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.

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.