Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.6.3.44 (P-glycoprotein)
 13,344 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Multicellular prostate tumor spheroids develop intrinsic P-glycoprotein (Pgp)-mediated multidrug resistance with the appearance of quiescent cell areas. We have investigated the effect of intracellular reactive oxygen species (ROS) on Pgp expression in large, quiescent and drug-resistant multicellular spheroids (diameter 250 +/- 50microm). Using the ROS-sensitive fluorescence dye 2;7;-dichlorodihydrofluorescein diacetate (H(2)DCFDA), we demonstrated that these tumor spheroids are characterized by reduced intracellular ROS compared with drug-sensitive small spheroids (diameter 60 +/- 20microm) consisting predominantly of proliferating cells. The prooxidants hydrogen peroxide, menadione and glyceraldehyde raised ROS in large tumor spheroids and significantly down-regulated Pgp within 24 hr. Comparable effects were achieved with the known Pgp-reversing agents sodium orthovanadate, quinidine and cyclosporin A but not with verapamil. Consequently, the retention and toxicity of the anthracycline doxorubicin was increased in tumor spheroids treated with prooxidants. Co-administration of prooxidants and the free radical scavenger ebselen did not alter Pgp levels, indicating that down-regulation of Pgp is mediated via ROS. Down-regulation of Pgp by H(2)O(2) was abolished when either forskolin, 8-Br-cAMP or IBMX, which raise intracellular cAMP levels, was co-administered, indicating that Pgp expression is regulated by protein kinase A (PKA). Furthermore, Pgp was down-regulated by the PKA inhibitors Rp-cAMPs and H89. Since prooxidants stimulated the growth of multicellular spheroids and down-regulated the cyclin-dependent kinase inhibitor p27(kip1), we conclude that ROS-mediated Pgp down-regulation may be paralleled by recruitment of drug-resistant quiescent cells in the depth of the tumor tissue for cell-cycle activity.
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PMID:Redox regulation of P-glycoprotein-mediated multidrug resistance in multicellular prostate tumor spheroids. 1062 88

The efficacy of the epipodophyllotoxins VP-16 and VM-26 is limited by the occurrence of drug resistance in the tumor cell population. Cellular insensitivity to drugs that stabilize the cleavable complex is frequently expressed as multidrug resistance (MDR). In some cell lines, overexpression of MDR-1/P-glycoprotein or the multidrug resistance associated protein (MRP) has been demonstrated and implicated as the mechanism of resistance. Typically, these cells have reduced drug accumulation, secondary to increased drug efflux. In other cell lines, an atypical MDR phenotype has been identified, with the predominant mechanism of resistance shown to be qualitative and/or quantitative changes in the levels and activity of topoisomerase II. For VP-16, increased expression of MDR-1 or MRP and alterations in topoisomerase II have been shown to confer tolerance. To further understand resistance to VP-16, T98G-VP(1000) was initially isolated as a single clone from parental cell, T98G, by exposure to VP-16. Subsequently, a population of cells from this subline was exposed to three-fold higher drug concentration allowing stable sublines to be established at higher extracellular drug concentration. Characterization of the resistant sublines demonstrates the adaptation that occurs with advancing drug concentrations during in vitro selections. Reduced topoisomerase II mRNA and protein levels were observed in the initial isolate. This reduction was accompanied by a decrease in topoisomerase II activity and cellular growth rate and was associated with 47-fold resistance to topoisomerase II poisons. With advancing resistance, MRP expression increased, with increased VP-16 efflux and reduced accumulation. This adaptation allowed for partial restoration of topoisomerase II activity secondary to increased expression and hyperphosphorylation, with a resultant increase in growth rate. In this cell line, hyperphosphorylation coincided with increased casein kinase II mRNA protein levels, without increased PKC protein levels, suggesting a role for this kinase in the acquired hyperphosphorylation. In this cell line, hyperphosphorylation mediated the increased activity despite a fall in topoisomerase II protein levels secondary to an acquired 615 bp deletion in one topoisomerase II allele, which resulted in reduced protein levels. In this subline, high levels of resistance were attained as a result of synergism between the reduced topoisomerase II levels and MRP overexpression. These studies demonstrate how cellular adaptation to increasing drug pressure occurs and how more than one mechanism can contribute to the resistant phenotype when increasing selecting pressure is applied. Reduced expression of topoisomerase II is sufficient to confer substantial resistance early in the selection process, with synergy from additional mechanisms helping to confer high levels of resistance.
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PMID:Increased phosphorylation of DNA topoisomerase II in etoposide resistant mutants of human glioma cell line. 1072 8

Intrinsic expression of the multidrug resistance (MDR) transporter P-glycoprotein (Pgp) may be regulated by reactive oxygen species (ROS). A transient expression of Pgp was observed during the growth of multicellular tumor spheroids. Maximum Pgp expression occurred in tumor spheroids with a high percentage of quiescent, Ki-67-negative cells, elevated glutathione levels, increased expression of the cyclin-dependent kinase inhibitors p27Kip1 and p21WAF-1 as well as reduced ROS levels and minor activity of the mitogen-activated kinase (MAPK) members c-Jun amino-terminal kinase (JNK), extracellular signal-regulated kinase ERK1,2, and p38 MAPK. Raising intracellular ROS by depletion of glutathione with buthionine sulfoximine (BSO) or glutamine starvation resulted in down-regulation of Pgp and p27Kip1, whereas ERK1,2 and JNK were activated. Down-regulation of Pgp was furthermore observed with low concentrations of hydrogen peroxide and epidermal growth factor, indicating that ROS may regulate Pgp expression. The down-regulation of Pgp following BSO treatment was abolished by agents interfering with receptor tyrosine kinase signaling pathways, i.e. the protein kinase C inhibitors bisindolylmaleimide I (BIM-1) and Ro-31-8220, the p21ras farnesyl protein transferase inhibitor III, the c-Raf inhibitor ZM 336372 and PD98059, which inhibits ERK1,2 activation. ROS involved as second messengers in receptor tyrosine kinase signaling pathways may act as negative regulators of Pgp expression.
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PMID:Down-regulation of intrinsic P-glycoprotein expression in multicellular prostate tumor spheroids by reactive oxygen species. 1127 18

The effects of cell cycle inhibition on the expression of the multidrug resistance transporter P-glycoprotein (P-gp) as well as of the cyclin-dependent kinase (CDK) inhibitors p27(Kip1) and p21(WAF-1) were investigated in DU-145 prostate tumor spheroids. With increasing spheroid size the number of cells in the G0/G1 phase augmented, whereas the number of cells in the G2/M phase and the S phase of the cell cycle declined. The number of G0/G1 cells was elevated after incubation with either mimosine, staurosporine or serum-free medium. Mitomycin C and roscovitine increased the number of S phase cells. Roscovitine additionally increased cells in the G2/M phase. Incubation in serum-free medium upregulated p21(WAF-1), p27(Kip1) and P-gp. Mimosine treatment resulted in upregulation of p27(Kip1) and P-gp, whereas p21(WAF-1) remained unchanged. Upon roscovitine treatment p27(Kip1) and p21(WAF-1) were downregulated, whereas P-gp was unaltered. Mitomycin C treatment resulted in downregulation of p27(Kip1) and p21(WAF-1); no significant change in P-gp levels was observed. Staurosporine induced upregulation of p21(WAF-1) whereas p27(Kip1) remained unaltered. P-gp was downregulated upon staurosporine treatment, which was owing to an elevation of intracellular reactive oxygen species by this compound. It is concluded that upregulation of P-gp in G0/G1 phase cells requires coexpression of the CDK inhibitor p27(Kip1) but not the CDK inhibitor p21(WAF-1).
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PMID:Modulation of intrinsic P-glycoprotein expression in multicellular prostate tumor spheroids by cell cycle inhibitors. 1190 40

Effect of specific inhibitors of extracellular-signal regulated protein kinase (ERK) pathway, PD98059 and U0126, on P-glycoprotein (Pgp)-mediated vincristine resistance of L1210/VCR cells was investigated. Both test inhibitors significantly reduced the survival of L1210/VCR cells in the presence of vincristine and this was associated with a decrease of LC50 values to vincristine from 2.65+/-0.43 to 0.67+/-0.28 micromol/l and to 0.69+/-0.09 micromol/l after treatment with 50 micromol/l PD98059 and 25 micromol/l UO126, respectively. Moreover, the effects of PD98059 are connected also with an increased intracellular accumulation of radiolabeled vincristine in resistant L1210/VCR cells in concentration dependent manner. The results of this study demonstrate that inhibitors of ERK signaling pathway are reversal agents of vincristine resistance in L1210/VCR cells. The precise mechanism of PD98059 and U0126 action in modulation of MDR is not resolved yet, but the role of ERK-mediated phosphorylation cascade could be considered.
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PMID:Reversal effect of specific inhibitors of extracellular-signal regulated protein kinase pathway on P-glycoprotein mediated vincristine resistance of L1210 cells. 1198 53

The multidrug resistance proteins P-glycoprotein (Pgp) and MRP1 are drug-efflux pumps. In this study, we compared the nucleotide triphosphatase activities of the isolated N-terminal nucleotide binding domains (NBD1) of Pgp and MRP1, and explored the potential role of the phosphorylation target domain of Pgp on the regulation of Pgp NBD1 ATPase activity. We found that: (1) the NBD1s of Pgp and MRP1 have ATPase and GTPase activities, (2) the K(m)s of Pgp NBD1 for ATP and GTP hydrolysis are identical, while the K(m) of MRP1 NBD1 for ATP is lower than that for GTP, and (3) phosphorylation of MLD by PKA or PKC produces a marginal increase of V(max) for ATP hydrolysis, without affecting the affinity for ATP. These results show efficient GTP hydrolysis by the NBD1s of Pgp and MRP1, and a minor role of phosphorylation in the control of Pgp NBD1 ATPase activity.
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PMID:Nucleotide triphosphatase activity of the N-terminal nucleotide-binding domains of the multidrug resistance proteins P-glycoprotein and MRP1. 1216 30

The first series of synthetic 1-aza-9-oxafluorenes with cytostatic activities in the micromolar range was evaluated as cyclin-dependent kinase (CDK1) inhibitors. Activity was found to be selective in comparison to the inhibition of other kinases within the CDK family. Compounds were shown to inhibit the membrane-efflux pump P-glycoprotein responsible for multidrug resistance in cancer cells. First structure-activity relationships are discussed.
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PMID:Evaluation of the first cytostatically active 1-aza-9-oxafluorenes as novel selective CDK1 inhibitors with P-glycoprotein modulating properties. 1259 68

Overexpression of Bcl-2 plays a role in the development of drug resistance in leukemia and other apoptosis-prone tumors. Raf isoforms areserine/threonine kinases that act as signal transducers in cascades initiated by many growth factors and mitogens. Raf isoform activation has been linked to drug resistance in leukemia. In this study we investigated effects of Bcl-2 and Raf-1 on doxorubicin-induced growth inhibition of MCF-7 breast cancer cells. In the absence of doxorubicin, overexpression of Bcl-2 or a constitutively active form of Raf-1 in MCF-7 cells did not affect proliferation rate. Overexpression of Bcl-2 increased resistance of MCF-7 cells to doxorubicin in 2-day, 5-day, and 8-week assays. Analysis of doxorubicin sensitivity of individual MCF/Bcl-2 clones showed that doxorubicin resistance was positively correlated with level of Bcl-2 overexpression. Overexpression of constitutively active Raf-1 also increased resistance to doxorubicin. Induction of Raf-1 activity in MCF-7 cells overexpressing Bcl-2 resulted in greater doxorubicin resistance than induction of Raf-1 activity in MCF-7 cells lacking Bcl-2 overexpression. Furthermore, levels of P-glycoprotein mRNA were increased in MCF-7 cells overexpressing a constitutively active Raf-1. MCF-7 cells overexpressing constitutively active Raf-1 were also more resistant to paclitaxel, which, like doxorubicin, is a substrate of P-glycoprotein. These observations suggest both independent and overlapping roles for Raf-1 and Bcl-2 oncogenes in the resistance to growth inhibition by doxorubicin.
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PMID:Raf-1 and Bcl-2 induce distinct and common pathways that contribute to breast cancer drug resistance. 1263 22

Elemental mercury (Hg0) is a highly toxic chemical with increasing public health concern. Although the lung receives the highest exposure to Hg0 vapor, it is resistant to Hg0 toxicity relative to the kidney and brain. In an earlier study, exposure of rats to 4 mg Hg0 vapor/m3, 2 h per day for 10 days, did not produce pathological alterations in the lung but increased metallothionein and glutathione S-transferase in the kidney. This study was undertaken to examine pulmonary gene expression associated with Hg0 vapor inhalation. Total RNA was extracted from lung tissues of rats, previously exposed to air or Hg0 vapor, and subjected to microarray analysis. Hg0 vapor exposure increased the expression of genes encoding inflammatory responses, such as chemokines, tumor necrosis factor-alpha (TNFalpha), TNF-receptor-1, interleukin-2 (IL-2), IL-7, prostaglandin E2 receptor, and heat-shock proteins. As adaptive responses, glutathione S-transferases (GST-pi, mGST1), metallothionein, and thioredoxin peroxidase were all increased in response to Hg exposure. Some transporters, such as multidrug resistance-associated protein (MRP), P-glycoprotein, and zinc transporter ZnT1, were also increased in an attempt to reduce pulmonary Hg load. The expression of transcription factor c-jun/AP-1 and PI3-kinases was suppressed, while the expression of protein kinase-C was increased. Expression of epidermal fatty acid-binding protein was also enhanced. Real-time RT-PCR and Western blot analyses confirmed the microarray results. In summary, genomic analysis revealed an array of gene alterations in response to Hg0 vapor exposure, which could be important for the development of pulmonary adaptation to Hg during Hg0 vapor inhalation.
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PMID:Genomic analysis of the rat lung following elemental mercury vapor exposure. 1273 Jun 25

The level of protein phosphorylation is known to affect the properties of various membrane proteins. We have previously shown that GTP is capable of greatly enhancing the phosphorylation by [gamma-32P]ATP of P-glycoprotein (Pgp) from KB-V1 cells (3). Investigating the possibility of a general modulation of [gamma-32P]ATP plasma membrane protein phosphorylation, we found that phosphorylation of other membrane proteins are also modulated by various combinations of [ATP + GTP]. The ATP/GTP ratio giving the highest phosphorylation level depended on the protein studied. Modulation of the [gamma-32P]ATP-mediated phosphorylation of numerous membrane proteins requires hydrolysis of both ATP and GTP. ADP and GDP also increased [gamma-32P]ATP-driven phosphorylation but to a lesser extent than GTP. This plasma membrane endogenous phosphorylation activity was neither inhibited by specific inhibitors of protein kinase C, nor by inhibitors of cAMP- or cGMP-dependent protein kinases or of casein kinase II, respectively. Mastoparan, a G-protein regulator, increased the phosphorylation of some proteins that were already enhanced by the presence of [ATP + GTP] mixtures, especially proteins migrating in gels at the same position as P-glycoprotein.
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PMID:Modulation by the ATP/GTP ratio of the phosphorylation level of P-glycoprotein and of various plasma membrane proteins of KB-V1 multidrug resistant cells. 1289 16


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