Glucose and zinc-depletion stimulate beta cell expression of ZnT3, a zinc transporter, whereas ZnT3 KO mice have decreased beta cell capacity

Background: Zinc transporter (ZnT)-proteins regulate zinc fluxes into beta-cell vesicles and recently ZnT8 was identified as a major risk factor for developing type 2 diabetes. Contrasting other ZnTs, ZnT3 is upregulated by high glucose levels as well as by zinc depletion in ȕ-cells. We here confirm the glucose dependence of ZnT3 and show by electron microscopy that ZnT3 is located in beta-cell cytoplasm. Further, we report the first results of experiments examining glucose metabolism in ZnT3 KO mice. Methods: Ins1-E rat beta-cells were exposed to 2, 5 or 16 mM glucose for 24 hrs. ZnT3 mRNA was measured by RT-PCR. Measurements were normalized to a mean of three house-keeping genes. Insulin secretion was measured by Elisa. For electron microscopy, INS1-E cells were fixed in paraformaldehyde and examined for the presence of ZnT3 by immunogold staining with a specific goat-rabbit antibody. To determine if knocking out the ZnT3 gene affects whole body glucose metabolism, twelve weeks old ZnT3 KO mice derived from the C57B16 mouse strain were compared to wild type mice after five days of treatment with intraperitoneal streptozotocin 50mg/kg daily or saline. After two weeks of recovery, tail blood glucose levels were measured after a 14 hrs fast with a One Touch Ultra Meter. Results: We confirmed that, relative to 5 mM glucose, 2 mM glucose caused a significant down-regulation of ZnT-3 and 16 mM an up-regulation (p < 0.01). Insulin secretion was increased at 16 mM glucose. By electron microscopy we could identify the presence of ZnT3 in the cytoplasm of INS1- E cells. No signifant changes in blood glucose levels were found between ZnT3 KO- and WT mice. However, after treatment with streptozotozin, KO mice had significantly higher blood glucose levels compared with shaminjected KO mice (p < 0.05) and streptozotocin-treated WT mice (p < 0.01) (Figure). Conclusion: This study identifies ZnT3 as a pivotal regulator of beta-cell zinc metabolism. Zinc ions have numerous roles in the islets as well as possible roles in the periphery. These include regulation of counter-regulatory hormones and beta-cell apoptosis as well as the formation of insulin crystals.