AP-1-Dependent miR-21 expression contributes to chemoresistance in cancer stem cell-like SP cells

The side population (SP) of cancer cells is a minor population of cells that has been identified in a variety of cancers and harbors many cancer stem cell (CSC)-like properties, such as self-renewal potential, tumorforming capacity, and chemoresistant phenotype. CSCs are regarded as the root of cancer origin and recurrence. Thus, new therapeutic approaches targeting these malignant cells have become the topic of ongoing research. However, the chemoresistant phenotype of CSCs makes it difficult to increase their sensitivity to anticancer drugs and to decrease the rate of cancer recurrence in patients. In this study, we analyzed the chemoresistant phenotype of SP cells derived from various cancer cell lines. Microarray analysis discriminated differential gene expression profiles between SP and non-SP cells. MicroRNA-21 (miR-21) and its upstream regulator activator protein-I (AP-1), composed of c-Jun and c-Fos family transcription factors, were found to be frequently upregulated in SP cells. Downregulation of tumor suppressor programmed cell death 4, one of the miR-21 target gene products, confirmed miR-21 overexpression in SP cells. Treatment of the cells with the AP-1 inhibitor SP600125 attenuated miR-21 levels and increased topotecan sensitivity. Furthermore, specific inhibition of miR-21 by an anti-miR-21 locked nucleic acid increased drug sensitivity and decreased colony forming ability. These findings define the critical role of miR-21 in maintenance of the chemoresistant phenotype of SP cells. Targeting miR-21 may provide a new strategy for cancer therapy by impairing resistance to chemotherapy in CSCs.