Evidence for cellular aging in long-term confluent cultures: heritable impairment of proliferation, accumulation of age pigments and their loss in neoplastic transformation

Age-Dependent RGS5 Loss in Pericytes Induces Cardiac Dysfunction and Fibrosis

BACKGROUND

Pericytes are capillary-associated mural cells involved in the maintenance and stability of the vascular network. Although aging is one of the main risk factors for cardiovascular disease, the consequences of aging on cardiac pericytes are unknown.

METHODS

In this study, we have combined single-nucleus RNA sequencing and histological analysis to determine the effects of aging on cardiac pericytes. Furthermore, we have conducted in vivo and in vitro analysis of RGS5 (regulator of G-protein signaling 5) loss of function and finally have performed pericytes-fibroblasts coculture studies to understand the effect of RGS5 deletion in pericytes on the neighboring fibroblasts.

RESULTS

Aging reduced the pericyte area and capillary coverage in the murine heart. Single-nucleus RNA sequencing analysis further revealed that the expression of Rgs5 was reduced in cardiac pericytes from aged mice. In vivo and in vitro studies showed that the deletion of RGS5 impaired cardiac function, induced fibrosis, and morphological changes in pericytes characterized by a profibrotic gene expression signature and the expression of different ECM (extracellular matrix) components and growth factors, for example, TGFB2 and PDGFB. Indeed, culturing fibroblasts with the supernatant of RGS5-deficient pericytes induced their activation as evidenced by the increased expression of αSMA (alpha smooth muscle actin) in a TGFβ (transforming growth factor beta)2-dependent mechanism.

CONCLUSIONS

Our results have identified RGS5 as a crucial regulator of pericyte function during cardiac aging. The deletion of RGS5 causes cardiac dysfunction and induces myocardial fibrosis, one of the hallmarks of cardiac aging.

SIRT6-PAI-1 axis is a promising therapeutic target in aging-related bone metabolic disruption

The mechanistic regulation of bone mass in aged animals is poorly understood. In this study, we examined the role of SIRT6, a longevity-associated factor, in osteocytes, using mice lacking Sirt6 in Dmp-1-expressing cells (cKO mice) and the MLO-Y4 osteocyte-like cell line. cKO mice exhibited increased osteocytic expression of Sost, Fgf23 and senescence inducing gene Pai-1 and the senescence markers p16 and Il-6, decreased serum phosphate levels, and low-turnover osteopenia. The cKO phenotype was reversed in mice that were a cross of PAI-1-null mice with cKO mice. Furthermore, senescence induction in MLO-Y4 cells increased the Fgf23 and Sost mRNA expression. Sirt6 knockout and senescence induction increased HIF-1α binding to the Fgf23 enhancer sequence. Bone mass and serum phosphate levels were higher in PAI-1-null aged mice than in wild-type mice. Therefore, SIRT6 agonists or PAI-1 inhibitors may be promising therapeutic options for aging-related bone metabolism disruptions.

Quality and Purity of L929 Cell Cultures: Isoenzyme Profiles As a Sensitive Marker of Cross-Contamination

Objective: Inappropriate handling of cells can generate modifications in the genomic DNA. The additional risk is cross-contamination. Isoenzyme analysis with gel agarose electrophoresis is a known, fast, and cheap technique for detection of species-specific isoforms of intracellular enzymes. The aim of the experimental work was to check if variations in the cell growth conditions can affect morphology and/or nuclear anomalies including micronuclei (MN) in the L929 cells; and to define how sensitive and selective is the classic gel agarose electrophoresis for analysis of isoforms of the selected enzymes to detect cross-contamination of L929 cultures with HeLa cells or with the related species, such as CHO-K1 cells, in the case of unavailability of the commercial kits. Methods: The experiments were done with use of the National Collection of Type Cultures clone 929 (L929)-mouse fibroblasts from subcutaneous connective tissue; HeLa-human cervix adenocarcinoma; and CHO-K1-epithelial-like hamster ovary cells. Cell morphology was evaluated with a light/fluorescence microscope. MN were determined with the cytokinesis-block micronucleus assay, and the isoenzyme analysis was performed using gel agarose electrophoresis. Results: As shown, the overgrown cultures result in a significant increase of the MN in the L929 cells. The band patterns for lactate dehydrogenate, glucose-6-phosphate dehydrogenase, or malate dehydrogenase allow identification of the single L929, HeLa, or CHO-K1 cell line and to detect the cross-contamination, even up to 0.4%. Conclusions: There can be no exceptions from the recommended cell culture conditions in the passage scheme. The sensitivity of the gel agarose separation depends on the cells and on the type of enzyme tested and seems to be sufficient in a quick screening of the CHO-K1, L929, or HeLa cell cultures through the possible mutual contamination.

Variation of 4 MV X-ray dose rate in fractionated irradiation strongly impacts biological endothelial cell response in vitro

PURPOSE

Even though X-ray beams are widely used in medical diagnosis or radiotherapy, the comparisons of their dose rates are scarce. We have recently demonstrated in vitro (clonogenic assay, cell viability, cell cycle, senescence) and in vivo (weight follow-up of animals and bordering epithelium staining of lesion), that for a single dose of irradiation, the relative biological effectiveness (RBE) deviates from 1 (up to twofold greater severe damage at the highest dose rate depending on the assay) when increasing the dose rate of high energy X-ray beams.

MATERIAL AND METHODS

To further investigate the impact of the dose rate on RBE, in this study, we performed in vitro fractionated irradiations by using the same two dose rates (0.63 and 2.5 Gy.min^-1) of high-energy X-rays (both at 4 MV) on normal endothelial cells (HUVECs). We investigated the viability/mortality, characterized radiation-induced senescence by using flow cytometry and measured gene analysis deregulations on custom arrays.

RESULTS

The overall results enlighten that, in fractionated irradiations when varying the dose rate of high-energy X-rays, the RBE of photons deviates from 1 (up to 2.86 for viability/mortality experiments performed 21 days postirradiation).

CONCLUSION

These results strengthen the interest of multiparametric analysis approaches in providing an accurate evaluation of the outcomes of irradiated cells in support of clonogenic assays, especially when such assays are not feasible.

Screening Method for Identifying Toxicants Capable of Inducing Astrocyte Senescence

Cellular senescence is a tumor-suppressive mechanism which leads to near irreversible proliferative arrest. However, senescent cells can cause tissue dysfunction, in large part because they express a senescence-associated secretory phenotype (SASP) involving secretion of, amongst other factors, proinflammatory cytokines known to compromise neuronal health. Therefore, established neurotoxicants may cause neurotoxicity in vivo, in part by triggering mitotic cells in the brain to undergo senescence and adopt an inflammatory SASP which in turn could cause deleterious effects to surrounding neurons. To begin to address this hypothesis, we examined whether we could screen known neurotoxicants for their ability to cause astrocytes (a mitotic cell type especially important for maintaining neuronal health) to undergo senescence. For this purpose, we utilized inducible pluripotent stem cell-derived human astrocytes and screened an 80 compound neurotoxicant library provided by the Biomolecular Screening Branch of the NIEHS National Toxicology Program. Here we present a screening method based on induction of the senescent marker, senescent-associated beta-galactosidase (SA-β-gal). We describe in detail an automated method for the unbiased quantitation of percentage of SA-β-gal + astrocytes. Although our results suggest that conducting an SA-β-gal senescence screen using human inducible pluripotent stem cell-derived astrocytes may be feasible, they also highlight challenges that likely preclude its adaptation to high-throughput. We also explore the possibility of using primary mouse astrocytes for this purpose and explain why this platform is problematic and very unlikely to yield meaningful results, even in small screens with compound replicates.

Promise and problems in relating cellular senescence in vitro to aging in vivo

According to the 'Hayflick limit', human fetal fibroblasts have a uniform, limited replicative lifespan of about 50 population doublings in cell culture. This concept was extrapolated to diverse cells in the body. It seemed to decrease with the age of the cell donor and, as a form of cell senescence, was thought to underlie the aging process. More discriminating analysis, however, showed that the fibroblasts decayed in a stochastic manner from the time of their explantation, at a rate that increased with the number of population doublings in culture. There was no consistent relation to the age of the donor. Despite the contradictory evidence, the original version of the Hayflick limit retained its general acceptance. Cell senescence was attributed to the absence of telomerase in the fibroblasts, which resulted in shortening of telomeres at each division until they fell below a critical length needed for further division. However, it is well established that stem cells in renewing tissues undergo many more than 50 divisions in a lifetime, without apparent senescence. Contrary to early findings of no telomerase in most tissues, their stem cells retain telomerase and presumably telomere length despite many divisions in vivo. Massive accumulation of lipofuscin granules occurs under stress in long term crowded cultures, but the granules dissipate on subculture or neoplastic transformation. The overall results indicate a critical disjunction between cell senescence in vitro and aging in vivo. By contrast, cell culture has been useful in showing a need for telomere capping in maintaining cell stability and viability. It may also provide information about the biochemical mechanism of lipofuscin production.

SSeCKS, a major protein kinase C substrate with tumor suppressor activity, regulates G(1)-->S progression by controlling the expression and cellular compartmentalization of cyclin D

SSeCKS, first isolated as a G(1)-->S inhibitor that is downregulated in src- and ras-transformed cells, is a major cytoskeleton-associated PKC substrate with tumor suppressor and kinase-scaffolding activities. Previous attempts at constitutive expression resulted in cell variants with truncated ectopic SSeCKS products. Here, we show that tetracycline-regulated SSeCKS expression in NIH 3T3 cells induces G(1) arrest marked by extracellular signal-regulated kinase 2-dependent decreases in cyclin D1 expression and pRb phosphorylation. Unexpectedly, the forced reexpression of cyclin D1 failed to rescue SSeCKS-induced G(1) arrest. Confocal microscopy analysis revealed cytoplasmic colocalization of cyclin D1 with SSeCKS. Because the SSeCKS gene encodes two potential cyclin-binding motifs (CY) flanking major in vivo protein kinase C (PKC) phosphorylation sites (Ser(507/515)), we addressed whether SSeCKS encodes a phosphorylation-dependent cyclin scaffolding function. Bacterially expressed SSeCKS-CY bound cyclins D1 and E, whereas K-->S mutations within either CY motif ablated binding. Activation of PKC in vivo caused a rapid translocation of cyclin D1 to the nucleus. Cell permeable, penetratin-linked peptides encoding wild-type SSeCKS-CY, but not K-->S or phospho-Ser(507/515) variants, released cyclin D1 from its cytoplasmic sequestration and induced higher saturation density in cyclin D1-overexpressor cells or rat embryo fibroblasts. Our data suggest that SSeCKS controls G(1)-->S progression by regulating the expression and localization of cyclin D1. These data suggest that downregulation of SSeCKS in tumor cells removes gating checkpoints for saturation density, an effect that may promote contact independence.

Relation of the slow growth phenotype to neoplastic transformation: possible significance for human cancer

Deletions are widely distributed over the genome in the most frequently occurring human cancers and are the most abundant genetic lesion found there. Deletions are highly correlated with the slow growth phenotype of mutated animal and human cells and result in chromosomal transposition when the retained ends are joined. Transpositions are only a minor source of mutation in rapidly multiplying bacteria but are a major cause of mutations in stationary bacteria. The NIH 3T3 line of mouse cells undergoes neoplastic transformation during prolonged incubation in a stationary state and expresses the slow growth phenotype on serial subculture at low density, suggesting a relation between transformation and chromosomal deletions. To further explore the relation between neoplastic transformation and the slow growth phenotype as a surrogate for deletions, two sublines of the NIH 3T3 cells with differing competence for transformation were serially subcultured in the stationary state at confluence and tested at each subculture for transformation and growth rate. Cell death in a fraction of the population and a heritable slowdown in proliferation of most of the survivors became increasingly pronounced with successive rounds of confluence. The reduction in growth rate was not proportional to the degree of transformation of the cultures, but all of the transformed cultures were slow growers at low density. All of the discrete colonies from cloning transformed cultures developed at a lower initial rate than control colonies under optimal conditions for growth, but they continued to grow at later stages, forming multilayered colonies under conditions that inhibited the further growth of the control colonies. The results suggest that prolonged incubation of NIH 3T3 cells in the stationary state results in growth-impairing deletions over a wide range of sites in the genome, but more restricted subsets of such lesions are responsible for neoplastic transformation. These findings provide dynamic, functional support in culture for the histopathological evidence that the quiescent state of cells associated with atrophy and fibrosis plays a significant role in the origin of some cancers in experimental animals and human beings.

Quantitative aspects of the selective killing of transformed cells by methotrexate in the presence of leucovorin

A quantitative study was made of the cytotoxicity of methotrexate (MTX) for nontransformed and transformed NIH 3T3 cells in the presence and absence of leucovorin. The study was preceded by an analysis of the growth rates of the cells at low and high population density combined with low and high concentrations of calf serum (CS). The reduced maximal growth rates of the transformed cells at low population densities relative to the nontransformed cells reinforced earlier evidence that heritable damage involving chromosome aberrations drives the process of transformation. When small numbers of transformed cells are cocultured with a large excess of nontransformed cells in the assay for transformed foci, the transformed cells were more readily killed by MTX than the nontransformed cells. The selectivity was increased when leucovorin (folinic acid) was present in the medium. The selective killing of the transformed cells actively multiplying in foci was most pronounced when the background of nontransformed cells had become confluent and their growth was inhibited. However, selectivity has also been demonstrated when transformed and nontransformed cells are growing at their maximum rates at low density despite the lower growth rate of the transformed cells under these conditions. The sensitivity of transformed cells in pure culture to MTX was lower during the first 3 d of subculture than in the following 6 d but decreased to zero a few d after net growth had ceased. The nontransformed cells were more susceptible to killing by MTX in Dulbecco's modified Eagle's medium (DMEM) than in MCDB 402, but the transformed cells were sensitive to MTX in both media. The high selectivity of MTX for transformed over nontransformed cells in MCDB 402 results from the presence of 1.0 microM leucovorin (5-formyltetrahydrofolate), a reduced form of the folic acid present in most other culture media. When leucovorin was added to DMEM with its high concentration of folic acid, the resistance to MTX of both nontransformed and transformed cells was greatly increased, but the selectivity of MTX for transformed cells was almost entirely lost. The results indicate that leucovorin protects nontransformed cells against concentrations of MTX that kill transformed cells, but the protection is dependent on the relative amounts of leucovorin to folic acid in the medium. The relative sensitivities of transformed and nontransformed cells in our system to MTX when both cell types are exhibiting their characteristic differential in growth behavior is similar to that described for tumor and normal cells in vivo. Since the unregulated growth behavior of the transformed, tumor-producing cells is efficiently and quantitatively measured in this system, it can be used to develop general principles of treatment and resolve questions of cytotoxic mechanism.

Clonal dynamics of progressive neoplastic transformation

In a recent study, we found that newly isolated clones of NIH 3T3 mouse cells undergo neoplastic transformation more readily than uncloned cultures from which they were derived. After eleven low-density passages (LDPs), most of the 29 clones produced lightly stained early-stage transformed foci when grown to confluence in a primary assay for transformation, and one of them consistently produced a few tiny dense foci. In the present work, six of the clones were kept in LDPs for 56 passages and assayed for focus formation at confluence at six passage levels. The clone that produced tiny dense foci switched to light foci during the LDPs, four others produced light foci at different passage levels, and one progressed from light to dense foci after the last passage. By contrast, all the clones progressed to dense focus formation in five or fewer serial repetitions of the assay at confluence. Because all but one of the clones underwent about half as many total divisions at each LDP as they did when grown to the stationary state at confluence, the latter is more efficient in eliciting progression than the exponential growth of the LDPs. Extension of the period at confluence of uncloned cultures results in the appearance of dense foci within light foci. Because the latter are localized clonal populations, the intrafocal progression reinforces the conclusion that clonal expansion favors transformation. We discuss the significance of these results for the clonal origin of human cancer and the increased incidence of cancer with age.

Differential expression of p53 tumor suppressor protein and IL-2 in activated T cells from elderly humans

Aging is associated with a decline in T cell proliferative responses and aberrations in cytokine production. In the present study, we examined if aging might alter the expression of the tumor-suppressor protein p53 and the retinoblastoma susceptibility gene product (Rb) as well as the levels of Bcl-2 in resting and activated human T cells. No significant differences were observed in the basal levels of p53 protein among resting T cells from young and elderly humans. After stimulation with anti-CD3 monoclonal antibody (mAb) OKT3 and phorbol myristate acetate (PMA), T cells from young humans exhibited severalfold increases in p53 protein expression compared with resting T cells. By contrast, T cells from a substantial portion of elderly humans failed to demonstrate significant increases in p53 in response to anti-CD3 plus PMA. No age-related alterations in the levels of Rb or Bcl-2 proteins were observed in resting or anti-CD3/PMA-stimulated T cells. To delineate whether the age-related reductions in p53 expression might be linked to decreased interleukin-2 (IL-2) production, we compared the expression of p53 and IL-2 in anti-CD3/PMA-stimulated T cells from elderly people. The results showed that impaired induction of p53 expression in activated T cells from certain elderly people could be observed without considerable impairments in IL-2 production. These observations suggest that age-related reductions in T cell expression of p53 may contribute to the decline of T cell competence independent of the impairments in IL-2 production.

Cell kinetics of hemopoietic colony-forming units in spleen (CFU-S) in young and old mice

The growth potential of the hemopoietic progenitor cells from young and old mice is similar. Previous studies of their cell-kinetics showed no significant differences between them when measured by the incorporation of tritiated thymidine ([3H]TdR). A different approach for exploring stem-cell kinetics in aged animals is provided by another method; the incorporation of bromodeoxyuridine (BrdUrd) during DNA synthesis followed by exposure to near-ultraviolet light (near-UV) kills BrdUrd labelled cells in DNA-synthesis (S-phase). This BrdUrd-near UV cytocide (BUUV) reveals the size of cycling fractions at the level of hemopoietic progenitor cells; it also demonstrates the velocity of the cells entering S-phase when cells are labeled by a continuous infusion of BrdUrd by an osmotic pump, followed by an appropriate colony-assay for each progenitor cell. We compared the cell kinetics of young and old hemopoietic progenitor cells (CFU-S) by this approach. Osmotic pumps were implanted subcutaneously in the backs of young (2 months) and old (22 months) male C57BL/6CrSlc mice for 2, 4, 8, 12, and 16 days to continuously infuse BrdUrd at a flow-rate of 1 mg/h per kg. Cells were harvested from femoral marrow of the infused mice, plated in non-coated bacterial plates, and exposed to a single dose of near-UV at 4000 J/m2. After BUUV cytocide, bone-marrow cells were assayed for 8- and 13-day CFU-S colonies. In the 8-day colonies, the cytocide fraction of CFU-S from young mice increased rapidly, whereas the fraction from old mice showed flatter curves. Both curves reached a plateau at 52.6% for young, and 43.9% for old mice, and then converged 4 days after labeling. In the 13-day colonies, the curve for the aged was much flatter than that for the young; however, the plateaus in both young and old are similar, but at much lower values than earlier, i.e. 24.5% and 16.0%, respectively. The size of the cycling fraction of progenitor cells was close in the two groups during the steady-state cell-cycle. However, the velocity of the cell cycle at a progenitor level was very different, old mice being much slower. Further, within the progenitors, the cell cycle was much slower in the primitive ones and became faster when the stem cells differentiated into mature progenitor cells.

Unmasking large and persistent reductions in proliferation rate of aging cells

We have reported that nontransformed sublines of NIH 3T3 cells that are incubated under the growth constraint of confluence for 10 d or longer exhibit heritable reductions of growth rate upon serial subculture at low density, which simulate the effects of aging in vivo on cell growth. There is also a marked increase in the likelihood of neoplastic transformation. After switching to a new batch of calf serum (CS), we found the reduced growth rate was no longer produced within the previously established timeframe. However, substitution of fetal bovine serum (FBS) for CS during the period of recovery from confluence or the following tests of growth rate resulted in profound inhibition of growth in cells serially subcultured from confluent cultures. In some cases, fewer than one in a thousand cells from subcultures of confluent cultures formed colonies in FBS although they cloned at relatively high efficiency in CS. The reduced growth in FBS was retained in the postconfluent subcultures after many generations of multiplication at low density in CS. Generally, similar results with individual variations were obtained with three other batches of FBS. The numbers of cells per 3-d colony initiated from subcultures of confluent cultures were lower than those of control cultures that had never been confluent. Supplementation of FBS-containing medium with CS fully restored the growth of the postconfluent subcultures to the rate in CS medium, indicating that there is a deficiency of growth factor(s) in FBS rather than the presence of an inhibitor. The results show that prolonged incubation at confluence induces a populationwide heritable increase in requirement for growth factor(s) in short supply in FBS. Because clonal studies have shown that the reduction in growth rate is irreversible and varies in degree from clone to clone, we propose it arises from damage to DNA at any of many different genetic loci or from chromosome aberrations. Such genetic damage is also consistent with the increased tendency for neoplastic transformation in subcultures from the long-term confluent cultures.

Cell aging in vivo and in vitro

It has become a staple assumption of biology that there is an intrinsic fixed limit to the number of divisions that normal vertebrate cells can undergo before they senesce, and this limit is in some way related to aging of the organism. The notion of such a limited replicative lifespan arose from the often repeated observation that diploid fibroblasts cannot proliferate indefinitely in monolayer culture, and that the number of divisions before senescence is directly related to the in vivo lifespan of different species. The in vitro evidence is countered by estimates that the number of cell divisions in some organs of rodents and man are one or more orders of magnitude higher than the in vitro limit, with no indication of the degenerative changes seen in culture. Serial transplantation experiments in animals also exhibit many more cell divisions than the in vitro studies, with some indicating an indefinite replicative lifespan. I present evidence that vertebrate cells are severely stressed by enzymatic dispersion and sustain cumulative damage during serial subcultivations. The evidence includes large increases in cell size and its heterogeneity, reductions in replicative efficiency at low seeding densities, appearance of abnormal structures in the cytoplasm, changes in metabolism to a common cell culture type, continuous loss of methyl groups and reiterated sequences from DNA, and a constant rate of decline of growth rate with passage. This evidence is complemented by the reduction induced in the replicative life span of diploid cells by a large array of treatments which have different primary targets in the cells. The most consistent and general observation of cell behavior in aging animals, with only a few exceptions, is a reduction in the rate of cell proliferation. This reduction is perpetuated when the cells are grown in culture, indicating it is an enduring and intrinsic property of the cells rather than a systemic effect of the aging organism. A similar heritable reduction in growth rate can be induced in established cell lines by prolonged incubation at quiescence. The reduction can be exaggerated by subculturing the quiescent cells under suboptimal conditions, just as the effects of age are exaggerated under stress. The constant decline of growth rate that occurs during serial passage of diploid cells may represent a similar decay of cell function. I propose that the limit on replicative lifespan is an artifact that reflects the failure of diploid cells to adapt to the trauma of dissociation and the radically foreign environment of cell culture. It is, however, a useful artifact that has given us much information about cell behavior under stressful conditions. The overall evidence indicates cell in vivo accumulate damage over a lifetime that results in gradual loss of differentiated function and growth rate accompanied by an increased probability for the development of cancer. Such changes are normally held to a minimum by the organized state of the tissues and homeostatic regulation of the organism. The rejection of an intrinsic limit on the number of cell divisions eliminates the need for a cellular clock, such as telomere length, that counts mitoses. I offer a heuristic explanation for the gradual reduction of cell function and growth capacity with age based on a cumulative discoordination of interacting pathways within and between cells and tissues. I also make a case for the use of established cell lines as model systems for studying heritable damage to cell populations that simulates the effects of aging in vivo, and represents a relatively unexplored area of cell biology.