The multidrug resistance protein breast cancer resistance protein (BCRP) protects adipose-derived stem cells against ischemic damage.

Adipose tissue-derived stem cells (ASCs) are promising candidates for regenerative therapy, like after myocardial infarction. However, when transplanted into the infarcted heart, ASCs are jeopardized by the ischemic environment. Interestingly, it has been shown that multidrug resistance (MDR) proteins like the breast cancer resistance protein (BCRP) and P-glycoprotein (P-gp) have a protective effect in haematopoietic stem cells. In ASC, however, only expression of BCRP was shown until now. In this study, we therefore analysed the expression and functional activity of BCRP and P-gp and their putative function in ischemia in ASC. BCRP and P-gp protein expression was studied over time (passages 2-6) using western blot analysis and immunohistochemical staining. MDR activity was analysed using protein-specific substrate extrusion assays. Ischemia was induced using metabolic inhibition. All analyses demonstrated protein expression and activity of BCRP in ASCs. In contrast, only minor expression of P-gp was found, without functional activity. BCRP expression was most prominent in early passage ASCs (p2) and decreased during culture. Finally, ischemia induced expression of BCRP. In addition, when BCRP was blocked, a significant increase in dead ASCs was found already after 1 h of ischemia. In conclusion, ASCs expressed BCRP, especially in early passages. In addition, we now show for the first time that BCRP protects ASCs against ischemia-induced cell death. These data therefore indicate that for transplantation of ASCs in an ischemic environment, like myocardial infarction, the optimal stem cell protective effect of BCRP theoretically will be achieved with early culture passages ASCs.

Expression of drug pathway proteins is independent of tumour type.

Current clinicopathological staging systems have the advantage of standardized criteria for assessing tumour stage, and a relationship between advancing tumour stage and poor prognosis has been established for most cancers. However, these tools have not led to clear criteria for therapy selection in individual patients. Indeed, the concept of therapy based on anatomical location seems quaint. Therefore, a representative drug pathway (irinotecan) was evaluated across common tumour types to test the hypothesis that pharmacological proteins are expressed independent of anatomical location. Many enzymes are involved in controlling the disposition of irinotecan, including the cellular target (TOP1), metabolism enzymes (CES2, UGT1A1, CYP3A4, CYP3A5), and cellular transporters of the anti-cancer agent (ABCB1, ABCC1, ABCC2, ABCC3, ABCC5, ABCG2). These 11 proteins were evaluated in tissue microarrays containing colon, breast, prostate, ovary, and lung cancers; brain tumours; melanoma; lymphoma; and selected normal tissues. A total of 255 tumours and 37 normal tissue samples were evaluable for all proteins. Linear discriminant analysis designed to predict the tissue type from the protein expression levels revealed a 49.6% misclassification rate, indicating that protein expression of this drug pathway is not associated with tissue type. Cluster analysis identified a variety of tumours with the same pharmacological profile. The anatomy independence of drug pathways stimulates efforts to move away from our traditional approaches to the selection of cancer therapy.

Hormonal regulation of renal multidrug resistance-associated proteins 3 and 4 (Mrp3 and Mrp4) in mice.

Multidrug resistance-associated proteins 3 and 4 (Mrp3 and Mrp4) are expressed at much higher levels in female than male kidney. Sex steroids and sex-specific growth hormone (GH) secretion patterns often mediate gender-predominant gene expression. Thus, three models were used to investigate potential endocrine regulation of Mrp3 and Mrp4: (1) gonadectomized (GNX) mice with 17beta-estradiol (E2) or 5alpha-dihydroxytestosterone (DHT) replacement; (2) hypophysectomized (HPX) mice receiving E2, DHT, or simulated male-pattern (MP) or female-pattern (FP) GH secretion; (3) lit/lit mice, which have a spontaneous mutation in the growth-hormone releasing-hormone (GHRH) receptor, with simulated MP- or FP-GH secretion. GNX and HPX decreased Mrp3 mRNA levels compared with intact females. In both respective models E2 administration increased Mrp3 expression in GNX and HPX mice. DHT markedly repressed Mrp3 from GNX+placebo levels, however, this was not observed in the HPX model. In lit/lit mice, Mrp3 expression was lower than in wild-type controls, and MP-GH and FP-GH simulation slightly increased Mrp3 expression. Whereas GNX increased Mrp4 in males to female levels, HPX actually increased Mrp4 expression in both genders +375% and +66%, respectively. In both models DHT markedly repressed Mrp4. Furthermore, Mrp4 was higher in lit/lit than wild-type male mice, and simulation of MP-GH secretion suppressed female-predominant Mrp4 expression. In conclusion, these data indicate that E2 contributes to higher Mrp3 mRNA expression in females, yet a role for androgens in Mrp3 repression cannot be discounted. In contrast, Mrp4 mRNA is higher in females due to repression by both DHT and MP-GH secretion in males.

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

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

Vascular colocalization of P-glycoprotein, multidrug-resistance associated protein 1, breast cancer resistance protein and major vault protein in human epileptogenic pathologies.

Multidrug transporters, such as P-glycoprotein (P-gp), multidrug-resistance associated protein 1 (MRP1) and breast cancer resistance protein (BCRP), are associated with multidrug resistance in cancers; other molecules, such as major vault protein (MVP), have a similar association with drug-resistant cancer. These proteins are postulated to generate drug resistance in epilepsy. They have been shown individually to be up-regulated in epileptogenic brain tissue. In any consideration of the function, inhibition or evasion of the activity of such proteins, the colocalization of such proteins needs to be understood. We systematically determined the presence of such colocalization, focusing on microvascular endothelium from epileptogenic human brain tissue. Double labelling immunofluorescence and confocal laser scanning microscopy were used to determine colocalization of P-gp, MRP1, BCRP and MVP in one case of hippocampal sclerosis and two cases of focal cortical dysplasia type IIb. Endothelial colocalization was examined with double labelling using antibodies to CD34 and Factor VIII. The presence of P-gp, BCRP and MVP in microvascular endothelium was confirmed. P-gp, BCRP and MVP colocalized in microvascular endothelium, though not all proteins appeared to be identically distributed within this tissue. MRP1 did not colocalize to endothelium. These findings were not unexpected but required formal confirmation. The demonstrated colocalization of P-gp, BCRP and MVP in microvascular endothelium in epileptogenic human brain tissue has important implications for functional experiments (including single knock-out mice studies), work with specific and broad-spectrum inhibitors of transport function, and any eventual trials of treatment of refractory epilepsy involving modulation of the function of these proteins.

Hypoxia-induced acidification causes mitoxantrone resistance not mediated by drug transporters in human breast cancer cells.

Hypoxia has clinically been associated with resistance to chemotherapy. The aim of this study was to investigate whether hypoxia induces resistance to doxorubicin and mitoxantrone, two common drugs in cancer treatment, in MCF-7 breast cancer cells, and SW1573 non-small lung cancer cells. In addition, the role of drug transporters P-gp, BCRP and MRP1 was analysed. Hypoxia induced resistance in MCF-7 cells to mitoxantrone shifted the IC(50) value from 0.09 microM (+/-0.01) to 0.54 microM (+/-0.06) under hypoxia, whereas survival of MCF-7 and SW1573 cells in the presence of doxorubicin was not altered. Accumulation of mitoxantrone and daunorubicin, a doxorubicin fluorescent homologue, appeared to be 5.3 and 3.2 times lower in MCF-7 cells, respectively. Cytotoxicity assays showed no increased functionality of the drug transporters P-gp, BCRP and MRP1 under hypoxia. In addition, protein levels of these drug transporters were not changed. Medium of the MCF-7 cells became more acidic under hypoxia thereby causing a decreased uptake of mitoxantrone. Hypoxia induces mitoxantrone resistance in MCF-7 cells not mediated by the three major MDR transporters. Hypoxia-induced acidification may cause this resistance by decreased cellular uptake together with a lowered cytotoxicity due to pH-dependent topoisomerase type II activity.

Acquired resistance of human T cells to sulfasalazine: stability of the resistant phenotype and sensitivity to non-related DMARDs.

BACKGROUND: A recent study from our laboratory showed that induction of the multidrug resistance related drug efflux pump ABCG2 contributed to acquired resistance of human T cells to the disease modifying antirheumatic drug (DMARD) sulfasalazine (SSZ). OBJECTIVES: To investigate the duration of SSZ resistance and ABCG2 expression after withdrawal of SSZ and rechallenging with SSZ, and to assess the impact of SSZ resistance on responsiveness to other DMARDs. METHODS: Human CEM cells (T cell origin) with acquired resistance to SSZ (CEM/SSZ) were characterised for (a) SSZ sensitivity and ABCG2 expression during withdrawal and rechallenge of SSZ, and (b) antiproliferative efficacy of other DMARDs. RESULTS: ABCG2 protein expression was stable for at least 4 weeks when CEM/SSZ cells were grown in the absence of SSZ, but gradually declined, along with SSZ resistance levels, to non-detectable levels after withdrawal of SSZ for 6 months. Rechallenging with SSZ led to a rapid (<2.5 weeks) resumption of SSZ resistance and ABCG2 expression as in the original CEM/SSZ cells. CEM/SSZ cells displayed diminished sensitivity to the DMARDs leflunomide (5.1-fold) and methotrexate (1.8-fold), were moderately more sensitive (1.6-2.0 fold) to cyclosporin A and chloroquine, and markedly more sensitive (13-fold) to the glucocorticoid dexamethasone as compared with parental CEM cells. CONCLUSION: The drug efflux pump ABCG2 has a major role in conferring resistance to SSZ. The collateral sensitivity of SSZ resistant cells for some other (non-related) DMARDs may provide a further rationale for sequential mono- or combination therapies with distinct DMARDs upon decreased efficacy of SSZ.

Development of sulfasalazine resistance in human T cells induces expression of the multidrug resistance transporter ABCG2 (BCRP) and augmented production of TNFalpha.

OBJECTIVE: To determine whether overexpression of cell membrane associated drug efflux pumps belonging to the family of ATP binding cassette (ABC) proteins contributes to a diminished efficacy of sulfasalazine (SSZ) after prolonged cellular exposure to this disease modifying antirheumatic drug (DMARD). METHODS: A model system of human T cells (CEM) was used to expose cells in vitro to increasing concentrations of SSZ for a period of six months. Cells were then characterised for the expression of drug efflux pumps: P-glycoprotein (Pgp, ABCB1), multidrug resistance protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2). RESULTS: Prolonged exposure of CEM cells to SSZ provoked resistance to SSZ as manifested by a 6.4-fold diminished antiproliferative effect of SSZ compared with parental CEM cells. CEM cells resistant to SSZ (CEM/SSZ) showed a marked induction of ABCG2/BCRP, Pgp expression was not detectable, while MRP1 expression was even down regulated. A functional role of ABCG2 in SSZ resistance was demonstrated by 60% reversal of SSZ resistance by the ABCG2 blocker Ko143. Release of the proinflammatory cytokine tumour necrosis factor alpha (TNFalpha) was threefold higher in CEM/SSZ cells than in CEM cells. Moreover, twofold higher concentrations of SSZ were required to inhibit TNFalpha release from CEM/SSZ cells compared with CEM cells. CONCLUSION: Collectively, ABCG2 induction, augmented TNFalpha release, and less efficient inhibition of TNFalpha production by SSZ may contribute to diminished efficacy after prolonged exposure to SSZ. These results warrant further clinical studies to verify whether drug efflux pumps, originally identified for their roles in cytostatic drug resistance, can also be induced by SSZ or other DMARDs.

Expression and cellular distribution of multidrug transporter proteins in two major causes of medically intractable epilepsy: focal cortical dysplasia and glioneuronal tumors.

The cell-specific distribution of multidrug resistance extrusion pumps was studied in developmental glioneuronal lesions, including focal cortical dysplasia (15 cases) and ganglioglioma (15 cases) from patients with medically intractable epilepsy. Lesional, perilesional, as well as normal brain regions were examined for the expression of the multidrug resistance gene 1 encoded P-glycoprotein (P-gp) and the multidrug resistance-associated protein 1 (MRP1) by immunocytochemistry. In normal brain MRP1 expression was below detection, whereas P-gp staining was present only in blood vessels. MRP1 and P-gp immunoreactivity was observed in dysplastic neurons of 11/15 cases of focal cortical dysplasia, as well as in the neuronal component of 14/15 ganglioglioma. Glial cells with astrocytic morphology within the lesion showed multidrug-resistant protein immunoreactivity (P-gp>MRP1). Moderate to strong MRP1 and P-gp immunoreactivity was observed in a population of large ballooned neuroglial cells. P-gp appeared to be most frequently expressed in glial fibrillary acidic protein-positive balloon cells (glial type), whereas MRP1 was more frequently expressed in microtubule-associated protein 2-positive balloon cells (neuronal type). In both types of lesions strong P-gp immunoreactivity was found in lesional vessels. Perilesional regions did not show increased staining in vessels or in neuronal cells compared with normal cortex. The predominant intralesional cell-specific distribution of multidrug transporter proteins supports the hypothesis of a constitutive overexpression as common mechanism underlying the intrinsic pharmaco-resistance to antiepileptic drugs of both malformative and neoplastic glioneuronal developmental lesions.

Lung resistance-related protein as a predictor of clinical outcome in advanced testicular germ-cell tumours.

This study was undertaken to investigate the expression and predictive value for outcome of multidrug resistance-associated (MDR) proteins P-glycoprotein (Pgp), MRP1, BCRP, and LRP, in advanced testicular germ-cell tumours (TGCT). Paraffin-embedded sections from 56 previously untreated patients with metastatic TGCT were immunostained for Pgp, MRP1, BCRP, and LRP. All patients received platinum-based chemotherapy after orchidectomy. Immunostaining was related to clinicopathological parameters, response to chemotherapy, and outcome. Strong and intermediate expressions of the different MDR-related proteins were: 27 and 41% (Pgp), 54 and 37% (MRP1), 86 and 7% (BCRP), and 14 and 29% (LRP). P-glycoprotein and MRP1 associated, respectively, to low AFP (P=0.026) and high LDH levels (P=0.014), whereas LRP expression associated with high beta-hCG levels (P=0.003) and stage IV tumours (P=0.029). No correlation was found between Pgp, MRP1, and BCRP expression and response to chemotherapy and survival. In contrast, patients with LRP-positive tumours (strong or intermediate expression) had shorter progression-free (P=0.0006) and overall survival (P=0.0116) than LRP-negative patients, even after individual log-rank adjustments by statistically associated variables. Our data suggest that a positive LRP immunostaining at the time of diagnosis in metastatic TGCT is associated with an adverse clinical outcome.

Induction of breast cancer resistance protein by the camptothecin derivative DX-8951f is associated with minor reduction of antitumour activity.

DX-8951f (exatecan mesylate), a new water-soluble derivative of camptothecin, is currently being evaluated in phase II clinical trials. Resistance may be acquired when treating cancer patients with DX-8951f. Therefore, we selected a subline of the human ovarian cancer cell line A2780 for resistance against DX-8951f to investigate possible mechanisms of resistance. This DX-8951f-resistant subline, designated 2780DX8 (resistance factor=9.3), displayed a typical cross-resistance pattern including compounds, such as topotecan (resistance factor =34), SN-38 (resistance factor =47), mitoxantrone (resistance factor =59) and doxorubicin (resistance factor =2.9), which have previously been associated with the expression of breast cancer resistance protein. 2780DX8 cells did not show changes in the topoisomerase I gene, in topoisomerase I protein levels or catalytic activity. Overexpression of breast cancer resistance protein could be detected, both at the mRNA and protein level, while staining for Pgp, MRP1, or LRP was negative. GF120918, an inhibitor of breast cancer resistance protein, was able to reverse the DX-8951f-induced resistance in 2780DX8 cells. In vivo experiments in well-established 2780DX8 human tumour xenografts demonstrated that the growth inhibition induced by CPT-11 was more affected by breast cancer resistance protein expression than that of DX-8951f. These data indicate for the first time that DX-8951f is able to induce breast cancer resistance protein as a mechanism of resistance. Breast cancer resistance protein, however, results in only minor reduction of antitumour activity of DX-8951f which is an advantage over topotecan and CPT-11/SN-38.

Increased expression of beta 2-microglobulin in multidrug-resistant tumour cells.

The rat monoclonal antibody LMR-12 was shown earlier to react with a plasma membrane protein, upregulated in multidrug-resistant cell lines. In this study, we observed distinct LMR-12 staining in 36 out of 55 non-drug-selected tumour cell lines, including melanomas, renal cell-, colon- and lung carcinomas, whereas in other tumour types, such as leukaemia and ovarian cancer, LMR-12 staining was generally low or absent. The cDNA encoding the LMR-12 antigen was isolated from a library of the multidrug-resistant human fibrosarcoma cell line HT1080/DR4 by expression cloning in MOP8 cells. Sequence analysis showed that the LMR-12 antigen is identical to the major histocompatibility complex class I molecule beta 2-microglobulin (beta2-m). The LMR-12/ beta2-m staining results were confirmed by mRNA microarray data from an independent National Cancer Institute study, as well as by newly obtained reverse transcriptase polymerase chain reaction data. Further analysis of the microarray data showed that beta2-m levels closely reflected levels of major histocompatibility complex class I heavy chains and the transporter associated with antigen processing. Since the ABC transporter associated with antigen processing was previously shown to contribute to multidrug-resistance, it may very well be that the observed LMR-12/ beta2-m levels are secondary to (elevated) levels of the transporter associated with antigen processing. A perspective arising from the present study is that drug resistant tumour cells may, by having elevated levels of major histocompatibility complex related molecules, be particular good candidates for alternative therapeutic therapies, such as cytotoxic T cell mediated immune-therapies.

Multidrug resistance related molecules in human and murine lung.

AIMS: Transporter proteins known to mediate multidrug resistance (MDR) in tumour cells--MDR1 P-glycoprotein (P-gp) and multidrug resistance related protein 1 (MRP1)--are thought to be involved in protecting the lungs against inhaled toxic pollutants. Recently, several new transporter family members have been identified--for example, MRP2, MRP3, and breast cancer resistance protein (BCRP). To study the possible contribution of these proteins and the earlier defined MDR1 and MDR3 P-gp molecules, MRP1, and the major vault protein (MVP) to lung functioning, their expression was analysed in normal lung tissue of humans and several animal species. METHODS: Frozen sections of normal lung tissues were examined for the expression of the multidrug resistance associated proteins, using an extended panel of monoclonal antibodies that specifically detect these proteins in immunohistochemical techniques. RESULTS: In line with earlier reports, the expression of MDR1 P-gp and MRP1 was readily detected in the apical and basolateral membranes, respectively, of the epithelial cell layers of the lungs. In addition, prominent cytoplasmic MVP staining was detected in these layers. In contrast, the recently discovered transporters were either undetectable or they were present at very low values in lung tissue. Immunohistochemical staining in tissues from mice, rats, and guinea pigs points to a strong evolutionary conservation for these transporter proteins. CONCLUSIONS: These results show that the "classic" MDR related molecules, MDR1 P-gp, MRP1, and MVP, should be considered the most important transporters in normal lung physiology. It will be of great interest to investigate differences in expression of both classic and newly defined transporters between normal individuals and-for example, patients with various bronchopulmonary pathological conditions.

Selection and characterisation of a phage-displayed human antibody (Fab) reactive to the lung resistance-related major vault protein.

The major vault protein is the main component on multimeric vault particles, that are likely to play an essential role in normal cell physiology and to be associated with multidrug resistance of tumour cells. In order to unravel the function of vaults and their putative contribution to multidrug resistance, specific antibodies are invaluable tools. Until now, only conventional major vault protein-reactive murine monoclonal antibodies have been generated, that are most suitable for immunohistochemical analyses. The phage display method allows for selection of human antibody fragments with potential use in clinical applications. Furthermore, cDNA sequences encoding selected antibody fragments are readily identified, facilitating various molecular targeting approaches. In order to obtain such human Fab fragments recognising major vault protein we used a large non-immunized human Fab fragment phage library. Phages displaying major vault protein-reactive Fabs were obtained through several rounds of selection on major vault protein-coated immunotubes and subsequent amplification in TG1 E coli bacteria. Eventually, one major vault protein-reactive clone was selected and further examined. The anti-major vault protein Fab was found suitable for immunohistochemical and Western blot analysis of tumour cell lines and human tissues. BIAcore analysis showed that the binding affinity of the major vault protein-reactive clone almost equalled that of the murine anti-major vault protein Mabs. The cDNA sequence of this human Fab may be exploited to generate an intrabody for major vault protein-knock out studies. Thus, this human Fab fragment should provide a valuable tool in elucidating the contribution(s) of major vault protein/vaults to normal physiology and cellular drug resistance mechanisms.

Detection of the Mr 110,000 lung resistance-related protein LRP/MVP with monoclonal antibodies.

The Mr 110,000 lung resistance-related protein (LRP), also termed the major vault protein (MVP), constitutes >70% of subcellular ribonucleoprotein particles called vaults. Overexpression of LRP/MVP and vaults has been linked directly to MDR in cancer cells. Clinically, LRP/MVP expression can be of value to predict response to chemotherapy and prognosis. Monoclonal antibodies (MAbs) against LRP/MVP have played a critical role in determining the relevance of this protein in clinical drug resistance. We compared the applicability of the previously described MAbs LRP-56, LMR-5, LRP, 1027, 1032, and newly isolated MAbs MVP-9, MVP-16, MVP-18, and MVP-37 for the immunodetection of LRP/MVP by immunoblotting analysis and by immunocyto- and histochemistry. The availability of a broader panel of reagents for the specific and sensitive immunodetection of LRP/MVP should greatly facilitate biological and clinical studies of vault-related MDR.

A novel 7-modified camptothecin analog overcomes breast cancer resistance protein-associated resistance in a mitoxantrone-selected colon carcinoma cell line.

We selected a mitoxantrone-resistant HT29 colon carcinoma cell line (HT29/MIT) that exhibited a very high degree of resistance to the selecting agent and marked resistance to topotecan and SN38, but limited resistance to doxorubicin. The development of drug resistance was independent of expression of P-glycoprotein or multidrug resistance-associated protein but was associated with high up-regulation of the breast carcinoma resistance protein (BCRP) as shown by Western blot analysis. BCRP overexpression was associated with a reduced intracellular accumulation of topotecan, a known substrate for BCRP. Conversely, a lipophilic 7-modified camptothecin analogue (ST1481) displayed a complete lack of cross-resistance in HT29/MIT cells, suggesting that the drug was not a substrate for BCRP because no defects in intracellular accumulation were found. This conclusion is consistent with the antitumor efficacy of ST1481 against a BCRP-expressing tumor. These results may have therapeutic implications because the antitumor efficacy of ST1481 is in part related to a good bioavailability after oral administration, and the drug is currently under Phase I clinical evaluation.

Multiple human vault RNAs. Expression and association with the vault complex.

Human vaults are intracellular ribonucleoprotein particles believed to be involved in multidrug resistance. The complex consists of a major vault protein (MVP), two minor vault proteins (VPARP and TEP1), and several small untranslated RNA molecules. Three human vault RNA genes (HVG1-3) have been described, and a fourth was found in a homology search (HVG4). In the literature only the association of hvg1 with vaults was shown in vivo. However, in a yeast three-hybrid screen the association of hvg1, hvg2, and hvg4 with TEP1 was demonstrated. In this study we investigated the expression and vault association of different vault RNAs in a variety of cell lines, including pairs of drug-sensitive and drug-resistant cells. HVG1-3 are expressed in all cell lines examined, however, none of the cell lines expressed HVG4. This probably is a consequence of the absence of essential external polymerase III promoter elements. The bulk of the vault RNA associated with vaults was hvg1. Interestingly, an increased amount of hvg3 was bound to vaults isolated from multidrug-resistant cell lines. Our findings suggest that vaults bind the RNA molecules with different affinities in different situations. The ratio in which the vault RNAs are associated with vaults might be of functional importance.

The wide spectrum of multidrug resistance 3 deficiency: from neonatal cholestasis to cirrhosis of adulthood.

BACKGROUND & AIMS: We have specified the features of progressive familial intrahepatic cholestasis type 3 and investigated in 31 patients whether a defect of the multidrug resistance 3 gene (MDR3) underlies this phenotype. METHODS: MDR3 sequencing, liver MDR3 immunohistochemistry, and biliary phospholipid dosage were performed. RESULTS: Liver histology showed a pattern of biliary cirrhosis with patency of the biliary tree. Age at presentation ranged from the neonatal period to early adulthood. Sequence analysis revealed 16 different mutations in 17 patients. Mutations were identified on both alleles in 12 patients and only on 1 allele in 5. Four mutations lead to a frame shift, 2 are nonsense, and 10 are missense. An additional missense mutation probably representing a polymorphism was found in 5 patients. MDR3 mutations were associated with abnormal MDR3 canalicular staining and a low proportion of biliary phospholipids. Gallstones or episodes of cholestasis of pregnancy were found in patients or parents. Children with missense mutations had a less severe disease and more often a beneficial effect of ursodeoxycholic acid therapy. CONCLUSIONS: At least one third of the patients with a progressive familial intrahepatic cholestasis type 3 phenotype have a proven defect of MDR3. This gene defect should also be considered in adult liver diseases.

Peptide transport by the multidrug resistance protein MRP1.

Small hydrophobic peptides were studied as possible substrates of the multidrug resistance protein (MRP)-1 (ABCC1) transmembrane transporter molecule. As observed earlier for P-glycoprotein- (Pgp; ABCB1) overexpressing cells, MRP1-overexpressing cells, including cells stably transfected with the MRP1 cDNA, showed distinct resistance to the cytotoxic peptide N-acetyl-Leu-Leu-norleucinal (ALLN). Resistance to this peptide and another toxic peptide derivative, which is based on a Thr-His-Thr-Nle-Glu-Gly backbone conjugated to butyl and benzyl groups (4A6), could be reversed by MRP1 inhibitors. The reduced toxicity of 4A6 in MRP1-overexpressing cells was found to be associated with lower accumulation of a fluorescein-labeled derivative of this peptide. Glutathione (GSH) depletion had a clear effect on resistance to ALLN but hardly affected 4A6 resistance. In a limited structure-activity study using peptides that are analogous to 4A6, MRP1-overexpressing cells were found to be resistant to these peptides as well. Remarkably, when selecting A2780 ovarian cancer cells for resistance to ALLN, even in the absence of Pgp blockers, resulting cell lines had up-regulated MRP1, rather than any of the other currently known multidrug resistance transporter molecules including Pgp, MRP2 (ABCC2), MRP3 (ABCC3), MRP5 (ABCCS), and the breast cancer resistance protein ABCG2. ALLN-resistant, MRP1-overexpressing cells were found to be cross-resistant to 4A6 and the classical multidrug resistance drugs doxorubicin, vincristine, and etoposide. This establishes MRP1 as a transporter for small hydrophobic peptides. More extensive structure-activity relationship studies should allow the identification of clinically useful peptide antagonists of MRP1.

Subcellular localization and distribution of the breast cancer resistance protein transporter in normal human tissues.

High expression of the Breast Cancer Resistance Protein (BCRP) gene has been shown to be involved in resistance to chemotherapeutic drugs. Knowledge of the localization of BCRP protein in normal tissues may help unravel the normal function of this protein. Therefore, we characterized the tissue distribution and cellular localization of BCRP in frozen sections of normal human tissues. For this purpose, we used the recently described monoclonal antibody BXP-34 and another independently developed monoclonal antibody directed against BCRP, BXP-21. Both monoclonal antibodies show specific BCRP plasma membrane staining on cytospins obtained from topotecan- or mitoxantrone-selected cell lines, as well as from BCRP-transfected cell lines. Immunoprecipitation experiments using either BXP-21 or BXP-34 yielded a clear M(r) 72,000 BCRP band from BCRP-overexpressing tumor cells. In the topotecan-selected T8 and mitoxantrone-selected MX3 tumor cell lines, BCRP turned out to be differentially glycosylated. In contrast to BXP-34, BXP-21 is able to detect the M(r) 72,000 BCRP protein on immunoblots and is reactive with BCRP in formalin-fixed, paraffin-embedded tissues. Using BXP-21 and BXP-34, prominent staining of BCRP was observed in placental syncytiotrophoblasts, in the epithelium of the small intestine and colon, in the liver canalicular membrane, and in ducts and lobules of the breast. Furthermore, BCRP was present in veinous and capillary endothelium, but not in arterial endothelium in all of the tissues investigated. In the tissues studied, the mRNA levels of BCRP were assessed using reverse transcription-PCR, and these corresponded with the levels of BCRP protein estimated from immunohistochemical staining. The presence of BCRP at the placental syncytiotrophoblasts is consistent with the hypothesis of a protective role of BCRP for the fetus. The apical localization in the epithelium of the small intestine and colon indicates a possible role of BCRP in the regulation of the uptake of p.o. administered BCRP substrates by back-transport of substrate drugs entering from the gut lumen. Therefore, it may be useful to attempt to modulate the uptake of p.o. delivered BCRP substrates, e.g., topotecan or irinotecan, by using a BCRP inhibitor. Clinical trials testing this hypothesis have been initiated in our institute.