Enhanced tumorigenicity of rat bladder squamous cell carcinoma cells after abrogation of gap junctional intercellular communication

GPX2 promotes development of bladder cancer with squamous cell differentiation through the control of apoptosis

Herein, we elucidated the molecular mechanisms and therapeutic potential of glutathione peroxidase 2 (GPX2) in bladder cancer. GPX2 expression gradually increased during progression from normal to papillary or nodular hyperplasia (PNHP) and urothelial carcinoma (UC) in a rat N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN)-induced bladder carcinogenesis model. GPX2 overexpression was more marked in UC with squamous differentiation (SqD) than in pure UC. Clinical intraepithelial lesions of papillary UC and invasive UC with SqD also had strong GPX2 expression in human radical cystectomy specimens. In addition, prognostic analysis using transurethral specimens revealed that low expression level of GPX2 predicted poor prognosis in patients with pure UC. Further, UC cell lines, BC31 and RT4, cultured in vitro also overexpressed GPX2. Knock-down of GPX2 induced significant inhibition of intracellular reactive oxygen species (ROS) production, in addition to significant growth inhibition and increased apoptosis with activation of caspase 3 or 7 in both BC31 and RT4 cells. Interestingly, tumor growth of BC31 cells subcutaneously transplanted in nude mice was significantly caused the induction of apoptosis, as well as inhibition of angiogenesis and SqD by GPX2 down-regulation. Our findings demonstrated that GPX2 plays an important role in bladder carcinogenesis through the regulation of apoptosis against intracellular ROS, and may be considered as a novel biomarker or therapeutic target in bladder cancer.

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.

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.

A mouse strain difference in tumorigenesis induced by biodegradable polymers

The use of poly-L-lactic acid (PLLA) surgical implants for repair of bone fractures has gained popularity in the past decade. The aim of this study was to evaluate the in vivo effect of PLLA plates on subcutaneous tissue in two mouse strains, BALB/cJ and SJL/J, which have higher and lower tumorigenicity, respectively. Gap-junctional intercellular communication and protein expression of connexin 43 were significantly suppressed, whereas secretion of transforming growth factor-beta1 and expression of extracellular matrix, insulin-like growth factor binding protein 3, and cysteine-rich intestinal protein 2 were significantly increased in PLLA-implanted BALB/cJ mice when compared with BALB/cJ controls. Finally, tumors were formed after implantation of cultured cells from the more-tumorigenic BALB/cJ, but not SJL/J, mice into nude mice.

Connexin expression and cell coupling fail to reverse the v-src transformed growth characteristics of a Cx43-/- cell line

Gap junctions, composed of connexins, have been shown to suppress transformation in a variety of malignancies and transformed cell types. In addition, transforming factors such as the src oncogene have been shown to directly phosphorylate some connexins (e.g., Cx43) and inhibit coupling. To investigate the role of gap junctions in cell transformsation by v-src, we utilized a clonal cell line derived from Cx43 knockout mice (KoA) that was immortalized, but not transformed. Transfection by v-src induced a marked transformed phenotype characterized by growth in low serum and anchorage-independent conditions. Subsequent transfections by Cx43, Cx32 or vector alone were then tested for their effects on growth. Activity of pp60v-src was confirmed in all transfectants as well as the ability of pp60v-src to phosphorylate Cx43 in several clones. Despite the documented effect of pp60v-src on Cx43 channel closure, modest coupling was still retained in many of the Cx43 and Cx32 transfectants. However, none of the four Cx43 transfected clones showed significant inhibitory effects on proliferation in either anchorage-independent or low serum growth conditions. Of the Cx32 clones, only one in five showed effects on growth in both assays, which was the same ratio observed for the control transfectants. Thus, based on the levels of expression achieved, which were comparable to endogenous levels in established cell lines, neither Cx43 nor Cx32 serve as effective suppressors of the transformed growth phenotype of this v-src expressing cell line.

Novel mechanism of tumorigenesis: Increased transforming growth factor-?1 suppresses the expression of connexin 43 in BALB/cJ mice after implantation of poly-L-lactic acid

Poly-L-lactic acid (PLLA) is a widely used promising material for surgical implants such as tissue-engineered scaffolds. In this study, we aimed to determine the in vivo effect of PLLA plates on the cellular function of subcutaneous tissue in the two mouse strains, BALB/cJ and SJL/J, higher and lower tumorigenic strains, respectively. Gap-junctional intercellular communication (GJIC) and the expression of connexin 43 (Cx43) protein were significantly suppressed, whereas the secretion of transforming growth factor-beta 1 (TGF-beta 1) level was significantly increased in PLLA-implanted BALB/cJ mice compared with BALB/cJ controls. However, no significant difference in TGF-beta 1 secretion was observed between the SJL/J-implanted and SJL/J control mice. We found for the first time that a significant difference was observed between the two strains; thus, the PLLA increased the secretion of TGF-beta 1 and suppressed the mRNA expression of Cx43 at the earlier stage after implantation into the higher-tumorigenic strain, BALB/cJ mice. This novel mechanism might have a vital role in the inhibition of GJIC and promote the tumorigenesis in BALB/cJ mice.

NBT-II carcinoma behaviour is not dependent on cell-cell communication through gap junctions

To study the mechanism(s) underlying the proliferation of heterogeneous cell populations within a solid tumour, the NBT-II rat bladder carcinoma system was used. It has been first investigated whether the different cell populations are coupled through gap junctions (GJIC). Cells overexpressing the Cx43 were generated to test for any tumour suppressive activity in vivo. To determine whether GJIC is essential for tumour proliferation and the establishment of a cooperative community effect, NBT-II cells that are incompetent for cell coupling were generated. The data report that (i) carcinoma cells expressing or not FGF-1 are coupled through GJIC in vitro and in coculture and express the gap junction protein Cx43, (ii) overexpression of Cx43 in these cells does not affect their in vitro coupling capacities and in vivo tumourigenic growth properties, (iii) inhibition of GJIC through antisense strategy has no in vivo obvious consequence on the tumour growth properties of the carcinoma, and (iv) the community effect between two carcinoma cell populations does not critically involve cell coupling through gap junctions.

A strategy for the suppression of tumorigenesis induced by biomaterials: Restoration of transformed phenotype of polyetherurethane-induced tumor cells by Cx43 transfection

Biomaterials such as polyetherurethans (PEUs) are the scaffolding, which is indispensable for the development of the bio-artificial organs. However, PEUs can induce tumors in subcutaneous implantation sites in rat. We have shown that the different inhibitory potential of gap junctional intercellular communication (GJIC) on the surface of the biomaterials, including PEUs, is a key step in determining the tumorigenic potential. Here we show that suppression of a gap junctional protein connexin 43 (Cx43) plays an important role in in vivo tumorigenesis induced by PEUs for the first time and that Cx43 transfection may be an effective strategy for preventing tumorigenesis induced by biomaterials. Rat tumor cell line U41 is derived from tumors in the subcutaneous implantation of PEU films. The GJIC and the expression of Cx43 were suppressed in U41. The restoration of normal phenotype, such as reduction of growth rate, recovery of contact inhibition and loss of colony formation ability in soft agar, was achieved by Cx43 transfection. These results strongly suggest that suppression of Cx43 expression plays an important role in the development of rat malignant fibrous histiocytoma (MFHC) caused by PEUs and that Cx43 transfection is effective for prevention of tumorigenesis induced by PEUs.

Differential effect of subcellular localization of communication impairing gap junction protein connexin43 on tumor cell growth in vivo

There is a large body of evidence suggesting the connexin gap junction proteins appear to act as tumor suppressors, and their tumor inhibitory effect is usually attributed to their main function of cell coupling through gap junctions. However, some cancer cells (e.g. the rat bladder carcinoma BC31 cell line) are cell-cell communication proficient. Using specific site-directed mutagenesis in the third membrane-spanning (3M) domain of connexin43 (Cx43), we abolished the intrinsic gap junction intercellular communication (GJIC) in BC31 cells either by closing the gap junctional channels or by disruption of the transport of connexin complexes to the lateral membrane. Clones of BC31 cells transfected with a dominant negative Cx43 mutant giving rise to gap junctional channels, permeable only for a small tracer (neurobiotin), displayed accelerated growth rate in vivo, showing the critical role of selective gap junctional permeability in the regulation of cell growth in vivo. The use of other dominant-negative mutants of Cx43 also suggested that the effect of impaired communication on the tumorigenicity of cancer cells depends on the subcellular location of connexin. Inhibition of intrinsic GJIC in BC31 cells by sequestering of Cx protein inside the cytoplasm, due to expression of dominant-negative transport-deficient Cx43 mutants, did not significantly enhance the growth of transfectants in nude mice, but occasionally slightly retarded it. In contrast, augmentation of GJIC in BC31 cells by forced expression of wild-type Cx43, or a communication-silent mutant, fully suppressed tumorigenicity of these cells. Overall, these results show that cell coupling is a strong, but not the sole, mechanism by which Cx suppresses growth of tumorigenic cells in vivo; a GJIC-independent activity of Cx proteins should be considered as another strong tumor-suppressive factor.