HER2-Associated Radioresistance of Breast Cancer Stem Cells Isolated from HER2-Negative Breast Cancer Cells

Purpose: To understand the role of HER2-associated signaling network in breast cancer stem cells (BCSC) using radioresistant breast cancer cells and clinical recurrent breast cancers to evaluate HER2-targeted therapy as a tumor eliminating strategy for recurrent HER2−/low breast cancers. Experimental Design: HER2-expressing BCSCs (HER2+/CD44+/CD24−/low) were isolated from radiation-treated breast cancer MCF7 cells and in vivo irradiated MCF7 xenograft tumors. Tumor aggressiveness and radioresistance were analyzed by gap filling, Matrigel invasion, tumor-sphere formation, and clonogenic survival assays. The HER2/CD44 feature was analyzed in 40 primary and recurrent breast cancer specimens. Protein expression profiling in HER2+/CD44+/CD24−/low versus HER2−/CD44+/CD24−/low BCSCs was conducted with two-dimensional difference gel electrophoresis (2-D DIGE) and high-performance liquid chromatography tandem mass spectrometry (HPLC/MS-MS) analysis and HER2-mediated signaling network was generated by MetaCore program. Results: Compared with HER2-negative BCSCs, HER2+/CD44+/CD24−/low cells showed elevated aldehyde dehydrogenase (ALDH) activity and aggressiveness tested by Matrigel invasion, tumor sphere formation, and in vivo tumorigenesis. The enhanced aggressive phenotype and radioresistance of the HER2+/CD44+/CD24−/low cells were markedly reduced by inhibition of HER2 via siRNA or Herceptin treatments. Clinical breast cancer specimens revealed that cells coexpressing HER2 and CD44 were more frequently detected in recurrent (84.6%) than primary tumors (57.1%). In addition, 2-D DIGE and HPLC/MS-MS of HER2+/CD44+/CD24−/low versus HER2−/CD44+/CD24−/low BCSCs reported a unique HER2-associated protein profile including effectors involved in tumor metastasis, apoptosis, mitochondrial function, and DNA repair. A specific feature of HER2–STAT3 network was identified. Conclusion: This study provides the evidence that HER2-mediated prosurvival signaling network is responsible for the aggressive phenotype of BCSCs that could be targeted to control the therapy-resistant HER2−/low breast cancer. Clin Cancer Res; 18(24); 6634–47. ©2012 AACR.