Breast cancer is a hormone-dependent malignancy and is the most prevalent cause of cancer-related mortality among females. Radiation therapy and chemotherapy are common treatments of breast cancer. However, tumor relapse and metastasis following therapy are major clinical challenges. The importance of B-lymphoma Moloney murine leukemia virus insertion region-1 (BMI-1) was implicated in cell proliferation, stem cell maintenance, and tumor initiation. We established radio- and temozolomide (TMZ)-resistant (IRC-R) MCF-7 and MDA-MB-231 cell lines to investigate the mechanism involved in therapeutic resistance. Cell proliferation and sphere number were dramatically elevated, and BMI-1 was remarkably upregulated, in IRC-R cells compared to parental cells. Silencing BMI-1 by RNA interference only affected the cell proliferation of IRC-R but not parental cells, suggesting the critical role of BMI-1 in radio- and TMZ resistance. We used a xenograft mice model to elucidate that BMI-1 was necessary in tumor development by assessing tumor volume and Ki67 expression. We found that Hedgehog (Hhg) signaling exerted synergized functions together with BMI-1, implicating the importance of BMI-1 in Hhg signaling. Downregulation of BMI-1 could be an effective strategy to suppress tumor growth, which supports the potential clinical use of targeting BMI-1 in breast cancer treatment.
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