Graduation Date

Spring 5-6-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Cancer Research

First Advisor

Dr. Hamid Band

Second Advisor

Dr. Vimla Band

Third Advisor

Dr. Joyce Solheim

Abstract

Triple-negative breast cancer (TNBC) comprises 10%-15% of all breast cancer cases, yet is clinically challenging due to lack of targeted therapies which leads to higher mortality. Molecular subtyping has identified the most aggressive subclasses of breast cancer to be enriched in components of caveolae. While caveolae have been linked to many biological processes, their precise role in TNBC is still poorly understood. EHD2, a member of the C-terminal EPS15-Homology Domain-containing (EHD) protein family, has emerged as a new regulator of caveolae dynamics and is essential to maintain a stable membrane pool of caveolae. Studies in model cells demonstrate that caveolae facilitate repair of plasma membrane injuries incurred under stressful conditions. Importantly, new evidence suggests the invasiveness of tumor cells makes them vulnerable to plasma membrane injuries and hence a robust repair mechanism is essential to protect injured tumor cells from cell death.

EHD2 is found to be highly expressed within the basal myoepithelial layer throughout mammary gland development. High EHD2 mRNA and protein expression was observed in human TNBC cell lines and positively correlated with Caveolin-1 and Caveolin-2 expression. Furthermore, by analyzing publicly available gene expression databases, we found that high expression of EHD2 mRNA correlated with lower probability of survival in TNBC patients. This led us to hypothesize that EHD2 is a marker and crucial regulator of tumorigenicity in TNBC. Using a cohort of 840 highly-annotated human breast cancer tissue samples, we discovered that high cytoplasmic expression of EHD2 marked TNBC cases, and served as a robust prognostic indicator of metastasis and lower patient survival. ShRNA-mediated knockdown of EHD2 in TNBC cell lines reduced invasiveness, growth under anchorage-independent conditions, and membrane repair ability in vitro. Using orthotopic implantation of human breast cancer cell lines in mouse mammary gland, we observed a dramatic abrogation of tumor growth and a reduction in metastasis of TNBC cell lines with EHD2 knockdown compared to their controls. These findings indicate that EHD2 is a novel clinical biomarker of poor prognosis in TNBC, and serves a novel role as a positive regulator of caveolae-dependent protection of plasma membrane against injury, enabling tumorigenicity and metastasis in TNBC. Our results support the potential of targeting EHD2 to develop therapeutic approaches against TNBC.

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