Involvement of reversible binding to alpha 2u-globulin in 1,4-dichlorobenzene-induced nephrotoxicity

Adverse outcome pathway (AOP): α2u-globulin nephropathy and kidney tumors in male rats

The National Research Council's vision of using adverse outcome pathways (AOPs) as a framework to assist with toxicity assessment for regulatory requirements of chemical assessment has continued to gain traction since its release in 2007. The need to expand the AOP knowledge base has gained urgency, with the U.S. Environmental Protection Agency's directive to eliminate reliance on animal toxicity testing by 2035. To meet these needs, our goal was to elucidate the AOP for male-rat-specific kidney cancer. Male-rat-specific kidney tumors occur through the ability of structurally diverse substances to induce α2u-globulin nephropathy (α2u-N), a well-studied mode of action (MoA) not relevant in humans that results in kidney tumor formation in male rats. An accepted AOP may help facilitate the differentiation from other kidney tumors MoAs. Following identification and review of relevant in vitro and in vivo literature, both the MIE and subsequent KEs were identified. Based on the weight of evidence from the various resources, the confidence in this AOP is high. Uses of this AOP include hazard identification, development of in vitro assays to determine if the MoA is through α2u-N and not relevant to humans resulting in decreased use of animals, and regulatory applications.

Reassessment of MTBE cancer potency considering modes of action for MTBE and its metabolites

A 1999 California state agency cancer potency (CP) evaluation of methyl tert-butyl ether (MTBE) assumed linear risk extrapolations from tumor data were plausible because of limited evidence that MTBE or its metabolites could damage DNA, and based such extrapolations on data from rat gavage and rat and mouse inhalation studies indicating elevated tumor rates in male rat kidney, male rat Leydig interstitial cells, and female rat leukemia/lymphomas. More recent data bearing on MTBE cancer potency include a rodent cancer bioassay of MTBE in drinking water; several new studies of MTBE genotoxicity; several similar evaluations of MTBE metabolites, formaldehyde, and tert-butyl alcohol or TBA; and updated evaluations of carcinogenic mode(s) of action (MOAs) of MTBE and MTBE metabolite's. The lymphoma/leukemia data used in the California assessment were recently declared unreliable by the U.S. Environmental Protection Agency (EPA). Updated characterizations of MTBE CP, and its uncertainty, are currently needed to address a variety of decision goals concerning historical and current MTBE contamination. To this end, an extensive review of data sets bearing on MTBE and metabolite genotoxicity, cytotoxicity, and tumorigenicity was applied to reassess MTBE CP and related uncertainty in view of MOA considerations. Adopting the traditional approach that cytotoxicity-driven cancer MOAs are inoperative at very low, non-cytotoxic dose levels, it was determined that MTBE most likely does not increase cancer risk unless chronic exposures induce target-tissue toxicity, including in sensitive individuals. However, the corresponding expected (or plausible upper bound) CP for MTBE conditional on a hypothetical linear (e.g., genotoxic) MOA was estimated to be ∼2 × 10(-5) (or 0.003) per mg MTBE per kg body weight per day for adults exposed chronically over a lifetime. Based on this conservative estimate of CP, if MTBE is carcinogenic to humans, it is among the weakest 10% of chemical carcinogens evaluated by EPA.

Lipid peroxidation and changes of trace elements in mice treated with paradichlorobenzene

Paradichlorobenzene (pDCB) has been used as a space deodorant and moth repellant, as well as an intermediate in the chemical industry. Given its broad applications and high volatility, considerable concern exists regarding the adverse health effects of pDCB in the home and the workplace. In this study, changes in lipid peroxidation, antioxidants, and trace element levels in the liver and kidney of pDCB-treated mice were investigated to determine their roles in toxicity. Mice were orally gavaged once daily for seven consecutive days with pDCB (0 (corn oil control), 450, and 900 mg/kg). The level of malondialdehyde (MDA), an end product of lipid peroxidation, markedly increased in the high-dose pDCB group in both the liver and kidney compared with the control group. Changes in hepatic levels of reduced glutathione (GSH) in the pDCB groups were indistinguishable from the control group, while renal levels of reduced GSH in the high-dose pDCB group were significantly lowered in comparison to the control and the low-dose groups. Superoxide dismutase (SOD) activity in the liver of mice treated with pDCB showed a downward trend, whereas there was no consistent trend associated with changes in SOD activity in the kidney. Additionally, renal iron levels in the high-dose pDCB group were significantly decreased compared with the low-dose group and the controls, whereas hepatic iron content in the low-dose pDCB group was significantly lower compared with the controls. Selenium and zinc levels in the kidney were both significantly decreased in the high-dose pDCB group vs. the control and low-dose groups. There were no treatment-induced changes in copper levels in either the kidney or liver. However, a significant increase was found in the liver zinc/copper ratio in the high-dose pDCB group vs. the controls. In addition, blood zinc levels showed a downward trend with increased pDCB dosage. These results suggest that pDCB toxicity is mediated by oxidative damage and tissue-specific alterations in trace element levels both in the liver and the kidney of mice.

Physiologically based pharmacokinetic model of methyl tertiary butyl ether and tertiary butyl alcohol dosimetry in male rats based on binding to alpha2u-globulin

Current physiologically based pharmacokinetic (PBPK) models for the fuel additive methyl tertiary butyl ether (MTBE) and its metabolite tertiary butyl alcohol (TBA) have not included a mechanism for chemical binding to the male rat-specific protein alpha2u-globulin, which has been postulated to be responsible for renal effects in male rats observed in toxicity and carcinogenicity studies with MTBE. The objective of this work was to expand the previously published models for MTBE to include binding to alpha2u-globulin in the kidney of male rats. In the model, metabolism of MTBE was assumed to occur only in the liver via two saturable pathways. TBA metabolism was assumed to occur only in the liver via one saturable, low-affinity pathway and to be inducible following repeated exposures. The binding of MTBE and TBA to alpha2u-globulin was modeled as saturable and competitive and was assumed to only affect the rate of hydrolysis of alpha2u-globulin in the kidney. The developed model characterized the differences in kidney concentrations of MTBE and TBA in male versus female rats from inhalation exposures to MTBE, as well as the observed changes in blood and tissue concentrations from repeated exposure to TBA. The model-predicted binding affinity of MTBE to alpha2u-globulin was greater than TBA, and the hydrolysis rate of chemically bound alpha2u-globulin was approximately 30% of the unbound protein. This PBPK model supports the role of MTBE and TBA binding to the male rat-specific protein alpha2u-globulin as essential for predicting concentrations of these chemicals in the kidney following exposure.

A mechanism-based cancer risk assessment for 1,4-dichlorobenzene

1,4-Dichlorobenzene (DCB) induced liver cancer in male and female B6C3F(1) mice in a gavage bioassay and in male and female BDF(1) mice in an inhalation bioassay. The weight of the evidence convincingly indicates that the mouse liver tumors induced by 1,4-DCB were via a nongenotoxic-mitogenic/promotional mode of action by forcing the growth of spontaneous precancerous lesions. Doses insufficient to exhibit mitogenic or promotional activity would not be expected to increase the risk of cancer. Benchmark dose modeling of the tumor response was conduced for the combined inhalation and oral gavage bioassay data sets based on an absorbed dose basis to establish the dose or airborne concentration corresponding to 1% extra risk. Assuming that as a point of departure and dividing by an uncertainty factor of 300, yielded a value of 0.1 ppm, representing a rational estimate of an airborne concentration for the human population below which there is unlikely to be any increased risk of cancer during a lifetime. In contrast, the default model that assumes a genotoxic mode of action estimates a one in one-million increased lifetime risk of cancer at an airborne concentration of 0.00004 ppm, some 2500-fold lower than the mechanism-based model and 1,875,000-fold lower than the no observed effect concentration for induced cancer of 75 ppm in the inhalation bioassay.

A comparative human health risk assessment of p-dichlorobenzene-based toilet rimblock products versus fragrance/surfactant-based alternatives

A comparison of the human health risk to consumers using one of two types of toilet rimblock products, either a p-dichlorobenzene-based rimblock or two newer fragrance/surfactant-based alternatives, was conducted. Rimblock products are designed for global use by consumers worldwide and function by releasing volatile compounds into indoor air with subsequent exposure presumed to be mainly by inhalation of indoor air. Using the THERdbASE exposure model and experimentally determined emission data, indoor air concentrations and daily intake values were determined for both types of rimblock products. Modeled exposure concentrations from a representative p-dichlorobenzene rimblock product are an order of magnitude higher than those from the alternative rimblock products due to its nearly pure composition and high sublimation rate. Lifetime exposure to p-dichlorobenzene or the subset of fragrance components with available RfD values is not expected to lead to non-cancer-based adverse health effects based on the exposure concentrations estimated using the THERdbASE model. A similar comparison of cancer-based effects was not possible as insufficient data were available for the fragrance components.

14C-labeled pulegone and metabolites binding to alpha2u-globulin in kidneys of male F-344 rats

Pulegone is a major constituent of pennyroyal oil and a minor component of peppermint oil. Pulegone is biotransformed to menthofuran and menthones (diastereomeric menthone and isomenthone) in pennyroyal and peppermint as well as in rodents. Pulegone and menthofuran are hepatotoxic to rodents, and menthones are less toxic. The metabolism and disposition of pulegone and menthofuran were previously studied in rodents, and higher concentrations of pulegone- and menthofuran-derived radioactivity were observed in male than female rat kidney. One explanation is the association of pulegone and metabolites with a male rat-specific protein, alpha2u-globulin. To test this hypothesis, male and female rats were dosed orally with 14C-labeled pulegone (80 mg/kg, 120 microCi/kg) or menthofuran (60 mg/kg, 120 microCi/kg) or menthones (80 mg/kg, 120 microCi/kg) in corn oil, and the kidney cytosol was prepared 24 h after dosing. An equilibrium dialysis experiment showed that in all three studies the radioactivity was associated with kidney cytosol proteins of male but not female rats. The chemicals present in the male rat kidney cytosol after dialysis were extracted with dichloromethane and characterized by high-performance liquid chromatography (HPLC) and gas chromatography/mass spectrometry (GC-MS). All parent compounds were detected, and the metabolites characterized included piperitone from pulegone or menthones treatment, menthones and possibly 8-hydroxymenthones from pulegone treatment, and mintlactones (diastereomeric mintlactone and isomintlactone) and 7a-hydroxymintlactone from menthofuran treatment. Analysis of the male rat kidney cytosol by a gel filtration column demonstrated that the retention was due to reversible binding of these chemicals with the male rat-specific protein alpha2u-globulin. However, binding of pulegone and/or metabolites to alpha2u-globulin did not produce accumulation of this protein in the kidney.

Metabolism and disposition of 2,2',4,4',5-pentabromodiphenyl ether (BDE99) following a single or repeated administration to rats or mice

The metabolism and disposition of 14C-labelled 2,2',4,4',5-pentabromodiphenyl ether (BDE99) were studied in F344 rats and B6C3F1 mice. Approximately 85% of a 1 micromol kg-1 oral dose was absorbed by male rats and mice. Within 24 h following oral doses ranging from 0.1 to 1000 micromol kg-1 to rats, 39-47% of the dose was excreted in the faeces (including 16% unabsorbed), up to 2% was excreted in the urine, and 34-38% remained in the tissues, mostly in adipose tissue. Mice excreted more in the urine and less in the faeces than rats. Tissue accumulation was observed following repeated dosing to rats. Two dihydrohydroxy-S-glutathionyl and two S-glutathionyl conjugates of BDE99, 2,4,5-tribromophenol glucuronide, two mono-hydroxylated BDE99 glucuronides, and three mono-hydroxylated tetrabromodiphenyl ether glucuronides were identified in male rat bile. 2,4,5-Tribromophenol and its glucuronide and sulfate conjugates, were identified in male rat urine. 2,4,5-Tribromophenol, one mono-hydroxylated tetrabromodiphenyl ether, and two mono-hydroxylated BDE99 were characterized in male rat faeces. BDE99 undergoes more extensive metabolism than does BDE47. Half of the absorbed oral dose in male rats was excreted in 10 days mostly as metabolites derived from arene oxide intermediates.

Semi-quantitative immunohistochemical analysis of male rat-specific alpha2u-globulin accumulation for chemical toxicity evaluation

We purified male rat urinary alpha(2u)-globulin, prepared the antibody in rabbits, and improved an immunohistochemical detection method using this antibody for male rat-specific alpha(2u)-globulin accumulation appearing as hyaline droplets in the kidneys. Our prepared antibody reacted specifically with alpha(2u)-globulin in both immunohistochemical and Western blotting analyses, furthermore, and the graded immuno-reactivities on the slide were well associated with computational image analyzing results. Using this method, we retrospectively analyzed the renal sections from the toxicity studies of 12 nephrotoxic chemicals, which had already been conducted under the Japanese Existing Chemicals Survey Program. We demonstrated that the hyaline droplets induced by treatment with 10 chemicals (1,4-dibromobenzene, dicyclopentadiene, 3,4-dimethylaniline, 1,4-dicyanobenzene, tetrahydrothiophene-1,1-dioxide, 1,3-dicyanobenzene, acenaphthene, 3,4-dichloro-1-butene, 3a,4,7,7a-tetrahydro-1H-indene and 3,5,5-trimethylhexan-1-ol) were directly associated with alpha(2u)-globulin accumulation. This immunohistochemical method is convenient for applying, even retrospectively, paraffin sections from general toxicity studies and could be useful for qualifying male rat-specific hyaline droplets consisting of alpha(2u)-globulin and renal risk in humans.

Carcinogenicity and chronic toxicity in mice and rats exposed by inhalation to para-dichlorobenzene for two years

Carcinogenicity and chronic toxicity of para-dichlorobenzene (p-DCB) were examined by exposing 50 BDF1 mice and 50 F344 rats of both sexes by inhalation to p-DCB vapor at a target concentration of 0 (control), 20, 75 or 300 ppm for 6 hr/day, 5 days/week and 2 years. Incidences of hepatocellular carcinomas, hepatoblastomas and hepatic histiocytic sarcomas in the 300 ppm-exposed male mice, and hepatocellular adenomas and carcinomas and hepatoblastomas in the 300 ppm-exposed female mice were increased. An increase in the incidences of most of those liver tumors was dose-related. No increase in tumor incidence was found in any p-DCB-exposed rat of either sex. Centrilobular hypertrophy of hepatocytes and papillary mineralization and pelvic urothelial hyperplasia of the kidney were noted in the 300 ppm-exposed male rats. Treatment- and age-related increases in incidences of the eosinophilic globules of the respiratory and olfactory epithelia in female rats and incidences of the respiratory metaplasia of the nasal gland epithelium in mice and rats and the olfactory epithelium in mice were noted. The nasal lesion was the most sensitive endpoint of chronic inhalation toxicity. Induction of the mouse hepatocarcinogenicity and lack of the rat nephrocarcinogenicity found in the present study were compared with the mouse liver tumors and the rat renal tumors reported by the NTP gavage study, and discussed in light of the estimated p-DCB uptake into the body through the inhalation and the oral administration.

Carcinogenicity and chronic toxicity of 1,4-dichloro-2-nitrobenzene in rats and mice by two years feeding

Carcinogenicity and chronic toxicity of 1,4-dichloro-2-nitrobenzene (DCNB) were examined by feeding each group of 50 F344 rats and 50 BDF1 mice of both sexes a DCNB-containing diet at a concentration of 0 (control), 320, 800 or 2,000 ppm (w/w) for 2 yr. In rats, incidences of hepatocellular adenomas and carcinomas and their combined incidence were increased in the 2,000 ppm-fed males, together with increased incidence of basophilic cell foci in the 800 and 2,000 ppm-fed males. A dose-related increase in combined incidences of renal cell adenomas and carcinomas was noted. Incidence of Zymbal gland adenomas tended to increase in the 2,000 ppm-fed males. In mice, incidences of hepatocellular adenomas in the 800 and 2,000 ppm-fed females and hepatocellular carcinomas in the 2,000 ppm-fed males and in the 800 and 2,000 ppm-fed females were increased. Incidence of hepatoblastomas was increased in all DCNB-fed males and in the 2,000 ppm-fed females. Signs of chronic toxicity were characterized by centrilobular hypertrophy of hepatocytes with nuclear atypia in mice, increased relative liver weight in rats, a dose-related increase in incidences of chronic progressive nephropathy with advanced grades of severity in male rats, and decreased hemoglobin concentration and hematocrit accompanied by increased bone marrow hematopoiesis in female rats. Carcinogenic activity of DCNB was evaluated for the three different tumors, and sensitive signs of the chronic toxicity were dis-

Thirteen-week inhalation toxicity of p-dichlorobenzene in mice and rats

Subchronic inhalation toxicity of p-dichlorobenzene (p-DCB) was examined by exposing BDF1 mice and F344 rats of both sexes (6 h/d and 5 d/wk) to inhalation of 25, 55, 120, 270 or 600 ppm (v/v) p-DCB vapor for 13 wk. The exposure to p-DCB vapor retarded the growth rate in the male mice, and induced hepatotoxicity in the mice and rats of both sexes and renal and hematological toxicity in the male rats. Hepatotoxicity was characterized by increased liver weight, hepatocellular hypertrophy, and increased serum levels of total cholesterol. Liver necrosis and increased serum levels of AST and ALT were observed in the exposed mice, whereas these changes, which indicate hepatocellular death, did not occur in any of the exposed rats. p-DCB-induced renal lesions occurred only in the male rats. Hyaline droplets were observed in the proximal tubular epithelial cells, and were stained positively with anti-alpha2u-globulin, suggesting excessive accumulation of alpha2u-globulin in the epithelial cells. Granular casts were formed in the tubular lumen, resulting from the necrotic desquamation of the renal tubular epithelium. Papillary mineralization in the renal pelvis and increased serum levels of BUN and creatinine were noted. These renal changes indicated alpha2u-globulin nephropathy. Decreases in red blood cell counts, hemoglobin concentration, hematocrit and mean corpuscular volume and increased spleen weight occurred in the exposed male rats. The NOAEL was 120 ppm for the hepatic endpoint in mice and for the renal endpoint in rats. The maximum tolerated dose for a 2-yr bioassay inhalation study of rodent carcinogenicity was estimated to be 300 ppm, based on the present results.

Chemically induced renal tubule tumors in the laboratory rat and mouse: review of the NCI/NTP database and categorization of renal carcinogens based on mechanistic information

The incidence of renal tubule carcinogenesis in male and female rats or mice with 69 chemicals from the 513 bioassays conducted to date by the NCI/NTP has been collated, the chemicals categorized, and the relationship between carcinogenesis and renal tubule hyperplasia and exacerbation of the spontaneous, age-related rodent disease chronic progressive nephropathy (CPN) examined. Where information on mechanism or mode of action exists, the chemicals have been categorized based on their ability to directly or indirectly interact with renal DNA, or on their activity via epigenetic pathways involving either direct or indirect cytotoxicity with regenerative hyperplasia, or exacerbation of CPN. Nine chemicals were identified as directly interacting with DNA, with six of these producing renal tubule tumors at high incidence in rats of both sexes, and in some cases also in mice. Ochratoxin A was the most potent compound in this group, producing a high tumor incidence at very low doses, often with metastasis. Three chemicals were discussed in the context of indirect DNA damage mediated by an oxidative free radical mechanism, one of these being from the NTP database. A third category included four chemicals that had the potential to cause DNA damage following conjugation with glutathione and subsequent enzymatic activation to a reactive species, usually a thiol-containing entity. Two chemicals were allocated into the category involving a direct cytotoxic action on the renal tubule followed by sustained compensatory cell proliferation, while nine were included in a group where the cell loss and sustained increase in renal tubule cell turnover were dependent on lysosomal accumulation of the male rat-specific protein, alpha2mu-globulin. In a sixth category, morphologic evidence on two chemicals indicated that the renal tumors were a consequence of exacerbated CPN. For the remaining chemicals, there were no pertinent data enabling assignment to a mechanistic category. Accordingly, these chemicals, acting through an as yet unknown mechanism, were grouped as either being associated with an enhancement of CPN (category 7, 16 chemicals), or not associated with enhanced CPN (category 8, 4 chemicals). A ninth category dealt with 11 chemicals that were regarded as producing increases in renal tubule tumors that did not reach statistical significance. A 10th category discussed 6 chemicals that induced renal tumors in mice but not in rats, plus 8 chemicals that produced a low incidence of renal tubule tumors in mice that did not reach statistical significance. As more mechanistic data are generated, some chemicals will inevitably be placed in different groups, particularly those from categories 7 and 8. A large number of chemicals in the series exacerbated CPN, but those in category 7 especially may be candidates for inclusion in category 6 when further information is gleaned from the relevant NTP studies. Also, new data on specific chemicals will probably expand category 5 as cytotoxicity and cell regeneration are identified as obligatory steps in renal carcinogenesis in more cases. Additional confirmatory outcomes arising from this review are that metastases from renal tubule tumors, while encountered with chemicals causing DNA damage, are rare with those acting through an epigenetic pathway, with the exception being fumonisin B1; that male rats and mice are generally more susceptible than female rats and mice to chemical induction of renal tubule tumors; and that a background of atypical tubule hyperplasia is a useful indicator reflecting a chemically associated renal tubule tumor response. With respect to renal tubule tumors and human risk assessment, chemicals in categories 1 and 2, and possibly 3, would currently be judged by linear default methods; chemicals in category 4 (and probably some in category 3) as exhibiting a threshold of activity warranting the benchmark approach; and those in categories 5 and 6 as representing mechanisms that have no relevance for extrapolation to humans.

Dose-related changes of oxidative stress and cell proliferation in kidneys of male and female F344 rats exposed to potassium bromate

It is still of importance to investigate renal carcinogenesis by potassium bromate (KBrO3), a by-product of water disinfection by ozonation, for assessment of the risk to man. Five female F344 rats in each group were given KBrO3 at a dose of 300 mg/kg by single i.g. intubation or at a dose of 80 mg/kg by single i.p. injection, and were killed 48 h after the administration for measurements of thiobarbituric acid-reactive substances (TBARS) and 8-oxodeoxyguanosine (8-oxodG) levels in the kidney. Both levels in the treated animals were significantly elevated as compared with the control values. In a second experiment, 5 male and female F344 rats in each group were administered KBrO3 at concentrations of 0, 15, 30, 60, 125, 250 and 500 ppm in the drinking water for 4 weeks. KBrO3 in the drinking water did not elevate TBARS in either sex at any of the doses examined, but 8-oxodG formation in both sexes at 250 ppm and above was significantly higher than in the controls. Additionally, the bromodeoxyuridine-labeling index for proximal convoluted tubules was significantly increased at 30 ppm and above in the males, and at 250 ppm and above in the females. Alpha2u-globulin accumulation in the kidneys of male rats was increased with statistical significance at 125 ppm and above. These findings suggest that DNA oxidation induced by KBrO3 may occur independently of lipid peroxidation and more than 250 ppm KBrO3 in the drinking water can exert a carcinogenic effect by way of oxidative stress.

1,4-Dichlorobenzene-Induced liver tumors in the mouse: evaluation of the role of chlorohydroquinones

1,4-Dichlorobenzene (1,4-DCB) is a wide-spread environmental contaminant and well-described hepatotoxicant for rats and mice. The prolonged oral or inhalation exposure to 1,4-DCB is associated with an increased frequency of hepatic tumors in mice, but not in rats. Evidence is lacking of direct genotoxicity with 1,4-DCB or its metabolites, and no generally accepted mechanism has been found to account for the increased numbers of 1,4-DCB-induced hepatic tumors in mice. No information is available on the carcinogenic effects of 1,4-DCB in humans. Here we consider evidence that the biotransformation of 1,4-DCB to substituted hydroquinone species contributes to hepatic adenoma and carcinoma formation in mouse liver. This phenomenon has implications for human carcinogenesis.

Captan impairs CYP-catalyzed drug metabolism in the mouse

To investigate whether the fungicide captan impairs CYP-catalyzed drug metabolism in murine liver, kidney and lung, the modulation of the regio- and stereo-selective hydroxylation of testosterone, including 6beta-(CYP3A), 6alpha-(CYP2A1 and CYP2B1) and 16alpha-(CYP2B9) oxidations was studied. Specific substrates as probes for different CYP isoforms such as p-nitrophenol (CYP2E1), pentoxyresorufin (CYP2B1), ethoxyresorufin (CYP1A1), aminopyrine (CYP3A), phenacetin and methoxyresorufin (CYP1A2), and ethoxycoumarin (mixed) were also considered. Daily doses of captan (7.5 or 15 mg/kg b.w., i.p.) were administered to different groups of Swiss Albino CD1 mice of both sexes for 1 or 3 consecutive days. While a single dose of this fungicide did not affect CYP-machinery, repeated treatment significantly impaired the microsomal metabolism; in the liver, for example, a general inactivating effect was observed, with the sole exception of testosterone 2alpha-hydroxylase activity which was induced up to 8.6-fold in males. In vitro studies showed that the mechanism-based inhibition was related to captan metabolites rather than the parental compound. In the kidney, both CYP3A- and CYP1A2-linked monooxygenases were significantly induced (2-fold) by this pesticide. Accelerated phenacetin and methoxyresorufin metabolism (CYP1A2) was also observed in the lung. Data on CYP3A (kidney) and CYP1A2 (kidney and lung) induction were corroborated by Western immunoblotting using rabbit polyclonal anti-CYP3A1/2 and CYP1A1/2 antibodies. By means of electron spin resonance (EPR) spectrometry coupled to a spin-trapping technique, it was found that the recorded induction generates a large amounts of the anion radical superoxide (O*2-) either in kidney or lung microsomes. These findings suggest that alterations in CYP-associated activities by captan exposure may result in impaired (endogenous) metabolism as well as of coadministered drugs with significant implications for their disposition. The adverse outcomes associated to CYP changes (e.g. cotoxicity, comutagenicity and promotion) may also have harmful consequences.

Evaluation of the in vivo interaction of methyl tert-butyl ether with alpha2u-globulin in male F-344 rats

Methyl tert-butyl ether (MTBE), a fuel additive blended into unleaded gasoline, decreases emissions of selected air pollutants. Exposure to MTBE causes a low incidence of renal tumors in male, but not female, F-344 rats. A number of chemicals that cause male rat-specific renal tumors also cause a syndrome unique to male rats referred to as alpha2u-globulin (alpha2u) nephropathy (alpha2u-N). Previous investigations have demonstrated that MTBE exposure induces a mild accumulation of alpha2u in male F-344 rats. The objective of the present study was to determine if MTBE, or a metabolite of MTBE, interacts with alpha2u in male rats administered MTBE orally. Eleven-week-old male and female F-344 rats were administered 750 mg [14C]MTBE/kg body wt or an equivalent volume of 10% emulphor orally for 4 consecutive days. Although [14C]MTBE-treated male rats exhibited a statistically significant increase in renal alpha2u concentration, the total radioactivity recovered was similar in kidney samples from [14C]MTBE-treated male and female rats. Further analysis of kidney cytosol prepared from [14C]MTBE-treated rats revealed that a slightly greater percentage of radioactivity coeluted on a G-25 gel filtration column with the total protein fraction from male rats than from female rats. Gel filtration (Sephadex G-75 column) and anion exchange chromatography, however, did not demonstrate any coelution of MTBE-derived radioactivity with the low-molecular-weight protein fraction or alpha2u fraction, respectively, in kidney cytosol prepared from [14C]MTBE-treated male or female rats. Further experimentation using a sealed vial equilibration system demonstrated that d-limonene oxide, a chemical with a high affinity for alpha2u, displaced MTBE in male, but not female, rat kidney samples following administration of MTBE. These findings provide indirect evidence that MTBE interacts with a male-specific protein such as alpha2u in male F-344 rats. Since the pathogenesis of alpha2u-N is dependent on the formation of a reversibly bound chemical-alpha2u complex, demonstration of an in vivo interaction of MTBE or one of its metabolites with alpha2u supports the alpha2u mechanism as a cause of MTBE-induced protein droplet nephropathy in male rats.

An evaluation of the carcinogenic hazard of 1,4-dichlorobenzene based on internationally recognized criteria

1,4-Dichlorobenzene (1,4-DCB) was shown to induce the formation of male rat renal tubule tumors and male and female mouse liver tumors when administered in a chronic bioassay. Since the original carcinogenicity findings, an extensive body of mechanistic information has been developed to elucidate the mode of action by which 1,4-DCB induces these effects and to evaluate the human relevance of the observed animal tumors. In addition, some regulatory and authoritative bodies (U.S. EPA and IARC) have developed rigorous scientific criteria for the amount and types of evidence needed to establish that a material causes kidney toxicity and tumors in male rats through a specific mechanism, alpha-2u-globulin nephropathy. This paper summarizes the mechanistic data developed for 1,4-DCB, which affords an understanding of the lack of human relevance of the male rat renal tubule tumors and mouse liver tumors; assesses that mechanistic data set utilizing the defined set of evaluation criteria formulated by U.S. EPA and IARC for alpha-2u-globulin nephropathy; and discusses the predictive power of mechanistic data developed to elucidate the mode of action of 1,4-DCB in inducing mouse liver tumors. Finally, there is a discussion of how some, but not all, regulatory and authoritative bodies have incorporated this substantial mechanistic data set for 1, 4-DCB into their cancer hazard evaluations and concluded that 1, 4-DCB presents little, if any, cancer hazard to humans.

Direct lysosomal uptake of alpha 2-microglobulin contributes to chemically induced nephropathy

BACKGROUND

An abnormal accumulation of alpha 2-microglobulin (alpha 2 mu) in kidney lysosomes of male rats has been described in the nephropathy resulting from exposure to a variety of chemicals. The increment in lysosomal levels of alpha 2 mu cannot be explained by a decrease in its proteolytic susceptibility. Because a portion of alpha 2 mu resides in the cytosol of kidney cells, we decided to analyze whether this cytosolic form also contributes to the abnormal lysosomal accumulation of alpha 2 mu after exposure to chemicals.

METHODS

Intact kidney lysosomes were isolated from untreated or 2,2,4-trimethylpentane (TMP) treated rats, and their ability to take up alpha 2 mu was compared.

RESULTS

alpha 2 mu can be directly transported into isolated lysosomes in the presence of the heat shock cognate protein of 73 kDa (hsc73). alpha 2 mu specifically binds to a lysosomal membrane glycoprotein of 96 kDa, previously identified as the receptor for the hsc73-mediated lysosomal pathway of protein degradation. In rats exposed to TMP, the specific lysosomal transport of alpha 2 mu increases, as well as the ability of lysosomes to directly transport other substrates for this pathway. The increased lysosomal transport is mainly due to an increase in the levels of the receptor protein in the lysosomal membrane.

CONCLUSIONS

The hsc73-mediated lysosomal pathway contributes to the normal degradation of alpha 2 mu in rat kidney and liver, and the activity of this pathway is increased after exposure to TMP. Our results suggest that the chemically induced accumulation of cytosolic alpha 2 mu in lysosomes is mediated by an increased rate of direct uptake into lysosomes.

Time course of enzyme induction in liver and kidneys and absorption, distribution and elimination of 1,4-dichlorobenzene in rats

Time course of enzyme induction was measured in Fischer344 rats treated daily at 150 and 600 mg 1,4-dichlorobenzene (1.4-DCB)/kg p.o. up to 28 days. The monoxygenases 7-ethoxycoumarin O-deethylase (ECOD), 7-ethoxyresorufin O-deethylase (EROD) and aldrin epoxidase (ALD) as well as the phase II enzymes; epoxide hydrolase (EH), glutathione S-transferase (GS-T) and glucuronyl transferase (GLU-T) were dose-dependently induced in the liver of males and females. A pronounced induction in the kidneys was measured at 600 mg/kg only for ECOD. After single oral administration of 100 and 1000 mg/kg bw and feeding of 100 and 1000 ppm (corresponding to approximately 10 and 100 mg/kg bw) to male Wistar rats for 28 days, the time course of 1,4-DCB and 2,5-DCP concentrations was investigated in plasma, adipose, hepatic and renal tissue. In addition, total urinary excretion of 2,5-DCP was determined. After single application, 1,4-DCB and 2,5-DCP were rapidly eliminated from the plasma and tissues, 40-60% of the dose administered was excreted as 2,5-DCP in the urine. There were no indications of cumulative effects after a feeding period of 28 days. The concentrations decreased in all tissues until the 7th day of study. Thereafter, there seems to be a steady state until the 28th day. A total of 7 days after the end of exposure, no more residues could be detected. Following long-term inhalation (450 and 3000 mg/m3) 1,4-DCB concentrations were highest in adipose tissues at 6 months followed by a marked decline at 18 months. 1,4-DCB and 2,5-DCP concentrations in plasma and liver were much lower but again with a peak at 6 months. When compared with published human data on measurements in plasma, urine, liver and adipose tissue the results suggest that there should be no hazard for the general population.

In vitro uptake of methyl tert-butyl ether in male rat kidney: use of a two-compartment model to describe protein interactions

Methyl tert-butyl ether (MTBE) is a gasoline additive that causes renal tumors in male rats. In the process of measuring chemical specific parameters necessary to develop a quantitative dosimetry model of MTBE in rats, the uptake of MTBE was found to be 5.5 times greater in male than in female F-344 rat kidney homogenate. The objectives of this study were to characterize the factor(s) that influences the high uptake of MTBE into male rat kidney in vitro and to develop a system to evaluate the interaction of MTBE with the male rat-specific protein, alpha 2u-globulin (alpha 2u). The uptake of MTBE in male, but not female, rat kidney homogenate was found to be dependent on protein and chemical concentrations. When [14C]MTBE was incubated with male rat kidney homogenate, radioactivity coeluted with the total protein fraction on a gel filtration column. An interaction between [14C]MTBE and male rat kidney proteins was not found under conditions of dialysis or anion exchange chromatography. A two-compartment vial equilibration model was used to assess the interaction between MTBE and alpha 2u. Using this system, the dissociation constant for MTBE and alpha 2u was estimated to be 2.15 x 10(-4) M, which is in the range of other chemicals known to bind to alpha 2u and cause alpha 2u-mediated nephropathy. d-Limonene oxide was used to validate this two-compartment vial equilibration system. These findings illustrate a technique useful in estimating the dissociation constant for a volatile chemical and a protein, as well as explain the process that contributes to the uptake of MTBE into male rat kidney homogenate in vitro. A description of the weak interaction between MTBE and alpha 2u will be used to refine a physiologically based pharmacokinetic model to describe the target tissue (kidney) concentrations of MTBE.

Species and strain differences in the hepatic cytochrome P450-mediated biotransformation of 1,4-dichlorobenzene

Our goal was to characterize possible species and strain differences in the hepatic microsomal biotransformation of 1,4-dichlorobenzene (1,4-DCB). Experiments compared extent of labeled 1,4-DCB conversion to oxidized metabolites, glutathione conjugates, and covalently bound metabolites by hepatic microsomes from humans, from male B6C3F1 mice, and from males of three rat strains (Fischer 344, Sprague-Dawley (SD), and Wistar). These rodents were selected for comparison because of their dissimilar responses to 1,4-DCB, notably, hepatocarcinogenicity in the B6C3F1 mouse but not the Wistar or Fischer rat, and nephrotoxicity and carcinogenicity in the Fischer rat. The species rank order for total in vitro conversion of 1,4-DCB was mouse > rat >> human. Conversion by microsomes from Fischer and Wistar rats was similar, whereas SD rats showed less biotransformation than the other two strains. Microsomes from the mouse produced most of the reactive metabolites as indicated by covalent binding to macromolecules (>20% of total metabolites formed). This covalent binding by mouse microsomes was extensively inhibited by ascorbic acid (AA), with a concomitant increase in hydroquinone formation, indicating an important role for benzoquinones as reactive metabolites. Phenobarbital pretreatment of rats enhanced the in vitro conversion of 1,4-DCB and the amount of covalent binding. Covalent binding for all rat microsomes was partly (33-79%) inhibited by AA. Addition of glutathione (GSH) plus AA further diminished the covalent binding with concomitant increased formation of the GSH-conjugated epoxide. Human microsomes produced the least reactive metabolites, with the majority (>70%) of this covalent binding prevented by GSH addition. The observed species differences, notably the more pronounced biotransformation of 1,4-DCB to reactive species including benzoquinones, could be factors in this compound's liver carcinogenicity in B6C3F1 mice but not other rodent species.

Methyl tert-butyl ether causes alpha2u-globulin nephropathy and enhanced renal cell proliferation in male Fischer-344 rats

Methyl tert-butyl ether (MTBE), a fuel additive blended into unleaded gasoline to decrease carbon monoxide emissions, induces renal tumors in male, but not female, rats exposed by inhalation to > or =3000 ppm MTBE. A number of chemicals that induce male rat-specific renal tumors also cause a syndrome unique to male rats referred to as alpha2u-globulin nephropathy (alpha2u-N). The objective of the present study was to determine if MTBE induces an alpha2u-N and renal cell proliferation in male F-344 rats. Male and female F-344 rats were exposed to MTBE vapors of 0, 413, 1516, or 3013 ppm for 6 hr/day for 10 consecutive days. Significant proximal tubule necrosis and protein droplet accumulation were observed in kidneys from male rats exposed to 1516 and 3013 ppm MTBE. Significantly greater labeling indices were observed in all groups of MTBE-exposed male rats. alpha2u-Globulin immunoreactivity was present in and confined to protein droplets in male rat kidney. A mild dose-related increase in alpha2u concentration in the kidney, as measured by an enzyme-linked immunosorbent assay, was observed in male rats exposed to MTBE, with a statistically significant increase in alpha2u concentration in male rats exposed to 3013 ppm MTBE. There was a strong positive correlation (r = 0.994) with exposure concentration between cell proliferation and alpha2u concentration in male rat kidney. No significant differences were observed in female rats for any of these responses. Further analysis of kidney cytosol failed to demonstrate the accumulation of any protein besides alpha2u in MTBE-exposed male rat kidney. These findings demonstrate that MTBE causes a mild induction of alpha2u-N and enhanced renal cell proliferation in male, but not female, F-344 rats, suggesting a role for alpha2u-N in renal tumorigenesis.

1,3,5-Trinitrobenzene-induced alpha-2u-globulin nephropathy

Male and female Fischer-344 (F-344) and male NCI-Black-Reiter (NBR) rats were dosed with 0, 35.5, or 71 mg 1,3,5-trinitrobenzene (TNB)/kg/day for 10 days. Male F-344 rats were dosed with TNB (0 and 35.5 mg/kg) for 20 and 30 days. Hematoxylin and eosin and Mallory-Heidenhain stains and alpha-2u-globulin and proliferating cell nuclear antigen immunohistochemical stains were performed on kidney sections. All treated male F-344 rats exhibited dose-related accumulation of hyaline droplets containing alpha-2u-globulin in proximal tubules. The kidney weights were significantly increased in male and female rats treated with TNB. Significant increases in cell proliferation in proximal tubules were observed in male F-344 rats. Renal changes observed in TNB-treated rats appeared identical to those from other chemicals that induce alpha-2u-globulin nephropathy in male rats. No hyaline droplet accumulation was found in female F-344 and male NBR rats at any doses. We can conclude that TNB induces dose-related exacerbation of hyaline droplets containing alpha-2u-globulin in male rat kidney and subsequent cell proliferation.

Kinetics and metabolism of 1,4-dichlorobenzene in male Wistar rats: no evidence for quinone metabolites

The biotransformation and kinetics of 1,4-dichlorobenzene (1,4-DCB) were studied in male Wistar rats at three oral dose levels (10, 50 and 250 mg/kg). The effect of induction of CYP2E1 by isoniazid on the kinetics and biotransformation was determined. Excretion was predominantly via the urine (78-85%) and to a small extent via the faeces (2-5%). The relative contributions of these routes were not dose dependent. Excretion via the bile ranged from less than 5% at the low dose level to 30% at the high dose level. The major biliary metabolite was the glucuronide of 2,5-dichlorophenol (2,5-DCP). The time point at which the plasma concentrations of the parent compound and the metabolites were maximal (TCmax) as well as the maximum concentrations (Cmax) increased with higher dose level. Induction by isoniazid resulted in a faster urinary elimination, whereas TCmax and Cmax were lower for induced rats. In addition, the area under the blood curve (AUC) was smaller and total clearance was higher for induced rats. 1,4-DCB was mainly metabolized to 2,5-DCP (ca. 90%), which was detected in the urine as its sulfate (50-60%), glucuronide (20-30%) and the free form (5-10%). Minor metabolites were the N-acetyl-cysteine-S-dihydro-hydroxy-1,4-dichlorobenzene and the corresponding dehydrated N-acetyl-cysteine-S-1,4-dichlorobenzene, which comprised ca. 10% of total metabolites. No hydroquinones were observed for the male Wistar rat, not even under conditions of induced oxidative metabolism.

Effects of four trihalomethanes on DNA strand breaks, renal hyaline droplet formation and serum testosterone in male F-344 rats

All four possible trihalomethanes (THMs) containing bromine and chlorine, as well as perchloroethylene (PCE), were evaluated for their ability to produce DNA strand breaks, alpha 2u-globulin rich renal deposits, and testosterone changes in male F-344 rats. Rats received daily equimolar doses (0.75 or 1.5 mmol/kg) of THMs or PCE (1000 mg/kg) in 4% Emulphor vehicle by oral gavage for 7 days. No significant DNA strand breaks were produced by any THM or PCE treatment. PCE treatment produced increased hyaline droplet formation in renal tubules. However, all THM treatments reduced or eliminated the appearance of renal hyaline droplets. All four THM treatments also produced a decrease in serum testosterone concentrations on day 7, which might account for decreased hyaline droplet formation. No significant increase in cell proliferation, measured by [3H]thymidine incorporation in vivo, appeared in this 1-week study.

Biotransformation and membrane transport in nephrotoxicity

The kidney is a frequent target organ for toxic effects of xenobiotics. In recent years, the molecular mechanisms responsible for the selective renal toxicity of many nephrotoxic xenobiotics have been elucidated. Accumulation by renal transport mechanisms, and thus aspects of renal physiology, plays an important role in the renal toxicity of some antibiotics, metals, and agents binding to low molecular weight proteins such as alpha(2u)-globulin. The accumulation by active transport of metabolites formed in other organs is involved in the kidney-specific toxicity of certain polyhaloalkanes, polyhaloalkenes, hydroquinones, and aminophenols. Other xenobiotics are selectively metabolized to reactive electrophiles by enzymes expressed in the kidney. This review summarizes the present knowledge on the mechanistic basis of target organ selectivity of these compounds.

Alpha 2u-globulins in the urine of male rats: a reliable indicator for alpha 2u-globulin accumulation in the kidney

Increases in kidney-type-alpha 2u-globulin (alpha G-K, molecular weight approximately 16 kDa) were detected in the urine of male adult rats treated with d-limonene by immunoblotting analysis using an antiserum which distinguishes native-type-alpha 2u-globulin (alpha G-N, molecular weight approximately 19 kDa) from alpha G-K. When male adult rats received d-limonene by gavage (0-300 mg/kg/day) for 14 consecutive days, dose-dependent increases in urinary excretion of alpha G-K were observed at a dosage level of more than 30 mg/kg/day. This was found to be directly correlated with alterations in the concentration of renal alpha G-K as well as the accumulation of hyaline droplets in proximal convoluted tubule (PCT) epithelial cells in the kidneys. Marked elevation of urinary alpha G-K was also noted following oral treatment of adult male rats with 2,2,4-trimethylpentane (TMP), 1,4-dichlorobenzene (DCB), decalin and isophorone (ISP) by gavage (1.5 mmol/kg/day) for 7 consecutive days, again in association with increased concentrations of renal alpha G-K and hyaline droplet accumulation in renal PCT epithelial cells. However, no such increases in urinary alpha G-K were observed for male adult rats treated with nephrotoxic chemicals such as puromycin aminonucleoside (PAN) (15 mg/kg/day, s.c., 14 consecutive days) or hexachloro-1,3-butadiene (HCBD) (100 mg/kg/day, p.o., 5 consecutive days), lacking the ability to cause kidney accumulation of the hyaline droplets and alpha G-K. The findings in this study thus indicate that measurement of urinary alpha G-K can give a reliable estimates not only of the potential to cause renal accumulation of alpha 2u-globulin but also of its magnitude.

Induced regenerative cell proliferation in livers and kidneys of male F-344 rats given chloroform in corn oil by gavage or ad libitum in drinking water

These studies were designed to establish the dose response relationships for the induction of cytolethality and regenerative cell proliferation in the liver and kidneys of male F-344 rats given chloroform by gavage or in drinking water. Rats were administered oral doses of 0, 10, 34, 90 or 180 mg/kg/day chloroform dissolved in corn oil by gavage for 4 days or for 5 days/week for 3 weeks. A second group of rats was given chloroform ad libitum in the drinking water at concentrations of 0, 60, 200, 400, 900 or 1800 ppm for 4 days or 3 weeks. Bromodeoxyuridine (BrdU) was administered via an implanted osmotic pump 3.5 days prior to necropsy to label cells in S-phase. Cells having incorporated BrdU were visualized in tissue sections immunohistochemically and the labelling index (LI) evaluated as the percentage of S-phase cells. Rats treated with 90 or 180 mg/kg/day by gavage for 4 days had mild to moderate degeneration of renal proximal tubules and centrilobular hepatocytes. These alterations were absent or slight after 3 weeks of treatment. LI were increased in the kidney cortex only in the rats treated with 180 mg/kg/day for 4 days. A dose-dependent increase in LI was seen in rat liver after 4 days of treatment with 90 and 180 mg/kg/day by gavage, but the LI remained elevated after 3 weeks of treatment only at the 180 mg/kg/day dose. When chloroform was administered in the drinking water, no microscopic alterations were seen in the kidneys after 4 days of treatment. As a general observation, rats treated for 3 weeks with 200 ppm chloroform and greater had slightly increased numbers of focal areas of regenerating renal proximal tubular epithelium and cell proliferation than were noted in the controls, but no clear dose response relationship was evident. However, the overall renal LI was not increased at any dose or time point. Similarly, only mild hepatocyte vacuolation was observed in rats given 1800 ppm chloroform in the water for 3 weeks with no increase in the hepatic LI at any time point, even though the rats were consuming chloroform at a rate of 106 mg/kg/day at the 1800 ppm drinking water concentration. These data indicate more severe hepatic and renal toxicity when chloroform is administered by gavage than in the drinking water and a different pattern of regenerative proliferation in the kidney.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative metabolism of the renal carcinogen 1,4-dichlorobenzene in rat: identification and quantitation of novel metabolites

1. The metabolism of 1,4-dichlorobenzene has been studied in the male and female Fisher 344 rat over 72 h after oral administration of 14C-1,4-dichlorobenzene (900 mg = 96.8 microCi/kg). No covalent binding of radioactivity could be detected in samples of liver, kidney, lung and spleen. The major route of excretion was with urine accounting for 41.3% of the dose for male and 37.8% of the dose for female rat within 72 h after dosing. 2. Urinary metabolites of 1,4-dichlorobenzene were identified and quantified. The major metabolites identified in the urine of both the male and female rat, were the sulphate and glucuronide of 2,5-dichlorophenol. Minor amounts of 2,5-dichlorohydroquinone were excreted as an unidentified conjugate. 3. 2-(N-acetyl-cysteine-S-yl)-1,4-dichlorobenzene and 2-(N-acetyl-cysteine-S-yl)-2,3-dihydro-3-hydroxy-1,3-hydroxy-1,4-dich lorobenzen e were minor metabolites excreted in the urine of both sexes. 4. A novel biotransformation pathway for 1,4-dichlorobenzene may be postulated, leading to the urinary excretion of a mercapturic acid of chlorophenol. 5. No marked differences in the distribution and excretion of metabolites of 1,4-dichlorobenzene were observed between the male and female Fisher 344 rat.

Occupational hydrocarbon exposure and chronic nephropathy

This review aims at discussing the questions raised by the hydrocarbon-related chronic nephropathy and its possible consequence, the hydrocarbon-related chronic renal failure. It has been attempted to adopt the point of view of the clinician. Therefore, the most important part of the review is devoted to a presentation and an analysis of the available data on humans. The main features of the available studies on human subjects are presented, their conclusions discussed in the light of the possible methodological flaws, and practical conclusions drawn. After a discussion of the main difficulties encountered for selecting the suitable exposure indicator, the studies are discussed in order of decreasing quality of the study design (cohort, case-control, cross-sectional studies, and the case reports). It is concluded that a great deal of controversies about chronic hydrocarbon-related nephropathy is explained by differences in the study design and that hydrocarbon-induced nephropathy is probably more than a mere hypothesis, although a causal relationship has not yet been proven. Finally, some practical consequences for dealing with a hydrocarbon-exposed patient diagnosed with a kidney disease and the need for further research are discussed.

Toxicokinetic and mechanistic considerations in the interpretation of the rodent bioassay

When chemicals that are nongenotoxic in conventional assays produce increases in tumor incidence in rodents in chronic bioassays, the determination of the significance of these data for human safety is a challenging task. An important first step in this process is consideration of available data on the mechanism of action and biological properties of the chemical as well as pharmacokinetic and metabolism data in the species showing the response. In recent years, there has been an increase in the understanding of so-called "secondary mechanisms" of carcinogenesis (e.g., thyroid tumors in rats following exposure to enzyme inducers). Application of these data may assist in determination of human risk. There are 2 important questions that will be explored and developed: Are there biological effects produced in the test species that could explain the increase in tumor incidence, and will these effects be reproduced in humans? What is the exposure to the chemical that is associated with the increase in tumors, and how does this relate to exposure in humans?

Risk assessment of d-limonene: an example of male rat-specific renal tumorigens

The naturally occurring food constituent d-limonene has been found to cause tumors at high doses only in the kidney of the male rat in association with the development of hyaline droplet nephropathy. In contrast, neither kidney tumors nor the associated nephropathy have been found in female rats or mice at much higher doses. Adult male rats produce large quantities of a specific low-molecular-weight protein in the liver, which is known as alpha 2U-globulin (alpha 2U-g). With administration of sufficient doses of d-limonene to male rats, this protein has been found to accumulate excessively in the P2 segment cells of renal proximal tubules, resulting in hyaline droplet formation as a manifestation of protein overload. Hyaline droplet accumulation is the first stage in a unique sequence of nephropathic lesions (also known as alpha 2U-g nephropathy), including granular casts in the outer medulla and linear mineralization in the papilla. The mechanism underlying protein accumulation appears to be the reversible binding of chemical to alpha 2U-g with subsequent prolongation of its half-life in the tubule cell. In the case of d-limonene, the minor metabolite d-limonene-1,2-oxide has been shown to be the primary chemical species that binds reversibly to alpha 2U-g, impeding the normal process of lysosomal proteinase degradation of alpha 2U-g. The ensuing nephropathy is associated with a sustained increase in compensatory renal tubule cell proliferation, which provides the putative mechanistic link with renal tumor formation possibly through tumor promotion of spontaneously initiated cells or enhanced spontaneous mutagenesis. This proposed mechanism has been supported by additional information, including negative genotoxicity tests for d-limonene and its oxide metabolites, experimentally verified tumor promotion, and enhanced cell proliferation primarily in P2 segment tubule cells in male F344 rats, but no such effects in the alpha 2U-g-deficient NBR rat. The mechanism of d-limonene tumor development does not appear to be possible in humans since neither the quantity nor the type of protein that binds d-limonene or d-limonene-1,2-oxide is present. The deduction that the renal tumors induced in male rats are not relevant to human carcinogenicity in the hazard evaluation step of risk assessment completes the evaluation of human risk for d-limonene. Consequently, it can be concluded that d-limonene does not pose any carcinogenic or nephrotoxic risk to humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Interpretation of male rat renal tubule tumors

Based on an analysis of recent scientific studies, a Technical Panel of the U.S. Environmental Protection Agency's (EPA) Risk Assessment Forum recently advised EPA risk assessors against using information on certain male rat renal tubule tumors to assess human risk under conditions specified in a new Forum report. Risk assessment approaches generally assume that chemicals producing tumors in laboratory animals are a potential cancer hazard to humans. For most chemicals, including classical rodent kidney carcinogens such as N-ethyl-N-hydroxyethylnitrosamine, this extrapolation remains appropriate. Some chemicals, however, induce accumulation of alpha 2u-globulin (alpha 2u-g), a low molecular weight protein, in the male rat kidney. The alpha 2u-g accumulation initiates a sequence of events that appears to lead to renal tubule tumor formation. Female rats and other laboratory mammals administered the same chemicals do not accumulate low molecular weight protein in the kidney, and they do not develop renal tubule tumors. Because humans appear to be more like other laboratory animals than like the male rat, in this special situation, the male rat is not a good model for assessing human risk. The Forum report stresses the need for full scrutiny of a substantial set of data to determine when it is reasonable to presume that renal tumors in male rats are linked to a process involving alpha 2u-g accumulation and to select appropriate procedures for estimating human risks under such circumstances.

A possible role for cell proliferation in potassium bromate (KBrO3) carcinogenesis

Accumulation of alpha 2u-globulin and induction of cell proliferation were examined in kidneys of rats exposed to KBrO3, KBr or NaBrO3 in their drinking water. Hyaline droplets observed after KBrO3 or NaBrO3 administration to male rats were specifically immunostained for alpha 2u-globulin. Increases in cell proliferation were found in the proximal tubules of male rats given KBrO3 or NaBrO3 but not KBr for 2, 4, and 8 weeks. No such change was evident in KBrO3-treated female rats or the distal tubules of any treated animal. The concordance between hyaline droplet accumulation and increased cell turnover suggests that KBrO3- and NaBrO3-induced cell replication in kidneys of male rats may result from alpha 2u-globulin nephropathy. Considering the fact that KBrO3 has genotoxic potential involving oxidative stress, we hypothesize that the induced cell proliferation might predominantly play an additive role in its carcinogenesis. Furthermore, the present data, showing similar effects of NaBrO3 on the rat kidney, are of direct significance to its risk assessment.

Acute hepatic and renal toxicity of dichlorobenzene isomers in Fischer 344 rats

Studies were conducted to examine acute hepatic and renal toxicity of dichlorobenzene (DCB) structural isomers. Male Fischer 344 (F344) rats were injected with 2, 3 or 4 mmol kg-1 of 1,2-DCB, 1,3-DCB or 1,4-DCB (o-, m-, p-). Pair-fed control (PFC) animals were injected (i.p.) with corn oil (1 ml kg-1). Hepatic and renal toxicity was quantitated 24 h after injection of DCB or vehicle. Plasma transaminase (ALT/GPT) activity was increased (P < 0.05) by 1,2-DCB as a function of dose administered. Centrilobular necrosis was observed in rats treated with 1,2-DCB while morphology was relatively normal in rats treated with m- or p-DCB. Administration of (2 or 4 mmol kg-1) 1,3-DCB or 1,4-DCB did not alter kidney weight or blood urea nitrogen (BUN) levels. Renal cortical slice accumulation of p-aminohippurate (PAH) was decreased (P < 0.05) by (2 and 4 mmol kg-1) 1,3-DCB and (3 and 4 mmol kg-1) 1,2-DCB while accumulation of the cation tetraethylammonium (TEA) was decreased by 4 mmol kg-1 1,4-DCB. (TEA). The results of these studies demonstrated that ortho substitution enhanced hepatic and renal toxicity. The results also would suggest that the liver was more sensitive than the kidney for DCB toxicity.

Differences in alpha 2u-globulins increased in male rat kidneys following treatment with several alpha 2u-globulin accumulating agents: cystein protease(s) play(s) an important role in production of kidney-type-alpha 2u-globulin

Effects of alpha 2u-globulin accumulating agents on alpha 2u-globulins in rat kidneys were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting analysis. Treatment of male animals with decalin (150 mg/kg), 2,2,4-trimethylpentane (50 mg/kg), isophorone (150 mg/kg), d-limonene (150 mg/kg) or 1,4-dichlorobenzene (150 mg/kg) by gavage for 14 consecutive days in each case resulted in a marked intensification of a protein band corresponding to kidney-type-alpha 2u-globulin, with a molecular mass calculated to be approximately 16 kDa. However, intraperitoneal treatment with leupeptin and E-64 (two times 0.07 mmol/kg, for each), well known cystein protease inhibitors, while only slightly increasing this kidney-type-alpha 2u-globulin band, caused the intensification of a approximately 19-kDa molecular mass protein band which was revealed to be a native-type-alpha 2u-globulin by SDS-PAGE and immunoblotting. These results indicated that at least two types of alpha 2u-globulin can be increased in male rat kidney by chemical treatment. Moreover, cystein protease(s) appear(s) to play an important role in the degradation of alpha 2u-globulin and particularly in the conversion of native-type-alpha 2u-globulin to kidney-type-alpha 2u-globulin in rat kidneys.

Alpha 2u-globulin is the only member of the lipocalin protein superfamily that binds to hyaline droplet inducing agents

The rate-limiting step in chemically induced, male rat-specific hyaline droplet nephropathy is the reversible binding of a xenobiotic to alpha 2u-globulin. In this study, equilibrium saturation binding experiments were conducted to evaluate the in vitro binding of d-limonene-1,2-oxide (dLO) and 2,4,4-trimethyl-2-pentanol (TMP-OH) to alpha 2u-globulin and members of the alpha 2u-globulin protein superfamily. Both dLO and TMP-OH bound to alpha 2u-globulin, with Scatchard analysis yielding dissociation constants of 5.6 and 6.4 x 10(-7) M, respectively. The Bmax for binding (nmol bound/mg protein) was 50.7 and 61.1 for dLO and TMP-OH, respectively, yielding a molar ratio of approximately 1 for both ligands. The ability of dLO and TMP-OH to bind to human-derived alpha 1-acid glycoprotein, rat-derived retinol-binding protein, human protein-1, and bovine beta-lactoglobulin was also studied. These superfamily proteins are generally abundant in plasma, are freely filtered across the glomerulus, and can bind a wide range of ligands. However, neither dLO nor TMP-OH bound to any of the superfamily proteins. In contrast, under identical experimental conditions, alpha 1-acid glycoprotein did bind progesterone (Kd = 10(-6) M), whereas both beta-lactoglobulin and retinol-binding protein bound retinol (Kd = 10(-8) M for both proteins). These results indicate that, under conditions where alpha 2u-globulin superfamily proteins bind to established ligands, the proteins do not interact with hyaline droplet inducing agents. Thus, the interaction between male rat-specific nephrotoxicants and alpha 2u-globulin is unique to this protein. More importantly, these results provide direct evidence that the presence of the alpha 2u-globulin superfamily proteins does not predispose humans to develop hyaline droplet nephropathy and renal cancer from this class of chemicals.

An alternative hypothesis on the role of chemically induced protein droplet (alpha 2u-globulin) nephropathy in renal carcinogenesis

Based on associations between the accumulation of protein droplets containing alpha 2u-globulin in proximal tubular epithelial cells and increased incidences of renal tubular neoplasms in male rats, it has been suggested that the carcinogenicity of chemicals that cause alpha 2u-globulin nephropathy is unique to animals that synthesize this protein. Chemicals that caused alpha 2u-globulin nephropathy and renal carcinogenicity in male rats have not been shown to produce renal tumors in animals that lack the capability for hepatic alpha 2u-globulin synthesis, including female rats, male NBR rats, or mice of either sex. Because humans do not synthesize alpha 2u-globulin it has been suggested that chemicals which cause renal toxicity associated with alpha 2u-globulin accumulation do not pose an increased cancer risk to humans. In this review on the association between alpha 2u-globulin nephropathy and renal carcinogenesis, it is apparent that (a) there are data inconsistent with the hypothesis linking these occurrences, (b) alternative mechanisms of renal toxicity and carcinogenicity are plausible, (c) data on quantitative dose-response correspondences between the various stages of alpha 2u-globulin nephropathy and renal carcinogenicity are limited, and (d) a greater understanding of the molecular changes occurring during renal carcinogenesis is needed before assuming that the current hypothesis is correct. Future research aimed at resolving issues raised in this paper should help determine whether or not the association between alpha 2u-globulin nephropathy and renal carcinogenesis represents a cause-and-effect relationship.

Cell proliferation induced in the kidneys and livers of rats and mice by short term exposure to the carcinogen p-dichlorobenzene

Cell proliferation in the kidneys and livers of rats and mice exposed short-term to p-dichlorobenzene (p-DCB) was evaluated by immunohistochemical measurement of bromodeoxyuridine (BrdU) incorporation into nuclei of DNA-synthesizing cells. p-DCB was given by gavage at two doses up to 600 mg/kg body weight for 4 days. The cumulative fraction of proliferating cells was increased in the proximal tubule epithelial cells of male rats at the high dose, but not at the low dose nor in females at either dose using gamma-glutamyl transferase reaction to identify tubular cells. Also, no increase in cell proliferation was found in mouse kidneys. The fractions of proliferating cells in the livers of rats and mice of both sexes were also increased. The increased cell proliferation in only male rat kidney and in the livers of mice of both sexes correlates with the reported carcinogenic effects of p-DCB in those tissues. However, the finding that p-DCB also induced cell proliferation in the livers of rats of both sexes, which were not a site of p-DCB-induced tumors in bioassays, and in female mice at the low dose, which was not affected by an increase in tumors, reveals a lack of concordance and indicates that acute induction of cell proliferation is not sufficient to lead to carcinogenesis.

Biochemical basis for mouse resistance to hyaline droplet nephropathy: Lack of relevance of the α2u-globulin protein superfamily in this male rat-specific syndrome

It is well-established that binding of a chemical to alpha 2u-globulin is the rate-limiting step in the development of male rat-specific hyaline droplet nephropathy. Mice synthesize mouse urinary protein (MUP), a protein which is very similar to alpha 2u-globulin, but this protein does not render the mouse sensitive to a similar renal toxicity. Therefore, the purpose of the present study was to determine the biochemical basis for mouse resistance to hyaline droplet nephropathy. Male Fischer 344 rats and B6C3F1 mice excreted 12.24 +/- 0.60 and 14.88 +/- 0.99 mg of alpha 2u-globulin and MUP daily, indicating that quantitative differences in protein excretion were not involved in the species specificity of the nephropathy. With d-limonene as a model hyaline droplet inducing agent, both rat and mouse liver microsomes oxidized the terpene to its 1,2-epoxide (the metabolite that binds reversibly to alpha 2u-globulin in vivo), demonstrating that metabolic differences do not determine the mouse resistance to this lesion. In spite of the formation of the epoxide intermediate, no binding of [14C]d-limonene equivalents to mouse kidney proteins was observed. In contrast, about 40% of the d-limonene equivalents in male rat kidney was reversibly bound to renal proteins. The renal reabsorption of alpha 2u-globulin and MUP was markedly different, as rats reabsorbed about 60% of the total filtered load of alpha 2u-globulin, but MUP was not reabsorbed by the mouse kidney. Given the absence of MUP in mouse kidney, in vitro equilibrium saturation binding studies were also conducted to determine whether MUP could bind the epoxide metabolite. alpha 2u-Globulin bound [14C]d-limonene-1,2-oxide with an apparent Kd of 4 x 10(-7) M. However, under identical experimental conditions, MUP failed to bind the epoxide. These data indicate that two major biochemical differences between alpha 2u-globulin and MUP contribute to mouse resistance to hyaline droplet nephropathy. Under both in vivo and in vitro conditions, MUP does not bind d-limonene-1,2-oxide, the rate-limiting step in the development of the nephropathy. However, even if MUP did bind the epoxide, the fact that it is not reabsorbed into the mouse kidney precludes its involvement in a syndrome involving renal protein overload. Finally, the absence of an interaction between d-limonene, a model hyaline droplet inducer, and the protein most similar to alpha 2u-globulin suggests that no other protein in the alpha 2u-globulin superfamily is likely to cause hyaline droplet nephropathy in other species.

Quantitation of urinary alpha 2u-globulin and albumin by reverse-phase high performance liquid chromatography

A rapid, reproducible, and sensitive high-performance liquid chromatography (HPLC) method for the quantitation of alpha 2u-globulin, the major urinary protein excreted by adult male rats, and albumin has been developed. Total urinary proteins, isolated by a simple Sephadex G-25 gel filtration step, are separated and quantitated by reverse-phase HPLC on a C4 Macrosphere 300 column. The proteins are separated and eluted with a two-step gradient of acetonitrile in aqueous trifluoroacetic acid. Detection limits of 9 and 25 micrograms/mL of urine were established for albumin and alpha 2u-globulin, respectively. Quantitation of urinary excretion of the two proteins in young adult male and female rats and aging male rats showed that values obtained with this method compared favorably with values from previously developed immunological techniques. To quantitate total urinary protein excretion, we modified the Bradford protein assay to use rat urinary protein as standard. Given the established importance of alpha 2u-globulin in the development of male rat-specific nephrotoxicity and nephrocarcinogenicity, these methods should be useful for studying the renal handling of this protein under normal and nephrotoxic conditions.

The liver, kidney, and thyroid toxicity of chlorinated benzenes

The acute toxicity of a number of chlorinated benzenes, ranging from monosubstituted to pentasubstituted benzenes, was studied in rats. Toxic effects on the liver, the kidneys, and the thyroid were monitored after a single ip administration of 1, 2, or 4 mmol/kg monochlorobenzene (MCB), 1,2-dichlorobenzene (1,2-DICB), 1,4-dichlorobenzene (1,4-DICB), 1,2,4-trichlorobenzene (1,2,4-TRCB), and pentachlorobenzene (PECB). Due to its low solubility, 1,2,4,5-tetrachlorobenzene (1,2,4,5-TECB) was tested at a highest dose of 0.8 mmol/kg. 1,2-DICB and 1,2,4-TRCB produced the most severe hepatotoxic effects when compared with an equimolar dose of the other chlorinated benzenes, as determined by plasma ALT profile and histopathological changes after 72 hr. MCB was considerably less hepatotoxic. Severe degenerative damage to the kidney was only observed in a few rats treated with 1,2,4-TRCB. However, protein droplets in the tubular epithelial cells were observed at 72 hr after administration of 1,4-DICB, 1,2,4-TRCB, 1,2,4,5-TECB, and PECB. In the latter two groups, these protein droplets were still observed 9 days after administration. All chlorinated benzenes tested excluding MCB induced a reduction in plasma thyroxine levels. The extent of decrease in plasma thyroxine was more severe in rats treated with 1,2,4-TRCB or PECB and correlated well with the relative binding affinities of the phenolic metabolites to the plasma transport protein for thyroxine, i.e., transthyretin. The present study indicates that the establishment of a structure-activity relationship with regard to toxicity depends on the sensitivity of the respective target organs. In the series of (poly)chlorinated benzenes studied, ranging from mono- to pentachlorobenzene, the most severe effects on liver, kidney, and thyroid were observed for 1,2,4-substitution.

Male rat specific nephrotoxicity resulting from subchronic administration of hexachlorobenzene

Male rats are more sensitive to the nephrocarcinogenic effect of hexachlorobenzene (HCB) than are female rats. The purpose of this study was to shed light on this phenomenon by investigating mechanisms of subchronic nephrotoxicity of HCB. Groups of rats were administered HCB in corn oil (po) at 100 mg/kg, 5 days per week for 15 days or at 50 mg/kg, 5 days per week for 50 days. Urine was collected on Days 1, 8, and 15 for the 15-day treatment and on Day 50 for the 50-day treatment. Glucosuria, proteinuria, and enzymuria (gamma-glutamyl transpeptidase) were measured to assess renal function. Twenty-four hours after the last HCB treatment, the animals were killed and kidneys were removed for histopathological evaluation. Urine analyses showed no indication of renal dysfunction in treated animals compared to controls during the 15-day treatment. However, histology of male rat kidneys revealed degenerative and regenerative cellular foci accompanied by an increased accumulation of protein droplets in epithelial cells of the proximal tubules. The same histological observations were also made in male rats after a 50-day HCB treatment but this time they were accompanied by renal function alterations. In female rats, no such renal functional or histological alterations were observed. The histopathological observations in male rats correspond well with the protein droplet nephropathy; the latter is characteristic of the accumulation in kidney cells of alpha 2u-globulin probably caused by the reversible binding of a chemical to alpha 2u that renders the protein indigestible to kidney proteases. alpha 2u-Globulin was measured in the cytosol of male rats and was found to be increased 11-fold compared to controls. Also, HCB was found to be bound reversibly to alpha 2u. These results suggest that HCB induces a male rat specific nephropathy that could explain the higher incidence of kidney tumors in male rats compared to female rats.