Graduation Date

Fall 12-15-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Cancer Research

First Advisor

Prof. Jing Jenny Wang

Second Advisor

Prof. Jennifer D. Black

Abstract

Colorectal cancer (CRC) is the second most common cause of cancer related deaths in the United States, mainly due to metastasis to the distant organ sites. However, the molecular basis of CRC metastasis is poorly understood. Therefore, identification and characterization of novel potential anti-cancer therapeutic targets CRC is of urgent need. Utilizing a 2D-DIGE proteomics approach ezrin was identified as a protein that is differentially expressed between primary colon tumors xenografts, orthotopically implanted in athymic nude mice, and corresponding and liver metastatic deposits. Ezrin, a cytoskeletal protein belonging to the ezrin–radixin–moesin (ERM) family plays important roles in cell motility, invasion and metastasis. However, its function in CRC is not well characterized. Increased phosphorylation of ezrin at the T567 site (termed here as p-ezrin T567) was observed in liver metastasis as compared to the primary tumors in both orthotopic xenograft mouse models and human CRC patient specimens. Inhibition of ezrin activation by siRNA, shRNA, pharmacological inhibitor or an ezrin phospho-deficient T567A mutant significantly increased cell death associated with downregulation of inhibitors of apoptosis (IAP) proteins, XIAP and survivin which have been linked to aberrant cell survival and metastasis. In addition inhibition of the IGF1R signaling pathway by the humanized recombinant IGF1R monoclonal antibody MK-0646 in subcutaneous xenografts resulted in the inhibition of p-ezrin T567. Furthermore, TGFβ/Smad3 signaling inhibits ezrin phosphorylation at T567. These results suggest that ezrin is a downstream effector of the IGF1R and TGFβ signaling pathways.

Ezrin is a known cAMP dependent A-kinase Anchoring Protein (AKAP). We demonstrate that the knockdown or inhibition of ezrin activates PKA in a cAMP-independent manner leading and leads to decreased expression of XIAP and survivin and CRC cell death. Interestingly, ezrin activation by hyperphosphorylation at T567 also activates PKA, but in cAMP-dependent manner leading to CRC cell survival and upregulation of XIAP and survivin. These results were further confirmed with ezrin T567 phospho-mimetic (T567D) and phospho-deficient (T567A) mutants which demonstrated PKA activation by cAMP-dependent and independent mechanisms respectively leading to different CRC cell fates. Hence, there are two opposing pathways mediated by ezrin in which PKA is activated but results in different cell fates.

Therefore, ezrin might be a novel target that could be utilized for the development of new therapeutic strategies. Further understanding of the mechanisms by which PKA is activated and its differential role in CRC cell fates could have potential impact for CRC treatment.

Available for download on Friday, December 07, 2018

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