Connexin genes as tumour suppressor genes?

Oral Cancer Cells Differ From Normal Oral Epithelial Cells in Tissuelike Organization and in Response to Lycopene Treatment: An Organotypic Cell Culture Study

We established distinctive monolayer and organotypic cell culture techniques to assess possible differences in cross-talk and spatial and structural organization of oral cancer cells compared with normal oral cells and also to evaluate possible differential responses of the cells to carotenoids. In monolayers, we investigated the effect of lycopene on the proliferation of an established oral cancer cell line, KB-1, and compared it with a primary cell line obtained from normal oral mucosa. Lycopene exerted a significant inhibitory effect on KB-1 cell proliferation inducing a dose-dependent downregulation of proliferating cell nuclear antigen (PCNA) associated with upregulation of connexin-43 (Cx-43) expression, whereas in the normal oral mucosal cells lycopene did not affect either PCNA expression, which was very low, or the expression of Cx-43, which was basically very high. Lycopene significantly inhibited the formation of colonies induced by the carcinogen 3-methylcholanthrene (MCA) on normal oral cells and almost completely abrogated the hyperplastic effect induced by MCA. KB-1 cells and normal oral epithelial cells in the organotypic cell culture method differed in their stratification and intercellular adhesion patterns as well as in the expression profile of cytokeratins, vimentin, and Cx-43. Lycopene induced Cx-43 expression in KB-1 cells grown by the organotypic raft method, similar to KB-1 cells grown as monolayers. We conclude that lycopene is a promising chemopreventive, pro-differentiating, and anticarcinogenic agent. No adverse effects of lycopene were detected in normal cells cultured in either monolayer or organotypic systems.

Gap junction intercellular communication propagates cell death in cancerous cells

Gap junction intercellular communication (GJIC) or cell coupling has an important function in maintaining tissue homeostasis and is thus a critical factor in the life and death balance of cells. While the role of GJIC in cell growth regulation has been much studied, its involvement in apoptosis remains unclear. In this study we elucidated the possibility that cell death is propagated via gap junctions, employing the rat bladder carcinoma cell line BC31. BC31 cells proliferate quickly, are tumorigenic, and are well-coupled via gap junctions that contain the gap junction protein Connexin43 (Cx43). In addition, these cells are predisposed to spontaneous death by apoptosis, particularly upon achieving confluency. We found that many dying BC31 cells express Cx43 just as their non-apoptotic counterparts do. Furthermore, Cx43 in apoptotic cells could be functionally competent, supporting coupling of these cells with their non-apoptotic neighbors, and as a result, clusters of coordinately dying cells were observed. The role of Cx43 and GJIC in propagating cell death was shown by analysing clones of BC31 cells expressing a mutant of Cx43 that is a dominant negative inhibitor of GJIC, and by using beta-glycyrrhetinic acid to inhibit intrinsic cell coupling in BC31 cells: in both cases the formation of clusters of dying cells was abrogated, and the intensity of cell death was considerably decreased. These results suggest that GJIC spreads cell-killing signals initially generated by a single cell that spontaneously initiates apoptosis, into healthy surrounding cells, thus increasing the level of cell death. Treatment of BC31 cells with the sleep-inducing lipid Oleamide, which selectively restricts gap junction permeability to Ca(2+) ions, did not abrogate coordinated cell death by clusters, indicating that Ca(2+) ions are the most probable cell-killing signals spread through gap junctions.