Low-energy laser irradiation affects satellite cell proliferation and differentiation in vitro

Low-energy laser (He-Ne) irradiation was found to promote skeletal muscle regeneration in vivo. In this study, its effect on the proliferation and differentiation of satellite cells in vitro was evaluated. Primary rat satellite cells were irradiated for various time periods immediately after preparation, and thymidine incorporation was determined after 2 days in culture. Laser irradiation affected thymidine incorporation in a bell-shaped manner, with a peak at 3 s of irradiation. Three seconds of irradiation caused an induction of cell-cycle regulatory proteins: cyclin D1, cyclin E and cyclin A in an established line of mouse satellite cells, pmi28, and proliferating cell nuclear antigen (PCNA) in primary rat satellite cells. The induction of cyclins by laser irradiation was compatible with their induction by serum refeeding of the cells. Laser irradiation effect on cell proliferation was dependent on the rat's age. At 3 weeks of age, thymidine incorporation in the irradiated cells was more than twofold higher than that in the controls, while at 6 weeks of age this difference had almost disappeared. Myosin heavy chain (MHC) protein levels were twofold lower in the irradiated than in the control cells, whereas the proliferation of the irradiated cells was twofold higher. Fusion percentage was lower in the irradiated compared to non-irradiated cells. In light of these data, the promoting effect of laser irradiation on skeletal muscle regeneration in vivo may be due to its effect on the activation of early cell-cycle regulatory genes in satellite cells, leading to increased proliferation and to a delay in cell differentiation.