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Peroxiredoxin II (Prx II), an antioxidant enzyme in the Prx family, reduces oxidative stress by decreasing the intracellular ROS levels. Osteoblast differentiation is promoted by bone morphogenetic protein 2 (BMP2), which upregulates the expression of osteoblast differentiation marker genes, through Smad1/5/9 phosphorylation. We found that Prx II expression was increased by a high dose of lipopolysaccharide (LPS) but was not increased by a low dose of LPS. Prx II itself caused a decrease in the osteogenic gene expression, alkaline phosphatase (ALP) activity, and Smad1/5/9 phosphorylation induced by BMP2. In addition, BMP2-induced osteogenic gene expression and ALP activity were higher in Prx II knockout (KO) cells than they were in wild-type (WT) cells. These inhibitory effects were mediated by protein phosphatase 2A Cα (PP2A Cα), which was increased and is known to induce the dephosphorylation of Smad1/5/9. The overexpression of Prx II increased the expression of PP2A Cα, and PP2A Cα was not expressed in Prx II KO cells. Moreover, PP2A Cα reduced the level of BMP2-induced osteogenic gene expression and Smad1/5/9 phosphorylation. LPS inhibited BMP2-induced Smad1/5/9 phosphorylation and the suppressed phosphorylation was restored by the PP2A inhibitor okadaic acid (OA). Bone phenotype analyses using microcomputed tomography (μCT) revealed that the Prx II KO mice had higher levels of bone mass than the levels of the WT mice. We hypothesize that Prx II has a negative role in osteoblast differentiation through the PP2A-dependent dephosphorylation of Smad1/5/9.

Kisspeptin-10 (KP-10) acts as a tumor metastasis suppressor via its receptor, G-protein-coupled receptor 54 (GPR54). The KP-10-GPR54 system plays an important role in embryonic kidney development. However, its function in osteoblast differentiation is unknown. Osteoblast differentiation is controlled by a range of hormones and cytokines, such as bone morphogenetic protein (BMPs), and multiple transcription factors, such as Runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP), and Distal-less homeobox 5 (Dlx5). In the present study, KP-10-treatment significantly increased the expression of osteogenic genes, including mRNA and protein levels of BMP2, in C3H10T1/2 cells. Moreover, KP-10 induced BMP2-luc activity and increased phosphorylation of Smad1/5/9. In addition, NFATc4 specifically mediated KP-10-induced BMP2 gene expression. However, KP-10 treatment did not induce expression of the BMP2 and Runx2 genes in GPR54−/− cells. To examine whether KP-10 induced secretion of BMP2 to the culture medium, we used the conditioned-medium (C.M) of KP-10 treated medium on C3H10T1/2 cells. Dlx5 and Runx2 expressions were higher in GPR54−/− cells treated with C.M than in those treated with KP-10. These results demonstrate that BMP2 protein has an autocrine effect upon KP-10 treatment. Taken together, these findings suggest that KP-10/GPR54 signaling induces osteoblast differentiation via NFATc4-mediated BMP2 expression.

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