Decline of poly(ADP-ribosyl)ation during in vitro senescence in human diploid fibroblasts

Protein Poly(ADP-ribosyl)ation System: Changes in Development and Aging as well as due to Restriction of Cell Proliferation

It is well known that the number of dividing cells in an organism decreases with age. The average rate of cell division in tissues and organs of a mature organism sharply decreases, which is probably a trigger for accumulation of damage leading to disturbance of genome integrity. This can be a cause for the development of many age-related diseases and appearance of phenotypic and physiological signs of aging. In this connection, the protein poly(ADP-ribosyl)ation system, which is activated in response to appearance of various DNA damage, attracts great interest. This review summarizes and analyzes data on changes in the poly(ADP-ribosyl)ation system during development and aging in vivo and in vitro, and due to restriction of cell proliferation. Special attention is given to methodological aspects of determination of activity of poly(ADP-ribose) polymerases (PARPs). Analysis of relevant publications and our own data has led us to the conclusion that PARP activity upon the addition of free DNA ends (in this review referred to as stimulated PARP activity) is steadily decreasing with age. However, the dynamics of PARP activity measured without additional activation of the enzyme (in this review referred to as unstimulated activity) does not have such a clear trend: in many studies, the presented differences are statistically non-significant, although it is well known that the number of unrepaired DNA lesions steadily increases with aging. Apparently, the cell has additional regulatory systems that limit its own capability of reacting to DNA damage. Special attention is given to the influence of the cell proliferative status on PARP activity. We have systematized and analyzed data on changes in PARP activity during development and aging of an organism, as well as data on differences in the dynamics of this activity in the presence/absence of additional stimulation and on cellular processes that are associated with activation of these enzymes. Moreover, data obtained in different models of cellular aging are compared.

Misregulation of poly(ADP-ribose) polymerase-1 activity and cell type-specific loss of poly(ADP-ribose) synthesis in the cerebellum of aged rats

Protein modification by ADP-ribose polymers is a common regulatory mechanism in eukaryotic cells and is involved in several aspects of brain physiology and physiopathology, including neurotransmission, memory formation, neurotoxicity, ageing and age-associated diseases. Here we show age-related misregulation of poly(ADP-ribose) synthesis in rat cerebellum as revealed by: (i) reduced poly(ADP-ribose) polymerase-1 (PARP-1) activation in response to enzymatic DNA cleavage, (ii) altered protein poly(ADP-ribosyl)ation profiles in isolated nuclei, and (iii) cell type-specific loss of poly(ADP-ribosyl)ation capacity in granule cell layer and Purkinje cells in vivo. In particular, although PARP-1 could be detected in virtually all granule cells, only a fraction of them appeared to be actively engaged in poly(ADP-ribose) synthesis and this fraction was reduced in old rat cerebellum. NAD(+), quantified in tissue homogenates, was essentially the same in the cerebellum of young and old rats suggesting that in vivo factors other than PARP-1 content and/or NAD(+) levels may be responsible for the age-associated lowering of poly(ADP-ribose) synthesis. Moreover, PARP-1 expression was substantially down-regulated in Purkinje cells of senescent rats.

From Weismann's theory to present day gerontology 1889-2003

From Weismann's theory to present day gerontology--Weismann's theory was based on the concept that through natural selection the division potential of somatic cells become finite thus limiting the regeneration of the soma and the life span of the organism. Indeed, the somatic cells of some animals have a finite division potential but what became apparent is that the implications for aging are more complex. Experiments showed that at each cell division the genetic information received by each daughter cell differs; cells are this way progressively modified through division creating a functional drift that is responsible in part for the continuous modifications going on in the organism from its very beginning to its extinction. Comparative biology showed that the finite or the infinite division potential of somatic cells has a complex connotation with developmental characteristics of the respective organism with implications for longevity that are far from being understood.

Iron chelation-induced senescence-like growth arrest in hepatocyte cell lines: association of transforming growth factor beta1 (TGF-beta1)-mediated p27Kip1 expression

Iron is essential for cellular proliferation in all organisms. When deprived of iron, the growth of cells is invariably inhibited. However, the mechanism involved remains largely unclear. In the present study, we have observed that subcytotoxic concentrations of desferroxamine mesylate (DFO), an iron chelator, specifically inhibited the transition from G1 to S-phase of Chang cells, a hepatocyte cell line. This was accompanied by the appearance of senescent biomarkers, such as enlarged and flattened cell morphology, senescence-associated beta-galactosidase activity and reduced expression of poly(ADP-ribose) polymerase. Concomitantly, p27Kip1 (where Kip is kinase-inhibitory protein) was induced markedly, whereas other negative cell-cycle regulators, such as p21Cip1 (where Cip is cyclin-dependent kinase-interacting protein), p15INK4B and p16INK4A (where INK is inhibitors of cyclin-dependent kinase 4), were not, implying its association in the G1 arrest. Furthermore, the induction of p27Kip1 was accompanied by an increased level of transforming growth factor beta1 (TGF-beta1) mRNA. When neutralized with an anti-(TGF-beta1) antibody, p27Kip1 induction was completely abolished, indicating that TGF-beta1 is the major inducer of p27Kip1. Finally, DFO-induced senescence-like arrest was found to be independent of p53, since cell-cycle arrest was still observed with two p53-negative cell lines, Huh7 and Hep3B cells. In conclusion, DFO induced senescence-like G1 arrest in hepatocyte cell lines and this was associated with the induction of p27Kip1 through TGF-beta1, but was independent of p53.

The effect of a ceramide analog, N-acetylsphingosine on the induction of proliferation and IL-2 synthesis in T cells from young and old F344 rats

Ceramide is a physiological mediator of extracellular signals that control various cellular functions, including proliferation and apoptosis. In the present study, we examined the effects of cell-permeable ceramide analog, N-acetyl-sphingosine (C(2)-ceramide) on the induction of proliferation and interleukin-2 (IL-2) synthesis in T cells from young and old rats. Splenic T cells from 6- and 24-month-old Fischer 344 rats were treated with C(2)-ceramide and then incubated with anti-CD3 antibody for 24 or 48 h. The induction of proliferation and IL-2 production by anti-CD3 was significantly (P<0.001) lower in T cells from old rats compared to T cells from young rats. C(2)-ceramide treatment resulted in suppression of proliferation and IL-2 production in a concentration-dependent manner. The suppressive effect of C(2)-ceramide on proliferation and IL-2 production was greater in T cells from old rats than T cells from young rats. We investigated whether this decreased responsiveness was due to induction of program cell death (apoptosis) and found that there was a significant increase in DNA fragmentation in C(2)-ceramide treated and anti-CD3 stimulated T cells from both young and old rats. The increase in DNA fragmentation was paralleled with an increase in caspase-3 activation. C(2)-ceramide-induced caspase-3 activation and DNA fragmentation was significantly (P<0.5) higher in stimulated T cells from old rats compared to stimulated T cells from young rats. These results suggest that the sphingomyelin-ceramide signaling pathway may play an important regulatory role in the well-documented age-related decline in immune function.

Ceramide induces expression of the senescence histochemical marker, beta-galactosidase, in human fibroblasts

We recently showed that ceramide is elevated in senescence and that when administered to low-passage cells induces biochemical changes characteristic of senescence. The in situ histochemical marker beta-galactosidase (beta-Gal) has provided an important tool in the study of cellular senescence. We investigated the ability of ceramide to induce the expression of beta-Gal and correlated this with cell proliferation. We find that D-e-C6-ceramide, induces the expression of acidic beta-Gal in fetal lung-derived Wi-38 human diploid fibroblasts. Our results show that this induction is: (1) time and concentration dependent; and (2) reversible upon ceramide removal. We also find that concomitant with the onset of beta-Gal staining, DNA synthesis is blocked. These conditions are reversible. The induction of beta-Gal expression is specific to C6-ceramide. We discuss a potential role of beta-Gal in the regulation of senescence. Although signal transduction of senescence is still not fully understood, this new evidence strengthens the hypothesis that ceramide plays a key role in signaling down stream biochemical changes in cellular senescence.

Synthesis, modifications, and turnover of proteins during aging

Slowing down of bulk protein synthesis is one of the most commonly observed biochemical changes during aging. The implications and consequences of slower rates of protein synthesis are manifold, including a decrease in the availability of enzymes for the maintenance, repair, and normal metabolic functioning of the cell, an inefficient removal of inactive, abnormal, and damaged macromolecules in the cell, the inefficiency of the intracellular and intercellular signalling pathways, and a decrease in the production and secretion of hormones, antibodies, neurotransmitters, and the components of the extracellular matrix. Age-related changes in the activity, specificity, and stability of a large number of proteins have been reported. However, the molecular mechanisms responsible for such alterations are still poorly understood. Studies on various components of the protein synthetic machinery have revealed a decline in the efficiency and accuracy of ribosomes, an increase in the levels of rRNA and tRNA, and a decrease in the amounts and activities of elongation factors. Because posttranslational modifications of proteins determine their activity and stability, alterations in the extent and level of various modifications such as phosphorylation, methylation, ADP-ribosylation, oxidation, glycation, and conformational changes during aging are being studied. Changes in the regulation of protein synthesis, posttranslational modifications, and protein turnover are crucial determinants of age-related decline in the maintenance, repair, and survival of the organism.

Poly(ADP-ribose) polymerase activity in mononuclear leukocytes of 13 mammalian species correlates with species-specific life span

Poly(ADP-ribosyl)ation is a eukaryotic posttranslational modification of proteins that is strongly induced by the presence of DNA strand breaks and plays a role in DNA repair and the recovery of cells from DNA damage. We compared poly(ADP-ribose) polymerase (PARP; EC 2.4.2.30) activities in Percoll gradient-purified, permeabilized mononuclear leukocytes from mammalian species of different maximal life span. Saturating concentrations of a double-stranded octameric oligonucleotide were applied to provide a direct and maximal stimulation of PARP. Our results on 132 individuals from 13 different species yield a strong positive correlation between PARP activity and life span (r = 0.84; P << 0.001), with human cells displaying approximately 5 times the activity of rat cells. Intraspecies comparisons with both rat and human cells from donors of all age groups revealed some decline of PARP activity with advancing age, but it was only weakly correlated. No significant polymer degradation was detectable under our assay conditions, ruling out any interference by poly(ADP-ribose) glycohydrolase activity. By Western blot analysis of mononuclear leukocytes from 11 species, using a crossreactive antiserum directed against the extremely well-conserved NAD-binding domain, no correlation between the amount of PARP protein and the species' life spans was found, suggesting a greater specific enzyme activity in longer-lived species. We propose that a higher poly(ADP-ribosyl)ation capacity in cells from long-lived species might contribute to the efficient maintenance of genome integrity and stability over their longer life span.

Protein synthesis, posttranslational modifications, and aging

Posttranslational modifications of proteins are involved in determining their activities, stability, and specificity of interaction. More than 140 major and minor modifications of proteins have been reported. Of these, only a few have been studied in relation to the aging of cells, tissues, and organisms. These include phosphorylation, methylation, ADP-ribosylation, oxidation, glycation, and deamidation. Several of these modifications occur on proteins involved in crucial cellular processes, such as DNA synthesis, protein synthesis, protein degradation, signal transduction, cytoskeletal organization, and the components of extracellular matrix. Some of the modifications are the markers of abnormal and altered proteins for rapid degradation. Others make them less susceptible to degradation by normal proteolytic enzymes, and hence these accumulate during aging.