ALP activity decreased in compressed PDL cells obtained from severe orthodontically root resorption

Alkaline phosphatase (ALP) plays an important role in periodontal ligament (PDL) itself and alveolar bone in relation of calcification. However, very little is known about the relationship between external apical root resorption (EARR) during orthodontic treatment and ALP.We examined the effect of compressive force (CF) on ALP activity in human PDL (HPDL) cells from patients with severe orthodontically induced EARR. ALP activity, PGE2 and IL-1β release were determined. The decrease of ALP and the increase of PGE2 and IL-1β were greater in the severe root resorption (SRR) group than in the non-resorption (NR) group (p < 0.001, two-way repeated measured ANOVA). NS-398, selective cyclooxygenase-2 (COX-2) inhibitor, and/or IL-1β antibody partially prevented the decrease in ALP activity. The presence of both factors together prevented almost completely the decrease in ALP activity to the control level (p < 0.001).Therefore, the decrease in ALP activity was mediated mainly by PGE2 and IL-1β produced by compressed PDL cells obtained from SRR. These results suggest that the decrease of ALP in compressed HPDL cells may be highly involved in the incidence of severe EARR during orthodontic treatment.