Cloning, in vitro expression, and tissue distribution of a human prostaglandin transporter cDNA(hPGT)

Characterization of prostaglandin E1 transport in rat renal brush-border membrane

Transport of prostaglandin E(1) (PGE(1)) was investigated in rat renal brush-border membrane vesicles. The uptake of [(3)H]PGE(1) was sensitive to osmosis and temperature. This uptake was saturable and mediated by high-affinity (K(m)=2.1 microM)/low-capacity (V(max)=17.4 pmol/mg protein/30 sec) and low-affinity (K(m)=526.5 microM)/high-capacity (V(max)=1,032.5 pmol/mg protein/30 sec) transport systems. [(3)H]PGE(1) uptake was Na(+)-independent and inhibited by various eicosanoids including PGE(2) and PGF(2alpha). Bromcresol green and sulfobromophthalein, potent inhibitors of prostaglandin transporter (PGT), significantly decreased [(3)H]PGE(1) uptake. Uptake was also inhibited by indomethacin and probenecid, which reportedly have little effect on PGT. Benzylpenicillin and taurocholate decreased the uptake of [(3)H]PGE(1). Like p-[(14)C]aminohippurate (PAH) uptake by vesicles, the uptake of [(3)H]PGE(1) was stimulated by an inside-positive membrane potential, created by applying an inward K(+) gradient and valinomycin. However, the uptake of [(3)H]PGE(1) was not inhibited by PAH, suggesting that PAH and PGE(1) are transported by separate transport systems. [(3)H]PGE(1) uptake was not stimulated by outwardly directed gradients of Cl(-) nor unlabeled PGE(1), indicating that an anion exchanger may not be involved in PGE(1) transport. These findings suggest that the transport of PGE(1) in rat renal brush-border membrane is mediated by specific transport system(s), at least in part, by a potential-sensitive transport system.

The role of monocarboxylate transporters in uptake of lactic acid in HeLa cells

This study was aimed to identify the monocarboxylate transporters (MCTs) in HeLa cells and to delineate their role in transportation of L-lactic acid. The functional role of MCTs in lactic acid transport was evaluated at various mucosal pHs (4.5-7.4) or in the presence of various loading doses (0.2-2mM) of lactic acid, MCT substrates (nicotinic acid, n-butyric acid, etc.) and inhibitors (alpha-cyano-4-hydroxycinnamate and para-chloromercuribenzoic acid). The molecular properties of MCTs were characterized using reverse transcription-polymerase chain reaction (RT-PCR). The uptake rate of lactic acid by HeLa cells significantly increased from 0.353+/-0.052 to 1.103+/-0.196 micromol/mg protein as the extra-cellular pH changed from 7.4 to 4.5, indicating that activities of MCT were mediated through H(+)-linked mechanism. The uptake profile of lactic acid followed the saturable process with the K(m) value of 0.53 mM. The uptake rate of lactic acid is concentration dependent and is reduced in the presence of MCT inhibitors. MCT isoforms 1, 5 and 6 in HeLa cells were identified by RT-PCR. HeLa cell line can be used as an effective screening tool for intravaginally administered drugs targeted toward MCT.

Depot-specific prostaglandin synthesis in human adipose tissue: a novel possible mechanism of adipogenesis

Despite the magnitude of the obesity epidemic, the mechanisms that contribute to increases in fat mass and to differences in fat depots are still poorly understood. Prostanoids have been proposed as potent adipogenic hormones, e.g. metabolites of prostaglandin J2 (PGJ2) bind and activate PPARgamma. We hypothesize that an altered expression of enzymes in PGJ2 synthesis may represent a novel pathogenic mechanism in human obesity. We characterized adipose depot-specific expression of enzymes in PGJ2 synthesis, prostaglandin transporter and PPARgamma isoforms. Paired omental and subcutaneous adipose tissue samples were obtained from 26 women undergoing elective abdominal surgery and gene expression examined in whole tissue and cultured preadipocytes using an Affymetrix cDNA microarray technique and validated with quantitative real-time PCR. All enzymes involved in prostaglandin synthesis were expressed in both adipose tissues. Expression of prostaglandin synthase-1 (PGHS1), prostaglandin D synthase (PTGDS), human prostaglandin transporter (hPGT) and PPARgamma2 was higher in OM adipose tissue compared to SC, whereas 17beta-hydroxysteroid dehydrogenase 5 (AKR1C3) showed predominance in SC adipose tissue. In SC adipose tissue, PGHS1 mRNA expression increased with BMI. The differential, depot-specific expression of key enzymes involved in transport, synthesis and metabolism of prostaglandins may have an important impact upon fat cell biology and may help to explain some of the observed depot-specific differences. In addition, the positive correlation between PGHS1 and BMI offers the novel hypothesis that the regulation of PG synthesis may have a role in determining fat distribution in human obesity.

Predominant contribution of organic anion transporting polypeptide OATP-B (OATP2B1) to apical uptake of estrone-3-sulfate by human intestinal Caco-2 cells

Human organic anion transporting polypeptide OATP-B (OATP2B1) is a pH-sensitive transporter expressed in the apical membranes of small intestinal epithelial cells. In this study, we have examined the contribution of OATP-B to the uptake of [3H]estrone-3-sulfate in Caco-2 cells in comparison with those of its homologs OATP-D (OATP3A1) and OATP-E (OATP4A1). Immunocytochemical study revealed that OATP-B is expressed in the apical membranes of Caco-2 cells. The uptake of [3H]estrone-3-sulfate by Caco-2 cells was Na+-independent and inhibited by several organic anions. It showed biphasic saturation kinetics with Km values of 1.81 microM and 1.40 mM. The uptake of [3H]estrone-3-sulfate by human embryonic kidney (HEK) 293 cells stably expressing OATP-B (HEK293/OATP-B) was also Na+-independent and inhibited by several organic anions. The Km value for estrone-3-sulfate uptake by OATP-B (1.56 microM) was close to that for the high-affinity component observed in Caco-2 cells. The mRNA expression level of OATP-B was higher than that of OATP-D or OATP-E in Caco-2 cells and in human jejunum biopsies from healthy volunteers. The values of [3H]estrone-3-sulfate uptake normalized to OATP-B mRNA expression were similar in Caco-2 cells and HEK293/OATP-B cells. The specific activity of OATP-B per mRNA expression was much higher than that of OATP-D and OATP-E. [3H]Estrone-3-sulfate uptake by membrane vesicles prepared from HEK293/OATP-B cells exhibited an overshoot phenomenon in the presence of an inwardly directed H+ gradient, suggesting that an H+ gradient is the driving force of estrone-3-sulfate transport by OATP-B. These results suggest that OATP-B is predominantly responsible for the apical uptake of estrone-3-sulfate in Caco-2 cells.

Anandamide Uptake Is Consistent with Rate-limited Diffusion and Is Regulated by the Degree of Its Hydrolysis by Fatty Acid Amide Hydrolase

The uptake of arachidonoyl ethanolamide (anandamide, AEA) in rat basophilic leukemia cells (RBL-2H3) has been proposed to occur via a saturable transporter that is blocked by specific inhibitors. Measuring uptake at 25 s, when fatty acid amide hydrolase (FAAH) does not appreciably affect uptake, AEA accumulated via a nonsaturable mechanism at 37 degrees C. Interestingly, saturation was observed when uptake was plotted using unbound AEA at 37 degrees C. Such apparent saturation can be explained by rate-limited delivery of AEA through an unstirred water layer surrounding the cells (1). In support of this, we observed kinetics consistent with rate-limited diffusion at 0 degrees C. Novel transport inhibitors have been synthesized that are either weak FAAH inhibitors or do not inhibit FAAH in vitro (e.g. UCM707, OMDM2, and AM1172). In the current study, none of these purported AEA transporter inhibitors affected uptake at 25 s. Longer incubation times illuminate downstream events that drive AEA uptake. Unlike the situation at 25 s, the efficacy of these inhibitors was unmasked at 5 min with appreciable inhibition of AEA accumulation correlating with partial inhibition of AEA hydrolysis. The uptake and hydrolysis profiles observed with UCM707, VDM11, OMDM2, and AM1172 mirrored two selective and potent FAAH inhibitors CAY10400 and URB597 (at low concentrations), indicating that weak inhibition of FAAH can have a pronounced effect upon AEA uptake. At 5 min, the putative transport inhibitors did not reduce AEA uptake in FAAH chemical knock-out cells. This strongly suggests that the target of UCM707, VDM11, OMDM2, and AM1172 is not a transporter at the plasma membrane but rather FAAH, or an uncharacterized intracellular component that delivers AEA to FAAH. This system is therefore unique among neuro/immune modulators because AEA, an uncharged hydrophobic molecule, diffuses into cells and partial inhibition of FAAH has a pronounced effect upon its uptake.

Functional characterization of prostaglandin transporter and terminal prostaglandin synthases during decidualization of human endometrial stromal cells

BACKGROUND

Decidualization of endometrial stromal cells is essential for successful implantation and pregnancy. Prostaglandins (PG) have been shown to be required for the initiation and maintenance of decidualization in animal models. The transport of PG across the plasma membrane is mediated by carriers such as prostaglandin transporter (PGT). Our recent data have shown the expression of human PGT (hPGT) in the endometrium during the menstrual cycle. The objective of the present study was to characterize hPGT in decidualized stromal cells.

METHODS AND RESULTS

Human endometrial stromal cells were treated with a combination of cAMP and medroxyprogesterone acetate to induce decidualization. Decidualization was confirmed by morphological differentiation and increased secretion of prolactin. A large increase in hPGT mRNA level, as measured by real-time PCR analysis, was observed in decidual cells compared with control. Similarly, a 2-fold up-regulation of hPGT and 3-12-fold increase in PG biosynthetic enzymes were obtained at the protein level. Decidual cells exhibited a higher isotopic PGE2 uptake and greater intracellular PG levels than control.

CONCLUSIONS

The higher uptake of PG by decidual cells is highly likely to be mediated via hPGT. PGT is a newly identified regulator of PG action at the cellular level and likely contributes to the regulation of PG action in female reproductive processes.

Coordinate control of prostaglandin E2 synthesis and uptake by hyperosmolarity in renal medullary interstitial cells

During water deprivation, prostaglandin E(2) (PGE(2)), formed by renal medullary interstitial cells (RMICs), feedback inhibits the actions of antidiuretic hormone. Interstitial PGE(2) concentrations represent the net of both PGE(2) synthesis by cyclooxygenase (COX) and PGE(2) uptake by carriers such as PGT. We used cultured RMICs to examine the effects of hyperosmolarity on both PG synthesis and PG uptake in the same RMIC. RMICs expressed endogenous PGT as assessed by mRNA and immunoblotting. RMICs rapidly took up [(3)H]PGE(2) to a level 5- to 10-fold above background and with a characteristic time-dependent "overshoot." Inhibitory constants (K(i)) for various PGs and PGT inhibitors were similar between RMICs and the cloned rat PGT. Increasing extracellular hyperosmolarity to the range of 335-485 mosM increased the net release of PGE(2) by RMICs, an effect that was concentration dependent, maximal by 24 h, reversible, and associated with increased expression of COX-2. Over the same time period, there was decreased cell-surface activity of PGT due to internalization of the transporter. With continued exposure to hyperosmolarity over 7-10 days, PGE(2) release remained elevated, COX-2 returned to baseline, and PGT-mediated uptake became markedly reduced. Our findings suggest that hyperosmolarity induces coordinated changes in COX-2-mediated PGE(2) synthesis and PGT-mediated PGE(2) uptake in RMICs.

15-deoxy Delta12,14-prostaglandin J2 suppresses transcription by promoter 3 of the human thromboxane A2 receptor gene through peroxisome proliferator-activated receptor gamma in human erythroleukemia cells

In humans, thromboxane (TX) A2 signals through two receptor isoforms, thromboxane receptor (TP)alpha and TPbeta, which are transcriptionally regulated by distinct promoters, Prm1 and Prm3, respectively, within the single TP gene. The aim of the current study was to investigate the ability of the endogenous peroxisome proliferator-activated receptor (PPAR)gamma ligand 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) to regulate expression of the human TP gene and to ascertain its potential effects on the individual TPalpha and TPbeta isoforms. 15d-PGJ2 suppressed Prm3 transcriptional activity and TPbeta mRNA expression in the platelet progenitor megakaryocytic human erythroleukemia (HEL) 92.1.7 cell line but had no effect on Prm1 or Prm2 activity or on TPalpha mRNA expression. 15d-PGJ2 also resulted in reductions in the overall level of TP protein expression and TP-mediated intracellular calcium mobilization in HEL cells. 15d-PGJ2 suppression of Prm3 transcriptional activity and TPbeta mRNA expression was found to occur through a novel mechanism involving direct binding of PPARgamma-retinoic acid X receptor (RXR) heterodimers to a PPARgamma response element (PPRE) composed of two imperfect hexameric direct repeat (DR) sequences centred at -159 and -148, respectively, spaced by five nucleotides (DR5). These data provide direct evidence for the role of PPARgamma in the regulation of human TP gene expression within the vasculature and point to further critical differences in the modes of transcriptional regulation of TPalpha and TPbeta in humans. Moreover, these data highlight a further link between enhanced risk of cardiovascular disease in diabetes mellitus associated with increased synthesis and action of thromboxane A2 (TXA2).

Comparative analysis of a BAC contig of porcine chromosome 13q31-q32 and human chromosome 3q21-q22

Background

The gene(s) encoding the ETEC F4ab/ac receptors, involved in neonatal diarrhoea in pigs (a disease not yet described in humans), is located close to the TF locus on Sscr13. In order to reveal and characterize possible candidate genes encoding these receptors, a porcine physical map of the TF region is indispensable.

Results

A contig of 33 BAC clones, covering approximately 1.35 Mb surrounding the TF locus on Sscr13q31-q32, was built by chromosome walking. A total of 22,552 bp from the BAC contig were sequenced and compared with database sequences to identify genes, ESTs and repeat sequences, and to anchor the contig to the syntenic region of the human genome sequence (Hsap3q21-q22). The contig was further annotated based on this human/porcine comparative map, and was also anchored to the Sanger porcine framework map and the integrated map of Sscr13 by RH mapping.

Conclusion

The annotated contig, containing 10 genes and 2 ESTs, showed a complete conservation of linkage (gene order and orientation) with the human genome sequence, based on 46 anchor points. This underlines the importance of the human/porcine comparative map for the identification of porcine genes associated with genetic defects and economically important traits, and for assembly of the porcine genome sequence.

Interaction with PDZK1 is required for expression of organic anion transporting protein 1A1 on the hepatocyte surface

Although many organic anion transport protein (Oatp) family members have PDZ consensus binding sites at their C termini, the functional significance is unknown. In the present study, we utilized rat Oatp1a1 (NM_017111) as a prototypical member of this family to examine the mechanism governing its subcellular trafficking. A peptide corresponding to the C-terminal 16 amino acids of rat Oatp1a1 was used to affinity-isolate interacting proteins from rat liver cytosol. Protein mass fingerprinting identified PDZK1 as the major interacting protein. This was confirmed by immunoprecipitation of an Oatp1a1-PDZK1 complex from cotransfected 293T cells as well as from native rat liver membrane extracts. Oatp1a1 bound predominantly to the first and third PDZ binding domains of PDZK1, whereas the high density lipoprotein receptor, scavenger receptor B type I binds to the first domain. Although it is possible that PDZK1 forms a complex with these two integral membrane proteins, this did not occur, suggesting that as yet undescribed factors lead to selectivity in the interaction of these protein ligands with PDZK1. Oatp1a1 protein expression was near normal in PDZK1 knock-out mouse liver. However, it was located predominantly in intracellular structures, in contrast to its normal basolateral plasma membrane distribution. Plasma disappearance of the Oatp1a1 ligand [35S]sulfobromophthalein was correspondingly delayed in knock-out mice. These studies show a critical role for oligomerization of Oatp1a1 with PDZK1 for its proper subcellular localization and function. Because its ability to transport substances into the cell requires surface expression, this must be considered in any assessment of physiologic function.

Molecular mechanisms of reduced glutathione transport: role of the MRP/CFTR/ABCC and OATP/SLC21A families of membrane proteins

The initial step in reduced glutathione (GSH) turnover in all mammalian cells is its transport across the plasma membrane into the extracellular space; however, the mechanisms of GSH transport are not clearly defined. GSH export is required for the delivery of its constituent amino acids to other tissues, detoxification of drugs, metals, and other reactive compounds of both endogenous and exogenous origin, protection against oxidant stress, and secretion of hepatic bile. Recent studies indicate that some members of the multidrug resistance-associated protein (MRP/CFTR or ABCC) family of ATP-binding cassette (ABC) proteins, as well as some members of the organic anion transporting polypeptide (OATP or SLC21A) family of transporters contribute to this process. In particular, five of the 12 members of the MRP/CFTR family appear to mediate GSH export from cells namely, MRP1, MRP2, MRP4, MRP5, and CFTR. Additionally, two members of the OATP family, rat Oatp1 and Oatp2, have been identified as GSH transporters. For the Oatp1 transporter, efflux of GSH may provide the driving force for the uptake of extracellular substrates. In humans, OATP-B and OATP8 do not appear to transport GSH; however, other members of this family have yet to be characterized in regards to GSH transport. In yeast, the ABC proteins Ycf1p and Bpt1p transport GSH from the cytosol into the vacuole, whereas Hgt1p mediates GSH uptake across the plasma membrane. Because transport is a key step in GSH homeostasis and is intimately linked to its biological functions, GSH export proteins are likely to modulate essential cellular functions.

Tissue distribution and ontogeny of mouse organic anion transporting polypeptides (Oatps)

Organic anion-transporting polypeptides (Oatps) are Na(+)-independent solute carriers for cellular uptake of organic compounds. The purpose of this study is to determine 1) the constitutive mRNA expression of the 15 mouse Oatp genes in 12 tissues, 2) whether there are gender differences in Oatp expression, and 3) the ontogenic expression of Oatps in liver and kidney. The mRNA expression of the 15 mouse Oatps was quantified using the branched DNA technique. Oatp1a1, 1a4, 1b2, and 2b1 are expressed in liver at relatively high levels, with Oatp1b2 being exclusively expressed in liver. Oatp1a1, 1a6, 3a1, and 4c1 are highly expressed in kidney. Oatp1a4 and 1c1 are highly expressed in brain. Oatp1a5, 6b1, 6c1, and 6d1 are predominant in testes. Oatp2a1, 4a1, and 5a1 are predominantly expressed in placenta. In liver, expression of Oatp1a1 was male-predominant, whereas expression of Oatp1a4 and 1a6 was female-predominant. In kidney, expression of Oatp1a1, 3a1, and 4c1 was higher in males than in females. Hepatic expression of Oatp1a1, 1a4, 1a6, 1b2, and 2b1 gradually increased after birth and reached adult levels by 6 weeks of age. Only Oatp2a1 was expressed at adult levels at birth. In kidney, expression of mouse Oatp1a1, 1a6, and 3a1 was lower at birth than at 6 weeks of age, whereas expression of mouse Oatp1a4, 2a1, and 2b1 was similar at birth and at 6 weeks of age. These data on the tissue distribution and ontogenic expression of mouse Oatps will aid in understanding the pharmacokinetics and toxicokinetics of drugs and other chemicals.

Functional characteristics and pharmacokinetic significance of kidney-specific organic anion transporters, OAT-K1 and OAT-K2, in the urinary excretion of anionic drugs

During the last decade, cDNA cloning has identified various gene families of drug transporters, and pharmacokinetic studies of drugs based on the molecular characteristics of transporters have advanced. We cloned and characterized two organic anion transporters OAT-K1 and OAT-K2 from the rat kidney. The expression of both transporters was limited to the kidney, especially the brush-border membranes of proximal tubules, with an apparent molecular mass of 40 kDa. Using MDCK or LLC-PK1 cells stably expressing OAT-K1, posttranslational cleavage was suggested to affect the membrane localization and functional characteristics; 50 kDa with multispecificity in the apical membrane of MDCK cells and 70 kDa with methotrexate specific transport in the basolateral membrane of LLC-PK1 cells. A wide variety of anionic compounds including methotrexate are bidirectionally transported via OAT-K1 and OAT-K2 across the apical membrane in the MDCK-transfectants. The urinary secretion of methotrexate was depressed in 5/6 nephrectomized rats in association with the selective loss of OAT-K1 and OAT-K2 expression, and both transporters were suggested to be target molecules for methotrexate-folinic acid rescue. In this review, recent advances in the study of OAT-K1 and OAT-K2 were summarized in comparison with other transporters.

Localization of organic anion transporting polypeptides in the rat and human ciliary body epithelium

PURPOSE

To identify and localize the expression of multispecific organic anion transporting polypeptides (Oatps/OATPs) in the ciliary body epithelium and to investigate their possible involvement in the transport of the antiglaucoma agent unoprostone.

METHODS

Oatps/OATPs were detected by immunoblot analysis and by immunofluorescence microscopy in homogenized and fixed rat and human ciliary body samples using specific polyclonal antibodies. Transport of 3H-labelled unoprostone was measured in Oatp/OATP expressing Xenopus laevis oocytes.

RESULTS

Immunoblots of ciliary body extracts were positive for rat Oatp1a4, Oatp1a5 and Oatp1b2 and for human OATP1A2, OATP1C1, OATP2B1, OATP3A1 and OATP4A1. Confocal immunofluorescence microscopy localized Oatp1a4 and Oatp1b2 as well as all immunoblot positive human OATPs at the basolateral plasma membrane of the non-pigmented rat and human ciliary body epithelium, respectively. However, for human OATPs additional regional differences in expression were found with OATP1A2 and OATP1C1 being expressed only in the pars plana of human ciliary body epithelium. Furthermore, OATP1C1, OATP3A1 and OATP4A1 were also expressed at the basolateral plasma membrane of the pars plana pigmented epithelium. And finally, deesterified unoprostone carboxylate was found to be transported by OATP1A2, OATP2B1 and OATP4A1 with approximate K(m)-values of 93, 91 and 132 microm, respectively.

CONCLUSIONS

Several multispecific organic anion transporting polypeptides are expressed at the basolateral plasma membrane of the non-pigmented, and to a lesser extent also of the pigmented, epithelium in rat and human ciliary body. These Oatps/OATPs can account for the previously suggested 'liver-like' transport functions of mammalian ciliary body epithelium.

Transport of prostaglandin E1 across the blood-brain barrier in rats

The transport of prostaglandin E(1) (PGE(1)) across the blood-brain barrier (BBB) was characterized using an in-situ rat brain perfusion technique. The uptake of [(3)H]PGE(1) was not affected by shortchain monocarboxylic acids (butyric acid and valeric acid). On the other hand, uptake of [(3)H]PGE(1) was significantly inhibited by medium-chain monocarboxylic acids such as hexanoic acid, enanthic acid and octanoic acid. These medium-chain monocarboxylic acids showed a more potent inhibitory effect on [(3)H]PGE(1) uptake with increasing number of carbon atoms. In contrast, there was no decrease in [(3)H]PGE(1) transport by any dicarboxylic acids with 5-8 carbon atoms. Valproic acid decreased [(3)H]PGE(1) uptake, whereas p-aminohippuric acid, a substrate for the organic anion transporter family, did not inhibit [(3)H]PGE(1) transport. Bromocresol green, an inhibitor of prostaglandin transporter (PGT), strongly decreased [(3)H]PGE(1) transport across the BBB. In addition, digoxin and taurocholate, substrates for organic anion transporting polypeptide subtype 2 (Oatp2), significantly inhibited [(3)H]PGE(1) uptake. RT-PCR analysis revealed that PGT mRNA and Oatp2 mRNA are expressed in a capillary-rich fraction from rat brain. Thus, it is suggested that PGE(1) transport across the BBB is mediated by some specific transport systems, possibly by the members of the Oatp family.

The organic anion transporter (OATP) family

In the last decade, many organic anion transporters have been isolated, characterized their distribution and substrates. The recently identified organic anion transporter family OATP (organic anion transporting polypeptide)/LST (liver-specific transporter) family, transport bile acids, hormones as well as eicosanoids, various compounds (BSP, HMG-CoA reductase inhibitor, angiotensin converting enzyme inhibitor, etc.). The isolation of the family revealed that not only hydrophilic compounds, drugs and hormones of lipophilic nature need a membrane transport system to penetrate cell membrane. In this family, the nomenclature becomes very complicated and the physiological role of this family is still unclear except about few organs such as the brain, liver and kidney. Even in such organs, the co-existence of the OATP/LST family and similar substrate specificity hamper the progress and clear characterization identifying the real role of the transporter family. Here, recent progress and an insight of this field are reviewed.

The two-step model of prostaglandin signal termination: in vitro reconstitution with the prostaglandin transporter and prostaglandin 15 dehydrogenase

Termination of prostaglandin (PG) signaling has been proposed to involve carrier-mediated uptake across the plasma membrane followed by cytoplasmic oxidation. Here, we tested this hypothesis directly by coexpressing the PG uptake carrier prostaglandin transporter (PGT) in various cell types with and without human PG 15 dehydrogenase (PG15DH). In HeLa cells, which express neither PGT nor PG15DH, exogenously added PGE2 or PGF2alpha were rapidly oxidized to the 13, 14-dihydro, 15-keto metabolites only when PGT and PG15DH were coexpressed, directly confirming the two-step hypothesis. Cells expressing PG15DH that were broken open formed more PG metabolites than cells in which the PGs could gain access to PG15DH only via PGT. Similar results were obtained using the human prostate cancer cell line LNCaP, in which endogenous PG15DH is induced after exposure to dihydrotestosterone. Because PGT in vivo is expressed in renal collecting duct epithelia, we also expressed PGT in Madin-Darby canine kidney cells grown on filters, where it mediated both the active uptake of PGE2 across the apical membrane and the transepithelial transport of PGE2 to the basolateral compartment. When PG15DH was coexpressed with PGT in these epithelial monolayers, about half of the PGE2 taken up apically was oxidized to 13, 14-dihydro, 15-keto-PGE2, which in turn exited the cells nondirectionally into both the apical and basolateral compartments. Our data represent reconstitution of the longstanding model of PG metabolism consisting of sequential carrier-mediated PG uptake, cytoplasmic oxidation, and diffusional efflux of the PG metabolite.

Pharmacogenomics of the OATP and OAT families

Drug disposition is highly dependent on the interplay between drug metabolism and transport in organs such as the intestine, kidney, and liver. Genetically determined variation in drug transporter function or expression is now increasingly recognized to have a significant role as a determinant of intersubject variability in drug response. Similar to the discoveries of functional genetic variations in drug efflux transporters, such as multi-drug resistance proteins 1 and 2, there have been considerable advances in the identification of single nucleotide polymorphisms in transporters that facilitate cellular drug uptake. Among the uptake transporters, members of the organic anion-transporting polypeptides and organic anion transporters can mediate the cellular uptake of a large number of structurally divergent compounds. Accordingly, functionally relevant polymorphisms in these transporters may contribute to interindividual and interethnic variability in drug disposition and response. In this review, recent progress relating to pharmacogenomics of organic anion transporters will be outlined along with a compilation of currently known genetic polymorphisms.

Detrimental effects of prostaglandin F2α on preimplantation bovine embryos

Two studies were performed to determine effects of prostaglandin F2alpha (PGF2alpha) on continued development of pre-compacted (in vitro-produced) and compacted (in vivo-derived) bovine embryos. In Experiment 1, pre-compacted embryos were placed in KSOM media supplemented with polyvinyl alcohol (0.3%) and assigned to the following treatments: (1) control; (2) PGF-1 (1 ng/mL PGF2alpha); (3) PGF-10 (10 ng/mL PGF2alpha); (4) PGF-100 (l00 ng/mL PGF2alpha); or (5) PGE-5 (5 ng/mL PGE2). Following 4 days of incubation in assigned treatments, continued development of pre-compacted embryos to blastocysts was reduced by addition of PGF2alpha in culture medium (P = 0.002). Development did not differ between control and PGE2 treatments (P > 0.10). In Experiment 2, compacted morula' s were placed in KSOM-PVA supplemented media and assigned to one of four treatments: (1) control; (2) PGF-0.1 (0.1 ng/mL PGF2alpha); (3) PGF-1 (1 ng/mL PGF2alpha); and (4) PGF-10 (10 ng/mL PGF2alpha). After 24h in culture, embryos were washed and placed in KSOM-BSA (0.5%) without PGF2alpha for an additional 48 h until assessment for development. Continued development of compacted morula to blastocyst was not affected by addition of PGF2alpha to the culture medium (P > 0.10). However, hatching rates of embryos cultured with PGF2alpha were lower (P = 0.05). In conclusion, it is suggested that PGF2alpha has a direct negative effect on continued embryonic development of pre-compacted and compacted bovine embryos.

Nuclear prostaglandin receptors: role in pregnancy and parturition?

The key regulatory role of prostanoids [prostaglandins (PGs) and thromboxanes (TXs)] in the maintenance of pregnancy and initiation of parturition has been established. However, our understanding of how these events are fine-tuned by the recruitment of specific signaling pathways remains unclear. Whereas, initial thoughts were that PGs were lipophilic and would easily cross cell membranes without specific receptors or transport processes, it has since been realized that PG signaling occurs via specific cell surface G-protein coupled receptors (GPCRs) coupled to classical adenylate cyclase or inositol phosphate signaling pathways. Furthermore, specific PG transporters have been identified and cloned adding a further level of complexity to the regulation of paracrine action of these potent bioactive molecules. It is now apparent that PGs also activate nuclear receptors, opening the possibility of novel intracrine signaling mechanisms. The existence of intracrine signaling pathways is further supported by accumulating evidence linking the perinuclear localization of PG synthesizing enzymes with intracellular PG synthesis. This review will focus on the evidence for a role of nuclear actions of PGs in the regulation of pregnancy and parturition.

Molecular characterization of human and rat organic anion transporter OATP-D

We have isolated and characterized a novel human and rat organic anion transporter subtype, OATP-D. The isolated cDNA from human brain encodes a polypeptide of 710 amino acids (Mr 76,534) with 12 predicted transmembrane domains. The rat clone encodes 710 amino acids (Mr 76,821) with 97.6% amino acid sequence homology with human OATP-D. Human and rat OATP-D have moderate amino acid sequence homology with LST-l/rlst-1, the rat oatp family, the prostaglandin transporter, and moatl/MOAT1/KIAA0880/OATP-B. Phylogenetic tree analysis revealed that OATP-D is branched in a different position from all known organic anion transporters. OATP-D transports prostaglandin E1 (Km 48.5 nM), prostaglandin E2 (Km 55.5 nM), and prostaglandin F2,, suggesting that, functionally, OATP-D encodes a protein that has similar characteristics to those of the prostaglandin transporter. Rat OATP-D also transports prostaglandins. The expression pattern of OATP-D mRNA was abundant mainly in the heart, testis, brain, and some cancer cells. Immunohistochemical analysis further revealed that rat OATP-D is widely expressed in the vascular, renal, and reproductive system at the protein level. These results suggest that OATP-D plays an important role in translocating prostaglandins in specialized tissues and cells.

Identification and characterization of a cyclooxygenase-like enzyme from Entamoeba histolytica

The intestinal protozoan parasite Entamoeba histolytica remains a significant cause of morbidity and mortality worldwide. However, almost nothing is known about the molecules secreted by the parasite that modulate host immune responses or epithelial barrier function in the colon. Herein, we describe the isolation and characterization of a cyclooxygenase (COX)-like enzyme in E. histolytica that is responsible for the biosynthesis of prostaglandin (PG)E2. PGE2 produced by ameba was constitutive but highly dependent on exogenous arachidonic acid substrate. COX-like activity and the immunoreactive protein were localized to the nuclear fraction of E. histolytica. The COX-like protein (72 kDa) was microsequenced and cloned by reverse transcriptase PCR. Ameba COX showed little homology with COX-1/2 enzymes from different species at the nucleotide and amino acid levels. Surprisingly, the arachidonate-binding domain and heme-coordinating and catalytic sites, which are conserved in other species, were absent in ameba. Ameba COX expressed in Escherichia coli demonstrated COX-like enzyme activity in vitro by converting arachidonic acid into PGE2 but not into PGD2 or PGF2alpha. COX activity was inhibited with 1 mM aspirin but not with indomethacin or COX-1/2-specific inhibitors. Taken together, these studies reveal that E. histolytica produces PGE2, by means of a previously undescribed ancestral COX-like enzyme, which could play a major role in pathogenesis and immune evasion.

Organic anion transporting polypeptides of the OATP/ SLC21 family: phylogenetic classification as OATP/ SLCO superfamily, new nomenclature and molecular/functional properties

The organic anion transporting polypeptides (rodents: Oatps, human: OATPs) form a superfamily of sodium-independent transport systems that mediate the transmembrane transport of a wide range of amphipathic endogenous and exogenous organic compounds. Since the traditional SLC21 gene classification does not permit an unequivocal and species-independent identification of genes and gene products, all Oatps/OATPs are newly classified within the OATP/ SLCO superfamily and subdivided into families (>/=40% amino acid sequence identity), subfamilies (>/=60% amino acid sequence identity) and individual genes and gene products according to their phylogenetic relationships and chronology of identification. Implementation of this new classification and nomenclature system occurs in agreement with the HUGO Gene Nomenclature Committee (HGNC). Among 52 members of the OATP/ SLCO superfamily, 36 members have been identified so far in humans, rat and mouse. The latter are clustered within 6 (out of 12) families (OATP1-OATP6) and 13 subfamilies. Oatps/OATPs represent 12 transmembrane domain proteins and contain the superfamily signature D-X-RW-(I,V)-GAWW-X-G-(F,L)-L. Although species divergence, multispecificity and wide tissue distribution are common characteristics of many Oatps/OATPs, some members of the OATP/ SLCO superfamily are highly conserved during evolution, have a high substrate specificity and exhibit unique cellular expression in distinct organs. Hence, while Oatps/OATPs with broad substrate specificity appear to play an important role in the bioavailability, distribution and excretion of numerous exogenous amphipathic organic anionic compounds, Oatps/OATPs with a narrow spectrum of transport substrates may exhibit more specific physiological functions in distinct organs.

Molecular cloning and spatio-temporal expression of the prostaglandin transporter: a basis for the action of prostaglandins in the bovine reproductive system

Prostaglandins (PGs) play important roles in mammalian reproductive function through autocrine, paracrine, and endocrine actions. However, they predominate as charged anions and diffuse poorly across the plasma membrane. Recently, a PG transporter (PGT) has been found to mediate PG transport across cell membranes. In ruminants, endometrial PGs are transported by a vascular pathway to the ovary to regress or rescue the corpus luteum. There is no report on the role of PGT in the reproductive functions of any species. We have cloned and characterized the bovine PGT (bPGT) that transports different PGs in the following affinity order: PGE2 = PGF2alpha >/= PGD2 much greater than arachidonate. bPGT mRNA and protein are expressed in endometrium, myometrium, and the utero-ovarian plexus (UOP) during the estrous cycle. The level of bPGT expression is higher in endometrium and UOP on the side of corpus luteum between days 13 and 18 of the estrous cycle. bPGT protein is localized in endometrial stroma, luminal epithelial cells, myometrial smooth muscle cells, and vascular smooth muscle cells of uterine vein and artery. In UOP, bPGT is selectively expressed in vascular smooth muscle cells of uterine vein and ovarian artery. Spatio-temporal expression of bPGT in uterine tissues and UOP supports a significant role of bPGT in cellular and compartmental transport of PGs to mediate the endocrine action at the time of luteolysis or establishment of pregnancy in bovine. This study describes and proposes a role of PGT in the regulation of reproductive processes.

Inhibition of prostaglandin D2 clearance in rat hepatocytes by the thromboxane receptor antagonists daltroban and ifetroban and the thromboxane synthase inhibitor furegrelate

Prostanoids, i.e. prostaglandins and thromboxane, regulate liver-specific functions both in homeostasis and during defense reactions. For example, prostanoids are released from Kupffer cells, the resident liver macrophages, in response to the inflammatory mediator anaphylatoxin C5a, and mediate an enhanced glucose output from hepatocytes as energy supply. In perfused rat livers, the thromboxane receptor antagonist daltroban enhanced C5a-induced prostanoid overflow and reduced glucose output. It was the aim of this study to elucidate whether daltroban interfered with prostanoid release from Kupffer cells or prostanoid clearance by hepatocytes, and/or whether it directly influenced prostanoid-dependent glucose metabolism in these cells. In perfused rat livers, daltroban enhanced prostaglandin (PG)D(2) overflow not only after infusion of C5a (15-fold), but also after PGD(2) (10-fold). Neither daltroban nor another receptor antagonist, ifetroban, or the thromboxane synthase inhibitor furegrelate enhanced prostanoid release from Kupffer cells. In contrast, all inhibitors reduced clearance, i.e. uptake and degradation, of PGD(2) by hepatocytes: within 5 min uptake of 1 nmol/L PGD(2) was reduced from 43+/-5 fmol (controls) to 22+/-6 fmol (daltroban), 24+/-6 fmol (ifetroban) and 21+/-6 fmol (furegrelate). PGD(2) in the medium was reduced to 39+/-7% in the controls, but remained at 93+/-9%, 93+/-11% and 60+/-3% in the presence of the inhibitors. PGD(2)-dependent glucose output in the perfused liver or activation of glycogen phosphorylase in isolated hepatocytes remained unaffected by daltroban. These data clearly demonstrate that the thromboxane-inhibitors reduced PGD(2) clearance by hepatocytes, presumably by inhibition of prostanoid transport into the cells. In contrast, they did not interfere with PGD(2)-dependent glucose metabolism, suggesting an independent mechanism for the inhibition of glucose output from the liver.

The human multidrug resistance protein MRP4 functions as a prostaglandin efflux transporter and is inhibited by nonsteroidal antiinflammatory drugs

Prostaglandins are involved in a wide variety of physiological and pathophysiological processes, but the mechanism of prostaglandin release from cells is not completely understood. Although poorly membrane permeable, prostaglandins are believed to exit cells by passive diffusion. We have investigated the interaction between prostaglandins and members of the ATP-binding cassette (ABC) transporter ABCC [multidrug resistance protein (MRP)] family of membrane export pumps. In inside-out membrane vesicles derived from insect cells or HEK293 cells, MRP4 catalyzed the time- and ATP-dependent uptake of prostaglandin E1 (PGE1) and PGE2. In contrast, MRP1, MRP2, MRP3, and MRP5 did not transport PGE1 or PGE2. The MRP4-mediated transport of PGE1 and PGE2 displayed saturation kinetics, with Km values of 2.1 and 3.4 microM, respectively. Further studies showed that PGF1alpha, PGF2alpha, PGA1, and thromboxane B2 were high-affinity inhibitors (and therefore presumably substrates) of MRP4. Furthermore, several nonsteroidal antiinflammatory drugs were potent inhibitors of MRP4 at concentrations that did not inhibit MRP1. In cells expressing the prostaglandin transporter PGT, the steady-state accumulation of PGE1 and PGE2 was reduced proportional to MRP4 expression. Inhibition of MRP4 by an MRP4-specific RNA interference construct or by indomethacin reversed this accumulation deficit. Together, these data suggest that MRP4 can release prostaglandins from cells, and that, in addition to inhibiting prostaglandin synthesis, some nonsteroidal antiinflammatory drugs might also act by inhibiting this release.

Prosurvival and antiapoptotic effects of PGE2 in radiation injury are mediated by EP2 receptor in intestine

The biological activities of PGE(2) are mediated through EP receptors (EP(1)-EP(4)), plasma membrane G protein-coupled receptors that differ in ligand binding and signal-transduction pathways. We investigated gastrointestinal EP(2) receptor expression in adult mice before and after radiation injury and evaluated intestinal stem cell survival and crypt epithelial apoptosis after radiation injury in EP(2) null mice. EP(2) was expressed throughout the gut. Intestinal EP(2) mRNA increased fivefold after gamma-irradiation. Crypt survival was diminished in EP(2)-/- mice (4.06 crypts/cross section) compared with wild-type littermates (8.15 crypts/cross section). Radiation-induced apoptosis was significantly increased in EP(2)-/- mice compared with wild-type littermates. Apoptosis was 1.6-fold higher in EP(2) (-/-) mice (5.9 apoptotic cells/crypt) than in wild-type mice (3.5 apoptotic cells/crypt). The EP(2) receptor is expressed in mouse gastrointestinal epithelial cells and is upregulated following radiation injury. The effects of PGE(2) on both crypt epithelial apoptosis and intestinal crypt stem cell survival are mediated through the EP(2) receptor.

Nuclear prostaglandin signaling system: biogenesis and actions via heptahelical receptors

Prostaglandins are ubiquitous lipid mediators that play pivotal roles in cardiovascular homeostasis, reproduction, and inflammation, as well as in many important cellular processes including gene expression and cell proliferation. The mechanism of action of these lipid messengers is thought to be primarily dependent on their interaction with specific cell surface receptors that belong to the heptahelical transmembrane spanning G protein-coupled receptor superfamily. Accumulating evidence suggests that these receptors may co-localize at the cell nucleus where they can modulate gene expression through a series of biochemical events. In this context, we have recently demonstrated that prostaglandin E2-EP3 receptors display an atypical nuclear compartmentalization in cerebral microvascular endothelial cells. Stimulation of these nuclear EP3 receptors leads to an increase of eNOS RNA in a cell-free isolated nuclear system. This review will emphasize these findings and describe how nuclear prostaglandin receptors, notably EP3 receptors, may affect gene expression, specifically of eNOS, by identifying putative transducing elements located within this organelle. The potential sources of lipid ligand activators for these intracellular sites will also be addressed. The expressional control of G-protein-coupled receptors located at the perinuclear envelope constitutes a novel and distinctive mode of gene regulation.

The presence of xenobiotic transporters in rat placenta

Understanding the role of transporters in placental handling of xenobiotics across the maternal-fetal interface is essential to evaluate the pharmacological and toxicological potential of therapeutic agents, drugs of abuse, and other xenobiotics to which the mother is exposed during pregnancy. Therefore, the purpose of this study was to assess mRNA levels of various transporters in placenta and to compare these to levels in maternal liver and kidney, predominant organs of excretion, to determine which transporters are likely to have a role in xenobiotic transfer within the placenta. During late stage pregnancy, relative amounts of mRNA levels of 40 genes representing 11 families/group of transporters were assessed in placenta with respect to relative maternal liver and kidney mRNA levels. Members of the following transporter families were assessed: three multidrug resistance (Mdr), six multidrug resistance-associated protein (Mrp), eight organic anion-transporting polypeptide (Oatp), three organic anion transporters (Oat), five organic cation transporters (Oct), two bile acid transporters (Na(+)/taurocholate-cotransporting polypeptide and bile salt export protein), four metal (ZnT1, divalent metal transporter 1, Menkes and Wilsons), a prostaglandin, two peptide, two sterolin, and four nucleoside transporters. Of the 40 genes evaluated, 16 [Mdr1a and 1b, Mrp1 and 5, Oct3 and Octn1, Oatp3 and 12, four metal, a prostaglandin, AbcG8, equilibrative nucleoside transporter 1 (ENT1), and ENT2] were expressed in placenta at concentrations similar to or higher than in maternal liver and kidney. The abundance of these mRNA transcripts in placenta suggests a role for these transporters in placental transport of xenobiotics and supports their role in the transport of endogenous substances.

Drug transport proteins in the liver

Together with drug metabolising enzymes, transmembrane transporters are important determinants of drug metabolism and drug clearance by the liver. Hepatic uptake of organic anions, cations, prostaglandins and bile salts is supported by dedicated transporter proteins in the basolateral (sinusoidal) membrane of hepatocytes: OATPs, OATs, OCTs, PGTs and NTCP, respectively. ATP-binding cassette (ABC) transporter proteins in the canalicular membrane of hepatocytes mediate the hepatic efflux of drugs, bile salts and metabolites against a steep concentration gradient from liver to bile. This transport is driven by ATP hydrolysis. Drugs, endogenous metabolites, bile salts and cytokines affect the expression levels of these transporters. They act through a family of ligand-activated transcription factors, the nuclear hormone receptors. Consequently, the levels of the various transporter proteins are subject to genetic polymorphism in the encoding genes as well as in these transcription factors. Adverse drug reactions may be caused by genetic or disease-induced variations of transporter expression or drug-drug interactions at the level of these transporters.

The superfamily of organic anion transporting polypeptides

Organic anion transporting polypeptides (Oatps/OATPs) form a growing gene superfamily and mediate transport of a wide spectrum of amphipathic organic solutes. Different Oatps/OATPs have partially overlapping and partially distinct substrate preferences for organic solutes such as bile salts, steroid conjugates, thyroid hormones, anionic oligopeptides, drugs, toxins and other xenobiotics. While some Oatps/OATPs are preferentially or even selectively expressed in one tissue such as the liver, others are expressed in multiple organs including the blood-brain barrier (BBB), choroid plexus, lung, heart, intestine, kidney, placenta and testis. This review summarizes the actual state of the rapidly expanding OATP superfamily and covers the structural properties, the genomic classification, the phylogenetic relationships and the functional transport characteristics. In addition, we propose a new species independent and open ended nomenclature and classification system, which is based on divergent evolution and agrees with the guidelines of the Human Genome Nomenclature Committee.

Transporters and xenobiotic disposition

Metabolism alone does not adequately account for the observed intersubject variability in drug disposition or response. Carrier-mediated processes, or transporters, are increasingly recognized to be importantly involved in drug absorption, distribution, and excretion. Thus for many drugs, transport and metabolism must be considered together to better predict drug disposition in vivo. Accordingly, this review will outline relevant background information regarding drug transporters and the role of such transporters in the drug disposition process.

Prostaglandin transport

Newly synthesized prostaglandins (PGs) efflux from cells by simple diffusion, driven by pH and the membrane potential. Metabolic clearance requires energy-dependent uptake across the plasma membrane, followed by cytoplasmic oxidation. Several PG carriers have been cloned and characterized. PGT is broadly expressed in cyclooxygenase (COX)-positive cells, appears to be a lactate/PG exchanger, and is coordinately regulated with COX. By analogy with neurotransmitter release and re-uptake, PGT may regulate pericellular PG levels via re-uptake. PGT may also direct PGs towards and/or away from specific sets of PG receptors. Other members of the OATP transporter family also catalyze PG uptake; these are variably expressed and have variable affinities for PGs. The OATs are alpha-ketoglutarate/organic anion exchangers that accept PGs; these probably represent the uptake step in renal and hepatic PG degradation and excretion. Finally, certain glutathione-conjugated leukotrienes and PGs are actively extruded from cells by the MRPs; these may also play a role in metabolic clearance of PGs.

Natural ligands of PPARgamma: are prostaglandin J(2) derivatives really playing the part?

The peroxisome proliferator-activated receptor (PPAR) family was discovered from an orphan nuclear receptor approach, and thereafter, three subtypes were identified, namely PPARalpha, PPARbeta or PPARgamma and PPARgamma. The two former seem to regulate lipid homeostasis, whereas the latter is involved, among others, in glucose homeostasis and adipocyte differentiation. PPARs were pharmacologically characterised first using peroxisome proliferators such as clofibrates, which demonstrate moderate affinity (efficiency at micromolar concentrations) and low PPARalpha/delta versus PPARgamma specificity. Hence, several laboratories have started the search for potent and subtype-specific natural PPAR activators. In this respect, prostaglandin (PG)-related compounds were identified as good PPARgamma agonists with varying specificity, the most notable PPAR ligand being 15-deoxy-Delta12-14-PGJ2 (15d-PGJ2). Recently, an oxidized phosphatidylcholine was identified as a potent alternative (patho)physiological natural ligand of PPARgamma. In the present review, we discuss the different PPARgamma-dependent and -independent biological effects of the PG PPARgamma ligands and the concern about their low potency in molecular models as compared with thiazolidinediones (TZDs), a family of potent (nanomolar) synthetic PPARgamma ligands. Finally, the oxidized lipids are presented as a novel and interesting alternative for discovering potent PPARgamma activators in order to understand more in details the implications of PPARgamma in various pathophysiological conditions.

Thyroid hormone transporters: recent advances

Thyroid hormones, being hydrophobic, were thought to enter target cell membranes by passive diffusion. However, recent studies have documented the existence of numerous organic anion transport systems, about half of which also transport thyroid hormones into (and possibly out of) a variety of target cells. Several of the genes encoding thyroid hormone transporters have been characterized by means of molecular approaches. Here, we discuss the classification of thyroid hormone transporters, with emphasis on how they are influenced by their ionic milieu and what their symported organic anions are.

Prostaglandin transporter PGT is expressed in cell types that synthesize and release prostanoids

PGT is a broadly expressed transporter of prostaglandins (PGs) and thromboxane that is energetically poised to take up prostanoids across the plasma membrane. To gain insight into the function of PGT, we generated mouse monoclonal antibody 20 against a portion of putative extracellular loop 5 of rat PGT. Immunoblots of endogenous PGT in rat kidney revealed a 65-kDa protein in a zonal pattern corresponding to PG synthesis rates (papilla congruent with medulla > cortex). Immunocytochemically, PGT in rat kidneys was expressed in glomerular endothelial and mesangial cells, arteriolar endothelial and muscularis cells, principal cells of the collecting duct, medullary interstitial cells, medullary vasa rectae endothelia, and papillary surface epithelium. Proximal tubules, which are known to take up and metabolize PGs, were negative. Immunoblotting and immunocytochemistry revealed that rat platelets also express abundant PGT. Coexpression of the PG synthesis apparatus (cyclooxygenase) and PGT by the same cell suggests that prostanoids may undergo release and reuptake.

Identification of lactate as a driving force for prostanoid transport by prostaglandin transporter PGT

We previously characterized the prostaglandin (PG) transporter PGT as an exchanger in which [(3)H]PGE(2) influx is coupled to the efflux of a countersubstrate. Here, we cultured HeLa cells that stably expressed human PGT under conditions known to favor glycolysis (glucose as a carbon source) or oxidative phosphorylation (glutamine as a carbon source) and studied the effect on PGT-mediated [(3)H]PGE(2) influx. PGT-expressing cells grown in glutamine exhibited a 2- to 4-fold increase in [(3)H]PGE(2) influx compared with the antisense control, whereas cells grown in glucose exhibited a 14-fold increase. In the presence of 10 vs. 25 mM glucose during the uptake, there was a dose-dependent increment in [(3)H]PGE(2) influx. Cis inhibition of [(3)H]PGE(2) influx occurred with lactate at physiological concentrations (apparent K(m) = 48 +/- 12 mM). Preloading with lactate caused a dose-dependent trans stimulation of PGT-mediated [(3)H]PGE(2) uptake, and external lactate caused trans stimulation of PGT-mediated [(3)H]PGE(2) release. Together, these data are consistent with PGT-mediated PG-lactate exchange. Cells engaged in glycolysis would then be poised energetically for prostanoid uptake by means of PGT.

Expression of PGT in MDCK cell monolayers: polarized apical localization and induction of active PG transport

The PG transporter (PGT) is expressed in subapical vesicles in the kidney collecting duct. To gain insight into the possible function of the PGT in this tubule segment, we tagged rat PGT with green fluorescent protein at the COOH terminus and generated stable PGT-expressing Madin-Darby canine kidney cell lines. When grown on permeable filters, green fluorescent protein-PGT was expressed predominantly at the apical membrane. Although the basal-to-apical transepithelial flux of [(3)H]PGE(2) was little changed by PGT expression, the apical-to-basolateral flux was increased 100-fold compared with wild-type cells. Analysis of driving forces revealed that this flux represents PGT-mediated active transepithelial PGE(2) transport. We propose that endogenous PGT is exocytically inserted into the collecting duct apical membrane, where it could control the concentration of luminal PGs.

Transporter-mediated Drug Interactions

Since 1994, researchers have isolated various genes encoding transporter proteins involved in drug uptake into and efflux from tissues that play key roles in the absorption, distribution and secretion of drugs in animals and humans. The pharmacokinetic characteristics of drugs that are substrates for these transporters are expected to be influenced by coadministered drugs that work as inhibitors or enhancers of the transporter function. This review deals with recent progress in molecular and functional research on drug transporters, and then with transporter-mediated drug interactions in absorption and secretion from the intestine, secretion from the kidney and liver, and transport across the blood-brain barrier in humans. Although the participation of the particular transporters in observed drug-drug interactions can be difficult to confirm in humans, this review focuses mainly on pharmacokinetic interactions of clinically important drugs.

Intestinal drug efflux: formulation and food effects

The intestine, primarily regarded as an absorptive organ, is also prepared for the elimination of certain organic acids, bases and neutral compounds depending on their affinity to intestinal carrier systems. Several of the transport systems known to mediate efflux in the major clearing organs--liver and kidney--are also expressed in the intestine. Examples of secretory transporters in the intestine are P-glycoprotein, members of the multidrug resistance associated protein family, breast cancer resistance protein, organic cation transporters and members of the organic anion polypeptide family. In this communication, the P-glycoprotein mediated intestinal secretion of talinolol, a model compound showing metabolic stability, has been investigated in the jejunum, ileum and colon of rat intestine by single-pass perfusion. A model has been developed which demonstrates an increase in carrier-mediated secretion in the order jejunum<ileum<colon. Furthermore, the potency of common excipients in peroral drug products towards inhibition of P-gp mediated secretion has been investigated using a radioligand-binding assay and transport studies in Caco-2 cell monolayers. Finally, evidence is provided which demonstrates that constituents of grapefruit juice not only may influence intestinal drug metabolism, but can also interfere with secretory transport systems, leading to a new and yet undescribed mechanism in drug-food interactions.

Functional characterization of human organic anion transporting polypeptide B (OATP-B) in comparison with liver-specific OATP-C

PURPOSE

To assess the functional characteristics of human organic anion transporter B (OATP-B) in comparison with those of the known, liver-specific OATP-C.

METHODS

OATP-B or -C was expressed in HEK293 cells or Xenopus oocytes, and uptakes of estradiol-17beta-glucuronide and estrone-3-sulfate were measured using radiolabeled compounds.

RESULTS

OATP-C transported both estrone-3-sulfate and estradiol-17beta-glucuronide, whereas OATP-B transported only the former. OATP-C-mediated uptake of estrone-3-sulfate exhibited biphasic saturation kinetics, whereas transports of estradiol-17beta-glucuronide by OATP-C and estrone-3-sulfate by OATP-B followed single-saturation kinetics. Inhibition kinetics showed that only the high-affinity site for estrone-3-sulfate on OATP-C was shared with glucuronide conjugates. Uptake of [3H]estrone-3-sulfate by OATP-B was inhibited by sulfate conjugates but not by glucuronide conjugates, whereas its uptake by OATP-C was inhibited by both types of conjugates.

CONCLUSIONS

OATP-B accepted sulfate conjugates of steroids but not glucuronide conjugates, whereas OATP-C transported both types of steroid conjugates. Transport of estrone-3-sulfate by OATP-B and -C followed single- and biphasic-saturation kinetics, respectively, and the high-affinity site on OATP-C was the same as that for estradiol-17beta-glucuronide. Other OATPs, OATP-A and OATP-8, reportedly exhibit different preferences for steroid conjugates, and the specific recognition of sulfate conjugates seems to be unique to OATP-B.

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Prostaglandin F(2alpha) receptor-dependent regulation of prostaglandin transport

Prostaglandin (PG) F(2alpha) may act on its G protein-coupled receptor (FP) or be imported intracellularly via a transporter, which has high affinity for PGF(2alpha) and PGE(2), but not prostacyclin (PGI(2)). In cells overexpressing the epitope-tagged FP together with the human prostaglandin transporter (hPGT), stimulation of the FP with PGF(2alpha) (1 nM-1 microM), or the less potent FP agonist, the isoprostane 8,12-iso-iPF(2alpha)-III, inhibited prostaglandin uptake via the hPGT. This effect was abolished by pretreatment of the cells with cholera toxin, but not with pertussis toxin. Furthermore, two dominant negative constructs directed against Galpha(s) partially blocked FP-mediated regulation of hPGT function, also suggesting Galpha(s) involvement in this phenomenon. Surprisingly, neither an activator (dibutyryl cyclic AMP) nor an inhibitor (H89) of cyclic AMP-dependent protein kinase had any effect on FP-mediated inhibition of hPGT activity. Furthermore, although PGF(2alpha) increases intracellular cyclic AMP via Galpha(s) activation, it does not induce phosphorylation of the transporter, excluding a role of cyclic AMP-dependent protein kinase in hPGT regulation. Activation of the PGI(2) receptor, which is also coupled to Galpha(s), does not regulate hPGT activity, despite markedly augmenting adenylate cyclase activation. In conclusion, activation of the FP reduces intracellular import of prostaglandins for metabolic inactivation, increasing prostanoid availability for membrane receptor activation. This effect seems to be mediated via Galpha(s), independent of adenylate cyclase and cyclic AMP-dependent protein kinase activation.

LST-2, a human liver-specific organic anion transporter, determines methotrexate sensitivity in gastrointestinal cancers

BACKGROUND & AIMS

One approach to the development of targeted cancer chemotherapy exploits increased uptake of the agent into neoplastic cells. In this scenario, higher concentrations of the agent in cancer cells are responsible for differential killing, whereas the low concentration in normal human cells decreases side effects. The aim of this study was to isolate an organic anion transporter that is weak in normal cells, but abundantly expressed in cancer cells, to deliver the anticancer drugs to the cells.

METHODS

A human liver complementary DNA (cDNA) library was screened with liver-specific transporter (LST)-1 cDNA as a probe. Northern blot analyses were performed using the isolated cDNA (termed LST-2). An LST-2-specific antibody was raised, and immunohistochemical analyses including immunoelectron microscopy were performed. Xenopus oocyte expression system was used for functional analysis. We also established a permanent cell line that consistently expresses LST-2 to examine the relationship between methotrexate uptake and sensitivity.

RESULTS

The isolated cDNA, LST-2, has 79.7% of overall homology with human LST-1. LST-2 exclusively expressed in the liver under normal conditions and its immunoreactivity was highest at the basolateral membrane of the hepatocytes around the central vein. Although its weak expression in the liver, LST-2 is abundantly expressed in the gastric, colon, and pancreatic cancers. On the other hand, the LST-1 was only detected in a hepatic cell line. LST-2 transports methotrexate in a saturable and dose-dependent manner. Furthermore, introduction of the LST-2 gene into mammalian cells potentiates sensitivity to methotrexate.

CONCLUSIONS

LST-2 is one of the prime candidate molecules for determining methotrexate sensitivity and may be a good target to deliver anticancer drugs to the gastrointestinal cancers.

Identification of thyroid hormone transporters in humans: different molecules are involved in a tissue-specific manner

We have recently identified that rat organic anion transporters, polypeptide2 (oatp2) and oatp3, both of which transport thyroid hormones. However, in humans the molecular organization of the organic anion transporters has diverged, and the responsible molecule for thyroid hormone transport has not been clarified, except for human liver-specific transporter (LST-1) identified by us. In this study we isolated and characterized a novel human organic anion transporter, OATP-E from human brain. The isolated complementary DNA encodes a polypeptide of 722 amino acids with 12 transmembrane domains. A rat counterpart, oatp-E, was also identified. Homology analysis and the phylogenetic tree analysis revealed that OATP-E/oatp-E is a subfamily of the organic anion transporter. Human OATP-E transported 3,3',5-triiodo-L-thyronine (K(m), 0.9 microM), thyronine, and rT(3) in a Na(+)-independent manner. Although the clone was isolated from the brain, OATP-E messenger RNA was abundantly expressed in various peripheral tissues. The rat counterpart, oatp-E, also transported 3,3',5-triiodo-L-thyronine. In addition, in this study we revealed that human OATP, which is exclusively expressed in the brain, transported 3,3',5-triiodo-L-thyronine (K(m), 6.5 microM), T(4) (K(m), 8.0 microM), and rT(3). These data suggest that in humans, several different molecules are involved in transporting thyroid hormone: OATP in the brain, LST-1 in the liver, and OATP-E in peripheral tissues.

Genomic organization and tissue-specific expression of splice variants of mouse organic anion transporting polypeptide 2

cDNAs that code for mouse organic anion transporting polypeptide 2 (oatp2) have been cloned. At least three forms of mouse oatp2 cDNAs containing the same coding sequence were isolated. The common coding sequence is for a protein of 670 amino acids with 12 putative transmembrane domains. The deduced amino acid sequence of the mouse oatp2 shares 89% identity with the reported rat oatp2. Cloning and analysis of mouse oatp2 gene indicates that these isoforms are alternatively spliced products from the same gene. Heterogeneity was observed in the 5'-untranslated region of the cDNAs. Two of the three isoforms lacked the noncoding exon 3 sequence. Northern-blot hybridization analysis using the exon 3-specific probes demonstrated that mouse oatp2 mRNA containing exon 3 sequence is expressed in heart and lung, whereas exon 1-, 2-, and 17-specific probes detected mRNA only in brain and liver. The mouse oatp2 gene consists of 17 exons, including three noncoding exons, and 16 introns. All of the introns are flanked by GT-AG splice sequences except for intron 10 that is flanked by GC-AG splice sequence.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Synthetic modification of prostaglandin f(2alpha) indicates different structural determinants for binding to the prostaglandin F receptor versus the prostaglandin transporter

Several principles governing the binding of E series prostaglandins to EP receptors have emerged in recent years. The C-1 carboxyl group binds electrostatically to a conserved arginine residue in the seventh transmembrane segment of the receptor. Prostaglandin E analogs involving bioisosteric replacements of the carboxyl group, such as acylsulfonamide, are also active. In addition, structurally similar esters may also exhibit similar affinity, presumably by virtue of hydrogen bonding. Other regions of the substrate molecule appear to bind to other domains of EP receptors, either via hydrophobic interactions or by hydrogen bonding. Less information is available about the structural requirements for substrate binding to FP receptors. Prostanoids also bind to the prostaglandin transporter PGT. In this case, a conserved C-1 carboxyl group is critically important, since C-1 esters exhibit little affinity. Here we examined the binding of chemically diverse PGF(2alpha) structural analogs to the FP receptor and compared these with binding by the PG transporter. PGT recognized a wide range of anionic substituents. In contrast, the carboxylic acid group was essential for optimal binding to the FP receptor, since replacement by larger moieties with a similar pK(a), such as acylsulfonamide and tetrazole, substantially decreased binding affinity. Interestingly, insertion of cyclic substituents in the omega chain increased binding to the FP receptor but reduced affinity for PGT, and substitution for the 15-hydroxyl group produced only a modest reduction in FP receptor binding, but eliminated binding by PGT. Because extracellular PGF(2alpha) may compete for binding between FP receptors and PGT, these findings have implications for designing PGF(2alpha) analogs for treating disease states.

Tax-dependent stimulation of G1 phase-specific cyclin-dependent kinases and increased expression of signal transduction genes characterize HTLV type 1-transformed T cells

Human T cell leukemia virus protein induces T cells to permanent IL-2-dependent growth. These cells occasionally convert to factor independence. The viral oncoprotein Tax acts as an essential growth factor of transformed lymphocytes and stimulates the cell cycle in the G(1) phase. In T cells and fibroblasts Tax enhances the activity of the cyclin-dependent kinases (CDK) CDK4 and CDK6. These kinases, which require binding to cyclin D isotypes for their activity, control the G(1) phase. Coimmunoprecipitation from these cells revealed that Tax associates with cyclin D3/CDK6, suggesting a direct activation of this kinase. The CDK stimulation may account in part for the mitogenic Tax effect, which causes IL-2-dependent T cell growth by Tax. To address the conversion to IL-2-independent proliferation and to identify overexpressed genes, which contribute to the transformed growth, the gene expression patterns of HTLV-1-transformed T cells were compared with that of peripheral blood lymphocytes. Potentially overexpressed cDNAs were cloned, sequenced, and used to determine the RNA expression. Genes found to be up-regulated are involved in signal transduction (STAT5a, cyclin G(1), c-fgr, hPGT) and also glycoprotein synthesis (LDLC, ribophorin). Many of these are also activated during T cell activation and implicated in the regulation of growth and apoptosis. The transcription factor STAT5a, which is involved in IL-2 signaling, was strongly up-regulated only in IL-2-independent cells, thus suggesting that it contributes to factor-independent growth. Thus, the differentially expressed genes could cooperate with the Tax-induced cell cycle stimulation in the maintenance of IL-2-dependent and IL-2-independent growth of HTLV-transformed lymphocytes.