The effect of oxidative stress on the cellular uptake and nuclear translocation of extracellular superoxide dismutase (EC-SOD) was investigated. EC-SOD was incorporated from conditioned medium of stable EGSOD expressing CHO-EK cells into 3T3-L1 cells within 15 min. The uptake was clearly inhibited by the addition of heparin at a concentration of 0.4 mug/ml. Treatment of the 3T3-L1 cells with H2O2 (5 mM for 5 min), followed by incubation with CHO-EK medium downregulated the uptake of EGSOD. Nuclear translocation of the incorporated EGSOD was clearly enhanced by H2O2 treatment following incubation with the CHO-EK medium. EGSOD is the only anti-oxidant enzyme which is known at this time to be actively transported into nuclei. The results obtained here suggest that the upregulation of the nuclear translocation of EGSOD by oxidative stress might play a role in the mechanism by which the nucleus is protected against oxidative damage of genomic DNA. (C) 2003 Elsevier Science (USA). All rights reserved.