The cell cycle associated change of the Ki‐67 reactive nuclear antigen expression

The change of human nuclear antigen expression in proliferating cells recognized by a monoclonal antibody, Ki‐67, during the cell cycle was investigated in HeLa S3 cells using a bivariate‐flow‐cytometric analysis. The antigen was immunocytochemically stained with FITC, and DNA was stained with propidium iodide (PI). The expression of the antigen increased with cell‐cycle progression, especially in the latter half of S‐phase and reached a maximum at G2M‐phase, although its content varied greatly from cell to cell. The cells in which DNA synthesis was inhibited by treatment with hydroxyurea increased markedly in the antigen expression (as compared to untreated cells). Treatment with adriamycin also elevated the antigen content. After digestion with DNase I, but not after RNase treatment, FITC fluorescence from the antigen disappeared. These results suggest that the Ki‐67 antigen is bound to DNA and its expression does not depend on DNA replication. Although the biological implications of the antigen remain unresolved, the antigen may be considered to be essential for maintaining the proliferating state of cells.