Background/Purpose: SH3BP2, an adaptor protein, is dominantly expressed in immune cells including macrophages and regulates intracellular signaling pathways. Gain-of-function mutations in SH3BP2 cause human genetic disorder cherubism, which is characterized by jaw bone destruction. We have previously reported that SH3BP2 plays an essential role in osteoclastogenesis and controls inflammatory bone destruction in murine arthritis models (Arthritis Rheumatol. 2015, 67(3):656-67; J Bone Miner Res. 2014, 29(12):2618-35). SH3BP2 protein levels are regulated by the degradation process mediated by Tankyrase 1/2. Tankyrase 1/2, poly (ADP ribose)polymerases, has recently been reported to degrade several proteins such as SH3BP2 and beta-catenin, however the involvement of Tankyrase in osteoclastogenesis is not well clarified. In this study, we investigated the role of Tankyrase in osteoclast differentiation and function.

Methods: Primary murine bone marrow-derived macrophages and murine pre-osteoclastic RAW264.7 cells were treated with either RANKL or TNF in the presence of Tankyrase inhibitors (IWR-1 and XAV-939) and Wnt inhibitors (IWP-2 and C59). SH3BP2 expression levels in the cells were determined by western blotting. Osteoclast differentiation was evaluated by TRAP-positive multinucleated cells (TRAP+ MNCs) formation and osteoclast-associated gene expression. Osteoclastic function was determined by resorption assay. Nuclear localization of NFATc1, a master regulator for osteoclastogenesis, was evaluated by immunofluorescence staining. 

Results: SH3BP2 protein levels were elevated in Tankyrase inhibitors-treated cells but not in Wnt inhibitors-treated cells. Tankyrase inhibitors enhanced both RANKL- and TNF-induced TRAP+ MNCs formation and osteoclast-associated genes (Oscar, Acp5, and Ctsk) expression in a concentration-dependent manner. Mineral resorbing activity in response to RANKL was significantly enhanced in Tankyrase inhibitors-treated cells. NFATc1 nuclear localization was significantly induced in the Tankyrase inhibitors-treated cells. The effects of the Tankyase inhibitors on osteoclastogenesis were comparable with the culture of primary bone marrow-derived macrophages from SH3BP2 gain-of-function mouse model (P416R SH3BP2 cherubism mutation knock-in mouse).

Conclusion: These data suggest that inhibition of Tankyrase activity enhances osteoclast differentiation via elevated SH3BP2 expression but not through Wnt pathway. Tankyrase is a novel regulator of osteoclastogenesis, therefore controlling Tankyrase activity could be a therapeutic option for bone destructive disorders including rheumatoid arthritis and osteoporosis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/tankyrase-regulates-osteoclastogenesis-via-sh3bp2-expression/