Nitric oxide-induced genotoxicity, mitochondrial damage, and apoptosis in human lymphoblastoid cells expressing wild-type and mutant p53

Nitric oxide (NO.) is mutagenic and, under appropriate conditions of exposure, also induces apoptosis in many in vitro and in vivo experimental models. Biochemical and cellular mechanisms through which NO. induces apoptosis are incompletely understood, but involve p53/mitochondria-dependent signaling pathways. In this study, we exposed human lymphoblastoid cells harboring either wild-type (TK6 cells) or mutant p53 (WTK-1 cells) to NO. delivered by diffusion through Silastic tubing. Cells were exposed for 2 h at constant rates of 100-533 nM/s, similar to levels estimated to occur in vivo in inflamed tissues. DNA double-strand breaks and fragmentation detected 8-48 h after NO. treatment were more extensive in TK6 cells than in WTK-1 cells, whereas NO.-induced mutant fractions in both HPRT and TK1 genes were significantly lower in TK6 cells than in WTK-1 cells (P < 0.01-0.05). Treatment of TK6 cells with NO. caused extensive apoptosis, but this response was delayed and greatly reduced in magnitude in WTK-1 cells. Mitochondrial membrane depolarization and cytochrome c release were induced in both cell types. However, elevation of apoptotic protease-activating factor-1 (Apaf-1) protein and reduction of X-chromosome-linked inhibitor of apoptosis (XIAP) protein were observed only in TK6 cells. These results indicate that p53 status is an important modulator of NO.-induced mutagenesis and apoptosis, and suggest that levels of the Apaf-1 and XIAP proteins, but not mitochondrial depolarization and cytochrome c release, are regulated by p53 in these human lymphoblastoid cells. Thus, Apaf-1 and XIAP may play important roles in the regulation of p53-mediated apoptotic responses.