Enterococcus faecalis induces apoptosis and pyroptosis of human osteoblastic MG63 cells via the NLRP3 inflammasome

Aim: To investigate the ability of E. faecalis to cause apoptosis and pyroptosis of human osteoblastic MG63 cells and whether the inflammasome is involved in this process.

Methodology: MG63 cells were infected with E. faecalis at a multiplicity of infection (MOI) of 10, 100, 500 and 1000 for 6 and 12 h. The cell proliferation was determined with the Cell Counting Kit-8. Apoptosis and pyroptosis after E. faecalis infection was evaluated by flow cytometry and a lactate dehydrogenase (LDH) cytotoxicity assay kit. Then, MG63 cells transfected with specific small interfering RNAs (siRNAs) targeting the NLR family pyrin domain-containing 3 (NLRP3) gene were infected with E. faecalis and subjected to the LDH release and apoptotic cell assays. One-way anova test and Student-Newman-Keuls test were used to evaluate the differences amongst groups in each experiment. An independent sample t-test was performed to analyse the differences between the 12- and 6-h time-points. P < 0.05 was regarded as significant.

Results: E. faecalis induced apoptosis and inflammatory cell death (pyroptosis) of MG63 cells in a MOI dose-dependent manner. The NLRP3 inflammasome and caspase-1 were activated in E. faecalis-infected MG63 cells. However, the percentages of induced apoptotic and pyroptic cell death decreased significantly when the NLRP3 inflammasome was downregulated using siRNAs (P < 0.05).

Conclusions: E. faecalis promoted apoptosis and pyroptosis of MG63 cells via the NLRP3 inflammasome. This indicates that E. faecalis infection may result in the delayed reconstruction of periapical lesions, and NLRP3 is the potential target of new intracanal therapeutic agents.