Protein kinase C alpha-mediated phosphorylation of PIM-1L promotes the survival and proliferation of acute myeloid leukemia cells.

FMS-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD) is a constitutively active mutant of FLT3 and causes 20%-30% of acute myeloid leukemia (AML) cases. FLT3-ITD upregulates the proviral integration site for Moloney murine leukemia virus 1 (PIM-1) expression and promotes the proliferation of AML cells. In this study, we investigated the role of protein kinase C (PKC)-mediated phosphorylation on the expression and function of PIM-1L. Drug screening in leukemia cell lines revealed that sotrastaurin (a PKC inhibitor) suppressed the proliferation of the FLT3-ITD-positive AML cell line MV4-11 but not of K562, HL60, or KG-1a cells, similar to SGI-1776 (a PIM-1/FLT3 inhibitor) and quizartinib (an FLT3 inhibitor). Sotrastaurin decreased the expression of pro-survival protein myeloid cell leukemia (MCL-1) and the phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), both of which are downstream effectors of PIM-1. PKCα directly phosphorylated Ser65 of PIM-1L, which is a long isoform of PIM-1. The PKCα-mediated phosphorylation stabilized PIM-1L. The phosphorylation-mimicked mutant, PIM-1L-S65D, was more stable and showed higher kinase activity than PIM-1L-S65A. Expression of PIM-1L-wildtype or -S65D reduced sotrastaurin-mediated apoptosis and growth inhibition in MV4-11 transfectants. These results suggest that PKCα directly upregulates PIM-1L, resulting in promotion of the survival and proliferation of AML cells.