Gap junction proteins connexin32 and connexin43 partially acquire growth-suppressive function in HeLa cells by deletion of their C-terminal tails

Analysis of the function and therapeutic strategy of connexin 43 from its subcellular localization

Connexins (Cxs) are a family of transmembrane proteins located in the plasma membrane of human cells, among which connexin 43 (Cx43) is abundantly expressed in various types of human cells. Cx43, encoded by the gap junction protein alpha 1 (GJA1) gene, assembles into a hexameric structure in the Golgi apparatus and translocates to the plasma membrane to form hemichannels (Hcs), which pair with those of the cells in contact with each other and form gap junction intercellular communication (GJIC). The role of Cx43 as a connexin localized at the plasma membrane to perform channel functions is well recognized in previous studies, but recent studies have found that it can also be localized in the nucleus, mitochondria, or present in extracellular vesicles (EVs) and tunneling nanotubes (TNTs). Cx43 in the nucleus is involved in gene transcription regulation, cytoskeleton formation, cell migration and adhesion. Cx43 in mitochondria is involved in mitochondrial respiration-related functions, and Cx43 in extracellular vesicles and tunneling nanotubes is involved in distant cellular information exchange. It is because of the diverse distribution of subcellular localization of Cx43 that it is possible to explore the corresponding functions by analyzing its localization. In this review, we summarize the important roles of Cx43 in disease development from the perspective of subcellular localization, and provide new ideas for Cx43 as a therapeutic target and the search for related pathological mechanisms.

Role of Connexin 43 phosphorylation on Serine-368 by PKC in cardiac function and disease

Intercellular communication mediated by gap junction channels and hemichannels composed of Connexin 43 (Cx43) is vital for the propagation of electrical impulses through cardiomyocytes. The carboxyl terminal tail of Cx43 undergoes various post-translational modifications including phosphorylation of its Serine-368 (S368) residue. Protein Kinase C isozymes directly phosphorylate S368 to alter Cx43 function and stability through inducing conformational changes affecting channel permeability or promoting internalization and degradation to reduce intercellular communication between cardiomyocytes. Recent studies have implicated this PKC/Cx43-pS368 circuit in several cardiac-associated diseases. In this review, we describe the molecular and cellular basis of PKC-mediated Cx43 phosphorylation and discuss the implications of Cx43 S368 phosphorylation in the context of various cardiac diseases, such as cardiomyopathy, as well as the therapeutic potential of targeting this pathway.

Non-canonical roles of connexins

Gap junctions mediate cellular communication and homeostasis by controlling the intercellular exchange of small and hydrophilic molecules and ions. Gap junction channels are formed by the docking of 2 hemichannels of adjacent cells, which in turn are composed of 6 connexin subunits. Connexin proteins as such can also control the cellular life cycle independent of their channel activities. This has been most demonstrated in the context of cell growth and cell death. Different mechanisms are involved mainly related to direct interaction with cell growth or cell death regulators, but also implying effects on the expression of cell growth and cell death regulators. The present paper focuses on these atypical roles of connexin proteins.

Acetylation of C-terminal lysines modulates protein turnover and stability of Connexin-32

BACKGROUND

The gap junction protein, Connexin32 (Cx32), is expressed in various tissues including liver, exocrine pancreas, gastrointestinal epithelium, and the glia of the central and peripheral nervous system. Gap junction-mediated cell-cell communication and channel-independent processes of Cx32 contribute to the regulation of physiological and cellular activities such as glial differentiation, survival, and proliferation; maintenance of the hepatic epithelium; and axonal myelination. Mutations in Cx32 cause X-linked Charcot-Marie-Tooth disease (CMT1X), an inherited peripheral neuropathy. Several CMT1X causing mutations are found in the cytoplasmic domains of Cx32, a region implicated in the regulation of gap junction assembly, turnover and function. Here we investigate the roles of acetylation and ubiquitination in the C-terminus on Cx32 protein function. Cx32 protein turnover, ubiquitination, and response to deacetylase inhibitors were determined for wild-type and C-terminus lysine mutants using transiently transfected Neuro2A (N2a) cells.

RESULTS

We report here that Cx32 is acetylated in transfected N2a cells and that inhibition of the histone deacetylase, HDAC6, results in an accumulation of Cx32. We identified five lysine acetylation targets in the C-terminus. Mutational analysis demonstrates that these lysines are involved in the regulation of Cx32 ubiquitination and turnover. While these lysines are not required for functional Cx32 mediated cell-cell communication, BrdU incorporation studies demonstrate that their relative acetylation state plays a channel-independent role in Cx32-mediated control of cell proliferation.

CONCLUSION

Taken together these results highlight the role of post translational modifications and lysines in the C-terminal tail of Cx32 in the fine-tuning of Cx32 protein stability and channel-independent functions.

Intrinsic Oncogenic Function of Intracellular Connexin26 Protein in Head and Neck Squamous Cell Carcinoma Cells

It has long been known that the gap junction is down-regulated in many tumours. One of the downregulation mechanisms is the translocation of connexin, a gap junction protein, from cell membrane into cytoplasm, nucleus, or Golgi apparatus. Interestingly, as tumours progress and reinforce their malignant phenotype, the amount of aberrantly-localised connexin increases in different malignant tumours including oesophageal squamous cell carcinoma, thus suggesting that such an aberrantly-localised connexin should be oncogenic, although gap junctional connexins are often tumour-suppressive. To define the dual roles of connexin in head and neck squamous cell carcinoma (HNSCC), we introduced the wild-type connexin26 (wtCx26) or the mutant Cx26 (icCx26) gene, the product of which carries the amino acid sequence AKKFF, an endoplasmic reticulum-Golgi retention signal, at the C-terminus and is not sorted to cell membrane, into the human FaDu hypopharyngeal cancer cell line that had severely impaired the expression of connexin during carcinogenesis. wtCx26 protein was trafficked to the cell membrane and formed gap junction, which successfully exerted cell-cell communication. On the other hand, the icCx26 protein was co-localised with a Golgi marker, as revealed by immunofluorescence, and thus was retained on the way to the cell membrane. While the forced expression of wtCx26 suppressed both cell proliferation in vitro and tumorigenicity in mice in vivo, icCx26 significantly enhanced both cell proliferation and tumorigenicity compared with the mock control clones, indicating that an excessive accumulation of connexin protein in intracellular domains should be involved in cancer progression and that restoration of proper subcellular sorting of connexin might be a therapeutic strategy to control HNSCC.

The SH3-binding domain of Cx43 participates in loop/tail interactions critical for Cx43-hemichannel activity

Connexin 43 (Cx43) hemichannels establish local signaling networks via the release of ATP and other molecules, but their excessive opening may result in cell death. Hence, the activity of Cx43-hemichannels ought to be critically controlled. This involves interactions between the C-terminal tail (CT) and the cytoplasmic loop (CL), more particularly the L2 domain within CL. Previous work revealed an important role for the last nine amino acids of the Cx43 CT by targeting the L2 domain, as these nine amino acids were sufficient to restore the activity of CT-truncated Cx43-hemichannels. However, we discovered that deletion of the last 19 amino acids of the CT only partially lowered the binding to the L2 domain, indicating that a second L2-binding region is present in the CT. We here provide evidence that the SH3-binding domain is another CT region that targets the L2 domain. At the functional level, the SH3-binding domain was able to restore the activity of CT-truncated Cx43-hemichannels and alleviate the inhibition of full-length Cx43-hemichannels by high intracellular Ca2+ concentration ([Ca2+]i) as demonstrated by various approaches including patch clamp studies of unitary Cx43-hemichannel activity. Finally, we show that in full-length Cx43-hemichannels, deletion of either the SH3-binding domain or the CT9 region suppresses the hemichannel activity, while deletion of both domains completely annihilates the hemichannel activity. These results demonstrate that the Cx43 SH3-binding domain, in addition to the CT9 region, critically controls hemichannel activity at high [Ca2+]i, which may be involved in pathological hemichannel opening.

The connexin 43 C-terminus: A tail of many tales

Connexins are chordate gap junction channel proteins that, by enabling direct communication between the cytosols of adjacent cells, create a unique cell signalling network. Gap junctional intercellular communication (GJIC) has important roles in controlling cell growth and differentiation and in tissue development and homeostasis. Moreover, several non-canonical connexin functions unrelated to GJIC have been discovered. Of the 21 members of the human connexin family, connexin 43 (Cx43) is the most widely expressed and studied. The long cytosolic C-terminus (CT) of Cx43 is subject to extensive post-translational modifications that modulate its intracellular trafficking and gap junction channel gating. Moreover, the Cx43 CT contains multiple domains involved in protein interactions that permit crosstalk between Cx43 and cytoskeletal and regulatory proteins. These domains endow Cx43 with the capacity to affect cell growth and differentiation independently of GJIC. Here, we review the current understanding of the regulation and unique functions of the Cx43 CT, both as an essential component of full-length Cx43 and as an independent signalling hub. We highlight the complex regulatory and signalling networks controlled by the Cx43 CT, including the extensive protein interactome that underlies both gap junction channel-dependent and -independent functions. We discuss these data in relation to the recent discovery of the direct translation of specific truncated forms of Cx43. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Role of Specificity Protein-1 and Activating Protein-2 Transcription Factors in the Regulation of the Gap Junction Protein Beta-2 Gene in the Epididymis of the Rat

In prepubertal rats, connexin 26 (GJB2) is expressed between adjacent columnar cells of the epididymis. At 28 days of age, when columnar cells differentiate into adult epithelial cell types, Gjb2 mRNA levels decrease to barely detectable levels. There is no information on the regulation of GJB2 in the epididymis. The present study characterized regulation of the Gjb2 gene promoter in the epididymis. A single transcription start site at position −3829 bp relative to the ATG was identified. Computational analysis revealed several TFAP2A, SP1, and KLF4 putative binding sites. A 1.5-kb fragment of the Gjb2 promoter was cloned into a vector containing a luciferase reporter gene. Transfection of the construct into immortalized rat caput epididymal (RCE-1) cells indicated that the promoter contained sufficient information to drive expression of the reporter gene. Deletion constructs showed that the basal activity of the promoter resides in the first −230 bp of the transcriptional start site. Two response elements necessary for GJB2 expression were identified: an overlapping TFAP2A/SP1 site (−136 to −126 bp) and an SP1 site (−50 bp). Chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assays confirmed that SP1 and TFAP2A were bound to the promoter. ChIP analysis of chromatin from young and pubertal rats indicated that TFAP2A and SP1 binding decreased with age. SP1 and TFAP2A knockdown indicated that SP1 is necessary for Gjb2 expression. DNA methylation did not appear to be involved in the regulation of Gjb2 expression. Results indicate that SP1 and TFAP2A regulate Gjb2 promoter activity during epididymal differentiation in rat.

Proteomic Analysis of Connexin 43 Reveals Novel Interactors Related to Osteoarthritis

We have previously reported that articular chondrocytes in tissue contain long cytoplasmic arms that physically connect two distant cells. Cell-to-cell communication occurs through connexin channels termed Gap Junction (GJ) channels, which achieve direct cellular communication by allowing the intercellular exchange of ions, small RNAs, nutrients, and second messengers. The Cx43 protein is overexpressed in several human diseases and inflammation processes and in articular cartilage from patients with osteoarthritis (OA). An increase in the level of Cx43 is known to alter gene expression, cell signaling, growth, and cell proliferation. The interaction of proteins with the C-terminal tail of connexin 43 (Cx43) directly modulates GJ-dependent and -independent functions. Here, we describe the isolation of Cx43 complexes using mild extraction conditions and immunoaffinity purification. Cx43 complexes were extracted from human primary articular chondrocytes isolated from healthy donors and patients with OA. The proteomic content of the native complexes was determined using LC-MS/MS, and protein associations with Cx43 were validated using Western blot and immunolocalization experiments. We identified >100 Cx43-associated proteins including previously uncharacterized proteins related to nucleolar functions, RNA transport, and translation. We also identified several proteins involved in human diseases, cartilage structure, and OA as novel functional Cx43 interactors, which emphasized the importance of Cx43 in the normal physiology and structural and functional integrity of chondrocytes and articular cartilage. Gene Ontology (GO) terms of the proteins identified in the OA samples showed an enrichment of Cx43-interactors related to cell adhesion, calmodulin binding, the nucleolus, and the cytoskeleton in OA samples compared with healthy samples. However, the mitochondrial proteins SOD2 and ATP5J2 were identified only in samples from healthy donors. The identification of Cx43 interactors will provide clues to the functions of Cx43 in human cells and its roles in the development of several diseases, including OA.

Connexins: junctional and non-junctional modulators of proliferation

Mounting evidence indicates that dysregulation of gap junctions and their structural subunits-connexins-often occurs in, and sometimes causes, a variety of proliferative disorders, including cancer. Connexin-mediated regulation of cell proliferation is complex and may involve modulation of gap junction intercellular communication (GJIC), hemichannel signalling, or gap junction-independent paths. However, the exact mechanisms linking connexins to proliferation remain poorly defined and a number of contradictory studies report both pro- and anti-proliferative effects, effects that often depend on the cell or tissue type or the microenvironment. The present review covers junctional and non-junctional regulation of proliferation by connexins, with a particular emphasis on their association with cancer.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Intercellular redistribution of cAMP underlies selective suppression of cancer cell growth by connexin26

Connexins (Cx), which constitute gap junction intercellular channels in vertebrates, have been shown to suppress transformed cell growth and tumorigenesis, but the mechanism(s) still remain largely speculative. Here, we define the molecular basis by which Cx26, but less frequently Cx43 or Cx32, selectively confer growth suppression on cancer cells. Functional intercellular coupling is shown to be required, producing partial blocks of the cell cycle due to prolonged activation of several mitogenic kinases. PKA is both necessary and sufficient for the Cx26 induced growth inhibition in low serum and the absence of anchorage. Activation of PKA was not associated with elevated cAMP levels, but appeared to result from a redistribution of cAMP throughout the cell population, eliminating the cell cycle oscillations in cAMP required for efficient cell cycle progression. Cx43 and Cx32 fail to mediate this redistribution as, unlike Cx26, these channels are closed during the G2/M phase of the cell cycle when cAMP levels peak. Comparisons of tumor cell lines indicate that this is a general pattern, with growth suppression by connexins occurring whenever cAMP oscillates with the cell cycle, and the gap junction remain open throughout the cell cycle. Thus, gap junctional coupling, in the absence of any external signals, provides a general means to limit the mitotic rate of cell populations.

Connexin43 reduces melanoma growth within a keratinocyte microenvironment and during tumorigenesis in vivo

Connexins (Cx) have been identified as tumor suppressors or enhancers, a distinction that appears to be dependent on the type and stage of disease. However, the role of connexins in melanoma tumorigenesis and their status during cancer onset and progression remain controversial and unclear. Here, we show that the aggressive B16-BL6 mouse melanoma cell line expresses low basal levels of Cx26 and Cx43, rendering them gap junctional intercellular communication-deficient as elucidated by immunofluorescence, Western blotting, and dye transfer studies. Following ectopic expression of green fluorescent protein-tagged Cx26 and Cx43 in these connexin-deficient melanomas, punctate gap junction-like plaques were evident at sites of cell-cell apposition, and the incidence of dye transfer was significantly increased similar to connexin-rich keratinocytes. We found that the expression of Cx43, but not Cx26, significantly reduced cellular proliferation and anchorage-independent growth from control melanomas, whereas migration was unaffected. Additionally, melanomas expressing Cx43 displayed significantly reduced growth within the in situ-like microenvironment of keratinocytes, despite a lack of heterocellular gap junctional intercellular communication between the two cell types. Furthermore, when grown in vivo in the chicken chorioallantoic membrane, primary tumors derived from Cx43-expressing melanomas were significantly smaller than controls, whereas Cx26-expressing melanomas produced tumors similar to controls. Collectively, these results suggest that Cx43, and not Cx26, can act as a tumor suppressor during melanoma tumorigenesis.

Negatively charged residues (Asp378 and Asp379) in the last ten amino acids of the C-terminal tail of Cx43 hemichannels are essential for loop/tail interactions

Connexin 43 (Cx43)-hemichannel activity is controlled by intramolecular interactions between cytoplasmic loop and C-terminal tail. We previously identified the last 10 amino acids of the C-terminal tail of Cx43 as essential for Cx43-hemichannel activity. We developed a cell-permeable peptide covering this sequence (TAT-Cx43CT). In this study, we examined the critical molecular determinants in TAT-Cx43CT to restore Cx43-hemichannel activity. Using amino acid substitutions in TAT-Cx43CT, we identified the two aspartate (Asp378 and Asp379) and two proline (Pro375 and Pro377) residues as critical for TAT-Cx43CT activity, since TAT-Cx43CT(DD/AA) and TAT-Cx43CT(PP/GG) did not overcome the inhibition of Cx43-hemichannel activity induced by thrombin, micromolar cytoplasmic Ca(2+) concentration or truncation of Cx43 at M(239). Consistent with this, we found that biotin-Cx43CT(DD/AA) was much less efficient than biotin-Cx43CT to bind the purified CL domain of Cx43 in surface plasmon resonance experiments. In conclusion, we postulate that Asp378 and Asp379 in the C-terminal part of Cx43 are essential for loop/tail interactions in Cx43 hemichannels, while Pro375 and Pro377 may help to properly coordinate the critical Asp residues.

Channel-independent influence of connexin 43 on cell migration

In this review we focus on the role of connexins, especially of Cx43, as modulators of migration - a fundamental process in embryogenesis and in physiologic functions of the adult organism. This impact of connexins is partly mediated by their function as intercellular channels but an increasing number of studies support the view that at least part of the effects are truly independent of the channel function. The channel-independent function comprises extrinsic guidance of migrating cells due to connexin mediated cell adhesion as well as intracellular processes. Cx43 has been shown to exert effects on migration by interfering with receptor signalling, cytoskeletal remodelling and tubulin dynamics. These effects are mainly dependent on the presence of the carboxyl tail of Cx43. The molecular basis of this channel-independent connexin function is still not yet fully understood but early results open an exciting view towards new functions of connexins in the cell. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Non-channel functions of connexins in cell growth and cell death

Cellular communication mediated by gap junction channels and hemichannels, both composed of connexin proteins, constitutes two acknowledged regulatory platforms in the accomplishment of tissue homeostasis. In recent years, an abundance of reports has been published indicating functions for connexin proteins in the control of the cellular life cycle that occur independently of their channel activities. This has yet been most exemplified in the context of cell growth and cell death, and is therefore as such addressed in the current paper. Specific attention is hereby paid to the molecular mechanisms that underpin the cellular non-channel roles of connexin proteins, namely the alteration of the expression of tissue homeostasis determinants and the physical interaction with cell growth and cell death regulators. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Connexin 43 signaling enhances the generation of Foxp3+ regulatory T cells

Despite their importance for the functioning of the immune system, thymic development and peripheral maintenance of Foxp3(+) regulatory T (T(R)) cells are poorly understood. We have found that connexin 43 (Cx43), expressed by thymic T(R) cells progenitors, supports T(R) development. Mice with deletion of the Cx43 gene induced in T cells produce only few T(R) cells and had increased proportion of activated T cells in the lymph nodes, suggesting impaired peripheral tolerance. Reduction of the T(R) cell numbers was accompanied by increased presence of CD4(+)CD25(+)GITR(+)Foxp3(-) T cells, which did not produce inflammatory cytokines and lost suppressor function. These results strongly argue that we have discovered a novel signaling pathway, controlled by Cx43, that enhances the generation of T(R) cells. We propose that a possible mechanism of Cx43 activity is by regulating Foxp3 expression in T(R) lineage cells.

Connexins and the gap in context

Gap junctions (GJ) can no longer be thought of as simple channel forming structures that mediate intercellular communication. Hemi-channel and channel-independent functions of connexins (Cxs) have been described and numerous Cx interacting partners have been uncovered ranging from enzymes to structural and scaffolding molecules to transcription factors. With the growing number of Cx partners and functions, including well-documented roles for Cxs as conditional tumor suppressors, it has become essential to understand how Cxs are regulated in a context-dependent manner to mediate distinct functions. In this review we will shed light on the tissue and context-dependent regulation and function of Cxs and on the importance of Cx-interactions in modulating tissue-specific function. We will emphasize how the context-dependent functions of Cxs can help in understanding the impact of Cx mis-expression on cancer development and, ultimately, explore whether Cxs can be used as potential therapeutic targets in cancer treatment. In the end, we will address the need for developing relevant assays for studying Cx and GJ functions and will highlight how advances in bioengineering tools and the design of 3D biological platforms can help studying gap junction function in real time in a non-intrusive manner.

Connexins: sensors and regulators of cell cycling

It is nowadays well established that gap junctions are critical gatekeepers of cell proliferation, by controlling the intercellular exchange of essential growth regulators. In recent years, however, it has become clear that the picture is not as simple as originally anticipated, as structural precursors of gap junctions can affect cell cycling by performing actions not related to gap junctional intercellular communication. Indeed, connexin hemichannels also foresee a pathway for cell growth communication, albeit between the intracellular compartment and the extracellular environment, while connexin proteins as such can directly or indirectly influence the production of cell cycle regulators independently of their channel activities. Furthermore, a novel set of connexin-like proteins, the pannexins, have lately joined in as regulators of the cell proliferation process, which they can affect as either single units or as channel entities. In the current paper, these multifaceted aspects of connexin-related signalling in cell cycling are reviewed.

The carboxyl tail of Cx43 augments p38 mediated cell migration in a gap junction-independent manner

The expression of connexin 43 (Cx43) has been shown to correlate with an enhanced migration of several cell types such as glioma or neural crest cells, but the mechanism remains unclear. We studied whether Cx43 also affects migration in non-neural cells and whether or not this is related to gap junction formation. Therefore, we analysed the migratory activity of HeLa cells under conditions of controlled connexin (Cx) expression. The expression of Cx43 enhanced their migration significantly as compared to Cx deficient wild-type cells. Expression of only the carboxyl tail of Cx43 (Cx43CT, AA 257-382) without channel forming capacity enhanced migration similarly as the full length protein. In contrast, the expression of the N-terminal part of Cx43 (Cx43NT, AA 1-257), which partially retained the gap junction channel function of Cx43, did not increase migration. The enhanced cell migration of HeLa cells expressing either full length Cx43 or the Cx43CT was associated with an increased activation of the p38 MAP kinase. The additional incubation with a specific inhibitor of p38 activation diminished the migration of HeLa-Cx43 cells to levels of control transfected cells. As a proof of concept, we studied whether Cx43 also modulates the migration of endothelial progenitor cells (EPC) which play an important role in angiogenesis. In these cells, which expressed Cx43 as the only connexin, the downregulation of Cx43 by siRNA resulted in a significantly decreased migration. These results demonstrate that expression of Cx43 augments migration via modulation of p38 MAP kinase activity. The carboxyl tail of Cx43 plays an essential role in this signalling pathway which is independent of gap junction function.

Intramolecular loop/tail interactions are essential for connexin 43-hemichannel activity

Connexin-assembled gap junctions (GJs) and hemichannels coordinate intercellular signaling processes. Although the regulation of connexins in GJs has been well characterized, the molecular determinants controlling connexin-hemichannel activity are unresolved. Here we investigated the regulation of Cx43-hemichannel activity by actomyosin contractility and intracellular [Ca(2+)] ([Ca(2+)](i)) using plasma membrane-permeable TAT peptides (100 μM) designed to interfere with interactions between the cytoplasmic loop (CL) and carboxy-terminal (CT) in primary bovine corneal endothelial cells and HeLa, C6 glioma, and Xenopus oocytes ectopically expressing Cx43. Peptides corresponding to the last 10 CT aa (TAT-Cx43CT) prevented the inhibition of Cx43-hemichannel activity by contractility/high [Ca(2+)](i), whereas a reverse peptide (TAT-Cx43CTrev) did not. These effects were independent of zonula occludens-1, a cytoskeletal-associated Cx43-binding protein. In contrast, peptides corresponding to CL (TAT-L2) inhibited Cx43-hemichannel responses, whereas a mutant peptide (TAT-L2(H126K/I130N)) did not inhibit. In these assays, TAT-Cx43CT acted as a scaffold for TAT-L2 and vice versa, a finding supported by surface plasmon resonance measurements. Loop/tail interactions appeared essential for Cx43-hemichannel activity, because TAT-Cx43CT restored the activity of nonfunctional hemichannels, consisting of either Cx43 lacking the C-terminal tail (Cx43(M239)) or intact Cx43 ectopically expressed in Xenopus oocytes. We conclude that intramolecular loop/tail interactions control Cx43-hemichannel activity, laying the basis for developing hemichannel-specific blockers.

Inhibition of connexin 43 gap junction channels by the endocrine disruptor ioxynil

Gap junctions are intercellular plasma membrane domains containing channels that mediate transport of ions, metabolites and small signaling molecules between adjacent cells. Gap junctions play important roles in a variety of cellular processes, including regulation of cell growth and differentiation, maintenance of tissue homeostasis and embryogenesis. The constituents of gap junction channels are a family of trans-membrane proteins called connexins, of which the best-studied is connexin 43. Connexin 43 functions as a tumor suppressor protein in various tissue types and is frequently dysregulated in human cancers. The pesticide ioxynil has previously been shown to act as an endocrine disrupting chemical and has multiple effects on the thyroid axis. Furthermore, both ioxynil and its derivative ioxynil octanoate have been reported to induce tumors in animal bioassays. However, the molecular mechanisms underlying the possible tumorigenic effects of these compounds are unknown. In the present study we show that ioxynil and ioxynil octanoate are strong inhibitors of connexin 43 gap junction channels. Both compounds induced rapid loss of connexin 43 gap junctions at the plasma membrane and increased connexin 43 degradation. Ioxynil octanoate, but not ioxynil, was found to be a strong activator of ERK1/2. The compounds also had different effects on the phosphorylation status of connexin 43. Taken together, the data show that ioxynil and ioxynil octanoate are potent inhibitors of intercellular communication via gap junctions.

Connexin 43 regulates epicardial cell polarity and migration in coronary vascular development

Connexin 43 knockout (Cx43 KO) mice exhibit conotruncal malformations and coronary artery defects. We observed epicardial blisters in the Cx43 KO hearts that suggest defects in epicardial epithelial-mesenchymal transformation (EMT), a process that generates coronary vascular progenitors. Analysis using a three-dimensional collagen gel invasion assay showed that Cx43 KO epicardial cells are less invasive and that, unlike wild-type epicardial cells, they fail to organize into thin vessel-like projections. Examination of Cx43 KO hearts using Wt1 as an epicardial marker revealed a disorganized pattern of epicardial cell infiltration. Time-lapse imaging and motion analysis using epicardial explants showed a defect in directional cell migration. This was associated with changes in the actin/tubulin cytoskeleton. A defect in cell polarity was indicated by a failure of the microtubule-organizing center to align with the direction of cell migration. Forced expression of Cx43 constructs in epicardial explants showed the Cx43 tubulin-binding domain is required for Cx43 modulation of cell polarity and cell motility. Pecam staining revealed early defects in remodeling of the primitive coronary vascular plexuses in the Cx43 KO heart. Together, these findings suggest an early defect in coronary vascular development arising from a global perturbation of the cytoarchitecture of the cell. Consistent with this, we found aberrant myocardialization of the outflow tract, a process also known to be EMT dependent. Together, these findings suggest cardiac defects in the Cx43 KO mice arise from the disruption of cell polarity, a process that may be dependent on Cx43-tubulin interactions.

Ca(2+) regulation of connexin 43 hemichannels in C6 glioma and glial cells

Connexin hemichannels have a low open probability under normal conditions but open in response to various stimuli, forming a release pathway for small paracrine messengers. We investigated hemichannel-mediated ATP responses triggered by changes of intracellular Ca(2+) ([Ca(2+)](i)) in Cx43 expressing glioma cells and primary glial cells. The involvement of hemichannels was confirmed with gja1 gene-silencing and exclusion of other release mechanisms. Hemichannel responses were triggered when [Ca(2+)](i) was in the 500nM range but the responses disappeared with larger [Ca(2+)](i) transients. Ca(2+)-triggered responses induced by A23187 and glutamate activated a signaling cascade that involved calmodulin (CaM), CaM-dependent kinase II, p38 mitogen activated kinase, phospholipase A2, arachidonic acid (AA), lipoxygenases, cyclo-oxygenases, reactive oxygen species, nitric oxide and depolarization. Hemichannel responses were also triggered by activation of CaM with a Ca(2+)-like peptide or exogenous application of AA, and the cascade was furthermore operational in primary glial cells isolated from rat cortex. In addition, several positive feed-back loops contributed to amplify the responses. We conclude that an elevation of [Ca(2+)](i) triggers hemichannel opening, not by a direct action of Ca(2+) on hemichannels but via multiple intermediate signaling steps that are adjoined by distinct signaling mechanisms activated by high [Ca(2+)](i) and acting to restrain cellular ATP loss.

Connexin 43 contributes to differentiation of retinal pigment epithelial cells via cyclic AMP signaling

Retinal pigment epithelium (RPE) cells play important roles in the visual system that supports neurosensory retina homeostasis. Connexin (Cx) 43-mediated gap-junctional intercellular communication (GJIC) participates in the regulation of retinal organogenesis, but much of the function of Cx43 on the differentiation of RPE cells is unclear. Here, we report the involvement of Cx43 in RPE differentiation. Knockdown of Cx43 in RPE cells dramatically inhibited the differentiation, whereas Cx43-overexpression successfully induced RPE cell differentiation under de-differentiation conditions. From the experiments using GJIC inhibitors and C-terminus-truncated mutant of Cx43, it was clearly demonstrated that the regulation of RPE cell differentiation by Cx43 did not result from Cx43-mediated GJIC. The RPE cell differentiation induced by Cx43-overexpression was abolished by a cAMP antagonist. In contrast, the treatment with forskolin and phosphodiesterase inhibitor rolipram induced RPE cell differentiation under de-differentiation conditions. These findings indicate that Cx43 contributes to RPE differentiation via cAMP signaling.

Cytoplasmic accumulation of connexin32 protein enhances motility and metastatic ability of human hepatoma cells in vitro and in vivo

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.

Control of intracellular localization and function of Cx43 by SEMA3F

Connexin genes are considered to form a family of tumor-suppressor genes. However, the mechanism of connexin-mediated growth control is not well understood. We now provide several lines of evidence which suggest that SEMA3F, a member of the class 3 semaphorin family, which is also reported to be a tumor suppressor, controls the intracellular localization and function of connexin 43 (Cx43). We employed a series of rat liver epithelial cell lines, among which we previously found that the level of expression of malignant phenotypes (IAR20 < IAR27E < IAR6-1 < IAR27F) is inversely related to that of gap junctional intercellular communication (GJIC). When we immunostained SEMA3F and Cx43 in these cell lines, the extent of immunostaining in the plasma membrane of both proteins decreased in the order of IAR20 > IAR27E > IAR6-1 > IAR27F, suggesting a close relationship between Cx43 and SEMA3F. Further studies revealed a partial colocalization of SEMA3F and Cx43 in the plasma membrane of IAR20 cells. We also found that both SEMA3F and Cx43 moved from the cytoplasm to the plasma membrane in a mouse papilloma cell line when E-cadherin became functional after transferring the cells from low- to high-calcium conditions. When SEMA3F gene expression was inhibited by siRNA in IAR20 cells, Cx43 localization in the plasma membrane and GJIC ability were reduced. Moreover, we found that SEMA3F binds with the cytoplasmic loop domain of Cx43, employing the yeast two-hybrid complementation and screening assays. Taken together, these results strongly suggest that SEMA3F directly associates with Cx43 and controls its intracellular localization and function.

Connexin hemichannels and gap junction channels are differentially influenced by lipopolysaccharide and basic fibroblast growth factor

Gap junction (GJ) channels are formed by two hemichannels (connexons), each contributed by the cells taking part in this direct cell-cell communication conduit. Hemichannels that do not interact with their counterparts on neighboring cells feature as a release pathway for small paracrine messengers such as nucleotides, glutamate, and prostaglandins. Connexins are phosphorylated by various kinases, and we compared the effect of various kinase-activating stimuli on GJ channels and hemichannels. Using peptides identical to a short connexin (Cx) amino acid sequence to specifically block hemichannels, we found that protein kinase C, Src, and lysophosphatidic acid (LPA) inhibited GJs and hemichannel-mediated ATP release in Cx43-expressing C6 glioma cells (C6-Cx43). Lipopolysaccharide (LPS) and basic fibroblast growth factor (bFGF) inhibited GJs, but they stimulated ATP release via hemichannels in C6-Cx43. LPS and bFGF inhibited hemichannel-mediated ATP release in HeLa-Cx43 cells, but they stimulated it in HeLa-Cx43 with a truncated carboxy-terminal (CT) domain or in HeLa-Cx26, which has a very short CT. Hemichannel potentiation by LPS was inhibited by blockers of the arachidonic acid metabolism, and arachidonic acid had a potentiating effect like LPS and bFGF. We conclude that GJ channels and hemichannels display similar or oppositely directed responses to modulatory influences, depending on the balance between kinase activity and the activity of the arachidonic acid pathway. Distinctive hemichannel responses to pathological stimulation with LPS or bFGF may serve to optimize the cell response, directed at strictly controlling cellular ATP release, switching from direct GJ communication to indirect paracrine signaling, or maximizing cell-protective strategies.

Regulation of Connexin-43-Mediated Growth Inhibition by a Phosphorylatable Amino-Acid is Independent of Gap Junction-Forming Ability

The ability of the gap junction phosphoprotein connexin-43 (Cx43) to inhibit DNA synthesis in primary cardiomyocytes is regulated by serine (S) 262, a protein kinase C phosphorylation site that also affects metabolic coupling. We have now examined if the S262-regulated growth suppression is operating in transformed cells and if so whether it depends on gap junction channel forming ability. Serine 262 became phosphorylated in response to protein kinase C stimulation in HEK293 cells transiently expressing either Cx43 or the non-channel-forming carboxy-terminal tail of Cx43 (Cx43CT). Expression of either wild type Cx43 or Cx43CT inhibited DNA synthesis, as did their mutated versions simulating lack of phosphorylation by carrying an S262-to-alanine substitution. The ability to inhibit DNA synthesis was eliminated when expressing mutated versions of either Cx43 or Cx43CT simulating constitutive phosphorylation by carrying an S262-to-aspartate substitution. We conclude that S262 phosphorylation cancels growth inhibition by Cx43 independently of channel-forming ability.

Connexin32 as a tumor suppressor gene in a metastatic renal cell carcinoma cell line

Connexin genes expressing gap junction proteins have tumor-suppressive effects on primary cancers with certain cell specificity, but the suppressive effects on metastatic cancers are still conflicting. In this study, we show that connexin32 (Cx32) has a strong tumor-suppressive effect on a human metastatic renal cell carcinoma cell line (Caki-1 cell). Cx32 expression in Caki-1 cells reduced in vitro malignant phenotypes of the cells such as anchorage independency and invasion capacity. Furthermore, the Cx32 expression drastically reduced the development of Caki-1 cells in nude mice. We also determined that Cx32 reduced the malignant phenotypes in Caki-1 cells mainly through the inactivation of Src signaling. Especially, Cx32-dependent inactivation of Src decreased the production of vascular epithelial growth factor (VEGF) via the suppression of signal transducers and activators of transcription 3 (Stat3) activation, and we confirmed this result using short interfering RNA. In nude mice, Cx32-transfected Caki-1 cells showed lower serum level of VEGF comparing mock transfectant, and the development of the cells in nude mice positively related to the VEGF level. These data suggest that Cx32 acts as a tumor suppressor gene in Caki-1 cells and that the tumor-suppressive effect partly depends on the inhibition of Src-Stat3-VEGF signal pathway.

Negative growth control of renal cell carcinoma cell by connexin 32: possible involvement of Her-2

Connexin (Cx) genes have negative growth effects on tumor cells with certain cell specificity. We have previously reported that Cx32 is specifically downregulated in human renal cell carcinoma cell (RCC) lines as well as cancerous regions of kidneys and that the Cx is expressed in the progenitor cells of the carcinoma. However, the precise role of Cx32 in growth control of RCC cells remains unknown. In this study, we examined whether Cx32 could act in growth control against a human RCC cell, Caki-2 cell. In order to estimate the cell growth control, we established Caki-2 cells that have stable expression of Cx32 genes. Cx32 expression in Caki-2 cells induced contact inhibition of growth and reduced anchorage-independent growth ability, but did not significantly affect lag phase growth rates. This growth control by Cx32 was dependent on the inhibition of the cell-cycle transition from G1 to S phase at high cell density, and the inhibition of the cell-cycle transition related to the suppression of Her-2 activation. Furthermore, the suppression of Cx32 expression in Caki-2 cells by short interfering RNA induced the activation of Her-2. These data suggest that Cx32 has negative growth control of Caki-2 cells, partly due to the inhibition of the Her-2 activation.

Regulation of gap junctions by tyrosine protein kinases

Most of the gap junction proteins are regulated in part by post-translational phosphorylation. Phosphorylation has been shown to be important in gap junction assembly and turnover, and for channel function in the resting state. Connexin phosphorylation may be altered by the activation of intracellular signaling pathways in response to growth factors, tumor promoters, activated oncogenes, hormones and inflammatory mediators. In some instances altered phosphorylation has been associated with changes in connexin function and in other cases appears to be associated with changes in the levels of the connexin protein and/or mRNA. This review focuses on the role of tyrosine protein kinases in the regulation of gap junctions. The literature is most extensive for connexin43 and those studies are reviewed here. A great deal has been learned in recent years about how connexin43 is regulated by tyrosine kinase-dependent signaling pathways. These pathways are often complex and to some extent are cell type- and stimulus-dependent. Although considerable progress has been made in unraveling the cellular pathways that regulate connexin function, significant challenges remain to be addressed in identifying additional phosphorylation sites and determining the stoichiometries of the phosphorylation events that regulate connexin function and it's interaction with other cellular proteins.

Evidence for a role of the N-terminal domain in subcellular localization of the neuronal connexin36 (Cx36)

The expression and functional properties of connexin36 (Cx36) have been investigated in two neuroblastoma cell lines (Neuro2A, RT4-AC) and primary hippocampal neurons transfected with a Cx36-enhanced green fluorescent protein (EGFP) expression vector. Transfected cells express Cx36-EGFP mRNA, and Cx36-EGFP protein is localized in the perinuclear area and cell membrane. Upon differentiation of cell lines, Cx36-EGFP protein was detectable in processes with both axonal and dendritic characteristics. Small gap junction plaques were found between adjacent cells, and electrophysiological recordings demonstrated that the electrical properties of these gap junctions were virtually indistinguishable from those reported for native Cx36. Mutagenesis of Cx36 led to the identification of a structural element that interferes with normal protein localization. In contrast, site directed mutagenesis of putative protein phosphorylation motifs did not alter subcellular localization. This excludes phosphorylation/dephosphorylation as a major regulatory step in Cx36 protein transport.

Influence of transfection with connexin 26 gene on malignant potential of human hepatoma cells

We investigated the effect of transfection with connexin (Cx) 26 gene on the malignant potential of PLC/PRF/5 hepatoma cells, observing changes in their morphological features, alpha-fetoprotein (AFP) expression, cell proliferation and apoptosis in vitro, and their tumor growth in vivo. Fluorescence-activated cell sorting (FACS) analysis showed that 10.6% of PLC/PRF/5 hepatoma cells transfected with Cx26 cDNA expressed excessive Cx26, and the spread of lucifer yellow was wider in the colony of stable transfectants (PLC/Cx26) after its microinjection than in control. Nucleo-cytoplasmic (N/C) ratio was significantly lower in PLC/Cx26 (P < 0.0001). Cell proliferation assay showed significantly lower numbers in PLC/Cx26 on day 10 after seeding than in control (P = 0.0039), and AFP level /10(5) cells was significantly lower in medium of PLC/Cx26 (P = 0.0039). The number of proliferating cell nuclear antigen (PCNA)-positive cells was less in PLC/Cx26 in vitro than in control (P = 0.0039), and single-stranded DNA (ssDNA)-positive cells were more abundant in the colony of PLC/Cx26 (P = 0.029). Tumor volume in SCID mice was significantly smaller in the group of PLC/Cx26 than in the control (P < 0.01) throughout the observation period, and tumor weight of PLC/Cx26 was significantly lower (P = 0.0019) week 9 after inoculation. Transfection with Cx26 cDNA inhibited dedifferentiation, suppressed cell proliferation, and apoptosis was induced. Tumor growth of PLC/Cx26 was retarded. These findings suggest that transfection with Cx26 gene into human hepatoma cells reduces their malignant potential.

Connexin 26 gene therapy of human bladder cancer: induction of growth suppression, apoptosis, and synergy with Cisplatin

The connexin 26 (Cx26) gene encodes a protein involved in gap junctional intercellular communication and is a putative tumor suppressor. We constructed a Cx26 adenovirus vector (Ad-Cx26) and used it to infect human bladder cancer cell lines UM-UC-3, UM-UC-6, UM-UC-14, and T24. Infection with Ad-Cx26 suppressed the growth of these cell lines in vitro and prevented tumor formation in vivo. Cell cycle accumulation or arrest at the G(1) phase was noted in UM-UC-3 cells and at the G(2)/M phase in UM-UC-6, UM-UC-14, and T24 cells. Apoptosis was noted in UM-UC-3, UM-UC-6, and UM-UC-14 cells both in vitro and in vivo. These effects were not seen with control adenovirus (Ad-CTR) or mock infection. Ad-Cx26 did not significantly alter the growth of the immortalized normal human bladder cell line SV-HUC. Direct injection of Ad-Cx26 into established UM-UC-3 and UM-UC-14 tumors in nude mice resulted in Cx26 expression, apoptosis, and significantly decreased growth compared with Ad-CTR treated tumors. Delayed resumption of tumor growth was associated with loss of Cx26 expression. Combination therapy with Ad-Cx26 and cisplatin resulted in decreased growth in vitro compared with either agent alone. We explored combination therapy with Ad-Cx26 and cisplatin to improve the in vivo efficacy of Cx26 gene therapy. In vivo therapy with Ad-Cx26 and cisplatin resulted in long-term suppression of tumor growth. These data demonstrate that combining gene and chemotherapy can result in dramatic synergy in vivo.

Control of intracellular movement of connexins by E-cadherin in murine skin papilloma cells

The gap junctional intercellular communication-deficient mouse skin papilloma cell line P3/22 expresses Cx43 but not E-cadherin. The E-cadherin gene-transfected cells (P3E1) communicate in a calcium-dependent manner and they were used to study how E-cadherin restores the function of connexins. At low calcium, Cx43 molecules remain in the cytoplasm of P3E1 cells and appear at cell-cell contact areas only in high-calcium medium. While Cx43 is unphosphorylated in P3E1 cells in low-calcium medium, two phosphorylated bands appeared at high calcium. However, when Cx26, which has no C-terminal tail that can undergo phosphorylation, was expressed in P3E1 cells, this connexin also moved to the plasma membrane after the calcium shift and partly colocalized with Cx43, suggesting that C-terminal phosphorylation is not essential for E-cadherin-mediated intracellular transport of connexins. In low calcium, both Cx26 and Cx43 remained and colocalized in the endoplasmic reticulum. As early as 30 min after the shift to high-calcium medium, both Cx43 and Cx26 began to accumulate in the Golgi apparatus. Intracellular movement of connexins to the cytoplasmic membrane at high calcium was effectively blocked by cytochalasin D and brefeldin A. These results suggest that E-cadherin junction formation at high calcium leads to formation of actin cables, which directly or indirectly transport connexins from the cytoplasm to the cell-cell contact membranes via the Golgi apparatus.