Mardiyantoro Fredy, Chiba Norika, Seong Chang-Hwan, Tada Ryohei, Ohnishi Tomokazu, Nakamura Norifumi, Matsuguchi Tetsuya
Department of Oral and Maxillofacial Surgery, Kagoshima University Graduate School of Medical and Dental Sciences, 8-35-1 Sakuragaoka, Kagoshima 890-8544, Japan.
Department of Oral Biochemistry, Kagoshima University Graduate School of Medical and Dental Sciences, 8-35-1 Sakuragaoka, Kagoshima 890-8544, Japan.
J Biochem. 2025 Feb 5;177(2):121-131. doi: 10.1093/jb/mvae080.
Osteopontin (OPN) is expressed in various cell types including osteoblasts. OPN expression level is robustly increased during osteoblast differentiation. Although OPN was initially found as a secretory protein (sOPN), recent reports identified the intracellular isoform of OPN (iOPN). Distinct functions of each OPN isoform in osteoblasts, however, are not well established. Here, using the Tet-On inducible expression system, we examined the role of each OPN isoform during osteoblast differentiation. Induced overexpression of wild type OPN (wtOPN), which includes both sOPN and iOPN, significantly increased matrix mineralization and osteogenic marker gene expression during osteogenic differentiation induced by either ascorbic acid or bone morphogenetic protein (BMP) 9. In contrast, these osteogenic differentiation processes were significantly inhibited by the specific overexpression of iOPN. Furthermore, the addition of recombinant OPN or neutralizing anti-OPN antibody to the culture medium exerted promotive or inhibitory effect on osteoblast differentiation, respectively. These data strongly indicate that iOPN exerts inhibitory effects on osteoblast differentiation, whereas sOPN exerts positive effects. We also found that the secretion process of OPN is positively regulated by c-Jun N-terminal kinase (JNK) activity in osteoblasts.
骨桥蛋白（OPN）在包括成骨细胞在内的多种细胞类型中表达。在成骨细胞分化过程中，OPN的表达水平会显著升高。尽管OPN最初被发现是一种分泌蛋白（sOPN），但最近的报道鉴定出了OPN的细胞内异构体（iOPN）。然而，每种OPN异构体在成骨细胞中的独特功能尚未完全明确。在此，我们使用Tet-On诱导表达系统，研究了每种OPN异构体在成骨细胞分化过程中的作用。野生型OPN（wtOPN，包括sOPN和iOPN）的诱导过表达在抗坏血酸或骨形态发生蛋白（BMP）9诱导的成骨分化过程中显著增加了基质矿化和成骨标记基因的表达。相比之下，iOPN的特异性过表达显著抑制了这些成骨分化过程。此外，向培养基中添加重组OPN或中和性抗OPN抗体分别对成骨细胞分化产生促进或抑制作用。这些数据有力地表明，iOPN对成骨细胞分化起抑制作用，而sOPN起积极作用。我们还发现，OPN的分泌过程在成骨细胞中受到c-Jun氨基末端激酶（JNK）活性的正向调节。
