Farnesoid X receptor activation by bile acids suppresses lipid peroxidation and ferroptosis.

Ferroptosis is a regulated cell death modality that occurs upon iron-dependent lipid peroxidation. Recent research has identified many regulators that induce or inhibit ferroptosis; yet, many regulatory processes and networks remain to be elucidated. In this study, we performed a chemical genetics screen using small molecules with known mode of action and identified two agonists of the nuclear receptor Farnesoid X Receptor (FXR) that suppress ferroptosis, but not apoptosis or necroptosis. We demonstrate that in liver cells with high FXR levels, knockout or inhibition of FXR sensitized cells to ferroptotic cell death, whereas activation of FXR by bile acids inhibited ferroptosis. Furthermore, FXR inhibited ferroptosis in ex vivo mouse hepatocytes and human hepatocytes differentiated from induced pluripotent stem cells. Activation of FXR significantly reduced lipid peroxidation by upregulating the ferroptosis gatekeepers GPX4, FSP1, PPARα, SCD1, and ACSL3. Together, we report that FXR coordinates the expression of ferroptosis-inhibitory regulators to reduce lipid peroxidation, thereby acting as a guardian of ferroptosis.

Kinetics of di(2-ethylhexyl) phthalate (DEHP) and mono(2-ethylhexyl) phthalate in blood and of DEHP metabolites in urine of male volunteers after single ingestion of ring-deuterated DEHP.

The plasticizer di(2-ethylhexyl) phthalate (DEHP) is suspected to induce antiandrogenic effects in men via its metabolite mono(2-ethylhexyl) phthalate (MEHP). However, there is only little information on the kinetic behavior of DEHP and its metabolites in humans. The toxikokinetics of DEHP was investigated in four male volunteers (28-61y) who ingested a single dose (645±20µg/kg body weight) of ring-deuterated DEHP (DEHP-D(4)). Concentrations of DEHP-D(4), of free ring-deuterated MEHP (MEHP-D(4)), and the sum of free and glucuronidated MEHP-D(4) were measured in blood for up to 24h; amounts of the monoesters MEHP-D(4), ring-deuterated mono(2-ethyl-5-hydroxyhexyl) phthalate and ring-deuterated mono(2-ethyl-5-oxohexyl) phthalate were determined in urine for up to 46h after ingestion. The bioavailability of DEHP-D(4) was surprisingly high with an area under the concentration-time curve until 24h (AUC) amounting to 50% of that of free MEHP-D(4). The AUC of free MEHP-D(4) normalized to DEHP-D(4) dose and body weight (AUC/D) was 2.1 and 8.1 times, that of DEHP-D(4) even 50 and 100 times higher than the corresponding AUC/D values obtained earlier in rat and marmoset, respectively. Time courses of the compounds in blood and urine of the volunteers oscillated widely. Terminal elimination half-lives were short (4.3-6.6h). Total amounts of metabolites in 22-h urine are correlated linearly with the AUC of free MEHP-D(4) in blood, the parameter regarded as relevant for risk assessment.

Styrene-7,8-oxide burden in ventilated, perfused lungs of mice and rats exposed to vaporous styrene.

Styrene (ST) is an important industrial chemical. In long-term inhalation studies, ST-induced lung tumors in mice but not in rats. To test the hypothesis that the lung burden by the reactive metabolite styrene-7,8-oxide (SO) would be most relevant for the species-specific tumorigenicity, we investigated the SO burden in isolated lungs of male Sprague-Dawley rats and in-situ prepared lungs of male B6C3F1 mice ventilated with air containing vaporous ST and perfused with a modified Krebs-Henseleit buffer (37 degrees C). Styrene vapor concentrations were determined in air samples collected in the immediate vicinity of the trachea. They were almost constant during each experiment. Styrene exposures ranged from 50 to 980 ppm (rats) and from 40 to 410 ppm (mice). SO was quantified from the effluent perfusate. Lungs of both species metabolized ST to SO. After a mathematical translation of the ex-vivo data to ventilation and perfusion conditions as they are occurring in vivo, a species comparison was carried out. At ST concentrations of up to 410 ppm, mean SO levels in mouse lungs ranged up to 0.45 nmol/g lung, about 2 times higher than in rat lungs at equal conditions of ST exposure. We conclude that the species difference in the SO lung burden is too small to consider the genotoxicity of SO as sufficient for explaining the fact that only mice developed lung tumors when exposed to ST. Another cause is considered as driving force for lung tumor development in the mouse.

Propylene oxide in blood and soluble nonprotein thiols in nasal mucosa and other tissues of male Fischer 344/N rats exposed to propylene oxide vapors--relevance of glutathione depletion for propylene oxide-induced rat nasal tumors.

High concentrations of propylene oxide (PO) induced inflammation in the respiratory nasal mucosa (RNM) of rodents. Concentrations > or =300 ppm caused nasal tumors. In order to investigate if glutathione depletion could be relevant for these effects, we determined in PO exposed male Fischer 344/N rats PO in blood and soluble nonprotein SH-groups (NPSH) in RNM and other tissues. Rats were exposed once (6 h) to PO concentrations between 0 and 750 ppm, and repeatedly for up to 20 days (6 h, 5 days/week) to concentrations between 0 and 500 ppm. At the end of the exposures, PO in blood and NPSH in tissues were determined. PO in blood was dependent on concentration and duration of exposure. After the 1-day exposures, NPSH depletion was most distinctive (RNM > liver > lung). Compared to controls, NPSH levels were 43% at 50 ppm PO in RNM and 16% at > or =300 ppm in both RNM and liver. Lung NPSH fell linearly to 20% at 750 ppm. After repeated exposures over 3 and 20 days to 5, 25, 50, 300, and 500 ppm, NPSH losses were less pronounced. At both time points, NPSH were 90%, 70%, 50%, 30%, and 30% of the control values in RNM. Liver NPSH decreased to 80% and 50% at 300 and 500 ppm, respectively. After 20 days, lung NPSH declined to 70% (300 ppm) and 50% (500 ppm). We conclude that continuous, severe perturbation of GSH in RNM following repeated high PO exposures may lead to inflammatory lesions and cell proliferation, critical steps on the path towards tumorigenicity.

Novel and existing data for a future physiological toxicokinetic model of ethylene and its metabolite ethylene oxide in mouse, rat, and human.

The olefin ethylene is a ubiquitously found gas. It originates predominantly from plants, combustion processes and industrial sources. In mammals, inhaled ethylene is metabolized by cytochrome P450-dependent monooxygenases, particularly by cytochrome P450 2E1, to ethylene oxide, an epoxide that directly alkylates proteins and DNA. Ethylene oxide was mutagenic in vitro and in vivo in insects and mammals and carcinogenic in rats and mice. A physiological toxicokinetic model is a most useful tool for estimating the ethylene oxide burden in ethylene-exposed rodents and humans. The only published physiological toxicokinetic model for ethylene and metabolically produced ethylene oxide is discussed. Additionally, existing data required for the development of a future model and for testing its predictive accuracy are reviewed and extended by new gas uptake studies with ethylene and ethylene oxide in B6C3F1 mice and with ethylene in F344 rats.

Ethylene oxide in blood of ethylene-exposed B6C3F1 mice, Fischer 344 rats, and humans.

The gaseous olefin ethylene (ET) is metabolized in mammals to the carcinogenic epoxide ethylene oxide (EO). Although ET is the largest volume organic chemical worldwide, the EO burden in ET-exposed humans is still uncertain, and only limited data are available on the EO burden in ET-exposed rodents. Therefore, EO was quantified in blood of mice, rats, or 4 volunteers that were exposed once to constant atmospheric ET concentrations of between 1 and 10 000 ppm (rodents) or 5 and 50 ppm (humans). Both the compounds were determined by gas chromatography. At ET concentrations of between 1 and 10 000 ppm, areas under the concentration-time curves of EO in blood (µmol × h/l) ranged from 0.039 to 3.62 in mice and from 0.086 to 11.6 in rats. At ET concentrations ≤ 30 ppm, EO concentrations in blood were 8.7-fold higher in rats and 3.9-fold higher in mice than that in the volunteer with the highest EO burdens. Based on measured EO concentrations, levels of EO adducts to hemoglobin and lymphocyte DNA were calculated for diverse ET concentrations and compared with published adduct levels. For given ET exposure concentrations, there were good agreements between calculated and measured levels of adducts to hemoglobin in rats and humans and to DNA in rats and mice. Reported hemoglobin adduct levels in mice were higher than calculated ones. Furthermore, information is given on species-specific background adduct levels. In summary, the study provides most relevant data for an improved assessment of the human health risk from exposure to ET.

Kinetics of ethylene and ethylene oxide in subcellular fractions of lungs and livers of male B6C3F1 mice and male fischer 344 rats and of human livers.

Ethylene (ET) is metabolized in mammals to the carcinogenic ethylene oxide (EO). Although both gases are of high industrial relevance, only limited data exist on the toxicokinetics of ET in mice and of EO in humans. Metabolism of ET is related to cytochrome P450-dependent mono-oxygenase (CYP) and of EO to epoxide hydrolase (EH) and glutathione S-transferase (GST). Kinetics of ET metabolism to EO and of elimination of EO were investigated in headspace vessels containing incubations of subcellular fractions of mouse, rat, or human liver or of mouse or rat lung. CYP-associated metabolism of ET and GST-related metabolism of EO were found in microsomes and cytosol, respectively, of each species. EH-related metabolism of EO was not detectable in hepatic microsomes of rats and mice but obeyed saturation kinetics in hepatic microsomes of humans. In ET-exposed liver microsomes, metabolism of ET to EO followed Michaelis-Menten-like kinetics. Mean values of V(max) [nmol/(min·mg protein)] and of the apparent Michaelis constant (K(m) [mmol/l ET in microsomal suspension]) were 0.567 and 0.0093 (mouse), 0.401 and 0.031 (rat), and 0.219 and 0.013 (human). In lung microsomes, V(max) values were 0.073 (mouse) and 0.055 (rat). During ET exposure, the rate of EO production decreased rapidly. By modeling a suicide inhibition mechanism, rate constants for CYP-mediated catalysis and CYP inactivation were estimated. In liver cytosol, mean GST activities to EO expressed as V(max)/K(m) [µl/(min·mg protein)] were 27.90 (mouse), 5.30 (rat), and 1.14 (human). The parameters are most relevant for reducing uncertainties in the risk assessment of ET and EO.

1,2:3,4-Diepoxybutane in blood of male B6C3F1 mice and male Sprague-Dawley rats exposed to 1,3-butadiene.

The important industrial chemical 1,3-butadiene (BD; CAS Registry Number: 106-99-0) is a potent carcinogen in B6C3F1 mice and a weak one in Sprague-Dawley rats. This difference is mainly attributed to the species-specific burden by the metabolically formed 1,2:3,4-diepoxybutane (DEB). However, only limited data exist on the DEB blood burden of rodents at BD concentrations below 100 ppm. Considering this, DEB concentrations were determined in the blood of mice and rats immediately after 6h exposures to various constant concentrations of BD of between about 1 and 1200 ppm. Immediately after its collection, blood was injected into a vial that contained perdeuterated DEB (DEB-D(6)) as internal standard. Plasma samples were prepared and treated with sodium diethyldithiocarbamate that derivatized metabolically produced DEB and DEB-D(6) to their bis(dithiocarbamoyl) esters, which were then analyzed by high performance liquid chromatography coupled with an electrospray ionization tandem mass spectrometer. DEB concentrations in blood versus BD exposure concentrations in air could be described by one-phase exponential association functions. Herewith calculated (±)-DEB concentrations in blood increased in mice from 5.4 nmol/l at 1 ppm BD to 1860 nmol/l at 1250 ppm BD and in rats from 1.2 nmol/l at 1 ppm BD to 92 nmol/l at 200 ppm BD, at which exposure concentration 91% of the calculated DEB plateau concentration in rat blood was reached. This information on the species-specific blood burden by the highly mutagenic DEB helps to explain why the carcinogenic potency of BD in rats is low compared to that in mice.

Quantitative investigation on the metabolism of 1,3-butadiene and of its oxidized metabolites in once-through perfused livers of mice and rats.

The industrial chemical 1,3-butadiene (BD) is a potent carcinogen in mice and a weak one in rats. This difference is generally related to species-specific burdens by the metabolites 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane (DEB), and 3,4-epoxy-1,2-butanediol (EBD), which are all formed in the liver. Only limited data exist on BD metabolism in the rodent liver. Therefore, metabolism of BD, its epoxides, and the intermediate 3-butene-1,2-diol (B-diol) was studied in once-through perfused livers of male B6C3F1 mice and Sprague-Dawley rats. In BD perfusions, predominantly EB and B-diol were found (both species). DEB and EBD were additionally detected in mouse livers. Metabolism of BD showed saturation kinetics (both species). In EB perfusions, B-diol, EBD, and DEB were formed with B-diol being the major metabolite. Net formation of DEB was larger in mouse than in rat livers. In both species, hepatic clearance (Cl(H)) of EB was slightly smaller than the perfusion flow. In DEB perfusions, EBD was formed as a major metabolite. Cl(H) of DEB was 61% (mouse) and 73% (rat) of the perfusion flow. In the B-diol-perfused rat liver, EBD was formed as a minor metabolite. Cl(H) of B-diol was 53% (mouse) and 34% (rat) of the perfusion flow. In EBD-perfused rat livers, Cl(H) of EBD represented only 22% of the perfusion flow. There is evidence for qualitative species differences with regard to the enzymes involved in BD metabolism. The first quantitative findings in whole livers showing intrahepatic first-pass metabolism of BD and EB metabolites will improve the risk estimation of BD.

Concentrations of the propylene metabolite propylene oxide in blood of propylene-exposed rats and humans--a basis for risk assessment.

Propylene (PE) was not carcinogenic in long-term studies in rodents. However, its biotransformation to propylene oxide (PO) raises questions about a carcinogenic risk. PO alkylates macromolecules, is a direct mutagen, and caused tumors in rodents at high concentrations. In order to acquire knowledge on the species-specific PO concentrations in blood resulting from PE exposure, we exposed male Fischer 344/N rats in closed exposure chambers to constant PE concentrations, between 20.1 and 3000 ppm (7 h at least), and four male volunteers to mean constant PE concentrations of 9.82 and 23.4 ppm (180 min) in inhaled air. In the animal experiments, PE and PO were measured in the chamber atmosphere, PE by gas chromatography with flame ionization detection (GC/FID), PO by GC/FID or GC with mass-selective detection (GC/MSD). In the human studies, PE was measured in inhaled and exhaled air by GC/FID. PO was quantified by GC/MSD from exhaled breath collected in gasbags. Blood concentrations of PO were calculated based on the measured PO concentrations in air using the blood-to-air partition coefficients of 60 (rat) and 66 (human). In rats, PO blood concentrations ranged from 53 nmol/l at 20.1 ppm PE to 1750 nmol/l at 3000 ppm PE. In humans, mean blood concentrations of PO were 0.44 and 0.92 nmol/l at mean PE concentrations of 9.82 and 23.4 ppm, respectively. These findings should be taken into consideration when estimating the carcinogenic risk of PE to humans based on carcinogenicity studies in PE- or PO-exposed rats.