The forkhead transcription factor FOXM1 coordinates expression of cell cycle-related genes and plays a pivotal role in tumorigenesis and cancer progression. We previously showed that FOXM1 acts downstream of 14-3-3¿ signaling, the elevation of which correlates with a more aggressive tumor phenotype. However, the role that FOXM1 might play in engendering resistance to endocrine treatments in estrogen receptor-positive (ER+) patients when tumor FOXM1 is high, has not been clearly defined.MethodsWe analyzed FOXM1 protein expression by immunohistochemistry (IHC) in 501 ER-positive breast cancers. We also mapped genome-wide FOXM1, extracellular-regulated kinase 2 (ERK2) and ER¿ binding events by chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq), in hormone-sensitive and resistant breast cancer cells after tamoxifen treatment. These binding profiles were integrated with gene expression data from cells before and after FOXM1 knockdown to highlight specific FOXM1 transcriptional networks. We also modulated the levels of FOXM1 and newly discovered FOXM1-regulated genes and examined their impact on the cancer stem-like cell population, and on cell invasiveness and resistance to endocrine treatments.ResultsFOXM1 protein expression was high in 20% of the tumors, and this correlated with a significantly reduced survival in these patients (P¿=¿0.003, log rank Mantel-Cox). ChIP-seq analyses revealed that FOXM1 binding sites were enriched at the transcription start site of genes involved in cell cycle progression, maintenance of stem cell properties, and invasion and metastasis, which are all associated with a poor prognosis in ER¿-positive patients treated with tamoxifen. Integration of binding profiles with gene expression highlighted FOXM1 transcriptional networks controlling cell proliferation, stem cell properties, invasion and metastasis. Increased expression of FOXM1 was associated with an expansion of the cancer stem-like cell population and with increased cell invasiveness and resistance to endocrine treatments. Use of a selective FOXM1 inhibitor proved very effective in restoring endocrine therapy sensitivity and decreasing breast cancer aggressiveness.ConclusionsCollectively, our findings uncover novel roles for FOXM1 and FOXM1-regulated genes in promoting cancer stem-like cell properties and therapy resistance, and highlight the relevance of FOXM1 as a therapeutic target to be considered for reducing invasiveness and enhancing breast cancer response to endocrine treatments. PMID:25213081