Glucose Regulates Free Cytosolic Zn(2+) Concentration, Slc39 (ZiP), and Metallothionein Gene Expression in Primary Pancreatic Islet β-Cells

Zn(2+) is an important cofactor for insulin biosynthesis and storage in pancreatic β-cells. Correspondingly, polymorphisms in the SLC30A8 gene, encoding the secretory granule Zn(2+) transporter ZnT8, are associated with type 2 diabetes risk. Using a genetically engineered (FRET)-based sensor (eCALWY-4), we show here that elevated glucose time-dependently increases free cytosolic Zn(2+) ([Zn(2+)](cyt)) in mouse pancreatic β-cells. These changes become highly significant (853 ± 96 pm versus 452 ± 42 pm, p < 0.001) after 24 h and are associated with increased expression of the Zn(2+) importer family members Slc39a6, Slc39a7, and Slc39a8, and decreased expression of metallothionein 1 and 2. Arguing that altered expression of the above genes is not due to altered [Zn(2+)](cyt), elevation of extracellular (and intracellular) [Zn(2+)] failed to mimic the effects of high glucose. By contrast, increases in intracellular cAMP prompted by 3-isobutyl-1-methylxanthine and forskolin partially mimicked the effects of glucose on metallothionein, although not ZiP, gene expression. Modulation of intracellular Ca(2+) and insulin secretion with pharmacological agents (tolbutamide and diazoxide) suggested a possible role for changes in these parameters in the regulation of Slc39a6 and Slc39a7 but not Slc39a8, nor metallothionein expression. In summary, 1) glucose induces increases in [Zn(2+)](cyt), which are then likely to facilitate the processing and/or the storage of insulin and its cocrystallization with Zn(2+), and 2) these increases are associated with elevated expression of zinc importers. Conversely, a chronic increase in [Zn(2+)](cyt) following sustained hyperglycemia may contribute to β-cell dysfunction and death in some forms of diabetes.