Mammalian cell fusion. VI. Regulation of mitosis in binucleate HeLa cells

In two different cell fusion experiments a synchronized population of HeLa cells, prelabeled with ³H-TdR, was fused with an unlabeled one using inactivated Sendai virus. In the first experiment, HeLa cells in early G2 phase which were exposed to either 4 °C, cycloheximide, actinomycin D or X-irradiation were fused separately with untreated and more advanced G2 cells. A comparison of the rates of mitotic accumulation (in the presence of Colcemid) for the various classes of mono- and binucleate cells revealed that the hybrid (binucleate) cells were intermediate between those of the advanced and the retarded parental types indicating that the chromosome condensing factors of the advanced component were diluted as a result of such fusion. The manner in which the retarding effects of actinomycin D and cycloheximide were reversed in the hybrid cells suggested that proteins had a major role as chromosome condensing factors in the G2 mitotic transition. In the second experiment, when S phase HeLa cells were fused with those in G2, the resulting heterophasic (S/G2) binucleate cells reached mitosis at about the same time as the homophasic (S/S) cells of the lagging parent indicating a complete dominance of the S over the G2 with regard to their progress towards mitosis. However, the addition of Mg²⁺ (2 × 10^-2 M of MgCl₂) to the medium helped the G2 nuclei to enter mitosis asynchronously, which consequently induced premature chromosome condensation (PCC) in the S phase component. These data suggested that in the heterophasic (S/G2) binucleate cells the S phase component caused decondensation of the G2 chromatin thus blocking it from entering into mitosis. This effect which did not appear to be dose-dependent could be neutralized and the G2 nuclei relieved from this repression by an external supply of Mg²⁺ ions.