Proliferative capacity of erythropoietic stem cell lines and aging: an overview

Molecular signaling and clinical implications in the human aging-cancer cycle

It is well documented that aging is associated with cancer, and likewise, cancer survivors display accelerated aging. As the number of aging individuals and cancer survivors continues to grow, it raises additional concerns across society. Therefore, unraveling the molecular mechanisms of aging in tissues is essential to developing effective therapies to fight the aging and cancer diseases in cancer survivors and cancer patients. Indeed, cellular senescence is a critical response, or a natural barrier to suppress the transition of normal cells into cancer cells, however, hypoxia which is physiologically required to maintain the stem cell niche, is increased by aging and inhibits senescence in tissues. Interestingly, oxygen restriction or hypoxia increases longevity and slows the aging process in humans, but hypoxia can also drive angiogenesis to facilitate cancer progression. In addition, cancer treatment is considered as one of the major reasons that drive cellular senescence, subsequently followed by accelerated aging. Several clinical trials have recently evaluated inhibitors to eliminate senescent cells. However, some mechanisms of aging typically can also retard cancer cell growth and progression, which might require careful strategy for better clinical outcomes. Here we describe the molecular regulation of aging and cancer in crosstalk with DNA damage and hypoxia signaling pathways in cancer patients and cancer survivors. We also update several therapeutic strategies that might be critical in reversing the cancer treatment-associated aging process.

Programmed ageing: decline of stem cell renewal, immunosenescence, and Alzheimer's disease

The characteristic maximum lifespan varies enormously across animal species from a few hours to hundreds of years. This argues that maximum lifespan, and the ageing process that itself dictates lifespan, are to a large extent genetically determined. Although controversial, this is supported by firm evidence that semelparous species display evolutionarily programmed ageing in response to reproductive and environmental cues. Parabiosis experiments reveal that ageing is orchestrated systemically through the circulation, accompanied by programmed changes in hormone levels across a lifetime. This implies that, like the circadian and circannual clocks, there is a master 'clock of age' (circavital clock) located in the limbic brain of mammals that modulates systemic changes in growth factor and hormone secretion over the lifespan, as well as systemic alterations in gene expression as revealed by genomic methylation analysis. Studies on accelerated ageing in mice, as well as human longevity genes, converge on evolutionarily conserved fibroblast growth factors (FGFs) and their receptors, including KLOTHO, as well as insulin-like growth factors (IGFs) and steroid hormones, as key players mediating the systemic effects of ageing. Age-related changes in these and multiple other factors are inferred to cause a progressive decline in tissue maintenance through failure of stem cell replenishment. This most severely affects the immune system, which requires constant renewal from bone marrow stem cells. Age-related immune decline increases risk of infection whereas lifespan can be extended in germfree animals. This and other evidence suggests that infection is the major cause of death in higher organisms. Immune decline is also associated with age-related diseases. Taking the example of Alzheimer's disease (AD), we assess the evidence that AD is caused by immunosenescence and infection. The signature protein of AD brain, Aβ, is now known to be an antimicrobial peptide, and Aβ deposits in AD brain may be a response to infection rather than a cause of disease. Because some cognitively normal elderly individuals show extensive neuropathology, we argue that the location of the pathology is crucial - specifically, lesions to limbic brain are likely to accentuate immunosenescence, and could thus underlie a vicious cycle of accelerated immune decline and microbial proliferation that culminates in AD. This general model may extend to other age-related diseases, and we propose a general paradigm of organismal senescence in which declining stem cell proliferation leads to programmed immunosenescence and mortality.

The Achilles' heel of cancer survivors: fundamentals of accelerated cellular senescence

Recent improvements in cancer treatment have increased the lifespan of pediatric and adult cancer survivors. However, cancer treatments accelerate aging in survivors, which manifests clinically as the premature onset of chronic diseases, such as endocrinopathies, osteoporosis, cardiac dysfunction, subsequent cancers, and geriatric syndromes of frailty, among others. Therefore, cancer treatment-induced early aging accounts for significant morbidity, mortality, and health expenditures among cancer survivors. One major mechanism driving this accelerated aging is cellular senescence; cancer treatments induce cellular senescence in tumor cells and in normal, nontumor tissue, thereby helping mediate the onset of several chronic diseases. Studies on clinical monitoring and therapeutic targeting of cellular senescence have made considerable progress in recent years. Large-scale clinical trials are currently evaluating senotherapeutic drugs, which inhibit or eliminate senescent cells to ameliorate cancer treatment-related aging. In this article, we survey the recent literature on phenotypes and mechanisms of aging in cancer survivors and provide an up-to-date review of the major preclinical and translational evidence on cellular senescence as a mechanism of accelerated aging in cancer survivors, as well as insight into the potential of senotherapeutic drugs. However, only with time will the clinical effect of senotherapies on cancer survivors be visible.

Emerging Evidence of the Significance of Thioredoxin-1 in Hematopoietic Stem Cell Aging

The United States is undergoing a demographic shift towards an older population with profound economic, social, and healthcare implications. The number of Americans aged 65 and older will reach 80 million by 2040. The shift will be even more dramatic in the extremes of age, with a projected 400% increase in the population over 85 years old in the next two decades. Understanding the molecular and cellular mechanisms of ageing is crucial to reduce ageing-associated disease and to improve the quality of life for the elderly. In this review, we summarized the changes associated with the ageing of hematopoietic stem cells (HSCs) and what is known about some of the key underlying cellular and molecular pathways. We focus here on the effects of reactive oxygen species and the thioredoxin redox homeostasis system on ageing biology in HSCs and the HSC microenvironment. We present additional data from our lab demonstrating the key role of thioredoxin-1 in regulating HSC ageing.

Biology of premature ageing in survivors of cancer

Over 30 million cancer survivors exist worldwide. Survivors have an earlier onset and higher incidence of chronic comorbidities, including endocrinopathies, cardiac dysfunction, osteoporosis, pulmonary fibrosis, secondary cancers and frailty than the general population; however, the fundamental basis of these changes at the cellular level is unknown. An electronic search was performed on Embase, Medline In-Process & Other Non-Indexed Citations, and the Cochrane Central Register of Controlled Trials. Original articles addressing the cellular biology of ageing and/or the mechanisms of cancer therapies similar to ageing mechanisms were included, and references of these articles were reviewed for further search. We found multiple biological process of ageing at the cellular level and their association with cancer therapies, as well as with clinical effects. The direct effects of various chemotherapies and radiation on telomere length, senescent cells, epigenetic modifications and microRNA were found. We review the effects of cancer therapies on recognised hallmarks of ageing. Long-term comorbidities seen in cancer survivors mimic the phenotypes of ageing and likely result from the interaction between therapeutic exposures and the underlying biology of ageing. Long-term follow-up of cancer survivors and research on prevention strategies should be pursued to increase the length and quality of life among the growing population of cancer survivors.

ICRP Publication 131: Stem Cell Biology with Respect to Carcinogenesis Aspects of Radiological Protection

This report provides a review of stem cells/progenitor cells and their responses to ionising radiation in relation to issues relevant to stochastic effects of radiation that form a major part of the International Commission on Radiological Protection's system of radiological protection. Current information on stem cell characteristics, maintenance and renewal, evolution with age, location in stem cell 'niches', and radiosensitivity to acute and protracted exposures is presented in a series of substantial reviews as annexes concerning haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. This foundation of knowledge of stem cells is used in the main text of the report to provide a biological insight into issues such as the linear-no-threshold (LNT) model, cancer risk among tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age. Knowledge of the biology and associated radiation biology of stem cells and progenitor cells is more developed in tissues that renew fairly rapidly, such as haematopoietic tissue, intestinal mucosa, and epidermis, although all the tissues considered here possess stem cell populations. Important features of stem cell maintenance, renewal, and response are the microenvironmental signals operating in the niche residence, for which a well-defined spatial location has been identified in some tissues. The identity of the target cell for carcinogenesis continues to point to the more primitive stem cell population that is mostly quiescent, and hence able to accumulate the protracted sequence of mutations necessary to result in malignancy. In addition, there is some potential for daughter progenitor cells to be target cells in particular cases, such as in haematopoietic tissue and in skin. Several biological processes could contribute to protecting stem cells from mutation accumulation: (a) accurate DNA repair; (b) rapidly induced death of injured stem cells; (c) retention of the DNA parental template strand during divisions in some tissue systems, so that mutations are passed to the daughter differentiating cells and not retained in the parental cell; and (d) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the niche. DNA repair mainly occurs within a few days of irradiation, while stem cell competition requires weeks or many months depending on the tissue type. The aforementioned processes may contribute to the differences in carcinogenic radiation risk values between tissues, and may help to explain why a rapidly replicating tissue such as small intestine is less prone to such risk. The processes also provide a mechanistic insight relevant to the LNT model, and the relative and absolute risk models. The radiobiological knowledge also provides a scientific insight into discussions of the dose and dose-rate effectiveness factor currently used in radiological protection guidelines. In addition, the biological information contributes potential reasons for the age-dependent sensitivity to radiation carcinogenesis, including the effects of in-utero exposure.

Chemotherapy and Stem Cell Transplantation Increase p16INK4a Expression, a Biomarker of T-cell Aging

The expression of markers of cellular senescence increases exponentially in multiple tissues with aging. Age-related physiological changes may contribute to adverse outcomes in cancer survivors. To investigate the impact of high dose chemotherapy and stem cell transplantation on senescence markers in vivo, we collected blood and clinical data from a cohort of 63 patients undergoing hematopoietic cell transplantation. The expression of p16^INK4a, a well-established senescence marker, was determined in T-cells before and 6 months after transplant. RNA sequencing was performed on paired samples from 8 patients pre- and post-cancer therapy. In patients undergoing allogeneic transplant, higher pre-transplant p16^INK4a expression was associated with a greater number of prior cycles of chemotherapy received (p = 0.003), prior autologous transplantation (p = 0.01) and prior exposure to alkylating agents (p = 0.01). Transplantation was associated with a marked increase in p16^INK4a expression 6 months following transplantation. Patients receiving autologous transplant experienced a larger increase in p16^INK4a expression (3.1-fold increase, p = 0.002) than allogeneic transplant recipients (1.9-fold increase, p = 0.0004). RNA sequencing of T-cells pre- and post- autologous transplant or cytotoxic chemotherapy demonstrated increased expression of transcripts associated with cellular senescence and physiological aging. Cytotoxic chemotherapy, especially alkylating agents, and stem cell transplantation strongly accelerate expression of a biomarker of molecular aging in T-cells.

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Peripheral blood T-cell senescence, as measured by the marker p16^INK4a, increases following autologous or allogeneic HSCT.

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RNAseq of T-cells post- auto HSCT or chemotherapy show increased expression of transcripts associated with senescence.

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Autologous HCT in particular induces a stronger effect on Tcell p16^INK4a expression than any other environmental stimulus tested to date.

Human chronological aging is associated with increased expression of markers of cellular aging (senescence). Cancer chemotherapy can produce frailty syndromes – recipients of cancer treatment may experience physiological changes ordinarily seen in individuals of more advanced chronological age. In our study, we found that a well-known marker of cellular senescence, p16^INK4a, increased in patients following autologous or allogeneic hematopoietic cell transplantation. Expression of p16^INK4a was higher in patients exposed to greater amounts of chemotherapy before transplant and those exposed to specific types of chemotherapy. These findings may ultimately influence clinical decision-making for patients with diseases that are commonly treated with transplantation.

Cellular and molecular longevity pathways: the old and the new

Human lifespan has been increasing steadily during modern times, mainly due to medical advancements that combat infant mortality and various life-threatening diseases. However, this gratifying longevity rise is accompanied by growing incidences of devastating age-related pathologies. Understanding the cellular and molecular mechanisms that underlie aging and regulate longevity is of utmost relevance towards offsetting the impact of age-associated disorders and increasing the quality of life for the elderly. Several evolutionarily conserved pathways that modulate lifespan have been identified in organisms ranging from yeast to primates. Here we survey recent findings highlighting the interplay of various genetic, epigenetic, and cell-specific factors, and also symbiotic relationships, as longevity determinants. We further discuss outstanding matters within the framework of emerging, integrative views of aging.

Emerging models and paradigms for stem cell ageing

Ageing is accompanied by a progressive decline in stem cell function, resulting in less effective tissue homeostasis and repair. Here we discuss emerging invertebrate models that provide insights into molecular pathways of age-related stem cell dysfunction in mammals, and we present various paradigms of how stem cell functionality changes with age, including impaired self-renewal and aberrant differentiation potential.

In vivo biocompatibility study of electrospun chitosan microfiber for tissue engineering

In this work, we examined the biocompatibility of electrospun chitosan microfibers as a scaffold. The chitosan microfibers showed a three-dimensional pore structure by SEM. The chitosan microfibers supported attachment and viability of rat muscle-derived stem cells (rMDSCs). Subcutaneous implantation of the chitosan microfibers demonstrated that implantation of rMDSCs containing chitosan microfibers induced lower host tissue responses with decreased macrophage accumulation than did the chitosan microfibers alone, probably due to the immunosuppression of the transplanted rMDSCs. Our results collectively show that chitosan microfibers could serve as a biocompatible in vivo scaffold for rMDSCs in rats.

Sirtuin 1, stem cells, aging, and stem cell aging

PURPOSE OF REVIEW

New discoveries focused on mitochondrial metabolism and gene silencing and their regulation by the sirtuin family of protein deacetylases is stimulating new ideas on how to improve geriatric medicine. Information about sertuins in stem cell biology is scarce. We consider recent information on sirtuin 1, its role in aging and metabolism in several species and tissues, and attempt to anticipate how it might influence stem cell aging.

RECENT FINDINGS

Calorie restriction lengthens lifespan, in part, due to mitochondrial metabolism reorganization through sirtuin 1/peroxisome proliferator-activated receptor gamma-coactivator-1alpha-regulated mitochondrial biogenesis. This reduces radical oxygen species levels that cause macromolecule damage, a major contributor to aging. Little is known about these processes in stem cells, whose longevity is implicated in human aging. Recent work indicates that sirtuin 1 influences growth-factor responses and maintenance of stem cells. Sirtuin 1 is required for calorie restriction-induced lifespan extension in mice, and calorie restriction upregulates sirtuin 1 in humans. Sirtuin 1 also appears to influence lineage/cell-fate decisions of stem cells via redox status.

SUMMARY

The same thermodynamic and biochemical mechanisms linked to aging in somatic cells may also work in stem cells. Developments in mitochondrial biology and new drug development based on this knowledge are finding their way into the clinic (i.e. diabetes) and may illuminate new ways of manipulating and using stem cells in medicine.

Gene therapy approaches for stem cell protection

Cytotoxic exposure of bone marrow and other non-hematopoietic organs containing self-renewing stem cell populations is associated with damage to the supportive microenvironment. Recent evidence indicates that radical oxygen species resulting from the initial oxidative stress persist for months after ionizing irradiation exposure of tissues including oral cavity, esophagus, lung and bone marrow. Antioxidant gene therapy using manganese superoxide dismutase plasmid liposomes has provided organ-specific radiation protection associated with delay or prevention of acute and late toxicity. Recent evidence has suggested that manganese superoxide dismutase transgene expression in cells of the organ microenvironment contributes significantly to the mechanism of protection. Incorporating this knowledge into designs of novel approaches for stem cell protection is addressed in the present review.

Hematopoietic stem cell exhaustion impacted by p18 INK4C and p21 Cip1/Waf1 in opposite manners

Transplantation-associated stress can compromise the hematopoietic potential of hematopoietic stem cells (HSCs). As a consequence, HSCs may undergo "exhaustion" in serial transplant recipients, for which the cellular and molecular bases are not well understood. Hematopoietic exhaustion appears to be accelerated in the absence of p21(Cip1/Waf1) (p21), a cyclin-dependent kinase inhibitor (CKI) in irradiated hosts. Our recent study demonstrated that unlike loss of p21, deletion of p18(INK4C) (p18), a distinct CKI, results in improved long-term engraftment, largely because of increased self-renewing divisions of HSCs in vivo. We show here that HSCs deficient in p18 sustained their competitiveness to wild-type HSCs from unmanipulated young mice, and retained multilineage differentiation potential after multiple rounds of serial bone marrow transfer over a period of more than 3 years. Further, p18 absence significantly decelerated hematopoietic exhaustion caused by p21 deficiency. Such an effect was shown to occur at the stem cell level, likely by a counteracting mechanism against the cellular senescence outcome. Our current study provides new insights into the distinct impacts of these cell-cycle regulators on HSC exhaustion and possibly HSC aging as well under proliferative stress, thereby offering potential pharmacologic targets for sustaining the durability of stressed HSCs in transplantation or elderly patients.

Bmi-1 promotes neural stem cell self-renewal and neural development but not mouse growth and survival by repressing the p16Ink4a and p19Arf senescence pathways

Bmi-1 is required for the post-natal maintenance of stem cells in multiple tissues including the central nervous system (CNS) and peripheral nervous system (PNS). Deletion of Ink4a or Arf from Bmi-1(-/-) mice partially rescued stem cell self-renewal and stem cell frequency in the CNS and PNS, as well as forebrain proliferation and gut neurogenesis. Arf deficiency, but not Ink4a deficiency, partially rescued cerebellum development, demonstrating regional differences in the sensitivity of progenitors to p16Ink4a and p19Arf. Deletion of both Ink4a and Arf did not affect the growth or survival of Bmi-1(-/-) mice or completely rescue neural development. Bmi-1 thus prevents the premature senescence of neural stem cells by repressing Ink4a and Arf, but additional pathways must also function downstream of Bmi-1.

HOXB4 overexpression mediates very rapid stem cell regeneration and competitive hematopoietic repopulation

OBJECTIVE

Hox transcription factors have emerged as important regulators of hematopoiesis. In particular, we have shown that overexpression of HOXB4 in mouse bone marrow can greatly enhance the level of hematopoietic stem cell (HSC) regeneration achieved at late times (> 4 months) posttransplantation. The objective of this study was to resolve if HOXB4 increases the rate and/or duration of HSC regeneration, and also to see if this enhancement was associated with impaired production of end cells or would lead to competitive reconstitution of all compartments.

METHODS

Retroviral vectors were generated with the GFP reporter gene +/- HOXB4 to enable the isolation and direct tracking of transduced cells in culture or following transplantation. Stem cell recovery was measured by limit dilution assay for long-term competitive repopulating cells (CRU).

RESULTS

HOXB4-overexpressing cells have enhanced growth in vitro, as demonstrated by their rapid dominance in mixed cultures and their shortened population doubling time. Furthermore, HOXB4-transduced cells have a marked competitive repopulating advantage in vivo in both primitive and mature compartments. CRU recovery in HOXB4 recipients was extremely rapid, reaching 25% of normal by 14 days posttransplant or some 80-fold greater than control transplant recipients, and attaining normal numbers by 12 weeks. Mice transplanted with even higher numbers of HOXB4-transduced CRU regenerated up to but not beyond the normal CRU levels.

CONCLUSIONS

HOXB4 is a potent enhancer of primitive hematopoietic cell growth, likely by increasing self-renewal probability but without impairing homeostatic control of HSC population size or the rate of production and maintenance of mature end cells.

Aging and cancer: issues of basic and clinical science

The majority of patients with cancer in the United States are more than 70 years old. Despite the increased understanding of the molecular bases for both oncogenesis and aging, the overlap of cancer and aging at that level remains a wide-open research domain. Similarly, at the clinical level, there is also an increased awareness of the need for more information about the influence of host age on the development of tumors, on the growth and spread of the disease, and on treatment expectations. In this review, we have attempted to frame questions regarding cancer and aging from the perspective of biogerontology and geriatric medicine. An increased effort to address the issues of aging is of paramount importance at all levels of cancer investigation.

The aging of hematopoietic stem cells

We have purified hematopoietic stem cells (HSCs) from the bone marrow of old mice and compared their properties to HSCs in young and middle-aged mice. Single, reconstituting HSCs (by limit dilution) from old and young mice exhibited indistinguishable progenitor activities in vivo. HSCs were five times as frequent in the bone marrow of old mice; however, HSCs from old mice were only one-quarter as efficient at homing to and engrafting the bone marrow of irradiated recipients. HSCs in young and middle-aged mice rarely were in the S/G2/M phases of the cell cycle, but HSCs in old mice were frequently in cycle. We speculate that the unexpected proliferation of HSCs in old mice might be related to the increased incidence of leukemia in old mice. HSCs change with age, but it is unknown whether these changes are determined intrinsically or caused by the aging of their environment.

Telomerase activity in hematopoietic cells is associated with self-renewal potential

It has been proposed that the biological clock underlying the limited division potential of eukaryotic cells is telomere length. We assayed telomerase activity in single cells of the hematopoietic and immune systems. We examined hematopoietic stem cells at four stages of differentiation, lineage-committed progenitors, and mature myeloid and lymphoid cells. The frequency of telomerase-expressing cells within each population was proportional to the frequency of cells thought to have self-renewal potential. Among bone marrow hematopoietic stem cells, 70% exhibited detectable telomerase activity. The telomerase-expressing somatic cells observed in this study are not thought to be immortal, and expression was not correlated with cell cycle distribution or differentiation state. This study demonstrates that the developmental characteristic most consistently associated with telomerase expression is self-renewal potential.

Oxidative and other DNA damages as the basis of aging: a review

DNA damages occur continuously in cells of living organisms. While most of these damages are repaired, some accumulate. In particular, there is evidence for DNA damage accumulation in non-dividing cells of mammals. These accumulated DNA damages probably interfere with RNA transcription. We consider that the decline in the ability of DNA to serve as a template for gene expression is the primary cause of aging. Oxidative DNA damages are among the best documented and prevalent DNA damages and are likely to be a prominent cause of aging.

Cycling patterns of hemopoietic stem cell subpopulations in young and old BDF1 mice

We have previously reported that two or more different subpopulations of bone marrow stem cells exist in mice as determined by cycling status of day-8 and day-14 CFU-S in long term bromodeoxyuridine (BrdU) infusion studies. In the present report, comparisons between cycling of stem cell subpopulations in old and young mice show that, while the general patterns persist, there are some statistically significant differences between corresponding time points of early and late CFU-S cycling patterns in young and old BDF1 mice. In both populations of CFU-S there exist cells which do not enter cycle over a five week period. The method which we have employed allowed the cycling measurements to be made in unstimulated steady-state bone marrow cell populations, since no cell death is caused in vivo.

Multipotent stem cell (CFU-S) numbers and circadian variations in aging mice

The multipotent stem cell (CFU-S) numbers were studied in aging female C3H mice (16, 21 and 26 months old, respectively) versus young controls (3 months old). Using the spleen colony technique, the d-8 CFU-S numbers were measured every 3 h during the 24-h period at three different times of the year. Prominent circadian variations were found in young mice. The peak and trough values were significantly different also in aging mice, although the peak-trough differences were declining. When comparing young and old mice at different times of the 24-h period, the CFU-S numbers were sometimes significantly different, but often not. The 24-h mean values were consistently declining during aging. Young mice had different circadian variation patterns and 24-h mean values when examined at different times of the year. It is concluded that the d-8 CFU-S numbers decline in aging mice. Conflicting reports may partly be due to neglect of physiological variations.

Age-related differences and circadian and seasonal variations of myelopoietic progenitor cell (CFU-GM) numbers in mice

The effect of aging on the myelopoietic progenitor cell (CFU-GM) numbers in mice was studied with special emphasis on biological rhythms in hemopoiesis. Female C3H mice, 16-, 21- and 26-month-old, were investigated versus 3-month-old controls every 3 hours during the 24-h period at three different times of the year. Strong circadian rhythms were observed in both CFU-GM concentration and content in the femur of all age-groups. The amplitudes and the 24-h mean values were declining in mice aged 21 and 26 months. From 16 months of age, a significant advance of circadian peak phases was observed. The overlapping was variable during the 24-h period. The present results may explain previous inconsistencies regarding myelopoietic progenitor cell numbers in aging mice. Some seasonal differences in rhythmicity patterns were also observed. Fitting of original data to single sinus functions was highly significant, although important details sometimes were obscured.

Effect of age on some properties of mice erythrocytes

The hematological parameters of young (2-month-old) and old (2-year-old) mice were compared. No differences could be detected with the exception of an increased percentage of reticulocytes in the old animals suggesting that anemia in senescent mice does not occur. Red blood cell mean half-life in old mice was 8 +/- 0.8 days compared to 12 +/- 1 days in young mice. This reduced survival of red blood cell is not due to a different rate of cell phagocytosis in the reticulohistiocytic system of young and old animals since erythrocytes from young mice have the same mean half-life when injected both in young and old animals and vice versa. Thus, the old mice have a reduced red cell life-span but the same hematocrit of the young, suggesting that old animals possess a chronologically younger population of erythrocytes than do young animals. This has been confirmed by measuring the specific activities of some red blood cell age-dependent enzymes (hexokinase, glucose-6-phosphate dehydrogenase, pyruvate kinase) that were found to be higher in the older animals, and by the separation of erythrocytes into different density (age) groups by Percoll/albumin density gradient centrifugation. However, the erythrocytes osmotic fragility, and the cellular contents of adenine and pyridine nucleotides, as well as the content of 2,3-diphosphoglycerate and reduced glutathione, show that circulating erythrocytes in old animals constitute an heterogeneous cell population whose properties cannot be explained on the basis of a chronologically younger erythrocyte population. Furthermore, evaluation of cell components in hemopoietic tissues have shown an increased porportion of erythroid precursor cells in old animals confirming that old mice compensate for reduced red cell survival with an increased erythropoiesis.

Rhythmic variations of different hemopoietic cell lines and maturation stages in aging mice

Non-proliferative and proliferative myeloid, and lymphoid and erythroid bone marrow cells were studied in aging female C3H mice. A chronobiological approach was used and mice aged 16, 21 and 26 months were examined vs. 3 month-old mice every 3 h during the 24-h period in three different experiments. Significant circadian fluctuations were observed in most of the cell populations, even in the oldest mice. The rhythmicity patterns might be different at different times of the year, and in young mice seasonal fluctuations in the 24-h mean values were observed. The absolute numbers of the 24-h means seemed to be highest at 21 months of age in all cell lines and maturation stages. Sinus function fitting indicated a decline of the amplitudes in aging mice. Minor age-related phase-differences were indicated in some populations. However, the fitting of original data to single sinus functions was variable and often obscured important features in the cell number variations. The present investigation illustrates the importance of performing time-sequence studies in hematology.

Alterations of cell cycle distribution in the bone marrow of aging mice measured by flow cytometry

The effect of aging on the cell cycle distribution in bone marrow cells was measured by flow cytometry with special reference to the lability in bone marrow physiology. Female C3H mice were examined every 3 h during a 24-h period at the age of 16, 21 and 26 months, vs 3-month-old control mice. Considerable circadian fluctuations were found in the different cell cycle phases in young mice. The rhythmicity patterns were different when comparing different months. In aging mice the variations were dampened, while consistent age-related phase shifts were not seen. The maximal 24-h mean numbers of cells in all three cell phases, but especially the S and G2 phases were reached at 21 months. The relative number of S and G2 phases were significantly reduced in 26-month-old mice, indicating an age-related shift of the proliferative capacity. The present findings are discussed in relation to age-related changes in granulopoiesis and the increase of myelotoxic effects during cancer chemotherapy in aging individuals.

Hematopoietic stem cells in elderly people

Marrow stem cells from old and young donors have often been compared by measuring concentration and numbers of CFU-S in mice. However, little information is available on hematopoietic stem cells in elderly people. This present study was undertaken to investigate age-associated changes in the concentrations of granulocyte-macrophage progenitor cell (CFU-C) and erythrocyte progenitor cell (CFU-E) in human bone marrow. We examined CFU-C from 101 subjects and CFU-E from 26 subjects by age, ranging from 28 to 95 years, and found there was no difference in the concentration of CFU-C but a slight decrease of the concentration in CFU-E in elderly subjects.

Effect of haemopoietic stem-cell proliferation regulators on early and late spleen colony-forming cells

An inhibitor and stimulator of CFU-s proliferation can be obtained from haemopoietic tissue containing, respectively, relatively quiescent CFU-s (e.g. normal bone marrow) and proliferating CFU-s (e.g. regenerating bone marrow). Their effects on the proliferative behaviour of steady-state and regenerating marrow CFU-s, which produce colonies 7, 10 and 12 days post-transplantation have been investigated. The results demonstrate changing sensitivities of CFU-s to inhibitor and stimulator as they progress through a developmental age structure, 'Older' CFU-s (producing early spleen colonies) are more sensitive to stimulator, 'Younger' CFU-s (producing late spleen colonies) are more sensitive to inhibitor.

Age-related changes in glucose metabolizing enzymes in spleen, thymus, and pulmonary lavage cells from F344 rats

Healthy male Fischer 344 rats were sampled at 6, 12, 18, and 24 months of age. There was no gross pathological evidence or deviations in body weight, hematology, or clinical chemistry that were indicative of disease. Mixed populations of thymus, spleen, and pulmonary cells were obtained for enzymatic analyses. Key enzymes from the hexose monophosphate shunt, glycolysis and the tricarboxylic acid cycle were evaluated to determine if there were tissue-specific or pathway-specific changes that occurred during aging. The enzyme responses among the tissues were not consistent during the aging process. Generally the activities of the glucose metabolizing enzymes in thymus and pulmonary lavage cells decreased with age whereas they increased in the spleen cells. Between 18 and 24 months enzymes representative of all three glucose metabolic pathways decreased in pulmonary lavage cells, whereas the decreases in thymic cells were mainly restricted to glycolytic enzymes. By contrast there were two- to ten-fold increases during aging in all of the splenic enzymes measured except malate dehydrogenase. The alterations in tissue enzyme activities probably reflected the changing cellular populations during aging, and in the thymic and pulmonary lavage cellular environment resulted in a loss of energy production by glucose oxidation, compared to the vigorous activity maintained in spleen.

Effects of in vivo indomethacin treatment in aging mice

The effects of in vivo treatment with indomethacin were assessed in aging (22-36 month old) as compared to young (2-3 month old)F1 (C3H/ebJ x C57BL/6J) mice. Old mice manifested erythropenia, which was not apparent in the young. In leukocyte level there was a more rapid decrease in the old, following an initial increase in both age groups. The response of peripheral blood leukocytes (PBL) of both young and old mouse groups to mitogens decreased after treatment with indomethacin, yet in the old a significant increase (3- to 4-fold) was subsequently noted. Similarly, splenocyte response to PHA decreased initially and then increased in both age groups. This dynamics of changes was not observed in ConA-stimulated splenocytes of the old mice. Levels of IL-1 activity in culture supernatants of adherent splenocytes from indomethacin treated old mice were higher than those of the untreated age-mates. Conversely, in the young, in vivo treatment with indomethacin led to reduced IL-1 levels. The pattern of effects of indomethacin thus seems to be altered in aging.

Long-term erythropoietic repopulating ability of old, young, and fetal stem cells

It is possible that erythropoietic stem cells do not age. This would mean that stem cells from old donors can function as well as those from young or fetal donors. The competitive repopulation assay has been used to test long-term stem cell function by directly comparing how well competing stem cells repopulate a recipient and produce differentiated cell types. C57BL/6J (B6) mice were used as donors, while recipients and competitors were WBB6F1 hybrids with genetically distinguishable hemoglobin. Lethally irradiated young WBB6F1 recipients were given a mixture of 2.5 X 10(6) cells from B6 old marrow, young marrow, or fetal liver donors; each recipient also received a standard dose of 1 X 10(6) marrow cells from a pool of young WBB6F1 competitors. Surprisingly, the old marrow cells competed the best in repopulating the recipients. This pattern was maintained even after recovery from sublethal irradiation, a treatment that severely stresses stem cells. This stress was demonstrated when sublethal irradiation caused a 20-fold decline in repopulating ability measured using hemoglobin markers, and a 3- to 7-fold decline using chromosome markers. Stem cells from old marrow competed better than young or fetal cells in similar experiments using immunologically crippled recipients or using unirradiated W/Wv recipients that are immunologically intact. In both types of recipients, the advantage of old marrow cells again persisted after recovery from sublethal irradiation. Other genotypes were tested, and marrow cells from old B6CBAF1 donors competed better than those from young donors of that genotype. However, marrow cells from young CBA donors completed better than those from old CBA donors. These results support the hypothesis that stem cells do not age, and suggest that regulatory changes with age promote rapid stem cell repopulation in B6 and B6CBAF1 mice, but inhibit it in CBA mice.

Loss of stem cell repopulating ability upon transplantation. Effects of donor age, cell number, and transplantation procedure

Long-term functional capacities of marrow cell lines were defined by competitive repopulation, a technique capable of detecting a small decline in repopulating abilities. There was little or no difference between cells from old and young donors, but a single serial transplantation caused a large decline in repopulating ability. Varying the numbers of marrow cells transplanted into the initial carrier from 10(5) to 10(7) did not alter the ability of the carrier's marrow cells to repopulate in competition with previously untransplanted cells. This ability was improved only in carriers that had received 10(8) marrow cells, although deleterious effects of transplantation were still present. These effects were not solely caused by cell damage from the transplantation procedure, because transplantation by parabiosis, or recovery from sublethal irradiation without transplantation, reduced repopulating abilities as much as transplanting 10(5) to 10(7) marrow cells. The transplantation effect also was not caused solely by irradiation, because the same effect appeared in unirradiated W/Wv carriers. The transplantation effect was more pronounced when donors were identified by hemoglobin type than by chromosome markers, implying that nonerythroid cell lines may be less affected by transplantation than erythroid precursor cells. When the effects of a lifetime of normal function and a single transplantation were compared, the latter caused 3-7 times more decline in repopulating abilities of phytohemagglutinin-responsive cell precursors, and at least 10-20 times more decline in erythroid cell precursors. Stem cell lines can be serially transplanted at least five times before losing their ability to repopulate and save lethally irradiated recipients or to cure genetically anemic mice. Therefore, if transplantation causes an acceleration of the normal aging process, these figures suggest that stem cells should be able to function normally through at least 15-50 life spans.

Self-renewal and differentiation capacity of bone marrow and fetal liver stem cells

An operational definition of the pluripotent stem cell (CFC-S) requires that it have both the capacity for self-renewal and the potential for differentiation into more than one class of formed blood elements. Because the CFC-S compartment is heterogeneous, younger stem cells would be expected to be less committed to differentiation and have a higher rate of self-renewal; whereas, older stem cells would be more committed to differentiation and have a lower rate of self-renewal. In this study, the self-renewal capacity versus the differentiation potential of adult bone marrow and fetal liver stem cells were compared. The self-renewal potential was estimated by determining the number of CFC-S which develop during growth in the spleen of femur of primary recipients. The differentiation potential was estimated by determining the total number of nucleated cells or committed progenitor cells (GM-CFU-C and BFU-E) which develop during growth in the spleen or femur of primary recipients. In order to circumvent possible differences in self-renewal or differentiation pressures due to the presence of differing numbers of CFC-S, an equivalent number of bone marrow and fetal liver CFC-S were allowed to seed the spleen and femur. While both adult bone marrow and fetal liver stem cells showed an extensive capacity for self-renewal, fetal liver CFC-S displayed a greater potential for self-renewal in both the spleen and femur and at all growth intervals measured when compared to adult bone marrow CFC-S. In contrast, no differences were seen in the number of nucleated cells or committed stem cells found per CFC-S when comparing adult bone marrow and fetal liver stem cells.

Selective decline in differentiating capacity of immunohemopoietic stem cells with aging

Bone marrow (BM) from young (3 months) and old (24 months) C57BL/6J mice were tested for the total number of colony forming units, which remain unchanged with age. The BM from both groups was used to reconstitute syngeneic, lethally irradiated mice that were 3 months' old. The reconstituted mice were followed for a period of 12 months for their ability to generate cell-mediated responses in mixed lymphocytic cultures and cultures containing the T mitogens--concanavalin A and phytohemagglutinin. For the first 8 months, mice given BM from young or old mice responded to a similar degree. Later, cellular immune responses of the mice reconstituted with BM from old mice declined markedly compared to those reconstituted with BM from young mice, although there was no detectable difference between the two groups in the hematopoietic compartment.

Mouse erythropoietic stem cell lines function normally 100 months: loss related to number of transplantations

Marrow stem cell lines from old and young donors in parallel experiments were transplanted into genetically anemic W/WV recipients. These recipients were populated and their anemias were cured by stem cell lines from WCB6F1 or C57BL/6 dorons that had been repeatedly transplanted up to five times at annual intervals into successive W/WV recipients. Old marrow cell lines produced erythrocytes normally for as long as 2600 to 3000 days. However, after three to four serial transplantations many stem cell lines failed to cure at least two-thirds of the recipients, and all failed by transplantation six. This decline occurred in a similar pattern whether the original stem line donor was old or young. Two experiments suggested that the decline was caused by the transplantation procedure: (1) chromosomally marked donor cells from old and young donors permanently populated lymph nodes in lethally irradiated recipients after the first transplantation, but under the same conditions cell lines from the same donors transplanted a second time were substantially infiltrated by regenerating recipient cells; (2) the ability to compete with the same chromosomally marked cell line in populating irradiated recipients declined markedly in both old and young marrow stem cell lines that had been previoulsy transplanted.