Previous work has shown that FLT3-ITD mediated leukemogenesis is associated with increased expression of PIM1 and PIM2 serine/threonine kinases. Here we show that retroviral expression of FLT3-ITD could not compensate impaired clonogenic in vitro growth of PIM1−/− bone marrow cells. Induction of a lethal myelo- and lymphoproliferative disorder by FLT3-ITD in vivo was independent of PIM2, but rather unexpectedly, lethally irradiated recipients could not be reconstituted with FLT3-ITD expressing bone marrow cells lacking PIM1. Transplants of CSFE-labeled PIM1−/− cells revealed an impaired homing capacity to bone marrow and spleen. Expression of lower surface CXCR4 levels (while maintaining normal total CXCR4 levels) in PIM1−/− bone marrow cells was associated with significantly reduced migration towards a CXCL12 gradient and impaired CXCL12-mediated intracellular Ca2+ release. Using siRNA-mediated knockdown, a small molecule PIM inhibitor, expression of a dominant-negative acting PIM1 mutant or re-expression of PIM1 in knockout cells, we observed that PIM1 activity was critical for CXCR4 surface expression. In vitro kinase assays and masspectrometric analysis further revealed that PIM1 directly phosphorylated serine 339 located in the CXCR4 intracellular domain known to be essential for proper receptor recycling. Interestingly, in leukemic blasts from acute myeloid leukemia (AML) patients, we found an association of increased PIM1 expression and high-level of surface CXCR4. In addition, treatment of the cells with a small molecule PIM inhibitor resulted in decreased surface CXCR4 expression in some patients. Our work suggests that PIM1 exerts its oncogenic activity not only by supporting proliferation and survival but also by regulation of cell homing and migration through direct modification of the CXCL12/CXCR4 axis. As CXCR4 is a key mediator of cancer stem cell homing and metastasis, targeting of PIM1 may offer new therapeutic avenues against tumor progression and relapse.

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