Reactive Oxygen Species-Mediated Diabetic Heart Disease: Mechanisms and Therapies

Significance: Diabetic heart disease (DHD) is the primary cause of mortality in people with diabetes. A significant contributor to the development of DHD is the disruption of redox balance due to reactive oxygen species (ROS) overproduction resulting from sustained high glucose levels. Therapies specifically focusing on the suppression of ROS will hugely benefit patients with DHD. Recent Advances: In addition to the gold standard pharmacological therapies, the recent development of gene therapy provides an exciting avenue for developing new therapeutics to treat ROS-mediated DHD. In particular, microRNAs (miRNAs) are gaining interest due to their crucial role in several physiological and pathological processes, including DHD. Critical Issues: miRNAs have many targets and differential function depending on the environment. Therefore, a proper understanding of the function of miRNAs in specific cell types and cell states is required for the successful application of this technology. In the present review, we first provide an overview of the role of ROS in contributing to DHD and the currently available treatments. We then discuss the newer gene therapies with a specific focus on the role of miRNAs as the causative factors and therapeutic targets to combat ROS-mediated DHD. Future Directions: The future of miRNA therapeutics in tackling ROS-mediated DHD is dependent on a complete understanding of how miRNAs behave in different cells and environments. Future research should also aim to develop conditional miRNA therapeutic platforms capable of switching on and off in response to disruptions in the redox state. Antioxid. Redox Signal. 36, 608-630.

Diabetes induces dysregulation of microRNAs associated with survival, proliferation and self-renewal in cardiac progenitor cells

AIMS/HYPOTHESIS

Diabetes mellitus causes a progressive loss of functional efficacy in stem cells, including cardiac progenitor cells (CPCs). The underlying molecular mechanism is still not known. MicroRNAs (miRNAs) are small, non-coding RNA molecules that regulate genes at the post-transcriptional level. We aimed to determine if diabetes mellitus induces dysregulation of miRNAs in CPCs and to test if in vitro therapeutic modulation of miRNAs would improve the functions of diabetic CPCs.

METHODS

CPCs were isolated from a mouse model of type 2 diabetes (db/db), non-diabetic mice and human right atrial appendage heart tissue. Total RNA isolated from mouse CPCs was miRNA profiled using Nanostring analysis. Bioinformatic analysis was employed to predict the functional effects of altered miRNAs. MS analysis was applied to determine the targets, which were confirmed by western blot analysis. Finally, to assess the beneficial effects of therapeutic modulation of miRNAs in vitro and in vivo, prosurvival miR-30c-5p was overexpressed in mouse and human diabetic CPCs, and the functional consequences were determined by measuring the level of apoptotic cell death, cardiac function and mitochondrial membrane potential (MMP).

RESULTS

Among 599 miRNAs analysed in mouse CPCs via Nanostring analysis, 16 miRNAs showed significant dysregulation in the diabetic CPCs. Using bioinformatics tools and quantitative real-time PCR (qPCR) validation, four altered miRNAs (miR-30c-5p, miR-329-3p, miR-376c-3p and miR-495-3p) were identified to play an important role in cell proliferation and survival. Diabetes mellitus significantly downregulated miR-30c-5p, while it upregulated miR-329-3p, miR-376c-3p and miR-495-3p. MS analysis revealed proapoptotic voltage-dependent anion-selective channel 1 (VDAC1) as a direct target for miR-30c-5p, and cell cycle regulator, cyclin-dependent protein kinase 6 (CDK6), as the direct target for miR-329-3p, miR-376c-3p and miR-495-3p. Western blot analyses showed a marked increase in VDAC1 expression, while CDK6 expression was downregulated in diabetic CPCs. Finally, in vitro and in vivo overexpression of miR-30c-5p markedly reduced the apoptotic cell death and preserved MMP in diabetic CPCs via inhibition of VDAC1.

CONCLUSIONS/INTERPRETATION

Our results demonstrate that diabetes mellitus induces a marked dysregulation of miRNAs associated with stem cell survival, proliferation and differentiation, and that therapeutic overexpression of prosurvival miR-30c-5p reduced diabetes-induced cell death and loss of MMP in CPCs via the newly identified target for miR-30c-5p, VDAC1.

Cardiac β-adrenergic responsiveness of obese Zucker rats: The role of AMPK

NEW FINDINGS

What is the central question of the study? Is the reduced signalling of AMP-activated protein kinase (AMPK), a key regulator of energy homeostasis in the heart, responsible for the reduced β-adrenergic responsiveness of the heart in obesity? What is the main finding and its importance? Inhibition of AMPK in isolated hearts prevented the reduced cardiac β-adrenergic responsiveness of obese rats, which was accompanied by reduced phosphorylation of AMPK, a proxy of AMPK activity. This suggests a direct functional link between β-adrenergic responsiveness and AMPK signalling in the heart, and it suggests that AMPK might be an important target to restore the β-adrenergic responsiveness in the heart in obesity.

ABSTRACT

The obesity epidemic impacts heavily on cardiovascular health, in part owing to changes in cardiac metabolism. AMP-activated protein kinase (AMPK) is a key regulator of energy homeostasis in the heart and is regulated by β-adrenoceptors (β-ARs) in normal conditions. In obesity, chronic sympathetic overactivation leads to impaired cardiac β-AR responsiveness, although it is unclear whether AMPK signalling, downstream of β-ARs, contributes to this dysfunction. Therefore, we aimed to determine whether reduced AMPK signalling is responsible for the reduced β-AR responsiveness in obesity. In isolated hearts of lean and obese Zucker rats, we tested β-AR responsiveness to the β₁ -AR agonist isoprenaline (ISO, 1 × 10^-10 to 5 × 10^-8  m) in the absence and presence of the AMPK inhibitor, compound C (CC, 10 μm). The β₁ -AR expression and AMPK phosphorylation were assessed by Western blot. β-Adrenergic responsiveness was reduced in the hearts of obese rats (logEC₅₀ of ISO-developed pressure dose-response curves: lean -8.53 ± 0.13 × 10^x  m versus obese -8.35 ± 0.10 × 10^x  m ; P < 0.05 lean versus obese, n = 6 per group). This difference was not apparent after AMPK inhibition (logEC₅₀ of ISO-developed pressure curves: lean CC -8.19 ± 0.12 × 10^x  m versus obese CC 8.17 ± 0.13 × 10^x  m, P < 0.05, n = 6 per group). β₁ -Adrenergic receptor expression and AMPK phosphorylation were reduced in hearts of obese rats (AMPK at Thr¹⁷² : lean 1.73 ± 0.17 a.u. versus lean CC 0.81 ± 0.13 a.u., and obese 1.18 ± 0.09 a.u. versus obese CC 0.81 ± 0.16 a.u., P < 0.05, n = 6 per group). Thus, a direct functional link between β-adrenergic responsiveness and AMPK signalling in the heart exists, and AMPK might be an important target to restore the reduced cardiac β-adrenergic responsiveness in obesity.

Uric acid: a modulator of prostate cells and activin sensitivity

Elevated serum uric acid (SUA) or urate is associated with inflammation and gout. Recent evidence has linked urate to cancers, but little is known about urate effects in prostate cancer. Activins are inflammatory cytokines and negative growth regulators in the prostate. A hallmark of prostate cancer progression is activin insensitivity; however, mechanisms underlying this are unclear. We propose that elevated SUA is associated with prostate cancer counteracting the growth inhibitory effects of activins. The expression of activins A and B, urate transporter GLUT9 and tissue urate levels were examined in human prostate disease. Intracellular and secreted urate and GLUT9 expression were assessed in human prostate cancer cell lines. Furthermore, the effects of urate and probenecid, a known urate transport inhibitor, were determined in combination with activin A. Activin A expression was increased in low-grade prostate cancer, whereas activin B expression was reduced in high-grade prostate cancer. Intracellular urate levels decreased in all prostate pathologies, while GLUT9 expression decreased in benign prostatic hyperplasia, prostatitis and high-grade prostate cancer. Activin responsive LNCaP cells had higher intracellular and lower secreted urate levels than activin-insensitive PC3 cells. GLUT9 expression in prostate cancer cells was progressively lower than in prostate epithelial cells. Elevated extracellular urate was growth promoting in vitro, which was abolished by the gout medication probenecid, and it antagonized the growth inhibitory effects of activins. This study shows for the first time that a change in plasma or intracellular urate levels, possibly involving GLUT9 and a urate efflux transporter, has an impact on prostate cancer cell growth, and that lowering SUA levels in prostate cancer is likely to be therapeutically beneficial.

Increased Efferent Cardiac Sympathetic Nerve Activity and Defective Intrinsic Heart Rate Regulation in Type 2 Diabetes

Elevated sympathetic nerve activity (SNA) coupled with dysregulated β-adrenoceptor (β-AR) signaling is postulated as a major driving force for cardiac dysfunction in patients with type 2 diabetes; however, cardiac SNA has never been assessed directly in diabetes. Our aim was to measure the sympathetic input to and the β-AR responsiveness of the heart in the type 2 diabetic heart. In vivo recording of SNA of the left efferent cardiac sympathetic branch of the stellate ganglion in Zucker diabetic fatty rats revealed an elevated resting cardiac SNA and doubled firing rate compared with nondiabetic rats. Ex vivo, in isolated denervated hearts, the intrinsic heart rate was markedly reduced. Contractile and relaxation responses to β-AR stimulation with dobutamine were compromised in externally paced diabetic hearts, but not in diabetic hearts allowed to regulate their own heart rate. Protein levels of left ventricular β1-AR and Gs (guanine nucleotide binding protein stimulatory) were reduced, whereas left ventricular and right atrial β2-AR and Gi (guanine nucleotide binding protein inhibitory regulatory) levels were increased. The elevated resting cardiac SNA in type 2 diabetes, combined with the reduced cardiac β-AR responsiveness, suggests that the maintenance of normal cardiovascular function requires elevated cardiac sympathetic input to compensate for changes in the intrinsic properties of the diabetic heart.

Impaired relaxation despite upregulated calcium-handling protein atrial myocardium from type 2 diabetic patients with preserved ejection fraction

BACKGROUND

Diastolic dysfunction is a key factor in the development and pathology of cardiac dysfunction in diabetes, however the exact underlying mechanism remains unknown, especially in humans. We aimed to measure contraction, relaxation, expression of calcium-handling proteins and fibrosis in myocardium of diabetic patients with preserved systolic function.

METHODS

Right atrial appendages from patients with type 2 diabetes mellitus (DM, n = 20) and non-diabetic patients (non-DM, n = 36), all with preserved ejection fraction and undergoing coronary artery bypass grafting (CABG), were collected. From appendages, small cardiac muscles, trabeculae, were isolated to measure basal and β-adrenergic stimulated myocardial function. Expression levels of calcium-handling proteins, sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) and phospholamban (PLB), and of β1-adrenoreceptors were determined in tissue samples by Western blot. Collagen deposition was determined by picro-sirius red staining.

RESULTS

In trabeculae from diabetic samples, contractile function was preserved, but relaxation was prolonged (Tau: 74 ± 13 ms vs. 93 ± 16 ms, non-DM vs. DM, p = 0.03). The expression of SERCA2a was increased in diabetic myocardial tissue (0.75 ± 0.09 vs. 1.23 ± 0.15, non-DM vs. DM, p = 0.007), whereas its endogenous inhibitor PLB was reduced (2.21 ± 0.45 vs. 0.42 ± 0.11, non-DM vs. DM, p = 0.01). Collagen deposition was increased in diabetic samples. Moreover, trabeculae from diabetic patients were unresponsive to β-adrenergic stimulation, despite no change in β1-adrenoreceptor expression levels.

CONCLUSIONS

Human type 2 diabetic atrial myocardium showed increased fibrosis without systolic dysfunction but with impaired relaxation, especially during β-adrenergic challenge. Interestingly, changes in calcium-handling protein expression suggests accelerated active calcium re-uptake, thus improved relaxation, indicating a compensatory calcium-handling mechanism in diabetes in an attempt to maintain diastolic function at rest despite impaired relaxation in the diabetic fibrotic atrial myocardium. Our study addresses important aspects of the underlying mechanisms of diabetes-associated diastolic dysfunction, which is crucial to developing new therapeutic treatments.

Organic anion transporter 3 (OAT3) and renal transport of the metal chelator 2,3-dimercapto-1-propanesulfonic acid (DMPS)

Recent investigations involving intact rabbit renal proximal tubules indicated that organic anion transporter 3 (OAT3) may be involved in the transport of 2,3-dimercapto-1-propanesulfonic acid (DMPS). Therefore, we evaluated the interaction of OAT3 with DMPS to determine the effect of OAT3 on basolateral DMPS uptake. We used stably transfected HEK293 cells expressing human and rabbit orthologs of the exchanger OAT1 and OAT3. Using 6-carboxyfluorescein (6-CF) as a substrate, the IC50 determinations for reduced DMPS (DMPSH) revealed a stronger interaction with OAT1 than with OAT3 (rbOAT1, 123.3 +/- 13.7; hOAT1, 85.1 +/- 8.8; rbOAT3, 171.7 +/- 22.3; and hOAT3, 172.2 +/- 36.4 micromol/L). However, inhibition of 6-CF uptake by the oxidized form of DMPS (DMPSS), the main form of DMPS in the blood, showed a greater affinity for OAT3 (rbOAT1, 237.4 +/- 23; hOAT1, 104.6 +/- 13.1; rbOAT3, 52.4 +/- 7.6; and hOAT3, 31.6 +/- 6.6 micromol/L). To determine whether DMPSH and DMPSS are substrates for OAT3, we performed efflux studies with [14C]glutarate and inwardly directed gradients of glutarate. The inhibitors trans-stimulated the efflux of [14C]glutarate, suggesting that OAT3 may be able to transport both forms of DMPS. On the basis of the substantial interaction of OAT3 with DMPSS, we conclude that OAT3 represents the dominant basolateral player in renal detoxification processes resulting from use of DMPS.

Molecular evidence for an involvement of organic anion transporters (OATs) in aristolochic acid nephropathy

Aristolochic acid (AA), present in Aristolochia species, is the major causative agent in the development of severe renal failure and urothelial cancers in patients with AA nephropathy. It may also be a cause of Balkan endemic nephropathy. Epithelial cells of the proximal tubule are the primary cellular target of AA. To study whether organic anion transporters (OATs) expressed in proximal tubule cells are involved in uptake of AA, we used human epithelial kidney (HEK293) cells stably expressing human (h) OAT1, OAT3 or OAT4. AA potently inhibited the uptake of characteristic substrates, p-aminohippurate for hOAT1 and estrone sulfate for hOAT3 and hOAT4. Aristolochic acid I (AAI), the more cytotoxic and genotoxic AA congener, exhibited high affinity for hOAT1 (K(i)=0.6 microM) as well as hOAT3 (K(i)=0.5 microM), and lower affinity for hOAT4 (K(i)=20.6 microM). Subsequently, AAI-DNA adduct formation (investigated by (32)P-postlabelling) was used as a measure of AAI uptake. Significantly higher levels of adducts occurred in hOAT-expressing cells than in control cells: this effect was abolished in the presence of the OAT inhibitor probenecid. In Xenopus laevis oocytes hOAT-mediated efflux of p-aminohippurate was trans-stimulated by extracellular AA, providing further molecular evidence for AA translocation by hOATs. Our study indicates that OATs can mediate the uptake of AA into proximal tubule cells and thereby participate in kidney cell damage by this toxin.

Functional differences in steroid sulfate uptake of organic anion transporter 4 (OAT4) and organic anion transporting polypeptide 2B1 (OATP2B1) in human placenta

Human trophoblasts depend on the supply of external precursors such as dehydroepiandrosterone-3-sulfate (DHEA-S) and 16alpha-OH-DHEA-S for synthesis of estrogens. Recently, we have characterized the uptake of DHEA-S by isolated mononucleated trophoblasts and identified different transporter polypeptides involved in this process. Immunohistochemistry of 1st and 3rd trimester placenta detected organic anion transporter 4 (OAT4) and organic anion transporting polypeptide 2B1 (OATP2B1, former name OATP-B) in cytotrophoblast membranes and at the basal surface of the syncytiotrophoblast, indicating that both transporter polypeptides are involved in placental uptake of foetal derived steroid sulfates. In the present study we have characterized and compared the kinetics of DHEA-S and estrone sulfate (E(1)S) uptake by these transporters stably expressed in FlpIn -HEK293 cells using the Flp recombinase-mediated site-specific recombination. Uptake of E(1)S by OAT4- and OATP2B1-transfected cells was highly increased compared to the non-transfected cells. In contrast, DHEA-S uptake was only highly increased in OAT4 (40 times), but only weakly enhanced in OATP2B1 cells. The uptake of DHEA-S and E(1)S by OAT4 was partly Na(+)-dependent (about 50%), whereas uptake of DHEA-S by OATP2B1 was Na(+)-independent. Kinetic analysis of the initial uptake rates of E(1)S by OAT4 and OATP2B1 gave very similar values for K(m) (about 20microM) and V(max) (about 600pmol/(minxmg protein)). In contrast, the affinity of DHEA-S towards OATP2B1 was about 10 times lower (K(m)>200microM) then for OAT4 (K(m)=29microM). Our results suggest different physiological roles of the two transporter polypeptides in placental uptake of foetal derived steroid sulfates. OATP2B1 seems not to be involved in de novo synthesis of placental estrogens but may contribute to the clearance of estrogen sulfates from foetal circulation.

Identification of a new urate and high affinity nicotinate transporter, hOAT10 (SLC22A13)

The orphan transporter hORCTL3 (human organic cation transporter like 3; SLC22A13) is highly expressed in kidneys and to a weaker extent in brain, heart, and intestine. hORCTL3-expressing Xenopus laevis oocytes showed uptake of [(3)H]nicotinate, [(3)H]p-aminohippurate, and [(14)C]urate. Hence, hORCTL3 is an organic anion transporter, and we renamed it hOAT10. [(3)H]Nicotinate transport by hOAT10 into X. laevis oocytes and into Caco-2 cells was saturable with Michaelis constants (K(m)) of 22 and 44 microm, respectively, suggesting that hOAT10 may be the molecular equivalent of the postulated high affinity nicotinate transporter in kidneys and intestine. The pH dependence of hOAT10 suggests p-aminohippurate(-)/OH(-), urate(-)/OH(-), and nicotinate(-)/OH(-) exchange as possible transport modes. Urate inhibited [(3)H]nicotinate transport by hOAT10 with an IC(50) value of 759 microm, assuming that hOAT10 represents a low affinity urate transporter. hOAT10-mediated [(14)C]urate uptake was elevated by an exchange with l -lactate, pyrazinoate, and nicotinate. Surprisingly, we have detected urate(-)/glutathione exchange by hOAT10, consistent with an involvement of hOAT10 in the renal glutathione cycle. Uricosurics, diuretics, and cyclosporine A showed substantial interactions with hOAT10, of which cyclosporine A enhanced [(14)C]urate uptake, providing the first molecular evidence for cyclosporine A-induced hyperuricemia.

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.

Human renal organic anion transporter 4 operates as an asymmetric urate transporter

Human organic anion transporter 4 (hOAT4) is located at the apical membrane of proximal tubule cells and involved in renal secretion and reabsorption of endogenous substances as well as many drugs and xenobiotics. This study reevaluated the physiologic role, transport mode, and driving forces of hOAT4. 6-Carboxyfluorescein (6-CF) uptake into HEK293 cells that stably expressed hOAT4 was saturable, resulting in a K(m) of 108 muM. 6-CF as well as [(3)H]estrone sulfate ([(3)H]ES) accumulation by HEK293-hOAT4 cells were abolished by ES, dehydroepiandrosterone sulfate, sulfinpyrazone, benzbromarone, and probenecid, whereas several OA, including p-aminohippurate (PAH), lactate, pyrazinoate, nicotinate, glutarate, and the diuretic hydrochlorothiazide (HCTZ) exhibited a slight or a NS inhibitory effect. PAH and glutarate are not taken up by HEK293-hOAT4 cells, but they trans-stimulated 6-CF and [(3)H]ES uptake, indicating an asymmetric interaction of hOAT4 with these substrates. In chloride-free medium, HEK293-hOAT4-mediated [(3)H]PAH efflux was almost abolished, whereas addition of ES restored it comparable to Ringer solution, consistent with a physiologic ES/PAH or PAH/Cl(-) exchange mode of hOAT4. Moreover, an acidification of the uptake medium increased 6-CF as well as [(3)H]ES uptake, which was reduced by nigericin, suggesting that hOAT4 also can operate as an OA/OH(-) exchanger. hOAT4 facilitates substantial uptake of [(14)C]urate, which was elevated 2.6-fold by intracellular HCTZ. Thus, hOAT4 is the long-postulated, low-affinity apical urate anion exchanger that facilitates HCTZ-associated hyperuricemia.

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.

Uptake of Chemically Reactive, DNA-Damaging Sulfuric Acid Esters into Renal Cells by Human Organic Anion Transporters

The procarcinogen 1-methylpyrene is activated by hepatic enzymes via 1-hydroxymethylpyrene to 1-sulfooxymethylpyrene (1-SMP), a highly reactive and mutagenic metabolite. Previously, high levels of 1-SMP DNA adducts were observed in rat kidneys after intraperitoneal administration of 1-hydroxymethylpyrene or 1-SMP. This study examined whether organic anion transporters (OAT) that are expressed at the basolateral membrane of proximal tubule cells are involved in uptake of SMP. Human epithelial kidney (HEK293) cells that stably express human OAT1 (hOAT1) and hOAT3 were used. Stable isomers of 1-SMP, (2-SMP and 4-SMP) competitively inhibited the uptake of characteristic substrates p-aminohippurate for hOAT1 and estrone sulfate for hOAT3. Both inhibitors exhibited high affinity for hOAT1 (K(i) = 4.4 microM for 2-SMP; K(i) = 5.1 microM for 4-SMP) as well as hOAT3 (K(i) = 1.9 microM for 2-SMP; K(i) = 2.1 microM for 4-SMP). The uptake rate of 4-SMP (at a concentration of 10 microM) by hOAT1- and hOAT3-expressing cells was 3.0 and 1.6 times higher, respectively, than in control cells. Uptake of the reactive isomer 1-SMP was investigated using as the end point the level of DNA adducts that were formed in the cells. After exposure to 1-SMP (10 microM), the DNA adduct level was 4.6 and 3.0 times higher in hOAT1- and hOAT3-expressing cells, respectively, than in control cells. The enhanced DNA adduct formation in hOAT-expressing cells was abolished in the presence of the OAT inhibitor probenecid. This study indicates that OAT can mediate the basolateral uptake of reactive sulfuric acid esters into proximal tubule cells and thereby participate in kidney cell damage by these compounds.

Murine renal organic anion transporters mOAT1 and mOAT3 facilitate the transport of neuroactive tryptophan metabolites

Tryptophan metabolites such as kynurenate (KYNA), xanthurenate (XA), and quinolinate are considered to have an important impact on many physiological processes, especially brain function. Many of these metabolites are secreted with the urine. Because organic anion transporters (OATs) facilitate the renal secretion of weak organic acids, we investigated whether the secretion of bioactive tryptophan metabolites is mediated by OAT1 and OAT3, two prominent members of the OAT family. Immunohistochemical analyses of the mouse kidneys revealed the expression of OAT1 to be restricted to the proximal convoluted tubule (representing S1 and S2 segments), whereas OAT3 was detected in almost all parts of the nephron, including macula densa cells. In the mouse brain, OAT1 was found to be expressed in neurons of the cortex cerebri and hippocampus as well as in the ependymal cell layer of the choroid plexus. Six tryptophan metabolites, including the bioactive substances KYNA, XA, and the serotonin metabolite 5-hydroxyindol acetate inhibited [3H] p-aminohippurate (PAH) or 6-carboxyfluorescein (6-CF) uptake by 50–85%, demonstrating that these compounds interact with OAT1 as well as with OAT3. Half-maximal inhibition of mOAT1 occurred at 34 μM KYNA and 15 μM XA, and it occurred at 8 μM KYNA and 11.5 μM XA for mOAT3. Quinolinate showed a slight but significant inhibition of [3H]PAH uptake by mOAT1 and no alteration of 6-CF uptake by mOAT3. [14C]-Glutarate (GA) uptake was examined for both transporters and demonstrated differences in the transport rate for this substrate by a factor of 4. Trans-stimulation experiments with GA revealed that KYNA and XA are substrates for mOAT1. Our results support the idea that OAT1 and OAT3 are involved in the secretion of bioactive tryptophan metabolites from the body. Consequently, they are crucial for the regulation of central nervous system tryptophan metabolite concentration.

Functional expression of pig renal organic anion transporter 3 (pOAT3)

With the cloning of pig renal organic anion transporter 1 (pOAT1) (Biochimie 84 (2002) 1219) we set up a model system for comparative studies of cloned and natively isolated membrane located transport proteins. Meanwhile, another transport protein involved in p-aminohippurate (PAH) uptake on the basolateral side of the proximal tubule cells was identified, designated organic anion transporter 3 (OAT3). To explore the contribution of pOAT1 to the PAH clearance in comparison to OAT3, it was the aim of this study to extend our model by cloning of the pig ortholog of OAT3. Sequence comparisons of human organic anion transporter 3 (hOAT3) with the expressed sequence tag (EST) database revealed a clone and partial sequence of the pig renal organic anion transporter 3 (pOAT3) ortholog. Sequencing of the entire open reading frame resulted in a protein of 543 amino acid residues encoded by 1632 base pairs (EMBL Acc. No. AJ587003). It showed high homologies of 81%, 80%, 76%, and 77% to the human, rabbit, rat, and mouse OAT3, respectively. A functional characterization of pOAT3 in Xenopus laevis oocytes yielded an apparent Km (Kt) for [3H]estrone sulfate of 7.8 +/- 1.3 microM. Moreover, pOAT3 mediated [3H]estrone sulfate uptake was almost abolished by 0.5 mM of glutarate, dehydroepiandosterone sulfate, or probenecid consistent with the hallmarks of OAT3 function.

Presence of organic anion transporters 3 (OAT3) and 4 (OAT4) in human adrenocortical cells

Since the organic anion transporter-1 (OAT1) has been implicated in cortisol release from bovine and rat adrenal zona fasciculata cells, we addressed the question of whether OATs are present in human adrenal cortical cells. In the human adrenal cell line NCI-H295R, 24-h cortisol secretion increased up to 30-fold on exposure to forskolin. Incubation of forskolin-treated cells for 24 h with the OAT substrates probenecid, p-aminohippurate (PAH), glutarate or cimetidine inhibited cortisol release partly. RT-PCR did not reveal expression of human OAT1 and OAT2, but OAT3 and OAT4 mRNAs were detected in both NCI-H295R cells and human adrenal tissue. When human OAT3 (hOAT3) and hOAT4 were expressed in Xenopus laevis oocytes, only hOAT3 showed [3H]cortisol uptake in excess of that of water-injected control oocytes. Cortisol uptake via OAT3 was saturable with an apparent Kt of 2.4 microM. In NCI-H295R cells, [3H]estrone sulphate uptake was saturable, cis-inhibited by OAT substrates and trans-stimulated by preloading with glutarate or cortisol. Likewise, [3H]PAH uptake was cis-inhibited by estrone sulphate and trans-stimulated by preloading the cells with PAH, glutarate or cortisol, indicating functional expression of OATs in the plasma membrane of NCI-H295R cells.

Relative contribution of OAT and OCT transporters to organic electrolyte transport in rabbit proximal tubule

We compared the characteristics of several cloned rabbit organic electrolyte (OE) transporters expressed in cultured cells with their behavior in intact rabbit renal proximal tubules (RPT) to determine the contribution of each to basolateral uptake of the weak acid ochratoxin A (OTA) and the weak base cimetidine (CIM). The activity of organic anion transporters OAT1 and OAT3 proved to be distinguishable because OAT1 had a high affinity for PAH ( Ktof 20 μM) and did not support estrone sulfate (ES) transport, whereas OAT3 had a high affinity for ES ( Ktof 4.5 μM) and a weak interaction with PAH (IC50> 1 mM). In contrast, both transporters robustly accumulated OTA. Intact RPT also accumulated OTA, with OAT1 and OAT3 each responsible for ∼50%: ES and PAH each reduced uptake by ∼50%, and the combination of the two eliminated mediated OTA uptake. The weak base CIM was transported by OAT3 ( Ktof 80 μM) and OCT2 ( Ktof 2 μM); OCT1 had a comparatively low affinity for CIM, and CIM uptake by OAT1 was equivocal. Intact RPT accumulated CIM, with TEA and ES reducing CIM uptake by 20 and 75%, respectively, suggesting that OAT3 plays a quantitatively more significant role in CIM uptake in the early proximal tubule than OCT1/2. In single S2 segments of RPT, ES and TEA each blocked ∼50% of CIM uptake. Thus the fractional contribution of different OE transporters to renal secretion is influenced by their affinity for substrate and relative expression level in RPT.

EXPRESSION STUDIES AND FUNCTIONAL CHARACTERIZATION OF RENAL HUMAN ORGANIC ANION TRANSPORTER 1 ISOFORMS

The human organic anion transporter 1 (hOAT1) facilitates the basolateral entry of organic anions such as endogenous metabolites, xenobiotics, and drugs into the proximal tubule cells. In the present study we investigated the general occurrence of hOAT1 isoforms in the kidneys and performed functional characterizations. Kidney specimens of 10 patients were analyzed by reverse transcription-polymerase chain reaction. We detected hOAT1-2 as the main transcript in almost all patients, and weak transcripts of hOAT1-1, hOAT1-3, and hOAT1-4 in many of them. An evaluation of the renal distribution showed all four mRNAs mostly restricted to the cortex. Western blot analysis of membrane fractions from two kidney specimens yielded two bands corresponding to the observed mRNA expression, suggesting hOAT1-3 and hOAT1-4 to be expressed on the protein level in vivo. This observation is further supported by immunofluorescence analyses of all four cloned hOAT1 isoforms transiently transfected in COS 7 cells. Functional characterizations did not show any transport activity of hOAT1-3 and hOAT1-4 for the tested substrates. Cotransfection studies of each of them with hOAT1-1 did not alter fluorescein uptake indicating no regulatory impact of these isoforms. Further functional comparisons of hOAT1-1 and hOAT1-2 in fluorescein uptake studies exhibited almost identical affinities for fluorescein with Michaelis constants of 11.6 +/- 3.7 microM (hOAT1-1) and 11.9 +/- 6.4 microM (hOAT1-2), and similar sensitivities to inhibition by p-aminohippurate [IC(50): 16 microM (hOAT1-1), 10 microM (hOAT1-2)], urate [IC(50): 440 microM (hOAT1-1), 385 microM (hOAT1-2)], and furosemide (IC(50): 14 microM (hOAT1-1), 20 microM (hOAT1-2)], implying functional equivalence.

Rat renal cortical OAT1 and OAT3 exhibit gender differences determined by both androgen stimulation and estrogen inhibition

In rats, the secretion of p-aminohippurate (PAH) by the kidney is higher in males (M) than in females (F). The role of the major renal PAH transporters, OAT1 and OAT3, in the generation of these gender differences, as well as the responsible hormones and mechanisms, has not been clarified. Here we used various immunocytochemical methods to study effects of gender, gonadectomy, and treatment with sex hormones on localization and abundance of OAT1 and OAT3 along the rat nephron. Both transporters were localized to the basolateral membrane: OAT1 was strong in proximal tubule S2 and weak in the S3 segments, whereas OAT3 was stained in proximal tubule S1 and S2 segments, thick ascending limb, distal tubule, and in principal cells along the collecting duct. Gender differences in the expression of both transporters in adult rats (M > F) were observed only in the cortical tubules. OAT1 in the cortex was strongly reduced by castration in adult M, whereas the treatment of castrated M with testosterone, estradiol, or progesterone resulted in its complete restitution, further depression, or partial restitution, respectively. In adult F, ovariectomy weakly increased, whereas estradiol treatment of ovariectomized F strongly decreased, the expression of OAT1. The expression of OAT3 in the M and F cortex largely followed a similar pattern, except that ovariectomy and progesterone treatment showed no effect, whereas in other tissue zones gender differences were not observed. In prepubertal rats, the expression of OAT1 and OAT3 in the kidney cortex was low and showed no gender differences. Our data indicate that gender differences in the rat renal cortical OAT1 and OAT3 (M > F) appear after puberty and are determined by both a stimulatory effect of androgens (and progesterone in the case of OAT1) and an inhibitory effect of estrogens.

Mutation of amino acid 475 of rat organic cation transporter 2 (rOCT2) impairs organic cation transport

Protein sequence alignments revealed one amino acid position, where organic cation transporters (OCTs, aspartate (D) at position 475 of rOCT2) and organic anion transporters (OATs, arginine (R) at position 466 of rOAT1) are charged oppositely. To address the impact of this amino acid for protein function we cloned rat organic cation transporter 2 (rOCT2), the renal electrogenic cation transporter of the basolateral side of proximal tubule cells. Site-directed mutagenesis was used to generate rOCT2-D475R (rOCT2-mut). Heterologous expression of rOCT2 wild-type (rOCT2-wt) in A6 cells resulted in a significant uptake of the fluorescent organic cation 4-(4-dimethylaminostyryl)-N-methylpyridinium (ASP(+)). Accordingly, rOCT2-wt-transfected COS 7 cells showed an almost fourfold uptake of 25 microM [(14)C]-TEA, whereas rOCT2-mut did not exhibit any uptake of [(14)C]-TEA. These data indicate that rOCT2 transports both ASP(+) and TEA and that aspartate at position 475 of rOCT2 plays a critical role in transport function.

Regulation of sodium-dicarboxylate cotransporter-3 from winter flounder kidney by protein kinase C

The sodium dicarboxylate cotransporter located at the basolateral side supplies renal proximal tubule cells with Krebs cycle intermediates and maintains the driving force for the exchange of organic anions like PAH against alpha-ketoglutarate through the organic anion transporter-1. Recently, we cloned sodium dicarboxylate cotransporter-3 from winter flounder kidney (fNaDC-3). To understand the regulation of fNaDC-3, we preincubated fNaDC-3-expressing oocytes with PMA, a PKC activator. PMA dose and time dependently inhibited fNaDC-3-mediated succinate uptake. Simultaneous preincubation of fNaDC-3-expressing oocytes with 50 nM PMA and either staurosporine or RO 31-8220 for 30 min attenuated PKC-mediated inhibition of succinate uptake. Site-directed mutagenesis of the five putative PKC sites (S7, T167, S174, T188, and S396) resulted in no change in PKC-mediated inhibition of the transporter. In electrophysiological studies performed at -60 mV, the K0.5 for succinate was not significantly affected (56 +/- 13 vs. 42 +/- 19 microM), but DeltaImax was reduced from -139 +/- 49 to -20 +/- 8 nA by PMA (50 nM, 30 min). Immunofluorescence analysis of fNaDC-3-expressing oocytes revealed that PMA leads to an endocytosis of fNaDC-3 protein. In conclusion, fNaDC-3 expressed in oocytes is downregulated by PMA through endocytosis. PKC consensus sites appear not to be important for this process.

Human Organic Anion Transporter 3 (hOAT3) can Operate as an Exchanger and Mediate Secretory Urate Flux

BACKGROUND/AIMS

Renal secretion of organic anions is critically dependent on their basolateral uptake against the electrochemical gradient. Due to their localization, two transporters are likely involved, namely OAT1 and OAT3. While OAT1 as an exchanger clearly operates in the secretory direction, OAT3 in its previously supposed mode as a uniporter should move anionic substrates from cell to blood. It would thus dissipate gradients established by OAT1 of common OAT1/OAT3 substrates. In the present study we therefore reinvestigated the driving forces of human OAT3.

METHODS

The human OAT3 obtained Xenopus laevis oocyte expression system, hOAT3-mediated transport of estrone sulfate (ES) and dicarboxylates was assayed for cis-inhibition and/or trans-stimulation in both the uptake and efflux direction.

RESULTS

hOAT3-mediated efflux of glutarate (GA), can be significantly trans-stimulated by a variety of ions with high cis-inhibitory potency, including GA (282%), alpha-ketoglutarate (476%), p-aminohippurate (179%), and, most notably, urate (167%). Urate cis-inhibited ES uptake with an IC(50) close to normal serum urate concentrations.

CONCLUSION

These data indicate that OAT3 does not represent a uniporter but operates as an organic ion%dicarboxylate exchanger similar to OAT1, and may mediate renal urate secretion.

Molecular evidence of organic ion transporters in the rat adrenal cortex with adrenocorticotropin-regulated zonal expression

Experimental evidence suggested that secretion of steroid hormones from adrenocortical cells involves carrier-mediated transport: Cortisol release from, and uptake of p-[3H]aminohippurate into, bovine adrenocortical cells showed properties of the renal p-[3H]aminohippurate/anion exchanger OAT1. Other poly-specific transporters such as organic anion-transporting polypeptides (oatps) and organic cation transporters (OCTs) could also be involved in steroid hormone release. A homology-cloning procedure was established to detect these transporters in rat adrenal gland cDNA. PCR revealed the presence of OAT1, oatp1, oatp2, and oatp3. In situ hybridization localized OAT1 in the outer zona fasciculata, oatp3 in the zona glomerulosa, and oatp1 and oatp2 in the inner zona fasciculata and outer zona reticularis. An OCT2-specific probe produced signals in the zona glomerulosa and outer zona fasciculata. Pretreatment of rats with ACTH increased the expression of OAT1 mRNA that spread to all zones, and hypophysectomy strongly decreased it. A less pronounced regulation was detected for OCT2 and oatp3. Specific antibodies confirmed the localization of OAT1 in the outer zona fasciculata, supporting a possible role of OAT1 in cortisol release. The zonated distribution of transporters furthermore suggest that oatp1-3 and OCT2 may be important for the endocrine function of rat adrenocortical cells.

RT-PCR-based evidence for the in vivo stimulation of renal tubularp-aminohippurate (PAH) transport by triiodothyronine (T3) or dexamethasone (DEXA) in kidney tissue of immature and adult rats

Our previous studies have shown that a pre-treatment of rats with triiodothyronine (T3) or dexamethasone (DEXA) increases renal PAH excretion significantly. This stimulation was accompanied by an enhanced protein synthesis within the renal cortex. To explore the molecular basis for this sub-chronic induction process, we investigated the stimulation of PAH accumulation in renal cortical slices as well as the expression level of organic anion transporter 1 (OAT1), the recently cloned renal basolateral PAH-transporter, using RT-PCR techniques under the applied conditions. 10- and 55-day-old Han:WIST rats were treated in vivo with T3 (20 microg/100 g b.wt.) or DEXA (60 microg/100 g b.wt.), both for 3 days, once daily. Renal cortical slices were incubated for 2 hours in Cross-Taggart medium and PAH uptake into kidney tissue was measured time dependently (slice to medium ratio, QS/M). The accumulation capacity is comparable between immature and mature rats (control-QS/M: 6.7 +/- 0.1 vs. 6.9 +/- 0.2, respectively). Both age groups showed a significant increase of PAH accumulation capacity after T3 treatment (10-day-old rats: 15.0 +/- 0.2; 55-day-old rats: 11.7 +/- 1.3). After DEXA pre-treatment, PAH accumulation was only slightly changed (10-day-old rats: 5.9 +/- 0.2; 55-day-old rats: 8.2 +/- 1.3). Semi-quantitative measurements of OAT1 mRNA expression level showed a significant increase of OAT1 mRNA after pre-treatment with both T3 and DEXA in the two age groups. Thus, this is the first evidence that T3 and DEXA pre-treatment induces the expression of OAT1.

Characterization and identification of steroid sulfate transporters of human placenta

Human trophoblasts depend on the supply of external precursors, such as dehydroepiandrosterone-3-sulfate (DHEA-S) and 16 alpha-OH-DHEA-S, for synthesis of estrogens. The aim of the present study was to characterize the uptake of DHEA-S by isolated mononucleated trophoblasts (MT) and to identify the involved transporter polypeptides. The kinetic analysis of DHEA-(35)S uptake by MT revealed a saturable uptake mechanism (K(m) = 26 microM, V(max) = 428 pmol x mg protein(-1) x min(-1)), which was superimposed by a nonsaturable uptake mechanism (diffusion constant = 1.2 microl x mg protein(-1) x min(-1)). Uptake of [(3)H]DHEA-S by MT was Na(+) dependent and inhibited by sulfobromophthalein (BSP), steroid sulfates, and probenecid, but not by steroid glucuronides, unconjugated steroids, conjugated bile acids, ouabain, p-aminohippurate (PAH), and bumetanide. MT took up [(35)S]BSP, [(3)H]estrone-sulfate, but not (3)H-labeled ouabain, estradiol-17beta-glucuronide, taurocholate, and PAH. RT-PCR revealed that the organic anion-transporting polypeptides OATP-B, -D, -E, and the organic anion transporter OAT-4 are highly expressed, and that OATP-A, -C, -8, OAT-3, and Na(+)-taurocholate cotransporting polypeptide (NTCP) are not or are only lowly expressed in term placental tissue and freshly isolated and cultured trophoblasts. Immunohistochemistry of first- and third-trimester placenta detected OAT-4 on cytotrophoblast membranes and at the basal surface of the syncytiotrophoblast. Our results indicate that uptake of steroid sulfates by isolated MT is mediated by OATP-B and OAT-4 and suggest a physiological role of both carrier proteins in placental uptake of fetal-derived steroid sulfates.

Molecular characterization of the renal organic anion transporter 1

Organic anions of diverse chemical structures are secreted in renal proximal tubules. The first step in secretion, uptake of organic anions across the basolateral membrane of tubule cells, is mediated for the polyspecific organic anion transporter 1 (OAT1), which exchanges extracellular organic anions for intracellular alpha-ketoglutarate or glutarate. OAT1 orthologs cloned from various species show 12 putative transmembrane domains and possess several sites for potential post-translational modification. The gene for the human OAT1 is located on chromosome 11q13.1 and is composed of 10 exons. Alternative splicing within exon 9 gives rise to four variants, two of which (OAT1-1 and OAT1-2) are functional. Following heterologous expression in Xenopus laevis oocytes, flounder renal OAT1 transported p-aminohippurate, glutarate, several diuretics, and the nephrotoxic agent ochratoxin A. Two cationic amino acid residues, lysine 394 and arginine 478, were found to be important for interaction with glutarate. Anionic neurotransmitter metabolites and the heavy-metal chelator, 2,3-dimercaptopropane sulfonate, interacted with the rabbit renal OAT1, which is expressed in kidneys and the retina.

Interaction of cysteine conjugates with human and rabbit organic anion transporter 1

Organic anion (OA) transport mediates accumulation of the zwitterionic nephrotoxic cysteine S-conjugates S-dichlorovinylcysteine (DCVC) and S-chlorotrifluoroethylcysteine (CTFC) in the rabbit renal proximal tubule (RPT). Although these cysteine conjugates are nephrotoxic to the human RPT, neither the role of OA transport nor the specific OA transport pathway(s) involved in cysteine conjugate accumulation are known. Since the OAT1 transporter has the characteristics of para-aminokippurate (PAH) transport that closely correlate to the native RPT, we examined the interaction of DCVC, CTFC, and the nontoxic benzothiazolylcysteine (BTC) with PAH transport mediated by human OAT1 and rabbit Oat1 expressed in Chinese hamster ovary and COS7 heterologous expression systems, respectively. Although the K(m) values for PAH uptake by hOAT1 and rbOat1 (8.9 +/- 3.6 and 20.7 +/- 8 microM, respectively) were 5- to 10-fold less than the K(m) for peritubular PAH transport into rabbit RPT, the IC(50) values for DCVC, CTFC, and BTC inhibition of PAH uptake mediated by either hOAT1 or rbOat1 were similar between these two transporters and to the IC(50) values for these conjugates measured in rabbit RPT. The IC(50) for inhibition of hOAT1- and rbOat1-mediated PAH uptake by the hydrophobic conjugate BTC was more than 5-fold lower than the IC(50) values seen with DCVC and CTFC, suggesting that hydrophobicity increases the affinity of OAT1 for cysteine conjugates. Finally, preloading cells transfected with hOAT1 with BTC significantly trans-stimulated the uptake of PAH, consistent with the conclusion that BTC and, hence, other cysteine S-conjugates are substrates for hOAT1.

Cloning of the pig renal organic anion transporter 1 (pOAT1)

A pig kidney cDNA library was screened for the porcine ortholog of the multispecific organic anion transporter 1 (pOAT1). Several positive clones were isolated resulting in two alternatively spliced cDNA clones of pOAT1 (pOAT1 and pOAT1A). pOAT1-cDNAs consist of 2126 or 1895 base pairs (EMBL Acc. No. AJ308234 and AJ308235) encoding 547 or 533 amino acid residue proteins with 89, 87, 83 and 81% homology to the human, rabbit, rat, and mouse OAT1, respectively. Heterologous expression of pOAT1 in Xenopus laevis oocytes revealed an apparent K(m) for [3H]PAH of 3.75 +/- 1.6 microM. [3H]PAH uptake mediated by pOAT1 was abolished by 0.5 mM glutarate or 1 mM probenecid. Functional characterization of pOAT1A did not show any affinity for [3H]PAH. In summary, we cloned two alternative splice variants of the pig ortholog of organic anion transporter 1. One splice form (pOAT1) showed typical functional characteristics of organic anion transporter 1, whereas the second form appears not to transport PAH.

Interaction of the metal chelator 2,3-dimercapto-1-propanesulfonate with the rabbit multispecific organic anion transporter 1 (rbOAT1)

The metal chelator DMPS (2,3-dimercapto-1-propanesulfonate) is used to treat heavy metal intoxication because it increases renal excretion of these toxins, which are accumulated in proximal tubule cells. To evaluate the involvement of the organic anion transporter 1 (OAT1) in the renal flux of DMPS, we examined the effect of DMPS on transport mediated by the rabbit ortholog of OAT1 and compared these characteristics with those observed in intact isolated rabbit proximal tubules. The rabbit OAT1 (rbOAT1) cDNA consisted of 2124 base pairs encoding a protein of 551 amino acids. Heterologous expression in COS-7 cells revealed rbOAT1-mediated transport of p-aminohippurate (PAH; K(t) = 16 microM). A 1 mM concentration of unlabeled PAH, alpha-ketoglutarate, urate, or probenecid inhibited [(3)H]PAH uptake by 70 to 90%. cis-Inhibition and trans-stimulation experiments using several Krebs cycle intermediates implicated alpha-ketoglutarate as the main intracellular exchange anion. Reduced DMPS inhibited rbOAT1-mediated fluorescein transport with an apparent K(i) of 102 microM. These characteristics paralleled those observed in isolated rabbit proximal tubules. PAH was transported into nonperfused single proximal tubule S(2) segments with a K(t) of 76 microM. DMPS inhibited FL uptake into single tubule segments with a K(i-app) of 71 microM. Fluorescein efflux from preloaded tubules was trans-stimulated by 1 mM PAH and 1 mM DMPS, consistent with DMPS entry into tubule cells by rbOAT1. In summary, rbOAT1 mediates basolateral uptake of DMPS into proximal tubule cells, implicating this process in the detoxification process of heavy metals in the kidneys.

Molecular physiology of renal p-aminohippurate secretion

Renal proximal tubules secrete various organic anions, including drugs and p-aminohippurate (PAH). Uptake of PAH from blood into tubule cells occurs by exchange with intracellular alpha-ketoglutarate and is mediated by the organic anion transporter 1. PAH exit into tubule lumen is species specific and may involve ATP-independent and -dependent transporters.

Genomic structure and in vivo expression of the human organic anion transporter 1 (hOAT1) gene

The human organic anion transporter 1 (hOAT1) plays a key role in the secretion of an array of potentially toxic organic anions including many clinically important drugs. Here we report on the genomic cloning of hOAT1. A human genomic library was used for screening of a PAC (P1 artificial chromosome) clone applying PCR techniques. Sequencing of several restriction subclones and of a PCR-generated clone revealed that the hOAT1 gene spans 8.2 kb and is composed of 10 exons divided by 9 introns. RT-PCR studies in a human kidney specimen led to the detection of two new splice variants, hOAT1-3 and hOAT1-4, showing a 132-bp in-frame deletion. Using fluorescence in situ hybridization (FISH) we mapped the hOAT1 gene as a single signal to chromosome 11q13.1-q13.2. Additionally, 600 bp of the 5' flanking region was analyzed, illustrating the probable transcription start site at nt -280, a NF-kappaB-site at nt -397 and several putative transcription factor binding sites.

Detection of different viral strains of hepatitis B virus in chronically infected children after seroconversion from HBsAg to anti-HBs indicating viral persistence

BACKGROUND/AIMS

Seroconversion to anti-HBs or the loss of HBsAg is usually associated with complete elimination of the replicative hepatitis B virus. Usually in these patients hepatitis B virus DNA (HBV DNA) becomes undetectable. Routine controls of patients who underwent anti-HBs seroconversion by more sensitive tests showed that in some cases the virus persisted in the patient. Therefore the aim of our study was to evaluate if virus persistence could also be found in children with chronic hepatitis B after anti-HBs seroconversion. The virus pool should be characterized before and after seroconversion.

METHODS

Viral DNA was extracted from nine HBsAg negative or anti-HBs positive sera of children, previously diagnosed as chronic HBsAg carriers. HBV DNA was amplified by polymerase chain reaction. Subsequently the nucleotide sequences of the polymerase chain reaction product in the a-determinant region (aa 121-161) were analyzed on an automatic fluorescent sequencer.

RESULTS

In the sera of seven children, HBV DNA was detected in the HBsAg negative phase of the HBV infection. Mutations in codons 122, 125, 127, 131, 134, 143, 159 and 161 of the S gene could be documented, resulting in amino acid changes. In three patients the sequence analysis revealed changes in the HBV genotype from genotype A (serotype adw) to genotype D (serotype ayw) during seroconversion to anti-HBs.

CONCLUSIONS

These data demonstrate that persistence of the hepatitis B virus can also occur in HBsAg negative and anti-HBs positive children. After loss of HBsAg, no specific HBV variant was identified. Although a conclusive explanation for the selection process cannot be provided, it remains a fact that the 'surviving' viral strain was mostly represented by genotype D.

Selection of a precore mutant after vertical transmission of different hepatitis B virus variants is correlated with fulminant hepatitis in infants

The incidence of perinatal transmission of hepatitis B virus (HBV) depends on the HBeAg/anti-HBe status of the mother. While children of HBeAg-positive mothers have a 90% probability of acquiring a chronic hepatitis B virus carrier state, babies of anti-HBe-positive mothers are more likely to develop fulminant hepatitis within the first 3 to 4 months of life. There is evidence that precore (pre-C) mutations of the HBV can be associated with fulminant hepatitis. The pre-C region was therefore examined in sera from nine infants with fulminant hepatitis after vertical transmission, one HBeAg-positive and seven anti-HBe-positive mothers by polymerase chain reaction (PCR) and direct sequence analysis. In five mother/infant pairs the virus populations were characterized in addition by analysing clones of the amplified products. All mothers were infected with two or four variants of HBV with mutations at different positions of the preC genome including position 1896, which results in a stop codon. While the precore stop codon was detected in a portion of the virus populations of the HBeAg-positive and of four anti-HBe-positive mothers the dominating viral strain was represented by the wild type virus in three. In contrast, the virus populations of all babies showed the 1896 precore variant as the prevalent virus strain during the phase of active disease. In the surviving baby only wild type sequences were detected after recovery. Subtype ayw was found in all mothers and infants and adw2 was present in three mothers and in the surviving child. The findings suggest that all mothers carried a wild type HBV population with a certain number of different HBV variants. After transmission of the mixed virus population a selection process was started in the baby. The association of subtype ayw with the precore mutations and with the fatal outcome of the hepatitis B might be the result of a directed selection of this variant with a particular advantage in the viral life cycle.