Epidermal growth factor- and stress-induced loss of gap junctional communication is mediated by ERK-1/ERK-2 but not ERK-5 in rat liver epithelial cells

Extracellular regulated kinase 5 mediates osteoporosis through modulating viability and apoptosis of osteoblasts in ovariectomized rats

Postmenopausal osteoporosis is a common condition characterized by the increase and activation of osteoclasts. The present study aimed to investigate the effects of extracellular signal-regulated kinase (ERK) 5 (ERK-5) on postmenopausal osteoporosis by regulating the biological behaviors of osteoblasts. Sprague-Dawley (SD) rats were ovariectomized to develop an osteoporosis model. A lentivirus packaging system was employed to generate lentiviruses capable of up- or down-regulating the expression of ERK-5 in ovariectomized rats. The femoral biomechanical properties, bone mineral density (BMD), contents of calcium (Ca), phosphorus (P) and alkaline phosphatase (ALP) and bone turnover markers in rats, as well as viability, cycle and apoptosis of osteoblasts and ALP activity in osteoblasts were measured in the ovariectomized rats so as to explore the functional significance of ERK-5 in postmenopausal osteoporosis. The femoral mechanical strength of ovariectomized rats was enhanced by overexpression of ERK-5. Meanwhile femoral BMD, and bone metabolism were increased, and bone turnover normalized in the ovariectomized rats when ERK-5 was overexpressed. Lentivirus-mediated ERK-5 overexpression in osteoblasts was observed to inhibit osteoblast apoptosis, and promote viability, accompanied with increased ALP activity. Taken together, ERK-5 could decelerate osteoblast apoptosis and improve postmenopausal osteoporosis by increasing osteoblast viability. Thus, our study provides further understanding on a promising therapeutic target for postmenopausal osteoporosis.

Connexin 43 in the development and progression of breast cancer: What's the connection? (Review)

Connexin 43 is a prominent gap junction protein within normal human breast tissue. Thus far, there have been a number of research studies performed to determine the function of connexin 43 in breast tumor formation and progression. Within primary tumors, research suggests that the level of connexin 43 expression in breast tumors is altered when compared to normal human breast tissue. While some reports indicate that connexin 43 levels decrease, other evidence suggests that connexin 43 levels are increased and protein localization shifts from the plasma membrane to the cytoplasm. In either case, the prevailing theory is that breast tumor cells have reduced gap junction intercellular communication within primary tumors. The current consensus appears to be that the loss of connexin 43 gap junction intercellular communication is an early event in malignancy, with the possibility of gap junction restoration in the event of metastasis. However, additional evidence is needed to support the latter claim. The purpose of this report is to review the connexin 43 literature that describes studies using human tissue samples, in order to evaluate the function of connexin 43 protein in normal human breast tissue as well as the role of connexin 43 in human breast tumor formation and metastatic progression.

1,4-naphthoquinones: from oxidative damage to cellular and inter-cellular signaling

Naphthoquinones may cause oxidative stress in exposed cells and, therefore, affect redox signaling. Here, contributions of redox cycling and alkylating properties of quinones (both natural and synthetic, such as plumbagin, juglone, lawsone, menadione, methoxy-naphthoquinones, and others) to cellular and inter-cellular signaling processes are discussed: (i) naphthoquinone-induced Nrf2-dependent modulation of gene expression and its potentially beneficial outcome; (ii) the modulation of receptor tyrosine kinases, such as the epidermal growth factor receptor by naphthoquinones, resulting in altered gap junctional intercellular communication. Generation of reactive oxygen species and modulation of redox signaling are properties of naphthoquinones that render them interesting leads for the development of novel compounds of potential use in various therapeutic settings.

EGF induces efficient Cx43 gap junction endocytosis in mouse embryonic stem cell colonies via phosphorylation of Ser262, Ser279/282, and Ser368

Gap junctions (GJs) traverse apposing membranes of neighboring cells to mediate intercellular communication by passive diffusion of signaling molecules. We have shown previously that cells endocytose GJs utilizing the clathrin machinery. Endocytosis generates cytoplasmic double-membrane vesicles termed annular gap junctions or connexosomes. However, the signaling pathways and protein modifications that trigger GJ endocytosis are largely unknown. Treating mouse embryonic stem cell colonies - endogenously expressing the GJ protein connexin43 (Cx43) - with epidermal growth factor (EGF) inhibited intercellular communication by 64% and activated both, MAPK and PKC signaling cascades to phosphorylate Cx43 on serines 262, 279/282, and 368. Upon EGF treatment Cx43 phosphorylation transiently increased up to 4-fold and induced efficient (66.4%) GJ endocytosis as evidenced by a 5.9-fold increase in Cx43/clathrin co-precipitation.

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.

Rapid changes in connexin-43 in response to genotoxic stress stabilize cell-cell communication in corneal endothelium

PURPOSE

To determine how corneal endothelial (CE) cells respond to acute genotoxic stress through changes in connexin-43 (Cx43) and gap junction intercellular communication (GJIC).

METHODS

Cultured bovine CE cells were exposed to mitomycin C or other DNA-damaging agents. Changes in the levels, stability, binding partners, and trafficking of Cx43 were assessed by Western blot analysis and immunostaining. Live-cell imaging of a Cx43-green fluorescent protein (GFP) fusion protein was used to evaluate internalization of cell surface Cx43. Dye transfer and fluorescent recovery after photobleaching (FRAP) assessed GJIC.

RESULTS

After genotoxic stress, Cx43 accumulated in large gap junction plaques, had reduced zonula occludens-1 binding, and displayed increased stability. Live-cell imaging of Cx43-GFP plaques in stressed CE cells revealed reduced gap junction internalization and degradation compared to control cells. Mitomycin C enhanced transport of Cx43 from the endoplasmic reticulum to the cell surface and formation of gap junction plaques. Mitomycin C treatment also protected GJIC from disruption after cytokine treatment.

DISCUSSION

These results show a novel CE cell response to genotoxic stress mediated by marked and rapid changes in Cx43 and GJIC. This stabilization of cell-cell communication may be an important early adaptation to acute stressors encountered by CE.

Loss of gap junctional intercellular communication in rat lung epithelial cells exposed to carbon or silica-based nanoparticles

The aim of this study was to investigate whether fine and ultrafine carbon black (fC and ufC), and fine and ultrafine silica (fS, ufS) particles affect gap junctional intercellular communication (GJIC) in rat lung epithelial cells. Exposure of cells to subcytotoxic doses of ufC, fS and ufS resulted in a 63%, 59% and 77% reduction of GJIC, respectively, as determined in a dye transfer assay. In contrast to ufC, fC did not significantly alter GJIC. Changes in subcellular localization of the major gap junction protein in RLE cells, connexin-43 (Cx43), and of β-catenin were observed in cells exposed to ufC, fS or ufS. The loss of GJIC was counteracted by N-acetyl cysteine and was largely prevented by specific inhibitors of epidermal growth factor receptor-dependent signaling, pointing to the crucial role of two known major mediators of nanoparticle action, namely reactive oxygen species and membrane-receptor signaling, in particle-induced modulation of GJIC.

Mechanisms of gap junction traffic in health and disease

Gap junctions (GJs) allow direct communication between cells. In the heart, GJs mediate the electrical coupling of cardiomyocytes and as such dictate the speed and direction of cardiac conduction. A prominent feature of acquired structural heart disease is remodeling of GJ protein expression and localization concomitant with increased susceptibility to lethal arrhythmias, leading many to hypothesize that the two are causally linked. Detailed understanding of the cellular mechanisms that regulate GJ localization and function within cardiomyocytes may therefore uncover potential therapeutic strategies for a significant clinical problem. This review will outline our current understanding of GJ cell biology with the intent of highlighting cellular mechanisms responsible for GJ remodeling associated with cardiac disease.

HuR regulates gap junctional intercellular communication by controlling beta-catenin levels and adherens junction integrity

Gap junctional intercellular communication (GJIC) plays a critical role in the regulation of tissue homeostasis and carcinogenesis and is modulated by the levels, subcellular localization, and posttranslational modification of gap junction proteins, the connexins (Cx). Here, using oval cell-like rat liver epithelial cells, we demonstrate that the RNA-binding protein HuR promotes GJIC through two mechanisms. First, HuR silencing lowered the levels of Cx43 protein and Cx43 messenger RNA (mRNA), and decreased Cx43 mRNA half-life. This regulation was likely due to the direct stabilization of Cx43 mRNA by HuR, because HuR associated directly with Cx43 mRNA, a transcript that bears signature adenylate-uridylate-rich (AU-rich) and uridylate-rich (U-rich) sequences in its 3'-untranslated region. Second, HuR silencing reduced both half-life and the levels of beta-catenin mRNA, also a target of HuR; accordingly, HuR silencing lowered the levels of whole-cell and membrane-associated beta-catenin. Coimmunoprecipitation experiments showed a direct interaction between beta-catenin and Cx43. Small interfering RNA (siRNA)-mediated depletion of beta-catenin recapitulated the effects of decreasing HuR levels: it attenuated GJIC, decreased Cx43 levels, and redistributed Cx43 to the cytoplasm, suggesting that depletion of beta-catenin in HuR-silenced cells contributed to lowering Cx43 levels at the membrane. Finally, HuR was demonstrated to support GJIC after exposure to a genotoxic agent, doxorubicin, or an inducer of differentiation processes, retinoic acid, thus pointing to a crucial role of HuR in the cellular response to stress and in physiological processes modulated by GJIC.

CONCLUSION

HuR promotes gap junctional intercellular communication in rat liver epithelial cells through two related regulatory processes, by enhancing the expression of Cx43 and by increasing the expression of beta-catenin, which, in turn, interacts with Cx43 and is required for proper positioning of Cx43 at the plasma membrane.