Investigation of transport mechanisms and regulation of intracellular Zn2+ in pancreatic alpha-cells.
During insulin secretion, pancreatic alpha-cells are exposed to Zn(2+) released from insulin-containing secretory granules. Although maintenance of Zn(2+) homeostasis is critical for cell survival and glucagon secretion, very little is known about Zn(2+)-transporting pathways and the regulation of Zn(2+) in alpha-cells. To examine the effect of Zn(2+) on glucagon secretion and possible mechanisms controlling the intracellular Zn(2+) level ([Zn(2+)](i)), we employed a glucagon-producing cell line (alpha-TC6) and mouse islets where non-beta-cells were identified using islets expressing green fluorescent protein exclusively in beta-cells. In this study, we first confirmed that Zn(2+) treatment resulted in the inhibition of glucagon secretion in alpha-TC6 cells and mouse islets in vitro. The inhibition of secretion was not likely via activation of K(ATP) channels by Zn(2+). We then determined that Zn(2+) was transported into alpha-cells and was able to accumulate under both low and high glucose conditions, as well as upon depolarization of cells with KCl. The nonselective Ca(2+) channel blocker Gd(3+) partially inhibited Zn(2+) influx in alpha-TC cells, whereas the L-type voltage-gated Ca(2+) channel inhibitor nitrendipine failed to block Zn(2+) accumulation. To investigate Zn(2+) transport further, we profiled alpha-cells for Zn(2+) transporter transcripts from the two families that work in opposite directions, SLC39 (ZIP, Zrt/Irt-like protein) and SLC30 (ZnT, Zn(2+) 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 Zn(2+) mobilization within alpha-cells. 2,2'-Dithiodipyridine (-SH group oxidant), menadione (superoxide generator), and SIN-1 (3-morpholinosydnonimine) (peroxynitrite generator) all increased [Zn(2+)](i) in alpha-cells. Together these results demonstrate that Zn(2+) inhibits glucagon secretion, and it is transported into alpha-cells in part through Ca(2+) channels. Zn(2+) transporters and the redox state also modulate [Zn(2+)](i).
Gyulkhandanyan
Armen V
AV
Department of Physiology, University of Toronto, Toronto, Ontario, Canada.
Lu
Hongfang
H
Lee
Simon C
SC
Bhattacharjee
Alpana
A
Wijesekara
Nadeeja
N
Fox
Jocelyn E Manning
JE
MacDonald
Patrick E
PE
Chimienti
Fabrice
F
Dai
Feihan F
FF
Wheeler
Michael B
MB
D000818:Animals; D027682:Cation Transport Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002460:Cell Line; D005682:Gadolinium / Q000494:pharmacology; D005934:Glucagon / Q000378:metabolism; D050416:Glucagon-Secreting Cells / Q000166:cytology / Q000378:metabolism*; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D017136:Ion Transport / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008822:Mice, Transgenic; D016877:Oxidants / Q000494:pharmacology; D022142:Secretory Vesicles / Q000378:metabolism; D015032:Zinc / Q000378:metabolism*
D027682:Cation Transport Proteins; D007328:Insulin; D016877:Oxidants; D005934:Glucagon; D005682:Gadolinium; D015032:Zinc