Biol. Pharm. Bull. 30(1) 27—31 (2007)

droxide formulations are currently used to accelerate the formation of hard tissue (dentin bridge) on the exposed pulp surface. Though this pulp-conserving coverage technique is practiced widely, hard tissue formation often does not progress smoothly towards a cure, necessitating pulp extraction. As dental pulp conservation in the root canal is considered to yield the best long-term outcome, and pulp extraction has a major impact on the life of the affected tooth, various pulp-conservation treatments are being sought. Recently, the clinical use of lasers has increased, and lasers are now employed frequently even for endodontic treatment. In particular, short wavelength Nd:YAG lasers and Ga-Al-As lasers are used for coagulation of deep tissue in the body, as well as for surgical treatment, hard tissue formation, and root canal therapy. Against this background, we have been studying the anti-inflammatory effect of GaAl-As laser irradiation when used as an adjuvant to accelerate hard tissue formation. Melcer et al. reported that dentin bridge formation on exposed pulp surfaces in monkeys was significantly more pronounced one month after CO2 laser irradiation than in non-irradiated controls. Utsunomiya reported that dentin bridge formation was observed earlier on exposed pulp surfaces in dogs at one week after Ga-Al-As laser irradiation than in non-irradiated controls. In contrast, research involving cultured HDP cells has shown that when cultured cells were exposed to laser irradiation, calcified nodule formation increased after long-term culture, and alkaline phosphatase (ALP) activity was significantly accelerated. We have investigated the effect of heating during Ga-Al-As laser irradiation on ALP activity and reported that the effects attributable to photochemical energy were greater than those attributable to thermal factors. However, issues such as cell-propagated signaling immediately after laser irradiation and hard tissue-related gene expression have received little attention. One well-known process of free radical production in the body is the transition from superoxide anions (O2 ) to hydrogen peroxide (H2O2), and thereby to hydroxyl radicals ( ·OH). Additional application of ultraviolet or X-ray stimulation during this process leads to significant production of active oxygen. Kashima-Tanaka et al. reported that ·OH was created from H2O2 by argon laser irradiation. Excess production of active oxygen species is considered to result in substances causing various diseases, but recent attention has focused on the fact that differing quantities of production have bactericidal and other bio-defensive roles, as well as enhancing cell-proliferative activity, and information signaling. Nishida et al. have reported that ·OH produced from H2O2 play a role in cell signaling by activation of cell membrane G-protein. Lee et al. have also reported that H2O2 significantly increases ALP activity and the formation of calcified nodules in MDPC-23 cells. Oxidative stress from active oxygen momentarily induces synthesis of heat shock proteins (HSP) as a cellular-level defense against damage. Among these, HSP27 is known to be expressed specifically by osteoblasts, and Kozawa et al. reported that HSP27 is intimately involved in osteoblastic hard tissue formation. Accordingly, our research has focused on free radicals as one mechanism for the transmission of laser photochemical energy to cells, and we have performed in vitro investigations involving laser irradiation of cultured HDP cells. Specifically, we investigated the effect of ·OH on HSP27 expression, together with the expression of BMP genes and proteins, and both the gene expression and activity of ALP, which are differentiation markers of accelerated hard tissue formation and calcified nodule formation.