Abstract 4371: PELP1-TFAP2C crosstalk promotes endocrine resistance in breast cancer cells
Authors: Liu Junhao , Viswanadhapalli Suryavathi , Li Mengxing , Pratap Uday P. , Tang Weiwei , Liu Zexuan , Luo Yiliao , Altwegg Kristin A. , Li Xiaonan , Sareddy Gangadhara R. , Tekmal Rajeshwar R. , Vadlamudi Ratna K. 
Published: September 2020

Abstract
Background: A significant proportion of estrogen receptor positive (ER+) breast cancer (BC) initially respond to antiestrogens or aromatase inhibitors but become therapy resistant-BC. Development of effective therapies for endocrine therapy resistant BC represent the highest unmet need. PELP1 is a nuclear receptor coregulator, commonly overexpressed in BC, contributes to therapy resistance and correlate with poor survival. TFAP2C (transcription factor AP-2 gamma) is a known regulator of ER activity and high expression of TFAP2C is associated with a decreased response to Fulvestrant. The objective of this study is to examine the significance of PELP1-TFAP2C crosstalk in the development of breast cancer therapy resistance.
Methods: We used yeast two-hybrid screen to identify PELP1 interacting transcription factors. Interaction of PELP1 and TFAP2C was confirmed by immunoprecipitation and GST pull down assays. Functional significance of the cross talk was tested using CellTiter-Glo, MTT and colony formation assays in the presence or absence of endocrine therapy. Mechanistic studies were conducted using shRNA, overexpression, Western blotting, reporter gene assays, RT-qPCR, ChIP-seq and RNA-seq analysis. Biological significance of PELP1 and TFAP2C in endocrine therapy resistance was examined using overexpression and under expression models of PELP1 and TFAP2C in multiple BC models including MCF7 and ZR75.
Results: Yeast based screening of a mammary gland cDNA expression library using PELP1 as the bait identified TFAP2C as a novel interacting protein of PELP1. Immunoprecipitation assays using multiple BC cell lysates confirmed the interaction of PELP1 with TFAP2C. Using various deletions of PELP1, and by using GST pull down assays, we identified N-terminal 400-600aa region of PELP1 as the major interaction site for TFAP2C. Using RNA-seq of PELP1 knockdown BC model cells, we predicted TFAP2C as an enriched transcription factor in PELP1 regulated genes. The GSEA results from RNA-seq showed TFAP2C and PELP1 induce a subset of common genes. Reporter gene assays confirmed that PELP1 functions as a coactivator of TFAP2C. Mechanistic studies showed that TFAP2C activates both AKT and ERK pathways in ER+ cell lines, while knock down of PELP1 attenuated these effects. Overexpression of TFAP2C contributed to increased cell proliferation and endocrine therapy resistance in MCF7 and ZR75 models, while knock down of PELP1 attenuated these effects. Utilizing ZR75-TFAP2C xenograft models with WT PELP1 or PELP1 knock down, we provided genetic evidence that endogenous PELP1 is essential for TFAP2C drive n breast cancer progression in vivo.
Conclusions: Collectively, our studies demonstrated that PELP1 functions as a coactivator of TFAP2C in modulating a set of ER target genes, TFAP2C functions as a transcription factor of PELP1 regulated genes and blocking the PELP1-TFAP2C axis could have therapeutic utility for treating therapy resistance. Supported by VA grant I01BX004545 (R.K. Vadlamudi)
Citation Format: Junhao Liu, Suryavathi Viswanadhapalli, Mengxing Li, Uday P. Pratap, Weiwei Tang, Zexuan Liu, Yiliao Luo, Kristin A. Altwegg, Xiaonan Li, Gangadhara R. Sareddy, Rajeshwar R. Tekmal, Ratna K. Vadlamudi. PELP1-TFAP2C crosstalk promotes endocrine resistance in breast cancer cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4371.