Bone Cell Growth Regulation by Runx2/Cbfa1

我们的关键发现是，成骨细胞成熟促进转录因子Runx2控制着成骨前细胞的细胞周期进程，细胞周期进程通过转录和转录后机制调节三个不同细胞周期阶段(即G0/G1、G1/S和M/G1转变)的Runx2活性。Runx2在有丝分裂过程中发挥着关键的表观遗传功能，当正常成骨细胞监测细胞外信号以启动细胞周期进程时，它在G1早期表达。我们认为，正常成骨细胞中Runx2的选择性蛋白降解是进入S时相之前Runx2表达急剧下调的关键。 我们建议的中心假设是，Runx2多肽基序(特异性目标1)和Runx2泛素化(特异性目标2)在成骨细胞周期的G1晚期介导其选择性降解，以通过激活假定的细胞周期检查点来防止细胞生长抑制。 特定目标1将检查Runx2多肽基序，这些基序代表了启动蛋白质/蛋白质相互作用的假定分子继而的一部分，对于Runx2的降解是必需的。具体目标2将确定破坏Runx2稳定的E3泛素连接酶，并表征支持泛素连接的与成骨细胞相关的新辅助因子(例如，F-box蛋白)。

We have made the key discoveries that Runx2, the osteoblast maturation promoting transcription factor, controls cell cycle progression in pre-osteoblastic cells, and progression through the cell cycle modulates Runx2 activity at three different cell cycle stages (i.e., the G0/G1, G1/S and M/G1 transitions) by both transcriptional and post-transcriptional mechanisms. Runx2 performs a critical epigenetic function during mitosis and its expression in early G1 occurs when normal osteoblasts monitor extracellular cues for competency to initiate cell cycle progression. We propose that selective proteolytic degradation of Runx2 in normal osteoblasts is critical for acute downregulation of Runx2 prior to entry into S phase. The central hypothesis of our proposal is that Runx2 peptide motifs (Specific Aim 1) and ubiquitination of Runx2 (Specific Aim 2) mediate its selective degradation during late G1 in the osteoblast cell cycle to prevent cell growth suppression through activation of a putative cell cycle checkpoint. Specific Aim 1 will examine what Runx2 peptide motifs, which represent part of a putative molecular relay that stages protein/protein interactions, are required for Runx2 degradation. Specific Aim 2 will identify the E3 ubiquitin ligase that destabilizes Runx2 and characterize novel osteoblast-related co-factors (e.g., F-box proteins) that support ubiquitin ligation.