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DOI: 10.1055/s-0034-1374963
Regulation of thioredoxin interacting protein by FFA1 ligands under hyperglycemic conditions
Background and aim: Multiple observations suggest that glucose-dependent stimulation of thioredoxin interacting protein (TxNIP) expression contributes to glucotoxicity. TxNIP inhibits thioredoxin thereby increasing the mitochondrial oxidative stress. TxNIP is transcriptionally activated by ChREBP, while FOXO1 antagonizes ChREBP binding to the TxNIP promoter. Glucose increases nuclear accumulation of ChREBP. Palmitic acid, a ligand for free fatty acid receptor-1 (FFA1), activates FOXO1 in a PKCdelta-dependent manner and has been found to inhibit TxNIP expression. Previously, we observed that a synthetic agonist for FFA1 exhibits anti-apoptotic properties. The present study investigates whether FFA1 ligands regulate TxNIP expression and examines a possible involvement of PKCdelta and FOXO1.
Methods: Protein expression was analysed by real time RT-PCR and western blotting. FOXO1 and PKCdelta expressions were inhibited using siRNA techniques.
Results: At 11 mM glucose, palmitic acid reduced mRNA levels of TxNIP in INS-1E cells. Similarly, low levels of TxNIP mRNA were measured in INS-1E cells in which nuclear accumulation of FOXO1 was induced by overexpression of PKCdelta. The FFA1-agonist TUG-469 (3 – 10µM) mimicked the effect of palmitic acid and decreased, while the FFA1-antagonist TUG-761 (10µM) increased TxNIP expression. Although TUG-469 induced nuclear accumulation of FOXO1 in 48% of the cells, reduction of neither FOXO1 (by 80%) nor PKCdelta (by 60%) reversed the effect of TUG-469 on TxNIP mRNA.
Conclusion: These observations suggest that TUG-469 inhibits TxNIP expression through a mechanism independent of the PKCdelta-mediated nuclear accumulation of FOXO1. Nevertheless, TxNIP inhibition may contribute to the anti-apoptotic property of TUG-469.