Minute oxidative stress is sufficient to induce apoptotic death of NIT-1 insulinoma cells

When cultured NIT-1 cells were subjected to a low level of oxidative stress (30 mu M hydrogen peroxide for 15 min at 37 degrees C) several of their lysosomes ruptured, as demonstrated by intravital staining with the lysosomotropic weak base acridine orange. Such rupture is due to intralysosomal, iron-catalyzed oxidative reactions, since it was largely prevented by previous endocytotic uptake of desferrioxamine. The resultant limited leakage of lysosomal hydrolytic enzymes into the cytosol could be important for an apoptotic-type degradation/fragmentation process within initially intact plasma membranes. In contrast, extensive lysosomal rupture leads to necrosis. The development of the damage process was followed by light- and electron microscopy; and by the TUNEL-reaction. As a result of the applied oxidative stress, which is comparable to that expected to occur within the microenvironment surrounding activated macrophages under oxidative burst (e.g. during autoimmune insulitis), about 90% of the cells eventually died due to post-apoptotic secondary necrosis. The few surviving cells phagocytosed the debris from their fragmented neighbours and began to divide about 24 h after the insult. Thus the sensitivity to oxidative stress varies, perhaps as a consequence of varying amounts of intralysosomal redox-active iron, as we have found to be the case in several other cellular systems. Since the NIT-1 cells are highly differentiated, and in many ways like beta cells, we consider our result to be of value for the understanding of beta-cell death during the development of insulin-dependent (Type I) diabetes mellitus (IDDM).