The Cytokine Interleukin-1β Reduces the Docking and Fusion of Insulin Granules in Pancreatic β-Cells, Preferentially Decreasing the First Phase of Exocytosis

Abstract

The prediabetic period in type I diabetes mellitus is characterized by the loss of first phase insulin release. This might be due to islet infiltration mediated by mononuclear cells and local release of cytokines, but the mechanisms involved are unknown. To determine the role of cytokines in insulin exocytosis, we have presently utilized total internal reflection fluorescence microscopy (TIRFM) to image and analyze the dynamic motion of single insulin secretory granules near the plasma membrane in live ␤-cells exposed for 24 h to interleukin (IL)-1␤ or interferon (IFN)-␥. Immunohistochemistry observed via TIRFM showed that the number of docked insulin granules was decreased by 60% in ␤-cells treated with IL-1␤, while it was not affected by exposure to IFN-␥. This effect of IL-1␤ was paralleled by a 50% reduction in the mRNA and the number of clusters of SNAP-25 in the plasma membrane. TIRF images of single insulin granule motion during a 15-min stimulation by 22 mM glucose in IL-1␤-treated ␤-cells showed a marked reduction in the fusion events from previously docked granules during the first phase insulin release. Fusion from newcomers, however, was well preserved during the second phase of insulin release of IL-1␤-treated ␤-cells. The present observations indicate that IL-1␤, but not IFN-␥, has a preferential inhibitory effect on the first phase of glucose-induced insulin release, mostly via an action on previously docked granules. This suggests that ␤-cell exposure to immune mediators during the course of insulitis might be responsible for the loss of first phase insulin release.Accumulating evidence over the past 30 years suggests that ␤-cells are destroyed by an autoimmune process in type 1 diabetes mellitus (T1D) 1 (1). The prediabetic period in humans is characterized by the presence of islet cell autoantibodies (2), which seem to have a close correlation with histological evidence of autoimmunity (3). There are already subtle changes in ␤-cell function during this period, including both disproportionately elevated proinsulin/insulin levels (4, 5) and a preferential loss of the first phase insulin secretion in response to an intravenous glucose challenge (6 -9). Of note, ␤-cell suppression precedes ␤-cell death in T1D, as suggested by results obtained in islets isolated from prediabetic non-obese diabetic mice (10, 11) or from a patient who died immediately after diagnosis of T1D (12). This initial ␤-cell functional suppression might be due to exposure to cytokines. Indeed, our previous observations (13) indicate that islet exposure to cytokines under in vitro conditions reproduce the disproportionately elevated proinsulin/insulin levels observed in prediabetic patients. Moreover, microarray analysis of purified ␤-cells or insulin-producing INS-1 cells cultured in the presence of IL-1 ␤ or IL-1␤ ϩ IFN-␥ showed inhibition of the expression of several genes involved in the exocytosis of insulin granules, including SNAP-25, and Rab3 (14,15). This raised the intriguing possibility that cytokines could also co...

Proteomics Analysis of Cytokine-induced Dysfunction and Death in Insulin-producing INS-1E Cells

“…insulin, pancreas/duodenum homeobox 1 (pdx-1) and insulin gene enhancer protein (isl-1)). Several of these genes were confirmed at the protein level by ELISA, Western blotting, or other methods (5,57,58). The majority of these proteins, however, were not detected in the present study.…”

Histone Deacetylase (HDAC) Inhibition as a Novel Treatment for Diabetes Mellitus

“…Cytokines inhibit both first-and second-phase insulin release through decreased expression of insulin (128) and proteins that are important for insulin secretion (129). Interestingly, the cytokinemediated reduction in acute glucosestimulated insulin secretion, which mainly depends on release of preformed insulin vesicles and less on de novo insulin transcription, translation and processing, is unaffected by HDACi (27).…”

Global profiling of genes modified by endoplasmic reticulum stress in pancreatic beta cells reveals the early degradation of insulin mRNAs

“…Beta cell exposure to the cytokines IFN-γ and IL-1β, which induce ER stress [15], results in the inhibition of expression of Pcsk1/2, impairing the conversion of pro-insulin into insulin [43,44], and the decreased expression of Pdx-1 and Isl-1 [26]. These cytokines also decrease the expression of SNAP-25, VAMP-2 and Rab3a, whose protein products are involved in granule fusion with the beta cell membrane [22,45], preferentially decreasing the first phase of exocytosis [45]. The present data suggest that these effects of cytokines are, at least in part, secondary to ER stress.…”