Naked cuticle homolog 2 (NKD2) has been recognized as an antagonist of Wnt/β-catenin signaling and a tumor suppressor. The role of NKD2 in osteoblast and osteoclast differentiation and the mechanism are not fully understood. In this study, we identified the up-regulation of NKD2 during osteoblast and adipocyte differentiation. Functional experiments revealed that NKD2 stimulated osteoblast differentiation and suppressed adipocyte formation. Furthermore, NKD2 down-regulated the expression of receptor activator of nuclear factor-κB ligand in bone marrow mesenchymal stem cells and inhibited osteoclast formation from osteoclast precursor cells. Mechanistic investigations revealed that the regulation of osteoblast and adipocyte differentiation by NKD2 involved Wnt/β-catenin and tuberous sclerosis complex subunit 1 (TSC1)/mechanistic target of rapamycin complex 1 (mTORC1) signaling pathways. Unlike in undifferentiated mesenchymal cells where NKD2 promoted Dishevelled-1 degradation, in the cells differentiating toward osteoblasts or adipocytes NKD2 down-regulated secreted frizzled related protein 1/4 expression and failed to destabilize Dishevelled-1, thereby activating Wnt/β-catenin signaling. Moreover, NKD2 bound to TSC1 and inhibited mTORC1 signaling. Further investigation uncovered an interplay between TSC1/mTORC1 and Wnt/β-catenin signaling pathways. Finally, transplantation of NKD2-overexpressing bone marrow mesenchymal stem cells into the marrow of mice increased osteoblasts, reduced osteoclasts and marrow fat, and partially prevented bone loss in ovariectomized mice. This study provides evidence that NKD2 in mesenchymal stem/progenitor cells reciprocally regulates the differentiation of osteoblasts and adipocytes by modulating Wnt/β-catenin and mTORC1 pathways and inhibits osteoclast formation by down-regulating receptor activator of nuclear factor-κB ligand. It suggests that NKD2 up-regulation may ameliorate postmenopausal bone loss.

裸角质层同源物 2 (NKD2) 已被认为是 Wnt/β-连环蛋白信号传导的拮抗剂和肿瘤抑制因子。NKD2在成骨细胞和破骨细胞分化中的作用及其机制尚不完全清楚。在这项研究中，我们发现了 NKD2 在成骨细胞和脂肪细胞分化过程中的上调。功能实验表明，NKD2 刺激成骨细胞分化并抑制脂肪细胞形成。此外，NKD2下调骨髓间充质干细胞中核因子-κB配体受体激活剂的表达，并抑制破骨细胞前体细胞形成破骨细胞。机制研究表明，NKD2 对成骨细胞和脂肪细胞分化的调节涉及 Wnt/β-catenin 和结节性硬化症复合物亚基 1 (TSC1)/雷帕霉素复合物机械靶点 1 (mTORC1) 信号通路。与在未分化间充质细胞中 NKD2 促进 Dishevelled-1 降解不同，在分化为成骨细胞或脂肪细胞的细胞中，NKD2 下调分泌型卷曲相关蛋白 1/4 的表达，并且无法破坏 Dishevelled-1 的稳定性，从而激活 Wnt/β-catenin 信号传导。此外，NKD2 与 TSC1 结合并抑制 mTORC1 信号传导。进一步的研究发现了 TSC1/mTORC1 和 Wnt/β-catenin 信号通路之间的相互作用。最后，将NKD2过表达的骨髓间充质干细胞移植到小鼠骨髓中可以增加成骨细胞数量，减少破骨细胞和骨髓脂肪，并部分防止卵巢切除小鼠的骨质流失。这项研究提供了证据，表明间充质干细胞/祖细胞中的NKD2通过调节Wnt/β-catenin和mTORC1通路相互调节成骨细胞和脂肪细胞的分化，并通过下调核因子-κB配体的受体激活剂抑制破骨细胞形成。这表明 NKD2 上调可能会改善绝经后骨质流失。