Author ORCID Identifier

https://orcid.org/0000-0001-6732-1640

Year of Publication

2022

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department/School/Program

Molecular and Cellular Biochemistry

First Advisor

Dr. Tianyan Gao

Abstract

Protein phosphorylation defines one of the most important regulatory mechanisms in cell signaling. PTPRF, protein tyrosine phosphatase receptor type F, belongs to the class I R2A subfamily of protein tyrosine phosphatases (PTP). The overall objective of this dissertation is to investigate the molecular mechanisms by which PTPRF regulates normal and cancer stem cells by controlling Wnt signaling.

Dysregulation of Wnt signaling promotes the initiation and progression of colorectal cancer (CRC). We first determined the functional important of PTPRF in regulating Wnt signaling in CRC. Combining cell culture, 3D tumor organoid and xenograft models, results from our study establish PTPRF as a positive regulator of the Wnt pathway and an oncogenic PTP in CRC.

To understand how PTPRF protein expression is regulated, we identify that NEDD4L, an E3 ubiquitin ligase, controls PTPRF protein stability and membrane localization. NEDD4L ubiquitinates PTPRF at the plasma membrane to induce PTPRF internalization. Functionally, NEDD4L blocks the ability of PTPRF to activate Wnt signaling. This study identifies NEDD4L-dependent ubiquitination of PTPRF as a novel mechanism that fine-tunes the regulation of Wnt signaling.

To define the molecular mechanisms by which PTPRF promotes Wnt activation, we discover that PTPRF activates the Wnt pathway by controlling the phosphorylation of Cav1. PTPRF-mediated dephosphorylation of Cav1-Y14 leads to prolonged signalosome retention at the plasma membrane, enhanced pLRP6 and downstream Wnt activation. Using PTPRF KO mouse model, we show that PTPRF supports Wnt signaling in vivo to enhance intestinal stem cell function. This study identifies Cav1 as a novel substrate of PTPRF and a functional connection between PTPRF and Wnt signalosome.

Collectively, our studies demonstrate that PTPRF plays a key role in promoting Wnt signaling to support normal and cancer stem cell functions and provide a rationale for targeting PTPRF in Wnt-driven diseases.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2022.267

Funding Information

1. National Science Foundation Graduate Research Fellowship (#1839289) from 2019-2022.

2. National Institute of Health grant R01 CA133429 2018-2022

3. National Institute of Health grant R01 CA208343 2018-2022

Available for download on Friday, August 02, 2024

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