Key Points• JAK3-mediated phosphorylation of EZH2 resulted in EZH2 oncogenic function independent of its enzymatic activity.• Targeted inhibition of JAK3 may be a promising treatment in NK/TL through suppressing noncanonical EZH2 activity.The best-understood mechanism by which EZH2 exerts its oncogenic function is through polycomb repressive complex 2 (PRC2)-mediated gene repression, which requires its histone methyltransferase activity. However, small-molecule inhibitors of EZH2 that selectively target its enzymatic activity turn out to be potent only for lymphoma cells with EZH2-activating mutation. Intriguingly, recent discoveries, including ours, have placed EZH2 into the category of transcriptional coactivators and thus raised the possibility of noncanonical signaling pathways. However, it remains unclear how EZH2 switches to this catalytic independent function. In the current study, using natural killer/T-cell lymphoma (NKTL) as a disease model, we found that phosphorylation of EZH2 by JAK3 promotes the dissociation of the PRC2 complex leading to decreased global H3K27me3 levels, while it switches EZH2 to a transcriptional activator, conferring higher proliferative capacity of the affected cells. Gene expression data analysis also suggests that the noncanonical function of EZH2 as a transcriptional activator upregulates a set of genes involved in DNA replication, cell cycle, biosynthesis, stemness, and invasiveness. Consistently, JAK3 inhibitor was able to significantly reduce the growth of NKTL cells, in an EZH2 phosphorylation-dependent manner, whereas various compounds recently developed to inhibit EZH2 methyltransferase activity have no such effect. Thus, pharmacological inhibition of JAK3 activity may provide a promising treatment option for NKTL through the novel mechanism of suppressing noncanonical EZH2 activity. (Blood. 2016;128(7):948-958)

Stat3 is a member of the signal transducer and activator of transcription family, which is important for cytokine signaling as well as for a number of cellular processes including cell proliferation, anti-apoptosis and immune responses. In recent years, evidence has emerged suggesting that Stat3 also participates in cell invasion and motility. However, how Stat3 regulates these processes remains poorly understood. Here, we find that loss of Stat3 expression in mouse embryonic fibroblasts leads to an elevation of Rac1 activity, which promotes a random mode of migration by reducing directional persistence and formation of actin stress fibers. Through rescue experiments, we demonstrate that Stat3 can regulate the activation of Rac1 to mediate persistent directional migration and that this function is not dependent on Stat3 transcriptional activity. We find that Stat3 binds to βPIX, a Rac1 activator, and that this interaction could represent a mechanism by which cytoplasmic Stat3 regulates Rac1 activity to modulate the organization of actin cytoskeleton and directional migration.

The interleukin (IL)-6-type cytokines play major roles in a variety of biological processes by signaling through a common receptor subunit, glycoprotein (gp) 130. We performed yeast two-hybrid screening to identify new binding partners of the activated gp130 and the associated Janus kinases. LMO4, a LIM domain-containing protein that belongs to a family of oncogenes, was identified in this assay. Further studies show that LMO4 associates with gp130 and Janus kinase1 in several mammalian cell types. It also interacts with proteintyrosine phosphatase 2 (SHP2) and suppressor of cytokine signaling 3 (SOCS3). The binding domains involved in these interactions were mapped, and the interactions were shown to be in a direct manner by in vitro binding assays. It is likely that LMO4 exists in the gp130 complex. The cellular localization of LMO4 was detected primarily in the nucleus with a substantial amount also detected in the cytoplasm in several cell types. The interleukin (IL 1 )-6-type cytokines belong to a cytokine family that plays major roles in hematopoiesis, immune responses, inflammation, and neural development by regulating the expression of their target genes (1, 2). IL-6 regulates the production of acute phase proteins in hepatocytes and stimulates differentiation of B and T cells and proliferation of keratinocytes, mesangial, and myeloma/plasmacytoma cells. The IL-6 cytokine family transduces their signals via a common receptor subunit, glycoprotein 130 (gp130). In the case of IL-6, binding of the ligand to the non-signaling ␣-receptor subunit recruits the ␤-receptor subunits, gp130, which form homodimers. The receptor dimerization leads to phosphorylation and activation of the non-receptor tyrosine kinases of the JAK family that are pre-associated with gp130. JAKs in turn phosphorylate specific tyrosine residues in the cytoplasmic portion of gp130, which serve as docking sites for signaling molecules containing Src homology 2 (SH2) domains. Six tyrosine residues are phosphorylated in gp130 upon ligand stimulation, leading to the activation of two major pathways (for viewed, see Ref.3). The four phosphotyrosine residues at the C-terminal region of gp130 serve as recruitment sites for Stat3 (4), whereas phosphorylation of its second membrane-proximal tyrosine residue leads to recruitment of SHP2 (3). SHP2 serves as an adaptor protein mediating the activation of the Ras-Rafmitogen-activated protein kinase signaling pathway. As a protein-tyrosine phosphatase, SHP2 also plays a negative regulatory role in IL-6 signaling (5, 6). The same site also mediates binding of the IL-6-induced feedback inhibitor SOCS3, which attenuates IL-6 signaling by inhibiting JAKs as well as by triggering protein degradation (7,8).The LIM domain, characterized by a double zinc finger structure, was originally identified in the homeodomain transcription factors Lin-11, Isl-1, and Mec-3, which have established roles in the central nervous system, and was subsequently found in a variety of proteins with diverse functions. The LIMcontain...