Threshold Effects of Nitric Oxide-Induced Toxicity and Cellular Responses in Wild-Type and p53-Null Human Lymphoblastoid Cells

Toxicity induced by nitric oxide (NO¥) has been extensively investigated in many in vitro and in vivo experimental models. Recently, our laboratories found that both concentration and cumulative total dose are critical determinants of cell death caused by NO¥. Here, we report results of studies designed to define total dose thresholds and threshold effects for several NO¥-induced toxicity and cellular responses and to determine impacts of p53 on them. We exposed human lymphoblastoid TK6 cells harboring wild-type p53 and isogenic p53-null NH32 cells to NO ¥ delivered by a membrane delivery system. Cells were exposed at a steady state concentration of 0.6 íM for varying lengths of time to deliver increasing cumulative doses (expressed in units of íM min), and several end points of cytotoxicity and mutagenesis were quantified. Threshold doses for NO¥-induced cytotoxicity were 150 íM min in TK6 cells and 300 íM min in NH32 cells, respectively. Threshold doses for NO¥-induced apoptosis were identical to those for cytotoxicity, but mitochondrial depolarization thresholds were lower than those for cytotoxicity and apoptosis in both cell types. To gain insight into underlying mechanisms, cells of both types were exposed to sublethal (33 % of cytotoxicity threshold), cytotoxicity threshold, or toxic (twice the cytotoxicity threshold) doses of NO¥. In TK6 cells (p53), the sublethal threshold dose induced DNA double-strand breaks, but nucleobase deamination products (xanthine, hypoxanthine, and uracil) in DNA were increase