PP2A-activating Drugs Enhance FLT3 Inhibitor Efficacy through AKT Inhibition-Dependent GSK-3β-Mediated c-Myc and Pim-1 Proteasomal Degradation

Fms-like tyrosine-like kinase 3 internal tandem duplication (FLT3-ITD) exists in acute myeloid leukemia (AML) in 30% of patients that is connected with short disease-free survival. FLT3 inhibitor effectiveness is bound and transient but could be enhanced by multitargeting of FLT3-ITD signaling pathways. FLT3-ITD drives both STAT5-dependent transcription of oncogenic Pim-1 kinase and inactivation within the tumor-suppressor protein phosphatase 2A (PP2A), and FLT3-ITD, Pim-1, and PP2A all regulate the c-Myc oncogene. We studied mechanisms of action of cotreatment of FLT3-ITD-expressing cells with FLT3 inhibitors and PP2A-activating drugs (PADs), that are in development. PADs, including FTY720 and DT-061, enhanced FLT3 inhibitor growth suppression and apoptosis induction in FLT3-ITD-expressing cell lines and first AML cells in vitro and MV4-11 growth suppression in vivo PAD and FLT3 inhibitor cotreatment individually downregulated c-Myc and Pim-1 protein through enhanced proteasomal degradation. c-Myc and Pim-1 downregulation was preceded by AKT inactivation, didn’t come in cells expressing myristoylated (constitutively active) AKT1, and it is because AKT inhibition. AKT inactivation introduced to activation of GSK-3ß, and GSK-3ß inhibition blocked downregulation of both c-Myc and Pim-1 by PAD and FLT3 inhibitor cotreatment. GSK-3ß activation elevated c-Myc proteasomal degradation through c-Myc phosphorylation on T58 infection with c-Myc with T58A substitution, stopping phosphorylation, blocked downregulation of c-Myc by PAD and FLT3 inhibitor cotreatment. GSK-3ß also phosphorylated Pim-1L/Pim-1S on S95/S4. Thus, PADs enhance effectiveness of FLT3 inhibitors in FLT3-ITD-expressing cells utilizing a novel mechanism involving AKT inhibition-dependent GSK-3ß-mediated elevated c-Myc DT-061 and Pim-1 proteasomal degradation.