Purinergic control of AMPK activation by ATP released through connexin 43 hemichannels - pivotal roles in hemichannel-mediated cell injury

Connexin-43 in Cancer: Above and Beyond Gap Junctions!

Connexin-43 (Cx43) is the most characterized gap junction protein, primarily involved in the Gap Junctional Intercellular Communication (GJIC) between adjacent cells to facilitate molecule exchange and the formation of a signaling network. It is increasingly evident that the importance of Cx43 is not only limited to its GJIC function, but rather includes its role in connecting the intracellular and extracellular environment by forming membrane hemichannels, as well as its intracellular signaling function mediated by its C-terminal tail (Cx43-CT). Notably, Cx43 has been implicated in a variety of cancers, with earlier notions suggesting a tumor-suppressor function, whereas new studies shed light on its pro-tumorigenic role. Moreover, apart from GJIC-based activities, the relevance of the non-canonical functions of Cx43 in tumor progression is being actively studied. This review provides an analysis of the current research on the pro-tumorigenic roles of Cx43, with a focus on Cx43-CT interactions and the function of hemichannels in cancer progression. A better understanding of the multifaceted functions of Cx43 in cancer biology could foster its recognition as a pivotal target for the development of innovative therapeutic strategies.

Connexin 43 confers chemoresistance through activating PI3K

Circumventing chemoresistance is crucial for effectively treating cancer including glioblastoma, a lethal brain cancer. The gap junction protein connexin 43 (Cx43) renders glioblastoma resistant to chemotherapy; however, targeting Cx43 is difficult because mechanisms underlying Cx43-mediated chemoresistance remain elusive. Here we report that Cx43, but not other connexins, is highly expressed in a subpopulation of glioblastoma and Cx43 mRNA levels strongly correlate with poor prognosis and chemoresistance in this population, making Cx43 the prime therapeutic target among all connexins. Depleting Cx43 or treating cells with αCT1–a Cx43 peptide inhibitor that sensitizes glioblastoma to the chemotherapy temozolomide–inactivates phosphatidylinositol-3 kinase (PI3K), whereas overexpression of Cx43 activates this signaling. Moreover, αCT1-induced chemo-sensitization is counteracted by a PI3K active mutant. Further research reveals that αCT1 inactivates PI3K without blocking the release of PI3K-activating molecules from membrane channels and that Cx43 selectively binds to the PI3K catalytic subunit β (PIK3CB, also called PI3Kβ or p110β), suggesting that Cx43 activates PIK3CB/p110β independent of its channel functions. To explore the therapeutic potential of simultaneously targeting Cx43 and PIK3CB/p110β, αCT1 is combined with TGX-221 or GSK2636771, two PIK3CB/p110β-selective inhibitors. These two different treatments synergistically inactivate PI3K and sensitize glioblastoma cells to temozolomide in vitro and in vivo. Our study has revealed novel mechanistic insights into Cx43/PI3K-mediated temozolomide resistance in glioblastoma and demonstrated that targeting Cx43 and PIK3CB/p110β together is an effective therapeutic approach for overcoming chemoresistance.

Glycyrrhetinic Acid Protects Renal Tubular Cells against Oxidative Injury via Reciprocal Regulation of JNK-Connexin 43-Thioredoxin 1 Signaling

Background and Objective: The incidence of chronic kidney disease (CKD) is steadily increasing. Although renal tubular epithelium injury is closely correlated with the prognosis of CKD, the underlying mechanism is not fully understood and therapeutic strategies are limited. The main bioactive component of the Chinese medicine herb, glycyrrhiza, is 18α-glycyrrhetinic acid (Ga), which is also a pharmacological inhibitor of gap junctions. Our previous studies indicated that Ga is able to ameliorate renal cell injury. The present study explored the regulatory role of Ga in redox signaling in renal tubular epithelial cells with oxidative injury. Methods: Rat renal tubular epithelial cells, NRK-52E, were incubated with Px-12, a thioredoxin inhibitor, to mimic thioredoxin deficiency and induce oxidative injury in vitro. A Cell Counting Kit-8 was used to analyze cell viability while a reactive oxygen species (ROS)/superoxide (O₂ ^-) fluorescence probe was employed to determine oxidative stress. Apoptosis was evaluated using DT-mediated dUTP nick end labeling/4,6-diamidino-2-phenylindole staining and cleaved caspase 3 protein analysis. Western blot analysis was used to analyze the expression of specific proteins while siRNA transfection was performed to downregulate targeted proteins. Results: Inhibition of thioredoxin 1 by Px-12 triggered renal tubular cell oxidative injury as evidenced by morphological change, loss of cellular viability, over production of ROS and O₂ ^-, and appearance of cleaved caspase-3. Ga significantly attenuated cell oxidative injury, as indicated by the parameters mentioned above. Px-12 induced phosphorylation of c-Jun N-terminal kinase (JNK) and subsequently the expression of connexin 43 (Cx43) in NRK-52E cells. Ga and the JNK inhibitor, sp600125, markedly suppressed Px-12-induced generation of intracellular ROS and O₂ ^-. Inhibition of JNK improved Px-12-elicited NRK-52E cell injury. Moreover, sp600125 inhibited Cx43 expression. After downregulation of Cx43 via Cx43 siRNA transfection, the phosphorylation of JNK was markedly reduced. Furthermore, Ga restored the expression of thioredoxin 1 inhibited by Px-12. Conclusion: ROS-JNK-Cx43-thioredoxin 1 signaling plays a crucial role in renal tubular cell injury. JNK is involved in the regulation of thioredoxin 1 and Cx43, and Cx43 reciprocally regulates thioredoxin 1. Inhibition of gap junctions by Ga alleviated renal tubular oxidative injury via improvement of thioredoxin 1-mediated redox signaling.

Gap junctions amplify TRPV4 activation-initiated cell injury via modification of intracellular Ca2+ and Ca2+-dependent regulation of TXNIP

The elevated intracellular Ca²⁺ and oxidative stress are well-reported mechanisms behind renal tubular epithelial injury initiated by various insults. Given that TRPV4 and connexin43 (Cx43) channels are activated by a wide range of stimuli and regulate both intracellular Ca²⁺ and redox status, we speculated an involvement of these channels in renal tubular cell injury. Here, we tested this possibility and explored the potential underlying mechanisms. Our results demonstrated that exposure of renal tubular epithelial cells to aminoglycoside G418 led to cell death, which was attenuated by both TRPV4 and gap junction (Gj) inhibitor. Activation of TRPV4 caused cell damage, which was associated with an early increase in Cx43 expression and function. Inhibition of Cx43 with chemical inhibitor or siRNA largely prevented TRPV4 activation-induced cell damage. Further analysis revealed that TRPV4 agonists elicited a rise in intracellular Ca²⁺ and caused a Ca²⁺-dependent elevation in TXNIP (a negative regulator of the antioxidant thioredoxin). In the presence of Gj inhibitor, however, these effects of TRPV4 were largely prevented. The depletion of intracellular Ca²⁺ with Ca²⁺ chelator BAPTA-AM or downregulation of TXNIP with siRNA significantly alleviated TRPV4 activation-initiated cell injury. Collectively, our results point to a critical involvement of TRPV4/Cx43 channel interaction in renal tubular cell injury through mechanisms involving a synergetic induction of intracellular Ca²⁺ and oxidative stress. Channel interactions could be an important mechanism underlying cell injury. Targeting channels could have therapeutic potential for the treatment of acute tubular cell injury.

Connexin Hemichannels Contribute to the Activation of cAMP Signaling Pathway and Renin Production

Connexin hemichannels play an important role in the control of cellular signaling and behaviors. Given that lowering extracellular Ca²⁺, a condition that activates hemichannels, is a well-characterized stimulator of renin in juxtaglomerular cells, we, therefore, tested a potential implication of hemichannels in the regulation of renin in As4.1 renin-secreting cells. Lowering extracellular Ca²⁺ induced hemichannel opening, which was associated with cAMP signaling pathway activation and increased renin production. Blockade of hemichannels with inhibitors or downregulation of Cxs with siRNAs abrogated the activation of cAMP pathway and the elevation of renin. Further analysis revealed that cAMP pathway activation was blocked by adenylyl cyclase inhibitor SQ 22536, suggesting an implication of adenyl cyclase. Furthermore, the participation of hemichannels in the activation of the cAMP signaling pathway was also observed in a renal tubular epithelial cell line NRK. Collectively, our results characterized the hemichannel opening as a presently unrecognized molecular event involved in low Ca²⁺-elicited activation of cAMP pathway and renin production. Our findings thus provide novel mechanistic insights into the low Ca²⁺-initiated cell responses. Given the importance of cAMP signaling pathway in the control of multiple cellular functions, our findings also highlight the importance of Cx-forming channels in various pathophysiological situations.

Paracellular permeability changes induced by multi-walled carbon nanotubes in brain endothelial cells and associated roles of hemichannels

Multi-walled carbon nanotubes (MWCNTs) have promising applications in neurology depending on their unique physicochemical properties. However, there is limited understanding of their impacts on brain microvascular endothelial cells, the cells lining the vessels and maintaining the low and selective permeability of the blood-brain barrier. In this study, we examined the influence of pristine MWCNT (p-MWCNT) and carboxylated MWCNT (c-MWCNT) on permeability and tight junction tightness of murine brain microvascular endothelial cells, and investigated the potential mechanisms in the sight of hemichannel activity. Treatment with p-MWCNT for 24 h at subtoxic concentration (20 μg/mL) decreased the protein expression of occludin, disrupted zonula occludens-1 continuity, and elevated monolayer permeability as quantified by transendothelial electrical resistance and paracellular flux of 4000 Da fluorescein isothiocyanate-dextran conjugates. Moreover, p-MWCNT exposure also increased hemichannel activity with upregulated protein expression and altered subcellular localization of connexin (Cx)43 and pannexin (Panx)1. p-MWCNT-induced elevation in endothelial permeability could be prevented by hemichannel inhibitor carbenoxolone and peptide blocker of Cx43 and Panx1, indicating the crucial role of activated Cx43 and Panx1 hemichannels. Furthermore, Cx43 and Panx1 hemichannel-mediated ATP release might be involved in p-MWCNT-induced rise in endothelial permeability. In contrast, the above effects caused by p-MWCNT were not observed in cells treated with c-MWCNT, the functionalized form with more stable dispersion and a lower tendency to aggregate. Our study contributes further understanding of the impact of MWCNTs on brain endothelial tightness and permeability, which may have important implications for the safety application of MWCNTs in nanomedicine.

Regulation of miR-1-Mediated Connexin 43 Expression and Cell Proliferation in Dental Epithelial Cells

Many genes encoding growth factors, receptors, and transcription factors are induced by the epithelial-mesenchymal interaction during tooth development. Recently, numerous functions of microRNAs (miRNAs) are reportedly involved in organogenesis and disease. miRNAs regulate gene expression by inhibiting translation and destabilizing mRNAs. However, the expression and function of miRNAs in tooth development remain poorly understood. This study aimed to analyze the expression of miRNAs produced during tooth development using a microarray system to clarify the role of miRNAs in dental development. miR-1 showed a unique expression pattern in the developing tooth. miR-1 expression in the tooth germ peaked on embryonic day 16.5, decreasing gradually on postnatal days 1 and 3. An in situ hybridization assay revealed that miR-1 is expressed at the cervical loop of the dental epithelium. The expression of miR-1 and connexin (Cx) 43, a target of miR-1, were inversely correlated both in vitro and in vivo. Knockdown of miR-1 induced the expression of Cx43 in dental epithelial cells. Interestingly, cells with miR-1 downregulation proliferated slower than the control cells. Immunocytochemistry revealed that Cx43 in cells with miR-1 knockdown formed both cell-cell gap junctions and hemichannels at the plasma membrane. Furthermore, the rate of ATP release was higher in cells with miR-1 knockdown than in control cells. Furthermore, Cx43 downregulation in developing molars was observed in Epiprofin-knockout mice, along with the induction of miR-1 expression. These results suggest that the expression pattern of Cx43 is modulated by miR-1 to control cell proliferation activity during dental epithelial cell differentiation.

Effects of N-acetylcysteine on the energy status and antioxidant capacity in heart and liver of cold-stressed broilers

OBJECTIVE

Cold stress induces oxidative damage and impairs energy status of broilers. N-acetylcysteine (NAC) exhibits antioxidant properties and modulates energy metabolism of animals. This study was conducted to investigate the effects of NAC on energy status and antioxidant capacity of heart and liver in the cold-stressed broilers.

METHODS

The experiment consisted of 4 treatments in a 2×2 factorial arrangement with two diets (basal diet or plus 0.1% NAC) and two ambient temperatures (thermoneutral [conventional ambient temperature] or cold stress [10°C±1°C during days 15 to 42]).

RESULTS

No ascites were seen in cold-stressed broilers. NAC did not attenuate the impaired growth performance of stressed birds. However, NAC decreased plasma asparagine but increased aspartate levels in cold-stressed birds (p<0.05). NAC reduced hepatic adenosine triphosphate (ATP) but elevated adenosine diphosphate contents in unstressed birds (p< 0.05). The hepatic ratio of adenosine monophosphate (AMP) to ATP was increased in birds fed NAC (p<0.05). NAC decreased plasma malondialdehyde (MDA) level and cardiac total superoxide dismutase (T-SOD) activity in unstressed birds, but increased hepatic activities of T-SOD, catalase and glutathione peroxidase in stressed birds (p<0.05). NAC down-regulated hepatic AMP-activated protein kinase but up-regulated cardiac heme-oxigenase mRNA expression in stressed birds, and decreased expression of hepatic peroxisome proliferatoractivated receptor coactivator-1α as well as hypoxia-inducible factor-1α in liver and heart of birds.

CONCLUSION

Dietary NAC did not affect energy status but enhanced the hepatic antioxidant capacity by increasing the activities of antioxidant enzymes in cold-stressed broilers.

Effects of Cadmium on ZO-1 Tight Junction Integrity of the Blood Brain Barrier

Cadmium (Cd) is a highly toxic environmental pollutant released from the smelting and refining of metals and cigarette smoking. Oral exposure to cadmium may result in adverse effects on a number of tissues, including the central nervous system (CNS). In fact, its toxicity has been related to neurological disorders, as well as neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. Under normal conditions, Cd barely reaches the brain in adults because of the presence of the blood-brain barrier (BBB); however, it has been demonstrated that Cd-dependent BBB alteration contributes to pathogenesis of neurodegeneration. However, the mechanism underlying Cd-dependent BBB alteration remain obscure. Here, we investigated the signaling pathway of Cd-induced tight junction (TJ), F-actin, and vimentin protein disassembly in a rat brain endothelial cell line (RBE4). RBE4 cells treated with 10 μM cadmium chloride (CdCl2) showed a dose- and time-dependent significant increase in reactive oxygen species (ROS) production. This phenomenon was coincident with the alteration of the TJ zonula occludens-1 (ZO-1), F-actin, and vimentin proteins. The Cd-dependent ROS increase elicited the upregulation of GRP78 expression levels, a chaperone involved in endoplasmic reticulum (ER) stress that induces caspase-3 activation. Further signal profiling by the pannexin-1 (PANX1) specific inhibitor 10Panx revealed a PANX1-independent increase in ATP spillage in Cd-treated endothelial cells. Our results point out that a ROS-dependent ER stress-mediated signaling pathway involving caspase-3 activation and ATP release is behind the BBB morphological alterations induced by Cd.

Connexin43 Is Required for the Effective Activation of Spleen Cells and Immunoglobulin Production

Gap junctions (Gjs), formed by specific protein termed connexins (Cxs), regulate many important cellular processes in cellular immunity. However, little is known about their effects on humoral immunity. Here we tested whether and how Gj protein connexin43 (Cx43) affected antibody production in spleen cells. Detection of IgG in mouse tissues and serum revealed that wild-type (Cx43^+/+) mouse had a significantly higher level of IgG than Cx43 heterozygous (Cx43^+/−) mouse. Consistently, spleen cells from Cx43^+/+ mouse produced more IgG under both basal and lipopolysaccharide (LPS)-stimulated conditions. Further analysis showed that LPS induced a more dramatic activation of ERK and cell proliferation in Cx43^+/+ spleen cells, which was associated with a higher pro-oxidative state, as indicated by the increased NADPH oxidase 2 (NOX2), TXNIP, p38 activation and protein carbonylation. In support of a role of the oxidative state in the control of lymphocyte activation, exposure of spleen cells to exogenous superoxide induced Cx43 expression, p38 activation and IgG production. On the contrary, inhibition of NOX attenuated the effects of LPS. Collectively, our study characterized Cx43 as a novel molecule involved in the control of spleen cell activation and IgG production. Targeting Cx43 could be developed to treat certain antibody-related immune diseases.

[Inhibitory effect of connexin43 protein on autophagy in cisplatin-resistant testicular cancer I-10 cells]

OBJECTIVE

To investigate the effect of connexin43 (Cx43) protein on autophagy in cisplatin (DDP)-resistant testicular cancer I-10 cells.

METHODS

The expression of Cx43 proteins in testicular cancer I-10 cells and I-10/DDP cells were detected with Western blotting. I-10/DDP cells were transfected with a full- length mouse Cx43 vector (mCx43) via Lipofectamine2000, the empty vector or Lipofectamine2000 (blank control group), and the changes in the expressions of LC3 and p62 proteins were determined with Western blotting. mCherry-GFP-LC3B transfection and transmission electron microscopy were used to analyze the changes in autophagy of the cells with Cx43 overexpression.

RESULTS

Cx43 was significantly decreased in I-10/DDP cells compared with I-10 cells (P < 0.01). Transfection of the I-10/DDP cells with mCx43 vector resulted in significantly increased Cx43 expression in the cells (P < 0.01) and caused significantly decreased expression of LC3-Ⅱ (P < 0.01) and increased expression of p62 (P < 0.05) as compared with the negative control cells. Both transmission electron microscopy and mCherry-GFP-LC3B transfection showed that the number of autophagosomes was obviously reduced in mCx43-transfected cells as compared with the negative control cells.

CONCLUSIONS

Cx43 inhibits autophagy in cisplatin-resistant testicular cancer I-10 /DDP cells.

Mechanisms of Spontaneous Electrical Activity in the Developing Cerebral Cortex-Mouse Subplate Zone

Subplate (SP) neurons exhibit spontaneous plateau depolarizations mediated by connexin hemichannels. Postnatal (P1-P6) mice show identical voltage pattern and drug-sensitivity as observed in slices from human fetal cortex; indicating that the mouse is a useful model for studying the cellular physiology of the developing neocortex. In mouse SP neurons, spontaneous plateau depolarizations were insensitive to blockers of: synaptic transmission (glutamatergic, GABAergic, or glycinergic), pannexins (probenecid), or calcium channels (mibefradil, verapamil, diltiazem); while highly sensitive to blockers of gap junctions (octanol), hemichannels (La3+, lindane, Gd3+), or glial metabolism (DLFC). Application of La3+ (100 μM) does not exert its effect on electrical activity by blocking calcium channels. Intracellular application of Gd3+ determined that Gd3+-sensitive pores (putative connexin hemichannels) reside on the membrane of SP neurons. Immunostaining of cortical sections (P1-P6) detected connexins 26, and 45 in neurons, but not connexins 32 and 36. Vimentin-positive glial cells were detected in the SP zone suggesting a potential physiological interaction between SP neurons and radial glia. SP spontaneous activity was reduced by blocking glial metabolism with DFLC or by blocking purinergic receptors by PPADS. Connexin hemichannels and ATP release from vimentin-positive glial cells may underlie spontaneous plateau depolarizations in the developing mammalian cortex.

Connexins in cancer: bridging the gap to the clinic

Gap junctions comprise arrays of intercellular channels formed by connexin proteins and provide for the direct communication between adjacent cells. This type of intercellular communication permits the coordination of cellular activities and plays key roles in the control of cell growth and differentiation and in the maintenance of tissue homoeostasis. After more than 50 years, deciphering the links among connexins, gap junctions and cancer, researchers are now beginning to translate this knowledge to the clinic. The emergence of new strategies for connexin targeting, combined with an improved understanding of the molecular bases underlying the dysregulation of connexins during cancer development, offers novel opportunities for clinical applications. However, different connexin isoforms have diverse channel-dependent and -independent functions that are tissue and stage specific. This can elicit both pro- and anti-tumorigenic effects that engender significant challenges in the path towards personalised medicine. Here, we review the current understanding of the role of connexins and gap junctions in cancer, with particular focus on the recent progress made in determining their prognostic and therapeutic potential.

Eliciting α7-nAChR exerts cardioprotective effects on ischemic cardiomyopathy via activation of AMPK signalling

Our previous studies have reported that agonist of α7 nicotinic acetylcholine receptors prevented electrophysiological dysfunction of rats with ischaemic cardiomyopathy (ICM) by eliciting the cholinergic anti‐inflammatory pathway (CAP). Adenosine monophosphate‐activated protein kinase (AMPK) signalling is widely recognized exerting cardioprotective effect in various cardiomyopathy. Here, we aimed to investigate whether the protective effects of the CAP are associated with AMPK signalling in ICM. In vivo, coronary artery of rats was ligated for 4 weeks to induce the ICM and then treated with PNU‐282987 (CAP agonist) and BML‐275 dihydrochloride (AMPK antagonist) for 4 weeks. In vitro, primary macrophages harvested from rats were induced inflammation by Lipopolysaccharide (LPS) treatment and then treated with PNU‐282987 and BML‐275 dihydrochloride. In vivo, exciting CAP by PUN‐282987 elicited an activation of AMPK signalling, alleviated ventricular remodeling, modified the cardiac electrophysiological function, reduced the cardiac expression of collagens and inflammatory cytokines and maintained the integrity of ultrastructure in the ischemic heart. However, the benefits of CAP excitation were blunted by AMPK signaling antagonization. In vitro, excitation of the CAP was observed inhibiting the nuclear transfer of NF‐κB p65 of macrophages and promoting the transformation of Ly‐6C^high macrophages into Ly‐6C^low macrophages. However, inhibiting AMPK signalling by BML‐275 dihydrochloride reversed the CAP effect on LPS‐treated macrophages. Finally, our findings suggest that eliciting the CAP modulates the inflammatory response in ICM through regulating AMPK signalling.

Connexin 43 hemichannels and pannexin-1 channels contribute to the α-synuclein-induced dysfunction and death of astrocytes

Diverse studies have suggested that cytoplasmic inclusions of misfolded α-synuclein in neuronal and glial cells are main pathological features of different α-synucleinopathies, including Parkinson's disease and dementia with Lewy bodies. Up to now, most studies have focused on the effects of α-synuclein on neurons, whereas the possible alterations of astrocyte functions and neuron-glia crosstalk have received minor attention. Recent evidence indicates that cellular signaling mediated by hemichannels and pannexons is critical for astroglial function and dysfunction. These channels constitute a diffusional route of communication between the cytosol and the extracellular space and during pathological scenarios they may lead to homeostatic disturbances linked to the pathogenesis and progression of different diseases. Here, we found that α-synuclein enhances the opening of connexin 43 (Cx43) hemichannels and pannexin-1 (Panx1) channels in mouse cortical astrocytes. This response was linked to the activation of cytokines, the p38 MAP kinase, the inducible nitric oxide synthase, cyclooxygenase 2, intracellular free Ca²⁺ concentration ([Ca²⁺ ]_i ), and purinergic and glutamatergic signaling. Relevantly, the α-synuclein-induced opening of hemichannels and pannexons resulted in alterations in [Ca²⁺ ]_i dynamics, nitric oxide (NO) production, gliotransmitter release, mitochondrial morphology, and astrocyte survival. We propose that α-synuclein-mediated opening of astroglial Cx43 hemichannels and Panx1 channels might constitute a novel mechanism involved in the pathogenesis and progression of α-synucleinopathies.

Connexin 43 Hemichannel Activity Promoted by Pro-Inflammatory Cytokines and High Glucose Alters Endothelial Cell Function

The present work was done to elucidate whether hemichannels of a cell line derived from endothelial cells are affected by pro-inflammatory conditions (high glucose and IL-1β/TNF-α) known to lead to vascular dysfunction. We used EAhy 926 cells treated with high glucose and IL-1β/TNF-α. The hemichannel activity was evaluated with the dye uptake method and was abrogated with selective inhibitors or knocking down of hemichannel protein subunits with siRNA. Western blot analysis, cell surface biotinylation, and confocal microscopy were used to evaluate total and plasma membrane amounts of specific proteins and their cellular distribution, respectively. Changes in intracellular Ca²⁺ and nitric oxide (NO) signals were estimated by measuring FURA-2 and DAF-FM probes, respectively. High glucose concentration was found to elevate dye uptake, a response that was enhanced by IL-1β/TNF-α. High glucose plus IL-1β/TNF-α-induced dye uptake was abrogated by connexin 43 (Cx43) but not pannexin1 knockdown. Furthermore, Cx43 hemichannel activity was associated with enhanced ATP release and activation of p38 MAPK, inducible NO synthase, COX₂, PGE₂ receptor EP₁, and P2X₇/P2Y₁ receptors. Inhibition of the above pathways prevented completely the increase in Cx43 hemichannel activity of cells treated high glucose and IL-1β/TNF-α. Both synthetic and endogenous cannabinoids (CBs) also prevented the increment in Cx43 hemichannel opening, as well as the subsequent generation and release of ATP and NO induced by pro-inflammatory conditions. The counteracting action of CBs also was extended to other endothelial alterations evoked by IL-1β/TNF-α and high glucose, including increased ATP-dependent Ca²⁺ dynamics and insulin-induced NO production. Finally, inhibition of Cx43 hemichannels also prevented the ATP release from endothelial cells treated with IL-1β/TNF-α and high glucose. Therefore, we propose that reduction of hemichannel activity could represent a strategy against the activation of deleterious pathways that lead to endothelial dysfunction and possibly cell damage evoked by high glucose and pro-inflammatory conditions during cardiovascular diseases.

Novel approach to temozolomide resistance in malignant glioma: connexin43-directed therapeutics

Resistance of malignant glioma, including glioblastoma (GBM), to the chemotherapeutic temozolomide (TMZ) remains a key obstacle in treatment strategies. The gap junction protein connexin43 (Cx43) has complex roles in the establishment, progression, and persistence of malignant glioma. Recent findings demonstrate that connexins play an important role in the microenvironment of malignant glioma and that Cx43 is capable of conferring chemotherapeutic resistance to GBM cells. Carboxyl-terminal Cx43 peptidomimetics show therapeutic promise in overcoming TMZ resistance via mechanisms that may include modulating junctional activity between tumor cells and peritumoral cells and/or downstream molecular signaling events mediated by Cx43 protein binding. High levels of intra-tumor and inter-tumor heterogeneity make it difficult to clearly define specific populations for Cx43-targeted therapy; hence, development of in vitro models that better mimic the microenvironment of malignant glioma, and the incorporation of patient-derived stem cells, could provide opportunities for patient-specific drug screening. This review summarizes recent advances in understanding the roles of Cx43 in malignant glioma, with a special focus on tumor microenvironment, TMZ resistance, and therapeutic opportunity offered by Cx43 peptidomimetics.

P2Y6 Receptor Activation Promotes Inflammation and Tissue Remodeling in Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a disease with a poor prognosis and very few available treatment options. The involvement of the purinergic receptor subtypes P2Y2 and P2X7 in fibrotic lung disease has been demonstrated recently. In this study, we investigated the role of P2Y6 receptors in the pathogenesis of IPF in humans and in the animal model of bleomycin-induced lung injury. P2Y6R expression was upregulated in lung structural cells but not in bronchoalveolar lavage (BAL) cells derived from IPF patients as well as in animals following bleomycin administration. Furthermore, BAL fluid levels of the P2Y6R agonist uridine-5'-diphosphate were elevated in animals with bleomycin-induced pulmonary fibrosis. Inflammation and fibrosis following bleomycin administration were reduced in P2Y6R-deficient compared to wild-type animals confirming the pathophysiological relevance of P2Y6R subtypes for fibrotic lung diseases. Experiments with bone marrow chimeras revealed the importance of P2Y6R expression on lung structural cells for pulmonary inflammation and fibrosis. Similar effects were obtained when animals were treated with the P2Y6R antagonist MRS2578. In vitro studies demonstrated that proliferation and secretion of the pro-inflammatory/pro-fibrotic cytokine IL-6 by lung fibroblasts are P2Y6R-mediated processes. In summary, our results clearly demonstrate the involvement of P2Y6R subtypes in the pathogenesis of pulmonary fibrosis. Thus, blocking pulmonary P2Y6 receptors might be a new target for the treatment of IPF.

The pivotal role of extracellular signal-regulated kinase in gap junction-mediated regulation of TXNIP

Gap junctions (GJs) play a major role in the control of cell structure, function, and metabolism. However, the molecular mechanisms involved are still poorly understood. Given that thioredoxin-interacting protein (TXNIP) regulates a broad range of cellular processes, we tested the possible involvement of TXNIP. Disruption of GJs with several chemical GJ inhibitors or connexin43 (Cx43) siRNA potently suppressed TXNIP, which was preceded by an activation of extracellular signal-regulated kinase (ERK). Inhibition of ERK or its upstream kinase with chemical inhibitors prevented the reduction of TXNIP. On the contrary, activation of ERK with mitogens or phosphatase inhibitors reproduced the suppressive effects of GJs. Further analysis revealed that dysfunction of GJs promoted TXNIP phosphorylation, ubiquitination, and degradation, whereas inhibition of ERK exerted the opposite effects. Moreover, inhibition of GJs elevated Glut1 and enhanced cell resistance to ER stress in a similar way to TXNIP downregulation. Collectively, our study thus characterizes ERK-mediated suppression of TXNIP as a presently unreported mechanism by which GJs regulate cell behaviors.

A Multicenter Randomized Controlled Trial Evaluating a Cx43-Mimetic Peptide in Cutaneous Scarring

The transmembrane protein Cx43 has key roles in fibrogenic processes including inflammatory signaling and extracellular matrix composition. aCT1 is a Cx43 mimetic peptide that in preclinical studies accelerated wound closure, decreased inflammation and granulation tissue area, and normalized mechanical properties after cutaneous injury. We evaluated the efficacy and safety of aCT1 in the reduction of scar formation in human incisional wounds. In a prospective, multicenter, within-participant controlled trial, patients with bilateral incisional wounds (≥10 mm) after laparoscopic surgery were randomized to receive acute treatment (immediately after wounding and 24 hours later) with an aCT1 gel formulation plus conventional standard of care protocols, involving moisture-retentive occlusive dressing, or standard of care alone. The primary efficacy endpoint was average scarring score using visual analog scales evaluating incision appearance and healing progress over 9 months. There was no significant difference in scar appearance between aCT1- or control-treated incisions after 1 month. At month 9, aCT1-treated incisions showed a 47% improvement in scar scores over controls (Vancouver Scar Scale; P = 0.0045), a significantly higher Global Assessment Scale score (P = 0.0009), and improvements in scar pigmentation, thickness, surface roughness, and mechanical suppleness. Adverse events were similar in both groups. aCT1 has potential to improve scarring outcome after surgery.

Connexin43 hemichannels contributes to the disassembly of cell junctions through modulation of intracellular oxidative status

Connexin (Cx) hemichannels regulate many cellular processes with little information available regarding their mechanisms. Given that many pathological factors that activate hemichannels also disrupts the integrity of cellular junctions, we speculated a potential participation of hemichannels in the regulation of cell junctions. Here we tested this hypothesis. Exposure of renal tubular epithelial cells to Ca²⁺-free medium led to disassembly of tight and adherens junctions, as indicated by the reduced level of ZO-1 and cadherin, disorganization of F-actin, and severe drop in transepithelial electric resistance. These changes were preceded by an activation of Cx43 hemichannels, as revealed by extracellular efflux of ATP and intracellular influx of Lucifer Yellow. Inhibition of hemichannels with chemical inhibitors or Cx43 siRNA greatly attenuated the disassembly of cell junctions. Further analysis using fetal fibroblasts derived from Cx43 wide-type (Cx43^+/+), heterozygous (Cx43^+/-) and knockout (Cx43^-/-) littermates showed that Cx43-positive cells (Cx43^+/+) exhibited more dramatic changes in cell shape, F-actin, and cadherin in response to Ca²⁺ depletion, as compared to Cx43-null cells (Cx43^-/-). Consistently, these cells had higher level of protein carbonyl modification and phosphorylation, and much stronger activation of P38 and JNK. Hemichannel opening led to extracellular loss of the major antioxidant glutathione (GSH). Supplement of cells with exogenous GSH or inhibition of oxidative sensitive kinases largely prevented the above-mentioned changes. Taken together, our study indicates that Cx43 hemichannels promote the disassembly of cell junctions through regulation of intracellular oxidative status.

•

The mechanisms about the coordinated regulation of cell junctions are obscure.

•

Ca²⁺ depletion activates hemichannels and disrupts cell junctions.

•

Hemichannel opening exaggerates oxidative stress via efflux of GSH.

•

Blocking hemichannels attenuates oxidative stress and cell junction disassembly.

•

Hemichannels regulate cell junctions via modulation of intracellular redox status.

AMPK Suppresses Connexin43 Expression in the Bladder and Ameliorates Voiding Dysfunction in Cyclophosphamide-induced Mouse Cystitis

Bladder voiding dysfunction is closely related to local oxidation, inflammation, and enhanced channel activities. Given that the AMP-activated protein kinase (AMPK) has anti-oxidative, anti-inflammatory and channel-inhibiting properties, we examined whether and how AMPK affected bladder activity. AMPK activation in rat bladder smooth muscle cells (BSMCs) using three different AMPK agonists resulted in a decrease in connexin43 (Cx43) expression and function, which was associated with reduced CREB phosphorylation, Cx43 promoter activity and mRNA expression, but not Cx43 degradation. Downregulation of CREB with siRNA increased Cx43 expression. A functional analysis revealed that AMPK weakened BSMC contraction and bladder capacity. AMPK also counteracted the IL-1β- and TNFα-induced increase in Cx43 in BSMCs. In vivo administration of the AMPK agonist AICAR attenuated cyclophosphamide-initiated bladder oxidation, inflammation, Cx43 expression and voiding dysfunction. Further analysis comparing the responses of the wild-type (Cx43(+/+)) and heterozygous (Cx43(+/-)) Cx43 mice to cyclophosphamide revealed that the Cx43(+/-) mice retained a relatively normal micturition pattern compared to the Cx43(+/+) mice. Taken together, our results indicate that AMPK inhibits Cx43 in BSMCs and improves bladder activity under pathological conditions. We propose that strategies that target AMPK can be developed as novel therapeutic approaches for treating bladder dysfunction.

Upregulation of connexin43 contributes to PX-12-induced oxidative cell death

Thioredoxin (Trx) is a small redox protein that underlies aggressive tumor growth and resistance to chemotherapy. Inhibition of Trx with the chemical inhibitor PX-12 suppresses tumor growth and induces cell apoptosis. Currently, the mechanism underlying the therapeutic actions of PX-12 and the molecules influencing cell susceptibility to PX-12 are incompletely understood. Given that connexin43 (Cx43), a tumor suppressor, regulates tumor cell susceptibility to chemotherapy, we examined the possible involvement of Cx43 in PX-12-induced cell death. Exposure of cells to PX-12 led to a loss of cell viability, which was associated with the activation of oxidative sensitive c-Jun N-terminal kinase (JNK). Inhibition of JNK or supplement of cells with anti-oxidants prevented the cell-killing action of PX-12. The forced expression of Cx43 in normal and tumor cells increased cell sensitivity to PX-12-induced JNK activation and cell death. In contrast, the downregulation of Cx43 with siRNA or the suppression of gap junctions with chemical inhibitors attenuated JNK activation and enhanced cell resistance to PX-12. Further analysis revealed that PX-12 at low concentrations induced a JNK-dependent elevation in the Cx43 protein, which was also preventable by supplementing the cells with anti-oxidants. Our results thus indicate that Cx43 is a determinant in the regulation of cell susceptibility to PX-12 and that the upregulation of Cx43 may be an additional mechanism by which PX-12 exerts its anti-tumor actions.

Connexin 43 Inhibition Sensitizes Chemoresistant Glioblastoma Cells to Temozolomide

Resistance of glioblastoma (GBM) to the front-line chemotherapeutic agent temozolomide (TMZ) continues to challenge GBM treatment efforts. The repair of TMZ-induced DNA damage by O-6-methylguanine-DNA methyltransferase (MGMT) confers one mechanism of TMZ resistance. Paradoxically, MGMT-deficient GBM patients survive longer despite still developing resistance to TMZ. Recent studies indicate that the gap junction protein connexin 43 (Cx43) renders GBM cells resistant to TMZ through its carboxyl terminus (CT). In this study, we report insights into how Cx43 promotes TMZ resistance. Cx43 levels were inversely correlated with TMZ sensitivity of GBM cells, including GBM stem cells. Moreover, Cx43 levels inversely correlated with patient survival, including as observed in MGMT-deficient GBM patients. Addition of the C-terminal peptide mimetic αCT1, a selective inhibitor of Cx43 channels, sensitized human MGMT-deficient and TMZ-resistant GBM cells to TMZ treatment. Moreover, combining αCT1 with TMZ-blocked AKT/mTOR signaling, induced autophagy and apoptosis in TMZ-resistant GBM cells. Our findings suggest that Cx43 may offer a biomarker to predict the survival of patients with MGMT-independent TMZ resistance and that combining a Cx43 inhibitor with TMZ could enhance therapeutic responses in GBM, and perhaps other TMZ-resistant cancers.

Suppression of cell membrane permeability by suramin: involvement of its inhibitory actions on connexin 43 hemichannels

BACKGROUND AND PURPOSE

Suramin is a clinically prescribed drug for treatment of human African trypanosomiasis, cancer and infection. It is also a well-known pharmacological antagonist of P2 purinoceptors. Despite its clinical use and use in research, the biological actions of this molecule are still incompletely understood. Here, we investigated the effects of suramin on membrane channels, as exemplified by its actions on non-junctional connexin43 (Cx43) hemichannels, pore-forming α-haemolysin and channels involved in ATP release under hypotonic conditions.

EXPERIMENTAL APPROACH

Hemichannels were activated by removing extracellular Ca(2+) . The influences of suramin on hemichannel activities were evaluated by its effects on influx of fluorescent dyes and efflux of ATP. The membrane permeability and integrity were assessed through cellular retention of preloaded calcein and LDH release.

KEY RESULTS

Suramin blocked Cx43 hemichannel permeability induced by removal of extracellular Ca(2+) without much effect on Cx43 expression and gap junctional intercellular communication. This action of suramin was mimicked by its analogue NF023 and NF449 but not by another P2 purinoceptor antagonist PPADS. Besides hemichannels, suramin also significantly blocked intracellular and extracellular exchanges of small molecules caused by α-haemolysin from Staphylococcus aureus and by exposure of cells to hypotonic solution. Furthermore, it prevented α-haemolysin- and hypotonic stress-elicited cell injury.

CONCLUSION AND IMPLICATIONS

Suramin blocked membrane channels and protected cells against toxin- and hypotonic stress-elicited injury. Our finding provides novel mechanistic insights into the pharmacological actions of suramin. Suramin might be therapeutically exploited to protect membrane integrity under certain pathological situations.

Specific deletion of AMP-activated protein kinase (α1AMPK) in murine oocytes alters junctional protein expression and mitochondrial physiology

Oogenesis and folliculogenesis are dynamic processes that are regulated by endocrine, paracrine and autocrine signals. These signals are exchanged between the oocyte and the somatic cells of the follicle. Here we analyzed the role of AMP-activated protein kinase (AMPK), an important regulator of cellular energy homeostasis, by using transgenic mice deficient in α1AMPK specifically in the oocyte. We found a decrease of 27% in litter size was observed in ZP3-α1AMPK-/- (ZP3-KO) female mice. Following in vitro fertilization, where conditions are stressful for the oocyte and embryo, ZP3-KO oocytes were 68% less likely to pass the 2-cell stage. In vivo and in cumulus-oocyte complexes, several proteins involved in junctional communication, such as connexin37 and N-cadherin were down-regulated in the absence of α1AMPK. While the two signalling pathways (PKA and MAPK) involved in the junctional communication between the cumulus/granulosa cells and the oocyte were stimulated in control oocytes, ZP3-KO oocytes exhibited only low phosphorylation of MAPK or CREB proteins. In addition, MII oocytes deficient in α1AMPK had a 3-fold lower ATP concentration, an increase in abnormal mitochondria, and a decrease in cytochrome C and PGC1α levels, suggesting perturbed energy production by mitochondria. The absence of α1AMPK also induced a reduction in histone deacetylase activity, which was associated with an increase in histone H3 acetylation (K9/K14 residues). Together, the results of the present study suggest that absence of AMPK, modifies oocyte quality through energy processes and oocyte/somatic cell communication. The limited effect observed in vivo could be partly due to a favourable follicle microenvironment where nutrients, growth factors, and adequate cell interaction were present. Whereas in a challenging environment such as that of in vitro culture following IVF, the phenotype is revealed.