Abstract 510: Activation of PP2A by SET antagonism destabilizes c-Myc and inhibits breast cancer growth.
Authors: Janghorban Mahnaz , Oddo Jessica , Risom Tyler , Farrell Amy , Vitek Michael P. , Sears Rosalie C. , Christensen Dale J. 
Published: August 2014

Abstract
Background: Protein phosphorylation plays an important role in regulating cellular signaling and deregulation of this process can contribute to many aberrant cellular behaviors, including those that contribute to cancer. It has been demonstrated that many oncogenes and proto-oncogenes are kinases, and that uncontrolled elevated levels of their activity lead to tumorigenic transformation.
Protein phosphatase 2A (PP2A) is a ubiquitously expressed heterotrimeric Serine-Threonine (S/T) phosphatase that is responsible for 30-50% of cellular S/T protein phosphatase activity. PP2A regulates numerous signaling pathways, including stem cell self-renewal, proliferation, differentiation, migration, cell survival, and apoptosis. The PP2A holoenzyme has 3 subunits: a catalytic (C) subunit, a structural (A) subunit, and a variable regulatory (B) subunit that directs the PP2A holoenzyme to a specific substrates. PP2A loss of activity is important for cell transformation and it has been shown that B56α, B56γ, and PR72/PR130 are involved in human cell transformation. Importantly, PP2A-B56α negatively regulates the c-Myc transcription factor that is a potent oncoprotein, by decreasing its protein stability.
The oncoprotein SET is an endogenous inhibitor of PP2A that is overexpressed in many cancers. The role that SET plays in the development, progression and metastasis of breast cancer has not been reported.
Methods: SET expression was analyzed by qRT-PCR in human breast tumors and compared to that in matched adjacent normal. SET was knocked down in MDA-MB-231 cells and tested for xenografts experiments. SET was also stably knocked down in MDA-MB-231 cells using two different shRNAs. Clonal cell lines with reduced SET expression were assayed for cell proliferation and soft agar in vitro. PP2A activation was measured following treatment with OP449, a SET inhibitor, in breast cancer cells lines. OP449’s effect on the growth and tumorigenic potential of these cells was also measured. Lysates of cells treated with OP449 have been used to analyze the effect of SET inhibition on c-Myc protein stability and promoter binding.
Results: Our data suggest that SET, is overexpressed in 50-60% of breast cancers. Treatment with OP449 and knockdown of SET reduces tumorigenic potential of some breast cancer cell lines in vitro and in vivo. OP449 treatment or SET knockdown also reduces S62-phosphorylated Myc levels and OP449 decreases Myc target gene promoter binding. OP449 also inhibits tumor growth in a Myc-driven transgenic mouse model of HER2-dependent breast cancer.
Conclusion: Activating PP2A through SET inhibition reduces tumorigenic potential of some breast cancer cell lines. This can be caused in part by decreasing Myc protein levels. Because of the ubiquitous expression of PP2A in eukaryotic cells and its important role in blocking transformation, we propose that activating PP2A can be a novel approach for breast cancer therapy.
Citation Format: Mahnaz Janghorban, Jessica Oddo, Tyler Risom, Amy Farrell, Michael P. Vitek, Rosalie C. Sears, Dale J. Christensen. Activation of PP2A by SET antagonism destabilizes c-Myc and inhibits breast cancer growth. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 510. doi:10.1158/1538-7445.AM2013-510