Cu,Zn superoxide dismutase in vascular cells: changes during cell cycling and exposure to hyperoxia

Conditions that generate reactive oxygen species elevate Cu,Zn superoxidase dismutase (SOD) in endothelial cells (EC) concomitant with decreased cellular proliferation. The current studies were undertaken with both vascular EC and smooth muscle cells (SMC) to compare the influences of cellular proliferation with those of hyperoxia on induction of Cu,Zn SOD. To assess cell cycling alone, EC and SMC were growth arrested, then released from arrest. Cell cycling was monitored by [3H]thymidine incorporation, counts, and flow cytometry. SOD catalytic activity was measured spectrophotometrically and SOD protein by enzyme-linked immunosorbent assay. A digoxigenin-labeled probe was used to quantify SOD mRNA by Northern analysis. EC reached the S phase of the cell cycle in 18 h and completed one cycle in 24-30 h, whereas SMC took 24-30 h to reach the S phase and 48 h to complete one cycle. Cu,Zn SOD mRNA for both EC and SMC was very low during the Go/G1 phase, peaked during the S phase, and then reverted to lower values as cells progressed through their cycles. Cu,Zn SOD activity and immunoprotein content showed corresponding changes to those of mRNA. Exposure to hyperoxia (95% O2) delayed the entry of released cells into the S phase of the cell cycle and blocked the cells in the S or G2 phase, but induced Cu,Zn SOD mRNA before the S phase and caused persistence of elevation of Cu,Zn SOD mRNA as cells progressed through their cycles. Exposure to hyperoxia also induced Cu,Zn SOD mRNA in growth-arrested cells within 24-48 h. Thus our studies support roles for both cells cycle dependency and reactive oxygen species in the induction of Cu,Zn SOD.