Facial jaw muscle is involved in the occurrence, development, treatment and maintenance of maxillofacial deformities. The structure and function of this tissue can be altered by changes in external stimuli, and orthodontists can regulate its reconstruction using orthopedic forces. The PI3K/Akt signaling pathway is most well-known for its biological functions in cell proliferation, survival and apoptosis. In the present study, the effects of the PI3K/Akt signaling pathway in cyclic stretch-induced myoblast apoptosis were investigated. For this purpose, L6 rat myoblasts were cultured under mechanical stimulation and treated with the PI3K kinase inhibitor, LY294002, to elucidate the role of the PI3K/Akt signaling pathway. Cells were stained with Hoechst 33258 to visualize morphological changes and apoptosis of myoblasts, and western blotting was performed to detect expression of Akt, phosphorylated (p)-Akt (Ser473), glycogen synthase kinase 3 beta (GSK-3 beta) and p-GSK-3 beta (Ser9). After addition of PI3K inhibitor, the expression of total Akt and GSK-3 beta did not significantly differ among groups; however, the levels of p-Akt and p-GSK-3 beta were lower in inhibitor-treated groups than in those treated with loading stress alone. In addition, the rate of apoptosis in myoblasts subjected to cyclic stretch increased in a time-dependent manner, peaking at 24 h. Collectively, it was also demonstrated that the PI3K/Akt/GSK-3 beta pathway plays an important role in stretch-induced myoblast apoptosis.

Aim: Mechanical strain plays a crucial role in bone formation and remodeling. Hypoxia-inducible factor (HIF)-1α and TWIST are upstream of master regulators of osteogenesis, including runt-related transcription factor 2 (RUNX2) and bone morphogenetic proteins (BMPs). This study investigated the effect of the HIF-1α-TWIST pathway on cyclic mechanical stretch-induced osteogenic differentiation of rat bone marrow mesenchymal stem cells (BMSCs) and the underlying mechanism. Materials and Methods: BMSCs were isolated from bone marrow derived from the femurs and humeri of Sprague-Dawley rats. Osteogenic differentiation of BMSCs was induced by applying cyclic mechanical stretch using the Flexcell Tension System. HIF-1α and TWIST were knocked down using recombinant lentiviral vectors. Osteogenic differentiation was evaluated by real-time qPCR, western blotting, and the alkaline phosphatase (ALP) activity assay. Results: Cyclic mechanical stretch increased ALP activity and expression of HIF-1α and TWIST in BMSCs. Knockdown of HIF-1α decreased TWIST expression in stretched BMSCs. Moreover, knockdown of HIF-1α or TWIST enhanced cyclic mechanical stretch-induced osteogenic differentiation of BMSCs. In addition, knockdown of TWIST increased expression of RUNX2 and BMP2 in stretched BMSCs. Conclusions: The HIF-1α-TWIST signaling pathway inhibits cyclic mechanical stretch-induced osteogenic differentiation of BMSCs. This finding may facilitate cell and tissue engineering for clinical applications.