Abstract PS17-34: Proto-oncogene PELP1 interactions with SETDB1 contribute to aberrant activation of AKT1 in breast cancer
Authors: Liu Zexuan , Liu Junhao , Tang Weiwei , Pratap Uday P. , Altwegg Kristin A. , Li Xiaonan , Viswanadhapalli Suryavathi , Vadlamudi Ratna 
Published: February 2021

Abstract
BACKGROUND: Hyperactivation of PI3K/Akt signaling is implicated in breast cancer (BCa) progression. SETDB1, a methyltransferase, is also implicated in BCa, however, the mechanism remains elusive. Recent studies have shown SETDB1 can methylate non-histone substrates such as Akt1 and contribute to its aberrant activation, thus leading to tumor progression. PELP1 is a proto-oncogene which is overexpressed in BCa and participates in both the nuclear and extra nuclear functions of various nuclear receptors. In a subset of BCa, PELP1 uniquely localizes in the cytoplasm and contributes to endocrine therapy resistance. The objective of this study is to characterize the significance of PELP1 in SETDB1 mediated Akt signaling in BCa progression.
METHODS: Using yeast two-hybrid screening, we identified PELP1 as a novel interacting protein of SETDB1. Functional significance of cross-talk was tested using MTT, proliferation, stemness and colony formation assays. Mechanistic studies were conducted using immunoprecipitation, RNA-seq, shRNA, overexpression, Western blotting, and RT-qPCR. Biological significance of PELP1 and SETDB1 in endocrine therapy resistance was also examined.
RESULTS: Analyses of TCGA databases showed that SETDB1 is highly expressed in BCa and associated with poor clinical outcome. Further, SETDB1 expression is positively correlated with PELP1 expression in BCa (r=0.30, p&lt;0.0001). Immunoprecipitation assays using multiple BCa cell lysates confirmed the interaction of SETDB1 with PELP1. Using two different shRNAs targeting SETDB1 and multiple BCa model cells, we provided evidence that SETDB1 plays an important role in the proliferation of BCa cells. SETDB1 upregulation is sufficient to accelerate proliferation and PELP1 knockdown attenuated SETDB1 oncogenic functions. SETDB1 overexpression contributed to resistance to tamoxifen treatment, while PELP1 knockdown re-sensitized cells to therapy. RNA-seq identified Akt signaling pathways were activated by SETDB1 in BCa. Mechanistic studies showed that SETDB1 overexpression increased Akt phosphorylation and its downstream signaling, while PELP1 knock down attenuated SETDB1 mediated Akt activation in both MCF7 and ZR75 models. Furthermore, in BCa model cells that uniquely express PELP1 in cytoplasm, knockdown of SETDB1 reduced activation of Akt and its downstream pathways.
CONCLUSIONS: Our study results suggest that the PELP1/SETDB1 interactome plays an important role in aberrant Akt activation. Drugs that target PELP1/SETDB1 axis may be useful in modulating aberrant Akt signaling. Supported by VA grant I01BX004545.
Citation Format: Zexuan Liu, Junhao Liu, Weiwei Tang, Uday P. Pratap, Kristin A. Altwegg, Xiaonan Li, Suryavathi Viswanadhapalli, Ratna Vadlamudi. Proto-oncogene PELP1 interactions with SETDB1 contribute to aberrant activation of AKT1 in breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS17-34.