HMGB1-induced inflammatory response promotes bone healing in murine tooth extraction socket.

High mobility group box 1 (HMGB1) is a non-histone DNA-binding protein that is secreted into the extracellular milieu in response to inflammatory stimuli. The secreted HMGB1 has been suggested to mediate various inflammatory diseases. However, it is still unknown whether HMGB1 is involved in a healing process in the tooth extraction socket, the tissue containing gingival epithelium, and alveolar bone that is exposed to oral bacteria. In this study, we constructed a murine tooth extraction model with anti-HMGB1 neutralization antibody administration and observed the inflammatory response and bone healing process in tooth extraction sockets by molecular imaging of myeloperoxidase (MPO) activity, histological analysis, and quantitative RT-PCR. The translocation of HMGB1 from the nucleus to the cytoplasm in gingival epithelial cells and inflammatory cells was inhibited by anti-HMGB1 antibody administration. The MPO activity around the tooth extraction socket was significantly reduced, and the numbers of CD31- and CD68-positive cells were significantly lower in the anti-HMGB1 antibody treatment samples than in the control samples. The TRAP-positive cells, osteocalcin positive cells, and the neoplastic bone area were significantly lower in anti-HMGB1 antibody treatment samples than in control samples. The expression levels of IL-1ß and VEGF-A were also decreased in anti-HMGB1 antibody treatment samples compared to that in control samples. Secreted HMGB1 induced initial acute inflammation and inflammatory cells recruitment after tooth extraction. HMGB1 was associated with angiogenesis and bone remodeling by osteoclast and osteoblast activation and promoted bone healing in the tooth extraction socket.

Aggregatibacter actinomycetemcomitans regulates the expression of integrins and reduces cell adhesion via integrin α5 in human gingival epithelial cells.

Gingival epithelial cells form a physiological barrier against bacterial invasion. Excessive bacterial invasion destroys the attachment between the tooth surface and the epithelium, resulting in periodontitis. Integrins play a significant role in cell attachment; therefore, we hypothesized that bacterial infection might decrease the expressions of these integrins in gingival epithelial cells, resulting in reduced cell adhesion. Immortalized human gingival epithelial cells were co-cultured with Aggregatibacter actinomycetemcomitans Y4 (Aa Y4), and the gene expression levels of IL-8, proliferating cell nuclear antigen (PCNA), and integrins (α2, α3, α5, ß4, and ß6) were measured using quantitative reverse transcription polymerase chain reaction. Expression of PCNA and integrins, except integrin α5, was significantly downregulated, while expression of IL-8 and integrin α5 was significantly upregulated in the cells co-cultured with Aa Y4. The number of adherent cells significantly decreased when co-cultured with Aa Y4, as determined using cell adhesion assays. In the cells co-cultured with Aa Y4 and an integrin α5 neutralizing antibody, there was no effect on the expression of IL-8 and PCNA, while the expressions of integrins α2, α3, ß4, and ß6, and the number of adherent cells did not decrease. The number of invading bacteria in the cells was reduced in the presence of the antibody and increased in the presence of TLR2/4 inhibitor. Therefore, integrin α5 might be involved in Aa Y4 invasion into gingival epithelial cells, and the resulting signal transduction cascade reduces cell adhesion by decreasing the expression of integrins, while the TLR2/4 signaling cascade regulates IL-8 expression.