Role of NADP⁺-dependent isocitrate dehydrogenase (NADP ⁺-ICDH) on cellular defence against oxidative injury by γ-rays

Purpose: To investigate the regulation of NADPH-producing isocitrate dehydrogenase (ICDH) in cytosol (IDPc) and mitochondria (IDPm) upon γ-ray irradiation, and the roles of IDPc and IDPm in the protection against cellular damage induced by γ-ray irradiation. Materials and methods: Changes of IDPc and IDPm proteins upon γ-ray irradiation to NIH3T3 cells were analysed by immunoblotting. To increase or decrease the expression of IDPc or IDPm, NIH 3T3 cells were stably transfected with mouse IDPc or IDPm cDNA in either the sense or the antisense direction. The transfected cells with either increased or decreased IDPc or IDPm were exposed to γ-rays, and the levels of reactive oxygen species generation, protein oxidation and lipid peroxidation were measured. Results: Both IDPc and IDPm activities were induced by γ-ray in NIH3T3 cells. Cells with decreased expression of IDPc or IDPm had elevated reactive oxygen species generation, lipid peroxidation and protein oxidation. Conversely, overproduction of IDPc or IDPm protein partially protected the cells from oxidative damage induced by γ-ray irradiation. Conclusions: The protective role of IDPc and IDPm against γ-ray-induced cellular damage can be attributed to elevated NADPH, reducing equivalents needed for recycling reduced glutathione in the cytosol and mitochondria. Thus, a primary biological function of the ICDHs may be production of NADPH, which is a prerequisite for some cellular defence systems against oxidative damage.