The results obtained suggest that the changes of amount of RANKL and OPG may be involved in bone resorption as a response to compression force.

The ligand receptor activator of NFkappaB (RANKL) plays an important role in osteoclast formation. However, very little is known about the relationship between external apical root resorption during orthodontic treatment and RANKL. We hypothesized that compressive force is responsible for RANKL formation and up-regulation of osteoclastogenesis in periodontal ligament (PDL) cells from patients with severe orthodontically induced external apical root resorption. RANKL and osteoprotegerin (OPG) production, TRAP-positive cells, and resorptive pits were determined. The increase of RANKL and the decrease of OPG were greater in the severe root resorption group than in the non-resorption group. The numbers of TRAP-positive cells and resorptive pits were also increased in the severe root resorption group than in the non-resorption group. These results support the hypothesis that the compressed PDL cells obtained from tissues with severe external apical root resorption may produce a large amount of RANKL and up-regulate osteoclastogenesis.

Our results suggest that human pulp fibroblasts may be involved in the progress of inflammation in pulp tissue during orthodontic tooth movement, as they produced large amounts of IL-1 beta, IL-6, and TNF-alpha following stimulation with neuropeptides.

Low-energy laser irradiation (Ga-Al-As semiconductor laser, output 50 mW) was applied to rat osteoclast precursor cells for 1, 3, 6, or 10 min at 24-h intervals during the culture period. The number of tartrate-resistant acid phosphatase positive multinucleate cells was increased by approximately 1.3-fold in the 3- and 6-min irradiation groups. Further, osteoclasts appeared on day 2 in the laser irradiation groups but not until day 3 in the control groups. In immunohistochemical staining for receptor activator of NF-kappaB (RANK), the laser irradiation groups showed significantly greater amounts of staining in comparison with the control group on days 2 and 3. Reverse transcription-polymerase chain reaction results also showed that the expressions of RANK were upregulated. In the pit formation assay, resorption pits were significantly more abundant in the laser irradiation groups than in the controls. The present results suggest that low-energy laser irradiation facilitates differentiation and activation of osteoclasts via RANK expression.

Cathepsin is a typical and well-characterized lysosomal cysteine protease that, under pathological conditions, is involved in tissue destruction. A recent immunocytochemical study demonstrated that cathepsins B (CAB) and L (CAL) were localized in the periodontal ligament (PDL) of the rat molar, and they were expressed in compressed sites during experimental tooth movement. Further, we demonstrated previously that the levels of CAB and CAL in gingival crevicular fluid increased during orthodontic tooth movement. Therefore, CAB and CAL may play important roles in the process of collagen degradation during orthodontic tooth movement, and our in vitro study examined the secretion of CAB and CAL in PDL cells following mechanical stress. PDL cells were subjected to 0.5, 1.0, 2.0, or 3.0 g/cm2 of compression force or an increase in surface area by tension force of 0.28%, 0.95%, 1.72%, or 2.50% for 24 hr. For detection of CAB and CAL in conditioned medium, commercially available ELISA kits were used. We found compression and tension significantly increased the secretions of both CAB and CAL in PDL cells, which were exhibited in a time- and force magnitude-dependent manner. The compression-stimulated secretion of CAB was increased approximately 3-fold and that of CAL 4-fold, as compared with the control. Further, tension-stimulated secretion of CAB was increased by 1.5-fold and that of CAL 2-fold compared with the control. When analyzed using a semiquantitative polymerase chain reaction assay, CAB and CAL mRNA were increased in response to both compression and tension forces. These findings demonstrated that mechanical stress (compression and tension forces) causes an increase in secretion of CAB and CAL in PDL cells in vitro.