Low-level laser irradiation (LLLI) has been shown to induce bone formation and osteoblast differentiation both in vivo and in vitro. However, the molecular mechanism by which LLLI stimulates osteoblast differentiation is still unclear. The aim of the present study was to examine whether Ga-Al-As laser irradiation could enhance BMP2-induced alkaline phosphatase (ALP) activity in C2C12 cells. Laser irradiation at 0.5 W for 20 min enhanced BMP2-induced ALP activity. Laser treatment alone did not affect ALP activity. To exclude the effect of pH or temperature changes during irradiation, we shortened the exposure time to 2 min, with various levels of laser power. At 2.5 W, irradiation stimulated BMP2-induced ALP activity but not cell proliferation, whereas 1 or 5 W laser power did not induce any significant effects. Irradiation stimulated BMP2-induced phosphorylation of Smad1/5/8 and BMP2 expression, but had no effect on the expression of inhibitory Smads 6 and 7, BMP4, or insulin-like growth factor 1. Laser irradiation enhanced Smad-induced Id1 reporter activity as well as expression of bone morphogenetic protein (BMP)-induced transcription factors such as Id1, Osterix, and Runx2. Laser irradiation also stimulated BMP-induced expressions of type I collagen, osteonectin, and osteocalcin mRNA, markers of osteoblasts. This enhancement of BMP2-induced ALP activity and Smad phosphorylation by laser irradiation was also observed in primary osteoblasts. These results suggest that LLLI accelerates the differentiation of BMP-induced osteoblasts by stimulating the BMP/Smad signaling pathway.
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