title: The role of NF-κB regulator Bcl-3 in the skeleton
Creators: name: Jaffery, Hussain
Publisher: name: University of Glasgow
Year: 2018
Subjects: subject: Bcl-3, NF-κB, bone, osteoblasts, osteoclasts, osteogenesis, osteoclastogenesis, osteoarthritis, p50, skeleton, miRNA, RANKL, OPG, dwarfism, bone mineral density.

description: Bone and joint erosion and fragility fractures are associated with osteoarthritis and osteoporosis, and represent a major unmet clinical problem. In health, the balance between chondrocytes, osteoblasts and osteoclasts is a dynamic process under tight regulation. In disease, regulation is uncontrolled resulting in overt skeletal dysfunction and bone loss. NF-κB (nuclear factor κ-light-chain-enhancer of activated B cells) is a master regulator of cellular function and is an essential element in the development and homeostasis of the skeletal system. As such it is a critical controller of chondrocyte, osteoblast and osteoclast differentiation and function. Bcl-3 (B-cell lymphoma 3-encoded protein) is an atypical IκB protein and via its selective interaction with p50 and p52 homodimers of NF-κB is a regulator of cellular function. As a regulator of NF-κB, the role of Bcl-3 was hypothesised to be critical in affecting skeletal health. With the aim of defining the role of Bcl-3 in the skeleton, mice deficient in Bcl-3 (Bcl-3-/-) were compared to wild-type (Wt) mice. Ex vivo phenotypic analysis of Bcl-3-/- mice and in vitro cellular differentiation assays of Bcl-3-/- chondrocytes, osteoblasts and osteoclasts were performed. In neonatal Bcl-3-/- mice, multiple phenotypes were discovered, including dwarfism and increased mineral density. Morphometric analysis of developing and adult mice showed that 20-week Bcl-3-/- mice had a profoundly divergent phenotype from Wt. Male 20-week Bcl-3-/- mice had increased bone density in the trabecular and cortical regions and increased biomechanical strength, compared to Wt, implicating increased bone formation. Female 20-week Bcl-3-/- mice did not differ from Wt in the trabecular region; however, they possessed decreased bone area in cortical bone. Bone turnover rates of 20-week Bcl-3-/- males were lower than in Wt, implicating an accelerated rate of bone resorption, following peak bone density. While no differences in chondrocyte differentiation were found, Bcl-3-/- osteoblasts were found to have accelerated osteogenesis. Complementarily, simulated overexpression with Bcl-3 mimetic peptide restricted osteoblast differentiation. An expression repertoire of marker genes and miRNAs reflected increased RUNX2 (Runt-related transcription factor 2) activity in early-differentiating Bcl-3-/- osteoblasts. Compared to Wt, Bcl-3-/- osteoblasts had an altered whole-transcriptomic profile in early osteogenesis, with multiple NF-κB-driven osteogenic gene clusters identified. Osteoblasts increased expression of RANKL (RANK ligand) and decreased OPG (osteoprotegerin) expression, involved in signalling cross-talk with osteoclasts. Bcl-3-/- osteoclasts had increased intrinsic osteoclastogenesis and resorption, while treatment with Bcl-3 peptide restricted osteoclast differentiation and function. The ex vivo phenotypic results were complementary to in vitro assays, showing increased bone turnover. The lack of Bcl-3 in both, osteoblasts and osteoclasts, increased their differentiation and activities. Thus, Bcl-3 functions as an inhibitor of NF-κB-driven early osteogenesis and osteoclastogenesis. These findings help identify Bcl-3 as a novel target for the treatment of diseases involving skeletal pathology.