Organic anion transporting polypeptide-C mediates arsenic uptake in HEK-293 cells

Chronic Kidney Disease and Exposure to Nephrotoxic Metals

Chronic kidney disease (CKD) is a common progressive disease that is typically characterized by the permanent loss of functional nephrons. As injured nephrons become sclerotic and die, the remaining healthy nephrons undergo numerous structural, molecular, and functional changes in an attempt to compensate for the loss of diseased nephrons. These compensatory changes enable the kidney to maintain fluid and solute homeostasis until approximately 75% of nephrons are lost. As CKD continues to progress, glomerular filtration rate decreases, and remaining nephrons are unable to effectively eliminate metabolic wastes and environmental toxicants from the body. This inability may enhance mortality and/or morbidity of an individual. Environmental toxicants of particular concern are arsenic, cadmium, lead, and mercury. Since these metals are present throughout the environment and exposure to one or more of these metals is unavoidable, it is important that the way in which these metals are handled by target organs in normal and disease states is understood completely.

Mechanisms underlying the toxic effects of antimony species in human embryonic kidney cells (HEK-293) and their comparison with arsenic species

Antimony cytotoxicity was assessed in human embryonic kidney cells (HEK-293). Uptake, mitochondrial respiratory activity, ROS generation and diffusional kinetics were measured using fluorescence recovery after photobleaching (FRAP). Furthermore, the toxic effect induced by Sb was compared with As toxicity in regard to ROS generation and diffusional kinetics, which provides information on the protein aggregation process. Our results show a favored uptake of Sb(III) and a more severe effect, decreasing the mitochondrial activity more than in the presence of Sb(V). In comparison with As, the Sb species did not generate a significant increase in ROS generation, which was observed with As(III) and As(V). FRAP analysis yielded important information on the diffusion and binding dynamics of live cells in presence of these metalloids. The mobile fraction showed a strong decrease with the As species and Sb(III). The diffusion rate and the k_off-rate were significantly decreased for the As and Sb species but were more strong in the presence of As(III).

Cellular arsenic transport pathways in mammals

Natural contamination of drinking water with arsenic results in the exposure of millions of people world-wide to unacceptable levels of this metalloid. This is a serious global health problem because arsenic is a Group 1 (proven) human carcinogen and chronic exposure is known to cause skin, lung, and bladder tumors. Furthermore, arsenic exposure can result in a myriad of other adverse health effects including diseases of the cardiovascular, respiratory, neurological, reproductive, and endocrine systems. In addition to chronic environmental exposure to arsenic, arsenic trioxide is approved for the clinical treatment of acute promyelocytic leukemia, and is in clinical trials for other hematological malignancies as well as solid tumors. Considerable inter-individual variability in susceptibility to arsenic-induced disease and toxicity exists, and the reasons for such differences are incompletely understood. Transport pathways that influence the cellular uptake and export of arsenic contribute to regulating its cellular, tissue, and ultimately body levels. In the current review, membrane proteins (including phosphate transporters, aquaglyceroporin channels, solute carrier proteins, and ATP-binding cassette transporters) shown experimentally to contribute to the passage of inorganic, methylated, and/or glutathionylated arsenic species across cellular membranes are discussed. Furthermore, what is known about arsenic transporters in organs involved in absorption, distribution, and metabolism and how transport pathways contribute to arsenic elimination are described.

Toxic trace elements at gastrointestinal level

Many trace elements are considered essential [iron (Fe), zinc (Zn), copper (Cu)], whereas others may be harmful [lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As)], depending on their concentration and chemical form. In most cases, the diet is the main pathway by which they enter our organism. The presence of toxic trace elements in food has been known for a long time, and many of the food matrices that carry them have been identified. This has led to the appearance of legislation and recommendations concerning consumption. Given that the main route of exposure is oral, passage through the gastrointestinal tract plays a fundamental role in their entry into the organism, where they exert their toxic effect. Although the digestive system can be considered to be of crucial importance in their toxicity, in most cases we do not know the events that occur during the passage of these elements through the gastrointestinal tract and of ascertaining whether they may have some kind of toxic effect on it. The aim of this review is to summarize available information on this subject, concentrating on the toxic trace elements that are of greatest interest for organizations concerned with food safety and health: Pb, Cd, Hg and As.

SLCO1B1 variants and urine arsenic metabolites in the Strong Heart Family Study

Arsenic species patterns in urine are associated with risk for cancer and cardiovascular diseases. The organic anion transporter coded by the gene SLCO1B1 may transport arsenic species, but its association with arsenic metabolites in human urine has not yet been studied. The objective of this study is to evaluate associations of urine arsenic metabolites with variants in the candidate gene SLCO1B1 in adults from the Strong Heart Family Study. We estimated associations between % arsenic species biomarker traits and 5 single-nucleotide polymorphisms (SNPs) in the SLCO1B1 gene in 157 participants, assuming additive genetics. Linear regression models for each SNP accounted for kinships and were adjusted for sex, body mass index, and study center. The minor allele of rs1564370 was associated with lower %MMA (p = .0003) and higher %DMA (p = .0002), accounting for 8% of the variance for %MMA and 9% for %DMA. The rs1564370 minor allele homozygote frequency was 17% and the heterozygote frequency was 43%. The minor allele of rs2291075 was associated with lower %MMA (p = .0006) and higher %DMA (p = .0014), accounting for 7% of the variance for %MMA and 5% for %DMA. The frequency of rs2291075 minor allele homozygotes was 1% and of heterozygotes was 15%. Common variants in SLCO1B1 were associated with differences in arsenic metabolites in a preliminary candidate gene study. Replication of this finding in other populations and analyses with respect to disease outcomes are needed to determine whether this novel candidate gene is important for arsenic-associated disease risks.

OATPs, OATs and OCTs: the organic anion and cation transporters of the SLCO and SLC22A gene superfamilies

The human organic anion and cation transporters are classified within two SLC superfamilies. Superfamily SLCO (formerly SLC21A) consists of organic anion transporting polypeptides (OATPs), while the organic anion transporters (OATs) and the organic cation transporters (OCTs) are classified in the SLC22A superfamily. Individual members of each superfamily are expressed in essentially every epithelium throughout the body, where they play a significant role in drug absorption, distribution and elimination. Substrates of OATPs are mainly large hydrophobic organic anions, while OATs transport smaller and more hydrophilic organic anions and OCTs transport organic cations. In addition to endogenous substrates, such as steroids, hormones and neurotransmitters, numerous drugs and other xenobiotics are transported by these proteins, including statins, antivirals, antibiotics and anticancer drugs. Expression of OATPs, OATs and OCTs can be regulated at the protein or transcriptional level and appears to vary within each family by both protein and tissue type. All three superfamilies consist of 12 transmembrane domain proteins that have intracellular termini. Although no crystal structures have yet been determined, combinations of homology modelling and mutation experiments have been used to explore the mechanism of substrate recognition and transport. Several polymorphisms identified in members of these superfamilies have been shown to affect pharmacokinetics of their drug substrates, confirming the importance of these drug transporters for efficient pharmacological therapy. This review, unlike other reviews that focus on a single transporter family, briefly summarizes the current knowledge of all the functionally characterized human organic anion and cation drug uptake transporters of the SLCO and the SLC22A superfamilies.

Arsenic and antimony transporters in eukaryotes

Arsenic and antimony are toxic metalloids, naturally present in the environment and all organisms have developed pathways for their detoxification. The most effective metalloid tolerance systems in eukaryotes include downregulation of metalloid uptake, efflux out of the cell, and complexation with phytochelatin or glutathione followed by sequestration into the vacuole. Understanding of arsenic and antimony transport system is of high importance due to the increasing usage of arsenic-based drugs in the treatment of certain types of cancer and diseases caused by protozoan parasites as well as for the development of bio- and phytoremediation strategies for metalloid polluted areas. However, in contrast to prokaryotes, the knowledge about specific transporters of arsenic and antimony and the mechanisms of metalloid transport in eukaryotes has been very limited for a long time. Here, we review the recent advances in understanding of arsenic and antimony transport pathways in eukaryotes, including a dual role of aquaglyceroporins in uptake and efflux of metalloids, elucidation of arsenic transport mechanism by the yeast Acr3 transporter and its role in arsenic hyperaccumulation in ferns, identification of vacuolar transporters of arsenic-phytochelatin complexes in plants and forms of arsenic substrates recognized by mammalian ABC transporters.

Direct effects of arsenic trioxide on action potentials in isolated cardiac tissues: importance of the choice of species, type of cardiac tissue and perfusion time

INTRODUCTION

The aim of the present study was to evaluate direct/acute effects of arsenic trioxide on action potentials (APs) in isolated cardiac tissues, and to investigate if the choice of species and tissue and the duration of the perfusion play a role in arsenic-induced acute/direct prolongation of AP/QT.

METHODS AND RESULTS

Direct electrophysiological effects of arsenic trioxide were measured in cardiac tissues isolated from four different species using micro-electrode recording. Arsenic (after 30 to 95 min perfusion at 10 μM) significantly prolonged APD(90), increased triangulation of the AP and elicited early afterdepolarizations (EADs) only in isolated guinea-pig and dog Purkinje fibers but not in rabbit and porcine (minipig) Purkinje fibers. Arsenic induced a prolongation of the APD(90) and increases in triangulation and the occurrence of EADs was not observed in papillary muscles of guinea-pigs and rabbits. Arsenic at 4 increasing concentrations from 0.1 μM to 10 μM at the standard perfusion-time of 15 min per concentration, and after a continuous 90-min perfusion at 1 μM and 1 Hz did not induce these direct effects on APD(90), triangulation and EADs in isolated guinea-pig Purkinje fibers, but it at 1 µM elicited EADs in 2 out of 7 preparations after 90 min at 0.2 Hz.

DISCUSSION

The present study demonstrates that the choice of species and cardiac tissue as well as perfusion-time play important roles in arsenic-induced direct/acute effects on APD(90) and induction of EADs in vitro.

Genetic polymorphisms of OATP transporters and their impact on intestinal absorption and hepatic disposition of drugs

There is convincing evidence that many organic anion transporting polypeptide (OATP) transporters influence the pharmacokinetics and pharmacological efficacy of their substrate drugs. Each OATP family member has a unique combination of tissue distribution, substrate specificity and mechanisms of gene expression. Among them, OATP1B1, OATP1B3 and OATP2B1 have been considered as critical molecular determinants of the pharmacokinetics of a variety of clinically important drugs. Liver-specific expression of OATP1B1 and OATP1B3 contributes to the hepatic uptake of drugs from the portal vein, and OATP2B1 may alter their intestinal absorption as well as hepatic extraction. Accordingly, changes in function and expression of these three OATPs owing to genetic polymorphisms may lead to altered pharmacological effects, including decreased drug efficacy and increased risk of adverse effects. Association of genetic polymorphisms in OATP genes with alterations in the pharmacokinetic properties of their substrate drugs has been reported; however, there still exists a degree of discordance between the reported outcomes in different clinical settings. For better understanding of the clinical relevance of genetic polymorphisms of OATP1B1, OATP1B3 and OATP2B1, the present review focuses on the association of the genotypes of these OATPs with in vitro activity changes and in vivo clinical outcomes of substrate drugs.

Arsenic-glutathione conjugate transport by the human multidrug resistance proteins (MRPs/ABCCs)

Millions of people world-wide are chronically exposed to inorganic forms of the environmental toxicant arsenic in drinking water. This has led to a public health crisis because arsenic is a human carcinogen, and causes a myriad of other adverse health effects. In order to prevent and treat arsenic-induced toxicity it is critical to understand the cellular handling of this metalloid. A large body of literature describes the importance of the cellular tripeptide glutathione (γ-Glu-Cys-Gly,GSH/GS) in the excretion of arsenic. The triglutathione conjugate of arsenite [As(III)(GS)(3)] and the diglutathione conjugate of monomethylarsonous acid [MMA(III)(GS)(2)] have been isolated from rat bile and mouse urine, and account for the majority of excreted arsenic, suggesting these are important transportable forms. The ATP-binding cassette (ABC) transporter proteins, multidrug resistance protein 1 (MRP1/ABCC1) and the related protein MRP2 (ABCC2), are thought to play an important role in arsenic detoxification through the cellular efflux of arsenic-GSH conjugates. Current knowledge on the cellular handling of arsenic with a special emphasis on the transport pathways of the arsenic-GSH conjugates As(III)(GS)(3), MMA(III)(GS)(2), and dimethylarsenic glutathione DMA(III)(GS), as well as, the seleno-bis(S-glutathionyl) arsinium ion [(GS)(2)AsSe](-) are reviewed.

Organic anion transporting polypeptide 1B1: a genetically polymorphic transporter of major importance for hepatic drug uptake

The importance of membrane transporters for drug pharmacokinetics has been increasingly recognized during the last decade. Organic anion transporting polypeptide 1B1 (OATP1B1) is a genetically polymorphic influx transporter expressed on the sinusoidal membrane of human hepatocytes, and it mediates the hepatic uptake of many endogenous compounds and xenobiotics. Recent studies have demonstrated that OATP1B1 plays a major, clinically important role in the hepatic uptake of many drugs. A common single-nucleotide variation (coding DNA c.521T>C, protein p.V174A, rs4149056) in the SLCO1B1 gene encoding OATP1B1 decreases the transporting activity of OATP1B1, resulting in markedly increased plasma concentrations of, for example, many statins, particularly of active simvastatin acid. The variant thereby enhances the risk of statin-induced myopathy and decreases the therapeutic indexes of statins. However, the effect of the SLCO1B1 c.521T>C variant is different on different statins. The same variant also markedly affects the pharmacokinetics of several other drugs. Furthermore, certain SLCO1B1 variants associated with an enhanced clearance of methotrexate increase the risk of gastrointestinal toxicity by methotrexate in the treatment of children with acute lymphoblastic leukemia. Certain drugs (e.g., cyclosporine) potently inhibit OATP1B1, causing clinically significant drug interactions. Thus, OATP1B1 plays a major role in the hepatic uptake of drugs, and genetic variants and drug interactions affecting OATP1B1 activity are important determinants of individual drug responses. In this article, we review the current knowledge about the expression, function, substrate characteristics, and pharmacogenetics of OATP1B1 as well as its role in drug interactions, in parts comparing with those of other hepatocyte-expressed organic anion transporting polypeptides, OATP1B3 and OATP2B1.

Hepatic OATP1B transporters and nuclear receptors PXR and CAR: interplay, regulation of drug disposition genes, and single nucleotide polymorphisms

Drug uptake transporters are now increasingly recognized as clinically relevant determinants of variable drug responsiveness and unexpected drug-drug interactions. Emerging evidence strongly suggests members of the organic anion transporting polypeptide (OATP) family appear to be particularly important to the disposition of many drugs in clinical use today. Specifically, the liver-enriched OATP1B subfamily members OATP1B1 and OATP1B3 exhibit broad substrate specificity and the ability to transport drugs which are ligands for xenobiotic sensing nuclear receptors such as the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Accordingly, OATP1B transporters may indirectly regulate expression of drug metabolism genes via modulation of the intracellular concentration of PXR and CAR ligands. Moreover, a number of functionally important single nucleotide polymorphisms (SNPs) in OATP1B transporters have been described. In this review, a brief summary of known SNPs in PXR and CAR will be followed by an in-depth outline of OATP1B1 and OATP1B3 transporters particularly in relation to the known SNPs in these OATPs and the interplay between OATP1B transporters with PXR and CAR, both in vitro and in vivo.

Xenobiotic transporters of the human organic anion transporting polypeptides (OATP) family

1. The organic anion transporting polypeptides (humans OATP; other species Oatp) belong to the SLCO gene superfamily of transporters and are twelve transmembrane domain glycoproteins expressed in various epithelial cells. Some OATPs/Oatps are expressed in a single organ, while others are expressed ubiquitously. 2. The functionally characterized members mediate sodium-independent transport of a variety of structurally independent, mainly amphipathic organic compounds, including bile salts, hormones and their conjugates, toxins, and various drugs. 3. This review summarizes the general features and the substrates of the eleven human OATPs. Furthermore, it reviews what is known about the mechanism of their multispecificity, their predicted structure, their role in drug-food interactions, and their role in cancer. 4. Finally, some open questions are raised that need to be addressed to advance OATP research in the near future.

Interplay of conjugating enzymes with OATP uptake transporters and ABCC/MRP efflux pumps in the elimination of drugs

BACKGROUND

Biliary excretion is a major elimination route of many drugs and their metabolites. Hepatobiliary elimination is a vectorial process involving uptake transporters in the basolateral hepatocyte membrane, possibly Phase I and Phase II metabolizing enzymes, and ATP-dependent efflux pumps in the apical hepatocyte membrane.

OBJECTIVES

Because many drugs and their metabolites are anions, this review focuses on transporters involved in their hepatocellular uptake (members of the organic anion transporting polypeptide (OATP) family) and biliary elimination (apical conjugate efflux pump ABCC2/MRP2).

METHODS

The molecular and functional characteristics of the human OATP and ABCC/MRP transporters are presented, including a detailed overview of endogenous and drug substrates. Examples illustrate the interplay of transporters with Phase II conjugating enzymes. Model systems to study the vectorial transport of organic anions are also discussed.

RESULTS/CONCLUSIONS

OATP uptake transporters, conjugating enzymes, and ABCC2/MRP2 work in concert to enable the hepatobiliary elimination of anionic drugs and their metabolites. It is increasingly important to understand how genetic variants of these transporters and enzymes influence the interindividual variability of drug elimination.

Arsenic trioxide in environmentally and clinically relevant concentrations interacts with calcium homeostasis and induces cell type specific cell death in tumor and non-tumor cells

While arsenic compounds are known as environmental toxicants (especially in drinking water) and as carcinogens, some arsenic compounds, like arsenic trioxide (As2O3), are clinically used in humans to treat some forms of cancer (e.g. leukemia). Although arsenic compounds have been studied intensively, their interactions with living cells are still not fully elucidated. We have previously proposed that modulation of intracellular calcium ([Ca2+]i) homeostasis induced by As2O3 could be an important mechanism to induce cytotoxicity. Here we demonstrate, using human cell models (neuroblastoma (SY-5Y) or embryonic kidney cells (HEK)) and confocal microscopy in combination with the calcium sensitive dye fluo 4-AM, that As2O3 interferes with calcium signaling at low (environmentally and clinically relevant concentrations of 100 pM to 1 microM). Within this concentration range, As2O3 had cell type specific cytotoxic effects, with neuroblastoma cells being more sensitive to As2O3 than HEK 293. In addition, by staining with Hoechst 33347 and counting micronucleated cells as well as apoptotic nuclei, As2O3 was found to increase the rate of apoptosis and DNA damage, which was also cell type specific. These results indicate that the As2O3-induced cell death could be triggered or mediated by [Ca2+]i signals and suggest that low concentrations of As2O3 are able to interfere with specific physiological processes in diverse cell models.