Different regulation of thymidine kinase during the cell cycle of normal versus DNA tumor virus-transformed cells.

We compared the cell cycle regulation of thymidine kinase (TK) after centrifugal elutriation in normal human and mouse cells (primary cells, diploid fibroblasts) with its expression in cells transformed with different DNA tumor viruses. Normal cells showed a rise of TK enzyme activity near the G1/S boundary, which peaked in S phase, and in G2 returned approximately to the level of G1. Conversely, in cells derived from viral transformation, TK activity remained high throughout S and G2 phases, although it was induced to a comparable extent at the onset of DNA replication. In addition, transformed cells exhibited much more enzyme activity during all phases of the cell cycle. The observed differences in expression were due neither to different rates of protein turnover nor to differences in enzyme stability. Enzyme activity was always totally paralleled by the protein level. In all normal cells, the pattern of TK mRNA variation during the cell cycle was similar to that of enzyme activity. In all transformed lines, however, mRNA levels were higher and did not fluctuate throughout the cell cycle. Recently we showed (Ogris et al., 1993) that the E2F binding site present in the TK promoter is a target for trans activation of the TK gene by polyoma virus large T antigen. Using cells expressing this antigen under the control of a hormone-inducible promoter, we were able to switch TK cell cycle expression from the normal to the transformed status. Obviously, DNA tumor viruses suppress transcriptional down-regulation of the endogenous DNA precursor pathway enzyme TK during the eukaryotic cell cycle, maybe to improve conditions for their own replication.