A measure of the mitotic index: studies of the abundance and half‐life of p34cdc2in cultured cells and normal and neoplastic tissues

Abstract

The cdc2 gene encodes a protein kinase, p34^cdc2, that is essential for mitosis, and is present at high levels in dividing cells. Classical studies of the levels of this protein in dividing and resting cells used antibodies that cross-react with other members of the CDK family, in particular with CDK2. We have therefore re-examined the abundance of p34^cdc2 in a variety of tissues and cell lines, using a highly specific, epitope-mapped monoclonal antibody that does not react with CDK2. We observed high levels of p34^cdc2 in proliferating cells, especially those in neoplastic tissues. Cells that have withdrawn from the cell cycle have low or undetectable levels. At the end of mitosis, the level of p34^cdc2 declines, with simple first-order kinetics, with a half-life which is never less than 6 h and is more typically about 18 h. The persistence of p34^cdc2 after the last cell division is comparable to that of PCNA, a commonly used marker of proliferation. The immunochemical detection of p34^cdc2 provides an accurate, reliable and meaningful measure of the proliferative activity of cells in tissues. We suggest that p34^cdc2 should be considered as the most authentic molecular marker of the mitotic index.