Distinct concentration-dependent oxidative stress profiles by cadmium in a rat kidney proximal tubule cell line

Levels and chemical species of reactive oxygen/nitrogen species (ROS/RNS) determine oxidative eustress and distress. Abundance of uptake pathways and high oxygen consumption for ATP-dependent transport makes the renal proximal tubule particularly susceptible to cadmium (Cd²⁺)-induced oxidative stress by targeting ROS/RNS generation or antioxidant defence mechanisms, such as superoxide dismutase (SOD) or H₂O₂-metabolizing catalase (CAT). Though ROS/RNS are well-evidenced, the role of distinct ROS profiles in Cd²⁺ concentration-dependent toxicity is not clear. In renal cells, Cd²⁺ (10-50 µM) oxidized dihydrorhodamine 123, reaching a maximum at 2-3 h. Increases (up to fourfold) in lipid peroxidation by TBARS assay and H₂O₂ by Amplex Red were evident within 30 min. ROS and loss in cell viability by MTT assay with 50 µM Cd²⁺ could not be fully reversed by SOD mimetics Tempol and MnTBAP nor by SOD1 overexpression, whereas CAT expression and α-tocopherol were effective. SOD and CAT activities were attenuated below controls only with >6 h 50 µM Cd²⁺, yet augmented by up to 1.5- and 1.2-fold, respectively, by 10 µM Cd²⁺. Moreover, 10 µM, but not 25-50 µM Cd²⁺, caused 1.7-fold increase in superoxide anion (O₂^•-), detected by dihydroethidium, paralled by loss in cell viability, that was abolished by Tempol, MnTBAP, α-tocopherol and SOD1 or CAT overexpression. H₂O₂-generating NADPH oxidase 4 (NOX4) was attenuated by ~50% with 10 µM Cd²⁺ at 3 h compared to upregulation by 50 µM Cd²⁺ (~1.4-fold, 30 min), which was sustained for 24 h. In summary, O₂^•- predominates with low-moderate Cd²⁺, driving an adaptive response, whereas oxidative stress by elevated H₂O₂ at high Cd²⁺ triggers cell death signaling pathways.Highlights Different levels of reactive oxygen species are generated, depending on cadmium concentration. Superoxide anion predominates and H₂O₂ is suppressed with low cadmium representing oxidative eustress. High cadmium fosters H₂O₂ by inhibiting catalase and increasing NOX4 leading to oxidative distress. Superoxide dismutase mimetics and overexpression were less effective with high versus low cadmium. Oxidative stress profile could dictate downstream signalling pathways.