PIM1 Protein Kinase regulates PRAS40 phosphorylation and mTOR activity in FDCP1 cells

Abstract

PIM1 is a serine /threonine kinase that has diverse biological roles in cell survival,

proliferation and differentiation.   PIM1 has been implicated in early transformation and

tumor progression in haematopoietic malignancies and prostate carcinomas.  The ability

of PIM1 to regulate these processes is thought to be in part secondary to its activity in

stimulating 4EBP1 phosphorylation and enhancement of protein synthesis.  Because

4EBP1 is an mTOR substrate, we have investigated how PIM1 might regulate this latter

enzyme.   We have examined the ability of PIM1 to modulate PRAS40, a protein known

to negatively regulate mTOR activity in FDP1 cells. Upon phosphorylation, PRAS40

dissociates from the mTOR complex and increases mTOR kinase activity. We find that

enforced overexpression of PIM1 increases PRAS40 phosphorylation at Thr246, an AKT

phosphorylation site, whether grown in complete media or deprived of IL-3 and serum.

The increase in PRAS40 phosphorylation was independent of AKT activation and not

inhibited by wortmannin.  In vitro kinase assays indicate that the PIM1 protein kinase is

capable of directly phosphorylating Thr246 in PRAS40. PIM1 protein kinase

overexpression reduced the association of PRAS40 with mTOR, and increased the mTOR

directed phosphorylation of 4EBP1 and p70S6Kinase. Treatment of FDCP1  cells

transfected with PIM1 (FD/mpim44) with small molecule inhibitors of PIM1 kinase

activity reduced both PRAS40  and 4EBP1 phosphorylation. These results suggest that

PIM1 regulates mTOR activity through phosphorylation of PRAS40.  Thus, increases in

mTOR activity mediated by the PIM protein kinase may have the potential to control cell

growth.