Aging is an inherently stochastic process, and its hallmark is heterogeneity between organisms, cell types, and clonal populations, even in identical environments. The replicative lifespan of primary human cells is telomere dependent; however, its heterogeneity is not understood. We show that mitochondrial superoxide production increases with replicative age in human fibroblasts despite an adaptive UCP-2–dependent mitochondrial uncoupling. This mitochondrial dysfunction is accompanied by compromised [Ca2+]i homeostasis and other indicators of a retrograde response in senescent cells. Replicative senescence of human fibroblasts is delayed by mild mitochondrial uncoupling. Uncoupling reduces mitochondrial superoxide generation, slows down telomere shortening, and delays formation of telomeric γ-H2A.X foci. This indicates mitochondrial production of reactive oxygen species (ROS) as one of the causes of replicative senescence. By sorting early senescent (SES) cells from young proliferating fibroblast cultures, we show that SES cells have higher ROS levels, dysfunctional mitochondria, shorter telomeres, and telomeric γ-H2A.X foci. We propose that mitochondrial ROS is a major determinant of telomere-dependent senescence at the single-cell level that is responsible for cell-to-cell variation in replicative lifespan.

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The ovine beta-lactoglobulin gene is expressed efficiently and at high levels in the mammary gland of transgenic mice. In contrast, when this gene is linked to a second gene construct comprising a mammalian cDNA or a CAT reporter sequence it fails to be expressed in the majority of transgenic lines generated. We suggest that mammalian cDNAs and prokaryotic reporter sequences can serve as active foci for gene silencing in the mammalian genome.

The observed effects of prolonged in vivo administration of progesterone will minimize the ability of the uterus to contract as a synctium and the ability of peripheral blood leukocytes to migrate into the myometrium during parturition. We suggest that these are putative mechanisms by which progesterone might prevent preterm birth in women at high risk.