Rapid postexposure decay of alpha 2u-globulin and hyaline droplets in the kidneys of gasoline-treated male rats

Toxicological Assessment of Higher Olefins in OECD TG 422 Repeated Dose and Reproductive /Developmental Toxicity Screening Tests in Han Wistar Rats

Higher olefins (HO) are used primarily as intermediates in the production of other chemicals, such as polymers, fatty acids, plasticizer alcohols, surfactants, lubricants, amine oxides, and detergent alcohols. The potential toxicity of five HO (i.e., 1-Octene, Nonene, Decene, Hexadecene, and 1-Octadecene) with carbon ranging from C8 to C18 was examined in a combined repeated dose and reproduction/developmental toxicity screening study (OECD TG 422). These five HO were administered to Han Wistar rats by gavage at 0 (controls), 100, 300, and 1000 mg/kg bw/day. As a group of substances, adaptive changes in the liver (liver weight increase without pathological evidence), as well as increased kidney weight in male rats, were observed in HO with carbon numbers from C8 to C10. The overall systemic no observed adverse effect level (NOAEL) for all HO was determined at 1000 mg/kg bw/day. In the reproductive/developmental toxicity assessment, offspring viability, size, and weights were reduced in litters from females treated with Nonene at 1000 mg/kg bw/day. The overall no observed effects level (NOEL) for reproductive toxicity was considered to be 300 mg/kg bw/day for Nonene and 1000 mg/kg bw/day for the other four HO, respectively. These data significantly enrich the database on the toxicity of linear and branched HO, allowing comparison with similar data published on a range of linear and branched HO. Comparisons between structural class and study outcome provide further supportive data in order to validate the read-across hypothesis as part of an overall holistic testing strategy.

Renal biomarker changes associated with hyaline droplet nephropathy in rats are time and potentially compound dependent

Alpha 2u-globulin mediated hyaline droplet nephropathy (HDN) is a male rat specific lesion induced when a compound or metabolite binds to alpha 2u-globulin. The objective of this study was to investigate if the newer and more sensitive renal biomarkers would be altered with HDN as well as be able to distinguish between HDN and oxidative stress-induced kidney injury. Rats were dosed orally for 7 days to determine (1) if HDN (induced by 2-propanol or D-limonene) altered the newer renal biomarkers and not BUN or creatinine, (2) if renal biomarkers could distinguish between HDN and oxidative stress-induced kidney injury (induced by potassium bromate), (3) sensitivity of HDN-induced renal biomarker changes relative to D-limonene dose, and (4) reversibility of HDN and renal biomarkers, using vehicle or 300 mg/kg/day D-limonene with 7 days of dosing and necropsies scheduled over the period of Days 8-85. HDN-induced renal biomarker changes in male rats were potentially compound specific: (1) 2-propanol induced mild HDN without increased renal biomarkers, (2) potassium bromate induced moderate HDN with increased clusterin, and (3) D-limonene induced marked HDN with increased αGST, μGST and albumin. Administration of potassium bromate did not result in oxidative stress-induced kidney injury, based on histopathology and renal biomarkers creatinine and BUN. The compound D-limonene induced a dose dependent increase in HDN severity and renal biomarker changes without altering BUN, creatinine or NAG: (1) minimal induction of HDN and no altered biomarkers at 10 mg/kg/day, (2) mild induction of HDN with increased αGST and μGST at 50 mg/kg/day and (3) marked induction of HDN with increased αGST, μGST and albumin at 300 mg/kg/day. HDN induced by D-limonene was reversible, but with a variable renal biomarker pattern over time: Day 8 there was increased αGST, μGST and albumin; on Day 15 increased clusterin, albumin and Kim-1. In summary, HDN altered the newer and more sensitive renal biomarkers in a time and possibly compound dependent manner.

Assessment in rats of the reproductive toxicity of gasoline from a gasoline vapor recovery unit

Gasoline (CAS 86290-81-5) is one of the world's largest volume commercial products. Although numerous toxicology studies have been conducted, the potential for reproductive toxicity has not been directly assessed. Accordingly, a two-generation reproductive toxicity study in rats was conducted to provide base data for hazard assessment and risk characterization. The test material, vapor recovery unit gasoline (68514-15-8), is the volatile fraction of formulated gasoline and the material with which humans are most likely to come in contact. The study was of standard design. Exposures were by inhalation at target concentrations of 5000, 10 000, and 20 000 mg/m(3). The highest exposure concentration was approximately 50% of the lower explosive limit and several orders of magnitude above anticipated exposure during refueling. There were no treatment-related clinical or systemic effects in the parental animals, and no microscopic changes other than hyaline droplet nephropathy in the kidneys of the male rats. None of the reproductive parameters were affected, and there were no deleterious effects on offspring survival and growth. The potential for endocrine modulation was also assessed by analysis of sperm count and quality as well as time to onset of developmental landmarks. No toxicologically important differences were found. Therefore, the NOAEL for reproductive toxicity in this study was > or =20 000 mg/m(3). The only systemic effects, in the kidneys of the male rats, were consistent with an alpha-2 u-globulin-mediated process. This is a male rat-specific effect and not relevant to human health risk assessment.

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.

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.

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.

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.

Sex difference in inhalation toxicity of p-dichlorobenzene (p-DCB) in rats

The organ distribution and toxicity of p-dichlorobenzene (p-DCB) were compared in male and female rats after inhalation of 500 ppm of p-DCB for 24 h in a whole-body chamber. Concentrations of p-DCB in the serum, liver, kidney and fatty tissues were measured by gas chromatography at intervals during and up to 24 h after the treatment. Though no significant differences in the serum levels were observed between male and female rats, the p-DCB values in the livers of female rats were significantly higher than those of male rats. Conversely, significantly higher levels were found in the kidneys of male than of female rats. The distribution results thus appeared to correlate with the fact that nephrotoxic changes were observed only in male rats and that the appearance of minor hepatotoxic changes was limited to females.

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.