Cancer Biology and Signal Transduction PIM Kinases Are Essential for Chronic Lymphocytic Leukemia Cell Survival (PIM2/3) and CXCR4-Mediated Microenvironmental Interactions (PIM1)

Overexpression of the CXCR4 receptor is a hallmark of chronic lymphocytic leukemia (CLL) and is important for CLL cell survival, migration, and interaction with their protective microenvironment. In acute myelogenous leukemia (AML), PIM1 was shown to regulate the surface expression of the CXCR4 receptor. Here, we show that PIM (proviral integration site for Moloney murine leukemia virus) kinases 1–3 are overexpressed and that the CXCR4 receptor is hyperphosphorylated on Ser339 in CLL compared with normal lymphocytes. Furthermore, CXCR4 phosphorylation correlates with PIM1 protein expression and PIM1 transcript levels in CLL. PIM kinase inhibition with three different PIM kinase inhibitors induced apoptosis in CLL cells independent of the presence of protective stromal cells. In addition, PIM inhibition caused dephosphorylation of the CXCR4 receptor on Ser339, resulting in enhanced ligand-dependent CXCR4 internalization and reduced re-externalization after withdrawal of CXCL12. Furthermore, PIM inhibition in CLL cells blocked CXCR4 functions, such as migration toward CXCL12or CXCL12-induced extracellular signal–regulated kinase (ERK) phosphorylation. In concordance, pretreatment of CLL cells with PIM kinase inhibitors strongly reducedhomingofCLL cells toward the bonemarrowand the spleen ofRag2 / gc / mice in vivo. Interestingly, the knockdownof PIMkinases inCLL cells demonstrated diverging functions,with PIM1 regulatingCXCR4 surface expression andPIM2andPIM3as important for the survival ofCLL cells.Our results show that PIM kinase inhibitors are an effective therapeutic option for CLL, not only by impairing PIM2/3mediatedCLL cell survival, but also by blocking the PIM1/CXCR4-mediated interaction ofCLL cellswith their protective microenvironment. Mol Cancer Ther; 13(5); 1231–45. 2014 AACR.