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V Ling

Publications and source records attributed to V Ling.

At least 19 recordsLinked to original sources
PubMed1999

Complete sequence determination of the mouse and human CTLA4 gene loci: cross-species DNA sequence similarity beyond exon borders.

CTLA4 (CD152), a receptor for the B7 costimulatory molecules (CD80 and CD86), is considered a fundamental regulator of T-cell activation. In this paper, we present the complete primary structure of the mouse and human CTLA4 gene loci. Sequence comparison between the mouse and the human CTLA4 gene loci revealed a high degree of sequence conservation both for homologous noncoding regions (65-78% identity) and for coding regions (72-98% identity), with an overall score of 71% over the entire length of the two genes. Of the CTLA4 genomic regions aligned, five simple repetitive elements were found in the mouse locus, whereas two simple repetitive sequences were localized on the human locus. RNA blot analysis of mouse and human primary tissues indicated that both CTLA4 and T-cell receptor transcripts were found in most organs with generally higher levels in lymphoid tissues. The conservation of CTLA4 gene patterning raises the possibility that constrained gene evolution of CTLA4 may be linked to conserved transcriptional control of this locus.

Abatacept↗
PubMed1999

A human nucleobase transporter-like cDNA (SLC23A1): member of a transporter family conserved from bacteria to mammals.

A family of related polytopic membrane proteins that mediate the transport of nucleobases has been extended to Homo sapiens by the cloning of a full-length human cDNA that encodes a nucleobase transporter-like protein. The protein is predicted to contain 11-14 transmembrane-spanning regions, exhibits 20-28% overall sequence identity to fungal and bacterial transporters, and contains a conserved signature motif found in this family. Fluorescence in situ hybridization localized the gene (HGMW-approved symbol SLC23A1) to human chromosome 20p13. Human nucleobase transporter-like mRNA was present in all tissues examined, with lower levels found in heart, skeletal muscle, and ovary. Expression of the 60-kDa cDNA-encoded protein was demonstrated by an in vitro transcription-translation approach. The identification of this nucleobase transporter-like protein will allow the further elucidation of the interaction of human cells with physiological nucleobases and pharmacologically important drugs such as 5-F-uracil, dideoxynucleosides, and acyclic nucleosides.

Amino Acid Sequence↗
PubMed1999

Ligand-mediated tertiary structure changes of reconstituted P-glycoprotein. A tryptophan fluorescence quenching analysis.

Ligand-dependent changes in accessibility of purified P-glycoprotein, functionally reconstituted in liposomes, were investigated by fluorescence measurements. Trp quenching experiments provided evidence that P-glycoprotein adopts different tertiary structures upon binding of drug substrates in the absence and presence of MgATP and its nonhydrolyzable analog, MgATPgammaS. Five anthracycline derivatives were tested as drug substrates: daunorubicin, 4'-epi-doxorubicin, iododoxorubicin, 4-demethoxy-daunorubicin, and methoxy-morpholino-doxorubicin. Among them, daunorubicin and 4'-epi-doxorubicin have been shown to be rejected outside the multidrug-resistant cells, whereas the three others have been shown to accumulate in multidrug-resistant cells overexpressing P-glycoprotein and therefore retain their cytotoxic activity. A small conformational change was associated with nucleotide binding and amplified after nucleotide hydrolysis. Different conformational states were adopted by P-glycoprotein upon the addition of the anthracycline derivatives in the absence and presence of MgATP or MgATPgammaS. These conformational changes are shown to be related to the nature of the antitumor agents and more precisely to their capacity to accumulate in resistant cells. These data also suggest that the cytotoxicity of iododoxorubicin and 4-demethoxy-daunorubicin is related to the fact they are not transported by P-glycoprotein. On the contrary, methoxy-morpholino-doxorubicin cytotoxicity may be explained in terms of its rapid reincorporation into the plasma membrane after being transported by P-glycoprotein.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1999

A mammalian lysosomal membrane protein confers multidrug resistance upon expression in Saccharomyces cerevisiae.

Mouse transporter protein (MTP) is a highly conserved polytopic membrane protein present in mammalian lysosomes and endosomes. The role of MTP in regulating the in vivo subcellular distribution of numerous structurally distinct small molecules has been examined in this study by its expression in a drug-sensitive strain of the yeast Saccharomyces cerevisiae. Surprisingly, the expression of MTP in membranes of an intracellular compartment resulted in a cellular resistance or hypersensitivity to a range of drugs that included nucleoside and nucleobase analogs, antibiotics, anthracyclines, ionophores, and steroid hormones. The intracellular bioavailability of steroid hormones was altered by MTP, as determined using an in vivo glucocorticoid receptor-driven reporter assay in yeast, suggesting that the MTP-regulated drug sensitivity arose due to a change in the subcellular compartmentalization of steroid hormones and other drugs. MTP-regulated drug sensitivity in yeast was blocked to varying degrees by compounds that inhibit lysosomal function, interfere with intracellular cholesterol transport, or modulate the multidrug resistance phenotype of mammalian cells. These results indicate that MTP is involved in the subcellular compartmentalization of diverse hydrophobic small molecules and contributes to the inherent drug sensitivity or resistance of the mammalian cell.

Base Sequence↗
PubMed1999

Identification and characterization of a mammalian mitochondrial ATP-binding cassette membrane protein.

Membrane proteins of the ATP-binding cassette (ABC) superfamily are involved in the transport of diverse substrates across organellar and plasma membranes of the mammalian cell. Most human ABC proteins identified to date are associated with genetically linked diseases or clinically relevant phenotypes. We describe a new human half-molecule ABC protein, designated M-ABC1, that contains a predicted single membrane and ATP-binding cassette domain. M-ABC1 is localized to membranes of the mitochondria and its transcript is expressed in all tissues. The N-terminal region of the M-ABC1 protein was shown to function independently as a mitochondrial signal sequence by its ability to target the green fluorescent protein to the mitochondria. The monomeric 60 kDa M-ABC1 protein was chemically crosslinked in vivo into a major protein species of 120-130 kDa, thereby confirming that M-ABC1 exists within a higher ordered ABC protein complex. A dominant negative repression approach using M-ABC1 protein with site-directed mutations in its Walker A motif revealed that the mutant protein was rapidly degraded and indicated that the intact Walker A motif of M-ABC1 was required for its stability. The identification of M-ABC1 extends the known distribution of members of the ABC protein family into the mammalian mitochondrion.

ATP Binding Cassette Transporter 1↗
PubMed1999

Regulation and expression of multidrug resistance (MDR) transcripts in the intestinal epithelium.

A paucity of information exists on the regulation of gene expression in the undifferentiated intestine. The intestinal epithelium is one of the few normal tissues expressing the multidrug resistance (MDR) genes that confer the multidrug resistant phenotype to a variety of tumours. Expression of mdr1a has been observed in the primitive rat intestinal epithelial cell line, IEC-18. It is hypothesized that characterization of MDR gene expression in IEC-18 cells will provide insight into gene regulation in undifferentiated intestinal cells. A series of hamster mdr1a promoter deletion constructs was studied in IEC-18 and a region with 12-13-fold enhancer activity was identified. This region was shown to function in an orientation- and promoter context-independent manner, specifically in IEC-18 cells. Unexpectedly, Northern probing revealed a greater expression of mdr1b than mdr1a in IEC-18 cells. A quantitative reverse transcription polymerase chain reaction assay was used to compare the relative expression of MDR genes in IEC cells, fetal intestine, and in the undifferentiated and differentiated components of adult intestinal epithelium. MDR transcript levels in IEC cells were found to resemble those of fetal intestine and small intestinal crypts, where a conversion from mixed mdr1a/mdr1b to predominantly mdr1a expression occurs as cells mature. This work describes two contributions to the field of gene regulation in the undifferentiated intestine--first, the initial characterization of a putative mdr1a enhancer region with specificity for primitive intestinal cells and secondly, the first report of mdr1b detection in the intestine and its expression in primitive cell types.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1999

Stimulation of P-glycoprotein-mediated drug transport by prazosin and progesterone. Evidence for a third drug-binding site.

P-glycoprotein is a plasma membrane protein of mammalian cells that confers multidrug resistance by acting as a broad-specificity, ATP-dependent efflux transporter of diverse lipophilic neutral or cationic compounds. Previously, we identified two positively cooperative drug-binding sites of P-glycoprotein involved in transport [Shapiro, A. B. & Ling, V. (1997) Eur. J. Biochem. 250, 130-137]. The H site is selective for Hoechst 33342 and colchicine. The R site is selective for rhodamine 123 and anthracyclines. Substrate binding to one site stimulates transport by the other. In this paper, we show that prazosin and progesterone stimulate the transport of both Hoechst 33342 and rhodamine 123. Rhodamine 123 and prazosin (or progesterone) in combination stimulate Hoechst 33342 transport in an additive manner. In contrast, Hoechst 33342 and either prazosin or progesterone interfere with each other, so that the stimulatory effect of the combination on rhodamine 123 transport is less than that of each individually. Non-P-glycoprotein-specific effects of prazosin on membrane fluidity and permeability were excluded. These results indicate the existence of a third drug-binding site on P-glycoprotein with a positive allosteric effect on drug transport by the H and R sites. This allosteric site appears to be one of the sites of photoaffinity labeling of P-glycoprotein by [125I]iodoarylazidoprazosin [Safa, A. R., Agresti, M., Bryk, D. & Tamai, I. (1994) Biochemistry 33, 256-265] and is likely not to be capable of drug transport.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

Taxol resistance mediated by transfection of the liver-specific sister gene of P-glycoprotein.

The sister gene of P-glycoprotein (Spgp) is a liver-specific ATP-binding cassette protein highly related to the P-glycoprotein (Pgp) family (S. Childs et al, Cancer Res., 55: 2029-2034, 1995). Spgp appears to be related to the Pgp family by an ancient duplication occurring before the division of fish and mammals. P-Glycoproteins have diverse functions including broad specificity multidrug resistance in cell lines and tumors, detoxification of tissues such as the intestine and blood-brain barrier, and phosphatidylcholine transport in liver. Spgp is a Mr approximately 170,000 glycosylated plasma membrane protein localized to the canalicular surface of hepatocytes in the rat liver. The full-length cDNA of Spgp was isolated from rat, and its expression was characterized in situ and in transfected cells. The expression of Spgp correlates with the differentiation of hepatocytes and is seen only in late liver development. It is not observed in hepatoma cell lines. The physiological function of Spgp in liver is unknown, but it maps to 2q31 in humans, in the vicinity of liver transport disorders for bile acids and cholesterol. Spgp may therefore be involved in some aspect of bile acid or cholesterol metabolism. Spgp transfectants have a low level resistance to Taxol but not to other drugs that form part of the multidrug resistance phenotype. This resistance is reversible by the Pgp-reversing agents cyclosporin A, PSC833, and verapamil, suggesting a conservation in some functions of Pgps across large evolutionary distance.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

Functional intracellular P-glycoprotein.

Efflux of chemotherapy drugs by P-glycoprotein (P-gp) at the plasma membrane is thought to be a major cause of cancer multidrug resistance. In this report, we show by flow cytometry that P-gp also concentrates large amounts of 2 different drugs, Hoechst 33342 and daunorubicin, within a cytoplasmic compartment of multidrug resistant CHRC5 cells. A quantitative assay of Hoechst 33342 revealed that cytoplasmic sequestration by P-gp in CHRC5 cells accounted for about half of the amount of Hoechst 33342 accumulated by the drug-sensitive parental Aux BI cells. Daunorubicin sequestered in the cytoplasm of CHRC5 cells could be released by inhibiting P-gp function with cyclosporin A, resulting in cell death. A likely site of drug sequestration is P-gp-containing cytoplasmic vesicles, in which the P-gp is oriented so that drugs are transported and concentrated in the interior of the vesicles. P-gp was detected in the membranes of cytoplasmic vesicles of CHRC5 cells by confocal immunofluorescence microscopy and immunoelectron microscopy with anti-P-gp monoclonal antibodies (MAbs). Vesicular localization of daunorubicin was observed by epifluorescence microscopy. The origin and nature of the P-gp-containing vesicles are unknown, but they do not correspond to endocytic vesicles. Our results directly demonstrate that chemosensitizer-induced release of drugs sequestered in cytoplasmic vesicles by P-gp can be used to overcome multidrug resistance.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

Embryonic stem cells and embryoid bodies express lymphocyte costimulatory molecules.

Despite the importance of the costimulatory proteins B7-1 (CD80), B7-2 (CD86), and their counterreceptors CD28 and CTLA-4 (CD154) in the regulation of T cell proliferation in the adult immunological system, the initial appearance of these proteins during embryonic development has not been investigated. Using in vitro cultures of undifferentiated mouse embryonic stem (ES) cells and differentiating embryoid bodies as a model of very early embryonic development, we examined these cells for the presence of mRNA and protein corresponding to the B7 and CD28 families of costimulatory molecules. By flow cytometry, a stochastically regulated subpopulation of B7-1+ cells comprising 33% of total cells was detected in ES cell cultures, while negligible staining was found for B7-2, CTLA-4, and CD28. When ES cells were differentiated into embryoid bodies for 12 days, a CD45+ subpopulation of embryoid body cells were found to stain positively for B7-1, B7-2, and CD28. RT-PCR confirmed cell staining data by revealing amplification products corresponding to B7-1, B7-2, and CD28 in corresponding samples. Very low levels of CTLA-4 amplification products were found in all samples; however, surface staining of CTLA-4 was never detected. The functional capacity of ES cell B7-1 to bind its ligand was verified by the ability of the soluble fusion protein CTLA-4-Ig to bind ES cells and the ability of this reagent to block anti-B7-1 antibody binding in cell based competition assays. These results demonstrate that expression of costimulatory molecules arises very early during in vitro development and suggests that the early embryonic environment may utilize cellular signaling systems analogous to those seen in the immune system.

Abatacept↗
PubMed1998

Transport of LDS-751 from the cytoplasmic leaflet of the plasma membrane by the rhodamine-123-selective site of P-glycoprotein.

P-glycoprotein is an ATP-dependent transporter of an extremely wide variety of lipophilic compounds. We showed previously [Shapiro, A. B. & Ling, V. (1997a) Eur. J. Biochem. 250, 130-137] that P-glycoprotein contains two drug transporting sites, dubbed H (for Hoechst 33342-selective) and R (for rhodamine-123-selective), that interact with positive cooperativity. The H site transports 2-[2-(4-ethoxyphenyl)-6-benzimidazolyl]-6-(1-methyl-4-piperazyl)be nzimidazole (Hoechst 33342) from the cytoplasmic leaflet of the plasma membrane to the aqueous extracellular medium [Shapiro, A. B. & Ling, V. (1997b) Eur. J. Biochem. 250, 122-129]. The environment from which the R site transports its substrates is unknown. In this paper, we used the fluorescent DNA dye 2-[4-[4-(dimethylamino)phenyl]-1,3-butadienyl]-3-ethylbenzothiazolium perchlorate (LDS-751), a substrate of the R site, to address this issue. LDS-751 which, like Hoechst 33342, exhibits lipid-dependent fluorescence and slow transleaflet diffusion, allowed us to use the same methodology that we used for the H site to study the location of the R site. As with Hoechst 33342, the specific initial rate of LDS-751 transport by P-glycoprotein-rich, isolated plasma membrane vesicles from CH(R)B30 cells was directly proportional to the amount of membrane-bound LDS-751 and inversely proportional to the concentration of free, aqueous LDS-751. This result demonstrates that the R site of P-glycoprotein transports LDS-751 out of the lipid membrane. The slight decrease, instead of an increase, in the initial rate of active transport of LDS-751 with the amount of time elapsed for slow diffusion of LDS-751 from the cytoplasmic leaflet to the extracellular leaflet indicates that the R site of P-glycoprotein removes LDS-751 from the cytoplasmic leaflet of the plasma membrane. Thus, both known drug-transporting sites of P-glycoprotein remove their substrates from the cytoplasmic leaflet. Since all of the P-glycoprotein substrates we have examined so far are recognized by one or both of the two known drug-transporting sites, these two sites in the cytoplasmic leaflet of the plasma membrane may be able to account for all substrate transport by P-glycoprotein.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

Stoichiometry of coupling of rhodamine 123 transport to ATP hydrolysis by P-glycoprotein.

In order to describe the transport mechanism of P-glycoprotein, it is essential to know the coupling ratio, i.e. the moles substrate transported/mole ATP hydrolyzed. P-glycoprotein couples ATP hydrolysis at two ATP-binding sites to transport of a wide variety of neutral or cationic lipophilic compounds. Previously published coupling ratios have fallen within the range 0.02-0.8 mol substrate transported/mol ATP hydrolyzed. We studied the energetics of transport by P-glycoprotein, performing quantitative measurements of the rates of ATP hydrolysis and transport of rhodamine 123 by P-glycoprotein, using isolated P-glycoprotein-rich plasma membrane vesicles. The continuous fluorescence-based assay of rhodamine 123 transport allowed accurate measurement of initial transport rates. Since we measured uptake of rhodamine 123 into the vesicles as a loss of fluorescence, we avoided the problem of high background due to substrate binding to the membranes. The coupling ratio of the transport reaction increased as the rhodamine 123 concentration increased, showing that the basal ATPase activity of P-glycoprotein was progressively recruited for rhodamine 123 transport. Both of the previously identified transport sites of P-glycoprotein [Shapiro, A. B. & Ling, V. (1997a) Eur J. Biochem. 250, 130-137] were involved in transport of saturating concentrations of rhodamine 123. At saturating rhodamine 123 and 0.3 mM ATP, the coupling ratio was 0.83, suggesting a mechanistic coupling ratio of 1. Interestingly, the coupling ratio decreased as the ATP concentration increased so that, at 1.5 mM, close to the cytoplasmic concentration of ATP, the coupling ratio was 0.57. The physiological significance of this effect is not yet understood.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

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.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

A gene encoding a liver-specific ABC transporter is mutated in progressive familial intrahepatic cholestasis.

The progressive familial intrahepatic cholestases (PFIC) are a group of inherited disorders with severe cholestatic liver disease from early infancy. A subgroup characterized by normal serum cholesterol and gamma-glutamyltranspeptidase (gammaGT) levels is genetically heterogeneous with loci on chromosomes 2q (PFIC2) and 18q. The phenotype of the PFIC2-linked group is consistent with defective bile acid transport at the hepatocyte canalicular membrane. The PFIC2 gene has now been identified by mutations in a positional candidate, BSEP, which encodes a liver-specific ATP-binding cassette (ABC) transporter, sister of p-glycoprotein (SPGP). The product of the orthologous rat gene has been shown to be an effective bile acid transporter in vitro. These data provide evidence that SPGP is the human bile salt export pump (BSEP).

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

How much insulin-like growth factor I (IGF-I) circulates? Impact of standardization on IGF-I assay accuracy.

There is a significant systematic difference between the normal range obtained from ethylenediamine tetraacetate plasma samples using the Genentech total insulin-like growth factor I (IGF-I) RIA and normal ranges for other total IGF-I RIAs. To determine whether the quality of the assay standard was the cause of this systematic difference, we analyzed commercially available preparations of recombinant human IGF-I (rhIGF-I) typical of those used as IGF-I immunoassay standards along with our own well characterized rhIGF-I assay standard. For the commercial standards, high performance liquid chromatography-derived purities were low, and some vendor-assigned protein concentrations were inconsistent with values from quantitative amino acid analysis. The Genentech rhIGF-I assay standard was highly pure and quantitatively correct. However, the poor quality of some commercial rhIGF-I preparations was not the primary reason for the systematic discrepancy between the Genentech total IGF-I RIA normal range and most other normal ranges. Most assays for total IGF-I are calibrated against the WHO International Reference Reagent (IRR) for IGF-I Immunoassays (87/518). The Genentech total IGF-I RIA is not calibrated against WHO IRR 87/518. The protein content assigned to WHO IRR 87/518 was a consensus value from a multicenter collaborative study. Physicochemical analyses showed that WHO IRR 87/518 is Met(-1)-IGF-I of low purity (44%), and that the assigned protein content is higher than the value determined by quantitative amino acid analysis. Thus, assays that are calibrated against WHO IRR 87/518 will report total IGF-I concentrations in excess of actual values. We believe that calibration against WHO IRR 87/518 is the cause of the systematic discrepancy between the Genentech IGF-I assay normal range and most other normal ranges, and that much of the plasma IGF-I concentration data in the literature are of questionable accuracy.

Adult↗
PubMed1998

The mechanism of ATP-dependent multidrug transport by P-glycoprotein.

Experiments with purified P-glycoprotein (Pgp) reconstituted into proteoliposomes have conclusively demonstrated that Pgp is an ATP-dependent drug transporter. Detailed biochemical analyses of drug transport by Pgp are beginning to yield a clearer picture of its mechanism. Working with Pgp-rich plasma membrane vesicles from CHRB30 cells, we have recently clarified several aspects of the drug transport mechanism. A major question about drug transport by Pgp is how it can recognize a vast array of unrelated chemical compounds as substrates. The substrate Hoechst 33342 is fluorescent in the lipid bilayer but not in aqueous solution. This property enabled us to show that Pgp transports Hoechst 33342 out of the lipid bilayer, not the aqueous phase. Because Hoechst 33342, like all Pgp substrates, is lipophilic its concentration in the bilayer greatly exceeds its concentration in the aqueous medium. High local substrate concentrations may allow for broad substrate recognition by one or more relatively low affinity binding site(s) within the lipid bilayer. Another fundamental question about Pgp is the number of drug binding sites it possesses. We have found evidence for at least two sites for drug binding and transport that interact in a positively cooperative manner. Initial rates of transport of two Pgp substrates, Hoechst 33342 and Rhodamine 123 by ChRB30 plasma membrane vesicles were measured. Each dye stimulated transport of the other. Additionally, colchicine stimulated Rhodamine 123 transport and inhibited Hoechst 33342 transport. Anthracyclines such as daunorubicin and doxorubicin had the reverse effect. Vinblastine, etoposide, and actinomycin D inhibited transport of both dyes. We interpret these results as follows. One site (R) preferentially recognizes Rhodamine 123, doxorubicin and daunorubicin. The other site (H) preferentially recognizes Hoechst 33342 and colchicine. Vinblastine, actinomycin D, and etoposide interact equally with both sites. Binding of drug at the R site stimulates transport of Hoechst 33342 by the H site and binding of drug at the H site stimulates transport of Rhodamine 123 by the R site. The existence of two drug binding sites on Pgp with different specificities is another way in which Pgp may expand the range of substrates it can transport. A third essential detail of the drug transport mechanism of Pgp is the ratio of substrate molecules transported per ATP hydrolyzed. By comparing the initial rate of Rhodamine 123 transport with the rate of ATP hydrolysis at saturating Rhodamine 123 concentration, we found that, under suitable conditions, Pgp is capable of transporting one Rhodamine 123 molecule per ATP molecule hydrolyzed.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

P-glycoprotein expression: critical determinant in the response to osteosarcoma chemotherapy.

BACKGROUND: Fewer than 20% of patients with bone cancer who are treated with surgery alone are cured. Even with the best current treatment, surgery combined with chemotherapy, only 60%-80% of patients with nonmetastatic bone cancer and 10% of patients with metastatic bone cancer are cured. Thus far, the reason for treatment failure in the nonresponding subset has not been identified. It has been hypothesized that P-glycoprotein, which confers multidrug resistance, might be the cause. We sought to determine whether the expression of P-glycoprotein is associated with poor treatment outcome in osteosarcoma. METHODS: In a retrospective study, we correlated P-glycoprotein expression with the outcome of conventional chemotherapy in 62 consecutive, clinically staged patients diagnosed as having osteosarcoma between 1980 and 1989. RESULTS: P-glycoprotein was overexpressed in 27 patients but not in another 34 patients, and expression was ambiguous in the sample from one patient. At a median follow-up of 8.9 years, the 34 patients whose tumors did not express P-glycoprotein had significantly better relapse-free rates than the 27 subjects whose tumors expressed the protein (87% versus 0%; P<.00001) and had improved survival rates (94% versus 35%; P<.00001). Among the 46 patients who received chemotherapy before surgery, the 23 whose tumors were negative for P-glycoprotein showed significantly better long-term outcomes (P<.00002), although differences in tumor necrosis in response to therapy were only of borderline significance (P = .057). CONCLUSIONS: P-glycoprotein expression does correlate with treatment failure in patients with osteosarcoma. This correlation raises the possibility that inhibiting the action of P-glycoprotein as part of therapy for this disease would improve outcome.

ATP Binding Cassette Transporter, Subfamily B, Mem↗