Low dose radiation upregulates Ras/p38 and NADPH oxidase in mouse colon two months after exposure

Exposure to ionizing is known to cause persistent cellular oxidative stress and NADPH oxidase (Nox) is a major source of cellular oxidant production. Chronic oxidative stress is associated with a myriad of human diseases including gastrointestinal cancer. However, the roles of NADPH oxidase in relation of long-term oxidative stress in colonic epithelial cells after radiation exposure are yet to be clearly established.

Mice were exposed either to sham or to 0.5 Gy γ radiation, and NADPH oxidase, oxidative stress, and related signaling pathways were assessed in colon samples 60 days after exposure. Radiation exposure led to increased expression of colon-specific NADPH oxidase isoform, Nox1, as well as upregulation of its modifiers such as Noxa1 and Noxo1 at the mRNA and protein level. Co-immunoprecipitation experiments showed enhanced binding of Rac1, an activator of NADPH oxidase, to Nox1. Increased 4-hydroxynonenal, 8-oxo-dG, and γH2AX along with higher protein carbonylation levels suggest increased oxidative stress after radiation exposure. Immunoblot analysis demonstrates upregulation of Ras/p38 pathway, and Gata6 and Hif1α after irradiation. Increased staining of β-catenin, cyclinD1, and Ki67 after radiation was also observed.

In summary, data show that exposure to a low dose of radiation was associated with upregulation of NADPH oxidase and its modifiers along with increased Ras/p38/Gata6 signaling in colon. When considered along with oxidative damage and proliferative markers, our observations suggest that the NADPH oxidase pathway could be playing a critical role in propagating long-term oxidative stress after radiation with implications for colon carcinogenesis.