To understand the role of HER2-associated signaling network in breast cancer stem cells (BCSCs); using radiation-resistant breast cancer cells and clinical recurrent breast cancers to evaluate HER2-targeted therapy as a tumor eliminating strategy for recurrent HER2−/low breast cancers.
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 radiation resistance 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 2-D DIGE and HPLC-MS/MS analysis and HER2-mediated signaling network was generated by MetaCore™ program.
Compared to 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 co-expressing 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.
This study provides the evidence that HER2-mediated pro-survival signaling network is responsible for the aggressive phenotype of breast cancer stem cells that could be targeted to control the therapy-resistant HER2−/low breast cancer.