Tumor suppressor dosage regulates stem cell dynamics during aging

Synthesis and Biological Evaluation of Indole-2-Carboxamides with Potent Apoptotic Antiproliferative Activity as EGFR/CDK2 Dual Inhibitors

The apoptotic antiproliferative actions of our previously reported CB1 allosteric modulators 5-chlorobenzofuran-2-carboxamide derivatives VIIa–j prompted us to develop and synthesise a novel series of indole-2-carboxamide derivatives 5a–k, 6a–c, and 7. Different spectroscopic methods of analysis were used to validate the novel compounds. Using the MTT assay method, the novel compounds were examined for antiproliferative activity against four distinct cancer cell lines. Compounds 5a–k, 6a–c, and 7 demonstrated greater antiproliferative activity against the breast cancer cell line (MCF-7) than other tested cancer cell lines, and 5a–k (which contain the phenethyl moiety in their backbone structure) demonstrated greater potency than 6a–c and 7, indicating the importance of the phenethyl moiety for antiproliferative action. Compared to reference doxorubicin (GI₅₀ = 1.10 µM), compounds 5d, 5e, 5h, 5i, 5j, and 5k were the most effective of the synthesised derivatives, with GI₅₀ ranging from 0.95 µM to 1.50 µM. Compounds 5d, 5e, 5h, 5i, 5j, and 5k were tested for their inhibitory impact on EGFR and CDK2, and the results indicated that the compounds tested had strong antiproliferative activity and are effective at suppressing both CDK2 and EGFR. Moreover, the studied compounds induced apoptosis with high potency, as evidenced by their effects on apoptotic markers such as Caspases 3, 8, 9, Cytochrome C, Bax, Bcl2, and p53.

Novel 1,2,4-triazole derivatives as apoptotic inducers targeting p53: Synthesis and antiproliferative activity

A series of novel thiazolo[3,2-b][1,2,4]-triazoles 3a-n has been synthesized and evaluated in vitro as potential antiproliferative. Compounds 3b-d exhibited significant antiproliferative activity. Compound 3b was the most potent with Mean GI₅₀ 1.37 µM comparing to doxorubicin (GI₅₀ 1.13 µM). The transcription effects of 3b, 3c and 3d on the p53 were assessed and compared with the reference doxorubicin. The results revealed an increase of 15-27 in p53 level compared to the test cells and that p53 protein level of 3b, 3c and 3d was significantly inductive (1419, 571 and 787 pg/mL, respectively) in relation to doxorubicin (1263 pg/mL). The docking study of the new compounds 3a-n revealed high binding scores for the new compounds toward p53 binding domain in MDM2. The docking analyses revealed the highest affinities for compounds 3b-d which induced p53 activity in MCF-7 cancer cells. Compound 3b which exhibited the highest antiproliferative activity and induced the highest increase in p53 level in MCF-7 cells showed also the highest affinity to MDM2.

Phorbol myristate acetate induces cellular senescence in rat microglia in vitro

The present study aimed to establish a cellular model to test the hypothesis that oncogene-induced senescence (OIS) is triggered by aging-related activation of microglia. Primary microglia were incubated with phorbol 12-myristate 13-acetate (PMA), and β-galactosidase (β-Gal) staining was applied to subsequent assessment of cellular senescence. Moreover, flow cytometry was employed for examinations of cell cycle arrest and senescence-associated proteins, p53 and p21 were measured by western blotting. Furthermore, examination of tumor necrosis factor α (TNF-α) and interleukin-1β (IL-1β) were carried out with microglia supernatants undergoing age-related degenerative diseases in the nervous system, using ELISA. PC12 cells were co-cultured with microglia activated by aging-related alteration(s) to evaluate whether apoptosis was increased in PC12 cells. Cellular senescence-associated β-Gal staining showed that microglial β-Gal expression gradually increased with prolonged PMA stimulation. Microglia in the group receiving 72 h of PMA stimulation displayed the highest percentage of cells arrested in G0/G1, the highest amount of senescence-associated expression of p53 and p21, and the most prominent secretion of TNF-α and IL-1β. In comparison with controls, an increase of apoptotic PC12 cells was detected, which were co-cultured with aging microglia. Taken together, microglia tend to undergo senescence after PMA treatment, suggesting that microglial senescence is associated with inactivation of certain oncogenes.

Crosstalk Between Vitamin D and p53 Signaling in Cancer: An Update

It has now been convincingly shown that vitamin D and p53 signaling protect against spontaneous or carcinogen-induced malignant transformation of cells. The vitamin D receptor (VDR) and the p53/p63/p73 proteins (the p53 family hereafter) exert their effects as receptors/sensors that turn into transcriptional regulators upon stimulus. While the p53 clan, mostly in the nucleoplasm, responds to a large and still growing number of alterations in cellular homeostasis commonly referred to as stress, the nuclear VDR is transcriptionally activated after binding its naturally occurring biologically active ligand 1,25-dihydroxyvitamin D with high affinity. Interestingly, a crosstalk between vitamin D and p53 signaling has been demonstrated that occurs at different levels, has genome-wide implications, and is of high importance for many malignancies, including non-melanoma skin cancer. These interactions include the ability of p53 to upregulate skin pigmentation via POMC derivatives including alpha-MSH and ACTH. Increased pigmentation protects the skin against UV-induced DNA damage and skin photocarcinogenesis, but also inhibits cutaneous synthesis of vitamin D. A second level of interaction is characterized by binding of VDR and p53 protein, an observation that may be of relevance for the ability of 1,25-dihydroxyvitamin D to increase the survival of skin cells after UV irradiation. UV irradiation-surviving cells show significant reductions in thymine dimers in the presence of 1,25-dihydroxyvitamin D that are associated with increased nuclear p53 protein expression and significantly reduced NO products. A third level of interaction is documented by the ability of vitamin D compounds to regulate the expression of the murine double minute (MDM2) gene in dependence of the presence of wild-type p53. MDM2 has a well-established role as a key negative regulator of p53 activity. Finally, p53 and its family members have been implicated in the direct regulation of the VDR. This review gives an update on some of the implications of the crosstalk between vitamin D and p53 signaling for carcinogenesis in the skin and other tissues, focusing on a genome-wide perspective.

Novel tertiary sulfonamides as potent anti-cancer agents

For adult women in the United States, breast cancer is the most prevalent form of cancer. Compounds that target dysregulated signal transduction can be efficacious anti-cancer therapies. A prominent signaling pathway frequently dysregulated in breast cancer cells is the Wingless-related integration site (Wnt) pathway. The purpose of the work was to optimize a "hit" from a screening campaign. 76,000 compounds were tested in a Wnt transcription assay and revealed potent and reproducible "hit," compound 1. Medicinal chemistry optimization of 1 led to more potent and drug-like molecules, 19, 24 and 25 (i.e., Wnt pathway IC₅₀ values = 11, 18 and 7 nM, respectively). The principal results showed compounds 19, 24 and 25 were potent anti-proliferative agents in breast cancer cell lines, MCF-7 (i.e., IC₅₀ values = 10, 7 and 4 nM, respectively) and MDA-MB 231 (i.e., IC₅₀ values = 13, 13 and 16 nM, respectively). Compound 19 synergized anti-proliferation with chemotherapeutic Doxorubicin in vitro. A major conclusion was that compound 19 enhanced anti-proliferation of Doxorubicin in vitro and in a xenograft animal model of breast cancer.

Naked mole rats can undergo developmental, oncogene-induced and DNA damage-induced cellular senescence

Cellular senescence is an important anticancer mechanism that restricts proliferation of damaged or premalignant cells. Cellular senescence also plays an important role in tissue remodeling during development. However, there is a trade-off associated with cellular senescence as senescent cells contribute to aging pathologies. The naked mole rat (NMR) (Heterocephalus glaber) is the longest-lived rodent that is resistant to a variety of age-related diseases. Remarkably, NMRs do not show aging phenotypes until very late stages of their lives. Here, we tested whether NMR cells undergo cellular senescence. We report that the NMR displays developmentally programmed cellular senescence in multiple tissues, including nail bed, skin dermis, hair follicle, and nasopharyngeal cavity. NMR cells also underwent cellular senescence when transfected with oncogenic Ras. In addition, cellular senescence was detected in NMR embryonic and skin fibroblasts subjected to γ-irradiation (IR). However, NMR cells required a higher dose of IR for induction of cellular senescence, and NMR fibroblasts were resistant to IR-induced apoptosis. Gene expression analyses of senescence-related changes demonstrated that, similar to mice, NMR cells up-regulated senescence-associated secretory phenotype genes but displayed more profound down-regulation of DNA metabolism, transcription, and translation than mouse cells. We conclude that the NMR displays the same types of cellular senescence found in a short-lived rodent.

Survival of irradiated recipient mice after transplantation of bone marrow from young, old and "early aging" mice

Bone marrow transplantation is used to examine survival, hematopoietic stem cell function and pathology in recipients of young and old wild type bone marrow derived stem cells (BMDSCs) as well as cells from p53-based models of premature aging. There is no difference in the long term survival of recipients of 8 week-old p53+/m donor cells compared to recipients of 8 week-old wild-type (WT) donor cells (70 weeks) or of recipients of 16-18 weeks-old donor cells from either p53+/m or WT mice. There is shorter survival in recipients of older versus younger WT donor bone marrow, but the difference is only significant when comparing 8 and 18 week-old donors. In the p44-based model, short term survival/engraftment is significantly reduced in recipients of 11 month-old p44 donor cells compared to 4 week-old p44 or wild type donor cells of either age; mid-life survival at 40 weeks is also significantly less in recipients of p44 cells. BMDSCs are readily detectable within recipient bone marrow, lymph node, intestinal villi and liver sinusoids, but not in epithelial derived cells. These results indicate that recipients of young BMDSCs may survive longer than recipients of old bone marrow, but the difference is marginal at best.

Host age is a systemic regulator of gene expression impacting cancer progression

Aging is the major determinant of cancer incidence, which, in turn, is likely dictated in large part by processes that influence the progression of early subclinical (occult) cancers. However, there is little understanding of how aging informs changes in aggregate host signaling that favor cancer progression. In this study, we provide direct evidence that aging can serve as an organizing axis to define cancer progression-modulating processes. As a model system to explore this concept, we employed adolescent (68 days), young adult (143 days), middle-aged (551 days), and old (736 days) C57BL/6 mice as syngeneic hosts for engraftment of Lewis lung cancer to identify signaling and functional processes varying with host age. Older hosts exhibited dysregulated angiogenesis, metabolism, and apoptosis, all of which are associated with cancer progression. TGFβ1, a central player in these systemic processes, was downregulated consistently in older hosts. Our findings directly supported the conclusion of a strong host age dependence in determining the host tumor control dynamic. Furthermore, our results offer initial mechanism-based insights into how aging modulates tumor progression in ways that may be actionable for therapy or prevention.

Age-related Deterioration of Hematopoietic Stem Cells

Aging is the process of system deterioration over time in the whole body. Stem cells are self-renewing and therefore have been considered exempt from the aging process. Earlier studies by Hayflick showed that there is an intrinsic limit to the number of divisions that mammalian somatic cells can undergo, and cycling kinetics and ontogeny-related studies strongly suggest that even the most primitive stem cell functions exhibit a certain degree of aging. Despite these findings, studies on the effects of aging on stem cell functions are inconclusive. Here we review the age-related properties of hematopoietic stem cells in terms of intrinsic and extrinsic alterations, proliferative potential, signaling molecules, telomere and telomerase, senescence and cancer issues, regenerative potential and other indications of stem cell aging are discussed in detail.

Tumor suppression in skin and other tissues via cross-talk between vitamin D- and p53-signaling

P53 and its family members have been implicated in the direct regulation of the vitamin D receptor (VDR). Vitamin D- and p53-signaling pathways have a significant impact on spontaneous or carcinogen-induced malignant transformation of cells, with VDR and p53 representing important tumor suppressors. VDR and the p53/p63/p73 proteins all function typically as receptors or sensors that turn into transcriptional regulators upon stimulus, with the main difference being that the nuclear VDR is activated as a transcription factor after binding its naturally occurring ligand 1,25-dihydroxyvitamin D with high affinity while the p53 family of transcription factors, mostly in the nucleoplasm, responds to a large number of alterations in cell homeostasis commonly referred to as stress. An increasing body of evidence now convincingly demonstrates a cross-talk between vitamin D- and p53-signaling that occurs at different levels, has genome-wide implications and that should be of high importance for many malignancies, including non-melanoma skin cancer. One interaction involves the ability of p53 to increase skin pigmentation via POMC derivatives including alpha-MSH and ACTH. Pigmentation protects the skin against UV-induced DNA damage and skin carcinogenesis, yet on the other hand reduces cutaneous synthesis of vitamin D. A second level of interaction may be through the ability of 1,25-dihydroxyvitamin D to increase the survival of skin cells after UV irradiation. UV irradiation-surviving cells show significant reductions in thymine dimers in the presence of 1,25-dihydroxyvitamin D that are associated with increased nuclear p53 protein expression, and significantly reduced NO products. A third level of interaction is documented by the ability of vitamin D compounds to regulate the expression of the murine double minute 2 (MDM2) gene in dependence of the presence of wild-type p53. MDM2 has a well-established role as a key negative regulator of p53 activity. Finally, p53 and family members have been implicated in the direct regulation of VDR. This overview summarizes some of the implications of the cross-talk between vitamin D- and p53-signaling for carcinogenesis in the skin and other tissues.

p53-dependent delayed effects of radiation vary according to time of irradiation of p53 + / - mice

We previously reported that in p53 (+ / -) mice that had been given a whole-body dose of 3 Gy at 8 weeks of age, p53-dependent delayed effects of radiation, as manifested in T-cell receptor (TCR) variant fractions (VF) instability in mouse splenocytes, were biphasic, namely, induction of TCR-VF mutation reappeared at 44 weeks. The manifestation of the delayed effects and the measures of biological markers varied according to the timing of irradiation. We also reported that the decrease in function of the p53 gene was related to the effects of a delayed mutation. In the present study, we investigated the functions and mutations of the p53 gene in old age for p53 (+ / -) mice following irradiation at various ages. p53 (+ / -) mice were given a whole-body dose of 3 Gy at 8, 28 or 40 weeks of age. There were significant differences for all variables tested at 8 weeks of age. This was similarly the case for mice irradiated at 28 weeks of age, in which there were also significant differences in TCR VF and the percentage of apoptosis. In mice irradiated at 40 weeks of age, there were significant differences for all considered variables except for the p53 allele. We demonstrated that the different patterns of delayed mutation of the p53 gene at 56 weeks of age depended on the age at which mice had undergone 3-Gy whole-body irradiation. Our conclusions are limited to variation in p53-dependent delayed effects according to the time of irradiation.

Vitiligo: a possible model of degenerative diseases

Vitiligo is characterized by the progressive disappearance of pigment cells from skin and hair follicle. Several in vitro and in vivo studies show evidence of an altered redox status, suggesting that loss of cellular redox equilibrium might be the pathogenic mechanism in vitiligo. However, despite the numerous data supporting a pathogenic role of oxidative stress, there is still no consensus explanation underlying the oxidative stress-driven disappear of melanocytes from the epidermis. In this study, in vitro characterization of melanocytes cultures from non-lesional vitiligo skin revealed at the cellular level aberrant function of signal transduction pathways common with neurodegenerative diseases including modification of lipid metabolism, hyperactivation of mitogen-activated protein kinase (MAPK) and cAMP response element-binding protein (CREB), constitutive p53-dependent stress signal transduction cascades, and enhanced sensibility to pro-apoptotic stimuli. Notably, these long-term effects of subcytotoxic oxidative stress are also biomarkers of pre-senescent cellular phenotype. Consistent with this, vitiligo cells showed a significant increase in p16 that did not correlate with the chronological age of the donor. Moreover, vitiligo melanocytes produced many biologically active proteins among the senescence-associated secretory phenotype (SAPS), such as interleukin-6 (IL-6), matrix metallo proteinase-3 (MMP3), cyclooxygenase-2 (Cox-2), insulin-like growth factor-binding protein-3 and 7 (IGFBP3, IGFBP7). Together, these data argue for a complicated pathophysiologic puzzle underlying melanocytes degeneration resembling, from the biological point of view, neurodegenerative diseases. Our results suggest new possible targets for intervention that in combination with current therapies could correct melanocytes intrinsic defects.

Age and space irradiation modulate tumor progression: implications for carcinogenesis risk

Age plays a major role in tumor incidence and is an important consideration when modeling the carcinogenesis process or estimating cancer risks. Epidemiological data show that from adolescence through middle age, cancer incidence increases with age. This effect is commonly attributed to a lifetime accumulation of cellular, particularly DNA, damage. However, during middle age the incidence begins to decelerate and, for many tumor sites, it actually decreases at sufficiently advanced ages. We investigated if the observed deceleration and potential decrease in incidence could be attributed to a decreased capacity of older hosts to support tumor progression, and whether HZE [high atomic number (Z), high energy (E)] radiation differentially modulates tumor progression in young vs. middle-age hosts, issues that are relevant to estimating carcinogenesis risk for astronauts. Lewis lung carcinoma (LLC) cells were injected into syngeneic mice (143 and 551 days old), which were then subject to whole-body (56)Fe irradiation (1 GeV/amu). Three findings emerged: (1) among unirradiated animals, substantial inhibition of tumor progression and significantly decreased tumor growth rates were seen for middle-aged mice compared to young mice, (2) whole-body (56)Fe irradiation inhibited tumor progression in both young and middle-aged mice (with greater suppression seen in case of young animals), with little effect on tumor growth rates, and (3) (56)Fe irradiation suppressed tumor progression in young mice to a degree that was not significantly different than transiting from young to middle-aged. Thus, (56)Fe irradiation acted similar to aging with respect to tumor progression. We further investigated the molecular underpinnings driving the radiation modulation of tumor dynamics in young and middle-aged mice. Through global gene expression analysis, the key players, FASN, AKT1 and the CXCL12/CXCR4 complex, were determined to be contributory. In sum, these findings demonstrated a reduced capacity of middle-aged hosts to support the progression phase of carcinogenesis and identify molecular factors that contribute to HZE radiation modulation of tumor progression as a function of age.

Notch and the p53 clan of transcription factors

Notch 1 to 4 and the p53 clan, comprising p53, p63 and p73 plus numerous isoforms thereof, are gene transcription regulators that are critically involved in various aspects of cell differentiation, stem cell maintenance and tumour suppression. It is thus perhaps no surprise that extensive crosstalk between the Notch and p53 pathways is implemented during these processes. Typically, Notch together with p53 and even more so with transactivation competent p63 or p73, drives differentiation, whereas Notch combined with transactivation impaired p63 or p73 helps maintain undifferentiated stem cell compartments. With regard to cancer, it seems that Notch acts as a tumour suppressor in cellular contexts where Notch signalling supports p53 activation and both together can bring on its way an anti-proliferative programme of differentiation, senescence or apoptosis. In contrast, Notch often acts as an oncoprotein in contexts where it suppresses p53 activation and activity and where differentiation is unwanted. It is no accident that the latter pathways-the inhibition by Notch of p53 and differentiation-are operative in somatic stem cells as well as in tumour cells.

The p53 tumor suppressor protein regulates hematopoietic stem cell fate

The p53 tumor suppressor protein is a key transcription factor that regulates several signaling pathways involved in the cell's response to stress. Through stress-induced activation, p53 accumulates and triggers the expression of target genes that protect the genetic integrity of all cells including hematopoietic stem cells (HSCs). These protective mechanisms include cell-cycle arrest, DNA repair, induction of apoptosis, or initiation of senescence. In addition to its function under stress conditions, p53 has important functions during steady-state hematopoiesis, regulating HSC quiescence and self-renewal. In addition, it appears that p53 levels affect HSC competition for the hematopoietic niche, with the less p53 activated HSCs preferentially surviving. The specific genes and precise mechanisms underlying p53's effects on normal HSCs are slowly being clarified. p53 also plays an important role in leukemia stem cell (LSC) behavior, with p53 loss affecting drug resistance and disease progression. Pharmacologic activation of p53 function could overcome the adverse impact of p53 inactivation in LSCs. Thus, understanding the p53 regulatory mechanisms active in HSCs and LSCs may promote the development of new therapeutic strategies that could eliminate the population of largely quiescent LSCs.

p63 maintains keratinocyte proliferative capacity through regulation of Skp2-p130 levels

p63 is a master regulator of proliferation and differentiation in stratifying epithelia, and its expression is frequently altered in carcinogenesis. However, its role in maintaining proliferative capacity remains unclear. Here, we demonstrate that hypoproliferation and loss of differentiation in organotypic raft cultures of primary neonatal human foreskin keratinocytes (HFKs) depleted of the α and β isoforms of p63 result from p53-p21-mediated accumulation of retinoblastoma (Rb) family member p130. Hypoproliferation in p63-depleted HFKs can be rescued by depletion of p53, p21(CIP1) or p130. Furthermore, we identified the gene encoding S-phase kinase-associated protein 2 (Skp2), the recognition component of the SCF(Skp2) E3 ubiquitin ligase, as a novel target of p63, potentially influencing p130 levels. Expression of Skp2 is maintained by p63 binding to a site in intron 2 and mRNA levels are downregulated in p63-depleted cells. Hypoproliferation in p63-depleted cells can be restored by re-expression of Skp2. Taken together, these results indicate that p63 plays a multifaceted role in maintaining proliferation in the mature regenerating epidermis, in addition to being required for differentiation.

Therapeutic targeting of p53 by small molecules

Aberrant p53 function is one of the major requirements for tumor development. Reactivation of p53 function by small molecules is a promising strategy to combat cancer due to potent tumor suppressor activities of p53. Recent developments in p53 biology reveal that manipulation of p53 function might pave way to a long cancer-free life. A number of small molecules which rescue p53 function by different mechanisms, acting upstream of p53 or targeting the p53 protein itself have been identified. Notably, these molecules trigger different biological outcomes, suggesting that it might be feasible to direct p53-mediated response in a desired way. In this review I discuss the latest developments in the search for small molecules which rescue p53 function by targeting the p53 protein.

20 years studying p53 functions in genetically engineered mice

Cell and molecular biological studies of p53 functions over the past 30 years have been complemented in the past 20 years by studies that use genetically engineered mice. As expected, mice that have mutant Trp53 alleles usually develop cancers of various types more rapidly than their counterparts that have wild-type Trp53 genes. These mouse studies have been instrumental in providing important new insights into p53 tumour suppressor function. Such studies have been facilitated by the development of increasingly sophisticated genetic engineering approaches, which allow the more precise manipulation of p53 structure and function in a mammalian model.

p53--a Jack of all trades but master of none

Cancers are rare because their evolution is actively restrained by a range of tumour suppressors. Of these p53 seems unusually crucial as either it or its attendant upstream or downstream pathways are inactivated in virtually all cancers. p53 is an evolutionarily ancient coordinator of metazoan stress responses. Its role in tumour suppression is likely to be a relatively recent adaptation, which is only necessary when large, long-lived organisms acquired the sufficient size and somatic regenerative capacity to necessitate specific mechanisms to reign in rogue proliferating cells. However, such evolutionary reappropriation of this venerable transcription factor entails compromises that restrict its efficacy as a tumour suppressor.

Tumour suppression by p53: a role for the DNA damage response?

Loss of p53 function occurs during the development of most, if not all, tumour types. This paves the way for genomic instability, tumour-associated changes in metabolism, insensitivity to apoptotic signals, invasiveness and motility. However, the nature of the causal link between early tumorigenic events and the induction of the p53-mediated checkpoints that constitute a barrier to tumour progression remains uncertain. This Review considers the role of the DNA damage response, which is activated during the early stages of tumour development, in mobilizing the tumour suppression function of p53. The relationship between these events and oncogene-induced p53 activation through the ARF pathway is also discussed.

Aging in the lympho-hematopoietic stem cell compartment

Cells of the immune system are progeny of a single primitive cell type, the hematopoietic stem cell (HSC). Aging in most strains of mice is associated with a reduction in HSC frequency and a reduction in HSC function. Aged HSCs demonstrate reduced differentiation toward the lymphoid lineage, and this might be a relevant factor influencing immunosenescence. The molecular mechanisms of HSC aging need to be determined in more detail, but current studies have identified, among others, a role for telomere dysfunction in inducing cell intrinsic checkpoints and environmental alterations, which both skews and reduces stem cell differentiation and function. Reverting or ameliorating aging of HSCs might be a crucial step to restoring immuno-competence in the elderly.

The ever-lengthening arm of p53

p53 is a pivotal bulwark against cancer, but exactly how it suppresses tumors remains elusive, in part because it modulates such diverse biological processes via so many downstream pathways. In a recent issue of Cell, Godar et al. (2008) now identify another string to p53's anticancer bow-repression of the CD44 cell-surface glycoproteins that coordinate many attributes of tumor progression.

Toward a control theory analysis of aging

Aging is due to the accumulation of damage over time that affects the function and survival of the organism; however, it has proven difficult to infer the relative importance of the many processes that contribute to aging. To address this, here we outline an approach that may prove useful in analyzing aging. In this approach, the function of the organism is described as a set of interacting physiological systems. Degradation of their outputs leads to functional decline and death as a result of aging. In turn, degradation of the system outputs is attributable to changes at the next hierarchical level down, the cell, through changes in cell number or function, which are in turn a consequence of the metabolic history of the cell. Within this framework, we then adapt the methods of metabolic control analysis (MCA) to determine which modifications are important for aging. This combination of a hierarchical framework and the methodologies of MCA may prove useful both for thinking about aging and for analyzing it experimentally.

Altered mammary gland development in the p53+/m mouse, a model of accelerated aging

The tumor suppressor p53 is important for inhibiting the development of breast carcinomas. However, little is known about the effects of increased p53 activity on mammary gland development. Therefore, the effect of p53 dosage on mammary gland development was examined by utilizing the p53+/m mouse, a p53 mutant which exhibits increased wild-type p53 activity, increased tumor resistance, a shortened longevity, and a variety of accelerated aging phenotypes. Here we report that p53+/m virgin mice exhibit a defect in mammary gland ductal morphogenesis. Transplants of mammary epithelium into p53+/m recipient mice demonstrate decreased outgrowth of wild-type and p53+/m donor epithelium, suggesting systemic or stromal alterations in the p53+/m mouse. Supporting these data, p53+/m mice display decreased levels of serum IGF-1 and reduced IGF-1 signaling in virgin glands. The induction of pregnancy or treatment of p53+/m mice with estrogen, progesterone, estrogen and progesterone in combination, or IGF-1 stimulates ductal outgrowth, rescuing the p53+/m mammary phenotype. Serial mammary epithelium transplants demonstrate that p53+/m epithelium exhibits decreased transplant capabilities, suggesting early stem cell exhaustion. These data indicate that appropriate levels of p53 activity are important in regulating mammary gland ductal morphogenesis, in part through regulation of the IGF-1 pathway.

Changes in DNA repair during aging

DNA is a precious molecule. It encodes vital information about cellular content and function. There are only two copies of each chromosome in the cell, and once the sequence is lost no replacement is possible. The irreplaceable nature of the DNA sets it apart from other cellular molecules, and makes it a critical target for age-related deterioration. To prevent DNA damage cells have evolved elaborate DNA repair machinery. Paradoxically, DNA repair can itself be subject to age-related changes and deterioration. In this review we will discuss the changes in efficiency of mismatch repair (MMR), base excision repair (BER), nucleotide excision repair (NER) and double-strand break (DSB) repair systems during aging, and potential changes in DSB repair pathway usage that occur with age. Mutations in DNA repair genes and premature aging phenotypes they cause have been reviewed extensively elsewhere, therefore the focus of this review is on the comparison of DNA repair mechanisms in young versus old.