Abstract 3080: Characterization of steroid receptor mediated activation of GLI as driving force of breast cancer growth
Authors: Massah Shabnam , Guo Maria , Foo Jane , Buttyan Ralph , Cherkasov Artem , Lallous Nada 
Published: April 2023

Abstract
Background: Hedgehog (Hh) signaling pathway plays a fundamental role in the early stages of development by regulating morphogenesis. GLI transcription factors are drivers of this signaling pathway by regulating the expression of growth related genes. Hyperactivation of GLI proteins have been associated with several cancers including medulloblastoma, glioblastoma, ovarian, prostate and breast cancers. Our study in breast cancer (BCa) suggests transcriptional activation of GLI3 in estrogen receptor alpha (ERα) positive cells upon estradiol stimulation. Loss of ERα greatly decreases GLI3 protein stability and stimulation with estradiol significantly increases GLI3 stability in ER-positive BCa cells. We have discovered that GLI3 forms nuclear complexes with ERα upon estradiol stimulation and that the loss of GLI3 reduces BCa cells growth. Therefore, we hypothesize that ERα-GLI3 complex orchestrates BCa transcriptome required for cell growth and development. Our study focuses on characterizing ERα mediated activation of GLI3 in BCa cells and the possible compensation by other steroid receptors such as the androgen receptor (AR) and the glucocorticoid receptor (GR) in ER-negative BCa cells.
Method: We examined ERα- GLI transcriptional activity by RNA sequencing upon stimulation with estradiol and inhibition of GLI activity by GANT61 in ER-positive MCF7 cells. We also identified ERα domains that are essential for GLI3 binding by immunoprecipitation and proximity ligation assay (PLA). To identify ERα-GLI3 interactome, we have optimized Rapid Immunoprecipitation Mass Spectrometry of Endogenous protein (RIME). We also examined the role of GLI3 in ER-negative BCa cell growth and characterized the role of AR and GR in activating GLI3 by luciferase reporter assay.Results: Inhibition of GLI DNA binding by GANT61 reduced the expression of GLI regulated (CDC20, CDK1, UBE2C) and modified expression of ERα regulated (FOXM1, SPC24, KIF20A, BIRC5) genes, suggesting cooperative role of ERα-GLI3 in mediating growth and metastasis. The optimized RIME assay suggests immunoprecipitation of ERα with GLI3 in BCa cells upon estradiol induction. Immunoprecipitation and PLA studies suggest that ERα-N terminal, DNA binding and C-terminal domains interact with GLI3. We also showed that knockdown of GLI3 reduces growth in ER negative BCa cells and that AR or GR stimulation by dihydrotestosterone and dexamethasone respectively, are important for GLI3 transcriptional activity.
Conclusion: Collectively, our results suggest a new role of steroid receptors in regulating GLI oncogenic transcriptional activity in BCa, leading to new therapeutic possibilities for patients.
Citation Format: Shabnam Massah, Maria Guo, Jane Foo, Ralph Buttyan, Artem Cherkasov, Nada Lallous. Characterization of steroid receptor mediated activation of GLI as driving force of breast cancer growth [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3080.