Insulinoma cells in culture show pronounced sensitivity to alloxan-induced oxidative stress

In a model system of cultured J-774 cells we have previously shown that alloxan in moderate concentration is toxic only in the presence of a reducing agent with the production of hydrogen peroxide. The cytotoxicity was found to involve lysosomal destabilization. In the present study the cytotoxic effects of (i) alloxan alone, (ii) a combination of alloxan and cysteine or (iii) hydrogen peroxide were investigated in two established insulinoma cell lines (HIT-T15 and RINm5F), and compared with the effects on J-774 cells. The protective effects of desferrioxamine and catalase, and the intracellular levels of reduced glutathione and activities of the enzymes glutathione peroxidase, glutathione reductase and catalase were also studied. HIT and RIN cells showed about 10 times greater sensitivity than J-774 cells against exposure to either alloxan and cysteine, or hydrogen peroxide. All cell types were relatively insensitive to alloxan alone. Preincubation with desferrioxamine and addition of catalase provided efficient protection against cytotoxicity and lysosomal destabilization. HIT and RIN cells has less capacity to degrade hydrogen peroxide and lower levels of glutathione peroxidase than J-774 cells. The lysosomal stability in all three cell lines was directly correlated to their viability. We conclude that HIT and RIN cells have weak antioxidative defence systems resulting in enhanced lysosomal vulnerability when they are exposed to alloxan and cysteine, which produce hydrogen peroxide extracellularly. The degree of cytotoxicity seems to be dependent on cellular capacity to degrade hydrogen peroxide and the lysosomal content of reactive iron.