Investigation of the effect of yeast NADH dehydrogenase (NDI1) on the radiation-induced bystander response in HCT116 cells with wild-type p53 function.

Purpose: Radiation-induced bystander effects describe the biological effects in unirradiated cells induced by signals from nearby radiation-targeted cells. Although the bystander effect has been well demonstrated in cell cultures, the mechanisms underlying how bystander signaling induces mitochondrial dysfunction and metabolic alterations are not fully understood. Of interest to this study is the impairment of mitochondrial Complex I's function in non-targeted mammalian cells.

Methods: NADH dehydrogenase from Saccharomyces cerevisiae (yeast) (NDI1) has been shown to restore functionality to mitochondria where complex I is inactive. In this study, NDI1 or an empty vector control was transfected into bystander human colon carcinoma cells with wild-type p53 function (HCT116 p53^+/+) to investigate the effect heterologous complex has on cellular response following direct and indirect Cs-137 gamma irradiation.

Results: HCT116 p53^+/+ were successfully transfected with either the empty vector control or NDI1 vector. NDI1 expressing cells showed no significant decrease in clonogenic survival after exposure to irradiated cell conditioned medium compared to controls.

Discussion: Our study suggests the alternative respiratory enzyme is able to prevent the reduction in colony formation in bystander cells. Yeast NDI1 expression may present a method to study the mechanisms by which bystander signals modulate Complex I activity. However, we cautiously note the results are preliminary in nature, and this work is an exploratory step toward mechanistic validation rather than conclusive evidence. There is a need for further validation to confirm mitochondrial localization and function of NDI1.