| Objective: Synthetic therapy is a main principle to treat the malignant tumor nowadays, the combination between radiotherapy and chemotherapy is the widest mode in the clinic. The application of combination between them can improve the therapeutic effect of carcinoma, which has proved in the clinic. But the sensitivity to chemotherapeutics of carcinoma is often reduced in patients who have received prior radiotherapy, the resistance mechanisms is not yet clear. Radiotherapy can induce overexpression of Drug-Resistance Gene and its protein, which may be one important reason. In this study simulating a clinical conventional fractionated method to irradiate the nasopharyngeal squamous cell carcinoma CNE-1 cell. To examine the change of sensitivity of nasopharyngeal squamous cell carcinoma CNE-1 cell to cisplatin(DDP)before x-ray irradiation and following x-ray irradiation, in addition, to examine the change in the expression of multi-drug resistance gene (MDR1), P-glycoprotein (P-gp), multi-drug resistance-associated protein gene (MRP gene) and multi-drug resistance-associated protein (MRP) with the time in CNE-1 cell before x-ray irradiation and following x-ray irradiation, for investigating the relationship between radiotherapy and multi-drug resistance of nasopharyngeal squamous cell carcinoma CNE-1 cell .Methods:The CNE-1 cell line was cultured in the flask, exponentially growing cells were exposed to X-ray radiation simulating the clinical way and the total dose was 20Gy(2Gy/day/fraction, 5 fractions weekly for 2 weeks consecutively). Collecting cells before x-ray irradiation and the 7th,14th,21st,28th and the 35th day after x-ray irradiation respectively for detection. The IC50 (inhibitory concentration of drug that reduces cell survival by 50%)of cells was examined by MTT method. The expression of MDR1 and MRP gene was examined in CNE-1 cell by reverse transcriptional polymerase chain reaction (RT-PCR), P-gp and MRP were detected by Western blotting.Results: IC50 of CNE-1 was 1.11±0.05μg/ml before radiation by MTT, IC50 heightened on the 7th,14th,21st,28th and the 35th day after radiation, and its value as follows 1.57±0.12μg/ml,2.65±0.15μg/ml,4.33±0.17μg/ml,3.72±0.19μg/ml and 2.76±0.20μg/ml(P<0.05). The numerical value of IC50 was the biggest at the 21stday, and then began to degrade after the 21st day. Semiquantitative value of MDR1 and MRP gene of CNE-1 before radiation by RT-PCR was 0.47±0.02 and 0.41±0.02 respectively. The expression of MDR1 on the 7th,14th,21st,28th and the 35th day after radiation was obviously heightened, and its value as follows 0.60±0.03,0.90±0.08,1.03±0.09,0.80±0.05 and 0.67±0.02(P<0.05), the expression of MRP gene on the 7th,14th,21st,28th and the 35th day after radiation was obviously heightened, and its value as follows 0.63±0.05,1.01±0.06,1.33±0.08,0.99±0.05 and 0.75±0.03(P<0.05). Semiquantitative value of P-gp and MRP of CNE-1 before radiation by Western blotting was 15.55±1.02 and 55.48±1.91 respectively. The expression of P-gp on the 14th,21st,28th and the 35th day after radiation was obviously heightened, and its value as follows 26.98±1.62,35.12±2.11,29.85±1.83 and 23.35±1.84, the expression of MRP on the 7th,14th,21st,28th and the 35th day after radiation was obviously heightened, and its value as follows 71.13±3.4,93.60±3.95,100.75±3.67,106.17±4.06 and 89.12±3.05(P<0.05). The expression of multi-drug resistance gene and its protein was step up gradually initially then began to degrade.Conclusions: There is low expression of drug-resistance gene (MDR1, MRP gene) and drug-resistance protein (P-gp, MRP) before x-irradiation, which explains there already existes endogenous resistance at some extent in the CNE-1 cells. The sensitivity of nasopharyngeal squamous cell carcinoma CNE-1 cell to cisplatin after x-irradiation degrades and the cells produces resistance to cisplatin. The expression of MDR1, MRP gene, P-gp and MRP after x-irradiation is higher than before x-irradiation, which indicates x-irradiation induces acquired resistance in the CNE-1 cells. The expression of drug-resistance genes and proteins is step up gradually initially then begins to degrade.
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