Date of Degree

5-2019

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biology

Advisor

Jill Bargonetti

Committee Members

Patricia Rockwell

Olorunseun Ogunwobi

Anjana Saxena

Michael Overholtzer

Subject Categories

Biology | Cancer Biology | Cell and Developmental Biology

Keywords

MDMX, MDM2, CTC, Metastasis, TNBC

Abstract

Many human breast cancers overexpress the E3 ubiquitin ligase MDM2 and its homolog MDMX. Expression of MDM2 and MDMX occurs in both estrogen receptor α positive (ER+) and triple negative breast cancer (TNBC). We and others have reported that estrogen activated MDM2 strongly promotes proliferation in ER+ T47D breast cancer cells in a p53-independent manner. Whether MDM2 elicits in vivo p53-independent proliferative functions in T47D breast cancer cells has not been determined. Furthermore it has been shown that ectopic expression of MDM2 targets E-Cadherin for degradation thus leading to increased cell migration and invasion. Therefore we assessed the in vivo function of MDM2 in an ER+ mtp53 (L194F) expressing T47D orthotopic mouse model and demonstrated that MDM2 knockdown considerably attenuated primary tumor growth. However depletion of MDM2 did not reduce tumor invasiveness and failed to increase E-Cadherin levels. Conversely in mtp53 (R280K) expressing MDA-MB-231 cells, which belong to claudin-low/TNBC subtype, inducible MDM2 knockdown reduced migration without changing cell proliferation. This indicates a context dependent role of MDM2 in breast cancer progression. Co-overexpression of mdm2 and mdmx has been documented in the Claudin-low/TNBC subtype, and high levels of MDMX in TNBC cell lines confer tumorigenic functions. Therefore we generated constitutive MDM2 or MDMX knockdown cell lines that could be tracked by GFP expression to compare their biological outcomes in proliferation, migration and in vivo metastatic potential. While MDMX knockdown showed a decreased tumor volume in NSG mice at the early stage of tumor development, MDM2 knockdown did not change tumor size. Importantly MDM2 or MDMX knockdown significantly reduced the number of circulating tumor cells (CTCs) and lung foci formation. To explore the molecular targets of MDM2 and MDMX, we carried out qRT-PCR tumor metastasis array card screening with cDNA from primary tumors. Subsequent validation identified MDMX down-regulation in MDA-MB-231 orthotopic tumors correlated with a significant decrease in cxcr4 transcript and a trend of reduction for ptgs2 transcript. To determine if cxcr4 and ptgs2 were directly upregulated by MDMX, we assessed their transcript levels upon MDMX knockdown in two-dimensional cultured cells and observed no association between MDMX, cxcr4 and ptgs2. This suggests MDMX is involved in indirect regulation of cxcr4 only in the tumor microenvironment. Our data positions MDM2 and MDMX as context specific regulators in breast cancer development and highlights the importance in considering their differential functions for targeted therapies.

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