Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1996.0001q.x

Connections with connexins: the molecular basis of direct intercellular signaling

Saez, Juan C., Viviana M. Berthoud, Maria C. Branes, Agustin D. Martinez, and Eric C. Beyer. Plasma Membrane Channels Formed by Connexins: Their Regulation and Functions. Physiol Rev 83: 1359-1400,...

Plasma Membrane Channels Formed by Connexins: Their Regulation and Functions

Inactivation of the mouse Msh2 gene results in mismatch repair deficiency, methylation tolerance, hyperrecombination, and predisposition to cancer

Alkylating agents and compounds converted metabolically to alkylating agents form an important class of carcinogens and mutagens that includes a number of environmentally important /V-nitrosocompounds. A number of alkylating agents are also useful antitumor drugs. There are many factors determining the sensitivity to the toxic, mutagenic, and carcinogenic poten tial of these compounds (1-4).2 One of these is the capacity to

/pdf/mammalian-o6-alkylguanine-dna-alkyltransferase-regulation-58rg79j6q8.pdf

Mammalian O6-alkylguanine-DNA alkyltransferase: regulation and importance in response to alkylating carcinogenic and therapeutic agents.

The role of carotenoids in the prevention of human pathologies.

Gap-junctional intercellular communication (GJIC) in 20 primary human liver tumors with different degrees of malignancy has been studied at the functional and molecular levels. When GJIC capacity was determined by dye-transfer assay performed directly with freshly removed tumor tissue, significant reduction was found in all samples, regardless of their morphology. In addition, a selective lack of GJIC between tumor and surrounding non-tumorous cells was observed in some cases, probably due to the physical separation between them resulting from encapsulation of tumors. There was, however, no essential change in the level of expression of the major liver gap-junction protein, connexin (cx) 32, in liver tumors as measured by Northern and Western blot analyses. Immunohistochemical study revealed aberrant localization of cx 32 in the majority of malignant liver tumors. Instead of cytoplasmic membrane localization at intercellular contacts, cx 32 was detected mainly either intracytoplasmically or in plasma membrane free from contact with other cells. We did not detect any mutation in the coding sequence of the cx 32 gene from any of the human liver tumors we tested. Thus it is likely that the aberrant localization of cx 32 in tumor cells is due to disruption of the mechanisms for establishment of this protein into gap-junction plaques, rather than to structural abnormality of the cx 32 protein itself. Another member of the connexin family, cx 43, not detectable in non-tumorigenic hepatocytes, was expressed in several tumors, especially in invasive areas, but was detected in only a few tumor cells and was localized intracytoplasmically, suggesting that cx 43 protein is not involved in GJIC in the tumors.

Altered homologous and heterologous gap‐junctional intercellular communication in primary human liver tumors associated with aberrant protein localization but not gene mutation of connexin 32

Two types of intercellular communication (humoral and cell contact-mediated) are involved in control of cellular function in multicellular organisms, both of them mediated by membrane-embedded proteins. Involvement of aberrant humoral communication in carcinogenesis has been well documented and genes coding for some growth factors and their receptors have been classified as oncogenes. More recently, cell contact-mediated communication has been found to have an important role in carcinogenesis, and some genes coding for proteins involved in this type of communication appear to form a family of tumor-suppressor genes. Both homologous (among normal or (pre-)cancerous cells) as well as heterologous (between normal and (pre)cancerous cells) communications appear to play important roles in cell growth control. Gap junctional intercellular communication (GJIC) is the only means by which multicellular organisms can exchange low molecular weight signals directly from within one cell to the interior of neighboring cells. GJIC is altered by many tumor-promoting agents and in many human and rodent tumors. We have recently shown that liver tumor-promoting agents inhibit GJIC in the rat liver in vivo. Molecular mechanisms which could lead to aberrant GJIC include: (1) mutation of connexin genes; (2) reduced and/or aberrant expression of connexin mRNA; (3) aberrant localization of connexin proteins, i.e., intracytoplasmic rather than in the cytoplasmic membrane; and (4) modulation of connexin functions by other proteins, such as those involved in extracellular matrix and cell adhesion. Whilst mutations of the cx 32 gene appear to be rare in tumors, cx 37 gene mutations have been reported in a mouse lung tumor cell line. Our results suggest that aberrant connexin localization is rather common in cancer cells and that possible molecular mechanisms include aberrant phosphorylation of connexin proteins and lack of cell adhesion molecules. Studies on transfection of connexin genes into tumor cells suggest that certain connexin genes (e.g., cx 26, cx 43 and cx 32) act as tumor-suppressor genes.

Intercellular communication and carcinogenesis

We have examined whether alterations of simple (CA)n DNA repeats, as observed in human colon cancers, occur during human gastric carcinogenesis and whether such alterations reflect genomic instability that could lead to other genetic changes. A total of 22 gastric cancer samples were analyzed: 15 well or moderately differentiated adenocarcinomas, 6 signet-ring cell carcinomas, and 1 poorly differentiated adenocarcinoma. When (CA)n repeat sequences were examined at 10 loci, one adenocarcinoma showed a loss of repeat sequences at five loci, three adenocarcinomas gained a repeat at one locus, and one adenocarcinoma had new, repeated sequences at five loci. Three samples showed mutations in the p53 gene, two in exon 5 (both GC to AT transition at a CpG dinucleotide) and one in exon 7 (AT to GC transition). Only one sample with a p53 mutation also showed altered (CA)n repeats. A putative tumor suppressor gene, connexin 32, was not altered as assessed by single-strand conformation polymorphism analysis. These results suggest that genomic instability revealed by (CA)n repeat changes does not seem to contribute to induction of point mutations in p53 or connexin 32 genes but may participate in loss of heterozygosity at APC/MCC loci. The results are consistent with the hypothesis that different mechanisms are involved in the gain and loss of (CA)n repeats.

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Alterations of (CA)n DNA Repeats and Tumor Suppressor Genes in Human Gastric Cancer

Genetic and epigenetic changes of intercellular communication genes during multistage carcinogenesis.

Malignant cells usually show altered gap junctional intercellular communication and are often associated with aberrant expression or localization of connexins. Transfection of connexin genes into tumorigenic cells restores normal cell growth, suggesting that connexins form a family of tumour suppressor genes. Some studies have also shown that specific connexins may be necessary to control growth of specific cell types. Although we have found that genes encoding connexin32 (Cx32; beta 1), Cx37 (alpha 4) and Cx43 (alpha 1) are rarely mutated in tumours, our recent studies suggest that methylation of the connexin gene promoter may be a mechanism by which connexin gene expression is down-regulated in certain tumors. We have produced various dominant negative mutants of the genes encoding Cx26 (beta 2), Cx32 and Cx43, some of which prevent the growth control exerted by the corresponding wild-type genes. A decade ago, we proposed a method to enhance killing of cancer cells by diffusion of therapeutic agents through gap junctions. Recently, we and others have shown that gap junctional intercellular communication is responsible for the bystander effect seen in herpes simplex virus thymidine kinase/ganciclovir gene therapy. Thus, connexin genes can exert dual effects in tumour control: tumour suppression and a bystander effect for cancer therapy.

Nikolai Mironov

Papers

Altered homologous and heterologous gap‐junctional intercellular communication in primary human liver tumors associated with aberrant protein localization but not gene mutation of connexin 32

Intercellular communication and carcinogenesis

Alterations of (CA)n DNA Repeats and Tumor Suppressor Genes in Human Gastric Cancer

Genetic and epigenetic changes of intercellular communication genes during multistage carcinogenesis.

Connexins in tumour suppression and cancer therapy.