Low-Level Laser Irradiation Stimulates RANKL-Induced Osteoclastogenesis via the MAPK Pathway in RAW 264.7 Cells  
[PERSON]  
1 Department of Oral and Maxillofacial Surgery, School of Dentistry, Pusan National University,  
Yangsan 50612, Korea; [EMAIL_ADDRESS]  
[PERSON]  
2 Dental and Life Science Institute, School of Dentistry, Pusan National University, Yangsan 50612, Korea; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Oral and Maxillofacial Surgery, School of Dentistry, Pusan National University,  
Yangsan 50612, Korea; [EMAIL_ADDRESS]  
[PERSON]  
2 Dental and Life Science Institute, School of Dentistry, Pusan National University, Yangsan 50612, Korea; [EMAIL_ADDRESS]  
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RANKL and its receptor RANK on the osteoclast induces the recruitment of cytoplasmic tumor necrosis factor receptor-associated factor 6 (TRAF6) activation, and this leads to the activation of downstream molecules, such as NFATc1, NF-kB, mitogen-activated protein kinases (MAPKs), c-Fos, and c-Jun [10; 11; 12; 13]. These transcription factors induce the complex responses of various genes, allowing the osteoclast progenitor cells to mature into multi-nuclear osteoclasts [14]. Mature osteoclasts degrade bone matrix proteins and inorganic components of bone by releasing various types of protein, including: Calcitonin receptor, tartrate resistant acid phosphatase (TRAP), cathepsin K, and matrix metalloproteinase-9 (MMP-9) [15; 16].  
Among medical applications using lasers, low-level laser therapy (LLLT) is recognized as an effective medical tool, and its development and research are growing rapidly [17; 18]. Medical applications of LLLT include the treatment of a multitude of conditions that require tissue repair, pain relief, inflammation treatment, and the restoration of tissue dysfunction [19]. When LLLT irradiates a living body, the photon LLLT light source is absorbed by intracellular pigments or photoreceptors to produce the following effects: Increased capillary production, cartilage production, nerve cell regeneration, muscle regeneration with relief of muscle atrophy, reduction in skin inflammation and swelling, and the promotion of collagen and bone formation [20; 21; 22; 23]. It was found that ATP synthesis and cytochrome c oxidase activity increased after LLLT irradiation in isolated mitochondria. This means that most of the energy-absorbing molecules for light are proteins, many of which are elements found in mitochondria, such as cytochromes [24]. Although dental applications of LLLT are not well documented compared to musculoskeletal applications, studies have recently been conducted in several major dental specialties, including endodontic, periodontal, and orthodontic treatment, and in maxillofacial surgery [25]. In particular, LLLT has been shown to be effective in managing chronic pain-related hard and soft tissue lesions in the maxillofacial region [26; 27]. Despite many laboratory and clinical reports, it is still controversial due to uncertainty about the underlying molecular and cellular mechanisms by which photons in LLLTs affect cells, i.e., the physical signals of light transformed into biological signals [28; 29; 30].  
Numerous studies have reported that LLLT stimulates osteoblasts or osteoblast-like cells and has a positive effect on bone regeneration [31; 32; 33; 34]. However, few studies have analyzed the molecular biology involved when applying LLLT to osteoclasts, and conclusions presented in studies have been controversial [35]. Therefore, in this study, we investigated the effect of LLLT on osteoclast differentiation using RAW 264.7 cells, and confirmed, at the cellular level, how much energy is appropriate when applying LLLT.