Regulation of 2-acetylaminofluorene-and 3-methylcholanthrene--mediated induction of multidrug resistance and cytochrome P450IA gene family expression in primary hepatocyte cultures and rat liver

Delay in hepatocyte proliferation and prostaglandin D2 synthase expression for cholestasis due to endotoxin during partial hepatectomy in rats

Infection is a frequent complication of liver transplantation or partial hepatectomy (PH) and sometimes results in cholestasis. We examined factors involved in infection‑induced cholestasis after PH, employing a rat PH model and lipopolysaccharide (LPS) as a bacterial toxin. Male Sprague‑Dawley rats were subjected to 70% PH and/or LPS injection, and tissues were harvested at 0, 24, 72 and 168 h. Gene expression was analyzed by microarray analysis and reverse transcription‑quantitative polymerase chain reaction, and protein levels and localization were analyzed by western blotting and immunohistochemistry, respectively. Plasma bile acid levels were significantly higher in the LPS + PH group than in the PH group. Ribonucleotide reductase regulatory subunit M2 and proliferating cell nuclear antigen peaked at 24 and 72 h in the PH group and LPS + PH group, respectively, indicating a delay in cell proliferation in the latter group. The sodium‑dependent taurocholate cotransporting polypeptide and organic‑anion‑transporting polypeptide 1a1 and 1a2 were reduced in the PH group at 24 h, and were not further decreased in the LPS + PH group. Chemokine ligand 9 (Cxcl9), a chemokine involved in M2 macrophage polarization, increased after 24 h in the LPS and the LPS + PH groups. The number and shape of Cxcl9‑positive cells were similar to CD163‑positive cells, suggesting that such cells produced the chemokine. Hematopoietic prostaglandin D2 synthase (Ptgds2) was only detected in hepatocytes of the LPS + PH group exhibiting a delay in cell proliferation. Thus, Kupffer cells activated with LPS were suggested to be responsible for a delay in hepatocyte proliferation after PH.

Impact of Breast Cancer Resistance Protein Expression on the In Vitro Efficacy of Anticancer Drugs in Pancreatic Cancer Cell Lines

Breast cancer resistance protein (BCRP) overexpression confers multidrug resistance to cancer cells, and the efficacy of anticancer drugs has been reported to be significantly affected by BCRP in cell lines transfected with BCRP or selected with drugs. It is unclear whether the in vitro efficacy of anticancer drugs is affected by endogenous BCRP, although cancer cell line panels consisting of defined tumor cell lines with endogenous BCRP have been used to screen for anticancer drugs in the pharmaceutical industry. We assessed the impact of BCRP expression on efficacy of anticancer drugs using pancreatic cancer cell lines expressing varying levels of endogenous BCRP. Pancreatic cancer cell lines were selected from the Cancer Cell Line Encyclopedia (CCLE). The EC₅₀ of 7-ethyl-10-hydroxycamptothecin (SN-38), topotecan, and mitoxantrone decreased in the presence of a BCRP inhibitor in PANC-1 and AsPC-1 cells, which exhibit high BCRP expression. However, no significant alterations in EC₅₀ were observed in HPAF-II, SW 1990, and MIA PaCa-2, which show moderate or low BCRP expression. The shift of EC₅₀ of anticancer drugs with and without a BCRP inhibitor increased with an increase of BCRP mRNA expression levels; however, the shift was obvious only in cells highly expressing BCRP. Thus, the in vitro efficacy of anticancer drugs on cell proliferation may be minimally affected by BCRP in most pancreatic cancer cell lines, considering that 72% of pancreatic cancer cell lines in CCLE show moderate or low BCRP expression. The effect of BCRP should be carefully evaluated in pancreatic cell lines that highly express BCRP.

Gender differences in detoxification metabolism of polycyclic aromatic hydrocarbon (chrysene) in scallop Chlamys farreri during the reproduction period

This study aims to investigate the effects of chrysene (CHR) on biotransformation and detoxification responses of mature scallop Chlamys farreri during the reproduction period. Scallops were exposed to 0.2, 0.8 and 3.2 μg/L CHR for 21 days; at day 10 scallops were induced to spawn. At days 1, 3, 6, 10, 11, 15 and 21, enzymatic activities of 7-ethoxyresorufin O-deethylase (EROD) and glutathione-s-transferase (GST), related mRNA expression levels of CYP1A1, GST-pi and P-glycoprotein (P-gp) in digestive glands and CHR bioaccumulation in tissues were examined by separately analyzing male and female scallops. During the pre-spawn period, CHR concentrations of the treated groups in tissues except the hemolymph increased rapidly. Levels of enzymatic activities and related gene expressions were all induced by the exposure to CHR for females and males. GST activity and GST-pi mRNA expression showed a good time- and dose-dependent relationship only in males, and P-gp mRNA expression exhibited a dose-dependent manner in both sexes. During the post-spawn period, spawning caused significant reductions of bioaccumulation in tissues but the gill and hemolymph. Enzymatic activities and related gene expressions were for females significantly depressed at day 21 at 0.8 or 3.2 μg/L CHR. Overall, females accumulated more CHR than males, while males were more sensitive than females to CHR exposure in gene expressions and enzyme activities. P-gp mRNA expression seemed to be a potential biomarker for PAH exposure. These results will offer the information on CHR biotransformation in this species, and ensure the influence of gender and reproductive status on PAH detoxification metabolism.

Current advances on ABC drug transporters in fish

Most members of the large ATP-binding cassette (ABC) gene family are transporters involved in substrate translocation across biological membranes. In eukaryotes, ABC proteins functioning as drug transporters are located in the plasma membrane and mediate the cellular efflux of a wide range of organic chemicals, with some transporters also transporting certain metals. As the enhanced expression of ABC drug transporters can confer multidrug resistance (MDR) to cancers and multixenobiotic resistance (MXR) to organisms from polluted habitats, these ABC family members are also referred to as MDR or MXR proteins. In mammals, ABC drug transporters show predominant expression in tissues involved in excretion or constituting internal or external body boundaries, where they facilitate the excretion of chemicals and their metabolites, and limit chemical uptake and penetration into "sanctuary" sites of the body. Available knowledge about ABC proteins is still limited in teleost fish, a large vertebrate group of high ecological and economic importance. Using transport activity measurements and immunochemical approaches, early studies demonstrated similarities in the tissue distribution of ABC drug transporters between teleosts and mammals, suggesting conserved roles of the transporters in the biochemical defence against toxicants. Recently, the availability of teleost genome assemblies has stimulated studies of the ABC family in this taxon. This review summarises the current knowledge regarding the genetics, functional properties, physiological function, and ecotoxicological relevance of teleostean ABC transporters. The available literature is reviewed with emphasis on recent studies addressing the tissue distribution, substrate spectrum, regulation, physiological function and phylogenetic origin of teleostean ABC transporters.

Evaluation of hydro-alcoholic extract of Eclipta alba for its multidrug resistance reversal potential: an in vitro study

The development of multidrug resistance (MDR) causes problems in the chemotherapy of human cancer. The present study was designed to evaluate and establish the role of Eclipta alba as MDR reversal agent using multidrug resistant hepatocellular carcinoma cell line (DR-HepG2). To develop DR-HepG2, hepatocellular carcinoma cell line (HepG2) was transfected with 2-Acetylaminofluorene (AAF) and Aflatoxin B1 (AFB). Cytotoxic effects of the Eclipta alba hydroalcoholic extract (EAE) and standard anti-ancer drug Doxorubicin (DOX) were determined in DR-HepG2 and the parental cells HepG2 using MTT assay. The expression level of MDR1 gene and P-glycoprotein (P-gp) level was analyzed by RT-PCR and western blotting. From the present investigation, it was found that EAE (10 and 20 μg/ml) could significantly inhibit cell proliferation in DR-HepG2 whereas DOX (0.5 μg/ml) could not because of enhancement effect of MDR1/P-gp. This study demonstrated for the first time the antiproliferative activities of EAE in multidrug resistant DR-HepG2 cells. The findings revealed that Eclipta alba components are effective inhibitors of MDR1/P-gp.

Altered expression of the xenobiotic transporter P-glycoprotein in liver and liver tumours of mummichog Fundulus heteroclitus from a creosote-contaminated environment

P-glycoproteins (Pgps) are involved in efflux of xenobiotics from drug-resistant cell lines and tumours, and in excretion of toxicants from normal tissues. Recently, investigators have proposed that Pgp activity contributes to resistance or tolerance of certain aquatic species to pollutants. In the present study using immunoblot and immunohistochemical techniques, we found elevation of Pgp in liver and liver tumours of creosote-resistant mummichog from a contaminated site in the Elizabeth River, Virginia. Immunoblots of mummichog liver extracts showed an immunoreactive band at 170 kDa and indicated two- to three-fold elevation of Pgp in livers of resistant fish relative to those from a reference site. Laboratory exposures of reference site fish to a model PAH (3-methylcholanthrene), however, produced no increase in liver Pgp levels as measured by immunoblot. Normal mummichog liver sections showed specific immunohistochemical staining for Pgp on the canalicular surface of hepatocytes. In the majority of hepatic neoplasms we observed a high level of over-expression and altered patterns of Pgp expression. However we did not observe Pgp over-expression in early proliferative lesions. Elevation of Pgp in livers and liver tumoursof these resistant mummichog may contribute to their survival in a heavily contaminated environment.

Assessment of toxic effects of triclosan on the swordtail fish (Xiphophorus helleri) by a multi-biomarker approach

The toxic effects of triclosan (TCS) on the swordtail fish (Xiphophorus helleri) were assessed based on various biomarkers including enzymatic activities of ethoxyresorufin O-deethylase (EROD), erythromycin N-demethylase (ERND) and glutathione-s-transferase (GST) and mRNA expression levels of CYP1A, CYP3A, glutathione S-transferase (GST) and P-glycoprotein (P-gp). The acute toxicity test showed the LC(50) value of 1.47 mg L(-1) for TCS. The mRNA expressions of CYP1A, CYP3A, GST and P-gp showed dose-effect relationships in female swordtail fish when exposed to TCS, These mRNA expression levels were found more sensitive to TCS exposure than the enzymatic activities of EROD, ERND and GST do. In addition, the male fish displayed higher gene expression levels and more dramatic changes in enzyme activities than the females did. Our data further demonstrated that TCS was a typical inducer to Phase I and Phase II metabolism enzymes and genes, suggesting it is a potential ecotoxicological risk to aquatic ecosystems.

Sustained repression and translocation of Ntcp and expression of Mrp4 for cholestasis after rat 90% partial hepatectomy

BACKGROUND & AIMS

To clarify the mechanism of persistent cholestasis after massive hepatectomy, the relationship between such cholestasis and the expression and localization of organic anion transporters for bile acids was examined in a rat model.

METHODS

Male Sprague-Dawley rats were subjected to 90% hepatectomy, and tissues were harvested at 0, 1, 3, and 7 days for microarray analysis, quantitative real-time polymerase chain reaction (RT-PCR), Western blotting, and immunohistochemistry to examine the expression of multidrug resistance protein 4 (Mrp4), bile salt export pump (Bsep), and sodium-dependent taurocholate cotransporting polypeptide (Ntcp).

RESULTS

Persistently elevated levels of serum bile acids were observed at days 3 and 7. RT-PCR and Western blotting indicated that the expression of Mrp4, a bile acid export pump located in the basolateral membrane, was increased at day 3. The expression of Ntcp, a transporter used to uptake bile acids from the sinusoids, was significantly decreased throughout the period. The levels of Bsep, an export pump localized to the canalicular membrane, were unchanged. Immunohistochemistry revealed the localization of Mrp4 and Bsep in the basolateral and canalicular membranes, respectively. On the other hand, at days 3 and 7, Ntcp was localized in the cytoplasm and was hardly detected in the basolateral membrane.

CONCLUSIONS

These results suggested that the sustained repression and translocation of Ntcp and the expression of Mrp4 at the basolateral membrane seem to be responsible for the high blood bile acids levels after massive hepatectomy.

C-Phycocyanin inhibits 2-acetylaminofluorene-induced expression of MDR1 in mouse macrophage cells: ROS mediated pathway determined via combination of experimental and In silico analysis

We studied the effects of C-Phycocyanin (C-PC), a biliprotein from Spirulina platensis on the 2-acetylaminofluorene (2-AAF)-induced expression of MDR1, encoded by the multidrug resistance (MDR1) gene, in mouse macrophage cell line (RAW 264.7). Our experimental and In silico studies revealed a significant inhibition of 2-AAF-induced expression of MDR1 protein in C-PC treated mouse macrophage cell line. MDR1 induction by 2-AAF was dependent on ROS (reactive oxygen species)-Akt (protein kinase B)-NF-kappaB (Nuclear factor kappa B) signaling pathway. Generation of ROS, phosphorylation of Akt and corresponding nuclear translocation of NF-kappaB, the events that play a major role in the induction of MDR1 expression, were decreased significantly in C-PC treated cells. NADPH oxidase inhibitor, DPI (Diphenyl iodide), and pharmacological inhibitor of Akt, Akt inhibitor IV, also showed a reduction in MDR1 expression, although not to the same extent as C-PC mediated inhibition of MDR1 expression. To further understand the mechanism, we created a computational model of the detailed ROS-Akt-NF-kappaB pathway. C-PC was modeled purely as a ROS scavenger and this representation matched the experimental trends accurately. Also the ROS levels determined through In silico investigation showed that C-PC was more effective in reduction of MDR1 expression than inhibitors of NADPH oxidase and Akt. Our experimental and In silico studies collectively suggest that 2-AAF induces MDR1 by ROS dependent pathway and C-PC is a potential negative regulator of MDR1 expression. This down regulation of MDR1 expression, induced by xenobiotics such as 2-AAF, suggests C-PC's usefulness in overcoming the drug resistance in cellular systems.

The role of pregnane X receptor in 2-acetylaminofluorene-mediated induction of drug transport and -metabolizing enzymes in mice

Activation of the pregnane X receptor (PXR) mediates the induction of several drug transporters and -metabolizing enzymes. In vitro studies have reported that several of these genes are induced after exposure to the hepatocarcinogen, 2-acetylaminofluorene (2-AAF). Thus, we hypothesized that PXR may play a role in the in vivo induction of gene expression by 2-AAF. We examined the expression of the drug-metabolizing enzymes CYP1A2 and CYP3A11 and the drug transporters breast cancer resistance protein (BCRP), MRP2, and OATP2. Wild-type (PXR+/+) and PXR-null (PXR-/-) C57BL/6 mice were injected daily for 7 days with 150 or 300 mg/kg 2-AAF suspended in corn oil (i.p.), whereas the control group received corn oil vehicle. Levels of mRNA isolated from liver were measured by reverse transcription-polymerase chain reaction and normalized to beta-actin. Treatment of PXR+/+ mice resulted in a dose-dependent 2- to 4-fold induction (p<0.001) of MRP2, OATP2, BCRP, CYP3A11, and CYP1A2, but no induction was observed in PXR-/- mice. Induction of PXR mRNA was observed in the 2-AAF-treated PXR+/+ mice. Furthermore, a dose-dependent increase in CYP3A4 promoter construct activity was observed in HepG2 cells cotransfected with human or rat PXR, indicating that 2-AAF does indeed activate PXR. These results suggest that PXR is responsible for 2-AAF-mediated induction of drug efflux transporters and biotransformation enzymes in the liver. Moreover, novel findings demonstrate that PXR plays a role in regulation of the drug efflux transporter, BCRP, in mice.

Identification of the Hepatic Efflux Transporters of Organic Anions Using Double-Transfected Madin-Darby Canine Kidney II Cells Expressing Human Organic Anion-Transporting Polypeptide 1B1 (OATP1B1)/Multidrug Resistance-Associated Protein 2, OATP1B1/Multidrug Resistance 1, and OATP1B1/Breast Cancer Resistance Protein

Until recently, it was generally believed that the transport of various organic anions across the bile canalicular membrane was mainly mediated by multidrug resistance-associated protein 2 (MRP2/ABCC2). However, a number of new reports have shown that some organic anions are also substrates of multidrug resistance 1 (MDR1/ABCB1) and/or breast cancer resistance protein (BCRP/ABCG2), implying MDR1 and BCRP could also be involved in the biliary excretion of organic anions in humans. In the present study, we constructed new double-transfected Madin-Darby canine kidney II (MDCKII) cells expressing organic anion-transporting polypeptide 1B1 (OATP1B1)/MDR1 and OATP1B1/BCRP, and we investigated the transcellular transport of four kinds of organic anions, estradiol-17beta-d-glucuronide (EG), estrone-3-sulfate (ES), pravastatin (PRA), and cerivastatin (CER), to identify which efflux transporters mediate the biliary excretion of compounds using double-transfected cells. We observed the vectorial transport of EG and ES in all the double transfectants. MRP2 showed the highest efflux clearance of EG among these efflux transporters, whereas BCRP-mediated clearance of ES was the highest in these double transfectants. In addition, two kinds of 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors, CER and PRA, were also substrates of all these efflux transporters. The rank order of the efflux clearance of PRA mediated by each transporter was the same as that of EG, whereas the contribution of MDR1 to the efflux of CER was relatively greater than for PRA. This experimental system is very useful for identifying which transporters are involved in the biliary excretion of organic anions that cannot easily penetrate the plasma membrane.

Expression of MRP2 and MRP3 during liver regeneration after 90% partial hepatectomy in rats1

BACKGROUND

Small-for-size grafts often cause persistent conjugated hyperbilirubinemia in the recipient after adult-to-adult living donor liver transplantation, but the cause has not yet been clarified. In physiologic status, bilirubin is excreted from hepatocytes to the bile canaliculus by means of multidrug resistance protein (MRP) 2 and, in particular circumstances, by means of MRP3 to the sinusoidal space. The aim of this study was to research whether there is any change in bilirubin excretion pattern during liver regeneration with reference to expression of MRP2 and MRP3.

METHODS

Sprague-Dawley rats underwent sham operation (n=37), 70% hepatectomy (n=38), or 90% hepatectomy (n=37). The degree of liver regeneration, total and direct bilirubin, protein synthesis, and interleukin (IL)-6 were serially assessed. Expression of MRP2 and MRP3 were semiquantified by Western blotting.

RESULTS

The proliferating cell nuclear antigen labeling index indicated rapid liver regeneration after 70% and 90% hepatectomy. Serum levels of total and direct bilirubin increased significantly (P<0.05), and conjugated hyperbilirubinemia was proved only in the 90% hepatectomy group. Coagulation factor VII dipped but increased as early as 12 to 24 hr postoperatively in both hepatectomy groups. Plasma IL-6 levels were significantly increased in the 90% hepatectomy group (P<0.05). Expression of MRP2 was decreased and MRP3 was expressed at 36 and 72 hr postoperatively in the 90% hepatectomy group, whereas no change was observed in MRP expression in the 70% hepatectomy group.

CONCLUSIONS

During liver regeneration after critical hepatectomy such as 90% hepatectomy, decrease of MRP2 and expression of MRP3 may play an important role in postoperative hyperbilirubinemia.

Transcriptional regulation of ABC drug transporters

P-glycoprotein, the founding member of the ATP-binding cassette (ABC) family of drug transporters, was first identified almost three decades ago and shown to confer resistance to multiple chemotherapeutic agents when overexpressed in human tumors. Subsequent years have witnessed a tremendous effort to characterize the function and regulation of P-glycoprotein, initially spurred by the hope that its inhibition was the key to overcoming clinical resistance to multiple anticancer agents. However, the identification of MRP1, another member of the ABC drug transporter family, led to the realization that the multidrug resistance (MDR) phenotype is considerably more complex than initially believed. Indeed, at the present time at least 10 members of the ABC transporter family have been implicated in an MDR phenotype, and it is likely that more will be added to this list as studies progress. With this complexity comes the imperative to improve our understanding of the function of individual transporters, as well as to delineate the mechanisms underlying their expression in normal and tumor cells, particularly those that may be amenable to therapeutic intervention. Several articles within this volume address the structure and function of drug transporters. This review will focus on our current understanding of the regulation of ABC drug transporters at the level of transcription.

Farnesoid X receptor activates transcription of the phospholipid pump MDR3

The human multidrug resistance gene MDR3 encodes a P-glycoprotein that belongs to the ATP-binding cassette transporter family (ABCB4). MDR3 is a critical trans-locator for phospholipids across canalicular membranes of hepatocytes, evidenced by the fact that human MDR3 deficiencies result in progressive familial intrahepatic cholestasis type III. It has been reported previously that MDR3 expression is modulated by hormones, cellular stress, and xenobiotics. Here we show that the MDR3 gene is trans-activated by the farnesoid X receptor (FXR) via a direct binding of FXR/retinoid X receptor alpha heterodimers to a highly conserved inverted repeat element (a FXR response element) at the distal promoter (-1970 to -1958). In FXR trans-activation assays, both the endogenous FXR agonist chenodeoxycholate and the synthetic agonist GW4064 activated the MDR3 promoter. Deletion or mutation of this inverted repeat element abolished FXR-mediated MDR3 promoter activation. Consistent with these data, MDR3 mRNA was significantly induced by both chenodeoxycholate and GW4064 in primary human hepatocytes in time- and dose-dependent fashions. In conclusion, we demonstrate that MDR3 expression is directly up-regulated by FXR. These results, together with the previous report that the bile salt export pump is a direct FXR target, suggest that FXR coordinately controls secretion of bile salts and phospholipids. Results of this study further support the notion that FXR is a master regulator of lipid metabolism.

Differential modulation of cytochrome P-450 1A and P-glycoprotein expression by aryl hydrocarbon receptor agonists and thyroid hormone in Xenopus laevis liver and intestine

Several defence mechanisms, such as cytochrome P450 1A (CYP1A) enzymes and P-glycoprotein (Pgp), may influence the intracellular concentration and consequently the toxicity of xenobiotics. The parallel expression of CYP1A and Pgp has been investigated in mammals and, to a lesser extent in fish, in search for evidence of co-ordinated responses to xenobiotic exposure. The aryl hydrocarbon receptor (AHR) agonists are well known CYP1A inducers but some of them resulted not to have a uniquely defined action on Pgp levels in mammalian and fish species. To the best of our knowledge, no detailed studies have been carried out so far on amphibians Xenopus laevis. For this reason, in this work, the time dependent responses of the hepatic CYP1A and Pgp, to the prototypical CYP1A inducers, benzo(a)pyrene (B(a)P), 3-methylcholanthrene (3MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in X. laevis have been assessed at the protein level and compared. The responsiveness of Xenopus intestinal Pgp to these compounds has also been analysed, as the epithelial cells lining the lumen of intestine represent another preferential site of Pgp expression. In addition, since the thyroid hormone has been demonstrated to down regulate the mdr gene during Xenopus development and in primary culture of Xenopus intestinal epithelial cells, the effects of 3,3',5-triiodo-L-thyronine (T(3)) on CYP1A and Pgp protein levels have been investigated in adult organisms. Western blot evidenced that a single injection of B(a)P (100 mg/kg), 3MC (20 mg/kg), and TCDD (3 microg/kg) elicited a statistically significant induction of hepatic CYP1A at all time points considered (72, 120 and 168 h) which decreased in time. The same trend of liver CYP1A induction was observed in T(3) treated Xenopus (15 microg/kg). Unlike CYP1A induction, the modulation of hepatic and intestinal Pgp expression exhibits an heterogeneous pattern. The basal levels of hepatic and intestinal Pgp were not statistically significant affected by treatments. In particular, the hepatic Pgp levels seem not to be induced by TCDD and T(3) at all times considered in comparison to control. For the first time the modulation of CYP1A and Pgp levels by B(a)P, 3MC and in particular by TCDD and T(3) in Xenopus has been demonstrated and the results herewith indicate that the two target defence mechanisms respond to AHR agonists in a dissimilar way in terms of proteins induction in Xenopus. Moreover, these data suggest additional experiments in order to clarify the complex mechanism, which adjusts the parallel expression of CYP1A and Pgp in Xenopus.

Pregnane X receptor-dependent and -independent effects of 2-acetylaminofluorene on cytochrome P450 3A23 expression and liver cell proliferation

The arylamide 2-acetylaminofluorene (AAF) is a powerful carcinogen displaying a marked promoting activity, also known to regulate expression of liver detoxifying proteins. In this study we identified CYP3A23, a major inducible cytochrome P-450 (CYP) isoform, as an AAF target in hepatocytes. Indeed, exposure to AAF of primary rat hepatocytes resulted in a marked up-regulation of CYP3A23 expression at both mRNA and protein levels. Using CYP3A23 reporter gene constructs, we further demonstrated that AAF activated the CYP3A23 Direct Repeat 3 (DR3) promoter element interacting with the nuclear pregnane X receptor (PXR). Moreover, the PXR antagonist ecteinascidin-743 fully suppressed AAF-related CYP3A23 induction. Low doses of AAF inhibiting DNA synthesis in hepatocytes however failed to trigger PXR-related CYP3A23 induction and PXR-negative epithelial liver cells remained sensitive to the mito-inhibitory effects of AAF. Such data indicate that AAF up-regulates CYP3A23 through PXR activation but does not require PXR for exerting its carcinogenic promoting properties based on inhibition of cell growth.

Superinduction of P-glycoprotein messenger RNA in vivo in the presence of transcriptional inhibitors

P-Glycoprotein (P-gp) is comprised of a small family of plasma membrane proteins, and its presence in high amounts often correlates with multidrug resistance in cultured cells. Dramatically increased levels of a single member of P-gp mRNA (pgp2) have been observed in experimental liver carcinogenesis models, during liver regeneration, upon culturing of hepatocytes and in the uterus of pregnant animals. In all cases, the increase in mRNA level appears to be the result of an increase in mRNA half-life (stability). Previously, we have used transcriptional inhibitors alpha-amanitin and actinomycin D to measure P-gp mRNA half-life in normal liver and in liver tumors. We showed that the level of all three P-gp mRNAs decreased with time in the presence of transcriptional inhibitors, yielding measured half-lives of less than 2 h in liver but greater than 12 h in liver tumors. This observation raised the possibility that regulation of P-gp mRNA stability plays a role in liver carcinogenesis. In the present study, we measured P-gp mRNA half-life in other normal tissues to determine if a short P-gp mRNA half-life is unique to the liver. Our study reveals that in contrast to liver, measured P-gp mRNA half-lives in most tissues examined are greater than 12 h. Moreover, we observed an unexpected, marked increase in the level of pgp2 mRNA with time after injection of transcriptional inhibitors. This can only be explained if the transcriptional inhibitors directly or indirectly inhibit the normally high degradation rate of pgp2 mRNA, resulting in the superinduction of this mRNA. These findings have implications for our understanding of the regulation of P-gp gene expression and drug resistance in vivo.

Regional expression of multidrug resistance genes in genetically epilepsy-prone rat brain after a single audiogenic seizure

PURPOSE

The multidrug resistance (mdr) gene family encodes the drug transport macromolecule P-glycoprotein (P-gp), which contributes to the functionality of the blood-brain barrier. Recent evidence suggests that P-gp-mediated drug extrusion may play a facilitatory role in refractory epilepsy. We investigated the regional expression of mdr genes in genetically epilepsy-prone rat (GEPR) brain after a single audiogenic seizure.

METHODS

Three groups of adult male GEPRs (n = 5/group) were exposed to a seizure-inducing audiogenic stimulus and killed at 4 h, 24 h, and 7 days thereafter. A further group (n = 5) served as a stimulus-naïve control. Expression of mdr1a and mdr1b in distinct anatomic brain regions (cortex, midbrain, pons/medulla, hippocampus) was determined by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in the presence of competitive internal standards.

RESULTS

When compared with control, mdr1a expression in cortex and midbrain was significantly (p < 0.05) increased at 24 h after a single audiogenic seizure. Cortical mdr1a expression remained elevated at 7 days after stimulus. In contrast, mdr1a expression in pons/medulla and hippocampus was unchanged. The mdr1b isoform was quantifiable in hippocampus alone and not influenced by seizure activity.

CONCLUSIONS

These findings suggest that acute seizures in the GEPR can induce the expression of mdr genes. The pattern of increased expression appears to follow the anatomic pathway of audiogenic seizures in these animals, with initiation in the midbrain and propagation to the cortex. Further studies are required to investigate the effects of recurrent seizure activity and to characterise mdr expression in other experimental seizure models.

Expression of P-glycoprotein and cytochrome p450 1A in intertidal fish (Anoplarchus purpurescens) exposed to environmental contaminants

Whether P-glycoproteins (P-gps) like those which confer multidrug resistance in tumor cell lines are important in adaptation to chemicals in natural populations of vertebrates exposed to contaminant mixtures is the focus of this study. P-gp expression was examined in the intertidal fish high cockscomb blenny (Anoplarchus purpurescens) exposed to crude oil or pulp mill effluent. The relationship between P-gp expression and cytochrome p450 1A (CYP1A) induction also was investigated. Immunohistochemical (IHC) analysis revealed that levels of P-gp expression in the bile canaliculi were three- to five-fold greater in oil exposed fish than in control fish. Levels of P-gp expression were highly correlated with hepatic CYP1A levels previously measured in these fish. In fish from sites near pulp mills, P-gp expression in freshly caught fish did not correlate with proximity to pulp mills. However, hepatic P-gp expression levels in freshly caught fish were 14-fold higher than in fish from those sites that were depurated in clean water for 6 weeks. CYP1A levels were also elevated in liver of freshly caught as compared with depurated fish. Expression of neither CYP1A nor P-gp was elevated in depurated fish exposed to sediment and food from within the original pulp mill effluent stream. Depurated fish, which were injected with the aryl hydrocarbon receptor (AHR) agonist ss-naphthoflavone (BNF) showed an expected induction of CYP1A but no induction of P-gp. These results suggest that in blennies, unlike CYP1A, P-gp expression is not regulated by the AHR pathway; although P-gp and CYP1A both may be induced by some compounds in petroleum and unidentified xenobiotics at field sites. While our data indicate that CYP1A and P-gp are not coordinately regulated, these proteins may play complementary roles in cellular detoxification. Thus the elevation of P-gp activity may be an important mechanism of multixenobiotic resistance for organisms, such as intertidal fish, which are commonly exposed to anthropogenic contaminants and naturally occurring toxins.

Heterogeneity of ductular reactions in adult rat and human liver revealed by novel expression of deleted in malignant brain tumor 1

The regenerative capacity of mammalian adult liver reflects the ability of a number of cell populations within the hepatic lineage to take action. Limited information is available regarding factors and mechanisms that determine the specific lineage level at which liver cells contribute to liver repair as well as the fate of their progeny in the hostile environment created by liver injury. In the present study, we attempted to identify novel molecules preferentially involved in liver regeneration by recruitment of transit-amplifying, ductular (oval) cell populations. With a subtractive cDNA library screening approach, we identified 48 enriched, nonredundant gene products associated with liver injury and oval cell proliferation in the adult rat liver. Of these, only two, namely alpha-fetoprotein and a novel transcript with high homology to human DMBT1 (deleted in malignant brain tumor 1), were specifically associated with the emergence of ductular (oval) cell populations in injured liver. Subsequent cloning and characterization of the rat DMBT1 homologue revealed a highly inducible expression in ductular reactions composed of transit-amplifying ductular (oval) cells, but not in ductular reactions after ligation of the common bile duct. In human liver diseases, DMBT1 was expressed in ductular reactions after infection with hepatitis B and acetaminophen intoxication, but not in primary biliary cirrhosis, primary sclerosing cholangitis, and obstruction of the large bile duct. The expression heterogeneity in ductular reactions and multiple functions of DMBT1 homologues point to intriguing roles in regulating not only tissue repair but also fate decision and differentiation paths of specific cell populations in the hepatic lineage.

Expression of P-glycoprotein in killifish (Fundulus heteroclitus) exposed to environmental xenobiotics

P-glycoproteins (P-gp) are transmembrane efflux flippases that prevent the cellular accumulation of moderately hydrophobic compounds and are responsible for certain multidrug resistance phenotypes in tumor cell lines and human patients. We investigated whether P-gps could be involved in a contaminant resistant phenotype observed in a population of fish exposed over generations to high levels of planar halogenated aromatic hydrocarbons (PHAHs). Hepatic and intestinal epithelial P-gp expression was examined by immunoblot and immunohistochemistry in killifish (Fundulus heteroclitus) from New Bedford Harbor, MA (NBH), a Superfund site highly contaminated with PHAHs, and from Scorton Creek on Cape Cod, MA (SC), a relatively unpolluted site. The NBH population has developed resistance to the toxicity of PHAHs. Hepatic P-gp levels were more than 40% greater in fish freshly collected from SC than in fish freshly collected from NBH. When killifish from either site were maintained in clean water for up to 78 days to permit depuration of bioaccumulated contaminants, hepatic P-gp levels decreased approximately 50% by day 8. P-glycoprotein expression was detected in the intestinal epithelium in 55% of freshly collected NBH fish. However, depurated NBH fish and freshly caught and depurated SC fish rarely expressed P-gp in the intestine. In an effort to determine whether environmental chemicals at the two sites might contribute to altered P-gp expression, depurated fish were exposed either to sediment collected from SC or 2,3,7,8-tetrachlorodibenzofuran, a contaminant found at the NBH site and a model aryl hydrocarbon receptor agonist. Neither exposure affected hepatic P-gp levels in killifish. Elevated intestinal P-gp in NBH fish might counter the absorption of P-gp substrates/inducers and thus limit the amount of these compounds reaching the liver, which might account for the lower hepatic P-gp levels in NBH fish compared to SC fish. The differences in hepatic P-gp levels (SC>NBH) and intestinal P-gp (NBH>SC) in freshly collected fish also might reflect environmental exposure to different anthropogenic contaminants or microbial, algal, plant or other natural products via the water column, sediment, or diet at each site.

Development of a real-time in vivo transcription assay: application reveals pregnane X receptor-mediated induction of CYP3A4 by cancer chemotherapeutic agents

We report the development of a rapid real-time assay that measures the transcription of luciferase reporter genes in transduced mouse hepatic cells in vivo. Luciferase activity is noninvasively measured by whole-body optical imaging within hours of the hydrodynamic injection of as little as 1 microg of naked DNA. Transcription of genes introduced as linearized DNA can be serially assayed for weeks in each animal. Transcription was quantified by extracorporal monitoring of bioluminescence as well as or better than by traditional in vitro bioluminescence assay. Our assay allows the measurement of transcription as it occurs, under the most informative biological conditions (i.e., in a living, intact organ). Furthermore, it substantially reduces the cost, time, and number of animals required for analysis of gene expression. The utility of the method is demonstrated in the discovery that topotecan and etoposide are ligands of pregnane X receptor that induce CYP3A4 transcription.

Induction of P-glycoprotein, glutathione-S-transferase and cytochrome P450 in rat liver by atrazine

We studied the effects of intraperitoneally administered atrazine on two hepatic neoplastic markers, P-glycoprotein (P-gp), and glutathione-S-transferase (GST), and several phase I drug-metabolizing enzyme cytochrome P450 (CYP) subfamilies in hepatic microsomes and cytosol of Fischer rats. The P-gp content was increased after 24 h of atrazine administration at 50 mg/kg, and maximum P-gp induction was observed at 300 mg/kg for 3 days. GST-P was induced at a lower dose than P-gp, from 10 mg/kg, but no other form of GST, such as GST1A1, was induced by the same dose. Among the CYP families, CYP1A2 was highly and CYP2B was slightly induced by atrazine while the CYP3A content remained unchanged. The liver plasma membrane marker alkaline phosphatase (AP) was not induced by the same doses. The inductions of P-gp, GST-P and CYP1A2 observed may explain some of the reported tumor-promoting properties and toxicity of atrazine in vivo.

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.

Transcellular transport of organic anions across a double-transfected Madin-Darby canine kidney II cell monolayer expressing both human organic anion-transporting polypeptide (OATP2/SLC21A6) and Multidrug resistance-associated protein 2 (MRP2/ABCC2)

Human organic anion transporting polypeptide 2 (OATP2/SLC21A6) and multidrug resistance-associated protein 2 (MRP2/ABCC2) play important roles in the vectorial transport of organic anions across hepatocytes. In the present study, we have established a double-transfected Madin-Darby canine kidney (MDCK II) cell monolayer, which expresses both OATP2 and MRP2 on basal and apical membranes, respectively. The basal-to-apical transport of 17 beta estradiol 17 beta-d-glucuronide (E(2)17 beta G), pravastatin, and leukotriene C(4) (LTC(4)), which are substrates of OATP2 and MRP2, was significantly higher than that in the opposite direction in the double-transfected cells. Such vectorial transport was also observed for taurolithocholate sulfate, which is transported by rat oatp1 and Mrp2. The K(m) values of E(2)17 beta G and pravastatin for the basal-to-apical flux were 27.9 and 24.3 microm, respectively, which were comparable with those reported for OATP2. Moreover, the MRP2-mediated export of E(2)17 beta G across the apical membrane was not saturated. In contrast, basal-to-apical transport of estrone-3-sulfate and dehydroepiandrosterone sulfate, which are significantly transported by OATP2, but not by MRP2, was not stimulated by MRP2 expression. The double-transfected MDCK II monolayer expressing both OATP2 and MRP2 may be used to analyze the hepatic vectorial transport of organic anions and to screen the transport profiles of new drug candidates.

Differential regulation of P-glycoprotein genes in primary rat hepatocytes by collagen sandwich and drugs

P-glycoprotein (Pgp) is a small family of plasma membrane proteins, which are capable of transporting substrates across cell membranes. Class I and II Pgp are able to transport drugs and have been shown to mediate multidrug resistance (MDR). Class III Pgp is a long chain phospholipid transporter and does not mediate MDR. The regulation of all three Pgp genes is still poorly understood. For instance, it is not clear if the three Pgp genes are co-regulated or differentially regulated by external stimuli. This study examined the effect of drugs and collagen sandwich system on expression and transcription of all the three Pgp genes in primary rat hepatocytes. Consistent with previous findings, dramatic overexpression (25-fold) of Class II Pgp mRNA was seen, upon culturing of hepatocytes onto a single layered collagen gel. Hepatocytes sandwiched between two layers of collagen gel exhibited decreased (4.5-fold) Class II Pgp mRNA expression as compared to the single layer system. Treatment of hepatocytes cultured on the single layer collagen system with cytoskeletal disrupting (cytochalasin D, colchicine) but not cytoskeletal stabilizing (phalloidin, taxol) drugs, suppressed Class II Pgp expression. In all cases, no change in Class II Pgp transcription was observed as demonstrated by nuclear run-on studies. This suggests that collagen configuration and drugs affect Class II Pgp mRNA expression predominantly through post-transcriptional mechanisms. In contrast, parallel increases in mRNA expression and transcription of Class I Pgp gene were observed upon culturing of hepatocytes, in the collagen sandwich system, and treatment with some drugs (cytochalasin D, colchicine, and phalloidin). This suggests that Class I Pgp gene is regulated primarily via transcriptional mechanisms by these stimuli. On the other hand, Class III Pgp gene appears to be post-transcriptionally co-regulated with Class II Pgp gene by treatment with the drugs, while collagen configuration affected both transcription and post-transcription of Class III Pgp gene. Finally, dose-dependent studies using cycloheximide provided further evidence that the two MDR-associated genes are not co-regulated. This study has implications for future studies on the molecular mechanisms of Pgp gene regulation.

P-glycoprotein in the catfish intestine: inducibility by xenobiotics and functional properties

The p-glycoprotein (pgp)-mediated multixenobiotic resistance (MXR) mechanism of aquatic animals has been associated with protection against pollution. Recent studies in mammals suggest that intestinal pgp may modulate intestinal bioavailability of dietary xenobiotics. In order to further delineate this mechanism in the catfish, these studies: (1) examined the pgp-related distribution in the intestine and liver of catfish, (2) evaluated the MXR response following exposure to various dietary xenobiotics and a prototypic pgp inducer and (3) evaluated pgp functional activity in membrane vesicles, using prototypic substrates and inhibitors. For this purpose, catfish were exposed in vivo to the pgp inducer vincristine (VIN), and the xenobiotics beta-naphthoflavone (BNF), benzo[a]pyrene (BaP), and 3,4,3',4'-tetrachlorobiphenyl (TCB). Membrane vesicles, prepared from liver and intestine (proximal and distal sections) of control and exposed catfish, were subjected to SDS PAGE, Western Blot, and detection with the pgp C219 monoclonal antibody. Transport activity was evaluated in vitro using the pgp substrate [3H]vinblastine (VBL), and the pgp inhibitor verapamil (VP). Immunoblot studies demonstrated a pgp-related protein of approximately 170 kDa in the intestine and liver of catfish. This protein appears to be very susceptible to degradation, and was present in higher levels in the liver, in comparison to the intestine, where regional differences were not observed. Dietary exposure to the pgp substrate VIN, or the xenobiotics BNF, BaP, and TCB, did not appear to affect pgp-related reactivity. Transport studies with VBL indicate that the pgp-related protein of the catfish intestine displays classic pgp-mediated multidrug resistance (MDR) characteristics, such as energy-dependency, and sensitivity to VP. These studies suggest that the pgp-related protein in the catfish intestine and liver is not only immunochemically, but also functionally related to the mammalian MDR. Moreover, the results presented indicate that pgp-related reactivity and transport in intestinal vesicles of catfish may be influenced by factors including method sensitivity, sample collection, sample preparation, and immunoblot conditions.

Prochloraz and nonylphenol diethoxylate inhibit an mdr1-like activity in vitro, but do not alter hepatic levels of P-glycoprotein in trout exposed in vivo

P-glycoproteins (P-gps) encoded by multidrug resistance 1 (mdr1) genes are ATP-dependent transporters located in the cytoplasmic membrane which mediate the efflux of a broad spectrum of hydrophobic compounds from the cell. The tissue distribution of P-gps suggests their role in the organismal defense against xenobiotics by effecting xenobiotic excretion and reducing xenobiotic uptake. In the present work, the interaction of P-gp(s) in the liver and in primary cultured hepatocytes of rainbow trout with two model pollutants was studied - the imidazole fungicide prochloraz and the alkylphenolic surfactant nonylphenol diethoxylate (NP2EO). Using a monoclonal antibody (mAB C219) directed against a conserved P-gp epitope, an immunoreactive protein of 160 kDa was detected in immunoblots of liver extracts from control trout. In sections of control trout livers, immunohistochemistry with the mAB C219 resulted in specific staining of bile canaliculi. In juvenile trout exposed for 7 days to sublethal concentrations of prochloraz (0.027 microM; 0.27 microM) or NP2EO (0.32 microM; 1.30 microM), no changes in levels of hepatic P-gp(s) were found in immunoblot and immunochemical investigations. The efflux of the fluorescent mdr 1 substrate rhodamine 123 (Rh123) from cultured isolated trout hepatocytes was partly inhibited by verapamil and vinblastine, compounds known to interfere with mdr 1-dependent transport. This demonstrates the presence of a mdr1-like mechanism in trout liver which is probably involved in the biliary excretion of hydrophobic xenobiotics. Non-cytotoxic concentrations of prochloraz and NP2EO were tested for effects on the efflux of Rh123 from trout hepatocytes. Prochloraz was a potent inhibitor of the mdr1-like mechanism, being effective at 0.3 microM and above. NP2EO inhibited Rh123 efflux only at the highest concentration tested (31.6 microM). The accumulation and elimination of 14C-prochloraz by cultured trout hepatocytes was not affected by mdr 1-type substrates (Rh123, vinblastine) and a mdr 1 inhibitor (verapamil). This shows that prochloraz is, despite its inhibitory potency, not a substrate of the mdr1-like mechanism in trout liver. The inhibition by prochloraz and NP2EO of the md r1-like mechanism in trout hepatocytes suggests that water pollutants can interfere with P-gp-function in fish and thus may impair the organismal defense against xenobiotics.

Aromatic hydrocarbon receptor (AhR).AhR nuclear translocator- and p53-mediated induction of the murine multidrug resistance mdr1 gene by 3-methylcholanthrene and benzo(a)pyrene in hepatoma cells

The mouse multidrug resistance gene family consists of three genes (mdr1, mdr2, and mdr3) encoding P-glycoprotein. We show that the expression of mdr1 is increased at the transcriptional level upon treatment of the hepatoma cell line Hepa-1c1c7 with the polycyclic aromatic hydrocarbon 3-methylcholanthrene (3-MC). This increase is not observed in the aromatic hydrocarbon receptor (AhR)-defective TAOc1BP(r)c1 and the AhR nuclear translocator (Arnt)-defective BP(r)c1 variants, demonstrating that the induction of mdr1 by 3-MC requires AhR.Arnt. We show that the mdr1 promoter (-1165 to +84) is able to activate the expression of a reporter gene in response to 3-MC in Hepa-1c1c7 but not in BP(r)c1 cells. Deletion analysis indicated that the region from -245 to -141 contains cis-acting sequences mediating the induction, including a potential p53 binding sequence. 3-MC treatment of the cells increased the levels of p53 and induced p53 binding to the mdr1 promoter in an AhR.Arnt-dependent manner. Mutations in the p53 binding site abrogated induction of mdr1 by 3-MC, indicating that p53 binding to the mdr1 promoter is essential for the induction. Benzo(a)pyrene, a polycyclic aromatic hydrocarbon and AhR ligand, which, like 3-MC, is oxidized by metabolizing enzymes regulated by AhR.Arnt, also activated p53 and induced mdr1 transcription. 2,3,7,8-Tetrachlorodibenzo-p-dioxin, an AhR ligand resistant to metabolic breakdown, had no effect. These results indicate that the transcriptional induction of mdr1 by 3-MC and benzo(a)pyrene is directly mediated by p53 but that the metabolic activation of these compounds into reactive species is necessary to trigger p53 activation. The ability of the anticancer drug and potent genotoxic agent daunorubicin to induce mdr1 independently of AhR.Arnt further supports the proposition that mdr1 is transcriptionally up-regulated by p53 in response to DNA damage.

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

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

2-acetylaminofluorene up-regulates rat mdr1b expression through generating reactive oxygen species that activate NF-kappa B pathway

Overexpression of multidrug resistance genes and their encoded P-glycoproteins is a major mechanism for the development of multidrug resistance in cancer cells. The hepatocarcinogen 2-acetylaminofluorene (2-AAF) efficiently activates rat mdr1b expression. However, the underlying mechanisms are largely unknown. In this study, we demonstrated that a NF-kappa B site on the mdr1b promoter was required for this induction. Overexpression of antisense p65 and I kappa B alpha partially abolished the induction. We then delineated the pathway through which 2-AAF activates NF-kappa B. 2-AAF treatment led to the increase of intracellular reactive oxygen species (ROS) which causes activation of IKK kinases, degradation of I kappa B beta (but not I kappa B alpha), and increase in NF-kappa B DNA binding activity. Consistent with the idea that ROS may participate in mdr1b regulation, antioxidant N-acetylcysteine inhibited the induction of mdr1b by 2-AAF. Overproduction of a physiological antioxidant glutathione (GSH) blocked the activation of IKK kinase complex and NF-kappa B DNA binding. Based on these results, we conclude that 2-AAF up-regulates mdr1b through the generation of ROS, activation of IKK kinase, degradation of I kappa B beta, and subsequent activation of NF-kappa B. This is the first report that reveals the specific cis-elements and signaling pathway responsible for the induction of mdr1b by the chemical carcinogen 2-AAF.

Unaltered expression of multidrug resistance transporters in polycyclic aromatic hydrocarbon-resistant rat liver cells

Rat liver epithelial cells resistant to the chemical carcinogen 3MC, termed F258/3MC cells and generated by long-term exposure of parental F258 cells to the PAH, were characterized, especially with respect to expression of multidrug resistance transporters such as P-glycoprotein, MRP1 and MRP2. F258/3MC cells were found to be cross-resistant to other PAHs such as BP and dimethylbenz(a)anthracene but remained sensitive to known substrates of multidrug resistance efflux pumps such as doxorubicin and vincristine. They did not display either decreased cellular PAH accumulation or increased PAH efflux. In addition, P-glycoprotein and MRP2 mRNA levels were not, or only barely detected, in F258/3MC cells and in their parental counterparts whereas these PAH-resistant and sensitive cells showed closed levels of MRP1 mRNAs and activity. Moreover, P-gp- and MRP1-overexpressing cells were shown to display similar accumulation and efflux of BP than those found in P-gp- and MRP1-negative control cells. These data therefore suggest that multidrug resistance transporters do not contribute to PAH resistance in PAH-selected liver cells.

Induction of P-glycoprotein mRNA transcripts by cycloheximide in animal tissues: evidence that class I Pgp is transcriptionally regulated whereas class II Pgp is post-transcriptionally regulated

P-glycoprotein (Pgp) are a small family of plasma membrane proteins capable of transporting substrates across cell membranes. Class I and class II Pgp are able to transport drugs and have been shown to mediate multidrug resistance (MDR). Class III Pgp is a long chain phospholipid transporter and does not mediate MDR. The expression and regulation of Pgp genes in animal tissues are not well understood. In this study, the protein synthesis inhibitor cycloheximide was used as a tool to understand Pgp gene expression and regulation in animal tissues. The sensitive RNase protection assay was used to detect changes in Pgp mRNA levels and nuclear run-on assay was used to determine whether transcription or post-transcription is important. The results showed that cycloheximide significantly induced class II Pgp expression in all tissues examined. This was predominantly through post-transcriptional effect. In contrast, the relatively modest increase in class I Pgp expression by cycloheximide was found to be mainly due to increased transcriptional activity. On the other hand, cycloheximide induced class III Pgp expression in some tissues while caused decay of class III Pgp mRNA in other tissues. The transcriptional and post-transcriptional mechanisms exerted by cycloheximide on Pgp genes are discussed. These findings have implications for our understanding of gene regulation in animal tissues and MDR reversal strategies in vivo.

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

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

Endotoxin downregulates hepatic expression of P-glycoprotein and MRP2 in 2-acetylaminofluorene-treated rats

In liver, the ATP-dependent transporters P-glycoprotein (PGP) and multidrug resistance protein-2 (MRP2) are involved in the secretion of numerous drugs and toxins in bile. Although constitutive levels of PGP and MRP-2 are decreased in rat liver after exposure to endotoxin, it is possible that induced forms of these transporters may be alternately affected. In vitro, the hepatocarcinogen, 2-acetylaminofluorene (AAF) induces expression of PGP and MRP2. Thus, we examined the influence of endotoxin on the expression of PGP and MRP2 in AAF-treated rats. Expression of PGP and MRP2 was analyzed on Westerns and by RT-PCR in livers obtained from endotoxin and control groups. In vivo, AAF treatment significantly induced PGP/mdr1 expression and imposed a significant reduction in the expression of spgp. MRP2 protein and mRNA levels were not altered by AAF administration. Endotoxin administration to both AAF-treated and non-AAF-treated rats elicited significant reductions in the protein and mRNA expression of MRP2 and PGP (P < 0.05). Our data indicate that endotoxin suppresses the overexpression of PGP and constitutive expression of MRP2 in AAF-treated rats. Furthermore, in vivo administration of AAF, which maximally induces PGP does not induce MRP2.

Association of tamoxifen biliary excretion rate with prior tamoxifen exposure and increased mdr1b expression

ATPase transporter proteins are commonly found in the hepatocyte canalicular membrane. Some of these, in particular the multidrug resistance (mdr1b) gene, have been previously demonstrated to be inducible genes. In this study, we found that tamoxifen induced expression of the mdr1b gene in the liver up to 40-fold after 14 days' exposure to tamoxifen in the diet at a concentration of 420 ppm. As tamoxifen and its metabolites are primarily excreted into the bile, we investigated if the increased expression of mdr1b in the liver following tamoxifen exposure had any effect on its excretion in rats. We found that the excretion of tamoxifen and its metabolites into bile was increased from 8 +/- 1% to 51 +/- 18% (mean +/- SD) of an administered dose of 180 nmol/kg over a collection period of 3 hr in rats that had received tamoxifen (35 mg/kg) orally for 12 days (plus a 3-day rest) prior to the experiment. These data suggest that prolonged treatment with tamoxifen may result in lower serum and tumour concentrations, due to a self-mediated enhancement of excretion via mdr1b gene-encoded P-glycoprotein. This may have implications for other drugs sharing the same route of excretion and co-administered with tamoxifen.

Induction of cytochrome P450 (CYP)1A1, CYP1A2, and CYP3A4 but not of CYP2C9, CYP2C19, multidrug resistance (MDR-1) and multidrug resistance associated protein (MRP-1) by prototypical inducers in human hepatocytes

Human hepatocytes cultured serum-free for up to 6 weeks were used to study expression and induction of enzymes and membrane transport proteins involved in drug metabolism. Phase I drug metabolizing enzymes cytochrome P450 (CYP)1A1, CYP1A2, CYP2C9, CYP2C19, CYP2E1, and CYP3A4 were detected by Western blot analyses and, when appropriate, by enzymatic assays for ethoxyresorufin-O-deethylase(EROD)-activity and testosterone-6beta-hydroxylase(T6H)-activity. Expression of the membrane transporter multi-drug resistance protein (P-glycoprotein, MDR-1), multidrug resistance-associated protein (MRP-1), and lung-resistance protein (LRP) was maintained during the culture as detected by RT-PCR and Western blot analyses. Model inducers like rifampicin, phenobarbital, or 3-methylcholanthrene and beta-naphtoflavone were able to induce CYP1A or CYP3A4 as well as EROD or T6H activities for up to 30 days. CYP2C9, CYP2C19 and CYP2E1 expression was maintained but not inducible for 48 days. Also, rifampicin and phenobarbital were unable to increase MDR-1 and MRP-1 protein levels significantly.

Multidrug-resistance transporters

P-glycoprotein was initially isolated due to its role in multidrug resistance to cancer chemotherapeutics. Recent work, however, makes it increasingly apparent that this transporter is also involved in the pharmacokinetics of many drugs. P-gp is strategically expressed in the luminal epithelial cells of organs often associated with drug absorption and disposition, for example, hepatocyte canalicular membrane, renal proximal tubules, and the intestinal mucosa. P-gp is also expressed in the endothelial cells comprising the blood-brain barrier. This localization clearly suggests the potential for this protein to serve as a protective mechanism against entry of toxic xenobiotics and also suggests that P-gp is well situated to participate in the removal of therapeutic agents. Numerous investigations with drugs such as digoxin, etoposide, cyclosporine, vinblastine, Taxol, loperamide, dom-peridone, and ondansteron demonstrate that P-gp has an important role in determining the pharmacokinetics of substrate drugs. Pharmacological modulation of P-gp function to increase drug bioavailability, both on a organismal and a cellular level, is one approach currently being explored to enhance therapeutic effectiveness. This approach is not without potential collateral consequences given the wide tissue distribution of P-gp. While animals deficient in P-gp are viable and without obvious abnormalities, the pharmacokinetics and toxic consequences of several compounds are significantly altered in these animals. Thus blockade of the protective P-gp barrier in humans may have adverse effects on substrate drugs. In particular, this situation may arise when several compounds which may be substrates compete for P-gp-mediated transport. Additional multidrug transporters, notably MRP and family members, have been identified and may also determine the fate of pharmaceuticals. Further understanding the physiological role of each of the multidrug transporters is critical for determining their role in pharmacokinetics and for evaluating the consequences of modification of their activities. Such information is also important in the development of novel drugs which may be substrates for these transporters.

Interspecies differences in P-glycoprotein mediated activity of multixenobiotic resistance mechanism in several marine and freshwater invertebrates

The presence and function of the P-glycoprotein mediated multixenobiotic resistance (MXR) mechanism was demonstrated in numerous aquatic organisms. The aim of this study was to investigate whether in aquatic organisms exists the inherent, species-specific basal level of MXR activity. Here the results of the direct comparison of the basal (noninduced) level of MXR activity measured in several marine (Mytilus galloprovincialis, Monodonta turbinata, Patella lusitanica) and freshwater (Dreissena polymorpha, Viviparus viviparus, Anodonta cygnea) molluscs species are presented. The primary criterion for the assessment and quantification of the basal level of MXR activity was the ratio (R) between the accumulation or efflux of the fluorescent model MXR substrates (rhodamine B or rhodamine 123) in or from the gills, measured with and in the absence of model MXR inhibitors verapamil or cyclosporin A. Significantly different levels of MXR activity were found in the species investigated. These levels generally show a relatively good correlation with the level of pollution present in their natural habitats. Considering these results a conclusion was reached that in aquatic organisms indeed exist the different inherent, species-specific levels of MXR activity. The identified levels might be, at least partly, responsible either for the resistance to, or for the sensitivity of a particular species to organic pollution.

Multixenobiotic resistance as a cellular defense mechanism in aquatic organisms

Multixenobiotic resistance in aquatic organisms exposed to natural toxins or anthropogenic contaminants is a phenomenon analogous to multidrug resistance in mammalian tumor cell lines tolerant of anti-cancer drugs. Multidrug resistance is commonly due to the elevated expression of transmembrane P-glycoproteins (P-gp) which actively transport a wide variety of structurally and functionally diverse compounds. The purpose of this review is to place aquatic ecotoxicological data in context of the larger multidrug resistance field of study. Information on P-glycoproteins structure, mechanism of transport, and substrate specificity gained through traditional mammalian and cell culture models is examined in conjunction with recent work on aquatic species exposed to xenobiotics both in the field and in the laboratory. The physiological function of P-glycoproteins is explored through studies of gene knockout models and expression patterns in normal tissues and tumors. The effect of xenobiotic exposures on P-gp activity and protein titer is examined in wild and captive populations of aquatic invertebrates and vertebrates. Substrate overlap and evidence of co-expression of phase I detoxification enzymes (e.g. cytochromes P450) and P-gp are presented. The role of P-gp chemosensitizers as environmental pollutants and the ecotoxicological consequences of P-gp inhibition are highlighted. The overwhelming evidence suggests that P-glycoproteins provide aquatic organisms with resistance to a wide range of natural and anthropogenic toxins.

Characterization of inducible nature of MRP3 in rat liver

We found previously that expression of multidrug resistance-associated protein (MRP) 3 is induced in a mutant rat strain (Eisai hyperbilirubinemic rats) whose canalicular multispecific organic anion transporter (cMOAT/MRP2) function is hereditarily defective and in normal Sprague-Dawley (SD) rats after ligation of the common bile duct. In the present study, the inducible nature of MRP3 was examined, using Northern and Western blot analyses, in comparison with that of other secondary active [Na(+)-taurocholic acid cotransporting polypeptide (Ntcp), organic anion transporting polypeptide 1 (oatp1), and organic cation transporter (OCT1)] and primary active [P-glycoprotein (P-gp), cMOAT/MRP2, and MRP6] transporters. alpha-Naphthylisothiocyanate treatment and common bile duct ligation induced expression of P-gp and MRP3, whereas expression of Ntcp, oatp1, and OCT1 was reduced by the same treatment. Although expression of MRP3 was also induced by administration of phenobarbital, that of cMOAT/MRP2, MRP1, and MRP6 was not affected by any of these treatments. Moreover, the mRNA level of MRP3, but not that of P-gp, was increased in SD rats after administration of bilirubin and in Gunn rats whose hepatic bilirubin concentration is elevated because of a defect in the expression of UDP-glucuronosyl transferase. However, the MRP3 protein level was not affected by bilirubin administration. Although the increased MRP3 mRNA level was associated with the increased concentration of bilirubin and/or its glucuronides in mutant rats and in SD rats that had undergone common bile duct ligation or alpha-naphthylisothiocyanate treatment, we must assume that factor(s) other than these physiological substances are also involved in the increased protein level of MRP3.

Differential regulation of canalicular multispecific organic anion transporter (cMOAT) expression by the chemopreventive agent oltipraz in primary rat hepatocytes and in rat liver

Expression of the canalicular multispecific organic anion transporter (cMOAT), an efflux pump involved in biliary secretion of xenobiotics, was investigated in rat hepatocytes exposed to the chemopreventive agent oltipraz. Northern blotting indicated that this compound increased cMOAT mRNA levels in primary cultured hepatocytes. Such an induction of cMOAT transcripts was demonstrated to be dose-dependent and started as early as 4 h treatment; in addition, western blotting showed increased levels of 190 kDa cMOAT in oltipraz-treated primary rat hepatocytes when compared with their untreated counterparts. In contrast, administration of oltipraz to rats failed to enhance hepatic cMOAT mRNA and protein amounts whereas it was found to induce liver expression of glutathione S-transferase P1, a well-known oltipraz-regulated drug metabolizing enzyme. These data therefore suggest that cMOAT up-regulation occurring in rat hepatocytes in response to oltipraz may be restricted to in vitro situations and is therefore unlikely to be directly involved in the in vivo chemopreventive properties of oltipraz.

Inhibition of P-glycoprotein by D-alpha-tocopheryl polyethylene glycol 1000 succinate (TPGS)

PURPOSE

To investigate whether d-alpha-tocopheryl polyethylene glycol 1000 succinate (TPGS) functions as an inhibitor of P-glycoprotein (P-gp), the multidrug resistance transporter.

METHODS

Two assays were used to measure the function of TPGS on P-gp function. First, we examined the ability of TPGS to modulate the cytotoxicity of established, cytotoxic, P-glycoprotein substrates. Parental NIH 3T3 cells and NIH 3T3 cells transfected with the human MDR1 cDNA (G185) were exposed to doxorubicin, paclitaxel, colchicine, vinblastine and 5-fluorouracil (5FU) in the presence or absence of TPGS. Cytotoxicity was assessed with the MTT assay. Second, polarized transport of the P-gp substrates rhodamine 123 (R123), paclitaxel and vinblastine was measured using the human intestinal HCT-8 and Caco-2 cell lines grown in Transwell dishes. Drug flux was measured by liquid scintillation counting or fluorescence spectroscopy of the media.

RESULTS

G185 cells were 27-135 fold more resistant to the cytotoxic drugs doxorubicin, vinblastine, colchicine and paclitaxel than the parental NIH 3T3 cells. In contrast 5FU, which is not a P-gp substrate, is equally cytotoxic to parental and G185 cells. Co-administration of TPGS enhanced the cytotoxicity of doxorubicin, vinblastine, paclitaxel, and colchicine in the G185 cells to levels comparable to the parental cells. TPGS did not increase the cytotoxicity of 5FU in the G185 cells. Using a polarized epithelial cell transport assay, TPGS blocked P-gp mediated transport of R123 and paclitaxel in a dose responsive manner.

CONCLUSIONS

These data demonstrate that TPGS acts as a reversal agent for P-glycoprotein mediated multidrug resistance and inhibits P-gp mediated drug transport. These results suggest that enhanced oral bioavailability of drugs co-administered with TPGS may, in part, be due to inhibition of P-glycoprotein in the intestine.

3-Hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors (statins) induce hepatic expression of the phospholipid translocase mdr2 in rats

BACKGROUND & AIMS

Biliary cholesterol secretion is coupled to that of phospholipids in a process controlled by mdr2 P-glycoprotein activity and bile salt secretion. Statins, the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors, have been shown to affect hepatobiliary lipid secretion in rats. The aim of this study was to relate the effects of statins on bile formation to the expression of mdr2 and other hepatic adenosine triphosphate-dependent transport proteins involved in bile formation in rats.

METHODS

Rats received simvastatin- or pravastatin-containing chow continuously for 5 days. In one group of rats, simvastatin treatment was withdrawn 9-12 hours before the end of the experiment to induce biliary cholesterol hypersecretion (rebound). Bile and liver tissue were collected for lipid analysis, and hepatic messenger RNA (mRNA) and protein levels were studied by reverse-transcription polymerase chain reaction, immunoblotting, and immunohistochemistry.

RESULTS

Simvastatin feeding did not alter biliary bile salt secretion. Secretion of phospholipids and cholesterol was stimulated by 74% and 90%, respectively, in the simvastatin-continuous group and by 72% and 235%, respectively, in the rebound group compared with controls. mdr2 mRNA levels increased only in the continuous group. mdr2 protein levels increased in both simvastatin-fed groups. Induction was most pronounced in periportal hepatocytes. mdr1b mRNA levels were moderately increased in both simvastatin-fed groups. Levels of other hepatic transport proteins did not change. Similar results were obtained in pravastatin-fed rats.

CONCLUSIONS

Statins increase expression of mdr2 and mdr1b in rats, revealing a novel effect of these commonly used drugs.

Reactive oxygen species participate in mdr1b mRNA and P-glycoprotein overexpression in primary rat hepatocyte cultures

P-glycoproteins encoded by multidrug resistance type 1 (mdr1) genes mediate ATP-dependent efflux of numerous lipophilic xenobiotics, including several anticancer drugs, from cells. Overexpression of mdr1-type transporters in tumour cells contributes to a multidrug resistance phenotype. Several factors shown to induce mdr1 overexpression (UV irradiation, epidermal growth factor, tumour necrosis factor alpha, doxorubicin) have been associated with the generation of reactive oxygen species (ROS). In the present study, primary rat hepatocyte cultures that exhibit time-dependent overexpression of the mdr1b gene were used as a model system to investigate whether ROS might participate in the regulation of intrinsic mdr1b overexpression. Addition of H2O2 to the culture medium resulted in a significant increase in mdrlb mRNA and P-glycoprotein after 3 days of culture, with maximal (approximately 2-fold) induction being observed with 0.5-1 mM H2O2. Furthermore, H2O2 led to activation of poly(ADP-ribose) polymerase, a nuclear enzyme activated by DNA strand breaks, indicating that ROS reached the nuclear compartment. Thus, extracellularly applied H2O2 elicited intracellular effects. Treatment of rat hepatocytes with the catalase inhibitor 3-amino-1,2,4-triazole (2-4 mM for 72 h or 10 mM for 1 h following the hepatocyte attachment period) also led to an up-regulation of mdrlb mRNA and P-glycoprotein expression. Conversely, antioxidants (1 mM ascorbate, 10 mM mannitol, 2% dimethyl sulphoxide, 10 mM N-acetylcysteine) markedly suppressed intrinsic mdr1b mRNA and P-glycoprotein overexpression. Intracellular steady-state levels of the mdrl substrate rhodamine 123, determined as parameter of mdr1-type transport activity, indicated that mdr1-dependent efflux was increased in hepatocytes pretreated with H2O2 or aminotriazole and decreased in antioxidant-treated cells. The induction of mdr1b mRNA and of functionally active mdr1-type P-glycoproteins by elevation in intracellular ROS levels and the repression of intrinsic mdrlb mRNA and P-glycoprotein overexpression by antioxidant compounds support the conclusion that the expression of the mdr1b P-glycoprotein is regulated in a redox-sensitive manner.