Mechanisms of fracture healing in aged mice, which is dependent on COX-2 derived PGE_2

摘要：COX-2诱导的PGE2可由机械刺激提供，在老年小鼠骨折修复过程中对软骨内重塑阶段有重要影响。它不仅能协调VEGFA和基质金属蛋白酶等下游分子，而且还能调控成纤维细胞生长因子-2。简介：骨折愈合随年龄增长而减慢。为了研究COX-2在老年COX-2基因敲除小鼠延迟修复过程中的作用，我们应用了低强度脉冲超声(LIPUS)，我们先前报道过诱导COX-2并用于临床加速修复过程。方法：在一岁的COX-2KO小鼠及其野生型仔鼠上，用微聚焦X射线计算机断层扫描、免疫组织化学和mRNA分析来表征闭合性稳定型股骨骨折愈合的延迟。通过从骨折后第4天开始应用EP2和EP4联合激动剂，证实了从对照和LIPUS处理的骨痂的缺损处拯救到野生型骨痂的水平。结果：至第10天，软骨形成相对正常，此后修复过程减慢，LIPUS不再加速修复过程。然而，这种治疗有效地增加了野生型窝产仔的软骨内重建率，这是一个在衰老过程中延长的过程。在基因敲除的愈伤组织中，VEGFA和MMP-9等关键成分的转录在第7天完全缺失，无论暴露在LIPUS中。此外，在LIPUS未进一步升高的情况下，基因敲除组的基础成纤维细胞生长因子2水平显著升高。结论：COX-2对老年小鼠骨折愈合是必不可少的，如果没有COX-2，软骨内重塑将停止，充满间充质组织的骨折间隙将持续存在。除了失去下游VEGFA和基质金属蛋白酶外，增加的成纤维细胞生长因子-2可能进一步延缓重塑过程。

Abstract : COX-2-induced PGE2, which can be provided by mechanical stimulation, crucially affects endochondral remodeling phase in fracture repair of aged mice. It not only orchestrates downstream molecules such as VEGFa and matrix metalloproteinases, but also controls FGF-2. Introduction : Fracture healing slows with age. In order to characterize the role of COX-2 in the delayed repair process in aged COX-2 knockout mice, we applied low-intensity pulsed ultrasound (LIPUS), which, we previously reported, induces COX-2 and is clinically used to accelerate repair processes. Methods : In one-yr-old COX-2 KO mice and their wild type littermates, micro focused X-ray computed tomography, (immuno) histochemical and mRNA analyses were conducted to characterize delay in healing of closed stabilized femur fracture. By administering combined EP2 and EP4 agonists from day 4 post fracture, rescue from the defect of both control and the LIPUS treated callus was confirmed to the level of wild type counterparts. Results : Cartilage formed relatively normally up to day 10. Thereafter, the repair process slowed down and was not accelerated by LIPUS. The treatment, however, effectively increased the rate of endochondral remodeling in the wild type littermates, a process prolonged in senescence. In the knockout callus, transcripts of such critical components as VEGFa and MMP-9 were entirely missing at day 7, regardless of exposure to LIPUS. In addition, the basal FGF-2 level was significantly elevated in the knockouts without further elevation by LIPUS. Conclusion : COX-2 is indispensable for fracture healing in aged mice ; if absent, endochondral remodeling is halted and the fracture gap filled with mesenchymal tissue persists. In addition to the loss of downstream VEGFa and matrix metalloproteinases, increased FGF-2 is likely to further delay the remodeling process.