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Division of Pulmonary and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA
1Correspondence: Johns Hopkins Asthma and Allergy Center, 5501 Hopkins Bayview Circle, 4B.77, Baltimore, MD 21224-6801, USA. E-mail: drgarcia{at}jhmi.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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30 min after PTX and were dependent on intrinsic ADP-ribosyltransferase activity, as neither the cell binding ß-oligomer subunit or a genetically engineered PTX mutant (devoid of ADP-ribosyltransferase activity) altered TER. PTX-induced barrier dysfunction was associated with mild increases in F-actin stress fiber formation and causally linked to p38 MAP kinase activities. PTX-mediated p38 MAP kinase activation did not involve either p42/p44 ERK, p60src, Rho family of GTPases, or phosphatidylinositol-3' kinase pathways. PTX-mediated decreases in TER were temporally linked to phosphorylation of the actin binding proteins Hsp27 and caldesmon, known substrates for the Ser/Thr kinase MAPKAP2, whose activity is regulated by p38 MAP kinase. In addition to defining novel signaling pathways involved in PTX-induced respiratory pathophysiology, these data suggest that the direct cell-activating effects of PTX be carefully considered as a potential limitation to its use as a tool in signal transduction analysis.—Garcia, J. G. N., Wang, P., Schaphorst, K. L., Becker, P. M., Borbiev, T., Liu, F., Birukova, A., Jacobs, K., Bogatcheva, N., Verin, A. D. Critical involvement of p38 map kinase in pertussis toxin-induced cytoskeletal reorganization and lung permeability.
Key Words: ADP-ribosyltransferase • ß-oligomer • endothelial cell • transendothelial electrical resistance • HSP 27 • caldesmon
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. One product of this infection, pertussis toxin (PTX), exerts a virulence factor effect by catalyzing the ADP-ribosylation of guanine binding G-proteins, a finding that has led to its wide use as a tool for examination of G-protein-regulated cellular signaling pathways. The PTX holotoxin is comprised of a ß-oligomer binding subunit and a S1 ADP-ribosyltransferase fragment whose targets include an arginine residue present on the 
subunit of Gi and Go subclasses of heterotrimeric G-proteins. As S1-mediated ADP-ribosylation effectively uncouples the G protein from its coupled receptor (GPCR), disruption of a signaling pathway by PTX is presumptive evidence of G-protein-mediated regulation. Direct effects of PTX on cellular function, i.e., in the absence of exogenous ligand/GPCR stimulation, have also been noted in numerous cell systems, suggesting potential tonic regulation by PTX of diverse second messenger signaling cascades such as Ca2+ mobilization (2)
, cAMP synthesis, diacylglycerol generation (3)
, tyrosine phosphorylation (4
, 5)
, and Ser/Thr phosphatase activity (6)
.
The exact mechanism by which PTX produces respiratory toxicity in humans is unknown but clearly involves a breach of lung cellular barriers with upper and lower airway edema formation. Intraperitoneally administered PTX directly increases lung weight gain in rodents consistent with lung cell activation and vascular barrier dysfunction (7
, 8)
, although the exact mechanism for this edemagenic response was not identified. We previously noted PTX to potently and directly stimulate increases in albumin permeability across confluent bovine pulmonary artery endothelium in vitro (9
, 10)
without increases in cytosolic Ca2+, myosin light chain kinase activity (11
12
13)
, or membrane phospholipases such as phosphatidylinositol-specific phospholipase C or phospholipase D (14
15
16)
. However, there was strong biochemical evidence of PTX-induced protein kinase C activation (10)
; PKC inhibition attenuated the extent of PTX-induced endothelial cell barrier dysfunction (10)
and reduced the lung weight gain in mice (8)
. Although these results suggest PTX may represent a novel model of endothelial cell permeability, the exact PTX targets and downstream effectors responsible for evoking changes in the endothelial cell cytoskeleton, gap formation, and permeability remain to be defined. The mitogen-activated MAP kinase family of serine/threonine protein kinases, a major signal system by which eukaryotic cells transduce extracellular signals to elicit intracellular responses may be important participants in these responses (17
, 18)
. We recently reported that PTX is a rapid and robust stimulus of p42/p44 MAP kinase (ERK) activity in endothelium, but ADP-ribosyltransferase activity is not required for ERK activation (19)
. The p38 MAP kinases have been shown to target several cytoskeletal proteins and, by inference, to participate in cell motility, apoptosis, stress responses, and inflammation (20
, 21)
. Whether PTX alters lung barrier properties via MAP kinase activities has not been previously addressed.
Using measurements of human lung endothelial cell monolayer electrical resistance, a highly sensitive index of paracellular gap formation and barrier dysfunction and the isolated perfused ferret lung model, we have confirmed in vitro and in vivo our prior observations that PTX mediates lung barrier dysfunction. Neither the PTX ß-oligomer nor a holotoxin mutant lacking ADP-ribosyltransferase activity altered transmonolayer resistance, indicating that the permeability response depends entirely on G-protein modification. We further show that p38 MAP kinase activation is involved in PTX-induced barrier disruption, actin cytoskeletal rearrangement, and increased phosphorylation of the actin binding proteins Hsp27 and caldesmon, known MAP kinase targets. Together, these studies provide mechanistic information on the pathogenesis of direct PTX-induced lung toxicity and increases in vascular permeability. Given the extent of direct cellular activation evoked by PTX, a cautious approach should be considered when using PTX to examine selective G-protein-linked signal transduction pathways.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Cell cultures
Bovine pulmonary artery endothelial cells (BPAEC) were obtained frozen at 16 passages from American Type Tissue Culture Collection (Rockville, MD; CCL 209) and used at passages 19–24 (11)
. BPAEC were cultured in DMEM supplemented with 20% (v/v) fetal bovine serum (FBS), 0.1% endothelial cell growth supplement, 1% antibiotic and antimycotic solution, and 1% nonessential amino acids. Human umbilical vein endothelial cells (HUVEC) were purchased from Clonetics (Walkersville, MD) at passage 1 and used at passages 5–6. HUVEC were cultured in M199 supplemented with 20% (v/v) FBS, 0.15% endothelial cell growth supplement, 1% antibiotic and antimycotic solution, and 0.2% heparin. Cells were cultured in HYQ RPMI 1640 media supplemented with 10% FBS, 1% antibiotic and antimycotic solution, and 1% nonessential amino acids. All cell cultures were maintained at 37°C in a humidified atmosphere of 5% CO2-95% air and grew to contact-inhibited monolayers. Cells from each primary flask were detached with 0.05% trypsin, resuspended in fresh culture media, and passaged into appropriate size flasks or dishes.
Measurement of endothelial cell electrical resistance
Lung endothelial and epithelial cellular barrier properties were measured using the highly sensitive biophysical assay (22
23
24)
with an electrical cell substrate impedance sensing system (Applied Biophysics, Troy, NY). Cells were cultured on several small gold electrodes (10-4 cm2) and culture media was used as the electrolyte. The total electrical resistance was measured dynamically across the monolayer and was determined by the combined resistance between the basal surface of the cell and the electrode, reflective of focal adhesion, and the resistance between cells. Changes in electrical resistance represent alterations in cell–cell adhesion and/or cell–matrix adhesion. As cells adhere and spread out on the microelectrode, the transmonolayer electrical resistance (TER) increases (maximal at confluence) whereas cell retraction, rounding, or loss of adhesion is reflected by a decrease in TER (25)
. The small gold electrodes and the larger counter electrodes (1 cm2) are connected to a phase-sensitive lock-in amplifier (5301A; EG&G Instruments Corp., Princeton, NJ) with a built-in differential preamplifier (5316A; EG&G Instruments). A 1 V, 4000 Hz AC signal was supplied through a 1-M
resistor to approximate a constant current source. Voltage and phase data were stored and processed with a Pentium 100 MHZ computer that controlled the output of the amplifier and relay switches to different electrodes. Experiments were conducted only on wells that achieved >1000 ohms (10 microelectrodes per well) of steady-state resistance. Resistance was expressed by the in-phase voltage (proportional to the resistance) that was normalized to the initial voltage and expressed as a fraction of the normalized resistance value as described previously (23)
. For some experiments, total resistance was resolved into components reflecting resistance to current flow beneath the cell layer () and resistance to current flow between adjacent cells (Rb) using the method of Giaever and Keese, which models the endothelial monolayer mathematically (26)
. Thus, changes in reflect alterations in the net state of cell–matrix adhesion whereas changes in Rb reflect alterations in the integrity of cell–cell adhesion.
MAP kinase activation assay
Endothelial or epithelial cell monolayers in 35 mm dishes grown to 100% confluence were challenged with agonists. After stimulation, cells were rinsed once with PBS, lysed with 150 µL of boiling lysis buffer (10 mM Tris-HCl, pH 7.4, 1% SDS, 1 mM sodium orthovanadate), heated to boiling for 5 min, and centrifuged for 5 min. The protein concentration of the resulting supernatant was determined using BCA Protein Assay Reagent. A common feature of the MAP kinase isoforms is the phosphorylation of threonine and tyrosine residues by a dual specificity Ser/Thr MAPK kinase. The p38 MAP kinase activity present in these samples was assessed by Western immunoblotting analysis with phospho-p38 MAP kinase-specific antibody as recently described (19)
. To assess the total amount of p38 MAP kinase, the blot was stripped using Re-blot Western blot recycling kit (Chemicon, Temecula, CA) and subjected to Western immunoblotting analysis with pan-p38 MAP kinase antibody. The p38 MAP kinase activity was also measured by immune complex kinase assay using GST-ATF-2 as a substrate. After stimulation, cells from 35 mm dishes were rinsed once with ice-cold PBS and lysed with 150 µL of immunoprecipitation buffer (20 mM Tris, pH 7.5, 150 mM NaCl, 1 mM EDTA, 1 mM EGTA, 1% Triton X-100, 0.5% Nonidet P-40, 20 mM NaF, 0.2 mM sodium orthovanadate) supplemented with 0.5% protease inhibitor mixtures for 30 min at 4°C. The cells were then scraped off the dishes, homogenized by passing through a 26 gauge syringe three times, and centrifuged for 10 min. Soluble cell lysates (100 µL) containing 100 µg of total proteins were incubated with immobilized phospho-p38 MAP kinase monoclonal antibody overnight at 4°C. The immunoprecipitates were washed three times with immunoprecipitation buffer and three times with kinase buffer (25 mM Tris-HCl, pH 7.5, 5 mM ß-glycerol-phosphate, 2 mM DTT, 0.1 mM sodium orthovanadate, 10 mM MgCl2). The pellets were suspended in 45 µL of kinase buffer containing 200 µM ATP and 2 µg ATF-2 fusion protein and incubated for 30 min at 30°C. The reaction was terminated by adding 15 µL of boiling 4x Laemmli sample buffer. The samples were boiled for 5 min and centrifuged for 5 min. The p38 MAP kinase activity within the resulting supernatant was analyzed by Western immunoblotting with phospho-ATF-2-specific antibody.
Infection of endothelial cells with adenovirus encoding dominant negative p38 MAP kinase
Adenoviruses expressing the p38 MAPK dominant negative (TGY
AGF) mutant and a replication-deficient control virus were a generous gift from Dr. W. T. Gerthoffer (Reno, NV). Cultured endothelial cells at 70–80% confluency were exposed to either recombinant or control adenovirus at 10–40 moi for 1 h in DMEM (Gibco BRL, Grand Island, NY) containing 2% FBS. The virus-containing medium was replaced with virus-free DMEM with 10% FBS and endothelial cells were analyzed after 30 h.
ADP-ribosylation of cell proteins
Bacterial toxin-catalyzed ADP-ribosylation of lung endothelial or epithelial cell homogenates was measured by incorporation of [32P]-NAD (10–20 µCi/mL in ribosylation mixture) as previously described (14
, 15)
. Pertussis toxin was used at a final concentration of 300 ng/mL and preactivated with 20 mM dithiothreitol. ADP-ribosylated proteins were separated by 1-D SDS-PAGE gels, located by autoradiography, and identity was confirmed by Western immunoblotting analysis.
HSP27 and caldesmon immunoprecipitation
Confluent endothelial cell monolayers in 35 mm dishes were labeled with [32P]-orthophosphate (0.25 mCi/plate) for 2.5 h in phosphate-free DMEM. After challenge with PTX, cells were rinsed twice with 1 mL media and twice with 1 mL PBS. Then 100 µL of cold immunoprecipitation buffer (1% Triton X-100, 150 mM NaCl, 10 mM Tris pH 7.4, 1 mM EDTA, 1 mM EGTA, 0.2 mM sodium orthovanadate, 0.5% Nonidet P-40) supplemented with 0.5% protease inhibitor mixtures was scraped off the dishes; cells were incubated with 170 µL of cold immunoprecipitation buffer and 30 µL of Omnisorb for 30 min at 4 °C and centrifuged for 15 min. The resulting supernatant was incubated with 1 µL of monoclonal anti-caldesmon antibody (Chemicon, Temecula, CA) or monoclonal Hsp 27 antibody (StressGen Biotechnologies, Victoria, BC, Canada) for 1 h at 4°C and with 30 µL of Omnisorb for 30 min at 4°C with constant agitation. After centrifugation (1 min), pellets were washed with 1 mL cold immunoprecipitation buffer, resuspended in 100 µL 2x Laemmli sample buffer, and boiled for 5 min. After centrifugation (5 min), supernatants were loaded onto a SDS-PAGE gel, transferred to nitrocellulose membrane, and exposed to X-Omat film (Kodak, Rochester, NY).
Cytoskeletal visualization by immunofluorescent microscopy
Cultured cells grown on glass coverslips were fixed in 3.7% formaldehyde solution in PBS for 10 min at 4°C and washed three times with PBS. The cells were permeabilized with 0.2% Triton X-100 in PBST for 5 min, washed three times with PBS, and blocked with 2% BSA in PBST for 20 min. Incubation with mouse anti-caldesmon antibody or mouse anti HA antibody diluted 1:100 with blocking solution was performed for 1 h at room temperature. After three washes with PBS, cells were incubated with anti-mouse secondary antibody (1:300) conjugated with fluorescent dye Alexa 488 (Molecular Probes, Eugene, OR) for 1 h at room temperature. Actin filaments were visualized by staining cells with Texas Red-conjugated phalloidin (Molecular Probes) for 1 h at room temperature. The coverslips were mounted and analyzed using Nikon video imagine system consisting of a phase contrast inverted microscope equipped with objectives and filters for immunofluorescence and connected to a digital camera and image processor. The images were recorded and saved on a Pentium II PC as TIFF files compatible with Adobe Photoshop 4.0.
Isolated perfused ferret lung preparation
Commercially available ferrets were anesthetized with pentobarbital (50 mg/kg IP), ventilated (20 breaths/min, tidal volume 12 mL/kg) via tracheostomy with warmed, humidified gas containing 28% O2, then rapidly exsanguinated as described previously (27)
. After exsanguination, the ventilatory gas mixture was switched to 16% O2-5% CO2, ventilatory rate adjusted to 10 breaths/min, and end-expiratory pressure of 3 mm Hg was added. Lungs were isolated by insertion of cannulas into the left atrium via the left ventricle and the pulmonary artery via the right ventricle, then residual blood was flushed from the lungs with physiological salt solution (PSS) containing 5 mM dextrose, 3 g/dL porcine albumin, and 2 g/dL Ficoll (27)
. Isolated lungs were perfused at constant flow (50 mL·kg-1·min-1) for 60 min with PSS (control, n=3) or PSS containing 300 ng/mL PTX (n=3). Pulmonary arterial (Ppa), left atrial (Pla), and airway (Paw) pressures were continuously monitored (Grass Model 7) with Statham P50 transducers referenced to the left atrium. Glucose concentration and pH were monitored throughout the perfusion period and did not differ between preparations.
After 60 min of extracorporeal perfusion, the pulmonary vasculature was filled with PSS containing washed ferret red blood cells (Hct 20%); pulmonary arterial and left atrial cannulas were connected to a common reservoir containing the same solution. The reservoir was pressurized to 30 mm Hg for 20 min, then samples were rapidly withdrawn from the left atrial cannula for measurement of red blood cell and albumin concentration (27)
. Osmotic reflection coefficient for albumin (95 alb) was calculated iteratively from the rate of change of albumin concentration relative to red blood cell concentration (27)
.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, 10)
. To extend these findings, we assessed PTX-induced endothelial cell barrier dysfunction using real-time measurements of TER (28)
as we have noted with other permeability-producing agents (22
, 23)
. After a 20 min lag time in which electrical resistance did not change, PTX (at concentrations 
100 ng/mL) consistently induced significant declines in TER that remained sustained for several hours in bovine pulmonary artery (Fig. 1
A), and human umbilical vein endothelial cells (Fig. 1B
). Similar responses were noted in cultured human lung epithelium challenged with PTX (data not shown). This dramatic PTX-mediated reduction in TER was attributable to reduced cell–cell adhesion (Rb) without declines in cell–matrix adhesion () (Fig. 1C
). PTX-induced endothelial cell permeability was also confirmed in vivo using isolated perfused ferret lungs (Fig. 2
) where the estimated osmotic reflection coefficient for albumin (
alb) in control lungs was 0.70, a value similar to that obtained for uninjured isolated ferret lungs (27)
. In contrast, after 60 min of perfusion with PTX, alb decreased to a mean value of 0.32 (Fig. 2)
in association with significant lung weight gain and consistent with PTX-mediated increased vascular permeability and endothelial cell barrier dysfunction. Pulmonary artery pressures (Ppa, Paw) remained constant throughout the initial 30 min of PTX perfusion, but gradually increased during the final 30 min of PTX perfusion and airway with obvious edema fluid formation in the tracheal cannula.
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The onset of PTX-induced barrier dysfunction occurred in a temporal sequence comparable to the time course of PTX-mediated G ADP-ribosylation in human and bovine endothelium we noted earlier (14
, 15
, 19)
. To further explore this linkage, endothelial cell lysates were retrieved after defined durations of PTX exposure (15–120 min) and used to determine maximal in vitro PTX-induced ADP-ribosylation. These experiments confirmed that PTX catalyzes the ADP-ribosylation of 40 kDa G proteins in endothelial cells (Fig. 3
) and that maximal ADP-ribosylation occurs after 120 min of PTX exposure. More important, the earliest detectable PTX-induced ADP-ribosyltransferase activity temporally coincided with the PTX-mediated reduction in TER that began after 30 min (Fig. 1)
. Two complementary strategies were used to further examine whether ADP-ribosyltransferase activity is necessary for PTX-induced endothelial cell barrier dysfunction. In contrast to the substantial decline in TER produced by the native wild-type PTX holotoxin, Fig. 4
demonstrates that neither the purified S1 subunit (A protomer), the purified S2–6 subunit complex (ß-oligomer), nor the addition of a genetically engineered PTX mutant protein with site-directed mutations (two) to completely eliminate S1 ADP-ribosyltransferase activity (29)
altered bovine endothelial cell (Fig. 4A
) or human endothelial cell (Fig. 4B
). These results strongly indicate that the deleterious PTX effects on endothelial barrier properties are completely dependent on ADP-ribosylation of G.
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Pertussis toxin induces significant actin rearrangement in endothelium
We and others have clearly demonstrated the critical involvement of actin cytoskeletal rearrangement in endothelial cell barrier regulation (12
, 13
, 53)
. Human and bovine endothelium were next challenged with PTX and stained to visualize polymerized F-actin by immunofluorescent microscopy. Compared to vehicle controls (Fig. 5
A, panel 1) PTX-induced endothelial cell activation is accompanied by the modest development of stress fibers that span the cell after 30 min of PTX challenge (Fig. 5A
, panel 2) whereas the S1 PTX mutant holotoxin, which does not exhibit ADP-ribosyltransferase activity, failed to induce stress fiber formation (Fig. 5A
, panel 3). Quantification of actin stress fiber formation induced by PTX verified the increase in F-actin beginning at 15 min. (Fig. 5B
). These results suggest that PTX-induced cytoskeletal rearrangement, known to be essential in barrier regulation (13
, 30)
, is dependent on ADP-ribosyltransferase activity.
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Pertussis toxin induces rapid p38 MAP kinase activation
PTX-induced endothelial cell permeability proceeds in a PKC-dependent manner (10)
and PKC is known to enhance several important effector systems, including members of the MAP kinase family. We have recently demonstrated that PTX produces robust activation of p42/p44 ERK MAP kinase in bovine endothelium in a PKC-dependent manner (19)
, and Fig. 6
A demonstrates that pertussis toxin holotoxin increases p38 MAP kinase activity in a dose-dependent fashion. However, whereas the S1 PTX mutant and the ß-oligomer increase ERK activity (19)
, neither reagent increases p38 MAP kinase activation (Fig. 6B
). This was evidenced by detection of phosphorylated p38 MAP kinase; peak p38 activity was noted 60 min after PTX, persisting for up to 180 min after PTX challenge, a time course consistent with PTX-mediated G-protein ADP-ribosylation (Fig. 3)
and alterations in transmonolayer electrical resistance (Fig. 1)
. PTX was a far more potent stimulus for p38 MAP kinase activation than other permeability agents such as the PKC-activating phorbol esters or the procoagulant serine protease thrombin (Fig. 6C
). PTX-stimulated p38 MAP kinase activation was unaffected by either inhibition of ß
G-protein subunit signaling produced by transfection with a ßARK minigene (data not shown) or by pharmacologic inhibition of phosphatidylinositol-3' kinase activity or p60src activity (Fig. 6D
). To determine whether p38 MAP kinase activation may be a consequence of cytoskeletal rearrangement, we assessed the effect of several pharmacologic inhibitors of actin microfilament and microtubule rearrangement or disruption. The p38 MAP kinase activation evoked by PTX was unaffected by complementary strategies to decrease Rho and Rho kinase activation (Fig. 7
A, B). Furthermore, microfilament disruption with cytochalasin B or microtubule stabilization with taxol, also failed to alter PTX-induced p38 MAPK activation (Fig. 7B, C
).
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Pertussis toxin-induced cytoskeletal rearrangement and barrier dysfunction involves p38 MAP kinase activation
Temporal kinetic analysis suggested that PTX-induced ERK activation peaks (19)
well before the onset of PTX-mediated reductions in TER (Fig. 1)
. In contrast, p38 MAP kinase activity more closely coincides with the maximal decreases in TER across confluent endothelial cell monolayers and persists for several hours. Having carefully assessed the regulation of p38 MAPK activity, we directly assessed whether ERK and p38 MAP kinase activities represent relevant PTX downstream effectors involved in lung vascular barrier regulation by using specific pharmacologic inhibitors UO 126 (ERK kinase inhibitor) and SB 203580 (p38 MAP kinase inhibitor) (33)
before PTX challenge. ERK inhibition failed to attenuate the PTX-mediated reduction in electrical resistance (Fig. 8
A). The p38 MAP kinase inhibitor SB 203580 significantly inhibited PTX-induced barrier dysfunction (Fig. 8B, C
), suggesting that p38 MAP kinase activity, but not ERK, is an important participant in the barrier dysfunction observed after PTX. These results were further confirmed in endothelial cells infected with either control adenoviral vector or a plasmid encoding dominant/negative p38 MAP kinase (Fig. 8D
). Pretreatment of human endothelium with SB 203580 also abolished PTX-induced stress fiber formation (Fig. 9
). These results are again consistent with the notion that PTX-mediated p38 MAP kinase activation is intimately involved in subsequent stress fiber formation and barrier dysfunction after PTX challenge.
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Pertussis toxin increases caldesmon and Hsp 27 phosphorylation in a p38 MAP kinase- dependent manner
Phosphorylation of Hsp 27 and caldesmon, two actin binding cytoskeletal proteins and known p38 MAP kinase targets, has been postulated as molecular switches that may facilitate actomyosin contraction without an increase in myosin light chain phosphorylation (34
, 35)
. We next examined whether caldesmon or Hsp 27 phosphorylation is temporally linked to intercellular gap formation and actin cytoskeleton remodeling as shown in PKC-dependent models of endothelial cell permeability (34)
. Immunoprecipitation of caldesmon from PTX-challenged bovine endothelium revealed a substantial increase in caldesmon phosphorylation from 10 min through 60 min (Fig. 10
A), with the increase in caldesmon phosphorylation (60 min) attenuated by p38 MAP kinase inhibition (Fig. 10B
). Similarly, Hsp27 immunoprecipitation after PTX revealed increased phosphorylation, which was p38 MAPK dependent (Fig. 1C
). Consistent with its actin binding properties, immunofluorescent studies demonstrated that caldesmon spatially colocalized with cortical actin under basal conditions but demonstrated marked translocation to newly formed stress fibers after PTX challenge (Fig. 11
). Taken together, these results suggest that PTX reduces cell–cell adhesion and junctional integrity by significant p38 MAP kinase-dependent cytoskeletal rearrangement, leading to paracellular gap formation and barrier dysfunction, and may involve the stimulated phosphorylation of specific cytoskeletal targets such as Hsp27 and caldesmon.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, 8)
. In this study, we have explored potential signaling pathways by which PTX increases lung permeability by focusing on mechanisms of endothelial cell activation and barrier dysfunction. Consistent with our earlier reports (9
, 10)
, PTX proved a potent agonist for disassembly of endothelial cell junctions, resulting in intercellular gap formation, significant increases in macromolecule permeability, and decreases in transmonolayer electrical resistance. We previously noted that PTX-induced endothelial cell activation and barrier dysfunction were not associated with increases in cytosolic Ca2+ or in myosin light chain kinase-dependent contraction (10)
. Although the PTX targets that alter endothelial cell barrier properties are largely unknown (12
, 36)
, we have further demonstrated the ability of PTX to significantly impair endothelial cell barriers in vitro and in vivo, with ADP-ribosylation and p38 MAP kinase activation required for this response.
The early upstream signaling events by which PTX induces endothelial cell p38 MAP kinase activation are unknown. However, our data are entirely consistent with a striking divergence in the requirements for PTX-mediated ERK and p38 MAP kinase activities. Gi ADP-ribosylation after PTX treatment ensued well after maximal ERK activation (5 min) (19)
and actually coincided with significant down-regulation of ERK activities. In contrast, the time course of p38 MAP kinase activation was temporally linked to the level of Gi ADP-ribosylation as well as PTX-mediated alterations in TER. These data strongly argue that the function of the single S1 A protomer subunit, which contains the ADP-ribosyltransferase but cannot efficiently enter the cell, is required for the full effect of PTX on endothelial permeability. This notion was rigorously confirmed with the S1 mutant PTX holotoxin and the purified ß-oligomer, which are devoid of ADP-ribosylation activity and failed to reproduce the native wild-type holotoxin effects on either TER or p38 MAP kinase activation, although stimuli were proficient in inducing ERK activation (19)
. The ß-oligomer, which allows the toxin to bind target cells through as yet uncharacterized surface receptors, contains five subunits and dramatically increases the efficiency of S1 entry (29
, 37
, 38)
. Carbohydrate moieties have been reported as crucial components of ß-oligomer binding sites, and ADP-ribosylation-independent activation of signaling pathways by the ß-oligomer has been reported involving PKC, Ca2+, and tyrosine kinases (3
, 5
, 6
, 39)
. Integrin receptors (CD11/CD18) have been suggested as PTX binding sites (40)
, but it is unknown whether integrin ligation by PTX occurs on the endothelial or epithelial cell surface. Current studies are under way to explore this interesting notion.
The postreceptor events that link PTX-evoked extracellular signals with the p38 MAP kinase pathway have not been defined. Recently, overexpression of the pp125 focal adhesion kinase-related tyrosine kinase (known as RAFTK or PYK2) was found to lead to ERK and p38 MAP kinase activation whereas a dominant/negative PYK2 mutant reduced p38 MAP kinase activation elicited by cytotoxic agents such as methyl methane sulfonate (41)
. This alternate mode of p38 MAP kinase activation is unlikely to be involved in PTX signaling, however, as PYK2 is only activated by extracellular signals that increase intracellular Ca2+ and pertussis toxin challenge in endothelium does not alter intracellular levels of Ca2+ (10
, 15)
. Although not reported here, our prior results evaluating PTX-mediated p42/44 ERK activation (19)
are consistent with the notion that upstream signaling involving Ras GTPase is unlikely to play a key role in p38 MAP kinase activation after PTX challenge. In these studies, PTX-mediated ERK activation was attenuated by either MEK or PKC inhibition but without measurable increases in Ras or Raf-1 activities (19)
. The S1 PTX mutant and the ß-oligomer were comparable to the PTX holotoxin in their induction of ERK activity, again suggesting that toxin binding is sufficient to activate ERK in a Raf-1- and Ras-independent manner.
The five known p38 MAP kinase isoforms (, ß, ß2, , and 
) share a common TGY motif (T180 and Y182 phosphorylation is associated with enhanced enzymatic activity) and p38 MAP kinase is a well-recognized participant in regulating inflammatory responses, including cytokine secretion and apoptosis (31
, 32)
. Osmotic stress, endotoxin, DNA-damaging agents, Rho family GTPases (Rac, cdc 42, p21-associated kinase), and proinflammatory cytokines (interleukin 1) and tumor necrosis factor all have the capacity to activate p38 MAP kinase (42)
. Recent studies indicate the p38 MAPK isoform is the predominant isoform present in endothelial cells (43)
and is specifically inhibited by the pyridenyl imidazole compound SB 203580 (33)
. Evaluation of signaling events potentially involved in PTX-induced p38 MAP kinase activation failed to define a significant role for Ras, p60src, phosphatidylinositol-3' kinase activities, or Rho kinase (Fig. 7A
) (19)
. Despite reports that Rac GTPase mediates p38 MAP kinase activation, our preliminary data suggest, that Rac GTPases are also unlikely to be an important intermediate in the PTX-mediated p38 MAP kinase activation pathway (data not shown), as PTX did not reproducibly activate Rac. This suggests that MKK4, a Rac GTPase-sensitive, p38 MAPK-activating kinase, is unlikely to be involved; MKK3 or MKK6, which are not regulated by Rac GTPase (31)
, are more likely to participate in PTX signaling in the endothelium. Since ADP-ribosylation of a Gi or Go heterotrimeric protein appears to be essential, we speculate the existence of a PTX-sensitive Gi protein that regulates p38 MAP kinase through an as yet undefined mechanism. We further speculate that G-protein inactivation/uncoupling by PTX results in the release of tonic inhibition of MKK-regulated p38 MAP kinase activity.
We also noted that the onset of p38 MAP kinase activity began as ERK activation was rapidly reduced, consistent with the notion that direct cross-talk between different MAP kinase pathways may exist (44)
. Diperoxovanadate-induced p38 MAP kinase activation was reported to exert an inhibitory effect on MEK-1 activity in baboon aortic smooth muscle cells, which was not due to direct phosphorylation (45
, 46)
. Although we did not rigorously evaluate the extent of direct cross-talk between ERK and p38 MAP kinase in PTX-stimulated endothelium, this is unlikely to be a consequence of PTX-mediated ADP-ribosylation, as the S1 mutant devoid of ADP-ribosylation transferase activity) stimulated ERK but failed to significantly increase p38 MAP kinase activity.
We found that PTX was a potent stimulus for p38 MAPK-dependent Ser/Thr phosphorylation and attempted to define potential downstream targets that may be involved in PTX-mediated lung toxicity and barrier dysfunction. Analysis of electrical resistance vectors after PTX (Fig. 1D
) indicated that disassembly of cell–cell junctions is likely the primary pathway of barrier compromise. Subsequent studies revealed paracellular gaps and a profound actin cytoskeletal rearrangement and stress fiber formation (Fig. 5)
, findings consistent with increased tension development. Recently, p42/p44 ERK MAP kinase activation was reported to increase the activity of the smooth muscle myosin light chain kinase (MLCK) isoform, providing a potential linkage to rapid tension development (47
, 48)
. We previously cloned a unique nonmuscle endothelial cell MLCK isoform (49)
and found several consensus sites for Ser/Thr phosphorylation catalyzed by proline-directed kinases such as ERK and p38 MAP kinases (50)
. Despite the inherent attractiveness of this hypothesis, we consistently failed to identify an increase in endothelial (10)
or epithelial cell MLCK activity after PTX and have yet to identify a role for either ERK or p38 MAP kinase in endothelial cell MLCK regulation (J. G. N. Garcia and A. D. Verin, unpublished observations). Appealing alternate p38 MAPK targets include the 27 kDa heat shock protein Hsp27 and the 77 kDa nonmuscle caldesmon, two actin binding proteins that have been proposed to function as molecular switches in the regulation of nonmuscle and smooth muscle contraction (34
, 35)
. We have previously shown PKC and MAP kinase activation with either phorbol esters or thrombin to increase endothelial cell caldesmon phosphorylation (24
, 34)
, and have now demonstrated that PTX also produces significant caldesmon phosphorylation catalyzed by p38 MAP kinases. Caldesmon was spatially localized with filamentous actin staining throughout PTX stimulation, initially as a component of the dense cortical actin ring and later in association with newly formed stress fibers. Although our experiments do not allow us to firmly establish a causal relationship between Hsp27 and caldesmon phosphorylation and PTX-induced barrier dysfunction, these events occur in a p38 MAP kinase-dependent manner. Activation of p38 MAP kinase is critical to subsequent PTX-mediated physiological derangement as SB 203580 attenuated Hsp27 and caldesmon phosphorylation as well as PTX-induced cytoskeletal rearrangement and barrier dysfunction. MAP kinases, Hsp27 and caldesmon have been found in close association with focal adhesions, strongly implicating p38 MAP kinase as a critical participant in transducing the cytoskeletal signals that result in barrier dysfunction (data not shown).
In addition to defining the need for careful assessment of direct PTX effects in future studies of signal transduction pathways, we have explored upstream signaling events and downstream targets involved in PTX-induced lung endothelial cell activation and barrier regulation. We have now identified the requirement for a signaling cascade involving ADP-ribosylation of Gi and p38 MAP kinase activities for the full permeability-producing response. Evaluation of potential p38 MAP kinase targets that produce endothelial and epithelial cell cytoskeletal rearrangement leading to paracellular gap formation and permeability yield two actin binding proteins, Hsp27 and caldesmon, whose phosphorylation represent potential molecular switches for endothelial cell contractile regulation. However, we further speculate the existence of a PTX-sensitive barrier protective Gi protein that regulates lung cellular barrier function with tonic suppression of p38 MAP kinase activity. ADP-ribosylation of this barrier regulatory Gi protein by PTX results in p38 MAP kinase activation, which via subsequent actin cytoskeletal rearrangement represents an essential component by which PTX signals increased lung permeability and subsequent respiratory pathophysiology in humans. Future studies using PTX in the examination of G-protein-linked signal transduction pathways should acknowledge this potent direct effect of PTX on cell activation.
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ACKNOWLEDGMENTS |
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Received for publication November 27, 2001.
Revision received March 6, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, ß, , and in inflammatory cell lineages. J. Immunol. 162,4246-4252This article has been cited by other articles:
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|
|
|
|
 |
|
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|
|
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|
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|
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|
|
|
|
 |
|
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|
|
|
|
|
|
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|
|
|
|
 |
|
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|
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|
|
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|
|
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|
|
|
|
|
|
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