Melatonin levels in human diabetic dental pulp tissue and its effects on dental pulp cells under hyperglycaemic conditions.

Abstract

To investigate melatonin (MEL) levels in human dental pulp tissue (hDP) in type 2 diabetic (T2D) participants and the underlying molecular mechanisms of its effects in human dental pulp cells (hDPCs) under hyperglycaemia. The study included 16 healthy and 16 T2D participants who underwent vital pulp extirpation for hDP and four healthy participants undergoing third molar extraction for hDPCs analyses. MTT and NRU were used as tests for cytotoxicity. The pulp tissue levels of MEL, inducible NO synthase (iNOS) and superoxide dismutase (SOD) activity, as well as iNOS, histone acetyltransferase p300 (p300) and SOD activity levels in hDPCs incubated with MEL (0.1 and 1.0mmolL-1 ) under normoglycaemia and hyperglycaemia were measured by enzyme-linked immunosorbent assay. Comparisons between the two groups were made by unpaired t-tests or Mann-Whitney test whilst the chi-square test was used for dichotomous variables. To compare more groups, the Kruskal-Wallis test with Dunn's multiple comparison was used, whilst Spearman correlation was used to assess association between two variables. Melatonin was decreased (124.30±21.6 vs. 240.0±19.1 pgmL-1 , P<0.01), whilst iNOS levels increased (0.92±0.08 vs. 0.32±0.09ngmL-1 , P<0.01) in hDP from T2D compared to nondiabetic participants. In hDPCs, MEL (0.1 and 1.0mmolL-1 ) had no cytotoxicity. Incubation with 1.0mmolL-1 of MEL (24h) decreased hyperglycaemia-induced increases of iNOS (0.34±0.01ngmL-1 vs. 0.40±0.01ngmL-1 , P<0.01) and p300 (11.59±0.58ngmL-1 vs. 16.12±0.39ngmL-1 , P<0.01), and also, increased SOD activity (87.11±3.10% vs. 68.56±3.77%, P<0.01) to the levels comparable to the normoglycaemic; iNOS and p300 protein expression levels showed strong positive correlation under hyperglycaemia (Spearman r=0.8242, P<0.001). Type 2 diabetic participants had decreased MEL in hDP. At pharmacological concentrations, MEL is not cytotoxic for hDPCs and normalizes iNOS and SOD activity levels in hyperglyceamic hDPCs suggesting its antioxidant and protective effects in human dental pulp tissue under hyperglycaemia.