DIFFERENT REGULATION OF THYMIDINE KINASE DURING THE CELL-CYCLE OF NORMAL VERSUS DNA TUMOR VIRUS-TRANSFORMED COILS

We compared the cell cycle regulation of thymidine kinase (TK) after c entrifugal elutriation in normal human and mouse cells (primary cells, diploid fibroblasts) with its expression in cells transformed with di fferent DNA tumor viruses. Normal cells showed a rise of TK enzyme act ivity near the G(1)/S boundary, which peaked in S phase, and in G(2) r eturned approximately to the level of G(1). Conversely, in cells deriv ed from viral transformation, TK activity remained high throughout S a nd G(2) phases, although it was induced to a comparable extent at the onset of DNA replication. In addition, transformed cells exhibited muc h more enzyme activity during all phases of the cell cycle. The observ ed differences in expression were due neither to different rates of pr otein turnover nor to differences in enzyme stability. Enzyme activity was always totally paralleled by the protein level. In all normal cel ls, the pattern of TK mRNA variation during the cell cycle was similar to that of enzyme activity. In all transformed lines, however, mRNA l evels were higher and did not fluctuate throughout the cell cycle. Rec ently we showed (Ogris et al., 1993) that the E2F binding site present in the TK promoter is a target for transactivation of the TK gene by polyoma virus large T antigen. Using cells expressing this antigen und er the control of a hormone-inducible promoter, we were able to switch TK cell cycle expression from the normal to the transformed status. O bviously, DNA tumor viruses suppress transcriptional down-regulation o f the endogenous DNA precursor pathway enzyme TK during the eukaryotic cell cycle, maybe to improve conditions for their own replication.