The long-acting GLP-1 analogue liraglutide regulates zinc transporter mRNA in β cells

Background and Aims: Zinc transporters (ZnT)-proteins regulate zinc flux into β-cell vesicles. β-cells depend on zinc for crystallization of insulin. Zinc efflux, which is also regulated by ZnTs controls ȕ-cell mass and glucagon secretion. Recently, the zinc transporter gene ZnT8 was identified as a novel risk factor for type 2 diabetes. We examined whether glucose levels, zinc depletion, GLP-1 or liraglutide affect ZnT mRNA expression. Materials and Methods: The rat ȕ-cell line INS1-E was exposed to 2, 5 or 16 mM glucose for 24 hrs. For zinc depletion the INS1-E cells were exposed to 100 ȝM DEDTC for 24 hrs. GLP-1 and liraglutide was added at 5 or 16 nM glucose (GLP-1 1 or 10 nM; liraglutide 10 or 100 nM) for 24 hrs. ZnT mRNA was measured by RT-PCR. Insulin was measured by Elisa. Regulations of genes were considered significant with p-values for relative expressions < 0.05. Results: Relative to 5 mM glucose, 2 mM glucose caused a down-regulation of ZnT-3 and an up-regulation of most other ZnT mRNA. 16 mM caused ZnT3 up-regulation and down-regulation of other ZnT mRNA. Results were significant for ZnT3, ZnT5 and ZnT8, all transporters with high expression levels in ȕ-cells. Zinc chelation lowered insulin expression and content and insignificant – insulin secretion along with a ZnT3 up-regulation. Liraglutide augmented glucose stimulated ZnT3 expression (1.73 fold increase), stimulated insulin expression and eliminated the glucose dependant downregulation of ZnT7 and ZnT8. Conclusion: ZnT mRNA in ȕ-cells responds to changes in glucose and zinc, supporting the hypothesis that ZnTs participate in insulin processing and that zinc trafficking in ȕ-cells is regulated. Furthermore, long-term (liraglutide) but not short-term (GLP-1) stimulation of the GLP-1 receptor has significant influences on ZnT mRNA expression indicating that this system is a potential target for pharmacological manipulation. (Fig. 1)