With an array of activating and inhibitory receptors, natural killer (NK) cells can specifically eradicate infected and transformed cells. Target cell killing is achieved through directed release of lytic granules. Recognition of target cells also induces production of chemokines and cytokines that can coordinate immune responses. Upon contact with susceptible cells, a multiplicity of activating receptors can induce signals for adhesion. Engagement of the integrin leukocyte functional antigen-1 mediates firm adhesion, provides signals for granule polarization and orchestrates the structure of an immunological synapse that facilitates efficient target cell killing. Other activating receptors apart from leukocyte functional antigen-1 signal for lytic granule exocytosis, a process that requires overcoming a threshold for activation of phospholipase C-γ, which in turn induces STIM1- and ORAI1-dependent store-operated Ca²⁺ entry as well as exocytosis mediated by the SNARE-containing protein syntaxin-11 and regulators thereof. Cytokine and chemokine release follows a different secretory pathway which also requires phospholipase C-γ activation and store-operated Ca²⁺ entry. Recent studies of human NK cells have provided insights into a hierarchy of effector functions that result in graded responses by NK cell populations. Responses display cellular heterogeneity and are influenced by environmental cues. This review highlights recent knowledge gained on the molecular pathways for and regulation of NK cell activation.

Hypoxia changes the responses of cancer cells to many chemotherapy agents, resulting in chemoresistance. The underlying molecular mechanism of hypoxia-induced drug resistance remains unclear. Pim-1 is a survival kinase which phosphorylates Bad at serine 112 to antagonize drug induced apoptosis. Here we show that hypoxia increases Pim-1 in a HIF-1α-independent manner. Inhibition of Pim-1 function by dominant negative Pim-1 dramatically restores the drug sensitivity to apoptosis induced by chemotherapy under hypoxic conditions in both in vitro and in vivo tumor models. Introduction of siRNAs for Pim-1 also resensitizes cancer cells to chemotherapy drugs under hypoxic conditions, while forced over-expression of Pim-1 endowes solid tumor cells with resistance to cisplatin, even under normoxia. Dominant negative Pim-1 prevents a decrease in mitochondrial transmembrane potential in solid tumor cells, which is normally induced by CDDP, followed by the reduced activity of Caspase-3 and -9, indicating that Pim-1 participates in hypoxia-induced drug resistance through the stabilization of mitochondrial transmembrane potential. Our results demonstrate that Pim-1 is a pivotal regulator involved in hypoxia-induced chemoresistance. Targeting Pim-1 may improve the chemotherapeutic strategy for solid tumors.

The seventh pathogenic fusion gene FIP1L1-RARA was isolated from a t(4;17)-positive acute promyelocytic leukemiaThe majority of acute promyelocytic leukemia (APL) cases are characterized by the expression of the chimeric fusion gene PML-RARA. Although the PML-RARA fusion gene is detected in more than 95% of APL cases, RARA has also been found to fuse with other partner genes in some APL variants. To date, five such partner genes have been reported: PLZF, NPM, NuMA, Stat5b and PRKAR1A. 1,2These fusion gene products however, must meet a number of common prerequisites for APL pathogenesis to ensue. The RARA gene portion of the fusion gene products ought to be from exon 3, and the fusion gene products must form homodimers as well as repress retinoic acid-responsive transcriptional activity.3,4 We hereby report the cloning of a seventh fusion gene from an APL variant and the functional characterization of its product.A 90 year-old woman was clinically diagnosed for APL. The karyotype was 47, XX, t(4;17)(q12;q21), +8. FISH analysis showed that 94% of the bone marrow cells had the RARA split signal without the PML-RARA fusion signal ( Figure 1A).To identify the 5'-fusion partner of RARA, we adopted the 5'-RACE method (5'-Full RACE Core Set, Takara Bio) according to the manufacturer's instructions. Briefly, the reverse primer 5'-GCGCTTTGCGCACCT-3' was designed, which was complimentary for exon 3 of the RARA gene. Following reverse transcription using total mRNA from the patient's bone marrow cells, the cDNA obtained was ligated by T4 RNA ligase. The ligated product was amplified by the nested polymerase chain reaction (PCR). PCR primer sequences were as follows: 1st PCR primers (5'-CTGCAGAAGTGCTTTGAAGT-3', 5'-CACCTTGTTGATGATGCAGT-3') and 2 nd PCR primers (5'-GAGTGCTCTGAGAGCTACAC-3', 5'-CGGTGA-CACGTGTACACCAT-3'). The products obtained were cloned and sequenced directly. As a result, FIP1L1 was identified as the fusion partner of RARA ( Figure 1B). The RARA portion in this case starts, as expected, from exon 3 and is fused to exon 15 of FIP1L1. While cloning the full length FIP1L1-RARA, we isolated three isoforms of FIP1L1-RARA; all of these isoforms are in-frame ( Figure 1C). We also confirmed the mRNA expression of RARA-FIP1L1 by means of RT-PCR analysis (data not shown). FIP1L1 is known to form a fusion gene with PDGFRA that causes chronic eosinophilic leukemia.5 In a similar fashion to FIP1L1-PDGFRA, which produces several isoforms caused by alternative splicing, the isoforms of FIP1L1-RARA also seemed to be generated by alternative splicing of the FIP1L1 portion.6 FIP1L1-RARA was recently isolated from a case of juvenile myelomonocytic leukemia (JMML).7 In the JMML case, as in our case, the fusion gene was generated between exon 15 of FIP1L1 and exon 3 of RARA. At the moment, the reason why FIP1L1-RARA causes two different phenotypes of leukemia is unknown, nevertheless we propose two hypotheses. One possibility is that the difference in cell lineage derived from the identical fusion gene may be due to some addition...