Tohjima Emiko, Inoue Daisuke, Yamamoto Nobuchika, Kido Shinsuke, Ito Yuji, Kato Shuji, Takeuchi Yasuhiro, Fukumoto Seiji, Matsumoto Toshio
Department of Medicine and Bioregulatory Sciences, University of Tokushima Graduate School of Medicine, Tokushima, Japan.
J Bone Miner Res. 2003 Aug;18(8):1461-70. doi: 10.1359/jbmr.2003.18.8.1461.
Expression of an osteogenic cytokine, IL-11, is decreased in SAMP6. We show here that IL-11 transcription largely depends on AP-1 transcription factors, activities of which are decreased in SAMP6 as well as aged ICR mice. Therefore, diminished AP-1 activity and the resultant decline in IL-11 expression may play a role in impaired bone formation in the aged.
Evidence suggests that impaired osteoblastogenesis contributes to aging-associated osteopenia. The P6 strain of senescence-accelerated mice (SAM) is an animal model of senile osteoporosis, which exhibits low bone mass caused by impaired bone formation. Bone marrow stromal cells from SAMP6 show decreased osteoblastogenesis and increased adipogenesis. We previously demonstrated that these abnormalities of SAMP6 stromal cells may be attributed to decreased expression of interleukin (IL)-11.
In this study, we attempted to determine the molecular mechanism of decreased IL-11 expression by SAMP6 stromal cells by cloning and analyzing the mouse IL-11 gene promoter.
We found that two tandem activating protein-1 (AP-1) sites that reside immediately upstream of TATA box play critical roles in IL-11 gene transcription. Gel shift analysis showed that binding activity to the IL-11 AP-1 sites was reduced in SAMP6 stromal cell nuclear extracts. Among multiple components of AP-1 transcription factors, Jun D binding was particularly decreased. Furthermore, decreased Jun D binding and IL-11 expression by stromal cells was also observed in aged mice of the ICR strain. Therefore, decreased AP-1 activity and a resultant decline in IL-11 expression by bone marrow stromal cells may play a role in impaired bone formation in the aged.
成骨细胞因子IL-11在SAMP6小鼠中的表达降低。我们在此表明，IL-11转录很大程度上依赖于AP-1转录因子，而在SAMP6小鼠以及老年ICR小鼠中，这些转录因子的活性均降低。因此，AP-1活性降低以及由此导致的IL-11表达下降可能在衰老过程中骨形成受损中起作用。
有证据表明，成骨细胞生成受损会导致与衰老相关的骨质减少。衰老加速小鼠（SAM）的P6品系是老年性骨质疏松症的动物模型，其表现出由于骨形成受损导致的低骨量。来自SAMP6的骨髓基质细胞显示成骨细胞生成减少，脂肪生成增加。我们之前证明，SAMP6基质细胞的这些异常可能归因于白细胞介素（IL）-11表达降低。
在本研究中，我们试图通过克隆和分析小鼠IL-11基因启动子来确定SAMP6基质细胞中IL-11表达降低的分子机制。
我们发现位于TATA框上游紧邻的两个串联激活蛋白-1（AP-1）位点在IL-11基因转录中起关键作用。凝胶迁移分析表明，SAMP6基质细胞核提取物中与IL-11 AP-1位点的结合活性降低。在AP-1转录因子的多个组分中，Jun D的结合尤其减少。此外，在ICR品系的老年小鼠中也观察到基质细胞中Jun D结合和IL-11表达降低。因此，骨髓基质细胞中AP-1活性降低以及由此导致的IL-11表达下降可能在衰老过程中骨形成受损中起作用。
