Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:1.5.1.3 (dihydrofolate reductase)
 5,819 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Interactions between cisplatin (CDDP) and irradiation are of potential significance for the combined modality treatment of cancer. Previous data have indicated that following in vitro exposure to X-irradiation certain tumour cells expressed resistance to CDDP. To identify parameters associated with this CDDP resistance, the human ovarian carcinoma cell line SK-OV-3/P was pre-exposed to fractionated X-irradiation (total dose: 50 Gy) in vitro. The resultant subline (SK-OV-3/DKR-10) proved 2-fold resistant to CDDP, but not to acute X-irradiation. Consistent with unaltered dihydrofolate reductase and thymidylate synthase activities, SK-OV-3/DXR-10 cells were neither cross-resistant to methotrexate nor to 5-fluorouracil. Verapamil (6.6 microM) significantly (P less than 0.05) enhanced CDDP-induced cytotoxicity in the resistant DXR-10 subline, but not in the parental cells. Total glutathione levels were significantly (P less than 0.01) lower in the resistant subline and BSO pretreatment failed to influence cytotoxicity, whilst related enzyme activities were not consistently modified in the SK-OV-3/DXR-10 cells. Resistance in these cells was associated with significantly decreased cisplatin uptake (P less than 0.002). Immediately following drug exposure the total platination level of the DNA, quantitated immunochemically, was higher (P less than 0.05) in the resistant subline indicative of increased tolerance to DNA damage. After an 18 h post-treatment incubation the parental cell line appeared proficient in the removal of the intrastrand adduct Pt-AG, but deficient in removing the major adduct Pt-GG and the difunctional Pt-(GMP)2 lesion, whilst the DXR-10 resistant subline appeared proficient in removal of all four Pt-DNA adducts. DNA polymerases alpha and beta activities, however, were comparable in both cell lines. These data implicate both enhanced repair and increased tolerance of DNA damage as mechanisms of resistance to CDDP resulting from in vitro exposure of a human ovarian carcinoma cell line to fractionated X-irradiation.
 ...
PMID:Mechanisms associated with the expression of cisplatin resistance in a human ovarian tumor cell line following exposure to fractionated X-irradiation in vitro. 163 88

Trimetrexate, a lipid-soluble analogue of methotrexate, appears to enter mammalian cells by passive diffusion, thus circumventing the methotrexate transport system which is frequently a subject for alterations leading to methotrexate resistance. Using a single-step selection protocol with trimetrexate, we have isolated 45 clonal variants and found the majority of them to be selectively resistant to lipophilic antifolates while retaining their sensitivity to methotrexate and drugs involved in multidrug resistance. The majority of spontaneously induced trimetrexate-resistant clones showed a change in neither the mRNA levels of dihydrofolate reductase (24 of 30) and P-glycoprotein (26 of 30) nor their gene copy numbers, whereas a small fraction of clones (4 of 30) showed multidrug resistance gene amplification and P-glycoprotein mRNA overexpression. gamma-Irradiation prior to selection markedly enhanced the frequency of trimetrexate resistance (100-fold after 1000 rads). None of the gamma-ray-induced trimetrexate-resistant clones (0 of 15) had evidence of dihydrofolate reductase and multidrug resistance gene amplification and/or overexpression. Flow cytometry data on trimetrexate-resistant clones showed no defect in the transport of trimetrexate. Verapamil, a modulator of the multidrug resistance phenotype, had no cytotoxic effect on parental and trimetrexate-resistant clones. However, when present with trimetrexate, verapamil (0.3-0.6 microM) reversed the lipophilic antifolate-resistant phenotype in clones that had invariant levels of P-glycoprotein and dihydrofolate reductase. This selective resistance to lipid-soluble antifolates was initially unstable but became stable after continued drug-selective growth. Two-dimensional gel electrophoresis showed some differences in protein(s) that may potentially be associated with this phenotype of selective resistance to lipophilic antifolates. We conclude that a gamma-radiation-enhanceable, verapamil-reversible, stable phenotype of selective resistance to lipid-soluble antifolates frequently emerges which requires neither the amplification nor the overexpression of dihydrofolate reductase or multidrug resistance genes.
 ...
PMID:A phenotype conferring selective resistance to lipophilic antifolates in Chinese hamster ovary cells. 167 47

We are studying mechanisms of resistance to hydrophilic and lipophilic antifolates in cultured mammalian cells. We determined the cytotoxicity of methotrexate and the lipid-soluble antifolate trimetrexate to various human carcinoma cells and their doxorubicin-resistant sublines. These multidrug-resistant cells were 17-fold to 26-fold more resistant to trimetrexate but as sensitive to methotrexate as their parental cells. Verapamil and quinidine, which are known to modulate the degree of pleiotropic drug resistance, reversed the cross-resistance to trimetrexate but did not alter methotrexate toxicity in multidrug-resistant cells. By flow cytometry, we show that multidrug-resistant Chinese hamster ovary cells retained eightfold more fluorescein-methotrexate bound to dihydrofolate reductase upon competition with trimetrexate than the drug-sensitive cells did. However, methotrexate displaced fluorescein-methotrexate equally well in sensitive and multidrug-resistant cells. Furthermore, verapamil produced dose-dependent displacement of fluorescein-methotrexate from the multidrug-resistant cells in the presence of low concentrations of trimetrexate but had no significant effect on displacement of fluorescein-methotrexate from sensitive cells. Hamster cells that overproduce dihydrofolate reductase by 93-fold were 145-fold and 96-fold more resistant to methotrexate and trimetrexate, respectively, than their sensitive parental cells. This form of antifolate resistance was not altered by verapamil or quinidine. We conclude that the cross-resistance to trimetrexate in cells that do not overproduce dihydrofolate reductase is associated with the multidrug-resistant phenotype. Possible implications of trimetrexate resistance in cancer chemotherapy are discussed.
 ...
PMID:Cross-resistance to the lipid-soluble antifolate trimetrexate in human carcinoma cells with the multidrug-resistant phenotype. 252 20

Trimetrexate (TMQ) is a lipophilic antifolate shown to have antitumor activity in humans. TMQ-resistant sublines of the MOLT-3 human acute lymphoblastic leukemia cell line were developed and were designated as MOLT-3/TMQ200, MOLT-3/TMQ800, and MOLT-3/TMQ2500 based on degrees of resistance to TMQ. The TMQ resistance was accompanied by 5- to 7-fold increases in dihydrofolate reductase activity and markedly reduced cellular TMQ accumulation. Methotrexate accumulation was not impaired in TMQ-resistant cells. TMQ retention (efflux) was unchanged in these TMQ-resistant cells. Verapamil enhanced the TMQ accumulation in the resistant cells to the level seen in the parent cells but had no effects on the TMQ retention. These sublines were cross-resistant not only to methotrexate but also to vincristine, doxorubicin, daunorubicin, and mitoxantrone. There was no cross-resistance to bleomycin or cisplatin. Resistance to vincristine, doxorubicin, daunorubicin, and mitoxantrone was reversed by verapamil. TMQ resistance was only minimally reversed by verapamil and methotrexate resistance not affected at all. Both cellular accumulation and retention of vincristine and daunorubicin in the TMQ-resistant cells were markedly decreased. Verapamil enhanced both accumulation and retention of the drug. Plasma membrane fractions of the TMQ-resistant cells analyzed by urea-sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by staining with Coomassie Blue revealed the presence of a distinct band with a molecular weight of 170,000. Immunoblot analysis with 125I-labeled monoclonal antibody raised against P-glycoprotein of multidrug-resistant Chinese hamster ovary cells (C219) cross-reacted with the Mr 170,000 protein of the TMQ-resistant cells. These results show that the TMQ-resistant cells displayed not only decreased TMQ uptake and increased dihydrofolate reductase but also characteristics associated with a classical multidrug-resistant phenotype. Multidrug resistance includes lipophilic antifolate.
 ...
PMID:Multidrug resistance in a human leukemic cell line selected for resistance to trimetrexate. 257 16