Combined effects of proinflammatory cytokines and intermittent cyclic mechanical strain in inhibiting osteogenicity in human periodontal ligament cells

Abstract

Mechanical strain plays an important role in bone formation and resorption during orthodontic tooth movement. The mechanism has not been fully studied, and the process becomes complex with increased amounts of periodontal patients seeking orthodontic care. Our aims were to elucidate the combined effects of proinflammatory cytokines and intermittent cyclic strain (ICS) on the osteogenic capacity of human periodontal ligament cells. Cultured human periodontal ligament cells were exposed to proinflammatory cytokines (interleukin-1β 5 ng/mL and tumor necrosis factor-α 10 ng/mL) for 1 and 5 days, and ICS (0.5 Hz, 12% elongation) was applied for 4 h per day. The autocrine of inflammatory cytokines was measured by enzyme-linked immunosorbent assay. The expression of osteoblast markers runt-related transcription factor 2 and rabbit collagen type I was determined using real-time polymerase chain reaction and Western blot. The osteogenic capacity was also detected by alkaline phosphatase (ALP) staining, ALP activity, and alizarin red staining. We demonstrated that ICS impaired the osteogenic capacity of human periodontal ligament cells when incubated with proinflammatory cytokines, as evidenced by the low expression of ALP staining, low ALP activity, reduced alizarin red staining, and reduced osteoblast markers. These data, for the first time, suggest that ICS has a negative effect on the inductive inhibition of osteogenicity in human PDL cells mediated by proinflammatory cytokines.

Effects of mechanical forces on osteogenesis and osteoclastogenesis in human periodontal ligament fibroblasts

“…Of the forces used to investigate the tissues, 13 studies investigated compressive force only, 6,7,12,13,15,16,18,19,23,27,28,29,31 whereas ten focused on tensile force 8-11,14,20,22,24,25,30 and two focused on fluid shear stress. 21,26 Another study used both compression and tension approaches 17 (Table II).…”

“…Of these, six used the Flexcell strain unit, which exerts a vacuum on the underside of culture plates, thereby stretching the plate and the attached cells. 14,17,22,24,25,30 Four studies also applied substrate deformation-based approaches using other stretching apparatuses with mechanisms similar to that of the Flexcell system. 8,10,11,20 One study used a tension incubator to exert a continuous or intermittent tensile force.…”

“…9,17 Other studies have shown, however, that periodontal ligament fibroblasts exposed to tensile strain exhibited higher levels of TNF-α expression. 24,25 An increased tensile force resulting in 12% elongation was shown to induce TNF-α and IL-1β production, whereas a 1.5% elongation decreased TNF-α production. Although orthodontic tooth movement-induced bone remodelling is associated with an inflammatory process, the inflammatory responses of periodontal ligament fibroblasts to tensile force may be strain magnitude-dependent because low magnitudes were found to have an anti-inflammatory effect.…”

“…Two studies reported upregulated IL-1β production and reduced osteogenesis in periodontal ligament fibroblasts subjected to tensile strain. 24,25 Tensile force induces an inflammatory response during orthodontic tooth movement, as shown by increased IL-1β levels at sites of periodontal inflammation. This phenomenon is consistent with the finding that the IL-1β level in gingival crevicular fluid increases one hour after the application of orthodontic force.…”

“…Notably, the tensile force created by a negative pressure model differs from that of a substrate deformation-based model and the two are not comparable. 9,24,25…”