Intracellular and extracellular factors influencing the genotoxicity of nitric oxide and reactive oxygen species.

A number of factors affect cellular responses to nitric oxide (NO•) and reactive oxygen species (ROS), including their source, concentration, cumulative dose, target gene and biological milieu. This limits the extrapolation of data to in vivo pathological states in which NO• and ROS may be important. The present study investigated lethality and mutagenesis in the HPRT and TK1 genes of human lymphoblastoid TK6 cells exposed to NO• and ROS derived from two delivery methods: A reactor system and a Transwell™ co-culture. The delivery of NO• into the medium at controlled steady-state concentrations (given in µM/min) and the production of NO• and ROS by activated macrophages, resulted in a time-dependent decrease in total cell numbers, and an increase in mutation frequency (MF), compared with untreated controls. This increase in MF was effectively suppressed by N-methyl-L-arginine monoacetate. Single base substitutions were the most common type of spontaneous and NO• induced mutations in HPRT, followed by exon exclusions and small deletions in both delivery systems. Among the single base pair substitutions, an equal frequency of four types of single base substitutions were identified in TK6 cells exposed to NO• delivered by the reactor system, whereas G:C to T:A transversions and A:T to G:C transitions were more frequent in the co-culture system. Taken together, these results demonstrate that both the delivery method of NO• and ROS, and the target genes are determinants of observed cytotoxic and mutagenic responses, indicating that these parameters need to be considered in assessing the potential effects of NO• and ROS in vivo.