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

Chaperone-mediated autophagy

Continuous renewal of intracellular components is required to preserve cellular functionality. In fact, failure to timely turnover proteins and organelles leads often to cell death and disease. Different pathways contribute to the degradation of intracellular components in lysosomes or autophagy. In this review, we focus on chaperone-mediated autophagy (CMA), a selective form of autophagy that modulates the turnover of a specific pool of soluble cytosolic proteins. Selectivity in CMA is conferred by the presence of a targeting motif in the cytosolic substrates that, upon recognition by a cytosolic chaperone, determines delivery to the lysosomal surface. Substrate proteins undergo unfolding and translocation across the lysosomal membrane before reaching the lumen, where they are rapidly degraded. Better molecular characterization of the different components of this pathway in recent years, along with the development of transgenic models with modified CMA activity and the identification of CMA dysfunction in different severe human pathologies and in aging, are all behind the recent regained interest in this catabolic pathway.

Metabolism and disposition of juglone in male F344 rats

The metabolism and disposition of 14C-labelled juglone in male F344 rats following oral, intravenous and dermal administration were studied. Approximately 40-50% of an oral dose (0.1 to 10 mg kg-1) and less than 20% of the dermal dose (4 mg kg-1) were absorbed within 24 h. Most of the oral dose was excreted in faeces and urine within 24 h and only 1-3% remained in the tissues. High concentrations of juglone-derived radioactivity were found in kidney for all three dosing routes. The accumulation in kidney can be attributed to covalent binding of juglone and/or metabolites to cytosolic protein. Five metabolites were identified in the urine of rats treated with an oral dose: 1,4,5-trihydroxynaphthalene di-glucuronide, 1,4,5-trihydroxynaphthalene mono-glucuronide mono-sulfate, 2-sulfo-2,3-dihydrojuglone, 4,8-dihydroxy-1-tetralone mono-glucuronide and 1,4,5-trihydroxynaphthalene mono-glucuronide. Liver microsomal incubations of juglone in the presence of NAD(P)H and UDP-glucuronic acid gave rise to two 1,4,5-trihydroxynaphthalene mono-glucuronides.

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

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

Chaperone-mediated autophagy in aging and disease

Different mechanisms target intracellular components for their degradation into lysosomes through what is known as autophagy. In mammals, three main forms of autophagy have been described: macroautophagy, microautophagy, and chaperone-mediated autophagy (CMA). CMA is the only autophagic pathway that allows selective degradation of soluble proteins in lysosomes. In contrast to the other mammalian forms of autophagy, CMA does not require vesicle formation or major changes in the lysosomal membrane. Instead, substrate proteins directly cross the lysosomal membrane to reach the lumen, where they are rapidly degraded. The substrate proteins are targeted to the lysosomal membrane by recognition of a targeting motif (a KFERQ-like motif), by a chaperone complex, consisting of hsc70 and its cochaperones, in the cytoplasm. Once at the lysosomal membrane, the protein interacts with a lysosomal receptor for this pathway, lysosomal associated membrane protein type 2A (LAMP-2A), and it is translocated across the membrane into the lysosomal lumen assisted by a lysosome resident chaperone. These two characteristics--selectivity and direct substrate translocation--determine the particular role of CMA in different physiological and pathological conditions. In this chapter, we cover current findings on the molecular mechanisms for CMA and the possible pathophysiological relevance of this selective lysosomal degradation.

Pathophysiology of chaperone-mediated autophagy

In contrast to the classically described "in bulk" lysosomal degradation, the first evidence for selective degradation of cytosolic proteins in lysosomes was presented more than 20 years ago. Throughout this time, we have gained a better understanding about this process, now known as chaperone-mediated autophagy (CMA). The identification of new substrates for CMA and novel components, in both the cytosol and the lysosomes, along with better insights on how CMA is regulated, have all helped to shape the possible physiological roles of CMA. We review here different intracellular functions of CMA that arise from its unique characteristics when compared to other forms of autophagy. In view of these functions, we discuss the relevance of the changes in CMA activity in aging and in different pathological conditions.

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.

Differential expression of cytosolic proteins in the rat kidney cortex and medulla: preliminary proteomics

The rodent kidney is a target of many xenobiotics and is typified by regionally specific structure and function. This renders distinct regions of the kidney differentially susceptible to toxic exposure and effect. To characterize these differences at the proteome level, protein patterns from male rat kidney cortex and medulla cytosols were examined by two-dimensional electrophoresis (2-DE) and image analysis and prominent proteins identified immunologically or by matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) and electrospray/ionization-tandem mass spectrometry (ESI-MS/MS) sequence tag identification. An average of 727 protein spots were resolved and matched to the cortex cytosol reference pattern, and 716 in the medulla. Of this total, 127 proteins were found to differ in abundance (86 higher in cortex; 41 higher in medulla) (P < 0.001). Of those proteins that were detectable in both cortex and medulla, the abundance of 97 differed significantly while 30 proteins were found to be unique to one region or the other (26 in cortex, 4 in medulla). Twenty protein spots were identified and their regional differences are discussed. These results both confirm and expand our understanding of the molecular heterogeneity characterizing structurally and functionally distinct regions of the kidney and serve as a useful foundation for future nephrotoxicologic studies.

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.

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.

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.

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.

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.

Male rat specific nephrotoxicity resulting from subchronic administration of hexachlorobenzene

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