Expression and inducibility of alpha, pi, and Mu glutathione S-transferase protein and mRNA in murine lung

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Antioxidant strategies in respiratory medicine

Pulmonary oxidant stress plays an important pathogenetic role in disease conditions including acute lung injury/adult respiratory distress syndrome (ALI/ARDS), hyperoxia, ischemia-reperfusion, sepsis, radiation injury, lung transplantation, COPD, and inflammation. Reactive oxygen species (ROS), released from activated macrophages and leukocytes or formed in the pulmonary epithelial and endothelial cells, damage the lungs and initiate cascades of pro-inflammatory reactions propagating pulmonary and systemic stress. Diverse molecules including small organic compounds (e.g. gluthatione, tocopherol (vitamin E), flavonoids) serve as natural antioxidants that reduce oxidized cellular components, decompose ROS and detoxify toxic oxidation products. Antioxidant enzymes can either facilitate these antioxidant reactions (e.g. peroxidases using glutathione as a reducing agent) or directly decompose ROS (e.g. superoxide dismutases [SOD] and catalase). Many antioxidant agents are being tested for treatment of pulmonary oxidant stress. The administration of small antioxidants via the oral, intratracheal and vascular routes for the treatment of short- and long-term oxidant stress showed rather modest protective effects in animal and human studies. Intratracheal and intravascular administration of antioxidant enzymes are being currently tested for the treatment of acute oxidant stress. For example, intratracheal administration of recombinant human SOD is protective in premature infants exposed to hyperoxia. However, animal and human studies show that more effective delivery of drugs to cells experiencing oxidant stress is needed to improve protection. Diverse delivery systems for antioxidants including liposomes, chemical modifications (e.g. attachment of masking pegylated [PEG]-groups) and coupling to affinity carriers (e.g. antibodies against cellular adhesion molecules) are being employed and currently tested, mostly in animal and, to a limited extent, in humans, for the treatment of oxidant stress. Further studies are needed, however, in order to develop and establish effective applications of pulmonary antioxidant interventions useful in clinical practice. Although beyond the scope of this review, antioxidant gene therapies may eventually provide a strategy for the management of subacute and chronic pulmonary oxidant stress.

Age-specific effects on rat lung glutathione and antioxidant enzymes after inhaling ultrafine soot

Vehicle exhaust is rich in polycyclic aromatic hydrocarbons (PAHs) and is a dominant contributor to urban particulate pollution (PM). Exposure to PM is linked to respiratory and cardiovascular morbidity and mortality in susceptible populations, such as children. PM can contribute to the development and exacerbation of asthma, and this is thought to occur because of the presence of electrophiles in PM or through electrophile generation via the metabolism of PAHs. Glutathione (GSH), an abundant intracellular antioxidant, confers cytoprotection through conjugation of electrophiles and reduction of reactive oxygen species. GSH-dependent phase II detoxifying enzymes glutathione peroxidase and glutathione S-transferase facilitate metabolism and conjugation, respectively. Ambient particulates are highly variable in composition, which complicates systematic study. In response, we have developed a replicable ultrafine premixed flame particle (PFP)-generating system for in vivo studies. To determine particle effects in the developing lung, 7-day-old neonatal and adult rats inhaled 22 μg/m(3) PFP during a single 6-hour exposure. Pulmonary GSH and related phase II detoxifying gene and protein expression were evaluated 2, 24, and 48 hours after exposure. Neonates exhibited significant depletion of GSH despite higher initial baseline levels of GSH. Furthermore, we observed attenuated induction of phase II enzymes (glutamate cysteine ligase, glutathione reductase, glutathione S-transferase, and glutathione peroxidase) in neonates compared with adult rats. We conclude that developing neonates have a limited ability to deviate from their normal developmental pattern that precludes adequate adaptation to environmental pollutants, which results in enhanced cytotoxicity from inhaled PM.

Constitutive expression and phenobarbital modulation of drug metabolizing enzymes and related nuclear receptors in cattle liver and extra-hepatic tissues

In humans and rodents, phenobarbital (PB) induces hepatic and extra-hepatic drug metabolizing enzymes (DMEs) through the activation of specific nuclear receptors (NRs). In contrast, few data about PB transcriptional effects in veterinary species are available. The constitutive expression and modulation of PB-responsive NR and DME genes, following an oral PB challenge, were investigated in cattle liver and extra-hepatic tissues (duodenum, kidney, lung, testis, adrenal and muscle). Likewise to humans and rodents, target genes were expressed to a lower extent compared to the liver with few exceptions. Phenobarbital significantly affected hepatic CYP2B22, 2C31, 2C87, 3A and UDP-glucuronosyltransferase 1A1-like, glutathione S-transferase A1-like and sulfotransferase 1A1-like (SULT1A1-like) mRNAs and apoprotein amounts; in extra-hepatic tissues, only duodenum showed a significant down-regulation of SULT1A1-like gene and apoprotein. Nuclear receptor mRNAs were never affected by PB. Presented data are the first evidence about the constitutive expression of foremost DME and NR genes in cattle extra-hepatic tissues, and the data obtained following a PB challenge are suggestive of species-differences in drug metabolism; altogether, these information are of value for the extrapolation of pharmacotoxicological data among species, the characterization of drug-drug interactions as well as the animal and consumer's risk caused by harmful residues formation.

Protein thiol oxidation in murine airway epithelial cells in response to naphthalene or diethyl maleate

Naphthalene (NA) is a semivolatile aromatic hydrocarbon to which humans are exposed from a variety of sources. NA results in acute cytotoxicity to respiratory epithelium in rodents. Cytochrome P450-dependent metabolic activation to form reactive intermediates and loss of soluble cellular thiols (glutathione) are critical steps in NA toxicity, but the precise mechanisms by which this chemical results in cellular injury remain unclear. Protein thiols are likely targets of reactive NA metabolites. Loss of these, through adduction or thiol oxidation mechanisms, may be important underlying mechanisms for NA toxicity. To address the hypothesis that loss of thiols on specific cellular proteins is critical to NA-induced cytotoxicity, we compared reduced to oxidized thiol ratios in airway epithelial cell proteins isolated from lungs of mice treated with NA or the nontoxic glutathione depletor, diethyl maleate (DEM). At 300 mg/kg doses, NA administration resulted in a greater than 85% loss of glutathione levels in the airway epithelium, which is similar to the loss observed after DEM treatment. Using differential fluorescent maleimide labeling followed by 2DE separation of proteins, we identified more than 35 unique proteins that have treatment-specific differential sulfhydryl oxidation. At doses of NA and DEM that produce similar levels of glutathione depletion, Cy3/Cy5 labeling ratios were statistically different for 16 nonredundant proteins in airway epithelium. Proteins identified include a zinc finger protein, several aldehyde dehydrogenase variants, beta-actin, and several other structural proteins. These studies show distinct patterns of protein thiol alterations with the noncytotoxic DEM and the cytotoxic NA.

Glutathione transferase pi plays a critical role in the development of lung carcinogenesis following exposure to tobacco-related carcinogens and urethane

Human cancer is controlled by a complex interaction between genetic and environmental factors. Such environmental factors are well defined for smoking-induced lung cancer; however, the roles of specific genes have still to be elucidated. Glutathione transferase pi (GSTP) catalyzes the detoxification of electrophilic diol epoxides produced by the metabolism of polycyclic aromatic hydrocarbons such as benzo[a]pyrene (BaP), a common constituent of tobacco smoke. Activity-altering polymorphisms in Gstp have therefore been speculated to be potential risk modifiers in lung cancer development. To clearly establish a role for GSTP in lung tumorigenesis, we investigated whether deletion of the murine Gstp genes (Gstp1 and Gstp2) alters susceptibility to chemically induced lung tumors following exposure to BaP, 3-methylcholanthrene (3-MC), and urethane. Gstp-null mice were found to have substantially increased numbers of adenomas relative to wild-type mice following exposure to all three compounds (8.3-, 4.3-, and 8.7-fold increase for BaP, 3-MC, and urethane, respectively). In Gstp-null mice, the capacity of pulmonary cytosol to catalyze conjugation of the BaP diol epoxide was significantly reduced. Concomitant with this, a significant increase in the level of BaP DNA adducts was measured in the lungs of null animals; however, no increase in DNA adducts was measured in the case of 3-MC exposure, suggesting that an alternative protective pathway exists. Indeed, significant differences in pulmonary gene expression profiles were also noted between wild-type and null mice. This is the first report to establish a clear correlation between Gstp status and lung cancer in vivo.

Tissue, species, and environmental differences in absolute quantities of murine mRNAs coding for alpha, mu, omega, pi, and theta glutathione S-transferases

This article reports the first absolute quantitative analysis of expression patterns of murine transcripts (Gsta1/2, Gsta3, Gsta4, Gstm1, Gstm2, Gstm3, Gsto1, Gstp1/2, Gstt1, Gstt2) coding for most glutathione S-transferases (GSTs) of alpha, mu, omega, pi, and theta classes. We examine how the steady-state numbers of transcripts are modulated in association with: three animal organs (liver, kidney, and lung) where extensive detoxification occurs; two species (Mus musculus and Mus spretus) representing common laboratory and aboriginal mice; and two genetic and animal living conditions (wild-derived inbred animals and free-living mice). Moreover, quantitations performed examine how the pulmonary steady-state Gst mRNA amounts are affected in M. musculus by paraquat (a superoxide generator), and in M. spretus by dwelling at a polluted area. The results point to complex tissue-, species-, and life condition-dependent expression of the investigated transcripts. Among others, they show: i) the ubiquity of most transcripts, except Gstm3 mRNA that was virtually absent or at very low amounts (< or = 0.001 molecules/pg) in kidney and lung of M. spretus; ii) unique expression profiles for each transcript and mouse organ examined; iii) outstanding species-specific differences in basal amounts of most Gst mRNAs, this effect being most apparent in the case of Gsta1/2, Gsta3, Gstm2, Gsto1, Gstt1, and Gstt2; iv) paraquat-induced upregulation of most Gst mRNAs, with the notable exception of those coding for theta class GSTs; v) a tendency for mice dwelling at a wildlife reserve of having lower and more homogeneous Gsta3 mRNA levels than those collected in an anthropogenic environment.

Disruption of the Glutathione Transferase Pi Class Genes

Glutathione transferases are a multi-gene family of enzymes responsible for the metabolism of a wide range of both endogenous and exogenous substrates. These polymorphic enzymes, which form part of an adaptive response to chemical and oxidative stress, are widely distributed and ubiquitously expressed and are subject to regulation by a number of structurally unrelated chemicals. One of these enzymes, GST P, has been the focus of much research in recent years in relation to its involvement in the etiology of disease, particularly cancer. As part of our research efforts into GST P, we have developed a mouse line that lacks this enzyme and have used this model to investigate the consequences of the absence of GST P on tumorigenesis, drug metabolism, and toxicity.

Propylthiouracil alters the expression and activity of glutathione-dependent enzymes in the mouse olfactory mucosa

Propylthiouracil (PTU) is an anti-thyroid drug that reportedly can impair olfactory function in humans and mice. In the mouse, PTU treatment disrupts survival and differentiation of olfactory precursor neurons. While the mechanism responsible for this effect is not understood, it is suspected that these changes are consequent to localized toxicity due to PTU metabolism. In vitro and in vivo studies in other biological systems demonstrate that PTU can significantly alter glutathione S-transferase (GST) enzyme expression and activity. The localization of GST biotransformation enzymes in basal cells, sustentacular cells and Bowman's glands of the olfactory mucosa suggests that these cells play a significant role in olfactory physiology. This study investigated the effects of PTU treatment, T(4) replacement therapy and thyroidectomy on GST expression, GST and glutathione peroxidase (GSH-PX) activity in mouse olfactory tissue. One month treatment with PTU revealed a significant decrease in expression of GST alpha (37%) as identified by Western blot analysis. In contrast, no change in GST mu expression was observed after 1 month of treatment. Concomitant treatment with T(4) caused a significant induction of GST alpha, and mu isozymes. GST enzyme activity significantly decreased after 1 month of PTU treatment (53%) and remained suppressed, despite the presence of exogenous T(4). GSH-PX activity significantly decreased after 1 month of PTU treatment (30%) and remained at control levels with T(4) supplementation. Thyroidectomy caused a 25% reduction in olfactory GST alpha expression. Total GST and GSH-PX activity were not altered in these animals. Supplementation with T(4) in thyroidectomized animals prevented the suppression of GST alpha expression. These results suggest that the combined action of localized PTU toxicity and altered levels of circulating thyroid hormone contribute to PTU-mediated abnormalities in the olfactory system.

Membrane association of glutathione S-transferase mGSTA4-4, an enzyme that metabolizes lipid peroxidation products

Lipid peroxidation products have signaling functions and at higher concentrations are toxic and may trigger cell death. The compounds are metabolized predominantly by glutathione S-transferases exemplified by mGSTA4-4, an enzyme highly efficient in glutathione conjugation of 4-hydroxyalkenals, and possessing glutathione peroxidase activity toward phospholipid hydroperoxides. mGSTA4-4 belongs to the predominant group of "canonical" glutathione S-transferases that are soluble and generally localized in the cytoplasm. The intracellular localization of mGSTA4-4 was examined in hepatocytes of normal mouse liver and in transfected HepG2 cells by fluorescence microscopy and digital deconvolution. mGSTA4-4 was found to be predominantly localized at or near the plasma membrane in transfected HepG2 cells, as well as in hepatocytes endogenously expressing the protein. In vitro, mGSTA4-4 associated with liposomes, and this interaction was potentiated when the liposomes contained negatively charged phospholipids. Mutating lysine 115 to glutamic acid resulted in a loss of the plasma membrane targeting of mGSTA4-4 as well as in a significant reduction of its binding to liposomes in vitro. These data suggest preferential targeting of mGSTA4-4 to the plasma membrane that may contain the major substrate(s) for this enzyme. Lysine 115 is critically important for the membrane association of mGSTA4-4, most likely by entering into an electrostatic interaction with negatively charged phospholipid headgroups.

Development of phase II xenobiotic metabolizing enzymes in differentiating murine clara cells

Glutathione S-transferases (GSTs) and epoxide hydrolases (EHs) protect cells from exogenous insult by detoxifying electrophilic compounds. Little is known about these enzyme systems during postnatal lung development. This study was designed to help establish whether the heightened neonatal susceptibility of the lung to bioactivated cytotoxicants is the result of inadequate ability to detoxify reactive intermediates. We compared the distribution of immunoreactive protein and enzymatic activity of GSTs and EHs in isolated distal airways during pre- and postnatal development in lungs of mice from 16 days gestation to 9 weeks postnatal age (adult). GST alpha, mu, and pi class protein expression in fetal and postnatal lung varied by isozyme and age. Isozymes alpha and mu are expressed at low levels before birth, high levels on postnatal day 7, low levels between postnatal days 14 and 21, high levels at postnatal day 28, and slightly lower levels in adults. Immunoreactive protein of isozyme pi has a peak expression on gestational day 18 and again on postnatal day 4, is undetectable at postnatal day 21, and is at peak levels in the adult mouse lung. GST activity in distal airways increased with age. Microsomal EH protein expression increased in intensity with age, while activity was similar in airways from all ages. We conclude that in the mouse lung (1) cellular expression of glutathione S-transferase varies by age and isozyme and does not increase with increasing age, (2) airway glutathione S-transferase activity increases with increasing age and does not correlate with immunoreactive protein expression, and (3) airway microsomal epoxide hydrolase activity does not increase, even though immunoreactive protein expression does increase with age.

Proteomic analysis of simulated occupational jet fuel exposure in the lung

We analyzed protein expression in the cytosolic fraction prepared from whole lung tissue in male Swiss-Webster mice exposed 1 h/day for seven days to aerosolized JP-8 jet fuel at concentrations of 1000 and 2500 mg/m3, simulating military occupational exposure. Lung cytosol samples were solubilized and separated via large scale, high resolution two-dimensional electrophoresis (2-DE) and gel patterns scanned, digitized and processed for statistical analysis. Significant quantitative and qualitative changes in tissue cytosol proteins resulted from jet fuel exposure. Several of the altered proteins were identified by peptide mass fingerprinting, confirmed by sequence tag analysis, and related to impaired protein synthetic machinery, toxic/metabolic stress and detoxification systems, ultrastructural damage, and functional responses to CO2 handling, acid-base homeostasis and fluid secretion. These results demonstrate a significant but comparatively moderate JP-8 effect on protein expression and corroborate previous morphological and biochemical evidence. Further molecular marker development and mechanistic inferences from these observations await proteomic analysis of whole tissue homogenates and other cell compartment, i.e., mitochondria, microsomes, and nuclei of lung and other targets.