Short-Term Curcumin Gavage Sensitizes Insulin Signaling in Dexamethasone-Treated C57BL/6 Mice1-3

Abstract

Background: Long-term dietary curcumin (>12 wk) improves metabolic homeostasis in obese mice by sensitizing insulin signaling and reducing hepatic gluconeogenesis. Whether these occur only secondary to its chronic anti-inflammatory and antioxidative functions is unknown. Objective: In this study, we assessed the insulin sensitization effect of short-term curcumin gavage in a rapid dexamethasone-induced insulin resistance mouse model, in which the chronic anti-inflammatory function is eliminated. Methods: Six-week-old male C57BL/6 mice received an intraperitoneal injection of dexamethasone (100 mg/kg body weight) or phosphate-buffered saline every day for 5 d, with or without simultaneous curcumin gavage (500 mg/kg body weight). On day 7, insulin tolerance tests were performed. After a booster dexamethasone injection and curcumin gavage on day 8, blood glucose and insulin concentrationsweremeasured. Liver tissues were collected on day 10 for quantitative polymerase chain reaction and Western blotting to assess gluconeogenic gene expression, insulin signaling, and the expression of fibroblast growth factor 21 (FGF21). Primary hepatocytes from separate, untreated C57BL/6 mice were used for testing the in vitro effect of curcumin treatment. Results: Dexamethasone injection impaired insulin tolerance (P < 0.05) and elevated ambient plasma insulin concentrations by;2.7-fold (P < 0.01). Concomitant curcumin administration improved insulin sensitivity and reduced hepatic gluconeogenic gene expression. The insulin sensitization effect of curcumin was demonstrated by increased stimulation of S473 phosphorylation of protein kinase B (P < 0.01) in the dexamethasone-treatedmouse liver, aswell as the repression of glucose production in primary hepatocytes (P < 0.001). Finally, curcumin gavage increased FGF21 expression by 2.1-fold in the mouse liver (P < 0.05) and curcumin treatment increased FGF21 expression in primary hepatocytes. Conclusion: These observations suggest that the early beneficial effect of curcumin intervention in dexamethasonetreated mice is the sensitization of insulin signaling, involving the stimulation of FGF21 production, a known insulin sensitizer.