Low-power laser irradiation (LPLI) has been found to induce various biological effects and cellular processes. Also, LPLI has been shown to promote fracture repair. Until now, it has been unclear how LPLI promotes bone formation and fracture healing. The aim of this study was to investigate the potential mechanism of LPLI-mediated enhancement of bone formation using mouse bone marrow mesenchymal stem cells (D1 cells). D1 cells were irradiated daily with a gallium-aluminum-arsenide (GaAlAs) laser at dose of 0, 1, 2, or 4 J/cm2. The lactate dehydrogenase (LDH) assay showed no cytotoxic effects of LPLI on D1 cells, and instead, LPLI at 4 J/cm2 significantly promoted D1 cell proliferation. LPLI also enhanced osteogenic differentiation in a dose-dependent manner and moderately increased expression of osteogenic markers. The neutralization experiments indicated that LPLI regulated insulin-like growth factor 1 (IGF1) and bone morphogenetic protein 2 (BMP2) signaling to promote cell proliferation and/or osteogenic differentiation. In conclusion, our study suggests that LPLI may induce IGF1 expression to promote both the proliferation and osteogenic differentiation of D1 cells, whereas it may induce BMP2 expression primarily to enhance osteogenic differentiation.
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