Activated oncogenes induce premature cellular senescence, a permanent state of 
proliferative arrest in primary rodent and human fibroblasts. Recent studies 
suggest that generation of reactive oxygen species (ROS) is involved in 
oncogenic Ras-induced premature senescence. However, the signaling mechanism 
controlling this oxidant-mediated irreversible growth arrest is not fully 
understood. Here, we show that through the Ras/MEK pathway, Ras oncogene 
up-regulated the expression of superoxide-generating oxidases, Nox1 in rat REF52 
cells and Nox4 in primary human lung TIG-3 cells, leading to an increase in 
intracellular level of ROS. Ablation of Nox1 and Nox4 by small interfering RNAs 
(siRNAs) blocked the RasV12 senescent phenotype including β-galactosidase 
activity, growth arrest and accumulation of tumor suppressors such as p53 and 
p16Ink4a. This suggests that Nox-generated ROS transduce senescence signals by 
activating the p53 and p16Ink4a pathway. Furthermore, Nox1 and Nox4 siRNAs 
inhibited both Ras-induced DNA damage response and p38MAPK activation, whereas 
overexpression of Nox1 and Nox4 alone was able to induce senescence. The 
involvement of Nox1 in Ras-induced senescence was also confirmed with embryonic 
fibroblasts derived from Nox1 knockout mice. Together, these findings suggest 
that Nox1- and Nox4-generated ROS play an important role in Ras-induced 
premature senescence, which may involve DNA damage response and p38MAPK 
signaling pathways.