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Breast cancer stem cells (BCSCs) are more resistant to radiation and chemotherapy than breast cancer cells.Hence,even though BCSCs make up less than 2% of breast cancer cells, they are regarded as the root of breast cancer metastasis, recurrence and chemotherapy failure.MicroRNAs (miRNAs) are a class of small non-coding RNA, which are participated in many physiological processes, including cellular proliferation, differentiation and apoptosis.Studies have shown that miRNAs are involved in self-renewal and differentiation of stem cells.MiRNAs may also involved in the process of mammary epithelium stem cells (MaSCs) mutating into BCSCs.To explore the important role of miRNAs in BCSCs, BCSCs (ESA+CD44+CD24-/low) and MaSCs (MUC1-ESA+) were isolated from MCF-7 and MCF-10A respectively.Then, microRNA microarray was performed to scan BCSCs-related miRNAs.The results have shown that BCSCs have 25 miRNAs and 17 miRNAs of different expression level compared with MCF-7 and MaSCs respectively.Bioinformatic analysis has shown that genes which were possibly regulated by these miRNAs are involved in stem cell function maintenance.We selected miR-21 and miR-122a from those miRNAs for further study.MiR-21 expression level showed a tendency of in MaSCs < in BASCs < in MCF-7 and miR-122a showed a tendency of in BASCs > in MaSCs > in MCF-7.Using luciferase reporter vector, we demonstrated that pre-miR-21 siginificantly blocked the expression of PDCD4, and pre-miR-122a siginificantly blocked the expression of G3BP2.