Cheng Shaohong, Aghajanian Patrick, Pourteymoor Sheila, Alarcon Catrina, Mohan Subburaman
Musculoskeletal Disease Center, Veterans Affairs Loma Linda Healthcare System, 11201 Benton Street, Loma Linda, CA 92357, USA.
Department of Medicine, Loma Linda University, Loma Linda, CA 92354, USA.
Sci Rep. 2016 Oct 24;6:35748. doi: 10.1038/srep35748.
Endochondral ossification plays an important role in the formation of the primary ossification centers (POCs) and secondary ossification centers (SOCs) of mammalian long bones. However, the molecular mechanisms that regulate POC and SOC formation are different. We recently demonstrated that Prolyl Hydroxylase Domain-containing Protein 2 (Phd2) is a key mediator of vitamin C effects on bone. We investigated the role of Phd2 on endochondral ossification of the epiphyses by conditionally deleting the Phd2 gene in osteoblasts and chondrocytes. We found that the deletion of Phd2 in osteoblasts did not cause changes in bone parameters in the proximal tibial epiphyses in 5 week old mice. In contrast, deletion of Phd2 in chondrocytes resulted in increased bone mass and bone formation rate (normalized to tissue volume) in long bone epiphyses, indicating that Phd2 expressed in chondrocytes, but not osteoblasts, negatively regulates secondary ossification of epiphyses. Phd2 deletion in chondrocytes elevated mRNA expression of hypoxia-inducible factor (HIF) signaling molecules including Hif-1α, Hif-2α, Vegfa, Vegfb, and Epo, as well as markers for chondrocyte hypertrophy and mineralization such as Col10, osterix, alkaline phosphatase, and bone sialoprotein. These data suggest that Phd2 expressed in chondrocytes inhibits endochondral ossification at the epiphysis by suppressing HIF signaling pathways.
软骨内骨化在哺乳动物长骨的初级骨化中心（POC）和次级骨化中心（SOC）形成中起重要作用。然而，调节POC和SOC形成的分子机制有所不同。我们最近证明，含脯氨酰羟化酶结构域蛋白2（Phd2）是维生素C对骨骼作用的关键介质。我们通过有条件地删除成骨细胞和软骨细胞中的Phd2基因，研究了Phd2在骨骺软骨内骨化中的作用。我们发现，在5周龄小鼠的胫骨近端骨骺中，成骨细胞中Phd2的缺失并未导致骨参数发生变化。相反，软骨细胞中Phd2的缺失导致长骨骨骺的骨量和骨形成率（相对于组织体积）增加，这表明在软骨细胞而非成骨细胞中表达的Phd2对骨骺的次级骨化起负调节作用。软骨细胞中Phd2的缺失提高了缺氧诱导因子（HIF）信号分子的mRNA表达，包括Hif-1α、Hif-2α、Vegfa、Vegfb和Epo，以及软骨细胞肥大和矿化的标志物，如Col10、osterix、碱性磷酸酶和骨唾液蛋白。这些数据表明，软骨细胞中表达的Phd2通过抑制HIF信号通路抑制骨骺处的软骨内骨化。
