Cellular replicative capacity correlates primarily with species body mass not longevity.

Although the limited replicative capacity of human fibroblasts in culture is frequently used as a model for aging, a question of major interest is whether the relationship between in vitro fibroblast proliferative capacity and species longevity is primary or secondary to a relationship with species body size. In this report we establish that body mass is the primary correlative of proliferative potential rather than species life-span

Modulation of replicative senescence of diploid human cells by nuclear ERK signaling.

Normal somatic cells have a limited replicative lifespan, and serial subcultivation ultimately results in senescence. Senescent cells are irreversibly growth-arrested and show impaired responses to mitogens. Activation of the ERK signaling pathway, an absolute requirement for cell proliferation, results in nuclear relocalization of active ERKs, an event impaired in senescent fibroblasts. This impairment coincides with increased activity of the nuclear ERK phosphatase MKP2. Here we show that replicative lifespan can be altered by changes in nuclear ERK activity. Ectopic expression of MKP2 results in premature senescence. In contrast, knock-down of MKP2 expression, through transduction of MKP2 sequence-specific short hairpin RNA, or expression of the phosphatase resistant ERK2(D319N) mutant, abrogates the effects of increased endogenous MKP2 levels and senescence is postponed. Nuclear targeting of ERK2(D319N) significantly augments its effects and the transduced cultures show higher than 60% increase in replicative lifespan compared with cultures transduced with wt ERK2. Long-lived cultures senesce with altered molecular characteristics and retain the ability to express c-fos, and Rb is maintained in its inactive form. Our results support that MKP2-mediated inactivation of nuclear ERK2 represents a key event in the establishment of replicative senescence. Although it is evident that senescence can be imposed through multiple mechanisms, restoration of nuclear ERK activity can bypass a critical senescence checkpoint and, thus, extend replicative lifespan

Replicative senescence: a critical review.

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo

Is DNA double strand breaks recognition related to longevity?

In mammals, species lifespan can vary by more than 100 fold (shrew 2 years, bowhead whale 211 years). Despite considerable research, the cellular mechanisms that make this variation possible remain unclear. In regard to these mechanisms, several predictions can be made. First, they must impact fundamental biochemical processes. Second, they would be expected to be related to structural differences between species at the cellular level. Furthermore, the goal would be to find significant correlation between cellular differences and the life span magnitude. As a tool to investigate these mechanisms, we have developed a series of skin fibroblast cell lines derived from mammalian species with a wide variation in lifespan (man, cow, bat, dog, mouse etc.). Using these lines, we have previously shown that the reported dependence of replicative capacity on longevity1 is most likely due to the dependence of replicative capacity on body mass, which is itself correlated with longevity2. Therefore, comparative studies of longevity must address the influence of body mass. The fact that DNA-PKcs and Ku 80 ablation in mice reduces average lifespan approximately 25% and 50% respectively and that Ku 80 null mice display symptoms of premature aging supports the potential role of these nuclear proteins in the aging process. DNA-PKcs and Ku are key proteins in double strand damage recognition. So we tested the capacity of skin fibroblast nuclear extracts from different mammalian species to bind DNA double strand breaks using an electrophoresis super-shift method that we have previously developed and that is now widely used in the field of DNA damage/repair3. Our results indicate that Ku-dependant DNA double strand break recognition increases exponentially with longevity and suggest that an enhanced ability to detect critical DNA damage may be a key requirement for longevity

Significant Correlation of Species Longevity with DNA Double Strand Break-Recognition but not with Telomere Length

The identification of the cellular mechanisms responsible for the wide differences in species lifespan remains one of the major unsolved problems of the biology of aging. We measured the capacity of nuclear protein to recognize DNA double strand breaks (DSB) and telomere length of skin fibroblasts derived from mammalian species that exhibit wide differences in longevity. Our results indicate DNA DSB recognition increases exponentially with longevity. Further, an analysis of the level of Ku80 protein in human, cow, and mouse suggests that Ku levels vary dramatically between species and these levels are strongly correlated with longevity. In contrast mean telomere length appears to decrease with increasing longevity of the species, although not significantly. These findings suggest that an enhanced ability to bind to DNA-ends may be important for longevity. A number of possible roles for increased levels of Ku and DNA-PKcs are discussed