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T W Gant

Publications and source records attributed to T W Gant.

At least 19 recordsLinked to original sources
PubMed2001

Gene expression and amplification in breast carcinoma cells with intrinsic and acquired doxorubicin resistance.

The multidrug resistance (MDR) phenotype is a major cause of cancer treatment failure. Here the expressions of 4224 genes were analysed for association with intrinsic or acquired doxorubicin (DOX) resistance. A cluster of overexpressed genes related to DOX resistance was observed. Included in this cluster was ABCB1 the P-glycoprotein transporter protein gene and MMP1 (Matrix Metalloproteinase 1), indicative of the invasive nature of resistant cells, and the oxytocin receptor (OXTR), a potential new therapeutic target. Overexpression of genes associated with xenobiotic transformation, cell transformation, cell signalling and lymphocyte activation was also associated with DOX resistance as was estrogen receptor negativity. In all carcinoma cells, compared with HBL100 a putatively normal breast epithelial cell line, a cluster of overexpressed genes was identified which included several keratins, in particular keratins 8 and 18 which are regulated through the ras signalling pathway. Analysis of genomic amplifications and deletions revealed specific genetic alterations common to both intrinsic and acquired DOX resistance including ABCB1, PGY3 (ABCB4) and BAK. The findings shown here indicate new possibilities for the diagnosis of DOX resistance using gene expression, and potential novel therapeutic targets for pharmacological intervention.

Antineoplastic Agents↗
PubMed2001

Regulation of MDR1 promoter activity in human breast carcinoma cells by protein kinase C isozymes alpha and theta.

Increased levels of the protein kinase C (PKC) isoenzymes alpha and theta occur in conjunction with MDR1 gene expression in cells and tissues that have acquired a multidrug resistance (MDR) phenotype. Studies using PKC activators or antisense strategies against PKC suggest that activation of PKC engenders MDR1 gene transcription. In this study the potential roles of PKC-alpha and PKC-theta in MDR1 gene transcriptional regulation were explored. Human-derived MCF-7 breast cancer cells that lack constitutive expression of PKC-alpha or PKC-theta at detectable levels were transfected with full-length PKC-alpha or PKC-theta genes driven by the ecdysone promoter. Stable transfectants were selected by use of the appropriate antibiotics. Treatment of these cells with ponasterone A induced expression of PKC that was catalytically active and underwent translocation and down-regulation on exposure to 12-O-tetradecanoyl-13-phorbol acetate (TPA). These cells were used to analyse PKC-mediated regulation of the MDR1 promoter by further transient transfection with either 1073 bp of the MDR1 gene promoter or deletion fragments thereof to -8 bp, each linked to a chloramphenicol acetyl transferase (CAT) reporter gene. In PKC-alpha expressing cells TPA caused activation of all promoter fragments to -29 bp. This finding suggests that TPA-inducible MDR1 transcription mediated through the TPA responsive factor early growth response 1 (EGR-1) in this region of the promoter may be due to activation of PKC-alpha. In contrast, PKC-theta activated only two MDR1 fragments, -982 and -612 bp. The effect of TPA on reporter gene expression was attenuated by the PKC inhibitor GF 109203X. These data suggest that MDR1 promoter transcription can be regulated by PKC-alpha and PKC-theta. The results support the search for therapeutic strategies directed specifically against PKC-alpha to ameliorate resistance of tumours against cytotoxic agents.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2000

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.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1999

Site-specific tamoxifen-DNA adduct formation: lack of correlation with mutational ability in Escherichia coli.

We have mapped sites of tamoxifen adduct formation, in the lacI gene using the polymerase STOP assay, following reaction in vitro with alpha-acetoxytamoxifen and horseradish peroxidase (HRP)/H(2)O(2) activated 4-hydroxytamoxifen. For both compounds, most adduct formation occurred on guanines. However, one adenine, within a run of guanines, generated a strong polymerase STOP site with activated 4-hydroxytamoxifen, and a weaker STOP site with alpha-acetoxytamoxifen at the same location. In Escherichia coli the lac I gene reacted with 4-hydroxytamoxifen was more likely to be mutated (2 orders of magnitude) than when reacted with alpha-acetoxytamoxifen, despite the greater DNA adduct formation by alpha-acetoxytamoxifen. This correlates with the greater predicted ability of activated 4-hydroxytamoxifen adducts to disrupt DNA structure than alpha-acetoxytamoxifen adducts. For lac I reacted with activated 4-hydroxytamoxifen, a hot spot of base mutation was located in the region of the only adenosine adduct. No mutational hot spots were observed with alpha-acetoxytamoxifen. Our data clearly shows a lack of correlation between gross adduct number, as assayed by (32)P-postlabeling and mutagenic potential. These data indicate the importance of minor adduct formation in mutagenic potential and further that conclusions regarding the mutagenicity of a chemical may not be reliably derived from the gross determination of adduct formation.

Bacteriophage T4↗
PubMed1999

Tamoxifen induces G:C-->T:A mutations in the cII gene in the liver of lambda/lacI transgenic rats but not at 5'-CpG-3' dinucleotide sequences as found in the lacI transgene.

Tamoxifen, a rat liver carcinogen, can induce mutations in the lacI gene in the livers of lambda/lacI transgenic rats. However, the presence of persistent tamoxifen adducts on the liver DNA raises the possibility that some contribution to the mutagenesis from ex vivo mutations during the in vitro lacI assay cannot be ruled out. To address this issue, mutagenesis at the cII gene of the transgenic shuttle vector was determined using a selection based assay which is unaffected by the presence of tamoxifen-DNA adducts. Female lambda/lacI transgenic rats were dosed orally with tamoxifen (20 mg/kg body wt) daily for 6 weeks, causing a 3.2-fold increase in the mutant frequency (MF) in the cII gene compared with that obtained with solvent treated animals. This was similar to the MF found previously at the lacI gene and confirms that tamoxifen is mutagenic in vivo. The major class of mutation induced by tamoxifen in the cII gene was G:C-->T:A transversions as was found previously in the lacI gene. However, in the one unreplicated study of mutations in the p53 gene of liver tumours induced by tamoxifen, no G:C-->T:A transversions were found; possible differences between mutagenesis in normal and tumour tissues are explored. The major proportion of the G:C-->T:A transversions occurred at 5'-CpG-3' dinucleotide (CpG) sites in the lacI gene, but not at such sites in the cII gene. The methylation of CpG sites greatly enhances the targeting of deoxyguanosine by carcinogens, thus this finding might be explained by differences in the methylation patterns at their respective CpG sites; however, nothing is known about the methylation status of either the lacI nor the cII gene in this transgenic rat. This study raises the important issue of which target genes (mammalian or transgenic) should be used as endpoints in mammalian mutagenesis assays.

Aflatoxin B1↗
PubMed1999

Detection of tamoxifen-DNA adducts on lacI genes using DNA polymerase stop assay.

BACKGROUND: Tamoxifen forms DNA adducts in rat liver and causes an increased mutation frequency at the lacI genes in the livers of lambda/lacI transgenic rats. Although an elevated occurrence of endometrial cancer is found in a small proportion of breast cancer patients treated with tamoxifen, there is conflicting evidence on whether or not low levels of DNA adducts are formed in humans. METHODS: Based on the finding that the progression of DNA/RNA polymerases on templates might be blocked by bulky DNA adducts, we successfully developed and used a polymerase stop assay to map the sites of adduct formation in the target lacI gene following its reaction in vitro with alpha-acetoxytamoxifen and horseradish peroxidase/H2O2 (HRP/H2O2) activated 4-hydroxytamoxifen. RESULTS: Using a T4 DNA polymerase stop assay, adduct formation in the lacI gene of the plasmid constructs, after the reaction in vitro with alpha-acetoxytamoxifen and HRP/H2O2 activated 4-hydroxytamoxifen, was found to mainly occur with guanines. In particular, one site of adenosine adduction was found on a triplet of adenosines located between two runs of guanines. CONCLUSION: The success of our development of DNA polymerase stop assay to map the sites of tamoxifen-DNA adducts formation will be very useful for the investigation of the mutagenicity/carcinogenicity of tamoxifen. The mutagenic potential of the tamoxifen adducted bases shall be further examined by transfecting the adducted plasmids into suitable human cell lines. Also, further investigations of the sequence specificity in specific oncogenes and tumor suppressor genes may be useful to explore the relationship between the occurrence of human endometrial cancer and tamoxifen treatment.

Animals↗
PubMed1998

The mechanism of trans-activation of the MDR1 gene by human T-cell leukemia virus.

Overexpression of P-glycoprotein (P-gp), the protein product of the multidrug resistance gene (MDR1), confers a drug resistant phenotype on cells. We have recently demonstrated that the MDR1 promoter is transcriptionally activated by the HTLV-I tax protein, providing an explanation for the development of drug resistance in HTLV-I infections. Here we report that HTLV-I mediated MDR1 activation is dependent on the presence of an NF-IL6-binding site located between base pairs -148 and -141 relative to the transcription start site. This finding opens up the possibility of moderating P-gp expression through interference with NF-IL6 binding to its trans recognition element and subsequent repression of MDR1 transcription.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

Enhanced MDR1 gene expression in human T-cell leukemia virus-I-infected patients offers new prospects for therapy.

Overexpression of P-glycoprotein (P-gp), the protein product of the multidrug resistance gene (MDR1), confers a drug resistant phenotype on cells. This phenotype is reminiscent of human T-cell leukemia virus (HTLV)-transformed leukemic cells, for which no consistently effective chemotherapeutic regime has been found. The presence of an active multiple drug resistance (MDR) phenotype in freshly isolated peripheral blood mononuclear cells (PBMC) from HTLV-I-infected subjects was investigated. Significant P-gp-mediated efflux activity and enhanced MDR1 mRNA expression was observed in nine of 10 HTLV-infected subjects. The development of MDR phenotypes was found to be independent of disease type or status with significant MDR activities being observed in adult T-cell leukemia (ATL), HTLV-associated myelopathy (HAM)/tropical spastic paraparesis (TSP), and asymptomatic HTLV-infected individuals. P-gp-mediated drug efflux was also found to be restricted to CD3+ T-cell populations. Furthermore, we show the novel finding that the MDR1 gene promoter is transcriptionally activated by the HTLV-I tax protein, suggesting a molecular basis for the development of drug resistance in HTLV-I infections. These observations open up the possibility of new chemotherapeutic approaches to HTLV-associated diseases through the use of P-gp inhibitors.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

Induction of hepatic mrp2 (cmrp/cmoat) gene expression in nonhuman primates treated with rifampicin or tamoxifen.

The multidrug resistance protein 2 (Mrp2) also called canalicular multidrug resistance protein (cMrp) or canalicular multispecific organic anion transporter (cMoat) is a transmembrane export pump located at the canalicular domain of hepatocytes. Mrp2 transports a broad spectrum of organic anions including glucuronides, glutathione conjugates, and organic sulphates into bile. Based on previous observations in rat hepatocytes, the inducibility of mrp2 gene expression in primate liver was investigated in rhesus monkeys treated with tamoxifen or rifampicin. It was found that treatment with tamoxifen (25 mg/kg per day; over 7 days) or rifampicin (15 mg/kg per day; over 7 days) leading to an induction of cytochrome P450 3A4, resulted in a strong increase in mrp2 mRNA in the liver of male and female rhesus monkeys. On the protein level, tamoxifen also was a highly effective inducer, while rifampicin showed some inducing effect in a female and was inactive in a male monkey. In sections of paraffin-embedded liver tissue, immunofluorescence staining confirmed the results of Western blot analysis. Induced Mrp2 was localized to the canalicular domain of the hepatocytes. In conclusion, our data show inducibility of mrp2 gene expression in the liver of primates which may represent an adaptive response aimed at the enhanced biliary elimination of the inducing drugs and/or their metabolites.

ATP-Binding Cassette Transporters↗
PubMed1998

Regulation of c-fos transcription by chemopreventive isoflavonoids and lignans in MDA-MB-468 breast cancer cells.

Isoflavonoids and lignans are diet constituents with chemopreventive properties. We compared the ability of the isoflavonoids genistein and equol, the lignans enterodiol, enterolactone and nordihydroguaiaretic acid (NDGA) and the lignan metabolite methyl p-hydroxyphenyllactate to interfere with mitogenic and tumour promotional signal transduction pathways. Their effects on c-fos mRNA levels after induction by either epidermal growth factor (EGF) or the tumour promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) was measured in human breast cancer-derived MDA-MB-468 cells. Of the six agents, only genistein decreased EGF-induced, c-fos transcription (by 63% compared to control at 100 mumol/l). In contrast, both genistein and equol at 100 mumol/l decreased TPA-induced c-fos levels, by 75 and 67%, respectively. NDGA and methyl p-hydroxyphenyllactate did not inhibit TPA mediated c-fos transcription and enterolactone and enterodiol had only a weak inhibitory effect. NDGA at 0.1-10 mumol/l increased c-fos mRNA levels. None of the agents inhibited protein kinase C and only genistein inhibited EGF receptor-linked protein tyrosine kinase obtained from MDA-MB-468 cells, with an IC50 of 60 mumol/l. NDGA and genistein arrested cell colony formation potently, genistein was 15-fold more growth-inhibitory than equol. The results suggest that both genistein and equol interfere similarly with TPA-induced signal transduction pathways. Inhibition by genistein of EGF-induced c-fos mRNA transcription is probably related to its interruption of EGF receptor-linked protein tyrosine kinase, whereas genistein-induced growth arrest is not. If ability to antagonise phorbol ester effects is important for chemopreventive efficacy, equol and genistein might be equi-efficacious chemopreventors, whereas enterolactone, enterodiol and NDGA should be much less potent. If phorbol ester antagonism together with antimitogenic activity determine optimal chemopreventive activity of this type of agent, genistein would be more potent than equol.

Breast Neoplasms↗
PubMed1997

Co-ordinate loss of protein kinase C and multidrug resistance gene expression in revertant MCF-7/Adr breast carcinoma cells.

The aim of this study was to investigate the link between protein kinase C (PKC) and multidrug resistance (mdr) phenotype. The expression of both was studied in doxorubicin-resistant MCF-7/Adr cells as they reverted to the wild-type phenotype when cultured in the absence of drug. The following parameters were measured in cells 4, 10, 15, 20 and 24 weeks after removal of doxorubicin; (1) sensitivity of the cells towards doxorubicin; (2) levels of P-glycoprotein (P-gp) and MDR1 mRNA; (3) levels and cellular localization of PKC isoenzyme proteins alpha, theta and epsilon; and (4) gene copy number of PKC-alpha and MDR1 genes. Cells lost their resistance gradually with time, so that by week 24 they had almost completely regained the drug sensitivity seen in wild-type MCF-7 cells. P-gp levels measured by Western blot mirrored the change in doxorubicin sensitivity. By week 20, P-gp had decreased to 18% of P-gp protein levels at the outset, and P-gp was not detectable at week 24. Similarly, MDR1 mRNA levels had disappeared by week 24. MCF-7/Adr cells expressed more PKCs-alpha and -theta than wild-type cells and possessed a different cellular localization of PKC-epsilon. The expression and distribution pattern of these PKCs did not change for up to 20 weeks, but reverted back to that seen in wild-type cells by week 24. MDR1 gene amplification remained unchanged until week 20, but then was lost precipitously between weeks 20 and 24. The PKC-alpha gene was not amplified in MCF-7/Adr cells. The results suggest that MCF-7/Adr cells lose MDR1 gene expression and PKC activity in a co-ordinate fashion, consistent with the existence of a mechanistic link between MDR1 and certain PKC isoenzymes.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1996

Multidrug resistance gene expression in rodents and rodent hepatocytes treated with mitoxantrone.

Overexpression of P-glycoprotein in tumor cells can represent a severe drawback for cancer chemotherapy. P-glycoprotein acts as an efflux transporter for a variety of chemotherapeutic agents. It is encoded by multidrug resistance (mdr) genes of the subfamily 1 in humans (MDR1) and rodents (mdr1a and 1b). Because mdr1 gene expression is inducible in cultured rat hepatocytes and in rat liver with chemical carcinogens such as 2-acetylaminofluorene or aflatoxin B1, which form DNA-binding electrophiles during their metabolism, we investigated whether the DNA-damaging chemotherapeutic drug mitoxantrone may induce multidrug resistance in rodents and in hepatocytes in primary culture. In H4IIE rat hepatoma cells stably transfected with a luciferase construct containing the rat mdr1b promoter, mitoxantrone caused a concentration-dependent increase in promoter activity. Mdr1 gene expression in cultured rat hepatocytes was enhanced at mitoxantrone concentrations greater than or equal to 0.1 microM and in mouse hepatocytes at 5 microM. In hepatocytes from both species, a correlation was found between mdr1 induction and the inhibition of protein synthesis. In vivo, mitoxantrone was a very powerful inducer of mdr1 gene expression in rat liver and small intestine. In rat kidney, induction of mRNA was lower, and a marginal effect was seen in lung. In contrast with rats, no significant induction of mdr1 gene expression was obtained in mouse liver. Probably as a consequence of inhibition of protein synthesis, mitoxantrone did not lead to a pronounced elevation of P-glycoprotein levels in rat liver and kidney.

Animals↗
PubMed1996

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.

Animals↗
PubMed1996

Comparison of staurosporine and four analogues: their effects on growth, rhodamine 123 retention and binding to P-glycoprotein in multidrug-resistant MCF-7/Adr cells.

The potent kinase inhibitor staurosporine and its protein kinase C (PKC)-selective analogue CGP 41251 are known to sensitise cells with the multidrug resistance (MDR) phenotype mediated by P-glycoprotein (P-gp) to cytotoxic agents. Here four PKC-selective staurosporine cogeners, CGP 41251, UCN-01, RO 31 8220 and GF 109203X, were compared with staurosporine in terms of their MDR-reversing properties and their susceptibility towards P-gp-mediated drug efflux from MCF-7/Adr cells. Staurosporine was the most potent and the bisindolylmaleimides RO 31 8220 and GF 109203X the least potent cytostatic agents. When compared with MCF-7 wild-type cells, MCF-7/Adr cells were resistant towards the growth-arresting properties of RO 31 8220 and UCN-01, with resistance ratios of 12.6 and 7.0 respectively. This resistance could be substantially reduced by inclusion of the P-gp inhibitor reserpine. The ratios for GF 109203X, staurosporine and CGP 41251 were 1.2, 2.0 and 2.9 respectively, and they were hardly affected by reserpine. These results suggest that RO 31 8220 and UCN-01 are avidly transported by P-gp but that the other compounds are not. Staurosporine and CGP 41251 at 10 and 20 nM, respectively, decreased efflux of the P-gp probe rhodamine 123 (R123) from MCF-7/Adr cells, whereas RO 31 8220 and GF 109203X at 640 nM were inactive. CGP 41251 was the most effective and GF 109203X the least effective inhibitor of equilibrium binding of [3H]vinblastine to its specific binding sites, probably P-gp, in MCF-7/Adr cells. Overall, the results imply that for this class of compound the structural properties that determine susceptibility towards P-gp-mediated substrate transport are complex. Comparison with ability to inhibit PKC suggests that the kinase inhibitors affect P-gp directly and not via inhibition of PKC. Among these compounds CGP 41251 was a very potent MDR-reversing agent with high affinity for P-gp and least affected by P-gp-mediated resistance, rendering it an attractive drug candidate for clinical development.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1996

Regulation of P-glycoprotein 1 and 2 gene expression and protein activity in two MCF-7/Dox cell line subclones.

The MCF-7 doxorubicin-resistant cell line MCF-7/Dox has been used extensively for studies of the multidrug resistance phenomenon. Using fluorescence-activated cell sorting (FACS), these cells were separated into two populations on the basis of rhodamine 123 (R123) accumulation. We designated these as low P-glycoprotein (LP-gp) and high P-gp (HP-gp) cells on the basis of their P-gp content. Using the reverse transcriptase polymerase chain reaction technique controlled by homologous internal standards, we analysed levels of MDR1 and MDR2 mRNA in each cell type. LP-gp and HP-gp cells had MDR1 mRNA levels of 2.17 +/- 0.17 and 6.65 +/- 2.29 amol ng-1 total RNA respectively, compared with 0.00088 +/- 0.00005 amol ng-1 in wild-type MCF-7 cells (MCF-7/WT). MCF-7/WT cells additionally contained 0.023 +/- 0.016 amol ng-1 of MDR2 mRNA, which was unchanged in LP-gp cells, but lower than in HP-gp cells, which contained 0.42 +/- 0.08 amol ng-1. Both LP-gp and HP-gp cells contained increased copies of the MDR1 gene. However, the degree of gene amplification did not correlate with the changes in MDR1 mRNA levels, indicating further regulatory levels of gene expression. The level of P-gp detected by MRK 16 correlated with R123 accumulation. HP-gp cells expressed a 10-fold higher level of P-gp1 than LP-gp cells. However, there was only a 3-fold increase in MDR1 mRNA level in HP-gp cells compared with LP-gp cells. These data suggest that some regulation of P-gp1 expression also occurred at the post-translational level. Phosphorylation of P-gp by protein kinase C (PKC)-alpha is necessary for its activity. Our analysis of PKC-alpha, 0 and epsilon isozyme levels, and subcellular distribution, shows a co-regulation of expression with P-gp, suggesting a necessary role for PKC in P-gp regulation.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1996

Intrinsic multidrug class 1 and 2 gene expression and localization in rat and human mammary tumors.

P-glycoproteins (P-gp), the protein products of the multidrug resistant (mdr) genes, are often overexpressed in tumors that are resistant to chemotherapy. In this study, we measured intrinsic (preexisting) mdr1a, -1b, and -2 gene expression in N-nitroso-N-methylurea-induced rat mammary tumors. Using immunohistochemistry and in situ hybridization, we determined the localization of mdr1b mRNA and P-gp. We compared these results with a similar morphologic analysis of untreated human breast carcinomas. The predominantly expressed member of the mdr gene family in normal rat mammary tissue was mdr2. In rat mammary tumors, mdr2 expression was only inconsistently and slightly increased (1.4-fold on average). In contrast, expression of the class 1 genes (mdr1a and mdr1b) were consistently increased to a much greater extent. The average increase of mdr1b gene expression was greater than that of mdr1a (on average, 12.9 compared with 2.4, respectively). This indicated a differential regulation of these two closely related genes in these tumors. Immunostaining and in situ analysis showed that mdr mRNA and P-gp were expressed in a minority of neoplastic epithelial cells located in defined areas of the tumor, often on the interface of the neoplastic epithelium and stroma. Single epithelial cells that had invaded the stroma also expressed P-gp. In human breast carcinomas, the patterns of mdr mRNA and P-gp expression were similar to those in rat N-nitroso-N-methylurea-induced mammary tumors.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1995

In vivo induction of liver P-glycoprotein expression by xenobiotics in monkeys.

P-glycoprotein (pgp), the protein product of the multidrug resistance (mdr) gene family, can confer a multidrug resistance (mdr) phenotype to cells in which it is expressed. One member of the pgp family, pgp2, is located on the hepatocyte biliary pole where it may have a role in biliary excretion. Using primates we sought to determine if mdr gene expression and pgp levels were affected by xenobiotics excreted via the bile in man. Five drugs were studied in male and female rhesus monkeys: erythromycin, rifampicin, tamoxifen, diethylstilbesterol (DES), and probenecid. For each xenobiotic, with the exception of DES, an increase in mdr2 mRNA was observed. The results suggest that expression of mdr2 is responsive to xenobiotics, or their metabolites, that require biliary excretion. We speculate that the mdr2 gene may be a member of a class of xenobiotic responsive genes coding for proteins that actively excrete xenobiotics and/or their metabolites into the bile.

ATP Binding Cassette Transporter, Subfamily B, Mem↗