Increased synthesis and degradation of DNA topoisomerase I during the initial phase of human T lymphocyte proliferation.
The regulation of DNA topoisomerase I was studied in human T lymphocytes following phytohemagglutinin (PHA) stimulation. As T lymphocytes began to enter the S phase 24 h after stimulation, there was a rapid increase in DNA topoisomerase I mRNA. The level of DNA topoisomerase I mRNA increased continuously over the next 18 h and peaked (> 50-fold increase) 42 h after stimulation with PHA. A concomitant increase in DNA topoisomerase I protein was also observed. However, the maximal increase in DNA topoisomerase I protein was only 6-fold. To explain the quantitative difference between the mRNA and protein levels, we investigated the change in the rates of DNA topoisomerase I protein synthesis versus degradation in human T lymphocytes following PHA stimulation. The increase in the mRNA parallels the increase in protein synthesis. However, the half-life of the enzyme protein was reduced to 9 h in proliferating T lymphocytes compared to a half-life of 36 h in resting lymphocytes. These results indicate that, in addition to the growth-regulated increase in the expression of DNA topoisomerase I, there was also a concomitant increase in the degradation of DNA topoisomerase I protein.
Hwong
C L
CL
Institute of Biochemistry, National Yang-Ming Medical College, Shih-Pai, Taipei, Republic of China.
Chen
C Y
CY
Shang
H F
HF
Hwang
J
J
D015152:Blotting, Northern; D002455:Cell Division; D004247:DNA / Q000096:biosynthesis; D004264:DNA Topoisomerases, Type I / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007163:Immunosorbent Techniques; D008213:Lymphocyte Activation; D010835:Phytohemagglutinins / Q000494:pharmacology; D012333:RNA, Messenger / Q000378:metabolism; D013601:T-Lymphocytes / Q000166:cytology / Q000502:physiology*
D010835:Phytohemagglutinins; D012333:RNA, Messenger; D004247:DNA; D004264:DNA Topoisomerases, Type I