| Background:Chemotherapy is the main treatment for leukemia and other tumors. With the widely using of chemotherapy drugs, drug resistance is becoming increasingly prominent. Resistance to anticancer drugs has become one of the major reasons of the failure of tumor treatment. Multidrug resistance (MDR) describes a phenomenon of cross-resistance of tumor cells to several structurally unrelated chemotherapeutic agents after exposing to a single cytotoxic drug. The current mechanisms of tumor drug resistance focus on the proteins related to multidrug resistance. Several proteins have been found be related to MDR, such as P-glycoprotein (P-gp), multidrug resistance related protein (MRP), lung resistance related protein (LRP), breast cancer multidrug resistance related protein (BCRP), topoisomerase II (TOP II) and glutathione-S-transferase (GST). Although the genesis of MDR has been extendedly investigated, an efficient agent to overcome drug-resistance has not been available. The mechanisms of MDR remain unclear and require further study.MicroRNA(miRNA) is one kind of small noncoding RNA molecules (19nt-23nt) which regulates protein expression by binding 3'-UTRs(3' untranslated regions) of target mRNA of protein-coding genes, resulting in cleavage or translation repression of target mRNA. MiRNAs have been widely identified in plants and animals, and is conserved in sequence between distantly related organisms. MiRNA plays an important role in various cellular processes, such as cell differentiation, proliferation and apoptosis. It is one of regulators of individual development, metabolism, viral infection and the genesis, development of tumor. Recently,more and more attention has been paid on the role of miRNA in tumor multidrug resistance. Several multidrug resistance-related miRNAs of breast cancer, ovarian cancer, gastric cancer have been reported. In this study, for the first time,we will screen and search the multidrug resistance-associated miRNAs of leukemia by using microRNA microarry in combination with Real time PT-PCR.Objective: Screen and search the miRNA, which differentially express in K562 cell line and it's adriaraycin -resistant cells --K562/A02 cell line by using microRNA Microarray in combination with Real time RT-PCR.Methods: Total RNA of K562 cells and K562/A02 cells was isolated by Trizol. Small RNAs (<300nt) were isolated using a YM-100 Microcon centrifugal filter (from Millipore), which of K562 and K562/A02 cells were Labeled with Cy3 and Cy5 respectively. Then the miRNA was hybridized on microRNA Microarray. Original image signal were collected and digitized. The differentially expressed miRNA were screened and identified through comparison between the two chips. The results of microarray were confirmed by Real time RT-PCR.Results: The results of microRNA microarray show that 22 miRNAs expressed differentially between K562 and K562/A02 cells (P<0.01) .As compared to K562 cells, expression of miR-221, miR-155 and miR-451 was up-regulated in K562/A02 cells by more than two folds, while expression of miR-98, miR-181a, let-7f, let-7g, miR-424 and miR-563 was down-regulated by more than two folds. MiR-221, miR-181a, miR-451 and let-7f were further confirmed by Real time RT-PCR. The results of Real time RT-PCR were consistent with that of microarray.Conclusion: MiR-98, miR-221, miR-181a, miR-155, miR-424, miR-451, let-7f, let-7g and miR-563 were differentially expressed between K562 and K562/A02 cells. Our results suggest that these differentially expressed miRNAs particularly miR-451 and let-7f may play an important role in the mechanisms of leukemia MDR.
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