Defective gap junctional intercellular communication in the carcinogenic process

Gap junctions are membrane structures made of intercellular channels which permit the diffusion from cytoplasm to cytoplasm of small hydrophilic molecules. Nearly 40 years ago, the loss of functional gap junctions has been described in cancer cells and led to the hypothesis that such type of intercellular communication is involved in the carcinogenesis process. From this time, a lot of data has been accumulated confirming that gap junctions are frequently decreased or absent in cancer cells whatever their tissue and species origins. Here, we review such data by insisting on the possible links existing between altered gap-junctional intercellular communication capacity (or the altered expression of their constitutive proteins, the connexins) and the stages of cancer progression in various cancer models. Then, we analyse particular aspects of the disturbance of connexin-mediated communication in cancer such as the cytoplasmic localization of connexins, the lack of heterologous communication between cancer cells and normal cells, the role of connexin gene mutations in cancer. In a separate part of the review, we also analyse the disturbance of gap-junctional intercellular communication during the late stages of cancer (invasion and metastasis processes).     

Connexins and their channels in cell growth and cell death

Direct communication between cells, mediated by gap junctions, is nowadays considered as an indispensable mechanism in the maintenance of cellular homeostasis. In fact, gap junctional intercellular communication is actively involved in virtually all aspects of the cellular life cycle, ranging from cell growth to cell death. For a long time, it was believed that this was merely a result of the capacity of gap junctions to control the direct intercellular exchange of essential cellular messengers. However, recent data show that the picture is more complicated than initially thought, as structural precursors of gap junctions, connexins and gap junction hemichannels, can affect the cellular homeostatic balance independently of gap junctional intercellular communication. In this paper, we summarize the current knowledge concerning the roles of connexins and their channels in the control of cellular homeostasis, with the emphasis on cell growth and cell death. We also briefly discuss the role of gap junctional intercellular communication in carcinogenesis and the potential use of connexins as tools for cancer therapy.     

The modulation of gap-junctional intercellular communication by lipid rafts

Lipid rafts are specific microdomains of plasma membrane which are enriched in cholesterol and sphingolipids. These domains seem to favour the interactions of particular proteins and the regulation of signalling pathways in the cells. Recent data have shown that among the proteins, which are preferentially localized in lipid rafts, are connexins that are the structural proteins of gap junctions. Since gap junctional intercellular communication is involved in various cellular processes and pathologies such as cancer, we were interested to review the various observations concerning this specific localization of connexins in lipid rafts and its consequences on gap junctional intercellular communication capacity. In particular, we will focus our discussion on the role of the lipid raft-connexin connection in cancer progression. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.     

Phosphorylation of connexin43 induced by Src: regulation of gap junctional communication between transformed cells

Cx43 is a widely expressed gap junction protein that mediates communication between many cell types. In general, tumor cells display less intercellular communication than their nontransformed precursors. The Src tyrosine kinase has been implicated in progression of a wide variety of cancers. Src can phosphorylate Cx43, and this event is associated with the suppression of gap junction communication. However, Src activates multiple signaling pathways that can also affect intercellular communication. For example, serine kinases including PKC and MAPK are downstream effectors of Src that can also phosphorylate Cx43 and disrupt gap junctional communication. In addition, Src can affect the expression of other proteins that may affect intercellular communication. Indeed, disruption of gap junctions by Src appears to be complex. It has become clear that Src can affect Cx43 activity by multiple mechanisms. Here, we review how Src may orchestrate events that regulate intercellular communication mediated by Cx43.     

Endocytosis and post-endocytic sorting of connexins

The connexins constitute a family of integral membrane proteins that form intercellular channels, enabling adjacent cells in solid tissues to directly exchange ions and small molecules. These channels assemble into distinct plasma membrane domains known as gap junctions. Gap junction intercellular communication plays critical roles in numerous cellular processes, including control of cell growth and differentiation, maintenance of tissue homeostasis and embryonic development. Gap junctions are dynamic plasma membrane domains, and there is increasing evidence that modulation of endocytosis and post-endocytic trafficking of connexins are important mechanisms for regulating the level of functional gap junctions at the plasma membrane. The emerging picture is that multiple pathways exist for endocytosis and sorting of connexins to lysosomes, and that these pathways are differentially regulated in response to physiological and pathophysiological stimuli. Recent studies suggest that endocytosis and lysosomal degradation of connexins is controlled by a complex interplay between phosphorylation and ubiquitination. This review summarizes recent progress in understanding the molecular mechanisms involved in endocytosis and post-endocytic sorting of connexins, and the relevance of these processes to the regulation of gap junction intercellular communication under normal and pathophysiological conditions. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.     

Early responses to mechanical load in tendon: role for calcium signaling, gap junctions and intercellular communication

Tendon and other connective tissue cells are subjected to diverse mechanical loads during daily activities. Thus, fluid flow, strain, shear and combinations of these stimuli activate mechanotransduction pathways that modulate tissue maintenance, repair and pathology. Early mechanotransduction events include calcium (Ca2+) signaling and intercellular communication. These responses are mediated through multiple mechanisms involving stretch-activated channels, voltage-activated channels such as Ca(v)1, purinoceptors, adrenoceptors, ryanodine receptor-mediated Ca2+ release, gap junctions and connexin hemichannels. Calcium, diacylglycerol, inositol (1,4,5)-trisphosphate, nucleotides and nucleosides play intracellular and/or extracellular signaling roles in these pathways. In addition, responses to mechanical loads in tendon cells vary among species, tendon type, anatomic location, loading conditions and other factors. This review includes a synopsis of the immediate responses to mechanical loading in connective tissue cells, particularly tenocytes. These responses involve Ca2+ signaling, gap junctions and intercellular communication.     

Gap junctions and cancer: implications and perspectives

Gap junctions are made of intercellular channels which permit the diffusion from cytoplasm to cytoplasm of small hydrophilic molecules (<1,200 Da) such as ions, sugars, amino acids, nucleotides, second messengers (calcium, inositol triphosphate, etc.). Since their discovery in the early sixties, several groups have described the loss of their function in cancer cells. The accumulation of such data led to the hypothesis that gap junctions are involved in the carcinogenesis process. This assumption has been confirmed by data establishing that gap junctional intercellular communication is inhibited by most of the tumor promoters and that the restoration of such a communication, by transfection of cDNAs encoding gap junction proteins (connexins), inhibits the aberrant growth rates of tumorigenic cells. Despite these important informations, several fundamental questions remain still open. First, we do not know how gap junctions mediate such a tumor suppressor effect and whether it may depend either on the cell type or on the connexin type. Moreover, most of the data concerning a possible involvement of gap junctions in carcinogenesis have been obtained from in vitro and animal models. The very few results which have been currently collected from human tumors are not sufficient to have a clear idea concerning the real involvement of gap junctions in sporadic human cancers. These points as well as other unresolved questions about the role of gap junctional intercellular communication in carcinogenesis are mentioned. To bring some answers, some prospects are proposed with the objective to use gap junctions for increasing the effect of anticancer therapies.     

Eph receptor and ephrin function in breast,gut, and skin epithelia

Epithelial cells are tightly coupled together through specialized intercellular junctions, including adherens junctions, desmosomes, tight junctions, and gap junctions. A growing body of evidence suggests epithelial cells also directly exchange information at cell-cell contacts via the Eph family of receptor tyrosine kinases and their membrane-associated ephrin ligands. Ligand-dependent and -independent signaling via Eph receptors as well as reverse signaling through ephrins impact epithelial tissue homeostasis by organizing stem cell compartments and regulating cell proliferation, migration, adhesion, differentiation, and survival. This review focuses on breast, gut, and skin epithelia as representative examples for how Eph receptors and ephrins modulate diverse epithelial cell responses in a context-dependent manner. Abnormal Eph receptor and ephrin signaling is implicated in a variety of epithelial diseases raising the intriguing possibility that this cell-cell communication pathway can be therapeutically harnessed to normalize epithelial function in pathological settings like cancer or chronic inflammation.     

Eph receptor and ephrin function in breast,gut, and skin epithelia

Epithelial cells are tightly coupled together through specialized intercellular junctions, including adherens junctions, desmosomes, tight junctions, and gap junctions. A growing body of evidence suggests epithelial cells also directly exchange information at cell-cell contacts via the Eph family of receptor tyrosine kinases and their membrane-associated ephrin ligands. Ligand-dependent and -independent signaling via Eph receptors as well as reverse signaling through ephrins impact epithelial tissue homeostasis by organizing stem cell compartments and regulating cell proliferation, migration, adhesion, differentiation, and survival. This review focuses on breast, gut, and skin epithelia as representative examples for how Eph receptors and ephrins modulate diverse epithelial cell responses in a context-dependent manner. Abnormal Eph receptor and ephrin signaling is implicated in a variety of epithelial diseases raising the intriguing possibility that this cell-cell communication pathway can be therapeutically harnessed to normalize epithelial function in pathological settings like cancer or chronic inflammation.     

Changes of gap junctional cell-cell communication in overactive detrusor in rats

To evaluate the changes in intercellular communication through gap junctions in detrusor overactivity (DO), we studied 23 adult female Wistar rats with DO after partial outflow obstruction (DO group) and 13 sham-operated rats (control group). The two groups were compared by means of urodynamics, light and electron microscopy, expression of Cx40, Cx43, and Cx45 mRNA genes with RT-PCR, Cx43 protein with Western blot analysis, and functional intercellular communication with scrape loading dye transfer (SLDT) and fluorescence recovery after photobleaching (FRAP). The number of gap junctions and the expression of connexin mRNA and Cx43 protein were increased in DO rats, and intercellular communication through gap junctions increased after 6 wk of partial outflow obstruction as assessed with SLDT and FRAP techniques. The findings provide a theoretical rationale for using Cx43 antagonists and gap junction inhibitors in the treatment of patients with overactive detrusor secondary to partial bladder outflow obstruction.     

间隙连接蛋白在卵泡发育过程中的作用

间隙连接广泛分布于各种组织细胞中,由其构成的通道允许小分子信号物质在相邻细胞间直接传递,在细胞间的通讯方面起着非常重要的作用。间隙连接由连接蛋白(Cx)组成,目前已经发现Cx家族有20多个成员[1],它们在相邻细胞间组成同种或异种间隙连接,调控着细胞的增殖和分化。在哺乳动物卵泡发育过程中,卵母细胞与周围的颗粒细胞之间形成的缝隙连接,介导胞间通讯,对生殖细胞迁移、卵母细胞减数分裂能力恢复、颗粒细胞分层、卵泡成腔、黄体形成、促性腺激素信号传递有非常重要的调节作用。本文根据近年来相关的研究报道,对卵泡发育过程中间隙连接的作用进行综述。     

7-Ketocholesterol modulates intercellular communication through gap-junction in bovine lens epithelial cells

Background  

Connexin43 (Cx43) is an integral membrane protein that forms intercellular channels called gap junctions. Intercellular communication in the eye lens relies on an extensive network of gap junctions essential for the maintenance of lens transparency. The association of Cx43 with cholesterol enriched lipid raft domains was recently demonstrated. The objective of this study is to assess if products of cholesterol oxidation (oxysterols) affect gap junction intercellular communication (GJIC).     

Effects of cAMP on intercellular coupling and osteoblast differentiation

Bone-forming cells are organized in a multicellular network interconnected by gap junctions. Direct intercellular communication via gap junctions is an important component of bone homeostasis, coordinating cellular responses to external signals and promoting osteoblast differentiation. The cAMP pathway, a major intercellular signal transduction mechanism, regulates osteoblastic function and metabolism. We investigated the effects of this second messenger on junctional communication and on the expression of differentiation markers in human HOBIT osteoblastic cells. Increased levels of cAMP induce posttranslational modifications (i.e., phosphorylations) of connexin43 and enhancement of gap junction assembly, resulting in an increased junctional permeance to Lucifer yellow and to a positive modulation of intercellular Ca(2+) waves. Increased intercellular communication, however, was accompanied by a parallel decrease of alkaline phosphatase activity and by an increase of osteocalcin expression. cAMP-dependent stimulation of cell-to-cell coupling induces a complex modulation of bone differentiation markers.     

Regulation of mouse oocyte growth: probable nutritional role for intercellular communication between follicle cells and oocytes in oocyte growth

Intercellular communication, as determined by two different assay procedures, was established in vitro between mouse oocytes free of adhering follicle cells and monolayers of either follicle or 3T3 cells. Both of these cell types are known to be able to form homologous gap junctions, and follicle cells naturally form heterologous gap junctions with oocytes in vivo. Monolayers of L cells that are communication deficient did not establish intercellular communication with oocytes as determined by the two different assays for intercellular communication. The diameter of oocytes cultured for 4 days in medium or on monolayers of L cells decreased markedly, 9.7 and 13.1 micron, respectively. In contrast, oocytes cultured for 4 days on follicle cell monolayers increased on the average about 4.7 micron in diameter. Oocytes cultured for 4 days on monolayers of 3T3 cells decreased slightly in diameter, i.e., 2.1 micron. Results from these experiments support a nutritional role for intercellular communication between follicle cells and oocytes in oocyte growth.     

Gap junctions in human synovial cells and tissue

Our objective was to establish the existence of intercellular communication through gap junctions in synovial lining cells and in primary and passaged cultures of human synovial cells. Communication between cells was assessed using the nystatin perforated-patch method, fluorescent dye transfer, immunochemistry, transmission electron microscopy, and immunoblotting. Functional gap junctions were observed in primary and passaged cultures and were based on measurements of the transient current response to a step voltage. The average resistance between cells in small aggregates was 300 +/- 150 MOmega. Gap junctions were also observed between synovial lining cells in tissue explants; the size of the cell network in synovial tissue was estimated to be greater than 40 cells. Intercellular communication between cultured cells and between synovial lining cells was confirmed by dye injection. Punctate fluorescent regions were seen along intercellular contacts between cultured cells and in synovial membranes in cells and tissue immunostained for connexin43. The presence of the protein was verified in immunoblots. Regular 2-nm intermembrane gap separations characteristic of gap junctions were seen in transmission electron micrographs of synovial biopsies. The results showed that formation of gap-junction channels capable of mediating ionic and molecular communication was a regular feature of synovial cells, both in tissue and in cultured cells. The gap junctions contained connexin43 protein and perhaps other proteins. The physiological purpose of gap junctions in synovial cells is unknown, but it is reasonable to anticipate that intercellular communication serves some presently unrecognized function.     

Regulation of gap junction intercellular communication by the ubiquitin system

Intercellular communication via gap junctions plays a critical role in numerous cellular processes, including the control of cell growth and differentiation, maintenance of tissue homeostasis and embryonic development. Gap junctions are aggregates of intercellular channels that enable adjacent cells in solid tissues to directly exchange ions and small molecules. These channels are formed by a family of integral membrane proteins called connexins, of which the best studied is connexin43. Connexins have a high turnover rate in most tissue types, and degradation of connexins is considered to be a tightly regulated process. Post-translational modification of connexins by ubiquitin is emerging as an important event in the regulation of connexin degradation. Ubiquitination is involved in endoplasmic reticulum-associated degradation of connexins as well as in trafficking of connexins to lysosomes. At both the endoplasmic reticulum and the plasma membrane, ubiquitination of connexins is strongly affected by changes in the extracellular environment. There is increasing evidence that the regulation of connexin ubiquitination might be an important mechanism for rapidly modifying the level of functional gap junctions at the plasma membrane, under both normal and pathological conditions. This review discusses the current knowledge about the regulation of intercellular communication via gap junctions by ubiquitination of connexins.     

Gap junctions and connexin expression in the normal and pathological central nervous system

Gap junctions are widely expressed in the various cell types of the central nervous system. These specialized membrane intercellular junctions provide the morphological support for direct electrical and biochemical communication between adjacent cells. This intercellular coupling is controlled by neurotransmitters and other endogenous compounds produced and released in basal as well as in pathological situations. Changes in the expression and the function of connexins are associated with number of brain pathologies and lesions suggesting that they could contribute to the expansion of brain damages. The purpose of this review is to summarize data presently available concerning gap junctions and the expression and function of connexins in different cell types of the central nervous system and to present their physiopathological relevance in three major brain dysfunctions: inflammation, epilepsy and ischemia.     

A novel form of cellular communication among thymic epithelial cells: intercellular calcium wave propagation

We here describe intercellular calcium waves as a novel form of cellular communication among thymic epithelial cells. We first characterized the mechanical induction of intercellular calcium waves in different thymic epithelial cell preparations: cortical 1-4C18 and medullary 3-10 thymic epithelial cell lines and primary cultures of thymic "nurse" cells. All thymic epithelial preparations responded with intercellular calcium wave propagation after mechanical stimulation. In general, the propagation efficacy of intercellular calcium waves in these cells was high, reaching 80-100% of the cells within a given confocal microscopic field, with a mean velocity of 6-10 µm/s and mean amplitude of 1.4- to 1.7-fold the basal calcium level. As evaluated by heptanol and suramin treatment, our results suggest the participation of both gap junctions and P2 receptors in the propagation of intercellular calcium waves in thymic nurse cells and the more prominent participation of gap junctions in thymic epithelial cell lines. Finally, in cocultures, the transmission of intercellular calcium wave was not observed between the mechanically stimulated thymic epithelial cell and adherent thymocytes, suggesting that intercellular calcium wave propagation is limited to thymic epithelial cells and does not affect the neighboring thymocytes. In conclusion, these data describe for the first time intercellular calcium waves in thymic epithelial cells and the participation of both gap junctions and P2 receptors in their propagation. gap junctions; connexin43; P2 receptors; intercellular communication