High glucose exacerbates the inflammatory response in gingival fibroblasts through oxidative stress and mitochondrial DNA release

To investigate if high glucose (HG) exacerbates Porphyromonas gingivalis (P.g) lipopolysaccharide (LPS)-induced inflammatory response in human gingival fibroblasts (HGFs) and to explore the underlying mechanisms. To provide a basis for the mechanism of diabetes aggravating periodontitis.

HGFs were divided into four groups: the control group (basal medium), the LPS group (treated with 5 μg/mL P.g-LPS for 24 h), the HG group (treated with 25 mmol/L glucose for 24 h), and the HG+LPS group (treated with 25 mmol/L glucose + 5 μg/mL P.g-LPS for 24 h). After culturing for 24 h in the respective media, the cells were harvested for experiments. Intracellular reactive oxygen species (ROS) and mitochondrial reactive oxygen species (mtROS) were detected using 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) and MitoSOX Red staining, respectively. Fluorescence intensity was analyzed by confocal fluorescence microscopy and directly measured in cell suspension. Immunofluorescence was used to detect changes in mitochondrial DNA (mtDNA) content of HGFs. Real-time fluorescence quantitative PCR was used to detect the content of mtDNA in cytoplasm and cell supernatant. Protein expression of the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway was assessed by western blot, while mRNA expression levels of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6) were detected by PCR.

Compared to the control group, both the LPS group and the HG group exhibited a significant increase in ROS and mtROS, with a more pronounced elevation in the HG+LPS group, demonstrating a synergistic effect (ROS: F = 396.5, P < 0.001; mtROS: F = 29.38, P < 0.001, CI < 1). The cytoplasmic mtDNA content was significantly elevated in the LPS group, with a more marked increase in the HG+LPS group (F = 27.85, P < 0.001). The supernatant mtDNA levels were significantly higher in both the LPS and HG groups, with a more pronounced elevation in the HG+LPS group (F = 15.26, P < 0.001). The phosphorylated proteins p-STING, p-TBK1, and p-P65 in the cGAS-STING pathway showed varying degrees of activation in the LPS and HG groups, reaching the highest levels in the HG+LPS group (p-STING: F = 52.67, P < 0.001; p-TBK1: F = 15.67, P = 0.001; p-P65: F = 9.83, P = 0.005), while p-IRF3 showed no significant differences among the groups (P = 0.072). Pro-inflammatory cytokine TNF-α was significantly higher in the HG+LPS group compared to the control group (F = 15.05, P < 0.001), and IL-1β increased in both the LPS and HG groups, with a more pronounced rise in the HG+LPS group (F = 30.98, P < 0.001). IL-6 showed no significant differences among the groups (P = 0.847).

High glucose and LPS act synergistically to enhance oxidative stress, accompanied by increased mtDNA release, which activates the cGAS-STING pathway, thereby amplifying the inflammatory response in HGFs.