Involvement of mitochondria in oxidative stress-induced cell death in mouse zygotes

Accumulation of reactive oxygen species during aging leads to programmed ce ll death (PCD) in many cell types but has not been explored in mammalian fe rtilized eggs, in which mitochondria are "immature," in contrast to "mature " mitochondria in somatic cells. We characterized PCD in mouse zygotes indu ced by either intensive (1 mM for 1.5 h) or mild (200 mu M for 15 min) hydr ogen peroxide (H2O2) treatment. Shortly after intensive treatment, zygotes displayed PCD, typified by cell shrinkage, cytochrome c release from mitoch ondria, and caspase activation, then terminal deoxynucleotidyl transferase- mediated dUTP nick end labeling (TUNEL) staining in condensed pronuclei. On the other hand, after mild treatment, zygotes arrested developmentally and showed neither cytochrome c release nor caspase activation over 48 h; unti l 72 h, 46% zygotes exhibited TU NEL staining, and 88% of zygotes lost plas ma membrane integrity. Interestingly, mild oxidative treatment induced a de cline in mitochondrial membrane potential and disruption of the mitochondri al matrix. Taken together, these results suggest that oxidative stress caus ed by H2O2 induces PCD in mouse zygotes and that mitochondria are involved in the early phase of oxidative stress-induced PCD. Furthermore, mitochondr ial malfunction also may contribute to cell cycle arrest, followed by cell death, triggered by mild oxidative stress.