Fermented noni (Morinda citrifolia L.) fruit juice improved oxidative stress and insulin resistance under the synergistic effect of Nrf2/ARE pathway and gut flora in db/db mice and HepG2 cells

Oxidative stress interferes with blood glucose homeostasis, leading to insulin resistance (IR) and hyperglycemia, which eventually induce type 2 diabetes (T2DM). Fermented noni (Morinda citrifolia L.) fruit juice (FNJ) is a traditional folk medicine in Polynesia as well as a functional food supplement with a variety of health benefits. In this study, the efficacy and mechanism of FNJ in improving oxidative stress and IR were investigated under the synergistic effect of Nrf2/ARE pathway and gut flora in vivo and in vitro. In vitro, insulin resistant HepG2 cells were established with 50 mM high glucose, and 25 μM rosiglitazone (ROSI) was used as a positive control. In vivo, male C57BLKS mice in the CON group were fed a normal diet and C57BLKS/J-db/db (db/db) mice were treated with 10 mg kg^-1 d^-1 ROSI and 6.5 and 13 mL kg^-1 d^-1 FNJ for 8 weeks, respectively. The results showed that 3-5% FNJ markedly improved IR by increasing glucose uptake and consumption and ameliorated oxidative stress via up-regulating the expression of related antioxidant enzymes such as SOD and GPx and attenuating ROS and MDA in IR-HepG2 cells. In vivo, db/db mice exhibited hyperglycemia, hyperinsulinemia, and oxidative stress, whereas FNJ significantly inverted those conditions. Moreover, FNJ supplementation notably increased the gut microbiota diversity of T2DM mice and remodeled the gut microbial composition to suppress microbial disorder. Specifically, FNJ increased the ratio of F/B and lowered the Proteobacteria abundance at the phylum level. The db/db mice were enriched in Enterobacteriaceae, Klebsiella, and Klebsiella-pneumonia, while FNJ feeding significantly up-regulated Rikenellaceae. In addition, western blotting and qPCR experiments showed that FNJ promoted the expression of Nrf2, HO-1, IRS1, PI3K, AKT, and GLUT4 and reduced the JNK level both in vivo and vitro. These results confirmed that FNJ had a protective effect on oxidative stress and IR by activating Nrf2/ARE signaling pathway and regulating gut flora in db/db mice and IR-HepG2 cells.