Abstract 5415: STAT3 and GLI1/tGLI1 oncogenic transcription factors interact to promote the aggressiveness of triple-negative and HER2-enriched breast cancers
Authors: Sirkisoon Sherona R. , Carpenter Richard L. , Tadas Rimkus , Ashley Anderson , Harrison Alexandria , Lange Allison M. , Jin Guangxu , Watabe Kounosuke , Lo Hui-Wen 
Published: August 2018

Abstract
Patients with triple-negative (TN) and HER2-enriched (HER2) breast cancers have high rates of metastasis, which accounts for 90% mortality for these patients, highlighting the need for improved therapeutic targets. The oncogenic transcription factors, STAT3, GLI1, and truncated GLI1 (tGLI1) are associated with poor clinical outcomes for breast cancer patients. However, physical or functional interactions between STAT3 and GLI1/tGLI1 have never been investigated. The purpose of this study is to determine whether the JAK2/STAT3 and GLI1/tGLI1 pathways are concurrently activated in TN and HER2 breast cancers. We examined 47 node-positive breast tumor samples using immunohistochemical staining and found that p-STAT3 (Y705), GLI1, and tGLI1 were co-overexpressed in 64% of TN breast cancers, 68% of HER2 breast cancers, and in 65% of lymph node metastases. Gene Set Enrichment Analysis of 710 breast tumors revealed that the activation signatures for STAT3 and GLI1/tGLI1 are co-enriched in TN and HER2 subtypes, but not in the luminal subtypes. Furthermore, Kaplan-Meier and log-rank analyses revealed that breast tumors with high levels of STAT3 and GLI1/tGLI1 co-activation were associated with shortened metastasis-free survival compared to those with lower levels. To determine whether these two pathways functionally cooperate, we examined whether STAT3 and GLI1/tGLI1 formed complexes by IP-WB and observed that STAT3 interacted with both GLI1 and tGLI1. ChIP assay showed that these complexes bind to both consensus GLI1- and STAT3-binding sites. We further observed that co-overexpression of STAT3 and GLI1/tGLI1 resulted in significant activation of a promoter controlled by GLI1-binding sites. We next analyzed three ChIP-Seq datasets to identify gene promoters that can be occupied by the STAT3-GLI1/tGLI1 complexes, and found 36 potential target genes. Validation of these target genes by RT-qPCR and Kaplan-Meier analysis resulted in three genes that are upregulated by STAT3-GLI1 and/or STAT3-tGLI1, namely, R-Ras2, Cep70, and UPF3A, and correlated with poor survival. These novel transcriptional targets of STAT3 and GLI1/tGLI1 are implicated in breast tumorigenesis, angiogenesis, and metastasis suggesting that co-activation of STAT3 and GLI1/tGLI1 may promote aggressive breast cancers. Lastly, co-overexpression of STAT3 and GLI1/tGLI1 increased the mammosphere-forming ability of breast cancer cells and interestingly, only the STAT3-tGLI1 complex allowed mammosphere formation of immortalized mammary epithelial cells. In conclusion, our study reports, for the first time, that the physical and functional interactions between STAT3 and GLI1/tGLI1 oncogenic transcription factors lead to gene co-activation, stem-like phenotype of breast cancer cells, and unfavorable prognosis for patients with triple-negative and HER2 breast cancers.
Citation Format: Sherona R. Sirkisoon, Richard L. Carpenter, Rimkus Tadas, Anderson Ashley, Alexandria Harrison, Allison M. Lange, Guangxu Jin, Kounosuke Watabe, Hui-Wen Lo. STAT3 and GLI1/tGLI1 oncogenic transcription factors interact to promote the aggressiveness of triple-negative and HER2-enriched breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5415.