Zinc, the pancreas, and diabetes: insights from rodent studies and future directions

In vivo expression and functional characterization of the zinc transporter ZnT8 in glucose-induced insulin secretion

Insulin-secreting pancreatic beta cells are exceptionally rich in zinc. In these cells, zinc is required for zinc-insulin crystallization within secretory vesicles. Secreted zinc has also been proposed to be a paracrine and autocrine modulator of glucagon and insulin secretion in pancreatic alpha and beta cells, respectively. However, little is known about the molecular mechanisms underlying zinc accumulation in insulin-containing vesicles. We previously identified a pancreas-specific zinc transporter, ZnT-8, which colocalized with insulin in cultured beta cells. In this paper we studied its localization in human pancreatic islet cells, and its effect on cellular zinc content and insulin secretion. In human pancreatic islet cells, ZnT-8 was exclusively expressed in insulin-producing beta cells, and colocalized with insulin in these cells. ZnT-8 overexpression stimulated zinc accumulation and increased total intracellular zinc in insulin-secreting INS-1E cells. Furthermore, ZnT-8-overexpressing cells display enhanced glucose-stimulated insulin secretion compared with control cells, only for a high glucose challenge, i.e. >10 mM glucose. Altogether, these data strongly suggest that the zinc transporter ZnT-8 is a key protein for both zinc accumulation and regulation of insulin secretion in pancreatic beta cells.

The effect of Zn(II) and streptozotocin administration in the mouse brain

Streptozotocin is a natural antibiotic produced by Streptomyces achromogenes able to induce diabetes in experimental animals. Among various toxic properties, streptozotocin is a potent source for reactive oxygen species. In this paper, we report the biological response of brain, upon treatment with streptozotocin in terms of metal ions dismetabolism and metallothionein expression. In addition, important information on the preventive effect of zinc in eliciting the pharmacological effect of the drug are reported, in relation to the effective role of the metal ions in inducing metallothionein synthesis. In the brain, streptozotocin treatment affects mostly the hippocampus and cerebellum as shown by a high GAFP and MT-I-II immunopositivity of glial cells. The Zn pre-treatment reduces significantly, as a general effect, the occurrence of hyperglycaemic status. At the brain level, the observed astrocytosis is strongly reduced. The high inducibility of MT represents a rapid and convenient response able to prevent the deleterious effects consequent to the oxidative stress. All together these results support the efficacy of the Zn treatment in order to prevent streptozotocin effects, including brain tissues.

Approaches towards endogenous pancreatic regeneration

The phenomenon of pancreatic regeneration in mammals has been well documented. It has been shown that pancreatic tissue is able to regenerate in several species of mammal after surgical insult. This tissue is also known to have the potential to maintain or increase its beta-cell mass in response to metabolic demands during pregnancy and obesity. Since deficiency in beta-cell mass is the hallmark of most forms of diabetes, it is worthwhile understanding pancreatic regeneration in the context of this disease. With this view in mind, this article aims to discuss the potential use in clinical strategies of knowledge that we obtained from studies carried out in animal models of diabetes. Approaches to achieve this goal involve the use of biomolecules, adult stem cells and gene therapy. Various molecules, such as glucagon-like peptide-1, beta-cellulin, nicotinamide, gastrin, epidermal growth factor-1 and thyroid hormone, play major roles in the initiation of endogenous islet regeneration in diabetes. The most accepted hypothesis is that these molecules stimulate islet precursor cells to undergo neogenesis or to induce replication of existing beta-cells, emphasizing the importance of pancreas-resident stem/progenitor cells in islet regeneration. Moreover, the potential of adult stem cell population from bone marrow, umbilical cord blood, liver, spleen, or amniotic membrane, is also discussed with regard to their potential to induce pancreatic regeneration.