Phenothiazines Induce Apoptosis in T-Cell Acute Lymphoblastic Leukemia by Activating the Phosphatase Activity of the PP2A Tumor Suppressor

Abstract 3558

Despite the use of intensive and toxic treatment regimens, T-cell acute lymphoblastic leukemia (T-ALL) remains fatal in 25% of children and 60% of adults, highlighting the need for novel therapeutic strategies. We performed two complementary small molecule screens to identify novel T-ALL therapeutic agents: i) an in vivo zebrafish screen for small molecules toxic to MYC-overexpressing thymocytes, and ii) an in vitro screen for drugs that synergize with NOTCH inhibitors in human T-ALL cell lines. Hits common to both screens included perphenazine, an FDA-approved phenothiazine antipsychotic. Perphenazine potently induced mitochondrial apoptosis and had antileukemic activity in zebrafish T-ALL, in human T-ALL cell lines, and in primary human T-ALL lymphoblasts. However, the molecular target mediating the antileukemic activity of perphenazine appeared to be unrelated to any of its known cellular targets. To identify the relevant biologic target of perphenazine, we developed a novel method for drug target discovery, termed activity correlation proteomics, which combines native protein extraction of isotope-labeled proteomes, fluorous affinity chromatography, and quantitative mass spectrometry proteomics to relate phenotypic activities of drugs with their binding targets. Using activity correlation proteomics, we identified protein phosphatase 2A (PP2A) as the protein whose binding affinity for different phenothiazines most closely correlated with their antileukemic activity. Perphenazine was shown to activate PP2A in T-ALL cells, as assessed by rapid drug-induced dephosphorylation of multiple PP2A substrates including MYC, AKT, p70S6K, ERK, and BAD, and its effects were phenocopied by FTY720, a PP2A activator. Moreover, shRNA knockdown of PP2A scaffolding or catalytic subunits attenuated the activity of perphenazine, indicating that PP2A is required for its antileukemic activity. Our findings thus establish a novel strategy for pharmacologic activation of the PP2A tumor suppressor, and highlight its therapeutic potential in T-ALL and other cancers driven by hyperphosphorylated PP2A substrates.