Loss of a critical neutral protease in ageing WI-38 cells

In-vivo screening implicates endoribonuclease Regnase-1 in modulating senescence-associated lysosomal changes

Accumulation of senescent cells accelerates aging and age-related diseases, whereas preventing this accumulation extends the lifespan in mice. A characteristic of senescent cells is increased staining with β-galactosidase (β-gal) ex vivo. Here, we describe a progressive accumulation of β-gal staining in the model organism C. elegans during aging. We show that distinct pharmacological and genetic interventions targeting the mitochondria and the mTORC1 to the nuclear core complex axis, the non-canonical apoptotic, and lysosomal-autophagy pathways slow the age-dependent accumulation of β-gal. We identify a novel gene, rege-1/Regnase-1/ZC3H12A/MCPIP1, modulating β-gal staining via the transcription factor ets-4/SPDEF. We demonstrate that knocking down Regnase-1 in human cell culture prevents senescence-associated β-gal accumulation. Our data provide a screening pipeline to identify genes and drugs modulating senescence-associated lysosomal phenotypes.

Asymmetric distribution of DNA between daughter cells with final symmetry breaking during aging of human fibroblasts

Human fibroblasts proliferating in vitro go through functional modifications, lose progressively their capacity to divide, and enter finally a post-mitotic state. These events are supposed to reproduce the developmental steps taking place in vivo during aging of the organism. The gradual changes occurring through proliferation are incompatible with an even distribution of the genetic material during cell division. We measured the amount of DNA on pairs of daughter cells at different population doubling levels of human fibroblasts. It was found that at each doubling in a significant fraction of cells, the distribution of DNA between sister cells is asymmetric. The cell system is in a steady state through the different phases of the fibroblast population life span; then during the last mitoses when the cells enter the terminal phase IV there is symmetry breaking with a phase transition, the cells settling into a new state.

Senescence-associated beta-galactosidase is lysosomal beta-galactosidase

Replicative senescence limits the proliferation of somatic cells passaged in culture and may reflect cellular aging in vivo. The most widely used biomarker for senescent and aging cells is senescence-associated beta-galactosidase (SA-beta-gal), which is defined as beta-galactosidase activity detectable at pH 6.0 in senescent cells, but the origin of SA-beta-gal and its cellular roles in senescence are not known. We demonstrate here that SA-beta-gal activity is expressed from GLB1, the gene encoding lysosomal beta-D-galactosidase, the activity of which is typically measured at acidic pH 4.5. Fibroblasts from patients with autosomal recessive G(M1)-gangliosidosis, which have defective lysosomal beta-galactosidase, did not express SA-beta-gal at late passages even though they underwent replicative senescence. In addition, late passage normal fibroblasts expressing small-hairpin interfering RNA that depleted GLB1 mRNA underwent senescence but failed to express SA-beta-gal. GLB1 mRNA depletion also prevented expression of SA-beta-gal activity in HeLa cervical carcinoma cells induced to enter a senescent state by repression of their endogenous human papillomavirus E7 oncogene. SA-beta-gal induction during senescence was due at least in part to increased expression of the lysosomal beta-galactosidase protein. These results also indicate that SA-beta-gal is not required for senescence.

High glucose-induced replicative senescence: point of no return and effect of telomerase

Primary human cells enter senescence after a characteristic number of population doublings (PDs). In the current study, human skin fibroblasts were propagated in culture under 5.5mM glucose (normoglycemia); addition of 16.5mM D-glucose to a concentration of 22 mM (hyperglycemia); and addition of 16.5mM L-glucose (osmotic control). Hyperglycemia induced premature replicative senescence after 44.42+/-1.5 PDs compared to 57.9+/-3.83 PDs under normoglycemia (p<0.0001). L-Glucose had no effect, suggesting that the effect of hyperglycemia was not attributed to hyperosmolarity. Activated caspase-3 measurement showed a significantly higher percentage of apoptotic cells in high glucose medium. Telomerase overexpression circumvented the effects of hyperglycemia on replicative capacity and apoptosis. The "point of no return," beyond which hyperglycemia resulted in irreversible progression to premature replicative senescence, occurred after exposure to hyperglycemia for as few as 20 PDs. These results may provide a biochemical basis for the relationship between hyperglycemia and those complications of diabetes, which are reminiscent of accelerated senescence.

Senescence-associated (beta)-galactosidase reflects an increase in lysosomal mass during replicative ageing of human endothelial cells

Senescence-associated (beta)-galactosidase is widely used as a biomarker of replicative senescence. However, it remains unknown whether this is a distinct enzyme active at pH 6, and differentially expressed in senescence, or a manifestation of an increase in the classic acid lysosomal (beta)-galactosidase. Here we have investigated the origin of senescence-associated-(beta)-galactosidase activity by modifying the intracellular and lysosomal pH of young and senescent human umbilical vein endothelial cells and examining the effect of these manipulations on the levels of activity, using a flow cytometric assay. Lysosomal alkalinisation with chloroquine or bafilomycin A(1), as well as equilibration of the intracellular milieu to pH 6 with nigericin, caused a profound (92-99%) inhibition of the total intracellular (beta)-galactosidase activity. However, independent of pH alterations, senescent cells showed levels of (beta)-galactosidase activity three- to sixfold higher than young cells. This increase in activity occurred in parallel to an increase in (beta)-galactosidase protein levels. Acridine Orange staining revealed an increase in lysosomal content with replicative age, which correlated with the increase in (beta)-galactosidase. These findings demonstrate that senescence-associated (beta)-galactosidase is a manifestation of residual lysosomal activity at a suboptimal pH, which becomes detectable due to the increased lysosomal content in senescent cells.

Increase of oxidatively modified protein is associated with a decrease of proteasome activity and content in aging epidermal cells

For the process of aging in epidermal cells to be characterized, the status of oxidized and damaged protein accumulation and removal by the proteasome has been investigated. Modified protein content and proteasome activity were assayed in lysates of epidermal cells from donors of different ages. Increased levels of oxidized proteins, glycated proteins, and proteins modified by the lipid peroxidation product 4-hydroxy-2-nonenal were observed in cells from old donors. At the same time, a decline of chymotrypsin-like and peptidylglutamyl-peptide hydrolase activities of the proteasome was found in aging keratinocytes. This age-related decline of the proteasome peptidase activities can be explained, at least in part, by a decreased proteasome content as observed by immunoblotting and enzyme-linked immunosorbent assay. In keratinocyte cultures, a decrease of proteasome activity and content was observed upon serial passaging. In cultures, as well as in skin, an inverse relationship was found between the aging marker 1-galactosidase and the proteasome content. These results suggest that proteasome is downregulated during replicative senescence as well as in aged cells in vivo, possibly resulting in the accumulation of modified proteins.

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.

A biomarker that identifies senescent human cells in culture and in aging skin in vivo

Normal somatic cells invariably enter a state of irreversibly arrested growth and altered function after a finite number of divisions. This process, termed replicative senescence, is thought to be a tumor-suppressive mechanism and an underlying cause of aging. There is ample evidence that escape from senescence, or immortality, is important for malignant transformation. By contrast, the role of replicative senescence in organismic aging is controversial. Studies on cells cultured from donors of different ages, genetic backgrounds, or species suggest that senescence occurs in vivo and that organismic lifespan and cell replicative lifespan are under common genetic control. However, senescent cells cannot be distinguished from quiescent or terminally differentiated cells in tissues. Thus, evidence that senescent cells exist and accumulate with age in vivo is lacking. We show that several human cells express a beta-galactosidase, histochemically detectable at pH 6, upon senescence in culture. This marker was expressed by senescent, but not presenescent, fibroblasts and keratinocytes but was absent from quiescent fibroblasts and terminally differentiated keratinocytes. It was also absent from immortal cells but was induced by genetic manipulations that reversed immortality. In skin samples from human donors of different age, there was an age-dependent increase in this marker in dermal fibroblasts and epidermal keratinocytes. This marker provides in situ evidence that senescent cells may exist and accumulate with age in vivo.

Inhibition of DNA polymerases by tripeptide derivative protease inhibitors

Benzyloxycarbonyl(Z)-Leu-Leu-Leu-al and dansyl(Dns)-Leu-Leu-Leu-CH2Cl, well known as protease inhibitors, effectively inhibit the activities of DNA polymerases alpha, beta and gamma from rat liver and pol I from Escherichia coli, but the ability of these inhibitors to inhibit terminal deoxynucleotidyl transferase (TdT) is weak. The mode of inhibition by these tripeptide analogues is non-competitive with dNTP. The Ki values for Z-Leu-Leu-Leu-al and Dns-Leu-Leu-Leu-CH2Cl are 6.25 x 10(-5) M and 6.56 x 10(-5) M, respectively.

Cell surface oligosaccharide modulation during differentiation: V. Partial characterization of the regulated surface during substrate adhesion and spreading

In previous studies with IMR-90 human fetal lung fibroblasts, it was shown that modulation of a small finite number of unique cell surface oligosaccharide structural specificities (as defined by reaction with specific lectins and monoclonal antibodies) define the cellular senescence phenotype. The cell surface oligosaccharide display can be characterized by assessing the epitope density and conformational arrangement of three or four individual carbohydrate specificities. Development of the senescence morphological phenotype was preceded by specific alterations in the cell surface oligosaccharide display. The senescence-dependent changes in these displays are primarily related to the binding affinity (the Kij of the Scatchard analyses) rather than the epitope density (the binding capacity, Rij of the Scatchard analyses). These alterations involve rearrangements within specific carbohydrate classes. In the course of these studies, the observation was made that low-density and contact-inhibited growth-retarded Phase II cells showed similar surface modulation of the oligosaccharide display. This suggests a broader significance for the date in growth regulation. These data suggest a structural/functional relationship between cellular senescence mechanisms and growth control in general. In this study we have investigated the possible role of cell surface oligosaccharide regulation in substrate interactions. Trypsinized cells were studied during substrate adherence, spreading and initiation of growth on control and poly-L-ornithine-treated polystyrene substrate. The cell surface oligosaccharide display was characterized using both biological assays and NMR-based measurements of the mobility of water at the cell surface. Results show that trypsinization does not remove or diminish the oligosaccharide display. The initial adherence of the cells to unmodified substrate is mediated by oligosaccharides. Cellular spreading results in specific changes in the display. Initiation of growth corresponds to further specific changes in the display. These data suggest a mechanistic connection between the cell surface oligosaccharide display and growth control of these fibroblasts.

Changes in the cell surface of human diploid fibroblasts during cellular aging

The electrophoretic mobility of 13 human diploid cell strains, TIG-1, TIG-2, TIG-3, TIG-7, WI-38, IMR-90, MRC-5, MRC-9, TIG-1H, TIG-1L, TIG-2M, TIG-2B, and TIG-3S, which were established from different tissues of human embryos, was studied at different passages. The net negative surface charge of the cells was characteristic for each cell strain and decreased significantly during the in vitro aging of the cells. The decrease in the net negative charge of the cells correlated well with the decrease in cell density throughout the life span of the cells. A strict linear correlation between the electrophoretic mobility and the number of cells harvested at each passage was obtained for all the human diploid cell strains. Moreover, almost the same linear regression coefficient of the cells was obtained among these cell strains. Therefore, the net negative surface charge of human diploid cell strains could serve as a cell surface marker for in vitro cellular aging.

The effects of bestatin, a microbial aminopeptidase inhibitor, on epidermal growth factor-induced DNA synthesis and cell division in primary cultured hepatocytes of rats

We investigated the effects of microbial protease inhibitors, in particular the aminopeptidase inhibitor bestatin, on DNA synthesis and cell division induced by epidermal growth factor (EGF) in hepatocytes. Although bestatin did not significantly affect binding of EGF to hepatocytes, it inhibited EGF-induced DNA synthesis and cell division. DNA synthesis in rat hepatocytes was maximal 24-26 h after EGF addition to the medium. The time required for maximal DNA synthesis was not affected if bestatin was removed less than 12 h after addition, but synthesis was partially inhibited if bestatin was added to the medium several hours after EGF addition, depending on the time of bestatin addition. Our results suggest that bestatin arrests the new cell cycle induced by EGF at about 12 h after the initiation. Considering also our results obtained by employing other protease inhibitors, we concluded that specific proteases play important roles in hepatocyte DNA synthesis and cell division induced by EGF.

The influence of cathepsin B and leupeptin on potentially lethal damage repair in mammalian cells

Cell response to irradiation depends on many micro-environmental and intracellular factors. It is known that proteinases control many physiological functions and are also involved in progression of the cell cycle. They also could be involved in cell response to irradiation. In this work the influence of cathepsin B, which is one of the important lysosomal proteinases, and one of its inhibitors, leupeptin, on the potentially lethal damage repair (PLDR) was studied. Chinese hamster V79 cells were irradiated with gamma rays in the plateau-phase of growth. Immediately after irradiation cathepsin B or leupeptin were added to the growth medium. Four hours later, a determined sufficient period of time for maximal PLDR, the cells were replated to assess survival and mutation induction. Mutation frequency was determined at the hypoxanthine-guanine phosphoribosyltransferase (HGPRT) locus using resistance to 6-thioguanine (6-TG). Simultaneously, the activity of cysteine, aspartic and serine proteinases were determined at different postirradiation intervals. The results show that when plateau-phase cells were incubated with cathepsin B during the postirradiation interval strong inhibition of PLDR was observed, accompanied with a reduced number of 6-TG resistant mutants. If leupeptin was added, more modest inhibition of PLDR was observed, accompanied with only slight reduction in the mutation frequency. The addition of cathepsin B or leupeptin to irradiated cells modified the activities of intracellular proteinases. As the highest alterations in proteinase activities were observed at the time when maximum repair of DNA lesions occurred, the biological consequences could involve a series of sequential steps in intracellular proteinase activities.

Proteinases and their inhibitors in cells and tissues

A large body of evidence has been assembled to indicate the substantial importance of proteolytic processes in various physiological functions. It has recently become clear too that endo-acting peptide bond hydrolases provisionally characterized and classified at present as serine, cysteine, aspartic and metallo together with unknown catalytic mechanism proteinases sometimes act in cascades. They are controlled by natural proteinase inhibitors present in cells and body fluids. In the first part of the present monograph the author was concerned to present an overview on the morphological and physiological approach to localization, surveying reaction principles and methods suitable for visualization of proteolytic enzymes and their natural and synthetic inhibitors. In the second part the roles played by proteinases have been summarized from the point of view of cell biology. The selection of earlier and recent data reviewed on the involvement of proteolysis in the behavior of individual cells reveals that enzymes, whether they be exogeneous or intrinsic, can be effective and sensitive modulators of cellular growth and morphology. There exists a close correlation between malignant growth and degradation of cells. It appears likely that as yet unknown or at least so far inadequately characterized factors that influence the survival or the death of cells may turn out to be proteinases. The causal role of extracellular proteolysis in cancer cell metastases, in stopping cancer cell growth and in cytolysis remains for further investigated. Ovulation, fertilization and implantation are basic biological functions in which proteolytic enzymes play a key role. The emergence of new approaches in reproductive biology and a growing factual basis will inevitably necessitate a reevaluation of present knowledge of proteolytic processes involved. The molecular aspects of intracellular protein catabolism have been discussed in terms of the inhibition of lysosomal and/or non-lysosomal protein breakdown. Peptide and protein hormone biosynthesis and inactivation are still at the centre of interest in cell biology, and a number of proteinases have been implicated in both processes. A number of conjectures partly based on the author's own work have been discussed which suggest the possibility of the involvement of proteolysis in exocytosis and endocytosis. The author's optimistic conclusion is that through the common action of biochemists, cell biologists, cytochemists, and pharmacologists the mystery of cellular proteolysis is beginning to be solved.

Culture conditions found to minimize false positive diagnosis of lysosomal storage disorders

The effect of culture conditions on the ultrastructure and enzyme activities of cultured skin fibroblast cells relevant to the diagnosis of lysosomal storage disorders are reported. The parameters examined were: pH of the culture media, type of media, increasing cell passage, and day of harvest. Ultrastructural changes were defined in terms of the number of lysosome-like inclusion bodies per cell according to a method devised in our laboratory and proven reliable in the detection of affected individuals. Our biochemical results included determination of enzyme activities of beta-hexosaminidase, alpha-mannosidase, beta-glucuronidase-lysosomal enzymes, arylsulfatase C, a microsomal marker, and 5' nucleotidase, a plasma membrane marker. Our results indicate that the cellular ultrastructure is more sensitive than enzyme activity to changes in culture conditions. The resulting ultrastructural "artifacts" observed under certain conditions were severe enough to result in a mistaken diagnosis. Due to certain difficulties we had previously encountered in heterozygote cultures (for lysosomal storage disorders) of amniotic cells, we decided to examine heterozygote cultures of skin fibroblasts. From these (preliminary) studies it seems that an elevation in the pH over the physiologic levels in the culture media may help to define between normal individuals and affected heterozygotes. On the basis of our results, we recommend that to minimize false positive ultrastructural results for the diagnosis of lysosomal storage disorders, cultures be grown in minimal essential medium, the pH of the medium carefully monitored to remain below 7.4, examining the cultures not later than cell Passage 8 and no later than Day 10 after subculture.

Changes in negative surface charge of human diploid fibroblasts, TIG-1, during in vitro aging

The electrophoretic mobility of human diploid fibroblasts, TIG-1, was studied at different passages. The net negative surface charge of the cells decreased from -1.658 +/- 0.108 micron/s/V/cm at an early passage (15 population doublings, PD) to -1.173 +/- 0.116 at the final passage (67 PD) in 1/15 M phosphate buffer supplemented with 5.4% glucose. The decrease was slow at 15-45 PD, but was rapid at 45-67 PD. The net negative surface charge of small cells in the late passage populations was not different from that of larger cells in this population, and was significantly lower than that of small cells in the middle passage populations. The distribution of the mobilities of cells in each passage was independent of the size of the individual cells, and the mean value was distinct for the passage number. The viability of the cells was retained during the assay of electrophoretic mobility under these conditions. These results indicate that the net negative surface charge of human diploid fibroblasts represents a cell surface maker for in vitro cellular age in the population.

Cell-layer-associated proteolytic cleavage of the telopeptides of type I collagen in fibroblast culture

In human skin fibroblast cultures a fraction of the procollagen that was processed to collagen and remained in the cell layer was further proteolytically modified by removal of both N- and C-terminal telopeptides. The proteolytic activity was associated with the cell layer, since secreted collagens were found always to contain intact telopeptides. The inclusion of neutral polymers, which caused the accumulation of the collagen in the cell layer [Bateman, Cole, Pillow & Ramshaw (1986) J. Biol. Chem. 261, 4198-4203], made the telopeptide cleavage more apparent in those cells which expressed the proteolytic activity. The extent of this cleavage was variable from cell culture to cell culture and between experiments with the same fibroblast line. The proteolytic activity was pH-dependent; cleavage was greatest at a culture-medium pH of 7.5 and 8.0 and was completely inhibited at a culture-medium pH of 7.0 and 6.5. The activity was significantly inhibited by soybean trypsin inhibitor, an elastase-specific inhibitor (N-acetylalanylalanylprolylvalylchloromethane) and the thrombin inhibitor hirudin. This cell-associated proteolytic activity may play a role in collagen degradation by removing the telopeptides, which are the primary sites of collagen cross-linking, thus destabilizing the collagen matrix sufficiently to render it susceptible to further proteolytic breakdown.

Degradation of peptides and proteins of different sizes by homogenates of human MRC5 lung fibroblasts. Aged cells have a decreased ability to degrade shortened proteins

The degradation of haemoglobin and haemoglobin-derived peptide fragments by homogenates of MRC5 fibroblasts has been investigated. Results show that the smaller fragments were degraded more rapidly than larger substrates at both pH 5.5 and pH 7.5. Only the smallest of the soluble cyanogen bromide peptides (Mr 3500) was degraded at pH 7.5. Degradation at pH 5.5 proceeded more rapidly than that at pH 7.5 for all substrates tested but was more marked with the larger substrates. Homogenates prepared from aged cells degraded puromycin peptides and, to a lesser extent, cyanogen bromide peptides at a slower rate, at pH 7.5, than those prepared from younger cells. We suggest that cytosolic degradation is less selective and at least one cytosolic proteolytic activity decreases as cells age.

Further evidence for a cell surface proteinase essential to the growth of cultured fibroblasts

Specific antibodies and protein proteinase inhibitors will inhibit cell-surface proteinase activity on human fibroblasts and cause a concomitant inhibition of DNA synthesis and of cell multiplication. An insolubilized proteinase inhibitor also inhibits cell multiplication. The same reagents partially inhibit the multiplication of mouse L cells, both in monolayer and suspension culture, and inhibit the mitogenic effect of epidermal growth factor (EGF) on both types of cell.

Significance of glycosidases and phosphatases in Cercopithecus and Macaca cells

When selected ratios of different glycosidases and phosphatases from primary monkey kidney cells or from monkey kidney cell lines are presented graphically, characteristic patterns do evolve. Three different subtypes of Vero cells show similar glycosidase patterns. The Vero subtypes tested show glycosidase patterns that are closely similar to those of primary cells of Cercopithecus aethiops. Glycosidase patterns of BS-C-1 and CV-1 cells are less similar to those of primary Cercopithecus cells than are those of Vero cells. Primary kidney cells from Macaca cynomolgus show significantly different glycosidase patterns compared with those of different Cercopithecus cells. The distinct glycosidase patterns can be used to classify the tested cell lines in relation to each other.

Altered degradation of intracellular proteins in aging human fibroblasts

Confluent cultures of fibroblasts at different population doubling levels were incubated with [14C]leucine for 2 days and with [3H]leucine for 2 h to label long-lived and short-lived proteins, respectively. Proteolysis was then measured in the presence of excess unlabeled leucine to prevent reutilization of the isotope. Catabolism of long-lived proteins was reduced in senescent cells when measured in media without fetal bovine serum, insulin, fibroblast growth factor, or dexamethasone. In contrast, degradation of short-lived proteins was increased in senescent cells but only when measured in the presence of serum, hormones, and growth factors. Further experiments with cells of varying ages indicate that in unsupplemented medium half-lives of long-lived proteins lengthened by as much as 20 min per population doubling and in supplemented media half-lives of short-lived proteins decreased by 4 min per population doubling. The reduced catabolism of long-lived proteins in senescent cells cannot be explained by age-related changes in protein secretion or cell death during degradation measurements. These alterations in proteolysis may have major effects on protein content and composition in senescent cells.

Antibody affinity chromatography of human proteinases and related proteins

Antibodies specific for a purified proteinase from the cell-surface of human leukocytes were used to prepare an antibody affinity chromatography column. This column bound a variety of proteins from extracts of human cells and tissues and from human body fluids, indicating that proteins immunologically related to the leukocyte proteinase are widespread in the human body. For human urine and a human fibroblast extract, a single protein species with serine proteinase activity was eluted from the column in each case. Small quantities of a heterogeneous protein fraction were also weakly bound to the column from an extract of mouse submaxillary glands.

Membrane proteinases from normal and neoplastic tissues in man and the rat

Plasma membranes were purified from rat liver, muscle and sarcoma tissues and from human liver and hepatoma tissues. The plasma membranes all contained DFP-sensitive, neutral proteolytic activity. Plasma membranes from all normal tissues contained a single DFP-binding protein of apparent molecular weight 68,000. Only the plasma membranes from tumour tissue contained a plasminogen activator; the DFP-binding proteins from these membranes were more diverse than those from the normal samples. The rat liver plasma membrane proteinase was purified. It was a labile enzyme sensitive to inhibition by DFP and by calcium ions, and with a broad substrate specificity. A similar protein was the sole DFP-binding protein in rat liver microsomes. This and the properties of the enzyme suggested a possible role in the processing and secretion of newly-synthesized protein.

Glycosaminoglycans and the control of cell surface proteinase activity

It is postulated that the metabolically variable fine structure of pericellular heparan glycosaminoglycans affects the ability of these molecules to influence cell proliferation-associated proteinase-catalysed reactions occurring at cell surfaces. Evidence suggesting the possibility of a wide repertoire of glycosaminoglycan-mediated positive and negative effects on such reactions is reviewed. It is suggested that clinical administration of compounds related chemically to heparins might usefully modulate cell proliferation-associated proteinase activity.

Comparison between proliferative changes and nuclear events during ageing of human fibroblasts in vitro

Age-related alterations in nucleoproteins were examined in resting human embryonic fibroblasts at various population doubling levels (PDL) with three techniques for electron microscopy. Conventional glutaraldehyde fixation showed nuclear modifications consistent with previously published studies. Miller's technique permitted the detection of transcription complexes in young as well as in old cells. The technique involving the ultrathin sectioning of partially loosened nucleoproteins revealed at high PDL a marked rarefaction of the chromatin threads and the appearance of large parts of the lamina densa devoid of attached threads. In addition, the nucleolar filamentous masses were considerably less tufty than at earlier PDL, suggesting a decrease in the ribosomal transcriptional activity. These changes in nucleoprotein organization were rare before the 40th PDL, then increased to about 5% of the nuclei when the percentage of cells initiating DNA during the first 24 h remained at low levels. They were the rule for the last 6 to 7 PDL and coincided with the time when the maximal densities started to fall rapidly. They appeared after treatment with a loosening medium at neutral and at alkaline pH.

Synthesis of sialoglycoconjugates during dimethylsulfoxide-induced erythrodifferentiation of friend leukemia cells

We have previously shown that erythroid differentiation of Friend murine leukemia cells by dimethylsulfoxide results in a decrease in sialic acid content and net negative surface charge. The mechanism responsible for the decrease in sialic acid content was examined by measuring the synthesis of sialic acid from N-acetylmannosamine and its catabolic removal from sialoconjugates during the maturation process. A decrease in the incorporation of N-[3H]acetylmannosamine into sialoglycoconjugates occurred as early as 12 h after exposure to dimethylsulfoxide. Radioactivity incorporated into sialoglycoconjugates was relatively stable in untreated and dimethyl-sulfoxide-treated cells, implying that catabolic removal of sialic acid residues was not a factor in the decreased surface sialic acid content of differentiated erythroleukemia cells. In addition, no difference existed between control and treated cells in sialyltransferase activity. Significant decreases occurred, however, in the incorporation of radioactivity from N-[3H]acetylmannosamine into N-acetylneuraminic acid, CMP-N-acetylneuraminic acid and a material tentatively identified as N-acetylmannosamine-6-phosphate, 48 h after the addition of dimethylsulfoxide. The decrease in sialic acid biosynthesis in differentiated erythroleukemia cells was reflected by an 83% decrease in the amount of radioactively-labeled sialic acid released by neuraminidase treatment of cells exposed to dimethylsulfoxide. These findings are consistent with a cellular aging phenomenon triggered by the polar solvent-induced differentiation of the leukemic cells into more mature forms.

Ganglioside changes during cell aging in human diploid fibroblast TIG-1

Gangliosides of human diploid fibroblast, TIG-1, derived from fetal lung were analyzed during cell aging. The major gangliosides of TIG-1 cells were GM3 and GD3. During cell aging, GM3 and other gangliosides were decreased or below detection level. Lipid-bound sialic acid per mg protein was 2 micrograms at the early, middle and late passages, but between the middle and late passage it was twofold higher than the other passages.

Preliminary evidence that thrombin may mimic a naturally occurring proteinase in cultured cells

A neutral proteinase has been purified from the membranes of human leukocytes. Antibodies to this enzyme inhibit its proteolytic activity, and inhibit the growth of cultured human fibroblasts. This growth inhibition is apparently reversed by added thrombin.

Ageing and the fusion sensitivity potential of human cells in culture: relation to tissue origin, donor age, and in vitro culture level and condition

Cell fusion was induced by inactivated Sendai virus in different human diploid cell lines. These were derived from the kidney, lung or skin originating from embryos or adult donors and representing predominantly epithelioid cells (kidney) and fibroblastic cells (lung and skin). The fusion sensitivity (FS) potentials of these cell lines were determined and related to various aspects of cell ageing. In case of the fibroblastic lines, an inverse relation was demonstrated both to the culture age in vitro and to the donor age. The FS potential of embryonal fibroblasts decreased some 40-50% during the in vitro cultivation. In comparison to exponentially growing fibroblasts, the FS potentials were higher in cells in the stationary phase of growth. This was shown to correlate well with the fact that the life-span in calendar time also increased in cultures predominantly grown in the stationary phase. In the case of the kidney cells, the FS potentials were some 50% higher than those of fibroblasts. Since the cellular manifestation of the FS potential most likely primarily resides in the cell membrane-cytoskeleton structure, the results emphasize the importance of this system in relation to ageing.

Specificity patterns of different types of human fucosidase. Recognition of a certain region of the pyranose ring in sugars by the enzymes

Data on the hydrolysis of fucosides, galactosides and arabinosides by different types of fucosidase are presented. The comparative study of the splitting of alpha-L-fucoside and beta-D-arabinoside with a similar structure at C1--C4 of the pyranose ring and the preparation of the enzyme by affinity chromatography showed that both substrates were hydrolysed by the same enzyme, alpha-L-fucoside(beta-D-arabinoside)hydrolase. The analogous investigation of the hydrolysis of beta-D-fucoside, beta-D-galactoside and alpha-L-arabinoside with a similar structure at C1--C4 of the pyranose ring demonstrated that these glycosides were split by the same enzyne, beta-D-fucoside(beta-D-galactoside, alpha-L-arabinoside)hydrolase, the activity of which is decreased dramatically in GM1-gangliosidosis. The data obtained support the assumption that the specific action of different types of fucosidase is due to recognition by these enzymes of C1--C4 of the pyranose ring in the corresponding substrates. The problems of differential diagnosis of some glycosidoses (focosidosis, GM1-gangliosidosis and Fabry disease) are discussed on the basis of the data obtained.

Changes in cell-surface glycosaminoglycans in human diploid fibroblasts during in vitro aging

Changes in glycosaminoglycans during in vitro aging were investigated in human diploid fibroblasts. The cells were found to produce predominantly hyaluronate and smaller amounts of chondroitin 4-sulfate, chondroitin 6-sulfate, dermatan sulfate and heparan sulfate. Accumulation of heparan sulfate on the cell surface was notable during aging. Total glycosaminoglycan production in preconfluent culture did not change with population doubling level (PDL), while in confluent culture a decline in glycosaminoglycan production was observed. In contrast with this, heparan sulfate on the cell surface increased as a function of PDL in both confluent and preconfluent cultures. The distribution pattern of heparan sulfate in medium and cell surface indicated that the increase in heparan sulfate on the cell surface could be attributed to an increased accumulation on the cell surface, but not to an elevated production. Thus, we conclude that the increased accumulation of heparan sulfate on the cell surface might be involved in an age-related alteration in the cell membrane.

A new cell surface marker of aging in human diploid fibroblasts

The relationship of cell surface changes to proliferative decline of human diploid fibroblasts was investigated using the concanavalin A-mediated red blood cell adsorption assay. The amount of the red blood cells adsorbed to human diploid fibroblasts via concanavalin A increased continuously from the early phases of cell passage up through cell senescence, while the amount of 3H-concanavalin A binding did not change to a significant extent. The red blood cell adsorption is not a function of cell cycle phase and time spent in culture. Cocultivation of young cells with old cells also did not affect the adsorption capacity of respective cells. Thus, the concanavalin A-mediated red blood cell adsorption can be expected to serve as a new cell surface marker for aging in vitro. Using this marker, it was revealed that transient cell size or 3H-thymidine incorporating capacity di not have a direct relationship with the division age of a cell. Small rapidly dividing cells in old populations resemble large slowly dividing or nondividing cells of the same populations and differ from small rapidly dividing cells in young populations, in terms of cell surface properties.

Sialyltransferase activities of aging diploid fibroblasts

Sialyltransferase activity and cell-cell adhesion rates of aging WI-38 cells were studied to determine the possible basis for a previously described decrease in membrane bound sialic acid and loss of proliferation of senescent cells. Ectosialyltransferase was demonstrated on the surface of both young and old WI-38 cells. The sialyltransferase assays consist of an enzyme source which is either the surface of intact cells (ectoenzyme) or a Triton X-100 cell homogenate, the nucleotide sialic acid donor (cytidine monophosphate-N-acetylneuraminic acid), and an asialo-acceptor which may be endogenous to the enzyme preparation or may be added exogenously. When sialyltransferase activity is measured in the absence of exogenous acceptors, there is a greater amount of sialic acid transferred by odl cells. However, when exogenous acceptors are provided, the amount of transfer is stimulated to a greater extent in young cells equalizing the amount of sialic acid incorporated into young and old cells. This suggests that there are fewer asialoglycoproteins and that acceptor concentration is a limiting factor in assays of young cell sialyltransferase. The end result of this may be the previously described decreased amount of membrane-bound sialic acid of old cells. A change in the adhesiveness of old cells is described which may be related to the altered cell surface.

On the aging of organisms and their cells

An explanation of organismic ageing based on a limited division capacity of dividing cells is difficult to reconcile with much of the available data. The physiology of cells in ageing organisms tends, on the contrary, to suggest that organisms age as a result of degeneration in their non-dividing cell populations. Senescence in these non-mitotic cells resmebles the ageing of the non-dividing fraction of cell cultures clonally senescing, or maintained in long-term quiescence in vitro. As cultures of diploid human fibroblasts senesce there is an accumulation of non-dividing cells. Alterations in these post-mitotic cells can explain the senescent properties of late passage cultures. It is proposed that during the in vitro senescence of fibroblast cultures, cell ageing results from, as opposed to causes, the absence of mitosis. Cell ageing may primarily result from changes in the chromatin induced by the non-mitotic state.