Use of antibodies to probe membrane glycoproteins associated with drug-resistant J774.2 cells

Induction of pgp3 expression and reversion of the multidrug resistance phenotype in 9-OH-ellipticine-resistant Chinese hamster lung fibroblasts transfected with the MYC oncogene

Chinese hamster lung cells resistant to the DNA topoisomerase II inhibitor 9-OH-ellipticine (DC-3F/9-OH-E) are cross resistant to various drugs through the expression of the MDR phenotype. The myc oncogene was approximately 10-fold amplified and 20-fold overexpressed in parental DC-3F cells as compared with DC-3F/9-HO-E cells. Transfection of the resistant cells with a mouse c-myc gene did not alter the resistance to topoisomerase II inhibitors and, in cells with a low multidrug (MDR) expression, reversed this phenotype. Northern and Western blot analyses revealed an increased expression of pgp1 in the DC-3F/9-OH-E cells, which was not modified in the myc-transfected clones. However, myc expression in these clones resulted in an increased expression of pgp3, roughly in proportion to the level of myc expression. Transfection of the DC-3F/9-OH-E cells with the human MDR3 gene, homologous to pgp3, also resulted in the reversion of the MDR phenotype. These results show that (1) expression of the transfected myc gene positively regulates pgp3 expression but has no effect on pgp1; (2) when observed, reversion of the MDR phenotype is proportional to the levels of myc and pgp3 expression; and (3) this reversion, resulting from pgp3 expression, is associated with a decreased functional activity of the pgp1 protein and might require an appropriate balance of pgp1 and pgp3 expression.

Involvement of membrane bleomycin-binding sites in bleomycin cytotoxicity

The authors have recently shown the existence of bleomycin (BLM)-binding sites at the surface of DC-3F cells. In order to study the involvement of these sites in the sensitivity of the cells to bleomycin several BLM-resistant cell lines from DC-3F cells were analysed. These mutants were obtained by electrotransfection of the Sh ble gene (D/BlmI cells) or the Sh ble-beta Gal fusion gene (D/BlmII cells) and/or by continuous culture in the presence of BLM (D/BlmIR and D/Blm40 cells). The resistance levels of the D/BlmII and D/Blm40 cells were 50- and 22-fold, respectively, determined at the EC50 level. The D/BlmI cells were only 2-fold resistant, whereas D/BlmIR cells were so resistant that almost no cytotoxicity was detected up to 200 microM BLM external concentration. Electropermeabilization was used in an attempt to bypass the plasma membrane of the cells and permit the distinction between internal resistance and membrane resistance. The former was observed when the products of the transfected genes were present. With respect to membrane resistance, differences were detected in the number of BLM-binding sites in several mutant cell lines, which could account for the differences in cell sensitivity to BLM. This suggests that the BLM-binding sites found at the cell surface may play a crucial role in BLM internalization and consequently in its cytotoxicity.

Progesterone and its metabolites: the potent inhibitors of the transporting activity of P-glycoprotein in the adrenal gland

P-glycoprotein (P-gp) is a transmembrane glycoprotein responsible for the multidrug resistant (MDR) phenotype in various cancer cells. It has been shown that P-gp transports various kinds of anti-cancer agents as well as hydrophobic chemicals. Although P-gp is also expressed in normal human tissues, such as liver, kidney, and adrenal gland, its function and transporting substrates in these tissues are still unknown. In previous work, we demonstrated that some compounds in human plasma modulate the transporting activity of P-gp. We also found that P-gp is expressed at a high level in the bovine adrenal gland and that this tissue contains large amount of compounds which inhibit the transporting activity of P-gp. We purified such compounds from the adrenal gland by monitoring the ability to enhance the accumulation of [3H]vincristine in MDR cells. Two major compounds were purified and identified as progesterone and pregnenolone by nuclear magnetic resonance (NMR) analysis. Progesterone was the most potent and abundant compound that inhibited the transporting activity of P-gp among the compounds extracted from bovine adrenal gland with methanol. We also found that six authentic progesterone metabolites in the 5 beta-metabolic pathway but none in the 5 alpha-metabolic pathway were able to enhance the accumulation of [3H]vincristine in MDR cells and to inhibit [3H]azidopine photolabeling of P-gp in the adrenal gland. These results indicate that some progesterone metabolites can interact with P-gp and that stereoisomerism around carbon 5 of the progesterone metabolites is important for them to be recognized by P-gp.

Biochemical and genetic characterization of the multidrug resistance phenotype in murine macrophage-like J774.2 cells

The development of multidrug resistance (MDR) in malignant tumors is a major obstacle to the treatment of many cancers. MDR sublines have been derived from the J774.2 mouse macrophage-like cell line and utilized to characterize the phenotype at the biochemical and genetic level. Two isoforms of the drug resistance-associated P-glycoprotein are present and distinguishable both electrophoretically and pharmacologically. Genetic analysis has revealed the presence of a three-member gene family; expression of two of these genes, mdr1a and mdr1b, is associated with MDR whereas the expression of the third, mdr2, is not. Studies of these three genes have revealed similarities and differences in the manner in which they are regulated at the transcriptional level, and have suggested that post-transcriptional effects may also be important.

Glycosylation of P-glycoprotein in a multidrug-resistant KB cell line, and in the human tissues

P-glycoprotein (P-gp) is thought to transport anti-cancer drugs and to be responsible for the multidrug-resistant (MDR) phenotype. Immunohistochemistry reveals that P-gp is also expressed in normal human tissues, such as the adrenal gland, kidney, liver, and the capillary endothelium of the brain and testis. However, little is known about the structural and functional variations of P-gp in these tissues. With immunoblotting and photoaffinity labeling, we found that the molecular mass of P-gp in these tissues varied between 130-140 kDa. To clarify the post-translational modification of P-gp, we studied the biosynthesis of P-gp in a human multidrug-resistant cell line (KB-C2). We found that P-gp was produced in KB-C2 cells as a 125 kDa precursor and was slowly processed (t1/2 = 45-60 min) to the mature form of 140 kDa. In the presence of tunicamycin, a 120 kDa form of P-gp was synthesized and this form was no longer processed. Treating the 125 kDa precursor form with endo-beta-N-acetylglucosaminidase H (Endo H) and the 140 kDa mature form with N-glycanase diminished the molecular size of P-gp to that of the tunicamycin-treated form. N-Glycanase almost completely removed [3H]glucosamine labeling from P-gp. These data indicate that the major modification of P-gp is N-linked glycosylation. P-gps from KB-C2 cells, kidney and adrenal gland had a different lectin-binding capacity. There seems to be a variety of N-linked glycosylations in tissue and tumor P-gps.

Structural analysis of the mouse mdr1a (P-glycoprotein) promoter reveals the basis for differential transcript heterogeneity in multidrug-resistant J774.2 cells

In multidrug-resistant mouse J774.2 cells, the differential overproduction of functionally distinct phosphoglycoprotein isoforms reflects the amplification or transcriptional activation or both of two mdr gene family members, mdr1a and mdr1b. The mdr1a gene is a complex transcriptional unit whose expression is associated with multiple transcript sizes. Independently selected multidrug-resistant J774.2 cell lines differentially overexpress either 4.6- and 5.0-kilobase (kb) or 4.7- and 5.1-kb mdr1a transcripts. However, abundant overproduction of the mdr1a gene product was observed only in cell lines which overexpressed the 4.6- and 5.0-kb mRNAs. In order to determine the basis for mdr1a transcript heterogeneity and the relationship between transcript size and steady-state mdr1a protein levels, genomic and cDNA sequence analyses of the 5' and 3' ends of the mdr1a gene were carried out. Promoter sequence analysis and primer extension mapping indicated that mdr1a transcripts were differentially initiated from two putative promoters to generate either 5.1- and 4.7-kb or 5.0- and 4.6-kb transcripts in four multidrug-resistant J774.2 cell lines. Sequence analysis of 3' cDNA variants and a 3' genomic fragment revealed that the 5.1- and 5.0-kb mRNAs had identical 3'-untranslated regions which differed from those of the 4.7- and 4.6-kb mRNAs as a result of the utilization of a more downstream alternative poly(A) addition signal. Transcript initiation from the putative upstream promoter correlated with a 70 to 85% decrease in steady-state mdr1a protein levels relative to transcript levels. In addition, the identification of putative AP-1 and AP-2 promoter elements suggests a possible role for protein kinase A and protein kinase C in the regulation of mdr1a. The implications of these findings for mdr gene expression and regulation are discussed.

Structural analysis of the mouse mdr1a (P-glycoprotein) promoter reveals the basis for differential transcript heterogeneity in multidrug-resistant J774.2 cells

In multidrug-resistant mouse J774.2 cells, the differential overproduction of functionally distinct phosphoglycoprotein isoforms reflects the amplification or transcriptional activation or both of two mdr gene family members, mdr1a and mdr1b. The mdr1a gene is a complex transcriptional unit whose expression is associated with multiple transcript sizes. Independently selected multidrug-resistant J774.2 cell lines differentially overexpress either 4.6- and 5.0-kilobase (kb) or 4.7- and 5.1-kb mdr1a transcripts. However, abundant overproduction of the mdr1a gene product was observed only in cell lines which overexpressed the 4.6- and 5.0-kb mRNAs. In order to determine the basis for mdr1a transcript heterogeneity and the relationship between transcript size and steady-state mdr1a protein levels, genomic and cDNA sequence analyses of the 5' and 3' ends of the mdr1a gene were carried out. Promoter sequence analysis and primer extension mapping indicated that mdr1a transcripts were differentially initiated from two putative promoters to generate either 5.1- and 4.7-kb or 5.0- and 4.6-kb transcripts in four multidrug-resistant J774.2 cell lines. Sequence analysis of 3' cDNA variants and a 3' genomic fragment revealed that the 5.1- and 5.0-kb mRNAs had identical 3'-untranslated regions which differed from those of the 4.7- and 4.6-kb mRNAs as a result of the utilization of a more downstream alternative poly(A) addition signal. Transcript initiation from the putative upstream promoter correlated with a 70 to 85% decrease in steady-state mdr1a protein levels relative to transcript levels. In addition, the identification of putative AP-1 and AP-2 promoter elements suggests a possible role for protein kinase A and protein kinase C in the regulation of mdr1a. The implications of these findings for mdr gene expression and regulation are discussed.

Multidrug resistance gene expression is controlled by steroid hormones in the secretory epithelium of the uterus

The multidrug resistance (mdr) gene family has been shown to encode a membrane glycoprotein, termed the P-glycoprotein, which functions as a drug efflux pump with broad substrate specificity. This multigene family is expressed in a tissue-specific fashion in a wide variety of normal and neoplastic tissues. The regulation of mdr gene expression in normal tissues is not understood. We have recently shown that mdr mRNA and the P-glycoprotein increases dramatically in the secretory luminal and glandular epithelium of the gravid murine uterus. This observation has suggested that mdr gene expression in the uterus is controlled by the physiologic changes associated with pregnancy. This report now demonstrates that mdr mRNA and P-glycoprotein are induced at high levels in the uterine secretory epithelium by the combination of estrogen and progesterone, the major steroid hormones of pregnancy. This regulation of mdr gene expression in the uterus does not require any other contribution from the fetus or placenta. The data indicate that this gene locus is hormonally responsive to estrogen and progesterone in the uterine secretory epithelium, suggesting an important and physiologically regulated role during pregnancy.

Use of recombinant P-glycoprotein fragments to produce antibodies to the multidrug transporter

Multidrug-resistance of human cancer cells may result from expression of a 170,000 dalton multidrug efflux pump called P-glycoprotein. To identify this multidrug transporter, and to study its structure and function, we have generated polyclonal rabbit antibodies against the amino-terminal and carboxy-terminal halves of the molecule using recombinant protein fragments produced in Escherichia coli. Two recombinant P-glycoprotein fragments, representing amino acids 140-228 and 919-1280, were overproduced in Escherichia coli by an inducible T7 expression system, gel-purified and injected into rabbits. Both antisera specifically immunoprecipitate 3H-azidopine and 35S-methionine labeled P-glycoprotein from multidrug-resistant cells and detect P-glycoprotein on Western blots with high sensitivity. Because these antisera were raised against epitopes in the amino- and carboxy-terminal halves of P-glycoprotein, they should be useful as research tools to define the function of these two halves of the molecule.

The products of the mdr1a and mdr1b genes from multidrug resistant murine cells have similar degradation rates

Two vinblastine-resistant sublines of the murine macrophage-like cell line J774.2, J7.V1-1 and J7.V3-1, overproduce unique forms of P-glycoprotein that are encoded by distinct mdr genes, mdr1b and mdr1a, respectively. Degradation rates of the two P-glycoprotein isoforms were measured by immunoprecipitation of P-glycoprotein. The half-life of immunoprecipitable P-glycoprotein from J7.V1-1 cells was 16.8 +/- 0.5 hours and from J7.V3-1 cells, 17.4 +/- 0.5 hours. This rate was not influenced by the presence of vinblastine in the growth medium. The data indicate that P-glycoproteins derived from distinct genes have similar degradation rates.

P-glycoproteins encoded by mdr 1b in murine gravid uterus and multidrug resistant tumor cell lines are differentially glycosylated

There are 3 members of the multidrug-resistance gene family expressed in mouse. Only one of these, mdr 1b, and its gene product P-glycoprotein are induced to high levels in the mouse endometrium during pregnancy. It is shown here that P-glycoprotein in the gravid uterus is significantly larger (Mr 155,000) compared to P-glycoprotein encoded by mdr 1b in a murine multidrug-resistant cell line (Mr 140,000). However, both species co-migrate after enzymatic removal of N-linked sugars (Mr 125,000). These results demonstrate that differential glycosylation of the mdr 1b gene product contributes to molecular heterogeneity found in P-glycoprotein from normal and multidrug-resistant cells.

Biosynthesis, processing and half-life of P-glycoprotein in a human multidrug-resistant KB cell

The biosynthesis, processing, and half-life of the drug efflux pump, P-glycoprotein, were studied in human multidrug-resistant KB (KB-C2) cells selected for resistance to colchicine. An antibody directed against a synthetic oligopeptide corresponding to the amino-acid sequence (Glu-393-Lys-408) of P-glycoprotein from human mdr1 cDNA was prepared in rabbits. With immunoblotting and immunoprecipitation, we detected a 140-170 kDa protein in KB-C2 cells but not in parental sensitive KB cells. KB-C2 cells made a 125 kDa precursor that was slowly processed (t1/2 = 45 min) to the mature form of 140-150 kDa. The processing rate of P-glycoprotein was slower than that of low-density lipoprotein receptor. We detected another 160-180 kDa smear band, which might be a completely denatured form of P-glycoprotein. With immunoblotting, a minor band of high molecular mass (greater than 500 kDa) was also detected and this form increased after the cells were treated with chemical cross-linker, 1,5-difluoro-2,4-dinitrobenzene. The half-life of P-glycoprotein was long; no significant loss of P-glycoprotein was observed within 24 h after synthesis. Cells treated with tunicamycin produced a 120 kDa form of P-glycoprotein which was no longer processed but showed stability similar to that of the mature 140-150 kDa form. Agents that reverse multidrug resistance, phorbol ester and transport substrate did not affect the stability of P-glycoprotein.

Distinct P-glycoprotein precursors are overproduced in independently isolated drug-resistant cell lines

A family of P-glycoproteins are overproduced in multidrug-resistant cells derived from the murine macrophage-like line J774.2. To determine whether individual family members are overproduced in response to different drugs, the P-glycoprotein precursors in several independently isolated cell lines, which were selected for resistance to vinblastine or taxol, were compared. Individual cell lines selected with vinblastine overproduced P-glycoprotein precursors of either 120 or 125 kDa. Taxol-selected cell lines overproduced either the 125-kDa precursor or both precursors simultaneously. Two similar but distinct peptide maps for the mature P-glycoproteins were observed. These maps corresponded to each precursor regardless of the drug used for selection. One vinblastine-resistant cell line switched from the 125- to the 120-kDa precursor when grown in increasing concentrations of drug. This change coincided with the overexpression of a distinct subset of mRNA species that code for P-glycoprotein. It is concluded that precursor expression is not drug-specific. These data suggest that individual overproduced P-glycoprotein family members are translated as distinct polypeptides. The results may help to explain the diversity in the multidrug-resistant phenotype.

The gene encoding multidrug resistance is induced and expressed at high levels during pregnancy in the secretory epithelium of the uterus

A survey of the expression of the multidrug-resistance gene (mdr) in mouse tissues revealed that a mdr mRNA species is expressed at extremely high levels in the gravid uterus. mdr mRNA expression levels increase dramatically during pregnancy compared to the relatively low levels of expression observed in the nongravid uterus. In situ hybridization experiments revealed that the increased expression of the mdr mRNA is specifically localized to the secretory epithelial cells of the endometrium. Immunocytochemistry studies with a mdr glycoprotein-specific antiserum demonstrate that the mdr glycoprotein is predominantly localized to the luminal surface of the secretory epithelial cells. These results indicate that the mdr gene expression in the uterus is controlled by the physiologic changes associated with pregnancy. Our data are consistent with a potential role for the mdr glycoprotein in the transport of substrate across the secretory epithelium of the gravid uterus.