Abstract 2437: Identification of MDMX/MDM2 metastasis signaling pathways in breast cancer cells with mutant p53
Authors: Lee Rusia , Ellison Viola , Xiao Gu , Forbes Dominique , Leybengrub Pam , Kern Alexandra , Alexandre Falande , Annor George , Bargonetti Jill 
Published: June 2022

Abstract
Human breast cancers often overexpress MDM family proteins MDM2 and MDMX(MDM4). MDM2 is an E3 ubiquitin ligase and MDMX is an E4 that promotes ubiquitination of MDM2 targets (however this is an understudied area). The Cancer Genome Atlas (TCGA) shows high MDMX/MDM2 expression in all breast cancer subtypes. We identified that MDMX depletion reduces circulating tumor cells (CTCs) and metastasis of breast cancers with mtp53. Our ongoing objective is to determine the mechanisms of how MDMX promotes CTCs and metastasis, and the potential MDMX regulated ubiquitinated targets. In TNBC primary tumors, knockdown of mdmx correlates with reduced expression of the G protein coupled receptor, CXCR4. We used mtp53 expressing T47D ER+, and MDA-MB-468, and MDA-MB-231 metastatic TNBC cells and their derived CTC lines, and shRNA-mediated knockdown of MDM2 and MDMX derived lines, to address the potential MDMX targets. We assessed metastatic properties of migration and invasion changed by the knockdown of MDM2/MDMX in parental and CTC cell lines using multi-faceted cell biological and biochemical approaches. Depletion of MDM2 in T47D and MDA-MB-231 cells increased the level of MDMX, demonstrating a critical function of MDM2 for regulating MDMX. MDMX depletion surprisingly did not change MDM2 levels, but did reduce the migration of both MDA-MB-231 cells and their derived CTC lines. Interestingly, the parental MDA-MB-231 cells migrated more rapidly in scratch assays, compared to both their derived CTCs and MDMX-depleted CTCs, suggesting migration to be critical for intravasation, but not for latter cancer cell survival. Experiments are in progress to address if loss of MDMX perturbs CXCL12 ligand communication with the CXCR4 receptor in parental and CTC breast cancer lines. Preliminary data suggests activated MDMX expression correlates with the activation of these metastasis signal transduction pathways. The ubiquitination targets of MDMX/MDM2 in the parental and CTC cell lines are being defined by affinity purification in the presence, and absence, of MDMX/MDM2, and add back of purified MDMX/MDM2 to validate targets. As expected, purified MDMX increases MDM2 E3 ubiquitin ligase activity, and on its own MDMX is inert. Stable isotope labeling in cell culture suggested protein targets involved in DNA replication and repair pathways. This preliminary data informed our interest in studying DNA damage signaling by pharmacologically inhibiting the DNA repair protein PARP. We examined how MDMX/MDM2 pathways respond to DNA damage caused by PARP inhibition. Our preliminary data affirms regulation of the MDMX/MDM2 stress-response circuit by replication stress response pathways, and reveals the requirement of MDMX/MDM2 for their function in the studied cell lines. Disrupting MDMX function has potential to be beneficial for inhibition of breast cancer progression and metastasis. Supported by BCRF-20-011.
Citation Format: Rusia Lee, Viola Ellison, Gu Xiao, Dominique Forbes, Pam Leybengrub, Alexandra Kern, Falande Alexandre, George Annor, Jill Bargonetti. Identification of MDMX/MDM2 metastasis signaling pathways in breast cancer cells with mutant p53 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2437.