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

Abstract

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 p m versus 452 ± 42 p m, 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.