Trastuzumab resistance induces EMT to transform HER2(+) PTEN(-) to a triple negative breast cancer that requires unique treatment options.

Although trastuzumab is an effective treatment in early stage HER2(+) breast cancer the majority of advanced HER2(+) breast cancers develop trastuzumab resistance, especially in the 40% of breast cancers with loss of PTEN. However, HER2(+) breast cancer patients continue to receive trastuzumab regardless PTEN status and the consequence of therapy in these patients is unknown. We demonstrate that continued use of trastuzumab in HER2(+) cells with loss of PTEN induces the epithelial-mesenchymal transition (EMT) and transform HER2(+) to a triple negative breast cancer. These transformed cells exhibited mesenchymal morphology and gene expression markers, while parent HER2(+) cells showed epithelial morphology and markers. The transformed cells exhibited loss of dependence on ERBB family signaling (such as HER2, HER3, HER4, BTC, HRG, EGF) and reduced estrogen and progesterone receptors. Continued use of trastuzumab in HER2(+) PTEN(-) cells increased the frequency of cancer stem cells (CSCs) and metastasis potential. Strikingly, parental HER2(+) cells and transformed resistant cells respond to treatment differently. Transformed resistant cells were sensitive to chemical probe (sulforaphane) through inhibition of IL-6/STAT3/NF-κB positive feedback loop whereas parental HER2(+) cells did not respond. This data suggests that trastuzumab resistance in HER2(+) PTEN- breast cancer induces EMT and subtype switching, which requires unique treatment options.

CRLX101, an investigational camptothecin-containing nanoparticle-drug conjugate, targets cancer stem cells and impedes resistance to antiangiogenic therapy in mouse models of breast cancer.

Antiangiogenic therapies inhibit the development of new tumor blood vessels, thereby blocking tumor growth. Despite the advances in developing antiangiogenic agents, clinical data indicate that these drugs have limited efficacy in breast cancer patients. Tumors inevitably develop resistance to antiangiogenics, which is attributed in part to the induction of intra-tumoral hypoxia and stabilization of hypoxia-inducible factor 1α (HIF-1α), a transcription factor that promotes tumor angiogenesis, invasion, metastasis, and cancer stem cell (CSC) self-renewal. Here, we tested whether inhibiting HIF-1α can reverse the stimulatory effects of antiangiogenic-induced hypoxia on breast CSCs. Breast cancer cells grown under hypoxic conditions were treated with the dual topoisomerase-1 (TOPO-1) and HIF-1α inhibitor camptothecin and assessed for their CSC content. In a preclinical model of breast cancer, treatment with bevacizumab was compared to the combination treatment of bevacizumab with CRLX101, an investigational nanoparticle-drug conjugate with a camptothecin payload or CRLX101 monotherapy. While exposure to hypoxia increased the number of breast CSCs, treatment with CPT blocked this effect. In preclinical mouse models, concurrent administration of CRLX101 impeded the induction of both HIF-1α and CSCs in breast tumors induced by bevacizumab treatment. Greater tumor regression and delayed tumor recurrence were observed with the combination of these agents compared to bevacizumab alone. Tumor reimplantation experiments demonstrated that the combination therapy effectively targets the CSC populations. The results from these studies support the combined administration of dual TOPO-1- and HIF-1α-targeted agents like CRLX101 with antiangiogenic agents to increase the efficacy of these treatments.

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.

Antiangiogenic agents increase breast cancer stem cells via the generation of tumor hypoxia.

Antiangiogenic therapy has been thought to hold significant potential for the treatment of cancer. However, the efficacy of such treatments, especially in breast cancer patients, has been called into question, as recent clinical trials reveal only limited effectiveness of antiangiogenic agents in prolonging patient survival. New research using preclinical models further suggests that antiangiogenic agents actually increase invasive and metastatic properties of breast cancer cells. We demonstrate that by generating intratumoral hypoxia in human breast cancer xenografts, the antiangiogenic agents sunitinib and bevacizumab increase the population of cancer stem cells. In vitro studies revealed that hypoxia-driven stem/progenitor cell enrichment is primarily mediated by hypoxia-inducible factor 1α. We further show that the Akt/ß-catenin cancer stem cell regulatory pathway is activated in breast cancer cells under hypoxic conditions in vitro and in sunitinib-treated mouse xenografts. These studies demonstrate that hypoxia-driven cancer stem cell stimulation limits the effectiveness of antiangiogenic agents, and suggest that to improve patient outcome, these agents might have to be combined with cancer stem cell-targeting drugs.