Glutathione status and the renal elimination of inorganic mercury in the Mrp2(-/-) mouse

Protective activities of ellagic acid and urolithins against kidney toxicity of environmental pollutants: A review

Oxidative stress and inflammation are two possible mechanisms related to nephrotoxicity caused by environmental pollutants. Ellagic acid, a powerful antioxidant phytochemical, may have great relevance in mitigating pollutant-induced nephrotoxicity and preventing the progression of kidney disease. This review discusses the latest findings on the protective effects of ellagic acid, its metabolic derivatives, the urolithins, against kidney toxicity caused by heavy metals, pesticides, mycotoxins, and organic air pollutants. We describe the chelating, antioxidant, anti-inflammatory, antifibrotic, antiautophagic, and antiapoptotic properties of ellagic acid to attenuate nephrotoxicity. Furthermore, we present the molecular targets and signaling pathways that are regulated by these antioxidants, and suggest some others that should be explored. Nevertheless, the number of reports is still limited to establish the efficacy of ellagic acid against kidney damage induced by environmental pollutants. Therefore, additional preclinical studies on this topic are required, as well as the development of well-designed clinical trials.

The Aging Kidney-As Influenced by Heavy Metal Exposure and Selenium Supplementation

The aging process in the kidneys has been well studied. It is known that the glomerular filtration rate (GFR) declines with age in subjects older than 50–60 years. However, there is still insufficient knowledge regarding the response of the aged kidney to environmental toxicants such as mercury, cadmium, and lead. Here, we present a review on the functional decline and proposed mechanisms in the aging kidney as influenced by metal pollutants. Due to the prevalence of these toxicants in the environment, human exposure is nearly unavoidable. Further, it is well known that acute and chronic exposures to toxic metals may be detrimental to kidneys of normal adults, thus it may be hypothesized that exposure of individuals with reduced GFR will result in additional reductions in renal function. Individuals with compromised renal function, either from aging or from a combination of aging and disease, may be particularly susceptible to environmental toxicants. The available data appear to show an association between exposure to mercury, cadmium and/or lead and an increase in incidence and severity of renal disease in elderly individuals. Furthermore, some physiological thiols, as well as adequate selenium status, appear to exert a protective action. Further studies providing improved insight into the mechanisms by which nephrotoxic metals are handled by aging kidneys, as well as possibilities of therapeutic protection, are of utmost importance.

Prophylactic supplementation with selenium alters disposition of mercury in aged rats

Mercury (Hg) is a prevalent environmental toxicant to which older individuals are particularly susceptible. Selenium (Se) has been used as an antidote following exposure to Hg. However, little is known about the effect of prophylactic supplementation with Se on the handling of Hg. The current study was designed to test the hypothesis that oral pre-treatment with Se alters the corporal disposition of Hg and reduces the risk of Hg-induced toxicity. Young and aged rats were gavaged for 10 days with sodium selenite or saline. On day 11, rats were injected intravenously with 0.5 μmol HgCl₂·kg^-1·2 mL^-1 normal saline. After 24 h, rats were euthanized and organs and tissues were harvested for determination of Hg content. Accumulation of Hg in the kidney was reduced significantly by pre-treatment with Se in both young and aged rats. In the renal cortex, the magnitude of the reduction was greater in aged rats than in young rats but in the outer stripe of the outer medulla, the magnitude of the reduction was similar between groups of rats. Urinary excretion of Hg was also reduced in rats pre-treated with Se. In contrast, the hepatic and hematologic burden of Hg increased in rats pre-treated with Se. Fecal excretion of Hg was decreased significantly by pre-treatment with Se in young rats but not in aged rats. These data suggest that prophylactic supplementation with Se alters the corporal disposition of Hg in a way that may reduce Hg-induced toxicity in target organs.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Polymorphisms in potential mercury transporter ABCC2 and neurotoxic symptoms in populations exposed to mercury vapor from goldmining

BACKGROUND

Artisanal small-scale gold miners have high levels of mercury in human specimens often above recommended threshold values. There are differences reported in the manifestation of neurological symptoms of individuals with a comparable level of exposure to mercury, suggesting a genetic component influencing the susceptibility to mercury neurotoxicity.

OBJECTIVE

To investigate associations between polymorphisms (rs1885301, rs717620, rs2273697) in the potential Hg-transporter ABCC2 gene and neurological effects.

METHODS

968 participants from the Philippines, Indonesia, Tanzania and Zimbabwe were included in this study (age 12-59 years). For the statistical analysis the countries were categorized into Africa (Philippines, Indonesia) and Asia (Tanzania, Zimbabwe). Study participants were from three exposure groups: without mercury exposure from goldmining (n = 129); living in mercury-contaminated areas (n = 281) and mercury working exposure (n = 558). To identify chronic inorganic mercury intoxication we applied a modified neurological score sum including eight binary coded parameters (from anamnestic, clinical and neurophysiological examinations). Associations between genotype and neurological score sum, as well as between genotype and separate neurological tests (ataxia of gait, dysdiadochokinesia, ataxia heel to shin, pencil tapping test and matchbox test) were evaluated.

RESULTS

We found that ABCC2 genotype were associated with performance on certain neurological tests: for rs1885301, A-allele carriers in the African populations showed significantly worse performance than GG carriers on the pencil tapping test; whereas for rs2273697, A-allele carriers in the African and Asian populations showed a significantly better performance than GG carriers on this test. When including an interaction term between genotype and exposure subgroup, interaction effects were also seen for the pencil tapping test and matchbox tests for rs2273697 in Asia.

CONCLUSIONS

The results suggest that certain ABCC2 polymorphisms may influence the neurotoxic effects in mercury-burdened individuals. ABCC2 alleles associated with worse neurological performance in the present study. These alleles have previously been correlated with higher levels of urinary mercury concentrations in the same cohort. Taken together, these associations between ABCC2 genotype, mercury levels, and neurological effects support the hypothesis that some ABCC2 genotypes may have a higher neurotoxic potential, although further functional studies are needed to prove causation.

ABC Family Transporters

The transport of specific molecules across lipid membranes is an essential function of all living organisms. The processes are usually mediated by specific transporters. One of the largest transporter families is the ATP-binding cassette (ABC) family. More than 40 ABC transporters have been identified in human, which are divided into 7 subfamilies (ABCA to ABCG) based on their gene structure, amino acid sequence, domain organization, and phylogenetic analysis. Of them, at least 11 ABC transporters including P-glycoprotein (P-GP/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2) are involved in multidrug resistance (MDR) development. These ABC transporters are expressed in various tissues such as the liver, intestine, kidney, and brain, playing important roles in absorption, distribution, and excretion of drugs. Some ABC transporters are also involved in diverse cellular processes such as maintenance of osmotic homeostasis, antigen processing, cell division, immunity, cholesterol, and lipid trafficking. Several human diseases such as cystic fibrosis, sitosterolemia, Tangier disease, intrahepatic cholestasis, and retinal degeneration are associated with mutations in corresponding transporters. This chapter will describe function and expression of several ABC transporters (such as P-GP, BCRP, and MRPs), their substrates and inhibitors, as well as their clinical significance.

In vitro transport of methotrexate by Drosophila Multidrug Resistance-associated Protein

Methotrexate (MTX) is a widely used chemotherapeutic agent, immune suppressant and antimalarial drug. It is a substrate of several human ABC proteins that confer multidrug resistance to cancer cells and determine compartmentalization of a wide range of physiological metabolites and endo or xenobiotics, by their primary active transport across biological membranes. The substrate specificity and tissue distribution of these promiscuous human ABC transporters show a high degree of redundancy, providing robustness to these key physiological and pharmacological processes, such as the elimination of toxins, e.g. methotrexate from the body. A similar network of proteins capable of transporting methotrexate has been recently suggested to exist in Drosophila melanogaster. One of the key players of this putative network is Drosophila Multidrug-resistance Associated Protein (DMRP). DMRP has been shown to be a highly active and promiscuous ABC transporter, capable of transporting various organic anions. Here we provide the first direct evidence that DMRP, expressed alone in a heterologous system lacking other, potentially functionally overlapping D. melanogaster organic anion transporters, is indeed able to transport methotrexate. Our in vitro results support the hypothesized but debated role of DMRP in in vivo methotrexate excretion.

MRP2 and the Transport Kinetics of Cysteine Conjugates of Inorganic Mercury

Human exposure to mercuric species occurs regularly throughout the world. Mercuric ions may accumulate in target cells and subsequently lead to cellular intoxication and death. Therefore, it is important to have a thorough understanding of how transportable species of mercury are handled by specific membrane transporters. The purpose of the current study was to characterize the transport kinetics of cysteine (Cys)-S-conjugates of inorganic mercury (Cys-S-Hg-S-Cys) at the site of the multidrug resistance-associated transporter 2 (MRP2). In order to estimate the maximum velocity (V _max) and Michaelis constant (K _m) for the uptake of Cys-S-Hg-S-Cys mediated by MRP2, in vitro studies were carried out using radioactive Cys-S-Hg-S-Cys (5 μM) and inside-out membrane vesicles made from Sf9 cells transfected with MRP2. The V _max was estimated to be 74.3 ± 10.1 nmol mg protein^-1 30 s^-1 while the K _m was calculated to be 63.4 ± 27.3 μM. In addition, in vivo studies were utilized to measure the disposition of inorganic mercury (administered dose 0.5 μmol kg^-1 in 2 mL normal saline) over time in Wistar and TR¯ (Mrp2-deficient) rats. These studies measured the disposition of mercuric ions in the kidney, liver, and blood. In general, the data suggest that the initial uptake of mercuric conjugates into select target cells is rapid followed by a period of slower uptake and accumulation. Overall, the data indicate that MRP2 transports Cys-S-Hg-S-Cys in a manner that is similar to that of other MRP2 substrates.

Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter

ATP binding cassette type-C (ABCC) transporters move molecules across cell membranes upon hydrolysis of ATP; however, their coupling of ATP hydrolysis to substrate transport remains elusive. Drosophila multidrug resistance-associated protein (DMRP) is the functional ortholog of human long ABCC transporters, with similar substrate and inhibitor specificity, but higher activity. Exploiting its high activity, we kinetically dissected the catalytic mechanism of DMRP by using E₂-d-glucuronide (E₂G), the physiologic substrate of human ABCC. We examined the DMRP-mediated interdependence of ATP and E₂G in biochemical assays. We observed E₂G-dependent ATPase activity to be biphasic at subsaturating ATP concentrations, which implies at least 2 E₂G binding sites on DMRP. Furthermore, transport measurements indicated strong nonreciprocal cooperativity between ATP and E₂G. In addition to confirming these findings, our kinetic modeling with the Complex Pathway Simulator indicated a 10-fold decrease in the E₂G-mediated activation of ATP hydrolysis upon saturation of the second E₂G binding site. Surprisingly, the binding of the second E₂G allowed for substrate transport with a constant rate, which tightly coupled ATP hydrolysis to transport. In summary, we show that the second E₂G binding-similar to human ABCC2-allosterically stimulates transport activity of DMRP. Our data suggest that this is achieved by a significant increase in the coupling of ATP hydrolysis to transport.-Karasik, A., Ledwitch, K. V., Arányi, T., Váradi, A., Roberts, A., Szeri, F. Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter.

The chemical speciation, spatial distribution and toxicity of mercury from Tibetan medicine Zuotai，β-HgS and HgCl2 in mouse kidney

Zuotai, a famous Tibetan medicinal mixture containing β-HgS, has been used to combine with herbal remedies for treating diseases for more than 1 300 years. The target organ for inorganic mercury toxicity is generally considered to be the kidney. Therefore, it is crucial to reveal the chemical speciation, spatial distribution and potential nephrotoxicity of mercury from Zuotai in kidney. To date, this remains poorly understood. We used X-ray absorption spectroscopy (XAS) and micro X-ray fluorescence (μ-XRF) imaging based on synchrotron radiation to study mercury chemical forms and mercury special distribution in kidney after mice were treated orally with Zuotai, β-HgS or HgCl₂. Meanwhile, the histopathology of kidney was observed. Mice exposed with Zuotai showed kidney with significant proportion of mercury ions bound to sulfydryl biomolecules (e.g. Cys-S-Hg-S-Cys) plus some of unknown species, but without methylmercury cysteine, which is the same as β-HgS and HgCl₂. The mercury is mainly deposited in renal cortex in mouse treated with Zuotai, β-HgS or HgCl₂, but with a low level of mercury in medulla. The total mercury in kidney of mice treated with HgCl₂ was much higher than that of β-HgS, and the later was higher than that of Zuotai. And, HgCl₂ cause severe impairments in mouse kidney, but that was not observed in the Zuotai and β-HgS groups. Meanwhile, the bio-metals (Ca, Zn, Fe and Cu) micro-distributions in kidney were also revealed. These findings elucidated the chemical nature, spatial distribution and toxicity difference of mercury from Zuotai, β-HgS and HgCl₂ in mouse kidney, and provide new insights into the appropriate methods for biological monitoring.

Xenobiotic transporters and kidney injury

Renal proximal tubules are targets for toxicity due in part to the expression of transporters that mediate the secretion and reabsorption of xenobiotics. Alterations in transporter expression and/or function can enhance the accumulation of toxicants and sensitize the kidneys to injury. This can be observed when xenobiotic uptake by carrier proteins is increased or efflux of toxicants and their metabolites is reduced. Nephrotoxic chemicals include environmental contaminants (halogenated hydrocarbon solvents, the herbicide paraquat, the fungal toxin ochratoxin, and heavy metals) as well as pharmaceuticals (certain beta-lactam antibiotics, antiviral drugs, and chemotherapeutic drugs). This review explores the mechanisms by which transporters mediate the entry and exit of toxicants from renal tubule cells and influence the degree of kidney injury. Delineating how transport proteins regulate the renal accumulation of toxicants is critical for understanding the likelihood of nephrotoxicity resulting from competition for excretion or genetic polymorphisms that affect transporter function.

Zuotai and HgS differ from HgCl2 and methyl mercury in Hg accumulation and toxicity in weanling and aged rats

Mercury sulfides are used in Ayurvedic medicines, Tibetan medicines, and Chinese medicines for thousands of years and are still used today. Cinnabar (α-HgS) and metacinnabar (β-HgS) are different from mercury chloride (HgCl₂) and methylmercury (MeHg) in their disposition and toxicity. Whether such scenario applies to weanling and aged animals is not known. To address this question, weanling (21d) and aged (450d) rats were orally given Zuotai (54% β-HgS, 30mg/kg), HgS (α-HgS, 30mg/kg), HgCl₂ (34.6mg/kg), or MeHg (MeHgCl, 3.2mg/kg) for 7days. Accumulation of Hg in kidney and liver, and the toxicity-sensitive gene expressions were examined. Animal body weight gain was decreased by HgCl₂ and to a lesser extent by MeHg, but unaltered after Zuotai and HgS. HgCl₂ and MeHg produced dramatic tissue Hg accumulation, increased kidney (kim-1 and Ngal) and liver (Ho-1) injury-sensitive gene expressions, but such changes are absent or mild after Zuotai and HgS. Aged rats were more susceptible than weanling rats to Hg toxicity. To examine roles of transporters in Hg accumulation, transporter gene expressions were examined. The expression of renal uptake transporters Oat1, Oct2, and Oatp4c1 and hepatic Oatp2 was decreased, while the expression of renal efflux transporter Mrp2, Mrp4 and Mdr1b was increased following HgCl₂ and MeHg, but unaffected by Zuotai and HgS. Thus, Zuotai and HgS differ from HgCl₂ and MeHg in producing tissue Hg accumulation and toxicity, and aged rats are more susceptible than weanling rats. Transporter expression could be adaptive means to reduce tissue Hg burden.

The aging kidney and the nephrotoxic effects of mercury

Owing to advances in modern medicine, life expectancies are lengthening and leading to an increase in the population of older individuals. The aging process leads to significant alterations in many organ systems, with the kidney being particularly susceptible to age-related changes. Within the kidney, aging leads to ultrastructural changes such as glomerular and tubular hypertrophy, glomerulosclerosis, and tubulointerstitial fibrosis, which may compromise renal plasma flow (RPF) and glomerular filtration rate (GFR). These alterations may reduce the functional reserve of the kidneys, making them more susceptible to pathological events when challenged or stressed, such as following exposure to nephrotoxicants. An important and prevalent environmental toxicant that induces nephrotoxic effects is mercury (Hg). Since exposure of normal kidneys to mercuric ions might induce glomerular and tubular injury, aged kidneys, which may not be functioning at full capacity, may be more sensitive to the effects of Hg than normal kidneys. Age-related renal changes and the effects of Hg in the kidney have been characterized separately. However, little is known regarding the influence of nephrotoxicants, such as Hg, on aged kidneys. The purpose of this review was to summarize known findings related to exposure of aged and diseased kidneys to the environmentally relevant nephrotoxicant Hg.

Mercury sulfides are much less nephrotoxic than mercury chloride and methylmercury in mice

Mercury sulfides (α-HgS, β-HgS) are frequently included in traditional medicines. Mercury is known for nephrotoxicity, their safety is of concern. To address this question, mice were orally administrated with Zuotai (54% β-HgS, 30mg/kg), α-HgS (HgS, 30mg/kg), HgCl₂ (33.6mg/kg), or MeHgCl (3.1mg/kg) for 7days, and nephrotoxicity was examined. Animal body weights were decreased by HgCl₂ and to a lesser extent by MeHg, but unaltered after Zuotai and HgS. HgCl₂ and MeHg produced renal tubular vacuolation, interstitial inflammation and cell degeneration with protein cysts in the tubular lumen, while these pathological lesions were mild in Zuotai and HgS-treated mice. Electron microscopy showed that HgCl₂ and MeHg produced spotted swelling endothelium reticulum, while these lesions were mild or absent in Zuotai and HgS-treated mice. Renal Hg contents reached 250-300ng/mg kidney in HgCl₂ and MeHg groups as compared to 2-3ng/mg in Zuotai and HgS groups. The expression of kidney injury biomarkers, kidney injury molecule-1 (Kim-1) and neutrophil gelatinase-associated lipocalin (Ngal), were increased after HgCl₂ and MeHg, but unaltered after Zuotai and HgS. The expression of renal influx transporters Oat3 and Oatp4c1 was decreased, while the expression of renal efflux transporter such as Mrp2, Mrp4, and Mate2 was increased following HgCl₂ and MeHg. These gene expressions were unchanged after Zuotai and HgS. In summary, both α-HgS and β-HgS are less nephrotoxic than HgCl₂ and MeHg, indicating that chemical forms of mercury are a major determinant of mercury disposition and toxicity.

Mechanisms involved in the transport of mercuric ions in target tissues

Mercury exists in the environment in various forms, all of which pose a risk to human health. Despite guidelines regulating the industrial release of mercury into the environment, humans continue to be exposed regularly to various forms of this metal via inhalation or ingestion. Following exposure, mercuric ions are taken up by and accumulate in numerous organs, including brain, intestine, kidney, liver, and placenta. In order to understand the toxicological effects of exposure to mercury, a thorough understanding of the mechanisms that facilitate entry of mercuric ions into target cells must first be obtained. A number of mechanisms for the transport of mercuric ions into target cells and organs have been proposed in recent years. However, the ability of these mechanisms to transport mercuric ions and the regulatory features of these carriers have not been characterized completely. The purpose of this review is to summarize the current findings related to the mechanisms that may be involved in the transport of inorganic and organic forms of mercury in target tissues and organs. This review will describe mechanisms known to be involved in the transport of mercury and will also propose additional mechanisms that may potentially be involved in the transport of mercuric ions into target cells.

Toxicological significance of renal Bcrp: Another potential transporter in the elimination of mercuric ions from proximal tubular cells

Secretion of inorganic mercury (Hg(2+)) from proximal tubular cells into the tubular lumen has been shown to involve the multidrug resistance-associated protein 2 (Mrp2). Considering similarities in localization and substrate specificity between Mrp2 and the breast cancer resistance protein (Bcrp), we hypothesize that Bcrp may also play a role in the proximal tubular secretion of mercuric species. In order to test this hypothesis, the uptake of Hg(2+) was examined initially using inside-out membrane vesicles containing Bcrp. The results of these studies suggest that Bcrp may be capable of transporting certain conjugates of Hg(2+). To further characterize the role of Bcrp in the handling of mercuric ions and in the induction of Hg(2+)-induced nephropathy, Sprague-Dawley and Bcrp knockout (bcrp(-/-)) rats were exposed intravenously to a non-nephrotoxic (0.5 μmol · kg(-1)), a moderately nephrotoxic (1.5 μmol · kg(-1)) or a significantly nephrotoxic (2.0 μmol · kg(-1)) dose of HgCl2. In general, the accumulation of Hg(2+) was greater in organs of bcrp(-/-) rats than in Sprague-Dawley rats, suggesting that Bcrp may play a role in the export of Hg(2+) from target cells. Within the kidney, cellular injury and necrosis was more severe in bcrp(-/-) rats than in controls. The pattern of necrosis, which was localized in the inner cortex and the outer stripe of the outer medulla, was significantly different from that observed in Mrp2-deficient animals. These findings suggest that Bcrp may be involved in the cellular export of select mercuric species and that its role in this export may differ from that of Mrp2.

Target organ specific activity of drosophila MRP (ABCC1) moderates developmental toxicity of methylmercury

Methylmercury (MeHg) is a ubiquitous and persistent neurotoxin that poses a risk to human health. Although the mechanisms of MeHg toxicity are not fully understood, factors that contribute to susceptibility are even less well known. Studies of human gene polymorphisms have identified a potential role for the multidrug resistance-like protein (MRP/ABCC) family, ATP-dependent transporters, in MeHg susceptibility. MRP transporters have been shown to be important for MeHg excretion in adult mouse models, but their role in moderating MeHg toxicity during development has not been explored. We therefore investigated effects of manipulating expression levels of MRP using a Drosophila development assay. Drosophila MRP (dMRP) is homologous to human MRP1-4 (ABCC1-4), sharing 50% identity and 67% similarity with MRP1. A greater susceptibility to MeHg is seen in dMRP mutant flies, demonstrated by reduced rates of eclosion on MeHg-containing food. Furthermore, targeted knockdown of dMRP expression using GAL4>UAS RNAi methods demonstrates a tissue-specific function for dMRP in gut, Malpighian tubules, and the nervous system in moderating developmental susceptibility to MeHg. Using X-ray synchrotron fluorescence imaging, these same tissues were also identified as the highest Hg-accumulating tissues in fly larvae. Moreover, higher levels of Hg are seen in dMRP mutant larvae compared with a control strain fed an equivalent dose of MeHg. In sum, these data demonstrate that dMRP expression, both globally and within Hg-targeted organs, has a profound effect on susceptibility to MeHg in developing flies. Our findings point to a potentially novel and specific role for dMRP in neurons in the protection against MeHg. Finally, this experimental system provides a tractable model to evaluate human polymorphic variants of MRP and other gene variants relevant to genetic studies of mercury-exposed populations.