Low doses of gamma-rays can induce the expression of mdr gene

Up-regulation of P-glycoprotein is involved in the increased paclitaxel resistance in human esophageal cancer radioresistant cells

OBJECTIVE

Development of drug and radiation resistance is one of the major causes of cancer treatment failure with chemoradiotherapy. Whether radiotherapy affects drugs resistance in esophageal cancer cells remain to be determined. The purpose of the study was to investigate the change of drug-sensitivity and P-glycoprotein (P-gp) expression in ionization radiation-induced human esophageal cancer radioresistant cells.

MATERIALS AND METHODS

Radioresistant cells were established by means of continuous fractionated gamma-ray irradiation on human esophageal squamous cancer cell line EC9706. The radiosensitivity and drug-sensitivity between established radioresistant cells and parental cells were detected by a colony-forming assay and 3-(4,5)-dimethylthiahiazo(-z-y1)-3,5-di-phenytetrazoliumromide (MTT) assay, respectively. The expressions of multidrug resistance type 1 gene (MDR1) mRNA and protein for P-gp were detected by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot methods. The roles of P-gp activity in irradiation-induced drugs resistance were studied by using verapamil, an inhibitor of P-gp activity.

RESULTS

The esophageal cancer radioresistant cells showed an increased cisplatin or paclitaxel resistance. Compared with their parental cells, the expressions of MDR1 mRNA and protein for P-gp were increased significantly in radioresistant cells. Verapamil reduced paclitaxel resistance but had no effect on cisplatin resistance in human esophageal cancer radioresistant cells.

CONCLUSIONS

These results suggested that up-regulation of P-gp is involved in the increased paclitaxel resistance but not cisplatin resistance in ionization radiation-induced human esophageal squamous cancer radioresistant cells.

Expression of P-glycoprotein and multidrug resistance associated protein in Ehrlich ascites tumor cells after fractionated irradiation

PURPOSE

To characterize irradiated murine tumor cells with respect to drug resistance, drug kinetics, and ATPase activity, and to evaluate the possible role of P-glycoprotein (PGP) and murine multidrug resistance associated protein (Mrp1) in the drug-resistant phenotype of these cells.

METHODS AND MATERIALS

Sensitive Ehrlich ascites tumor cells (EHR2) were in vitro exposed to fractionated irradiation (60 Gy). Western blot analysis was performed for determination of PGP and Mrp1, reverse transcriptase-polymerase chain reaction (RT-PCR) for determination of mdr1a + b mRNA, and semiquantitative RT-PCR for Mrp1 mRNA. The clonogenic assay was applied to investigate sensitivity, whereas the steady-state drug accumulation of daunorubicin (DNR), 3H-vincristine (VCR), and 3H-etoposide (VP16) was measured by spectrofluorometry and scintillation counting, respectively. For determining of ATPase activity, the release of inorganic phosphate from ATP was quantified using a colorimetric method.

RESULTS

Compared with EHR2, the irradiated cell line EHR2/irr showed increased expression of PGP (threefold), Mrp1 (eightfold), and Mrp1 mRNA (sixfold), and a slight reduction of mdr1b mRNA, whereas mdr1a was present in EHR2 but could not be detected in EHR2/irr. EHR2/irr developed sixfold resistance to VP16, twofold resistance to vincristine, but remained sensitive to DNR. Addition of the PGP inhibitor, verapamil (VER) or depletion of glutathione by buthionine sulfoximine (BSO) partly reversed the resistance in EHR2/irr. In EHR2/irr, the steady-state accumulation of 3H-VCR and 3H-VP16 was significantly decreased as compared with EHR2, whereas the accumulation of DNR was unchanged. The ATPase activity of plasma membrane vesicles prepared from EHR2/irr cells was similar to that of wild-type EHR2 cells. The ATPase activity was neither stimulated by vinblastine nor VER.

CONCLUSION

Irradiation induced a multidrug-resistant phenotype in sensitive tumor cells. This phenotype was characterized by increased expression of Mrp1 mRNA, Mrp1, and PGP but decreased expression of mdr1a + b mRNA. The influence of irradiation on PGP and Mrp1 expression seemed to be different.

Cisplatin resistant glioblastoma cells may have increased concentration of urokinase plasminogen activator and plasminogen activator inhibitor type 1

Gliomas are the most common form of intrinsic primary brain tumors, that extensively invade the surrounding normal brain tissue. The failure of chemotherapy treatment of these tumors is chiefly attributed to drug-resistance. From human glioblastoma we developed two cell sublines resistant to cisplatin due to acute (AT cells) or continuous (CT cells) treatment with clinically relevant doses of cisplatin. We examined their sensitivity to different cytostatics by colorimetric MTT assay. The concentrations of urokinase plasminogen activator (uPA) and plasminogen activator inhibitor type 1 (PAI-1) were determined by the ELISA assay. The results reveal that both AT and CT cells became resistant to cisplatin and vincristine; AT cells became resistant also to etoposide. Both AT and CT cells did not significantly change their sensitivity to doxorubicin, 5-fluorouracil and chlorambucil. Concentrations of uPA and PAI-1 were increased in CT cells, with no change in AT cells. In the conditioned medium of both, AT and CT cells, the level of uPA were increased. No differences in concentrations of PAI-1 in the conditioned medium of these cells were found. Thus, our results show that drug-resistance of glioblastoma cells may be accompanied with the increased levels of markers for tumor invasion.