Epigallocatechin-3-gallate protects pro-inflammatory cytokine induced injuries in insulin-producing cells through the mitochondrial pathway

Abstract

Pro-inflammatory cytokine-mediated pancreatic β-cell dysfunction is a key pathological event in type 1 diabetes mellitus. There are few studies about the protection of epigallocatechin-3-gallate (EGCG) against pro-inflammatory cytokine-induced β-cell apoptosis. To examine the direct effects of EGCG on β-cells, insulin-producing RINm5F cells were exposed to a combination of recombinant interleukin-1 beta (IL-1β), tumor necrosis factor alpha (TNF-α), and interferon gamma (IFN-γ), with or without EGCG pretreatment for 24h. Cell death was monitored by the MTT assay. Glucose-stimulated insulin release was measured using radio immunoassay. Intracellular reactive oxygen species was examined with dichlorofluorescein (DCF) fluorescence by flow cytometry. To evaluate RINm5F cells mitochondrial function, change in mitochondrial membrane potential, intracellular ATP levels, and nitric oxide was assessed. The expression of cytochrome c, Bax, Bcl-2, and iNOS proteins was measured by western blotting. In the present study, EGCG pretreatment protected against cytokines inducing cell death and restored glucose stimulated-insulin secretion in RINm5F cells. EGCG reduced the cytokine-induced generation of reactive oxygen species, the loss of mitochondrial membrane potential (Δψm), the release of cytochrome c from the mitochondria, and translocation of Bax protein to the mitochondria from the cytosol. EGCG pretreatment prevented cytokine-induced iNOS overexpression and NO generation. In summary, pro-inflammatory cytokines lead to a reduction of glucose-induced insulin secretion, mitochondrial activity and viability in RINm5F cells. The pro-inflammatory cytokine-induced effects can be prevented by EGCG pretreatment via the mitochondrial pathway.