Response of a primary human fibroblast cell line to H2O2: senescence-like growth arrest or apoptosis?

Hydrogen peroxide has been shown to induce either apoptosis or features of senescence in different cultured cell lines. We now show that both processes can be induced in the same culture of primary human diploid fibroblasts and that the outcome of apoptosis or the senescence-like phenotype is determined by the H2O2 concentrations. At 50 and 100 microM, H2O2 predominantly induced the senescence-like state, characterized by a reduced rate of proliferation, an increased number of cells in G0-G1, typically enlarged and flattened morphology, and increased CIP1 and fibronectin expression. At 300 and 400 microM, H2O2 mainly triggered apoptosis. At the intermediate 200 microM H2O2, features of both senescence and apoptosis were observed in the same culture. Thus, the higher the H2O2 concentration, the higher the proportion of cells undergoing apoptosis, suggesting a key role of the level of damage in the choice of a cell population to enter apoptosis and/or the senescence-like state. Before the induction of one or the other process, cells entered a transient "shock state" characterized by a typical morphological change, cell cycle arrest in G0-G1, and the induction of CIP1 and BCL-2.