Phosphorylation of Runx2, induced by cyclic mechanical tension via ERK1/2 pathway, contributes to osteodifferentiation of human periodontal ligament fibroblasts

Occlusal force is an important stimulus for maintaining periodontal homeostasis. This is attributed to the quality of human periodontal ligament fibroblasts (hPDLFs) that could transfer occlusal force into biological signals modulating osteoblst differentiation. However, few studies investigated the mechanism of occlusal force-induced osteodifferentiation of hPDLFs. In our study, we used the cyclic mechanical tension (CMT) at 10% elongation with 0.5 Hz to mimic occlusal force, and explored its effects on osteogenesis of hPDLFs. Firstly, elevated expressions of several osteoblast marker genes (Runx2, ATF4, SP7, OCN, and BSP), as well as activated ERK1/2 pathway were detected during CMT loading for 1, 3, 6, 12, 18, and 24 h. To gain further insight into how CMT contributed to those effects, we focused on the classic ERK1/2-Runx2 pathway by inhibiting ERK1/2 and overexpressing Runx2. Our results reflected that Runx2 overexpression alone could induce osteodifferentiation of hPDLFs. Meanwhile, CMT loading could intensify while combined ERK1/2 blockage could weaken this process. Furthermore, we found that CMT promoted Runx2 transcription and phosphorylation via ERK1/2; protein level of phospho-Runx2 (p-Runx2), rather than Runx2, was in parallel with mRNA expressions of SP7, OCN, and BSP. Taken together, our study proved that p-Runx2, elevated by CMT via ERK1/2 pathway, is the predominate factor in promoting osteoblast differentiation of hPDLFs.