Cell Aging

Understanding the malleable determinants of cellular aging is critical to understanding human longevity. Telomeres may provide a pathway for exploring this question. Telomeres are the protective caps at the ends of chromosomes. The length of telomeres offers insight into mitotic cell and possibly organismal longevity. Telomere length has now been linked to chronic stress exposure and depression. This raises the question of mechanism: How might cellular aging be modulated by psychological functioning?

Human diploid fibroblasts, strain MRC-5, were sequentially irradiated with 60Co gamma rays at intervals during their in vitro lifespan. The results indicate that 3 or 6 doses of 1 Gy can increase lifespan, and the same was true for cells treated with 3 doses of 3 Gy. Higher doses (5 x 3 Gy) did reduce growth potential, suggesting either that mid-late passage cells become more sensitive to radiation, or that doses beyond a given threshold reduce population lifespan by multiple cellular hits. The life extension induced by gamma rays might be due to an induced hypermethylation of DNA.

Lymphocytes have a finite and predictable proliferative life span in culture similar to that observed in fibroblasts. In general, the senescence of human fibroblasts is inevitable and irreversible, but their proliferative life span can be extended by certain DNA tumor virus oncogenes, such as the large T antigen of the SV40 virus. Here, we show that human T lymphocytes (HTL) can be stably transfected with SV40 large T and that expression of T antigen extended the life span of T cell cultures.

Cellular senescence is a state of irreversible cell cycle arrest in which normal cells at the end of their lifespan fail to enter into DNA synthesis upon serum or growth factor stimulation. We examined whether proteins required for G1/S cell cycle progression were irreversibly down-regulated in senescent human fibroblasts. Both the 44- and 42-kDa forms of the MAP-kinase protein were expressed at similar levels in young and senescent cells.

For several decades simian virus 40 (SV40) early region genes have been used as a means of generating immortalized human cell lines; however, the molecular mechanisms of this process have begun to be understood only recently. SV40-induced immortalization proceeds via two phases. In the first phase ("lifespan extension"), cells continue proliferating for a limited number of population doublings beyond the point at which normal cells undergo senescence.

We have examined the effects of the naturally occurring dipeptide carnosine (beta-alanyl-L-histidine) on the growth, morphology, and lifespan of cultured human diploid fibroblasts. With human foreskin cells, HFF-1, and fetal lung cells, MRC-5, we have shown that carnosine at high concentrations (20-50 mM) in standard medium retards senescence and rejuvenates senescent cultures. These late-passage cultures preserve a nonsenescent morphology in the presence of carnosine, in comparison to the senescent morphology first described by Hayflick and Moorhead.

This article reviews the current understanding of the involvement of telomerase in in vitro immortalisation of human cells. In vitro immortalisation with DNA tumour viruses or chemicals usually occurs in two phases. The first stage is an extension of lifespan beyond that at which cells would normally senescence, after which the culture enters a period of crisis. The second stage involves the escape from crisis of a rare cell in the culture, which goes on to proliferate indefinitely.

Telomere loss has been proposed as a mechanism for counting cell divisions during aging in normal somatic cells. How such a mitotic clock initiates the intracellular signalling events that culminate in G1 cell cycle arrest and senescence to restrict the lifespan of normal human cells is not known. We investigated the possibility that critically short telomere length activates a DNA damage response pathway involving p53 and p21(WAF1) in aging cells.

Cell cycle checkpoints and tumor suppressor gene functions appear to be required for the maintenance of a stable genome in proliferating cells. In this study chromosomal destabilization was monitored in relation to telomere structure, lifespan control and G2 checkpoint function. Replicative senescence was inactivated in secondary cultures of human skin fibroblasts by expressing the human papillomavirus type 16 (HPV-16) E6 oncoprotein to inactivate p53. Chromosome aberrations were enumerated during in vitro aging of isogenic control (F5neo) and HPV-16E6-expressing (F5E6) fibroblasts.

Transfection of nearly senesced human fibroblasts with plasmids encoding HPV16 E6 protein or dominant-negative p53 mutants greatly increased their colony-forming ability. Isolated colonies with these plasmids showed extension of lifespan compared to those with a control plasmid. These data demonstrate that p53 plays a major role in senescence in normal human fibroblasts.