Roles of Gingipains in Periodontal Bone Loss

Cleavage of host cytokeratin-6 by lysine-specific gingipain induces gingival inflammation in periodontitis patients

BACKGROUND/PURPOSE

Lysine-specific gingipain (Kgp) is a virulence factor secreted from Porphyromonas gingivalis (P. gingivalis), a major etiological bacterium of periodontal disease. Keratin intermediate filaments maintain the structural integrity of gingival epithelial cells, but are targeted by Kgp to produce a novel cytokeratin 6 fragment (K6F). We investigated the release of K6F and its induction of cytokine secretion.

METHODS

K6F present in the gingival crevicular fluid of periodontal disease patients and in gingipain-treated rat gingival epithelial cell culture supernatants was measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometer-based rapid quantitative peptide analysis using BLOTCHIP. K6F in gingival tissues was immunostained, and cytokeratin 6 protein was analyzed by immunofluorescence staining and flow cytometry. Activation of MAPK in gingival epithelial cells was evaluated by immunoblotting. ELISA was used to measure K6F and the cytokines release induced by K6F. Human gingival fibroblast migration was assessed using a Matrigel invasion chamber assay.

RESULTS

We identified K6F, corresponding to the C-terminus region of human cytokeratin 6 (amino acids 359-378), in the gingival crevicular fluid of periodontal disease patients and in the supernatant from gingival epithelial cells cultured with Kgp. K6F antigen was distributed from the basal to the spinous epithelial layers in gingivae from periodontal disease patients. Cytokeratin 6 on gingival epithelial cells was degraded by Kgp, but not by Arg-gingipain, P. gingivalis lipopolysaccharide or Actinobacillus actinomycetemcomitans lipopolysaccharide. K6F, but not a scrambled K6F peptide, induced human gingival fibroblast migration and secretion of interleukin (IL)-6, IL-8 and monocyte chemoattractant protein-1. These effects of K6F were mediated by activation of p38 MAPK and Jun N-terminal kinase, but not p42/44 MAPK or p-Akt.

CONCLUSION

Kgp degrades gingival epithelial cell cytokeratin 6 to K6F that, on release, induces invasion and cytokine secretion by human gingival fibroblasts. Thus, Kgp may contribute to the development of periodontal disease.

Porphyromonas gingivalis-derived lysine gingipain enhances osteoclast differentiation induced by tumor necrosis factor-α and interleukin-1β but suppresses that by interleukin-17A: importance of proteolytic degradation of osteoprotegerin by lysine gingipain

Periodontitis is a chronic inflammatory disease accompanied by alveolar bone resorption by osteoclasts. Porphyromonas gingivalis, an etiological agent for periodontitis, produces cysteine proteases called gingipains, which are classified based on their cleavage site specificity (i.e. arginine (Rgps) and lysine (Kgps) gingipains). We previously reported that Kgp degraded osteoprotegerin (OPG), an osteoclastogenesis inhibitory factor secreted by osteoblasts, and enhanced osteoclastogenesis induced by various Toll-like receptor (TLR) ligands (Yasuhara, R., Miyamoto, Y., Takami, M., Imamura, T., Potempa, J., Yoshimura, K., and Kamijo, R. (2009) Lysine-specific gingipain promotes lipopolysaccharide- and active-vitamin D3-induced osteoclast differentiation by degrading osteoprotegerin. Biochem. J. 419, 159-166). Osteoclastogenesis is induced not only by TLR ligands but also by proinflammatory cytokines, including tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, and IL-17A, in inflammatory conditions, such as periodontitis. Although Kgp augmented osteoclastogenesis induced by TNF-α and IL-1β in co-cultures of mouse osteoblasts and bone marrow cells, it suppressed that induced by IL-17A. In a comparison of proteolytic degradation of these cytokines by Kgp in a cell-free system with that of OPG, TNF-α and IL-1β were less susceptible, whereas IL-17A and OPG were equally susceptible to degradation by Kgp. These results indicate that the enhancing effect of Kgp on cytokine-induced osteoclastogenesis is dependent on the difference in degradation efficiency between each cytokine and OPG. In addition, elucidation of the N-terminal amino acid sequences of OPG fragments revealed that Kgp primarily cleaved OPG in its death domain homologous region, which might prevent dimer formation of OPG required for inhibition of receptor activator of nuclear factor κB ligand. Collectively, our results suggest that degradation of OPG by Kgp is a crucial event in the development of osteoclastogenesis and bone loss in periodontitis.