The role of TXNIP in the hepatic glucose homeostasis

Abstract

Thioredoxin interacting protein (TXNIP) has multiple functions in several pathways involved in the reactive oxygen species (ROS) generation, apoptosis, inflammation and glucose metabolism. TXNIP is upregulated in the hyperglycemic state and represses glucose uptake into several peripheral tissues, resulting in a homeostatic imbalance of glucose. Although TXNIP has relevance to metabolic syndromes such as obesity and type I and II diabetes mellitus, the role and regulation of TXNIP in liver is unclear. To investigate a metabolic role of TXNIP in the liver, Ad-Txnip is administrated to normal mice and an intraperitoneal glucose tolerance test (IPGTT), insulin tolerance test (ITT), and pyruvate tolerance test (PTT) were performed. Overexpression of TXNIP resulted in an impaired glucose, insulin, and pyruvate tolerance in normal mice. After Ad-Txnip administration, the expression of genes involved in glucose metabolism, including glucose-6-phosphatase (G6pc) and glucokinase (Gck) were analysed using qPCR and western blot. Ad-Txnip transduction upregulated G6pc expression and caused a decrease in Gck levels in the liver of normal mice and primary hepatocytes. To understand increased G6pc expression in the liver as a result of TXNIP overexpression, pull down assays for TXNIP and small heterodimer partner (SHP) were performed and confirmed that TXNIP increased G6pc expression by forming a complex with SHP which is known to be a negative modulator of gluconeogenesis. To study for the regulation of Txnip gene expression, luciferase reporter assays and chromatin immunoprecipitation (ChIP) assays using the Txnip promoter were performed to elucidate the interrelationship between carbohydrate response element binding protein (ChREBP) and transcription factor E3 (TFE3) in the regulation of Txnip expression. Furthermore, Txnip expression in diabetic mouse models was decreased by Ad-Tfe3 administration, suggesting that TFE3 may play a negative role through competition with ChREBP at the E-box of the Txnip promoter. These findings demonstrated that TXNIP impairs glucose and insulin tolerance in mice by upregulating G6pc through interaction with SHP and modulating TXNIP expression.