Role of Cx43 phosphorylation and MAP kinase activation in EGF induced enhancement of cell communication in human kidney epithelial cells

Modulation of gap junction-associated Cx43 in neural stem/progenitor cells following traumatic brain injury

Restoration of learning and memory deficits following traumatic brain injury (TBI) is attributed, in part, to enhanced neural stem/progenitor cell (NSPCs) function. Recent findings suggest gap junction (GJ)-associated connexin 43 (Cx43) plays a key role in the cell cycle regulation and function of NSPCs and is modulated following TBI. Here, we demonstrate that Cx43 is up-regulated in the dentate gyrus following TBI and is expressed on vimentin-positive cells in the subgranular zone. To test the role of Cx43 on NSPCs, we exposed primary cultures to the α-connexin Carboxyl Terminal (αCT1) peptide which selectively modulates GJ-associated Cx43. Treatment with αCT1 substantially reduced proliferation and increased caspase 3/7 expression on NSPCs in a dose-dependent manner. αCT1 exposure also reduced overall expression of Cx43 and phospho (p)-Serine368. These findings demonstrate that Cx43 positively regulates adult NPSCs; the modulation of which may influence changes in the dentate gyrus following TBI.

Red paprika (Capsicum annuum L.) and its main carotenoids, capsanthin and β-carotene, prevent hydrogen peroxide-induced inhibition of gap-junction intercellular communication

This study was conducted to investigate the protective effect of red paprika extract (RPE) and its main carotenoids, namely, capsanthin (CST) and β-carotene (BCT), on the H2O2-induced inhibition of gap-junction intercellular communication (GJIC) in WB-F344 rat liver epithelial cells (WB cells). We found that pre-treatment with RPE, CST and BCT protected WB cells from H2O2-induced inhibition of GJIC. RPE, CST and BCT not only recovered connexin 43 (Cx43) mRNA expression but also prevented phosphorylation of Cx43 protein by H2O2 treatment. RPE attenuated the phosphorylation of ERK, p38 and JNK, whereas pre-treatment with CST and BCT only attenuated the phosphorylation of ERK and p38 and did not affect JNK in H2O2-treated WB cells. RPE, CST and BCT significantly suppressed the formation of reactive oxygen species (ROS) in H2O2-treated cells compared to untreated WB cells. These results suggest that dietary intake of red paprika might be helpful for lowering the risk of diseases caused by oxidative stress.

Connexin 43 and Its Hemichannels Mediate Hypoxia-Ischemia-Induced Cell Death in Neonatal Rats

Wistar rat pups had the left common carotid artery cut, and they were exposed to 8% oxygen with free access to food and water until they were killed at 1, 12, 24, and 48 hours after the hypoxia–ischemia (HI) insult. Connexin 43 (Cx43), hemichannel (HC1), and caspase 3 (Casp3) in cerebral HI tissues were examined by immunohistochemistry and Western blot analyses. Astrocytes cell line, astrocytes transduced with a retroviral empty vector (Psup astrocyte), or a Cx43-specific small hairpin RNA (shRNA) construct (shRNA astrocytes) was treated with oxygen–glucose deprivation (OGD) insult. The viability of astrocytes was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The results showed the expression of Cx43, HC1, and Casp3 in rats’ brain, and astrocytes and Psup astrocytes increased significantly after 24 hours of HI/OGD insult. Cell viability decreased after 24 hours of the insult. The results suggest that Cx43 and hemichannel are likely to mediate the astrocytic death after HI insult.

Epidermal growth factor regulates connexin 43 in the human epididymis: role of gap junctions in azoospermia

BACKGROUND

Gap junctions (GJs) allow for direct communication between adjacent cells. They are composed of connexons consisting of transmembrane proteins, connexins (Cxs). The objectives of this study were to determine if GJ proteins GJA1 (Cx43), GJB1 (Cx32) and GJB2 (Cx26) are present in the epididymis of men with a normal epididymis, to assess whether or not Cx expression and localization are altered in azoospermic patients, and to determine if epidermal growth factor (EGF) regulates GJA1 expression.

METHODS

Epididymides were obtained from men with localized testis cancer with active spermatogenesis and histologically normal epididymal tubule (group 1), men with non-obstructive azoospermia secondary to Sertoli-cell only syndrome (group 2) and from azoospermic men with normal spermatogenesis and epididymal obstruction (group 3). Epididymides were subdivided into three segments: caput, corpus and cauda. Quantitative real-time RT-PCR was performed to assess GJA1, GJB1, GJB2 and EGF receptor (EGFR) mRNA levels in epididymides from patients from each group (all n=3, except n=1 for caput blockage). A human caput epididymal cell line was then used to determine the role of EGFR signaling on the regulation of human epididymal GJA1.

RESULTS

Real-time RT-PCR analysis revealed that GJA1, GJB1, GJB2 and EGFR were expressed along the human epididymis. In the cauda epididymidis of group 2 and 3 men, we observed a significant decrease in GJA1 (P=0.0456 and P=0.0465, respectively) and GJB1 (P=0.0450 and P=0.0497, respectively) mRNA levels when compared with group 1 men. We also observed a decrease in EGFR mRNA levels (P=0.0358) in the cauda epididymidis of group 3 men when compared with group 1. Immunocytochemistry revealed that in the epididymis, GJA1 and EGFR were localized between basal and principal cells and between adjacent principal cells. In group 2 and 3 patients, however, we noted a dramatic increase in cytosolic immunostaining for both GJA1 and EGFR in both principal and basal cells. Using a human caput epididymal cell line derived from fertile men, we demonstrated that changes in GJA1 phosphorylation could be regulated by EGF (P=0.015) and the extracellular regulated kinase 1/2 signaling pathway (P=0.03). Furthermore, while the phosphoinositide-3-kinase (PI3K)/AKT signaling pathway did not alter GJA1 phosphorylation, treatment with PI3K/AKT inhibitor LY294002 significantly (P=0.024) inhibited the EGF-stimulated increase in GJA1 total protein levels at 24 h. Immunolocalization indicated that loss of PI3K/AKT signaling was associated with increased cytosolic localization of Cx43 in this cell line.

CONCLUSIONS

Together, these data suggest that in azoospermic men decreased expression of EGFR may be responsible for decreasing GJA1 levels and increasing its cytosolic localization via the PI3K/AKT signaling pathway.

Modulation of gap junction channels and hemichannels by growth factors

Gap junction hemichannels and cell-cell channels have roles in coordinating numerous cellular processes, due to their permeability to extra and intracellular signaling molecules. Another mechanism of cellular coordination is provided by a vast array of growth factors that interact with relatively selective cell membrane receptors. These receptors can affect cellular transduction pathways, including alteration of intracellular concentration of free Ca(2+) and free radicals and activation of protein kinases or phosphatases. Connexin and pannexin based channels constitute recently described targets of growth factor signal transduction pathways, but little is known regarding the effects of growth factor signaling on pannexin based channels. The effects of growth factors on these two channel types seem to depend on the cell type, cell stage and connexin and pannexin isoform expressed. The functional state of hemichannels and gap junction channels are affected in opposite directions by FGF-1 via protein kinase-dependent mechanisms. These changes are largely explained by channels insertion in or withdrawal from the cell membrane, but changes in open probability might also occur due to changes in phosphorylation and redox state of channel subunits. The functional consequence of variation in cell-cell communication via these membrane channels is implicated in disease as well as normal cellular responses.

Inhibition of gap junctional Intercellular communication in WB-F344 rat liver epithelial cells by triphenyltin chloride through MAPK and PI3-kinase pathways

Background

Organotin compounds (OTCs) have been widely used as stabilizers in the production of plastic, agricultural pesticides, antifoulant plaints and wood preservation. The toxicity of triphenyltin (TPT) compounds was known for their embryotoxic, neurotoxic, genotoxic and immunotoxic effects in mammals. The carcinogenicity of TPT was not well understood and few studies had discussed the effects of OTCs on gap junctional intercellular communication (GJIC) of cells.

Method

In the present study, the effects of triphenyltin chloride (TPTC) on GJIC in WB-F344 rat liver epithelial cells were evaluated, using the scrape-loading dye transfer technique.

Results

TPTC inhibited GJIC after a 30-min exposure in a concentration- and time-dependent manner. Pre-incubation of cells with the protein kinase C (PKC) inhibitor did not modify the response, but the specific MEK 1 inhibitor PD98059 and PI3K inhibitor LY294002 decreased substantially the inhibition of GJIC by TPTC. After WB-F344 cells were exposed to TPTC, phosphorylation of Cx43 increased as seen in Western blot analysis.

Conclusions

These results show that TPTC inhibits GJIC in WB-F344 rat liver epithelial cells by altering the Cx43 protein expression through both MAPK and PI3-kinase pathways.

Human adipose tissue-derived multipotent stem cells differentiate in vitro and in vivo into osteocyte-like cells

Cell-based therapies are used to treat bone defects. We recently described that human multipotent adipose-derived stem (hMADS) cells, which exhibit a normal karyotype, self renewal, and the maintenance of their differentiation properties, are able to differentiate into different lineages. Herein, we show that hMADS cells can differentiate into osteocyte-like cells. In the presence of a low amount of serum and EGF, hMADS cells express specific molecular markers, among which alkaline phosphatase, CBFA-1, osteocalcin, DMP1, PHEX, and podoplanin and develop functional gap-junctions. When loaded on a hardening injectable bone substitute (HIBS) biomaterial and injected subcutaneously into nude mice, hMADS cells develop mineralized woven bone 4 weeks after implantation. Thus hMADS cells represent a valuable tool for pharmacological and biological studies of osteoblast differentiation in vitro and bone development in vivo.

Müllerian anomalies, hearing loss, and connexin 26 mutations

In a study of the possible inter-relationship among müllerian anomalies, hearing loss, and connexin 26 mutations, we evaluated all women in whom müllerian anomalies were diagnosed on hysterosalpingography during a 6-year period (n = 24/519). Audiometric testing revealed five (22.7%) with unrecognized sensorineural hearing loss; however, on genetic evaluation, none harbored a connexin 26 mutation.

Reversal of the TPA-induced inhibition of gap junctional intercellular communication by Chaga mushroom (Inonotus obliquus) extracts: effects on MAP kinases

Chaga mushroom (Inonotus obliquus) has continued to receive attention as a folk medicine with indications for the treatment of cancers and digestive diseases. The anticarcinogenic effect of Chaga mushroom extract was investigated using a model system of gap junctional intercellular communication (GJIC) in WB-F344 normal rat liver epithelial cells. The cells were pre-incubated with Chaga mushroom extracts (5, 10, 20 microg/ml) for 24 h and this was followed by co-treatment with Chaga mushroom extracts and TPA (12-O-tetradecanoylphorbol-13-acetate, 10 ng/ml) for 1 h. The inhibition of GJIC by TPA (12-O-tetradecanoylphorbol-13-acetate), promoter of cancer, was prevented with treatment of Chaga mushroom extracts. Similarly, the increased phosphorylated ERK1/2 and p38 protein kinases were markedly reduced in Chaga mushroom extracts-treated cells. There was no change in the JNK kinase protein level, suggesting that Chaga mushroom extracts could only block the activation of ERK1/2 and p38 MAP kinase. The Chaga mushroom extracts further prevented the inhibition of GJIC through the blocking of Cx43 phosphorylation. Indeed cell-to-cell communication through gap junctional channels is a critical factor in the life and death balance of cells because GJIC has an important function in maintaining tissue homeostasis through the regulation of cell growth, differentiation, apoptosis and adaptive functions of differentiated cells. Thus Chaga mushroom may act as a natural anticancer product by preventing the inhibition of GJIC through the inactivation of ERK1/2 and p38 MAP kinase.

Connexin phosphorylation as a regulatory event linked to gap junction internalization and degradation

Gap junction proteins, connexins, are dynamic polytopic membrane proteins that exhibit unprecedented short half-lives of only a few hours. Consequently, it is well accepted that in addition to channel gating, gap junctional intercellular communication is regulated by connexin biosynthesis, transport and assembly as well as the formation and removal of gap junctions from the cell surface. At least nine members of the 20-member connexin family are known to be phosphorylated en route or during their assembly into gap junctions. For some connexins, notably Cx43, evidence exists that phosphorylation may trigger its internalization and degradation. In recent years it has become apparent that the mechanisms underlying the regulation of connexin turnover are quite complex with the identification of many connexin binding molecules, a multiplicity of protein kinases that phosphorylate connexins and the involvement of both lysosomal and proteasomal pathways in degrading connexins. This paper will review the evidence that connexin phosphorylation regulates, stimulates or triggers gap junction disassembly, internalization and degradation.

The effects of connexin phosphorylation on gap junctional communication

Gap junctions are specialized membrane domains composed of collections of channels that directly connect neighboring cells providing for the cell-to-cell diffusion of small molecules, including ions, amino acids, nucleotides, and second messengers. Vertebrate gap junctions are composed of proteins encoded by the "connexin" gene family. In most cases examined, connexins are modified post-translationally by phosphorylation. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the connexin "lifecycle", such as the trafficking, assembly/disassembly, degradation, as well as, the gating of gap junction channels. Since connexin43 (Cx43) is widely expressed in tissues and cell lines, we understand the most about how it is regulated, and thus, connexin43 phosphorylation is a major focus of this review. Recent reports utilizing new methodologies combined with the latest genome information have shown that activation of several kinases including protein kinase A, protein kinase C, p34(cdc2)/cyclin B kinase, casein kinase 1, mitogen-activated protein (MAP) kinase and pp60(src) kinase can lead to phosphorylation at 12 of the 21 serine and two of the six tyrosine residues in the C-terminal region of connexin43. In several cases, use of site-directed mutants of these sites have shown that these specific phosphorylation events can be linked to changes in gap junctional communication.

Mechanism of oleic acid-induced gap junctional disassembly in rat cardiomyocytes

This study investigated the mechanism of oleic acid (OA) on gap junctions and identified the protein kinase C (PKC) isoforms involved in OA-mediated gap junction disassembly in cardiomyocytes. Control cardiomyocytes showed continuous staining of the plasma membrane at cell-cell contact areas using antibodies reacting with connexin 43 (Cx43). The spontaneous contraction rate of cultured cardiomyocytes was reduced in a time-dependent manner by OA. In addition, Cx43 expression at cell-cell junction decreased, suggesting the disassembly of gap junction. Staining for PKC and PKCalpha, which were shown to colocalize with Cx43, also decreased with increased duration of OA treatment. The effects of OA on these distributional changes at cell junctions were reversed by 24 h incubation in fresh culture medium devoid of OA. Immunoprecipitation assays confirmed the biochemical binding between Cx43 and PKC/PKCalpha, and this protein interaction was not affected by OA. This may provide the basis for simultaneous detachment of Cx and PKC/PKCalpha from the cell-cell junction to the cytosol upon OA stimulation. Western blot analysis showed that OA-induced Cx43 Ser368 phosphorylation, and that this effect could be blocked by cotreatment with the general PKC inhibitor, calphostin C, the PKC inhibitor, eV1-2, or the Src kinase inhibitor, PP1, but not by the PKCalpha inhibitor, Gö6976. eV1-2 also prevented the OA-induced disassembly of gap junctions. Taken together, these data suggest that OA-induced Cx43 Ser368 phosphorylation is mediated by activation of PKC and Src kinase and might be responsible for OA-induced gap junctional disassembly.

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.

Effects of 18beta-glycyrrhetinic acid on the junctional complex and steroidogenesis in rat adrenocortical cells

Cellular junctions play important roles in cell differentiation, signal transduction, and cell function. This study investigated their function in steroid secretion by adrenal cells. Immunofluorescence staining revealed the presence of gap junctions and adherens junctions between adrenal cells. The major gap junction protein, connexin43, was seen as a linear dotted pattern of the typical gap junction plaques, in contrast to alpha-, beta-, and gamma-catenin, which were seen as continuous, linear staining of cell-cell adherens junction. Treatment with 18beta-glycyrrhetinic acid, a gap junction inhibitor, reduced the immunoreactivity of these proteins in a time- and dose-dependent manner, and caused the gap junction and adherens junction to separate longitudinally from the cell-cell contact sites, indicating the structural interdependency of these two junctions. Interestingly, 18beta-glycyrrhetinic acid stimulated a two- to three-fold increase in steroid production in these adrenal cells lacking intact cell junctions. These data raise the question of the necessity for cell communication for the endocrine function of adrenal cells. Pharmacological analyses indicated that the steroidogenic effect of 18beta-glycyrrhetinic acid was partially mediated by extracellular signal-related kinase and calcium/calmodulin-dependent kinase, a pathway distinct from the protein kinase A signaling pathway already known to mediate steroidogenesis in adrenal cells.

Connexin43 phosphorylation at S368 is acute during S and G2/M and in response to protein kinase C activation

Phorbol esters such as 12-O-tetradeconylphorbol-13-acetate (TPA) activate protein kinase C, increase Connexin43 (Cx43) phosphorylation, and decrease cell-cell communication via gap junctions in many cell types. Previous work has implicated protein kinase C (PKC) in the direct phosphorylation of Cx43 at S368, which results in a change in single channel behavior that contributes to a decrease in intercellular communication. We have examined Cx43 phosphorylation in several cell lines with an antibody specific for phosphorylated S368. We show that this antibody detects Cx43 only when it is phosphorylated at S368 and, consistent with previous results, TPA treatment causes a dramatic increase in phosphorylation at S368. However, in some cell types, the increased phosphorylation at S368 did not cause a detectable shift in migration as compared with the nonphosphorylated Cx43. Immunofluorescence showed increased S368 immunolabeling in cytoplasmic and plasma membrane structures in response to TPA. Immunoblot analysis of synchronized cells showed increased phosphorylation at S368 during S and G2/M phases of the cell cycle. S-phase cells contained more total Cx43 but assembled fewer functional gap junctional channels than G0-phase cells. Since M-phase cells also communicate poorly and contain few assembled gap junctions, phosphorylation at S368 appears to be negatively correlated with gap junction assembly. Thus, both gap junctional communication and S368 phosphorylation change during S phase and G2/M, implying that phosphorylation at S368 might play a role in key cell-cycle events.

Epidermal growth factor regulation of connexin 43 in cultured granulosa cells from preantral rabbit follicles

Connexin 43 (Cx43), a gap junction protein expressed in differentiated granulosa cells, is necessary for normal follicular development. Cx43 expression and regulation by epidermal growth factor (EGF) were characterized in immature rabbit granulosa cells. Cx43 mRNA was expressed in the granulosa cells of primary follicles, but was undetectable in primordial follicles. Abundant expression of Cx43 mRNA was maintained in the granulosa cells of growing follicles through maturity. Granulosa cells were isolated from early preantral follicles and maintained in monolayer cultures for 72 hr. After the first 24 hr of culture, they were maintained for 48 hr in serum-free medium supplemented with 0, 1, 5, or 10 ng/ml of mouse EGF. Granulosa cell proteins were isolated, solubilized, and evaluated for Cx43 by Western blot analysis using antibodies to rat Cx43. Relative amounts of Cx43 protein (both phosphorylated and nonphosphorylated) were increased (P < 0.05) by EGF in a dose-dependent manner. Northern blot analysis of RNA from cultured granulosa cells demonstrated increased amounts of Cx43 mRNA in the EGF treated cultures (10 ng EGF/ml) relative to controls (P < 0.03). In summary, Cx43 gap junctions are synthesized in granulosa cells following the onset of folliculogenesis in vivo and their expression is enhanced by EGF in vitro.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.

2',5'-Dihydroxychalcone down-regulates endothelial connexin43 gap junctions and affects MAP kinase activation

We examined the effect of 2',5'-dihydroxychalcone on connexin43 (Cx43) expression and gap-junctional communication in human umbilical vein endothelial cells (HUVEC). The result showed that expression of Cx43 is rapidly reduced by 2',5'-dihydroxychalcone in a dose-dependent manner, Concomitantly, the communication function, determined by fluorescence recovery after photobleaching (FRAP), is decreased. We further investigated whether the mitogen-activated protein (MAP) kinase and the degradation pathway of gap junctions are involved in these processes. Although the change of Cx43 is not affected by the level of fetal calf serum (FCS) used in the medium, activation of MAP kinase varies, depending on the FCS level. At a low level (0.5%), the chalcone inhibits the activation, like PD98059, a specific inhibitor of MAP kinase kinase. However, at a high level (20%), MAP kinase is activated. On the other hand, the chalcone's down-regulating effect on Cx43, while is totally blocked by protease inhibitors leupeptin and N-acetyl-leucyl-norleucinal (ALLN), persists in the presence of PD98059, We concluded that 2',5'-dihydroxychalcone down-regulates Cx43 expression and gap-junctional communication in the HUVEC via enhancement of the proteolysis pathway, and this compound possesses dual effects on MAP kinase activation.

Epidermal growth factor enhances expression of connexin 43 protein in cultured porcine preantral follicles

Connexin 43 (Cx43) and gap junctional coupling appear to play a critical role in early follicular development because absence of Cx43 disrupts progression of follicles beyond primary stages in transgenic mouse ovaries. Two experimental culture systems were used to determine whether epidermal growth factor (EGF) stimulates expression of Cx43 in early porcine follicular development. Ovarian explants were collected from 32- to 40-day-old gilts and cultured for 6 days on membrane inserts in Waymouth MB 752/1 medium supplemented with 0, 50, or 500 ng/ml mouse EGF. Western blot analysis demonstrated significant increases (P < 0.05) in relative amounts of Cx43 protein (both phosphorylated and nonphosphorylated) with 50 and 500 ng/ml of EGF as compared with control cultures. Preantral follicles were enzymatically isolated from 70- to 86-day-old gilts and cultured for 8 days in collagen matrices. Medium and EGF treatments were the same as previously described. Western blot analysis demonstrated a significant increase (P < 0.05) in relative amounts of Cx43 protein with 50 and 500 ng/ml of EGF as compared with control cultures. EGF increased expression of Cx43 protein in secondary preantral follicles in a dose-dependent manner, which suggests that EGF or similar growth factor molecules may modulate early folliculogenesis by stimulating expression of Cx43 gap junctions.

Increase in gap-junctional intercellular communications (GJIC) of normal human dermal fibroblasts (NHDF) on surfaces coated with high-molecular-weight hyaluronic acid (HMW HA)

Normal human dermal fibroblast (NHDF) cells were used to detect differences in gap-junctional intercellular communication (GJIC) by hyaluronic acid (HA), a linear polymer built from repeating disaccharide units that consist of N-acetyl-D-glucosamine (GlcNa) and D-glucuronic acid (GlcA) linked by a beta 1-4 glycosidic bond. The NHDF cells were cultured with different molecular weights (MW) of HA for 4 days. The rates of cell attachment in dishes coated with high-molecular-weight (HMW; 310 kDa or 800 kDa) HA at 2 mg/dish were significantly reduced at an early time point compared with low-molecular-weight (LMW; 4.8 kDa or 48 kDa) HA with the same coating amounts. HA-coated surfaces were observed by atomic force microscopy (AFM) under air and showed that HA molecules ran parallel in the dish coated with LMW HA and had an aggregated island structure in the dish coated with HMW HA surfaces. The cell functions of GJIC were assayed by a scrape-loading dye transfer (SLDT) method using a dye solution of Lucifer yellow. Promotion of the dye transfer was clearly obtained in the cell monolayer grown on the surface coated with HMW HA. These results suggest that HMW HA promotes the function of GJIC in NHDF cells. In contrast, when HMW HA was added to the monolayer of NHDF cells, the functions of GJIC clearly were lowered in comparison with the cells grown in the control dish or with those grown on the surface of HMW HA. Therefore it is concluded that the MW size of HA and its application method are important factors for generating biocompatible tissue-engineered products because of the manner in which the GJIC participates in cell differentiation and cell growth rate.

Determination of the epigenetic effects of ochratoxin in a human kidney and a rat liver epithelial cell line

Epidemiological studies have implicated ochratoxin A (OTA), a fungal metabolic-contaminant of animal and human food sources, in Balkan Endemic Nephropathy and renal tumors. Many environmental toxicants operate through nongenotoxic mechanisms that epigenetically control gene expression leading to a diseased state. Gap junctional intercellular communication (GJIC) plays a central role in the epigenetic control of genes in which alteration of normal GJIC has been implicated in many human pathologies, including cancer, teratogenesis, reproductive dysfunction and peripheral neuropathies. The cell proliferative stages of human diseases, such as cancer, also involves the induction of signal transduction pathways controlling the mitogenic steps, in which the mitogen activated protein kinases (MAPK), such as extracellular receptor kinase (ERK) and p38, are central to mitogenesis. We therefore determined the effects of OTA on GJIC and MAPK in a human kidney and rat liver epithelial cell line. OTA reversibly inhibited GJIC at noncytotoxic doses in the rat liver but not the human kidney cell line. Similarly, OTA was also a strong activator of MAPK, ERK and p38, in the rat liver cells but only weakly activated ERK and had no affect on p38 in the human kidney cell line. Another hallmark of human diseases is an abnormal alteration of apoptosis, also known as programmed cell death. We used our myc-transfected cell line, which exhibits higher levels of apoptosis, to test the effects of OTA on apoptosis. OTA greatly induced apoptosis in this cell line, which is contrary to the effects of most tumor promoters. In summary, OTA exhibits tumor promoting properties in the liver, but the effects of OTA on the human kidney epithelial cells suggested a lack of tumorigenic activity assuming that these epithelial cells, like the rat liver epithelial cells, are a primary target for carcinogens. These results also indicate that the nephrotoxicity of OTA either does not involve GJIC, assuming these epithelial cells play a vital role in kidney physiology, or that a more differentiated kidney cell type is the target for OTA toxicity, of which the role of GJIC remains unknown.

Epidermal growth factor down-regulates connexin-43 expression in cultured rat cortical astrocytes

Astrocytes are coupled via gap junction channels, predominantly formed by connexin-43 (Cx43), and contribute to neuronal function in the normal and diseased brain. In this study, we demonstrate that epidermal growth factor (EGF), applied to cortical astrocytes, results in a decrease in the expression of Cx43 mRNA and protein. We have further shown that the decrease is associated with the receptor tyrosine kinase pathway and the MEK inhibitor prevents EGF-stimulated down-regulation of Cx43 expression. These findings demonstrate a previously unknown function of EGF on cultured astrocytes, which may be relevant to the regulation of astrocytic growth and differentiation.

Pharmacological evidence for system-dependent involvement of protein kinase C isoenzymes in phorbol ester-suppressed gap junctional communication

Several phorbol esters are potent activators of protein kinase C. They down-regulate gap junctional intercellular communication and induce phosphorylation of connexin43, but the sensitivity and extent of responses vary much between systems. We asked whether the total protein kinase C enzyme activity or the protein kinase C isoenzyme constitution was of importance for such variations. Some fibroblastic culture systems were compared. It was concluded that the total protein kinase C enzyme activity did not determine the sensitivity to phorbol esters. Furthermore, the use of isotype-specific inhibitors of protein kinase C indicated that protein kinase C alpha, delta, and epsilon may be involved to different extents in different fibroblastic systems in the response to phorbol esters.

VEGF transiently disrupts gap junctional communication in endothelial cells

Vascular endothelial growth factor, VEGF, stimulates angiogenesis by directly acting on endothelial cells. The effects of VEGF are mediated by two tyrosine kinase receptors, VEGFR-1 (Flt-1) and VEGFR-2 (Flk-1/KDR) that are highly related to receptors of the platelet derived growth factor (PDGF) receptor family. We are interested in early signalling events downstream from VEGF receptors that affect blood vessel homeostasis. Endothelial cells form multiple types of cell-cell junctions that are required for cellular organization into complex networks. These junctions also regulate communication among adjacent cells. Stimulation by various growth factors such as epidermal growth factor (EGF) or PDGF has been shown to disrupt cell-cell junctions, consequently affecting cell-to-cell communication. We investigated gap junctional communication (GJC) by monitoring the transfer of a low molecular mass fluorescent tracer molecule between adjacent cells using immunofluorescence microscopy. VEGF maximally blocked GJC 15 minutes after growth factor administration. The cells resumed communication via gap junctions within 1–2 hours after treatment. This early effect of VEGF on communication correlated with changes in the phosphorylation state of one of the proteins involved in gap junction formation, connexin 43 (Cx43). The signalling mechanisms involved in this phenomenon depend on activation of VEGFR-2, impinge on a tyrosine kinase of the Src family and activate the Erk family of MAP kinases. The function of VEGF-mediated disruption of GJC might be to restrict an increase in endothelium permeability to the environment affected by local injury to blood vessels.

Phosphorylation of connexin43 and inhibition of gap junctional communication in 12-O-tetradecanoylphorbol-13-acetate-exposed R6 fibroblasts: minor role of protein kinase C beta I and mu

12-O:-tetradecanoylphorbol-13-acetate (TPA) inhibits gap junctional communication in many cell culture systems, but TPA-induced phosphorylation of the gap junction protein connexin43 (Cx43) varies much between systems. We have here studied whether these responses and their sensitivities can be correlated with total protein kinase C (PKC) enzyme activity and if specific PKC isoenzymes are involved. Rat R6 fibroblasts transfected with the cDNA sequence encoding PKC beta I (R6-PKC3) had a total PKC activity 7- to 16-fold higher than the corresponding control cells (R6-C1), depending on the selection pressure (G418 concentration). Still, R6-PKC3 cells were no more sensitive than R6-C1 cells to TPA-induced down-regulation of communication, except at the highest selection pressure (500 micrograms/ml G418). Thus, total PKC activity does not indicate absolute sensitivity of a cell system to TPA-induced suppression of communication, but within a certain cell system increasing PKC activity may enhance the sensitivity to TPA in this respect. The results also suggest that PKC beta I is of minor importance for TPA-induced regulation of communication. Experiments with the Lilly compound 379196, a PKC beta-specific inhibitor, further supported this conclusion. Except for PKC beta I in R6-PKC3 cells, both cell lines contained the TPA-responsive PKC isoenzymes alpha, delta, epsilon and mu. Long-term treatment with TPA caused strong down-regulation of PKC alpha, delta and epsilon, but little down-regulation of PKC mu. Concurrently, the cells became refractory to repeated exposure to TPA, indicating that PKC mu is of minor importance. Experiments with the general PKC inhibitor GF109203X and the PKC alpha (and beta/gamma) inhibitor Gö6976 suggested that both classical (alpha) and novel PKCs (delta and epsilon) might be involved in TPA-induced suppression of intercellular communication, while phosphorylation of Cx43 may mainly be mediated by PKC alpha in the present systems.

Upregulation of gap junction connexin 32 with epileptiform activity in the isolated mouse hippocampus

Gap junctions, which serve as intercellular channels providing direct cytoplasmic continuity and ionic current flow between adjacent cells, are constituted by connexin proteins. Using an in vitro model of bicuculline-induced epileptiform activity, we asked whether increased connexin levels occur during epileptiform activity in the intact whole hippocampus, freshly isolated from young (15-day-old) mouse brain. Exposure to bicuculline (10 microM), for 2-10 h, induced persistent changes in electrical activities that included enhanced spontaneous field activity (4 h), an epileptiform response to single electrical stimulation (6 h), and spontaneous epileptiform activity (6 h). These electrophysiological changes were not reversed by up to 60 min perfusion with normal artificial cerebrospinal fluid, but were greatly depressed by the gap junction uncoupler, carbenoxolone (120 microM, 10 min). Data from RNase protection assay and immunoblotting showed that among several detected gap junctions, only connexin 32 was affected. After 2-6 h exposure to bicuculline, the connexin 32 mRNA expression was upregulated to 2-3-fold control (P < 0.01), and its protein level was significantly elevated the following 6 h (P < 0.01), at which time electrophysiologically measured evidence of clearly epileptiform activity was apparent. In addition, the transcription factor, c-fos protein, but not the cAMP response element-binding protein, was also found to be increased at the early stage of bicuculline exposure (2 h) compared to control (P < 0.05).Thus, we have found that exposing the acutely isolated hippocampus to bicuculline, induced increased c-fos protein, followed by increased connexin 32 transcript and protein, and concurrently, persistent epileptiform activity that was depressed by carbenoxolone.

Regulation of gap junctions by phosphorylation of connexins

Gap junctions are a unique type of intercellular junction found in most animal cell types. Gap junctions permit the intercellular passage of small molecules and have been implicated in diverse biological processes, such as development, cellular metabolism, and cellular growth control. In vertebrates, gap junctions are composed of proteins from the "connexin" gene family. The majority of connexins are modified posttranslationally by phosphorylation, primarily on serine amino acids; however, phosphotyrosine has also been detected in connexin from cells coexpressing nonreceptor tyrosine protein kinases. Connexins are targeted by numerous protein kinases, of which some have been identified: protein kinase C, mitogen-activated protein kinase, and the v-Src tyrosine protein kinase. Phosphorylation has been implicated in the regulation of a broad variety of connexin processes, such as the trafficking, assembly/disassembly, degradation, as well as the gating of gap junction channels. This review examines the consequences of connexin phosphorylation for the regulation of gap junctional communication.

Regulation of Cx43 gap junctions: the gatekeeper and the password

Gap junctions are regulatable pores that connect the cytoplasms of neighboring cells. Hossain and Boynton focus on connexin 43 gap junctions and their regulation by changing the phosphorylation status of the COOH-terminal domain of connexin 43 or by altering protein-protein interactions in this region. The COOH-terminal domain of connexin 43 appears to be a key player in regulating gap junctional communication (GJC) because many divergent signals in many different cell types modify this domain to inhibit GJC.