Proteome analysis of proliferative response of bystander cells adjacent to cells exposed to ionizing radiation

Recently ( Cytometry 2003, 56A , 71–80), we reported that direct cell‐to‐cell contact is required for stimulating proliferation of bystander rat liver cells (WB‐F344) cocultured with irradiated cells, and neither functional gap junction intercellular communication nor long‐range extracellular factors appear to be involved in this proliferative bystander response (PBR). The molecular basis for this response is unknown. Confluent monolayers of WB‐F344 cells were exposed to 5‐Gray (Gy) of γ‐rays. Irradiated cells were mixed with unirradiated cells and co‐cultured for 24 h. Cells were harvested and protein expression was examined using 2‐DE. Protein expression was also determined in cultures of unirradiated and 5‐Gy irradiated cells. Proteins were identified by MS. Nucleophosmin (NPM)‐1, a multifunctional nucleolar protein, was more highly expressed in bystander cells than in either unirradiated or 5‐Gy irradiated cells. Enolase‐α, a glycolytic enzyme, was present in acidic and basic variants in unirradiated cells. In bystander and 5‐Gy irradiated cells, the basic variant was weakly expressed, whereas the acidic variant was overwhelmingly present. These data indicate that the presence of irradiated cells can affect NPM‐1 and enolase‐α in adjacent bystander cells. These proteins appear to participate in molecular events related to the PBR and suggest that this response may involve cellular defense, proliferation, and metabolism.