Mechanisms linking periodontitis and diabetes

Background + Aims

• Diabetes mellitus (DM) accelerates periodontal disease through hyperglycaemia-induced inflamm-ageing, a chronic, low-grade inflammatory state driven by cellular senescence and the senescence-associated secretory phenotype (SASP). Inflamm-aging disrupts gingival defence mechanisms, including epithelial and fibroblast resistance and immune responses to infection.
  – Cellular senescence is a state of permanent cell cycle arrest triggered by stressors like DNA damage or oxidative stress.
  – Senescent cells secrete pro-inflammatory factors collectively termed SASP, which include cytokines, chemokines, and proteases that disrupt tissue homeostasis, contribute to inflammation, impair regeneration, and promote neighbouring cell dysfunction.
• Previous research identified NLRC4-induced inflamm-ageing as a key pathological feature under hyperglycaemic conditions, impairing periodontal protection.
• This study employed diabetic mouse models and in vitro cell experiments to investigate the effect of macrophage pyroptosis in diabetic periodontitis, focusing on its impact on fibroblast senescence and periodontal inflamm-ageing. It explores the underlying mechanism involving NLRC4 phosphorylation.

Materials + Methods

• Three groups (control, diabetic, and metformin-treated diabetic) with 10 mice each were studied.
• Diabetic conditions were induced with streptozotocin (STZ), and metformin treatment was administered daily for eight weeks.
• Alveolar bone loss was measured using stereomicroscopy, and gingival tissues were analysed via histology and immunohistochemistry for markers like GSDMD and NLRC4.
• In vitro, RAW 264.7 macrophages and human gingival fibroblasts were cultured under hyperglycaemic conditions, with macrophages treated using small interfering RNA (siRNA) to target Gsdmd and Nlrc4. Conditioned medium from pyroptotic macrophages was applied to fibroblasts to assess paracrine inflamm-ageing induction.
• Advanced imaging (scanning electron microscopy), flow cytometry, and other biochemical techniques were used to evaluate pyroptosis and fibroblast senescence.
• Statistical analyses, including ANOVA and post hoc tests, validated the findings. The results identified NLRC4 phosphorylation as a key regulator of pyroptosis and its contribution to inflamm-ageing, with metformin demonstrating protective effects.

Results

• In diabetic mouse models, increased alveolar bone loss and elevated expression of pyroptosis markers, such as gasdermin D (GSDMD) and NLR family CARD domain-containing protein 4 (NLRC4), were observed.
• In vitro experiments with RAW 264.7, macrophages demonstrated that high glucose levels induced pyroptosis, characterised by cell membrane rupture and release of pro-inflammatory cytokines.
• Conditioned medium from these pyroptotic macrophages promoted senescence in human gingival fibroblasts, evidenced by increased senescence-associated β-galactosidase activity and expression of senescence markers.
• Knockdown of NLRC4 or treatment with metformin reduced macrophage pyroptosis and subsequent fibroblast senescence.
• These findings suggest that hyperglycaemia-induced macrophage pyroptosis contributes to periodontal inflamm-ageing by promoting fibroblast senescence, with NLRC4 playing a pivotal role in this process.

Limitations