Objectives: Diabetes is a potential risk factor of periodontal diseases. IL-18 and the adipokine leptin are elevated in type 2 diabetes mellitus (T2DM) and play a crucial role in immunity and inflammation. However, the relationship between IL-18 and leptin in periodontal diseases still remains unclear. The objective of this study was to investigate the effect of leptin on the regulation of IL-18 in human monocytes and the possible role of caspase-1 activation in this pathway.
Methods: Pro-monocytic THP-1 cells were differentiated into monocytes with vitamin D3 and incubated with leptin (1 mg/ml), ATP (6 mM) or lipopolysaccharide (LPS) from E. coli and P. gingivalis (both 1 ng/ml &100 ng/ml) for up to 48 hours in RPMI under standard conditions. IL-18 mRNA expression was measured using real-time RT-PCR, and IL-18 protein secretion by ELISA. Pro-IL-18 is processed intracellularly into active IL-18 by the action of caspase-1 associated with the inflammasome. The activation of caspase-1 was therefore measured by western blotting and caspase-1 activity assayed using caspase-specific colorimetric peptide.
Results: E.coli LPS, P.gingivalis LPS as well as leptin alone enhanced IL-18 release in monocytes. In addition, leptin significantly increased IL-18 release in P. gingivalis LPS treated cells but not in E.coli LPS treated-cells. In contrast, leptin had no effect on IL-18 mRNA levels in monocytes, suggesting leptin regulates IL-18 secretion at the post-translational level. Leptin increased the molecular activation of caspase-1 as demonstrated by increased levels of caspase-1 p10 in leptin treated-cells. Leptin also upregulated caspase-1 activity and ATP (which activates the inflammasome) significantly enhanced IL-18 release in leptin treated-cells.
Conclusion: Leptin promotes IL-18 secretion and this is associated with an activation of caspase-1 activity which may reflect post-translational regulation of IL-18 by this adipokine. Leptin-mediated enhancement of IL-18 synthesis may be important in the cross-susceptibility between diabetes and periodontal disease.