TSH, The Bone Suppressing Hormone

AbstractThe skeleton is a dynamic organ whose structural integrity depends on constant remodeling, controlled by many local and systemic factors. In this issue of Cell, Abe et al. (2003) identify thyroid-stimulating hormone (TSH) as an important regulator of this process

The IκB Function of NF-κB2 p100 Controls Stimulated Osteoclastogenesis

The prototranscription factor p100 represents an intersection of the NF-κB and IκB families, potentially serving as both the precursor for the active NF-κB subunit p52 and as an IκB capable of retaining NF-κB in the cytoplasm. NF-κB–inducing kinase (NIK) controls processing of p100 to generate p52, and thus NIK-deficient mice can be used to examine the biological effects of a failure in such processing. We demonstrate that treatment of wild-type osteoclast precursors with the osteoclastogenic cytokine receptor activator of NF-κB ligand (RANKL) increases both expression of p100 and its conversion to p52, resulting in unchanged net levels of p100. In the absence of NIK, p100 expression is increased by RANKL, but its conversion to p52 is blocked, leading to cytosolic accumulation of p100, which, acting as an IκB protein, binds NF-κB complexes and prevents their nuclear translocation. High levels of unprocessed p100 in osteoclast precursors from NIK−/− mice or a nonprocessable form of the protein in wild-type cells impair RANKL-mediated osteoclastogenesis. Conversely, p100-deficient osteoclast precursors show enhanced sensitivity to RANKL. These data demonstrate a novel, biologically relevant means of regulating NF-κB signaling, with upstream control and kinetics distinct from the classical IκBα pathway

NIK Stabilization in Osteoclasts Results in Osteoporosis and Enhanced Inflammatory Osteolysis

Maintenance of healthy bone requires the balanced activities of osteoclasts (OCs), which resorb bone, and osteoblasts, which build bone. Disproportionate action of OCs is responsible for the bone loss associated with postmenopausal osteoporosis and rheumatoid arthritis. NF-κB inducing kinase (NIK) controls activation of the alternative NF-κB pathway, a critical pathway for OC differentiation. Under basal conditions, TRAF3-mediated NIK degradation prevents downstream signaling, and disruption of the NIK:TRAF3 interaction stabilizes NIK leading to constitutive activation of the alternative NF-κB pathway.Using transgenic mice with OC-lineage expression of NIK lacking its TRAF3 binding domain (NT3), we now find that alternative NF-κB activation enhances not only OC differentiation but also OC function. Activating NT3 with either lysozyme M Cre or cathepsinK Cre causes high turnover osteoporosis with increased activity of OCs and osteoblasts. In vitro, NT3-expressing precursors form OCs more quickly and at lower doses of RANKL. When cultured on bone, they exhibit larger actin rings and increased resorptive activity. OC-specific NT3 transgenic mice also have an exaggerated osteolytic response to the serum transfer model of arthritis.Constitutive activation of NIK drives enhanced osteoclastogenesis and bone resorption, both in basal conditions and in response to inflammatory stimuli

NF-κB–inducing kinase controls lymphocyte and osteoclast activities in inflammatory arthritis

NF-κB is an important component of both autoimmunity and bone destruction in RA. NF-κB–inducing kinase (NIK) is a key mediator of the alternative arm of the NF-κB pathway, which is characterized by the nuclear translocation of RelB/p52 complexes. Mice lacking functional NIK have no peripheral lymph nodes, defective B and T cells, and impaired receptor activator of NF-κB ligand–stimulated osteoclastogenesis. We investigated the role of NIK in murine models of inflammatory arthritis using Nik–/– mice. The serum transfer arthritis model is initiated by preformed antibodies and required only intact neutrophil and complement systems in recipients. While Nik–/– mice had inflammation equivalent to that of Nik+/+ controls, they showed significantly less periarticular osteoclastogenesis and less bone erosion. In contrast, Nik–/– mice were completely resistant to antigen-induced arthritis (AIA), which requires intact antigen presentation and lymphocyte function but not lymph nodes. Additionally, transfer of Nik+/+ splenocytes or T cells to Rag2–/– mice conferred susceptibility to AIA, while transfer of Nik–/– cells did not. Nik–/– mice were also resistant to a genetic, spontaneous form of arthritis, generated in mice expressing both the KRN T cell receptor and H-2(g7). Thus, NIK is important in the immune and bone-destructive components of inflammatory arthritis and represents a possible therapeutic target for these diseases

RelA/p65 promotes osteoclast differentiation by blocking a RANKL-induced apoptotic JNK pathway in mice

Osteoclasts (OCs) function to reabsorb bone and are responsible for the bone loss associated with inflammatory arthritis and osteoporosis. OC numbers are elevated in most disorders of accelerated bone destruction, reflecting altered rates of precursor differentiation and apoptosis. Both of these processes are regulated by the JNK family of MAP kinases. In this study, we have demonstrated that the NF-κB subunit RelA/p65 inhibits JNK-mediated apoptosis during a critical period of commitment to the OC phenotype in response to the cytokine RANKL. This RelA/p65-mediated arrest of cell death led to enhanced OC differentiation. Hence, Rela–/– OC precursors displayed prolonged JNK activation in response to RANKL, and this was accompanied by an increase in cell death that prevented efficient differentiation. Although complete blockade of JNK activity inhibits osteoclastogenesis, both short-term blockade in RelA-deficient cultures and suppression of the downstream mediator, Bid rescued apoptosis and differentiation. These antiapoptotic effects were RelA specific, as overexpression of RelA, but not RelB, blocked apoptosis and rescued differentiation in Rela–/– precursors. Thus, RelA blocks a RANKL-induced, apoptotic JNK-Bid pathway, thereby promoting OC differentiation. Consistent with this, mice lacking RelA/p65 in the hematopoietic compartment were shown to have a deficient osteoclastogenic response to RANKL and were protected from arthritis-induced osteolysis

Increased activity of NT3.catK osteoclasts in vitro.

<p><b>A–B</b>. Control and NT3.catK OC precursors were cultured with 20 ng/ml RANKL on bone slices for 10 days. (A)Bone slices were fixed and stained with peroxidase-conjugated wheat germ agglutinin to visualize the resoprtion pits. (B) Pit area was quantitated on bone slices from A. n = 3 per condition. <b>C–E</b>. Preosteoclasts were generated on plastic using 100 ng/ml RANKL for 2 days, then lifted and replated on bone for culture with 100 ng/ml RANKL for an additional 2 days. (C) Media was collected for culture CTX assay. n = 3. (D) Bone slices were fixed and stained with Alexa 488 phalloidin to visualize actin rings. Scale bar, 100 µm. (E) Actin ring area was quantified. * <i>p<0.05</i>; ** <i>p<0.01</i> versus controls.</p