Oral infection is inflammatory disease caused by bacteria. A major component of gram negative bacteria membrane associated with inflammation is lipopolysaccharide (LPS). Currently, evidence presenting the combined effect of LPS and hypoxia to inflammatory response in human periodontal ligament cells (HPDLs) was yet lacking. Here, we studied whether the influence of oxygen on LPS-stimulated inflammatory cytokines in HPDLs. HPDLs were stimulated with LPS in normoxia and hypoxia for 24 h. The mRNA and protein expression of inflammatory cytokines were examined by polymerase chain reaction and enzyme-linked immunosorbent assay, respectively. The intracellular mechanisms of these effects were investigated by chemical inhibitors and small interfering RNA (siRNA). The results showed that LPS-stimulated IL-1β, IL-6, IL-8 in HPDLs in both hypoxia and normoxia. Hypoxia condition enhanced the effect of LPS-stimulated cytokines expression. Apigenin, the hypoxia-inducible factors (HIF)-1α inhibitor, totally prevented LPS-stimulated IL-1β expression in both normoxia and hypoxia. Similar to knockout HIF-1α gene expression by siRNA did \not prevent LPS-stimulated IL-1β expression. These data concluded that hypoxia increased virulence of LPS-stimulated IL-1β production in HPDLs.

Mechanical force was shown to promote IGF-1 expression in periodontal ligament both in vitro and in vivo. Though the mechanism of this effect has not yet been proved, here we investigated the molecular mechanism of intermittent mechanical stress on IGF-1 expression. In addition, the role of hypoxia on the intermittent compressive stress on IGF-1 expression was also examined. In this study, human periodontal ligament cells (HPDLs) were stimulated with intermittent mechanical stress for 24 hours. IGF-1 expression was examined by real-time polymerase chain reaction. Chemical inhibitors were used to determine molecular mechanisms of these effects. For hypoxic mimic condition, the CoCl2 supplementation was employed. The results showed that intermittent mechanical stress dramatically increased IGF-1 expression at 24 h. The pretreatment with TGF-β receptor I or TGF-β1 antibody could inhibit the intermittent mechanical stress-induced IGF-1 expression. Moreover, the upregulation of TGF-β1 proteins was detected in intermittent mechanical stress treated group. Correspondingly, the IGF-1 expression was upregulated upon being treated with recombinant human TGF-β1. Further, the hypoxic mimic condition attenuated the intermittent mechanical stress and rhTGF-β1-induced IGF-1 expression. In summary, this study suggests intermittent mechanical stress-induced IGF-1 expression in HPDLs through TGF-β1 and this phenomenon could be inhibited in hypoxic mimic condition.

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