The causes of periodontal disease are complex. Butyric acid, a metabolite of periodontopathic bacteria such as Porphyromonas gingivalis, acts as a histone deacetylase inhibitor that has a direct effect on mRNA expression. Butyric acid produced by Clostridium butyricum in the intestinal tract induces differentiation of regulatory T cells, thereby suppressing inflammation in the gut. Mice lacking Clostridium butyricum in the intestinal tract suffer from colitis. By contrast, butyric acid in the oral cavity worsens periodontal disease. Periodontal disease is a chronic condition in which periodontal tissue is exposed to virulence factors (such as butyric acid); however, no study has examined the effects of long-term exposure to butyric acid. The present study demonstrated that long-term exposure of human gingival fibroblasts (HGFs) to butyric acid induced cytostasis and apoptosis via the intrinsic and extrinsic pathways. Butyric acid inhibited the division of HGFs by altering expression of mRNAs encoding cyclins. Butyric acid induced apoptosis in HGFs via the intrinsic pathway, followed by activation of caspase 9; there was no DNA damage or p53 activation. Butyric acid also upregulated expression of TNF-alpha mRNA and protein by HGFs. Furthermore TNF-alpha induced apoptosis by activating caspase 8 (the extrinsic pathway) and by inducing production of pro-inflammatory cytokines. Taken together, the results show that butyric acid induced cytostasis and apoptosis in HGFs, accompanied by production of pro-inflammatory cytokines. It thus acts as a death ligand and plays a critical role as a prophlogistic substance. (C) 2016 Elsevier Inc. All rights reserved.