Notch reporter activity in breast cancer cell lines identifies a subset of cells with stem cell activity.

Developmental pathways such as Notch play a pivotal role in tissue-specific stem cell self-renewal as well as in tumor development. However, the role of Notch signaling in breast cancer stem cells (CSC) remains to be determined. We utilized a lentiviral Notch reporter system to identify a subset of cells with a higher Notch activity (Notch(+)) or reduced activity (Notch(-)) in multiple breast cancer cell lines. Using in vitro and mouse xenotransplantation assays, we investigated the role of the Notch pathway in breast CSC regulation. Breast cancer cells with increased Notch activity displayed increased sphere formation as well as expression of breast CSC markers. Interestingly Notch(+) cells displayed higher Notch4 expression in both basal and luminal breast cancer cell lines. Moreover, Notch(+) cells demonstrated tumor initiation capacity at serial dilutions in mouse xenografts, whereas Notch(-) cells failed to generate tumors. γ-Secretase inhibitor (GSI), a Notch blocker but not a chemotherapeutic agent, effectively targets these Notch(+) cells in vitro and in mouse xenografts. Furthermore, elevated Notch4 and Hey1 expression in primary patient samples correlated with poor patient survival. Our study revealed a molecular mechanism for the role of Notch-mediated regulation of breast CSCs and provided a compelling rationale for CSC-targeted therapeutics.

HER2 drives luminal breast cancer stem cells in the absence of HER2 amplification: implications for efficacy of adjuvant trastuzumab.

Although current breast cancer treatment guidelines limit the use of HER2-blocking agents to tumors with HER2 gene amplification, recent retrospective analyses suggest that a wider group of patients may benefit from this therapy. Using breast cancer cell lines, mouse xenograft models and matched human primary and metastatic tissues, we show that HER2 is selectively expressed in and regulates self-renewal of the cancer stem cell (CSC) population in estrogen receptor-positive (ER(+)), HER2(-) luminal breast cancers. Although trastuzumab had no effects on the growth of established luminal breast cancer mouse xenografts, administration after tumor inoculation blocked subsequent tumor growth. HER2 expression is increased in luminal tumors grown in mouse bone xenografts, as well as in bone metastases from patients with breast cancer as compared with matched primary tumors. Furthermore, this increase in HER2 protein expression was not due to gene amplification but rather was mediated by receptor activator of NF-κB (RANK)-ligand in the bone microenvironment. These studies suggest that the clinical efficacy of adjuvant trastuzumab may relate to the ability of this agent to target the CSC population in a process that does not require HER2 gene amplification. Furthermore, these studies support a CSC model in which maximal clinical benefit is achieved when CSC targeting agents are administered in the adjuvant setting. Cancer Res; 73(5); 1635-46. ©2012 AACR.

Activation of an IL6 inflammatory loop mediates trastuzumab resistance in HER2+ breast cancer by expanding the cancer stem cell population.

Although inactivation of the PTEN gene has been implicated in the development of resistance to the HER2 targeting antibody trastuzumab, the mechanisms mediating this resistance remain elusive. We generated trastuzumab resistant cells by knocking down PTEN expression in HER2 overexpressing breast cancer cell lines and demonstrate that development of trastuzumab resistance in these cells is mediated by activation of an IL6 inflammatory feedback loop leading to expansion of the cancer stem cell (CSC) population. Long term trastuzumab treatment generates highly enriched CSCs which display an EMT phenotype secreting over 100-fold more IL6 than parental cells. An IL6 receptor antibody interrupted this inflammatory feedback loop reducing the cancer stem cell population resulting in decreased tumor growth and metastasis in mouse xenographs. These studies demonstrate that trastuzumab resistance may be mediated by an IL6 inflammatory loop and suggest that blocking this loop may provide alternative strategy to overcome trastuzumab resistance.