Mechanism of hydrogen peroxide-induced cell cycle arrest in vascular smooth muscle

Reactive oxygen species such as hydrogen peroxide (H₂O₂) can positively and negatively modulate vascular smooth muscle cell (VSMC) growth. To investigate these paradoxical effects of H₂O₂, we examined its effect on apoptosis, cell cycle progression, and cell cycle proteins. High concentrations of H₂O₂ (500 μM to 1 mM) induced apoptosis, whereas moderate concentrations (100 μM) caused cell cycle arrest in G1. H₂O₂ (100 μM) blocked serum-stimulated cyclin-dependent kinase-2 (CDK2) activity, but not CDK4 activity, suggesting that cell cycle arrest occurred in part by inhibiting CDK2 activity. The serum-induced increase in cyclin A mRNA was also completely suppressed by H₂O₂, whereas cyclin D1 mRNA was not affected. In addition, H₂O₂ caused a dramatic increase in expression of the cell cycle inhibitor p21 mRNA (9.67 ± 0.94-fold at 2 h) and protein (8.75 ± 0.08-fold at 8 h), but no change in p27 protein. Finally, H₂O₂ transiently increased p53 protein levels (3.16 ±1.2-fold at 2 h). Thus, whereas high levels of H₂O₂ induce apoptosis, moderate concentrations of H₂O₂ coordinate a set of molecular events leading to arrest of VSMCs at the G1/S checkpoint of the cell cycle. These results provide insight into the mechanisms underlying positive and negative regulation of VSMC growth by H₂O₂ in vascular disease.