NPM-ALK Promotes Cell Cycle Progression through Activation of JNK and c-Jun in Anaplastic Large Cell Lymphoma.

Abstract

Anaplastic large cell lymphoma (ALCL) frequently carries the t(2;5)(p23;q35) resulting in aberrant expression of nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) chimeric protein. NPM-ALK mediates its oncogenic effects through phosphorylation of a number of proteins involved in known signal transduction pathways including PLC, PI3K-AKT and JAK-STAT. ALK+ ALCL cells also are known to overexpress c-Jun, a member of the activator protein-1 (AP-1) transcription factor family that controls cell proliferation, differentiation, growth and apoptosis. Phosphorylation of c-Jun at serine 73 and serine 63 residues substantially increases AP-1 transcriptional activity and the levels of c-Jun protein through an autoregulatory positive feedback loop. In this study, we hypothesized that NPM-ALK activates JNK which , in turn, phosphorylates and activates c-Jun, resulting in uncontrolled cell cycle progression in ALCL. 293T and Jurkat (T-acute lymphoblastic leukemia) cells were transfected with a vector expressing NPM-ALK with active kinase domain (pDest40-NPM-ALK) or a construct lacking NPM-ALK kinase activity (pDest40-K210R) or empty vector. Cells were harvested at 48 hours and analyzed for protein expression by Western blot analysis and for AP-1 activity by luciferase reporter assay. Two ALK+ ALCL cell lines Karpas 299 and SU-DHL-1, found to express high levels of serine phosphorylated and total c-Jun in immunoblots, were treated with JNK (SP600125), ERK (U0126), or ALK (WHI-P154) inhibitors or were transiently transfected with siRNAs specific for JNK1 and c-Jun. Cell proliferation was assessed by MTS assay, and cell cycle was analyzed by BrdU assay or propidium iodide staining and flow cytometry. Forced expression of NPM-ALK in 293T and Jurkat cells resulted in increased levels of JNK and c-Jun phosphorylation in immunoblots and a dramatic increase in AP-1 activity. Conversely, pharmacologic inhibition of ALK activity in Karpas 299 and SU-DHL1 resulted in a concentration-dependent decrease of JNK and c-Jun phosphorylation levels. Co-immunoprecipitation studies revealed that NPM-ALK physically binds to JNK1 and its upstream activator MKK7 in ALK+ ALCL cells. Selective inhibition of JNK, but not ERK, in Karpas 299 and SU-DHL1 decreased the level of c-Jun phosphorylation in a dose-dependent manner as shown by Western blot analysis and in vitro kinase assays. Inhibition of JNK by SP600125 or silencing of the JNK1 gene by siRNA also resulted in decreased cell proliferation associated with decreased AP-1 activity, cell cycle arrest mostly at G2 phase, and up-regulation of the cyclin-dependent inhibitor p21, a transcriptional target of c-Jun. Similarly, silencing of c-Jun by specific siRNA led to decreased S-phase fraction of cell cycle, which was associated with up-regulation of p21 and downregulation of cyclin D3. These findings reveal a novel function of NPM-ALK oncoprotein, phosphorylation and activation of JNK, which may contribute to uncontrolled cell cycle progression through activation of c-Jun. Modulation of JNK or c-Jun activity may be a target for therapy in patients with ALCL.