Differential effect of activators of protein kinase C on cytoskeletal changes in mouse and hamster eggs

Treatment of metaphase II-arrested hamster eggs with activators of protein kinase C has been reported to promote resumption of the cell cycle, second polar body emission, and pronucleus formation (G.I. Gallicano, S.M. Schwarz, R.W. McGaughey, and D.G. Capco, 1993, Dev. Biol. 156, 94-106). In contrast, we have not observed these responses in mouse eggs obtained from CF-1 mice treated with these activators. In this report, we evaluated if this difference was due to differences in the technique used for PKC stimulation in the two different laboratories or due to species differences. Metaphase II-arrested hamster or mouse eggs were treated with phorbol diesters for 5 min or with a membrane-permeable diacylglycerol for 1 hr. Treatment of hamster eggs resulted in (1) the formation of "second polar body-like structures" commencing 5 min after treatment and reaching a maximum by 20-40 min; (2) a remarkable increase in the staining of filamentous actin in the region of these polar body-like structures; and (3) the disassembly of spindle microtubules. A reduction in cdc2/cyclin B1 kinase activity, as assessed by a decrease in H1 kinase activity, as well as progression from metaphase to anaphase were not observed. Treatment of mouse eggs from either CF-1 or CD-1 mice with these activators of PKC did not result in the formation of these polar body-like structures, did not cause an increase in filamentous actin, and did not result in a reduction in histone H1 kinase activity. This treatment, however, did induce disassembly of the spindle microtubules and the formation of multiple "pronucleus-like structures" that were more discernible in eggs from CD-1 mice. We conclude that the "apparent" activation of hamster eggs by activators of PKC is due to the effect of these agents on the cytoskeleton, which gives rise to structures that appear similar to polar bodies, but without any evidence of cell cycle resumption. The different responses seen in mouse and hamster eggs are mainly due to differences in the sensitivity of the cytoskeleton to rearrangements induced by these agents.