MRP2 involvement in renal proximal tubular elimination of methylmercury mediated by DMPS or DMSA

Interaction of mercury species with proteins: towards possible mechanism of mercurial toxicology

The nature of the binding of mercurials (organic and inorganic) and their subsequent transformations in biological systems is a matter of great debate as several different hypotheses have been proposed and none of them has been conclusively proven to explain the characteristics of Hg binding with the proteins. Thus, the chemical nature of Hg-protein binding through the possible transportation mechanism in living tissues is critically reviewed herein. Emphasis is given to the process of transportation, and binding of Hg species with selenol-containing biomolecules that are appealing for toxicological studies as well as the advancement of environmental and biological research.

Lead and copper led to the dysregulation of bile acid homeostasis by impairing intestinal absorption in Bufo gargarizans larvae: An integrated metabolomics and transcriptomics approach

Bile acids, as metabolic regulators and signaling molecules, play key roles in the regulation of host metabolism and immune responses. Heavy metals such as lead (Pb) and copper (Cu) are widespread environmental pollutants that threaten public health. However, the effects of heavy metals on bile acid metabolism and the underlying molecular mechanisms remain unclear, particularly for ecologically important amphibian species. In the present research, the effects of exposure to environmentally-relevant concentrations of Pb (250 μg/L), Cu (50 μg/L), and a mixture of both (Mix) on bile acid metabolism and the underlying molecular mechanisms in the intestines of Bufo gargarizans larvae were comprehensively investigated using histopathology, metabolomics and transcriptomics analysis. Our results suggested that Pb and/or Cu caused histopathological damage to the intestine and liver, such as decreased intestinal epithelial cell height and dilated hepatic sinusoid. The total bile acid level was decreased in the Pb and Mix exposure groups but elevated in the Cu treatment. A significant decrease in the ratio of conjugated to unconjugated bile acids was present in all treatment groups. Also, the level of GCA was increased while TCA and TCDCA were decreased in all exposure groups. In addition, exposure to Pb and Cu altered the expression levels of genes related to intestinal absorption. For example, mrp2, mrp3 and aqp4 had higher expression in the Pb and Mix treatment groups, and aqp1 and mrp4 were increased in the Cu treatment group. Overall, we speculated that the dysregulation of bile acid homeostasis induced by Pb and Cu exposure may be due to impaired intestinal absorption. These findings raise further concerns about the hazards of Pb and/or Cu in influencing bile acid metabolism that might lead to the development of metabolic diseases and inflammatory disorders.

Environmental Fe, Ti, Al, Cu, Hg, Bi, and Si Nanoparticles in the Atrioventricular Conduction Axis and the Associated Ultrastructural Damage in Young Urbanites: Cardiac Arrhythmias Caused by Anthropogenic, Industrial, E-Waste, and Indoor Nanoparticles

Air pollution exposure is a risk factor for arrhythmia. The atrioventricular (AV) conduction axis is key for the passage of electrical signals to ventricles. We investigated whether environmental nanoparticles (NPs) reach the AV axis and whether they are associated with ultrastructural cell damage. Here, we demonstrate the detection of the shape, size, and composition of NPs by transmission electron microscopy (TEM) and energy-dispersive X-ray spectrometry (EDX) in 10 subjects from Metropolitan Mexico City (MMC) with a mean age of 25.3 ± 5.9 and a 71-year-old subject without cardiac pathology. We found that in every case, Fe, Ti, Al, Hg, Cu, Bi, and/or Si spherical or acicular NPs with a mean size of 36 ± 17 nm were present in the AV axis in situ, freely and as conglomerates, within the mitochondria, sarcomeres, lysosomes, lipofuscin, and/or intercalated disks and gap junctions of Purkinje and transitional cells, telocytes, macrophages, endothelium, and adjacent atrial and ventricular fibers. Erythrocytes were found to transfer NPs to the endothelium. Purkinje fibers with increased lysosomal activity and totally disordered myofilaments and fragmented Z-disks exhibited NP conglomerates in association with gap junctions and intercalated disks. AV conduction axis pathology caused by environmental NPs is a plausible and modifiable risk factor for understanding common arrhythmias and reentrant tachycardia. Anthropogenic, industrial, e-waste, and indoor NPs reach pacemaker regions, thereby increasing potential mechanisms that disrupt the electrical impulse pathways of the heart. The cardiotoxic, oxidative, and abnormal electric performance effects of NPs in pacemaker locations warrant extensive research. Cardiac arrhythmias associated with nanoparticle effects could be preventable.

Novel Insights into Mercury Effects on Hemoglobin and Membrane Proteins in Human Erythrocytes

Mercury (Hg) is a global environmental pollutant that affects human and ecosystem health. With the aim of exploring the Hg-induced protein modifications, intact human erythrocytes were exposed to HgCl₂ (1-60 µM) and cytosolic and membrane proteins were analyzed by SDS-PAGE and AU-PAGE. A spectrofluorimetric assay for quantification of Reactive Oxygen Species (ROS) generation was also performed. Hg²⁺ exposure induces alterations in the electrophoretic profile of cytosolic proteins with a significant decrease in the intensity of the hemoglobin monomer, associated with the appearance of a 64 kDa band, identified as a mercurized tetrameric form. This protein decreases with increasing HgCl₂ concentrations and Hg-induced ROS formation. Moreover, it appears resistant to urea denaturation and it is only partially dissociated by exposure to dithiothreitol, likely due to additional protein-Hg interactions involved in aggregate formation. In addition, specific membrane proteins, including band 3 and cytoskeletal proteins 4.1 and 4.2, are affected by Hg²⁺-treatment. The findings reported provide new insights into the Hg-induced possible detrimental effects on erythrocyte physiology, mainly related to alterations in the oxygen binding capacity of hemoglobin as well as decreases in band 3-mediated anion exchange. Finally, modifications of cytoskeletal proteins 4.1 and 4.2 could contribute to the previously reported alteration in cell morphology.

Pregnancy Alters Renal and Blood Burden of Mercury in Females

Methylmercury (CH₃Hg⁺), a common environmental toxicant, has serious detrimental effects in numerous organ systems. We hypothesize that a significant physiological change, like pregnancy, can alter the disposition and accumulation of mercury. To test this hypothesis, pregnant and non-pregnant female Wistar rats were exposed orally to CH₃Hg⁺. The amount of mercury in blood and total renal mass was significantly lower in pregnant rats than in non-pregnant rats. This finding may be due to expansion of plasma volume in pregnant rats and dilution of mercury, leading to lower levels of mercury in maternal blood and kidneys.

Disposition of methylmercury over time in a 75% nephrectomized rat model

Chronic kidney disease (CKD) is a highly relevant clinical condition that is characterized by the permanent loss of functional nephrons. Individuals with CKD may exhibit impaired renal clearance, which may alter corporal handling of metabolites and xenobiotics. Methylmercury (MeHg) is an important environmental toxicant to which humans are exposed to on a regular basis. Given the prevalence of CKD and ubiquitous presence of MeHg in the environment, it is important to understand how mercuric ions are handled in patients with CKD. Therefore, the purpose of the current study was to characterize the disposition of MeHg over time in a rat model of CKD (i.e., 75% nephrectomized (NPX) rats). Control and NPX rats were exposed intravenously (iv) to a non-nephrotoxic dose of MeHg (5 mg/kg) once daily for1, 2, or 3 d and the amount of MeHg in organs, blood, urine, and feces determined. The accumulation of MeHg in kidneys and blood of controls was significantly greater than that of NPX animals. In contrast, MeHg levels in brain and liver of controls were not markedly different from corresponding NPX rats. In all organs examined, accumulation of MeHg increased over the course of exposure, suggesting that urinary and fecal elimination are not sufficient to fully eliminate all mercuric ions. The current findings are important in that the disposition of mercuric ions in rats with normal renal function versus renal insufficiency following exposure to MeHg for a prolonged period differ and need to be taken into account with respect to therapeutic management.

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.

Oral exposure of pregnant rats to toxic doses of methylmercury alters fetal accumulation

Methylmercury (CH₃Hg⁺) is an environmental toxicant that may lead to significant pathologies in exposed individuals. The current study assessed the disposition and toxicological effects of 2.5 or 7.5mgkg^-1 CH₃Hg⁺, conjugated to cysteine (Cys; Cys-S-CH₃Hg) and administered orally to pregnant and non-pregnant Wistar and TR^- rats. Rats were euthanized on gestational day 20 and the content of mercury in each fetus, amniotic sac, and placenta was determined. The brain, liver, and kidneys were removed from each fetus for estimation of mercury content. From the dams, a sample of blood, kidneys, liver, and brain were removed at the time of euthanasia. The findings from this study indicate that pregnancy leads to significant changes in the handling of mercuric ions, particularly in the liver. Furthermore, there are significant differences in the handling of non-nephrotoxic and nephrotoxic doses of Cys-S-CH₃Hg by maternal and fetal organs.

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.

Novel Hg2+-induced nephropathy in rats and mice lacking Mrp2: evidence of axial heterogeneity in the handling of Hg2+ along the proximal tubule

The role of the multi-resistance protein 2 (Mrp2) in the nephropathy induced by inorganic mercuric mercury (Hg(2+)) was studied in rats (TR(-)) and mice (Mrp2(-/-)), which lack functional Mrp2, and control animals. Animals were exposed to nephrotoxic doses of HgCl2. Forty-eight or 24 hours after exposure, tissues were harvested and analyzed for Hg content and markers of injury. Histological analyses revealed that the proximal tubular segments affected pathologically by Hg(2+) were significantly different between Mrp2-deficient animals and controls. In the absence of Mrp2, cellular injury localized almost exclusively in proximal tubular segments in the subcapsular (S1) to midcortical regions (early S2) of the kidney. In control animals, cellular death occurred mainly in the proximal tubular segments in the inner cortex (late S2) and outer stripe of the outer medulla (S3). These differences in renal pathology indicate that axial heterogeneity exists along the proximal tubule with respect to how mercuric ions are handled. Total renal and hepatic accumulation of mercury was also greater in animals lacking Mrp2 than in controls, indicating that Mrp2 normally plays a significant role in eliminating mercuric ions from within proximal tubular cells and hepatocytes. Analyses of plasma creatinine, BUN, and renal expression of Kim-1 and Ngal tend to support the severity of the nephropathies detected histologically. Collectively, our findings indicate that a fraction of mercuric ions is normally secreted by Mrp2 in early portions of proximal tubules into the lumen and then is absorbed downstream in straight portions, where mercuric species typically induce toxic effects.

The putative multidrug resistance protein MRP-7 inhibits methylmercury-associated animal toxicity and dopaminergic neurodegeneration in Caenorhabditis elegans

Parkinson's disease (PD) is the most prevalent neurodegenerative motor disorder worldwide, and results in the progressive loss of dopamine (DA) neurons in the substantia nigra pars compacta. Gene-environment interactions are believed to play a significant role in the vast majority of PD cases, yet the toxicants and the associated genes involved in the neuropathology are largely ill-defined. Recent epidemiological and biochemical evidence suggests that methylmercury (MeHg) may be an environmental toxicant that contributes to the development of PD. Here, we report that a gene coding for the putative multidrug resistance protein MRP-7 in Caenorhabditis elegans modulates whole animal and DA neuron sensitivity to MeHg. In this study, we demonstrate that genetic knockdown of MRP-7 results in a twofold increase in Hg levels and a dramatic increase in stress response proteins associated with the endoplasmic reticulum, golgi apparatus, and mitochondria, as well as an increase in MeHg-associated animal death. Chronic exposure to low concentrations of MeHg induces MRP-7 gene expression, while exposures in MRP-7 genetic knockdown animals results in a loss of DA neuron integrity without affecting whole animal viability. Furthermore, transgenic animals expressing a fluorescent reporter behind the endogenous MRP-7 promoter indicate that the transporter is expressed in DA neurons. These studies show for the first time that a multidrug resistance protein is expressed in DA neurons, and its expression inhibits MeHg-associated DA neuron pathology.

Placental and fetal disposition of mercuric ions in rats exposed to methylmercury: role of Mrp2

Methylmercury is a prevalent environmental toxicant that can have deleterious effects on a developing fetus. Previous studies indicate that the multidrug resistance-associated protein 2 (Mrp2) is involved in renal and hepatic export of mercuric ions. Therefore, we hypothesize that Mrp2 is also involved in export of mercuric ions from placental trophoblasts and fetal tissues. To test this hypothesis, we assessed the disposition of mercuric ions in pregnant Wistar and TR(-) (Mrp2-deficient) rats exposed to a single dose of methylmercury. The amount of mercury in renal tissues (cortex and outer stripe of outer medulla), liver, blood, amniotic fluid, uterus, placentas and fetuses was significantly greater in TR(-) rats than in Wistar rats. Urinary and fecal elimination of mercury was greater in Wistar dams than in TR(-) dams. Thus, our findings suggest that Mrp2 may be involved in the export of mercuric ions from maternal and fetal organs following exposure to methylmercury.

Luminal transport of thiol S-conjugates of methylmercury in isolated perfused rabbit renal proximal tubules

Lumen-to-cell transport, cellular accumulation, and toxicity of L-cysteine (Cys), glutathione (GSH) and N-acetylcysteine (NAC) S-conjugates of methylmercury (CH(3)Hg(+)) were evaluated in isolated, perfused rabbit proximal tubular segments. When these conjugates were perfused individually through the lumen of S(2) segments of the proximal tubule it was found that Cys-S-CH(3)Hg and GSH-S-CH(3)Hg were transported avidly, while NAC-S-CH(3)Hg was transported minimally. In addition, 95% of the (203)Hg taken up by the tubular cells was associated with precipitable proteins of the tubule, while very little was found in the acid-soluble cytosol. No visual cellular pathological changes were observed during 30min of study. Luminal uptake of Cys-S-CH(3)Hg was temperature-dependent and inhibited significantly by the amino acids L-methionine and l-cystine. Rates of luminal uptake of GSH-S-CH(3)Hg were twice as great as that of Cys-S-CH(3)Hg and uptake was inhibited significantly (74%) by the presence of acivicin. When 2,3-bis(sulfanyl)propane-1-sulfonate (DMPS) was added to the bathing or luminal fluid, luminal uptake of Cys-S-CH(3)Hg was diminished significantly. Overall, our data indicate that Cys-S-CH(3)Hg is likely a transportable substrate of one or more amino acid transporters (such as system B(0,+) and system b(0,+)) involved in luminal absorption of L-methionine and L-cystine along the renal proximal tubule. In addition, GSH-S-CH(3)Hg appears to be degraded enzymatically to Cys-S-CH(3)Hg, which can then be taken up at the luminal membrane. By contrast NAC-S-CH(3)Hg and Cys-S-CH(3)Hg (in the presence of DMPS) are not taken up avidly at the luminal membrane of proximal tubular cells, thus promoting the excretion of CH(3)Hg(+) into the urine.

Relationships between the renal handling of DMPS and DMSA and the renal handling of mercury

Within the body of this review, we provide updates on the mechanisms involved in the renal handling mercury (Hg) and the vicinal dithiol complexing/chelating agents, 2,3-bis(sulfanyl)propane-1-sulfonate (known formerly as 2,3-dimercaptopropane-1-sulfonate, DMPS) and meso-2,3-bis(sulfanyl)succinate (known formerly as meso-2,3-dimercaptosuccinate, DMSA), with a focus on the therapeutic effects of these dithiols following exposure to different chemical forms of Hg. We begin by reviewing briefly some of the chemical properties of Hg, with an emphasis on the high bonding affinity between mercuric ions and reduced sulfur atoms, principally those contained in protein and nonprotein thiols. A discussion is provided on the current body of knowledge pertaining to the handling of various mercuric species within the kidneys, focusing on the primary cellular targets that take up and are affected adversely by these species of Hg, namely, proximal tubular epithelial cells. Subsequently, we provide a brief update on the current knowledge on the handling of DMPS and DMSA in the kidneys. In particular, parallels are drawn between the mechanisms participating in the uptake of various thiol S-conjugates of Hg in proximal tubular cells and mechanisms by which DMPS and DMSA gain entry into these target epithelial cells. Finally, we discuss factors that permit DMPS and DMSA to bind intracellular mercuric ions and mechanisms transporting DMPS and DMSA S-conjugates of Hg out of proximal tubular epithelial cells into the luminal compartment of the nephron, and promoting urinary excretion.

MRP proteins as potential mediators of heavy metal resistance in zebrafish cells

Acquired resistance of mammalian cells to heavy metals is closely relevant to enhanced expression of several multidrug resistance-associated proteins (MRP), but it remains unclear whether MRP proteins confer resistance to heavy metals in zebrafish. In this study, we obtained zebrafish (Danio rerio) fibroblast-like ZF4 cells with resistance to toxic heavy metals after chronic cadmium exposure and selection for 6months. These cadmium-resistant cells (ZF4-Cd) were maintained in 5μM cadmium and displayed cross-resistance to cadmium, mercury, arsenite and arsenate. ZF4-Cd cells remained the resistance to heavy metals after protracted culture in cadmium-free medium. In comparison with ZF4-WT cells, ZF4-Cd cells exhibited accelerated rate of cadmium excretion, enhanced activity of MRP-like transport, elevated expression of abcc2, abcc4 and mt2 genes, and increased content of cellular GSH. Inhibition of MRP-like transport activity, GSH biosynthesis and GST activity significantly attenuated the resistance of ZF4-Cd cells to heavy metals. The results indicate that some of MRP transporters are involved in the efflux of heavy metals conjugated with cellular GSH and thus play crucial roles in heavy metal detoxification of zebrafish cells.

Protective effects of Syzygium cumini seed extract against methylmercury-induced sistemic toxicity in neonatal rats

Syzygium cumini (L.) Skeels (Sc) belongs to the medicinal plants with an important source of phenolic compounds. Sc has been shown to possess antioxidant and anti-inflammatory properties. Methylmercury (MeHg), a highly toxic environmental pollutant, induces oxidative stress and dysfunction in many cell types. This study was aimed to evaluate the effect of aqueous seed extract of Sc (ASc) on MeHg-induced toxicity in rats. Two-day-old rats (P2) received a single dose of MeHg (10 mg/kg) and two doses of ASc (0.9 mg/kg) per os. After two days, the effects of the treatment were investigated in the cerebral cortex, hippocampus, kidney, liver and urine samples. Our results demonstrated that N-acetyl-β-D: -glucosaminidase (NAG) activity in the kidney and urine, the lipid peroxidation levels in the liver and kidney samples, as well as the adenosine deaminase (ADA) activity in the hippocampus, kidney and liver were higher in MeHg-group when compared to the control group. The administration of ASc reverted the toxic effects of MeHg. It is noteworthy to observe that the main compounds present in the ASc, as gallic acid (the major component), chlorogenic acid and rutin, might be the responsible for such benefit, since they were found to display antioxidant properties.

MRP2 and the handling of mercuric ions in rats exposed acutely to inorganic and organic species of mercury

Mercuric ions accumulate preferentially in renal tubular epithelial cells and bond with intracellular thiols. Certain metal-complexing agents have been shown to promote extraction of mercuric ions via the multidrug resistance-associated protein 2 (MRP2). Following exposure to a non-toxic dose of inorganic mercury (Hg²+), in the absence of complexing agents, tubular cells are capable of exporting a small fraction of intracellular Hg²+ through one or more undetermined mechanisms. We hypothesize that MRP2 plays a role in this export. To test this hypothesis, Wistar (control) and TR(-) rats were injected intravenously with a non-nephrotoxic dose of HgCl₂ (0.5 μmol/kg) or CH₃HgCl (5 mg/kg), containing [²⁰³Hg], in the presence or absence of cysteine (Cys; 1.25 μmol/kg or 12.5mg/kg, respectively). Animals were sacrificed 24 h after exposure to mercury and the content of [²⁰³Hg] in blood, kidneys, liver, urine and feces was determined. In addition, uptake of Cys-S-conjugates of Hg²+ and methylmercury (CH₃Hg+) was measured in inside-out membrane vesicles prepared from either control Sf9 cells or Sf9 cells transfected with human MRP2. The amount of mercury in the total renal mass and liver was significantly greater in TR⁻ rats than in controls. In contrast, the amount of mercury in urine and feces was significantly lower in TR⁻ rats than in controls. Data from membrane vesicles indicate that Cys-S-conjugates of Hg²+ and CH₃Hg+ are transportable substrates of MRP2. Collectively, these data indicate that MRP2 plays a role in the physiological handling and elimination of mercuric ions from the kidney.

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

Transport of inorganic mercury and methylmercury in target tissues and organs

Owing to the prevalence of mercury in the environment, the risk of human exposure to this toxic metal continues to increase. Following exposure to mercury, this metal accumulates in numerous organs, including brain, intestine, kidneys, liver, and placenta. Although a number of mechanisms for the transport of mercuric ions into target organs were proposed in recent years, these mechanisms have not been characterized completely. This review summarizes the current literature related to the transport of inorganic and organic forms of mercury in various tissues and organs. This review identifies known mechanisms of mercury transport and provides information on additional mechanisms that may potentially play a role in the transport of mercuric ions into target cells.

Seventy-five percent nephrectomy and the disposition of inorganic mercury in 2,3-dimercaptopropanesulfonic acid-treated rats lacking functional multidrug-resistance protein 2

In the present study, we evaluated the disposition of inorganic mercury (Hg(2+)) in sham-operated and 75% nephrectomized (NPX) Wistar and transport-deficient (TR(-)) rats treated with saline or the chelating agent meso-2,3-dimercaptosuccinic acid (DMSA). Based on previous studies, DMSA and TR(-) rats were used as tools to examine the potential role of multidrug-resistance protein 2 (MRP2) in the disposition of Hg(2+) during renal insufficiency. All animals were treated with a low dose (0.5 mumol/kg i.v.) of mercuric chloride (HgCl(2)). At 24 and 28 h after exposure to HgCl(2), matched groups of Wistar and TR(-) rats received normal saline or DMSA (intraperitoneally). Forty-eight hours after exposure to HgCl(2), the disposition of Hg(2+) was examined. A particularly notable effect of 75% nephrectomy in both strains of rats was enhanced renal accumulation of Hg(2+), specifically in the outer stripe of the outer medulla. In addition, hepatic accumulation, fecal excretion, and blood levels of Hg(2+) were enhanced in rats after 75% nephrectomy, especially in the TR(-) rats. Treatment with DMSA increased both the renal tubular elimination and urinary excretion of Hg(2+) in all rats. DMSA did not, however, affect hepatic content of Hg(2+), even in the 75% NPX TR(-) rats. We also show with real-time polymerase chain reaction that after 75% nephrectomy and compensatory renal growth, expression of MRP2 (only in Wistar rats) and organic anion transporter 1 is enhanced in the remaining functional proximal tubules. We conclude that MRP2 plays a significant role in the renal and corporal disposition of Hg(2+) after a 75% reduction of renal mass.

Multidrug efflux transporters limit accumulation of inorganic, but not organic, mercury in sea urchin embryos

Mercuric compounds are persistent global pollutants that accumulate in marine organisms and in humans who consume them. While the chemical cycles and speciation of mercury in the oceans are relatively well described, the cellular mechanisms that govern which forms of mercury accumulate in cells and why they persist are less understood. In this study we examined the role of multidrug efflux transport in the differential accumulation of inorganic (HgCl(2)) and organic (CH(3)HgCl) mercury in sea urchin (Strongylocentrotus purpuratus) embryos. We found that inhibition of MRP/ABCC-type transporters increases intracellular accumulation of inorganic mercury but had no effect on accumulation of organic mercury. Similarly, pharmacological inhibition of metal conjugating enzymes by ligands GST/GSH significantly increases this antimitotic potency of inorganic mercury, but had no effect on the potency of organic mercury. Our results point to MRP-mediated elimination of inorganic mercury conjugates as a cellular basis for differences in the accumulation and potency of the two major forms of mercury found in marine environments.

Effect of DMPS and DMSA on the placental and fetal disposition of methylmercury

Methylmercury (CH3Hg+) is a serious environmental toxicant. Exposure to this metal during pregnancy can cause serious neurological and developmental defects in a developing fetus. Surprisingly, little is known about the mechanisms by which mercuric ions are transported across the placenta. Although it has been shown that 2,3-dimercaptopropane-1-sulfonate (DMPS) and 2,3-dimercaptosuccinic acid (DMSA) are capable of extracting mercuric ions from various organs and cells, there is no evidence that they are able to extract mercury from placental or fetal tissues following maternal exposure to CH3Hg+. Therefore, the purpose of the current study was to evaluate the ability of DMPS and DMSA to extract mercuric ions from placental and fetal tissues following maternal exposure to CH3Hg+. Pregnant Wistar rats were exposed to CH3HgCl, containing [203Hg], on day 11 or day 17 of pregnancy and treated 24 h later with saline, DMPS or DMSA. Maternal organs, fetuses, and placentas were harvested 48 h after exposure to CH3HgCl. The disposition of mercuric ions in maternal organs and tissues was similar to that reported previously by our laboratory. The disposition of mercuric ions in placentas and fetuses appeared to be dependent upon the gestational age of the fetus. The fetal and placental burden of mercury increased as fetal age increased and was reduced by DMPS and DMSA, with DMPS being more effective. The disposition of mercury was examined in liver, total renal mass, and brain of fetuses harvested on gestational day 19. On a per gram tissue basis, the greatest amount of mercury was detected in the total renal mass of the fetus, followed by brain and liver. DMPS and DMSA reduced the burden of mercury in liver and brain while only DMPS was effective in the total renal mass. The results of the current study are the first to show that DMPS and DMSA are capable of extracting mercuric ions, not only from maternal tissues, but also from placental and fetal tissues following maternal exposure to CH3Hg+.