Abstract 615: PP2A activation as a strategy for treatment of breast cancer
Authors: Christensen Dale J. , Oddo Jessica , Farrell Amy S. , Janghorban Mahnaz J. , Vitek Michael P. , Sears Rosalie C. 
Published: June 2012

Background: Cells respond to stimuli by activating kinase signaling cascades that are counterbalanced by specific phosphatases so that the cell can return to a resting state. PP2A is a critical cellular phosphatase that negatively regulates multiple kinase cascades and displays strong tumor suppressor activity (Westermarck and Hahn, Trends Mol Med, 2008, 14:152-60). Inhibition of PP2A is essential for cell transformation and tumorigenesis (Chen et al., Cancer Cell, 2004, 5:127-36) and several physiological PP2A inhibitory proteins have been described. The oncoprotein SET is a potent inhibitor of PP2A that is overexpressed in cancer. We found that SET is overexpressed in 55-60% of primary breast cancer samples and identified peptides that antagonize SET, activate PP2A, reduce phosphorylation of oncoproteins, and inhibit xenograft tumor growth.

Methods: Primary breast cancer samples and matched normal adjacent tissue were evaluated for overexpression of SET using qPCR, immunofluorescence staining, and Western blotting. Cytotoxicity of PP2A activation in breast cancer cell lines was assessed using the MTT assay. PP2A activity was measured using an immunprecipitation assay and cellular correlates of dephosphorylation were measured by Western blotting. Tumors in NOD/SCID mice with human breast cancer cell lines implanted into the mammary fat pad were treated with SET antagonist or a vehicle control by subcutaneous injection.

Results: Assessment of SET expression was analyzed by qPCR from breast cancer samples and normal adjacent tissues. We observed greater than 3-fold increased expression of SET in 11 of 19 patients. We also observed SET overexpression in 7 of 13 triple negative breast cancer samples in a commercially available breast cancer cDNA array. SET overexpression was confirmed in breast cancer cell lines by qPCR and Western blotting. Antagonism of SET using COG449 inhibited growth of several cell lines arising from activation of PP2A. Activation of PP2A with COG449 reduced S62 phosphorylation of c-Myc and other known PP2A targets. Finally, subcutaneous treatment with 10 mg/kg COG449 significantly inhibited growth of xenografted tumors in the mammary fat pad of NOD/SCID mice. Importantly, in vivo studies in normal mice using COG449 shows no toxicity even at doses of 100 mg/kg.

Conclusions: We demonstrated SET overexpression in breast cancer samples and that SET antagonism with COG449 activated PP2A. This results in cytotoxicity in breast cancer cells in vitro and reduced phosphorylation of c-Myc S62 and other PP2A targets. PP2A activation by SET antagonism inhibited tumor growth in xenograft models at concentrations that are nontoxic in normal mice to demonstrate that PP2A activation is a novel approach to breast cancer therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 615. doi:10.1158/1538-7445.AM2011-615