[Effects of lipopolysaccharide-activated interferon gene stimulator signaling on periodontal ligament cells]

Objective: To investigate the effects of lipopolysaccharide (LPS)-activated stimulator of interferon genes (STING) signaling on the biological behavior of periodontal ligament cells and its mechanism of action. Methods: Human periodontal ligament cells (hPDLC) were divided into the PBS group and the LPS group by stimulated with phosphate-buffered saline (PBS) and LPS derived from Porphyromonas gingivalis (ATCC 33277) for 12 hours, respectively. The intracellular distribution of 8-hydroxydeoxyguanosine (8-OHdG), a marker of DNA damage, and the activation level of STING signaling were detected by immunofluorescence. The source of intracellular double-stranded DNA was detected by live-cell probes. The levels of osteogenic-related proteins, such as special protein 7 (SP7), bone morphogenetic protein 2 (BMP2), cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS), and STING were detected by Western blotting. The cell supernatants of the PBS group and the LPS group were collected, and the levels of inflammatory cytokines, such as interleukin (IL)-1β, IL-6, and interferon (IFN)-β, were detected by double-antibody sandwich enzyme-linked immunosorbent assay (ELISA). A total of 12 cgas knockout mice and 12 littermate wild-type mice were constructed. The maxillary second molars of the mice were ligated with silk or sham surgery, respectively. After 7 days of modeling, the mice were divided into littermate control sham surgery group, littermate control periodontitis group, cgas knockout sham surgery group, and cgas knockout periodontitis group, with 6 mice in each group. Micro-CT was used to collect image data, and three-dimensional reconstructions were performed on the maxillary samples of each group. The distance from the cemento-enamel junction to the alveolar bone crest (CEJ-ABC), bone volume fraction (BV/TV), and bone mineral density (BMD) in the model area were statistically analyzed using CTAn and CTVOX software. Frozen sectioning was used to obtain sections of the maxillary molars of each group of mice, and the signal intensities of cGAS and STING proteins were detected by immunofluorescence. Results: Immunofluorescence results showed that the fluorescence signal intensity of 8-OHdG outside the nucleus in the LPS group (4.09±0.24) was significantly higher than that in the PBS group (1.00±0.10) (t=20.33, P<0.001). The co-localization signal of mitochondrial marker TOM20 and 8-OHdG (8.56±0.53) were significantly higher than that of PBS group (1.00±0.09) (t=24.37, P<0.001). Live cell DNA probe detection showed that the signal intensity of double-stranded DNA in LPS group (3.23±0.12) was significantly stronger than that in PBS group (1.00±0.17) (t=18.30, P<0.001). Immunofluorescence demonstrated a significant increase in STING expression in hPDLC of the LPS group (t=6.42, P<0.001), and it was colocalized with the Golgi marker GM130. ELISA results showed that the abundance of IL-6, IFN-β, and IL-1β in the supernatant of the LPS group were higher than those of the PBS group (t=12.44, t=11.38, t=9.48, all P<0.001). Animal experiments confirmed that compared with the sham operation group [(207.61±38.09) and (238.97±45.90) μm], the CEJ-ABC in the periodontitis group [(420.31±35.32) and (405.16±35.51) μm] were increased (P<0.01), while the CEJ-ABC in the cgas knockout periodontitis group [(295.11±35.43) and (309.15±32.22) μm] were significantly lower than those in the control periodontitis group of the same litter (P<0.01). Compared with the sham operation group (45.84±6.41), the STING fluorescence signal in the periodontitis group (152.44±6.86) was significantly increased (P<0.001). Compared with the control periodontitis group of the same litter, the STING signal in the cgas knockout periodontitis group was significantly reduced (88.31±9.70) (P<0.001). Conclusions: LPS stimulation can activate the STING signal by generating mitochondrial-derived double-stranded DNA, stimulating hPDLC to secrete inflammatory cytokines and impairing osteogenic differentiation potential. Suppressing STING activation in animal models can reduce bone destruction in periodontitis.