Gingival Epithelial Cells Potently Release HMGB1: Implication in Periodontitis

Objectives: High mobility group box 1 (HMGB1), originally described as a DNA-binding protein, can be released extracellularly and functions as a late mediator of inflammatory responses. Chronic periodontitis is an inflammatory disease. However, the pathogenesis of this disease is not fully understood. The purpose of this study was to clarify the source of HMGB1 in chronic periodontitis tissues and subsequently to elucidate its inducible pathway.

Methods: Human healthy and inflamed gingival tissues were stained for HMGB1 by immunohistochemistry. A human gingival epithelial cell line, Ca9-22, was treated with TNF-a.The levels of HMGB1 in the culture supernatants of Ca9-22 were evaluated by Western blot. The phosphorylation of mitogen-activated protein kinases (MAPKs) in Ca9-22 was also examined. The signaling pathways involved in the release of HMGB1 were investigated using the specific MAPK inhibitors.

Results: HMGB1 was detected in the cytoplasm and nucleus of gingival epithelial cells in inflamed gingival tissues by immunohistochemical staining. TNF-a triggered the release of HMGB1 from Ca9-22 in a dose- and time-dependent manner. The HMGB1 release was blocked by TNF receptor 1 antibody. The molecular dialogue between TNF-a and gingival epithelial cells involved modulation of the activities of p38MAPK, JNK and p44/42. A specific inhibitor of p38MAPK (SB203580), but not inhibitors of ERK1/2 (U0126) or JNKs (SP600125), significantly reduced HMGB1 release.

Conclusion: We suggest that HMGB1 is mainly expressed in gingival epithelial cells in periodontal lesions and that gingival epithelial cells release HMGB1 possibly via p38MAPK signaling pathway in response to TNF-a. HMGB1 may be a key mediator involved in chronic inflammatory responses in periodontal disease. Further studies are still required to examine the roles of HMGB1 in periodontal pathology.