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

Duodenal secretion in humans mediated by the EP4 receptor subtype

AIM

Assessment of functional EP receptor subtypes involved in PGE2-induced secretion in human duodenum. The spectrum of activities by PGE2 in mammals, including cytoprotective bicarbonate secretion in duodenum, is mediated through four G protein-coupled receptor subtypes (EP1-EP4).

METHODS

Biopsies from the second part of duodenum from patients undergoing endoscopy were mounted in modified Ussing chambers. Basal and stimulated short circuit current (SCC) and slope conductance (SG) were measured. Dose-response relations for PGE2 and subtype receptors EP1/EP3 (sulprostone), EP2 (butaprost), and EP4 (1-OH PGE1) were assessed by cumulated doses of single agonists.

RESULTS

PGE2 caused a dose-dependent increase in SCC, maximum 101 +/- 20 microA cm(-2) with an EC50 of 35.6 +/- 5.8 nm (n = 3). Likewise 1-OH PGE1 caused a dose-dependent SCC increase, maximum 63.3 +/- 28.6 microA cm(-2) with an EC50 of 56.7 +/- 7.2 nm (n = 3). 1-OH PGE1 at 500 nm increased SCC by 18.0 +/- 3.0 microA cm(-2) (n = 10) and SG by 2.9 +/- 0.4 mS cm(-2) (n = 6). Sulprostone (n = 6) and butaprost (n = 6) had no effects on SCC or SG. SCC was inhibited 31.4 +/- 13.2% (n = 10) by bumetanide (25 microM serosa) and 18.6 +/- 5.8% (n = 10) by acetazolamide (250 microM lumen). Diphenylamine-2-carboxylate (DPC) (250 microM mucosa) and SITS (10 microM mucosa) had almost no effect.

CONCLUSIONS

Effects of PGE2 on secretion in the second part of human duodenum is mediated through the EP4 receptor and not through EP1, EP2, or EP3.

Coordinated expression of multidrug resistance-associated proteins (Mrps) in mouse liver during toxicant-induced injury

Following acute chemical injury, hepatocytes are generally more resistant to toxicant re-exposure. Alterations in expression of hepatobiliary transport systems may contribute to this resistance by preventing accumulation of potentially toxic chemicals. Previous data demonstrate the concomitant reduction of uptake transporter and induction of efflux transporter mRNA during chemical liver injury. The present study further characterizes the expression of multidrug resistance-associated proteins 1-4 (Mrp1-4), breast cancer resistance protein (Bcrp) and sodium-taurocholate co-transporting polypeptide (Ntcp) in mouse liver following administration of the hepatotoxicants acetaminophen (APAP) and carbon tetrachloride (CCl4). Mice received hepatotoxic doses of APAP (400 mg/kg), CCl4 (10 or 25 microl/kg), or vehicle, ip. Livers were collected at 6, 24, and 48 h for Western blot quantification and immunofluorescence analysis. Protein expression of Bcrp was unchanged with treatment. Ntcp levels were preserved in APAP-exposed livers and reduced to 30-50% of control after CCl4. Conversely, Mrp1-4 expression was differentially up-regulated. CCl4 increased Mrp1 (3.5-fold), Mrp2 (1.4-fold), and Mrp4 (26-fold) while reducing Mrp3 levels to 20% of control. Administration of APAP enhanced expression of Mrp2 (1.6-fold), Mrp3 (3.5-fold), and Mrp4 (16-fold). Immunostaining of liver sections obtained 48 h after hepatotoxicant treatment confirmed expression patterns of a subset of transporters (Bcrp, Ntcp, Mrp3, and Mrp4). Double immunofluorescence imaging demonstrated the simultaneous down-regulation of Ntcp and up-regulation of Mrp4 in hepatocytes adjacent to the central vein after CCl4. Altered expression of transporters may reduce the overall chemical burden of an injured liver during recovery and contribute to the resistance of hepatocytes to subsequent toxicant exposure.

In vivo RNA interference-mediated ablation of MDR1 P-glycoprotein

Multidrug resistance (MDR) remains a major obstacle to successful chemotherapeutic treatment of cancer and can be caused by overexpression of P-glycoprotein, the MDR1 gene product. To further validate a knockdown approach for circumventing MDR, we developed a P-glycoprotein inhibition strategy using short hairpin RNA interference (shRNAi) and now show efficacy and target specificity in vivo. Two of eight tested shRNAi constructs targeted against human MDR1 mRNA inhibited expression of P-glycoprotein by >90%, whereas control shRNAi had no effect. Ablation of P-glycoprotein in cells stably transduced with retroviral-mediated shRNAi was documented by Western blot and functionally confirmed by increased sensitivity of MDR1-transfected cells toward the cytotoxic drugs vincristine, paclitaxel, and doxorubicin as well as by transport of (99m)Tc-Sestamibi. shRNAi-mediated down-regulation of P-glycoprotein transport activity both in cultured cells and in tumor implants in living animals could be followed by direct noninvasive bioluminescence imaging using the Renilla luciferase fluorophore, coelenterazine, a known P-glycoprotein transport substrate. Furthermore, after somatic gene transfer by hydrodynamic infusion of a MDR1-Firefly luciferase (MDR1-FLuc) fusion construct into mouse liver, the effect of shRNAi delivered in vivo on P-glycoprotein-FLuc protein levels was documented with bioluminescence imaging using d-luciferin. ShRNAi against MDR1 reduced bioluminescence output of the P-glycoprotein-FLuc reporter 4-fold in vivo compared with mice treated with control or scrambled shRNAi. Targeted down-regulation of a somatically transferred P-glycoprotein-eGFP fusion reporter also was observed using fluorescence microscopy. Our results show that shRNAi effectively inhibited MDR1 expression and function in cultured cells, tumor implants and mammalian liver, documenting the feasibility of a knockdown approach to reversing MDR in vivo.

The multidrug resistance protein 5 (ABCC5) confers resistance to 5-fluorouracil and transports its monophosphorylated metabolites

5'-Fluorouracil (5-FU), used in the treatment of colon and breast cancers, is converted intracellularly to 5'-fluoro-2'-deoxyuridine (5-FUdR) by thymidine phosphorylase and is subsequently phosphorylated by thymidine kinase to 5'-fluoro-2'-dUMP (5-FdUMP). This active metabolite, along with the reduced folate cofactor, 5,10-methylenetetrahydrofolate, forms a stable inhibitory complex with thymidylate synthase that blocks cellular growth. The present study shows that the ATP-dependent multidrug resistance protein-5 (MRP5, ABCC5) confers resistance to 5-FU by transporting the monophosphate metabolites. MRP5- and vector-transfected human embryonic kidney (HEK) cells were employed in these studies. In 3-day cytotoxicity assays, MRP5-transfected cells were approximately 9-fold resistant to 5-FU and 6-thioguanine. Studies with inside-out membrane vesicles prepared from transfected cells showed that MRP5 mediates ATP-dependent transport of 5 micromol/L [(3)H]5-FdUMP, [(3)H]5-FUMP, [(3)H]dUMP, and not [(3)H]5-FUdR, or [(3)H]5-FU. The ATP-dependent transport of 5-FdUMP showed saturation with increasing concentrations and had a K(m) of 1.1 mmol/L and V(max) of 439 pmol/min/mg protein. Uptake of 250 micromol/L 5-FdUMP was inhibited by dUMP, cyclic nucleotide, cyclic guanosine 3',5'-monophosphate, amphiphilic anions such as probenecid, MK571, the phosphodiesterase inhibitors, trequinsin, zaprinast, and sildenafil, and by the chloride channel blockers, 5-nitro-2-(3-phenylpropylamino)-benzoic acid and glybenclamide. Furthermore, the 5-FU drug sensitivity of HEK-MRP5 cells was partially modulated to that of the HEK-vector by the presence of 40 micromol/L 5-nitro-2-(3-phenylpropylamino)-benzoic acid but not by 2 mmol/L probenecid. Thus, MRP5 transports the monophosphorylated metabolite of this nucleoside and when MRP5 is overexpressed in colorectal and breast tumors, it may contribute to 5-FU drug resistance.

Impairment of blood-cerebrospinal fluid barrier properties by retrovirus-activated T lymphocytes: reduction in cerebrospinal fluid-to-blood efflux of prostaglandin E2

The choroid plexus epithelium forms the interface between the blood and the CSF. In conjunction with the tight junctions restricting the paracellular pathway, polarized specific transport systems in the choroidal epithelium allow a fine regulation of CSF-borne biologically active mediators. The highly vascularized stroma delimited by the choroidal epithelium can be a reservoir for retrovirus-infected or activated immune cells. In this work, new insight in the implication of the blood-CSF barrier in neuroinfectious and inflammatory diseases is provided by using a differentiated cellular model of the choroidal epithelium, exposed to infected T lymphocytes. We demonstrate that T cells activated by a retroviral infection, but not non-infected cells, reduce the transporter-mediated CSF-to-blood efflux of organic anions, in particular that of the potent pro-inflammatory prostaglandin PGE2, via the release of soluble factors. A moderate alteration of the paracellular permeability also occurs. We identified the viral protein Tax, oxygenated free radicals, matrix-metalloproteinases and pro-inflammatory cytokines as active molecules released during the exposure of the epithelium to infected T cells. Among them, tumour necrosis factor and interleukin 1 are directly involved in the mechanism underlying the decrease in some choroidal organic anion efflux. Given the strong involvement of CSF-borne PGE2 in sickness behaviour syndrome, these data suggest that the blood-CSF barrier plays an important role in the pathophysiology of neuroinflammation and neuroinfection, via changes in the transport processes controlling the CSF biodisposition of PGE2.

Inefficient central nervous system delivery limits the use of ibuprofen in neurodegenerative diseases

Chronic use of non-steroidal anti-inflammatory drugs may reduce the risk or delay the onset of Alzheimer's disease. To date, only limited information exists on the brain distribution of these drugs. The objective of this study was to determine the absolute brain delivery of ibuprofen by using constant in vivo infusion in rats. Ibuprofen was infused to steady-state concentrations both in plasma and brain tissue. Ibuprofen levels in plasma and brain tissue were measured by RP-HPLC after the plasma and the brain samples were purified by protein precipitation and solid phase extraction, respectively. Results indicate that both plasma and brain concentrations reached steady-state within 6h, and that the brain to plasma ratio of ibuprofen was only 0.02. Thus, limited brain penetration prevents the possible use of ibuprofen in treating or preventing neurodegenerative disorders such as Alzheimer's disease.

The human multidrug resistance protein MRP5 transports folates and can mediate cellular resistance against antifolates

Members of the multidrug resistance protein family, notably MRP1-4/ABCC1-4, and the breast cancer resistance protein BCRP/ABCG2 have been recognized as cellular exporters for the folate antagonist methotrexate (MTX). Here we show that MRP5/ABCC5 is also an antifolate and folate exporter based on the following evidence: (a) Using membrane vesicles from HEK293 cells, we show that MRP5 transports both MTX (KM = 1.3 mmol/L and VMAX = 780 pmol per mg protein per minute) and folic acid (KM = 1.0 mmol/L and VMAX = 875 pmol per mg protein per minute). MRP5 also transports MTX-glu2 (KM = 0.7 mmol/L and VMAX = 450 pmol per mg protein per minute) but not MTX-glu3. (b) Both accumulation of total [3H]MTX and of MTX polyglutamates were significantly reduced in MRP5 overexpressing cells. (c) Cell growth inhibition studies with MRP5 transfected HEK293 cells showed that MRP5 conferred high-level resistance (>160-fold) against the antifolates MTX, GW1843, and ZD1694 (raltitrexed) in short-term (4 hours) incubations with high drug concentrations; this resistance was proportional to the MRP5 level. (d) MRP5-mediated resistance (8.5- and 2.1-fold) was also found in standard long-term incubations (72 hours) at low concentrations of ZD1694 and GW1843. These results show the potential of MRP5 to mediate transport of (anti)folates and contribute to resistance against antifolate drugs.

Distribution of the novel antifolate pemetrexed to the brain

Pemetrexed disodium is a novel antifolate that exhibits potent inhibitory effects on multiple enzymes in folate metabolism. Phase II/III clinical trials have shown that pemetrexed is effective against various solid tumors. Like methotrexate, pemetrexed may be useful in treatment of primary and secondary brain tumors. In this study, we examined the central nervous system (CNS) distribution of pemetrexed and the interaction with an organic anion transport inhibitor indomethacin. Male Wistar rats were administered pemetrexed by either single intravenous bolus or constant intravenous infusion. Unbound pemetrexed in blood and brain was measured by simultaneous arterial blood and frontal cortex microdialysis sampling. In the i.v. bolus experiments, indomethacin was administered by i.v. bolus (10 mg/kg) followed by i.v. infusion (0.1 mg/kg/h) in a crossover manner. In the infusion experiments, the same dose of indomethacin was administered after a steady state was reached for pemetrexed. CNS distributional kinetics was analyzed by compartmental and noncompartmental methods. Both bolus and infusion studies showed that pemetrexed has a limited CNS distribution. The mean area under concentration-time curve (AUC)(brain)/AUC(plasma) ratio of unbound pemetrexed was 0.078 +/- 0.038 in the i.v. bolus study. The pemetrexed steady-state brain-to-plasma unbound concentration ratio after i.v. infusion was 0.106 +/- 0.054. The distributional clearance into the brain was approximately 10% of the clearance out of the brain in both the compartmental and noncompartmental analyses. Indomethacin had no effect on either the brain-to-plasma AUC ratio or the steady-state brain-to-plasma concentration ratio. The distribution of pemetrexed into the brain is limited, and an efflux clearance process, such as an efflux transporter, may be involved.

ATP-binding cassette (ABC) transporters in normal and pathological lung

ATP-binding cassette (ABC) transporters are a family of transmembrane proteins that can transport a wide variety of substrates across biological membranes in an energy-dependent manner. Many ABC transporters such as P-glycoprotein (P-gp), multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP) are highly expressed in bronchial epithelium. This review aims to give new insights in the possible functions of ABC molecules in the lung in view of their expression in different cell types. Furthermore, their role in protection against noxious compounds, e.g. air pollutants and cigarette smoke components, will be discussed as well as the (mal)function in normal and pathological lung. Several pulmonary drugs are substrates for ABC transporters and therefore, the delivery of these drugs to the site of action may be highly dependent on the presence and activity of many ABC transporters in several cell types. Three ABC transporters are known to play an important role in lung functioning. Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene can cause cystic fibrosis, and mutations in ABCA1 and ABCA3 are responsible for respectively Tangier disease and fatal surfactant deficiency. The role of altered function of ABC transporters in highly prevalent pulmonary diseases such as asthma or chronic obstructive pulmonary disease (COPD) have hardly been investigated so far. We especially focused on polymorphisms, knock-out mice models and in vitro results of pulmonary research. Insight in the function of ABC transporters in the lung may open new ways to facilitate treatment of lung diseases.

Expression and localization of human multidrug resistance protein (ABCC) family members in pancreatic carcinoma

Pancreatic ductal adenocarcinoma is among the top 10 causes of death from cancer in industrialized countries. In comparison with other gastrointestinal malignancies, pancreatic cancer is one of the tumors most resistant to chemotherapy. An important mechanism of tumor multidrug resistance is increased drug efflux mediated by several transporters of the ABC superfamily. Especially BCRP (ABCG2), MDR1 P-glycoprotein (ABCB1) and members of the MRP (ABCC) family are important in mediating drug resistance. The MRP family consists of 9 members (MRP1-MRP9) with MRP1-MRP6 being best characterized with respect to protein localization and substrate selectivity. Here, we quantified the mRNA expression of BCRP and of all MRP family members in normal human pancreas and pancreatic carcinoma and analyzed the mRNA level of the transporters most abundantly expressed in pancreatic tissue, BCRP, MRP1, MRP3, MRP4 and MRP5, in 37 tissue samples. In addition, we determined the localization of the 4 MRP proteins in normal human pancreas and in pancreatic carcinoma. The expression of BCRP, MRP1 and MRP4 mRNA did not correlate with tumor stage or grading. On the other hand, the expression of MRP3 mRNA was upregulated in pancreatic carcinoma samples and was correlated with tumor grading. The MRP5 mRNA level was significantly higher in pancreatic carcinoma tissue compared to normal pancreatic tissue. These data suggest that MRP3 and MRP5 are involved in drug resistance of pancreatic tumors and that quantitative analysis of their expression may contribute to predict the benefit of chemotherapy in patients with pancreatic cancer.

Cellular export of drugs and signaling molecules by the ATP-binding cassette transporters MRP4 (ABCC4) and MRP5 (ABCC5)

Like other members of the multidrug resistance protein (MRP)/ABCC subfamily of ATP-binding cassette transporters, MRP4 (ABCC4) and MRP5 (ABCC5) are organic anion transporters. They have, however, the outstanding ability to transport nucleotides and nucleotide analogs. In vitro experiments using drug-selected or -transfected cells indicated that these transport proteins, when overexpressed, can lower the intracellular concentration of nucleoside/nucleotide analogs, such as the antiviral compounds PMEA (9-(2-phosphonylmethoxyethyl)adenine) or ganciclovir, and of anticancer nucleobase analogs, such as 6-mercaptopurine, after their conversion into the respective nucleotides. This may lead to an impaired ability of these compounds to inhibit virus replication or cell proliferation. It remains to be tested whether antiviral or anticancer chemotherapy based on nucleobase, nucleoside, or nucleotide precursors can be modulated by inhibition of MRP4 and MRP5. MRP4 also seems to be able to mediate the transport of conjugated steroids, prostaglandins, and glutathione. Furthermore, cyclic nucleotides (cyclic adenosine monophosphate and cyclic guanine monophosphate) are exported from cells by MRP4 and MRP5. This may modulate the intracellular concentration of these important mediators, besides the action of phosphodiesterases, as well as provide extracellular nucleotides for a possible paracrine action. In this line, tissue distribution and subcellular localization of MRP4 and MRP5 specifically in smooth muscle cells (MRP5), platelet-dense granules (MRP4), and nervous cells (MRP4 and MRP5), besides the capillary endothelium, point not only to a possible function of these transporters as exporters in cellular defense, but also to a physiological function in signaling processes.

The effect of Bcrp1 (Abcg2) on the in vivo pharmacokinetics and brain penetration of imatinib mesylate (Gleevec): implications for the use of breast cancer resistance protein and P-glycoprotein inhibitors to enable the brain penetration of imatinib in patients

Imatinib mesylate (signal transduction inhibitor 571, Gleevec) is a potent and selective tyrosine kinase inhibitor, which was shown to effectively inhibit platelet-derived growth factor-induced glioblastoma cell growth preclinically. However, in patients, a limited penetration of imatinib into the brain has been reported. Imatinib is transported in vitro and in vivo by P-glycoprotein (P-gp; ABCB1), which thereby limits its distribution into the brain in mice. Previously, imatinib was shown to potently inhibit human breast cancer resistance protein (BCRP; ABCG2). Here, we show that imatinib is efficiently transported by mouse Bcrp1 in transfected Madin-Darby canine kidney strain II (MDCKII) monolayers. Furthermore, we show that the clearance of i.v. imatinib is significantly decreased 1.6-fold in Bcrp1 knockout mice compared with wild-type mice. At t = 2 hours, the brain penetration of i.v. imatinib was significantly 2.5-fold increased in Bcrp1 knockout mice compared with control mice. We tested the hypothesis that P-gp and BCRP inhibitors, such as elacridar and pantoprazole, improve the brain penetration of imatinib. Firstly, we showed in vitro that pantoprazole and elacridar inhibit the Bcrp1-mediated transport of imatinib in MDCKII-Bcrp1 cells. Secondly, we showed that co-administration of pantoprazole or elacridar significantly reduced the clearance of i.v. imatinib in wild-type mice by respectively 1.7-fold and 1.5-fold. Finally, in wild-type mice treated with pantoprazole or elacridar, the brain penetration of i.v. imatinib significantly increased 1.8-fold and 4.2-fold, respectively. Moreover, the brain penetration of p.o. imatinib increased 5.2-fold when pantoprazole was co-administered in wild-type mice. Our results suggest that co-administration of BCRP and P-gp inhibitors may improve delivery of imatinib to malignant gliomas.

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.

INDUCTION OF THE MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN FAMILY OF TRANSPORTERS BY CHEMICAL ACTIVATORS OF RECEPTOR-MEDIATED PATHWAYS IN MOUSE LIVER

The multidrug resistance-associated proteins (Mrp) are ATP-dependent transporters that export a variety of conjugated and unconjugated compounds out of cells. There are nine identified Mrp transporters in humans, with murine orthologs for all except Mrp8. Because nuclear receptors mediate induction of phase I enzymes, Mrp transporter expression might be similarly regulated by these receptors to coordinate metabolism and export of chemicals from liver. To test the hypothesis that Mrp expression may be coordinately regulated with phase I enzyme expression in liver, 15 different compounds were given representing known transcriptionally mediated pathways: aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), constitutive androstane receptor (CAR), peroxisome proliferator-activated receptor alpha (PPARalpha), and nuclear factor-E2-related factor 2 (Nrf2). Each of these compounds induced expression of their respective target enzyme in liver, demonstrating that the chemical regimens were effective. The AhR ligands [2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyl 126 (PCB126), and beta-naphthoflavone] induced Mrp2, -3, -5, and -6 mRNA expression. The CAR activator 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) induced Mrp2, -3, -4, -6, and -7 mRNA expression. Mrp3 was also induced by two other CAR activators phenobarbital and diallyl sulfide, two PXR ligands, pregnenalone-16alpha-carbonitrile and spironolactone, and the PPARalpha ligands clofibrate, ciprofibrate, and diethylhexylphthalate. The Nrf2 activators (butylated hydroxyanisole, oltipraz, and ethoxyquin) induced Mrp2-6. In conclusion, a variety of mechanisms are suggested for Mrp3 induction, including AhR, CAR, PXR, PPARalpha, and Nrf2, whereas on a whole, a predominant role for AhR and Nrf2 in hepatic induction of the Mrp family was observed. Thus, these specific transcription factors are implicated in regulation of both drug metabolism and efflux transport.

Acetaminophen-sensitive prostaglandin production in rat cerebral endothelial cells

Acetaminophen is a widely used antipyretic and analgesic drug whose mechanism of action has recently been suggested to involve inhibitory effects on prostaglandin synthesis via a newly discovered cyclooxygenase variant (COX-3). Because COX-3 expression is high in cerebral endothelium, we investigated the effect of acetaminophen on the prostaglandin production of cultured rat cerebral endothelial cells (CECs). Acetaminophen dose-dependently inhibited both basal and LPS-induced PGE2 production in CECs with IC50 values of 15.5 and 6.9 μM, respectively. Acetaminophen also similarly inhibited the synthesis of 6-keto-PGF1α and thromboxane B2. LPS stimulation increased the expression of COX-2 but not COX-1 or COX-3. In addition, the selective COX-2 inhibitor NS398 (1 μM) was equally as effective as acetaminophen in blocking LPS-induced PGE2 production. Acetaminophen did not influence the expression of the three COX isoforms and the inducible nitric oxide synthase. In LPS-stimulated isolated cerebral microvessels, acetaminophen also significantly inhibited PGE2 production. Our results show that prostaglandin production in CECs during basal and stimulated conditions is very sensitive to inhibition by acetaminophen and suggest that acetaminophen acts against COX-2 and not COX-1 or COX-3. Furthermore, our findings support a critical role for cerebral endothelium in the therapeutic actions of acetaminophen in the central nervous system.

Interleukin-6-type cytokines upregulate expression of multidrug resistance-associated proteins in NHEK and dermal fibroblasts

Normal human epidermal keratinocytes (NHEK) and dermal fibroblasts express a cell-specific pattern of efflux transport proteins. Since regulatory mechanisms for these transporters in cells of the human skin were unknown, we analyzed the influence of inflammatory cytokines on the expression of multidrug resistance-associated proteins (MRP1, 3, 4, 5). Using real-time PCR, RT-PCR, cDNA microarray, immunostaining and efflux assays we demonstrated that stimulation of NHEK and primary human dermal fibroblasts with interleukin-6 (IL-6), in combination with its soluble alpha-receptor, or oncostatin M (OSM) for 24-72 h resulted in an upregulation of MRP expression and activity. Both cytokines induced a strong activation of signal transducer and activator of transcription (STAT)1 and STAT3 as well as the mitogen-activated protein kinase (MAPK) Erk1/2. OSM additionally activated proteinkinase B strongly. Using the MAPK/extracellular signal-regulated kinase kinase 1-specific inhibitor U0126 we could exclude a stimulatory effect of MAPK on MRP gene expression. Inhibition of the phosphatidylinositol 3-kinase, however, indicated that this pathway might be involved of OSM-mediated upregulation of MRP4 in dermal fibroblasts. Several inflammatory skin diseases show an enhanced expression of IL-6-type cytokines. Correspondingly, upregulation of MRP expression was found in lesional skin taken from patients with psoriasis and lichen planus.

Expression and immunolocalization of the multidrug resistance proteins, MRP1-MRP6 (ABCC1-ABCC6), in human brain

Multidrug resistance proteins (MRPs, symbol ABCC) are membrane glycoproteins that mediate the ATP-dependent export of organic anions, including cytotoxic and antiviral drugs, from cells. To identify MRP family members possibly involved in the intrinsic resistance of human brain to cytotoxic and antiviral drugs, we analyzed the expression and localization of MRP1-MRP6 in rapidly frozen perilesional samples of several regions of adult human brain obtained during neurosurgery. Quantitative polymerase chain reaction analysis showed expression of MRP1, MRP2, MRP3, MRP4, and MRP5 mRNA, whereas MRP6 mRNA was below detectability. However, immunofluorescence microscopy of cryosections from human brain showed no reactivity for the MRP2 or MRP3 proteins. The proteins MRP1, MRP4, and MRP5 were clearly localized by confocal laser scanning microscopy to the luminal side of brain capillary endothelial cells. The MRP4 and MRP5 proteins were also detected in astrocytes of the subcortical white matter. Notably, MRP5 protein was present in pyramidal neurons. MRP proteins may, thus, contribute to the cellular efflux of endogenous anionic glutathione or glucuronate conjugates (substrates for MRP1), cyclic nucleotides (substrates for MRP4 and MRP5), or glutathione (co-substrate for MRP1 and MRP4); in addition, they may play an important role in the resistance of the brain to several cytotoxic and antiviral drugs.

Pharmacogenomics of MDR and MRP subfamilies

Drug-metabolizing enzymes, drug transporters and drug targets play significant roles as determinants of drug efficacy and toxicity. Their genetic polymorphisms often affect the expression and function of their products and are expected to become surrogate markers to predict the response to drugs in individual patients. With the sequencing of the human genome, it has been estimated that approximately 500–1200 genes code for drug transporters and, recently, there have been significant and rapid advances in the research on the relationships between genetic polymorphisms of drug transporters and interindividual variation of drug disposition. At present, the clinical studies of multi-drug resistance protein 1 (MDR1, P-glycoprotein, ABCB1), which belongs to the ATP-binding cassette (ABC) superfamily, are the most comprehensive among the ABC transporters, but clinical investigations on other drug transporters are currently being performed around the world. MDR1 can be said to be the most important drug transporter, since clinical reports have suggested that it regulates the disposition of various types of clinically important drugs, but in vitro investigations or animal experiments have strongly suggested that the members of the multi-drug resistance-associated protein (MRP) subfamily can also become key molecules for pharmacotherapy. In addition to those, breast cancer resistance protein (BCRP, ABCG2), another ABC transporter, is well known as a key molecule of multi-drug resistance to several anticancer agents. However, this review focuses on the latest information on the pharmacogenetics of the MDR and MRP subfamilies, and its impact on pharmacotherapy is discussed.

Modulation of nuclear factor-kappa B activity by indomethacin influences A beta levels but not A beta precursor protein metabolism in a model of Alzheimer's disease

Epidemiological studies show that some nonsteroidal anti-inflammatory drugs, nonspecific inhibitors of the cyclooxygenase enzyme, reduce the incidence of Alzheimer's disease (AD). We determined the impact of two nonsteroidal anti-inflammatory drugs on A beta levels, deposition, and metabolism in a mouse model (the Tg2576) of AD-like amyloidosis. To this end, mice were treated with indomethacin and nimesulide continuously from 8 months of age until they were 15 months old. At the end of the study, indomethacin significantly reduced A beta(1-40) and A beta(1-42) levels in both cortex and hippocampus. This decrease was coincidental with a significant reduction of the nuclear factor (NF)-kappa B activity. By contrast, nimesulide had no effect on both A beta peptides and NF-kappa B. Consistently, mice receiving indomethacin, but no nimesulide, showed a significant reduction in the amyloid burden compared with placebo. Neither drug had an effect on plasma levels of A beta peptides or the A beta precursor protein metabolism. In vitro studies confirmed that genetic absence of this factor reduces the anti-amyloidogenic effect of indomethacin. These findings indicate that chronic administration of indomethacin by blocking the activation of the NF-kappa B significantly reduces the amyloid pathology in Tg2576 mice, and provide insights into the mechanisms by which this drug could slow progression of AD.

Transport of bile acids, sulfated steroids, estradiol 17-beta-D-glucuronide, and leukotriene C4 by human multidrug resistance protein 8 (ABCC11)

We previously determined that expression of human multidrug resistance protein (MRP) 8, a recently described member of the MRP family of ATP-binding cassette transporters, enhances cellular extrusion of cyclic nucleotides and confers resistance to nucleotide analogs (J Biol Chem 278:29509-29514, 2003). However, the in vitro transport characteristics of the pump have not been determined. In this study, the substrate selectivity and biochemical activity of MRP8 is investigated using membrane vesicles prepared from LLC-PK1 cells transfected with MRP8 expression vector. Expression of MRP8 is shown to stimulate the ATP-dependent uptake of a range of physiological and synthetic lipophilic anions, including the glutathione S-conjugates leukotriene C4 and dinitrophenyl S-glutathione, steroid sulfates such as dehydroepiandrosterone 3-sulfate (DHEAS) and estrone 3-sulfate, glucuronides such as estradiol 17-beta-D-glucuronide (E(2)17betaG), the monoanionic bile acids glycocholate and taurocholate, and methotrexate. In addition, MRP8 is competent in the in vitro transport of cAMP and cGMP, in accord with the results of our previously reported cellular studies. DHEAS, E(2)17betaG, and methotrexate were transported with K(m) and V(max) values of 13.0 +/- 0.8 microM and 34.9 +/- 9.5 pmol/mg/min, 62.9 +/- 12 microM and 62.0 +/- 5.2 pmol/mg/min, and 957 +/- 28 microM and 317 +/- 17 pmol/mg/min, respectively. Based upon the stimulatory action of DHEAS on uptake of E(2)17betaG, the attenuation of this effect at high DHEAS concentrations and the lack of reciprocal promotion of DHEAS uptake by E(2)17betaG, a model involving nonreciprocal constructive interactions between some transport substrates is invoked. These results suggest that MRP8 participates in physiological processes involving bile acids, conjugated steroids, and cyclic nucleotides and indicate that the pump has complex interactions with its substrates.

MAPPING OF MULTIDRUG RESISTANCE GENE 1 AND MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN ISOFORM 1 TO 5 mRNA EXPRESSION ALONG THE HUMAN INTESTINAL TRACT

Efflux transporters such as P-glycoprotein and multidrug resistance-associated proteins (MRPs) in the intestinal wall restrict intestinal drug transport. To overcome this limitation for enteral drug absorption, galenical targeting approaches have been proposed for site-specific luminal drug release in segments of the gut, where expression of the respective absorption-limiting transporter is minimal. Therefore, expression of multidrug resistance gene 1 (MDR1) and MRP1-5 was systematically investigated in 10 healthy subjects. Biopsies were taken from different segments of the gastrointestinal tract (from duodenum and terminal ileum, as well as ascending, transverse, descending, and sigmoid colon). Gene expression was investigated by quantitative real-time PCR (TaqMan). MRP3 appeared to be the most abundantly expressed transporter in investigated parts of the human intestine, except for the terminal ileum, where MDR1 showed the highest expression. The ranking of transporter gene expression in the duodenum was MRP3 >> MDR1 > MRP2 > MRP5 > MRP4 > MRP1. In the terminal ileum, the ranking order was as follows: MDR1 > MRP3 >> MRP1 approximately MRP5 approximately MRP4 > MRP2. In all segments of the colon (ascending, transverse, descending, and sigmoid colon), the transporter gene expression showed the following order: MRP3 >> MDR1 > MRP4 approximately MRP5 > MRP1 >> MRP2. We have shown, for the first time, systematic site-specific expression of MDR1 and MRP mRNA along the gastrointestinal tract in humans. All transporters showed alterations in their expression levels from the duodenum to sigmoid colon. The most pronounced changes were observed for MRP2, with high levels in the small intestine and hardly any expression in colonic segments. This knowledge may be useful to develop new targeting strategies for enteral drug delivery.

Differential Expression of Mouse Hepatic Transporter Genes in Response to Acetaminophen and Carbon Tetrachloride

Drug-metabolizing enzymes and membrane transporters are responsible for the detoxication and elimination of xenobiotics from the body. The goal of this study was to identify alterations in mRNA expression of various transport and detoxication proteins in mouse liver after administration of the hepatotoxicants, acetaminophen or carbon tetrachloride. Therefore, male C57BL/6 J mice received acetaminophen (APAP, 200, 300, or 400 mg/kg, ip) or carbon tetrachloride (CCl4, 10 or 25 microl/kg, ip). Plasma and liver samples were collected at 6, 24, and 48 h for assessment of alanine aminotransferase (ALT) activity, total RNA isolation, and histopathological analysis of injury. Heme oxygenase-1 (Ho-1), NAD(P)H quinone oxidoreductase-1 (Nqo1), organic anion-transporting polypeptides (Oatp1a1, 1a4 and 1b2), sodium/taurocholate-cotransporting polypeptide (Ntcp), and multidrug resistance-associated protein (Mrp 1-6) mRNA levels in liver were determined using the branched DNA signal amplification assay. Hepatotoxic doses of APAP and CCl4 increased Ho-1 and Nqo1 mRNA levels by 22- and 2.5-fold, respectively, and reduced Oatp1a1, 1a4, and Ntcp mRNA levels in liver. By contrast, expression of Mrps 1-4 was increased after treatment with APAP and CCl4. Notably, a marked elevation of Mrp4 mRNA expression was observed 24 h after APAP 400 mg/kg (5-fold) and CCl4 25 microl/kg (37-fold). Collectively, these expression patterns suggest a coordinated regulation of both transport and detoxification genes during liver injury. This reduction in expression of uptake transporters, as well as enhanced transcription of detoxication enzymes and export transporters may limit the accumulation of potentially toxic products in hepatocytes.

Development of nonsteroidal anti-inflammatory drug analogs and steroid carboxylates selective for human aldo-keto reductase isoforms: potential antineoplastic agents that work independently of cyclooxygenase isozymes

Human aldo-keto reductases (AKRs) regulate nuclear receptors by controlling ligand availability. Enzymes implicated in regulating ligand occupancy and trans-activation of the nuclear receptors belong to the AKR1C family (AKR1C1-AKR1C3). Nuclear receptors regulated by AKR1C members include the steroid hormone receptors (androgen, estrogen, and progesterone receptors) and the orphan peroxisome proliferator-activated receptor (PPARgamma). In human myeloid leukemia (HL-60) cells, ligand access to PPARgamma is regulated by AKR1C3, which diverts PGD(2) metabolism away from J-series prostanoids (Desmond et al., 2003). Inhibition of AKR1C3 by indomethacin, a nonsteroidal anti-inflammatory drug (NSAID), caused PPARgamma-mediated terminal differentiation of the HL-60 cells. To discriminate between antineoplastic effects of NSAIDs that are mediated by either AKR1C or cyclooxygenase (COX) isozymes, selective inhibitors are required. We report a structural series of N-phenylanthranilic acid derivatives and steroid carboxylates that selectively inhibit recombinant AKR1C isoforms but do not inhibit recombinant COX-1 or COX-2. The inhibition constants, IC(50), K(I) values, and inhibition patterns were determined for the NSAID analogs and steroid carboxylates against AKR1C and COX isozymes. Lead compounds, 4-chloro-N-phenylanthranilic acid and 4-benzoyl-benzoic acid for the N-phenylanthranilic acid analogs and most steroid carboxylates, exhibited IC(50) values that had greater than 500-fold selectivity for AKR1C isozymes compared with COX-1 and COX-2. Crystallographic and molecular modeling studies showed that the carboxylic acid of the inhibitor ligand was tethered by the catalytic Tyr55-OH(2)(+) and explained why A-ring substituted N-phenylanthranilates inhibited only AKR1C enzymes. These compounds can be used to dissect the role of the AKR1C isozymes in neoplastic diseases and may have cancer chemopreventive roles independent of COX inhibition.

Inhibition of hyaluronan export from human fibroblasts by inhibitors of multidrug resistance transporters

In a previous report we described the export of hyaluronan from Streptococcus pyogenes by an ABC transporter. Extending these findings a sequence homology search against human proteins revealed a strong homology to the multidrug resistance transporter ABC-B (MDR-1) and ABC-C (MRP 5). Using several inhibitors directed against these and other transporters, a decreased hyaluronan production in cell culture as well as in hyaluronan synthase activity in purified membrane fractions was observed. The inhibitory capacity (IC(50) concentrations) was compared the with reported IC(50)- or the K(i)-concentrations for individual transporters. These analyses revealed that hyaluronan is synthesized within the cytoplasm of mammalian cells and actively secreted into the pericellular space by energy dependent transport proteins. While inhibition of several transport proteins resulted in a decrease of hyaluronan export, inhibition of the MRP5 transporter was the most effective one to decrease hyaluronan in the cell culture supernatant indicating that hyaluronan export is one physiological role of this transport protein.

Mrp4 confers resistance to topotecan and protects the brain from chemotherapy

The role of the multidrug resistance protein MRP4/ABCC4 in vivo remains undefined. To explore this role, we generated Mrp4-deficient mice. Unexpectedly, these mice showed enhanced accumulation of the anticancer agent topotecan in brain tissue and cerebrospinal fluid (CSF). Further studies demonstrated that topotecan was an Mrp4 substrate and that cells overexpressing Mrp4 were resistant to its cytotoxic effects. We then used new antibodies to discover that Mrp4 is unique among the anionic ATP-dependent transporters in its dual localization at the basolateral membrane of the choroid plexus epithelium and in the apical membrane of the endothelial cells of the brain capillaries. Microdialysis sampling of ventricular CSF demonstrated that localization of Mrp4 at the choroid epithelium is integral to its function in limiting drug penetration into the CSF. The topotecan resistance of cells overexpressing Mrp4 and the polarized expression of Mrp4 in the choroid plexus and brain capillary endothelial cells indicate that Mrp4 has a dual role in protecting the brain from cytotoxins and suggest that the therapeutic efficacy of central nervous system-directed drugs that are Mrp4 substrates may be improved by developing Mrp4 inhibitors.

Human organic anion transporter MRP4 (ABCC4) is an efflux pump for the purine end metabolite urate with multiple allosteric substrate binding sites

The end product of human purine metabolism is urate, which is produced primarily in the liver and excreted by the kidney through a well-defined basolateral blood-to-cell uptake step. However, the apical cell-to-urine efflux mechanism is as yet unidentified. Here, we show that the renal apical organic anion efflux transporter human multidrug resistance protein 4 (MRP4), but not apical MRP2, mediates ATP-dependent urate transport via a positive cooperative mechanism (K(m) of 1.5 +/- 0.3 mM, V(max) of 47 +/- 7 pmol x mg(-1) x min(-1), and Hill coefficient of 1.7 +/- 0.2). In HEK293 cells overexpressing MRP4, intracellular urate levels were lower than in control cells. Urate inhibited methotrexate transport (IC50 of 235 +/- 8 microM) by MRP4, did not affect cAMP transport, whereas cGMP transport was stimulated. Urate shifted cGMP transport by MRP4 from positive cooperativity (K(m) and V(max) value of 180 +/- 20 microM and 58 +/- 4 pmol x mg(-1) x min(-1), respectively, Hill coefficient of 1.4 +/- 0.1) to single binding site kinetics (K(m) and V(max) value of 2.2 +/- 0.9 mM and 280 +/- 50 pmol x mg(-1) x min(-1), respectively). Finally, MRP4 could transport urate simultaneously with cAMP or cGMP. We conclude that human MRP4 is a unidirectional efflux pump for urate with multiple allosteric substrate binding sites. We propose MRP4 as a candidate transporter for urinary urate excretion and suggest that MRP4 may also mediate hepatic export of urate into the circulation, because of its basolateral expression in the liver.

Multidrug resistance protein 4 (ABCC4)-mediated ATP hydrolysis: effect of transport substrates and characterization of the post-hydrolysis transition state

Multidrug resistance protein 4 (MRP4/ABCC4), transports cyclic nucleoside monophosphates, nucleoside analog drugs, chemotherapeutic agents, and prostaglandins. In this study we characterize ATP hydrolysis by human MRP4 expressed in insect cells. MRP4 hydrolyzes ATP (Km, 0.62 mm), which is inhibited by orthovanadate and beryllium fluoride. However, unlike ATPase activity of P-glycoprotein, which is equally sensitive to both inhibitors, MRP4-ATPase is more sensitive to beryllium fluoride than to orthovanadate. 8-Azido[alpha-32P]ATP binds to MRP4 (concentration for half-maximal binding approximately 3 microm) and is displaced by ATP or by its non-hydrolyzable analog AMPPNP (concentrations for half-maximal inhibition of 13.3 and 308 microm). MRP4 substrates, the prostaglandins E1 and E2, stimulate ATP hydrolysis 2- to 3-fold but do not affect the Km for ATP. Several other substrates, azidothymidine, 9-(2-phosphonylmethoxyethyl)adenine, and methotrexate do not stimulate ATP hydrolysis but inhibit prostaglandin E2-stimulated ATP hydrolysis. Although both post-hydrolysis transition states MRP4.8-azido[alpha-32P]ADP.Vi and MRP4.8-azido[alpha-32P]ADP.beryllium fluoride can be generated, nucleotide trapping is approximately 4-fold higher with beryllium fluoride. The divalent cations Mg2+ and Mn2+ support comparable levels of nucleotide binding, hydrolysis, and trapping. However, Co2+ increases 8-azido[alpha-32P]ATP binding and beryllium fluoride-induced 8-azido[alpha-32P]ADP trapping but does not support steady-state ATP hydrolysis. ADP inhibits basal and prostaglandin E2-stimulated ATP hydrolysis (concentrations for half-maximal inhibition 0.19 and 0.25 mm, respectively) and beryllium fluoride-induced 8-azido[alpha-32P]ADP trapping, whereas Pi has no effect up to 20 mm. In aggregate, our results demonstrate that MRP4 exhibits substrate-stimulated ATP hydrolysis, and we propose a kinetic scheme suggesting that ADP release from the post-hydrolysis transition state may be the rate-limiting step during the catalytic cycle.

The nucleotide transporter MRP4 (ABCC4) is highly expressed in human platelets and present in dense granules, indicating a role in mediator storage

Platelet aggregation is initiated by the release of mediators as adenosine diphosphate (ADP) stored in platelet granules. Possible candidates for transport proteins mediating accumulation of these mediators in granules include multidrug resistance protein 4 (MRP4, ABCC4), a transport pump for cyclic nucleotides and nucleotide analogs. We investigated the expression of MRP4 in human platelets by immunoblotting, detecting a strong signal at 170 kDa. Immunofluorescence microscopy using 2 MRP4-specific antibodies revealed staining mainly in intracellular structures, which largely colocalized with the accumulation of mepacrine as marker for delta-granules and to a lower extent at the plasma membrane. Furthermore, an altered distribution of MRP4 was observed in platelets from a patient with Hermansky-Pudlak syndrome with defective delta-granules. Adenosine triphosphate (ATP)-dependent cyclic guanosine monophosphate (cGMP) transport codistributed with MRP4 detection in subcellular fractions, with highest activities in the dense granule and plasma membrane fractions. This transport was inhibited by dipyramidole, indomethacin, and MK571 with median inhibitory concentration (IC(50)) values of 12, 22, and 43 microM, and by ibuprofen. Transport studies with [(3)H]ADP indicated the presence of an orthovanadate-sensitive ADP transporting system, inhibited by dipyramidole, MK571, and cyclic nucleotides. The results indicate a function of MRP4 in platelet mediator storage and inhibition of MRP4 may represent a novel mechanism for inhibition of platelet function by some anti-inflammatory drugs.

The potential impact of drug transporters on nucleoside-analog-based antiviral chemotherapy

Several ATP-binding cassette (ABC) transporters can transport drugs out of cells against steep concentration gradients resulting in resistance to the drugs transported. Recent work has shown that at least three members of the family of human Multidrug Resistance-associated Proteins (MRPs), MRP4, 5 and 8, are able to transport some nucleoside-monophosphate analogs. This can result in resistance to the base, nucleoside or nucleotide precursors of these results, at least in cell lines with high levels of transporter. The affinity of these transporters for the nucleotide analogs studied thus far is relatively low (millimolar rather than micromolar), and this limits their potential impact on the resistance. We briefly review how ABC transporters in general, and MRPs in particular, could affect the disposition and cellular accumulation of antiviral compounds.

Enhanced expression of basolateral multidrug resistance protein isoforms Mrp3 and Mrp5 in rat liver by LPS

Lipopolysaccharide (LPS) induces hepatocellular down-regulation and endocytic retrieval of multidrug resistance protein 2 (Mrp2, Abcc2). Basolateral Mrp isoforms may compensate for the intracellular metabolic changes in cholestasis. Therefore, the effect of LPS on the zonal localization of Mrp2 and Mrp3 and the expression of Mrp3, Mrp4, Mrp5, and Mrp6 mRNA were investigated in rat liver. In normal rat liver Mrp3 was found in pericentral hepatocytes also expressing glutamine synthetase. In LPS-treated rat liver the decrease in Mrp2 protein was most pronounced in pericentral hepatocytes, with only minor down-regulation in periportal hepatocytes. Conversely, induction of Mrp3 was found in pericentral hepatocytes with a low expression of Mrp2. Furthermore, we found a strong induction of Mrp5 mRNA. Likewise, Mrp6 mRNA was up-regulated, however Mrp6 protein expression was not significantly altered. It is concluded that Mrp3 is inversely regulated to Mrp2 in a zonal pattern and may compensate for the LPS-induced loss of Mrp2 in the perivenous area. Induction of pericentral Mrp3 and up-regulation of Mrp5 mRNA may play an important role in the hepatocellular clearance of cholephilic substances and cyclic nucleotides accumulating after LPS treatment.

Differential multidrug resistance-associated protein 1 through 6 isoform expression and function in human intestinal epithelial Caco-2 cells

Multidrug resistance-associated protein (MRP) isoforms 1 through 6 mRNA are expressed in the human intestine and Caco-2 cells. In Caco-2 cells, the rank order for mRNA expression was MRP2 > or = MRP6 > MRP4 > or = MRP3 > MRP1 = MRP5. The functional expression of MRP-like activity was quantified as the efflux of the fluorescent probe calcein from confluent, polarized monolayers of Caco-2 cells. Calcein efflux was sensitive to temperature, energy depletion, and the MRP antagonist MK571 [3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid]. Calcein efflux across the apical membrane of Caco-2 cells exceeded that across the basolateral by approximately 2-fold, correlating with the apical localization of MRP2 visualized by immunocytochemical staining. T84 cells do not express MRP2 and show a predominance of basolateral calcein efflux over apical efflux. MRP3 was localized by immunocytochemical staining to the basolateral membrane. MRP1 staining was not localized to either membrane domain and MRP5 staining was not detected. Thus, basolateral calcein efflux may reflect a function of MRP3 or MRP4 and 6 inferred by their basolateral localization in other tissues. Basolateral, but not apical, calcein efflux was sensitive to glutathione depletion with buthioninesulfoximine, indicating that whereas MRP2-mediated apical efflux is independent of glutathione, basolateral efflux is glutathione-dependent. Benzbromarone, probenecid, pravastatin, and diclofenac were able to inhibit both apical and basolateral calcein efflux. The apical calcein efflux in Caco-2 cells was selectively sensitive to indomethacin and propranolol, but not verapamil or erythromycin, whereas the converse was observed for basal efflux. The differential pharmacological sensitivity of apical (MRP2) and basolateral calcein efflux provides tools for dissecting MRP isoform functional roles.

Nonsteroidal anti-inflammatory drugs potentiate 1-methyl-4-phenylpyridinium (MPP+)-induced cell death by promoting the intracellular accumulation of MPP+ in PC12 cells

In this study, we investigated the effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on 1-methyl-4-phenylpyridinium (MPP(+))-induced cell death in PC12 cells. Coincubation of PC12 cells with indomethacin, ibuprofen, ketoprofen, or diclofenac, but not aspirin or N-[2-(cyclohexyloxy)-4-nitrophenyl]methanosulfonamide (NS-398), significantly potentiated the MPP(+)-induced cell death. In contrast, these NSAIDs had no effect on rotenone-induced cell death. The potentiating actions of these NSAIDs were not suppressed by treatment with phenyl-N-butyl-nitrone, a radical scavenger; N-acetyl-l-cysteine, an antioxidant; Ac-DEVD-CHO, a selective caspase-3 inhibitor; or 2-chloro-5-nitro-N-phenylbenzamide (GW9662), a selective antagonist of peroxisome proliferator-activated receptor gamma. Furthermore, we observed that DNA fragmentation, which is one of the hallmarks of apoptosis, was not induced by coincubation with MPP(+) and NSAIDs. We confirmed that coincubation of PC12 cells with 30 microM MPP(+) and 100 microM indomethacin, ibuprofen, ketoprofen, or diclofenac led to a significant increase in the accumulation of intracellular MPP(+) compared with incubation with 30 microM MPP(+) alone. In addition, these NSAIDs markedly reduced the efflux of MPP(+) from PC12 cells. (3-(3-(2-(7-Chloro-2-quinolinyl) ethenyl) phenyl ((3-dimethyl amino-3oxo-propyl) thio) methyl) propanoic acid (MK 571), which is an inhibitor of multidrug resistance proteins (MRPs), mimicked the NSAIDs-induced effects, increasing cell toxicity and promoting the accumulation of MPP(+). Moreover, some types of MRPs' mRNA were detected in PC12 cells. These results suggest that some NSAIDs might cause a significant increase in the intracellular accumulation of MPP(+) via the suppression of reverse transport by the blockade of MRP, resulting in the potentiation of MPP(+)-induced cell death.

Rat multidrug resistance protein 4 (Mrp4, Abcc4): molecular cloning, organ distribution, postnatal renal expression, and chemical inducibility

In the present study, we report cloning of the rat Mrp4 cDNA. The cDNA is 4526 bp, containing a 3975 bp open reading frame. The deduced polypeptide has 1325 amino acids and is 83% and 91% identical to human MRP4 and mouse Mrp4, respectively. Phylogenetic analysis revealed that the cloned rat cDNA is closely related to human MRP4 and mouse Mrp4. Additionally, an alternatively spliced variant, 111 bp shorter than the full-length form, was cloned. Rat Mrp4 mRNA was detectable in 11 tissues examined, with levels being highest in kidney, and lowest in liver. Mrp4 mRNA levels in kidney were higher in males than females, and at birth were about half of adult levels. Mrp4 expression in liver and kidney of rats treated with six classes of microsomal enzyme inducers was examined. Mrp4 mRNA in liver was induced by two electrophile response element activators, namely ethoxyquin and oltipraz.

Quantitative Evaluation of the Drug-Drug Interactions between Methotrexate and Nonsteroidal Anti-Inflammatory Drugs in the Renal Uptake Process Based on the Contribution of Organic Anion Transporters and Reduced Folate Carrier

The present study examined the possible role of transporters in the drug-drug interactions between methotrexate (MTX) and nonsteroidal anti-inflammatory drugs (NSAIDs) in the renal uptake process of MTX. MTX is recognized by reduced folate carrier (RFC-1) and rat organic anion transporters (rOat1 and rOat3) as a substrate. Uptake of MTX by kidney slices was saturable and inhibited potently by dibromosulfophthalein. Folate and benzylpenicillin (PCG) inhibited the uptake by 30 to 40% and 40 to 50% of the total saturable uptake of MTX by kidney slices, respectively, whereas the effect of p-aminohippurate (PAH) was minimal at the concentration selective for rOat1. In contrast, the uptake of 5-methyltetrahydrofolate by the kidney slices was inhibited by MTX, folate, and dibromosulfophthalein, but not by PAH and PCG. These results suggest that rOat3 and RFC-1 are almost equally involved in the uptake of MTX by the kidney slices, whereas RFC-1 is responsible for the renal uptake of 5-methyltetrahydrofolate. NSAIDs, except salicylate, were potent inhibitors of rOat3 (K(i) of 1.3-19 microM), but weak inhibitors of RFC-1 (K(i) of 70-310 microM). This is in a good agreement with the biphasic inhibition profiles of NSAIDs for the uptake of MTX by kidney slices. These results suggest that the renal uptake of MTX is not so greatly affected by NSAIDs as expected from the inhibition of rOat3-mediated transport.

The ABCs of drug transport in intestine and liver: efflux proteins limiting drug absorption and bioavailability

Many orally administered drugs must overcome several barriers before reaching their target site. The first major obstacle to cross is the intestinal epithelium. Although lipophilic compounds may readily diffuse across the apical plasma membrane, their subsequent passage across the basolateral membrane and into blood is by no means guaranteed. Efflux proteins located at the apical membrane, which include P-glycoprotein (Pgp; MDR1) and MRP2, may drive compounds from inside the cell back into the intestinal lumen, preventing their absorption into blood. Drugs may also be modified by intracellular phase I and phase II metabolising enzymes. This process may not only render the drug ineffective, but it may also produce metabolites that are themselves substrates for Pgp and/or MRP2. Drugs that reach the blood are then passed to the liver, where they are subject to further metabolism and biliary excretion, often by a similar system of ATP-binding cassette (ABC) transporters and enzymes to that present in the intestine. Thus a synergistic relationship exists between intestinal drug metabolising enzymes and apical efflux transporters, a partnership that proves to be a critical determinant of oral bioavailability. The effectiveness of this system is optimised through dynamic regulation of transporter and enzyme expression; tissues have a remarkable capacity to regulate the amounts of protein both at transcriptional and post-transcriptional levels in order to maintain homeostasis. This review addresses the progress to date on what is known about the role and regulation of drug efflux mechanisms in the intestine and liver.

Modulation of β-amyloid metabolism by non-steroidal anti-inflammatory drugs in neuronal cell cultures

Alzheimer disease (AD) is characterized by cerebral deposits of beta-amyloid (Abeta) peptides, which are surrounded by neuroinflammatory cells. Epidemiological studies have shown that prolonged use of non-steroidal anti-inflammatory drugs (NSAIDs) reduces the risk of developing AD. In addition, biological data indicate that certain NSAIDs specifically lower Abeta42 levels in cultures of peripheral cells independently of cyclooxygenase (COX) activity and reduce cerebral Abeta levels in AD transgenic mice. Whether other NSAIDs, including COX-selective compounds, modulate Abeta levels in neuronal cells remains unexploited. Here, we investigated the effects of compounds from every chemical class of NSAIDs on Abeta40 and Abeta42 secretion using both Neuro-2a cells and rat primary cortical neurons. Among non-selective NSAIDs, flurbiprofen and sulindac sulfide concentration-dependently reduced the secretion not only of Abeta42 but also of Abeta40. Surprisingly, both COX-2 (celecoxib; sc-125) or COX-1 (sc-560) selective compounds significantly increased Abeta42 secretion, and either did not alter (sc-560; sc-125) or reduced (celecoxib) Abeta40 levels. The levels of betaAPP C-terminal fragments and Notch cleavage were not altered by any of the NSAIDs, indicating that gamma-secretase activity was not overall changed by these drugs. The present findings show that only a few non-selective NSAIDs possess Abeta-lowering properties and therefore have a profile potentially relevant to their clinical use in AD.