Abstract 389: Role of her3 signaling pathways in er+ and her2+ breast cancers

HER3 is an important molecule that may regulate oncogenic activity in estrogen receptor (ER)+ breast cancers, which accounts for about 70-80% of all breast cancers. HER3 gene expression is highest in ER+ or luminal tumors. Treatment with the ER degrader fulvestrant induces protein expression of HER3 in ER+ clinical breast tumors, suggesting that HER3 may have therapeutic value in luminal breast cancers. About 20% of all breast cancers diagnosed are HER2 positive (+). HER3 is as essential as HER2 for maintaining cell viability in HER2-overexpressing breast cancer cells. We aim to identify proteins that bind to HER3 upon therapeutic inhibition of HER2 or anti-endocrine therapy. We have found that ERα is upregulated in ER+ T47D and MCF-7 breast cancer cell lines expressing the activating hotspot T355I HER3 mutation compared to HER3 wild-type (WT) cells. We noted that ERα co-immunoprecipitated to HER3-T355I but not to HER3-WT T47D cells. Knocking down HER3 resulted in decreased ERα expression in both HER3-WT and HER3-T355I expressing T47D cells. We are currently analyzing the nuclear expression of endogenous HER3 in MCF-7, T47D, BT474, MDA-MB-361, MDA-MB-453 and SK-BR3 breast cancer cells. We are determining if stimulation with the HER3 ligand heregulin alters the subcellular location of HER3 or the ability to co-immunoprecipitate with ERα in cells expressing exogenous HER3-WT, exogenous HER3-T355I or endogenous HER3. We are using a proteomics approach to identify HER3-interacting proteins to determine how HER2 inhibition affects the repertoire of HER3 binding partners in HER2+ breast cancer cells. We immunopurified HER3 from BT474 cells treated with DMSO or with neratinib (24 hour treatment). We then identified enrichment of specific proteins by mass spectrometry. MALDI tandem mass spectrometry experiments identified non-muscle myosin-II heavy chain from HER3 immunoprecipitates treated 24 hours with neratinib treatment but not 24 hours of DMSO treatment in BT474 cells. Furthermore, our immunoprecipitation data indicated that there is more myosin-IIa bound to HER3 in response to neratinib treatment. Myosin-IIa has a key role in cytokinesis, cell migration, polarization, adhesion and metastasis. Thus, we aim to decipher the mechanism of action by which myosin-IIa interacts with HER3 and determining if downstream myosin/actin signaling could be a possible target in HER2 over-expressing breast cancer patients who show resistance to HER2 targeted therapy. In conclusion, we have identified two potential novel HER3 binding partners in breast cancer: ERα and myosin-IIa. Deciphering the function of HER3 binding partners has potential implication for understanding the invasive properties of ER+ and HER2+ breast cancers. Citation Format: Rosalin Mishra, Samar Alanazi, Hima Patel, Long Yuan, Joan T. Garrett. Role of her3 signaling pathways in er+ and her2+ breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 389.