Junctional adhesion molecule A promotes epithelial tight junction assembly to augment lung barrier function.

Epithelial barrier function is maintained by tight junction proteins that control paracellular fluid flux. Among these proteins is junctional adhesion molecule A (JAM-A), an Ig fold transmembrane protein. To assess JAM-A function in the lung, we depleted JAM-A in primary alveolar epithelial cells using shRNA. In cultured cells, loss of JAM-A caused an approximately 30% decrease in transepithelial resistance, decreased expression of the tight junction scaffold protein zonula occludens 1, and disrupted junctional localization of the structural transmembrane protein claudin-18. Consistent with findings in other organs, loss of JAM-A decreased ß1 integrin expression and impaired filamentous actin formation. Using a model of mild systemic endoxotemia induced by i.p. injection of lipopolysaccharide, we report that JAM-A(-/-) mice showed increased susceptibility to pulmonary edema. On injury, the enhanced susceptibility of JAM-A(-/-) mice to edema correlated with increased, transient disruption of claudin-18, zonula occludens 1, and zonula occludens 2 localization to lung tight junctions in situ along with a delay in up-regulation of claudin-4. In contrast, wild-type mice showed no change in lung tight junction morphologic features in response to mild systemic endotoxemia. These findings support a key role of JAM-A in promoting tight junction homeostasis and lung barrier function by coordinating interactions among claudins, the tight junction scaffold, and the cytoskeleton.

Rosiglitazone attenuates chronic hypoxia-induced pulmonary hypertension in a mouse model.

Chronic hypoxia contributes to pulmonary hypertension through complex mechanisms that include enhanced NADPH oxidase expression and reactive oxygen species (ROS) generation in the lung. Stimulation of peroxisome proliferator-activated receptor gamma (PPARgamma) reduces the expression and activity of NADPH oxidase. Therefore, we hypothesized that activating PPARgamma with rosiglitazone would attenuate chronic hypoxia-induced pulmonary hypertension, in part, through suppressing NADPH oxidase-derived ROS that stimulate proliferative signaling pathways. Male C57Bl/6 mice were exposed to chronic hypoxia (CH, Fi(O2) 10%) or room air for 3 or 5 weeks. During the last 10 days of exposure, each animal was treated daily by gavage with either the PPARgamma ligand, rosiglitazone (10 mg/kg/d) or with an equal volume of vehicle. CH increased: (1) right ventricular systolic pressure (RVSP), (2) right ventricle weight, (3) thickness of the walls of small pulmonary vessels, (4) superoxide production and Nox4 expression in the lung, and (5) platelet-derived growth factor receptor beta (PDGFRbeta) expression and activity and reduced phosphatase and tensin homolog deleted on chromosome 10 (PTEN) expression. Treatment with rosiglitazone prevented the development of pulmonary hypertension at 3 weeks; reversed established pulmonary hypertension at 5 weeks; and attenuated CH-stimulated Nox4 expression and superoxide production, PDGFRbeta activation, and reductions in PTEN expression. Rosiglitazone also attenuated hypoxia-induced increases in Nox4 expression in pulmonary endothelial cells in vitro despite hypoxia-induced reductions in PPARgamma expression. Collectively, these findings indicate that PPARgamma ligands attenuated hypoxia-induced pulmonary vascular remodeling and hypertension by suppressing oxidative and proliferative signals providing novel insights for mechanisms underlying therapeutic effects of PPARgamma activation in pulmonary hypertension.

The role of NADPH oxidase in chronic intermittent hypoxia-induced pulmonary hypertension in mice.

Obstructive sleep apnea, characterized by intermittent periods of hypoxemia, is an independent risk factor for the development of pulmonary hypertension. However, the exact mechanisms of this disorder remain to be defined. Enhanced NADPH oxidase expression and superoxide (O2(-).) generation in the pulmonary vasculature play a critical role in hypoxia-induced pulmonary hypertension. Therefore, the current study explores the hypothesis that chronic intermittent hypoxia (CIH) causes pulmonary hypertension, in part, by increasing NADPH oxidase-derived reactive oxygen species (ROS) that contribute to pulmonary vascular remodeling and hypertension. To test this hypothesis, male C57Bl/6 mice and gp91phox knockout mice were exposed to CIH for 8 hours per day, 5 days per week for 8 weeks. CIH mice were placed in a chamber where the oxygen concentration was cycled between 21% and 10% O2 45 times per hour. Exposure to CIH for 8 weeks increased right ventricular systolic pressure (RVSP), right ventricle (RV):left ventricle (LV) + septum (S) weight ratio, an index of RV hypertrophy, and thickness of the right ventricular anterior wall as measured by echocardiography. CIH exposure also caused pulmonary vascular remodeling as demonstrated by increased muscularization of the distal pulmonary vasculature. CIH-induced pulmonary hypertension was associated with increased lung levels of the NADPH oxidase subunits, Nox4 and p22phox, as well as increased activity of platelet-derived growth factor receptor beta and its associated downstream effector, Akt kinase. These CIH-induced derangements were attenuated in similarly treated gp91phox knockout mice. These findings demonstrate that NADPH oxidase-derived ROS contribute to the development of pulmonary vascular remodeling and hypertension caused by CIH.

Alcohol primes the airway for increased interleukin-13 signaling.

BACKGROUND: Using an experimental model of airway fibrosis following lung transplantation, we recently showed that chronic alcohol ingestion by donor rats amplifies airway fibrosis in the recipient. Associated with alcohol-mediated amplification of airway fibrosis is increased transforming growth factor beta-1(TGFbeta(1)) and alpha-smooth muscle actin expression. Other studies have shown that interleukin-13 (IL-13) modulates TGFbeta(1) signaling during experimentally-induced airway fibrosis. Therefore, we hypothesized that IL-13 is a component of alcohol-mediated amplification of pro-fibrotic mediators in the alcoholic lung. METHODS: To test this hypothesis, we analyzed tracheal epithelial cells and type II alveolar cells from control- or alcohol-fed rats, alcohol-treated mouse lung fibroblasts, and human bronchial epithelial cells in vitro for expression of various components of the IL-13 signaling pathway. Signaling via the IL-13 pathway was assessed by measuring levels of phosphorylated signal transducers and activators of transcription-6 (STAT6). In addition, we performed heterotopic tracheal transplantation using control-fed and alcohol-fed donor rats and analyzed tracheal allografts for expression of components of the IL-13 signaling pathway by RT-PCR and immunocytochemical analyses. RESULTS: Interleukin-13 expression was detected in type II alveolar epithelial cells and human bronchial epithelial cells, but not in lung fibroblasts. IL-13 expression was decreased in whole lung and type II cells in response to alcohol exposure. In all cell types analyzed, expression of IL-13 signaling receptor (IL-13R alpha(1)) mRNA was markedly increased. In contrast, mRNA and protein expression of the IL-13 decoy receptor (IL-13R alpha(2)) were decreased in all cells analyzed. Exposure to alcohol also increased STAT6 phosphorylation in response to IL-13 and lipopolysaccharide. CONCLUSIONS: Data from multiple cell types in the pulmonary system suggest that IL-13 and its receptors play a role in alcohol-mediated activation of pro-fibrotic pathways. Taken together, these data suggest that alcohol primes the airway for increased IL-13 signaling and subsequent tissue remodeling upon injury such as transplantation.

Na,K-ATPase expression is increased in the lungs of alcohol-fed rats.

BACKGROUND: Alcohol abuse independently increases the risk of developing the acute respiratory distress syndrome (ARDS), a disease characterized by diffuse alveolar epithelial damage, lung edema, and consequent severe hypoxemia. Chronic alcohol abuse increases alveolar epithelial permeability both in vitro and in vivo, in part due to altered tight junction formation. However, both alcohol-fed animals and otherwise healthy alcoholic humans do not have pulmonary edema at baseline, even though their lungs are highly susceptible to acute edematous injury in response to inflammatory stresses. This suggests that active fluid transport by the alveolar epithelium is preserved or even augmented in the alcoholic lung. Chronic alcohol ingestion increases expression of apical sodium channels in the alveolar epithelium; however, its effects on the Na,K-ATPase complex that drives sodium and fluid transport out of the alveolar space have not been examined. METHODS: Age- and gender-matched Sprague-Dawley rats were fed the Lieber-DeCarli liquid diet containing either alcohol or an isocaloric substitution (control diet) for 6 weeks. Gene and protein expression of lung Na,K-ATPase alpha1, alpha2, and beta1 subunits were quantified via real-time PCR and immunobiological analyses, respectively. Alcohol-induced, Na,K-ATPase-dependent epithelial barrier dysfunction was determined by calculating lung tissue wet:dry ratios following an ex vivo buffer-perfused challenge for 2 hours in the presence of ouabain (10(-4) M), a Na,K-ATPase inhibitor. RESULTS: Chronic alcohol ingestion significantly increased gene and protein expression of each Na,K-ATPase subunit in rat lungs. Immunohistochemical analyses of the alcoholic lung also revealed that protein expression of the Na,K-ATPase alpha1 subunit was increased throughout the alveolar epithelium. Additionally, lungs isolated from alcohol-fed rats developed more edema than comparably treated lungs from control-fed rats, as reflected by increased lung tissue wet:dry ratios. CONCLUSIONS: These findings indicate that chronic alcohol ingestion, which is known to increase alveolar epithelial paracellular permeability, actually increases the expression of Na,K-ATPase in the lung as a compensatory mechanism. This provides a potential explanation as to why the otherwise healthy alcoholic does not have evidence of pulmonary edema at baseline.

Alcohol ingestion by donors amplifies experimental airway disease after heterotopic transplantation.

RATIONALE: Obliterative bronchiolitis (OB) after lung transplantation is triggered by alloimmunity, but is ultimately mediated by transforming growth factor (TGF)-beta(1)-dependent airway fibrosis. OBJECTIVES: Chronic alcohol use increases TGF-beta(1) expression and renders the lung susceptible to injury. Therefore, we hypothesized that donor alcohol abuse could prime the lung allograft for OB, as many organ donors have a history of alcohol abuse. METHODS: Tracheas from control and alcohol-fed rats (8 wk) were heterotopically transplanted into recipients with varying degrees of alloimmune mismatch and analyzed for obliterative airway disease severity on Postoperative Day 21. MEASUREMENTS AND MAIN RESULTS: Although donor alcohol ingestion did not increase the number of antigen-presenting cells or infiltrating lymphocytes, it nevertheless increased allograft lumenal collagen content fourfold compared with allografts from control donors. In parallel, alcohol increased TGF-beta(1) and alpha-smooth muscle actin expression in allografts. Alcohol amplified airway disease even in isografts with minor alloimmune mismatches. In contrast, it did not cause any airway disease in isografts in a pure isogenic background, suggesting that a minimal alloimmune response is necessary to trigger alcohol-induced airway fibrosis. CONCLUSIONS: Although alloimmune inflammation is required to initiate airway disease, alcohol primes the allograft for greater TGF-beta(1) expression, myofibroblast transdifferentiation, and fibrosis than by alloimmune inflammation alone. This has serious clinical implications, as many lung donors have underlying alcohol abuse that may prime the allograft recipient for subsequent OB.

The role of donor chronic alcohol abuse in the development of primary graft dysfunction in lung transplant recipients.

Primary graft dysfunction (PGD) following lung transplantation is clinically similar to the acute respiratory distress syndrome. Because alcohol abuse independently increases the incidence of acute respiratory distress syndrome in at-risk individuals, we hypothesized that donor alcohol use is correlated with an increased risk of PGD. As a pilot study, we collected alcohol use histories using a validated instrument, the Alcohol Use Disorder Identification Test questionnaire, from 74 donors and correlated these with the development of PGD in corresponding recipients. Nineteen percent (14/74) of donors were classified as heavy alcohol users, as defined by the Alcohol Use Disorder Identification Test scores≥8. In the 1st 4 days post-transplantation, similar percentages of recipients developed grade 3 PGD on at least 1 day (heavy alcohol user=29% [4/14] versus lighter alcohol user=27% [16/60]); however, recipients receiving a lung from a heavy alcohol user were more likely to have multiple and consecutive days of grade 3 PGD, especially in the 1st 48 hours post-transplant. Both median length of stay in the intensive care unit and hospital were somewhat longer in the heavy alcohol user group (9 versus 7 days and 19.5 versus 17.5 days, respectively). If these preliminary findings are validated in a multi-center study, they would have important implications not only for our understanding of the pathophysiology of PGD but also for the development of novel treatments based on the evolving evidence from experimental and clinical studies on how alcohol abuse renders the lung susceptible to acute edematous injury.

Human immunodeficiency virus-1 transgene expression increases pulmonary vascular resistance and exacerbates hypoxia-induced pulmonary hypertension development.

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by increased pulmonary arterial resistance and vessel remodeling. Patients living with human immunodeficiency virus-1 (HIV-1) have an increased susceptibility to develop severe pulmonary hypertension (PH) irrespective of their CD4+ lymphocyte counts. While the underlying cause of HIV-PAH remains unknown, the interaction of HIV-1 proteins with the vascular endothelium may play a critical role in HIV-PAH development. Hypoxia promotes PH in experimental models and in humans, but the impact of HIV-1 proteins on hypoxia-induced pulmonary vascular dysfunction and PAH has not been examined. Therefore, we hypothesize that the presence of HIV-1 proteins and hypoxia synergistically augment the development of pulmonary vascular dysfunction and PH. We examined the effect of HIV-1 proteins on pulmonary vascular resistance by measuring pressure-volume relationships in isolated lungs from wild-type (WT) and HIV-1 Transgenic (Tg) rats. WT and HIV-1 Tg rats were exposed to 10% O2 for four weeks to induce experimental pulmonary hypertension to assess whether HIV-1 protein expression would impact the development of hypoxia-induced PH. Our results demonstrate that HIV-1 protein expression significantly increased pulmonary vascular resistance (PVR). HIV-1 Tg mice demonstrated exaggerated pulmonary vascular responses to hypoxia as evidenced by greater increases in right ventricular systolic pressures, right ventricular hypertrophy and vessel muscularization when compared to wild-type controls. This enhanced PH was associated with enhanced expression of HIF-1α and PCNA. In addition, in vitro studies reveal that medium from HIV-infected monocyte derived macrophages (MDM) potentiates hypoxia-induced pulmonary artery endothelial proliferation. These results indicate that the presence of HIV-1 proteins likely impact pulmonary vascular resistance and exacerbate hypoxia-induced PH.

Efficacy of oral ribavirin in lung transplant patients with respiratory syncytial virus lower respiratory tract infection.

BACKGROUND: Respiratory syncytial virus (RSV) can cause severe lower respiratory tract infection (LRI) and is a risk factor for the development of bronchiolitis obliterans syndrome (BOS) after lung transplantation (LTx). Currently, the most widely used therapy for RSV is inhaled ribavirin. However, this therapy is costly and cumbersome. We investigated the utility of using oral ribavirin for the treatment of RSV infection after LTx. METHODS: RSV was identified in nasopharyngeal swabs (NPS) or bronchoalveolar lavage (BAL) using direct fluorescent antibody (DFA) in 5 symptomatic LTx patients diagnosed with LRI. Data were collected from December 2005 and August 2007 and included: age; gender; type of LTx; underlying disease; date of RSV; pulmonary function prior to, during and up to 565 days post-RSV infection; need for mechanical ventilation; concurrent infections; and radiographic features. Patients received oral ribavirin for 10 days with solumedrol (10 to 15 mg/kg/day intravenously) for 3 days, until repeat NPS were negative. RESULTS: Five patients had their RSV-LRI diagnosis made at a median of 300 days post-LTx. Mean forced expiratory volume in 1 second (FEV(1)) fell 21% (p < 0.012) during infection. After treatment, FEV(1) returned to baseline and was maintained at follow-up of 565 days. There were no complications and no deaths with oral therapy. A 10-day course of oral ribavirin cost $700 compared with $14,000 for nebulized ribavirin at 6 g/day. CONCLUSIONS: Treatment of RSV after LTx with oral ribavirin and corticosteroids is well tolerated, effective and less costly than inhaled ribavirin. Further studies are needed to directly compare the long-term efficacy of oral vs nebulized therapy for RSV.

GM-CSF receptor expression and signaling is decreased in lungs of ethanol-fed rats.

Alcohol abuse dramatically increases the risk of acute lung injury. In an experimental rat model of ethanol-mediated susceptibility to lung injury, recombinant granulocyte/macrophage colony-stimulating factor (GM-CSF) restored alveolar epithelial barrier function both in vitro and in vivo, even during acute endotoxemia. These findings suggested that the alveolar epithelium, which secretes GM-CSF into the airway where it is required for alveolar macrophage maturation, likewise responds to GM-CSF priming in a receptor-mediated manner. In this study we determined that both the GM-CSF receptor alpha- and beta-subunits (GM-CSFRalpha and GM-CSFRbeta) are expressed throughout the rat airway epithelium and that this expression was significantly decreased in the alveolar epithelium following chronic ethanol ingestion (6 wk). In parallel, PU.1, the master transcription factor for GM-CSF signaling in hematopoietic cells, is also expressed in alveolar epithelial cells, and ethanol ingestion likewise decreased PU.1 protein expression and nuclear binding in the alveolar epithelium. Finally, GM-CSF signaling as reflected by PU.1 expression and nuclear binding was restored with recombinant GM-CSF treatment in vitro. We conclude that chronic ethanol ingestion decreases GM-CSF receptor expression and signaling in the lung epithelium. Consequently, we speculate that dampening of GM-CSF stimulation of the alveolar epithelium is responsible at least in part for the diverse functional defects that characterize the alcoholic lung and could be a therapeutic target in acute lung injury.

Sca-1 negatively regulates proliferation and differentiation of muscle cells.

Satellite cells are tissue-specific stem cells critical for skeletal muscle growth and regeneration. Upon exposure to appropriate stimuli, satellite cells produce progeny myoblasts. Heterogeneity within a population of myoblasts ensures that a subset of myoblasts readily differentiate to form myotubes, whereas other myoblasts remain undifferentiated and thus available for future muscle growth. The mechanisms that contribute to this heterogeneity in myoblasts are largely unknown. We show that satellite cells are Sca-1(neg) but give rise to myoblasts that are heterogeneous for sca-1 expression. The majority of myoblasts are sca-1(neg), rapidly divide, and are capable of undergoing myogenic differentiation to form myotubes. In contrast, a minority population is sca-1(pos), divides slower, and does not readily form myotubes. Sca-1 expression is not static but rather dynamically modulated by the microenvironment. Gain-of-function and loss-of-function experiments demonstrate that sca-1 has a functional role in regulating proliferation and differentiation of myoblasts. Myofiber size of sca-1 null muscles is altered in an age-dependent manner, with increased size observed in younger mice and decreased size in older mice. These studies reveal a novel system that reversibly modulates the myogenic behavior of myoblasts. These studies provide evidence that, rather than being a fixed property, myoblast heterogeneity can be modulated by the microenvironment.

Multiple roles of calcineurin in skeletal muscle growth.

Adequate muscle mass and function are critical for human health. Several types of skeletal muscle growth, including regeneration and growth after atrophy, can occur in the rehabilitative period after orthopaedic surgery. The molecular pathways regulating growth of skeletal muscle in adults are understood poorly. The role of calcineurin in regulating regeneration and growth of atrophied skeletal muscle is discussed. Understanding the molecular and cellular pathways that regulate such muscle growth may lead to the development of strategies to enhance muscle growth, thereby facilitating rehabilitation after orthopaedic surgery.

Skeletal muscle atrophy leads to loss and dysfunction of muscle precursor cells.

Atrophy of skeletal muscle leads to decreases in myofiber size and nuclear number; however, the effects of atrophic conditions on muscle precursor cells (MPC) are largely unknown. MPC lie outside myofibers and represent the main source of additional myonuclei necessary for muscle growth and repair. In the present study, we examined the properties of MPC after hindlimb suspension (HS)-induced atrophy and subsequent recovery of the mouse hindlimb muscles. We demonstrated that the number of MPC in atrophied muscles was decreased. RT-PCR analysis of cells isolated from atrophied muscles indicated that several mRNA characteristic of the myogenic program in MPC were absent. Cells isolated from atrophied muscles failed to properly proliferate and undergo differentiation into multinucleated myotubes. Thus atrophy led to a decrease in MPC and caused dysfunction in those MPC that remained. Upon regrowth of the atrophied muscles, these deleterious effects were reversed. Our data suggest that preventing loss or dysfunction of MPC may be a new pharmacological target during muscle atrophy.

Calcineurin differentially regulates maintenance and growth of phenotypically distinct muscles.

Adequate muscle mass is critical for human health. The molecular pathways regulating maintenance and growth of adult skeletal muscle are little understood. Calcineurin (CN) is implicated as a key signaling molecule in hypertrophy. Whether CN is involved in all forms of muscle growth or in different muscles is unknown. Here, we examine the role of CN in regulating maintenance of muscle size and growth of atrophied muscle in the soleus (slow) and plantaris (fast). The CN inhibitor cyclosporin A (CsA) differentially affects muscle growth and maintenance depending on muscle phenotype. The plantaris is more severely affected by CsA than the soleus in both growth conditions. One-week vs. 2-wk CsA treatment suggests that both CN-dependent and CN-independent growth occur in the atrophied soleus, whereas plantaris growth appears to be totally CN dependent. Our results suggest that CN regulates multiple types of muscle growth, depending both on muscle phenotype and stage of myofiber growth. Differential expression of components of the CN pathway occurs and may contribute to the differences between muscles.

Calcineurin initiates skeletal muscle differentiation by activating MEF2 and MyoD.

Skeletal muscle differentiation is characterized by withdrawal from the cell cycle, expression of muscle specific genes, fusion into multinucleated cells, and assembly of the contractile apparatus. Although many of the key regulatory elements have been identified, the factors that initiate the differentiation process are not well understood. The calcium-dependent phosphatase calcineurin plays an important regulatory role early in myogenesis, but the downstream effectors of calcineurin in differentiation are not known. Here, we show that calcium and calcineurin regulate expression of the myogenin gene at the level of transcription. The myogenin promoter contains two essential elements; an E-box and an A/T rich element that bind MRF and MEF2 transcription factors, respectively. Both of these elements are responsive to calcium and calcineurin. In differentiating myoblasts, MyoD is the major MRF protein that binds to the myogenin promoter E-box. Calcineurin activates MyoD indirectly by decreasing the expression of the Id inhibitory proteins, probably by down-regulating Egr-1 expression, an upstream activator of Id transcription. These results demonstrate that calcineurin regulates skeletal muscle differentiation by activating MEF2 and MyoD transcription factors leading to the induction of myogenin expression.