Regulation of thymidine kinase expression during cellular senescence.

As human diploid fibroblasts (HDFs) in culture senesce, the expression of thymidine kinase (TK) and the activity of its promoter become attenuated. Herein we analyze the cis-elements involved in transcriptional activation of the hTK promoter, and show that the Sp1 binding site located at -118/-113 and one CCAAT box located at either -71/-67 or -40/-36 are critical for maximal expression of hTK promoter activity in young IMR-90 HDFs. However, the DNA binding activities to TK-CCAAT and Sp1 were not defective in serum-stimulated senescent HDFs. On the other hand, treatment of young HDFs during the late G1 transition with a specific inhibitor of CDK2, roscovitine, blocked the induction of TK RNA expression. Because CDK2 remained inactive during serum stimulation in senescent HDFs, it is likely that the impairment of TK expression in senescent HDFs during serum stimulation is relevant to the inactivation of CDK2, rather than to the controlling mechanism at the level of NF-Y and Sp1 activity.