Functional Characterization of Protein Tyrosine Phosphatases in Tumorigenesis through Substrate Identification

<p>Tyrosine phosphorylation plays critical roles in literally every cellular process. Highlighting its importance, the dysregulation of tyrosine phospho-rylation leads to the pathogenesis of many human diseases including cancers. Tyrosine phosphorylation is regulated by two families of enzymes PTKs and PTPs. Despite the fact that many PTPs were found mutated in different cancers, the cellular functions of many PTPs are still largely unknown. This work focuses on the elucidation of cellular functions of two PTPs: PTPRT and PTPN14, both of which are mutated in multiple cancer types, including colorectal cancers.</p><p>We utilized a proteomic approach to search for substrates of PTPN14 in colorectal cancer cells. We identified and validated that p130Cas is a direct substrate of PTPN14 and PTPN14 dephosphorylates p130Cas at the tyrosine 128 residue. To dissect the role of pY128 p130Cas in colorectal tumorigenesis, we engineered p130Cas Y128F mutant knock-in colorectal cancer cell lines. The p130Cas Y128F mutant cells display reduced transformation ability <i>in vitro</i> and xenograft tumor growth <i>in vivo</i>. Mechanistically, the p130Cas Y128F mutant cells exhibit impaired Akt signaling through reduced binding to p85.</p><p>Our previous studies demonstrated that PTPRT functions as a tumor suppressor and directly dephosphorylates STAT3 at the tyrosine 705 residue. Although it is well-documented that pY705 STAT3 plays an oncogenic role in some cancers, the role of pY705 STAT3 in colorectal tumorigenesis is not well defined. To this end, we engineered STAT3 Y705F mutant KI colorectal cancer cell lines. We demonstrated that STAT3 Y705F mutant cancer cells, display reduced tumorigenicity both <i>in vivo</i> and <i>in vitro</i>. Significantly, we discovered a novel cross-talk between STAT3 and PLCɣ1. We also showed that the extracellular fragments of PTPRT mediate homophilic cell-cell interactions and that cancer-derived mutations in the MAM, Ig and FN III domains of PTPRT impair cell-cell adhesion, indicating that these PTPRT mutations are loss-of-function mutations.</p> <p>Taken together, these studies provide a novel method for substrate identification for PTPs in a systematic, unbiased way. Once candidate substrates are experimentally verified, the specific phosphosite that is subject to dephosphorylation by the PTP of interest can be genetically modified to study its cellular functions. With the improved understanding of the roles of PTPs in cells especially those related to tumorigenesis, we can explore the potential of PTPs as the targets for a new generation of anti-cancer drugs.</p>

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Genetics

Grantor dc:publisher

Case Western Reserve University School of Graduate Studies

Year dc:date

2013

Author dc:creator

• Zhang, Peng

Contributors dc:contributor

• Wang, Zhenghe

• Genetics

Statement dc:rights

• unrestricted
• This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

Language dc:language

English

Repository record dc:identifier

http://rave.ohiolink.edu/etdc/view?acc_num=case1365174835

OAI identifier oai:identifier

oai:etd.ohiolink.edu:case1365174835