Date Published: June 13, 2013
Publisher: Public Library of Science
Author(s): Morten Dall, Kirstine Calloe, Martin Haupt-Jorgensen, Jesper Larsen, Nicole Schmitt, Knud Josefsen, Karsten Buschard, Matthias G. von Herrath. http://doi.org/10.1371/journal.pone.0066474
In non-obese diabetic (NOD) mice, diabetes incidence is reduced by a gluten-free diet. Gluten peptides, such as the compound gliadin, can cross the intestinal barrier and may directly affect pancreatic beta cells. We investigated the effects of enzymatically-digested gliadin in NOD mice, INS-1E cells and rat islets. Six injections of gliadin digest in 6-week-old NOD mice did not affect diabetes development, but increased weight gain (20% increase by day 100). In INS-1E cells, incubation with gliadin digest induced a dose-dependent increase in insulin secretion, up to 2.5-fold after 24 hours. A similar effect was observed in isolated rat islets (1.6-fold increase). In INS-1E cells, diazoxide reduced the stimulatory effect of gliadin digest. Additionally, gliadin digest was shown to decrease current through KATP-channels. A specific gliadin 33-mer had a similar effect, both on current and insulin secretion. Finally, INS-1E incubation with gliadin digest potentiated palmitate-induced insulin secretion by 13% compared to controls. Our data suggest that gliadin fragments may contribute to the beta-cell hyperactivity observed prior to the development of type 1 diabetes.
Gluten is a wheat protein that confers elasticity to white bread, and it is universally present in the western diet. Gluten consists of two families of prolamins, known as gliadin and glutenin. Gliadin is a strongly hydrophobic glycoprotein, with a very poor solubility. This severely limits its enzymatic degradation, which results in the persistence of gliadin fragments in the gut and intestine. This has been reported to initiate subclinical inflammation in the intestinal mucosa . Up to 2% of Caucasians develop celiac disease, also known as gluten intolerance, which is an immune-mediated enteropathy. A variety of proline-rich, protease-resistant gliadin fragments are implicated in the pathogenesis of celiac disease, including a specific 33-mer peptide . Ex vivo, the 33-mer is transported into duodenal cells, where it escapes degradation . A gliadin 19-mer has also been described as a gliadin digestion product, and transport of the 19-mer across the epithelial barrier is well-established in untreated celiac disease patients . Non-degraded gliadin has been identified in breast milk of healthy mothers , providing evidence that gliadin can pass undigested from the intestine into the bloodstream. Patients with type 1 diabetes mellitus (T1D) have increased intestinal permeability , which suggests that gluten exposure may be increased in this group.
We have demonstrated that gliadin digest potentiates insulin secretion in INS-1E cells and rat islets, independently of glucose levels. This effect relies on closure of the ATP-sensitive potassium channels. The protease-resistant 33-mer gliadin fragment, which is generated in large quantities by enzymatic digestion of gliadin , may be the stimulatory component in the gliadin digest. Due to the observed weight gain in NOD mice following gliadin digest administration, we found it reasonable to investigate the effect of gliadin in vitro on beta cells. We hypothesised the increased weight was a consequence of increased insulin secretion, which induced a trophic effect in the insulin target tissues. Nevertheless, the observed mechanism remains unproven in vivo. Furthermore, gliadin digest injections did not accelerate the development of diabetes in NOD mice. This results is corroborated in parallel by the finding that a gluten enriched diet does not increase NOD diabetes incidence .