D. Schröder

IL-1β, IFN-γ and TNF-α increase vulnerability of pancreatic beta cells to autoimmune destruction

Abstract In the pathogenesis of type-1 diabetes insulin-producing β-cells are destroyed by cellular autoimmune processes. The locality of β-cell destruction is the inflamed pancreatic islet. During insulitis cytokines released from islet-infiltrating mononuclear cells affect β-cells at several levels. We investigated whether cytokine-induced β-cell destruction is associated with changes in the expression of the surface receptors intercellular adhesion molecule (ICAM)-1 and Fas. Islets from diabetes-prone and congenic diabetes-resistant BB rats were exposed to interleukin (IL)-1β alone or in combination with interferon (IFN)-γ plus tumour necrosis factor (TNF)-α. Cytokines decreased islet insulin content, suppressed glucose stimulated insulin secretion and generated enhanced amounts of nitric oxide and DNA-strand breaks. While no membrane alterations of IL-1β treated islets cells were detectable, the cytokine combination caused damage of cell membranes. Independent of diabetes susceptibility IL-1β treated islet β-cells expressed a significantly increased amount of ICAM-1 on their surfaces which was not further increased by IFN-γ+TNF-α. However, IL-1β induced Fas expression was significantly enhanced only on β-cells from diabetes-prone BB rats. From these results we suggest that IL-1β mediates the major stimulus for ICAM-1 induction which is possibly a necessary but not sufficient step in the process of β-cell destruction. Obviously, the additional enhancement of Fas expression on the surface of β-cells is important for destruction. The combined action of all three cytokines induced the expression of Fas on the β-cell surface independent of diabetes susceptibility, indicating that such a strong stimulus in vitro may induce processes different from the precise mechanisms of β-cell destruction in vivo.

Impact of metabolic activity of beta cells on cytokine-induced damage and recovery of rat pancreatic islets

The influence of beta cell activity on cytokineinduced functional and structural impairments as well as the ability of those damaged cells to recover were investigated. Rat islets cultured for 4 days in the presence of 5, 10, and 30 mmol/l glucose were exposed to interferon-γ (IFN, 500 U/ml) and tumor necrosis factor-α (TNF, 250 U/ml) for the last 24 h. After cytokine removal islets were allowed to recover spontaneously in culture medium containing 10 mmol/l glucose for a further 7 days. Cytokines significantly inhibited insulin release into culture medium, insulin storage, glucose-stimulated insulin secretion, protein, and DNA synthesis. In the presence of cytokines there was a six- to eightfold increase in nitrite production by the islets. The functional impairments were more pronounced in metabolically stimulated beta cells. In addition, cytokines caused membrane alterations as indicated by increased spontaneous chromium-51 release. The cytokines specifically induced the synthesis of two proteins (72 and 88 kDa, respectively). By immunoblotting, the 72-kDa protein was identified as heat shock protein. After a 1-week recovery period, insulin storage and stimulated insulin secretion of cytokine-treated islets were still significantly diminished. However, protein and DNA synthesis of cytokine-exposed islets returned to pre-exposure levels. In conclusion, high beta cell activity increases islet susceptibility to TNF+IFN. Cytokine-induced, longlasting, inhibitory effects are primarily directed to betacell-specific functions, while general vital cell functions clearly recover after cytokine removal. The induction of certain proteins and the increased protein synthesis and replication rate after cytokine removal might reflect activated repair processes.