Molecular Protective Actions of Myricetin against Interleukin-1β-Mediated Cytotoxicity in RINm5F β-Cells by Both Modulating Apoptosis and Inhibiting iNOS Expression Mechanisms

Myricetin, one of the flavonoids, has diverse beneficial effects in β-cells of pancreatic islet. Meanwhile, interleukin-1β (IL-1β), a main pro-inflammatory cytokine, is a key molecule leading to β-cell dysfunction. This study explores protective molecular mechanisms of myricetin against IL-1β-induced β-cells damage. In RINm5F β-cells, IL-1β decreased cell viability, and it increased levels of reactive oxygen species (ROS) and nitric oxide (NO). These phenomena were revered by myricetin treatment. Myricetin (30 μM) restored cell viability by reducing apoptosis, modulating the expression of apoptotic molecules, and inhibiting ROS accumulation. Myricetin inhibited IL-1β-induced NO generation, iNOS protein and mRNA expressions in a dose-dependent manner. Additionally, myricetin significantly suppressed IL-1β-mediated IκB phosphorylation and nuclear translocation of NF-κB, a key transcription factor necessary for iNOS transcription and it reduced IL-1β-mediated NF-κB reporter luciferase activity. Consistent with these findings, myricetin significantly inhibited IL-1β-induced iNOS promoter activity at transcriptional step. Interestingly, myricetin meaningfully deteriorated iNOS mRNA stability in actinomycin D chase and iNOS 3′-UTR construct studies at post-transcriptional step. Furthermore, cycloheximide chase study showed that myricetin accelerated iNOS protein instability at post-translational step. Our study suggests that myricetin can protect RINm5F β-cells exposed to IL-1β by inhibiting the apoptotic pathway and in particular, the inhibitory effect of myricetin on iNOS expression is exerted through at transcriptional, post-transcriptional, and post-translational control mechanisms. These results support that myricetin may have a protective effect against pancreatic β-cells dysfunction exposed to proinflammatory cytokine though this study was conducted in in vitro cell line.