Signaling involved in osteoblastic cell differentiation remains largely unknown. This study further investigates mechanisms involved in BMP-2-induced osteoblastic cell differentiation. We report that BMP-2 can activate JNK and p38 in osteoblastic cells and provide evidences that these MAP kinases have distinct roles in regulating alkaline phosphatase and osteocalcin expression. Introduction: Bone morphogenetic protein (BMP)-2 exerts many of its biological effects through activation of the Smad pathway. Cooperative interactions between the Smads and the stress-activated protein kinase (SAPK) p38 and c-Jun-NH2-terminal kinase (JNK) pathways have recently been observed in TGF-β signaling. Materials and Methods: Activation of mitogen-activated protein (MAP) kinases by BMP-2 and the role of these signaling pathways for cell differentiation induced by BMP-2 was investigated in mouse MC3T3-E1 and primary cultured calvaria-derived osteoblastic cells using immunoprecipitation, in vitro kinase assay and Western blot analysis, as well as specific MAP kinase inhibitors. Results: Associated with the rapid activation of Smads, BMP-2 barely affected extracellular-signal regulated kinase (ERK) activity, whereas it induced a transient activation of p38 and JNK. The role of p38 and JNK in mediating BMP-2-induced stimulation of osteoblastic cell differentiation was evaluated using the respective specific inhibitors SB203580 and SP600125. Inhibition of p38 by SB203580 was mainly associated with decreased alkaline phosphatase (ALP) activity, whereas inhibition of JNK by SP600125 was associated with a marked reduction in osteocalcin (OC) production induced by BMP-2. Corresponding alterations in ALP and OC mRNA levels were found in cells treated with BMP-2 and inhibitors, suggesting an implication of p38 and JNK pathways in BMP-2-induced osteoblastic cell differentiation at a transcriptional level. Conclusion: Data presented in this study describe p38 and JNK as new signaling pathways involved in BMP-2-induced osteoblastic cell differentiation with evidences for a distinct role of each MAP kinase in the control of alkaline phosphatase and osteocalcin expression.

Objective: Interleukin‐33 (IL‐33; or, IL‐1F11) was recently identified as the ligand of the IL‐1 family receptor T1/ST2. The aim of this study was to examine IL‐33 production in human and mouse joints and to investigate the role of IL‐33 and T1/ST2 in experimental arthritis.Methods: IL‐33 expression was examined in human synovial tissue, rheumatoid arthritis (RA) synovial fibroblasts, and arthritic mouse joints. Mice with collagen‐induced arthritis (CIA) were treated with blocking anti‐ST2 antibody or control antibody beginning at the onset of disease. Arthritis severity was assessed by clinical and histologic scoring. Draining lymph node (LN) cell responses were examined ex vivo, and joint messenger RNA (mRNA) was used for expression profiling.Results: IL‐33 was highly expressed in human RA synovium. In cultured synovial fibroblasts, IL‐33 expression was strongly induced by IL‐1β and/or tumor necrosis factor α. Furthermore, IL‐33 mRNA was detected in the joints of mice with CIA and increased during the early phase of the disease. Administration of a blocking anti‐ST2 antibody at the onset of disease attenuated the severity of CIA and reduced joint destruction. Anti‐ST2 antibody treatment was associated with a marked decrease in interferon‐γ production as well as with a more limited reduction in IL‐17 production by ex vivo–stimulated draining LN cells. Finally, RANKL mRNA levels in the joint were reduced by anti‐ST2 treatment.Conclusion: IL‐33 is produced locally in inflamed joints, and neutralization of IL‐33 signaling has a therapeutic effect on the course of arthritis. These observations suggest that locally produced IL‐33 may contribute to the pathogenesis of joint inflammation and destruction.

IL-36α (IL-1F6), IL-36β (IL-1F8), and IL-36γ (IL-1F9) are members of the IL-1 family of cytokines. These cytokines bind to IL-36R (IL-1Rrp2) and IL-1RAcP, activating similar intracellular signals as IL-1, whereas IL-36Ra (IL-1F5) acts as an IL-36R antagonist (IL-36Ra). In this study, we show that both murine bone marrow-derived dendritic cells (BMDCs) and CD4+ T lymphocytes constitutively express IL-36R and respond to IL-36α, IL-36β, and IL-36γ. IL-36 induced the production of proinflammatory cytokines, including IL-12, IL-1β, IL-6, TNF-α, and IL-23 by BMDCs with a more potent stimulatory effect than that of other IL-1 cytokines. In addition, IL-36β enhanced the expression of CD80, CD86, and MHC class II by BMDCs. IL-36 also induced the production of IFN-γ, IL-4, and IL-17 by CD4+ T cells and cultured splenocytes. These stimulatory effects were antagonized by IL-36Ra when used in 100- to 1000-fold molar excess. The immunization of mice with IL-36β significantly and specifically promoted Th1 responses. Our data thus indicate a critical role of IL-36R ligands in the interface between innate and adaptive immunity, leading to the stimulation of T helper responses.