Uncoupling of M-phase kinase activation from the completion of S-phase by heat shock

Chronic exposure of asynchronous HeLa cell cultures to 41.5°C leads to an accumulation of cells in the S-phase, spontaneous premature chromosome condensation, and loss of clonogenicity (M. A. Mackey, S. L. Anolik, and J. L. Roti Roti. Cancer Res., 52: 1101-1106, 1992). In this report, we show that increases in histone H1 kinase activity during 41.5°C exposure occur coincidentally with the appearance of premature chromosome condensation. Furthermore, this kinase activity is shown to be associated with M-phase kinase complexes containing cyclin B1. These increases in the activity of M- phase kinase were found to occur concomitantly with an elevation in cyclin B1 mRNA and an accumulation of cyclin B1 protein. Because cyclin B1 transcription begins in the S-phase, it is probable that the heat-induced delay in the S-phase allows the accumulation of abnormally high cyclin B1 levels. Elevated cyclin B1 levels could then account for the observed abrogation of the cell cycle checkpoint, which usually assures that mitosis does not proceed until DNA replication is complete. This involvement of M- phase kinase in heat-induced cytotoxicity demonstrates the importance of the coordinate regulation of the processes of DNA replication and entry into mitosis.