The etiology of periodontal disease is multifactorial. Exogenous stimuli such as bacterial pathogens can interact with toll-like receptors to activate intracellular calcium signaling in gingival epithelium and other tissues. The triggering of calcium signaling induces the secretion of pro-inflammatory cytokines such as interleukin-8 as part of the inflammatory response; however, the exact mechanism of calcium signaling induced by bacterial toxins when gingival epithelial cells are exposed to pathogens is unclear. Here, we investigate calcium signaling induced by bacteria and expression of inflammatory cytokines in human gingival epithelial cells. We found that peptidoglycan, a constituent of grampositive bacteria and an agonist of toll-like receptor 2, increases intracellular calcium in a concentration- dependent manner. Peptidoglycan-induced calcium signaling was abolished by treatment with blockers of phospholipase C (U73122), inositol 1,4,5-trisphosphate receptors, indicating the release of calcium from intracellular calcium stores. Peptidoglycan-mediated interleukin-8 expression was blocked by U73122 and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester). Moreover, interleukin-8 expression was induced by thapsigargin, a selective inhibitor of the sarco/endoplasmic reticulum calcium ATPase, when thapsigargin was treated alone or co-treated with peptidoglycan. These results suggest that the gram-positive bacterial toxin peptidoglycan induces calcium signaling via the phospholipase C/inositol 1,4,5-trisphosphate pathway, and that increased interleukin-8 expression is mediated by intracellular calcium levels in human gingival epithelial cells.
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