Tissue distribution and induction of the rat multidrug resistance-associated proteins 5 and 6

Multidrug resistance-associated proteins (Mrps) are ATP-dependent transporters which transport a wide variety of anionic and cationic compounds. The purpose of this study was to determine the tissue distribution of Mrp5 and 6 in male and female Sprague-Dawley rats in various tissues, and to investigate whether the expression is altered by cholestasis or administration of microsomal enzyme inducers (MEIs). These MEIs activate six different transcriptionally-mediated pathways, and their effects on Mrp5 and Mrp6 expression were determined. The effects of bile-duct ligation, a cholestasis model, on Mrp5 and 6 expression in male rats were quantified. Mrp5 had marked expression in adrenal gland, and moderate expression in cerebral cortex, cerebellum, and stomach. The MEIs polychlorinated biphenyl (PCB)126, phenobarbital, and PCB99 slightly repressed Mrp5, but no single class of receptor agonists induced or repressed Mrp5. Bile-duct ligation tended to increase Mrp5 expression, but was not statistically significant at a 3 day timepoint. Mrp6 expression was highest in intestine, liver, and kidney. Mrp6 was slightly repressed by phenobarbital, dexamethasone, and isoniazid, but no one class of receptor agonists induced or repressed Mrp6, and expression was also unchanged bile-duct ligation. In conclusion, Mrp5 in rats is most highly expressed in the adrenal gland, whereas Mrp6 is mainly expressed in excretory organs (liver, intestine, and kidney), suggesting markedly different functions. Hepatic mRNA levels of Mrp5 or Mrp6 do not seem to be coordinately regulated along with Phase I enzymes via receptor-mediated pathways, and are not part of the hepatoprotective upregulation of basolateral transporters that occurs during cholestasis.

Alterations in transporter expression in liver, kidney, and duodenum after targeted disruption of the transcription factor HNF1alpha

The transcription factor hepatocyte nuclear factor 1alpha (HNF1alpha) is involved in regulation of glucose metabolism and transport, and in the expression of several drug and bile acid metabolizing enzymes. Targeted disruption of the HNF1alpha gene results in decreased Cyp1a2, and Cyp2e1 expression, and increased Cyp4a1 and Cyp7a1 expression, suggesting these enzymes are HNF1alpha target genes. Since hepatic metabolism can be coordinately linked with drug and metabolite transport, this study aims to demonstrate whether HNF1alpha regulates expression of a variety of organic anion and cation transporters through utilization of an HNF1alpha-null mouse model. Expression of 32 transporters, including members of the Oat, Oatp, Oct, Mrp, Mdr, bile acid and sterolin families, was quantified in three different tissues: liver, kidney, and duodenum. The expression of 17 of 32 transporters was altered in liver, 21 of 32 in kidney, and 6 of 32 in duodenum of HNF1alpha-null mice. This includes many novel observations, including marked downregulation of Oats in kidney, as well as upregulation of many Mrp and Mdr family members in all three tissues. These data indicate that disruption of HNF1alpha causes a marked attenuation of several Oat and Oatp uptake transporters in liver and kidney, and increased expression of efflux transporters such as Mdrs and Mrps, thus suggesting that HNF1alpha is a central mediator in regulating hepatic, renal, and intestinal transporters.

Induction of Hepatic Transporters Multidrug Resistance-Associated Proteins (Mrp) 3 and 4 by Clofibrate Is Regulated by Peroxisome Proliferator-Activated Receptor α

Hepatic transporters play a vital role in the disposition of endogenous compounds and xenobiotics in the liver. The current study investigates the expression and regulation of hepatic efflux transporters in response to treatment with the peroxisome proliferator-activated receptor (PPAR)alpha agonist clofibrate (CFB). Changes in mRNA and protein levels for several hepatic transporters were assessed in male CD-1 mice after 10 days of CFB dosing (500 mg/kg i.p.). Administration of CFB up-regulated mRNA levels for breast cancer resistance protein (Bcrp) and multidrug resistance-associated proteins 3 and 4 (Mrp3 and Mrp4, respectively). Western blot analysis confirmed that CFB enhances protein expression of liver Bcrp, Mrp3, and Mrp4 in CD-1 mice. To further characterize the regulation of these hepatic transporters, CFB-mediated changes in transporter mRNA levels were assessed in wild-type (sv/129) and PPARalpha-null male mice. Wild-type mice treated with CFB showed similar changes in mRNA levels for all of these transporters, whereas the PPARalpha-null mice did not. Although protein expression of Mrp3 and Mrp4 in the wild-type mice correlated well with changes in mRNA levels, Bcrp protein was not up-regulated by CFB treatment. These results show that PPARalpha activation by CFB coordinately regulates the hepatic efflux transporters Mrp3 and Mrp4. Induction of Mrp3 and Mrp4 by CFB may alter the disposition of toxicants and xenobiotics that are substrates for these transporters.

trans-Stilbene oxide induces expression of genes involved in metabolism and transport in mouse liver via CAR and Nrf2 transcription factors

trans-Stilbene oxide (TSO) induces drug metabolizing enzymes in rat and mouse liver. TSO is considered a phenobarbital-like compound because it induces Cyp2B mRNA expression in liver. Phenobarbital increases Cyp2B expression in liver via activation of the constitutive androstane receptor (CAR). The purpose of this study was to determine whether TSO induces gene expression in mouse liver via CAR activation. TSO increased CAR nuclear localization in mouse liver, activated the human Cyp2B6 promoter in liver in vivo, and activated a reporter plasmid that contains five nuclear receptor 1 (NR1) binding sites in HepG2 cells. TSO administration increased expression of Cyp2b10, NAD(P)H:quinone oxidoreductase (Nqo1), epoxide hydrolase, heme oxygenase-1, UDP-glucuronosyl-transferase (Ugt) 1a6 and 2b5, and multidrug resistance-associated proteins (Mrp) 2 and 3 mRNA in livers from male mice. Cyp2b10 and epoxide hydrolase induction by TSO was decreased in livers from CAR-null mice, compared with wild-type mice, suggesting CAR involvement. In contrast, TSO administration induced Nqo1 and Mrp3 mRNA expression equally in livers from wild-type and CAR-null mice, suggesting that TSO induces expression of some genes through a mechanism independent of CAR. TSO increased nuclear staining of the transcription factor Nrf2 in liver, and activated an antioxidant/electrophile response element luciferase reporter construct that was transfected into HepG2 cells. In summary, in mice, TSO increases Cyp2b10 and epoxide hydrolase expression in mice via CAR, and potentially induces Nqo1 and Mrp3 expression via Nrf2. Moreover, our data demonstrate that a single compound can activate both CAR and Nrf2 transcription factors in liver.

Tissue distribution and ontogeny of organic cation transporters in mice

Organic cation transporters (Octs) play an important role in transporting cationic xeno- and endobiotics across biological membranes. Little is known about Octs in mice; therefore, the tissue distribution and developmental changes in the mRNA expression of Octs in mice were quantified. Oct1, Oct2, Oct3, Octn1, Octn2, and Octn3 mRNA expression was quantified in 14 tissues from male and female mice using the branched DNA signal amplification assay. Oct1 mRNA expression was highest in kidney, followed by liver. Oct2 mRNA was almost exclusively expressed in kidney, with male mice having twice that in female mice. The higher expression of Oct2 in male mice is due to testosterone. Oct3 mRNA was most highly expressed in placenta, ovary, and uterus, but was expressed at low levels in most tissues. Octn1 and Octn2 mRNA expression was similar, with the highest levels in kidney followed by small intestine. Octn3 mRNA was almost exclusively expressed in testes. The developmental expression of Oct1, Oct2, Octn1, and Octn2 mRNA in kidneys as well as Oct1 in liver was determined in young mice. Ontogenic expression data indicate that each of the Octs approached adult expression levels by about 3 weeks of age. The gender difference in Oct mRNA expression did not become apparent until day 30 after birth. The differences in tissue distribution of the Octs may play an important role in drug disposition to various tissues. Furthermore, low expression of the Octs in young animals may affect the pharmacokinetic behavior of drugs compared with that in adults.

Acquired cadmium resistance in metallothionein-I/II(-/-) knockout cells: role of the T-type calcium channel Cacnalpha1G in cadmium uptake

Metallothioneins (MTs) are cytoplasmic proteins that sequester certain divalent cations and are considered a primary cellular defense against the toxic transition metal cadmium (Cd(2+)). MT-I/II(-/-) knockout [MT(-/-)] cells are available and serve as an excellent tool to study non-MT-related mechanisms in metal tolerance. In the current study, Cd(2+)-resistant MT(-/-) (CdR) and CdR revertant (CdR-rev) cell lines were developed and characterized to investigate non-MT-mediated cellular protection mechanisms. Resistance to Cd(2+) was approximately 70-fold higher in CdR than the parental MT(-/-) cell line (IC(50) = 20 versus 0.3 microM, respectively) and was stable in the absence of Cd(2+) for 35 days. Accumulation of Cd(2+) by the CdR cell line was reduced by approximately 95% compared with parental cells, primarily because of a decreased Cd(2+) uptake. Cd(2+) uptake by the MT(-/-) parental cell line was independent of sodium, energy, and electrogenic potential. Uptake was saturable (K(m) = 65 nM; V(max) = 4.9 pmol/mg/min) and pH-dependent (maximal at pH 6.5-7). Potent inhibitors of Cd(2+) uptake included Zn(2+) (IC(50) = 7 microM), Mn(2+) (IC(50) = 0.4 microM), and the T-type Ca(2+) channel antagonist mibefradil (IC(50) = 5 microM), whereas other metals (including Fe(2+)) and L-type Ca(2+) channel antagonists had little effect. Immunoblot and real-time reverse transcription-polymerase chain reaction analysis indicated that the Cacnalpha(1G) T-type Ca(2+) channel was expressed at a reduced level in CdR compared with the parental MT(-/-) cell line, suggesting it is important for Cd(2+) uptake. The CdR1-rev cell line was found to have a Cd(2+) uptake and sensitivity level in between that of the CdR1 and MT(-/-) cell lines. Consistent with this was an intermediate expression of Cacnalpha(1G) in the CdR-rev cell line. These data suggest that decreased expression of Cacnalpha(1G) protects cells from Cd(2+) exposure by limiting Cd(2+) uptake.

Up-regulation of Mrp4 expression in kidney of Mrp2-deficient TR- rats

Multidrug resistance-associated proteins (Mrps) are a group of ATP-dependent efflux transporters for organic anions. Mrp2 and Mrp4 are co-localized to the apical (brush-border) membrane domain of renal proximal tubules, where they may function together in the urinary excretion of organic anions. Previous reports showed that urinary excretion of some organic anions is not impaired in transport-deficient (TR-) rats, which lack Mrp2, suggesting that up-regulation of other transporter(s) may compensate for the loss of Mrp2 function. The purpose of this study was to determine whether Mrp4 expression in kidney is altered in TR- rats. Mrp4 mRNA expression was quantified using the high-throughput branched DNA signal amplification assay. Mrp4 protein expression was determined by Western blot and immunohistochemical analysis. Mrp4 mRNA in kidney of TR- rats was 100% higher than normal Wistar rats. Western blot analysis showed a 200% increase in Mrp4 protein expression in kidney of the mutant rats compared to normal rats. Immunohistochemical analysis of Mrp4 protein demonstrated apical localization of Mrp4 on renal proximal tubules, and that the immunoreactivity was more intense in kidney sections from TR- rats than those from normal rats. In summary, the results of the present study demonstrate that renal Mrp4 expression is up-regulated in TR- rats, which may explain why urinary excretion of some organic anions remains normal in the mutant rats.

Regulation of hepatic transporters by xenobiotic receptors

Chemicals that increase expression of phase-I and -II biotransformation enzymes in liver, as well as enhance hepatic uptake and biliary excretion are often referred to as microsomal enzyme inducers (MEIs). Early studies suggested that drug metabolism might be coordinately regulated along with drug efflux from hepatocytes as a means for the liver to rid itself of foreign chemicals. Since then, the identification and characterization of nuclear receptors (NRs) has aided in understanding of how various MEIs enhance xeniobiotic uptake, biotransformation, and excretion. In addition, the NRs by which several classes of MEIs induce phase-I and -II drug metabolizing enzymes have been elucidated (i.e. AHR, CAR, PXR, PPARalpha, Nrf2). Several transporter families which mediate uptake of chemicals into liver and excretion of chemicals from liver into blood and/or bile have been cloned and identified. In general, the organic anion transporting polypeptide family (Oatps) along with Organic cation transporter 1 (Oct1) and Organic anion transporter 2 mediate uptake of a large number of xenobiotics from blood into liver. Conversely, Multidrug resistance proteins (Mdrs), Multidrug resistance-associated proteins (Mrps), and Breast cancer resistance protein (Bcrp) mediate efflux of xenobiotics from liver into bile or blood. Recent studies have demonstrated that MEIs increase expression of various Oatps, Mrps, and Mdrs in liver, and some occur via activation of nuclear receptors.

Retinoid X receptor alpha Regulates the expression of glutathione s-transferase genes and modulates acetaminophen-glutathione conjugation in mouse liver

Nuclear receptors, including constitutive androstane receptor, pregnane X receptor, and retinoid X receptor (RXR), modulate acetaminophen (APAP)-induced hepatotoxicity by regulating the expression of phase I cytochrome P450 (P450) genes. It has not been fully resolved, however, whether they regulate APAP detoxification at the phase II level. The aim of the current study was to evaluate the role of RXRalpha in phase II enzyme-mediated detoxification of APAP. Wild-type and hepatocyte-specific RXRalpha knockout mice were treated with a toxic dose of APAP (500 mg/kg i.p.). Mutant mice were protected from APAP-induced hepatotoxicity, even though basal liver glutathione (GSH) levels were significantly lower in mutant mice compared with those of wild-type mice. High-performance liquid chromatography analysis of APAP metabolites revealed significantly greater levels of APAP-GSH conjugates in livers and bile of mutant mice compared with those of wild-type mice. Furthermore, hepatocyte RXRalpha deficiency altered the gene expression profile of the glutathione S-transferase (Gst) family. Basal expression of 13 of 15 Gst genes studied was altered in hepatocyte-specific RXRalpha-deficient mice. This probably led to enhanced APAP-GSH conjugation and reduced accumulation of N-acetyl-p-benzoquinone imine, a toxic electrophile that is produced by biotransformation of APAP by phase I P450 enzymes. In conclusion, the data presented in this study define an RXRalpha-Gst regulatory network that controls APAP-GSH conjugation. This report reveals a potential novel strategy to enhance the detoxification of APAP or other xenobiotics by manipulating Gst activity through RXRalpha-mediated pathways.

Regulation of mouse organic anion-transporting polypeptides (Oatps) in liver by prototypical microsomal enzyme inducers that activate distinct transcription factor pathways

Drug-metabolizing enzymes and transporters are key factors that affect disposition of xenobiotics. Phase I enzyme induction by classes of microsomal enzyme inducers occurs via activation of transcription factors such as aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor alpha (PPARalpha), and nuclear factor erythroid 2-related factor 2 (Nrf2). However, regulation of organic anion-transporting polypeptide (Oatp) uptake transporters by these factors is poorly understood. Hepatic Oatp uptake of some chemicals must occur prior to biotransformation; thus, we hypothesize that expression of Oatps and biotransformation enzymes is coordinately regulated in liver. In the present study, the effects of known chemical activators of AhR, CAR, PXR, PPARalpha, and Nrf2 on the hepatic mRNA expression of mouse Oatps and drug-metabolizing enzymes were quantified by the branched DNA assay. All chemicals increased the expression of their well characterized target drug-metabolizing enzymes: AhR ligands increased Cyp1A1, CAR activators increased Cyp2B10, PXR ligands increased Cyp3A11, PPARalpha ligands increased Cyp4A14, and Nrf2 activators induced NAD(P)H:quinone oxidoreductase 1. AhR ligands (2,3,7,8-tetrachlorodibenzo-p-dioxin, polychlorinated biphenyl 126, and beta-naphthoflavone) increased Oatp2b1 and 3a1 mRNA expression in liver. CAR activators [phenobarbital, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, and diallyl sulfide] decreased Oatp1a1 mRNA expression. Two PXR ligands [pregnenolone-16alpha-carbonitrile (PCN) and spironolactone] increased Oatp1a4 mRNA expression in liver, whereas PXR ligands (PCN, spironolactone, and dexamethasone) and PPARalpha ligands (clofibrate, ciprofibrate, and diethylhexylphthalate) decreased Oatp1a1, 1b2, 2a1, and 2b1 mRNA expression in liver. Nrf2 activators (oltipraz, ethoxyquin, and butylated hydroxyanisole) down-regulated Oatp1a1 but up-regulated Oatp2b1 mRNA expression. Therefore, only a few transcription factor activators increased Oatp expression, and, surprisingly, many decreased Oatp expression.

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.

Rat and mouse differences in gender-predominant expression of organic anion transporter (Oat1-3; Slc22a6-8) mRNA levels

Organic anion transporters (Oats) mediate the initial step of active renal excretion, specifically substrate uptake into proximal tubule cells. Despite extensive characterization of rat Oats, mouse Oat expression patterns are virtually unknown. This study was designed to identify basal expression patterns of mouse Oat1 (Slc22a6), Oat2 (Slc22a7), and Oat3 (Slc22a8) mRNA, compare these patterns with those in rat, and characterize postnatal development of mouse Oat mRNA. Tissues were collected from adult male and female 129J and C57BL/6 mice, and male and female C57BL/6 mice 0 to 40 days of age. Oat mRNA levels were determined by branched DNA signal amplification. Mouse Oat1 mRNA was primarily expressed in kidney of both strains, with male predominance. Mouse Oat2 mRNA levels were highest in kidney of both strains without gender predominance. In both strains, Oat3 mRNA was highest in kidney, and liver expression was male-predominant. However, only 129J mice had higher Oat3 mRNA levels in female kidney than in male kidney. During postnatal development, both Oat1 and Oat2 mRNA levels began to rise after 25 days of age. Oat3 mRNA levels rose gradually from birth through 40 days of age. Oat2 mRNA increased 30-fold during the first 40 days, whereas Oat1 and Oat3 increased about 2-fold. The most notable species differences in Oat mRNA expression were a lack of Oat2 female predominance in mouse kidney and a less dramatic Oat3 male predominance in mouse liver. With the exception of a significant species difference in Oat2 expression, many similarities were found between rat and mouse Oat mRNA levels.

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.

Tissue distribution and hepatic and renal ontogeny of the multidrug resistance-associated protein (Mrp) family in mice

Analysis of the mouse genome has revealed eight multidrug resistance-associated (Mrp) transporters, with mouse homologs for all human MRPs except MRP8. Whereas MRP expression in tissues of humans and rats has been examined, no characterization exists for mice. Furthermore, the ontogeny of mouse Mrps is unknown, and such knowledge may be helpful in understanding age-related pharmacokinetics. Therefore, the purpose of this study was to quantitatively determine 1) expression of the Mrp family in 12 different tissues, 2) gender variations in Mrp expression in liver and kidney, and 3) whether Mrp expression is altered during development. Highest expression of the Mrp family members is as follows: Mrp1 in testes, ovary, and placenta; Mrp2 in intestine, followed by liver and kidney; Mrp3 in large intestine; Mrp4 in kidney; Mrp5 in brain, followed by lung and stomach; Mrp6 in liver; Mrp7 in testes, intestine, and kidney; and Mrp9 solely in testes. Gender differences in Mrp expression were observed: Mrp1, 3, and 4 in kidney, as well as Mrp1 and 4 in liver were female-predominant. Ontogeny of the four Mrps expressed in liver was as follows: Mrp2 and Mrp4 were expressed at adult levels at birth; Mrp3 reached adult levels at day 30, and Mrp6 was not expressed until day 10. In kidney, Mrp1 and Mrp5 were expressed at adult levels at birth, whereas Mrp2, 3, 4, and 6 generally increased over time. In conclusion, marked differences in expression of the individual Mrp family members exist in various tissues, with age, and with gender.

Lipopolysaccharide-mediated regulation of hepatic transporter mRNA levels in rats

The function of hepatic transporters is to move organic substances across sinusoidal and canalicular membranes. During extrahepatic cholestasis, transporters involved in the movement of substances from blood to bile, such as sodium/taurocholate-cotransporting polypeptide (Ntcp) and multidrug resistance protein 2 (Mrp2), are down-regulated, whereas others that transport chemicals from liver to blood, such as Mrp3, are up-regulated. Unlike extrahepatic cholestasis, where transporter expression responds to the stress of accumulating bile constituents, lipopolysaccharide (LPS)-induced intrahepatic cholestasis may be directly caused by alterations in transporter expression. The aim of this study was to quantitatively determine the effect of LPS on transporter expression and study the mechanism(s) by which LPS alters mRNA levels of major hepatic transporters in Sprague-Dawley rats. Hepatic mRNA levels of Mrp2, Mrp6, multiple drug resistance protein 1a (Mdr1a), organic anion-transporting polypeptide 1 (Oatp1), Oatp2, Oatp4, Ntcp, bile salt export pump, organic cation transporter 1 (Oct1), and organic anion transporter 3 (Oat3) were dramatically decreased, beginning approximately 6 h after LPS administration, whereas Mrp5 and Oat2 levels were unchanged. In contrast, LPS increased mRNA levels of Mrp1, Mrp3, and Mdr1b concurrently with the down-regulated transporters. Pretreatment with dexamethasone, which decreases the release of cytokines, reversed the reduction of Mdr1a, Oatp1, Oatp2, Oct1, and Ntcp mRNA following LPS administration. Furthermore, dexamethasone pretreatment also prevented the LPS-mediated increase in Mrp1, Mrp3, and Mdr1b, whereas pretreatment with aminoguanidine or gadolinium chloride, an inhibitor of inducible nitric oxide synthetase and a Kupffer cell toxicant, respectively, had no effect on the LPS-induced changes. The concurrent repression and induction of various transporters, as well as dexamethasone abatement of both LPS-mediated repression and induction, indicates that these responses may be mediated through similar pathways.

Expression and regulation of the sterol half-transporter genes ABCG5 and ABCG8 in rats

The ABCG5 and ABCG8 genes encode half-transporter proteins that heterodimerize to form a transporter of plant sterols and cholesterol. The purpose of this study was to examine the expression and regulation of ABCG5 and ABCG8 at the mRNA level in Sprague-Dawley rats. Both ABCG5 and ABCG8 mRNA were expressed primarily in rat small intestine and liver, and gender-specific differences in expression were observed. The effects of treatment with a battery of microsomal enzyme inducers on ABCG5 and ABCG8 mRNA were examined; most treatments had no effect, but of three PXR ligands, PCN was an effective inducer, spironolactone was repressive, and dexamethasone was ineffective. The effects of a 1% cholesterol diet on the regulation of rat ABCG5 and ABCG8 were also examined, and compared with those in C57BL/6 mice. Cholesterol caused a suppression of ABCG5 and ABCG8 mRNA in rat liver, but the same treatment increased the expression of these genes in mouse liver. ABCG5 and ABCG8 mRNA was also induced by cholesterol in rat ileum, but not mouse ileum. These results suggest variation between rats and mice in regulatory mechanisms controlling ABCG5 and ABCG8 expression, and may explain some differences in lipid metabolism observed between these two species.

Tissue distribution and hormonal regulation of the breast cancer resistance protein (Bcrp/Abcg2) in rats and mice

Breast cancer resistance protein (Bcrp/Abcg2) is a member of the ABC transporter family. The purpose of this study was to quantify Bcrp mRNA in rat and mouse tissues, and to determine whether there are gender differences in Bcrp mRNA expression. Rat Bcrp mRNA levels were high in intestine and male kidney, and intermediate in testes. Mouse Bcrp expression was highest in kidney, followed by liver, ileum, and testes. Male-predominant expression of Bcrp was observed in rat kidney and mouse liver. Furthermore, gonadectomy and hypophysectomy experiments were conducted to determine whether sex steroids and/or growth hormone are responsible for Bcrp gender-divergent expression patterns. Male-predominant expression of Bcrp in rat kidney appears to be due to the suppressive effect of estradiol, and male-predominant expression of Bcrp in mouse liver appears to be due to the inductive effect of testosterone.

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.

Dietary iron regulates intestinal cadmium absorption through iron transporters in rats

The intestinal absorption of cadmium (Cd) is influenced by body iron (Fe) status in laboratory animals and humans. In this study we investigated the role of the apical Fe transporter divalent metal transporter 1 (DMT1) and the basolateral Fe exporter metal transporter protein 1 (MTP1) in Cd absorption. Rats were divided into the following groups; an Fe-sufficient (FeS) control group that was fed an FeS diet for 4 weeks (FeS, 4 weeks); an Fe-deficient (FeD) group that was fed an FeD diet for 4 weeks (FeD, 4 weeks); an FeS control group that was fed an FeS diet for 8 weeks (FeS, 8 weeks); an FeD/FeS group that was fed an FeD diet for 4 weeks and then an FeS diet for the following 4 weeks (FeD/FeS, 4 weeks/4 weeks); and an FeD group that was fed an FeD diet for 8 weeks (FeD, 8 weeks). After the 4- and 8-week feeding periods, rats were given a single oral gavage of Cd and were sacrificed 24 h later. The FeD (4 weeks) group developed Fe deficient anemia, but the parameters returned to control levels in the FeD/FeS group (4 weeks/4 weeks). The Cd body burden was greater in FeD (4 weeks) rats compared to FeS control (4 weeks), but returned to control Cd levels in FeD/FeS (4 weeks/4 weeks) rats. In addition, the expression of DMT1 and MTP1 was induced by Fe deficiency in the duodenum of FeD (4 weeks) rats, but was down-regulated to control values in FeD/FeS (4 weeks/4 weeks) rats. The correlation between duodenal DMT1 and MTP1 expression and Cd body burden in rats suggests an important role of DMT1 and MTP1 in Cd absorption.

Lipopolysaccharide-Induced Down-Regulation of Organic Anion Transporting Polypeptide 4 (Oatp4; Slc21a10) Is Independent of Tumor Necrosis Factor- , Interleukin-1 , Interleukin-6, or Inducible Nitric Oxide Synthase

Organic anion transporting polypeptide 4 (Oatp4; Slc21a10) is expressed almost exclusively in liver, where it mediates uptake of a variety of compounds, including bile acids, as well as other endo- and xenobiotics, across hepatic sinusoidal membranes in a Na+-independent manner. Lipopolysaccharide (LPS) has been shown to decrease Oatp4 mRNA levels in a dose- and time-dependent manner in Toll-like receptor 4 (TLR4)-normal (C3H/OuJ) mice, but not in TLR4-mutant (C3H/HeJ) mice. Moreover, after LPS administration, serum concentrations of tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and interleukin-6 (IL-6) are markedly lower in TLR4-mutant mice than in TLR4-normal mice. Thus, TLR4 is considered an upstream mediator of LPS-induced decrease in mouse Oatp4 mRNA. LPS is thought to alter liver gene expression through LPS-induced cytokines or nitric oxide (NO). TNF receptor p55 (TNFRp55) and type I IL-1 receptor (IL-1RI) mediate the biological functions of TNF-alpha and IL-1beta, respectively. Therefore, to determine whether endogenous cytokines or NO are mediators of LPS-induced down-regulation of Oatp4, Oatp4 mRNA levels were determined in mice deficient in the TNFRp55, IL-1RI, IL-6, or inducible nitric oxide synthase (iNOS) after LPS administration. Mice homozygous for a targeted deletion of genes for TNFRp55, IL-1RI, IL-6, or iNOS exhibited similar decreases in Oatp4 mRNA levels as wild-type mice after LPS administration. Moreover, in mouse hepatoma cells, treatment with TNF-alpha, IL-1beta, or IL-6 individually or in combination did not suppress activity of mouse Oatp4 promoter (-4.8 kb to +30). Therefore, LPS-induced down-regulation of Oatp4 appears to be independent of TNF-alpha, IL-1beta, IL-6, or iNOS.

Promotion of Thyroid Tumors in Rats by Pregnenolone-16 -Carbonitrile (PCN) and Polychlorinated Biphenyl (PCB)

Pregnenolone-16alpha-carbonitrile (PCN) and Aroclor 1254 (PCB) both reduce serum thyroid hormone levels in rats, but only PCN consistently produces an increase in serum thyrotropin (TSH). PCN-mediated increases in TSH result in increased thyroid follicular cell proliferation and hyperplasia, which may represent early events on a morphological continuum leading to neoplasia. The purpose of this study was to assess whether PCN, a compound that increases serum TSH, and PCB, which does not increase TSH, promote thyroid tumors in a two-stage carcinogenesis model. Male SD rats were administered the thyroid tumor initiator diisopropanolnitrosamine (2.5 g/kg, sc), and after seven days were fed control diet, diet containing 1000 ppm PCN, or diet containing 100 ppm PCB for 19 weeks. Body weights were unaffected by PCN treatment, but were reduced 21% after 19 weeks of PCB treatment compared to control. PCN treatment significantly reduced serum T4 through week 3 before returning to control concentrations, whereas T4 levels following PCB treatment fell below detection limits by week 3 and remained drastically reduced through week 19. TSH concentrations in PCN-treated rats increased three-fold at week 2, then declined to near control values at week 19. After one week of PCB treatment, TSH concentrations reached nearly twice that of controls, and were sustained until week 6. The incidence of thyroid follicular cell proliferative lesions, including cystic and follicular hyperplasia, cystic and follicular adenoma, and follicular carcinoma, was significantly increased following PCN treatment, but not following PCB treatment. PCB treatment caused an increase in thyroid carcinomas (4 of 22 rats) not associated with the proliferative-type lesions produced by PCN, despite an increase in TSH serum concentrations. In conclusion, PCN appears to promote thyroid tumors in a manner consistent with known effects of excessive TSH stimulation. However, thyroid carcinomas stemming from PCB treatment indicate that separate mechanisms exist for the production of thyroid cancer in rodents by chemicals classically considered microsomal enzyme inducers.

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.

Bromobenzene-induced hepatotoxicity at the transcriptome level

Rats were exposed to three levels of bromobenzene, sampled at 6, 24, and 48 h, and liver gene expression profiles were determined to identify dose and time-related changes. Expression of many genes changed transiently, and dependent on the dose. Few changes were identified after 6 h, but many genes were differentially expressed after 24 h, while after 48 h, only the high dose elicited large effects. Differentially expressed genes were involved in drug metabolism (upregulated GSTs, mEH, NQO1, Mrps, downregulated CYPs, sulfotransferases), oxidative stress (induced HO-1, peroxiredoxin, ferritin), GSH depletion (induced GCS-l, GSTA, GSTM) the acute phase response, and in processes like cholesterol, fatty acid and protein metabolism, and intracellular signaling. Trancriptional regulation via the electrophile and sterol response elements seemed to mediate part of the response to bromobenzene. Recovery of the liver was suggested in response to BB by the altered expression of genes involved in protein synthesis and cytoskeleton rearrangement. Furthermore, after 48 h, rats in the mid dose group showed no toxicity, and gene expression patterns resembled the normal situation. For certain genes (e.g., CYP4A, metallothioneins), intraday variation in expression levels was found, regardless of the treatment. Selected cDNA microarray measurements were confirmed using the specific and sensitive branched DNA signal amplification assay.

Toxicokinetic and genomic analysis of chronic arsenic exposure in multidrug-resistance mdr1a/1b(-/-) double knockout mice

Multidrug-resistance gene knockout mdr1a/1b(-/-) mice, which are deficient in P-glycoproteins, are more sensitive than wild-type (WT) mice to acute arsenic toxicity. This study assessed toxic manifestations of chronic oral arsenic in mdr1a/1b(-/-) mice, including oxidative stress and altered gene expression, and investigated altered toxicokinetics as a potential basis of enhanced arsenic toxicity. Thus, mdr1a/1b(-/-) and WT mice were exposed to sodium arsenite (0-80 ppm as arsenic) in the drinking water for 10 weeks at which time hepatic arsenic accumulation, lipid peroxidation (LPO), redox status and change in gene expression level were assessed. All mice survived the arsenic exposure, but body weight gain in the highest dose group was reduced in both mdr1a/1b(-/-) and WT mice. Arsenic induced pathological changes, elevated LPO levels and enhanced glutathione S-transferase (GST) activity, in the liver to a greater extent in mdr1a/1b(-/-) than in WT mice. Arsenic also decreased Cu/Zn superoxide dismutase activity in both mdr1a/1b(-/-) and WT mice. The expressions of certain genes, such as those encoding cell proliferation, GST, acute-phase proteins and metabolic enzymes, were modestly altered in arsenic-exposed mice. The expression of cyclin D1, a potential hepatic oncogene, was enhanced in arsenic-exposed mdr1a/1b(-/-) mice only. At the highest level of exposure, hepatic arsenic content was higher in mdr1a/1b(-/-) than in WT mice, suggesting that enhanced accumulation due to transport deficiency may, in part, account for the enhanced toxicity in these mice. In summary, this study shows that chronic arsenic toxicity, including liver pathology and oxidative stress, is enhanced in mdr1a/1b(-/-) mice, possibly due to enhanced accumulation of arsenic as a result of transport system deficiency.