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.

Cadmium accumulation and detoxification by alveolar macrophages of cigarette smokers

STUDY OBJECTIVES

Cadmium (Cd) is a toxic metal associated with emphysema and lung cancer, which is present in both air pollution and cigarette smoke. Metallothionein (MT) is an inducible protein that binds and detoxifies cellular Cd. The goals of this study were to determine whether increased concentrations of Cd are present in alveolar macrophages (AMs) of cigarette smokers (CSMs) and to determine whether MT accumulated in response to the presence of Cd.

DESIGN

AMs were recovered by BAL from 10 healthy nonsmokers (NSMs) and 10 CSMs. The Cd content of the AMs was determined by inductively coupled plasma-mass spectrometry, and the MT content was determined using a Cd/hemoglobin radioassay (with (109)Cd).

MEASUREMENTS

Cd was detected in AMs recovered from all subjects, with higher mean (+/- SEM) concentrations in CSMs compared with those in NSMs (3.4 +/- 0.5 vs 1.3 +/- 0.2 ng/10(6) cells; p < 0.005). There was a correlation between current smoking history (cigarettes per day) and the AM content of Cd (r = 0.74; p < 0.05). The mean AM content of MT was similar in NSMs (1.2 +/- 0.2 microg/10(7) cells) and CSMs (1.0 +/- 0.2 microg/10(7) cells).

CONCLUSIONS

AMs in CSMs accumulate significant amounts of Cd without a concurrent increase in MT content, indicating greater saturation of MT. Increased Cd burden in alveolar cells could contribute to the development of lung diseases in CSMs.

INDUCTION OF MULTIDRUG RESISTANCE PROTEIN 3 (MRP3) IN VIVO IS INDEPENDENT OF CONSTITUTIVE ANDROSTANE RECEPTOR

We previously demonstrated that multidrug resistance protein 3 (Mrp3/ABCC3) is induced in rat liver by phenobarbital (PB) and several other microsomal enzyme inducers that induce cytochrome P450 2B (CYP2B). CYP2B is induced by constitutive androstane receptor (CAR)-retinoid X receptor (RXR) heterodimer binding to a phenobarbital-responsive promoter element in the CYP2B promoter. Hepatic mRNA levels of CYP2B and Mrp3 were measured in three models of altered CAR activity to determine whether CAR is also involved in the induction of Mrp3. In Wistar Kyoto rats, where males express higher CAR protein levels than females, the induction of CYP2B1/2 was significantly higher in males than in females by PB, diallyl sulfide, and trans-stilbene oxide but not oltipraz. Mrp3 was induced by each of these treatments, but in contrast to CYP2B1/2, to a similar magnitude in males and females. In male hepatocyte-specific RXRalpha-/- mice, CYP2B10 was not induced by diallyl sulfide or oltipraz but remained inducible by PB and trans-stilbene oxide after considering the decrease in basal CYP2B10 expression. Mrp3, however, was induced by PB, diallyl sulfide, trans-stilbene oxide and oltipraz in both wild-type and RXRalpha-/- mice. Additionally, constitutive expression of Mrp3 was significantly reduced in RXRalpha-/- mice. In CAR-/- mice, the robust induction of CYP2B10 by PB was completely absent. However, Mrp3 was equally induced both in wild-type and CAR-/- mice by PB. These data clearly demonstrate that induction of hepatic Mrp3 by PB and other microsomal enzyme inducers is CAR-independent and implies a role for RXRalpha in the constitutive expression of Mrp3.

CONSTITUTIVE EXPRESSION OF VARIOUS XENOBIOTIC AND ENDOBIOTIC TRANSPORTER mRNAs IN THE CHOROID PLEXUS OF RATS

The aim of this study was to quantitatively determine the constitutive expression levels of various transporter mRNAs in rat choroid plexus. To provide a reference for the relative expression levels, the expression of various transporter mRNAs in choroid plexus were compared with that in liver, kidney, and ileum. The mRNA levels of multidrug resistance protein (Mrp)1, 2, 3, 4, 5, and 6; multidrug resistance (Mdr)1a, 1b, and 2; organic anion transporting polypeptide (Oatp)1, 2, 3, 4, 5, 9, 12, and Oat-K (1/2); organic anion transporter (Oat)1, 2, and 3; organic cation transporter (Oct)1, 2, 3, N1, and N2; bile acid transporters sodium taurocholate cotransporting polypeptide (Ntcp), bile salt excretory protein (Bsep), and ileal bile acid transporter (Ibat); divalent metal transporter 1 (DMT1), Menke's and Wilson's metal transporters; equilibrative nucleotide transporters (Ent) 1 and 2, and constitutive nucleotide transporters (Cnt)1 and 2; peptide transporters (Pept)1 and 2; as well as ATP-binding cassette (Abc)G5 and 8 were measured in choroid plexus by the branched DNA signal amplification method. Mrp1, 4, and 5, Oatp3, Menke's transporter, DMT1, Ent1, and Pept2 mRNAs were expressed in choroid plexus at higher levels than in liver, kidney, or ileum. OctN1 and N2, Oatp2, Oat2 and 3, and Cnt1 and 2 mRNAs expressions were detectable in choroid plexus, but the levels were lower compared with that in liver, kidney, or ileum. The remaining transporters [Mrp2, Mrp3, Oct1, Oct2, Oatp1, Oatp4, Oatp5, Oatp12, Oat-K (1/2), Ntcp, Bsep, Ibat, Mdr1a, Mdr1b, Mdr2, Oat1, Ent2, Pept1, AbcG5, AbcG8] were expressed at very low levels in choroid plexus. The constitutive expression levels of different transporters in choroid plexus may provide an insight into the range of xenobiotics that can potentially be transported by the choroid plexus, thereby providing a means of xenobiotic detoxification in the brain.

Induction of multidrug resistance protein 3 in rat liver is associated with altered vectorial excretion of acetaminophen metabolites

Treatment with the microsomal enzyme inducer trans-stilbene oxide (TSO) can decrease biliary excretion of acetaminophen-glucuronide (AA-GLUC) and increase efflux of AA-GLUC into blood. The hepatic canalicular multidrug resistance protein (Mrp) 2 and sinusoidal protein Mrp3 transport AA-GLUC conjugates into bile and blood, respectively. Thus, TSO-induced alterations in the vectorial excretion of AA-GLUC may occur via increased hepatic Mrp3 levels. The goal of this study was to determine whether TSO, diallyl sulfide (DAS), and oltipraz (OLT) treatments can up-regulate Mrp3 protein expression, and whether treatment with DAS and OLT can correspondingly increase hepatovascular efflux of AA metabolites. Rats were administered phenobarbital, TSO, DAS, OLT, or vehicle for 4 days. Interestingly, all of the chemicals increased the plasma concentration and urinary excretion of AA-GLUC and decreased its biliary excretion. In control animals, approximately 77% and 23% of AA-GLUC was excreted into bile or urine, respectively, whereas with inducer-pretreated animals, <32% of AA-GLUC was excreted into bile and >68% was excreted into urine. Correspondingly, all of the compounds increased hepatic Mrp3 mRNA levels by 13- to 37-fold and protein levels by 2- to 6-fold, respectively. In conclusion, these studies correlate increased Mrp3 protein levels in liver with increased hepatovascular excretion of AA-GLUC and suggest that induction of Mrp3 affects the route of drug excretion.

Nuclear receptor, pregname X receptor, is required for induction of UDP-glucuronosyltranferases in mouse liver by pregnenolone-16 alpha-carbonitrile

The aim of this study was to determine the role of pregnane X receptor (PXR) in the induction of UDP-glucuronosyltransferases (UGTs) by pregnenolone-16 alpha-carbonitrile (PCN). Four- to six-month-old male wild-type and PXR-null mice received control or PCN-treated (1500 ppm) diet for 21 days. On day 22, livers were taken to prepare microsomes and total RNA to determine UGT activity and mRNA levels, respectively. In wild-type mice, PCN treatment significantly increased UGT activities toward bilirubin, 1-naphthol, chloramphenicol, thyroxine, and triiodothyronine. On control diet, the UGT activities toward the above substrates (except for 1-naphthol) in the PXR-null mice were significantly higher than those of wild-type mice. However, UGT activities in PXR-null mice were not increased by PCN. In agreement with the above findings, mRNA levels of mouse Ugt1a1 and Ugt1a9, which are involved in the glucuronidation of bilirubin and phenolic compounds, were increased about 100% in wild-type mice following PCN treatment, whereas the expression of Ugt1a2, 1a6, and 2b5 was not affected. In contrast, PCN treatment had no effect on the mRNA levels of these UGTs in PXR-null mice. Taken together, these results indicate that PCN treatment induces glucuronidation in mouse liver, and that PXR regulates constitutive and PCN-inducible expression of some UGTs.

Endocrine regulation of rat organic anion transporters

Messenger RNA levels of rat organic anion transporter 1 (Oat1; Slc22a6) and Oat2 (Slc22a7) in kidney and Oat3 (Slc22a8) in liver are gender-predominant. Oat1 and Oat3 are male-predominant, whereas Oat2 is female-predominant. Gonadectomized and hypophysectomized (HX) rats were studied to determine whether sex steroids and/or growth hormone (GH) are responsible for these gender-divergent patterns. GH was administered to HX rats by two daily injections (simulating male secretion) or continuous infusion (simulating female secretion). Oat1 mRNA levels, normally higher in male than female kidney, were lowered by gonadectomy and HX in male rats, but not in females. Additionally, GH injections or infusion did not alter Oat1 levels in HX rats. Oat2 mRNA levels, typically much higher in female than in male kidney, were unaffected by gonadectomy. However, HX dramatically decreased Oat2 in female kidney without altering male levels. GH administered by continuous infusion increased Oat2 in kidneys of both HX male and female rats, whereas injections had no affect. Gonadectomy reduced Oat3 mRNA levels in male livers without affecting levels in female livers. In contrast, HX decreased male and elevated female Oat3 mRNA. GH injections did not significantly change Oat3 mRNA levels in HX rats, but infusion decreased Oat3 mRNA in liver. In conclusion, androgens, but not GH, are responsible for the Oat1 mRNA gender difference in kidney; the female GH secretion pattern is responsible for the Oat2 mRNA gender difference in kidney; and both androgens and the female GH secretion pattern are responsible for the Oat3 mRNA gender difference in liver.

Induction of T(4) UDP-GT activity, serum thyroid stimulating hormone, and thyroid follicular cell proliferation in mice treated with microsomal enzyme inducers

The microsomal enzyme inducers phenobarbital (PB), pregnenolone-16 alpha-carbonitrile (PCN), 3-methylcholanthrene (3MC), and Aroclor 1254 (PCB) are known to induce thyroxine (T(4)) glucuronidation and reduce serum T(4) concentrations in rats. Also, microsomal enzyme inducers that increase serum TSH (i.e., PB and PCN) also increase thyroid follicular cell proliferation in rats. Little is known about the effects of these microsomal enzyme inducers on T(4) glucuronidation, serum thyroid hormone concentrations, serum TSH, and thyroid gland growth in mice. Therefore, we tested the hypothesis that microsomal enzyme inducers induce T(4) UDP-GT activity, resulting in reduced serum T(4) concentrations, as well as increased serum TSH and thyroid follicular cell proliferation in mice. B6C3F male mice were fed a control diet or a diet containing PB (600, 1200, 1800, or 2400 ppm), PCN (250, 500, 1000, or 2000 ppm), 3MC (62.5, 125, 250, or 500 ppm), or PCB (10, 30, 100, or 300 ppm) for 21 days. All four inducers increased liver weight and hepatic microsomal UDP-GT activity toward chloramphenicol, alpha-naphthol, and T(4). PB and PCB decreased serum total T(4), but PCN and 3MC did not. Serum thyroid stimulating hormone was markedly increased by PCN and 3MC treatments, and slightly increased by PB and PCB treatments. All four microsomal enzyme inducers dramatically increased thyroid follicular cell proliferation in mice. The findings suggest that PB, PCN, 3MC, and PCB disrupt thyroid hormone homeostasis in mice.

Tissue mRNA expression of the rat UDP-glucuronosyltransferase gene family

UDP-Glucuronosyltransferases (UGTs) are phase II biotransformation enzymes that glucuronidate numerous endobiotic and xenobiotic substrates. Glucuronidation increases the water solubility of the substrate and facilitates renal and biliary excretion of the resulting glucuronide conjugate. UGTs have been divided into two gene families, UGT1 and UGT2. Tissue distribution of UGTs has not been thoroughly examined, and such data could provide insight into the importance of individual UGT isoforms in specific tissues and to the pharmacokinetics and target organ toxicity of UGT substrates. Therefore, the aim of this study was to determine mRNA levels of rat UGT1 and UGT2 family members in liver, kidney, lung, stomach, duodenum, jejunum, ileum, large intestine, cerebellum, and cerebral cortex, as well as nasal epithelium for UGT2A1. Tissue levels of UGT mRNA were detected using branched DNA signal amplification analysis. Three UGT isoforms, UGT1A1, UGT1A6, and UGT2B12, were detected in many tissues, whereas distribution of other UGT isoforms was more tissue-specific. For example, UGT2A1 was detected predominantly in nasal epithelium. Additionally, UGT1A5, UGT2B1, UGT2B2, UGT2B3, and UGT2B6 were detected primarily in liver. Furthermore, detection of UGT1A2, UGT1A3, UGT1A7, and UGT2B8 was somewhat specific to gastrointestinal (GI) tract. However, not all of these UGTs were detected in all portions of the GI tract. UGT1A8 was unique in that it was barely detectable in any of the tissues examined. In conclusion, some UGT isoforms were expressed in multiple tissues, whereas other UGT isoforms were predominantly expressed in a certain tissue such as nasal epithelium, liver, or GI tract.

The presence of xenobiotic transporters in rat placenta

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

Cadmium absorption and its relationship to divalent metal transporter-1 in the pregnant rat

Increased intestinal absorption of essential nutrients is characteristic of pregnancy as the maternal gastrointestinal tract undergoes physiological and biochemical changes to accommodate the increased demand for essential nutrients by the fetus. Divalent metal transporter-1 (DMT-1) is primarily responsible for dietary iron uptake in the duodenum but also recognizes nonessential metals such as cadmium (Cd). Increased absorption of Cd has been reported in pregnant compared with nonpregnant mice; however, the mechanism is not understood. The purpose of this work was to determine whether Cd absorption is increased in pregnant compared with nonpregnant rats and whether this correlates with a time-dependent up-regulation of DMT-1 expression. Timed pregnant and nonpregnant female Sprague-Dawley rats were administered (109)Cd-labeled CdCl(2) by oral gavage on gestation day (gd) 19. Tissues were collected on gd 20 for (109)Cd assessment (values expressed as pmol Cd). Greater accumulation of (109)Cd was observed in duodenum than in jejunum and ileum in both pregnant and nonpregnant rats. However, the amount of Cd in small intestine was higher in pregnant than nonpregnant rats. Additionally, more Cd accumulated in the liver and kidney of pregnant than nonpregnant rats. DMT-1 mRNA levels were determined in duodenum, placenta, liver, and kidney with branched DNA signal amplification. DMT-1 mRNA levels were about sixfold higher in duodenum of (gd 21) pregnant than nonpregnant rats and the levels in pregnant rats increased from gd 15 through gd 21. The correlation between Cd absorption and DMT-1 expression observed in pregnant rats suggests a role for DMT-1 in the increased absorption of Cd during pregnancy.

Expression of rat Multidrug Resistance Protein 2 (Mrp2) in male and female rats during normal and pregnenolone-16alpha-carbonitrile (PCN)-induced postnatal ontogeny

The normal maturation of biliary organic anion excretion in newborn rats can be enhanced by microsomal enzyme-inducing chemical treatment, yet the mechanism for this phenomenon is not known. Multidrug Resistance Protein 2 (Mrp2) is a biliary efflux transporter that is inducible by select microsomal enzyme-inducing chemicals. Thus, the aims of this study were to compare the normal and pregnenolone-16alpha-carbonitrile (PCN)-induced postnatal ontogeny of Mrp2 in male and female rats. Mrp2 protein increased in an age-dependent manner in both sexes between 0 and 90 days of age. At birth, Mrp2 protein in both male and female rats was the same, approximately 70% of adult levels. Mrp2 protein in both sexes reached maximal expression levels that were higher than adult levels (male: days 25-40; female: day 45), then decreased to adult levels, at which age Mrp2 protein expression in male and female rats was the same. Second, male and female rats of various ages were treated with PCN (75 mg/kg, ip) or corn oil for 4 days, after which livers were removed and analyzed for Mrp2 protein and mRNA expression. PCN accelerated the expression of Mrp2 protein in male and female rats as early as 10 days of age, whereas, PCN did not affect male and female Mrp2 mRNA ontogeny. These data suggest that PCN increased Mrp2 protein by a sex-independent posttranscriptional mechanism.

Intestinal absorption of cadmium is associated with divalent metal transporter 1 in rats

The intestinal absorption of cadmium (Cd) increases when the body iron (Fe) stores are depleted. The depletion of Fe upregulates the expression of divalent metal transporter 1 (DMT1), which is located at the apical membrane of enterocytes lining the small intestine. DMT1 has been shown to transport Fe and other divalent metal ions in vitro. However, it is not known whether DMT1 mediates the intestinal absorption of Cd. To investigate DMT1 involvement in Cd absorption, rats were fed a diet for 4 weeks either deficient in Fe (FeD diet, 2-6 mg Fe/kg) or supplemented with Fe (FeS diet, 120 mg Fe/kg), followed by a single oral administration of 109 CdCl2. Body Fe status, hemoglobin, and tissue Cd concentration were determined at 48 h after Cd administration. Also, DMT1 mRNA levels were quantified in duodenum, kidney, and liver by the branched DNA signal amplification method. Animals fed the FeD diet exhibited a reduced body weight gain, depletion of body Fe, and Fe deficiency anemia. Tissue Cd concentration was significantly higher in FeD than in FeS diet-fed rats, especially in the duodenum. The amount of Cd retained in the body was 10-fold higher in rats fed the FeD diet than in those fed the FeS diet. DMT1 mRNA was highly expressed in duodenum and was 15-fold higher in the FeD diet group. The levels of DMT1 mRNA were significantly lower in kidney and liver than in duodenum, but were 30 and 40% higher, respectively, in rats fed the FeD diet than in rats fed the FeS diet. These findings suggest that functional DMT1 protein is likely upregulated in the small intestine at the mRNA level by body iron depletion and increases Cd uptake from the gastrointestinal tract with subsequent transfer of Cd to the circulation and body tissues. Furthermore, the data from this study may indicate that DMT1 is a nonspecific metal transporter, which can transport not only Fe, but probably the toxic metal as well.

Tissue distribution and chemical induction of multiple drug resistance genes in rats

Multiple drug resistance (mdr) genes encode P-glycoprotein, which is responsible for resistance to some cancer chemotherapeutic drugs and efflux of xenobiotics of cells. Thus, mdr can protect organs from xenobiotics. In rats, there are two mdr1 genes capable of xenobiotic transport, mdr1a and mdr1b. The purpose of this study was to determine the tissue distribution of rat mdr1a and mdr1b mRNA and whether microsomal enzyme inducers that increase phase I and II drug-metabolizing enzymes coordinately regulate mdr1a and/or mdr1b. The mRNA levels of mdr1a and mdr1b were determined using branched-DNA signal amplification technology. The highest level of expression of mdr1a mRNA was observed in the gastrointestinal tract, with levels increasing, respectively, from duodenum, jejunum, and ileum to large intestine. Expression levels of mdr1a mRNA in the cerebral cortex, cerebellum, kidney, lung, and liver were less than one-tenth of that in the ileum. The tissue distribution of mdr1b mRNA was similar to mdr1a with highest expression in the gastrointestinal tract but only about 3-fold higher than in most other tissues. The induction of mdr1a and mdr1b mRNA transcripts in liver, kidney, and ileum by treatment of rats with 18 chemicals representing aryl hydrocarbon receptor ligands, constitutive androstane receptor ligands, pregnane X receptor ligands, peroxisome proliferator-activated receptor ligands, electrophile-response-element activators, and CYP4502E1 inducers was assessed. Hepatic, renal, and intestinal expression of mdr1a and mdr1b mRNA were not significantly altered by treatment of rats with any of these classes of ligands. In conclusion, the primary expression of rat mdr1 genes is in the gastrointestinal tract where they are thought to function to decrease the absorption of some xenobiotics. Rat mdr1 gene expression is not readily increased by microsomal enzyme inducers in rats through coordinate mechanisms with phase I and II drug-metabolizing enzymes.

iNOS-null mice are not resistant to cadmium chloride-induced hepatotoxicity

Acute administration of cadmium (Cd) to rats results in hepatotoxicity. Recent reports indicate that Kupffer cells, the resident macrophages of the liver, participate in the manifestation of Cd-induced hepatotoxicity. Nitric oxide (NO) is a reactive nitrogen radical produced by activated Kupffer cells via the induction of inducible nitric oxide synthase (iNOS). Nitric oxide can combine with superoxide to form peroxynitrite, a molecule that may participate in the toxic mechanisms of hepatotoxins, such as acetaminophen and bacterial endotoxin. It has been speculated that Cd also may exert its hepatotoxicity, in part, via the production of NO by iNOS. Therefore, this study was undertaken to determine whether iNOS contributes to Cd-induced hepatotoxicity. Wild-type (WT) mice were administered selective iNOS inhibitors (AMT and 1400W) concurrently and 3 h after administration of a hepatotoxic dose of Cd (4.0 mg Cd/mg). Additionally, WT and iNOS-null (iNOS-KO) mice were dosed iv with saline or 2.0, 2.5, 3.0, 3.5 or 4.0 mg Cd/kg. Serum alanine aminotransferase (ALT) and sorbitol dehydrogenase (SDH) activities were quantified to assess liver injury. Administration of iNOS inhibitors failed to prevent Cd-induced hepatotoxicity. Also, Cd caused a dose-dependent increase in liver injury in both WT and iNOS-KO mice. The liver injury produced by Cd in the iNOS-KO mice was not different from that in WT at any dose. These data indicate that iNOS does not appear to mediate Cd-induced hepatotoxicity.

Analysis of strain difference in sensitivity to cadmium-induced hepatotoxicity in Fischer 344 and Sprague-Dawley rats

Acute administration of cadmium (Cd) in rats results in hepatotoxicity that appears to involve the activation of Kupffer cells and the subsequent production of proinflammatory chemokines and cytokines. However, the importance of these endogenous mediators in Cd-induced hepatotoxicity is unknown. Therefore, this study was conducted to define and utilize a rat strain difference in sensitivity to Cd-induced hepatotoxicity to elucidate the role of cytokines and chemokines in Cd-induced hepatotoxicity. Doses were selected from a dose-response study of the effect of Cd on serum alanine aminotransferase (ALT) and sorbitol dehydrogenase (SDH) activities. Hepatotoxic doses of 2.0 mg Cd/kg in Fischer 344 (F344) rats and 3.0 mg Cd/kg in Sprague-Dawley (SD) rats, as well as a relatively nontoxic dose of 2.0 mg Cd/kg in SD rats, were chosen for the time-course experiment. Blood and liver from F344 (saline or 2.0 mg Cd/kg iv) and SD rats (saline or 2.0 or 3.0 mg Cd/kg iv) were collected at 0, 1, 3, 6, 10, 18, 24, and 48 h after Cd administration. Cadmium treatment caused an increase in serum ALT and SDH by 3 h and peaked between 18 and 24 h in both strains. Hepatic Cd content, metallothionein (MT) induction, and nonprotein sulfhydryl (NPSH) content were quantified and determined to be consistent with dosing rather than strain differences. Total RNA samples isolated from liver samples were analyzed for chemokine (CINC-1 and MCP-1) and cytokine (TNF-alpha, IL-1beta, IL-6, and IL-10) mRNA levels by the Quantigene branched DNA signal amplification assay. Lipopolysaccharide treatment served as a positive control for chemokine and cytokine induction. After Cd administration, F344 rat livers did not contain higher levels or earlier induction of chemokine and cytokine mRNAs than SD rats. Therefore, this study demonstrates a strain difference in sensitivity to Cd-induced hepatotoxicity that appears to be unrelated to Cd, MT, NPSH, or cytokine expression.

Regulation of rat multidrug resistance protein 2 by classes of prototypical microsomal enzyme inducers that activate distinct transcription pathways

Microsomal enzyme inducers are capable of modulating biliary excretion of organic anions and bile flow, but the mechanism for modulation is unknown. Therefore, this study was designed (1) to determine the effects of microsomal enzyme inducers on protein and mRNA expression of rat multidrug resistance protein 2 (Mrp2), a canalicular organic anion transporter; and (2) to determine whether classes of microsomal enzyme inducers affect Mrp2 expression in similar manners, thus implying specific nuclear receptor-activated transcription pathways. Male Sprague-Dawley rats were treated with aryl hydrocarbon (Ah) receptor (AhR) ligands/cytochrome P450 (CYP) 1A inducers, constitutive androstane receptor (CAR) ligands/CYP2B inducers, pregnane-X receptor (PXR) ligands/CYP3A inducers, peroxisomal proliferator-activating receptor-alpha (PPARalpha) ligands/CYP4A inducers, antioxidant/electrophile response element (ARE/EpRE) ligands, CYP2E1 inducers, or control vehicle. Mrp2 protein levels were significantly increased by all 3 PXR ligands/CYP3A inducers (pregnenolone-16alpha-carbonitrile [PCN], spironolactone [SP], and dexamethasone [DEX]) and by both ARE/EpRE ligands (ethoxyquin [EQ] and oltipraz [OPZ]). In contrast, PPARalpha ligands/CYP4A inducers (clofibric acid [CLOF], di-(2-ethylhexyl)phthalate [DEHP], and perfluorodecanoic acid [PFDA]) tended to decrease Mrp2 protein levels. Mrp2 mRNA expression was not significantly affected by any microsomal enzyme inducer, though ARE/EpRE ligands tended to upregulate Mrp2 mRNA. In summary, this study demonstrates that Mrp2 protein levels are significantly increased by PXR ligands/CYP3A inducers and ARE/EpRE ligands, and appear to be decreased by PPARalpha ligands/CYP4A inducers by posttranscriptional mechanisms. Furthermore, these data suggest that measuring Mrp2 mRNA is not a good indicator for Mrp2 protein expression in vivo.

Gadolinium chloride pretreatment prevents cadmium chloride-induced liver damage in both wild-type and MT-null mice

The heavy metal cadmium (Cd) causes hepatotoxicity upon acute administration. Kupffer cells, the resident macrophages of the liver, have been suggested to play a role in Cd-induced hepatotoxicity. Gadolinium chloride (GdCl3) may prevent Cd-induced hepatotoxicity by suppressing Kupffer cells. However, GdCl3 also induces the Cd-binding protein, metallothionein (MT). Therefore, this study was conducted to determine whether GdCl3 prevents Cd-induced hepatotoxicity via the induction of MT. Hepatic MT and Kupffer cell counts were analyzed 24 h after wild-type (WT) mice were administered saline or 10, 30, or 60 mg GdCl3/kg. GdCl3 induced MT in a dose-dependent manner without affecting nonprotein sulfhydryl content. All examined doses of GdCl3 were effective at eliminating Kupffer cells from the liver. To examine the hepatoprotective effects of GdCl3, WT and MT-null mice were pretreated with saline or 10, 30, or 60 mg GdCl3 24 h prior to a hepatotoxic dose of Cd (2.5 mg Cd/kg). Blood and livers were removed 16 h later and analyzed for hepatotoxicity as well as MT, Cd, and Kupffer cell content. Hepatotoxicity was alleviated in both WT and MT-null mice that were pretreated with 30 or 60 mg GdCl3/kg, indicating that MT induction is not required for the hepatoprotective effects of GdCl3. Hepatic Cd content was not decreased by GdCl3, demonstrating that GdCl3 does not negatively affect Cd distribution to the liver. Kupffer cells were depleted at all three doses of GdCl3, whereas hepatoprotection was only observed at doses of 30 and 60 mg GdCl3/kg. This does not rule out Kupffer cells in the mechanism of Cd-induced hepatotoxicity, but it does suggest that GdCl3 exerts hepatoprotective effects on the liver aside from depleting Kupffer cells. In summary, these data substantially rule out MT induction and decrease the importance of Kupffer cells as mechanisms of GdCl3-induced protection from Cd-induced hepatotoxicity.

Tissue expression, ontogeny, and inducibility of rat organic anion transporting polypeptide 4

To date, organic anion transporting polypeptide 4 (Oatp4; Slc21a10) is known as a liver-specific and sodium-independent transporter that mediates transport of a variety of compounds. The purpose of this study was to determine whether Oatp4 mRNA expression is specific to the liver compared with Oatp1, 2, 3, or 5. In addition, the effect of gender and age was determined by assessing the expression of Oatp4 mRNA during the postnatal development of rats. Furthermore, to determine whether Oatp4 gene expression is coordinately modulated by drug-metabolizing enzyme inducers, male rats were administered chemicals known to induce the expression of drug-metabolizing enzymes through six mechanisms: the aryl hydrocarbon receptor, constitutive androstane receptor, pregnane X receptor, peroxisome proliferator-activated receptor, electrophile response element, or CYP2E1 inducers. The levels of Oatp1, 2, 3, 4, and 5 mRNA were measured using the branched DNA signal amplification technique. The tissue distribution of Oatp4 was almost exclusively expressed in liver in contrast to Oatp1, 2, 3, and 5. The hepatic expression of Oatp4 was low in newborn rats and increased gradually to the adult level with no significant difference between genders. The expression of Oatp4 was not consistently induced by any of the six groups of enzyme inducers. These findings continue to suggest that Oatp4 is expressed specifically in the liver. The preference of Oatp4 for endogenous compounds coupled with its refractory response to known drug-metabolizing enzyme inducers suggests that Oatp4 may be largely responsible for the homeostasis of endogenous rather than exogenous chemicals, including pharmaceuticals.

Induction of rat organic anion transporting polypeptide 2 by pregnenolone-16alpha-carbonitrile is via interaction with pregnane X receptor

The rat organic anion transporting polypeptide 2 (oatp2; Slc21a5) is a liver transporter that mediates the uptake of a variety of structurally diverse compounds, and has a high affinity for cardiac glycosides. Treatment of rats with pregnenolone-16alpha-carbonitrile (PCN), a ligand for the rodent pregnane X receptor (PXR), significantly enhances the rat oatp2 gene expression. To understand the molecular mechanism of oatp2 induction by PCN, rat oatp2 gene was cloned. The rat oatp2 gene consists of 16 exons; alternative splicing of the second noncoding exon gives rise to the two published rat oatp2 cDNAs. Approximately 8700 base pairs (bp) of the 5'-flanking region of the rat oatp2 gene were linked to the luciferase reporter gene and used in transient transfection assays in H4IIE cells. Treatment of PCN induced the expression of the reporter gene in a dose-dependent manner. Four potential PXR response elements (PXREs) were identified in the 5'-flanking region of the rat oatp2 gene. One element (DR3-1) is located approximately -5000 bp with three more (DR3-2, DR3-3, and DR3-4) clustered at about -8000 bp. Results from electrophoretic mobility shift assays showed that the PXR-retinoid X receptor alpha heterodimer binds to the DR3-2 with the highest affinity, to the DR3-4 and DR3-1 with a lower affinity, and weakly or not at all to the DR3-3. Furthermore, a series of partial deletions of the 5'-flanking region illustrated that both the proximal and distal clusters of PXREs are required for maximal induction of rat oatp2 by PCN. In conclusion, these data elucidate the molecular mechanism by which PCN treatment induces rat oatp2 gene expression. In addition, this study identifies rat oatp2 as a direct PXR-targeted gene and further supports the hypothesis that activation of PXR affects a network of genes that is involved in either metabolism or transport of drugs, steroids, and bile acids.

Gender-specific and developmental influences on the expression of rat organic anion transporters

Rat organic anion transporter 1 (Oat1), Oat2, and Oat3, members of the organic anion transporter family, transport some organic anions across cellular membranes. Previously, highest Oat1 and Oat3 mRNA expression was reported in kidney and Oat2 in liver. However, gender and developmental differences in Oat expression remain unknown. This study describes gender- and age-specific patterns of rat organic anion transporter expression in various tissues. Oat mRNA expression was evaluated in adult male and female Sprague-Dawley rat tissues, and developmental expression was also determined in kidneys of Sprague-Dawley rats ranging in age from days 0 through 45. Expression was quantified using branched-DNA signal amplification. Oat1 mRNA expression was primarily observed in kidney. Surprisingly, Oat2 mRNA expression was also highest in kidney rather than in liver. Moreover, considerably higher Oat2 levels were seen in female kidney as compared with male. Finally, Oat3 mRNA expression was highest in kidney of both genders, whereas a male-predominant pattern was observed in liver. At birth, all kidney Oat mRNA levels were low. Renal Oat1 expression gradually increased throughout development, approaching adult levels at 30 days of age, where at days 40 and 45 Oat1 levels were greater in males than females. Oat2 expression in kidney was minimal through day 30 but increased dramatically at day 35 in females only. Lastly, Oat3 mRNA expression in kidney matured earliest, rapidly increasing from birth through day 10. These data indicate that Oat mRNA expression is primarily localized to the kidney, and observed expression patterns may explain some previously recognized age- and gender-dependent toxicities associated with chemical exposure.