Augmented expression of cytosolic phospholipase A2 during phenotypic transformation of cultured type II pneumocytes

Current insights in the complexities underlying drug-induced cholestasis

Drug-induced cholestasis (DIC) poses a major challenge to the pharmaceutical industry and regulatory agencies. It causes both drug attrition and post-approval withdrawal of drugs. DIC represents itself as an impaired secretion and flow of bile, leading to the pathological hepatic and/or systemic accumulation of bile acids (BAs) and their conjugate bile salts. Due to the high number of mechanisms underlying DIC, predicting a compound's cholestatic potential during early stages of drug development remains elusive. A profound understanding of the different molecular mechanisms of DIC is, therefore, of utmost importance. Although many knowledge gaps and caveats still exist, it is generally accepted that alterations of certain hepatobiliary membrane transporters and changes in hepatocellular morphology may cause DIC. Consequently, liver models, which represent most of these mechanisms, are valuable tools to predict human DIC. Some of these models, such as membrane-based in vitro models, are exceptionally well-suited to investigate specific mechanisms (i.e. transporter inhibition) of DIC, while others, such as liver slices, encompass all relevant biological processes and, therefore, offer a better representation of the in vivo situation. In the current review, we highlight the principal molecular mechanisms associated with DIC and offer an overview and critical appraisal of the different liver models that are currently being used to predict the cholestatic potential of drugs.

The toxicology of air pollution predicts its epidemiology

The epidemiologic investigation has successively delineated associations of air pollution exposure with non-malignant and malignant lung disease, cardiovascular disease, cerebrovascular disease, pregnancy outcomes, perinatal effects and other extra-pulmonary disease including diabetes. Defining these relationships between air pollution exposure and human health closely parallels results of an earlier epidemiologic investigation into cigarette smoking and environmental tobacco smoke (ETS), two other particle-related exposures. Humic-like substances (HULIS) have been identified as a chemical component common to cigarette smoke and air pollution particles. Toxicology studies provide evidence that a disruption of iron homeostasis with sequestration of host metal by HULIS is a fundamental mechanistic pathway through which biological effects are initiated by cigarette smoke and air pollution particles. As a result of a common chemical component and a shared mechanistic pathway, it should be possible to extrapolate from the epidemiology of cigarette smoking and ETS to predict associations of air pollution exposure with human disease, which are currently unrecognized. Accordingly, it is anticipated that the forthcoming epidemiologic investigation will demonstrate relationships of air pollution with COPD causation, peripheral vascular disease, hypertension, renal disease, digestive disease, loss of bone mass/risk of fractures, dental disease, eye disease, fertility problems, and extrapulmonary malignancies.

Human lung injury following exposure to humic substances and humic-like substances

Among the myriad particles the human respiratory tract is exposed to, a significant number are distinctive in that they include humic substances (HS) and humic-like substances (HULIS) as organic components. HS are heterogeneous, amorphous, organic materials which are ubiquitous occurring in all terrestrial and aqueous environments. HULIS are a complex class of organic, macromolecular compounds initially extracted from atmospheric aerosol particles which share some features with HS including an aromatic, polyacidic nature. As a result of having a variety of oxygen-containing functional groups, both HS and HULIS complex metal cations, especially iron. Following particle uptake by cells resident in the lung, host iron will be sequestered by HS- and HULIS-containing particles initiating pathways of inflammation and subsequent fibrosis. It is proposed that (1) human exposures to HS and HULIS of respirable size (<10 µm diameter) are associated with inflammatory and fibrotic lung disease and (2) following retention of particles which include HS and HULIS, the mechanism of cell and tissue injury involves complexation of host iron. Human inflammatory and fibrotic lung injuries following HS and HULIS exposures may include coal workers' pneumoconiosis, sarcoidosis, and idiopathic pulmonary fibrosis as well as diseases associated with cigarette smoking and exposures to emission and ambient air pollution particles.

Review: Ferruginous Bodies: Implications in the Mechanism of Fiber and Particle Toxicity

Exposures to fibers and particles can be associated with several different lung injuries including bronchitis, bronchiolitis, pneumonitis, pleuritis, pulmonary alveolar proteinosis, pneumoconiosis, mesotheliomas, and lung cancers. The mechanism of biological effect exerted by fibers and particles has not been exactly defined. Exposures to all fibers and particles introduce a solid-liquid interface into the lower respiratory tract. These surfaces all have some concentration of oxygen-containing functional groups that demonstrate a capacity to coordinate iron. Radical generation is catalyzed by this metal resulting in a cascade of cell signaling, transcription factor activation, and mediator release. We propose that the ferruginous body (i.e., a fiber or particle with a coating of both protein and iron) provides direct evidence of a participation of iron in the biological effect of both fibers and particles. It is recommended that an identification of ferruginous bodies in the lung be regarded as support for a metal-catalyzed oxidative stress in the mechanism of cell and tissue injury.

Wood Smoke Particle Sequesters Cell Iron to Impact a Biological Effect

The biological effect of an inorganic particle (i.e., silica) can be associated with a disruption in cell iron homeostasis. Organic compounds included in particles originating from combustion processes can also complex sources of host cell iron to disrupt metal homeostasis. We tested the postulate that (1) wood smoke particle (WSP) sequesters host cell iron resulting in a disruption of metal homeostasis, (2) this loss of essential metal results in both an oxidative stress and biological effect in respiratory epithelial cells, and (3) humic-like substances (HULIS), a component of WSP, have a capacity to appropriate cell iron and initiate a biological effect. BEAS-2B cells exposed to WSP resulted in diminished concentrations of mitochondrial (57)Fe, whereas preincubation with ferric ammonium citrate (FAC) prevented significant mitochondrial iron loss after such exposure. Cellular oxidant generation was increased after WSP exposure, but this signal was diminished by coincubation with FAC. Similarly, exposure of BEAS-2B cells to 100 μg/mL WSP activated mitogen-activated protein (MAP) kinases, elevated NF-E2-related factor 2/antioxidant responsive element (Nrf2 ARE) expression, and provoked interleukin (IL)-6 and IL-8 release, but all these changes were diminished by coincubation with FAC. The biological response to WSP was reproduced by exposure to 100 μg/mL humic acid, a polyphenol comparable to HULIS included in the WSP that complexes iron. We conclude that (1) the biological response following exposure to WSP is associated with sequestration of cell iron by the particle, (2) increasing available iron in the cell diminished the biological effects after particle exposure, and (3) HULIS included in WSP can sequester the metal initiating the cell response.

Sodium-dependent bile salt transporters of the SLC10A transporter family: more than solute transporters

The SLC10A transporter gene family consists of seven members and substrates transported by three members (SLC10A1, SLC10A2 and SLC10A6) are Na(+)-dependent. SLC10A1 (sodium taurocholate cotransporting polypeptide [NTCP]) and SLC10A2 (apical sodium-dependent bile salt transporter [ASBT]) transport bile salts and play an important role in maintaining enterohepatic circulation of bile salts. Solutes other than bile salts are also transported by NTCP. However, ASBT has not been shown to be a transporter for non-bile salt substrates. While the transport function of NTCP can potentially be used as liver function test, interpretation of such a test may be complicated by altered expression of NTCP in diseases and presence of drugs that may inhibit NTCP function. Transport of bile salts by NTCP and ASBT is inhibited by a number of drugs and it appears that ASBT is more permissive to drug inhibition than NTCP. The clinical significance of this inhibition in drug disposition and drug-drug interaction remains to be determined. Both NCTP and ASBT undergo post-translational regulations that involve phosphorylation/dephosphorylation, translocation to and retrieval from the plasma membrane and degradation by the ubiquitin-proteasome system. These posttranslational regulations are mediated via signaling pathways involving cAMP, calcium, nitric oxide, phosphoinositide-3-kinase (PI3K), protein kinase C (PKC) and protein phosphatases. There appears to be species difference in the substrate specificity and the regulation of plasma membrane localization of human and rodent NTCP. These differences should be taken into account when extrapolating rodent data for human clinical relevance and developing novel therapies. NTCP has recently been shown to play an important role in HBV and HDV infection by serving as a receptor for entry of these viruses into hepatocytes.

Composition of air pollution particles and oxidative stress in cells, tissues, and living systems

Epidemiological studies demonstrated an association between increased levels of ambient air pollution particles and human morbidity and mortality. Production of oxidants, either directly by the air pollution particles or by the host response to the particles, appears to be fundamental in the biological effects seen after exposure to particulate matter (PM). However, the precise components and mechanisms responsible for oxidative stress following PM exposure are yet to be defined. Direct oxidant generation by air pollution particles is attributed to organic and metal components. Organic compounds generate an oxidative stress through redox cycling of quinone-based radicals, by complexing of metal resulting in electron transport, and by depletion of antioxidants by reactions between quinones and thiol-containing compounds. Metals directly support electron transport to generate oxidants and also diminish levels of antioxidants. In addition to direct generation of oxidants by organic and metal components, cellular responses contribute to oxidative stress after PM exposure. Reactive oxygen species (ROS) production occurs in the mitochondria, cell membranes, phagosomes, and the endoplasmic reticulum. Oxidative stress following PM exposure initiates a series of cellular reactions that includes activation of kinase cascades and transcription factors and release of inflammatory mediators, which ultimately lead to cell injury or apoptosis. Consequently, oxidative stress in cells and tissues is a central mechanism by which PM exposure leads to injury, disease, and mortality.

The role of the sodium-taurocholate cotransporting polypeptide (NTCP) and of the bile salt export pump (BSEP) in physiology and pathophysiology of bile formation

Bile formation is an important function of the liver. Bile salts are a major constituent of bile and are secreted by hepatocytes into bile and delivered into the small intestine, where they assist in fat digestion. In the small intestine, bile salts are almost quantitatively reclaimed and transported back via the portal circulation to the liver. In the liver, hepatocytes take up bile salts and secrete them again into bile for ongoing enterohepatic circulation. Uptake of bile salts into hepatocytes occurs largely in a sodium-dependent manner by the sodium taurocholate cotransporting polypeptide NTCP. The transport properties of NTCP have been extensively characterized. It is an electrogenic member of the solute carrier family of transporters (SLC10A1) and transports predominantly bile salts and sulfated compounds, but is also able to mediate transport of additional substrates, such as thyroid hormones, drugs and toxins. It is highly regulated under physiologic and pathophysiologic conditions. Regulation of NTCP copes with changes of bile salt load to hepatocytes and prevents entry of cytotoxic bile salts during liver disease. Canalicular export of bile salts is mediated by the ATP-binding cassette transporter bile salt export pump BSEP (ABCB11). BSEP constitutes the rate limiting step of hepatocellular bile salt transport and drives enterohepatic circulation of bile salts. It is extensively regulated to keep intracellular bile salt levels low under normal and pathophysiologic situations. Mutations in the BSEP gene lead to severe progressive familial intrahepatic cholestasis. The substrates of BSEP are practically restricted to bile salts and their metabolites. It is, however, subject to inhibition by endogenous metabolites or by drugs. A sustained inhibition will lead to acquired cholestasis, which can end in liver injury.

Disruption of Iron Homeostasis as a Mechanism of Biologic Effect by Ambient Air Pollution Particles

Several features of the clinical presentation and changes in physiology and pathology following exposure to many diverse ambient air pollution particles are comparable, suggesting a common mechanism for their biological effect. We propose that a mechanism of biological effect common to many ambient air pollution particles is a disruption of iron homeostasis in cells and tissues. Among traits shared by every particle-related lung injury is the introduction of a solid-liquid interface into the respiratory tract. All surfaces of particulate matter have some concentration of oxygen-containing functional groups. As a result of its electropositivity, Fe(3+) has a high affinity for oxygen-donor ligands and will react with these groups at the particle surface. Retained particles accumulate metal from available sources in a cell and tissue, and this complexed iron mediates oxidant generation. In addition to complexation onto the solid-liquid interface provided by the surface of particulate matter (PM), there are several alternative pathways by which metal homeostasis in the lower respiratory tract can be disrupted following exposure to ambient air pollution particles to affect an oxidative stress. Evidence suggests that disruption in iron homeostasis following exposures to ambient air pollution particles is an initial event in their biological effect. An association between metal equilibrium in the lower respiratory tract and biological effect in the lung could explain the observed differential toxicity of ultrafine, fine, and coarse particles and disparities in host susceptibility.

Iron homeostasis in the lung following asbestos exposure

Human exposure to asbestos can cause a wide variety of pulmonary diseases, including pneumoconiosis (i.e., asbestosis). This lung injury is mediated by oxidant generation which increases with the concentration of iron associated with the asbestos. Iron from host sources is complexed by the surface of these fibrous silicates following introduction into the lower respiratory tract. Using bronchoalveolar lavage from unexposed and exposed workers, we demonstrate that asbestos disrupts the normal iron homeostasis in the lungs. Based on these findings, we propose a model of oxidative stress and human lung injury after asbestos exposure.

Inhibition of bile acid transport across Na+/taurocholate cotransporting polypeptide (SLC10A1) and bile salt export pump (ABCB 11)-coexpressing LLC-PK1 cells by cholestasis-inducing drugs

Vectorial transport of bile acids across hepatocytes is a major driving force for bile flow, and bile acid retention in the liver causes hepatotoxicity. The basolateral and apical transporters for bile acids are thought to be targets of drugs that induce cholestasis. Previously, we constructed polarized LLC-PK1 cells that express both a major bile acid uptake transporter human Na+/taurocholate cotransporting polypeptide (SLC10A1) (NTCP) and the bile acid efflux transporter human bile salt export pump (ABCB 11) (BSEP) and showed that monolayers of such cells can be used to characterize vectorial transcellular transport of bile acids. In the present study, we investigated whether cholestasis-inducing drugs could inhibit bile acid transport in such cells. Because fluorescent substrates allow the development of a high-throughput screening method, we examined the transport by NTCP and BSEP of fluorescent bile acids as well as taurocholate. The aminofluorescein-tagged bile acids, chenodeoxycholylglycylamidofluorescein and cholylglycylamidofluorescein, were substrates of both NTCP and BSEP, and their basal-to-apical transport rates across coexpressing cell monolayers were 4.3 to 4.5 times those of the vector control, although smaller than for taurocholate. The well known cholestatic drugs, rifampicin, rifamycin SV, glibenclamide, and cyclosporin A, reduced the basal-to-apical transport and the apical efflux clearance of taurocholate across NTCP- and BSEP-coexpressing cell monolayers. Further analysis indicated that the drugs inhibited both NTCP and BSEP. Our study suggests that such coexpressing cells can provide a useful system for the identification of inhibitors of these two transport systems, including potential drug candidates.

The solute carrier family SLC10: more than a family of bile acid transporters regarding function and phylogenetic relationships

The solute carrier family 10 (SLC10) comprises two sodium-dependent bile acid transporters, i.e. the Na(+)/taurocholate cotransporting polypeptide (NTCP; SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT; SLC10A2). These carriers are essentially involved in the maintenance of the enterohepatic circulation of bile acids mediating the first step of active bile acid transport through the membrane barriers in the liver (NTCP) and intestine (ASBT). Recently, four new members of the SLC10 family were described and referred to as P3 (SLC10A3), P4 (SLC10A4), P5 (SLC10A5) and sodium-dependent organic anion transporter (SOAT; SLC10A6). Experimental data supporting carrier function of P3, P4, and P5 is currently not available. However, as demonstrated for SOAT, not all members of the SLC10 family are bile acid transporters. SOAT specifically transports steroid sulfates such as oestrone-3-sulfate and dehydroepiandrosterone sulfate in a sodium-dependent manner, and is considered to play an important role for the cellular delivery of these prohormones in testes, placenta, adrenal gland and probably other peripheral tissues. ASBT and SOAT are the most homologous members of the SLC10 family, with high sequence similarity ( approximately 70%) and almost identical gene structures. Phylogenetic analyses of the SLC10 family revealed that ASBT and SOAT genes emerged from a common ancestor gene. Structure-activity relationships of NTCP, ASBT and SOAT are discussed at the amino acid sequence level. Based on the high structural homology between ASBT and SOAT, pharmacological inhibitors of the ASBT, which are currently being tested in clinical trials for cholesterol-lowering therapy, should be evaluated for their cross-reactivity with SOAT.

Reconstructed mass-spectrometric pattern for characterization of carbon compounds in smoker's lung in situ

BACKGROUND

Cigarette smoke is a major anthropogenic pollutant and contributes to the permanent load of ambient particulate matter in the air, particularly indoors. It is the leading risk factor for premature loss of life due to chronic bronchitis, emphysema and lung cancer. Smoker's lung and graphite pneumoconiosis are pathological states characterized by the deposition of carbonaceous particles.

METHODS

Mass spectrometry was used to evaluate unstained lung sections obtained in vivo from a heavy smoker and a patient with occupationally acquired graphite pneumoconiosis.

RESULTS AND DISCUSSION

The composition of carbon compounds deposited in lung tissue samples is demonstrated here for the first time. Thirty carbonaceous-containing microareas from ten biopsies (three areas per biopsy) of lung tissues were analyzed mass-spectrometrically. In each case, the samples were taken from a smoker's lung or those demonstrating a graphite pneumoconiosis. The lung-tissue samples were selected by light microscopy before they were evaporated for mass spectrometry. First-order criteria were anionic and cationic mass peaks which occur within the mass patterns in lung tissues of smoker's lung, although not in graphite pneumoconiosis. Second-order criteria were mass peaks from smoker's lung with standard deviations SD < or = 14% of the mean value. First and second-order mass peaks matched the mass peaks of experimental cigarette-smoke condensate in 9 out of 11 peaks. A software program was developed that enabled fast, automated recognition of the typical mass peaks, and thereby confirmed the histological diagnosis of smoker's lung.

CONCLUSIONS

The analysis of carbonaceous particles within lung biopsies from a heavy smoker corresponded to the spectra of tobacco condensate and not to the investigated biopsies of graphite peneumoconiosis.

RECOMMENDATION AND OUTLOOK

The analyses were performed in order to find out whether mass-spectrometric criteria exist for the differentiation of carbonaceous lung-tissue deposits. Mass spectrometry may be a valuable tool in determining the composition of carbon compounds deposited in human lung tissue. So far, qualitative assessment of the composition of deposits in lung tissue is only possible after the patient is deceased (autopsy).

Characterization of cloned mouse Na+/taurocholate cotransporting polypeptide by transient expression in COS-7 cells

The mouse Na+/taurocholate cotransporting polypeptide transiently expressed in COS-7 cells caused sodium-dependent uptake of [3H]taurocholic acid with Km and Vmax values of 18 microM and 102 pmol/mg protein/min, respectively. This Km value is comparable to that for rat NTCP and higher than that for human NTCP. Substrate specificity was evaluated by measuring inhibitory effects of unlabeled bile acids on [3H]taurocholic acid transport.

Preferential distribution of long chain polyunsaturated fatty acids in phospatidyl ethanolamine fraction of guinea pig alveolar apical membranes

We investigated the fatty acid distribution in guinea pig alveolar apical membranes at different developmental stages. Fatty acid composition of the purified membranes isolated from guinea pig fetuses (at 65 day, term=68 day), neonates (day 1) and adult males was determined. The levels of arachidonic acid (AA) and docosahexaenoic acid (DHA) were higher in the adult guinea pig alveolar apical membrane phosphatidylethanolamine (PE) fraction (9. 3+/-2.2 and 2.9+/-1.0%, respectively) while in other phospholipids (PL) fractions their levels were low or absent (P<0.01). Furthermore, levels of AA and DHA in the PE fraction of apical membrane increased significantly from fetal (6.6+/-3.0 and 0.8+/-0.4%, respectively) to neonatal life (10.3+/-1.5 and 3.0+/-0.8%, respectively). Increase in the level of DHA (almost four-fold) was much more pronounced than that of AA (P<0.05). As for guinea pig alveolar membranes, EPA and AA were mostly present in the PE fraction in pulmonary adenocarcinoma derived cells (A549 cells), a parallel model of type II pneumocytes, with the levels of AA around three-fold greater than that of EPA, Binding of radiolabelled fatty acids to A549 cells showed no significant differences between the maximum uptake achieved for different fatty acids (AA, 1.7+/-0.2, EPA, 2.3+/-0.3, LA, 1.7+/-0.2, OA, 2.0+/-0.2nmol/mg protein, P>0.5). Once the fatty acids were taken up by these cells AA was mostly identifiable in the monoacylglycerol (MAG) fraction, whereas EPA was equally distributed between the MAG and PL fractions. Oleic acid was mainly present in the triglyceride (TAG) fraction whereas LA was evenly distributed between the TAG, MAG, and PL fractions. Our data demonstrate a preferential distribution of AA and DHA in PE fractions of alveolar apical membranes during development.

Role of phospholipase A2 in the cytotoxic effects of oxalate in cultured renal epithelial cells

BACKGROUND

Oxalate, a common constituent of kidney stones, is cytotoxic for renal epithelial cells. Although the exact mechanism of oxalate-induced cell death remains unclear, studies in various cell types, including renal epithelial cells, have implicated phospholipase A2 (PLA2) as a prominent mediator of cellular injury. Thus, these studies examined the role of PLA2 in the cytotoxic effects of oxalate.

METHODS

The release of [3H]-arachidonic acid (AA) or [3H]-oleic acid (OA) from prelabeled Madin-Darby canine kidney (MDCK) cells was measured as an index for PLA2 activity. The cell viability was assessed by the exclusion of ethidium homodimer-1.

RESULTS

Oxalate exposure (175 to 550 microM free) increased the release of [3H]-AA in MDCK cells but had no effect on the release of [3H]-OA. Oxalate-induced [3H]-AA release was abolished by arachidonyl trifluoromethyl ketone (AACOCF3), a selective inhibitor of cytosolic PLA2 (cPLA2), but was not affected by selective inhibitors of secretory PLA2 and calcium-independent PLA2. The [3H]-AA release could be demonstrated within 15 minutes after exposure to oxalate, which is considerably earlier than the observed changes in cell viability. Furthermore, AACOCF3 significantly reduced oxalate toxicity in MDCK cells.

CONCLUSIONS

Oxalate increases AA release from MDCK cells by a process involving cPLA2. In addition, based on the evidence obtained using a selective inhibitor of this isoform, it would appear that the activity of this enzyme is responsible, at least in part, for the cytotoxic effects of oxalate. The finding that oxalate can trigger a known lipid-signaling pathway may provide new insight into the initial events in the pathogenesis of nephrolithiasis.

Molecular cloning and functional characterization of two alternatively spliced Ntcp isoforms from mouse liver1

To isolate the murine Na+/taurocholate cotransporting polypeptide (Ntcp), we screened a mouse liver cDNA library and identified Ntcp1, encoding a 362 amino acid protein and Ntcp2, encoding a 317 amino acid protein which had a shorter C-terminal end. Both isoforms mediated saturable Na+-dependent transport of taurocholate when expressed in Xenopus laevis oocytes. Analysis of the gene revealed that Ntcp2 is produced by alternative splicing where the last intron is retained.

Arachidonic acid metabolism by adult human osteoblast-like cells exhibits sexually dimorphic characteristics

The eicosanoids, including prostaglandin E2 (PGE2) and other bioactive arachidonic acid metabolites, are important local mediators of bone remodeling. Presumably, the limited or excessive synthesis of the eicosanoids could compromise bone homeostasis. We have noted that the stimulated release of arachidonic acid by adult male donor derived human osteoblast-like (hOB) cells exceeded the stimulated release measured for female-derived hOB cells by 1.5-fold. Assays of PGE2 biosynthesis by cytokine-stimulated hOB cells also demonstrated a sex-linked difference, such that male hOB cell PGE2 production exceeded female cell production by 1.6-2.2-fold. The calcium-dependent cytoplasmic phospholipase A2 activity in subcellular fractions prepared from hOB cell homogenates was higher in both the cytosolic (1.6-fold) and particulate (1.5-fold) fractions from the male cells than in those prepared from female hOB cells, suggesting a molecular basis for the observed sexually dimorphic characteristics related to arachidonic acid metabolism by hOB cells. The relatively limited capacity of the female cells may limit needed intracellular and intercellular signaling during bone remodeling, thereby contributing to the development of bone pathology.

Altered expression and localization of 5-lipoxygenase accompany macrophage differentiation in the lung

The alveolar macrophage (AM) exhibits a greater capacity to synthesize bioactive leukotrienes from arachidonic acid than does its circulating precursor the peripheral blood monocyte. Macrophage differentiation in the lung entails cellular residence within both the pulmonary interstitial and alveolar compartments. In the present study, we sought to determine 1) whether this enhanced metabolic activity was acquired during maturation within the alveolar space and 2) the underlying mechanisms responsible for this upregulation. Rat AMs were separated by Percoll gradient centrifugation into four density-defined subpopulations thought to reflect their degree of maturation. On stimulation with a calcium ionophore, synthesis of leukotriene B4 increased with the degree of maturation, although it was diminished in the oldest subpopulation. This maturation-dependent upregulation was not explained by increases in arachidonic acid release but was associated with increased expression of 5-lipoxygenase (5-LO) protein as determined by immunoblot analysis. Whereas 5-LO is primarily cytosolic in monocytes, it is known to be primarily intranuclear in unfractionated AMs. Here, the localization of 5-LO was investigated by immunofluorescence microscopy and was found to be predominantly nuclear in all AM subpopulations; by contrast, the protein was cytosolic in interstitial macrophages isolated by mechanical and enzymatic lung digestion. These divergent localization patterns in AMs and interstitial macrophages were verified in situ by immunohistochemical staining of sections of normal rat lung. When unfractionated AMs were isolated and maintained in culture for 3 days, a shift in 5-LO distribution from nucleus to cytosol was observed. We conclude that 1) nuclear import of 5-LO occurs within the alveolar space and is reversible on removal from the alveolar milieu and 2) leukotriene synthetic capacity increases further during AM residence within the alveolar space as a result of a progressive increase in the amount of 5-LO protein.

Capacity for repeatable leukotriene generation after transient stimulation of mast cells and macrophages

Leukotriene (LT) synthesis is initiated by the enzyme 5-lipoxygenase (5-LO). Prolonged cell stimulation causes the translocation of 5-LO to the nuclear envelope and the synthesis of LT, with subsequent inactivation and persistent membrane association of 5-LO. In this study, we examined whether persistent membrane association of 5-LO, as well as the inactivation of 5-LO, could be prevented by shortening the length of cell stimulation or by blocking LT synthesis. As expected, stimulation of rat basophilic leukaemia (RBL) cells, a mast cell model, or alveolar macrophages (AMs) with calcium ionophore for 15 min caused 5-LO translocation, LT generation and the inactivation and persistent membrane association of 5-LO. When RBL cells or AMs instead were stimulated for 0.5-5 min, translocation of 5-LO and synthesis of LT still occurred. However, after washing and resting, the 5-LO enzyme returned to its original intracellular distribution. Furthermore these cells showed a retained capacity for LT synthesis on subsequent re-stimulation. Similar results were obtained when cells were stimulated with either formyl peptide or zymosan, instead of ionophore. In contrast, blockade of LT synthesis during the initial stimulation, with the selective inhibitors zileuton or MK-886, did not inhibit 5-LO translocation, inactivation or persistent membrane association resulting from prolonged cell stimulation. We conclude that, in long-lived immune cells, 5-LO translocation is reversible when cell stimulation is short, but persistent after prolonged stimulation. In addition 5-LO remains active and LT synthetic capacity is retained after transient stimulation, whereas significant inactivation of 5-LO occurs after prolonged stimulation. Finally, results with LT synthesis inhibitors indicate that inactivation and persistent membrane association of 5-LO can result independently of 5-LO activation.

Organic anion transporting polypeptide mediates organic anion/HCO3- exchange

Organic anion transporting polypeptide (oatp) is an integral membrane protein cloned from rat liver that mediates Na+-independent transport of organic anions such as sulfobromophthalein and taurocholic acid. Previous studies in rat hepatocytes suggested that organic anion uptake is associated with base exchange. To better characterize the mechanism of oatp-mediated organic anion uptake, we examined transport of taurocholate in a HeLa cell line stably transfected with oatp under the regulation of a zinc-inducible promoter (Shi, X., Bai, S., Ford, A. C., Burk, R. D., Jacquemin, E., Hagenbuch, B., Meier, P. J., and Wolkoff, A. W. (1995) J. Biol. Chem. 270, 25591-25595). Whereas noninduced transfected cells showed virtually no uptake of [3H]taurocholate, taurocholate uptake by induced cells was Na+-independent and saturable (Km = 19.4 +/- 3.3 microM; Vmax = 62.2 +/- 1.4 pmol/min/mg protein; n = 3). To test whether organic anion transport is coupled to HCO3- extrusion, we compared the rates of taurocholate-dependent HCO3- efflux from alkali-loaded noninduced and induced cells. Monolayers grown on glass coverslips were loaded with the pH-sensitive dye 2', 7'-bis(carboxyethyl)-5(6)-carboxyfluorescein; intracellular pH (pHi) was measured by excitation ratio fluorometry. Noninduced and induced cells were alkalinized to an equivalent pHi ( approximately 7.7) by transient exposure to a 50 mM HCO3-, Cl--free solution. In the absence of extracellular Cl- and taurocholate, isohydric reduction of superfusate HCO3- concentration from 50 to 25 mM resulted in an insignificant change in pHi over time (dpHi/dt) in both groups. Addition of 25 microM taurocholate to the superfusate led to a rapid fall in pHi in induced (-0.037 +/- 0.011 pH units/min to pHi of 7.41 +/- 0.14) but not in noninduced (0.003 +/- 0.006 pH units/min to pHi of 7.61 +/- 0.08) cells (p < 0.03). These data indicate that oatp-mediated taurocholate transport is Na+-independent, saturable, and accompanied by HCO3- exchange. We conclude that organic anion/base exchange is an important, potentially regulatable component of oatp function.

Translocation of cytosolic phospholipase A2 to the nuclear envelope elicits topographically localized phospholipid hydrolysis

Cytosolic phospholipase A2 (cPLA2) is a good candidate for mediating the agonist-stimulated release of arachidonic acid (AA) from membrane phospholipids. This enzyme undergoes a Ca(2+)-dependent translocation from the cytosol to a membrane site in a variety of cell types, and this site has recently been identified as the nuclear envelope in leucocytes. The functional correlate of this finding has not yet been established. The present study was therefore undertaken to determine whether translocation of cPLA2 to the nuclear envelope was associated with localized phospholipid hydrolysis at this site. Rat alveolar epithelial cells, previously shown to contain cPLA2, were prelabelled with [3H]AA and stimulated with the model agonist, ionophore A23187. Ionophore-induced AA release exhibited characteristics typical of a cPLA2-mediated response, in that it was Ca(2+)-dependent, sn-2 AA-selective, and inhibited by arachidonyl trifluoromethyl ketone. As determined by indirect immunofluorescence microscopic analysis as well as subcellular fractionation with immunoblotting, ionophore treatment resulted in a translocation of cPLA2 protein from the cytoplasm to the nuclear envelope. To determine whether the nuclear membrane was indeed the source of released AA, prelabelled cells were incubated in the presence or absence of A23187, after which the phospholipid radioactivity was quantified in nuclear and non-nuclear membrane fractions. [3H]AA was distributed in both nuclear and non-nuclear membrane phospholipids. Following A23187 stimulation, the loss of [3H]AA from nuclear membrane phospholipids accounted for 88.1 +/- 5.8% of the total loss from phospholipids and for 92.9 +/- 2.3% of the total [3H]AA released into the medium. These results demonstrate for the first time that agonist-stimulated translocation of cPLA2 to the nuclear envelope is associated with phospholipid hydrolysis which is preferentially localized to that site.

The functional expression of sodium-dependent bile acid transport in Madin-Darby canine kidney cells transfected with the cDNA for microsomal epoxide hydrolase

Previous studies have suggested that the enzyme microsomal epoxide hydrolase (mEH) is able to mediate sodium-dependent transport of bile acids such as taurocholate into hepatocytes (von Dippe, P., Amoui, M., Alves, C., and Levy, D.(1993) Am. J. Physiol. 264, G528-G534). In order to characterize directly the putative transport properties of the enzyme, a pCB6 vector containing the cDNA for this protein (pCB6-mEH) was transfected into Madin-Darby canine kidney (MDCK) cells, and stable transformants were isolated that could express mEH at levels comparable with the levels expressed in hepatocytes. Sodium-dependent transport of taurocholate was shown to be dependent on the expression of mEH and to be inhibited by the bile acid transport inhibitor 4,4'-diisothiocyanostilbene-2,2'disulfonic acid (DIDS), as well as by other bile acids. Kinetic analysis of this system indicated a Km of 26.3 microM and a Vmax of 117 pmol/mg protein/min. The Km value is essentially the same as that observed in intact hepatocytes. The transfected MDCK cells also exhibited sodium-dependent transport of cholate at levels 150% of taurocholate in contrast to hepatocytes where cholate transport is only 30% of taurocholate levels, suggesting that total hepatocyte bile acid transport is a function of multiple transport systems with different substrate specificities, where mEH preferentially transports cholate. This hypothesis is further supported by the observation that a monoclonal antibody that partially protects (26%) taurocholate transport from inhibition by DIDS in hepatocytes provides almost complete protection (88%) from DIDS inhibition of hepatocyte cholate transport, suggesting that taurocholate is also taken up by an alternative system not recognized by this antibody. Additional support for this concept is provided by the observation that the taurocholate transport system is almost completely protected (92%) from DIDS inhibition by this antibody in MDCK cells that express mEH as the only bile acid transporter. These results demonstrate that mEH is expressed on the surface of hepatocytes as well as on transfected MDCK cells and is able to mediate sodium-dependent transport of taurocholate and cholate.

Characterization of a secretory phospholipase A2 in human bronchoalveolar lavage fluid

Phospholipase A2 (PLA2) is a pivotal enzyme involved in the synthesis of the potent lipid inflammatory mediators platelet activating factor (PAF) and the eicosanoids. This study characterizes a PLA2 recovered in the bronchoalveolar lavage fluid (BALF) of healthy adult human subjects. Human BALF PLA2 exhibited characteristics of secretory PLA2s that include an activity that is acid stable, sensitive to reducing agents, and optimally requires millimolar calcium. BALF PLA2 showed marked selectivity for phosphatidylcholine containing arachidonic acid (AA) over linoleic or palmitic acids. Size exclusion chromatography showed the BALF PLA2 protein to be approximately 14 kDa in mass, consistent with it being a secretory form of PLA2. The biological significance of BALF PLA2 was tested by applying BALF concentrates to cultures of the human bronchial epithelial cell line BEAS 2B. Cultures of BEAS 2B cells treated with BALF concentrates released increased amounts of AA and produced higher levels of PAF. These data show that the lining fluid of the human respiratory tract normally contains a secretory PLA2, which may be involved in the formation of lipid inflammatory mediators in normal and pathophysiologic states in the lung.