Ankaflavin (AK) is an active compound having anti-inflammatory, anti-
cancer, antiatherosclerotic, and hypolipidemic effects. We have previously reported that AK acts as an
antioxidant and
antidiabetic drug; however, the mechanism by which AK prevents diabetes remains unknown.
Hyperglycemia is associated with protein glycation, which produces
advanced glycation end-products (AGEs).
Methylglyoxal (MG)-a metabolite of
carbohydrates-is believed to cause
insulin resistance by inducing
inflammation and pancreas damage. In this work, diabetes was induced in Wistar rats (4 weeks of age) by treating them with MG (600 mg/kg bw) for 4 weeks. We observed that AK (10mg/kg bw) exerted
peroxisome proliferator-activated receptor-γ (PPARγ) agonist activity, thereby enhancing
insulin sensitivity (as indicated by hepatic GLUT2 translocation, PTP1B suppression, and
glucose uptake) by downregulating
blood glucose and upregulating pancreatic and duodenal homeobox-1 and Maf-A expression and increasing
insulin production in MG-induced rats. However, these effects were abolished by the administration of
GW9662 (PPARγ antagonist), but the expression of hepatic
heme oxygenase-1 (HO-1) and
glutamate-cysteine ligase (GCL) was not suppressed in MG-induced rats. Therefore, the nuclear factor erythroid-related factor-2 (Nrf2) activation was investigated. AK did not affect hepatic Nrf2
mRNA or
protein expression but significantly increased Nrf2 phosphorylation (
serine 40), which was accompanied by increased transcriptional activation of hepatic HO-1 and GCL. These data indicated that AK protected rats from oxidative stress resulting from MG-induced
insulin resistance. In contrast, these effects were not detected when the rats were treated with the
antidiabetic drug rosiglitazone (10mg/kg bw). Moreover, we found that AK did not inhibit the generation of AGEs in vitro; however, the
glutathione (GSH) levels in liver and pancreas of MG-induced rats were elevated in rats administered AK. Therefore, we believe that GSH may lower the MG level, which attenuates the formation of AGEs in the serum, kidney, liver, and pancreas of MG-induced rats. We also found that AK treatment reduced the production of inflammatory factors, such as
tumor necrosis factor-α and interleukin-1β. Taken together, the results of our mechanistic study of MG-induced rats suggest that the protective effects of AK against diabetes are mediated by the upregulation of the signaling pathway of Nrf2, which enhances
antioxidant activity and serves as a PPARγ agonist to enhance
insulin sensitivity.