Molecular aspects of renal tubular handling and regulation of inorganic sulfate

Advances in the understanding of acetaminophen toxicity mechanisms: a clinical toxicology perspective

INTRODUCTION

Acetaminophen (paracetamol) is a commonly used analgesic and antipyretic agent, which is safe in therapeutic doses. Acetaminophen poisoning due to self-harm or repeated supratherapeutic ingestion is a common cause of acute liver injury. Acetylcysteine has been a mainstay of treatment for acetaminophen poisoning for decades and is efficacious if administered early. However, treatment failures occur if administered late, in 'massive' overdoses or in high-risk patients.

AREAS COVERED

This review provides an overview of the mechanisms of toxicity of acetaminophen poisoning (metabolic and oxidative phase) and how this relates to the assessment and treatment of the acetaminophen poisoned patient. The review focuses on how these advances offer further insight into the utility of novel biomarkers and the role of proposed adjunct treatments.

EXPERT OPINION

Advances in our understanding of acetaminophen toxicity have allowed the development of novel biomarkers and a better understanding of how adjunct treatments may prevent acetaminophen toxicity. Newly proposed adjunct treatments like fomepizole are being increasingly used without robust clinical trials. Novel biomarkers (not yet clinically available) may provide better assessment of these newly proposed adjunct treatments, particularly in clinical trials. These advances in our understanding of acetaminophen toxicity and liver injury hold promise for improved diagnosis and treatment.

Acetaminophen Metabolites on Presentation Following an Acute Acetaminophen Overdose (ATOM-7)

Acetaminophen (APAP) is commonly taken in overdose and can cause acute liver injury via the toxic metabolite NAPQI formed by cytochrome (CYP) P450 pathway. We aimed to evaluate the concentrations of APAP metabolites on presentation following an acute APAP poisoning and whether these predicted the subsequent onset of hepatotoxicity (peak alanine aminotransferase > 1,000 U/L). The Australian Toxicology Monitoring (ATOM) study is a prospective observational study, recruiting via two poison information centers and four toxicology units. Patients following an acute APAP ingestion presenting < 24 hours post-ingestion were recruited. Initial samples were analyzed for APAP metabolites, those measured were the nontoxic glucuronide (APAP-Glu) and sulfate (APAP-Sul) conjugates and NAPQI (toxic metabolite) conjugates APAP-cysteine (APAP-Cys) and APAP-mercapturate (APAP-Mer). The primary outcome was hepatotoxicity. In this study, 200 patients were included, with a median ingested dose of 20 g, 191 received acetylcysteine at median time of 5.8 hours post-ingestion. Twenty-six patients developed hepatotoxicity, one had hepatotoxicity on arrival (excluded from analysis). Those who developed hepatotoxicity had significantly higher total CYP metabolite concentrations: (36.8 μmol/L interquartile range (IQR): 27.8-51.7 vs. 10.8 μmol/L IQR: 6.9-19.5) and these were a greater proportion of total metabolites (5.4%, IQR: 3.8-7.7) vs. 1.7%, IQR: 1.3-2.6, P < 0.001)]. Furthermore, those who developed hepatotoxicity had lower APAP-Sul concentrations (49.1 μmol/L, IQR: 24.7-72.2 vs. 78.7 μmol/L, IQR: 53.6-116.4) and lower percentage of APAP-Sul (6.3%, IQR: 4.6-10.9 vs. 13.1%, IQR, 9.1-20.8, P < 0.001)]. This study found that those who developed hepatotoxicity had higher APAP metabolites derived from CYP pathway and lower sulfation metabolite on presentation. APAP metabolites may be utilized in the future to identify patients who could benefit from increased acetylcysteine or newer adjunct or research therapies.

Serum sulfate level and Slc13a1 mRNA expression remain unaltered in a mouse model of moderate vitamin D deficiency

Sulfate is essential for healthy foetal growth and neurodevelopment. The SLC13A1 sulfate transporter is primarily expressed in the kidney where it mediates sulfate reabsorption and maintains circulating sulfate levels. To meet foetal demands, maternal sulfate levels increase by twofold in pregnancy via upregulated SLC13A1 expression. Previous studies found hyposulfataemia and reduced renal Slc13a1 mRNA expression in rodent models with either severe vitamin D deficiency or perturbed vitamin D signalling. Here we investigated a mouse model of moderate vitamin D deficiency. However, serum sulfate level and renal Slc13a1 mRNA expression was not decreased by a moderate reduction in circulating vitamin D level. We confirmed that the mouse Slc13a1 5'-flanking region was upregulated by 1,25(OH)₂D₃ using luciferase assays in a cultured renal OK cell line. These results support the presence of a functional VDRE in the mouse Slc13a1 but suggests that moderate vitamin D deficiency does not impact on sulfate homeostasis. As sulfate biology is highly conserved between rodents and humans, we proposed that human SLC13A1 would be under similar transcriptional regulation by 1,25(OH)₂D₃. Using an online prediction tool we identified a putative VDRE in the SLC13A1 5'-flanking region but unlike the mouse Slc13a1 sequence, the human sequence did not confer a significant response to 1,25(OH)₂D₃ in vitro. Overall, this study suggests that moderate vitamin D deficiency may not alter sulfate homeostasis. This needs to be confirmed in humans, particularly during pregnancy when vitamin D and sulfate levels need to be maintained at high levels for healthy maternal and child outcomes.

Effect of supplemental methyl sulfonyl methane on performance, carcass and meat quality and oxidative status in chronic cyclic heat-stressed finishing broilers

Methyl sulfonyl methane (MSM) is available as a dietary supplement for human and has been associated with multiple health benefits such as reduction of oxidative stress. Heat stress (HS) is an environmental stressor challenging poultry production and known to inflict oxidative stress. We hypothesized that dietary MSM could attenuate HS-induced detrimental effects in broilers mediated by enhancement of antioxidant defenses. Hence, seven hundred ninety-two 1-day-old male Ross 308 broilers were allocated to 3 dietary treatments composed of corn-soybean meal diets with 0 (Ctrl), 1, or 2 g/kg MSM, with 12 replicates (22 birds each) per treatment for 39 d and subjected to a chronic cyclic HS model (temperature of 34°C and 52-58% relative humidity for 6 h daily) from d 24 to 39. MSM at 1 and 2 g/kg linearly increased daily gain and decreased feed-to-gain ratio compared with Ctrl in the grower phase (d 10-21, both P < 0.05). In the finisher phase (d 21-39) none of the performance and carcass indices were affected by treatment (P > 0.05). Nonetheless, data suggest reduced mortality by feeding MSM during HS. Also, during HS the diets with graded levels of MSM resulted in reduced rectal temperatures (P < 0.05) along with linearly decreased panting frequency on d 24 (P < 0.05). MSM supplemented birds showed a trend for linearly decreased thiobarbituric acid reactive substances of breast meat upon simulated retail display (P = 0.078). In addition, MSM administration linearly decreased lipid oxidation in plasma (d 25 and 39, P < 0.05) and breast muscle at d 23 (P < 0.05), concomitantly with linearly increased glutathione levels in erythrocytes (d 23 and 39, P < 0.05; d 25, P < 0.1) and breast muscle (d 23, P < 0.05; d 39, P < 0.1). In conclusion, MSM increased growth performance of broilers during grower phase, and exhibited positive effects on heat tolerance mediated by improved antioxidant capacity in broilers resulting in lower mortality in finisher phase.

Small Intestinal Absorption of Methylsulfonylmethane (MSM) and Accumulation of the Sulfur Moiety in Selected Tissues of Mice

The principal dietary sources of sulfur, the amino acids methionine and cysteine, may not always be consumed in adequate amounts to meet sulfur requirements. The naturally occurring organosulfur compound, methylsulfonylmethane (MSM), is available as a dietary supplement and has been associated with multiple health benefits. Absorption of MSM by the small intestine and accumulation of the associated sulfur moiety in selected tissues with chronic (8 days) administration were evaluated using juvenile male mice. Intestinal absorption was not saturated at 50 mmol, appeared passive and carrier-independent, with a high capacity (at least 2 g/d-mouse). The ³⁵S associated with MSM did not increase in serum or tissue homogenates between days 2 and 8, indicating a stable equilibrium between intake and elimination was established. In contrast, proteins isolated from the preparations using gel electrophoresis revealed increasing incorporation of ³⁵S in the protein fraction of serum, cellular elements of blood, liver, and small intestine but not skeletal muscle. The potential contributions of protein synthesis using labeled sulfur amino acids synthesized by the gut bacteria and posttranslational sulfation of proteins by incorporation of the labeled sulfate of MSM in 3'-phosphoadenosine 5'-phosphosulfate (PAPS) and subsequent transfer by sulfotransferases are discussed.

A Survey of Vitamin D Status in Patients with Degenerative Diseases of the Spine

STUDY DESIGN

Descriptive cross-sectional study.

PURPOSE

To determine the prevalence of vitamin D deficiency in patients with degenerative diseases of the spine about to undergo spinal surgery and the relations between such deficiency and potential risk factors.

OVERVIEW OF LITERATURE

Vitamin D has a major role in musculoskeletal system health maintenance. Recently, studies on degenerative diseases of the spine have shown a high prevalence of vitamin D deficiency in patients undergoing spine surgery.

METHODS

Serum levels of 25(OH)D were determined by an electrochemiluminescence detection assay. The other variables were determined through relevant questionnaires, and the data was analyzed through analysis of variance, t-test, chi-square and multivariate logistic regression analysis.

RESULTS

A total of 110 patients were enrolled in the study. The mean serum level of 25(OH)D was 27.45±18.75 ng/mL, and 44.5% of patients showed vitamin D deficiency (25(OH)D<20 ng/mL), with an additional 17.3% of patients having a serum level of 25(OH)D that was insufficient (20≤25(OH)D<30 ng/mL). The prevalence of vitamin D deficiency was significantly higher in the younger age group compared to the older age group (p<0.001) and the ones without a history of taking vitamin D supplements (p=0.013). Compared to men, women showed significantly higher levels of vitamin D (p=0.029).

CONCLUSIONS

A high prevalence of vitamin D deficiency is seen in patients with degenerative diseases of the spine. On the other hand, the conventional risk factors such as old age or female sex alone did not seem to be sufficient in determining the likelihood of deficiency. Thus, it is recommended that vitamin D deficiency prevention strategies comprise a broader spectrum of the population through which such degenerative diseases and their consequences may be prevented or delayed.

Relationship between vitamin D receptor gene (VDR) polymorphisms, vitamin D status, osteoarthritis and intervertebral disc degeneration

The vitamin D endocrine system is involved in bony and cartilaginous metabolisms and alterations in the homeostasis of this system could be associated to pathological conditions of cartilaginous tissue. In this context, the presence of polymorphisms in the vitamin D receptor gene (VDR), in association with the susceptibility to common osteochondral diseases, was largely investigated. The aim of this review was to summarize data present in literature, analyzing the association of the VDR polymorphisms, vitamin D status and knee cartilage and intervertebral disc pathologies, trying to suggest links between the different specific pathologies analyzed. Concerning the association between VDR polymorphisms and cartilaginous tissue diseases, we found controversial reports. However, the great majority of papers reported an association with lumbar disc degeneration, whereas about half of the studies found an association with osteoarthritis. A further association between VDR polymorphisms (in linkage disequilibrium) and the presence of specific characteristics of these diseases, in particular the formation of osteophytes, was evidenced. Finally, the influence of vitamin D status on these pathologies was evaluated, trying to evidence the relation between the presence of particular genetic variants in the VDR and vitamin D levels or to show whether a particular vitamin D status could predispose to the development or progression of such diseases, however, no significant associations were found. In the future, given the role of vitamin D system in the cartilaginous tissue metabolism, it could be interesting to perform functional and tissue specific studies to analyze the interplay between the different VDR variants and its ligand.

Genetic polymorphisms associated with intervertebral disc degeneration

BACKGROUND CONTEXT

Disc degeneration (DD) is a multifaceted chronic process that alters the structure and function of the intervertebral discs and can lead to painful conditions. The pathophysiology of degeneration is not well understood, but previous studies suggest that certain genetic polymorphisms may be important contributing factors leading to an increased risk of DD.

PURPOSE

To review the genetic factors in DD with a focus on polymorphisms and their putative role in the pathophysiology of degeneration. Elucidating the genetic components that are associated with degeneration could provide insights into the mechanism of the process. Furthermore, defining these relationships and eventually using them in a clinical setting may allow an identification and early intervention for those who are at a high risk for painful DD.

STUDY DESIGN

Literature review.

METHODS

This literature review focused on the studies concerning genetic polymorphisms and their associations with DD.

RESULTS

Genetic polymorphisms in 20 genes have been analyzed in association with DD, including vitamin D receptor, growth differentiation factor 5 (GDF5), aggrecan, collagen Types I, IX, and XI, fibronectin, hyaluronan and proteoglycan link protein 1 (HAPLN1), thrombospondin, cartilage intermediate layer protein (CILP), asporin, MMP1, 2, and 3, parkinson protein 2, E3 ubiquitin protein ligase (PARK2), proteosome subunit β type 9 (PSMB9), tissue inhibitor of metalloproteinase (TIMP), cyclooxygenase-2 (COX2), and IL1α, IL1β, and IL6. Each genetic polymorphism codes for a protein that has a functional role in the pathogenesis of DD.

CONCLUSIONS

There are known associations between several genetic polymorphisms and DD. Of the 20 genes analyzed, polymorphisms in vitamin D receptor, aggrecan, Type IX collagen, asporin, MMP3, IL1, and IL6 show the most promise as functional variants. Genetic studies are crucial for understanding the mechanism of the degeneration. This genetic information could eventually be used as a predictive model for determining a patient's risk for symptomatic DD.

Increased biological response to 1,25(OH)(2)D(3) in genetic hypercalciuric stone-forming rats

Genetic hypercalciuric stone-forming (GHS) rats, bred to maximize urine (U) calcium (Ca) excretion, have increased intestinal Ca absorption and bone Ca resorption and reduced renal Ca reabsorption, leading to increased UCa compared with the Sprague-Dawley (SD) rats. GHS rats have increased vitamin D receptors (VDR) at each of these sites, with normal levels of 1,25(OH)(2)D(3) (1,25D), indicating that their VDR is undersaturated with 1,25D. We tested the hypothesis that 1,25D would induce a greater increase in UCa in GHS rats by feeding both strains ample Ca and injecting 1,25D (25 ng · 100 g body wt(-1) · day(-1)) or vehicle for 16 days. With 1,25D, UCa in SD increased from 1.7 ± 0.3 mg/day to 24.4 ± 1.2 (Δ = 22.4 ± 1.5) and increased more in GHS from 10.5 ± 0.7 to 41.9 ± 0.7 (Δ = 29.8 ± 1.8; P = 0.003). To determine the mechanism of the greater increase in UCa in GHS rats, we measured kidney RNA expression of components of renal Ca transport. Expression of transient receptor potential vanilloid (TRPV)5 and calbindin D(28K) were increased similarly in SD + 1,25D and GHS + 1,25D. The Na(+)/Ca(2+) exchanger (NCX1) was increased in GHS + 1,25D. Klotho was decreased in SD + 1,25D and GHS + 1,25D. TRPV6 was increased in SD + 1,25D and increased further in GHS + 1,25D. Claudin 14, 16, and 19, Na/K/2Cl transporter (NKCC2), and secretory K channel (ROMK) did not differ between SD + 1,25D and GHS + 1,25D. Increased UCa with 1,25D in GHS exceeded that of SD, indicating that the increased VDR in GHS induces a greater biological response. This increase in UCa, which must come from the intestine and/or bone, must exceed any effect of 1,25D on TRPV6 or NCX1-mediated renal Ca reabsorption.

Association of the polymorphisms of vitamin D receptor and aggrecan genes with degenerative disc disease

The aim of this study was to investigate the association between the polymorphisms of the vitamin D receptor (VDR) and aggrecan genes and degenerative disc disease in young Turkish patients. Aggrecan and VDR proteins are the main components of bone and cartilage. In our study, the polymorphisms of the VDR and aggrecan genes were investigated in a total of 300 individuals regarding disc degeneration and herniation. An association was found in the patients having VDR gene TT, Tt, FF, and Ff genotypes with the protrusion type of disc herniation, whereas the patients having tt and ff genotypes were associated with extrusion/sequestration types of the disease. Also, an association was observed between TT and FF genotypes of the VDR gene and mild forms of disc degeneration; and tt, ff, and Ff genotypes and severe forms of the disease. There was also an association between shorter, normal, and longer alleles of the aggrecan gene and a protrusion type of disc herniation. An association was found between short alleles and multilevel and severe disc degeneration, as well as normal and long alleles and mild disc degeneration. This study revealed that the polymorphisms of the VDR and aggrecan genes are associated with disc degeneration and herniation.

Monitoring protein-protein interactions between the mammalian integral membrane transporters and PDZ-interacting partners using a modified split-ubiquitin membrane yeast two-hybrid system

PDZ-binding motifs are found in the C-terminal tails of numerous integral membrane proteins where they mediate specific protein-protein interactions by binding to PDZ-containing proteins. Conventional yeast two-hybrid screens have been used to probe protein-protein interactions of these soluble C termini. However, to date no in vivo technology has been available to study interactions between the full-length integral membrane proteins and their cognate PDZ-interacting partners. We previously developed a split-ubiquitin membrane yeast two-hybrid (MYTH) system to test interactions between such integral membrane proteins by using a transcriptional output based on cleavage of a transcription factor from the C terminus of membrane-inserted baits. Here we modified MYTH to permit detection of C-terminal PDZ domain interactions by redirecting the transcription factor moiety from the C to the N terminus of a given integral membrane protein thus liberating their native C termini. We successfully applied this "MYTH 2.0" system to five different mammalian full-length renal transporters and identified novel PDZ domain-containing partners of the phosphate (NaPi-IIa) and sulfate (NaS1) transporters that would have otherwise not been detectable. Furthermore this assay was applied to locate the PDZ-binding domain on the NaS1 protein. We showed that the PDZ-binding domain for PDZK1 on NaS1 is upstream of its C terminus, whereas the two interacting proteins, NHERF-1 and NHERF-2, bind at a location closer to the N terminus of NaS1. Moreover NHERF-1 and NHERF-2 increased functional sulfate uptake in Xenopus oocytes when co-expressed with NaS1. Finally we used MYTH 2.0 to demonstrate that the NaPi-IIa transporter homodimerizes via protein-protein interactions within the lipid bilayer. In summary, our study establishes the MYTH 2.0 system as a novel tool for interactive proteomics studies of membrane protein complexes.

Roles of Slc13a1 and Slc26a1 sulfate transporters of eel kidney in sulfate homeostasis and osmoregulation in freshwater

Sulfate is required for proper cell growth and development of all organisms. We have shown that the renal sulfate transport system has dual roles in euryhaline eel, namely, maintenance of sulfate homeostasis and osmoregulation of body fluids. To clarify the physiological roles of sulfate transporters in teleost fish, we cloned orthologs of the mammalian renal sulfate transporters Slc13a1 (NaSi-1) and Slc26a1 (Sat-1) from eel (Anguilla japonica) and assessed their functional characteristics, tissue localization, and regulated expression. Full-length cDNAs coding for ajSlc13a1 and ajSlc26a1 were isolated from a freshwater eel kidney cDNA library. Functional expression in Xenopus oocytes revealed the expected sulfate transport characteristics; furthermore, both transporters were inhibited by mercuric chloride. Northern blot analysis, in situ hybridization, and immunohistochemistry demonstrated robust apical and basolateral expression of ajSlc13a1 and ajSlc26a1, respectively, within the proximal tubule of freshwater eel kidney. Expression was dramatically reduced after the transfer of eels from freshwater to seawater; the circulating sulfate concentration in eels was in turn markedly elevated in freshwater compared with seawater conditions (19 mM vs. 1 mM). The reabsorption of sulfate via the apical ajSlc13a1 and basolateral ajSlc26a1 transporters may thus contribute to freshwater osmoregulation in euryhaline eels, via the regulation of circulating sulfate concentration.

Intestinal transport of an obdurate anion: oxalate

In this review, we focus on the role of gastrointestinal transport of oxalate primarily from a contemporary physiological standpoint with an emphasis on those aspects that we believe may be most important in efforts to mitigate the untoward effects of oxalate. Included in this review is a general discussion of intestinal solute transport as it relates to oxalate, considering cellular and paracellular avenues, the transport mechanisms, and the molecular identities of oxalate transporters. In addition, we review the role of the intestine in oxalate disease states and various factors affecting oxalate absorption.

Critical role of vitamin D in sulfate homeostasis: regulation of the sodium-sulfate cotransporter by 1,25-dihydroxyvitamin D3

As the fourth most abundant anion in the body, sulfate plays an essential role in numerous physiological processes. One key protein involved in transcellular transport of sulfate is the sodium-sulfate cotransporter NaSi-1, and previous studies suggest that vitamin D modulates sulfate homeostasis by regulating NaSi-1 expression. In the present study, we found that, in mice lacking the vitamin D receptor (VDR), NaSi-1 expression in the kidney was reduced by 72% but intestinal NaSi-1 levels remained unchanged. In connection with these findings, urinary sulfate excretion was increased by 42% whereas serum sulfate concentration was reduced by 50% in VDR knockout mice. Moreover, levels of hepatic glutathione and skeletal sulfated proteoglycans were also reduced by 18 and 45%, respectively, in the mutant mice. Similar results were observed in VDR knockout mice after their blood ionized calcium levels and rachitic bone phenotype were normalized by dietary means, indicating that vitamin D regulation of NaSi-1 expression and sulfate metabolism is independent of its role in calcium metabolism. Treatment of wild-type mice with 1,25-dihydroxyvitamin D3 or vitamin D analog markedly stimulated renal NaSi-1 mRNA expression. These data provide strong in vivo evidence that vitamin D plays a critical role in sulfate homeostasis. However, the observation that serum sulfate and skeletal proteoglycan levels in normocalcemic VDR knockout mice remained low in the absence of rickets and osteomalacia suggests that the contribution of sulfate deficiency to development of rickets and osteomalacia is minimal.

Role of tubular secretion and carbonic anhydrase in vertebrate renal sulfate excretion

The renal proximal tubule of vertebrates performs an essential role in controlling plasma SO42−concentration ([SO42−]). Although net tubular SO42−reabsorption is the predominate control process in terrestrial vertebrates, a facilitated secretory flux is also present. In contrast, marine teleosts obtain excess SO42−from drinking, and increased plasma [SO42−] is prevented predominately through net tubular secretion. Tubular SO42−secretion is accomplished by at least two electroneutral anion exchange processes in series. Movement of SO42−into the cell across the basolateral membrane is pH dependent, suggesting SO42−/OH−exchange. Luminal HCO3−and Cl−can facilitate SO42−movement out of the cell across the brush-border membrane. The molecular identities of the anion exchangers are unknown but are probably homologues of SO42−transporters in the mammalian SLC26 gene family. In all species tested, glucocorticoids increase renal SO42−excretion. Whereas glucocorticoids downregulate SO42−reabsorptive mechanisms in terrestrial vertebrates, they may also stimulate a mediated secretory flux. In the marine teleost, cortisol increases the level of SO42−/HCO3−exchange at the brush-border membrane, tubular carbonic anhydrase (CA) activity, CAII protein, and a proportion of tubular SO42−secretion that is CA dependent. CA activity is required for about one-half of this net SO42−secretion but is also required for about one-half of the net reabsorption in bird proximal epithelium. A CA-SO42−/anion exchanger metabolon arrangement is proposed that may speed both the secretory and reabsorptive processes.

The effect of cysteine analogues on the excretion of urinary sulphate in the rat following cysteine administration

A major pathway for the production of sulphate within the mammalian body is known to be via the oxidative degradation of the sulphur moiety within the amino acid, L-cysteine. The ability of two structurally similar sulphur-containing drugs, the anti-rheumatic agent, D-penicillamine, and the mucoactive compound, S-carboxymethyl-L-cysteine, to interfere with this sulphate production was investigated. Co-administration to the male rat of D-penicillamine (p.o.) and S-carboxymethyl-L-cysteine (p.o.) with [35S]-L-cysteine (i.p.) led to a significant decrease in the subsequent urinary elimination of inorganic sulphate whilst having no measurable effect on organic sulphate excretion. The co-administration of L-valine, an amino acid not containing sulphur, had no effect. It is not known where, within the complex sequence of events surrounding the degradation of cysteine to sulphate, that D-penicillamine or S-carboxymethyl-L-cysteine may interact.

Serines 260 and 288 are involved in sulfate transport by hNaSi-1

The low affinity Na+/sulfate cotransporter, NaSi-1, belongs to the SLC13 family that also includes the Na+/dicarboxylate cotransporters, NaDC. Two serine residues in hNaSi-1, at positions 260 and 288, are conserved in all of the sulfate transporters in the family whereas the NaDC contain alanine or threonine at those positions. Therefore, the functional roles of serines 260 and 288 in substrate and cation binding by hNaSi-1 were investigated. These two serine residues were first mutated to alanine and the mutants were characterized in Xenopus oocytes. Alanine substitution of Ser-260 resulted in increased Km values for both substrate and Na+ whereas alanine replacement at Ser-288 resulted in a broadened cation selectivity, indicating that these two serines might play important roles in cation and/or substrate binding of hNaSi-1. The two serines and 12 surrounding residues were further mutated to cysteine and studied using a thiol-reactive compound, [2-(trimethylammonium)ethyl]methane-thiosulfonate (MTSET). Four mutants surrounding Ser-260 (T257C, T259C, T261C, and L263C) were sensitive to MTSET inhibition. The sensitivity to MTSET was dependent on the presence of substrate, suggesting that the accessibility of these substituted cysteines depends on the conformational state of the transporter. Because the four residues are located in transmembrane domain 5, this transmembrane domain is likely to participate in the conformational movements during the transport cycle of hNaSi-1.

Transport of organic anions across the basolateral membrane of proximal tubule cells

Renal proximal tubules secrete diverse organic anions (OA) including widely prescribed anionic drugs. Here, we review the molecular properties of cloned transporters involved in uptake of OA from blood into proximal tubule cells and provide extensive lists of substrates handled by these transport systems. Where tested, transporters have been immunolocalized to the basolateral cell membrane. The sulfate anion transporter 1 (sat-1) cloned from human, rat and mouse, transported oxalate and sulfate. Drugs found earlier to interact with sulfate transport in vivo have not yet been tested with sat-1. The Na(+)-dicarboxylate cotransporter 3 (NaDC-3) was cloned from human, rat, mouse and flounder, and transported three Na(+) with one divalent di- or tricarboxylate, such as citric acid cycle intermediates and the heavy metal chelator 2,3-dimercaptosuccinate (succimer). The organic anion transporter 1 (OAT1) cloned from several species was shown to exchange extracellular OA against intracellular alpha-ketoglutarate. OAT1 translocated, e.g., anti-inflammatory drugs, antiviral drugs, beta-lactam antibiotics, loop diuretics, ochratoxin A, and p-aminohippurate. Several OA, including probenecid, inhibited OAT1. Human, rat and mouse OAT2 transported selected anti-inflammatory and antiviral drugs, methotrexate, ochratoxin A, and, with high affinities, prostaglandins E(2) and F(2alpha). OAT3 cloned from human, rat and mouse showed a substrate specificity overlapping with that of OAT1. In addition, OAT3 interacted with sulfated steroid hormones such as estrone-3-sulfate. The driving forces for OAT2 and OAT3, the relative contributions of all OA transporters to, and the impact of transporter regulation by protein kinases on renal drug excretion in vivo must be determined in future experiments.

Association between vitamin D receptor gene polymorphisms and tubular citrate handling in calcium nephrolithiasis

OBJECTIVES

Hypocitraturia is a risk factor for calcium nephrolithiasis. 1,25(OH)2D3 influences renal citrate handling and enhances citraturia. The aim of this study was to evaluate the relationship between vitamin D receptor (VDR) allelic variant and urinary citrate excretion in recurrent stone formers (SF) patients.

DESIGN

Case-control study.

SUBJECTS

A total of 220 recurrent calcium oxalate SF patients and 114 healthy control (C) subjects were enrolled for this study. Subjects with urinary tract infections, hyperparathyroidism, cystinuria >70 micromol/24 h, gouty diathesis, renal tubular acidosis, renal failure, chronic diarrhoeal states, intake of thiazide diuretics, angiotensin-converting enzyme (ACE)-inhibitors, glucocorticoids or oestrogens were excluded. A standard constant diet was given for 7 days. The 24-h urinary citrate excretion and the active tubular reabsorption of filtered citrate (Rcit) were evaluated. Hypocitraturia was defined as a urinary citrate excretion lower than 1.7 mmol day-1. Stone formers patients and C were genotyped for BsmI and TaqI VDR alleles. Contingency table chi-square tests were used to compare genotype frequencies in hypocitraturic SF patients, normocitraturic SF and C.

RESULTS

The prevalence of hypocitraturia in SF patients was 32.7% (72 of 200). Hypocitraturia in these patients resulted from excessive Rcit of a normal load of citrate. We found a different distribution (P < 0.05) of BsmI and TaqI VDR genotypes in hypocitraturic SF patients compared with normocitraturic SF and C. In particular, the prevalence of bb and TT VDR genotypes in hypocitraturic SF was significantly higher than in normocitraturic SF and C.

CONCLUSIONS

These results point to a genetic association between BsmI and TaqI VDR polymorphisms and idiopathic hypocitraturia in calcium-oxalate recurrent SF patients.