Cytokines induce deoxyribonucleic acid strand breaks and apoptosis in human pancreatic islet cells

We have previously observed that a 6-day exposure of human pancreatic islets to a combination of cytokines (interleukin-1beta 50 U/ml + tumour necrosis factor-alpha 1000 U/ml + interferon-gamma 1000 U/ml) severely impairs beta-cell functions. In the present study, we examined whether this condition affects DNA integrity and viability of human islet cells. Cells were studied after 3, 6, and 9 days of cytokine treatment by both single cell gel electrophoresis (the "comet assay," a sensitive method for detection of DNA strand breaks) and by a cytotoxicity assay using the DNA binding dyes Hoechst 33342 and propidium iodide as indices for the number of viable, necrotic, and apoptotic cells. Cytokine treatment for 6 and 9 days resulted in a 50% increase in comet length (P < 0.01 vs. controls), indicating DNA strand breaks, as well as in a significant increase in the number of apoptotic cells (P < 0.02 vs. controls), but not in the number of necrotic cells. The arginine analogs N(G)-nitro-L-arginine and N(G)-monomethyl-L-arginine prevented nitric oxide formation by the cytokines but did not interfere with cytokine-induced DNA strand breaks and apoptosis. The present data suggest that prolonged (6-9 days) exposure of human pancreatic islets to a mixture of cytokines induces DNA strand breaks and cell death by apoptosis. These deleterious effects of cytokines appear to be independent of nitric oxide generation.