Investigation of transport mechanisms and regulation of intracellular Zn2+ in pancreatic α-cells

Abstract

During insulin secretion, pancreatic α-cells are exposed to Zn 2+ released from insulin-containing secretory granules. Although maintenance of Zn2+ homeostasis is critical for cell survival and glucagon secretion, very little is known about Zn2+-transporting pathways and the regulation of Zn2+ in α-cells. To examine the effect of Zn2+ on glucagon secretion and possible mechanisms controlling the intracellular Zn2+ level ([Zn2+]i i), we employed a glucagon-producing cell line (α-TC6) and mouse islets where non-β-cells were identified using islets expressing green fluorescent protein exclusively in β-cells. In this study, we first confirmed that Zn2+ treatment resulted in the inhibition of glucagon secretion in α-TC6 cells and mouse islets in vitro. The inhibition of secretion was not likely via activation of KATP channels by Zn 2+. We then determined that Zn2+ was transported into α-cells and was able to accumulate under both low and high glucose conditions, as well as upon depolarization of cells with KCl. The nonselective Ca2+ channel blocker Gd3+ partially inhibited Zn 2+ influx in α-TC cells, whereas the L-type voltage-gated Ca2+ channel inhibitor nitrendipine failed to block Zn2+ accumulation. To investigate Zn2+ transport further, we profiled α-cells for Zn2+ transporter transcripts from the two families that work in opposite directions, SLC39 (ZIP, Zrt/Irt-like protein) and SLC30 (ZnT, Zn2+ transporter). We observed that Zip1, Zip10, and Zip14 were the most abundantly expressed Zips and ZnT4, ZnT5, and ZnT8 the dominant ZnTs. Because the redox state of cells is also a major regulator of [Zn 2+]i, we examined the effects of oxidizing agents on Zn2+ mobilization within α-cells. 2,2′-Dithiodipyridine (-SH group oxidant), menadione (superoxide generator), and SIN-1 (3-morpholinosydnonimine) (peroxynitrite generator) all increased [Zn 2+]i in α-cells. Together these results demonstrate that Zn2+ inhibits glucagon secretion, and it is transported into α-cells in part through Ca2+ channels. Zn2+ transporters and the redox state also modulate [Zn2+]i. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.