Integrated control of lung fluid balance

Translocation pathway of the intratracheally instilled C60 fullerene from the lung into the blood circulation in the mouse: possible association of diffusion and caveolae-mediated pinocytosis

Ultrafine particles are ubiquitous in ambient urban and indoor air from multiple sources and may contribute to adverse respiratory and cardiovascular diseases. Recently, it has been demonstrated that ultrafine particles (UFPs) are translocated from the lung into the systemic circulation. The exact pathway, however, for the translocation in the lung remains unclear. In this study, we examined the translocation pathway of intratracheally instilled C60 fullerene particles from the lung into the blood circulation in the mouse. Using light microscopy, aggregated particles of fullerene were observed in the capillary lumen in the lung and the pulmonary lymph nodes immediately after instillation. Electron microscopic analysis demonstrated an increased number of pinocytotic vesicles (caveolae) of various sizes in the type 1 alveolar epithelial cells (AEC) and endothelial cells; occasional caveolae containing some particulate substances were observed. In addition, particles of various sizes were observed throughout the structure of the air-blood barrier (ABB). These findings suggest that fullerene particles may pass the ABB by both diffusion and caveolae-mediated pinocytosis, resulting in immediate translocation into the systemic circulation.

Pulmonary vascular heterogeneity and the Starling hypothesis

It has generally been assumed that movement of fluid between the pulmonary microvasculature and surrounding tissues is governed by a "Starling" balance of hydrostatic and protein osmotic forces similar to that which prevails in the extremities. However, both recent and older observations suggest that the lungs are more resistant to edema formation than most other organs. Several structural aspects of the lung may account for protection of the airspaces from edema formation. The pulmonary microvasculature, which comprises >70% of the pulmonary circulatory bed, appears to be less permeable to fluid and electrolytes than the endothelium of the pulmonary arteries and veins and other microvascular exchange areas. This arrangement may help explain why early edema is confined to the perivascular and peribronchial regions and why lymphatics do not reach the alveoli. Unlike the peripheral vasculature, which is compressed by edema formation, the extra-alveolar vessels remain tethered open by airway distention, even when interstitial pressures rise above those in the vessels. This may also facilitate return of proteins to the circulation. Ultrafiltration of plasma may lower local protein concentrations in the interstitium, thereby slowing further edema formation. Transendothelial reabsorption of fluid may also be altered by vesicular transport.

Activation of sphingosine kinase-1 reverses the increase in lung vascular permeability through sphingosine-1-phosphate receptor signaling in endothelial cells

The lipid mediator sphingosine-1-phosphate (S1P), the product of sphingosine kinase (SPHK)-induced phosphorylation of sphingosine, is known to stabilize interendothelial junctions and prevent microvessel leakiness. Here, we investigated the role of SPHK1 activation in regulating the increase in pulmonary microvessel permeability induced by challenge of mice with lipopolysaccharide or thrombin ligation of protease-activating receptor (PAR)-1. Both lipopolysaccharide and thrombin increased mouse lung microvascular permeability and resulted in a delayed activation of SPHK1 that was coupled to the onset of restoration of permeability. In contrast to wild-type mice, Sphk1(-/-) mice showed markedly enhanced pulmonary edema formation in response to lipopolysaccharide and PAR-1 activation. Using endothelial cells challenged with thrombin concentration (50 nmol/L) that elicited a transient but reversible increase in endothelial permeability, we observed that increased SPHK1 activity and decreased intracellular S1P concentration preceded the onset of barrier recovery. Thus, we tested the hypothesis that released S1P in a paracrine manner activates its receptor S1P1 to restore the endothelial barrier. Knockdown of SPHK1 decreased basal S1P production and Rac1 activity but increased basal endothelial permeability. In SPHK1-depleted cells, PAR-1 activation failed to induce Rac1 activation but augmented RhoA activation and endothelial hyperpermeability response. Knockdown of S1P1 receptor in endothelial cells also enhanced the increase in endothelial permeability following PAR-1 activation. S1P treatment of Sphk1(-/-) lungs or SPHK1-deficient endothelial cells restored endothelial barrier function. Our results suggest the crucial role of activation of the SPHK1-->S1P-->S1P1 signaling pathway in response to inflammatory mediators in endothelial cells in regulating endothelial barrier homeostasis.

Regulation of endothelial junctional permeability

The endothelium is a semi-permeable barrier that regulates the flux of liquid and solutes, including plasma proteins, between the blood and surrounding tissue. The permeability of the vascular barrier can be modified in response to specific stimuli acting on endothelial cells. Transport across the endothelium can occur via two different pathways: through the endothelial cell (transcellular) or between adjacent cells, through interendothelial junctions (paracellular). This review focuses on the regulation of the paracellular pathway. The paracellular pathway is composed of adhesive junctions between endothelial cells, both tight junctions and adherens junctions. The actin cytoskeleton is bound to each junction and controls the integrity of each through actin remodeling. These interendothelial junctions can be disassembled or assembled to either increase or decrease paracellular permeability. Mediators, such as thrombin, TNF-alpha, and LPS, stimulate their respective receptor on endothelial cells to initiate signaling that increases cytosolic Ca2+ and activates myosin light chain kinase (MLCK), as well as monomeric GTPases RhoA, Rac1, and Cdc42. Ca2+ activation of MLCK and RhoA disrupts junctions, whereas Rac1 and Cdc42 promote junctional assembly. Increased endothelial permeability can be reversed with "barrier stabilizing agents," such as sphingosine-1-phosphate and cyclic adenosine monophosphate (cAMP). This review provides an overview of the mechanisms that regulate paracellular permeability.

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.

Delivery of nano-objects to functional sub-domains of healthy and failing cardiac myocytes

Cardiovascular disease, including heart failure, is one of the leading causes of mortality in the world. Delivery of nano-objects as carriers for markers, drugs or therapeutic genes to cellular organelles has the potential to sharply increase the efficiency of diagnostic and treatment protocols for heart failure. However, cardiac cells present special problems to the delivery of nano-objects, and the number of papers devoted to this important area is remarkably small. The present review discusses fundamental aspects, problems and perspectives in the delivery of nano-objects to functional sub-domains of failing cardiomyocytes. What size nano-objects can reach cellular sub-domains in failing hearts? What are the mechanisms for their permeation through the sarcolemma? How can we improve the delivery of nano-objects to the sub-domains? Answering these questions is fundamental to identifying cellular targets within the failing heart and the development of nanocarriers for heart-failure therapy at the cellular level.

Cdc42 regulates the restoration of endothelial adherens junctions and permeability

The endothelial adherens junction (AJ) complex consisting of VE-cadherin and its associated catenins is a major determinant of fluid, solute, and plasma protein permeability of the vessel wall endothelial barrier. Impairment of endothelial barrier function contributes to cardiovascular diseases such as vascular inflammation and atherosclerosis. Adherens junctions disassemble in response to proinflammatory mediators, producing an increase in endothelial permeability; however, AJs also have the capacity to reassemble, leading to restoration of endothelial barrier function. Activation of Cdc42, a member of the Rho family of monomeric GTPases, is an essential signal regulating reannealing of AJs and reversal of the increase in endothelial permeability. The possibility of activating Cdc42 therapeutically represents a novel approach to prevent inflammatory diseases resulting from breakdown of the endothelial barrier. This review summarizes recent findings concerning the role of Cdc42 in restoring endothelial barrier integrity.

Prevention of LPS-induced acute lung injury in mice by mesenchymal stem cells overexpressing angiopoietin 1

BACKGROUND

The acute respiratory distress syndrome (ARDS), a clinical complication of severe acute lung injury (ALI) in humans, is a leading cause of morbidity and mortality in critically ill patients. ALI is characterized by disruption of the lung alveolar-capillary membrane barrier and resultant pulmonary edema associated with a proteinaceous alveolar exudate. Current specific treatment strategies for ALI/ARDS are lacking. We hypothesized that mesenchymal stem cells (MSCs), with or without transfection with the vasculoprotective gene angiopoietin 1 (ANGPT1) would have beneficial effects in experimental ALI in mice.

METHODS AND FINDINGS

Syngeneic MSCs with or without transfection with plasmid containing the human ANGPT1 gene (pANGPT1) were delivered through the right jugular vein of mice 30 min after intratracheal instillation of lipopolysaccharide (LPS) to induce lung injury. Administration of MSCs significantly reduced LPS-induced pulmonary inflammation, as reflected by reductions in total cell and neutrophil counts in bronchoalveolar lavage (BAL) fluid (53%, 95% confidence interval [CI] 7%-101%; and 60%, CI 4%-116%, respectively) as well as reducing levels of proinflammatory cytokines in both BAL fluid and lung parenchymal homogenates. Furthermore, administration of MSCs transfected with pANGPT1 resulted in nearly complete reversal of LPS-induced increases in lung permeability as assessed by reductions in IgM and albumin levels in BAL (96%, CI 6%-185%; and 74%, CI 23%-126%, respectively). Fluorescently tagged MSCs were detected in the lung tissues by confocal microscopy and flow cytometry in both naïve and LPS-injured animals up to 3 d.

CONCLUSIONS

Treatment with MSCs alone significantly reduced LPS-induced acute pulmonary inflammation in mice, while administration of pANGPT1-transfected MSCs resulted in a further improvement in both alveolar inflammation and permeability. These results suggest a potential role for cell-based ANGPT1 gene therapy to treat clinical ALI/ARDS.

RhoGDI-1 Modulation of the Activity of Monomeric RhoGTPase RhoA Regulates Endothelial Barrier Function in Mouse Lungs

Rho family GTPases have been implicated in the regulation of endothelial permeability via their actions on actin cytoskeletal organization and integrity of interendothelial junctions. In cell culture studies, activation of RhoA disrupts interendothelial junctions and increases endothelial permeability, whereas activation of Rac1 and Cdc42 enhances endothelial barrier function by promoting the formation of restrictive junctions. The primary regulators of Rho proteins, guanine nucleotide dissociation inhibitors (GDIs), form a complex with the GDP-bound form of the Rho family of monomeric G proteins, and thus may serve as a nodal point regulating the activation state of RhoGTPases. In the present study, we addressed the in vivo role of RhoGDI-1 in regulating pulmonary microvascular permeability using RhoGDI-1 −/− mice. We observed that basal endothelial permeability in lungs of RhoGDI-1 −/− mice was 2-fold greater than wild-type mice. This was the result of opening of interendothelial junctions in lung microvessels which are normally sealed. The activity of RhoA (but not of Rac1 or Cdc42) was significantly increased in RhoGDI-1 −/− lungs as well as in cultured endothelial cells on downregulation of RhoGDI-1 with siRNA, consistent with RhoGDI-1–mediated modulation RhoA activity. Thus, RhoGDI-1 by repressing RhoA activity regulates lung microvessel endothelial barrier function in vivo. In this regard, therapies augmenting endothelial RhoGDI-1 function may be beneficial in reestablishing the endothelial barrier and lung fluid balance in lung inflammatory diseases such as acute respiratory distress syndrome.

Heparan sulfates mediate pressure-induced increase in lung endothelial hydraulic conductivity via nitric oxide/reactive oxygen species

We investigated the nonlinear dynamics of the pressure vs. hydraulic conductivity (Lp) relationship in lung microvascular endothelial cells and demonstrate that heparan sulfates, an important component of the endothelial glycocalyx, participate in pressure-sensitive mechanotransduction that results in barrier dysfunction. The pressure vs. Lp relationship was complex, possessing both time- and pressure-dependent components. Pretreatment of lung capillary endothelial cells with heparanase III completely abolished the pressure-induced increase in Lp. This extends our ( 7 ) previous observation regarding heparan sulfates as mechanotransducers for shear stress. Inhibition of nitric oxide (NO) synthase with l-NAME ( NG-nitro-l-arginine methyl ester HCl) and intracellular scavenging of reactive oxygen species (ROS) by TBAP [tetrakis-(4-benzoic acid) porphorin] significantly attenuated the pressure-induced Lp response. Intracellular NO/ROS were visualized using the fluorescent dye, 2′7′-dichlorofluorescein diacetate (DCFA), and cells demonstrated a pressure-induced increase in intracellular fluorescence. Heparanase pretreatment significantly reduced the pressure-induced increase in intracellular fluorescence, suggesting that cell-surface heparan sulfates directly participate in mechanotransduction that results in NO/ROS production and increased permeability. This is the first report to demonstrate a role for heparan sulfates in pressure-mediated mechanotransduction and barrier regulation. These observations may have important clinical implications during conditions where pulmonary microvascular pressure is elevated.

The alveolar-epithelial barrier: a target for potential therapy

During acute lung injury (ALI), the alveolar-capillary barrier is damaged, resulting in the accumulation of fluid and protein in the alveolar space characteristic of the acute respiratory distress syndrome (ARDS). Disordered epithelial repair may contribute to the development of fibrosis and worsen outcomes in patients who have lung injury. This article discusses novel emerging therapies based on these mechanisms that are designed to preserve the function and promote the repair of the alveolar epithelium in patients who have ALI/ARDS.

Conceptos actuales en la fisiopatología, monitorización y resolución del edema pulmonar

Pulmonary edema, both in its lesional as well as hydrostatic version, is a frequent cause of acute respiratory failure. From the pathophysiological point of view, the most important advance is undoubtedly the knowledge that the reabsorption process of pulmonary edema is an active process with energy consumption. This concept has revolutionized this field due to the possibility of finding substances or factors that stimulate or inhibit this reabsorption. Furthermore, in the monitoring field, significant advances have also been experimented due to the possibility of quantifying the edema in a simple and reliable way with transpulmonary thermodilution.

Cognate interaction between endothelial cells and T cells

Endothelial cells lining the blood vessels form a barrier between circulating immune cells and parenchymal tissue. While the molecular mechanisms involved in antigen-independent recruitment of leukocytes into infected tissue have been extensively studied, the mechanisms involving antigen-specific recruitment of T cells into tissue have remained largely elusive. Here I shall review the experimental evidence that endothelial cells function as antigen-presenting cells and in this function contribute first to regulation of immune responses and second, to antigen-specific recruitment of T cells.

Role of O2-sensitive K+ and Ca2+ channels in the regulation of the pulmonary circulation: Potential role of caveolae and implications for high altitude pulmonary edema

High altitude pulmonary edema (HAPE) is a potentially fatal complication in response to exposure to low O(2) at high altitudes. Hypoxia, by causing pulmonary vasoconstriction, increases pulmonary vascular resistance and pulmonary arterial pressure, both of which are features in the pathogenesis of HAPE. Uneven hypoxic pulmonary vasoconstriction is thought to be responsible for increased capillary pressure and leakage, resulting in edema. O(2)-sensitive ion channels are known to play pivotal roles in determining vascular tone in response to hypoxia. K(+), Ca(2+) and Na(+) channels are ubiquitously expressed in both endothelial and smooth muscle cells of the pulmonary microvasculature, subfamilies of which are regulated by local changes in P(O(2)). Hypoxia reduces activity of voltage-gated K(+) channels and down-regulates their expression leading to membrane depolarization, Ca(2+) influx in pulmonary artery smooth muscle cells (by activating voltage-dependent Ca(2+) channels) and vasoconstriction. Hypoxia up-regulates transient receptor potential channels (TRPC) leading to enhanced Ca(2+) entry through receptor- and store-operated Ca(2+) channels. Altered enrichment of ion channels in membrane microdomains, in particular in caveolae, may play a role in excitation-contraction coupling and perhaps in O(2)-sensing in the pulmonary circulation and thereby may contribute to the development of HAPE. We review the role of ion channels, in particular those outlined above, in response to low O(2) on vascular tone and pulmonary edema. Advances in the understanding of ion channels involved in the physiological response to hypoxia should lead to a greater understanding of the pathogenesis of HAPE and perhaps in the identification of new therapies.

Manganese and iron transport across pulmonary epithelium

Pathways mediating pulmonary metal uptake remain unknown. Because absorption of iron and manganese could involve similar mechanisms, transferrin (Tf) and transferrin receptor (TfR) expression in rat lungs was examined. Tf mRNA was detected in bronchial epithelium, type II alveolar cells, macrophages, and bronchus-associated lymphoid tissue (BALT). Tf protein levels in lung and bronchoalveolar lavage fluid did not change in iron deficiency despite increased plasma levels, suggesting that lung Tf concentrations are regulated by local synthesis in a manner independent of body iron status. Iron oxide exposure upregulated Tf mRNA in bronchial and alveolar epithelium, macrophages, and BALT, but protein was not significantly increased. In contrast, TfR mRNA and protein were both upregulated by iron deficiency. To examine potential interactions with lung Tf, rats were intratracheally instilled with (54)Mn or (59)Fe. Unlike (59)Fe, interactions between (54)Mn and Tf in lung fluid were not detected. Absorption of intratracheally instilled (54)Mn from the lungs to the blood was unimpaired in Belgrade rats homozygous for the functionally defective G185R allele of divalent metal transporter-1, indicating that this transporter is also not involved in pulmonary manganese absorption. Pharmacological studies of (54)Mn uptake by A549 cells suggest that metal uptake by type II alveolar epithelial cells is associated with activities of both L-type Ca(2+) channels and TRPM7, a member of the transient receptor potential melastatin subfamily. These results demonstrate that iron and manganese are absorbed by the pulmonary epithelium through different pathways and reveal the potential role for nonselective calcium channels in lung metal clearance.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Transport across the endothelium: regulation of endothelial permeability

An important function of the endothelium is to regulate the transport of liquid and solutes across the semi-permeable vascular endothelial barrier. Two cellular pathways controlling endothelial barrier function have been identified. The transcellular pathway transports plasma proteins of the size of albumin or greater via the process of transcytosis in vesicle carriers originating from cell surface caveolae. Specific signalling cues are able to induce the internalisation of caveolae and their movement to the basal side of the endothelium. Caveolin-1, the primary structural protein required for the formation of caveolae, is also important in regulating vesicle trafficking through the cell by controlling the activity and localisation of signalling molecules that mediate vesicle fission, endocytosis, fusion and finally exocytosis. An important function of the transcytotic pathways is to regulate the delivery of albumin and immunoglobulins, thereby controlling tissue oncotic pressure and host-defence. The paracellular pathway induced during inflammation is formed by gaps between endothelial cells at the level of adherens and tight junctional complexes. Paracellular permeability is increased by second messenger signalling pathways involving Ca2+ influx via activation of store-operated channels, protein kinase Calpha (PKCalpha), and Rho kinase that together participate in the stimulation of myosin light chain phosphorylation, actin-myosin contraction, and disruption of the junctions. In this review of the field, we discuss the current understanding of the signalling pathways regulating paracellular and transcellular endothelial permeability.

Perinatal changes in pulmonary vascular endothelial function

The pulmonary endothelium plays a crucial role in lung development and function during the perinatal period. Its 2 most important functions at this time are to help reduce pulmonary vascular resistance (PVR) in order to permit the entire cardiac output to pass through the lungs for the first time and to facilitate the clearance of lung fluid. In response to changes in environmental factors such as oxygen tension, blood flow, circulating cytokines, and growth factors, the endothelium synthesizes and/or extracts many vasoactive mediators such as endothelin-1 (ET-1), norepinephrine, angiotensin 1, thromboxane, prostacyclin (PGI(2)), and the endothelial-derived relaxing factor nitric oxide (NO). The endothelium acts as a transducer conveying information about environmental changes to the underlying smooth muscle cells (SMCs), which helps regulate their reactivity and pulmonary vascular tone. The endothelial layer also acts as a barrier, regulating the exchange of fluids and nutrients between blood components and the surrounding tissues. The purpose of this review is to demonstrate the importance of structural and functional changes in the pulmonary endothelium during the perinatal period and explain their role in the regulation of the pulmonary circulation in health and disease. We also highlight signalling pathways of some of the most important endothelium-derived factors and indicate potential targets for pharmacological intervention.

Control of intracellular trafficking of ICAM-1-targeted nanocarriers by endothelial Na+/H+ exchanger proteins

Targeting nanocarriers (NC) loaded by antioxidant enzymes (e.g., catalase) to endothelial cell adhesion molecules (CAM) alleviates oxidative stress in the pulmonary vasculature. However, antioxidant protection is transient, since CAM-targeted catalase is internalized, delivered to lysosomes, and degraded. To design means to modulate the metabolism and longevity of endothelial cell (EC)-targeted drugs, we identified and manipulated cellular elements controlling the uptake and intracellular trafficking of NC targeted to ICAM-1 (anti-ICAM/NC). BAPTA, thapsigargin, amiloride, and EIPA inhibited anti-ICAM/NC uptake by EC and actin rearrangements induced by anti-ICAM/NC (required for uptake), suggesting that member(s) of Na(+)/H(+) exchanger family proteins (NHE) regulate these processes. Consistent with this hypothesis, an siRNA specific for the plasmalemma NHE1, but not the endosome-associated NHE6, inhibited actin remodeling induced by anti-ICAM/NC and internalization. Anti-ICAM/NC binding to EC stimulated formation of a transient ICAM-1/NHE1 complex. One hour after uptake, ICAM-1 dissociated from NHE1, and anti-ICAM/NC were transported to NHE6-positive vesicles en route to lysosomes. Inhibition of PKC (an activator of intracellular NHE) accelerated nanocarrier lysosomal trafficking. In contrast, monensin, which enhances the endosomal sodium influx and proton efflux maintained by NHE6, inhibited delivery of anti-ICAM/NC to lysosomes by switching their trafficking to a plasma membrane recycling pathway. This markedly prolonged the protective effect of catalase-coated anti-ICAM/NC. Therefore, 1) NHE1 and NHE6 regulate distinct phases of anti-ICAM/NC uptake and trafficking; 2) pharmacological agents affecting these regulatory elements alter the itinerary of anti-ICAM/NC intracellular trafficking; and 3) these agents modulate duration of the therapeutic effects of targeted drugs.

Thrombin impairs alveolar fluid clearance by promoting endocytosis of Na+,K+-ATPase

Coagulation is an emerging area of interest in the pathogenesis and treatment of acute lung injury. Concentrations of the edemagenic coagulation protease thrombin are elevated in plasma and lavage fluids from afflicted patients. We explored the impact of thrombin on the formation and resolution of alveolar edema. Intravascularly applied thrombin inhibited active transepithelial 22Na transport in intact rabbit lungs, suppressing alveolar fluid clearance. Epithelial permeability was unaffected, whereas endothelial permeability was increased. In A549 human lung epithelial cells and in mouse primary alveolar type II cells, thrombin blocked ouabain-sensitive Na+,K+-ATPase-mediated 86Rb+ uptake, without altering amiloride-sensitive sodium currents. Furthermore, thrombin downregulated cell-surface expression of Na+,K+-ATPase, but not ENaC alpha and beta subunits. The endocytosis inhibitor phalloidin oleate blocked all thrombin-induced effects on sodium transport activity. Similarly, diphenyleneiodonium chloride, an inhibitor of reactive oxygen radical production, as well as a protein kinase C-zeta inhibitor, prevented these thrombin-induced effects. Thus, thrombin signaling via reactive oxygen species and protein kinase C-zeta promotes Na+,K+-ATPase endocytosis, resulting in loss of function. We propose here a dual role for thrombin in mediating disturbances to fluid balance in the lung: thrombin concomitantly provokes edema formation by increasing endothelial permeability, and inhibits alveolar edema resolution by blocking Na+,K+-ATPase function.

Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles

Although humans have been exposed to airborne nanosized particles (NSPs; < 100 nm) throughout their evolutionary stages, such exposure has increased dramatically over the last century due to anthropogenic sources. The rapidly developing field of nanotechnology is likely to become yet another source through inhalation, ingestion, skin uptake, and injection of engineered nanomaterials. Information about safety and potential hazards is urgently needed. Results of older biokinetic studies with NSPs and newer epidemiologic and toxicologic studies with airborne ultrafine particles can be viewed as the basis for the expanding field of nanotoxicology, which can be defined as safety evaluation of engineered nanostructures and nanodevices. Collectively, some emerging concepts of nanotoxicology can be identified from the results of these studies. When inhaled, specific sizes of NSPs are efficiently deposited by diffusional mechanisms in all regions of the respiratory tract. The small size facilitates uptake into cells and transcytosis across epithelial and endothelial cells into the blood and lymph circulation to reach potentially sensitive target sites such as bone marrow, lymph nodes, spleen, and heart. Access to the central nervous system and ganglia via translocation along axons and dendrites of neurons has also been observed. NSPs penetrating the skin distribute via uptake into lymphatic channels. Endocytosis and biokinetics are largely dependent on NSP surface chemistry (coating) and in vivo surface modifications. The greater surface area per mass compared with larger-sized particles of the same chemistry renders NSPs more active biologically. This activity includes a potential for inflammatory and pro-oxidant, but also antioxidant, activity, which can explain early findings showing mixed results in terms of toxicity of NSPs to environmentally relevant species. Evidence of mitochondrial distribution and oxidative stress response after NSP endocytosis points to a need for basic research on their interactions with subcellular structures. Additional considerations for assessing safety of engineered NSPs include careful selections of appropriate and relevant doses/concentrations, the likelihood of increased effects in a compromised organism, and also the benefits of possible desirable effects. An interdisciplinary team approach (e.g., toxicology, materials science, medicine, molecular biology, and bioinformatics, to name a few) is mandatory for nanotoxicology research to arrive at an appropriate risk assessment.