Culture media variation as related to in vitro aging of human fibroblasts: II. Effects on nucleolar number/cell, volume/nucleolus and total nucleolar volume/cell

Nucleolar localization of the Werner syndrome protein in human cells

Werner Syndrome (WS) is a human genetic disorder with many features of premature aging. The gene defective in WS (WRN) has been cloned and encodes a protein homologous to several helicases, including Escherichia coli RecQ, the human Bloom syndrome protein (BLM), and Saccharomyces cerevisiae Sgs1p. To better define the function of WRN protein we have determined its subcellular localization. Indirect immunofluorescence using polyclonal anti-human WRN shows a predominant nucleolar localization. Studies of WRN mutant cells lines confirmed the specificity of antibody recognition. No difference was seen in the subcellular localization of the WRN protein in a variety of normal and transformed human cell lines, including both carcinomas and sarcomas. The nucleolar localization of human WRN protein was supported by the finding that upon biochemical subcellular fractionation, WRN protein is present in an increased concentration in a subnuclear fraction enriched for nucleolar proteins. We have also determined the subcellular localization of the mouse WRN homologue (mWRN). In contrast to human WRN protein, mWRN protein is present diffusely throughout the nucleus. Understanding the function of WRN in these organisms of vastly differing lifespan may yield new insights into the mechanisms of lifespan determination.

The pathway of cell senescence: WI-38 cells arrest in late G1 and are unable to traverse the cell cycle from a true G0 state

Senescent human diploid fibroblasts have an undefined arrest state partially characterized by the differential expression of cell cycle-regulated genes and a failure to complete the mitogen-stimulated cascade of signalling events that lead to DNA synthesis. We present evidence that this arrest state precludes the entry of senescent fibroblasts into a normally reversible G0 or quiescent state. Both nuclear association kinetics and quinacrine dihydrochloride nuclear fluorescence show chromatin condensation patterns consistent with arrest in late G1 and exclusion of senescent cells from the G0 phase of the cell cycle. Steady-state thymidine kinase mRNA levels indicate that some of the signalling cascades initiated from a functional G0 state may be intact in senescent cells, at least qualitatively, and that this expression may represent an abortive attempt to complete pathways required for DNA replication. Taken together, the evidence suggests that growth arrest in senescent cells likely occurs in a physiologic state fundamentally distinct from that of the G0, quiescent state that is achieved by nonproliferating young cells. A full response to serum or growth factor addition, leading from quiescence to DNA synthesis, may require cells to initiate this traverse from a true G0 state. If so, senescent cells would be excluded from this pathway.

Accelerated aging and nucleolar fragmentation in yeast sgs1 mutants

The SGS1 gene of yeast encodes a DNA helicase with homology to the human WRN gene. Mutations in WRN result in Werner's syndrome, a disease with symptoms resembling premature aging. Mutation of SGS1 is shown to cause premature aging in yeast mother cells on the basis of a shortened life-span and the aging-induced phenotypes of sterility and redistribution of the Sir3 silencing protein from telomeres to the nucleolus. Further, in old sgs1 cells the nucleolus is enlarged and fragmented-changes that also occur in old wild-type cells. These findings suggest a conserved mechanism of cellular aging that may be related to nucleolar structure.

Chromosomal analysis in young vs. senescent human fibroblasts by fluorescence in situ hybridization: a selection hypothesis

Almost all previous studies on chromosomal analysis related to in vitro aging of human fibroblasts were done using only metaphase chromosomes. However, this procedure may provide only partial information since the aneuploidy presumably hidden in interphase cells would remain undetected. For this reason, we have analyzed aneuploidy both at interphase and at metaphase. Female (IMR-90) and male (IMR-91) cells were grown from the lowest to the highest population doubling levels and aneuploidy analysis was done by FISH with alpha-satellite DNA probes of 15 autosomes and two sex chromosomes. Our data on total aneuploidy in young cells indicate that significantly higher proportions of cells with aneuploidy can be detected at interphase than at metaphase. This presumably indicates that during active division of young cells, more aneuploid than diploid cells are selected against entry to mitosis. In contrast, interphase senescent cells from both strains show significantly fewer aneuploid nuclei than do young cells at interphase. This probably indicates that during senescence, there is greater selective pressure in the culture against long-term survival of aneuploid cells than against survival of diploid cells. Our study shows that cellular dynamics with respect to aneuploidy involving various chromosomes differs significantly at interphase and at mitosis during in vitro aging of human fibroblasts, and we propose a 'Selection Hypothesis' as an explanation to our findings.

Cell enlargement: one possible mechanism underlying cellular senescence

We previously demonstrated an inverse relationship between the G1 volume of human diploid fibroblast-like (HDFL) cells obtained from foreskin tissue and clonal replicative potential. On the basis of these results, we suggested that one process underlying in vitro senescence is a progressive increase in the mean cell volume of successive progeny within clonal lineages. We now report that the size of HDFL cells, as well as of chick embryo fibroblasts, can be increased in the virtual absence of cell division by culturing at low density and at low serum concentration (0.1-1.0%). Consequent to an increase in cell size, the replicative potential of the cells is reduced to the level of later-passage cells of similar size. By clonal analysis, the populations of enlarged cells contain up to three times as many nondividing cells as do controls. In the enlarged populations, the proportion of cells producing attenuated clones (four or fewer progeny) increases by about 30%, whereas the proportion of cells yielding greater than 32 cells declines by a similar percentage. These observations lead us to propose that replicative potential may be limited by cell size, which in turn may be regulated by a kinetic relationship between cellular growth and cell division cycles.