Superoxide dismutase and thioredoxin restore defective p34cdc2 kinase activation in mouse two-cell block

We recently showed that superoxide dismutase (SOD), a free radical scavenger, and thioredoxin, a potent protein disulfide reductase, release mouse two-cell stage developmental block in vitro. To elucidate the mechanism underlying the two-cell block and the effects of these enzymes, we studied the chronological changes in the kinase activity and the immunoblotting pattern of p34cdc2, a key regulator of the cell cycle, during the first and second cell cycles of the mouse embryonic development. In vivo embryos were freshly collected at fixed times, and in vitro embryos cultured from the pronuclear stage were also sampled with the same time-course. A marked elevation of p34cdc2 kinase activity was observed in vivo at 27-32 and 51 h after an injection of human chorionic gonadotropin. These times coincide with the M-phases of embryo cleavage. In vitro embryos showed high kinase activity during the M-phase of the first cleavage, but this activity was not elevated during the second cell cycle. The addition of recombinant human SOD (200 micrograms/ml) or thioredoxin from Escherichia coli (500 micrograms/ml) to the medium enabled kinase activation with a time course similar to that of in vivo embryos. The immunoblotting patterns suggested the dephosphorylation of p34cdc2 at the M-phase of the first and the second cleavages in vivo. Although p34cdc2 was dephosphorylated at the M-phase of the first cleavage and then rephosphorylated for embryos cultured in vitro, the second dephosphorylation was not observed during the second cell cycle. The addition of SOD or thioredoxin permitted the dephosphorylation at the M-phases of both the first and the second cleavage. These results suggest that one of the chief causes of two-cell block in vitro is the impairment in p34cdc2 dephosphorylation, recently shown to be catalyzed by the cdc25 homologue. This impairment is thought to be due to oxidative stress, because both SOD and thioredoxin are known to play a defensive role against it.