Replicative senescence

Figure 26.1 Immortalization of human cells Cells enter replicative senescence at mortality stage 1 (Ml Hayflick limit) after about 60 population doublings (PD). The protein p 16 accumulates in senescent cells. The simian virus 40 (SV40) large T antigen as well as the human papilloma virus (HPV) type 16-E6 and E7 proteins sequester the retinoblastoma protein (Rb) and/or p53 constitutively releases the transcription factor E2F. E2F induces expression proteins required for progression through Gl/S transition, thus the cells escape cell cycle arrest. At mortality stage 2 (M2), transformed cells must overcome senescence and crisis before they are immortalized. This is likely to involve the activation of telomerase either by the introduction of hTERT cDNA or by a genetic change that activates telomerase.

Cancer cells avoid replicative senescence by maintaining integrity of their chromosome ends through increased activity of which of the following enzymes ... 

After numerous passages, cells will no longer proliferate and cultures will begin to deteriorate. This property, replicative senescence, is thought to be a manifestation of a predetermined limit on the number of proliferative cycles available to a cell. During senescence of the culture, rare cells may arise that exhibit resistance, continue to proliferate, and overgrow the culture. 

Cell tines offer several advantages over primary cell cultures, such as an unlimited life-span and the lack of time-consuming isolation procedures. Additionally once established, they are often more stable than primary cells which are usually in a continuous state of de-differentiation. Thus, the majority of in vitro nephrotoxicity studies have been performed on renal epithelial cell tines. In normal somatic cells, telomeres, the tandemly repeated hexamers at the end of mammalian chromosomes, act as the cellular replicative clock [43] and shorten at each cell division. Once telomeres have exceeded a certain critical length, the so called "Hayflick limit" [44], the cell enters replicative senescence and no longer proliferates. Until recently the most widely used renal cell tines were those which arose from spontaneously acquired immortalization in culture. These cell tines include LLC-PK (Hampshire pig) [45,46], JTC-12 (cynomolgus monkey) [47] and OK (American opossum) [48] cells, which exhibit biochemical and antigenic characteristics suggestive of proximal... 

Since DNA in chromosomes is a linear molecule, problems arise when replication comes to the ends of the DNA. Synthesis of the lagging strand at each end of the DNA requires a primer so that replication can proceed in a 5 to 3 direction. This becomes impossible at the ends of the DNA and 50-100 bp is lost each time a chromosome replicates. Thus, at each mitosis of a somatic cell, the DNA in chromosomes becomes shorter and shorter. Ultimately, after a limited number of divisions, a cell enters a nondividing state, called replicative senescence, which may play an important role in biological aging. 

RPE/Bruch s Membrane Compromised RPE cell functions could seriously jeopardize photoreceptor health. RPE senescence has most widely been studied in the model of replicative senescence. It results from repeated division of the RPE cells in culture (Flood et al., 1980 Burke and Soref, 1988 Sheedlo et al., 1997). These studies have estabhshed a relationship between donor age and the location of the RPE cells (central versus peripheral) and replicative lifespan. There are also studies describing changes in gene expression or the alteration of enzymatic activities during the replicative senescence of RPE cells in vitro (Tombran-Tink et al., 1995). 

S-phase through a complex mechanism that involves cleavage of the C-strand. G-overhangs have been implicated in the structure of the telomere extremities to create the T-loop that protects chromosome ends from fusion and their degradation has been associated with the onset of replicative senescence and more recently to the deprotection of telomeres though inactivation of proteins from the shelterin complex. ... 

Nicole E. Mathon, Denise S. Malcolm, Marie C. Harrisingh, Lili Cheng, Alison C. Lloyd, Lack of Replicative Senescence in Normal Rodent Glia, Science, 291 (2001), 872-875. 

Goldstein S (1990). Replicative senescence the human fibroblast comes of age. Science. 249 1129-1133. 

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