Abstract 5188: MDM2 and MDMX promote p53-independent initiation of circulating tumor cells from triple negative breast cancers

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Abstract Many human breast cancers overexpress the E3 ubiquitin ligase MDM2 and its homolog MDMX. The Cancer Genome Atlas (TCGA) shows that high MDM2 and MDMX expression correlates with both estrogen receptor positive (ER+) and triple negative breast cancer (TNBC) subtypes. Overexpression of MDM2 and MDMX can drive breast cancer progression through molecular pathways that do not require wild-type p53 degradation. ER positive breast cancers often express wild-type p53 but 80% of TNBC express mutant p53 (mtp53). It has not been determined how MDM2 and MDMX expression in different sub-types of breast cancer, with mutant p53, influence proliferation and metastasis. We compared MDM2 and MDMX-dependent biological outcomes for mtp53 expressing ER+ T47D (mtp53 L194F) and TNBC cell type MDA-MB-231 (mtp53 R280K). In the ER+ cells no change was detected for real time cell migration or E-cadherin protein levels when MDM2 was knocked down by stable shRNA expression but cell proliferation was reduced. In support of this in vitro data, these MDM2 knockdown cells tested in an orthotopic xenograft model in NSG mice demonstrated no MDM2-mediated change in primary tumor invasiveness but did show a reduced tumor volume. Importantly, in the TNBC MDA-MB-231 cells, knocking down either MDM2 or MDMX did not reduce cell proliferation in vitro but greatly blocked cell migration. In support of this in vitro data, these MDA-MB-231 cells with either MDM2 or MDMX knockdown in NSG mice did not show a reduced primary tumor volume but showed a significant inhibition of the production of circulating tumor cells (CTCs). Our data shows that in different subtypes of breast cancers, MDM2 and MDMX can promote alternative p53-independent biological outcomes. The estrogen-activated MDM2 strongly promotes cell proliferation while MDM2 and MDMX overexpression in TNBC promotes circulating tumor cells and potentially metastasis. Understanding the alternative p53-independent roles of MDM2 and MDMX in ER+ and triple negative breast cancers will provide insight for the development of MDMs-targeted theranostics. Citation Format: Chong Gao, Gu Xiao, Ale Piersiga, Jiangtao Gou, Olorunseun Ogunwobi, Jill Bargonetti. MDM2 and MDMX promote p53-independent initiation of circulating tumor cells from triple negative 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 5188.