Metformin ameliorates experimental diabetic periodontitis independently of mammalian target of rapamycin (mTOR) inhibition by reducing NIMA-related kinase 7 (Nek7) expression

Background: Metformin, a classical treatment for diabetes mellitus, has shown sound anti-inflammatory effects. Emerging studies have focused on the mechanism underlying inflammation-related diabetic complications, such as diabetic periodontitis. Herein, we investigated the anti-inflammatory effects of metformin on the NIMA-related kinase 7 (Nek7)/nucleotide-binding oligomerization domain (NOD)-like receptor family pyrin domain containing 3 (NLRP3) pathway both in vivo and in vitro in experimental diabetic periodontitis.

Methods: All procedures were conducted in Porphyromonas gingivalis (Pg)-infected streptozotocin (STZ)-induced diabetic mice and in lipopolysaccharide (LPS)-treated RAW 264.7 cells under high-glucose conditions. A range of techniques were performed in this study: microcomputed tomography, western blotting, and immunofluorescence were used to analyze periodontal tissues. Enzyme-linked immunosorbent assay (ELISA) was for serum interleukin-1β (IL-1β) detection. Specific pharmacological inhibition was used to stimulate cells. Flow cytometry was implemented to analyze cell cycle.

Results: We found that metformin treatment can robustly ameliorate periodontal infection and tissue destruction and reduce blood glucose and serum IL-1β levels in mice with diabetic periodontitis. Moreover, gingival tissue exhibited less macrophage infiltration and decreased expression of Nek7, NLRP3, caspase-1, and mammalian target of rapamycin (mTOR), which were simultaneously observed in RAW 264.7 cell models stimulated with metformin. Metformin also affected the cell cycle in a dose-dependent way. Furthermore, after stimulation with the mTOR inhibitor rapamycin, additional metformin treatment could still downregulate Nek7/NLRP3.

Conclusions: Our research indicated that metformin significantly attenuated experimental diabetic periodontitis both in vivo and in vitro. Metformin suppressed the inflammatory state by inhibiting Nek7 expression to decrease NLRP3 inflammasome activity. Interestingly, mTOR inhibition was not involved in metformin-induced Nek7 downregulation. The observed Nek7 reduction could be related to metformin-mediated cell cycle arrest.