DNA topoisomerase I and II activities during cell proliferation and the cell cycle in cultured mouse embryo fibroblast (C3H 10T1/2) cells.
We have used C3H 10T1/2 cells to examine the regulation of topoisomerase activities during cell proliferation and the cell cycle. The specific activity of topoisomerase I was about 4-fold greater in proliferating (log phase) cells than in non-proliferating (confluent) cells. In synchronized cells, the bulk of the increased activity occurred during or just prior to S phase, depending upon the method of synchronization. A smaller increase in activity also occurred during G1 phase. The increase in activity during S phase was not altered by a hydroxyurea block at the G1/S phase boundary indicating that it is not directly coupled to DNA synthesis and is not the result of topoisomerase I gene dosage. The increase was inhibited by blocking cells at mid-G1 phase using isoleucine deprivation. Thus, the increase in activity during S phase is dependent on events occurring during mid- to late G1 phase. In contrast to the changes in topoisomerase I levels, the specific activity of topoisomerase II showed no detectable difference in proliferating vs non-proliferating cells. In addition, no detectable difference in topoisomerase II specific activity was seen in G1, S and M phases of the cell cycle. The differences in the activity profiles of the topoisomerases I and II during the cell cycle suggest that the two activities are regulated independently and may be required for different functions.
Tricoli
J V
JV
Sahai
B M
BM
McCormick
P J
PJ
Jarlinski
S J
SJ
Bertram
J S
JS
Kowalski
D
D
D000818:Animals; D002453:Cell Cycle; D002455:Cell Division; D002478:Cells, Cultured; D004261:DNA Replication; D004264:DNA Topoisomerases, Type I / Q000378:metabolism*; D004250:DNA Topoisomerases, Type II / Q000378:metabolism*; D007399:Interphase; D007532:Isoleucine / Q000172:deficiency; D051379:Mice; D008809:Mice, Inbred C3H
D007532:Isoleucine; D004264:DNA Topoisomerases, Type I; D004250:DNA Topoisomerases, Type II