Cystein proteinases (gingipains) from Porphyromonas gingivalis cleave a broad range of in-host proteins and are considered to be key virulence factors in the onset and development of adult periodontitis and host defense evasion. In periodontitis, an inflammatory disease triggered by bacterial infection, the production of hepatocyte growth factor (HGF) is induced not only by various factors derived from the host, such as inflammatory cytokines, but also by bacterial components. In this study we examined the possible enhanced production of HGF produced by human gingival fibroblasts upon stimulation with gingipains. Arginine-specific gingipain (Rgp) caused a marked production of HGF into the supernatant, the induction of HGF expression on the cell surface, and the up-regulation of HGF mRNA expression in a dose-dependent and an enzymatic activity-dependent manner. Because it has been reported that Rgp activated protease-activated receptors (PARs), we examined whether the induction of HGF triggered by Rgps on human gingival fibroblasts occurred through PARs. An RNA interference assay targeted to PAR-1 and PAR-2 mRNA revealed that gingipains-induced secretion of HGF was significantly inhibited by RNA interference targeted to PAR-1 and PAR-2. In addition, the Rgps-mediated HGF induction was completely inhibited by the inhibition of phospholipase C and was clearly inhibited by RNA interference targeted to p65, which is an NF-κB component. These results suggest that Rgps activated human gingival fibroblasts to secrete HGF in the inflamed sites and the mechanism (s) involved may actively participate in both inflammatory and reparative processes in periodontal diseases.