Txnip contributes to impaired glucose tolerance by upregulating the expression of genes involved in hepatic gluconeogenesis in mice.

AIMS/HYPOTHESIS: Thioredoxin-interacting protein (TXNIP) is upregulated in the hyperglycaemic state and represses glucose uptake, resulting in imbalanced glucose homeostasis. In this study, we propose a mechanism of how TXNIP impairs hepatic glucose tolerance at the transcriptional level.
METHODS: We administered adenoviral Txnip (Ad-Txnip) to normal mice and performed intraperitoneal glucose tolerance tests (IPGTT), insulin tolerance tests (ITT) and pyruvate tolerance tests (PTT). After Ad-Txnip administration, the expression of genes involved in glucose metabolism, including G6pc and Gck, was analysed using quantitative real-time PCR and western blot. To understand the increased G6pc expression in liver resulting from Txnip overexpression, we performed pull-down assays for TXNIP and small heterodimer partner (SHP). Luciferase reporter assays and chromatin immunoprecipitation 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.
RESULTS: Overabundance of TXNIP resulted in impaired glucose, insulin and pyruvate tolerance in normal mice. Ad-Txnip transduction upregulated G6pc expression and caused a decrease in Gck levels in the liver of normal mice and primary hepatocytes. TXNIP increased G6pc expression by forming a complex with SHP, which is known to be a negative modulator of gluconeogenesis. Txnip expression in mouse models of diabetes 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. CONCLUSIONS/
INTERPRETATION: We demonstrated that TXNIP impairs glucose and insulin tolerance in mice by upregulating G6pc through interaction with SHP.