[No authors listed]
The pineal hormone melatonin influences the secretion of insulin by pancreatic islets via the Gâproteinâcoupled melatonin receptors 1 and 2 that are expressed in pancreatic βâcells. Genomeâwide association studies indicate that melatonin receptor 1B (MTNR1B) single nucleotide polymorphisms are tightly associated with type 2 diabetes mellitus (T2DM). However, the underlying mechanism is unclear. Rafâ1 serves a critical role in the mitogenâactivated protein kinase (MAPK) pathways in βâcell survival and proliferation and, therefore, may be involved in the mechanism by which melatonin impacts on T2DM through MTNR1B. In the present study, the mRNA expression of the two mouse insulin genes Ins1 and Ins2 was investigated in MIN6 cells treated with different concentrations of melatonin, and insulin secretion was detected under the same conditions. Following the overexpression or silencing of MTNR1B, the activities of components of the MAPK signaling pathway, including Rafâ1 and ERK, were evaluated. The impact of MTNR1B knockdown on the melatoninâregulated insulin gene expression and insulin secretion were also investigated. The results demonstrated that exogenous melatonin inhibited the expression of insulin mRNA in the MIN6 cells. Insulin secretion by the MIN6 cells, however, was not affected by melatonin. The MAPK signaling pathway was inhibited in MIN6 cells by treatment with melatonin or the overexpression of MTNR1B. The knockdown of MTNR1B totally attenuated the regulating effect of melatonin on insulin gene expression. Additionally, the inductive effect of melatonin on the expression of insulin mRNA was attenuated when the activities of Rafâ1 or ERK were blocked using the chemical inhibitors GW5074 and U0126, respectively. It may be concluded that melatonin exerts an inhibitory effect on insulin transcription via MTNR1B and the downstream MAPK signaling pathway.
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