2705

GJB1

gap junction protein, beta 1, 32kDa;GJB1;gap junction protein, beta 1, 32kDa

GAP junction 28 kDa liver protein|connexin 32|connexin-32|gap junction beta-1 protein

Xq13.1

protein-coding

OBJECTIVE: The aim of this study was to investigate the expression of connexin (Cx) and the function of gap junction intercellular communication (GJIC) in the carcinogenesis, progression and metastasis of gastric cancers. METHODS: Immunohistochemistry was used to detect the expression of Cx32 and Cx43 proteins in tissue samples. Indirect immunofluorescence assay was used to investigate the expression of Cx32 and Cx43 proteins in several gastric cancer lines of various differentiation grades. The expression of Cx43 in samples of gastric cancer tissue, adjacent normal tissue and in the gastric cancer cell lines of various differentiation grades was detected by Western blot. Scrape-loading dye transfer (SLDT) technique was used to detect the function of gap junction intercellular communication (GJIC) in the various cell lines. RESULTS: In the normal gastric mucosa the expression rates of both Cx32 and Cx43 were 100%. In gastric cancers, the expression rates of Cx32 andCx43 were 49.5% (55/111) and 39.6% (44/111), respectively. There was a significant difference between their expression in normal and cancer tissues (P < 0.05). Age of the patients was not significantly correlated with the expression level of Cx32 and Cx43 (P > 0.05). Cx43 expression was significantly associated with the TMN stage, histological type, depth of infiltration and distant metastasis (P < 0.05), but Cx32 expression was not significantly correlated with depth of infiltration ( P > 0.05). In the cancer cell lines, a positive expression of Cx32 and Cx43 was detected in transfected human stomach mucosal cell line (CES-1) and human well differentiated stomach cancer cell line (N87), but negative in the poorly differentiated stomach cancer cell line (BGC-823) at all. Both Cx32 and Cx43 expression rates were 100% in the cell line GES-1. Cx32 expression rate was 49.0% and Cx43 expression rate was 55.0% in the cell line N87. But in the poorly differentiated cancer cell line BGC-823 both Cx32 and Cx43 expression was negative. GJIC function detection showed: GES-1 showed well GIJC function but no GIJC function in the cell lines N87 and BGC-823. The intensity of fluorescence was gradually decreasing from GES-1 cells to N87 cells and almost no fluorescence in BGC-823 cells. Western blotting showed that Cx43 expression in normal tissue was higher than that in gastric cancer tissue, and in the cell lines GES-1, N87 and BGC-823, the bands seemed decreasing progressively. There was very low expression in BGC-823 cells. CONCLUSION: The decreasing expression of connexin Cx32 and Cx43 is obviously correlated with the occurrence, development and metastatic potential of stomach cancers.

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.

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.

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.