These terminally differentiated, multinucleated cells are derived from precursors in the monocyte/macrophage lineage. The primary cytokine mediating OC differentiation is receptor activator of NF-kB ligand, a member of the TNF superfamily. RANKL, working via its receptor RANK, commits early precursors to the OC fate, and causes fusion of these preosteoclasts to generate mature multinucleated cells. OCs attach to the bone surface, via avb3 integrins, forming a tight sealing zone that delineates a resorptive lacuna into which acid and matrixdegrading enzymes are secreted. The actin ring is a distinctive cytoskeletal structure that OCs must form in order to generate a sealing zone. Many signaling pathways, including those downstream of RANKL, appear to contribute to actin ring formation, but specific transcriptional programs have not been defined. Even before the identification of RANKL, NF-kB was identified as an important BAY-60-7550 customer reviews pathway in the context of bone when it was found that mice lacking both the p50 and p52 subunits were osteopetrotic, with a complete absence of OCs. More recent studies have defined two distinct NF-kB pathways, both of which are activated by RANKL in osteoclast lineage cells. The primary role of the classical pathway is to allow survival of OC precursors. In contrast, the alternative or non-canonical NFkB pathway controls OC differentiation, but not survival. It is initiated by the upstream kinase NIK, and culminates in transcription of target genes by RelB/p52 NF-kB dimers. This pathway is negatively regulated at 2 levels, by the instability of NIK protein and the retention of RelB in the cytoplasm by p100. In unstimulated cells, NIK interacts with TRAF3, leading to ubiquitination by cIAPs and degradation by the proteosome, keeping total cellular NIK levels very low. Upon RANKL stimulation, TRAF3 is degraded and NIK is stabilized in the cell. NIK then promotes processing of p100 to p52 by the proteosome, leading to accumulation of active RelB/p52 dimers in the nucleus. We have previously shown that absence of NIK or RelB in OCs blocks osteoclastogenesis, in vitro, and pathological osteolysis in the context of inflammation and bone metastasis, but has little effect on basal bone homeostasis. However, in these studies utilizing the globally NIK-deficient mouse, the complete lack of OC differentiation, in vitro, and the effect of NIK deletion in other cell types, in vivo, limited our ability to fully delineate the role of NIK and the alternative NF-kB pathway in the OC lineage. Recently, constitutive activation of NIK – by direct mutation or mutation of its negative regulators cIAP1/2 and TRAF3 – has been identified in multiple myeloma. This aberrant NIK activation leads to increased cell survival and proliferation of malignant plasma cells. Although it did not cause myeloma in mice, transgenic expression of a constitutively active NIK in B cells caused growth factor independent B cell hyperplasia.