxidized beta-carotene inhibits gap junction intercellular communication in the human lung adenocarcinoma cell line A549

In addition to its antioxidant activity, beta-carotene (BC) is known to enhance gap junction intercellular communication (GJIC) by up-regulation of connexin 43 (Cx43), an action that may be important in its control of tumor growth. Surprisingly, two clinical trials on supplemental BC suggest that BC may increase lung cancer incidence in smokers. Recently, an animal study indicated that a very high dose of BC (50 mg/kg b.w./day for 5 days) decreases GJIC in rat liver, while a lower dose (5 mg/kg b.w./day) increases GJIC. It is unclear how high-doses of BC inhibit GJIC. In this study, we tested whether oxidized BC (OBC, obtained by heating BC at 60 degrees C in open air for 1 h) may inhibit GJIC. We incubated a human lung cancer cell line (A549) with OBC or BC at 2-10 microM for 5 days. Cell viability (by Trypan-blue assay), GJIC (by scrape-loading dye transfer) and Cx43 expression (by western blotting and immunocytochemical localization) were measured to investigate the effects of OBC and BC on GJIC and the possible mechanisms. The results show that OBC at concentrations lower than 10 microM did not significantly affect cell viability. However, OBC at 5 muM inhibited GJIC, whereas BC at 5 microM markedly increased GJIC. The loss of GJIC in A549 induced by OBC accompanied the aberrant localization and phosphorylation of connexin43 (Cx43). These changes in the expression of Cx43 induced by OBC were similar to those induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), a tumor promoter. Thus, our results suggest that in vivo inhibition of GJIC by a high dose of BC on GJIC is, at least in part, attributable to the effect of OBC.

Transient decrease in gap junction expression during compensatory renal growth in mice

OBJECTIVES

To examine gap junction expression during compensatory renal growth. Compensatory renal growth is a normal response to the removal of renal cells. Gap junctional intercellular communication plays important roles in homeostatic regulation, differentiation, and synchronization of cellular electronic and metabolic functions.

METHODS

The right ureters of 5-week-old ddy mice were dorsally incised and unilaterally obstructed, and the animals were killed at varying intervals. The control group underwent a sham operation. Freeze-fractured kidney tissue samples were studied using electron microscopy. The amount of proliferative cell nuclear antigen-positive cells and connexin 43 (Cx43) protein was studied by immunohistochemistry and Western blotting, respectively.

RESULTS

Proliferative cell nuclear antigen-positive cells in the renal tubules increased on days 1 and 2 after surgery that obstructed the ureter, and decreased to normal levels by day 14. The number of gap junctions significantly decreased on days 1 and 2, and then gradually increased to normal levels from days 3 to 14 after surgery. The amount of Cx43 protein in the renal tubules decreased until day 2 and recovered to the same level as that of the control by day 14 after surgery. Significantly, a hyperphosphorylated band of Cx43 in the control kidney was not detected in the operated kidney.

CONCLUSIONS

These data suggest that the gap junctional intercellular communication of renal tubular cells during compensatory renal growth after unilateral ureteral obstruction could be temporarily reduced concomitant with a decrease of the expression of a phosphorylated Cx43 protein in renal cortical tubular cells after unilateral ureteral obstruction of the contralateral kidney.

Cellular basis and clinical implications of biological markers in salivary tissues: their topological distribution in murine submandibular gland

Cell proliferation and apoptosis as well as cell-cell adhesion and communication are essential processes that assure cell survival, renewal and coordination. Since junctional proteins have a tumor suppressor activity, their immunohistochemical characterization has diagnostic and prognostic value. The purpose of this report is to review the role played by junctional and proliferation-related proteins in the salivary glands and to illustrate their immunohistochemical localisation in normal murine submandibular gland. Normal salivary gland tissue was obtained from normal adult male BALB/c mice. After immediate fixation in formalin and ethanol, the samples were immunohistochemically stained for E-cadherin (HECD-1), Bcl-2, Ki67 (MIB-1), connexin26 and connexin 32, beta-catenin and gamma-catenin. Their topological distribution and reactivity were evaluated by light microscopy. The nuclei of submandibular acinar cells exhibited low to moderate staining for Ki67, but no reaction was observed in ductal cells. Murine Bcl-2 was light to moderately expressed in the latero-basal domain of cells of submandibular acini but was only lightly expressed in striated and eosinophilic ducts. The lateral domain of acinar cells were heavily stained with anti-E-cadherin, while only low levels were expressed at the cellular surface of ducts. beta-Catenin was consistently and evenly distributed along the latero-apical boundaries of eosinophilic secretory duct cells as well as on the lateral domain of acinar cells. On the contrary, gamma-catenin was generally expressed at lower levels than beta-catenin, was not expressed in ductal cells and was only lightly stained on the lateral membranes of acinar cells. No expression of connexin 32 was observed in ducts but it was significantly expressed in a spotted pattern along the plasma membrane of acinic cells. Connexin 26 showed similar localization to that of connexin 32 but the staining was much more intense. Since these proteins have been reported to play key roles in maintaining homeostasis via control of cell growth, differentiation and death, their analysis in normal salivary tissue will hopefully contribute to the study of salivary tumorigenesis.

All-trans retinoic acid enhances gap junctional intercellular communication among renal epithelial cells in vitro treated with renal carcinogens

Epidemiological and clinical studies imply that retinoids have a chemopreventative action against cancer and can suppress the growth of cancer cells. The regulation of connexin (Cx) expression by retinoids varies among tissues and organs. In this study, we investigated whether all-trans retinoic acid (ATRA) upregulates gap junctional intercellular communication (GJIC) in renal epithelial cells exposed to renal carcinogens. Madin Darby canine kidney (MDCK) cells were incubated with ATRA for 3 days, then briefly exposed to 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or renal carcinogens potassium bromate (KBrO3) and dimethylnitrosamine (DMN). ATRA increased the expression of connexin 43 mRNA and protein without affecting Cx 43 phosphorylation and prevented inadequate Cx 43 localisation caused by TPA/KBrO3 or DMN. Consequently, ATRA prevented the disruption of GJIC in MDCK cells. These data suggest that ATRA enhanced GJIC by upregulating Cx 43 expression and that ATRA might be useful for prevention of renal cell carcinoma.