Background/Purpose: Tankyrase is a poly (ADP-ribose) polymerase that leads to ubiquitination and degradation of target proteins. Since tankyrase inhibitors increase Axin protein, a negative regulator of Wnt pathway, they are widely used as Wnt inhibitors. Tankyrase inhibitors have attracted the attention as a promising drug candidate for cancer and fibrotic diseases, in which Wnt pathways are critical in the pathogenesis. Tankyrase has recently been reported to degrade an adaptor protein SH3BP2 (SH3 domain-binding protein 2). We have previously shown that SH3BP2 gain-of-function mutation enhances RANKL-induced osteoclastogenesis in murine bone marrow-derived macrophages (BMMs). Though the interaction between tankyrase and SH3BP2 has been reported, it is not fully elucidated whether tankyrase is involved in bone metabolism. In this study, we investigated the effect of tankyrase inhibition in bone metabolism in vitro and in vivo.

Methods: Primary murine BMMs from wild-type (WT) mice and SH3BP2 knockout (KO) mice and human peripheral blood mononuclear cells (PBMCs) were treated with tankyrase inhibitors (IWR-1 or G007-LK) in the presence of RANKL. Osteoclasts formation, function and intracellular signaling were analyzed by TRAP staining, resorption assay and western blotting, respectively. Primary calvarial osteoblasts from WT mice and SH3BP2 KO mice were also cultured with tankyrase inhibitors. Osteoblast differentiation and intracellular signaling were analyzed by qPCR, Alizarin red staining and western blotting. To examine in vivo effect of the tankyrase inhibitor, 7-week-old WT male mice were treated with G007-LK for 4 weeks, and then tibias and lumber vertebras were analyzed by micro-CT.

Results: In murine BMMs and human PBMCs culture, both tankyrase inhibitors enhanced osteoclast formation and function. Tankyrase inhibitors increased SH3BP2 protein and augmented phosphorylation of Syk and nuclear localization of NFATc1 in response to RANKL. Moreover, in SH3BP2 KO BMMs culture, tankyrase inhibitors did not promote osteoclast formation, indicating that the osteoclast-inducing effect is mediated by SH3BP2. Next, in primary calvarial osteoblasts culture, tankyrase inhibitors significantly increased osteoblast-associated genes expression and enhanced mineralization even though they have Wnt inhibitory effect. The osteogenic effect of tankyrase inhibitors were diminished by SH3BP2 deficiency, suggesting increased SH3BP2 expression enhanced osteoblastogenesis by surpassing the Wnt inhibitory effect. Finally, in vivo experiment, the administration of G007-LK significantly decreased trabecular bone volume of both tibias and vertebras.

Conclusion: Tankyrase inhibition upregulates both osteoclastogenesis and osteoblastogenesis through the accumulation of SH3BP2. Our findings highlight the role of tankyrase as the novel regulator of bone metabolism. Also, we demonstrated that in vivo administration of the tankyrase inhibitor induces bone loss. This indicates that we should carefully evaluate the potential adverse effect on bone when tankyrase inhibitors are applied to patients with cancer or fibrotic diseases.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/tankyrasewnt-inhibitor-upregulates-osteoclastogenesis-and-osteoblastogenesis-via-sh3bp2/