During proliferative glomerulonephritis, the early phase of mesangiolysis is linked to increased nitric oxide (NO) production. NO. as well as superoxide (O2-) are inflammatory mediators that are generated by mesangial cells (MC) after cytokine stimulation. Added individually, both radicals induce MC apoptosis. However, the co-existence of a defined NO./O2- ratio is cross-protective. Apoptosis is characterized by specific features such as chromatin condensation, DNA strand breaks, and the occurrence of apoptotic regulating proteins. The tumor suppressor p53 and Bax (Bcl-2 associated protein x) are considered to be classical death promotors, which accumulate after toxic insults. To study p53 and Bax protein accumulation in NO. and/or O2(-)-induced apoptosis, we used the NO-donor S-nitrosoglutathione (GSNO) and the redox cycler 2,3-dimethoxy-1,4-naphtoquione (DMNQ). Both agonists initiated DNA fragmentation in a concentration dependent manner associated with transient p53 and Bax up-regulation. Co-generation of NO./O2- resulted not only in reduced DNA fragmentation, but also in decreased Bax accumulation. Comparable to the NO./O2- co-generation, cytokines failed to induce apoptosis. In contrast, cytokines in combination with pyrrolidine dithiocarbamate, which blocks endogenous superoxide dismutase, allowed p53 and Bax accumulation as well as DNA fragmentation. Our results demonstrate p53 and Bax as early components in NO. and O2(-)-induced rat MC apoptosis and point to the NO./O2- interaction as a naturally occurring cell defense mechanism.