Apical trypsin increases ion transport and resistance by a phospholipase C-dependent rise of Ca2+

House dust mite allergens induce Ca2+ signalling and alarmin responses in asthma airway epithelial cells

Type 2 inflammation in asthma develops with exposure to stimuli to include inhaled allergens from house dust mites (HDM). Features include mucus hypersecretion and the formation of pro-secretory ion transport characterised by elevated basal Cl- current. Studies using human sinonasal epithelial cells treated with HDM extract report a higher protease activated receptor-2 (PAR-2) agonist-induced calcium mobilisation that may be related to airway sensitisation by allergen-associated proteases. Herein, this study aimed to investigate the effect of HDM on Ca2+ signalling and inflammatory responses in asthmatic airway epithelial cells. Primary bronchial epithelial cells (hPBECs) from asthma donors cultured at air-liquid interface were used to assess electrophysiological, Ca2+ signalling and inflammatory responses. Differences were observed regarding Ca2+ signalling in response to PAR-2 agonist 2-Furoyl-LIGRLO-amide (2-FLI), and equivalent short-circuit current (Ieq) in response to trypsin and 2-FLI, in ALI-asthma and healthy hPBECs. HDM treatment led to increased levels of intracellular cations (Ca2+, Na+) and significantly reduced the 2-FLI-induced change of Ieq in asthma cells. Apical HDM-induced Ca2+ mobilisation was found to mainly involve the activation of PAR-2 and PAR-4-associated store-operated Ca2+ influx and TRPV1. In contrast, PAR-2, PAR-4 antagonists and TRPV1 antagonist only showed slight impact on basolateral HDM-induced Ca2+ mobilisation. HDM trypsin-like serine proteases were the main components leading to non-amiloride sensitive Ieq and also increased interleukin-33 (IL-33) and thymic stromal lymphopoietin (TSLP) from asthma hPBECs. These studies add further insight into the complex mechanisms associated with HDM-induced alterations in cell signalling and their relevance to pathological changes within asthma.

Leukemic cells resist lysosomal inhibition through the mitochondria-dependent reduction of intracellular pH and oxidants

Acidic lysosomes are indispensable for cancer development and linked to chemotherapy resistance. Chloroquine (CQ) and functional analogues have been considered as a potential solution to overcome the cancer progression and chemoresistance by inhibiting the lysosome-mediated autophagy and multidrug exocytosis. However, their anti-cancer efficacy in most clinical trials demonstrated modest improvement. In this study, we investigated the detailed mechanisms underlying the acquired resistance of K562 leukemic cells to CQ treatment. In response to 5-80 μM CQ, the lumen pH of endosomal-lysosomal system immediately increased and gradually reached dynamic equilibrium within 24 h. Leukemic cells produced more acidic organelles to tolerate 5-10 μM CQ. CQ (20-80 μM) concentration-dependently triggered cytosolic pH (pHi) rise, G0/G1 arrest, mitochondrial depolarization/fragmentation, and necrotic/apoptotic cell death. Oxidant induction by CQ was responsible for the mitochondria-dependent cytotoxicity and partial pHi elevation. Cells that survived the CQ cytotoxicity were accompanied with increased mitochondria. Under the 80 μM CQ challenge, co-treatment with the inhibitor of F₀ part of mitochondrial H⁺-ATP synthase, oligomycin (40 nM), prevented the elevation of oxidants as well as pHi, and attenuated stresses on mitochondria, cell survival, and cell proliferation. Besides, oligomycin-treated cells obviously displayed the lysosomal peripheralization and plasma membrane blebbing, suggesting that these cells were in process of lysosomal exocytosis and microvesicle release. Enhanced motion of these secretory processes allowed the cells to exclude CQ and repair necrotic injury. Together, the oxidant production and the proton dynamic interconnection among lysosomes, mitochondria, and cytosol are crucial for leukemic susceptibility to lysosomotropic chemotherapeutics.

Trypsin Depolarizes Pacemaker Potentials in Murine Small Intestinal Interstitial Cells of Cajal

Interstitial cells of Cajal (ICCs) generate pacemaker potentials in the gastrointestinal (GI) tract. In this study, the effects of trypsin on pacemaker potentials in murine small intestinal ICCs were examined. We used whole-cell patch-clamp analysis. The results of whole-cell patch-clamp analysis revealed that trypsin dose-dependently depolarized pacemaker potentials and decreased their amplitude. Treatments with the antagonists of neurokinin1 (NK1) and NK2 receptors (SR-140333 and SR-48968, respectively) slightly inhibited the trypsin-induced responses. However, treatment with the combination of SR-140333 and SR-48968 completely inhibited trypsin-induced responses. Trypsin slightly depolarized pacemaker potentials and increased their amplitude after the intracellular application of GDP-β-S. Additionally, incubation in external Ca2+-free solution inhibited trypsin-induced responses. In the presence of U-73122, staurosporine, Go6976, or xestospongin C, trypsin did not depolarize the pacemaker’s potentials. However, trypsin depolarized the pacemaker potentials in the presence of rottlerin. Finally, HC067047, a TRPV4 inhibitor, did not affect the trypsin-induced responses. These results suggest that trypsin depolarized pacemaker potentials through NK1 and NK2 receptors in the murine small intestinal ICCs, with this effect being dependent on the G protein, phospholipase C, protein kinase C, inositol triphosphate pathways, and extracellular Ca2+ but being independent of the TRPV4 pathway. Hence, trypsin-mediated GI motility regulation must be considered for prokinetic drug developments.

Exposure of human intestinal epithelial cells and primary human hepatocytes to trypsin-like serine protease inhibitors with potential antiviral effect

Human intestinal epithelial cell line-6 (HIEC-6) cells and primary human hepatocytes (PHHs) were treated with 3-amidinophenylalanine-derived inhibitors of trypsin-like serine proteases for 24 hours. It was proven that treatment with MI-1900 and MI-1907 was tolerated up to 50 μM in HIEC-6. These inhibitors did not cause elevations in extracellular H₂O₂ levels and in the concentrations of interleukin (IL)-6 and IL-8 and did not alter occludin distribution in HIEC-6. It was also found that MI-1900 and MI-1907 up to 50 μM did not affect cell viability, IL-6 and IL-8 and occludin levels of PHH. Based on our findings, these inhibitors could be safely applicable at 50 μM in HIEC-6 and in PHH; however, redox status was disturbed in case of PHH. Moreover, it has recently been demonstrated that MI-1900 prevents the replication and spread of the new SARS-CoV-2 in infected Calu-3 cells, most-likely via an inhibition of the membrane-bound host protease TMPRSS2.

Signaling pathways induced by serine proteases to increase intestinal epithelial barrier function

Changes in barrier function of the gastrointestinal tract are thought to contribute to the inflammatory bowel diseases Crohn's disease and ulcerative colitis. Previous work in our lab demonstrated that apical exposure of intestinal epithelial cell lines to serine proteases results in an increase in transepithelial electrical resistance (TER). However, the underlying mechanisms governing this response are unclear. We aimed to determine the requirement for proteolytic activity, epidermal growth factor receptor (EGFR) activation, and downstream intracellular signaling in initiating and maintaining enhanced barrier function following protease treatment using a canine intestinal epithelial cell line (SCBN). We also examined the role of phosphorylation of myosin regulatory light chain on the serine protease-induced increase in TER through. It was found that proteolytic activity of the serine proteases trypsin and matriptase is required to initiate and maintain the protease-mediated increase in TER. We also show that MMP-independent EGFR activation is essential to the sustained phase of the protease response, and that Src kinases may mediate EGFR transactivation. PI3-K and ERK1/2 signaling were important in reaching a maximal increase in TER following protease stimulation; however, their upstream activators are yet to be determined. CK2 inhibition prevented the increase in TER induced by serine proteases. The bradykinin B(2) receptor was not involved in the change in TER in response to serine proteases, and no change in phosphorylation of MLC was observed after trypsin or matriptase treatment. Taken together, our data show a requirement for ongoing proteolytic activity, EGFR transactivation, as well as downstream PI3-K, ERK1/2, and CK2 signaling in protease-mediated barrier enhancement of intestinal epithelial cells. The pathways mediating enhanced barrier function by proteases may be novel therapeutic targets for intestinal disorders characterized by disrupted epithelial barrier function.

Protease-activated receptor 2 signalling pathways: a role in pain processing

INTRODUCTION

Pain is a complex biological phenomenon that includes intricate neurophysiological, behavioural, psychosocial and affective components. Despite decades of pain research, many patients continue suffering from chronic pain that may be refractory to current medical regimens. Accumulating evidence has indicated an important role of protease-activated receptor 2 (PAR2) in the pathogenesis of pain, including inflammation, neuropathic and cancer pain.

AREAS COVERED

In this review, the role of the PAR2 signalling pathway in pain processes, basic mechanism of PAR2 activation and expression of PAR2 in the nervous system is covered. Furthermore, intracellular signalling pathways that are activated by PAR2 are also described.

EXPERT OPINION

The role of PAR2 in pain processing is becoming increasingly clear, and although causal implication remains to be established, PAR2 activation has been observed in several disease model systems. Since PAR2 is activated after nerve injury as well as by trypsin and related serine proteases, and PAR2 plays an important role in pain development and maintenance, exploring PAR2 and its corresponding signalling pathways will provide unfathomable knowledge in understanding the molecular basis of pain. This will also help to identify new targets for pharmacological intervention; however, in the context of potential PAR2-directed therapies, several aspects should be clarified.

Coupling of epithelial Na+ and Cl- channels by direct and indirect activation by serine proteases

The mammalian collecting duct (CD) is continuously exposed to urinary proteases. The CD expresses an epithelial Na(+) channel (ENaC) that is activated after cleavage by serine proteases. ENaC also exists at the plasma membrane in the uncleaved form, rendering activation by extracellular proteases an important mechanism for regulating Na(+) transport. Many exogenous and a small number of endogenous extracellular serine proteases have been shown to activate the channel. Recently, kallikrein 1 (KLK1) was shown to increase γENaC cleavage in the native CD indicating a possible direct role of this endogenous protease in Na(+) homeostasis. To explore this process, we examined the coordinated effect of this protease on Na(+) and Cl(-) transport in a polarized renal epithelial cell line (Madin-Darby canine kidney). We also examined the role of native urinary proteases in this process. Short-circuit current (I(sc)) was used to measure transport of these ions. The I(sc) exhibited an ENaC-dependent Na(+) component that was amiloride blockable and a cystic fibrosis transmembrane conductance regulator (CFTR)-dependent Cl(-) component that was blocked by inhibitor 172. Apical application of trypsin, an exogenous S1 serine protease, activated I(ENaC) but was without effects on I(CFTR). Subtilisin an exogenous S8 protease that mimics endogenous furin-type proteases activated both currents. A similar activation was also observed with KLK1 and native rat urinary proteases. Activation with urinary proteases occurred within minutes and at protease concentrations similar to those in the CD indicating physiological significance of this process. ENaC activation was irreversible and mediated by enhanced cleavage of γENaC. The activation of CFTR was indirect and likely dependent on activation of an endogenous apical membrane protease receptor. Collectively, these data demonstrate coordinated stimulation of separate Na(+) and Cl(-) transport pathways in renal epithelia by extracellular luminal proteases. They also indicate that baseline urinary proteolytic activity is sufficient to modify Na(+) and Cl(-) transport in these epithelia.

Allergic sensitization enhances anion current responsiveness of murine trachea to PAR-2 activation

Protease-activated receptor 2 (PAR-2) is a G protein-coupled receptor possibly involved in the pathogenesis of asthma. PAR-2 also modulates ion transport in cultured epithelial cells, but these effects in native airways are controversial. The influence of allergic inflammation on PAR-2-induced changes in ion transport has received little attention. Here, we studied immediate changes in transepithelial short circuit current (I (sc)) induced by PAR-2 activation in the tracheas of naive and allergic mice. Activation of PAR-2 with an apically added activation peptide (AP) induced a small increase in I (sc), while a much larger increase was observed following basolateral AP addition. In ovalbumin-sensitized and -challenged animals used as a model of allergic airway inflammation, the effect of basolateral AP addition was enhanced. Responses to basolateral AP in both naive and allergic mice were not decreased by blocking sodium absorption with amiloride or CFTR function with CFTR(inh)172 but were reduced by the cyclooxygenase inhibitor indomethacin and largely blocked (>80%) by niflumic acid, a calcium-activated chloride channels' (CaCC) blocker. Allergic mice also showed an enhanced response to ATP and thapsigargin. There was no change in mRNA expression of Par-2 or of the chloride channels Ano1 (Tmem16a) and Bestrophin 2 in tracheas from allergic mice, while mRNA levels of Bestrophin 1 were increased. In conclusion, basolateral PAR-2 activation in the mouse airways led to increased anion secretion through apical CaCC, which was more pronounced in allergic animals. This could be a protective mechanism aimed at clearing allergens and defending against mucus plugging.

Physiological effect of protein kinase C on ENaC-mediated lung liquid regulation in the adult rat lung

Tight control of lung liquid (LL) regulation is vital for pulmonary function. The aim of this work was to determine whether PKC activation is involved in the physiological regulation of LL volume in a whole lung preparation. Rat lungs were perfused with a modified Ringer solution, and the lumen was filled with the same solution without glucose. LL volume was measured during a control period and after modulating drugs were administered, and net LL transepithelial movement (J(v)) was calculated. When the PKC activator PMA (10(-5) M) and the Ca(2+) ionophore ionomycin (10(-6) M) were instilled into the lung together, J(v) was significantly reduced (P = 0.03). This reduction was blocked by the PKC inhibitor chelerythrine chloride (10(-6) M; P = 0.56) and by a second PKC inhibitor GF109203X (10(-5) M; P = 0.98). When PMA and ionomycin were added with the β-adrenergic agonist terbutaline, the terbutaline-induced increase in J(v) was abolished. Addition of PMA and ionomycin with the epithelial Na(+) channel (ENaC) blocker amiloride had no additional inhibitory effect. Together, these results suggest that PKC is likely to be involved in LL absorption, and the ability of PMA/ionomycin to block the terbutaline-induced increase in J(v) suggests that the downstream target of PKC is ENaC.

Matrix metalloproteinase-8 mediates the unfavorable systemic impact of local irradiation on pharmacokinetics of anti-cancer drug 5-Fluorouracil

Concurrent chemoradiation with 5-fluorouracil (5-FU) is widely accepted for cancer treatment. However, the interactions between radiation and 5-FU remain unclear. Here, we evaluated the influence of local irradiation on the pharmacokinetics of 5-FU in rats. The single-fraction radiation was delivered to the whole pelvic fields of Sprague-Dawley rats after computerized tomography-based planning. 5-FU at 100 mg/kg was prescribed 24 hours after radiation. A high-performance liquid chromatography system was used to measure 5-FU in the blood. Matrix metalloproteinase-8 (MMP-8) inhibitor I was administered to examine whether or not RT modulation of 5-FU pharmacokinetic parameters could be blocked. Compared with sham-irradiated controls, whole pelvic irradiation reduced the area under the concentration versus time curve (AUC) of 5-FU in plasma and, in contrast, increased in bile with a radiation dose-dependent manner. Based on protein array analysis, the amount of plasma MMP-8 was increased by whole pelvic irradiation (2.8-fold by 0.5 Gy and 5.3-fold by 2 Gy) in comparison with controls. Pretreatment with MMP-8 inhibitor reversed the effect of irradiation on AUC of 5-FU in plasma. Our findings first indicate that local irradiation modulate the systemic pharmacokinetics of 5-FU through stimulating the release of MMP-8. The pharmacokinetics of 5-FU during concurrent chemoradiaiton therapy should be rechecked and the optimal 5-FU dose should be reevaluated, and adjusted if necessary, during CCRT.

Effect of neutrophil elastase and its inhibitor EPI-hNE4 on transepithelial sodium transport across normal and cystic fibrosis human nasal epithelial cells

BACKGROUND

Hyperactivity of the epithelial sodium (Na+) channel (ENaC) and increased Na+ absorption by airway epithelial cells leading to airway surface liquid dehydration and impaired mucociliary clearance are thought to play an important role in the pathogenesis of cystic fibrosis (CF) pulmonary disease. In airway epithelial cells, ENaC is constitutively activated by endogenous trypsin-like serine proteases such as Channel-Activating Proteases (CAPs). It was recently reported that ENaC activity could also be stimulated by apical treatment with human neutrophil elastase (hNE) in a human airway epithelial cell line, suggesting that hNE inhibition could represent a novel therapeutic approach for CF lung disease. However, whether hNE can also activate Na+ reabsorption in primary human nasal epithelial cells (HNEC) from control or CF patients is currently unknown.

METHODS

We evaluated by short-circuit current (Isc) measurements the effects of hNE and EPI-hNE4, a specific hNE inhibitor, on ENaC activity in primary cultures of HNEC obtained from control (9) and CF (4) patients.

RESULTS

Neither hNE nor EPI-hNE4 treatments did modify Isc in control and CF HNEC. Incubation with aprotinin, a Kunitz-type serine protease inhibitor that blocks the activity of endogenous CAPs, decreased Isc by 27.6% and 54% in control and CF HNEC, respectively. In control and CF HNEC pretreated with aprotinin, hNE did significantly stimulate Isc, an effect which was blocked by EPI-hNE4.

CONCLUSIONS

These results indicate that hNE does activate ENaC and transepithelial Na+ transport in both normal and CF HNEC, on condition that the activity of endogenous CAPs is first inhibited. The potent inhibitory effect of EPI-hNE4 on hNE-mediated ENaC activation observed in our experiments highlights that the use of EPI-hNE4 could be of interest to reduce ENaC hyperactivity in CF airways.

Lipopolysaccharide hyperpolarizes guinea pig airway epithelium by increasing the activities of the epithelial Na(+) channel and the Na(+)-K(+) pump

Earlier, we found that systemic administration of lipopolysaccharide (LPS; 4 mg/kg) hyperpolarized the transepithelial potential difference (V(t)) of tracheal epithelium in the isolated, perfused trachea (IPT) of the guinea pig 18 h after injection. As well, LPS increased the hyperpolarization component of the response to basolateral methacholine, and potentiated the epithelium-derived relaxing factor-mediated relaxation responses to hyperosmolar solutions applied to the apical membrane. We hypothesized that LPS stimulates the transepithelial movement of Na(+) via the epithelial sodium channel (ENaC)/Na(+)-K(+) pump axis, leading to hyperpolarization of V(t). LPS increased the V(t)-depolarizing response to amiloride (10 μM), i.e., offset the effect of LPS, indicating that Na(+) transport activity was increased. The functional activity of ENaC was measured in the IPT after short-circuiting the Na(+)-K(+) pump with basolateral amphotericin B (7.5 μM). LPS had no effect on the hyperpolarization response to apical trypsin (100 U/ml) in the Ussing chamber, indicating that channel-activating proteases are not involved in the LPS-induced activation of ENaC. To assess Na(+)-K(+) pump activity in the IPT, ENaC was short-circuited with apical amphotericin B. The greater V(t) in the presence of amphotericin B in tracheas from LPS-treated animals compared with controls revealed that LPS increased Na(+)-K(+) pump activity. This finding was confirmed in the Ussing chamber by inhibiting the Na(+)-K(+) pump via extracellular K(+) removal, loading the epithelium with Na(+), and observing a greater hyperpolarization response to K(+) restoration. Together, the findings of this study reveal that LPS hyperpolarizes the airway epithelium by increasing the activities of ENaC and the Na(+)-K(+) pump.

K+ channel openers restore verapamil-inhibited lung fluid resolution and transepithelial ion transport

Background

Lung epithelial Na⁺ channels (ENaC) are regulated by cell Ca²⁺ signal, which may contribute to calcium antagonist-induced noncardiogenic lung edema. Although K⁺ channel modulators regulate ENaC activity in normal lungs, the therapeutical relevance and the underlying mechanisms have not been completely explored. We hypothesized that K⁺ channel openers may restore calcium channel blocker-inhibited alveolar fluid clearance (AFC) by up-regulating both apical and basolateral ion transport.

Methods

Verapamil-induced depression of heterologously expressed human αβγ ENaC in Xenopus oocytes, apical and basolateral ion transport in monolayers of human lung epithelial cells (H441), and in vivo alveolar fluid clearance were measured, respectively, using the two-electrode voltage clamp, Ussing chamber, and BSA protein assays. Ca²⁺ signal in H441 cells was analyzed using Fluo 4AM.

Results

The rate of in vivo AFC was reduced significantly (40.6 ± 6.3% of control, P < 0.05, n = 12) in mice intratracheally administrated verapamil. K_Ca3.1 (1-EBIO) and K_ATP (minoxidil) channel openers significantly recovered AFC. In addition to short-circuit current (Isc) in intact H441 monolayers, both apical and basolateral Isc levels were reduced by verapamil in permeabilized monolayers. Moreover, verapamil significantly altered Ca²⁺ signal evoked by ionomycin in H441 cells. Depletion of cytosolic Ca²⁺ in αβγ ENaC-expressing oocytes completely abolished verapamil-induced inhibition. Intriguingly, K_V (pyrithione-Na), K _Ca3.1 (1-EBIO), and K_ATP (minoxidil) channel openers almost completely restored the verapamil-induced decrease in Isc levels by diversely up-regulating apical and basolateral Na⁺ and K⁺ transport pathways.

Conclusions

Our observations demonstrate that K⁺ channel openers are capable of rescuing reduced vectorial Na⁺ transport across lung epithelial cells with impaired Ca²⁺ signal.

Protein kinase D isoforms are expressed in rat and mouse primary sensory neurons and are activated by agonists of protease-activated receptor 2

Serine proteases generated during injury and inflammation cleave protease-activated receptor 2 (PAR(2)) on primary sensory neurons to induce neurogenic inflammation and hyperalgesia. Hyperalgesia requires sensitization of transient receptor potential vanilloid (TRPV) ion channels by mechanisms involving phospholipase C and protein kinase C (PKC). The protein kinase D (PKD) serine/threonine kinases are activated by diacylglycerol and PKCs and can phosphorylate TRPV1. Thus, PKDs may participate in novel signal transduction pathways triggered by serine proteases during inflammation and pain. However, it is not known whether PAR(2) activates PKD, and the expression of PKD isoforms by nociceptive neurons is poorly characterized. By using HEK293 cells transfected with PKDs, we found that PAR(2) stimulation promoted plasma membrane translocation and phosphorylation of PKD1, PKD2, and PKD3, indicating activation. This effect was partially dependent on PKCepsilon. By immunofluorescence and confocal microscopy, with antibodies against PKD1/PKD2 and PKD3 and neuronal markers, we found that PKDs were expressed in rat and mouse dorsal root ganglia (DRG) neurons, including nociceptive neurons that expressed TRPV1, PAR(2), and neuropeptides. PAR(2) agonist induced phosphorylation of PKD in cultured DRG neurons, indicating PKD activation. Intraplantar injection of PAR(2) agonist also caused phosphorylation of PKD in neurons of lumbar DRG, confirming activation in vivo. Thus, PKD1, PKD2, and PKD3 are expressed in primary sensory neurons that mediate neurogenic inflammation and pain transmission, and PAR(2) agonists activate PKDs in HEK293 cells and DRG neurons in culture and in intact animals. PKD may be a novel component of a signal transduction pathway for protease-induced activation of nociceptive neurons and an important new target for antiinflammatory and analgesic therapies.

Safety and efficacy of alpha-1-antitrypsin augmentation therapy in the treatment of patients with alpha-1-antitrypsin deficiency

Alpha-1-antitrypsin deficiency (AATD), also known as alpha1-proteinase inhibitor deficiency, is an autosomal co-dominant condition. The genotypes associated with AATD include null, deficient, and dysfunctional alpha-1-antitrypsin (A1AT) variants, which result in low levels of circulating functional A1AT, unbalanced protease activity, and an increased risk of developing lung emphysema, the leading cause of morbidity in these patients. Furthermore, the most common abnormal genotype, Pi*ZZ may also cause trapping of abnormally folded protein polymers in hepatocytes causing liver dysfunction. A major focus of therapy for patients with lung disease due to AATD is to correct the A1AT deficiency state by augmenting serum levels with intravenous infusions of human plasma-derived A1AT. This strategy has been associated with effective elevations of A1AT levels and function in serum and lung epithelial fluid and observational studies suggest that it may lead to attenuation in lung function decline, particularly in patients with moderate impairment of lung function. In addition, an observational study suggests that augmentation therapy is associated with a reduction of mortality in subjects with AATD and moderate to severe lung impairment. More recent randomized placebo-controlled studies utilizing computer scan densitometry suggest that this therapy attenuates lung tissue loss. Augmentation therapy has a relative paucity of side effects, but it is highly expensive. Therefore, this therapy is recommended for patients with AATD who have a high-risk A1AT genotype with plasma A1AT below protective levels (11 μM) and evidence of obstructive lung disease. In this article, we review the published evidence of A1AT augmentation therapy efficacy, side effects, and safety profile.

Serine proteases decrease intestinal epithelial ion permeability by activation of protein kinase Czeta

Epithelial permeability to ions and larger molecules in the gut is essential for fluid balance, and its dysregulation contributes to intestinal pathology. We investigated the effect of digestive serine proteases on epithelial paracellular permeability. Trypsin, chymotrypsin, and elastase elicited sustained increases in transepithelial resistance (R(TE)) in polarized monolayers of three intestinal epithelial cell lines. This effect was reflected by decreases in paracellular conductances of Na+ and Cl- and a concomitant decrease in permeability to 3,000 molecular weight dextran. The enzyme activities of the proteases were required, yet activators of known protease-activated receptors (PARs) did not reproduce the effect of these proteases on R(TE). PKCzeta isoform-specific inhibitor significantly reduced the trypsin-induced increase in R(TE) whereas PKCzeta activity was increased in cells treated with trypsin and chymotrypsin compared with control cells; this activity was reduced to control levels in the presence of PKCzeta-specific inhibitor. Ca2+ chelators and pharmacological inhibitors of cell signaling support the role for PKCzeta in the protease-induced effect. Finally, we showed that treatment with the serine proteases increased occludin immunostaining and zonula occludin-1 coimmunoprecipitation with occludin in the detergent-insoluble fraction of cell lysates, and these increases were ablated by pretreatment with PKCzeta-specific inhibitor. This finding indicates increased insertion of occludin into the cell junctional complex. These data demonstrate a role for serine proteases in the facilitation of epithelial barrier function through a mechanism that is independent of PARs and is mediated by activation of PKCzeta.

Alpha(1)-antitrypsin inhibits epithelial Na+ transport in vitro and in vivo

A variety of studies have shown that Na(+) reabsorption across epithelial cells depends on the protease-antiprotease balance. Herein, we investigate the mechanisms by which alpha(1)-antitrypsin (A1AT), a major anti-serine protease in human plasma and lung epithelial fluid and lacking a Kunitz domain, regulates amiloride-sensitive epithelial Na(+) channel (ENaC) function in vitro and in vivo. A1AT (0.05 mg/ml = 1 microM) decreased ENaC currents across Xenopus laevis oocytes injected with human alpha,beta,gamma-ENaC (hENaC) cRNAs, and human lung Clara-like (H441) cells expressing native ENaC, in a partially irreversible fashion. A1AT also decreased ENaC single-channel activity when added in the pipette but not in the bath solutions of ENaC-expressing oocytes patched in the cell-attached mode. Incubation of A1AT with peroxynitrite (ONOO(-)), an oxidizing and nitrating agent, abolished its antiprotease activity and significantly decreased its ability to inhibit ENaC. Intratracheal instillation of normal but not ONOO(-)-treated A1AT (1 microM) in C57BL/6 mice also decreased Na(+)-dependent alveolar fluid clearance to the same level as amiloride. Incubation of either H441 cells or ENaC-expressing oocytes with normal but not ONOO(-)-treated A1AT decreased their ability to cleave a substrate of serine proteases. A1AT had no effect on amiloride-sensitive currents of oocytes injected with hENaC bearing Liddle mutations, presumably because these channels remain at the surface longer than the wild-type channels. These data indicate that A1AT may be an important modulator of ENaC activity and of Na(+)-dependent fluid clearance across the distal lung epithelium in vivo by decreasing endogenous protease activity needed to activate silent ENaC.

Alpha1-antitrypsin inhibits the activity of the matriptase catalytic domain in vitro

Matriptase is a type II transmembrane protease that is characterized by an N-terminal transmembrane and multiple extracellular domains, in addition to the conserved extracellular serine protease catalytic domain. The expression pattern of matriptase suggests that this protease may play broad roles in the biology of surface lining epithelial cells. In this study we report that alpha1-antitrypsin (AAT), an endogenous inhibitor of serine proteases, inhibits the catalytic domain of human recombinant matriptase in vitro. Co-incubation of AAT with matriptase (at a molar ratio 1:2) resulted in the formation of heat stable complexes, clearly seen in sodium dodecyl sulfate electrophoresis and Western blots. AAT was found to be a slow, tight-binding inhibitor of the catalytic domain of matriptase with a second order reaction rate constant of 0.31 x 10(3) M(-1)s(-1). Notably, the oxidized form of AAT, which lacks serine protease inhibitor activity, failed to generate matriptase complexes and to inhibit matriptase activity. Since matriptase is involved in a number of physiologic processes, including activation of epithelial sodium channels, our findings offer considerable new insights into new regulatory function of AAT in vivo.

A pure population of lung alveolar epithelial type II cells derived from human embryonic stem cells

Alveolar epithelial type II (ATII) cells are small, cuboidal cells that constitute approximately 60% of the pulmonary alveolar epithelium. These cells are crucial for repair of the injured alveolus by differentiating into alveolar epithelial type I cells. ATII cells derived from human ES (hES) cells are a promising source of cells that could be used therapeutically to treat distal lung diseases. We have developed a reliable transfection and culture procedure, which facilitates, via genetic selection, the differentiation of hES cells into an essentially pure (>99%) population of ATII cells (hES-ATII). Purity, as well as biological features and morphological characteristics of normal ATII cells, was demonstrated for the hES-ATII cells, including lamellar body formation, expression of surfactant proteins A, B, and C, alpha-1-antitrypsin, and the cystic fibrosis transmembrane conductance receptor, as well as the synthesis and secretion of complement proteins C3 and C5. Collectively, these data document the successful generation of a pure population of ATII cells derived from hES cells, providing a practical source of ATII cells to explore in disease models their potential in the regeneration and repair of the injured alveolus and in the therapeutic treatment of genetic diseases affecting the lung.

Adenosine regulation of alveolar fluid clearance

Adenosine is a purine nucleoside that regulates cell function through G protein-coupled receptors that activate or inhibit adenylyl cyclase. Based on the understanding that cAMP regulates alveolar epithelial active Na(+) transport, we hypothesized that adenosine and its receptors have the potential to regulate alveolar ion transport and airspace fluid content. Herein, we report that type 1 (A(1)R), 2a (A(2a)R), 2b (A(2b)R), and 3 (A(3)R) adenosine receptors are present in rat and mouse lungs and alveolar type 1 and 2 epithelial cells (AT1 and AT2). Rat AT2 cells generated and produced cAMP in response to adenosine, and micromolar concentrations of adenosine were measured in bronchoalveolar lavage fluid from mice. Ussing chamber studies of rat AT2 cells indicated that adenosine affects ion transport through engagement of A(1)R, A(2a)R, and/or A(3)R through a mechanism that increases CFTR and amiloride-sensitive channel function. Intratracheal instillation of low concentrations of adenosine (< or =10(-8)M) or either A(2a)R- or A(3)R-specific agonists increased alveolar fluid clearance (AFC), whereas physiologic concentrations of adenosine (> or =10(-6)M) reduced AFC in mice and rats via an A(1)R-dependent pathway. Instillation of a CFTR inhibitor (CFTR(inh-172)) attenuated adenosine-mediated down-regulation of AFC, suggesting that adenosine causes Cl(-) efflux by means of CFTR. These studies report a role for adenosine in regulation of alveolar ion transport and fluid clearance. These findings suggest that physiologic concentrations of adenosine allow the alveolar epithelium to counterbalance active Na(+) absorption with Cl(-) efflux through engagement of the A(1)R and raise the possibility that adenosine receptor ligands can be used to treat pulmonary edema.

Regulation of the epithelial Na+ channel by peptidases

Recent investigations point to an important role for peptidases in regulating transcellular ion transport by the epithelial Na(+) channel, ENaC. Several peptidases, including furins and proteasomal hydrolases, modulate ENaC maturation and disposal. More idiosyncratically, apical Na(+) transport by ENaC in polarized epithelia of kidney, airway, and gut is stimulated constitutively by one or more trypsin-family serine peptidases, as revealed by inhibition of amiloride-sensitive Na(+) transport by broad-spectrum antipeptidases, including aprotinin and bikunin/SPINT2. In vitro, the transporting activity of aprotinin-suppressed ENaC can be restored by exposure to trypsin. The prototypical channel-activating peptidase (CAP) is a type 1 membrane-anchored tryptic peptidase first identified in Xenopus kidney cells. Frog CAP1 strongly upregulates Na(+) transport when coexpressed with ENaC in oocytes. The amphibian enzyme's apparent mammalian orthologue is prostasin, otherwise known as CAP1, which is coexpressed with ENaC in a variety of epithelia. In airway cells, prostasin is the major basal regulator of ENaC activity, as suggested by inhibition and knockdown experiments. Other candidate regulators of mature ENaC include CAP2/TMPRSS4 and CAP3/matriptase (also known as membrane-type serine protease 1/ST14). Mammalian CAPs are potential targets for treatment of ENaC-mediated Na(+) hyperabsorption by the airway in cystic fibrosis (CF) and by the kidney in hypertension. CAPs can be important for mammalian development, as indicated by embryonic lethality in mice with null mutations of CAP1/prostasin. Mice with selectively knocked out expression of CAP1/prostasin in the epidermis and mice with globally knocked out expression of CAP3/matriptase exhibit phenotypically similar defects in skin barrier function and neonatal death from dehydration. In rats, transgenic overexpression of human prostasin disturbs salt balance and causes hypertension. Thus, several converging lines of evidence indicate that ENaC function is regulated by peptidases, and that such regulation is critical for embryonic development and adult function of organs such as skin, kidney, and lung.

Up-regulation of trypsin and mesenchymal MMP-8 during development of hyperoxic lung injury in the rat

Acute lung injury is marked by damage to alveolar-capillary barrier. High pulmonary levels of matrix-degrading serine proteinase trypsin and matrix metalloproteinases (MMP)-2, -8, and -9 have been shown in preterm infants with respiratory distress syndrome (RDS). We studied expression of trypsin and MMP-2, -8, and -9 in rats exposed to >95% oxygen for 24, 48, or 60 h. As demonstrated by zymography and Western immunoblotting, levels of trypsin and MMP-2, -8, and -9 in bronchoalveolar lavage fluid (BALF) sharply increased after 48 h of hyperoxia relative to normoxia controls. This coincided with increase in alveolar-capillary permeability, as indicated by increased protein concentration in BALF. Both neutrophil-derived 80-kD and mesenchymal cell-derived 60-kD MMP-8 isoforms were detected in BALF. Of them, mesenchymal-type MMP-8 predominated. In immunohistochemistry, alveolar epithelium showed strong trypsin expression at 48 and 60 h of hyperoxia, whereas it was predominantly negative in controls. MMP-8 was mostly expressed in macrophages. Marked up-regulation of trypsin and MMP-8 early during hyperoxic lung injury suggests that these enzymes play a role in the pathogenesis of acute lung injury and may therefore be potential targets for therapy of lung injury.

Prostasin regulates epithelial monolayer function: cell-specific Gpld1-mediated secretion and functional role for GPI anchor

Prostasin, a trypsinlike serine peptidase, is highly expressed in prostate, kidney, and lung epithelia, where it is bound to the cell surface, secreted, or both. Prostasin activates the epithelial sodium channel (ENaC) and suppresses invasion of prostate and breast cancer cells. The studies reported here establish mechanisms of membrane anchoring and secretion in kidney and lung epithelial cells and demonstrate a critical role for prostasin in regulating epithelial monolayer function. We report that endogenous mouse prostasin is glycosylphosphatidylinositol (GPI) anchored to the cell surface and is constitutively secreted from the apical surface of kidney cortical collecting duct cells. Using site-directed mutagenesis, detergent phase separation, and RNA interference approaches, we show that prostasin secretion depends on GPI anchor cleavage by endogenous GPI-specific phospholipase D1 (Gpld1). Secretion of prostasin by kidney and lung epithelial cells, in contrast to prostate epithelium, does not depend on COOH-terminal processing at conserved Arg(322). Using stably transfected M-1 cells expressing wild-type, catalytically inactive, or chimeric transmembrane (not GPI)-anchored prostasins we establish that prostasin regulates transepithelial resistance, current, and paracellular permeability by GPI anchor- and protease activity-dependent mechanisms. These studies demonstrate a novel role for prostasin in regulating epithelial monolayer resistance and permeability in kidney epithelial cells and, furthermore, show specifically that prostasin is a critical regulator of transepithelial ion transport in M-1 cells. These functions depend on the GPI anchor as well as the peptidase activity of prostasin. These studies suggest that cell-specific Gpld1- or peptidase-dependent pathways for prostasin secretion may control prostasin functions in a tissue-specific manner.

Protease-activated receptor regulation of Cl- secretion in Calu-3 cells requires prostaglandin release and CFTR activation

Human lung epithelial (Calu-3) cells were used to investigate the effects of protease-activated receptor (PAR) stimulation on Cl(-) secretion. Quantitative RT-PCR (QRT-PCR) showed that Calu-3 cells express PAR-1, -2, and -3 receptor mRNAs, with PAR-2 mRNA in greatest abundance. Addition of either thrombin or the PAR-2 agonist peptide SLIGRL to the basolateral solution of monolayers mounted in Ussing chambers produced a rapid increase in short-circuit current (I(sc): thrombin, 21 +/- 2 microA; SLIGRL, 83 +/- 22 microA), which returned to baseline within 5 min after stimulation. Pretreatment of monolayers with the cell-permeant Ca(2+)-chelating agent BAPTA-AM (50 microM) abolished the increase in I(sc) produced by SLIGRL. When monolayers were treated with the cyclooxygenase inhibitor indomethacin (10 microM), nearly complete inhibition of both the thrombin- and SLIGRL-stimulated I(sc) was observed. In addition, basolateral treatment with the PGE(2) receptor antagonist AH-6809 (25 microM) significantly inhibited the effects of SLIGRL on I(sc). QRT-PCR revealed that Calu-3 cells express mRNAs for CFTR, the Ca(2+)-activated KCNN4 K(+) channel, and the KCNQ1 K(+) channel subunit, which, in association with KCNE3, is known to be regulated by cAMP. Stimulation with SLIGRL produced an increase in apical Cl(-) conductance that was blocked in cells expressing short hairpin RNAs designed to target CFTR. These results support the conclusion that PAR stimulation of Cl(-) secretion occurs by an indirect mechanism involving the synthesis and release of prostaglandins. In addition, PAR-stimulated Cl(-) secretion requires activation of CFTR and at least two distinct K(+) channels located in the basolateral membrane.

The protease-activated receptor2 (PAR2)-prostaglandin E2-prostanoid EP receptor axis: a potential bronchoprotective unit in the respiratory tract?

Protease-activated receptor2 (PAR2) is a subtype of G protein-coupled receptor that is widely expressed within the respiratory tract. Stimulation of PAR2 by proteases such as trypsin and tryptase, or by small peptidic activators induces a complex array of effects within the airways. One such PAR2-mediated effect by basal airway epithelial cells is the generation of prostaglandin E2. Prostaglandin E2 produces a raft of anti-inflammatory effects within the airways, principally through the activation of the prostanoid EP2 and EP3 receptor subtypes. This article reviews the PAR2-prostaglandin E2-prostanoid EP receptor axis and discusses approaches through which its activation may provide beneficial effects in respiratory disease.

Neutrophil granule contents in the pathogenesis of lung injury

PURPOSE OF REVIEW

This review summarizes recent literature on the role of neutrophil granule contents in acute lung injury and the mechanisms by which these contribute to inflammatory tissue injury.

RECENT FINDINGS

Neutrophil products such as elastase, reactive oxygen species, and antimicrobial peptides can alter pulmonary cell function in a nondegradative fashion through activation of cell surface receptors or modulation of signal transduction pathways. These effects can be either beneficial or detrimental to the host.

SUMMARY

The primary function of neutrophils in the innate immune response--to contain and kill invading microbial pathogens--is achieved through a series of rapid and coordinated responses culminating in phagocytosis and intracellular killing of the pathogens. Neutrophils have a potent antimicrobial arsenal that includes oxidants, proteinases, and cationic peptides. Reactive oxygen species such as oxygen are produced by the phagocyte NADPH oxidase and are microbicidal. Granules within the neutrophil cytoplasm contain potent proteolytic enzymes and cationic proteins that can digest a variety of microbial substrates. These compounds are released directly into the phagosome, compartmentalizing both the pathogen and the cytotoxic products. Under pathological circumstances, however, unregulated release of microbicidal compounds into the extracellular space can paradoxically damage host tissues. Nonspecific inhibition of neutrophils is not clinically realistic, as it would leave the host vulnerable to infection. As the mechanisms of action of neutrophil granule contents are elucidated, therapeutic targets will be identified that will allow for suppression of neutrophils' detrimental effects while avoiding inhibition of their beneficial effects.

Heightened epithelial Na+ channel-mediated Na+ absorption in a murine polycystic kidney disease model epithelium lacking apical monocilia

The Tg737 degrees (rpk) autosomal recessive polycystic kidney disease (ARPKD) mouse carries a hypomorphic mutation in the Tg737 gene. Because of the absence of its protein product Polaris, the nonmotile primary monocilium central to the luminal membrane of ductal epithelia, such as the cortical collecting duct (CCD) principal cell (PC), is malformed. Although the functions of the renal monocilium remain elusive, primary monocilia or flagella on neurons act as sensory organelles. Thus we hypothesized that the PC monocilium functions as a cellular sensor. To test this hypothesis, we assessed the contribution of Polaris and cilium structure and function to renal epithelial ion transport electrophysiology. Properties of Tg737 degrees (rpk) mutant CCD PC clones were compared with clones genetically rescued with wild-type Tg737 cDNA. All cells were grown as polarized cell monolayers with similarly high transepithelial resistance on permeable filter supports. Three- to fourfold elevated transepithelial voltage (V(te)) and short-circuit current (I(sc)) were measured in mutant orpk monolayers vs. rescued controls. Pharmacological and cell biological examination of this enhanced electrical end point in mutant monolayers revealed that epithelial Na(+) channels (ENaCs) were upregulated. Amiloride, ENaC-selective amiloride analogs (benzamil and phenamil), and protease inhibitors (aprotinin and leupeptin) attenuated heightened V(te) and I(sc). Higher concentrations of additional amiloride analogs (ethylisopropylamiloride and dimethylamiloride) also revealed inhibition of V(te). Cell culture requirements and manipulations were also consistent with heightened ENaC expression and function. Together, these data suggest that ENaC expression and/or function are upregulated in the luminal membrane of mutant, cilium-deficient orpk CCD PC monolayers vs. cilium-competent controls. When the genetic lesion causes loss or malformation of the monocilium, ENaC-driven Na(+) hyperabsorption may explain the rapid emergence of severe hypertension in a majority of patients with ARPKD.