Posttranslational regulation of thioredoxin-interacting protein

Abstract

Thioredoxin-interacting protein (Txnip) is a metabolic regulator, which modulates insulin sensitivity and likely plays a role in type 2 diabetes. We studied the regulation of Txnip in 3T3-L1 adipocytes. Cells were incubated under different conditions and Txnip was measured by immunoblotting. We confirmed that high glucose markedly increases Txnip expression by promoting transcription. Insulin decreases Txnip protein levels. Rapamycin under most conditions decreased Txnip, suggesting that mTOR complex-1 is involved. The acute effects of insulin are mainly posttranscriptional; insulin (100 nM) accelerates Txnip degradation more than tenfold. This effect is cell type specific. It works in adipocytes, preadipocytes and in L6 myotubes but not in HepG2 or in HEK 293 cells or in a pancreatic β-cell line. The ubiquitin/proteasome pathway is involved. Degradation of Txnip occurred within 15 min in the presence of 3 nM insulin and overnight with 0.6 nM insulin. Proteasomal Txnip degradation is not mediated by a cysteine protease or an anti-calpain enzyme. Okadaic acid (OKA), an inhibitor of phosphoprotein phosphatases (pp), markedly reduced Txnip protein and stimulated its further decrease by insulin. The latter occurred after incubation with 1 or 1000 nM OKA, suggesting that insulin enhances the phosphorylation of a pp2A substrate. Incubation with 0.1 μM Wortmannin, a PI3 kinase inhibitor, increased Txnip protein twofold and significantly inhibited its insulin-induced decrease. Thus, while OKA mimics the effect of insulin, Wortmannin opposes it. In summary, insulin stimulates Txnip degradation by a PI3 kinase-dependent mechanism, which activates the ubiquitin/proteasome pathway and likely serves to mitigate insulin resistance.