Guo Fengjin, Han Xiaofeng, Wu Zhimeng, Cheng Zhi, Hu Qin, Zhao Yunpeng, Wang Yingxiong, Liu Chuanju
Department of Cell Biology and Genetics, Core Facility of Development Biology, Chongqing Medical University, Chongqing 400016, China
Department of Cell Biology and Genetics, Core Facility of Development Biology, Chongqing Medical University, Chongqing 400016, China.
J Cell Sci. 2016 Feb 15;129(4):717-28. doi: 10.1242/jcs.169623. Epub 2015 Nov 2.
Our previous research has shown that the spliced isoform of XBP1 (XBP1s) is an important downstream mediator of BMP2 and is involved in BMP2-stimulated chondrocyte differentiation. Herein, we report that ATF6 and its cleaved N-terminal cytoplasmic domain (known as ATF6a) are expressed in growth plate chondrocytes. We find that these proteins are differentially induced during BMP2-triggered chondrocyte differentiation. This differential expression probably results from the activation of the ATF6 gene by Runx2 and its repression by the Sox6 transcription factor. Runx2 and Sox6 act through their respective binding elements on the ATF6 gene. When overexpressed, ATF6 and ATF6a intensify chondrogenesis; our studies demonstrate that under the stimulation of ATF6 and ATF6a, chondrocytes tend to be hypertrophied and mineralized, a process leading to bone formation. By contrast, lowering expression of ATF6a by use of its specific siRNA suppresses chondrocyte differentiation. Moreover, ATF6a interacts with Runx2 and augments the Runx2-mediated hypertrophication of chondrocytes. Importantly, overexpression and knockdown of ATF6a during the chondrocyte hypertrophy process also led to altered expressions of IHH and PTHrP (also known as PTHLH). Taken together, these findings indicate that ATF6a favorably controls chondrogenesis and bone formation (1) by acting as a co-factor of Runx2 and enhancing Runx2-incited hypertrophic chondrocyte differentiation, and (2) by affecting IHH and PTHrP signaling.
我们之前的研究表明，XBP1的剪接异构体（XBP1s）是BMP2的重要下游介质，参与BMP2刺激的软骨细胞分化。在此，我们报道ATF6及其切割后的N端胞质结构域（称为ATF6a）在生长板软骨细胞中表达。我们发现这些蛋白在BMP2触发的软骨细胞分化过程中受到不同程度的诱导。这种差异表达可能是由于Runx2激活ATF6基因以及Sox6转录因子对其的抑制所致。Runx2和Sox6通过它们在ATF6基因上各自的结合元件发挥作用。当ATF6和ATF6a过表达时，会增强软骨形成；我们的研究表明，在ATF6和ATF6a的刺激下，软骨细胞倾向于肥大和矿化，这一过程导致骨形成。相比之下，使用其特异性siRNA降低ATF6a的表达会抑制软骨细胞分化。此外，ATF6a与Runx2相互作用并增强Runx2介导的软骨细胞肥大。重要的是，在软骨细胞肥大过程中过表达和敲低ATF6a也会导致IHH和PTHrP（也称为PTHLH）表达的改变。综上所述，这些发现表明，ATF6a通过以下方式有利地控制软骨形成和骨形成：（1）作为Runx2的辅助因子，增强Runx2诱导的肥大软骨细胞分化；（2）影响IHH和PTHrP信号通路。
