Cultured Cell Models and Replicative Senescence - Unlike cancer cells, diploid somatic cells typically have a finite capacity for propagation during serial culture, shown first for skin fibroblasts (Hayflick and Moorhead, 1961). After a finite number of subcultures (population doublings), cell division ceases and cultures are considered senescent. The ‘Hayflick’ phenomenon extends to many cell types, including vascular endothelia and lymphocytes (Campisi, 2005; Cristofalo et al, 2004; Hayflick, 2000). Although theend-phase cultures are considered ‘senescent,’ post-replicative cells survive many months if media are refreshed (Matsumura et al, 1979). In fact, senescent cultures are highly resistant to apoptosis. Resistance to apoptosis may link back to the insulin pathways that modulate life span (Fig. 1.3), because senescent cell cultures have decreased endocytic uptake of the IGF-binding protein IGFBP-3 (Hampel et al, 2005). It is cogent to Query I that senescent cultures of fibroblasts and other cell types show increased inflammatory factors including COX-2, IL-1, MMP-3, collagenase, TIMP-1 (tissue inhibitor of metaloproteases) (Han et al, 2004; Parrinello et al,2005; West et al, 1989; Zeng et al, 1996). These same changes arise in atheromas and, moreover, are blocked in senescing cultures by the COX-2 inhibitor NS398 (Han et al, 2004) (Chapter 2). Inflammatory factors secreted by replicatively senescent cells are implicated in focal tissue remodeling in the progression of pre-malignant cells (Campisi, 2005).
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| cultured cells (sumber: stemcells.nih.gov) |
Individual cell variations in replicative potential are associated with variable telomere length (Martin-Ruiz et al, 2004). It is not known if telomere heterogeneity causes the daughter cell differences in proliferative potential, which range from 0 (growth arrest) to 15 or more replications (Matsumura et al, 1985). Somatic cell replicative heterogeneity may contribute to the remarkable differences of individual life span in twins and in highly inbred worms (Finch and Kirkwood, 2000) (Section 5.2).
Contrary to earlier conclusions, adult age up to 92 years does not change the Hayflick limit of skin fibroblasts (Cristofalo et al, 2004; Goldstein et al, 1978; Smith et al, 2002). However, cells from embryos or children have 2-fold higher Hayflick limits (Martin, 1970). Thus, the major effect of age on the cell senescence model is before maturation. In vivo exposure to oxidative stress may be a factor in the reduced proliferation of cells from diabetics (Goldstein et al, 1979). Moreover, the standard protocol of culturing cells for aging studies in ambient air (20% oxygen) is grossly unphysiological. When mouse cells were ‘aged’ at 3% oxygen, closer to the tissue levels, their replicative potential was greatly increased (Parrinello et al, 2003). Nonethless, even under the standard culture conditions, in species comparisons, resistance of cultured fibroblasts to oxidative stress correlates with life span (Kapahi et al., 1999). We may anticipate fruitful further analysis of species differences in resistance to oxidative stress that might, in turn, inform about in vivo vulnerability to bystander effects from inflammatory processes.
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