Extracellular adenosine triphosphate regulates inflammatory responses of periodontal ligament cells

Background: Various stimuli, that is, mechanical stresses or inflammation, induce the release of adenosine triphosphate (ATP) by human periodontal ligament cells (HPDLCs). Extracellular adenosine triphosphate (eATP) affects HPDLCs' functions such as immunosuppressive action and inflammatory responses. Lipopolysaccharide (LPS) is the key factor involved in periodontal inflammation. However, the possible correlation and detailed mechanism of inflammation-mediated eATP by LPS and inflammatory cascade formation in HPDLCs is unclarified. This study aims to examine the role of eATP on the HPDLCs' responses concerning inflammatory actions after LPS treatment. Methods: HPDLCs were stimulated with Porphyromonas gingivalis LPS and polyinosinic:polycytidylic acid (poly I:C). The amount of ATP release was measured at different time points using a bioluminescence assay. HPDLCs were treated with eATP. The expression of pro-inflammatory and anti-inflammatory genes was determined. Specific P2X purinoreceptor 7 (P2X7) inhibitors (brilliant blue G [BBG] and KN62), a specific P2Y purinoreceptor 1 (P2Y1) inhibitors (MRS2179), calcium chelator (EGTA), protein kinase C (PKC) inhibitors, nuclear factor kappa-light-chain-enhancer of activated B cells (NF kappa B) activation inhibitors, and cyclic adenosinemonophosphate (cAMP)-dependent protein kinase A (PKA) inhibitors (H89 dihydrochloride) and activators (forskolin) were used to dissect the mechanism of eATP-induced HPDLCs' inflammatory responses. Results: LPS and poly I:C induced ATP release. A low concentration of eATP (50 mu M) increased pro-inflammatory genes (COX2, IL1B, IL6, IL8, IL12, and TNFA), while a high concentration (500 mu M) enhanced anti-inflammatory genes (IL4 and IL10). BBG, KN62, and NF kappa B activation inhibitors impeded eATP-induced pro-inflammatory genes. MRS2179 and H89 markedly suppressed eATP-induced anti-inflammatory genes. Forskolin induced IL4 and IL10. Conclusion: HPDLCs respond to LPS by releasing ATP. eATP has dosedependent dual functions on HPDLCs' inflammatory responses via different