Gender difference in the Oatp1-mediated tubular reabsorption of estradiol 17beta-D-glucuronide in rats

Exposure to a mixture of legacy, alternative, and replacement per- and polyfluoroalkyl substances (PFAS) results in sex-dependent modulation of cholesterol metabolism and liver injury

BACKGROUND

Epidemiological studies have shown Per- and polyfluoroalkyl substances (PFAS) to be associated with diseases of dysregulated lipid and sterol homeostasis such as steatosis and cardiometabolic disorders. However, the majority of mechanistic studies rely on single chemical exposures instead of identifying mechanisms related to the toxicity of PFAS mixtures.

OBJECTIVES

The goal of the current study is to investigate mechanisms linking exposure to a PFAS mixture with alterations in lipid metabolism, including increased circulating cholesterol and bile acids.

METHODS

Male and female wild-type C57BL/6J mice were fed an atherogenic diet used in previous studies of pollutant-accelerated atherosclerosis and exposed to water containing a mixture of 5 PFAS representing legacy, replacement, and alternative subtypes (i.e., PFOA, PFOS, PFNA, PFHxS, and GenX), each at a concentration of 2 mg/L, for 12 weeks. Changes at the transcriptome and metabolome level were determined by RNA-seq and high-resolution mass spectrometry, respectively.

RESULTS

We observed increased circulating cholesterol, sterol metabolites, and bile acids due to PFAS exposure, with some sexual dimorphic effects. PFAS exposure increased hepatic injury, demonstrated by increased liver weight, hepatic inflammation, and plasma alanine aminotransferase levels. Females displayed increased lobular and portal inflammation compared to the male PFAS-exposed mice. Hepatic transcriptomics analysis revealed PFAS exposure modulated multiple metabolic pathways, including those related to sterols, bile acids, and acyl carnitines, with multiple sex-specific differences observed. Finally, we show that hepatic and circulating levels of PFOA were increased in exposed females compared to males, but this sexual dimorphism was not the same for other PFAS examined.

DISCUSSION

Exposure of mice to a mixture of PFAS results in PFAS-mediated modulation of cholesterol levels, possibly through disruption of enterohepatic circulation.

Novel testing strategy for prediction of rat biliary excretion of intravenously administered estradiol-17β glucuronide

The aim of the present study was to develop a generic rat physiologically based kinetic (PBK) model that includes a novel testing strategy where active biliary excretion is incorporated using estradiol-17β glucuronide (E217βG) as the model substance. A major challenge was the definition of the scaling factor for the in vitro to in vivo conversion of the PBK-model parameter Vmax. In vitro values for the Vmax and Km for transport of E217βG were found in the literature in four different studies based on experiments with primary rat hepatocytes. The required scaling factor was defined based on fitting the PBK model-based predicted values to reported experimental data on E217βG blood levels and cumulative biliary E217βG excretion. This resulted in a scaling factor of 129 mg protein/g liver. With this scaling factor the PBK model predicted the in vivo data for blood and cumulative biliary E217βG levels with on average of less than 1.8-fold deviation. The study provides a proof of principle on how biliary excretion can be included in a generic PBK model using primary hepatocytes to define the kinetic parameters that describe the biliary excretion.

Risk to human health related to the presence of perfluorooctane sulfonic acid and perfluorooctanoic acid in food

The European Commission asked EFSA for a scientific evaluation on the risks to human health related to the presence of perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) in food. Regarding PFOS and PFOA occurrence, the final data set available for dietary exposure assessment contained a total of 20,019 analytical results (PFOS n = 10,191 and PFOA n = 9,828). There were large differences between upper and lower bound exposure due to analytical methods with insufficient sensitivity. The CONTAM Panel considered the lower bound estimates to be closer to true exposure levels. Important contributors to the lower bound mean chronic exposure were 'Fish and other seafood', 'Meat and meat products' and 'Eggs and egg products', for PFOS, and 'Milk and dairy products', 'Drinking water' and 'Fish and other seafood' for PFOA. PFOS and PFOA are readily absorbed in the gastrointestinal tract, excreted in urine and faeces, and do not undergo metabolism. Estimated human half-lives for PFOS and PFOA are about 5 years and 2-4 years, respectively. The derivation of a health-based guidance value was based on human epidemiological studies. For PFOS, the increase in serum total cholesterol in adults, and the decrease in antibody response at vaccination in children were identified as the critical effects. For PFOA, the increase in serum total cholesterol was the critical effect. Also reduced birth weight (for both compounds) and increased prevalence of high serum levels of the liver enzyme alanine aminotransferase (ALT) (for PFOA) were considered. After benchmark modelling of serum levels of PFOS and PFOA, and estimating the corresponding daily intakes, the CONTAM Panel established a tolerable weekly intake (TWI) of 13 ng/kg body weight (bw) per week for PFOS and 6 ng/kg bw per week for PFOA. For both compounds, exposure of a considerable proportion of the population exceeds the proposed TWIs.

Risk to human health related to the presence of perfluorooctane sulfonic acid and perfluorooctanoic acid in food

The European Commission asked EFSA for a scientific evaluation on the risks to human health related to the presence of perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) in food. Regarding PFOS and PFOA occurrence, the final data set available for dietary exposure assessment contained a total of 20,019 analytical results (PFOS n = 10,191 and PFOA n = 9,828). There were large differences between upper and lower bound exposure due to analytical methods with insufficient sensitivity. The CONTAM Panel considered the lower bound estimates to be closer to true exposure levels. Important contributors to the lower bound mean chronic exposure were 'Fish and other seafood', 'Meat and meat products' and 'Eggs and egg products', for PFOS, and 'Milk and dairy products', 'Drinking water' and 'Fish and other seafood' for PFOA. PFOS and PFOA are readily absorbed in the gastrointestinal tract, excreted in urine and faeces, and do not undergo metabolism. Estimated human half-lives for PFOS and PFOA are about 5 years and 2-4 years, respectively. The derivation of a health-based guidance value was based on human epidemiological studies. For PFOS, the increase in serum total cholesterol in adults, and the decrease in antibody response at vaccination in children were identified as the critical effects. For PFOA, the increase in serum total cholesterol was the critical effect. Also reduced birth weight (for both compounds) and increased prevalence of high serum levels of the liver enzyme alanine aminotransferase (ALT) (for PFOA) were considered. After benchmark modelling of serum levels of PFOS and PFOA, and estimating the corresponding daily intakes, the CONTAM Panel established a tolerable weekly intake (TWI) of 13 ng/kg body weight (bw) per week for PFOS and 6 ng/kg bw per week for PFOA. For both compounds, exposure of a considerable proportion of the population exceeds the proposed TWIs.

Comparison of uptake transporter functions in hepatocytes in different species to determine the optimal model for evaluating drug transporter activities in humans

A thorough understanding of species-dependent differences in hepatic uptake transporters is critical for predicting human pharmacokinetics (PKs) from preclinical data. In this study, the activities of organic anion transporting polypeptide (OATP/Oatp), organic cation transporter 1 (OCT1/Oct1), and sodium-taurocholate cotransporting polypeptide (NTCP/Ntcp) in cultured rat, dog, monkey and human hepatocytes were compared. The activities of hepatic uptake transporters were evaluated with respect to culture duration, substrate and species-dependent differences in hepatocytes. Longer culture duration reduced hepatic uptake transporter activities across species except for Oatp and Ntcp in rats. Comparable apparent Michaelis-Menten constant (K_m,app) values in hepatocytes were observed across species for atorvastatin, estradiol-17β-glucuronide and metformin. The K_m,app values for rosuvastatin and taurocholate were significantly different across species. Rat hepatocytes exhibited the highest Oatp percentage of uptake transporter-mediated permeation clearance (PS_inf,act) while no difference in %PS_inf,act of probe substrates were observed across species. The in vitro hepatocyte inhibition data in rats, monkeys and humans provided reasonable predictions of in vivo drug-drug interaction (DDIs) between atorvastatin/rosuvastatin and rifampin. These findings suggested that using human hepatocytes with a short culture time is the most robust preclinical model for predicting DDIs for compounds exhibiting active hepatic uptake in humans.

Influence of testosterone on phase II metabolism and availability of soy isoflavones in male Wistar rats

Genistein and daidzein are the main isoflavones in soy. Their potential beneficial or adverse effects in males like the prevention of prostate cancer or the impact on reproductive functions are controversially discussed. Major determinants of their bioactivity are the absorption and biotransformation of isoflavones. In this study, we focused on the influence of testosterone on plasma availability and phase II metabolism of isoflavones. Male Wistar rats, receiving an isoflavones rich diet, were randomized into three groups: Two groups were orchiectomized (ORX) at postnatal day (PND) 80 and treated for 11 days with testosterone propionate (TP) (ORX TP group) or a vehicle (ORX group) after a 7 days lasting hormonal decline. The third group served as control and remained intact. Rats were sacrificed at PND 98. ORX rats had reduced isoflavones plasma levels. Differently regulated mRNA expressions of transporters relevant for transport of phase II metabolites in liver and kidney may be responsible for this reduction, more precisely Slc10a1 and Slc21a1 in kidney as well as Slc22a8 in liver. While main phase II metabolites in intact rats were disulfates and sulfoglucuronides, the amount of sulfate conjugates was significantly diminished by ORX. In accordance with that, mRNA expression of different sulfotransferases was reduced in liver by ORX. The observed effects could be almost restored by TP treatment. In conclusion, testosterone, and likely further androgens, has a huge impact on phase II metabolism and availability of isoflavones by influencing the expression of different sulfotransferases and transporters.

Bisphenol A: Update on newly developed data and how they address NTP's 2008 finding of "Some Concern"

Bisphenol A (BPA) is a component of polycarbonate plastics and epoxy resins used in many commercial products including coatings and liners of food containers. Low levels of BPA can be detected in over 90% of human urine samples in the US, indicating that exposure to BPA is widespread. In 2008, the US National Toxicology Program's Center for the Evaluation of Risks to Human Reproduction (NTP-CERHR) expressed concerns regarding BPA's potential health effects, and suggested improved study designs and methodologies that they believed would address those concerns. This paper discusses some of the controversial issues surrounding BPA, summarizes the current regulatory status of BPA, reviews recent pharmacokinetic studies, and describes ongoing and planned research on the effects of BPA. In addition, we evaluate two papers studying BPA neurobehavioral effects, identified by the European Food Safety Authority and the German Federal Institute for Risk Assessment as being valid for use in risk assessment, to determine whether they address the NTP-CERHR methodological concerns. The data from these studies would likely be sufficient for NTP to lower its concern level for neurobehavioral effects of BPA. At this time, many regulatory agencies from around the world support the use of BPA in food contact materials.

Expression and function of renal and hepatic organic anion transporters in extrahepatic cholestasis

Obstructive jaundice occurs in patients suffering from cholelithiasis and from neoplasms affecting the pancreas and the common bile duct. The absorption, distribution and elimination of drugs are impaired during this pathology. Prolonged cholestasis may alter both liver and kidney function. Lactam antibiotics, diuretics, non-steroidal anti-inflammatory drugs, several antiviral drugs as well as endogenous compounds are classified as organic anions. The hepatic and renal organic anion transport pathways play a key role in the pharmacokinetics of these compounds. It has been demonstrated that acute extrahepatic cholestasis is associated with increased renal elimination of organic anions. The present work describes the molecular mechanisms involved in the regulation of the expression and function of the renal and hepatic organic anion transporters in extrahepatic cholestasis, such as multidrug resistance-associated protein 2, organic anion transporting polypeptide 1, organic anion transporter 3, bilitranslocase, bromosulfophthalein/bilirubin binding protein, organic anion transporter 1 and sodium dependent bile salt transporter. The modulation in the expression of renal organic anion transporters constitutes a compensatory mechanism to overcome the hepatic dysfunction in the elimination of organic anions.

Ochratoxin a and mitotic disruption: mode of action analysis of renal tumor formation by ochratoxin A

The mycotoxin and food contaminant ochratoxin A (OTA) is a potent renal carcinogen in rodents, but its mode of action (MoA) is still poorly defined. In 2006, the European Food Safety Authority concluded that there is a "lack of evidence for the existence of OTA-DNA adducts" and thus insufficient evidence to establish DNA reactivity as a MoA for tumor formation by OTA. In reviewing the available database on OTA toxicity, a MoA for renal carcinogenicity of OTA is developed that involves a combination of genetic instability and increased proliferative drive as consequences of OTA-mediated disruption of mitosis, whereby the organ- and site-specificity of tumor formation by OTA is determined by selective renal uptake of OTA into the proximal tubule epithelium. The proposed MoA is critically assessed with respect to concordance of dose-response of the suggested key events and tumor formation, their temporal association, consistency, and biological plausibility. Uncertainties, data gaps and needs for further research are highlighted.

Organic anion transporting polypeptide (Oatp) 1a1-mediated perfluorooctanoate transport and evidence for a renal reabsorption mechanism of Oatp1a1 in renal elimination of perfluorocarboxylates in rats

Organic anion transporting polypeptide (Oatp) 1a1 has been hypothesized to play a key role in rat renal reabsorption of perfluorooctanoate (PFO). We have investigated PFO uptake kinetics in Chinese Hamster Ovary (CHO) cells that have been stably transfected with the cDNA encoding Oatp1a1. The Oatp1a1-expressing CHO cells have been validated by their Oatp1a1 gene expression, estrone-3-sulfate (E3S) uptake kinetics, and the correlation between Oatp1a1 gene expression and E3S uptake activity that were both induced by the treatment of sodium butyrate. Oatp1a1-mediated PFO uptake underwent a saturable process with a K(m) value of 162.2+/-20.2microM, which was effectively inhibited by known Oatp1a1 substrates sulfobromophthalein and taurocholate, and a major flavonoid in grapefruit juice, naringin. The inhibition of Oatp1a1-mediated E3S uptake has been compared for linear perfluorocarboxylates with carbon chain lengths ranged from 4 to 12. There was no apparent inhibition by perfluorobutanoate and perfluoropentanoate at 1mM. Inhibition was observed for perfluorohexanoate at 1mM and the level of inhibition increased as the increase of the chain length up to perfluorodecanoate. The values of apparent inhibition constant (K(i,app)) were determined for perfluorocarboxylates with chain lengths between 6 and 10. The log values of K(i,app) exhibited a negative linear relationship to the chain lengths and a positive linear relationship to the log values of the total clearance of perfluorocarboxylates in male rats. This in vitro-to-in vivo correlation strongly supports a tubular reabsorptive role of Oatp1a1 in rat renal elimination of perfluorocarboxylates. Due to the sex-dependent expression of Oatp1a1 in rat kidney, Oatp1a1-mediated tubular reabsorption is suggested to be the mechanism for the sex-dependent renal elimination of PFO in rats.

Carnitine/organic cation transporter OCTN2 (Slc22a5) is responsible for renal secretion of cephaloridine in mice

Carnitine/organic cation transporter (OCTN) 2 (SLC22A5) plays a pivotal role in renal tubular reabsorption of carnitine, a vitamin-like compound, on apical membranes of proximal tubules, but its role in relation to therapeutic drugs remains to be clarified. The purpose of the present study was to elucidate the involvement of OCTN2 in renal disposition of a beta-lactam antibiotic, cephaloridine (CER), based on experiments with juvenile visceral steatosis (jvs) mice, which have a functional deficiency of the octn2 gene. Renal clearance of CER during constant intravenous infusion in wild-type mice was much higher than could be accounted for by glomerular filtration, but was decreased by increasing the infusion rate with minimal change in kidney-to-plasma concentration ratio, suggesting the existence of saturable transport mechanism(s) across the apical membranes. The plasma concentration profile and kidney-to-plasma concentration ratio after intravenous injection in jvs mice were higher than those in wild-type mice, whereas renal clearance in jvs mice was much lower than that in wild-type mice and could be accounted for by glomerular filtration. Uptake of CER by mouse OCTN2 was shown in Xenopus laevis oocytes expressing mouse OCTN2. The CER transport by OCTN2 exhibited saturation with K(m) of approximately 3 mM, which is similar to the renal CER concentration exhibiting saturation in renal clearance in vivo. The OCTN2-mediated CER transport was inhibited by carnitine and independent of Na(+) replacement in the medium. These results show OCTN2 on apical membranes of proximal tubules plays a major role in renal secretion of CER in mice.

Characterization of the mechanisms involved in the increased renal elimination of bromosulfophthalein during cholestasis: involvement of Oatp1

The kidneys and liver are the major routes for organic anion elimination. We have recently shown that acute obstructive jaundice is associated with increased systemic and renal elimination of two organic anions, p-aminohippurate and furosemide, principally excreted through urine. This study examined probable adaptive mechanisms involved in renal elimination of bromosulfophthalein (BSP), a prototypical organic anion principally excreted in bile, in rats with acute obstructive jaundice. Male Wistar rats underwent bile duct ligation (BDL rats). Pair-fed sham-operated rats served as controls. BSP renal clearance was performed by conventional techniques. Renal organic anion-transporting polypeptide 1 (Oatp1) expression was evaluated by immunoblotting and IHC. Excreted, filtered, and secreted loads of BSP were all higher in BDL rats compared with Sham rats. The higher BSP filtered load resulted from the increase in plasma BSP concentration in BDL rats, because glomerular filtration rate showed no difference with the Sham group. The increase in the secreted load might be explained by the higher expression of Oatp1 observed in apical membranes from kidneys of BDL animals. This likely adaptation to hepatic injury, specifically in biliary components elimination, might explain, at least in part, the huge increase in BSP renal excretion observed in this experimental model.

Xenobiotic transporters: ascribing function from gene knockout and mutation studies

Transporter-mediated absorption, secretion, and reabsorption of chemicals are increasingly recognized as important determinants in the biological activities of many xenobiotics. In recent years, the rapid progress in generating and characterizing mice with targeted deletion of transporters has greatly increased our knowledge of the functions of transporters in the pharmacokinetics/toxicokinetics of xenobiotics. In this introduction, we focus on functions of transporters learned from experiments on knockout mice as well as humans and rodents with natural mutations of these transporters. We limit our discussion to transporters that either directly transport xenobiotics or are important in biliary excretion or cellular defenses, namely multidrug resistance, multidrug resistance-associated proteins, breast cancer resistance protein, organic anion transporting polypeptides, organic anion transporters, organic cation transporters, nucleoside transporters, peptide transporters, bile acid transporters, cholesterol transporters, and phospholipid transporters, as well as metal transporters. Efflux transporters in intestine, liver, kidney, brain, testes, and placenta can efflux xenobiotics out of cells and serve as barriers against the entrance of xenobiotics into cells, whereas many xenobiotics enter the biological system via uptake transporters. The functional importance of a given transporter in each tissue depends on its substrate specificity, expression level, and the presence/absence of other transporters with overlapping substrate preferences. Nevertheless, a transporter may affect a tissue independent of its local expression by altering systemic metabolism. Further studies on the gene regulation and function of transporters, as well as the interrelationship between transporters and phase I/II xenobiotic-metabolizing enzymes, will provide a complete framework for developing novel strategies to protect us from xenobiotic insults.

Organic anion transporter 3 (Oat3/Slc22a8) knockout mice exhibit altered clearance and distribution of penicillin G

The interaction of renal basolateral organic anion transporter 3 (Oat3) with commonly used pharmacotherapeutics (e.g., NSAIDs, beta-lactams, and methotrexate) has been studied extensively in vitro. However, the in vivo role of Oat3 in drug disposition, in the context of other transporters, glomerular filtration, and metabolism, has not been established. Moreover, recent investigations have identified inactive human OAT3 polymorphisms. Therefore, this investigation was designed to elucidate the in vivo role of Oat3 in the disposition of penicillin G and prototypical substrates using an Oat3 knockout mouse model. Oat3 deletion resulted in a doubling of penicillin's half-life (P < 0.05) and a reduced volume of distribution (P < 0.01), together yielding a plasma clearance that was one-half (P < 0.05, males) to one-third (P < 0.001, females) of that in wild-type mice. Inhibition of Oat3 abolished the differences in penicillin G elimination between genotypes. Hepatic accumulation of penicillin was 2.3 times higher in male knockouts (P < 0.05) and 3.7 times higher in female knockouts (P < 0.001). Female knockouts also exhibited impaired estrone-3-sulfate clearance. Oat3 deletion did not impact p-aminohippurate elimination, providing correlative evidence to studies in Oat1 knockout mice that suggest Oat1 governs tubular uptake of p-aminohippurate. Collectively, these findings are the first to indicate that functional Oat3 is necessary for proper elimination of xenobiotic and endogenous compounds in vivo. Thus Oat3 plays a distinct role in determining the efficacy and toxicity of drugs. Dysfunctional human OAT3 polymorphisms or instances of polypharmacy involving OAT3 substrates may result in altered systemic accumulation of beta-lactams and other clinically relevant compounds.

Gender differences in kidney function

Sex hormones influence the development of female (F) and male (M) specific traits and primarily affect the structure and function of gender-specific organs. Recent studies also indicated their important roles in regulating structure and/or function of nearly every tissue and organ in the mammalian body, including the kidneys, causing gender differences in a variety of characteristics. Clinical observations in humans and studies in experimental animals in vivo and in models in vitro have shown that renal structure and functions under various physiological, pharmacological, and toxicological conditions are different in M and F, and that these differences may be related to the sex-hormone-regulated expression and action of transporters in the apical and basolateral membrane of nephron epithelial cells. In this review we have collected published data on gender differences in renal functions, transporters and other related parameters, and present our own microarray data on messenger RNA expression for various transporters in the kidney cortex of M and F rats. With these data we would like to emphasize the importance of sex hormones in regulation of a variety of renal transport functions and to initiate further studies of gender-related differences in kidney structure and functions, which would enable us to better understand occurrence and development of various renal diseases, pharmacotherapy, and drug-induced nephrotoxicity in humans and animals.

Implications of gender differences for human health risk assessment and toxicology

This paper from The Human Health working group of SGOMSEC 16 examines a broad range of issues on gender effects in toxicology. Gender differences in toxicology begin at the gamete and embryo stage, continuing through development and maturation and into old age. Sex influences exposure, toxicokinetics, and toxicodynamics. The effects of sex have often been overlooked in both epidemiology and toxicology. In addition to the obvious modifying effects of the sex hormones and conditions affecting the male and female reproductive organs and sex roles, both genetic and hormonal effects influence many aspects of life and toxic responses. All aspects of toxicology should consider gender-balanced designs so that a more comprehensive understanding of differences and similarities can be obtained. Differential gene expression is a new frontier in toxicology. Risk assessment should account for gender and life cycle differences. The biological basis for altered sex ratios observed in several populations should be sought in animal models, and expanded to other compounds that might exert sex-selective effects. Wherever possible and feasible, toxicologic and environmental epidemiological studies should be designed and have sufficient statistical power to quantify differential gender-based exposures and outcomes.

Bile acid transporters: structure, function, regulation and pathophysiological implications

Specific transporters expressed in the liver and the intestine, play a critical role in driving the enterohepatic circulation of bile acids. By preserving a circulating pool of bile acids, an important factor influencing bile flow, these transporters are involved in maintaining bile acid and cholesterol homeostasis. Enterohepatic circulation of bile acids is fundamentally composed of two major processes: secretion from the liver and absorption from the intestine. In the hepatocytes, the vectorial transport of bile acids from blood to bile is ensured by Na+ taurocholate co-transporting peptide (NTCP) and organic anion transport polypeptides (OATPs). After binding to a cytosolic bile acid binding protein, bile acids are secreted into the canaliculus via ATP-dependent bile salt excretory pump (BSEP) and multi drug resistant proteins (MRPs). Bile acids are then delivered to the intestinal lumen through bile ducts where they emulsify dietary lipids and cholesterol to facilitate their absorption. Intestinal epithelial cells reabsorb the majority of the secreted bile acids through the apical sodium dependent bile acid transporter (ASBT) and sodium independent organic anion transporting peptide (OATPs). Cytosolic ileal bile acid binding protein (IBABP) mediates the transcellular movement of bile acids to the basolateral membrane across which they exit the cells via organic solute transporters (OST). An essential role of bile acid transporters is evident from the pathology associated with their genetic disruption or dysregulation of their function. Malfunctioning of hepatic and intestinal bile acid transporters is implicated in the pathophysiology of cholestatic liver disease and the depletion of circulating pool of bile acids, respectively. Extensive efforts have been recently made to enhance our understanding of the structure, function and regulation of the bile acid transporters and exploring new potential therapeutics to treat bile acid or cholesterol related diseases. This review will highlight current knowledge about structure, function and molecular characterization of bile acid transporters and discuss the implications of their defects in various hepatic and intestinal disorders.

Renal expression of organic anion transporter OAT2 in rats and mice is regulated by sex hormones

The renal reabsorption and/or excretion of various organic anions is mediated by specific organic anion transporters (OATs). OAT2 (Slc22a7) has been identified in rat kidney, where its mRNA expression exhibits gender differences [females (F) > males (M)]. The exact localization of OAT2 protein in the mammalian kidney has not been reported. Here we studied the expression of OAT2 mRNA by RT-PCR and its protein by Western blotting (WB) and immunocytochemistry (IC) in kidneys of adult intact and gonadectomized M and F, sex hormone-treated castrated M, and prepubertal M and F rats, and the protein in adult M and F mice. In adult rats, the expression of OAT2 mRNA was predominant in the outer stripe (OS) tissue, exhibiting 1) gender dependency (F > M), 2) upregulation by castration and downregulation by ovariectomy, and 3) strong downregulation by testosterone and weak upregulation by estradiol and progesterone treatment. A polyclonal antibody against rat OAT2 on WB of isolated renal membranes labeled a approximately 66-kDa protein band that was stronger in F. By IC, the antibody exclusively stained brush border (BB) of the proximal tubule S3 segment (S3) in the OS and medullary rays (F > M). In variously treated rats, the pattern of 66-kDa band density in the OS membranes and the staining intensity of BB in S3 matched the mRNA expression. The expression of OAT2 protein in prepubertal rats was low and gender independent. In mice, the expression pattern largely resembled that in rats. Therefore, OAT2 in rat (and mouse) kidney is localized to the BB of S3, exhibiting gender differences (F > M) that appear in puberty and are caused by strong androgen inhibition and weak estrogen and progesterone stimulation.

Gender-specific expression of liver organic anion transporters in rat

BACKGROUND

Sex differences in drug pharmacokinetics have been well recognized and gender has been considered a risk factor for adverse events to medications. The aim of this study was to investigate the effect of gender on the expression of hepatocellular transport proteins involved in uptake and secretion of organic anions in rat.

MATERIALS AND METHODS

Expression of the rat liver organic anion transporting polypeptides (Oatps) and multidrug resistance proteins (Mrps) was analysed by reverse transcription polymerase chain reaction (RT-PCR), immunoblot analysis and immunofluorescence microscopy in male and female rats. Regulation of these transport proteins in response to the steroid dehydroepiandrosterone (DHEA) was investigated.

RESULTS

In untreated rats, protein expression significantly differed between genders being higher (Mrp2, Mrp3), comparable [Oatp1a1 (Oatp1); Oatp1b2 (Oatp4)] or lower [Oatp1a4 (Oatp2)] in female than in male rat. DHEA treatment over 3 days (100 mg d(-1)) led to a further increase in Mrp3 expression only in female rats. Mrp2 expression was not influenced by DHEA treatment. Oatp1a1 and Oatp1b2 were significantly down-regulated after DHEA treatment in both male and female rats. In contrast, Oatp1a4 was down-regulated in male rats only.

CONCLUSIONS

In rat, liver transport proteins of the Oatp and Mrp family are expressed and regulated in a gender-specific manner according to sexual differences in the hepatic metabolism of steroids and drugs. These findings may partly explain the well-known sex differences in hepatic handling of organic anions.

Apical/Basolateral Surface Expression of Drug Transporters and its Role in Vectorial Drug Transport

It is well known that transporter proteins play a key role in governing drug absorption, distribution, and elimination in the body, and, accordingly, they are now considered as causes of drug-drug interactions and interindividual differences in pharmacokinetic profiles. Polarized tissues directly involved in drug disposition (intestine, kidney, and liver) and restricted distribution to naive sanctuaries (blood-tissue barriers) asymmetrically express a variety of drug transporters on the apical and basolateral sides, resulting in vectorial drug transport. For example, the organic anion transporting polypeptide (OATP) family on the sinusoidal (basolateral) membrane and multidrug resistance-associated protein 2 (MRP2/ABCC2) on the apical bile canalicular membrane of hepatocytes take up and excrete organic anionic compounds from blood to bile. Such vectorial transcellular transport is fundamentally attributable to the asymmetrical distribution of transporter molecules in polarized cells. Besides the apical/basolateral sorting direction, distribution of the transporter protein between the membrane surface (active site) and the intracellular fraction (inactive site) is of practical importance for the quantitative evaluation of drug transport processes. The most characterized drug transporter associated with this issue is MRP2 on the hepatocyte canalicular (apical) membrane, and it is linked to a genetic disease. Dubin-Johnson syndrome is sometimes caused by impaired canalicular surface expression of MRP2 by a single amino acid substitution. Moreover, single nucleotide polymorphisms in OATP-C/SLC21A6 (SLCO1B1) also affect membrane surface expression, and actually lead to the altered pharmacokinetic profile of pravastatin in healthy subjects. In this review article, the asymmetrical transporter distribution and altered surface expression in polarized tissues are discussed.

Characterization of renal excretion mechanism for a novel diuretic, M17055, in rats

M17055 was developed as a novel diuretic that inhibits both Na(+), K(+), and 2Cl(-) cotransport at the thick ascending Henle's loop and Na(+) reuptake at the distal tubule. It is secreted at the renal proximal tubules. The purpose of the present study was to characterize the renal excretion mechanism of M17055. We used the renal cortical slices and brush border membrane vesicles (BBMVs) to investigate the transport mechanisms across the basolateral and brush border membranes, respectively. M17055 uptake by rat renal slices increased with time and was saturable. Several organic anions including probenecid, para-aminohippurate (PAH), and estrone-3-sulfate, decreased M17055 uptake. The uptake of M17055 was also observed into HEK293 cells expressing rat OAT1, and was inhibited by PAH. M17055 uptake by BBMVs was time-dependent, saturable, osmolarity-sensitive, and inhibited by several organic anions, but not by PAH. These results suggest that plural organic anion transport systems are involved in M17055 transport via both basolateral and brush border membranes of proximal tubule epithelial cells, a part of the renal uptake being mediated by OAT1.

Temporal expression profiles of organic anion transport proteins in placenta and fetal liver of the rat

Physiological cholestasis linked to immature hepatobiliary transport systems for organic anions occurs in rat and human neonates. In utero, the placenta facilitates vectorial transfer of certain fetal-derived solutes to the maternal circulation for elimination. We compared the ontogenesis of organic anion transporters in the placenta and the fetal liver of the rat to assess their relative abundance throughout gestation and to determine whether the placenta compensates for the late maturation of transporters in the developing liver. The mRNA of members of the organic anion transporting polypeptide (Oatp) superfamily, the multidrug resistance protein (Mrp) family, one organic anion transporter (OAT), and the bile acid carriers Na(+)-taurocholate cotransporting polypeptide (Ntcp) and bile salt export pump (Bsep) was quantified by real-time PCR. The most abundant placental transporters were Oatp4a1, whose mRNA increased 10-fold during gestation, and Mrp1. Mrp1 immunolocalized predominantly to epithelial cells of the endoplacental yolk sac, suggesting an excretory role that sequesters fetal-derived solutes in the yolk sac cavity, and faintly to the basal syncytiotrophoblast surface. The mRNA levels of Oatp2b1, Mrp3, and Bsep in the placenta exceeded those in the fetal liver until day 20 of gestation, suggesting that the fetus relies on placental clearance of substrates when expression in the developing liver is low. Mrp3 immunolocalized to the epithelium of the endoplacental yolk sac and less abundantly in the labyrinth zone and endothelium of the maternal arteries. The placental expression of Oatp1a1, Oatp1a4, Oatp1a5, Oatp1b2, Oat, Ntcp, Mrp2, and Mrp6 was low.

Impact of drug transporter studies on drug discovery and development

Drug transporters are expressed in many tissues such as the intestine, liver, kidney, and brain, and play key roles in drug absorption, distribution, and excretion. The information on the functional characteristics of drug transporters provides important information to allow improvements in drug delivery or drug design by targeting specific transporter proteins. In this article we summarize the significant role played by drug transporters in drug disposition, focusing particularly on their potential use during the drug discovery and development process. The use of transporter function offers the possibility of delivering a drug to the target organ, avoiding distribution to other organs (thereby reducing the chance of toxic side effects), controlling the elimination process, and/or improving oral bioavailability. It is useful to select a lead compound that may or may not interact with transporters, depending on whether such an interaction is desirable. The expression system of transporters is an efficient tool for screening the activity of individual transport processes. The changes in pharmacokinetics due to genetic polymorphisms and drug-drug interactions involving transporters can often have a direct and adverse effect on the therapeutic safety and efficacy of many important drugs. To obtain detailed information about these interindividual differences, the contribution made by transporters to drug absorption, distribution, and excretion needs to be taken into account throughout the drug discovery and development process.