Fusion of OTT to BSAC results in aberrant up-regulation of transcriptional activity.

OTT/RBM15-BSAC/MAL/MKL1/MRTF-A was identified as a fusion transcript generated by t(1;22)(p13;q13) in acute megakaryoblastic leukemia. Previous studies have shown that BSAC (basic, SAP, and coiled-coil domain) activates the promoters containing CArG boxes via interaction with serum response factor, and OTT (one twenty-two) negatively regulates the development of megakaryocytes and myeloid cells. However, the mechanism by which OTT-BSAC promotes leukemia is largely unknown. Here we show that OTT-BSAC, but not BSAC or OTT strongly activates several promoters containing a transcription factor Yin Yang 1-binding sequence. In addition, although BSAC predominantly localizes in the cytoplasm and its nuclear translocation is considered to be regulated by the Rho-actin signaling pathway, OTT-BSAC exclusively localizes in the nucleus. Moreover, OTT interacts with histone deacetylase 3, but this interaction is abolished in OTT-BSAC. Collectively, these functional and spatial changes of OTT and BSAC caused by the fusion might perturb their functions, culminating in the development of acute megakaryoblastic leukemia.

An antiapoptotic protein, c-FLIPL, directly binds to MKK7 and inhibits the JNK pathway.

Inhibition of NF-kappaB activation increases susceptibility to tumor necrosis factor (TNF)alpha-induced cell death, concurrent with caspases and prolonged c-Jun N-terminal kinase (JNK) activation, and reactive oxygen species (ROS) accumulation. However, the detailed mechanisms are unclear. Here we show that cellular FLICE-inhibitory protein (c-FLIP) is rapidly lost in NF-kappaB activation-deficient, but not wild-type fibroblasts upon TNFalpha stimulation, indicating that NF-kappaB normally maintains the cellular levels of c-FLIP. The ectopic expression of the long form of c-FLIP (c-FLIPL) inhibits TNFalpha-induced prolonged JNK activation and ROS accumulation in NF-kappaB activation-deficient fibroblasts. Conversely, TNFalpha induces prolonged JNK activation and ROS accumulation in c-Flip-/- fibroblasts. Moreover, c-FLIPL directly interacts with a JNK activator, MAP kinase kinase (MKK)7, in a TNFalpha-dependent manner and inhibits the interactions of MKK7 with MAP/ERK kinase kinase 1, apoptosis-signal-regulating kinase 1, and TGFbeta-activated kinase 1. This stimuli-dependent interaction of c-FLIPL with MKK7 might selectively suppress the prolonged phase of JNK activation. Taken that ROS promote JNK activation and activation of the JNK pathway may promote ROS accumulation, c-FLIPL might block this positive feedback loop, thereby suppressing ROS accumulation.

Genome wide analysis of TNF-inducible genes reveals that antioxidant enzymes are induced by TNF and responsible for elimination of ROS.

We recently showed that TNF induces accumulation of reactive oxygen species (ROS) that mediates necrosis in murine embryonic fibroblasts (MEFs) derived from TRAF2- and TRAF5-double deficient (DKO) mice. To elucidate the defects that subsequently cause accumulation of ROS in DKO MEFs, we compared gene expression profiles of wild-type and DKO MEFs before and after TNF stimulation using cDNA microarrays. Interestingly, many antioxidant enzymes are induced by TNF in wild-type MEFs, induction of these genes is impaired in DKO MEFs. Taken that TNF induces accumulation of ROS in DKO, but not wild-type MEFs, upregulation of antioxidant enzyme(s) might play a crucial role in elimination of ROS.

NF-kappaB inhibits TNF-induced accumulation of ROS that mediate prolonged MAPK activation and necrotic cell death.

NF-kappaB downregulates tumor necrosis factor (TNF)-induced c-Jun N-terminal kinase (JNK) activation that promotes cell death, but the mechanism is not yet fully understood. By using murine embryonic fibroblasts (MEFs) that are deficient in TNF receptor-associated factor (TRAF) 2 and TRAF5 (DKO) or p65 NF-kappaB subunit (p65KO), we demonstrate here that TNF stimulation leads to accumulation of reactive oxygen species (ROS), which is essential for prolonged mitogen-activated protein kinase (MAPK) activation and cell death. Interestingly, dying cells show necrotic as well as apoptotic morphological changes as assessed by electron microscopy and flow cytometry, and necrotic, but not apoptotic, cell death is substantially inhibited by antioxidant. Importantly, TNF does not induce ROS accumulation or prolonged MAPK activation in wild-type MEFs, indicating that TRAF-mediated NF-kappaB activation normally suppresses the TNF-induced ROS accumulation that subsequently induces prolonged MAPK activation and necrotic cell death

Phosphorylation of serine 276 is essential for p65 NF-kappaB subunit-dependent cellular responses.

Phosphorylation of several serine residues especially in the transactivation (TA) domain of p65 NF-kappaB subunit has been suggested to be important for its transcriptional activity. However, the responsible phosphorylation site of p65 remains controversial. To investigate the biological significance of phosphorylation and to determine the critical phosphorylation sites of p65, we reconstituted murine embryonic fibroblasts (MEFs) from p65(-/-) mice with various serine to alanine (SA)-substituted mutants of p65. Unexpectedly, mutants in the TA domain, including S529A, S536A, and S529A/S536A, completely rescued the defect of p65(-/-) MEFs as assessed by tumor necrosis factor (TNF)- or interleukin-1 (IL-1)-induced IL-6 production and protection from TNF-induced cell death. On the other hand, S276A mutant had an impaired ability to rescue these responses. Moreover, TNF-induced phosphorylation of p65 was severely impaired in S276A mutant, indicating that S276 is the major phosphorylation site of p65 and its phosphorylation is essential for p65-dependent cellular responses.

Identification of a novel transcriptional activator, BSAC, by a functional cloning to inhibit tumor necrosis factor-induced cell death.

Tumor necrosis factor (TNF) is a multifunctional cytokine, which induces proliferation or death in a cell type-dependent manner. We previously showed that murine embryonic fibroblasts (MEFs) from TNF receptor-associated factor 2 (Traf2) and Traf5 double-deficient (double knockout (DKO)) mice were highly susceptible to TNF-induced cell death. By functional cloning to rescue DKO MEFs from TNF-induced cell death, we have identified a novel gene, Bsac. BSAC is composed of N-terminal basic, SAP (SAF-A/B, Acinus, PIAS), and coiled-coil domains. BSAC is a nuclear protein, and overexpression of BSAC potently activates promoters containing A + T-rich sequences named CArG boxes. Domain mapping analysis revealed that both N-terminal basic and C-terminal proline-rich sequence are required for the transcriptional activity. Overexpression of BSAC in DKO MEFs partially inhibited TNF-induced cell death by suppressing activation of caspases. Interestingly, inhibition of TNF-induced cell death was not observed in DKO MEFs transfected with either N-terminal or C-terminal deletion mutant of BSAC, revealing an intimate correlation between transcriptional activity and antiapoptotic function. Recently, a human homologue of BSAC named MAL/MKL1 (megakaryocytic acute leukemia/megakaryoblastic leukemia-1) was identified as a fusion transcript generated by t(1,22) translocation in acute megakaryoblastic leukemia. Collectively, BSAC is a novel transcriptional activator with antiapoptotic function, which may be involved in the leukemogenesis.