Taeko Yokochi

Induction and activation of the transcription factor NFATc1 (NFAT2) integrate RANKL signaling in terminal differentiation of osteoclasts.

Signaling by RANKL is essential for terminal differentiation of monocytes/macrophages into osteoclasts. The TRAF6 and c-Fos signaling pathways both play important roles downstream of RANKL. We show here that RANKL selectively induces NFATc1 expression via these two pathways. RANKL also evokes Ca(2+) oscillations that lead to calcineurin-mediated activation of NFATc1, and therefore triggers a sustained NFATc1-dependent transcriptional program during osteoclast differentiation. We also show that NFATc1-deficient embryonic stem cells fail to differentiate into osteoclasts in response to RANKL stimulation, and that ectopic expression of NFATc1 causes precursor cells to undergo efficient differentiation without RANKL signaling. Thus, NFATc1 may represent a master switch for regulating terminal differentiation of osteoclasts, functioning downstream of RANKL.

RANKL maintains bone homeostasis through c-fos-dependent induction of interferon-β

Osteoclasts are cells of monocyte/macrophage origin that erode bone matrix: regulation of their differentiation is central to the understanding of the pathogenesis and treatment of bone diseases such as osteoporosis. Signalling by RANKL (receptor activator of NF-κB ligand), also known as Tnfsf11, is essential for the induction of osteoclast differentiation, and it must be strictly regulated to maintain bone homeostasis. But it is not known whether RANKL signalling to the cell interior is linked to any regulatory mechanisms. Here we show that RANKL induces the interferon-β (IFN-β) gene in osteoclast precursor cells, and that IFN-β inhibits the differentiation by interfering with the RANKL-induced expression of c-Fos, an essential transcription factor for the formation of osteoclasts. This IFN-β gene induction mechanism is distinct from that induced by virus, and is dependent on c-Fos itself. Thus an autoregulatory mechanism operates—the RANKL-induced c-Fos induces its own inhibitor. The importance of this regulatory mechanism for bone homeostasis is emphasized by the observation that mice deficient in IFN-β signalling exhibit severe osteopenia (loss of bone mass) accompanied by enhanced osteoclastogenesis. Our study places the IFN-β system in a new context, and may offer a molecular basis for the treatment of bone diseases.

Antiviral response by natural killer cells through TRAIL gene induction by IFN–α/β

Natural killer (NK) cells play an important role in early defense against viral infection. The cytotoxic activity of NK cells is increased by interferon‐α/β (IFN‐α/β), produceden masse in virally infected cells. However, the mechanism(s) by which IFN‐α/β contribute to the NK‐cell‐mediated antiviral response is not well understood. Here we provide evidence that the cytotoxicity of NK cells is enhanced by IFN‐α/β through induction of TNF‐related apoptosis‐inducing ligand (TRAIL). Isolation and analysis of the murine TRAIL promoter revealed the presence of an IFN‐stimulated response element (ISRE), which binds to the transcription factor ISGF3 (interferon stimulated gene factor‐3). This promoter is indeed activated by IFN‐β in ISGF3‐dependent manner. We also show that virally infected cells, but not uninfected cells, are susceptible to TRAIL‐mediated cytotoxicity in vitro, and that the TRAIL expressed in NK cells is indeed crucial in limiting virus replication in vivo. Thus, our study reveals a new molecular link between IFN‐α/β signaling and activation of NK cells in antiviral response of the host.

CD8+ T Cell–Mediated Skin Disease in Mice Lacking IRF-2, the Transcriptional Attenuator of Interferon-α/β Signaling

Abstract The balanced action of cytokines is known to be critical for the maintenance of homeostatic immune responses. Here, we report the development of an inflammatory skin disease involving CD8 + T cells, in mice lacking the transcription factor, interferon regulatory factor-2 (IRF-2). CD8 + T cells exhibit in vitro hyper-responsiveness to antigen stimulation, accompanied with a notable upregulation of the expression of genes induced by interferon-α/β (IFN-α/β). Furthermore, both disease development and CD8 + T cell abnormality are suppressed by the introduction of nullizygosity to the genes that positively regulate the IFN-α/β signaling pathway. IRF-2 may represent a unique negative regulator, attenuating IFN-α/β-induced gene transcription, which is necessary for balancing the beneficial and harmful effects of IFN-α/β signaling in the immune system.

Cross talk of the interferon‐α/β signalling complex with gp130 for effective interleukin‐6 signalling

Background Signalling cross talk provides a molecular basis for modulating a given signalling pathway by another, and it is often critical for regulating cellular responses elicited by cytokines. Previously, we reported on the critical role of the IFN‐α/β signalling complex, generated by spontaneously produced IFN‐α/β, in efficient IFN‐γ signalling.

Regulation of IFN‐α/β genes: evidence for a dual function of the transcription factor complex ISGF3 in the production and action of IFN‐α/β

Background: Efficient production of interferons (IFNs) in virally infected cells is an essential aspect of the host defence. The transcription factor complex ISGF3 (IFN‐stimulated gene factor 3) was originally identified as a critical mediator of the IFN signal; it is formed upon IFN receptor (IFNR) stimulation and binds to ISREs (IFN‐stimulated response elements) to activate IFN‐inducible genes. It has recently been shown that the DNA binding component of ISGF3, p48 (ISGF3γ) also binds to virus‐inducible elements in the IFN‐α/β genes, suggesting a potential new role of p48 in IFN production.

Requirement of the IFN-alpha/beta-induced CXCR3 chemokine signalling for CD8+ T cell activation.

Background: Activation of both CD4+ T and CD8+ T cells is triggered by the engagement of the T cell antigen receptor (TCR) with MHC/peptide complexes on antigen‐presenting cells. This process also requires other molecular interactions, which transmit co‐stimulatory signals to these T cells. To ensure an effective immune response, distinct T cell subsets may additionally employ unique mechanism(s) for efficient activation.

Functional dissection of the cytoplasmic subregions of the IL-2 receptor βc chain in primary lymphocyte populations

The interleukin 2 (IL‐2) receptor βc chain (IL‐2Rβc) is known to regulate the development and function of distinct lymphocyte populations. Thus far, the functions of the IL‐2Rβc cytoplasmic subregions have been studied extensively by using cultured cell lines; however, this approach has limitations with respect to their functions in distinct primary lymphocyte populations. In the present study, we generated mice each expressing a mutant form of an IL‐2Rβc transgene, lacking the cytoplasmic A‐ or H‐region, on an IL‐2Rβc null background. We show that lack of the H‐region, which mediates activation of the Stat5/Stat3 transcription factors, selectively affects the development of natural killer cells and T cells bearing the γδ T cell receptor. This region is also required for the IL‐2‐induced proliferation of T cells in vitro, by upregulating IL‐2Rα expression. In contrast, the A‐region, which mediates activation of the Src family protein tyrosine kinase (PTK) members, contributes to downregulation of the T cell proliferation function. The IL‐2Rβc null mutant mice develop severe autoimmune symptoms; these are all suppressed following the expression of either of the mutants, suggesting that neither the Stats nor the Src PTK members are required. Thus, our present approach offers new insights into the functions of these cytoplasmic subregions of the IL‐2Rβc chain.