Protein phosphatase 2A is a serine/threonine phosphatase involved in the regulation of many cellular processes. A confirmed tumor suppressor protein, PP2A is genetically altered or functionally inactivated in many cancers highlighting a need for its therapeutic reactivation. In this review we will discuss recent literature on PP2A: the elucidation of its structure and the functions of its subunits, and the identification of molecular lesions and post-translational modifications leading to its dysregulation in cancer. A final section will discuss the proteins and small molecules that modulate PP2A and how these might be used to target dysregulated forms of PP2A to treat cancers and other diseases.

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Primary prostate cancer is generally treatable by androgen deprivation therapy, however, later recurrences of castrate-resistant prostate cancer (CRPC) that are more difficult to treat nearly always occur due to aberrant reactivation of the androgen receptor (AR). In this study, we report that CRPC cells are particularly sensitive to the growth-inhibitory effects of reengineered tricyclic sulfonamides, a class of molecules that activate the protein phosphatase PP2A, which inhibits multiple oncogenic signaling pathways. Treatment of CRPC cells with small-molecule activators of PP2A (SMAP) decreased cellular viability and clonogenicity and induced apoptosis. SMAP treatment also induced an array of significant changes in the phosphoproteome, including most notably dephosphorylation of full-length and truncated isoforms of the AR and downregulation of its regulatory kinases in a dose-dependent and time-dependent manner. In murine xenograft models of human CRPC, the potent compound SMAP-2 exhibited efficacy comparable with enzalutamide in inhibiting tumor formation. Overall, our results provide a preclinical proof of concept for the efficacy of SMAP in AR degradation and CRPC treatment. A novel class of small-molecule activators of the tumor suppressor PP2A, a serine/threonine phosphatase that inhibits many oncogenic signaling pathways, is shown to deregulate the phosphoproteome and to destabilize the androgen receptor in advanced prostate cancer. .

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<p>Table S1. KSEA Analysis and the Mean FC Method in LNCaP treated with SMAP. Table S2. ANOVA with multiple comparisons and Dunnett''s post-hoc test for SMAP treated LNCaP colony formation assay. Table S3. ANOVA with multiple comparisons and Dunnett''s post-hoc test for SMAP treated 22Rv1 colony formation assay. Table S4. ANOVA with multiple comparisons and Dunnett''s post-hoc test for Annexin V staining of LNCap and 22Rv1 cells. Table S5. PPI of the phosphoproteomics dataset. Table S6. Kinase Substrate Database for LNCaP treated with SMAP. Table S7. ANOVA with multiple comparisons and Tukey''s post-hoc test for SMAP treatment in LNCaP cells phosphorylated to total AR ratio protein. Table S8. ANOVA for qRT-PCR RNA for AR targets in LNCaP and 22Rv1 cells in presence of SMAPs. Table S9. ANOVA with multiple comparisons and Dunnett''s post-hoc test for LNCaP and 22Rv1 cells treated with SMAP, bortezomib, and combination. Table S10. ANOVA with multiple comparisons and Tukey''s post-hoc test for relative AR protein expression in LNCaP- Small T stably expressing lines treated with SMAP. Table S11. ANOVA with multiple comparisons and Tukey''s post-hoc test for relative AR protein expression in LNCaP cells in FBS or CSS in presence or absence of R1881. Table S12. ANOVA for qRT-PCR RNA AR targets statistical analysis for LNCaP cells in FBS or CSS in presence or absence of R1881. Table S13. ANOVA with multiple comparisons and Dunnett''s post-hoc test for LnCaP/AR xenograft treatment study: vehicle control, SMAP 400mg/kg, SMAP 100mg/kg and MDV3100 100mg/kg. Table S14. ANOVA with multiple comparisons and Dunnett''s post-hoc test for castrated LNCaP/AR tumor volumes for individual treatments groups: Vehicle control, SMAP-2 100mg/kg and SMAP-2 30mg/kg. Table S15. ANOVA with multiple comparisons and Dunnett''s post-hoc test for TUNEL positive staining quantification from castrated LNCaP/AR tumors from individual treatments groups: Vehicle control, SMAP-2 30mg/kg and SMAP-2 100mg/kg. Table S16. ANOVA with multiple comparisons and Dunnett''s post-hoc test for PCNA positive staining quantification from castrated LNCaP/AR for individual treatments groups: Vehicle control, SMAP-2 100mg/kg and SMAP-2 30mg/kg.</p>