Continuous increase in phosphorylation of cytosolic thymidine kinase during proliferation of rat hepatoma JB1 cells.

Thymidine kinase (TK) is a protein closely associated with DNA replication. Here we show that, in contrast with the variation of TK activity, which changed parallel with S phase cell distribution during asynchronous culture of rat hepatoma JB1 cells, the serine residues of cytosolic TK protein was phosphorylated continuously in response to increasing G0/G1 phase cell distribution. The shifting of phosphorylation of TK protein during different cell growth conditions was further confirmed with the examination of the elution profile of cytosolic TK activity by anion-exchange high performance liquid chromatography (HPLC). HPLC analysis revealed that the rapid proliferating (62 h after asynchronous culture) rat hepatoma JB1 cells showed only the hyperphosphorylated form of cytosolic TK activity, while synchronously M phase-arrested JB1 cells showed hypo- and hyper-phosphorylated forms of cytosolic TK activity. These results suggested that the modulation of phosphorylation of cytosolic TK protein was cell cycle-dependent, and that the phosphorylation of cytosolic TK protein might be involved in the negative regulation of TK activity in vivo.