RNA and protein synthesis in cultured human fibroblasts derived from donors of various ages

Age-related decrease in the responsiveness of rat articular chondrocytes to EGF is associated with diminished number and affinity for the ligand of cell surface binding sites

The effect of age on the responsiveness of articular chondrocytes (AC) to epidermal growth factor (EGF) was examined. Cells were isolated by digesting cartilage fragments from the humeral and femoral heads of 21-day old, 8- and 14-month old rats with collagenase. The cells were cultured under standard conditions, as monolayers. DNA synthesis was measured by [3H]thymidine incorporation and cell proliferation by the DNA content of subconfluent cultures. [125I]EGF binding and the amounts of EGF and EGF-receptor mRNAs were determined using confluent cells. DNA synthesis was decreased with age of animals. EGF stimulated DNA synthesis in cultures in 1- and 8-month old rats at low serum concentrations (< 5%), and in cultures in 14-month old animals at high serum concentrations. It also increased 5-day DNA content of cultures compared to serum-treated controls but this effect was weak in cultures in 14-month old rats. The number of high affinity binding sites for [125I]EGF decreased from 37,800 in the 1-month old to 1950 in the 14-month old rat AC. The apparent dissociation constant (Kd) also decreased with age: 0.18 nmol/l in the 1-month old; 0.12 nmol/l in the 8-month old; and 0.07 nmol/l in the 14-month old cells. AC in older rats contained more EGF mRNA and less EGF-receptor mRNA. Incubation of the cells with EGF resulted in down regulation of the EGF- and upregulation of EGF-receptor mRNA expressions. These findings show the age-related quantitative and qualitative alterations in EGF and EGF-receptor which may account, at least in part, for the diminished responsiveness of senescent AC to EGF.

The biochemistry of mammalian senescence

Senescence is a process which, until quite recently, has been the subject of little scientific investigation. Even the word "senescence" is difficult to define, and complex methodological pitfalls have impeded progress. In the past few years, there have been exciting advances in understanding the physiological, cell biological, biochemical, and molecular biological nature of senescence. Changes in membrane function, protein synthesis, DNA structure (including glycosylation, altered tertiary structure, free-radical effects, and loss of telomeric DNA), and changes in gene regulation with age are reviewed. Recent work on changes in responses to transcriptional regulatory proteins and cellular senescence factors, some of which have been identified, is particularly promising and leads to the conclusion that senescence, at least in part, is a programmed process. Despite these advances, the fundamental cause of senescence remains elusive but might, as in the case of other biological processes which are phylogenetically widespread, turn out to be quite simple, and perhaps, even modifiable.

Protein synthesis and the components of protein synthetic machinery during cellular aging

The slowing down of protein synthesis is a change widely observed during the aging of organisms. It has also been claimed that a decline in the rate of protein synthesis occurs during cellular aging. However, the evidence in favour of this view is not clear-cut, and reliable estimates of rates of protein synthesis during cellular aging have yet to be made. Studies on various components of the protein synthetic machinery during cellular aging 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. Detailed studies on the structure and function of ribosomes, tRNA isoacceptor profiles, activities of aminoacyl-tRNA synthetases, levels and activities of initiation factors, rates of protein elongation, and the accuracy of protein synthesis will be needed before the molecular mechanisms of the regulation of protein synthesis during cellular aging can be understood.

Appearance of the terminal senescent cell population in human diploid fibroblasts analyzed by flow cytometry

We studied changes in the distribution pattern of relative RNA content during the in vitro aging of TIG-3 cells by flow cytometry (FACS III). Propidium iodide (PI) does not stain total cellular RNA, but it intercalates specifically into double-helical regions of both DNA and RNA. In applying this principle to RNA, we stained double-stranded RNA (dsRNA) in whole cells with PI after DNA digestion with DNase. The results showed that dsRNA distribution patterns were relatively constant at 7-75 population doublings (PD) but were significantly altered after 77 PD. The distribution patterns were similar as those for cell volume measured with a Coulter Counter. The total cellular dsRNA contents increased linearly at the senescent phase of their in vitro life span. In contrast, the mean dsRNA contents (50% dsRNA contents) rapidly increased to 77-79 PD, but decreased somewhat at 81-83 PD. Two-dimensional histograms of the dsRNA contents versus cell size were little altered from 25 PD to 75 PD. However, a population with relatively larger cell volume and weaker fluorescence intensity appeared and increased after 79 PD. This cell population group may be categorized as "terminal senescent cells" that no more divide in respect that the dsRNA content decreases in spite of the increase of total RNA content.

Cellular senescence revisited: a review

The field of cellular senescence (cytogerontology) is reviewed. The historical precedence for investigation in this field is summarized, and placed in the context of more recent studies of the regulation of cellular proliferation and differentiation. The now-classical embryonic lung fibroblast model is compared to models utilizing other cell types as well as cells from donors of different ages and phenotypes. Modulation of cellular senescence by growth factors, hormones, and genetic manipulation is contrasted, but newer studies in oncogene involvement are omitted. A current consensus would include the view that the life span of normal diploid cells in culture is limited, is under genetic control, and is capable of being modified. Finally, embryonic cells aging in vitro share certain characteristics with early passage cells derived from donors of increasing age.

Effects of aging on the recycling via the pentose cycle and on the kinetics of glycogen and protein metabolism in various organs of the rat

The rate of metabolic kinetics and the frequency of biological cycles may be correlated with the rate of aging and the maximum life-span potential. Therefore, investigations either into changes with age of such parameters within one species or into differences between species may give some information about the genetic programming of the aging process. Male Sprague-Dawley rats aged 3.5, 7, 12, 17, 23 and 33 months (m) were used to determine the changes with age of those metabolic pathways mentioned in the title, using the liver, kidney, brain, heart and the skeletal muscle. The maximum percentage of glucose utilization via the pentose pathway, compared to the total glucose utilization, was calculated after intravenous administration of D-[1-14C]- and D-[6-14C]glucose by the determination of the trioses (as lipids) 3 hours after the application. Glycogen kinetics was determined analogously. Total protein metabolism was observed using the essential amino acid L-[2,5-3H]histidine. The results indicate a decrease in the glucose utilization via the pentose pathway in the course of aging in liver, kidney, heart and skeletal muscle and a decrease from 3.5 months on in brain, a small but not significant change of the kinetics of glycogen metabolism (a lower turnover), and a reduced rate of protein synthesis in liver, kidney, heart and brain through an age of 23 months, followed by an elevated rate. Brain did not show any changes. The reduction of the pentose pathway may possibly be the cause of higher lipofuscin accumulation in the cells of some organs, lacking sufficient reduction equivalents for lipid metabolism. Furthermore, there could exist a connection with the reduced protein turnover, because less riboses are provided for the synthesis of nucleic acids.

Inhibition of collagen production in scleroderma fibroblast cultures by a connective tissue glycoprotein extracted from normal dermis

It was shown in a previous paper that a connective tissue glycoprotein (CTGP) extracted from normal rabbit dermis was able to inhibit total protein and collagen syntheses by normal dermis fibroblast cultures. In the present study, the effects of CTGP on scleroderma fibroblasts were investigated. [14C]Proline incorporation into total proteins of the supernatant was not significantly different from that found in controls. By contrast, the amount of collagen, expressed as percentage of total secreted protein, was far higher in scleroderma cultures than in normal ones (14.4% +/- 6.0% vs 4.6% +/- 0.9%). Addition of CTGP to the medium induced a concentration-dependent inhibition of [14C]proline incorporation into proteins from both control and scleroderma cells. In control cultures, no significant decrease of the percentage of collagen was observed, but over 60 micrograms/ml, both cytotoxic effects and inhibition of protein synthesis occurred. In scleroderma cultures, the inhibition was twice as effective on collagen as on noncollagen protein synthesis. The inhibition of collagen secretion was not related either to changes in collagen hydroxylation or to the intracellular catabolism of newly synthesized procollagen.