Glucose-Induced Nuclear Shuttling of ChREBP Is Mediated by Sorcin and Ca~(2+) Ions in Pancreatic β-Cells

摘要

Carbohydrate-responsive element-binding protein (ChREBP) is a regulator of pancreatic β-cell gene expression and an important mediator of glucotoxicity. Glucose increases the activity and nuclear localization of ChREBP by still ill-defined mechanisms. Here we reveal, using both MIN6 and primary mouse β-cells, a unique mechanism behind ChREBP nuclear translocation. At low glucose concentrations, ChREBP interacts with sorcin, a penta EF hand Ca~(2+) binding protein, and is sequestered in the cytosol. Sorcin overexpression inhibits ChREBP nuclear accumulation at high glucose and reduced the activity of L-type pyruvate kinase (L-PK) and TxNIP promoters, two well-characterized ChREBP target genes. Sorcin inactivation by RNA interference increases ChREBP nuclear localization and in vivo binding to the L-PK promoter at low glucose concentrations. Ca~(2+) influx was essential for this process since Ca~(2+) chelation with EGTA, or pharmacological inhibition with diazoxide and nifedipine, blocked the effects of glucose. Conversely, mobilization of intracellular Ca~(2+) with ATP caused the nuclear accumulation of ChREBP. Finally, sorcin silencing inhibited ATP-induced increases in intracellular Ca~(2+) and glucose-stimulated insulin secretion. We therefore conclude that sorcin retains ChREBP in the cytosol at low glucose concentrations and may act as a Ca~(2+) sensor for glucose-induced nuclear translocation and the activation of ChREBP-dependent genes.