The Effects of Laser Irradiation on Osteoblast and Osteosarcoma Cell Proliferation and Differentiation In Vitro Renno, A.C.M.1, McDonnell, P.A.2, Parizotto, N. A 1, Laakso, E- L.3 1Physical Therapy Department, Federal University of Sao Carlos, Sao Carlos, Brazil. Rod. Washinton Luiz, km 235, Sao Carlos, SP, 13565-2081, Brazil. 2School of Biomolecular and Biomedical Science, Griffith University, Nathan, Queensland, Australia. 3School of Physiotherapy and Exercise Science, Griffith University, Gold Coast, Queensland, Australia. Correspondence concerning this manuscript should be sent to: Ana Claudia Muniz Renno. Department of Physiotherapy, Federal University of S㯠Carlos. CEP 13565-905-S㯠Carlos, SP. Brasil. Phone: (0055-16) 3351-8345 / Fax: (0055-16) 33518630 E-mail: acmr_ft@yahoo.com.br Key words: alkaline phosphatase assay, cell proliferation, low-level laser therapy, osteoblast, osteosarcoma. Running title: low-level laser irradiation on healthy and carcinogenic bone cells Abstract Background and Objective: Some studies have shown that laser phototherapy is able to stimulate the osteogenesis of bone tissue, increasing osteoblast proliferation and accelerating fracture consolidation. It is suggested that laser may have a biostimulatory effect in tumor cells. However, the mechanism by which laser acts on cells is not fully understood. The aim of this study was to investigate the effects of 670 nm, 780 nm and 830 nm laser irradiation on cell proliferation of normal primary osteoblast (MC3T3) and malignant osteosarcoma (MG63) cell lines in vitro. Material and Methods: Neonatal, murine, calvarial, osteoblastic and human osteosarcoma cell lines were studied. A single laser irradiation was performed at three different wavelengths, at the energies of 0.5, 1, 5 and 10 J/cm2. Twenty-four hours after laser, cell proliferation and alkaline phosphatase assays were assessed. Results: Osteoblast proliferation increased significantly after 830 nm laser irradiation (at 10 J/cm2) but decreased after 780 nm laser irradiation (at 1, 5 and 10 J/cm2). Osteosarcoma cell proliferation increased significantly after 670 nm (at 5 J/cm2) and 780 nm laser irradiations (at 1, 5 and 10 J/cm2), but not after 830 nm laser irradiation. Alkaline phosphatase (ALP) activity in the osteoblast line was increased after 830 nm laser irradiation at 10J/cm2, whereas ALP activity in the osteosarcoma line was not altered, irrespective of laser wavelength or intensity. Conclusion: Based on the conditions of this study, we conclude that each cell line responds differently to specific wavelength and dose combinations. Further investigations are required to investigate the physiological mechanisms responsible for the contrasting outcomes obtained when using laser irradiation on cultured normal and malignant bone cells.