2,2,4-Trimethylpentane-induced nephrotoxicity. II. The reversible binding of a TMP metabolite to a renal protein fraction containing alpha 2u-globulin

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.

Dysfunction of chaperone-mediated autophagy in human diseases

Chaperone-mediated autophagy (CMA), one of the degradation pathways of proteins, is highly selective to substrates that have KFERQ-like motif. In this process, the substrate proteins are first recognized by the chaperone protein, heat shock cognate protein 70 (Hsc70), then delivered to lysosomal membrane surface where the single-span lysosomal receptor, lysosome-associated membrane protein type 2A (LAMP2A) can bind to the substrate proteins to form a 700 kDa protein complex that allows them to translocate into the lysosome lumen to be degraded by the hydrolytic enzymes. This degradation pathway mediated by CMA plays an important role in regulating glucose and lipid metabolism, transcription, DNA reparation, cell cycle, cellular response to stress and consequently, regulating many aging-associated human diseases, such as neurodegeneration, cancer and metabolic disorders. In this review, we provide an overview of current research on the functional roles of CMA primarily from a perspective of understanding and treating human diseases and also discuss its potential applications for diseases.

Subchronic Toxicity Study of Dicyclopentadiene Vapor in Rats

Fischer 344 rats were exposed by inhalation to 0, 1, 5 or 50 ppm dicyclopentadiene (DCPD) vapor 6 hr/day, 5 days/week for 13 weeks, followed by a 13-week recovery period. Animals were euthanized following completion of exposure at 2, 6, or 13 weeks and at postexposure weeks 4 or 13. No mortality, overt signs, body weight changes, hematologic or clinical chemistry values were related to DCPD exposure. In the high-exposure male rats, relative liver weights were significantly increased but with no accompanying histopathologic changes. Exposure to DCPD produced adverse kidney effects in male, but not female, rats as evidenced by the excretion of epithelial cells in the urine. Histologic changes were localized to the proximal tubules of the kidney and included increased accumulation of protein droplets, regenerative epithelium, and the presence of intraluminal proteinaceous material. In addition, several alterations in renal function were observed. Urinary Na+ excretion rates were decreased and urinary K+ excretion rates were increased throughout the exposure period; however, glucose was not present in the urine, and creatinine clearance was normal. The ability of the kidney to concentrate urine was also impaired. After the recovery period, many of the treatment-related kidney effects were not observed, including the presence of hyaline droplets in the proximal tubules and epithelial cells in the urine. These findings indicate an overall low degree of systemic toxicity following sub-chronic inhalation exposure of dicyclopentadiene at exposure levels up to 50 ppm. The only effect that was observed was a male rat-specific nephropathy that is characteristic of the hyaline droplet nephropathy produced by a diverse group of compounds.

Lysosomal Changes in Renal Proximal Tubular Epithelial Cells of Male Sprague Dawley Rats following Decalin Exposure

A histochemical stain for acid phosphatase served as a marker for lysosomal alterations in renal tubular cells associated with male rat hyaline droplet nephropathy. Morphometric analysis and quantitative histochemistry were used to compare the size and acid phosphatase stain reaction of lysosomes in tubular epithelial cells of treated and control animals. Decalin exposure increased the size and significantly ( p < 0.01) reduced the acid phosphatase stain intensity of individual lysosomes. However, there was no significant different ( p > 0.05) between the acid phosphatase stain intensity of treated and control animals when analyzed on a whole cell basis. The increase in size of the lysosomes without a proportional increase in whole cell acid phosphatase stain intensity indicates a dilution or a failure to accommodate in the acid phosphatase concentration (stain intensity/μm2) per lysosome. All acid phosphatase stain reaction product was contained within intact lysosomes, mitigating against the hypothesis of lysosomal enzyme leakage as the cause of cell death in decalin-induced alpha 2U globulin nephropathy.

Safety assessment of isoparaffins as used in cosmetics

The safety of isoparaffins as used in cosmetic products is reviewed in this safety assessment. These ingredients function mostly as solvents and also function as emollients in the 0001% to 90% concentration range. The Cosmetic Ingredient Review (CIR) Expert Panel has reviewed relevant animal and clinical data and concluded that these ingredients are safe in the present practices of use and concentration described in this safety assessment.

Sex-related differences in renal toxicodynamics in rodents

INTRODUCTION

An issue yet to be addressed, in the investigation of the xenobiotic toxicity, is a detailed characterization of the sex differences in toxicological responses. The 'sex issue' is particularly significant in nephrotoxicology as the kidney is a relevant target organ for xenobiotics and few studies have approached this subject in the past. There is a strong need to improve our understanding regarding the influence of sex in toxicology, given their increased requirement to establish the limits of exposure to chemicals in the environment and at work.

AREAS COVERED

In this review, the authors provide the reader with the current knowledge of sex differences in kidney toxicity for rats and mice. To make the review easier to consult, these studies have been organized according to the class of xenobiotic.

EXPERT OPINION

From the analysis of the present knowledge emerges a dramatic need for information on sex differences in xenobiotics toxicity. Although animals are reasonably good predictors of adverse renal effects in patients, there is need to identify alternative methods (e.g. in vitro/ex vivo) to better study sex differences in organ toxicity.

Disruption of glomerular cell-cell and cell-matrix interactions in hydrocarbon nephropathy

Environmental chemicals play an etiological role in greater than 50% of idiopathic glomerular diseases. The present studies were conducted to define mechanisms of renal cell-specific hydrocarbon injury. Female rats were given 10 mg/kg benzo(a)pyrene (BaP) once a week for 16 wk. Progressive elevations in total urinary protein, protein/creatinine ratios, and microalbuminuria were observed in rats treated with BaP for up to 16 wk. The nephropathic response involved early reductions in mesangial cell numbers and fibronectin levels by 8 wk, coupled to transient increases in podocyte cellularity. Changes in podocyte numbers subsided by 16 wk and correlated with rebound increases in mesangial cell numbers and fibronectin levels, along with increased α-smooth muscle actin and Cu/Zn superoxide dismutase and fusion of podocyte foot processes. In culture, mesangial cells were more sensitive than podocytes to hydrocarbon injury and expressed higher levels of inducible aryl hydrocarbon hydroxylase activity. Naïve mesangial cells exerted a strong inhibitory influence on podocyte proliferation under both direct and indirect coculture conditions, and this response involved a mesangial cell-derived matrix that selectively inhibited podocyte proliferation. These findings indicate that hydrocarbon nephropathy in rats involves disruption of glomerular cell-cell and cell-matrix interactions mediated by deposition of a mesangial cell-derived growth-inhibitory matrix that regulates podocyte proliferation.

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.

Species Differences in the Metabolism of (+)- and (−)-Limonenes and their Metabolites, Carveols and Carvones, by Cytochrome P450 Enzymes in Liver Microsomes of Mice, Rats, Guinea Pigs, Rabbits, Dogs, Monkeys, and Humans

(+)-Limonene is shown to cause renal toxicity in male rats, but not in female rats and other species of animals including mice, guinea pigs, rabbits, and dogs. We have previously shown that male-specific rat CYP2C11 (but not female-specific CYP2C12) is able to convert limonenes to carveols and perillyl alcohols (M. Miyazawa, M. Shindo, and T. Shimada: Chem. Res. Toxicol., 15, 15-20, 2002). Here, we investigated whether (+)- and (-)-limonene enantiomers are differentially metabolized by P450 enzymes in liver microsomes of mice, rats, guinea pigs, rabbits, dogs, monkeys, and humans. Limonene enantiomers were converted to respective carveols, perillyl alcohols, and carvones (oxidative metabolites of carveols) by liver microsomes of dogs, rabbits, and guinea pigs. Mice, rats, monkeys, and humans produced carveols and perilly alcohols, but not carvones. Reconstituted monooxygenase systems containing purified rabbit CYP1A2 and 2B4 and NADPH-P450 reductase were found to catalyze (+)-limonene to (+)-carveol, (+)-carvone, and (+)-perillyl alcohol, being more active with CYP2B4. When (+)-carveol and (+)-carvone were used as substrates, dogs, rabbits, and guinea pigs metabolized them to (+)-carvone and (+)-carveol, respectively. Again humans, monkeys, rats, and mice did not convert (+)-carveol to (+)-carvone, but metabolized (+)-carvone to (+)-carveol, with male rats having the highest rates. CYP2C enzymes were suggested to play major roles in metabolizing (+)-carveol to (+)-carvone and (+)-carvone to (+)-carveol by liver microsomes, since the activities were inhibited significantly by anti-human CYP2C9 antibodies in these animal species. Studies with recombinant P450 enzymes suggested that CYP2C9 and 2C19 in humans and CYP2C11 in untreated male rats were the major enzymes in metabolizing (+)-carvone. These results suggest that there are species-related differences in the metabolism of limonenes by P450 enzymes, particularly in the way from (+)-carveol to (+)-carvone. However, it remains unclear whether these differences in limonene metabolism by these animal species explain species-related differences in limonene-induced renal toxicity.

Hepatic alpha 2 mu-globulin localizes to the cytosol of rat proximal tubule cells

BACKGROUND

Alpha 2 mu-Globulin (A2), an 18.6 kD protein of hepatic origin, accumulates in the proximal tubule as an abundant, 15.5 kD cleavage product termed "A2-fragment" (A2-f). A2-f facilitates proximal tubule fatty acid oxidation, presumably by binding hydrophobic ligands. This requires some A2-f to enter the cytosol of the renal epithelial cell (REC). The localization of A2/A2-f in the proximal tubule cell was evaluated in this study.

METHODS

Immunoblot analysis of renal cortical homogenates separated by differential centrifugation and quantitative immunoelectron microscopy (IEM) was performed to localize A2/A2-f using an affinity-purified antibody that detects both proteins. To evaluate A2 as a physiologically relevant ligand, the accumulation of A2-f in the female rat kidney (normally devoid of A2-f) was examined after the induction of hepatic A2 synthesis. Ligand binding, uptake, and degradation assays were used to assess A2 processing by RECs in vitro.

RESULTS

Although A2 and A2-f were detected in the "lysosomal" fraction, only A2-f was found in the soluble protein fraction. IEM confirmed the presence of significant signal in the vesicular and lysosomal as well as the cytosolic compartments. In contrast, both beta 2 mu globulin (B2) and cathepsin B were restricted to endosomes. In the female rat, induction of hepatic A2 production resulted in A2-f accumulation in the renal cortex. In RECs in culture, uptake of A2 and B2 demonstrated nonsaturable, nondisplacable surface binding and similar uptake rates. Compared with B2, A2 was markedly resistant to degradation.

CONCLUSIONS

A fraction of A2 escapes lysosomal degradation, permitting A2-f to accumulate in the cytosol of the proximal tubule epithelial cell. A2 may represent an unusual example of a physiologic protein capable of accumulating in a distant cell type.

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.

Quantitation of alpha2u-globulin in rat kidney cytosol by capillary electrophoresis

The renal accumulation of alpha2u-globulin has been implicated in the tumorigenicity of many nongenotoxic chemicals to the kidney of the male rat. Several chemicals inducing renal tumors in the male rat were shown to bind to alpha2u-globulin. This binding impairs the renal degradation of alpha2u-globulin, resulting in lysosomal overload, cell death, increased cell proliferation, and, presumably, renal tumor formation. To support the role of alpha2u-globulin accumulation in the renal toxicity of a chemical, a demonstration of the accumulation of this protein in the kidney of the male rat is one prerequisite. Monoclonal antibodies to alpha2u-globulin are available for quantifying alpha2u-globulin content; however, the procedure is time-consuming and complicated. We developed a method for the quantitation of alpha2u-globulin in renal cytosol using capillary electrophoresis. Renal cytosol fractions were analyzed by capillary electrophoresis as protein-SDS complexes. Using alpha2u-globulin purified from urine of male rats, the limit of detection was 10 microg/ml sample in routine analyses. Excellent run to run reproducibility in migration time (CV </= 4%) and peak areas corresponding to alpha2u-globulin (CV </= 3%) after normalization to the internal standard was observed. Significant increases in renal alpha2u-globulin content (up to 85% of total protein content) compared to controls (approx 15%) were observed in kidney cytosol of rats treated with alpha2u-globulin nephropathy-inducing agents such as trimethylpentane or the alkylphosphonates dimethyl methylphosphonate and diethyl ethylphosphonate, but not in kidney cytosol of male rats treated with tris-(2-chloroethyl)phosphate or the nephrotoxic agent hexachlorobutadiene. A good correlation of the alpha2u-globulin contents determined by capillary electrophoresis and immunoblotting with an alpha2u-globulin-specific antibody (r2 = 0.997) was obtained. Capillary electrophoresis provides a simple, rapid, and highly reproducible quantitation of alpha2u-globulin accumulation for renal tumorigens and may assist in the risk assessment process for these chemicals.

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.

2-sec-butyl-4,5-dihydrothiazole is a ligand for mouse urinary protein and rat alpha 2u-globulin: physiological and toxicological relevance

Mouse urinary protein (MUP) and alpha 2u-globulin are structurally homologous proteins that belong to a superfamily of ligand-binding proteins and represent the major urinary proteins excreted by adult male mice and rats, respectively. Although a variety of xenobiotics bind to alpha 2u-globulin and produce a male rat-specific hyaline droplet nephropathy, no endogenous ligand for this protein has been identified. Despite extensive sequence homology. MUP does not bind to hyaline droplet-inducing agents. While performing experiments with purified MUP, we observed that it presented with a strong, distinctive odor reminiscent of mouse urine. To determine whether this odor was the result of contamination or degradation or was attributed to an endogenous ligand bound to the protein, the protein was subjected to thermal desorption and any released volatile compounds were detected with a gas chromatograph equipped with an external sniff port and mass spectrometer. With this approach, two odorous compounds were detected at the sniff port by a human observer, but only one was present in sufficient mass to allow identification. This compound, which presented with the characteristic odor, was subsequently identified as 2-sec butyl-4,5-dihydrothiazole (DHT) by GC/MS/matrix isolation IR and NMR analyses. The identification of DHT was confirmed by comparing the chromatographic and spectral properties to those of the synthesized authentic compound. In direct contrast, purified urinary alpha 2u-globulin did not present with an obvious odor, and no volatile ligands were detected on this protein. Although DHT is a major endogenous ligand for MUP, it was also found to competitively inhibit the binding of [14C]d-limonene-1,2-epoxide to alpha 2u-globulin with relatively high affinity (Ki = 2.3 microM). When dosed orally to F344 rats, DHT (1 mmol/kg for 3 days) caused the characteristic exacerbation of hyaline droplets in male rat kidneys and increased renal levels of immunoreactive alpha 2u-globulin about threefold over control levels. These results indicate that despite structural homology, MUP and alpha 2u-globulin are distinguished by the presence of a volatile endogenous ligand only on the former, a distinction that may reflect differences in the physiological functions of the two proteins. Furthermore, although DHT can bind to both MUP and alpha 2u-globulin, renal toxicity was only observed in rats, thereby emphasizing the unique toxicological properties of alpha 2u-globulin in the development of hyaline droplet nephropathy.

Mechanisms of chemically induced renal carcinogenesis in the laboratory rodent

Laboratory studies with classical renal carcinogens in the rat and mouse, as well as research investigation with some of the chemicals proving positive for the kidney in National Toxicology Program carcinogenicity bioassays, have demonstrated the existence of a range of diverse mechanisms underlying rodent kidney carcinogenesis. The classical carcinogens used as experimental models for studying renal tumor pathogenesis, such as the nitrosamines, are genotoxic and interact directly with DNA, forming DNA adducts with mutagenic potential. In contrast, potassium bromate and ferric nitrilotriacetate (Fe-NTA), also effective renal carcinogens, appear to cause indirect damage to DNA mediated by oxidative stress. A number of nongenotoxic chemicals are associated with epigenetic renal tumor induction in rodents, and the activity of these tends to involve prolonged stimulation of cell proliferation throughout the duration of exposure. This mode of action reflects a sustained regenerative response, either due to direct chemical toxicity to the tubule cells, as with chloroform, or to indirect cytotoxicity associated with lysosomal overload, as in alpha2u-globulin accumulation in male rats resulting from the administration of such chemicals as d-limonene and tetrachloroethylene. The histopathologic nature of hydroquinone renal carcinogenesis suggests that an additional epigenetic pathway to renal tubule tumor formation in rats may be through chemical-mediated exacerbation of, and interaction with, the age-related spontaneous renal disease, chronic progressive nephropathy. These various mechanistic pathways have implications for the nature of the induced cancer process with respect to tumor incidence, latency, malignancy, and sex predisposition.

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.

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.

A research to develop a predicting system of mammalian subacute toxicity. II. Single dose detailed toxicity studies

"Single dose detailed toxicity study" in rats was conducted with 6 chemical substances to establish a predicting system of subacute toxicity of non-congeneric industrial chemicals. In the detailed examinations, new and previously reported biological parameters were developed and utilized for the establishment and confirmation of the correlations for both qualitative and quantitative prediction of target organs and effects in repeated dose toxicity studies of non-congeners; the new parameters were concerned with liver, kidneys and blood and the established relating equations for prediction had a correlation coefficient of 0.83 (liver), 0.49 (kidneys) and 0.94 (blood), while the old parameters effectively confirmed the previous correlations. The serial changes in the study gave a biological explanation to the established relationships between acute and subacute toxicities in terms of extrapolation.

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.

Risk assessment and biological mechanisms: lessons learned, future opportunities

During the past decade, toxicological research has been dominated by two themes; investigations to elucidate the mechanisms of action of toxicants and studies to provide information to support improved assessments of human health risks. The conduct of mechanistic investigations was given an early impetus by advances in biochemistry and cell biology and, more recently, by related advances in molecular biology. Research to provide information for improved human health risk assessments was stimulated by the 1983 NAS/NRC report that provided a codified structure for conducting risk assessments. At first glance, it would appear that the two themes are closely related and, indeed, should represent parts of a joined theme. However, examination of the toxicology/risk assessment literature of the past decade indicates that this has not been the case. Reports of mechanistic studies infrequently indicate how the information can be used to provide improved estimates of human risk from exposure to toxicants. If reference is made, it is usually qualitative in nature. Neither is examination of the risk assessment literature reassuring. Mechanistic studies may be cited; however, the final step of the process, risk characterization, is usually dominated by the use of default options grounded in conservative interpretations of generic scientific knowledge. Two examples are reviewed that stand out as illustrations of how mechanistic information can be used to make a difference in risk assessments: (1) consideration of the alpha 2u-globulin-mediated mechanism for evaluating male rat data for relevance in assessing human risks of renal cancer and (2) the use of DNA-protein cross-links as an internal dose metric in cross-species extrapolation of nasal cancer risks from inhaled formaldehyde. This paper reviews past experience on these topics and suggests a strategy for increasing the use of mechanistic information in risk assessments. A key component of the strategy is to use the risk assessment process to identify research needs/opportunities that, if addressed, will reduce the use of default options, thereby reducing the uncertainties in risk assessments. Another component of the strategy is to identify a few chemicals anticipated to exert their effect via different mechanisms and whose mechanisms of dosimetry and disease pathogenesis can be investigated in-depth within a risk assessment framework; this identification will create prototype approaches as alternatives to the use of default options that have major impact on the outcome of the risk assessment process.

Toxicity and carcinogenicity of t-butyl alcohol in rats and mice following chronic exposure in drinking water

t-Butyl alcohol (TBA) was administered in drinking water to F344/N rats and B6C3F1 mice for two years using 60 animals/dose/sex/species. Male rats received doses of 0, 1.25, 2.5, or 5 mg/ml and females received 0, 2.5, 5, or 10 mg/ml, resulting in average daily doses of approximately 85, 195, or 420 mg TBA/kg body weight for males and 175, 330, or 650 mg/kg for females. Ten rats per group were evaluated after 15 months. Male and female mice received doses of 0, 5, 10, or 20 mg/ml, resulting in average daily doses of approximately 535, 1,035, or 2,065 mg TBA/kg body weight for males and 510, 1,015, or 2,105 mg/kg for females. Survival was significantly reduced in male rats receiving 5 mg/ml, female rats receiving 10 mg/ml, and male mice receiving 20 mg/ml. Long-term exposure to TBA produced increased incidences of renal tubule adenoma and carcinoma in male rats; transitional epithelial hyperplasia of the kidney in male and female rats; follicular cell adenoma of the thyroid in female mice; and follicular cell hyperplasia of the thyroid and inflammation and hyperplasia of the urinary bladder in male and female mice. In addition, a slight increase in follicular cell adenoma or carcinoma of the thyroid (combined) in male mice may have been related to the administration of TBA.

Two-generation reproduction toxicity study with isopropanol in rats

A two-generation reproduction toxicity study was conducted in rats with isopropanol. Thirty rats of each sex per group (P1) were dosed once daily by oral gavage with 0, 100, 500 or 1000 mg isopropanol kg-1 for at least 10 weeks prior to mating. Parental animals were mated within groups for up to 3 weeks. Parental females were dosed during mating, gestation and lactation; parental males were dosed during mating through delivery of their last litter sired. The P2 adults were selected from the F1 litters and were dosed for 10-13 weeks before mating to produce a single litter. Findings in the parental animals included increased lactation body weight gain in the mid- and high-dose females, increased liver and kidney weights in the mid- and high-dose groups of both sexes and centrilobular hepatocyte hypertrophy in some P2 males. There was also accumulation of hyaline droplets and other microscopic findings in the kidneys from the mid- and high-dose P1 males and from all treated groups of the P2 males. Increased mortality was observed in the high-dose F1 offspring during the early postnatal period, although no other clinical signs of toxicity were observed in the offspring of either generation. In addition, offspring body weight was reduced during the early postnatal period in the high-dose F1 males and in the high-dose F2 pups of both sexes. Eighteen out of 70 F1 weanlings in the 1000 mg kg-1 group died or were euthanized prior to P2 selection. No treatment-related post-mortem findings were observed in the offspring from either generation.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Studies on the renal transport of trimethylpentanoic acid metabolites of 2,2,4-trimethylpentane in rat renal cortical slices

2,2,4-Trimethylpentane (TMP), a nephrotoxic component of unleaded gasoline in male but not female rats, undergoes oxidative metabolism to yield 2,2,4- and 2,4,4-trimethylpentanol, pentanoic acid and 5-hydroxypentanoic acid. We have examined the effect of three of these pentanoic acid metabolites on the renal transport of the organic anion p-aminohippurate (PAH) and the organic cation tetraethylammonium (TEA) in renal cortical slices from male Fischer 344 rats. 2,4,4-Trimethylpentanoic acid, the major urinary metabolite in rats, produced a selective decrease in the accumulation of PAH without affecting TEA accumulation. Kinetic analysis showed that 2,4,4-trimethylpentanoic acid was a competitive inhibitor of the organic anion transport system, with a Ki of 4 mM. 2,4,4-Trimethyl-5-hydroxypentanoic acid also showed selective inhibition of PAH transport, while 2,2,4-trimethylpentanoic acid was less selective and reduced both PAH and TEA transport. Additional studies with radiolabeled 2,4,4-trimethylpentanoic acid showed that there was a time- and concentration-dependent accumulation of radioactivity into slices of renal cortex. However, experiments conducted at 4 degrees C and studies with metabolic inhibitors, or with an inhibitor of organic anion transport, indicated that little of the accumulated material was entering the cell. We conclude from these studies that the pentanoic acid metabolites formed from 2,2,4-trimethylpentane are not actively transported by the renal organic anion transport system. In summary, in vitro the pentanoic acid metabolites appear to bind to renal cortical tissue and thereby reduce the transport of PAH.

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.

Hyaline droplet nephropathy resulting from exposure to 3,5,5-trimethylhexanoyloxybenzene sulfonate

Acute oral dosing of 3,5,5-trimethylhexanoyloxybenzene sulfonate (THBS) to adult male and female rats causes a male rat-specific nephrotoxicity manifested as exacerbation of hyaline droplet formation. This chemical is structurally distinct from the volatile hydrocarbons known to cause male rat-specific kidney lesions. Therefore, to classify THBS as a hyaline droplet-inducing agent, experiments were conducted to determine whether [14C]THBS equivalents bound to alpha 2 mu-globulin and caused the protein to accumulate in male rat kidney cortex. Two-dimensional gel electrophoretic separation of male rat kidney proteins indicated that alpha 2u-globulin levels in kidney increased 24 hr after a single oral dose of THBS (500 mg/kg). Furthermore, a sex-dependent retention THBS was noted as there was approximately 10 times more THBS equivalents in male rat kidney than in female rat kidney. Equilibrium dialysis experiments indicated that 40% of THBS equivalents bound reversibly to male rat kidney proteins, whereas no interaction between THBS and female rat kidney proteins was detected. Specific binding of THBS to alpha 2mu-globulin was determined by anion-exchange HPLC after which metabolites in the alpha 2u-globulin fraction were identified by gas chromatography with parallel radioactivity-mass spectrometry and mass spectrometry-matrix isolation Fourier-transform infrared analysis. Four metabolites of THBS were found in this protein fraction, and the major component (approximately 70%) was identified as the cis gamma-lactone of 3,5,5-trimethylhexanoic acid. Experiments were also conducted in mice to determine whether THBS bound to any mouse kidney proteins, particularly mouse urinary protein. The results indicated that there was no interaction between THBS and mouse urinary protein, a protein which shares significant homology with alpha 2u-globulin. These results indicate that THBS treatment exacerbates hyaline droplet formation in male rat kidneys by binding to alpha 2mu-globulin, thereby causing the protein to accumulate in the renal cortex. The interaction between THBS and alpha 2mu-globulin appears to be unique to this male rat-specific protein as THBS does not interact with a very similar protein found in mice.

The human relevance of the renal tumor-inducing potential of d-limonene in male rats: implications for risk assessment

The monoterpene d-limonene is a naturally occurring chemical which is the major component in oil of orange. Currently, d-limonene is widely used as a flavor and fragrance and is listed to be generally recognized as safe (GRAS) in food by the Food and Drug Administration (21 CFR 182.60 in the Code of Federal Regulations). Recently, however, d-limonene has been shown to cause a male rat-specific kidney toxicity referred to as hyaline droplet nephropathy. Furthermore, chronic exposure to d-limonene causes a significant incidence of renal tubular tumors exclusively in male rats. Although d-limonene is not carcinogenic in female rats or male and female mice given much higher dosages, the male rat-specific nephrocarcinogenicity of d-limonene may raise some concern regarding the safety of d-limonene for human consumption. A considerable body of scientific data has indicated that the renal toxicity of d-limonene results from the accumulation of a protein, alpha 2u-globulin, in male rat kidney proximal tuble lysosomes. This protein is synthesized exclusively by adult male rats. Other species, including humans, synthesize proteins that share significant homology with alpha 2u-globulin. However, none of these proteins, including the mouse equivalent of alpha 2u-globulin, can produce this toxicity, indicating a unique specificity for alpha 2u-globulin. With chronic exposure to d-limonene, the hyaline droplet nephropathy progresses and the kidney shows tubular cell necrosis, granular cast formation at the corticomedullary junction, and compensatory cell proliferation. Both d-limonene and cis-d-limonene-1,2-oxide (the major metabolite involved in this toxicity) are negative in in vitro mutagenicity screens. Therefore, the toxicity-related renal cell proliferation is believed to be integrally involved in the carcinogenicity of d-limonene as persistent elevations in renal cell proliferation may increase fixation of spontaneously altered DNA or serve to promote spontaneously initiated cells. The scientific data base demonstrates that the tumorigenic activity of d-limonene in male rats is not relevant to humans. The three major lines of evidence supporting the human safety of d-limonene are (1) the male rat specificity of the nephrotoxicity and carcinogenicity; (2) the pivotal role that alpha 2u-globulin plays in the toxicity, as evidenced by the complete lack of toxicity in other species despite the presence of structurally similar proteins; and (3) the lack of genotoxicity of both d-limonene and d-limonene-1,2-oxide, supporting the concept of a nongenotoxic mechanism, namely, sustained renal cell proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of chemical binding to alpha 2u-globulin in vitro--evaluating structure-activity relationships

alpha 2u-Globulin (alpha 2u) has been shown to accumulate in the kidneys of male rats treated with 2,2,4-trimethylpentane (TMP). 2,4,4-Trimethyl-2-pentanol (TMP-2-OH), a metabolite of TMP, is found reversibly bound to alpha 2u isolated from the kidneys of these treated rats. The objectives of the following study were to characterize the ability of [3H]TMP-2-OH to bind to alpha 2u in vitro and to determine whether other compounds that cause this protein to accumulate have the same binding characteristics. Although compounds that have been shown to cause the accumulation of alpha 2u in male rat kidneys compete in vitro with [3H]TMP-2-OH for binding to alpha 2u, they do so to varying degrees. The binding affinity (Kd) of the [3H]TMP-2-OH-alpha 2u complex was calculated to be on the order of 10(-7) M. The inhibition constant values (Ki) determined for d-limonene, 1,4-dichlorobenzene, and 2,5-dichlorophenol were all in the range 10(-4) M, whereas the Ki values for isophorone, 2,4,4- or 2,2,4-trimethyl-1-pentanol, and d-limonene oxide were determined to be in the range 10(-6) and 10(-7) M, respectively. TMP and 2,4,4- and 2,2,4-trimethylpentanoic acid did not compete for binding. This suggests that other factors, besides binding, are involved in the accumulation of alpha 2u. In this study the ability of a chemical to bind to alpha 2u was used as a measure of biological activity to assess structure-activity relationships among the chemicals tested and known to cause the accumulation of alpha 2u. The results so far suggest that binding is dependent on both hydrophobic interactions and hydrogen bonding.

Hyaline droplet accumulation in rodent kidney proximal tubules: an association with histiocytic sarcoma

Since recognition during the last decade that certain renal carcinogens can initially cause an accumulation of hyaline (protein) droplets in proximal tubules of male rats, it has become appropriate to establish whether this phenomenon of protein overload can also occur in rodent kidneys unrelated to chemical treatment. Kidney tissue from a number of selected rodent studies held in the National Toxicology Program (NTP) or Food and Drug Administration (FDA) archives were evaluated for hyaline droplet accumulation in proximal tubules. The survey concentrated on rats and mice of both sexes bearing hematopoietic tumors, as our preliminary observations had suggested this direction of study. The tissues of 101 Sprague-Dawley, 25 Osborne-Mendel, and 70 Fischer 344 rats and 96 B6C3F1 mice were examined. These animals provided an assortment of tumors including histiocytic sarcoma, lymphocytic lymphoma, mononuclear cell leukemia, and sarcoma. Hyaline droplet accumulation, primarily involving the P2 segment of proximal tubules, was diagnosed in 96% of rats with histiocytic sarcoma (74/77 cases in Sprague-Dawley, 17/18 in Osborne-Mendels, 7/7 in Fischers) and in 55% of B6C3F1 mice with histiocytic sarcoma (18/33 cases). There appeared to be a qualitative correlation between hyaline droplet accumulation and degree of tumor burden. Thus, in cases negative for hyaline droplets, the tumor was often confined to a single location, while increasing involvement of proximal segments beyond P2 occurred with more extensive multi-organ dissemination of the tumor. By immunohistochemistry on 11 cases of rat and 8 cases of mouse histiocytic sarcoma, the protein in hyaline droplets was identified as lysozyme, a known major secretory product of monocytes and macrophages. The hyaline droplets were negative for alpha 1-antitrypsin, alpha 2u-globulin, rat or mouse immunoglobulin, and albumin. More sparsely scattered droplets and granules present in proximal tubules of Fischer rats with mononuclear cell leukemia were negative for lysozyme but positive for either iron or lipofuscin pigment. The study establishes a clear association between renal tubule hyaline droplet and lysozyme accumulation in rats and mice with histiocytic sarcoma. Hyaline droplets secondary to neoplasia should be distinguished from chemically-induced hyaline droplet nephropathy in the male rat involving alpha 2u-globulin.

Behavior of alpha 2u-globulin accumulating in kidneys of male rats treated with d-limonene: kidney-type alpha 2u-globulin in the urine as a marker of d-limonene nephropathy

Effects of d-limonene on alpha 2u-globulin in the kidneys, urine and serum were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting analysis. Treatment of male rats with d-limonene by gavage for 14 consecutive days (300 mg/kg/day) caused accumulation of hyaline droplets in renal proximal tubule cells, and a marked intensification of a protein band corresponding to the kidney-type alpha 2u-globulin, with a molecular weight calculated to be approximately 16 kDa. However, no significant changes in the serum alpha 2u-globulin (native-type) band, of approximately 19 kDa, were observed between treated rats and controls, suggesting that circulating alpha 2u-globulin levels were not affected by the d-limonene administration. While the molecular weight of the major alpha 2u-globulin in the urine from control rats was the same as that in the serum (native-type), marked increase in the protein band corresponding to kidney-type-alpha 2u-globulin was observed in the urine from treated rats. The results were indicative of elimination of alpha 2u-globulin from the kidney to urine, the appearance of kidney-type-alpha 2u-globulin in urine implying disruption or exfoliation of proximal tubule cells. Therefore, it is suggested that the presence of the alpha 2u-globulin (kidney-type) in the urine might be used as an indicator of chemically induced alpha 2u-globulin nephropathy.

Relationships between structure and induction of hyaline droplet accumulation in the renal cortex of male rats by aliphatic and alicyclic hydrocarbons

A rapidly growing list of hydrocarbons has been reported to induce morphological changes in the kidney of adult male rats, beginning with hyaline droplet accumulation (HDA) followed by the development of granular casts, later on chronic nephrosis as sequela, and finally renal adenomas and carcinomas. The present study focuses on identifying structure-based properties common to HDA-inducing aliphatics and cycloaliphatics. On the basis of rank-ordered activities reported in the literature, a calculated n-octanol-water partition coefficient above 3.5 and the presence of an isopentyl structural moiety appear to be associated with HDA-inducing activity in aliphatics. A binding site model for highly active aliphatics has been derived by superimposing their minimum energy conformations along the common isopentyl substructure and calculating the union volume of their respective van der Waal (VDW) volumes. Generalization of this model to include cycloaliphatics has been achieved by maximizing the steric overlap of the VDW volumes of the compounds with their binding site union volume. HDA-inducing cycloaliphatics are correctly identified on the basis of their negligible excess volume. This approach has been used to predict the HDA-inducing activity of previously untested compounds. Eighteen aliphatic/cycloaliphatic hydrocarbons were screened in a study on adult male Wistar rats treated with 250 mg/kg per day for 5 days. Azan-stained kidney sections were semiquantitatively evaluated for the presence of HDA. The predicted and observed HDA activities were in very good agreement.

Lysosomal changes in renal proximal tubular epithelial cells of male Sprague Dawley rats following decalin exposure

A histochemical stain for acid phosphatase served as a marker for lysosomal alterations in renal tubular cells associated with male rat hyaline droplet nephropathy. Morphometric analysis and quantitative histochemistry were used to compare the size and acid phosphatase stain reaction of lysosomes in tubular epithelial cells of treated and control animals. Decalin exposure increased the size and significantly (p less than 0.01) reduced the acid phosphatase stain intensity of individual lysosomes. However, there was no significant different (p greater than 0.05) between the acid phosphatase stain intensity of treated and control animals when analyzed on a whole cell basis. The increase in size of the lysosomes without a proportional increase in whole cell acid phosphatase stain intensity indicates a dilution or a failure to accommodate in the acid phosphatase concentration (stain intensity/microns 2) per lysosome. All acid phosphatase stain reaction product was contained within intact lysosomes, mitigating against the hypothesis of lysosomal enzyme leakage as the cause of cell death in decalin-induced alpha 2U globulin nephropathy.

Lysosomal degradation of alpha 2u-globulin and alpha 2u-globulin-xenobiotic conjugates

A diverse group of chemicals cause a male rat-specific nephrotoxicity in which alpha 2u-globulin accumulates in renal lysosomes. It has been suggested that these chemicals bind to the protein and decrease its degradation by lysosomal proteinases. To test this hypothesis, the lysosomal degradation of native alpha 2u-globulin and that to which d-limonene, d-limonene-1,2-oxide, isophorone, 1,4-dichlorobenzene, and 2,5-dichlorophenol were bound was studied. alpha 2u-Globulin was purified from male rat urine, and male rat renal cortical lysosomes, isolated by differential centrifugation, served as the proteolytic enzyme source. Pepstatin, an inhibitor of aspartic acid proteinases, and leupeptin, an inhibitor of cysteine proteinases, reduced alpha 2u-globulin degradation to 28 +/- 8 and 17 +/- 5% of control, respectively, whereas addition of both inhibitors decreased alpha 2u-globulin degradation to 8 +/- 1% of control values. These results indicate that both classes of endopeptidases are important in the degradation of alpha 2u-globulin. Under the incubation conditions used, 30% of native alpha 2u-globulin was degraded in a 4-hr period. Conjugates of the protein were made for in vitro binding experiments. Binding of d-limonene and 1,4-dichlorobenzene to alpha 2u-globulin did not alter the degradation of the protein, whereas binding of d-limonene-1,2-oxide, 2.5-dichlorophenol, and isophorone decreased alpha 2u-globulin degradation by 33%. These results indicate that not all chemicals which have been shown to bind in vivo to alpha 2u-globulin alter the in vitro lysosomal degradation of the protein. However, in all cases, one metabolite of each hyaline droplet inducer did alter degradation of alpha 2u-globulin, suggesting that a decrease in lysosomal degradation is involved in the accumulation of this protein in male rat kidney lysosomes.

A comparison of male rat and human urinary proteins: implications for human resistance to hyaline droplet nephropathy

alpha 2u-Globulin (alpha G), the major urinary protein of sexually mature male rats, is a key determinant of susceptibility to hyaline droplet nephropathy (HDN) induced by a variety of hydrocarbons in male rats. Arguments against extrapolating renal toxicity and carcinogenicity data for HDN-inducing toxicants from male rats to risk assessment for humans rely on the observation that humans do not express alpha G. Yet, human serum and urine are known to contain proteins coded for by the same gene family that also controls alpha G synthesis in the rat. Therefore, to understand some of the quantitative and qualitative differences between proteins of human and male rat urine which confer apparent resistance to HDN in humans, urinary proteins of male F344 rats (ca. 3 months old) and normal human males were compared by cation exchange, gel filtration, SDS-PAGE, and partially identified by Western blotting. We observed that (1) the protein content of human urine is only 1% that of male rat urine; (2) human urinary proteins, recovered by (NH4)2SO4 precipitation followed by dialysis, are primarily of high (greater than or equal to 75 kDa) molecular weight (MW) with minor components of 12-66 kDa; (3) male rat urine has little high-MW protein, but is rich in alpha G (18.5 kDa); (4) at pH 5, the most cationic fraction of human urinary protein constituted only about 4% of the total while the analogous fraction of rat urine, containing alpha G, contained 26% of total urinary protein; and (5) cationic (at pH 5.0) human urinary proteins included small amounts of proteins, e.g., alpha 1-acid glycoprotein, and alpha 1-microglobulin, which are products of the gene family coding for alpha G in rat. Thus, although humans excrete trace amounts of proteins similar to alpha G, the very low protein content of human urine, the relatively small proportion of cationic to total proteins, and the high MW of the most abundant human urinary proteins form a biological basis for suggesting that humans are not at risk for the type of fuel and solvent hydrocarbon-induced nephropathy, and the sequelae of such nephropathy, observed in male rats.

Elevated proliferation of proximal tubule cells and localization of accumulated alpha 2u-globulin in F344 rats during chronic exposure to unleaded gasoline or 2,2,4-trimethylpentane

In order to better characterize the pathogenesis of alpha 2u-globulin (alpha 2uG) nephropathy, cell proliferation was quantitated within the three proximal tubule segments of the kidney (P1, P2, and P3) and proximal tubule segments affected by chronic progressive nephrosis (CPN) in male and female F344 rats exposed to 10, 70, or 300 ppm unleaded gasoline (UG) or 50 ppm 2,2,4-trimethylpentane (TMP) from 3 to 50 weeks. The P2 segment of male rats exposed to UG or TMP responded with dose-related increases in cell turnover (up to 11-fold) that persisted during chronic exposure. This proliferative response closely paralleled the extent and severity of immunohistochemically detectable alpha 2uG in the P2 segment. Neither alpha 2uG nor cytotoxicity was evident in cells of the P1 or P3 segment; however, cell proliferation was increased (up to 8-fold) for up to 22 weeks of exposure in the P3 segment. Increased numbers of proximal tubules affected by CPN were found in males exposed to UG or TMP for 22 or 48 weeks, compared to controls. These lesions contained epithelial cells that were highly proliferative. Control or treated female rats exhibited neither alpha 2uG nephropathy nor increases in P2 or P3 cell turnover, and the extent of CPN was greatly reduced as compared to male rats. The results of this and related studies suggest that chronic cell proliferation associated with alpha 2uG nephropathy and CPN in male rats exposed to UG or isoparaffinic components of UG, such as TMP, may be responsible for the sex- and species-specific nephrocarcinogenic effects of UG.