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

Defective Osteoclastogenesis by IKKβ-null Precursors Is a Result of Receptor Activator of NF-κB Ligand (RANKL)-induced JNK-dependent Apoptosis and Impaired Differentiation*

It has been reported previously that inhibitory κB kinase (IKK) supports osteoclastogenesis through NF-κB-mediated prevention of apoptosis. This finding suggests that the ligand for receptor activator of NF-κB (RANKL), the master osteoclastogenic cytokine, induces apoptosis of osteoclast precursors (OCPs) in the absence of IKKβ/NF-κB competency. To validate this hypothesis, we sought to determine the pro-apoptotic signaling factors induced by RANKL in IKKβ-null osteoclast OCPs and to rescue osteoclast differentiation in the absence of IKKβ through their inhibition. To accomplish this, we generated mice that lack IKKβ in multiple hematopoietic lineages, including OCPs. We found that these mice possess both in vitro and in vivo defects in osteoclast generation, in concurrence with previous reports, and that this defect is a result of susceptibility to RANKL-mediated apoptosis as a result of gain-of-function of JNK activation. We demonstrate that differentiation of OCPs depends on IKKβ because reduced IKKβ mRNA expression correlates with impaired induction of osteoclast differentiation markers in response to RANKL stimulation. We further show that fine-tuned inhibition of JNK activation in these cells inhibits RANKL-induced apoptosis and restores the ability of IKKβ-null OCPs to become mature osteoclasts. Our data highlight the pro-osteoclastogenic and anti-apoptotic roles of IKKβ in OCPs and identify a pro-apoptotic mechanism activated within the RANK signalosome