Activation of distinct multidrug-resistance (P-glycoprotein) genes during rat liver regeneration and hepatocarcinogenesis

TrxR1, Gsr, and oxidative stress determine hepatocellular carcinoma malignancy

Thioredoxin reductase-1 (TrxR1)-, glutathione reductase (Gsr)-, and Nrf2 transcription factor-driven antioxidant systems form an integrated network that combats potentially carcinogenic oxidative damage yet also protects cancer cells from oxidative death. Here we show that although unchallenged wild-type (WT), TrxR1-null, or Gsr-null mouse livers exhibited similarly low DNA damage indices, these were 100-fold higher in unchallenged TrxR1/Gsr-double-null livers. Notwithstanding, spontaneous cancer rates remained surprisingly low in TrxR1/Gsr-null livers. All genotypes, including TrxR1/Gsr-null, were susceptible to N-diethylnitrosamine (DEN)-induced liver cancer, indicating that loss of these antioxidant systems did not prevent cancer cell survival. Interestingly, however, following DEN treatment, TrxR1-null livers developed threefold fewer tumors compared with WT livers. Disruption of TrxR1 in a marked subset of DEN-initiated cancer cells had no effect on their subsequent contributions to tumors, suggesting that TrxR1-disruption does not affect cancer progression under normal care, but does decrease the frequency of DEN-induced cancer initiation. Consistent with this idea, TrxR1-null livers showed altered basal and DEN-exposed metabolomic profiles compared with WT livers. To examine how oxidative stress influenced cancer progression, we compared DEN-induced cancer malignancy under chronically low oxidative stress (TrxR1-null, standard care) vs. elevated oxidative stress (TrxR1/Gsr-null livers, standard care or phenobarbital-exposed TrxR1-null livers). In both cases, elevated oxidative stress was correlated with significantly increased malignancy. Finally, although TrxR1-null and TrxR1/Gsr-null livers showed strong Nrf2 activity in noncancerous hepatocytes, there was no correlation between malignancy and Nrf2 expression within tumors across genotypes. We conclude that TrxR1, Gsr, Nrf2, and oxidative stress are major determinants of liver cancer but in a complex, context-dependent manner.

Spatiotemporal Changes in P-glycoprotein Levels in Brain and Peripheral Tissues Following Ischemic Stroke in Rats

P-glycoprotein (P-gp) is known to transport a diverse array of xenobiotics, including therapeutic drugs. A member of the ATP-binding cassette (ABC) transporter family, P-gp is a protein encoded by the gene Mdr1 in humans and Abcb1 in rodents (represented by 2 isoforms Abcb1a and Abcb1b). Lining the luminal and abluminal membrane of brain capillary endothelial cells, P-gp is a promiscuous efflux pump extruding a variety of exogenous toxins and drugs. In this study, we measured dynamic changes in Abcb1a and Abcb1b transcripts and P-gp protein in the brain, liver, and kidney after experimental stroke. P-glycoprotein has been shown to increase in brain endothelial cells following hypoxia in vitro or after exposure to proinflammatory cytokines. Using a rat model of ischemic stroke, we hypothesized that P-gp expression will be increased in the brain, liver, and kidney in response to neuroinflammation following ischemic stroke. Adult Sprague Dawley rats underwent middle cerebral artery occlusion (MCAO) for 90 minutes and were killed at 4, 14, 24, and 48 hours postreperfusion onset to determine the time course of P-gp expression. To mimic ischemia occurring at the blood-brain barrier, rat brain endothelial (RBE4) cells were subjected to hypoxia and low glucose (HLG) for 16 hours. Immunoblotting analyses showed P-gp increases in brain and liver following 90-minute MCAO, as well as in cultured RBE4 cells after 16-hour HLG treatment, but fluctuated in the kidney depending on the time point. The relative roles of each isoform in the protein expression were analyzed with quantitative reverse transcriptase polymerase chain reaction. Ischemic stroke leads to significant increases in P-gp levels not only in the brain but also in the liver. The increase in P-gp could dramatically reduce the bioavailability and efficacy of neuroprotective drugs. Therefore, P-gp represents a big hurdle to drug delivery to the ischemic brain.

Pharmacokinetics of drugs in adult living donor liver transplant patients: regulatory factors and observations based on studies in animals and humans

INTRODUCTION

Limited information is available on the pharmacokinetics of drugs in the donors and recipients following adult living donor liver transplantation (LDLT). Given that both the donors and recipients receive multiple drug therapies, it is important to assess the pharmacokinetics of drugs used in these patients.

AREAS COVERED

Pathophysiological changes that occur post-surgery and regulatory factors that may influence pharmacokinetics of drugs, especially hepatic drug metabolism and transport in both LDLT donors and the recipients are discussed. Pharmacokinetic data in animals with partial hepatectomy are presented. Clinical pharmacokinetic data of certain drugs in LDLT recipients are further reviewed.

EXPERT OPINION

It takes up to six months for the liver volume to return to normal after LDLT surgery. In the LDLT recipients, drug exposure generally is higher with lower clearance during early period post-transplant; lower initial dosages of immunosuppressants are used than deceased donor liver transplant recipients during the first six months post-transplantation. In animals, the activities of hepatic drug metabolizing enzymes and transporters are known to be altered differentially during liver regeneration. Future studies on the actual hepatic function with reference to drug metabolism, drug transport, and biliary secretion in both LDLT donors and recipients are required.

mRNA levels of related Abcb genes change opposite to each other upon histone deacetylase inhibition in drug-resistant rat hepatoma cells

The multidrug-resistant phenotype of tumor cells is acquired via an increased capability of drug efflux by ABC transporters and causes serious problems in cancer treatment. With the aim to uncover whether changes induced by epigenetic mechanisms in the expression level of drug transporter genes correlates with changes in the drug resistance phenotypes of resistant cells, we studied the expression of drug transporters in rat hepatoma cell lines. We found that of the three major rat ABC transporter genes Abcb1a, Abcb1b and Abcc1 the activity of only Abcb1b increased significantly in colchicine-selected, drug-resistant cells. Increased transporter expression in drug-resistant cells results primarily from transcriptional activation. A change in histone modification at the regulatory regions of the chromosomally adjacent Abcb1a and Abcb1b genes differentially affects the levels of corresponding mRNAs. Transcriptional up- and down-regulation accompany an increase in acetylation levels of histone H3 lysine 9 at the promoter regions of Abcb1b and Abcb1a, respectively. Drug efflux activity, however, does not follow tightly the transcriptional activity of drug transporter genes in hepatoma cells. Our results point out the need for careful analysis of cause-and-effect relationships between changes in histone modification, drug transporter expression and drug resistance phenotypes.

Increased expression of the multidrug resistance-associated protein 1 (MRP1) in kidney glomeruli of streptozotocin-induced diabetic rats

Oxidative stress has been linked to the podocytopathy, mes-angial expansion and progression of diabetic nephropathy. The major cell defence mechanism against oxidative stress is reduced glutathione (GSH). Some ABC transporters have been shown to extrude GSH, oxidised glutathione or their conjugates out of the cell, thus implying a role for these transporters in GSH homeostasis. We found a remarkable expression of mRNA for multidrug resistance-associated proteins (MRP/ABCC) 1, 3, 4 and 5 in rat glomeruli. Three weeks after induction of diabetes in glomeruli of streptozotocin-treated rats, we observed a decline in reduced GSH levels and an increase in the expression and activity of MRP1 (ABCC1). These lower GSH levels were improved by ex vivo treatment with pharmacological inhibitors of MRP1 activity (MK571). We conclude that increased activity of MRP1 in diabetic glomeruli is correlated with an inadequate adaptive response to oxidative stress.

Role of hepatic transporters in prevention of bile acid toxicity after partial hepatectomy in mice

The enterohepatic recirculation of bile acids (BAs) is important in several physiological processes. Although there has been considerable research on liver regeneration after two-thirds partial hepatectomy (PHx), little is known about how the liver protects itself against BA toxicity during regeneration. In this study, various BAs in plasma and liver, the composition of micelle-forming bile constituents, as well as gene expression of the main hepatobiliary transporters were quantified in sham-operated and PHx mice 24 and 48 h after surgery. PHx did not influence the hepatic concentrations of taurine-conjugated BAs (T-BA) but increased the concentration of glycine-conjugated (G-BA) and unconjugated BAs. Total BA excretion (microg x min(-1) x g liver wt(-1)) increased 2.4-fold and was accompanied by a 55% increase in bile flow after PHx. The plasma concentrations of T-BAs (402-fold), G-BAs (17-fold), and unconjugated BAs (500-fold) increased. The mRNA and protein levels of the BA uptake transporter Ntcp were unchanged after PHx, whereas the canalicular Bsep protein increased twofold at 48 h. The basolateral efflux transporter Mrp3 was induced at the mRNA (2.6-fold) and protein (3.1-fold) levels after PHx, which may contribute to elevated plasma BA and bilirubin levels. Biliary phospholipid excretion was nearly doubled in PHx mice, most likely owing to increased mRNA expression of the phospholipid transporter, Mdr2. In conclusion, the remnant liver after PHx excretes 2.5-fold more BAs and three times more phospholipids per gram liver than the sham-operated mouse liver. Upregulation of phospholipid transport may be important in protecting the biliary tract from BA toxicity during PHx.

Redox regulation of multidrug resistance in cancer chemotherapy: molecular mechanisms and therapeutic opportunities

The development of multidrug resistance to cancer chemotherapy is a major obstacle to the effective treatment of human malignancies. It has been established that membrane proteins, notably multidrug resistance (MDR), multidrug resistance protein (MRP), and breast cancer resistance protein (BCRP) of the ATP binding cassette (ABC) transporter family encoding efflux pumps, play important roles in the development of multidrug resistance. Overexpression of these transporters has been observed frequently in many types of human malignancies and correlated with poor responses to chemotherapeutic agents. Evidence has accumulated showing that redox signals are activated in response to drug treatments that affect the expression and activity of these transporters by multiple mechanisms, including (a) conformational changes in the transporters, (b) regulation of the biosynthesis cofactors required for the transporter's function, (c) regulation of the expression of transporters at transcriptional, posttranscriptional, and epigenetic levels, and (d) amplification of the copy number of genes encoding these transporters. This review describes various specific factors and their relevant signaling pathways that are involved in the regulation. Finally, the roles of redox signaling in the maintenance and evolution of cancer stem cells and their implications in the development of intrinsic and acquired multidrug resistance in cancer chemotherapy are discussed.

Species Differences of Inhibitory Effects on P‐glycoprotein‐mediateD Drug Transport

Previously, we clarified the species differences in P-glycoprotein (P-gp)-mediated drug transport activity using human MDR1, monkey MDR1, canine MDR1, rat MDR1a, rat MDR1b, mouse mdr1a, and mouse mdr1b transfected LLC-PK(1) cell lines. However, the species differences in the inhibitory effects on P-gp-mediated drug transport have not been clarified yet. The purpose of the present study was to evaluate the species differences in the inhibitory effects of typical P-gp inhibitors, quinidine and verapamil, on P-gp-mediated drug transport using MDR1 transfected cell lines. The transcellular transport of [(3)H]daunorubicin, [(3)H]digoxin, and [mebmt-beta-(3)H]cyclosporin A across monolayers of the MDR1 transfected cells were measured in the presence or absence of P-gp inhibitors. On daunorubicin transport, the relative IC(50) value (quinidine IC(50)/verapamil IC(50)) of human P-gp was 5.25 and those from other species ranged from 0.89 to 10.70. The transport of digoxin and cyclosporin A also showed different relative IC(50) values among human, monkey, canine, rat, and mouse P-gps. The present study revealed that species differences in the inhibitory effects on P-gp-mediated drug transport should not be disregarded among human, monkey, canine, rat, and mouse. This study will provide useful information for predicting drug interactions mediated by P-gp.

MDR1 gene polymorphisms and risk of recurrence in patients With hepatocellular carcinoma after liver transplantation

BACKGROUND AND OBJECTIVES

Recurrence of hepatocellular carcinoma (HCC) after liver transplantation (LT) remains a major cause of post-LT death. However, currently there is still lacking the markers to reliably predict recurrence. This study was undertaken to evaluate the association between three polymorphisms (C1236T, G2677A/T, C3435T) of Multidrug resistance 1 (MDR1) gene and the risk of recurrence after LT.

METHODS

Genomic DNA of 99 HCC patients undergoing LT was extracted from peripheral blood lymphocytes and genotyping was performed using polymerase chain reaction-restriction fragment length polymorphism assay. Cox proportional hazard model was used to estimate the hazard ratios associated with polymorphisms.

RESULTS

During a mean follow-up of 14.9 months, 49 patients experienced recurrence. The association between recurrence-free and 2677A carrier (carrying at least one variant A allele) was significant (P = 0.019). However, no significant association was observed in other polymorphisms. Patients with 2677A carrier conferred a 63% reduction in recurrence risk compared with 2677A non-carrier (odds ratio: 0.374; 95% confidence interval: 0.177-0.788; P = 0.010). The median recurrence-free survival for 2677A carrier group was significantly longer than that for 2677A non-carrier group (44.2 vs. 10.5 months, P = 0.015).

CONCLUSION

The polymorphism of MDR1 gene may be a valuable molecular marker for HCC recurrence after LT.

Differences in gene expression profiles in the liver between carcinogenic and non-carcinogenic isomers of compounds given to rats in a 28-day repeat-dose toxicity study

Some compounds have structural isomers of which one is apparently carcinogenic, and the other not. Because of the similarity of their chemical structures, comparisons of their effects can allow gene expression elicited in response to the basic skeletons of the isomers to be disregarded. We compared the gene expression profiles of male Fischer 344 rats administered by daily oral gavage up to 28 days using an in-house oligo microarray. 2-Acetylaminofluorene (2-AAF), 2,4-diaminotoluene (2,4-DAT), 2-nitropropane (2-NP), and 2-nitro-p-phenylenediamine (2-NpP) are hepatocarcinogenic. However, their isomers, 4-acetylaminofluorene (4-AAF), 2,6-diaminotoluene (2,6-DAT), 1-nitropropane (1-NP), and 4-nitro-o-phenylenediamine (4-NoP), are non-hepatocarcinogenic. Because of the limited carcinogenicity of 2-NpP, we attempted to perform two-parametric comparison analyses with (1) a set of 4 isomers: 2-AAF, 2,4-DAT, 2-NP, and 2-NpP as "carcinogenic", and 4-AAF, 2,6-DAT, 1-NP, and 4-NoP as "non-carcinogenic"; and (2) a set of 3 isomers: 2-AAF, 2,4-DAT, and 2-NP, as "carcinogenic", and 4-AAF, 2,6-DAT, and 1-NP as "non-carcinogenic". After ratio filtering and Welch's approximate t-test analysis, 54 and 28 genes were selected from comparisons between the sets of 3 and 4 isomers, respectively, for day 28 data. Using hierarchical clustering analysis with the 54 or 28 genes, 2-AAF, 2,4-DAT, and 2-NP clustered into a "carcinogenic" branch. 2-NpP was in the same cluster as 4-NoP and 4-AAF. This clustering corresponded to the previous finding that 2-NpP is not carcinogenic in male Fischer 344 rats, which indicates that comparing the differences in gene expression elicited by different isomers is an effective method of developing a prediction system for carcinogenicity.

Roles of reactive oxygen species in hepatocarcinogenesis and drug resistance gene expression in liver cancers

Hepatocellular carcinoma (HCC) has traditionally been an attractive system for cancer research because many animal HCC models are available. It is well known that liver tumors in animals can be induced by many different protocols, such as chronic hepatitis viral infections, carcinogens, toxins, steroid hormones, and dietary intervention. Although these different inducers have different cellular targets and modes of cytotoxic effects, their common denominator is the formation of reactive oxygen species (ROS). In this review, we present compelling evidence to support the hypothesis that ROS play important roles in hepatocarcinogenesis and the associated upregulation of drug resistance gene expression.

The role of prostaglandin E receptor-dependent signaling via cAMP in Mdr1b gene activation in primary rat hepatocyte cultures

Multidrug resistance (mdr) proteins of the mdr1 type function as multispecific xenobiotic transporters in hepatocytes. In the liver, mdr1 overexpression occurs during regeneration, cirrhosis, and hepatocarcinogenesis and may contribute to primary chemotherapy resistance. Cultured rat hepatocytes exhibit a time-dependent "intrinsic" increase in functional mdr1b expression, which depends on cyclooxygenase-catalyzed prostaglandin E(2) release. In the present study, the prostaglandin E (EP) receptor agonist misoprostol (1-10 microg/ml) further enhanced intrinsic mdr1b mRNA expression in primary rat hepatocytes. On the other hand, [1alpha(z),2beta,5alpha]-(+)-7-[5-[1,1'-(biphenyl)-4-yl]methoxy]-2-(4-morpholinyl)-3-oxocyclopentyl]-4-heptenoic acid (AH23848B) (30 microM), an antagonist of the cAMP-coupled EP4 receptor, and the protein kinase A (PKA) inhibitor, N-(2-[bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide (H89) (10 nM), repressed intrinsic mdr1b mRNA up-regulation, whereas the stable cAMP analog 8-bromo-cAMP (10 microM) and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) (100 microM) further enhanced intrinsic mdr1b expression. Primary rat hepatocytes, transiently transfected with reporter gene constructs controlled by mdr1b 5'-gene-flanking regions [-1074 to +154 base pairs (bp) or -250 to +154 bp], demonstrated pronounced mdr1b promoter activity, already without the addition of exogenous modulators. Nevertheless, activity was further stimulated by misoprostol, 8-bromo-cAMP, or IBMX. Cotransfection with expression vectors for PKI, an inhibitor protein of cAMP-dependent PKA, or KCREB, a dominant-negative mutant of the cAMP-responsive element-binding protein (CREB), decreased high-intrinsic mdr1b promoter activity. KCREB also counteracted misoprostol-induced mdr1b promoter activation. In conclusion, these data provide evidence for a pivotal role of EP receptor-stimulated, cAMP-dependent activation of PKA and CREB or CREB-related proteins in mdr1b gene activation in primary rat hepatocytes. Thus, these data might offer potential new target structures for the reversal of primary drug resistance, for example, of liver tumors.

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.

How does human stem cell therapy influence gene expression after liver injury? Microarray evaluation on a rat model

BACKGROUND

Tissue homeostasis is guaranteed by stem proliferating reserve, depending on dynamic changes in gene expression. A high plasticity is shown by the haematopoietic stem cells, potential source for liver regeneration.

AIM

We aimed to evaluate the gene expression modifications induced by human haematopoietic stem cell therapy after liver injury in rats.

SUBJECTS

Rats were sorted as follows: (A) human-haematopoietic stem cell injection after allyl alcohol liver damage; (B) only haematopoietic stem cell injection; (C) only allyl alcohol injection; and (D) sacrifice without any treatment.

METHODS

Livers, spleens and bone marrows were analysed with flow-cytometry. Livers were also studied by reverse-transcription PCR, histology, immunohistochemistry and microarray analysis; selected genes were confirmed by real-time PCR.

RESULTS

In subset A, haematopoietic stem cells were selectively recruited by liver, with respect to the group B, and they improved the liver regeneration process compared to group C. As regards microarrays, haematopoietic stem cell infusion upregulates 265 genes and downregulates 149 genes. Differentially regulated genes belong to a broad range of functional pathways, including proliferation, differentiation, adhesion/migration and transcripts related to oval-cell activation. Real-time PCR validated array results.

CONCLUSIONS

Our study confirmed the capacity of haematopoietic stem cells to contribute to liver regeneration. Moreover, microarray analysis led to the identification of genes whose regulation strongly correlates with a more efficient process of liver repair after haematopoietic stem cell injection.

Puromycin-based purification of rat brain capillary endothelial cell cultures. Effect on the expression of blood-brain barrier-specific properties

One of the main difficulties with primary rat brain endothelial cell (RBEC) cultures is obtaining pure cultures. The variation in purity limits the achievement of in vitro models of the rat blood-brain barrier. As P-glycoprotein expression is known to be much higher in RBECs than in any contaminating cells, we have tested the effect of five P-glycoprotein substrates (vincristine, vinblastine, colchicine, puromycin and doxorubicin) on RBEC cultures, assuming that RBECs would resist the treatment with these toxic compounds whereas contaminating cells would not. Treatment with either 4 microg/mL puromycin for the first 2 days of culture or 3 microg/mL puromycin for the first 3 days showed the best results without causing toxicity to the cells. Transendothelial electrical resistance was significantly increased in cell monolayers treated with puromycin compared with untreated cell monolayers. When cocultured with astrocytes in the presence of cAMP, the puromycin-treated RBEC monolayer showed a highly reduced permeability to sodium fluorescein (down to 0.75 x 10(-6) cm/s) and a high electrical resistance (up to 500 Omega x cm(2)). In conclusion, this method of RBEC purification will allow the production of in vitro models of the rat blood-brain barrier for cellular and molecular biology studies as well as pharmacological investigations.

Puromycin selectively increases mdr1a expression in immortalized rat brain endothelial cell lines

The blood-brain barrier (BBB) plays an important role in controlling the passage of molecules from blood to brain extracellular fluid. The multidrug efflux pump P-glycoprotein (P-gp) is highly expressed in the luminal membrane of brain endothelium and contributes to the formation of a functional barrier to lipid-soluble drugs such as anticancer agents. The mdr1a P-gp-encoding gene is exclusively expressed in the rodent BBB. Primary cultures of rat brain endothelial cells and GP8.3 cells showed a dramatic decrease in mdr1a mRNA level and some expression of mdr1b mRNA. GPNT cells, derived from GP8.3 cells after transfection with a puromycin resistance gene, were chronically treated with 5 microg/mL puromycin, a P-gp substrate. Compared with rat brain endothelial cells and GP8.3 cells, GPNT cells exhibited a very high level of expression of mdr1a mRNA together with a moderate level of mdr1b mRNA expression. Accordingly, P-gp expression and activity were strongly increased. When GP8.3 and puromycin-starved GPNT cells were treated with puromycin, mdr1a expression was selectively increased. High expression of mdr1a mRNA in GPNT cells may thus be related to the chronic treatment with puromycin. We conclude that GPNT cells may be used as a valuable rat in vitro model for studying the regulation of mdr1a expression at the BBB level.

Characteristic expression profiles induced by genotoxic carcinogens in rat liver

When applied in toxicological studies, the recently developed gene expression profiling techniques using microarrays, which brought forth the new field of toxicogenomics, facilitate the interpretation of a toxic compound's mechanism of action. In this study, we investigated whether genotoxic carcinogens at doses known to induce liver tumors in the 2-year rat bioassay deregulate a common set of genes in a short-term in vivo study and, if so, whether these deregulated genes represent defined biological pathways. Rats were dosed with the four genotoxic hepatocarcinogens dimethylnitrosamine (4 mg/kg/day), 2-nitrofluorene (44 mg/kg/day), aflatoxin B1 (0.24 mg/kg/day), and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK, 20 mg/kg/day). After treatment for up to 14 days, the expression profiles of the livers were analyzed on Affymetrix RG_U34A microarrays. Among the significantly upregulated genes were a set of target genes of the tumor suppressor protein p53, indicating a DNA damage response. Such a response was expected and, therefore, confirmed the validity of our approach. In addition, the gene expression changes suggest a specific detoxification response, the activation of proliferative and survival signaling pathways, and some cell structural changes. These responses were strong throughout the 14 day time course for 2-nitrofluorene and aflatoxin B1; in the case of dimethylnitrosamine and NNK, the effects were weakly detectable at day 1 and then increased with time. For dimethylnitrosamine and aflatoxin B1, which caused observable inflammation in vivo, we found a corresponding upregulation of inflammatory genes at the same time points. Thus, by the toxicogenomic analysis of short-term in vivo studies, we identified genes and pathways commonly deregulated by genotoxic carcinogens, which may be indicative for the early events in tumorigenesis and, thus, predictive of later tumor development.

Decreased expression of P-glycoprotein during differentiation in the human intestinal cell line Caco-2

The expression profile of the multidrug resistance (MDR) 1 gene product P-glycoprotein (Pgp) was examined during culture using Caco-2 cells as an in vitro model. Levels of MDR1 and cyclooxygenase 2 mRNA expression in Caco-2 cells were the highest on day 3 and decreased with days in culture, but the level of cyclooxygenase 1 was stable throughout the culture period. The stability of MDR1 mRNA was 7-fold higher on day 3 than on day 9, and the run-on assay suggested the transcription rate of the MDR1 gene on day 3 tended to be higher than on day 9. In addition, the expression of Pgp was comparable with that of MDR1 mRNA, but was inversely correlated with villin expression. The Pgp-mediated tacrolimus transport was the highest on day 1 and the lowest on day 11. These results suggested that the changeable mRNA stability rather than transcription rate of MDR1 contributed to its up-regulation during cell proliferation and down-regulation after post-confluent differentiation in Caco-2 cells. Therefore, the temporal induction and subsequent down-regulation of the enterocyte Pgp could affect bioavailability of several drugs during the regeneration of the intestinal wall.

Cyclosporine a augments P-glycoprotein expression in the regenerating rat liver

In the liver, the multidrug resistance (MDR) protein P-glycoprotein (P-gp) is physiologically expressed at the bile canalicular membrane, where it participates in the biliary excretion of various lipophilic drugs and xenobiotics. Previous studies showed that the immunosuppressive agent cyclosporine A (CsA) modulates P-gp and exerts a hepatotrophic influence in the regenerating liver. Hepatocytes isolated from regenerating rat liver, after 2/3 partial hepatectomy (PH 2/3), were used as an in vivo experimental model of cells with high proliferating activity in order to investigate whether CsA influences cellular levels of P-gp in those cells. Male Wistar rats were treated with CsA (20 mg/kg body weight) for 4 d preoperatively and 1 d postoperatively, and regenerating hepatocytes were isolated by collagenase perfusion 12, 24 and 48 h after PH 2/3. Flow cytometry and Western blotting studies with the monoclonal antibodies C494 and C219 showed that after PH 2/3, cellular levels of P-gp were initially suppressed, 12 h after PH 2/3, by 23%, but were significantly elevated thereafter, 24 and 48 h after PH 2/3 by 28% and 73%, respectively. In CsA pretreated animals, P-gp levels were increased even in normal hepatocytes by 34%, and an additional augmentation was seen in hepatocytes from 24 and 48 h regenerating livers (60% and 56%, respectively). In summary, we demonstrate for the first time that CsA has an additive effect on the expression of P-glycoprotein during liver regeneration in the rat. Therefore, induction of P-gp might also be considered in patients receiving CsA after liver transplantation for hepatocellular carcinoma and chemotherapy as an adjuvant treatment for the prevention of tumor recurrence.

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.

Induction of human MDR1 gene expression by 2-acetylaminofluorene is mediated by effectors of the phosphoinositide 3-kinase pathway that activate NF-kappaB signaling

The expression of P-glycoprotein encoded by the multidrug resistance (MDR1) gene is associated with the emergence of the MDR phenotype in cancer cells. Human MDR1 and its rodent homolog mdr1a and mdr1b are frequently overexpressed in liver cancers. However, the underlying mechanisms are largely unknown. The hepatocarcinogen 2-acetylaminofluorene (2-AAF) efficiently activates rat mdr1b expression in cultured cells and in Fisher 344 rats. We recently reported that activation of rat mdr1b in cultured cells by 2-AAF involves a cis-activating element containing a NF-kappaB binding site located -167 to -158 of the rat mdr1b promoter. 2-AAF activates IkappaB kinase (IKK), resulting in degradation of IkappaBbeta and activation of NF-kappaB. In this study, we report that 2-AAF could also activate the human MDR1 gene in human hepatoma and embryonic fibroblast 293 cells. Induction of MDR1 by AAF was mediated by DNA sequence located at -6092 which contains a NF-kappaB binding site. Treating hepatoma cells with 2-AAF activated phosphoinositide 3-kinase (PI3K) and its downstream effectors Rac1, and NAD(P)H oxidase. Transient transfection assays demonstrated that constitutively activated PI3K and Rac1 enhanced the activation of the MDR1 promoter by 2-AAF. Treatment of hepatoma cells with 2-AAF also activated another PI3K downstream effector Akt. Transfection of recombinant encoding a dominant activated Akt also enhanced the activation of MDR1 promoter activation by 2-AAF. These results demonstrated that 2-AAF up-regulates MDR1 expression is mediated by the multiple effectors of the PI3K signaling pathway.

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.

Multidrug resistance gene (MDR-1) expression as a useful prognostic factor in patients with human hepatocellular carcinoma after surgical resection

BACKGROUND

Multidrug resistance gene (MDR-1) overexpression has been correlated with tumor aggressiveness and worse prognosis in some human neoplasms. The aim of this study is to evaluate the clinical value of MDR-1 mRNA expression as a prognostic factor after surgical resection in human hepatocellular carcinoma (HCC).

METHODS

MDR-1 mRNA levels in tissue samples from 34 patients with HCC, who underwent surgical resection, were measured by quantitative northern blot analysis. We stratified these patients into two groups according to a ratio of MDR-1 mRNA levels of HCC to nontumorous tissue; MDR-1 mRNA ratio > or = 1.0 and < 1.0. The overall and disease-free survival rates were analyzed using multivariate regression analysis.

RESULTS

The median survival periods were 10.3 and 35.8 months for patients with the MDR-1 mRNA ratio > or = 1.0 and < 1.0, respectively, and the corresponding 5-year survival rates were 33 and 54%, respectively, P < 0.05. The multivariate analysis revealed that TNM stage and MDR-1 mRNA ratio were independent factors for predicting overall survival after surgical resection.

CONCLUSION

This study suggested that the measurement of the MDR-1 mRNA levels in HCC and nontumorous liver tissue might be a useful prognostic factor after surgical resection in patients with HCC.

mRna expression and transport characterization of conditionally immortalized rat brain capillary endothelial cell lines; a new in vitro BBB model for drug targeting

Brain capillary endothelial cell lines (TR-BBB) were established from a recently developed transgenic rat harboring temperature-sensitive simian virus 40 (ts SV 40) large T-antigen gene (Tg rat) and used to characterize the endothelial marker, transport activity, and mRNA expression of transporters and tight-junction strand proteins at the blood-brain barrier (BBB). These cell lines expressed active large T-antigen and grew well at 33 degrees C with a doubling-time of about 22-31 hr, but did not grow at 39 degrees C. TR-BBBs expressed the typical endothelial marker, von Willebrand factor, and exhibited acetylated low-density lipoprotein uptake activity. Although the gamma-glutamyltranspeptidase activity in TR-BBBs was approximately 13% of that of the brain capillary fraction of a normal rat, it was localized in the apical side, suggesting that it reflects the functional polarity of the in vivo BBB. The mRNA of tight-junction strand proteins such as claudine-5, occludin, and junctional adhesion molecule are expressed in TR-BBB13. Drug efflux transporter, P-glycoprotein, with a molecular weight of 170 kDa was expressed in all TR-BBBs and mdr 1a, mdr 1b, and mdr 2 mRNA were detected in TR-BBBs using RT-PCR. Moreover, mrp1 mRNA was expressed in all TR-BBBs. Influx transporter, GLUT-1, expressed at 55 kDa was revealed by Western blot analysis. It had 3-O-methyl-D-glucose (3-OMG) uptake activity which was concentration-dependent with a Michaelis-Menten constant of 9.86 +/- 1.20 mM. The mRNA of large neutral amino acid transporter, which consists of LAT-1 and 4F2hc was expressed in TR-BBBs. In conclusion, the conditionally immortalized rat brain capillary endothelial cell lines (TR-BBB) had endothelial makers, expressed mRNA for tight-junction strand proteins and the influx and efflux transporters and produced GLUT-1, which is capable of 3-OMG transport activity.

Conditionally immortalized retinal capillary endothelial cell lines (TR-iBRB) expressing differentiated endothelial cell functions derived from a transgenic rat

The objective of this study was to establish and characterize a retinal capillary endothelial cell line (TR-iBRB) from a newly developed transgenic rat harboring the temperature-sensitive simian virus 40 (SV 40) large T-antigen gene (Tg rat). Retinal capillary endothelial cells were isolated from a Tg rat and cultured in collagen-coated dishes at 37 degrees C for a period of 48 hr. Cells were subsequently cultured at 33 degrees C to activate the large T-antigen. At the third passage, cells were cloned by colony formation and isolated from other cells. Nine immortalized cell lines of retinal capillary endothelial cells (TR-iBRB1 approximately 9) were obtained from a Tg rat. These cell lines had a spindle-fiber shape morphology, expressed the typical endothelial marker, von Willebrand factor, and internalized acetylated-low density lipoprotein. Moreover, vascular endothelial growth factor (VEGF) receptor-2 was expressed in TR-iBRBs. TR-iBRBs expressed a large T-antigen and grew well at 33 degrees C with a doubling time of 19-21 hr. In contrast, cells did not grow at 37 and 39 degrees C due to the reduced expression of large T-antigen, supporting temperature-dependent cell growth. TR-iBRBs expressed GLUT1 and exhibited 3- O -methyl- D -glucose (3-OMG) uptake activity. This 3-OMG uptake was saturable with a Michaelis-Menten constant of 5.56 +/- 0.51 m M and a maximum uptake rate of 45.3 +/- 2.6 nmol min(-1) mg protein(-1). P-Glycoprotein, with a molecular weight of approximately 180 KDa, was expressed in TR-iBRBs. In addition, mdr 1a, mdr 1b and mdr 2 were detected in TR-iBRB2 using RT-PCR. In conclusion, conditionally immortalized retinal capillary endothelial cell lines were established from a transgenic rat harboring the temperature-sensitive SV 40 large T-antigen gene and these lines were shown to exhibit the properties of retinal capillary endothelial cells.

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.

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.

Expression and regulation of hepatic drug and bile acid transporters

Transport across hepatocyte plasma membranes is a key parameter in hepatic clearance and usually occurs through different carrier-mediated systems. Sinusoidal uptake of compounds is thus mediated by distinct transporters, such as Na(+)-dependent or Na(+)-independent anionic transporters and by some cationic transporters. Similarly, several membrane proteins located at the apical pole of hepatocytes have been incriminated in the excretion of compounds into the bile. Indeed, biliary elimination of anionic compounds, including glutathione S-conjugates, is mediated by MRP2, whereas bile salts are excreted by a bile salt export pump (BSEP) and Class I-P-glycoprotein (P-gp) is involved in the secretion of amphiphilic cationic drugs, whereas class II-P-gp is a phospholipid transporter. The expression of hepatic transporters and their activity are regulated in various situations, such as ontogenesis, carcinogenesis, cholestasis, cellular stress and after treatment by hormones and xenobiotics. Moreover, a direct correlation between a defect and the absence of transporter with hepatic disease has been demonstrated for BSEP, MDR3-P-gp and MRP2.

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.

Elevated expression of hepatic proliferative markers during early hepatocarcinogenesis in hepatitis-B virus transgenic mice lacking mdr1a-encoded P-glycoprotein

Recent studies have shown that expression levels of the multidrug resistance gene MDR1, which encodes the drug transporter P-glycoprotein, correlate with prognostic outcomes of certain tumor types. These findings suggest that expression of MDR1 may affect tumor behaviors. To address this issue further, we investigated the expression of mdr1a, a human MDR1 homolog, on the development of hepatocellular carcinoma in a transgenic mouse model carrying the liver-targeted expression of human hepatitis-B virus (HBV) surface antigen. The pathogenetic program was compared in HBV mice carrying either mdr1a(+/+) or mdr1a(-/-). We found that the expressions of proliferative activity markers, Ki67 nuclear antigen, and proliferating cell nuclear antigen were elevated in mdr1a(-/-) mice younger than 10 wk in comparison with those in the same age group of wild-type animals. Replication in the hepatic population as determined by bromodeoxyuridine incorporation tended to support observation that mdr1a(-/-) mice exhibited elevated labeling indices in this age group. Moreover, histologic staining and flow-cytometric analysis showed that the mdr1a(-/-) animals exhibited a higher cell population with polyploidy than did the mdr1a(+/+) counterparts of the same age. However, no significant differences in the expression of the liver-injury markers serum alanine transaminase and aspartate transaminase were observed. Although our results showed that absence of mdr1a expression is correlated with modest enhanced proliferative characteristics in the livers at stage before the development of hepatocellular carcinoma, the overall life spans between these two strains of mice were not significantly different. The implication of these findings to the role of P-glycoprotein in tumor development and cancer chemotherapy is discussed.

Molecular identification and functional characterization of Mdr1a in rat cholangiocytes

BACKGROUND & AIMS

The multidrug resistance P-glycoprotein 170 gene products (mdr1a and 1b) are glycosylated plasma membrane proteins that function as adenosine triphosphate-dependent transmembrane export pumps for lipophilic xenobiotics of widely different structure. We assessed whether these P-glycoproteins are functionally expressed in cholangiocytes.

METHODS

A reverse-transcription polymerase chain reaction was performed on RNA from a normal rat cholangiocyte cell line using mdr1-specific primers. Northern and Western blot analyses were performed on cholangiocytes immunoisolated from 2-week bile duct-ligated rats and cholangiocytes and isolated cholangiocyte membrane subfractions, respectively. Functional assays were performed in isolated bile duct units from bile duct-ligated rats and incubated with rhodamine 123, a P-glycoprotein substrate, with or without the P-glycoprotein inhibitors verapamil or GF120918.

RESULTS

A 400-base pair fragment with 99% homology to the cytosolic domain of rat intestinal mdr1a (5' 1953-2350 3') was identified that hybridized to a 5.2-kilobase RNA transcript in a normal rat cholangiocyte cell line, isolated rat cholangiocytes, and ileum. Western analysis localized mdr1 to the apical membrane of cholangiocytes. Confocal microscopy showed active secretion of rhodamine 123 into the lumen of isolated bile duct units that was abolished by vanadate and P-glycoprotein competitive antagonists, verapamil and GF120918, in a dose-dependent manner.

CONCLUSIONS

These findings provide the first molecular and functional evidence for the expression of mdr1a on the luminal membrane of cholangiocytes, where it may have a protective role.

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.

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.

Regulation of hepatic transport systems involved in bile secretion during liver regeneration in rats

We investigated the expression of hepatic transport systems involved in bile secretion during liver regeneration after partial hepatectomy (PH) in rats. Initial studies showed maximal BrdU incorporation 24 hours after PH. Therefore, transporter expression and bile secretion were analyzed in detail at this time. The mRNA levels of the multidrug resistance genes mdr1a and mrp1 slightly increased, whereas mdr1b mRNA levels showed an extensive increase after PH. The mRNA levels of the conjugate transporter, mrp2, decreased slightly, whereas mrp2 protein levels did not change. Bilirubin secretion did not change, but the biliary glutathione secretion markedly decreased and the hepatic GSH content increased. The messenger RNA levels of the bile salt uptake transporters ntcp, oatp1, and oatp2 and the bile salt exporter, bsep/spgp, all decreased with ntcp showing the most prominent decrease. Protein levels of ntcp dramatically decreased whereas oatp2 only slightly decreased. Oatp1 protein expression slightly increased and bsep/spgp protein levels did not change. Decreased levels of bile salt uptake systems were associated with a 10-fold increase in the plasma bile salt concentration, yet, bile flow and bile salt secretion were increased when expressed per gram liver and unaffected when expressed on the basis of body weight. In conclusion, during the initial phase of rat liver regeneration ntcp is down-regulated whereas other transporter proteins involved in bile secretion are only slightly affected. Despite increased serum bile salt levels the remnant liver is not cholestatic: bile flow is maintained by uptake of bile salts probably via oatp isoforms and their secretion via bsep/spgp.

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.

Increased P-glycoprotein messenger RNA stability in rat liver tumors in vivo

P-glycoproteins (Pgp) are comprised of a small family of plasma membrane proteins whose abundance in cultured cells is often associated with the multidrug resistance phenotype. Overexpression of Pgp has been observed in many types of human cancers, but the molecular basis for this overexpression has not been established. We have used primary monolayer cultures of adult rat hepatocytes and a stepwise model of rat liver carcinogenesis to study the regulation of Pgp gene expression. We observed a marked overexpression of Pgp, specifically the class II Pgp, in both systems. In addition, we observed that a number of unrelated genes including alpha-tubulin, beta-actin, gamma-actin, cytokeratin 8, cytokeratin 18, and c-myc are overexpressed in cultured hepatocytes, and they are also overexpressed during liver carcinogenesis and in transplantable tumors. Nuclear run-on assays showed no increase in the transcriptional activity of Pgp genes in transplantable liver tumors compared to normal liver. Studies of in vivo mRNA stability, however, revealed that all three Pgp mRNAs were relatively stable in transplantable liver tumors (t(1/2) > 12 h), in contrast to what was found in normal liver (t(1/2) < 2 h). In addition, mRNA for several other genes, including alpha-tubulin, c-myc, and cyclin D1, all appear to be stabilized in the tumors. These findings suggest that the overexpression of Pgp genes in rat liver tumors may be the result of a mechanism involving stabilization of a diverse group of mRNAs.

Induction of mdr1b mRNA and P-glycoprotein expression by tumor necrosis factor alpha in primary rat hepatocyte cultures

Mammalian liver exhibits expression of members of the family of multidrug resistance (mdr) transporters (P-glycoproteins). P-glycoprotein isoforms encoded by mdr1 genes participate in extrusion of an array of xenobiotics into the bile. Induction of mdr1b mRNA expression has been shown to occur in rat hepatocytes in response to hepatotrophic growth factors. As the cytokine tumor necrosis factor alpha (TNF-alpha) is known to exert a direct mitogenic effect on hepatocytes, its influence on mdr1b expression was investigated. In primary rat hepatocytes cultured in the absence of TNF-alpha, a time-dependent increase in basal expression of mdr1b mRNA and in immunodetectable P-glycoprotein was observed. In cells treated with TNF-alpha (4,000 U/ml) for 3 days, expression of mdr1b mRNA and of immunodetectable P-glycoprotein was induced approximately twofold. Moreover, intracellular steady-state levels of the mdr1 substrate rhodamine 123 were decreased in cells pretreated with TNF-alpha in comparison to controls, indicating an increase in functional transporter(s) mediating dye extrusion. Treatment of hepatocytes with antioxidants (1 mM ascorbic acid and 2% dimethyl sulfoxide) for 3 days markedly suppressed mdr1b mRNA and P-glycoprotein expression both in cells cultured in the presence of TNF-alpha and in the absence of the cytokine, but did not fully abolish mdr1b mRNA induction by TNF-alpha, supporting the notion that reactive oxygen species participate in regulation of basal mdr1b gene expression during hepatocyte culture. In conclusion, the present data indicate that by inducing mdr1b expression in hepatocytes, TNF-alpha may affect the capacity of the liver for extrusion or detoxification of endogenous or xenobiotic mdr1 substrates.

Altered expression of the growth and transformation suppressor PML gene in human hepatocellular carcinomas and in hepatitis tissues

The promyelocytic leukaemia (PML) gene, which encodes a transformation and growth suppressor, was first identified at a chromosomal translocation break point in acute promyelocytic leukaemia. To elucidate if PML may be involved in hepatocellular carcinoma (HCC), the expression of PML was analysed using immunohistochemistry in human HCC and hepatitis tissues. Our studies demonstrated overexpression of PML protein in the PML-oncogenic domain (POD) structure in 50% of HCC (11/22). Enhanced expression and cytoplasmic localisation of PML was associated with cirrhosis. Increased expression of PML was also found in liver abscesses. However, in colon metastasis to the liver, the expression of PML was moderate to low, although strong expression was seen in the surrounding interstitial cells, macrophages and lymphocytes, an indication of the inflammation process associated with tumour growth. Most interestingly, strong expression of PML was found in neoplastic cells at the periphery of the tumours, but progressively decreased in cells at the centre of the tumours, which may be associated with an altered transform phenotype or apoptosis. The altered expression of PML indicates that this nuclear protein may play an important role in cellular response to stress and inflammation, as well as in compensatory cell growth.

Modulation of P-glycoprotein expression by cytochrome P450 3A inducers in male and female rat livers

A strong overlap between P-glycoprotein (Pgp) and cytochrome P450 3A (CYP3A) substrates and modulators has been reported. To test the hypothesis that CYP3A and Pgp are coordinately regulated, we examined the effects of known inducers of CYP3A (triacetyloleandomycin, rifampicin, dexamethasone, pregnenolone 16alpha-carbonitrile) on Pgp expression in rat liver. We also investigated the gender-specific expression of Pgp and compared its response to dexamethasone between male and female rats. In male rats, western blot analyses showed that rifampicin and dexamethasone caused 50% and 5-fold increases in Pgp levels, respectively. RNase protection assays using gene-specific probes for the three Pgp isoforms revealed a 3-fold increase in mdr2 mRNA levels after dexamethasone administration and a 2-fold increase following rifampicin treatment. Triacetyloleandomycin and pregnenolone 16alpha-carbonitrile had no effect on Pgp expression and mRNA levels. We also observed that the basal level of Pgp was 40% lower in male rats than in females and that mdr2 mRNA levels in male rats were one-half those in females. As opposed to the results in male rats, dexamethasone reduced Pgp expression by approximately 60% and caused a 30% decrease in mdr2 mRNA levels in female rats. Mdr1a was not affected and mdr1b was not detected in female or male rats. We conclude that, at the dosage regimen used, CYP3A and Pgp responses to CYP3A inducers are regulated independently in rat liver. In addition, this study shows that Pgp expression and regulation are gender specific.

In vivo model systems in P-glycoprotein-mediated multidrug resistance

In this article we review the in vivo model systems that have been developed for studying P-glycoprotein-mediated multidrug resistance (MDR) in the preclinical setting. Rodents have two mdr genes, both of which confer the MDR phenotype: mdr 1a and mdr 1b. At gene level they show strong homology to the human MDR1 gene and the tissue distribution of their gene product is very similar to P-glycoprotein expression in humans. In vivo studies have shown the physiological roles of P-glycoprotein, including protection of the organism from damage by xenobiotics. Tumors with intrinsic P-glycoprotein expression, induced MDR or transfected with an mdr gene, can be used as syngeneic or xenogenic tumor models. Ascites, leukemia, and solid MDR tumor models have been developed. Molecular engineering has resulted in transgenic mice that express the human MDR1 gene in their bone marrow and in knockout mice missing a murine mdr gene. The data on pharmacokinetics, efficacy, and toxicity of chemosensitizers of P-glycoprotein in vivo are described. Results from studies using monoclonal antibodies directed against P-glycoprotein and other miscellaneous approaches for modulation of MDR are mentioned. The importance of in vivo studies prior to clinical trials is being stressed and potential pitfalls due to differences between species are discussed.

NF-kappaB-mediated induction of mdr1b expression by insulin in rat hepatoma cells

The expression of P-glycoproteins encoded by the mdr gene family is associated with the emergence of multidrug resistance phenotype in animal cells. However, the mechanisms controlling the expression of these genes have not been well elucidated. Here, we report that the expression of rat mdr1b gene in cultured H-4-II-E hepatoma cells can be induced by insulin. Transient transfection assays using reporter gene constructs containing various 5' mdr1b sequences showed that the sequence located between base pairs -243 and -163 is important for insulin's induction of mdr1b promoter activity. Further analyses revealed that a NF-kappaB-binding site (located between base pairs -167 and -158) is required for insulin-induced promoter activity. Gel mobility shift assay demonstrated that insulin stimulates the binding of nuclear p50/p65 subunits to the mdr1b NF-kappaB sequence. Cotransfection of plasmids expressing either the p50/p65 NF-kappaB subunits or Raf-1 kinase or both resulted in increased expression of the gene containing wild-type but not NF-kappaB site-mutated mdr1b promoter. Finally, expression of either the antisense p65 subunit of NF-kappaB or dominant negative Raf-1 kinase blocked insulin's induction of the mdr1b promoter activity. Taken together, our results suggest that the insulin-induced mdr1b expression is mediated by transcription factor NF-kappaB via the Raf-1 kinase signaling pathway.

Up-regulation of P-glycoprotein expression in rat liver cells by acute doxorubicin treatment

Expression of P-glycoprotein, a plasma-membrane glycoprotein involved in multidrug resistance and encoded by mdr genes, was investigated in cultured rat liver cells acutely exposed to doxorubicin. This anticancer drug was shown to increase mdr mRNA levels in a dose-dependent manner in both rat liver epithelial (RLE) cells and primary rat hepatocytes. This induction of mdr transcripts was detected as early as a 4-h exposure to doxorubicin used at 0.5 microg/ml. It occurred through increased expression of the mdr1 gene as assessed by northern blot analysis using rat mdr-gene-specific probes. In addition, RLE cells exposed to doxorubicin displayed an overexpression of a 140-kDa P-glycoprotein as demonstrated by western blotting. Moreover, doxorubicin-treated RLE cells displayed enhanced cellular efflux of the P-glycoprotein substrate rhodamine 123 that was inhibited by the P-glycoprotein blocker verapamil, thus providing evidence that doxorubicin-induced P-glycoprotein was functional in liver cells. Doxorubicin-mediated mdr mRNA induction was found to be fully inhibited by actinomycin D, thus indicating its dependence on RNA synthesis; it was demonstrated to be not associated with alteration of protein synthesis, suggesting it differed from the known mdr mRNA overexpression occurring in response to cycloheximide. In contrast to P-glycoprotein, other liver detoxification pathways such as cytochromes P-450 1A were not induced by doxorubicin treatment. These data indicate that doxorubicin can act as a potent acute inducer of functional P-glycoprotein in rat liver cells and therefore may modulate both chemosensitivity of hepatic cells and P-glycoprotein-mediated biliary secretion of xenobiotics.

P-glycoprotein induction in rat liver epithelial cells in response to acute 3-methylcholanthrene treatment

Expression of P-glycoprotein (P-gp), a plasma membrane glycoprotein involved in multidrug resistance and encoded by mdr genes, was investigated in nonparenchymal rat liver epithelial (RLE) cells in response to acute exposure to carcinogenic polycyclic aromatic hydrocarbons (PAHs). High levels of mdr mRNAs were evidenced by Northern blotting in two independent RLE cell lines after treatment by either 3-methylcholanthrene (MC) or benzo-(a)pyrene. MC-mediated mdr mRNA induction was demonstrated to be dose-dependent; it occurred through enhanced expression of the mdr 1 gene, as indicated by reverse transcriptase-polymerase chain reaction analysis using rat mdr gene-specific primers and paralleled an induction of a 140 kDa P-gp as demonstrated by Western blotting. In addition, MC-induced P-gp appeared to be fully functional because RLE cells exposed to MC displayed enhanced cellular efflux of rhodamine 123, a known P-gp substrate, compared to their untreated counterparts. Analysis of time-course induction revealed that mdr mRNA levels were maximally increased when RLE cells were treated for 48 to 96 hr and returned to low levels after the PAH was removed. In contrast to P-gp, both cytochrome P-450 1A1 and cytochrome P-450 1A2 were not detected after exposure to MC, thus indicating that these liver detoxification pathways are not coordinately regulated with P-gp in RLE cells. In addition, MC-mediated P-gp regulation was not associated with major cellular disturbances such as alteration of protein synthesis and, thereby, differed from the known mdr mRNA induction occurring in response to cycloheximide. Moreover, cotreatment with MC and cycloheximide led to a superinduction of mdr mRNAs, thus suggesting that the effects of the two xenobiotics were, at least partly, additive. In contrast to MC and benzo(a)pyrene, 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(e)pyrene were unable to increase P-gp expression. These results indicate that some PAHs can act as potent inducers of P-gp in RLE cells and may be interpreted as an adaptive reaction of these cells in lowering cellular accumulation of toxic drugs, including carcinogens transported by P-gp and, therefore, conferring protection on these compounds.

Adenoviral delivery of recombinant DNA into transgenic mice bearing hepatocellular carcinomas

To evaluate the applicability of recombinant adenoviral vectors in gene transfer to liver cancers, we infused the recombinant adenoviruses AD5CMV-LacZ and Ad5CMV-p53 through the portal veins into two lines of transgenic mice, one bearing the SV40 T antigen and the other the human hepatitis B viral envelope protein. These transgenic animals develop hepatocellular carcinomas (HCC) with predictable pathological manifestations. The levels of expression of the transgenes were dependent upon the viral doses. In all cases, high levels of expression were detected within 2 or 3 days after infusion, but were drastically reduced 7 days after infusion. Significant toxicities were found in the infused animals: > 80% of them died within 7 days after infusion with 10(10) pfu, and transgenic animals bearing HCC apparently were more sensitive to viral toxicity. Although a lower dose (10(9) pfu/animal) produced less toxicity, the levels of expression were substantially reduced (only about 10% of that in animals infused with 10(10) pfu). When Ad5CMV-p53 was infused into animals with nodular hyperplastic stage, the expression of the reporter gene seemed to distribute preferentially at the peripheries of the tumor nodules, and low levels of transgene expression were seen inside the nodules. In tumors in which necrotic lesions were evident, p53 was also expressed at the perpheries of the lesions. These distribution patterns were seen in both tumor models. There was no apparent suppression of tumor growth in the Ad5CMV-p53-infused animals. Our results suggest that alternative methods for gene therapy for HCC need to be explored.

Use of internally controlled reverse transcriptase-polymerase chain reaction for absolute quantitation of individual multidrug resistant gene transcripts in tissue samples

Research into the relative importance of P-glycoprotein (pgp) overexpression, in comparison to other mechanisms of multidrug resistance (mdr) phenotype development, has been hampered by difficulties of measurement in clinical samples. The small size, heterogeneity and often poor quality of clinical specimens renders the quantitation of mdr mRNA species by Northern analysis or RNAse protection difficult. The reverse transcriptase-polymerase chain reaction (RT-PCR) assay has both the sensitivity and specificity to make it suitable for the analysis of mdr mRNA levels in clinical samples. It also has a significant speed advantage over Northern and RNAse protection analysis. Unfortunately the variable nature of the reactions involved have made it difficult to obtain accurate and reproducible quantitative data. To overcome this difficulty we constructed internal RNA standards to each human and rat mdr mRNA species. When included in the RT-PCR reaction these internal standards allow both comparative and absolute quantitation of individual mdr family mRNA levels.