Background/Purpose: Tankyrase is a poly (ADP-ribose) polymerase that leads to ubiquitination and degradation of target proteins. Axin, a regulatory protein of Wnt/β-catenin signaling, is one of the target proteins of tankyrase, therefore tankyrase inhibitors function as a Wnt/β-catenin inhibitor. Tankyrase has recently been reported to degrade an adaptor protein SH3BP2 (SH3 domain-binding protein 2). We have previously reported that SH3BP2 gain-of-function mutation in murine bone marrow-derived macrophages enhances RANKL-induced osteoclastogenesis. Though the interaction between tankyrase and SH3BP2 has been reported, it is not fully elucidated whether tankyrase is involved in osteoclastogenesis. In this study, we investigated the role of tankyrase in bone metabolism.

Methods: Primary murine bone marrow-derived macrophages and murine preosteoclastic RAW264.7 cells were treated with RANKL in the presence of tankyrase inhibitors (IWR1 or XAV-939) and Wnt inhibitors (ICG001 or IWP2). Osteoclasts were visualized by a tartrate-resistant acid phosphatase (TRAP) staining, and TRAP-positive multinucleated cells (TRAP+ MNCs) were counted as osteoclasts. Expression levels of osteoclast-associated genes (Cathepsine K, Oscar, Acp5 and Dc-stamp) were measured by quantitative PCR analysis. Osteoclastic function was assessed by resorption assay. To determine the intracellular mechanisms, protein expression patterns of SH3BP2, NFATc1, and Syk were examined by western blotting.

Results:  Both IWR1 and XAV-939 enhanced RANKL-induced TRAP+ MNCs formation, osteoclast-associated genes expression, and mineral resorbing activity. SH3BP2 protein levels were elevated in cells treated with tankyrase inhibitors but not with Wnt inhibitors. Tankyrase inhibitors significantly augmented nuclear localization of NFATc1 and phosphorylation of Syk in response to RANKL. Finally, FK506, an NFATc1 inhibitor, fully abolished the promoting effect of tankyrase inhibitors on osteoclast formation.

Conclusion:  These findings suggest that the inhibition of tankyrase enhances osteoclastogenesis through activating Syk and NFATc1 via elevated SH3BP2 expression. Our findings highlight the undetermined effect of tankyrase inhibition in addition to its suppressive effect on Wnt/β-catenin signaling. Modulating tankyrase activity could be a novel therapeutic option of bone destructive diseases including osteoporosis and rheumatoid arthritis.

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