Expression of cytosolic NADP ⁺-dependent isocitrate dehydrogenase in melanocytes and its role as an antioxidant

Background: Cytosolic NADP ⁺-dependent ICDH (IDPc) has an antioxidant effect as a supplier of NADPH to the cytosol, which is needed for the production of glutathione. Objective: To evaluate the expression of IDPc in melanocytes and to elucidate its role as an antioxidant. Methods: The knock-down of IDPc expression in immortalized mouse melanocyte cell lines (melan-a) was performed using the short interfering RNA (siRNA)-targeted gene silencing method. After confirming the silencing of IDPc expression with mRNA and protein levels, viability, apoptosis and necrosis, as well as ROS production in IDPc-silenced melanocytes were monitored under conditions of oxidative stress and non-stress. Also, the ratio of oxidized glutathione to total glutathione was examined, and whether the addition of glutathione recovered cell viability, decreased by oxidant stress, was checked. Results: The expression of IDPc in both primary human melanocytes and melan-a cells was confirmed by Western blot and RT-PCR. The silencing of IDPc expression by transfecting IDPc siRNA in melan-a cells was observed by Western blotting and real-time RT-PCR. IDPc knock-down cells showed significantly decreased cell viability and an increased number of cells under apoptosis and necrosis. IDPc siRNA-treated melanocytes demonstrated a higher intensity of DCFDA after the addition of H ₂O ₂ compared with scrambled siRNA-treated melanocytes, and a lower ratio of reduced glutathione to oxidized glutathione were observed in IDPc siRNA transfected melanocytes. In addition, the addition of glutathione recovered cell viability, which was previously decreased after incubation with H ₂O ₂. Conclusions: This study suggests that decreased IDPc expression renders melanocytes more vulnerable to oxidative stress, and IDPc plays an important antioxidant function in melanocytes.