Fatal Epileptic Seizures in Mice Having Compromised Glutathione and Ascorbic Acid Biosynthesis

Reduced glutathione (GSH) and ascorbic acid (AA) are the two most abundant low-molecular-weight antioxidants in mammalian tissues. Gclm^KO knockout mice lack the gene encoding the modifier subunit of the rate-limiting enzyme in GSH biosynthesis; Gclm^KO mice exhibit 10-40% of normal tissue GSH levels and show no overt phenotype. Gulo^KO knockout mice, lacking a functional Gulo gene encoding L-gulono-γ-lactone oxidase, cannot synthesize AA and depend on dietary ascorbic acid for survival. To elucidate functional crosstalk between GSH and AA in vivo, we generated the Gclm^KO/Gulo^KO double-knockout (DKO) mouse. DKO mice exhibited spontaneous epileptic seizures, proceeding to death between postnatal day (PND)14 and PND23. Histologically, DKO mice displayed neuronal loss and glial proliferation in the neocortex and hippocampus. Epileptic seizures and brain pathology in young DKO mice could be prevented with AA supplementation in drinking water (1 g/L). Remarkably, in AA-rescued adult DKO mice, the removal of AA supplementation for 2-3 weeks resulted in similar, but more severe, neocortex and hippocampal pathology and seizures, with death occurring between 12 and 21 days later. These results provide direct evidence for an indispensable, yet underappreciated, role for the interplay between GSH and AA in normal brain function and neuronal health. We speculate that the functional crosstalk between GSH and AA plays an important role in regulating glutamatergic neurotransmission and in protecting against excitotoxicity-induced brain damage.

Cytochrome b5 reductase and the control of lipid metabolism and healthspan

Cytochrome b₅ reductases (CYB5R) are required for the elongation and desaturation of fatty acids, cholesterol synthesis and mono-oxygenation of cytochrome P450 enzymes, all of which are associated with protection against metabolic disorders. However, the physiological role of CYB5R in the context of metabolism, healthspan and aging remains ill-defined. We generated CYB5R-overexpressing flies (CYB5R-OE) and created a transgenic mouse line overexpressing CYB5R3 (CYB5R3-Tg) in the C57BL/6J background to investigate the function of this class of enzymes as regulators of metabolism and age-associated pathologies. Gender- and/or stage-specific induction of CYB5R, and pharmacological activation of CYB5R with tetrahydroindenoindole extended fly lifespan. Increased expression of CYB5R3 was associated with significant improvements in several metabolic parameters that resulted in modest lifespan extension in mice. Diethylnitrosamine-induced liver carcinogenesis was reduced in CYB5R3-Tg mice. Accumulation of high levels of long-chain polyunsaturated fatty acids, improvement in mitochondrial function, decrease in oxidative damage and inhibition of chronic pro-inflammatory pathways occurred in the transgenic animals. These results indicate that CYB5R represents a new target in the study of genes that regulate lipid metabolism and healthspan.

Mechanistic studies of the toxicity of zinc gluconate in the olfactory neuronal cell line Odora

Zinc is both an essential and potentially toxic metal. It is widely believed that oral zinc supplementation can reduce the effects of the common cold; however, there is strong clinical evidence that intranasal (IN) zinc gluconate (ZG) gel treatment for this purpose causes anosmia, or the loss of the sense of smell, in humans. Using the rat olfactory neuron cell line, Odora, we investigated the molecular mechanism by which zinc exposure exerts its toxic effects on olfactory neurons. Following treatment of Odora cells with 100 and 200μM ZG for 0-24h, RNA-seq and in silico analyses revealed up-regulation of pathways associated with zinc metal response, oxidative stress, and ATP production. We observed that Odora cells recovered from zinc-induced oxidative stress, but ATP depletion persisted with longer exposure to ZG. ZG exposure increased levels of NLRP3 and IL-1β protein levels in a time-dependent manner, suggesting that zinc exposure may cause an inflammasome-mediated cell death, pyroptosis, in olfactory neurons.

Branched-Chain Amino Acid Supplementation in Combination with Voluntary Running Improves Body Composition in Female C57BL/6 Mice

Exercise is an inexpensive intervention that may be used to reduce obesity and its consequences. In addition, many individuals who regularly exercise utilize dietary supplements to enhance their exercise routine and to accelerate fat loss or increase lean mass. Branched-chain amino acids (BCAAs) are a popular supplement and have been shown to produce a number of beneficial effects in rodent models and humans. Therefore, we hypothesized that BCAA supplementation would protect against high fat diet (HFD)-induced glucose intolerance and obesity in mice with and without access to exercise. We subjected 80 female C57BL/6 mice to a paradigm of HFD feeding, exercise in the form of voluntary wheel running, and BCAA supplementation in the drinking water for 16 weeks (n = 10 per group). Body weight was monitored weekly, while food and water consumption were recorded twice weekly. During the 5th, 10th, and 15th weeks of treatment, glucose tolerance and body composition were analyzed. Exercise significantly improved glucose tolerance in both control-fed and HFD-fed mice. BCAA supplementation, however, did not significantly alter glucose tolerance in any treatment group. While BCAA supplements did not improve lean to fat mass ratio in sedentary mice, it significantly augmented the effects of exercise on this parameter.

Dietary whey protein stimulates mitochondrial activity and decreases oxidative stress in mouse female brain

In humans and experimental animals, protein-enriched diets are beneficial for weight management, muscle development, managing early stage insulin resistance and overall health. Previous studies have shown that in mice consuming a high fat diet, whey protein isolate (WPI) reduced hepatosteatosis and insulin resistance due in part to an increase in basal metabolic rate. In the current study, we examined the ability of WPI to increase energy metabolism in mouse brain. Female C57BL/6J mice were fed a normal AIN-93M diet for 12 weeks, with (WPI group) or without (Control group) 100g WPI/L drinking water. In WPI mice compared to controls, the oxidative stress biomarkers malondialdehyde and 4-hydroxyalkenals were 40% lower in brain homogenates, and the production of hydrogen peroxide and superoxide were 25-35% less in brain mitochondria. Brain mitochondria from WPI mice remained coupled, and exhibited higher rates of respiration with proportionately greater levels of cytochromes a+a3 and c+c1. These results suggested that WPI treatment increased the number or improved the function of brain mitochondria. qRT-PCR revealed that the gene encoding a master regulator of mitochondrial activity and biogenesis, Pgc-1alpha (peroxisome proliferator-activated receptor-gamma coactivator-1alpha) was elevated 2.2-fold, as were the PGC-1alpha downstream genes, Tfam (mitochondrial transcription factor A), Gabpa/Nrf-2a (GA-binding protein alpha/nuclear respiratory factor-2a), and Cox-6a1 (cytochrome oxidase-6a1). Each of these genes had twice the levels of transcript in brain tissue from WPI mice, relative to controls. There was no change in the expression of the housekeeping gene B2mg (beta-2 microglobulin). We conclude that dietary whey protein decreases oxidative stress and increases mitochondrial activity in mouse brain. Dietary supplementation with WPI may be a useful clinical intervention to treat conditions associated with oxidative stress or diminished mitochondrial activity in the brain.

Mitochondrial targeting of mouse NQO1 and CYP1B1 proteins

Four dioxin-inducible enzymes--NAD(P)H: quinone oxidoreductase-1 (NQO1) and three cytochromes P450 (CYP1A1, CYP1A2 & CYP1B1)--are implicated in both detoxication and metabolic activation of various endobiotics and xenobiotics. NQO1 is generally regarded as a cytosolic enzyme; whereas CYP1 proteins are located primarily in endoplasmic reticulum (ER), CYP1A1 and CYP1A2 proteins are also targeted to mitochondria. This lab has generated Cyp1a1(mc/mc) and Cyp1a1(mtt/mtt) knock-in mouse lines in which CYP1A1 protein is targeted exclusively to ER (microsomes) and mitochondria, respectively. Comparing dioxin-treated Cyp1(+/+) wild-type, Cyp1a1(mc/mc), Cyp1a1(mtt/mtt), and Cyp1a1(-/-), Cyp1b1(-/-) and Nqo1(-/-) knockout mice, in the present study we show that [a] NQO1 protein locates to cytosol, ER and mitochondria, [b] CYP1B1 protein (similar to CYP1A1 and CYP1A2 proteins) traffics to mitochondria as well as ER, and [c] NQO1 and CYP1B1 targeting to mitochondrial or ER membranes is independent of CYP1A1 presence in that membrane.

Rodent toxicity and nongenotoxic carcinogenesis: knowledge-based human risk assessment based on molecular mechanisms

It is necessary to determine whether chemicals or drugs have the potential to pose a threat to human health. Chemicals that can damage DNA are detected in short-term assays, but the detection of nongenotoxic carcinogens relies upon bioassays in laboratory animals. However, there are marked differences between rodents and humans in response to nongenotoxic carcinogens, which makes the relevance of rodent data to human risk assessment questionable. Here, we address the background issues concerning rodent nongenotoxic carcinogenesis and then focus upon peroxisome proliferators, chloroform, and dioxins as examples of toxicants that cause rodent-specific oxidative stress, cell proliferation, and the suppression of apoptosis. In the case of peroxisome proliferators and dioxins, this response is receptor-mediated. The evidence presented suggests that, at least for some toxicants, the molecular mechanisms of the rodent carcinogenic responses do not operate in humans; this is discussed in the context of human risk assessment. Finally, consideration is given to incorporating mechanism-based information into risk assessment for regulatory purposes.

Distribution and systemic effects of intranasally administered 25 nm silver nanoparticles in adult mice

Previous work indicates that silver nanoparticles (AgNPs) given IP to mice alter the regulation of inflammation- and oxidative stress-related genes in brain. Here we assessed the distribution and toxic potential of AgNP following intranasal (IN) exposure. Adult male C57BL/6J mice received 25-nm AgNP (100 or 500 mg/kg) once IN. After 1 or 7 days, histopathology of selected organs was performed, and tissue reduced glutathione (GSH) levels were measured as an indicator of oxidative stress. Aggregated AgNP were found in spleen, lung, kidney, and nasal airway by routine light microscopy. Splenic AgNP accumulation was greatest in red pulp and occurred with modestly reduced cellularity and elevated hemosiderin deposition. Aggregated AgNP were not associated with microscopic changes in other tissues except for nasal mucosal erosions. Autometallography revealed AgNP in olfactory bulb and the lateral brain ventricles. Neither inflammatory cell infiltrates nor activated microglia were detected in brains of AgNP-treated mice. Elevated tissue GSH levels was observed in nasal epithelia (both doses at 1 day, 500 mg/kg at 7 days) and blood (500 mg/kg at 7 days). Therefore, IN administration of AgNP permits systemic distribution, produces reversible oxidative stress in the nose and in blood, and mildly enhances macrophage-mediated erythrocyte destruction in the spleen.

Glutathione-deficient mice are susceptible to TCDD-Induced hepatocellular toxicity but resistant to steatosis

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) generates both hepatocellular injury and steatosis, processes that involve oxidative stress. Herein, we evaluated the role of the antioxidant glutathione (GSH) in TCDD-induced hepatotoxicity. Glutamate-cysteine ligase (GCL), comprising catalytic (GCLC) and modifier (GCLM) subunits, is rate limiting in de novo GSH biosynthesis; GCLM maintains GSH homeostasis by optimizing the catalytic efficiency of GCL holoenzyme. Gclm(-/-) transgenic mice exhibit 10-20% of normal tissue GSH levels. Gclm(-/-) and Gclm(+/+) wild-type (WT) female mice received TCDD for 3 consecutive days and were then examined 21 days later. As compared with WT littermates, Gclm(-/-) mice were more sensitive to TCDD-induced hepatocellular toxicity, exhibiting lower reduction potentials for GSH, lower ATP levels, and elevated levels of plasma glutamic oxaloacetic transaminase (GOT) and γ-glutamyl transferase (GGT). However, the histopathology showed that TCDD-mediated steatosis, which occurs in WT mice, was absent in Gclm(-/-) mice. This finding was consistent with cDNA microarray expression analysis, revealing striking deficiencies in lipid biosynthesis pathways in Gclm(-/-) mice; qrt-PCR analysis confirmed that Gclm(-/-) mice are deficient in expression of several lipid metabolism genes including Srebp2, Elovl6, Fasn, Scd1/2, Ppargc1a, and Ppara. We suggest that whereas GSH protects against TCDD-mediated hepatocellular damage, GSH deficiency confers resistance to TCDD-induced steatosis due to impaired lipid metabolism.

Comparison of the effects of hexavalent chromium in the alimentary canal of F344 rats and B6C3F1 mice following exposure in drinking water: implications for carcinogenic modes of action

Exposure to high concentrations of hexavalent chromium (Cr[VI]) in drinking water is reported to induce oral mucosa tumors in F344 rats and intestinal tumors in B6C3F1 mice. To investigate the modes of action underlying these tumors, 90-day drinking water studies (with interim necropsy at day 8) were conducted with concentrations of 0.1-182 mg/l Cr(VI), administered as 0.3-520 mg/l sodium dichromate dihydrate. Blood and tissue samples were analyzed for chromium content, oxidative stress, iron levels, and gross and microscopic lesions. Results for the F344 rats are described herein and compared with results from B6C3F1 mice published previously. After 90 days of exposure, total chromium concentrations in the rat and mouse oral mucosae were comparable, yet significant dose-dependent decreases in the reduced-to-oxidized glutathione ratio (GSH/GSSG) were observed only in rats. In the duodenum, changes in GSH/GSSG were only observed in mice. Levels of 8-hydroxydeoxyguanosine were not increased in the oral or duodenal mucosae of either species. Glutathione levels were increased in the duodenum but decreased in the jejunum of both species, indicating potential differential responses in the intestinal segments. Histiocytic infiltration was observed in the duodenum of both species, yet duodenal cytokines were repressed in mice but increased in rats. Serum and bone marrow iron levels were more decreased in rats than mice. Collectively, these data suggest that Cr(VI)-induced carcinogenesis in the rodent alimentary canal involves oxidative stress; however, differences in histopathology, cytokines, and iron status suggest potential contributions from other factors as well.

Investigation of the mode of action underlying the tumorigenic response induced in B6C3F1 mice exposed orally to hexavalent chromium

Chronic ingestion of high concentrations of hexavalent chromium [Cr(VI)] in drinking water induces intestinal tumors in mice. To investigate the mode of action (MOA) underlying these tumors, a 90-day drinking water study was conducted using similar exposure conditions as in a previous cancer bioassay, as well as lower (heretofore unexamined) drinking water concentrations. Tissue samples were collected in mice exposed for 7 or 90 days and subjected to histopathological, biochemical, toxicogenomic, and toxicokinetic analyses. Described herein are the results of toxicokinetic, biochemical, and pathological findings. Following 90 days of exposure to 0.3–520 mg/l of sodium dichromate dihydrate (SDD), total chromium concentrations in the duodenum were significantly elevated at ≥ 14 mg/l. At these concentrations, significant decreases in the reduced-to-oxidized glutathione ratio (GSH/GSSG) were observed. Beginning at 60 mg/l, intestinal lesions were observed including villous cytoplasmic vacuolization. Atrophy, apoptosis, and crypt hyperplasia were evident at ≥ 170 mg/l. Protein carbonyls were elevated at concentrations ≥ 4 mg/l SDD, whereas oxidative DNA damage, as assessed by 8-hydroxydeoxyguanosine, was not increased in any treatment group. Significant decreases in the GSH/GSSG ratio and similar histopathological lesions as observed in the duodenum were also observed in the jejunum following 90 days of exposure. Cytokine levels (e.g., interleukin-1β) were generally depressed or unaltered at the termination of the study. Overall, the data suggest that Cr(VI) in drinking water can induce oxidative stress, villous cytotoxicity, and crypt hyperplasia in the mouse intestine and may underlie the MOA of intestinal carcinogenesis in mice.

Dietary whey protein lowers the risk for metabolic disease in mice fed a high-fat diet

Consuming a high-fat (HF) diet produces excessive weight gain, adiposity, and metabolic complications associated with risk for developing type 2 diabetes and fatty liver disease. This study evaluated the influence of whey protein isolate (WPI) on systemic energy balance and metabolic changes in mice fed a HF diet. Female C57BL/6J mice received for 11 wk a HF diet, with or without 100 g WPI/L drinking water. Energy consumption and glucose and lipid metabolism were examined. WPI mice had lower rates of body weight gain and percent body fat and greater lean body mass, although energy consumption was unchanged. These results were consistent with WPI mice having higher basal metabolic rates, respiratory quotients, and hepatic mitochondrial respiration. Health implications for WPI were reflected in early biomarkers for fatty liver disease and type 2 diabetes. Livers from WPI mice had significantly fewer hepatic lipid droplet numbers and less deposition of nonpolar lipids. Furthermore, WPI improved glucose tolerance and insulin sensitivity. We conclude that in mice receiving a HF diet, consumption of WPI results in higher basal metabolic rates and altered metabolism of dietary lipids. Because WPI mice had less hepatosteatosis and insulin resistance, WPI dietary supplements may be effective in slowing the development of fatty liver disease and type 2 diabetes.

Glutathione deficient C57BL/6J mice are not sensitized to ozone-induced lung injury

In this study we examined the role of the antioxidant glutathione (GSH) in pulmonary susceptibility to ozone toxicity, utilizing GSH deficient C57BL/6J mice that lack the expression of glutamate-cysteine ligase modifier subunit (GCLM). Gclm(-/-) knockout mice had 70% GSH depletion in the lung. Gclm(+/+) wild-type and Gclm(-/-) mice were exposed to either 0.3 ppm ozone or filtered air for 48h. Ozone-induced lung hyperpermeability, as measured by total protein concentration in bronchoalveolar lavage fluid, was surprisingly lower in Gclm(-/-) mice than in wild-type mice. Lung hyperpermeability did not correlate with the degree of neutrophilia or with inflammatory gene expression. Pulmonary antioxidant response to ozone, assessed by increased mRNA levels of metallothionein 1 and 2, alpha-tocopherol transporter protein, and solute carrier family 23 member 2 (sodium-dependent vitamin C transporter) was greater in Gclm(-/-) mice than in Gclm(+/+) mice. These results suggest that compensatory augmentation of antioxidant defenses in Gclm(-/-) mice may confer increased resistance to ozone-induced lung injury.

Inhibitor of kappaB kinase beta regulates redox homeostasis by controlling the constitutive levels of glutathione

Cytokine-activated inhibitor of kappaB kinase beta (IKKbeta) is a key mediator of immune and inflammatory responses, but recent studies suggest that IKKbeta is also required for tissue homeostasis in physiopathological processes. Here we report a novel role for IKKbeta in maintenance of constitutive levels of the redox scavenger GSH. Inactivation of IKKbeta by genetic or pharmacological means results in low cellular GSH content and marked reduction of redox potential. Similar to Ikkbeta(-/-) cells, Tnfr1(-/-) and p65(-/-) cells are also GSH-deficient. As a consequence, cells deficient in IKKbeta signaling are extremely susceptible to toxicity caused by environmental and pharmacological agents, including oxidants, genotoxic agents, microtubule toxins, and arsenic. GSH biosynthesis depends on the activity of the rate-limiting enzyme glutamate-cysteine ligase (GCL), consisting of a catalytic subunit (GCLC) and a modifier subunit (GCLM). We found that loss of IKKbeta signaling significantly reduces basal NF-kappaB activity and decreases binding of NF-kappaB to the promoters of Gclc and Gclm, leading to reduction of GCLC and GCLM expression. Conversely, overexpression of GCLC and GCLM in IKKbeta-null cells partially restores GSH content and prevents stress-induced cytotoxicity. We suggest that maintenance of GSH is a novel physiological role of the IKKbeta-NF-kappaB signaling cascade to prevent oxidative damage and preserve the functional integrity of the cells.

Early onset senescence occurs when fibroblasts lack the glutamate-cysteine ligase modifier subunit

Cellular senescence is the irreversible entry of cells into growth arrest. Senescence of primary cells in culture has long been used as an in vitro model for aging. Glutamate-cysteine ligase (GCL) controls the synthetic rate of the important cellular antioxidant glutathione (GSH). The catalytic subunit of GCL, GCLC, is catalytically active and essential for life. By contrast the modifier subunit of GCL, GCLM, is dispensable in mice. Although it is recognized that GCLM increases the rate of GSH synthesis, its physiological role is unclear. Herein, we show that loss of Gclm leads to premature senescence of primary murine fibroblasts as characterized by: (a) diminished growth rate, (b) cell morphology consistent with senescence, (c) increases in senescence-associated beta-galactosidase activity, and (d) cell cycle arrest at the G(1)/S and G(2)/M boundaries. These changes are accompanied by increased intracellular ROS, accumulation of DNA damage, and induction of p53 and p21 proteins. We also found that N-acetylcysteine increases intracellular GSH and prevents premature senescence in Gclm(-/-) cells. These results suggest that the control of GCLM, which in turn controls aspects of the cellular redox environment via GSH, is important in determining the replicative capacity of the cell.

Peroxiredoxin-6 protects against mitochondrial dysfunction and liver injury during ischemia-reperfusion in mice

Hepatic ischemia-reperfusion (I/R) injury is an important complication of liver surgery and transplantation. Mitochondrial function is central to this injury. To examine alterations in mitochondrial function during I/R, we assessed the mitochondrial proteome in C57Bl/6 mice. Proteomic analysis of liver mitochondria revealed 234 proteins with significantly altered expression after I/R. From these, 13 proteins with the greatest expression differences were identified. One of these proteins, peroxiredoxin-6 (Prdx6), has never before been described in mitochondria. In hepatocytes from sham-operated mice, Prdx6 expression was found exclusively in the cytoplasm. After ischemia or I/R, Prdx6 expression disappeared from the cytoplasm and appeared in the mitochondria, suggesting mitochondrial trafficking. To explore the functional role of Prdx6 in hepatic I/R injury, wild-type and Prdx6-knockout mice were subjected to I/R injury. Prdx6-knockout mice had significantly more hepatocellular injury compared with wild-type mice. Interestingly, the increased injury in Prdx6-knockout mice occurred despite reduced inflammation and was associated with increased mitochondrial generation of H(2)O(2) and dysfunction. The mitochondrial dysfunction appeared to be related to complex I of the electron transport chain. These data suggest that hepatocyte Prdx6 traffics to the mitochondria during I/R to limit mitochondrial dysfunction as a protective mechanism against hepatocellular injury.

Knock-in mouse lines expressing either mitochondrial or microsomal CYP1A1: differing responses to dietary benzo[a]pyrene as proof of principle

In the past, CYP1A1 protein was known to be located in the endoplasmic reticulum (ER; microsomes). More recently, CYP1A1 was shown also to be targeted to the inner mitochondrial membrane; mitochondrial import is dependent on NH(2)-terminal processing that exposes a cryptic targeting signal. It is interesting that microsomal and mitochondrial CYP1A1 enzymes exhibit different substrate specificities, electron donors, and inducer properties. To understand the physiological functions of microsomal versus mitochondrial CYP1A1, we have generated three knock-in lines by altering the CYP1A1 NH(2) terminus. Cyp1a1(mtt/mtt) mice encode an NH(2)-terminal 31-amino acid-truncated protein, deleting the ER-targeting signal and exposing the cryptic mitochondrial-targeting signal. Cyp1a1(mtp/mtp) mice encode a protein carrying L7N and L17N mutations; this mutant lacks the signal recognition particle (SRP)-binding site and subsequent ER-targeting, but requires proteolysis by a cytosolic peptidase for mitochondrial import. Cyp1a1(mc/mc) mice encode a microsomal protein having R34D and K39I mutations, which abolish the mitochondrial targeting signal. After dioxin or beta-naphthoflavone treatment of these mouse lines, the CYP1A1 protein was shown to be located in the mitochondria of the Cyp1a1(mtp/mtp) and Cyp1a1(mtt/mtt) lines and in microsomes of the Cyp1a1(mc/mc) line. To test for differences in function, we compared the response to dietary benzo[a]pyrene (BaP). After 18 days of daily oral BaP, wild-type and Cyp1a1(mc/mc) mice were completely protected, whereas Cyp1a1(-/-) and Cyp1a1(mtp/mtp) mice showed striking toxicity and compensatory up-regulation of CYP1A2 and CYP1B1 mRNA in several tissues. Our data support the likelihood that it is the microsomal rather than mitochondrial CYP1A1 enzyme that protects against oral BaP toxicity.

Tetrahydroindenoindole inhibits the progression of diabetes in mice

Diabetes is characterized by elevated fasting blood glucose (FBG) resulting from improper insulin regulation and/or insulin resistance. Herein we used female C57BL/6J mouse models for type 1 diabetes (streptozotocin [STZ] treatment) and type 2 diabetes (high-fat diet) to examine the ability of 4b,5,9b,10-tetrahydroindeno[1,2-b]indole (THII) to intervene in the progression of diabetes. THII (100 microM in drinking water) significantly diminished and partially reversed the increase in FBG levels produced by STZ. After 10 weeks on a high-fat diet, mice had normal FBG levels, but exhibited fasting hyperinsulemia and loss of glucose tolerance. THII significantly diminished these changes in glucose and insulin. In isolated liver mitochondria, THII inhibited succinate-dependent H(2)O(2) production, while in white adipose tissue, THII inhibited NADPH oxidase-mediated H(2)O(2) production and lipid peroxidation. Without intervention, such oxidative processes might otherwise promote diabetogenesis via inflammatory pathways. THII also increased O(2) consumption and lowered respiratory quotient (CO(2) produced/O(2) consumed) in vivo, indicating a greater utilization of fat for metabolic fuel. Increased metabolic utilization of fat correlated with a decrease in the rate of body weight gain in THII-treated mice fed the high-fat diet. We conclude that THII may retard the progression of diabetes via multiple pathways, including the inhibition of oxidative and inflammatory pathways.

Over-the-counter analgesics normalize blood glucose and body composition in mice fed a high fat diet

Type 2 diabetes (noninsulin-dependent diabetes mellitus) develops from a pre-diabetic condition that is characterized by insulin resistance and glucose intolerance, and is exacerbated by obesity. In this study, we compared the ability of over-the-counter analgesic drugs (OTCAD) [acetaminophen (APAP); ibuprofen (IBU); naproxen (NAP); aspirin (ASA)], to protect against the development of a pre-diabetic state in mice fed a high fat diet. After 10 weeks on the high fat diet, mice had normal fasting blood glucose (FBG) levels, but exhibited impaired glucose tolerance. Treatment with 20 mg OTCADs/kg body weight improved glucose tolerance, with the order of efficacy, APAP=ASA>IBU, while NAP proved ineffective. Mice fed the high fat diet also exhibited increases in weight gain associated with an increase in body fat. OTCADs prevented in part this increase in body fat, in the order of efficacy, APAP=IBU>NAP=ASA. In isolated liver mitochondria, OTCADs inhibited succinate-dependent H2O2 production, while in white adipose tissue, APAP inhibited NADPH-oxidase mediated H2O2 production and lipid peroxidation. Thus, OTCADs diminish pro-oxidant processes that might otherwise exacerbate inflammation and a pre-diabetic state. We conclude that OTCADs, especially APAP and IBU, may be valuable tools to delay or prevent the development of type 2 diabetes from a pre-diabetic condition.

Acetaminophen normalizes glucose homeostasis in mouse models for diabetes

Loss of pancreatic beta cell insulin secretion is the most important element in the progression of type 1 and type 2 diabetes. Since oxidative stress is involved in the progressive loss of beta cell function, we evaluated the potential for the over-the-counter analgesic drug and antioxidant, acetaminophen (APAP), to intervene in the diabetogenic process. We used mouse models for type 1 diabetes (streptozotocin) and type 2 diabetes (high-fat diet) to examine the ability of APAP to intervene in the progression of diabetes. In C57BL/6J mice, streptozotocin caused a dosage dependent increase in fasting blood glucose (FBG), from 100 to >600mg/dl. Daily APAP (20mg/kg BW, gastric gavage), significantly prevented and partially reversed the increase in FBG levels produced by streptozotocin. After 10 weeks on a high-fat diet, mice developed fasting hyperinsulemia and impaired glucose tolerance compared to animals fed a control diet. APAP largely prevented these changes in insulin and glucose tolerance. Furthermore, APAP prevented most of the increase in body fat in mice fed the high-fat diet. One protective mechanism for APAP is suggested by studies using isolated liver mitochondria, where low micromolar concentrations abolished the production of reactive oxygen that might otherwise contribute to the destruction of pancreatic beta-cells. These findings suggest that administration of APAP to mice, in a dosage used safely by humans, reduces the production of mitochondrial reactive oxygen and concomitantly prevents the development of type 1 and type 2 diabetes in established animal models.

Interaction between the catalytic and modifier subunits of glutamate-cysteine ligase

Glutamate-cysteine ligase (GCL) is the rate-limiting enzyme in the glutathione (GSH) biosynthesis pathway. This enzyme is a heterodimer, comprising a catalytic subunit (GCLC) and a regulatory subunit (GCLM). Although GCLC alone can catalyze the formation of l-gamma-glutamyl-l-cysteine, its binding with GCLM enhances the enzyme activity by lowering the K(m) for glutamate and ATP, and increasing the K(i) for GSH inhibition. To characterize the enzyme structure-function relationship, we investigated the heterodimer formation between GCLC and GCLM, in vivo using the yeast two-hybrid system, and in vitro using affinity chromatography. A strong and specific interaction between GCLC and GCLM was observed in both systems. Deletion analysis indicated that most regions, except a portion of the C-terminal region of GCLC and a portion of the N-terminal region of GCLM, are required for the interaction to occur. Point mutations of selected amino acids were also tested for the binding activity. The GCLC Cys248Ala/Cys249Ala and Pro158Leu mutations enzyme showed the same strength of binding to GCLM as did wild-type GCLC, yet the catalytic activity was dramatically decreased. The results suggest that the heterodimer formation may not be dependent on primary amino-acid sequence but, instead, involves a complex formation of the tertiary structure of both proteins.

A critical role for IkappaB kinase beta in metallothionein-1 expression and protection against arsenic toxicity

Arsenic is a widespread environmental toxic agent that has been shown to cause diverse tissue and cell damage and at the same time to be an effective anti-cancer therapeutic agent. The objective of this study is to explore the signaling mechanisms involved in arsenic toxicity. We show that the IkappaB kinase beta (IKKbeta) plays a crucial role in protecting cells from arsenic toxicity. Ikkbeta(-)(/)(-) mouse 3T3 fibroblasts have decreased expression of antioxidant genes, such as metallothionein 1 (Mt1). In contrast to wild type and IKKbeta-reconstituted Ikkbeta(-)(/)(-) cells, IKKbeta-null cells display a marked increase in arsenic-induced reactive oxygen species (ROS) accumulation, which leads to activation of the MKK4-c-Jun NH(2)-terminal kinase (JNK) pathway, c-Jun phosphorylation, and apoptosis. Pretreatment with the antioxidant N-acetylcysteine (NAC) and expression of MT1 in the Ikkbeta(-)(/)(-) cells prevented JNK activation; moreover, NAC pretreatment, MT1 expression, MKK4 ablation, and JNK inhibition all protected cells from death induced by arsenic. Our data show that two signaling pathways appear to be important for modulating arsenic toxicity. First, the IKK-NF-kappaB pathway is crucial for maintaining cellular metallothionein-1 levels to counteract ROS accumulation, and second, when this pathway fails, excessive ROS leads to activation of the MKK4-JNK pathway, resulting in apoptosis.

Hepatocyte-specific Gclc deletion leads to rapid onset of steatosis with mitochondrial injury and liver failure

Oxidative stress is considered to be a critical mediator in liver injury of various etiologies. Depletion of glutathione (GSH), the major antioxidant in liver, has been associated with numerous liver diseases. To explore the specific role of hepatic GSH in vivo, we targeted Gclc, a gene essential for GSH synthesis, so that it was flanked by loxP sites and used the albumin-cyclization recombination (Alb-Cre) transgene to disrupt the Gclc gene specifically in hepatocytes. Deletion within the Gclc gene neared completion by postnatal day (PND)14, and loss of GCLC protein was complete by PND21. Cellular GSH was progressively depleted between PND14 and PND28-although loss of mitochondrial GSH was less severe. Nevertheless, ultrastructural examination of liver revealed dramatic changes in mitochondrial morphology; these alterations were accompanied by striking decreases in mitochondrial function in vitro, cellular ATP, and a marked increase in lipid peroxidation. Plasma liver biochemistry tests from these mice were consistent with progressive severe parenchymal damage. Starting at PND21, livers from hepatocyte-specific Gclc knockout [Gclc(h/h)] mice showed histological features of hepatic steatosis; this included inflammation and hepatocyte death, which progressed in severity such that mice died at approximately 1 month of age due to complications from liver failure.

CONCLUSION

GSH is essential for hepatic function and loss of hepatocyte GSH synthesis leads to steatosis with mitochondrial injury and hepatic failure.

7H-dibenzo[c,g]carbazole metabolism by the mouse and human CYP1 family of enzymes

Found in tobacco smoke, fossil fuel and other organic combustion products, 7H-dibenzo[c,g]carbazole (DBC) is a potent mouse lung carcinogen and potential human carcinogen. Although the first hydroxylation is critical for determining activation versus detoxication, the enzymes responsible for site-specific hydroxylation of DBC are not known. We found that DBC-DNA adduct levels are significantly higher in aromatic hydrocarbon receptor null Ahr(-/-) mice, suggesting that the induction of Aromatic hydrocarbon receptor (AHR)-regulated genes, such as those in the CYP1 family, decrease DBC genotoxicity. Using knockout mice for Cyp1a1, Cyp1a2 and Cyp1b1, we showed that the major CYP1 enzymes that metabolize DBC are CYP1A1 in beta-naphthoflavone (BNF)-induced liver, CYP1A2 in non-induced liver, CYP1B1 and CYP1A1 in induced lung and none in non-induced lung. DBC metabolism by the human CYP1 enzymes was examined in vitro using Supersomestrade mark. Each mouse CYP1, as well as each human CYP1, has a unique DBC metabolite profile. Comparison of the metabolite profile in BNF-induced mice suggested that CYP1A1 primarily generates 1-OH, 2-OH and (5 + 6)-OH-DBC, whereas CYP1A2 generates primarily (5 + 6)-OH-DBC and CYP1B1 primarily generates 4-OH-DBC. This was similar to that observed in the human CYP1 enzymes. Most importantly, lung CYP1B1 is associated with forming 4-OH-DBC, the most potent metabolite leading to DBC-DNA adducts. These studies suggest that for non-pulmonary routes of exposure (i.e. skin, gastric, i.p.), low hepatic expression of CYP1A2 and CYP1A1, together with high expression levels of lung CYP1B1 and CYP1A1, may define a phenotype for high susceptibility to carcinogens such as DBC.

TCDD decreases ATP levels and increases reactive oxygen production through changes in mitochondrial F(0)F(1)-ATP synthase and ubiquinone

Mitochondria generate ATP and participate in signal transduction and cellular pathology and/or cell death. TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) decreases hepatic ATP levels and generates mitochondrial oxidative DNA damage, which is exacerbated by increasing mitochondrial glutathione redox state and by inner membrane hyperpolarization. This study identifies mitochondrial targets of TCDD that initiate and sustain reactive oxygen production and decreased ATP levels. One week after treating mice with TCDD, liver ubiquinone (Q) levels were significantly decreased, while rates of succinoxidase and Q-cytochrome c oxidoreductase activities were increased. However, the expected increase in Q reduction state following TCDD treatment did not occur; instead, Q was more oxidized. These results could be explained by an ATP synthase defect, a premise supported by the unusual finding that TCDD lowers ATP/O ratios without concomitant changes in respiratory control ratios. Such results suggest either a futile cycle in ATP synthesis, or hydrolysis of newly synthesized ATP prior to release. The TCDD-mediated decrease in Q, concomitant with an increase in respiration, increases complex 3 redox cycling. This acts in concert with glutathione to increase membrane potential and reactive oxygen production. The proposed defect in ATP synthase explains both the greater respiratory rates and the lower tissue ATP levels.

Induction of oxidative stress responses by dioxin and other ligands of the aryl hydrocarbon receptor

TCDD and other polyhalogenated aromatic hydrocarbon ligands of the aryl hydrocarbon receptor (AHR) have been classically considered as non-genotoxic compounds because they fail to be directly mutagenic in either bacteria or most in vitro assay systems. They do so in spite of having repeatedly been linked to oxidative stress and to mutagenic and carcinogenic outcomes. Oxidative stress, on the other hand, has been used as a marker for the toxicity of dioxin and its congeners. We have focused this review on the connection between oxidative stress induction and the toxic effects of fetal and adult dioxin exposure, with emphasis on the large species difference in sensitivity to this agent. We examine the roles that the dioxin-inducible cytochromes P450s play in the cellular and toxicological consequences of dioxin exposure with emphasis on oxidative stress involvement. Many components of the health consequences resulting from dioxin exposure may be attributable to epigenetic mechanisms arising from prolonged reactive oxygen generation.

For dioxin-induced birth defects, mouse or human CYP1A2 in maternal liver protects whereas mouse CYP1A1 and CYP1B1 are inconsequential

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) induces cleft palate and hydronephrosis in mice, when exposed in utero; these effects are mediated by the aryl hydrocarbon receptor. The Cyp1a1, Cyp1a2, and Cyp1b1 genes are up-regulated by the aryl hydrocarbon receptor. To elucidate their roles in dioxin-induced teratogenesis, we compared Cyp1a1(-/-), Cyp1a2(-/-), and Cyp1b1(-/-) knock-out mice with Cyp1(+/+) wild-type mice. Dioxin was administered (25 microg/kg, gavage) on gestational day 10, and embryos were examined on gestational day 18. The incidence of cleft palate and hydronephrosis was not significantly different in fetuses from Cyp1a1(-/-), Cyp1b1(-/-), and Cyp1(+/+) wild-type mice. To fetuses carried by Cyp1a2(-/-) dams, however, this dose of dioxin was lethal; this effect was absolutely dependent on the maternal Cyp1a2 genotype and independent of the embryonic Cyp1a2 genotype. Dioxin levels were highest in adipose tissue, mammary gland, and circulating blood of Cyp1a2(-/-) mothers, compared with that in the Cyp1(+/+) mothers, who showed highest dioxin levels in liver. More dioxin reached the embryos from Cyp1a2(-/-) dams, compared with that from Cyp1(+/+) dams. Fetuses from Cyp1a2(-/-) dams exhibited a approximately 6-fold increased sensitivity to cleft palate, hydronephrosis, and lethality. Using the humanized hCYP1A1_1A2 transgenic mouse (expressing the human CYP1A1 and CYP1A2 genes in the absence of mouse Cyp1a2 gene), the teratogenic effects of dioxin reverted to the wild-type phenotype. These data indicate that maternal mouse hepatic CYP1A2, by sequestering dioxin and thus altering the pharmacokinetics, protects the embryos from toxicity and birth defects; substitution of the human CYP1A2 trans-gene provides the same protection. In contrast, neither CYP1A1 nor CYP1B1 appears to play a role in dioxin-mediated teratogenesis.

Toward the evaluation of function in genetic variability: characterizing human SNP frequencies and establishing BAC-transgenic mice carrying the human CYP1A1_CYP1A2 locus

Interindividual differences in human CYP1A1 and CYP1A2 expression appear to be associated with variability in risk toward various types of environmental toxicity and cancer. These two genes are oriented head-to-head on human chromosome 15; the 23.3-kb spacer region might contain distinct regulatory regions for CYP1A1 and distinct regulatory regions for CYP1A2, or the regulatory regions for the two genes might overlap one another. From 24 unrelated subjects of five major, geographically-isolated subgroups, we resequenced both genes (all exons and all introns) plus some 3' flanking sequences and the entire spacer region (39.6 kb total); 85 SNPs were found, 49 of which were not currently in the National Center for Biotechnology Information (NCBI) database. Of the 57 double-hit SNPs, we carried out SNP-typing in 94 Africans, 96 Asians, and 83 Caucasians and found striking ethnic differences in SNP frequencies and haplotype evolution; the two CYP1A1 SNPs and the one CYP1A2 SNP that are most commonly used in epidemiological studies were shown not to be representative haplotype tag SNPs across these three human subgroups. Four BAC-transgenic mouse lines, carrying the human CYP1A2 and 15,190 bp of 5' flank, expressed only negligible basal or inducible CYP1A2 mRNA. A fifth BAC-transgenic mouse line, carrying both the human CYP1A1 and CYP1A2 genes and ample amounts of 3' flanking sequences, plus all of the spacer region--in the absence of the mouse Cyp1a1 or Cyp1a2 genes--expressed the human CYP1A1 and CYP1A2 mRNA, protein and enzyme activities in liver and nonhepatic tissues very similar to that of the mouse. Comparison of this hCYP1A1_1A2 transgenic line with hCYP1A1_1A2 lines carrying other common human haplotypes will enable us to evaluate function in human CYP1A1_CYP1A2 locus variability, with regard to toxicity and cancer caused by combustion products.

Comparison of mouse hepatic mitochondrial versus microsomal cytochromes P450 following TCDD treatment

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) induces cytochromes P450 (CYPs) such as CYP1A1 and CYP1A2 via activation of the aromatic hydrocarbon receptor (AHR). Herein we describe the TCDD-dependent enrichment of CYPs in liver microsomes and mitoplasts from C57BL/6J mice. TCDD-induced accumulation of CYP1A1 and CYP1A2 was observed in microsomes and mitoplasts after treatment with 15 microg TCDD/kg/d for 3d. While microsomal CYP1 proteins peaked at 1 week and diminished thereafter, mitoplast CYP1 proteins persisted 8 weeks at high levels. TCDD also induced microsomal CYP2A5, but not microsomal proteins immunoreactive to CYP2C11, CYP3A2 or CYP4A1 antibodies. Nevertheless, each of these proteins increased in mitoplasts following TCDD exposure. These results suggest that TCDD increases mitochondrial CYP immunoreactive proteins under the transcriptional control of the AHR, as well as CYPs that are not under AHR control. We speculate that such mitochondrial CYPs may be involved in the generation, or mitigation, of the well-known TCDD-inducible oxidative stress response.

Oral benzo[a]pyrene in Cyp1 knockout mouse lines: CYP1A1 important in detoxication, CYP1B1 metabolism required for immune damage independent of total-body burden and clearance rate

CYP1A1 and CYP1B1 metabolically activate many polycyclic aromatic hydrocarbons (PAHs), including benzo[a]pyrene, to reactive intermediates associated with toxicity, mutagenesis, and carcinogenesis. Paradoxically, however, Cyp1a1-/- knockout mice are more sensitive to oral benzo[a]pyrene exposure, compared with wild-type Cyp1a1+/+ mice (Mol Pharmacol 65:1225, 2004). To further investigate the mechanism for this enhanced sensitivity, Cyp1a1-/-, Cyp1a2-/-, and Cyp1b1-/- single-knockout, Cyp1a1/1b1-/- and Cyp1a2/1b1-/- double-knockout, and Cyp1+/+ wild-type mice were analyzed. After administration of oral benzo[a]pyrene (125 mg/kg/day) for 18 days, Cyp1a1-/- mice showed marked wasting, immunosuppression, and bone marrow hypocellularity, whereas the other five genotypes did not. After 5 days of feeding, steady-state blood levels of benzo[a]pyrene were approximately 25 and approximately 75 times higher in Cyp1a1-/- and Cyp1a1/1b1-/- mice, respectively, than in wild-type mice. Benzo[a]pyrene-DNA adduct levels were highest in liver, spleen, and marrow of Cyp1a1-/- and Cyp1a1/1b1-/- mice. Many lines of convergent data obtained with oral benzo[a]pyrene dosing suggest that: 1) inducible CYP1A1, probably in both intestine and liver, is most important in detoxication; 2) CYP1B1 in spleen and marrow is responsible for metabolic activation of benzo[a]pyrene, which results in immune damage in the absence of CYP1A1; 3) both thymus atrophy and hepatocyte hypertrophy are independent of CYP1B1 metabolism but rather may reflect long-term activation of the aryl hydrocarbon receptor; and 4) the magnitude of immune damage in Cyp1a1-/- and Cyp1a1/1b1-/- mice is independent of plasma benzo[a]pyrene and total-body burden and clearance. Thus, a balance between tissue-specific expression of the CYP1A1 and CYP1B1 enzymes governs sensitivity of benzo[a]pyrene toxicity and, possibly, carcinogenicity.

Genetic Differences in Lethality of Newborn Mice Treated In Utero with Coplanar versus Non-Coplanar Hexabromobiphenyl

Polybrominated biphenyl (PBB) exposure in humans is known to cause immunotoxicity and disorders related to the central nervous system. Coplanar PBBs bind to the aryl hydrocarbon receptor (AHR) in vertebrates. We compared the coplanar PBB, 3,3',4,4',5,5'-hexabromobiphenyl (cHBB), with its stereoisomer, the non-coplanar PBB, 2,2',4,4'6,6'-hexabromobiphenyl (ncHBB), using C57BL/6J (B6) inbred mice (having the high-affinity AHR) and congenic B6.D2-Ahr d mice (having the low-affinity AHR in a >99.8% C57BL/6J genetic background). Pregnant dams were treated i.p. with vehicle alone, cHBB, or ncHBB on gestational day 5 (GD 5). Unexpectedly, neonatal lethality within the first 72 h postpartum was significant in cHBB-treated B6 mice at doses as low as 2.5 mg/kg, whereas no deaths were seen in B6 pups whose mother had received ncHBB 100 mg/kg or in either B6.D2-Ahr d or Ahr(-/-) knockout mice whose mother had received cHBB 100 mg/kg. Histological and gross anatomical analyses of a battery of tissues in the mother or fetus at GD 18, as well as 24 h postpartum, revealed no significant differences, except for decreased thymus and spleen weights in cHBB-treated B6 GD 18 fetuses. Cross-fostering and genetics experiments confirmed the association of neonatal deaths principally with in utero (rather than lactational) exposure to cHBB, and also no paternal effect. For the end points of mouse neonatal lethality and immunotoxicity, cHBB appears to act through the high-affinity AHR receptor. Although dioxin in utero is well known to cause AHR-dependent cleft palate and hydronephrosis, cHBB did not; thus, chronic activation of the AHR appears to be necessary but not sufficient for AHR-mediated teratogenicity.

Theophylline pharmacokinetics: comparison of Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice, humanized hCYP1A1_1A2 knock-in mice lacking either the mouse Cyp1a1 or Cyp1a2 gene, and Cyp1(+/+) wild-type mice

OBJECTIVES

Pharmacokinetics of theophylline was investigated in Cyp1(+/+) wild-type mice, Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice, and humanized hCYP1A1_1A2 mice lacking either the mouse Cyp1a1 or Cyp1a2 gene.

METHODS AND RESULTS

Animals received a single dose of theophylline (8 mg/kg i.p.), either alone or pretreated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 10 microg/kg i.p.) 24 h prior to theophylline. We found that mouse or human CYP1A2 is the predominant enzyme for theophylline metabolism, the contribution of mouse or human CYP1A1 to theophylline metabolism is negligible, and another TCDD-inducible enzyme plays a minor role in 1-methyluric acid and 1,3-dimethyluric acid formation as well as enhanced theophylline clearance from the body. The half-life of elimination from plasma was more than four times longer in Cyp1a2(-/-) than Cyp1(+/+) mice and more than 10 times different after TCDD pretreatment. In humanized hCYP1A1_1A2 mice lacking the mouse Cyp1a2 gene, the half-life of elimination from plasma was two to three times longer than that in Cyp1(+/+) mice and four to five times different after TCDD pretreatment.

CONCLUSION

Replacement of mouse Cyp1a2 with a functional human CYP1A2 gene restored the ability to metabolize theophylline, and the metabolism changed to a humanized profile (i.e. 3-methylxanthine formation, not seen in the wild-type mouse). TCDD-pretreated hCYP1A1_1A2 Cyp1a2(-/-) mice exhibited enhanced theophylline metabolism and clearance, due to induction of the human CYP1A2 enzyme. Comparing the hCYP1A1_1A2 Cyp1a2(-/-) and wild-type mice with published clinical studies, we found theophylline clearance to be about 5 times and 12 times, respectively, greater than that reported in humans.

Glutamate cysteine ligase catalysis: dependence on ATP and modifier subunit for regulation of tissue glutathione levels

Glutamate cysteine ligase (GCL), which synthesizes gamma-glutamyl-cysteine (gamma-GC), is the rate-limiting enzyme in GSH biosynthesis. gamma-GC may be produced by the catalytic subunit GCLC or by the holoenzyme (GCLholo), which comprises GCLC and the modifier subunit GCLM. The Gclm(-/-) knock-out mouse shows tissue levels of GSH that are between 9 and 40% of the Gclm(+/+) wild-type mouse. In the present study, we used recombinant GCLC and GCLM and Gclm(-/-) mice to examine the role of GCLM on gamma-GC synthesis by GCLholo. GCLM decreased the Km for ATP by approximately 6-fold and, similar to other species, decreased the Km for glutamate and increased the Ki for feedback inhibition by GSH. Furthermore, GCLM increased by 4.4-fold the Kcat for gamma-GC synthesis; this difference in catalytic efficiency of GCLholo versus GCLC allowed us to derive a mathematical relationship for gamma-GC production and to determine the relative levels of GCLholo and GCLC; in homogenates of brain, liver, and lung, the ratio of GCLC to GCLholo was 7.0, 2.0, and 3.5, respectively. In kidney, however, the relationship between GCLC and GCLholo was complicated. Kidney contains GCLholo, free GCLC, and free GCLM, and free GCLC in kidney cannot interact with GCLM. Taken together, we conclude that, in most tissues, GCLM is limiting, suggesting that an increase in GCLM alone would increase gamma-GC synthesis. On the other hand, our results from kidney suggest that gamma-GC synthesis may be controlled post-translationally.

Butylhydroquinone protects cells genetically deficient in glutathione biosynthesis from arsenite-induced apoptosis without significantly changing their prooxidant status

Arsenic, first among the top environmentally hazardous substances, is associated with skin, lung, liver, kidney, prostate, and bladder cancer. Arsenic is also a cardiovascular and a central nervous system toxicant, and it has genotoxic and immunotoxic effects. Paradoxically, arsenic trioxide is used successfully in the treatment of acute promyelocytic leukemia and multiple myeloma. Arsenic induces oxidative stress, and its toxicity is decreased by free thiols and increased by glutathione depletion. To further characterize the role of glutathione and oxidative stress in the toxicity of arsenic, we have used fetal fibroblasts from Gclm(-/-) mice, which lack the modifier subunit of glutamate-cysteine ligase, the rate-limiting enzyme in glutathione biosynthesis. Gclm(-/-) mouse embryo fibroblasts (MEFs) are eight times more sensitive to arsenite-induced apoptotic death. Because of a dramatic decrease in glutathione levels, Gclm(-/-) MEFs have a high prooxidant status that is not significantly relieved by treatment with the phenolic antioxidant tBHQ; however, tBHQ blocks arsenite-induced apoptosis in both Gclm(+/+) and Gclm(-/-) cells, although it raises a significant antioxidant response only in Gclm(+/+) cells. Global gene expression profiles indicate that tBHQ is significantly effective in reversing arsenite-induced gene deregulation in Gclm(+/+) but not in Gclm(-/-) MEFs. This effect of tBHQ is evident in the expression of metalloproteases and chaperones, and in the expression of genes involved in DNA damage and repair, protein biosynthesis, cell growth and maintenance, apoptosis, and cell cycle regulation. These results suggest that regulation of glutathione levels by GCLM determines the sensitivity to arsenic-induced apoptosis by setting the overall ability of the cells to mount an effective antioxidant response.

alpha-Melanocortin and endothelin-1 activate antiapoptotic pathways and reduce DNA damage in human melanocytes

UV radiation is an important etiologic factor for skin cancer, including melanoma. Constitutive pigmentation and the ability to tan are considered the main photoprotective mechanism against sun-induced carcinogenesis. Pigmentation in the skin is conferred by epidermal melanocytes that synthesize and transfer melanin to keratinocytes. Therefore, insuring the survival and genomic stability of epidermal melanocytes is critical for inhibiting photocarcinogenesis, particularly melanoma, the most deadly form of skin cancer. The paracrine factors alpha-melanocortin and endothelin-1 are critical for the melanogenic response of cultured human melanocytes to UV radiation. We report that alpha-melanocortin and endothelin-1 rescued human melanocytes from UV radiation-induced apoptosis and reduced DNA photoproducts and oxidative stress. The survival effects of alpha-melanocortin and endothelin-1 were mediated by activation of the melanocortin 1 and endothelin receptors, respectively. Treatment of melanocytes with alpha-melanocortin and/or endothelin-1 before exposure to UV radiation activated the inositol triphosphate kinase-Akt pathway and increased the phosphorylation and expression of the microphthalmia-related transcription factor. Treatment with alpha-melanocortin and/or endothelin-1 enhanced the repair of cyclobutane pyrimidine dimers and reduced the levels of hydrogen peroxide induced by UV radiation. These effects are expected to reduce genomic instability and mutagenesis.

Glutathione Redox State Regulates Mitochondrial Reactive OxygenProduction

Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin) is poorly understood. Following one dose of TCDD (5 microg/kg body weight), mitochondrial succinate-dependent production of superoxide and H2O2 in mouse liver doubled at 7-28 days, then subsided by day 56; concomitantly, levels of GSH and GSSG increased in both cytosol and mitochondria. Cytosol displayed a typical oxidative stress response, consisting of diminished GSH relative to GSSG, decreased potential to reduce protein-SSG mixed disulfide bonds (type 1 thiol redox switch) or protein-SS-protein disulfide bonds (type 2 thiol redox switch), and a +10 mV change in GSSG/2GSH reduction potential. In contrast, mitochondria showed a rise in reduction state, consisting of increased GSH relative to GSSG, increases in type 1 and type 2 thiol redox switches, and a -25 mV change in GSSG/2GSH reduction potential. Comparing Ahr(-/-) knock-out and wild-type mice, we found that TCDD-induced thiol changes in both cytosol and mitochondria were dependent on the aromatic hydrocarbon receptor (AHR). GSH was rapidly taken up by mitochondria and stimulated succinate-dependent H2O2 production. A linear dependence of H2O2 production on the reduction potential for GSSG/2GSH exists between -150 and -300 mV. The TCDD-stimulated increase in succinate-dependent and thiol-stimulated production of reactive oxygen paralleled a four-fold increase in formamidopyrimidine DNA N-glycosylase (FPG)-sensitive cleavage sites in mitochondrial DNA, compared with a two-fold increase in nuclear DNA. These results suggest that TCDD produces an AHR-dependent oxidative stress in mitochondria, with concomitant mitochondrial DNA damage mediated, at least in part, by an increase in the mitochondrial thiol reduction state.

Genetically altered mice to evaluate glutathione homeostasis in health and disease

The tripeptide glutathione (GSH) is part of an integrated antioxidant system that protects cells and tissues from oxidative damage. Oxidative stress can result from exposure to excessive amounts of endogenous and exogenous electrophiles. Until recently, animal and cell model systems used to investigate the role of GSH in disease processes had employed chemical agents that deplete cellular GSH by inhibiting GSH synthesis or by reacting chemically with GSH. Such models have proven useful, but questions concerning nonspecific effects of such chemicals remain. Recently, our laboratories and others have developed mouse models with genetic deficiencies in enzymes of the GSH biosynthetic pathway. This review focuses on the regulation of GSH homeostasis and, specifically, the new GSH-deficient mouse models that have been developed. These models will improve our understanding of the role of GSH in animal and human diseases.

Metalloproteinases mediate mucin 5AC expression by epidermal growth factor receptor activation

Chronic obstructive pulmonary disease is marked by alveolar enlargement and excess production of airway mucus. Acrolein, a component of cigarette smoke, increases mucin 5AC (MUC5AC), a prevalent airway mucin in NCI-H292 cells by transcriptional activation, but the signal transduction pathways involved in acrolein-induced MUC5AC expression are unknown. Acrolein depleted cellular glutathione at doses of 10 muM or greater, higher than those sufficient (0.03 muM) to increase MUC5AC mRNA, suggesting that MUC5AC expression was independent of oxidative stress. In contrast, acrolein increased MUC5AC mRNA levels by phosphorylating epidermal growth factor receptor (EGFR) and mitogen-activated protein kinase 3/2, or MAPK 3/2(ERK1/2). Pretreating the cells with an EGFR-neutralizing antibody, or a metalloproteinase inhibitor, decreased the acrolein-induced MUC5AC mRNA increase. Small, interfering RNA directed against ADAM17 or MMP9 inhibited the acrolein-induced MUC5AC mRNA increase. Acrolein increased the release and subsequent activation of pro-MMP9. Acrolein increased MMP9 and decreased tissue inhibitor of metalloproteinase 3 (TIMP3), an endogenous inhibitor of ADAM17, transcripts. Together, these data suggest that acrolein induces MUC5AC expression via an initial ligand-dependent activation of EGFR mediated by ADAM17 and MMP9. In addition, a prolonged effect of acrolein may be mediated by altering MMP9 and TIMP3 transcription.

Cyp1a1(-/-) male mice: protection against high-dose TCDD-induced lethality and wasting syndrome, and resistance to intrahepatocyte lipid accumulation and uroporphyria

To study liver toxicity and uroporphyrin (URO) accumulation and urinary excretion, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent ligand for the aryl hydrocarbon receptor (AHR), is often used as the prototype. In this study, we asked the question how important is the role of CYP1A1 in causing TCDD toxicity. Using a single large intraperitoneal dose of TCDD (200 microg/kg) and following the response over an 8-week period, we found this dose: (a) was lethal in less than 4 weeks to Cyp1a1(+/+) males but not to Cyp1a1(-/-) males or to females of either genotype; (b) caused a wasting syndrome in Cyp1a1(+/+) but not Cyp1a1(-/-) mice; (c) resulted in thymic atrophy, regardless of gender or genotype; (d) decreased spleen size and caused leukocytopenia in males but not females of either genotype; (e) caused hepatocyte hypertrophy in Cyp1a1(+/+) more so than in Cyp1a1(-/-) mice; (f) increased intrahepatocyte lipids and total liver fat content in Cyp1a1(+/+) more than Cyp1a1(-/-) males and females; and (g) caused uroporphyria in Cyp1a1(+/+) males much more than Cyp1a1(+/+) females, or in Cyp1a1(-/-) mice. Contrary to Cyp1a2(-/-) knockout mice that exhibited 15 times less accumulation of TCDD in liver than Cyp1a1/1a2(+/+) wild-type mice, Cyp1a1(-/-) mice did not show this altered TCDD distribution-indicating that CYP1A2 but not CYP1A1 is the major hepatic TCDD-binding "sink". Our data demonstrate that CYP1A1 contributes to high-dose TCDD-induced toxicity, uroporphyria, and lethality.

Oral Exposure to Benzo[a]pyrene in the Mouse: Detoxication by Inducible Cytochrome P450 Is More Important Than Metabolic Activation

The cytochrome P450 (CYP1A1) enzyme metabolically activates many polycyclic aromatic hydrocarbons, including benzo[a]pyrene (BaP), to DNA- and protein-binding intermediates that are associated with toxicity, mutagenesis, and carcinogenesis. As a result, it is widely accepted that CYP1A1 potentiates the toxicity of this class of chemicals. In distinct contrast, we show here that CYP1A1 inducibility is essential in the detoxication of oral BaP. We compared Cyp1a1(-/-) knockout mice, having the genetic absence of the CYP1A1 enzyme, with Cyp1a1(+/+) wild-type mice. At an oral BaP dose of 125 mg/kg/day, Cyp1a1(-/-) mice died within 30 days whereas Cyp1a1(+/+) mice displayed no outward signs of toxicity. The rate of BaP clearance was 4-fold slower in Cyp1a1(-/-) than Cyp1a1(+/+) mice. The cause of death in Cyp1a1(-/-) mice receiving oral BaP seemed to be immunotoxicity, including toxic chemical depression of the bone marrow; some toxic effects in Cyp1a1(-/-) mice were noted at a BaP dose as low as 1.25 mg/kg/day. DNA post-labeling studies demonstrated dramatically higher BaP-DNA adduct levels in all Cyp1a1(-/-) tissues assayed, with the exception of the small intestine, which is probably a major site of BaP metabolism in Cyp1a1(+/+) mice. Different BaP-DNA adduct patterns were also observed between the two genotypes receiving oral BaP. Despite previous studies in vitro and in cell culture that have shown a participatory role for CYP1A1 in BaP toxicity, the present data indicate that, in the intact animal, inducible CYP1A1 is extremely important in detoxication and protection against oral BaP toxicity.

Uncoupling-mediated generation of reactive oxygen by halogenated aromatic hydrocarbons in mouse liver microsomes

Studying liver microsomes from 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced or vehicle-treated (noninduced) mice, we evaluated the in vitro effects of added chemicals on the production of reactive oxygen due to substrate/P450-mediated uncoupling. The catalase-inhibited NADPH-dependent H(2)O(2) production (luminol assay) was lower in induced than noninduced microsomes. The effects of adding chemicals (2.5 microM) in vitro could be divided into three categories: Group 1, highly halogenated and coplanar compounds that increased H(2)O(2) production at least 5-fold in induced, but not in noninduced, microsomes; Group 2, non-coplanar halogenated biphenyls that did not affect H(2)O(2) production; Group 3, minimally halogenated biphenyls and benzo[a]pyrene that decreased H(2)O(2) production. Molar consumption of NADPH and O(2) and molar H(2)O(2) production (o-dianisidine oxidation) revealed that Group 1 compounds mostly increased, Group 2 had no effect, and Group 3 decreased the H(2)O(2)/O(2) and H(2)O(2)/NADPH ratios. Microsomal lipid peroxidation (thiobarbituric acid-reactive substances) was proportional to H(2)O(2) production. Although TCDD induction decreased microsomal production of H(2)O(2), addition of Group 1 compounds to TCDD-induced microsomes in vitro stimulated the second-electron reduction of cytochrome P450 and subsequent release of H(2)O(2) production. This pathway is likely to contribute to the oxidative stress response and associated toxicity produced by many of these environmental chemicals.

Cyp1a2 protects against reactive oxygen production in mouse liver microsomes

H(2)O(2) production was evaluated in liver microsomes prepared from Cyp1a1/1a2(+/+) wild-type and Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice pretreated with 5 microg dioxin (TCDD)/kg body wt or vehicle alone. NADPH-dependent H(2)O(2) production in TCDD-induced microsomes from wild-type mice was about one-third of that in noninduced microsomes. In Cyp1a2(-/-) mice, H(2)O(2) production was the same for induced and noninduced microsomes, with levels significantly higher than those in wild-type mice. Cyp1a1(-/-) microsomes displayed markedly lower levels of H(2)O(2) production in both induced and noninduced microsomes, compared with those in wild-type and Cyp1a2(-/-) microsomes. The CYP1A2 inhibitor furafylline in vitro exacerbated microsomal H(2)O(2) production proportional to the degree of CYP1A2 inhibition, and the CYP2E1 inhibitor diethyldithiocarbamate decreased H(2)O(2) production proportional to the degree of CYP2E1 inhibition. Microsomal H(2)O(2) production was strongly correlated to NADPH-stimulated production of thiobarbituric acid-reactive substances, as well as to decreases in microsomal membrane polarization anisotropy, indicative of peroxidation of unsaturated membrane lipids. Our results suggest that possibly acting as an "electron sink," CYP1A2 might decrease CYP2E1-and CYP1A1-mediated H(2)O(2) production and oxidative stress. In this regard, CYP1A2 may be considered an antioxidant enzyme.

High glucose concentration in isotonic media alters caco-2 cell permeability

Caco-2 cell permeability was evaluated in isotonic media containing high (25 mM) or physiological (5.5 mM) glucose concentrations. Transepithelial electrical resistance (TEER) and membrane fluidity were measured to assess glucose-induced alterations in physical barrier properties. In parallel, distribution of the actin filament (F-actin) and zonula occludens-1 (ZO-1) proteins was assessed by confocal microscopy. Transepithelial fluxes of mannitol, hydrocortisone, digoxin, and glycyl sarcosine (Gly-Sar) that permeate the intestinal mucosa by various pathways were measured to quantify the effect of glucose-induced changes on Caco-2 cell permeability. High glucose decreased maximum TEER of cell monolayers by 47%, whereas membrane fluidity at the hydrophobic core and lipid/polar head interphase was significantly increased. F-actin distribution in high glucose cells appeared more diffuse while ZO-1 was unchanged. Mannitol and hydrocortisone fluxes across Caco-2 cells cultured in high glucose increased by 65% and 24%, respectively. In addition, high glucose decreased the maximum transport capacity (Vmax) of PepT-1. P-glycoprotein activity, however, was unchanged. In conclusion, high extracellular glucose concentration in isotonic media significantly alters physical barrier properties of Caco-2 cell monolayers, which predominantly affects transepithelial transport of solutes permeating the cell barrier by paracellular and transcellular passive diffusion and facilitated transport mediated by the proton-dependent oligopeptide transporter (PepT-1).

Balancer-Cre transgenic mouse germ cells direct the incomplete resolution of a tri-loxP-targeted Cyp1a1 allele, producing a conditional knockout allele

To generate conditional alleles, genes are commonly engineered to contain recognition sites for bacteriophage recombinases, such as Cre recombinase. When such motifs (lox sites) flank essential gene sequences, and provided that Cre recombinase is expressed, Cre recombinase will excise the flanked sequence-creating a conditional knockout allele. Targeted conditional alleles contain a minimum of three lox sites. It would be desirable to have Cre recombinase perform partial resolution (i.e., recombination some of the time between only the two lox sites flanking the marker gene). Here we report use of the commercially available Balancer2-Cre transgenic mouse line to carry out this function from a tri-loxP-site-containing cytochrome p450 1A1 (Cyp1a1) targeted allele. Such incomplete resolution of this complex locus occurred progressively with age in germ cells of male mice; the conditional Cyp1a1 gene was recovered in offspring from mice containing the targeted Cyp1a1 allele and the Cre recombinase transgene. Removal of the marker gene resulted in a conditional Cyp1a1 allele whose expression was indistinguishable from that of the wild-type allele.

Antioxidant perturbations in the olfactory mucosa of alachlor-treated rats

The chloracetanilide herbicide alachlor (2-chloro-2',6'-diethyl-N-(methoxymethyl)acetanilide) induces olfactory mucosal tumors in rats following chronic dietary exposure. Previous reports demonstrated that alachlor exposure was associated with depletion of glutathione (GSH) in liver in vivo and in vitro, but did not address this issue in the target tissue for the carcinogenic response. In this study we investigated a potential oxidative stress pathway in olfactory tissue by examining perturbations in olfactory mucosal antioxidants. Male Long-Evans rats were fed alachlor for up to 10 days (10-126 mg/kg per day), and intracellular reduced GSH and ascorbate levels were measured in olfactory mucosa. Both GSH and ascorbate rapidly decreased in olfactory mucosa following alachlor exposure, with a subsequent increase in both antioxidants to approximately 160% of control levels in the high dose group, and recovery of GSH to control levels in all groups by 10 days. Using Western blot analysis, we found that the modifier subunit of the rate-limiting enzyme in GSH synthesis, glutamate-cysteine ligase, increased in olfactory mucosa and remained elevated (126 mg/kg per day group). Two ascorbate transporters were detected by RT-PCR in olfactory mucosa, but neither appeared to be upregulated by alachlor exposure, and ascorbate synthesis was not stimulated in olfactory mucosa by alachlor treatment. Dietary exposure to alachlor depletes olfactory mucosa antioxidants, which may contribute to DNA damage and tissue-specific tumor formation.

4-aminobiphenyl-induced liver and urinary bladder DNA adduct formation in Cyp1a2(-/-) and Cyp1a2(+/+) mice

BACKGROUND

Metabolites of the potent human carcinogen 4-aminobiphenyl (ABP) induce oxidative stress and form DNA adducts that are associated with hepatic and urinary bladder toxicity and bladder tumorigenesis. Results of in vitro and cell culture studies have suggested that cytochrome P450 1A2 (CYP1A2) is the major metabolic activator of ABP. We used Cyp1a2(-/-) knockout mice to examine the role of CYP1A2 in ABP-DNA adduct formation in the liver and the bladder.

METHODS

Cyp1a2(+/+) wild-type and Cyp1a2(-/-) mice (total of four mice per group) were treated topically with 10 mg/kg ABP for various times, with or without pretreatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an inducer of CYP1A2 activity. We evaluated ABP-induced toxicity by carrying out quantitative histology (of the liver, skin, and bladder), oxidative stress by measuring hepatic thiol levels, and liver and bladder DNA adduct formation by using 32P-postlabeling. Data were analyzed by general linear models and analysis of variance. All statistical tests were two-sided.

RESULTS

At the experimental times selected, we observed no histologic evidence of toxicity in the liver, skin, or bladder. Overall, Cyp1a2(+/+) mice had fewer DNA adducts 24 hours after ABP treatment than similarly treated Cyp1a2(-/-) mice. Compared with male mice, female mice had more DNA adducts in the liver but fewer adducts in the bladder, regardless of Cyp1a2 genotype. TCDD pretreatment was associated with a decrease in ABP-DNA adduct levels overall. After 2 hours of ABP treatment, hepatic thiol levels underwent statistically significant declines of severalfold in Cyp1a2(+/+) and Cyp1a2(-/-) males and in Cyp1a2(-/-) females.

CONCLUSIONS

Contrary to our expectations, CYP1A2 expression was not associated with ABP-induced hepatic oxidative stress or with ABP-DNA adduct formation. Either CYP1A2 is not the major metabolic activator of ABP or other enzymes metabolically activate ABP in mice in the absence of CYP1A2.

Pharmacological rescue of the 14CoS/14CoS mouse: hepatocyte apoptosis is likely caused by endogenous oxidative stress

Whereas ch/ch wild-type mice and ch/14CoS heterozygotes are viable, 14CoS/14CoS mice homozygous for a 3800 kb deletion on chromosome 7 die during the first day postpartum. Death is caused by disruption of the fumarylacetoacetate hydrolase (Fah) gene; absence of FAH, final enzyme in the tyrosine catabolism pathway, leads to accumulation of reactive electrophilic intermediates. In this study, we kept 14CoS/14CoS mice alive for 60 d with oral 2-(2-nitro-4-trifluoromethyl-benzyol)-1,3-cyclohexanedione (NTBC), an inhibitor of p-hydroxyphenylpyruvate dioxygenase, second enzyme in the tyrosine catabolic pathway. The 70% of NTBC-treated 14CoS/14CoS mice that survived 60 d showed poor growth and developed corneal opacities, compared with ch/14CoS littermates; NTBC-rescued Fah(-/-) knockout mice did not show growth retardation or ocular toxicity. NTBC-rescued 14CoS/14CoS mice also exhibited a striking oxidative stress response in liver and kidney, as measured by lower GSH levels and mRNA induction of four genes: glutamate cysteine ligase catalytic (Gclc) and modifier (Gclm) subunits, NAD(P)H:quinone oxidoreductase (Nqo1), and heme oxygenase-1 (Hmox1). Withdrawal of NTBC for 24-48 h from rescued adult 14CoS/14CoS mice resulted in severe apoptosis of the liver, detected histologically and by cytochrome c release from the mitochondria, increased caspase 3-like activity, and further decreases in GSH content. In kidney, proximal tubular epithelial cells were abnormal. Human hereditary tyrosinemia type I (HT1), caused by mutations in the FAH gene, is an autosomal recessive disorder in which the patient usually dies of liver fibrosis and cirrhosis during early childhood; NTBC treatment is known to prolong HT1 children's lives-although liver fibrosis, cirrhosis, hepatocarcinoma, and corneal opacities sometimes occur. The mouse data in the present study are consistent with the possibility that endogenous oxidative stress-induced apoptosis may be the underlying cause of liver pathology seen in NTBC-treated HT1 patients.