Organization of DNA topoisomerase II isotypes during the cell cycle of human lymphocytes and HeLa cells

Abstract

We have monitored the organization of DNA topoisomerase II (Topo II) in relation to chromatin disaggregation during mitogen stimulation of lymphocytes and to the mitotic chromosome condensation cycle by immunofluorescence microscopy with isozyme-specific antibodies. Labelling for both Topo II alpha and Topo II beta was diffusely nucleoplasmic and non-nucleolar in resting lymphocytes and the pattern changed little during stimulation. Topo II alpha labelling intensity increased in parallel with the extent of cell stimulation, but a fraction of fully stimulated cells was labelled very brightly. Topo II beta labelling intensity was also greater in stimulated cells, but all partially and fully stimulated cells were labelled at the same, higher, intensity. In addition, anti-Topo II beta detected a few small spots within nucleoli of stimulated cells that coincided with regions containing fibrillarin. In lymphocytes and HeLa, chromosome association of Topo II alpha began in prophase and lasted throughout mitosis. In contrast, Topo II beta stayed nucleoplasmic in prophase, was diffusely cytoplasmic during mitosis, and was first detected post-mitotically in nuclei with decondensing chromosomes and a reformed nuclear envelope. The results are consistent with a role for Topo II alpha, but not for Topo II beta, in mitotic chromosome condensation, and indicate that the isotypes may play independent roles in the reorganization of chromatin structure during lymphocyte mitogenic activation.

Active DNA Topoisomerase IIα Is a Component of the Salt-stable Centrosome Core

“…However, such a mechanism would not be meaningful for a chromosomebound enzyme, such as topoisomerase II␣ (2,8), which gets equally distributed between daughter cells via the chromosomes. A hint at a possible biological meaning of centrosomal topoisomerase II␣ might be provided by our finding of the enzyme at centrosomes of quiescent peripheral blood lymphocytes, which are devoid of topoisomerase II␣-specific mRNA and do not have topoisomerase II␣ in the nucleus (30). The finding suggests a significantly extended life span of the cen- FIG.…”

Type II topoisomerase activities in both the G1 and G2/M phases of the dinoflagellate cell cycle

“…The only other known exception to the eukaryotic G 2 /M pattern is topoisomerase IIβ, which has no observable cell cycle variation in mammalian cells. However, topoisomerase IIβ is dispensable and its level only accounts for 20-30% of the mammalian enzyme (Akimitsu et al 2003;Chaly et al 1996;Woessner et al 1991). One possible function of topoisomerase II in the dinoflagellate G 2 /M phase is to promote chromosome segregation during mitosis, as it does in other eukaryotes.…”

Essential Mitotic Functions of DNA Topoisomerase IIα Are Not Adopted by Topoisomerase IIβ in Human H69 Cells

“…The two isoenzymes share a high degree of structural homology and have similar enzymatic properties (4). However, they show different patterns of spatial organization both on the level of the cell (5,6) and in tissues (7). In a given tissue the ␤-isoenzyme is found in virtually every cell, whereas the ␣-isoenzyme is restricted to proliferative compartments (7).…”

“…The ␣-form localizes to the inside of nucleoli (6) and clusters at centromeric regions (8), whereas the ␤-isoenzyme shows a reticular pattern in the vicinity but mostly outside of nucleoli (6). In mitosis the ␤-isoenzyme diffuses away from the chromatin, whereas the ␣-isoenzyme becomes up-regulated (9 -11) and binds tightly to the centromeres and the axes of the chromosome arms (5,6,12), where it displays a dynamic pattern, which changes as cells progress through mitosis (13). There are indications that the ␣-isoenzyme in its chromosome-bound state is mostly catalytically inactive, whereas the ␤-isoenzyme sustains a diffusible type II topoisomerase activity throughout the cell cycle (6).…”