Makita Naoyuki, Suzuki Mitsuhiro, Asami Shiori, Takahata Rintaroh, Kohzaki Daika, Kobayashi Sho, Hakamazuka Takashi, Hozumi Nobumichi
Research Institute for Biological Sciences, Tokyo University of Science, Chiba, Japan.
Gene. 2008 Apr 30;413(1-2):8-17. doi: 10.1016/j.gene.2007.12.025. Epub 2008 Jan 26.
Runx2 is a Runt domain transcription factor that transcriptionally regulates osteoblast differentiation and bone formation. In this study, we show that human chondro- and osteosarcoma cell lines, human mesenchymal stem cells (hMSC) and a human primary chondrocytes (HC), osteoblst cells (HOb) express an intact isoform (RUNX2wt) and 3 alternatively spliced isoforms (RUNX2Delta5, Delta7, and Delta5Delta7) that are generated by skipping exon 5 and/or exon 7. Two of the truncated forms of RUNX2 (RUNX2Delta5 and RUNX2Delta5Delta7) did not localize in the nucleus and had lost their DNA binding activity. In cotransfection experiments with an osteocalcin (OC) promoter construct, we confirmed that only RUNX2wt and RUNX2Delta7 could upregulate the OC promoter activity in the osteosarcoma cell line. In addition, the coactivator CBP/p300 enhanced the transcriptional activity of the OC promoter when coexpressed with RUNX2wt or RUNX2Delta7, but not when coexpressed with RUNX2Delta5 or RUNX2Delta5Delta7. In contrast, the corepressor HDAC3 only repressed the activation from the OC promoter when coexpressed with RUNX2wt. These results support the hypothesis that RUNX2 both up- and downregulates its target gene promoters, as exemplified by the OC gene, using various isoforms and context-dependent formation of transcriptional complexes.
Runx2是一种Runt结构域转录因子，可转录调控成骨细胞分化和骨形成。在本研究中，我们发现人软骨肉瘤和骨肉瘤细胞系、人间充质干细胞（hMSC）以及人原代软骨细胞（HC）、成骨细胞（HOb）表达一种完整的异构体（RUNX2wt）和3种可变剪接异构体（RUNX2Delta5、Delta7和Delta5Delta7），它们是通过外显子5和/或外显子7的跳跃产生的。RUNX2的两种截短形式（RUNX2Delta5和RUNX2Delta5Delta7）不在细胞核中定位，并且失去了它们的DNA结合活性。在用骨钙素（OC）启动子构建体进行的共转染实验中，我们证实只有RUNX2wt和RUNX2Delta7能够上调骨肉瘤细胞系中OC启动子的活性。此外，共激活因子CBP/p300与RUNX2wt或RUNX2Delta7共表达时增强了OC启动子的转录活性，但与RUNX2Delta5或RUNX2Delta5Delta7共表达时则没有。相反，共抑制因子HDAC3仅在与RUNX2wt共表达时抑制OC启动子的激活。这些结果支持了这样一种假说，即RUNX2使用各种异构体和转录复合物的上下文依赖性形成，对其靶基因启动子进行上调和下调，以骨钙素基因为例。
