Porphyromonas gingivalis Lipopolysaccharide Upregulates Insulin Secretion From Pancreatic β Cell Line MIN6

Background: A close association between periodontitis and diabetes has been demonstrated in human cross-sectional studies, but an exact relationship between periodontitis and prediabetes has not been established. Previous studies using animal model systems consistently have shown that hyperinsulinemia occurs in animals with periodontitis compared to animals with healthy periodontium (while maintaining normoglycemia). Because bacterial lipopolysaccharide (LPS) plays an important role in the pathogenesis of periodontitis, we hypothesized that LPS may stimulate insulin secretion through a direct effect on beta cell function. To test this hypothesis, pancreatic beta cell line MIN6 cells were used to determine the effect of Porphyromonas gingivalis (Pg) LPS on insulin secretion. Furthermore, expression of genes altered by Pg LPS in innate immunity and insulin-signaling pathways was determined. Methods: MIN6 cells were grown in medium with glucose concentration of normoglycemia (5.5 mM). Pg LPS was added to each well at final concentrations of 50, 200, and 500 ng/mL. Insulin secretion was measured using enzyme-linked immunosorbent assay. Gene expression levels altered by Pg LPS were determined by polymerase chain reaction (PCR) array for mouse innate and adaptive immunity response and mouse insulin-signaling pathways, and results were confirmed for specific genes of interest by quantitative PCR. Results: Pg LPS stimulated insulin secretion in the normoglycemic condition by approximate to 1.5- to 3.0-fold depending on the concentration of LPS. Pg LPS treatment altered the expression of several genes involved in innate and adaptive immune response and insulin-signaling pathway. Pg LPS upregulated the expression of the immune response-related genes cluster of differentiation 8a (Cd8a), Cd14, and intercellular adhesion molecule-1 (Icam1) by about two-fold. LPS also increased the expression of two insulin signaling-related genes, glucose-6-phosphatase catalytic subunit (G6pc) and insulin-like 3 (Insl3), by three-to four-fold. Conclusions: We have demonstrated for the first time that Pg LPS stimulates insulin secretion by pancreatic b cell line MIN cells. Pg LPS may have significant implications on the development of b cell compensation and insulin resistance in prediabetes in individuals with periodontitis.