RUNX3 is a transcription factor that is ubiquitously expressed in different tissues and has been shown to have diverse functions in many developmental procedures. Recently it has also been acknowledged that RUNX3 is involved as a tumor suppressor in many distinct cancers in different tissues. In this thesis, we will examine the regulation of this tumor suppressor in gastric cancer and breast cancer.
Chronic infection with cagA-positive Helicobacter pylori is the strongest risk factor for the development of gastric adenocarcinoma. The cagA gene product CagA is injected into gastric epithelial cells and disturbs cellular functions by physically interacting with and deregulating a variety of cellular signaling molecules. RUNX3 is expressed gastric epithelial tissues, and is frequently inactivated in gastric cancer. In the first part of the thesis, we showed that H. pylori infection inactivates the gastric tumor suppressor RUNX3 in a CagA-dependent manner. CagA directly associates with RUNX3 through a specific recognition of the PY motif of RUNX3 by a WW domain of CagA. Deletion of the WW domains of CagA or mutation of the PY motif in RUNX3 abolishes the ability of CagA to induce the ubiquitination and degradation of RUNX3, thereby extinguishing its ability to inhibit the transcriptional activation of RUNX3. This study identify RUNX3 as a novel cellular target of H. pylori CagA and also reveal a mechanism by which CagA functions as an oncoprotein by blocking the activity of gastric tumor suppressor RUNX3.
RUNX3 has also been known to be inactivated in breast cancer through dual mechanism of cytoplasmic mislocalization as well as promoter hypermethylation. Recent studies in our lab have shown that RUNX3 knockout mice have an approximate 20% increased chance of
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developing breast cancer compared to WT mice. At the same time, MCF7 cells expressing RUNX3 resulted in smaller tumor growth in a tumorigenicity assay compared to MCF7 cells expressing a control vector, further affirming RUNX3’s importance as a tumor suppressor in breast cancer. Pin1 is an isomerase that is over-expressed in human breast cancer. Pin1 specifically isomerizes only the Ser/Thr-Pro bonds in certain proteins, which allows it to act as a molecular switch controlling protein functions. In the second part of this thesis, we discuss the findings that Pin1 interacts specifically with tumor suppressor protein RUNX3 through 4 separate phosphorylated Ser/Thr-Pro motifs on the RUNX3 protein. Through this interaction with Pin1, the ubiquitination of RUNX3 is markedly enhanced, resulting in decreased protein stability. RUNX3 is therefore targeted to the 26S proteasome for degradation in the presence of Pin1. Our data shows a novel pathway through which tumor suppressor protein RUNX3 can be inactivated and hence regulated in breast tissues by Pin1
