Negative regulation of Pim-1 protein kinase levels by the B56β subunit of PP2A

The Pim protein kinases are serine threonine protein kinases that regulate important cellular signaling pathway molecules, and enhance the ability of c-Myc to induce lymphomas. We demonstrate that a cascade of events controls the cellular levels of Pim. We find that overexpression of the protein phosphatase ( PP) 2A catalytic subunit decreases the activity and protein levels of Pim-1. This effect is reversed by the application of okadaic acid, an inhibitor of PP2A, and is blocked by SV40 small T antigen that is known to disrupt B subunit binding to PP2A A and C subunits. Pim-1 can coimmunoprecipitate with th e PP2A regulatory B subunit, B56 beta, but not B56 alpha, c, delta, e or B55 alpha. Using short hairpin RNA targeted at B56b, we demonstrate that decreasing the level of B56b increases the half-life of Pim-1 from 0.7 to 2.8 h, and decreases the ubiquitinylation level of Pim-1. We also find that Pin1, a prolyl-isomerase, is capable of binding Pim-1 and leads to a decrease in the protein level of Pim-1. On the basis of these observations, we hypothesize that phosphorylated Pim-1 binds Pin1 allowing the interaction of PP2A through B56b. Dephosphorylation of Pim-1 then allows for ubiquitinylation and protein degradation of Pim-1.