Metallothionein and Catalase Sensitize to Diabetes in Nonobese Diabetic Mice

Abstract

It is widely proposed that reactive oxygen species (ROS) contribute to ␤-cell death in type 1 diabetes. We tested this in nonobese diabetic (NOD) mice using ␤-cell-specific overexpression of three antioxidant proteins: metallothionein (MT), catalase (Cat), or manganese superoxide dismutase (MnSOD). Unexpectedly, the cytoplasmic antioxidants, MT and catalase, greatly accelerated diabetes after cyclophosphamide and accelerated spontaneous diabetes in male NOD mice. This occurred despite the fact that they reduced cytokine-induced ROS production and MT reduced streptozotocin diabetes in NOD mice. Accelerated diabetes onset coincided with increased ␤-cell death but not with increased immune attack. Islets from MTNOD mice were more sensitive to cytokine injury. In vivo and in vitro studies indicated reduced activation of the Akt/pancreatic duodenal homeobox-1 survival pathway in MTNOD and CatNOD islets. Our study indicates that cytoplasmic ROS may have an important role for protecting the ␤-cell from autoimmune destruction. Diabetes 55:1592-1604, 2006

Molecular mechanisms for hyperinsulinaemia induced by overproduction of selenium-dependent glutathione peroxidase-1 in mice

“…Along with a modest decrease (24%) in pancreatic PDX1 phosphorylation (degradation) [25], the upregulation of Pdx1 gene expression enabled the OE mice to maintain a higher level of functional PDX1 protein than the WT mice to promote beta cell differentiation and insulin synthesis. While similar positive effects on PDX1 protein were produced by three dietary antioxidant supplements in C57BL/KsJ-db/db mice [23], overproduction of catalase in beta cells accelerated cytokine-induced PDX1 protein disappearance [9]. It is intriguing to see such different impacts on PDX1 protein by GPX1 and catalase, given the so similar catalytic functions.…”

“…Scale bar, 10 µm. Magnification, 40× insulin concentration nor insulin gene expression in islets was altered by overexpressing catalase by up to 50-fold [9], two forms of metallothionein by up to 30-fold [9,42] or three forms of superoxide dismutase enzymes by up to 10-fold [5,7]. Furthermore, overproduction of metallothionein and catalase in beta cells actually accelerated cytokineinduced beta cell death [9].…”

“…*p<0.05; †p≤0.06; ‡p≤0.07 for difference between genotypes transcription by protecting ROS-induced nuclear localisation of forkhead box O1 (an inhibitor of PDX1 transcription) [46]. The decreased levels of phosphorylated AKT on Thr-308 might attenuate H 2 O 2 -mediated phosphorylation (degradation) of PDX1 [25], but could also inhibit Pdx1 transcription via reduced phosphorylation of forkhead box O1 [9,47]. While the decrease of PTP1B protein with concomitant increases of PDX1 protein, beta cell mass and insulin synthesis in the OE islets resembled the phenotype of PTP1B-null mice [32], it does not support a possible increase of ROS inhibition of PTP1B as the ultimate mechanism for suppressing the AKT/PDX1 pathway by catalase overexpression [9].…”

“…However, overproduction of antioxidant proteins in animals, either specifically in pancreatic beta cells [5,6] or ubiquitously in various tissues [7,8], has generated highly variable or even negative phenotypes [9]. Most strikingly, we have observed spontaneous development of hyperglycaemia, hyperinsulinaemia, insulin resistance and obesity in mice overproducing Se-dependent cellular glutathione peroxidase-1 (GPX1), a major intracellular antioxidant enzyme [10].…”

“…In addition, Pdx1 transcription and PDX1 stability are affected by three key insulin signal proteins: c-jun terminal kinase (JNK), protein kinase B (AKT) and protein tyrosine phosphatase 1B (PTP1B) [20,31,32]. Because these proteins are highly sensitive to ROS [9,33,34], elevated GPX1 activity may impact on PDX1 via alteration of their production or phosphorylation.…”