Low-dose interleukin 1 and tumor necrosis factor individually stimulate insulin release but in combination cause suppression.

The macrophage-derived cytokines interleukin 1 (IL-1) and tumor necrosis factor (TNF) have direct effects on pancreatic beta cells and have been hypothesized to play important roles in the autoimmune beta cell lesion of type I diabetes because of two major effects on beta cells: altered insulin secretion and beta cell cytotoxicity. High doses of IL-1 are cytotoxic to beta cells and strongly inhibit insulin release; high-dose IL-1 plus TNF acts synergically to suppress further the insulin release. In contrast, we observed that the predominant effect of low-dose IL-1 and TNF when administered separately was the stimulation of insulin release. We therefore asked whether the combination of low-dose IL-1 plus TNF would act synergistically to stimulate or suppress insulin release. Studies were performed on cultured rat islets and both insulin release and cytotoxicity (51Cr release) were measured. After 2 days of culture, increasing doses of IL-1-25, 50, 75 and 100 ng/l--caused progressively increased cytotoxicity and impaired insulin release. In contrast, the lowest dose of IL-1 tested, 10 ng/l, increased insulin release but was still slightly cytotoxic. Tumor necrosis factor at doses of 10, 25, 62.5, 75 and 100 micrograms/l also was slightly cytotoxic but increased insulin release. The augmented insulin release declined progressively with increasing TNF dose. However the combination of insulin stimulatory doses of IL-1 (10 ng/l) and TNF (62.5 micrograms/l) suppressed insulin release. The effects of these two cytokines on insulin release demonstrated a similar pattern after 4 and 6 days of culture.(ABSTRACT TRUNCATED AT 250 WORDS)