Quercetin and quercetin-3-O-glucuronide are equally effective in ameliorating endothelial insulin resistance through inhibition of reactive oxygen species-associated inflammation

Scope Quercetin represents antioxidative/antiinflammatory flavonoids widely distributed in the human diet. Quercetin is efficiently metabolized during absorption to quercetin-3-O-glucuronide. This study aims to parallelly investigate whether quercetin and quercetin-3-O-glucuronide exert protection against palmitate (PA)-induced inflammation and insulin resistance in the endothelium. Methods and results Human umbilical vein endothelial cells were pretreated with quercetin and quercetin-3-O-glucuronide for 30 min, and then incubated with 100 M PA for 30 min or 12 h with or without insulin. PA stimulation led to reactive oxygen species (ROS) production with collapse of mitochondrial membrane potential (m). Quercetin and quercetin-3-O-glucuronide inhibited ROS overproduction and effectively restored m, demonstrating their chemorpotection of mitochondrial function through antioxidative actions. Also, quercetin and quercetin-3-O-glucuronide inhibited ROS-associated inflammation by inhibition of interleukin-6 and tumor necrosis factor- production with suppression of IKK/NF-B phosphorylation. Inflammation impaired insulin PI3K signaling and reduced insulin-mediated nitric oxide (NO) production. Quercetin and quercetin-3-O-glucuronide facilitated PI3K signaling by positive regulation of serine/tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and restoration of downstream Akt/eNOS activation, leading to an increased insulin-mediated NO level. Conclusion The above-mentioned evidence indicates that quercetin and quercetin-3-O-glucuronide are equally effective in inhibiting ROS-associated inflammation and ameliorating insulin resistant endothelial dysfunction by beneficial regulation of IRS-1 function.