The tumor suppressor 2A (PP2A) is a multi-enzyme complex that targets a variety of oncogenic signaling cascades such as Raf, MEK and AKT. Numerous reports have now emerged indicating that the inhibition of PP2A leads to the development of tumors including breast, gastric and lung and a range of hematological malignancies such as chronic and acute myeloid leukemia (CML and AML, respectively). We and others have shown that inhibition of PP2A is crucial for the oncogenic effects of leukemia associated receptor tyrosine kinases such as BCR/Abl, c-KIT and FLT3. Importantly when PP2A is reactivated using pharmacological agents (FTY720 and AAL(S)), growth and survival of these cells is inhibited. This approach targets molecules downstream of these oncogenic kinases and thus PP2A reactivation is designed to avoid resistance associated with kinase inhibitor therapy. As the mechanism of action of these PP2A activators is unknown, it is essential that we now determine the molecular targets of these compounds to generate more effective drugs. Using a chemical proteomics approach incorporating click chemistry linking the drugs to beads, unbiased pull down assays were performed in myeloid leukaemia cells and protein isolates were identified by mass spectrometry. Using this approach we have identified a novel cellular target, which is known to be overexpressed in a range of cancers, including leukemias. We have also shown that this protein associates with PP2A complexes in these cells, and thus may act as an endogenous inhibitor of PP2A activity. We have also performed phospho-proteomics and co-immunoprecipitation studies to identify protein changes in myeloid leukemia cells in response to PP2A activators. In addition to identifying known PP2A targets, such as Akt and ERK, we have also identified a number of novel protein changes, including altered expression of the SET oncoprotein. SET is a known inhibitor of PP2A, suggesting that FTY720 may act via inhibition of SET. Other PP2A regulatory proteins such as CIP2A, PME-1 and PP2A-B56alpha are also affected by FTY720/AAL(S) treatment. Our research findings are thus providing the vital clues necessary for determining the mechanism of action of clinically relevant PP2A activators, thereby moving us towards the provision of novel anti-cancer treatment regimes.
Citation Format: Matthew D. Dun, Amanda M. Smith, Richard G. S. Kahl, Nathan D. Smith, Anchit Khanna, Anthony S. Don, Hanish F. Toop, Jonathan C. Morris, Nicole M. Verrills. Unraveling the mechanism of action: drugs that activate the tumor suppressor 2A . [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 2038. doi:10.1158/1538-7445.AM2013-2038
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