Interleukin-6 (IL-6) is a multifunctional cytokine produced by various cells to regulate hematopoiesis, inflammation, immune responses, and bone homeostasis. IL-6 is also known to modulate the differentiation of osteoblasts and osteoclasts. IL-6 is believed to play a positive regulatory role in osteoclast differentiation by inducing the expression of receptor activator of NF-B ligand (RANKL) on the surface of osteoblasts: RANKL then interacts with RANK expressed on osteoclast progenitors, inducing osteoclast differentiation via the RANK signaling pathway, which involves NF-B, JNK, and p38. In this report, we demonstrate that IL-6 can also directly act on osteoclast progenitors to suppress their differentiation via an inhibition of RANK signaling pathways. IL-6 specifically suppressed RANK-mediated IB degradation and JNK activation. Microarray analysis revealed that costimulation with IL-6 and RANKL up-regulates the transcription of MKP1 and MKP7, which encode enzymes that dephosphorylate JNK, and down-regulates the transcription of Senp2 and Cul4A, which are related to the ubiquitin pathway. Thus, IL-6 directly acts on osteoclast progenitors and suppresses their differentiation by regulating the transcription of specific genes related to MAPK phosphatases and the ubiquitin pathway.Bone tissue is continuously remodeled under physiological conditions (1), but dysregulation of the balance between bone formation and resorption induces pathological conditions, including osteoporosis and osteosclerosis. Bone remodeling is maintained by two key cell populations, osteoblasts and osteoclasts, which are regulated by cytokines, hormones, and growth factors (2, 3). Among the cytokines, the interleukin-6 (IL-6) 3 family cytokines are known to modulate both osteoblast and osteoclast differentiation (3-5).IL-6 is a multifunctional cytokine produced by various cell types that regulates hematopoiesis, acute phase reactions, immune responses, and bone homeostasis (6 -8). The receptor for IL-6 consists of a ligand-binding subunit and a common signal-transducing subunit, gp130 (9). gp130 contains a number of tyrosine residues in its cytoplasmic region, as well as four copies of the YXXQ motif, which is required for the tyrosine phosphorylation of STAT3. Activated STAT3 dimerizes, enters the nucleus, and regulates the transcription of various genes that regulate cell survival, proliferation, and differentiation in a cell-specific manner (6, 8). Tyr-759 of gp130 is required for the tyrosine phosphorylation of Src homology 2 domain-containing tyrosine phosphatase (SHP)-2, which activates the MAPK ERK via a complex comprising SHP2, Gab1/2, and phosphatidylinositol 3-kinase p85 (10 -12).It has been reported that IL-6 together with soluble IL-6 receptor acts to induce the expression of receptor activator of NF-B ligand (RANKL) on the surface of osteoblasts. RANKL interacts with RANK, which is expressed on the surface of osteoclast progenitors. Coculture experiments have demonstrated that this interaction is indispensable for the differen...

No abstract

Mutations in the TRPS1 gene lead to the tricho-rhino-phalangeal syndrome, which is characterized by skeletal defects and abnormal hair development. The TRPS1 gene encodes an atypical member of the GATA-type family of transcription factors. Here we show that mice with a disrupted Trps1 gene develop a chondrodysplasia characterized by diminished chondrocyte proliferation and decreased apoptosis in growth plates. Our analyses revealed that Trps1 is a repressor of Stat3 expression, which in turn controls chondrocyte proliferation and survival by regulating the expression of cyclin D1 and Bcl2. Our conclusion is supported (i) by siRNA-mediated depletion of Stat3 in Trps1-deficient chondrocytes, which normalized the expression of cyclin D1 and Bcl2, (ii) by overexpression of Trps1 in ATDC5 chondrocytes, which diminished Stat3 levels and increased proliferation and apoptosis, and (iii) by mutational analysis of the GATA-binding sites in the Stat3 gene, which revealed that their integrity is critical for the direct association with Trps1 and for Trps1-mediated repression of Stat3. Altogether our findings identify Trps1 as a novel regulator of chondrocytes proliferation and survival through the control of Stat3 expression.

Mast cells are thought to participate in a variety of immune responses, such as parasite resistance and the allergic reaction. Mast cell development depends on stem cell factor (Kit ligand) and its receptor, c-Kit. Gab2 is an adaptor molecule containing a pleckstrin homology domain and potential binding sites for SH2 and SH3 domains. Gab2 is phosphorylated on tyrosine after stimulation with cytokines and growth factors, including KitL. Gab2-deficient mice were created to define the physiological requirement for Gab2 in KitL/c-Kit signaling and mast cell development. In Gab2-deficient mice, the number of mast cells was reduced markedly in the stomach and less severely in the skin. Bone marrow-derived mast cells (BM-MCs IntroductionMast cells are hematopoietic-lineage cells that participate in immunoglobulin (Ig)E-associated immune responses, including allergic reactions and parasite resistance (see Galli 1 for a review). It was recently shown that mast cells also participate in the innate immunity to bacterial infection, in which IgE may not be involved. 2 Genetic evidence indicates that Kit ligand (KitL) and its receptor, c-Kit, play essential roles in mast cell development. Mutations in the mouse Kit ligand and c-Kit genes (Steel and White spotting) lead to defects in the development of melanocytes, germ cells, erythroid cells, basophils, and mast cells. 3-5 c-Kit is a receptor-type tyrosine kinase that displays some homology with platelet-derived growth factor receptors. The binding of KitL to c-Kit induces the dimerization and transphosphorylation of c-Kit. Tyrosyl-phosphorylated c-Kit recruits signaling molecules containing the Src homology 2 (SH2) domain, such as phosphatidyl inositol (PI)-3 kinase, 6 phospholipase C␥1, 7,8 Grb2, and the Src kinase, 9 to c-Kit and initiates cytoplasmic signaling. In addition to KitL/c-Kit signaling, interleukin (IL)-3 is also involved in mast cell development. IL-3-deficient mice maintain a basal level of mast cells, whereas mast cells fail to expand in response to infection by the nematode Stronglyoides venezuelensis. 10 Double-mutant Kit W /Kit W-v , IL-3 Ϫ/Ϫ mice display a more severe reduction in mast cell and basophil expansion elicited by the nematode infection than do single-mutant mice. 10 The result suggests that IL-3 is not essential for the generation of mast cells in a resting state but that it is required for the increase in mast cells in the immune response elicited by parasites. On the other hand, KitL-mediated signals are required for the development of the basal level of mast cells.Gab2 is a member of the Gab/DOS family of adapter molecules, which contain a pleckstrin homology (PH) domain and potential binding sites for the SH2 and SH3 domains. [11][12][13][14] Gab2 is tyrosine phosphorylated on stimulation by growth factors, cytokines, and Tand B-cell antigen receptors, including KitL and IL-3, and phosphorylated Gab2 binds SHP-2 and p85 PI-3 kinase. 11,15,16 Overexpression of Gab2 enhances the activation of cytokine-dependent ERK mitogen-activated protei...

Signal transducer and activator of transcription 3 (STAT3) mediates signals of various growth factors and cytokines, including interleukin-6 (IL-6). In certain IL-6-responsive cell lines, the stat3 gene is autoregulated by STAT3 through a composite IL-6 response element in its promoter that contains a STAT3-binding element (SBE) and a cyclic AMP-responsive element. To reveal the nature and roles of the stat3 autoregulation in vivo, we generated mice that harbor a mutation in the SBE (stat3 mSBE ). The intact SBE was crucial for IL-6-induced stat3 gene activation in the spleen, especially in the red pulp region, the kidney, and both mature and immature T lymphocytes. The SBE was not required, however, for IL-6-induced stat3 gene activation in hepatocytes. T lymphocytes from the stat3 mSBE/mSBE mice were more susceptible to apoptosis despite the presence of IL-6 than those from wild-type mice. Consistent with this, IL-6-dependent activation of the Pim-1 and junB genes, direct target genes for STAT3, was attenuated in T lymphocytes of the stat3 mSBE/mSBE mice. Thus, the tissue-specific autoregulation of the stat3 gene operates in vivo and plays a role in IL-6-induced antiapoptotic signaling in T cells.

We generated a series of knockin mouse lines, in which the cytokine receptor gp130-dependent STAT3 and/or SHP2 signals were disrupted, by replacing the mouse gp130 gene with human gp130 mutant cDNAs. The SHP2 signal-deficient mice (gp130F759/F759 were born normal but displayed splenomegaly and lymphadenopathy and an enhanced acute phase reaction. In contrast, the STAT3 signal-deficient mice (gp130FXQ/FXXQ) died perinatally, like the gp130-deficient mice (gp130D/D). The gp130F759/F759 mice showed prolonged gp130-induced STAT3 activation, indicating a negative regulatory role for SHP2. Th1-type cytokine production and IgG2a and IgG2b production were increased in the gp130F759/F759 mice, while they were decreased in the gp130FXXQ/FXXQ immune system. These results indicate that the balance of positive and negative signals generated through gp130 regulates the immune responses.

By using a genome-wide N-ethyl-N-nitrosourea (ENU)-induced dominant mutagenesis screen in mice, a founder with low bone mineral density (BMD) was identified. Mapping and sequencing revealed a T to C transition in a splice donor of the collagen alpha1 type I (Col1a1) gene, resulting in the skipping of exon 9 and a predicted 18-amino acid deletion within the N-terminal region of the triple helical domain of Col1a1. Col1a1Jrt /þ mice were smaller in size, had lower BMD associated with decreased bone volume/tissue volume (BV/TV) and reduced trabecular number, and furthermore exhibited mechanically weak, brittle, fracture-prone bones, a hallmark of osteogenesis imperfecta (OI). Several markers of osteoblast differentiation were upregulated in mutant bone, and histomorphometry showed that the proportion of trabecular bone surfaces covered by activated osteoblasts (Ob.S/BS and N.Ob/BS) was elevated, but bone surfaces undergoing resorption (Oc.S/BS and N.Oc/BS) were not. The number of bone marrow stromal osteoprogenitors (CFU-ALP) was unaffected, but mineralization was decreased in cultures from young Col1a1 Jrt /þ versus þ/þ mice. Total collagen and type I collagen content of matrices deposited by Col1a1Jrt /þ dermal fibroblasts in culture was $40% and 30%, respectively, that of þ/þ cells, suggesting that mutant collagen chains exerted a dominant negative effect on type I collagen biosynthesis. Mutant collagen fibrils were also markedly smaller in diameter than þ/þ fibrils in bone, tendon, and extracellular matrices deposited by dermal fibroblasts in vitro. Col1a1Jrt /þ mice also exhibited traits associated with Ehlers-Danlos syndrome (EDS): Their skin had reduced tensile properties, tail tendon appeared more frayed, and a third of the young adult mice had noticeable curvature of the spine. Col1a1Jrt /þ is the first reported model of combined OI/EDS and will be useful for exploring aspects of OI and EDS pathophysiology and treatment.

The role of microRNAs (miRNAs) in vascular calcification is currently unclear. To examine how miRNAs are involved in vascular smooth muscle cell (VSMC) calcification, we explored the alteration of miRNAs in VSMC calcification in vitro and in vivo. Klotho homozygous mutant mice (kl/kl) display vascular calcification and have perturbations of calcium handling. We therefore hypothesized that the calcium perturbations in VSMCs could be mediated by miRNAs. Using an miRNA array analysis, we demonstrated that miRNAs are aberrantly expressed in the aortic media of 3-week-old kl/kl mice compared with wild-type (WT) mice. The expression levels of miR-135a*, miR-762, miR-714, and miR-712* in the aortic media of kl/kl mice were significantly higher than in WT mice. We used quantitative real-time reverse transcriptase polymerase chain reaction to further confirm that these miRNAs were increased in the aortic media of kl/kl mice and in cultured VSMCs treated with high phosphate and calcium. A search of the miRNA database indicated that the Ca 2 þ efflux proteins NCX1, PMCA1, and NCKX4 frequently appeared as potential targets of these miRNAs. The transfection of miRNA mimics into cultured VSMCs reduced the protein levels of each potential target. Conversely, miRNA inhibitors reduced phosphate and calcium-induced VSMC calcification. Furthermore, these inhibitors decreased the intracellular Ca 2 þ concentration in cultured VSMCs after treatment with phosphate and calcium. Our results suggest that increased expression of miR-135a*, miR-762, miR-714, and miR-712* in VSMCs may be involved in VSMC calcification by disrupting Ca 2 þ efflux proteins.