Connexins as therapeutic targets in lung disease

A Novel Cytochrome P450 2E1 Inhibitor Q11 Is Effective on Lung Cancer via Regulation of the Inflammatory Microenvironment

Lung cancer is the leading cause of death among all cancers. A persistent chronic inflammatory microenvironment is highly correlated with lung cancer. However, there are no anti-inflammatory agents effective against lung cancer. Cytochrome P450 2E1 (CYP2E1) plays an important role in the inflammatory response. Here, it is found that CYP2E1 is significantly higher in the peritumoral tissue of non-small cell lung cancer (NSCLC) patients and lung tumor growth is significantly impeded in Cyp2e1-/- mice. The novel CYP2E1 inhibitor Q11, 1-(4-methyl-5-thialzolyl) ethenone, is effective in the treatment of lung cancer in mice, which can inhibit cancer cells by changing macrophage polarization rather than directly act on the cancer cells. It is also clarify that the benefit of Q11 may associated with the IL-6/STAT3 and MAPK/ERK pathways. The data demonstrate that CYP2E1 may be a novel inflammatory target and that Q11 is effective on lung cancer by regulation of the inflammatory microenvironment. These findings provide a molecular basis for targeting CYP2E1 and illustrate the potential druggability of the CYP2E1 inhibitor Q11.

Intercellular Communication in Airway Epithelial Cell Regeneration: Potential Roles of Connexins and Pannexins

Connexins and pannexins are transmembrane proteins that can form direct (gap junctions) or indirect (connexons, pannexons) intercellular communication channels. By propagating ions, metabolites, sugars, nucleotides, miRNAs, and/or second messengers, they participate in a variety of physiological functions, such as tissue homeostasis and host defense. There is solid evidence supporting a role for intercellular signaling in various pulmonary inflammatory diseases where alteration of connexin/pannexin channel functional expression occurs, thus leading to abnormal intercellular communication pathways and contributing to pathophysiological aspects, such as innate immune defense and remodeling. The integrity of the airway epithelium, which is the first line of defense against invading microbes, is established and maintained by a repair mechanism that involves processes such as proliferation, migration, and differentiation. Here, we briefly summarize current knowledge on the contribution of connexins and pannexins to necessary processes of tissue repair and speculate on their possible involvement in the shaping of the airway epithelium integrity.

[Research Progress on the Pathogenesis of Lung Cancer Associated with Idiopathic Pulmonary Fibrosis]

Idiopathic pulmonary fibrosis (IPF) is the most common interstitial lung disease (ILD) of unknown causes, which is characterized by pulmonary fibrosis. The median survival period after diagnosis is about 2-4 years. In recent years, the incidence rate of lung cancer associated with IPF (IPF-LC) is increasing, and the prognosis is worse than that of IPF alone. Pulmonary fibrosis may be closely associated with the occurrence and development of lung cancer. Although the pathogenesis of IPF-LC is still unclear, the current research shows that there are similarities between the pathogenesis of these two diseases at molecular and cellular levels. At present, the research on the cellular and molecular mechanism of lung cancer related to pulmonary fibrosis has become the focus of researchers' attention. This article reviews the related literature, focusing on the latest status of the cellular and molecular mechanisms and treatment of IPF-LC, hoping to help clinicians understand IPF-LC.
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The Role of Connexin Hemichannels in Inflammatory Diseases

The connexin protein family consists of approximately 20 members, and is well recognized as the structural unit of the gap junction channels that perforate the plasma membranes of coupled cells and, thereby, mediate intercellular communication. Gap junctions are assembled by two preexisting hemichannels on the membranes of apposing cells. Non-junctional connexin hemichannels (CxHC) provide a conduit between the cell interior and the extracellular milieu, and are believed to be in a protectively closed state under physiological conditions. The development and characterization of the peptide mimetics of the amino acid sequences of connexins have resulted in the development of a panel of blockers with a higher selectivity for CxHC, which have become important tools for defining the role of CxHC in various biological processes. It is increasingly clear that CxHC can be induced to open by pathogen-associated molecular patterns. The opening of CxHC facilitates the release of damage-associated molecular patterns, a class of endogenous molecules that are critical for the pathogenesis of inflammatory diseases. The blockade of CxHC leads to attenuated inflammation, reduced tissue injury and improved organ function in human and animal models of about thirty inflammatory diseases and disorders. These findings demonstrate that CxHC may contribute to the intensification of inflammation, and serve as a common target in the treatments of various inflammatory diseases. In this review, we provide an update on the progress in the understanding of CxHC, with a focus on the role of these channels in inflammatory diseases.

Immune Stroma in Lung Cancer and Idiopathic Pulmonary Fibrosis: A Common Biologic Landscape?

Idiopathic pulmonary fibrosis (IPF) identifies a specific entity characterized by chronic, progressive fibrosing interstitial pneumonia of unknown cause, still lacking effective therapies. Growing evidence suggests that the biologic processes occurring in IPF recall those which orchestrate cancer onset and progression and these findings have already been exploited for therapeutic purposes. Notably, the incidence of lung cancer in patients already affected by IPF is significantly higher than expected. Recent advances in the knowledge of the cancer immune microenvironment have allowed a paradigm shift in cancer therapy. From this perspective, recent experimental reports suggest a rationale for immune checkpoint inhibition in IPF. Here, we recapitulate the most recent knowledge on lung cancer immune stroma and how it can be translated into the IPF context, with both diagnostic and therapeutic implications.

The Role of Connexin 43 in Lung Disease

The term lung disease describes a broad category of disorders that impair lung function. More than 35 million Americans have a preventable chronic lung disease with high mortality rates due to limited treatment efficacy. The recent increase in patients with lung disease highlights the need to increase our understanding of mechanisms driving lung inflammation. Connexins, gap junction proteins, and more specifically connexin 43 (Cx43), are abundantly expressed in the lung and are known to play a role in lung diseases. This review focuses on the role of Cx43 in pathology associated with acute respiratory distress syndrome (ARDS), chronic obstructive pulmonary disease (COPD) and asthma. Additionally, we discuss the role of Cx43 in preventing disease through the transfer of mitochondria between cells. We aim to highlight the need to better understand what cell types are expressing Cx43 and how this expression influences lung disease.

Contribution of Connexin Hemichannels to the Pathogenesis of Acute Lung Injury

Connexin (Cx) family members form hemichannels (HCs) and gap junctions (GJs). Biological functions of Cx HCs have not been adequately characterized due to the inability to selectively target HCs or GJs. Recently, we developed a 6-mer peptide mimetic (P5) of the first extracellular loop of Cx43 and showed that it can block the permeability of HCs but not GJs formed by Cx43. In this study, we further characterized the HC blocking property of P5 and investigated the role of Cx HCs in acute lung injury (ALI). We found that P5 administration decreased HC permeability, in pulmonary microvascular endothelial cells, HepG2 cells, and even Cx43-deficient astrocytes, which express different sets of Cxs, suggesting that P5 is a broad spectrum Cx HC blocker. In addition, P5 reduced HC permeability of alveolar cells in vivo. Moreover, P5 decreased endotoxin-induced release, by vascular endothelial cells in vitro, of high mobility group box protein 1 (HMGB1), a critical mediator of acute lung injury (ALI), and reduced HMGB1 accumulation in bronchoalveolar lavage fluid (BALF) of mice subjected to intratracheal endotoxin instillation. Furthermore, P5 administration resulted in a significant decrease in the concentrations of ALT, AST, and LDH in the BALF, the accumulation of leukocytes in alveoli, and the mortality rate of mice subjected to ALI. Wright-Giemsa staining showed that P5 caused similar reductions of both neutrophils and monocytes in BALF of ALI mice. Together, these results suggest that Cx HCs mediate HMGB1 release, augment leukocyte recruitment, and contribute to ALI pathology.

Effects of Connexin 32-Mediated Lung Inflammation Resolution During Liver Ischemia Reperfusion

BACKGROUND

Hepatic ischemia reperfusion (HIR) leads to a lung inflammatory response and subsequent pulmonary barrier dysfunction. The gap junction communication protein connexin 32 (Cx32), which is widely expressed in the lungs, participates in intercellular signaling. This study determined whether the communication protein Cx32 could affect pulmonary inflammation caused by HIR.

METHODS

Mice were randomly allocated into four groups (n = 8/group): (i) Cx32+/+ sham group; (ii) Cx32+/+ HIR model group; (iii) Cx32-/- sham group; and (iv) Cx32-/- HIR model group. Twenty-four hours after surgery, lung tissues were collected for bright field microscopy, western blot (Cx32, JAK2, p-JAK2, STAT3, p-STAT3), and immunofluorescence (ZO-1, 8-OHDG) analyses. The collected bronchoalveolar fluid was tested for levels of interleukin-6 (IL-6), matrix metalloproteinase 12 (MMP-12), and antitrypsin (α1-AT). Lung mmu-miR-26a/b expression was detected using a PCR assay.

RESULTS

Increased expression of Cx32 mRNA and protein was noted in the lungs after HIR. Cx32 deletion significantly aggravated pulmonary function from acute lung injury induced by HIR. In addition, Cx32 deletion decreased the protein level of ZO-1 (pulmonary function) and increased the level of the oxidative stress marker 8-OHDG in the lungs. Moreover, in the Cx32-/- HIR model group, the levels of IL-6 and MMP-12 in bronchoalveolar lavage fluid were significantly increased leading to activation of the JAK2/STAT3 pathway, and decreased α1-AT levels. Furthermore, we found mmu-miR-26a/b was significantly downregulated in the Cx32-/- HIR model group.

CONCLUSION

HIR leads to acute lung inflammatory injury. Cx32 deletion aggravates hepatic-derived lung inflammation, partly through blocking the transferring of mmu-miR-26a/b and leading to IL-6-related JAK2/STAT3 pathway activation.

Airborne PAHs inhibit gap junctional intercellular communication and activate MAPKs in human bronchial epithelial cell line

Inhalation exposures to polycyclic aromatic hydrocarbons (PAHs) have been associated with various adverse health effects, including chronic lung diseases and cancer. Using human bronchial epithelial cell line HBE1, we investigated the effects of structurally different PAHs on tissue homeostatic processes, namely gap junctional intercellular communication (GJIC) and MAPKs activity. Rapid (<1 h) and sustained (up to 24 h) inhibition of GJIC was induced by low/middle molecular weight (MW) PAHs, particularly by those with a bay- or bay-like region (1- and 9-methylanthracene, fluoranthene), but also by fluorene and pyrene. In contrast, linear low MW (anthracene, 2-methylanthracene) or higher MW (chrysene) PAHs did not affect GJIC. Fluoranthene, 1- and 9-methylanthracene induced strong and sustained activation of MAPK ERK1/2, whereas MAPK p38 was activated rather nonspecifically by all tested PAHs. Low/middle MW PAHs can disrupt tissue homeostasis in human airway epithelium via structure-dependent nongenotoxic mechanisms, which can contribute to their human health hazards.

Hypoxia‑induced internalization of connexin 26 and connexin 43 in pulmonary epithelial cells is involved in the occurrence of non‑small cell lung cancer via the P53/MDM2 signaling pathway

Reports have highlighted an association between connexins (CXs) or gap junction proteins and non-small cell lung cancer (NSCLC). In the present study, it was aimed to elucidate the regulatory mechanism of CX26 and CX43 under hypoxic conditions in NSCLC. Clinical samples were collected for analysis of CX26 and CX43 expression and clinical cancerization followed by quantification of CX26 and CX43 expression. Following the establishment of an in vitro hypoxia model, P53/murine double minute-2 (MDM2) signaling pathway-, proliferation- and epithelial-mesenchymal transition (EMT)-related genes were quantified to evaluate the influence of CX26 and CX43 on the biological functions of pulmonary epithelial cells in NSCLC. In addition, the proliferation and tumorigenicity of cancer cells were assessed by EdU staining and xenograft tumors, respectively. Decreased expression of CX26 and CX43 was found in cancer tissues compared with surrounding normal tissue. Hypoxia was shown to activate the P53/MDM2 axis and stimulate the downregulation, ubiquitination and degradation of CX26 and CX43, which were translocated from the membrane to the cytoplasm. Low levels of CX26 and CX43 were demonstrated to further promote EMT and the induction of the proliferation and tumorigenicity of cancer cells. These results were reflected by decreased E-cadherin expression and increased N-cadherin expression, along with increased cell migration, promoted cell proliferation ability and elevated relative protein expression of Oct4 and Nanog, and accelerated tumor growth, accompanied by a higher number of metastatic nodes. Taken together, the key observations of the present study demonstrate that the internalization of CX26 and CX43 promoted proliferation, EMT and migration and thus induced NSCLC via aberrant activation of the P53/MDM2 signaling pathway under hypoxic conditions.

Molecular Mechanisms of Pulmonary Fibrogenesis and Its Progression to Lung Cancer: A Review

Idiopathic pulmonary fibrosis (IPF) is defined as a specific form of chronic, progressive fibrosing interstitial pneumonia of unknown cause, occurring primarily in older adults, and limited to the lungs. Despite the increasing research interest in the pathogenesis of IPF, unfavorable survival rates remain associated with this condition. Recently, novel therapeutic agents have been shown to control the progression of IPF. However, these drugs do not improve lung function and have not been tested prospectively in patients with IPF and coexisting lung cancer, which is a common comorbidity of IPF. Optimal management of patients with IPF and lung cancer requires understanding of pathogenic mechanisms and molecular pathways that are common to both diseases. This review article reflects the current state of knowledge regarding the pathogenesis of pulmonary fibrosis and summarizes the pathways that are common to IPF and lung cancer by focusing on the molecular mechanisms.

Insight into the Role and Regulation of Gap Junction Genes in Lung Cancer and Identification of Nuclear Cx43 as a Putative Biomarker of Poor Prognosis

Direct intercellular communication, mediated by gap junctions formed by the connexin transmembrane protein family, is frequently dysregulated in cancer. Connexins have been described as tumour suppressors, but emerging evidence suggests that they can also act as tumour promoters. This feature is connexin- and tissue-specific and may be mediated by complex signalling pathways through gap junctions or hemichannels or by completely junction-independent events. Lung cancer is the number one cancer in terms of mortality worldwide, and novel biomarkers and therapeutic targets are urgently needed. Our objective was to gain a better understanding of connexins in this setting. We used several in silico tools to analyse TCGA data in order to compare connexin mRNA expression between healthy lung tissue and lung tumours and correlated these results with gene methylation patterns. Using Kaplan-Meier plotter tools, we analysed a microarray dataset and an RNA-seq dataset of non-small cell lung tumours in order to correlate connexin expression with patient prognosis. We found that connexin mRNA expression is frequently either upregulated or downregulated in lung tumours. This correlated with both good and poor prognosis (overall survival) in a clear connexin isoform-dependent manner. These associations were strongly influenced by the histological subtype (adenocarcinoma versus squamous cell carcinoma). We present an overview of all connexins but particularly focus on four isoforms implicated in lung cancer: Cx26, Cx30.3, Cx32 and Cx43. We further analysed the protein expression and localization of Cx43 in a series of 73 human lung tumours. We identified a subset of tumours that exhibited a unique strong nuclear Cx43 expression pattern that predicted worse overall survival (p = 0.014). Upon sub-stratification, the prognostic value remained highly significant in the adenocarcinoma subtype (p = 0.002) but not in the squamous carcinoma subtype (p = 0.578). This finding highlights the importance of analysis of connexin expression at the protein level, particularly the subcellular localization. Elucidation of the underlying pathways regulating Cx43 localization may provide for novel therapeutic opportunities.

Idiopathic Pulmonary Fibrosis and Lung Cancer: Mechanisms and Molecular Targets

Idiopathic pulmonary fibrosis (IPF) is the most common idiopathic interstitial pulmonary disease with a median survival of 2–4 years after diagnosis. A significant number of IPF patients have risk factors, such as a history of smoking or concomitant emphysema, both of which can predispose the patient to lung cancer (LC) (mostly non-small cell lung cancer (NSCLC)). In fact, IPF itself increases the risk of LC development by 7% to 20%. In this regard, there are multiple common genetic, molecular, and cellular processes that connect lung fibrosis with LC, such as myofibroblast/mesenchymal transition, myofibroblast activation and uncontrolled proliferation, endoplasmic reticulum stress, alterations of growth factors expression, oxidative stress, and large genetic and epigenetic variations that can predispose the patient to develop IPF and LC. The current approved IPF therapies, pirfenidone and nintedanib, are also active in LC. In fact, nintedanib is approved as a second line treatment in NSCLC, and pirfenidone has shown anti-neoplastic effects in preclinical studies. In this review, we focus on the current knowledge on the mechanisms implicated in the development of LC in patients with IPF as well as in current IPF and LC-IPF candidate therapies based on novel molecular advances.

Gjb4 serves as a novel biomarker for lung cancer and promotes metastasis and chemoresistance via Src activation

Most lung cancer patients are diagnosed late with metastasis, which is the major cause of cancer-related death and recurrent tumors that often exhibit chemoresistance. In the present study, we initially identified gap junction beta-4 protein (Gjb4) to be overexpressed in highly metastatic cancer cells selected by their enhanced binding to serum components. Overexpression or knockdown of Gjb4 increased or decreased lung metastasis of syngeneic mice, respectively. We found that Gjb4 expression was higher in lung tumors than normal tissues (p = 0.0026), and Gjb4 levels in blood buffy coat samples showed significant performance in diagnosing stage I-III (p = 0.002814) and stage IV (p < 0.0001) lung cancer. Moreover, high Gjb4 expression levels were correlated with poor prognosis (p = 1.4e-4) and recurrence (p = 1.9e-12). Using syngeneic mouse model, we observed that Gjb4 was able to promote tumor growth. High molecular weight serum fraction containing the major growth factor component IGF1 was able to induce Gjb4 via PKC pathway. Gjb4 activated Src signaling via MET, and overexpression of Gjb4 enhanced sphere-forming ability and anchorage-independent growth, which were reversed by inhibition of Src. In addition, we demonstrated that Gjb4-mediated Src activation enhanced chemoresistance of cancer cells toward gemcitabine and etoposide. The combination of Gjb4 knockdown, gemcitabine, and dasatinib further enhanced the inhibition of cancer cell viability. Together, our study has identified Gjb4 as a potential novel diagnostic and prognostic biomarker for lung cancer. Targeting Gjb4 may be exploited as a modality for improving lung cancer therapy.

Gap Junctions Are Involved in the Rescue of CFTR-Dependent Chloride Efflux by Amniotic Mesenchymal Stem Cells in Coculture with Cystic Fibrosis CFBE41o- Cells

We previously found that human amniotic mesenchymal stem cells (hAMSCs) in coculture with CF immortalised airway epithelial cells (CFBE41o- line, CFBE) on Transwell® filters acquired an epithelial phenotype and led to the expression of a mature and functional CFTR protein. In order to explore the role of gap junction- (GJ-) mediated intercellular communication (GJIC) in this rescue, cocultures (hAMSC : CFBE, 1 : 5 ratio) were studied for the formation of GJIC, before and after silencing connexin 43 (Cx43), a major component of GJs. Functional GJs in cocultures were inhibited when the expression of the Cx43 protein was downregulated. Transfection of cocultures with siRNA against Cx43 resulted in the absence of specific CFTR signal on the apical membrane and reduction in the mature form of CFTR (band C), and in parallel, the CFTR-dependent chloride channel activity was significantly decreased. Cx43 downregulation determined also a decrease in transepithelial resistance and an increase in paracellular permeability as compared with control cocultures, implying that GJIC may regulate CFTR expression and function that in turn modulate airway epithelium tightness. These results indicate that GJIC is involved in the correction of CFTR chloride channel activity upon the acquisition of an epithelial phenotype by hAMSCs in coculture with CF cells.

Association between connexin 40 and potassium voltage-gated channel subfamily A member 5 expression in the atrial myocytes of patients with atrial fibrillation

Structural and electrical remodeling within the atrium mediate the pathogenesis of atrial fibrillation (AF). Two key genes that sever a role in this remodeling are connexin 40 (Cx40) and potassium voltage-gated channel subfamily A member 5 (KCNA5), respectively. Electrical remodeling is considered to induce structural remodeling during AF. In the present study, the left atrial appendage section and atrial myocytes of patients with AF were evaluated. It was observed that Cx40 and KCNA5 mRNA (P<0.05) and protein (P<0.01) expression was significantly downregulated in AF compared with rheumatic heart disease. In addition, a positive correlation between the mRNA expression Cx40 and KCNA5 was observed in the atrial myocytes of patients with AF (P<0.05; r=0.42). The association between Cx40 and KCNA5 expression was subsequently investigated in primary cultured atrial myocytes using siRNA transfection. In atrial myocytes, downregulation of Cx40 inhibited the expression of KCNA5. Similarly, silencing of KCNA5 suppressed the expression of Cx40. These results indicate that synergistic regulation may occur between Cx40 and KCNA5 expression. Furthermore, the combined effects of electrical and structural remodeling in the atrial myocytes of patients with AF may contribute to the pathogenesis of AF.

Connexin43 Controls the Myofibroblastic Differentiation of Bronchial Fibroblasts from Patients with Asthma

Pathologic accumulation of myofibroblasts in asthmatic bronchi is regulated by extrinsic stimuli and by the intrinsic susceptibility of bronchial fibroblasts to transforming growth factor-β (TGF-β). The specific function of gap junctions and connexins in this process has remained unknown. Here, we investigated the role of connexin43 (Cx43) in TGF-β-induced myofibroblastic differentiation of fibroblasts derived from bronchoscopic biopsy specimens of patients with asthma and donors without asthma. Asthmatic fibroblasts expressed considerably higher levels of Cx43 and were more susceptible to TGF-β₁-induced myofibroblastic differentiation than were their nonasthmatic counterparts. TGF-β₁ efficiently up-regulated Cx43 levels and activated the canonical Smad pathway in asthmatic cells. Ectopic Cx43 expression in nonasthmatic (Cx43_low) fibroblasts increased their predilection to TGF-β₁-induced Smad2 activation and fibroblast-myofibroblast transition. Transient Cx43 silencing in asthmatic (Cx43^high) fibroblasts by Cx43 small interfering RNA attenuated the TGF-β₁-triggered Smad2 activation and myofibroblast formation. Direct interactions of Smad2 and Cx43 with β-tubulin were demonstrated by co-immunoprecipitation assay, whereas the sensitivity of these interactions to TGF-β₁ signaling was confirmed by Förster Resonance Energy Transfer analyses. Furthermore, inhibition of the TGF-β₁/Smad pathway attenuated TGF-β₁-triggered Cx43 up-regulation and myofibroblast differentiation of asthmatic fibroblasts. Chemical inhibition of gap junctional intercellular communication with 18 α-glycyrrhetinic acid did not affect the initiation of fibroblast-myofibroblast transition in asthmatic fibroblasts but interfered with the maintenance of their myofibroblastic phenotype. Collectively, our data identified Cx43 as a new player in the feedback mechanism regulating TGF-β₁/Smad-dependent differentiation of bronchial fibroblasts. Thus, our observations point to Cx43 as a novel profibrotic factor in asthma progression.

Connexins in endothelial barrier function - novel therapeutic targets countering vascular hyperpermeability

Prolonged vascular hyperpermeability is a common feature of many diseases. Vascular hyperpermeability is typically associated with changes in the expression patterns of adherens and tight junction proteins. Here, we focus on the less-appreciated contribution of gap junction proteins (connexins) to basal vascular permeability and endothelial dysfunction. First, we assess the association of connexins with endothelial barrier integrity by introducing tools used in connexin biology and relating the findings to customary readouts in vascular biology. Second, we explore potential mechanistic ties between connexins and junction regulation. Third, we review the role of connexins in microvascular organisation and development, focusing on interactions of the endothelium with mural cells and tissue-specific perivascular cells. Last, we see how connexins contribute to the interactions between the endothelium and components of the immune system, by using neutrophils as an example. Mounting evidence of crosstalk between connexins and other junction proteins suggests that we rethink the way in which different junction components contribute to endothelial barrier function. Given the multiple points of connexin-mediated communication arising from the endothelium, there is great potential for synergism between connexin-targeted inhibitors and existing immune-targeted therapeutics. As more drugs targeting connexins progress through clinical trials, it is hoped that some might prove effective at countering vascular hyperpermeability.

Expression and role of connexin-based gap junctions in pulmonary inflammatory diseases

Connexins are transmembrane proteins that can generate intercellular communication channels known as gap junctions. They contribute to the direct movement of ions and larger cytoplasmic solutes between various cell types. In the lung, connexins participate in a variety of physiological functions, such as tissue homeostasis and host defence. In addition, emerging evidence supports a role for connexins in various pulmonary inflammatory diseases, such as asthma, pulmonary hypertension, acute lung injury, lung fibrosis or cystic fibrosis. In these diseases, the altered expression of connexins leads to disruption of normal intercellular communication pathways, thus contributing to various pathophysiological aspects, such as inflammation or tissue altered reactivity and remodeling. The present review describes connexin structure and organization in gap junctions. It focuses on connexins in the lung, including pulmonary bronchial and arterial beds, by looking at their expression, regulation and physiological functions. This work also addresses the issue of connexin expression alteration in various pulmonary inflammatory diseases and describes how targeting connexin-based gap junctions with pharmacological tools, synthetic blocking peptides or genetic approaches, may open new therapeutic perspectives in the treatment of these diseases.

Connexin gap junction channels and chronic rhinosinusitis

BACKGROUND

Gap junction channels are formed by connexin (Cx) proteins. These channels facilitate communication between adjacent cells, and some have been implicated in acute and chronic inflammation. We investigated whether altered connexin expression could be associated with the inflammatory changes of the sinonasal mucosa that characterize chronic rhinosinusitis (CRS). Our aims were first to screen normal sinus mucosa to determine the expression profile of the connexin family of genes, and second to compare the level of expression of 3 key connexins (Cx26, Cx30, and Cx43) in CRS and normal sinus mucosa. These 3 connexins have been implicated in lower airway epithelial cell repair, as well as chronic and acute cutaneous wounds.

METHODS

Sinus mucosa biopsies were taken from 11 patients with CRS undergoing sinus surgery and from 7 controls with normal sinuses undergoing transnasal pituitary surgery. Gene expression study of the connexin family was performed using polymerase chain reaction (PCR). Subsequent targeted quantitative analyses were done using quantitative real-time PCR (qPCR) and fluorescent immunohistochemistry (IHC).

RESULTS

A total of 16 different connexin genes were expressed in the normal mucosa including Cx26, Cx30, and Cx43. The qPCR demonstrated increased abundance of Cx26 (p = 0.005), Cx30 (p = 0.07), and Cx43 (p = 0.04) in CRS compared to control mucosa. IHC confirmed significantly higher levels of Cx43 in CRS (p < 0.001).

CONCLUSION

The majority of the connexin family is expressed in normal sinus mucosa. Expression of 3 selected connexins was found elevated in CRS mucosa. Connexin gap junction modulation may offer a novel therapeutic target for CRS.

Connexin 43 Upregulation in Mouse Lungs during Ovalbumin-Induced Asthma

BACKGROUND

Connexin (Cx)-based gap junction channels play important roles in the inflammatory response. Cx43 is involved in the pathogenesis of some lung diseases such as acute lung injury. However, the Cx43 expression in asthma is unclear. In the present study, we used a murine model of ovalbumin (OVA)-induced allergic airway disease to examine the levels of Cx43 and analyze the relationship between Cx43 and airway inflammation in allergic airway disease.

METHODS

Asthma was induced in mice via sensitization and challenge with OVA. Cx43 mRNA and protein expression levels were investigated via QT-PCR, western blot, and immunohistochemistry 0 h, 8 h, 1 d, 2 d and 4 d after the first challenge. The relationship between Cx43 protein levels and inflammatory cell infiltration, cytokine levels was analyzed.

RESULTS

The OVA-induced mice exhibited typical pathological features of asthma, including airway hyper-responsiveness; strong inflammatory cell infiltration surrounding the bronchia and vessels; many inflammatory cells in the bronchoalveolar lavage fluid (BALF); higher IL-4, IL-5 and IL-13 levels; and high OVA specific IgE levels. Low Cx43 expression was detected in the lungs of control (PBS) mice. A dramatic increase in the Cx43 mRNA and protein levels was found in the asthmatic mice. Cx43 mRNA and protein expression levels increased in a time-dependent manner in asthma mice, and Cx43 was mostly localized in the alveolar and bronchial epithelial layers. Moreover, lung Cx43 protein levels showed a significant positive correlation with inflammatory cell infiltration in the airway and IL-4 and IL-5 levels in the BALF at different time points after challenge. Interestingly, the increase in Cx43 mRNA and protein levels occurred prior to the appearance of the inflammatory cell infiltration.

CONCLUSION

Our data suggest that there is a strong upregulation of Cx43 mRNA and protein levels in the lungs in asthma. Cx43 levels also exhibited a positive correlation with allergic airway inflammation. Cx43 may represent a target to treat allergic airway diseases in the future.

Airway Epithelial Cell Integrity Protects from Cytotoxicity of Pseudomonas aeruginosa Quorum-Sensing Signals

Cell-to-cell communication via gap junctions regulates airway epithelial cell homeostasis and maintains the epithelium host defense. Quorum-sensing molecules produced by Pseudomonas aeruginosa coordinate the expression of virulence factors by this respiratory pathogen. These bacterial signals may also incidentally modulate mammalian airway epithelial cell responses to the pathogen, a process called interkingdom signaling. We investigated the interactions between the P. aeruginosa N-3-oxo-dodecanoyl-L-homoserine lactone (C12) quorum-sensing molecule and human airway epithelial cell gap junctional intercellular communication (GJIC). C12 degradation and its effects on cells were monitored in various airway epithelial cell models grown under nonpolarized and polarized conditions. Its concentration was further monitored in daily tracheal aspirates of colonized intubated patients. C12 rapidly altered epithelial integrity and decreased GJIC in nonpolarized airway epithelial cells, whereas other quorum-sensing molecules had no effect. The effects of C12 were dependent on [Ca(2+)]i and could be prevented by inhibitors of Src tyrosine family and Rho-associated protein kinases. In contrast, polarized airway cells grown on Transwell filters were protected from C12 except when undergoing repair after wounding. In vivo during colonization of intubated patients, C12 did not accumulate, but it paralleled bacterial densities. In vitro C12 degradation, a reaction catalyzed by intracellular paraoxonase 2 (PON2), was impaired in nonpolarized cells, whereas PON2 expression was increased during epithelial polarization. The cytotoxicity of C12 on nonpolarized epithelial cells, combined with its impaired degradation allowing its accumulation, provides an additional pathogenic mechanism for P. aeruginosa infections.

Junctional abnormalities in human airway epithelial cells expressing F508del CFTR

Cystic fibrosis (CF) has a profound impact on airway physiology. Accumulating evidence suggests that intercellular junctions are impaired in CF. We examined changes to CF transmembrane conductance regulator (CFTR) function, tight junctions, and gap junctions in NuLi-1 (CFTR(wt/wt)) and CuFi-5 (CFTR(ΔF508/ΔF508)) cells. Cells were studied at air-liquid interface (ALI) and compared with primary human bronchial epithelial cells. On the basis of fluorescent lectin binding, the phenotype of the NuLi-1 and CuFi-5 cells at week 8 resembled that of serous, glycoprotein-rich airway cells. After week 7, CuFi-5 cells possessed 130% of the epithelial Na(+) channel activity and 17% of the CFTR activity of NuLi-1 cells. In both cell types, expression levels of CFTR were comparable to those in primary airway epithelia. Transepithelial resistance of NuLi-1 and CuFi-5 cells stabilized during maturation in ALI culture, with significantly lower transepithelial resistance for CuFi-5 than NuLi-1 cells. We also found that F508del CFTR negatively affects gap junction function in the airway. NuLi-1 and CuFi-5 cells express the connexins Cx43 and Cx26. While both connexins were properly trafficked by NuLi-1 cells, Cx43 was mistrafficked by CuFi-5 cells. Cx43 trafficking was rescued in CuFi-5 cells treated with 4-phenylbutyric acid (4-PBA), as assessed by intracellular dye transfer. 4-PBA-treated CuFi-5 cells also exhibited an increase in forskolin-induced CFTR-mediated currents. The Cx43 trafficking defect was confirmed using IB3-1 cells and found to be corrected by 4-PBA treatment. These data support the use of NuLi-1 and CuFi-5 cells to examine the effects of F508del CFTR expression on tight junction and gap junction function in the context of serous human airway cells.

The lung communication network

The different types of cells in the lung, from the conducting airway epithelium to the alveolar epithelium and the pulmonary vasculature, are interconnected by gap junctions. The specific profile of gap junction proteins, the connexins, expressed in these different cell types forms compartments of intercellular communication that can be further shaped by the release of extracellular nucleotides via pannexin1 channels. In this review, we focus on the physiology of connexins and pannexins and describe how this lung communication network modulates lung function and host defenses in conductive and respiratory airways.

Cardiac to cancer: connecting connexins to clinical opportunity

Gap junctions and their connexin components are indispensable in mediating the cellular coordination required for tissue and organ homeostasis. The critical nature of their existence mandates a connection to disease while at the same time offering therapeutic potential. Therapeutic intervention may be offered through the pharmacological and molecular disruption of the pathways involved in connexin biosynthesis, gap junction assembly, stabilization, or degradation. Chemical inhibitors aimed at closing connexin channels, peptide mimetics corresponding to short connexin sequences, and gene therapy approaches have been incredibly useful molecular tools in deciphering the complexities associated with connexin biology. Recently, therapeutic potential in targeting connexins has evolved from basic research in cell-based models to clinical opportunity in the form of human trials. Clinical promise is particularly evident with regards to targeting connexin43 in the context of wound healing. The following review is aimed at highlighting novel advances where the pharmacological manipulation of connexin biology has proven beneficial in animals or humans.

Novel concepts of acute lung injury and alveolar-capillary barrier dysfunction

In this review we summarize recent major advances in our understanding on the molecular mechanisms, mediators, and biomarkers of acute lung injury (ALI) and alveolar-capillary barrier dysfunction, highlighting the role of immune cells, inflammatory and noninflammatory signaling events, mechanical noxae, and the affected cellular and molecular entities and functions. Furthermore, we address novel aspects of resolution and repair of ALI, as well as putative candidates for treatment of ALI, including pharmacological and cellular therapeutic means.

Idiopathic pulmonary fibrosis: an altered fibroblast proliferation linked to cancer biology

The fibrotic process that characterizes idiopathic pulmonary fibrosis (IPF) is commonly considered the result of a recurrent injury to the alveolar epithelium followed by an uncontrolled proliferation of fibroblasts. However, based on considerable scientific evidence, it has been recently hypothesized that IPF might be considered a neoproliferative disorder of the lung because this disease exhibits several pathogenic features similar to cancer. Indeed, epigenetic and genetic abnormalities, altered cell-to-cell communications, uncontrolled proliferation, and abnormal activation of specific signal transduction pathways are biological hallmarks that characterize the pathogenesis of IPF and cancer. IPF remains a disease marked by a survival of 3 years, and little therapeutic progress has been made in the last few years, underlining the urgent need to improve research and to change our approach to the comprehension of this disease. The concept of IPF as a cancer-like disease may be helpful in identifying new pathogenic mechanisms that can be borrowed from cancer biology, potentially leading to different and more effective therapeutic approaches. Such vision will hopefully increase the awareness of this disease among the public and the scientific community.

Treatment with baicalin up-regulates the expression of connexion 26 and connexion 43 in human hepatocellular carcinoma cell line SMMC-7721

AIM: To investigate the effect of treatment with baicalin on the expression of connexion 26 (Cx26) and connexion 43 (Cx43) in human hepatocellular cell line SMMC-7721.

METHODS: SMMC-7721 cells were randomly divided into 4 groups: normal control group, low-dose (10 mg/L) baicalin group, medium-dose (20 mg/L) baicalin group, and high-dose (40 mg/L) baicalin group. Gap junction intercellular communication (GJIC) was measured by scrap loading/dye transfer assay (SL/DT). The expression of Cx26 and Cx43 mRNAs in SMMC-7721 cells was determined by RT-PCR. The expression of Cx26 protein was detected by Western blot, and that of Cx43 protein was detected by immunohistochemistry.

RESULTS: Compared to the normal control group, the expression of Cx26 mRNA and protein was significantly enhanced in SMMC-7721 cells treated with low, medium and high concentrations of baicalin (mRNA: 0.148 ± 0.111, 10.253 ± 0.222, 17.283 ± 0.024 vs 0.138 ± 0.111; all P < 0.05; protein: 0.516 ± 0.029, 0.759 ± 0.020, 1.019 ± 0.076 vs 0.367 ± 0.029; all P < 0.05). Compared to the normal control group, the expression of Cx43 mRNA showed no significant changes, but the expression of Cx43 protein was significantly enhanced in SMMC-7721 cells treated with different concentrations of baicalin.

CONCLUSION: Restoration or enhancement of GJIC induced by up-regulation of Cx26 and Cx43 is likely to be an important molecular mechanism by which baicalin inhibits tumor growth.