Aims/hypothesis: IL-1beta is a candidate mediator of apoptotic beta cell destruction, a process that leads to type 1 diabetes and progression of type 2 diabetes. IL-1beta activates beta cell c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK) and p38, all of which are members of the mitogen-activated protein kinase (MAPK) family. Inhibition of JNK prevents IL-1beta-mediated beta cell destruction. In mouse embryo fibroblasts and 3DO T cells, overexpression of the gene encoding growth arrest and DNA-damage-inducible 45beta (Gadd45b) downregulates pro-apoptotic JNK signalling. The aim of this study was to investigate if Gadd45b prevents IL-1beta-induced beta cell MAPK signalling and apoptosis.
Materials: Rat insulinoma INS-1E cells and mouse beta-TC3 cells stably expressing Gadd45b were generated. The effects of Gadd45b expression on signalling by JNK, ERK and p38 were assessed by Western blotting and kinase assays. Apoptosis rate was measured by terminal deoxynucleotidyl-mediated dUTP-biotin nick end-labelling (TUNEL) and an ELISA designed to detect apoptotic nucleosomes. Expression of endogenous Gadd45b mRNA was measured by RT-PCR.
Results: In INS-1E and beta-TC3 cells, expression of Gadd45b inhibited IL-1beta-induced activation of JNK and ERK, but augmented IL-1beta-mediated p38 activity. IL-1beta-induced nitric oxide production and decreases in insulin content and secretion were reduced by GADD45beta. IL-1beta-induced apoptosis was reduced by GADD45beta by up to 77%. Although IL-1beta stimulated the time-dependent induction of endogenous Gadd45b in INS-1E cells and rat islets, expression levels were lower in these cells than in IL-1beta-exposed NIH-3T3 and 3DO T cells.
Conclusions/interpretation: Inadequate induction of Gadd45b, which encodes a novel beta cell JNK and ERK inhibitor, may in part explain the pro-apoptotic response of beta cells to IL-1beta.