Osteoporosis is a common disease characterized by bone mass reduction, leading to an increased risk of bone fracture, and it is caused by an imbalance of osteoblastic bone formation and osteoclastic bone resorption. Current osteoporosis drugs aim to reduce the risk of bone fracture, either by increasing osteoblastic bone formation or decreasing osteoclastic bone resorption. However, osteoblasts and osteoclasts are closely coupled, such that any reagent altering the differentiation or activity of one eventually affects the other. This tight coupling between osteoblasts and osteoclasts not only limits the therapeutic efficacy but also threatens the safety of osteoporosis drugs. This review will discuss the biological mechanisms of action of currently approved medications for osteoporosis treatment, focusing on the osteoblast–osteoclast coupling.

Inhibition of Rho-associated coiled coil-containing kinase (ROCK) has been reported to promote differentiation of neuronal cells. Here, we examined the effect of Y-27632, a ROCK inhibitor, on the outgrowth of neurites in PC12 cells. Y-27632 caused a rapid induction of neurite outgrowth in PC12 cells in a time-dependent manner. The neurite outgrowth, triggered by Y-27632, was accompanied by Rac1 activation, and was attenuated by Rac1 inhibitor NSC23766, in a concentration-dependent manner. Y-27632 also induced an increase in the production of reactive oxygen species (ROS). Pretreatment with N-acetylcysteine, an ROS scavenger, inhibited the ROS generation and neurite outgrowth in response to Y-27632. These results indicate that the activation of Rac1 and the generation of ROS contribute to the neurite outgrowth triggered by Y-27632 in PC12 cells.

In the present study, we investigated the effect of staurosporine on the formation of cellular processes in human gingival fibroblasts and rat astrocytes. Staurosporine caused a rapid induction of process formation in human gingival fibroblasts and rat astrocytes in a concentration dependent manner. The process formation of human gingival fibroblasts and rat astrocytes was prevented by the pretreatment with N-acetylcysteine, suggesting that staurosporine-induced ROS production was responsible for the process formation. Colchicine, a microtubule depolymerizing agent, inhibited the staurosporine-induced process formation, whereas cytochalasin D, an actin filament breakdown agent, failed to suppress the formation of cellular processes. This result indicated that polymerization of microtubule, and not actin filament, was responsible for the formation of cellular processes induced by staurosporine. In support of this hypothesis, Western blot analysis was conducted using anti-tubulin antibody, and the results showed that the amount of polymerized microtubule was increased by the treatment with staurosporine while that of depolymerized beta-tubulin in soluble fraction was decreased. These results indicate that staurosporine induces ROSmediated, microtubule-dependent formation of cellular processes in human gingival fibroblasts and rat astrocytes.

H₂O₂, a member of reactive oxygen species (ROS), is known to be involved in the mediation of physiological functions in a variety of cell types. However, little has been known about the physiological role of H₂O₂in exocrine cells. Therefore, in the present study, the effect of H₂O₂on cholecystokinin (CCK)-evoked Cα²+ mobilization and amylase release was investigated in rat pancreatic acinar cells. Stimulation of the acinar cells with sulfated octapeptide form of CCK (CCK-8S) induced biphasic increase in amylase release. Addition of 30μM H₂O₂ enhanced amylase release caused by 10 pM CCK-8S, but inhibited the amylase release induced by CCK-8S at concentrations higher than 100 pM. An ROS scavenger, 10 μM Mn(III)tetrakis(4-benzoic acid)porphyrin chloride, increased amylase release caused by CCK-8S at concentrations higher than 100 pM, although lower concentrations of CCK-8S-induced amylase release was not affected. To examine whether the effect of H₂O₂on CCK-8S-induced amylase release was exerted via modulation of intracellular Cα²+ signaling, we measured the changes in intracellular Cα²+ concentration ([Cα²+]i) in fura-2 loaded acinar cells. Although 30 μM H₂O₂did not induce any increase in([Cα²+]i by itself, it increased the frequency and amplitude of([Cα²+]i oscillations caused by 10 pM CCK-8S. However, 30μM H₂O₂had little effect on 1 nM CCK-8S-induced increase in [Cα²+]i. ROS scavenger, 1 mM N-acetylcysteine, did not affect [Cα²+]i changes induced by 10 pM or 1 nM CCK-8S. Therefore, it was concluded that 30 μM H₂O₂ enhanced low concentration of CCK-8S-induced amylase release probably by increasing [Cα²+]i oscillations while it inhibited high concentration of CCK-8S-induced amylase release.