Vanderbilt Center for Bone Biology, Vanderbilt University Medical Center, 1225F Medical Research Building IV, Nashville, TN 37232, USA.
Development. 2009 Dec;136(24):4143-53. doi: 10.1242/dev.043281. Epub 2009 Nov 11.
Activating transcription factor 4 (Atf4) is a leucine-zipper-containing protein of the cAMP response element-binding protein (CREB) family. Ablation of Atf4 (Atf4(-/-)) in mice leads to severe skeletal defects, including delayed ossification and low bone mass, short stature and short limbs. Atf4 is expressed in proliferative and prehypertrophic growth plate chondrocytes, suggesting an autonomous function of Atf4 in chondrocytes during endochondral ossification. In Atf4(-/-) growth plate, the typical columnar structure of proliferative chondrocytes is disturbed. The proliferative zone is shortened, whereas the hypertrophic zone is transiently expanded. The expression of Indian hedgehog (Ihh) is markedly decreased, whereas the expression of other chondrocyte marker genes, such as type II collagen (Col2a1), PTH/PTHrP receptor (Pth1r) and type X collagen (Col10a1), is normal. Furthermore, forced expression of Atf4 in chondrocytes induces endogenous Ihh mRNA, and Atf4 directly binds to the Ihh promoter and activates its transcription. Supporting these findings, reactivation of Hh signaling pharmacologically in mouse limb explants corrects the Atf4(-/-) chondrocyte proliferation and short limb phenotypes. This study thus identifies Atf4 as a novel transcriptional activator of Ihh in chondrocytes that paces longitudinal bone growth by controlling growth plate chondrocyte proliferation and differentiation.
激活转录因子 4（Atf4）是 cAMP 反应元件结合蛋白（CREB）家族的一种含亮氨酸拉链的蛋白。在小鼠中敲除 Atf4（Atf4(-/-)）会导致严重的骨骼缺陷，包括骨化延迟和骨量低、身材矮小和四肢短小。Atf4 在增殖和预肥大生长板软骨细胞中表达，表明 Atf4 在软骨细胞中具有自主功能，参与软骨内骨化过程。在 Atf4(-/-)生长板中，增殖性软骨细胞的典型柱状结构受到干扰。增殖区缩短，而肥大区短暂扩大。Indian hedgehog（Ihh）的表达显著降低，而其他软骨细胞标记基因的表达，如 II 型胶原（Col2a1）、甲状旁腺素/甲状旁腺素受体（Pth1r）和 X 型胶原（Col10a1），则正常。此外，在软骨细胞中强制表达 Atf4 会诱导内源性 Ihh mRNA 的表达，并且 Atf4 直接结合 Ihh 启动子并激活其转录。这些发现表明，在小鼠肢体外植体中通过药理学方法重新激活 Hh 信号通路可以纠正 Atf4(-/-)软骨细胞增殖和短肢表型。因此，本研究确定 Atf4 是软骨细胞中 Ihh 的一种新型转录激活因子，通过控制生长板软骨细胞的增殖和分化来调节纵向骨生长。
