Limits to lifespan

Human placenta-derived mesenchymal stem cells stimulate neuronal regeneration by promoting axon growth and restoring neuronal activity

In the last decades, mesenchymal stem cells (MSCs) have become the cornerstone of cellular therapy due to their unique characteristics. Specifically human placenta-derived mesenchymal stem cells (hPMSCs) are highlighted for their unique features, including ease to isolate, non-invasive techniques for large scale cell production, significant immunomodulatory capacity, and a high ability to migrate to injuries. Researchers are exploring innovative techniques to overcome the low regenerative capacity of Central Nervous System (CNS) neurons, with one promising avenue being the development of tailored mesenchymal stem cell therapies capable of promoting neural repair and recovery. In this context, we have evaluated hPMSCs as candidates for CNS lesion regeneration using a skillful co-culture model system. Indeed, we have demonstrated the hPMSCs ability to stimulate damaged rat-retina neurons regeneration by promoting axon growth and restoring neuronal activity both under normoxia and hypoxia conditions. With our model we have obtained neuronal regeneration values of 10%-14% and axonal length per neuron rates of 19-26, μm/neuron. To assess whether the regenerative capabilities of hPMSCs are contact-dependent effects or it is mediated through paracrine mechanisms, we carried out transwell co-culture and conditioned medium experiments confirming the role of secreted factors in axonal regeneration. It was found that hPMSCs produce brain derived, neurotrophic factor (BDNF), nerve-growth factor (NGF) and Neurotrophin-3 (NT-3), involved in the process of neuronal regeneration and restoration of the physiological activity of neurons. In effect, we confirmed the success of our treatment using the patch clamp technique to study ionic currents in individual isolated living cells demonstrating that in our model the regenerated neurons are electrophysiologically active, firing action potentials. The outcomes of our neuronal regeneration studies, combined with the axon-regenerating capabilities exhibited by mesenchymal stem cells derived from the placenta, present a hopeful outlook for the potential therapeutic application of hPMSCs in the treatment of neurological disorders.

Multiparity increases the risk of diabetes by impairing the proliferative capacity of pancreatic β cells

Pregnancy imposes a substantial metabolic burden on women, but little is known about whether or how multiple pregnancies increase the risk of maternal postpartum diabetes. In this study, we assessed the metabolic impact of multiple pregnancies in humans and in a rodent model. Mice that underwent multiple pregnancies had increased adiposity, but their glucose tolerance was initially improved compared to those of age-matched virgin mice. Later, however, insulin resistance developed over time, but insulin secretory function and compensatory pancreatic β cell proliferation were impaired in multiparous mice. The β cells of multiparous mice exhibited aging features, including telomere shortening and increased expression of Cdkn2a. Single-cell RNA-seq analysis revealed that the β cells of multiparous mice exhibited upregulation of stress-related pathways and downregulation of cellular respiration- and oxidative phosphorylation-related pathways. In humans, women who delivered more than three times were more obese, and their plasma glucose concentrations were elevated compared to women who had delivered three or fewer times, as assessed at 2 months postpartum. The disposition index, which is a measure of the insulin secretory function of β cells, decreased when women with higher parity gained body weight after delivery. Taken together, our findings indicate that multiple pregnancies induce cellular stress and aging features in β cells, which impair their proliferative capacity to compensate for insulin resistance.

TGF-β1/IL-11/MEK/ERK signaling mediates senescence-associated pulmonary fibrosis in a stress-induced premature senescence model of Bmi-1 deficiency

To study whether TGF-β1/IL-11/MEK/ERK (TIME) signaling mediates senescence-associated pulmonary fibrosis (SAPF) in Bmi-1-deficient (Bmi-1-/-) mice and determines the major downstream mediator of Bmi-1 and crosstalk between p16INK4a and reactive oxygen species that regulates SAPF, phenotypes were compared among 7-week-old p16INK4a and Bmi-1 double-knockout, N-acetylcysteine (NAC)-treated Bmi-1-/-, Bmi-1-/-, and wild-type mice. Pulmonary fibroblasts and alveolar type II epithelial (AT2) cells were used for experiments. Human pulmonary tissues were tested for type Ι collagen, α-smooth muscle actin (α-SMA), p16INK4a, p53, p21, and TIME signaling by using enzyme-linked immunosorbent assay (ELISA). Our results demonstrated that Bmi-1 deficiency resulted in a shortened lifespan, ventilatory resistance, poor ventilatory compliance, and SAPF, including cell senescence, DNA damage, a senescence-associated secretory phenotype and collagen overdeposition that was mediated by the upregulation of TIME signaling. The signaling stimulated cell senescence, senescence-related secretion of TGF-β1 and IL-11 and production of collagen 1 by pulmonary fibroblasts and the epithelial-to-mesenchymal transition of AT2 cells. These processes were inhibited by anti-IL-11 or the MEK inhibitor PD98059. NAC treatment prolonged the lifespan and ameliorated pulmonary dysfunction and SAPF by downregulating TIME signaling more than p16INK4a deletion by inhibiting oxidative stress and DNA damage and promoting ubiquitin-proteasome degradation of p16INK4a and p53. Cytoplasmic p16INK4a accumulation upregulated MEK/ERK signaling by inhibiting the translocation of pERK1/2 (Thr202/Tyr204) from the cytoplasm to the nucleus in senescent fibroblasts. The accumulation of collagen 1 and α-SMA in human lungs accompanied by cell senescence may be mediated by TIME signaling. Thus, this signaling in aging fibroblasts or AT2 cells could be a therapeutic target for preventing SAPF.

P16 INK4a Deletion Ameliorated Renal Tubulointerstitial Injury in a Stress-induced Premature Senescence Model of Bmi-1 Deficiency

To determine whether p16 ^INK4a deletion ameliorated renal tubulointerstitial injury by inhibiting a senescence-associated secretory phenotype (SASP) in Bmi-1-deficient (Bmi-1 ^-/-) mice, renal phenotypes were compared among 5-week-old Bmi-1 and p16 ^INK4a double-knockout, and Bmi-1 ^-/- and wild-type mice. Fifth-passage renal interstitial fibroblasts (RIFs) from the three groups were analyzed for senescence and proliferation. The effect of Bmi-1 deficiency on epithelial-to-mesenchymal transition (EMT) was examined in Bmi-1-knockdown human renal proximal tubular epithelial (HK2) cells, which were treated with concentrated conditioned medium (CM) from the fifth-passage renal interstitial fibroblasts (RIFs) of above three group mice or with exogenous TGF-β1. Our results demonstrated that p16 ^INK4a deletion largely rescued renal aging phenotypes caused by Bmi-1 deficiency, including impaired renal structure and function, decreased proliferation, increased apoptosis, senescence and SASP, DNA damage, NF-κB and TGF-β1/Smad signal activation, inflammatory cell infiltration, and tubulointerstitial fibrosis and tubular atrophy. P16 ^INK4a deletion also promoted proliferation, reduced senescence and SASP of RIFs and subsequently inhibited EMT of Bmi-1-knockdown HK2 cells. TGF-β1 further induced the EMT of Bmi-1-knockdown HK2 cells. Thus, p16 ^INK4a positive senescent cells would be a therapeutic target for preventing renal tubulointerstitial injury.

Hyaluronan keeps mesenchymal stem cells quiescent and maintains the differentiation potential over time

Hyaluronan (HA), an abundant polysaccharide found in human bodies, plays a role in the mesenchymal stem cells (MSCs) maintenance. We had previously found that HA prolonged the lifespan, and prevented the cellular aging of murine adipose-derived stromal cells. Recently, we had also summarized the potential pathways associated with HA regulation in human MSCs. In this study, we used the human placenta-derived MSCs (PDMSC) to investigate the effectiveness of HA in maintaining the PDMSC. We found that coating the culture surface coated with 30 μg cm^-2 of HA (C) led to cluster growth of PDMSC, and maintained a higher number of PDMSC in quiescence compared to those grown on the normal tissue culture surface (T). PDMSC were treated for either 4 (short-term) or 19 (long-term) consecutive passages. PDMSC which were treated with HA for 19 consecutive passages had reduced cell enlargement, preserved MSCs biomarker expressions and osteogenic potential when compared to those grown only on T. The PDMSC transferred to T condition after long-term HA treatment showed preserved replicative capability compared to those on only T. The telomerase activity of the HA-treated PDMSC was also higher than that of untreated PDMSC. These data suggested a connection between HA and MSC maintenance. We suggest that HA might be regulating the distribution of cytoskeletal proteins on cell spreading in the event of quiescence to preserve MSC stemness. Maintenance of MSCs stemness delayed cellular aging, leading to the anti-aging phenotype of PDMSC.

Multipotent mesenchymal stromal cell therapy and risk of malignancies

Cell therapy with Multipotent Mesenchymal Stromal Cells (MSC) holds enormous promise for the treatment of a large number of degenerative and immune/inflammatory diseases. Their multilineage differentiation potential, immunoprivilege and capacity of promoting recovery of damaged tissues coupled with anti-inflammatory and immunosuppressive properties are the focus of a multitude of clinical studies currently underway. The recognized clinical potential of MSC repairing/immunomodulatory effects now encompasses graft-versus-host disease, hematologic malignancies, cardiovascular diseases, neurologic and inherited diseases, autoimmune diseases, organ transplantation, refractory wounds, and bone/cartilage defects among others. However, it has been suggested that both the need of extensive ex vivo culture for MSC clinical use, and their proangiogenic, anti-apoptotic and immunomodulatory properties may act together as tumor promoters, raising significant safety concerns. This paper will review the available data on in vitro MSC maldifferentiation and the ability of MSC to sustain tumor growth in vivo, with the aim to clarify whether MSC-based therapeutic approaches may carry actual risk of malignancies.

First comparative transcriptomic analysis of wild adult male and female Lutzomyia longipalpis, vector of visceral leishmaniasis

Leishmaniasis is a vector-borne disease with a complex epidemiology and ecology. Visceral leishmaniasis (VL) is its most severe clinical form as it results in death if not treated. In Latin America VL is caused by the protist parasite Leishmania infantum (syn. chagasi) and transmitted by Lutzomyia longipalpis. This phlebotomine sand fly is only found in the New World, from Mexico to Argentina. However, due to deforestation, migration and urbanisation, among others, VL in Latin America is undergoing an evident geographic expansion as well as dramatic changes in its transmission patterns. In this context, the first VL outbreak was recently reported in Argentina, which has already caused 7 deaths and 83 reported cases. Insect vector transcriptomic analyses enable the identification of molecules involved in the insect's biology and vector-parasite interaction. Previous studies on laboratory reared Lu. longipalpis have provided a descriptive repertoire of gene expression in the whole insect, midgut, salivary gland and male reproductive organs. Nevertheless, the study of wild specimens would contribute a unique insight into the development of novel bioinsecticides. Given the recent VL outbreak in Argentina and the compelling need to develop appropriate control strategies, this study focused on wild male and female Lu. longipalpis from an Argentine endemic (Posadas, Misiones) and a Brazilian non-endemic (Lapinha Cave, Minas Gerais) VL location. In this study, total RNA was extracted from the sand flies, submitted to sequence independent amplification and high-throughput pyrosequencing. This is the first time an unbiased and comprehensive transcriptomic approach has been used to analyse an infectious disease vector in its natural environment. Transcripts identified in the sand flies showed characteristic profiles which correlated with the environment of origin and with taxa previously identified in these same specimens. Among these, various genes represented putative targets for vector control via RNA interference (RNAi).

Roles and mechanisms of cellular senescence in regulation of tissue homeostasis

Cellular senescence is the state of irreversible cell cycle arrest that can be induced by a variety of potentially oncogenic stimuli and has therefore long been considered to suppress tumorigenesis, acting as a guardian of homeostasis. However, surprisingly, emerging evidence reveals that senescent cells also promote secretion of a series of inflammatory cytokines, chemokines, growth factors and matrix remodeling factors, which alter the local tissue environment and contribute to chronic inflammation and cancer. This newly identified senescence phenotype, termed the senescence-associated secretory phenotype (SASP) or the senescence-messaging secretome (SMS), is induced by DNA damage that promotes the induction of cellular senescence. All of these senescence-associated secreting factors are involved in homeostatic disorders such as cancer. Therefore, it is quite possible that accumulation of senescent cells during the aging process in vivo might contribute to age-related increases in homeostatic disorders. In this review, current knowledge of the molecular and cellular biology of cellular senescence is introduced, focusing on its positive and negative roles in controlling tissue homeostasis in vivo.

Senescence regulation by the p53 protein family

p53, a guardian of the genome, exerts its tumor suppression activity by regulating a large number of downstream targets involved in cell cycle arrest, DNA repair, apoptosis, and cellular senescence. Although p53-mediated apoptosis is able to kill cancer cells, a role for cellular senescence in p53-dependent tumor suppression is becoming clear. Mouse studies showed that activation of p53-induced premature senescence promotes tumor regression in vivo. However, p53-mediated cellular senescence also leads to aging-related phenotypes, such as tissue atrophy, stem cell depletion, and impaired wound healing. In addition, several p53 isoforms and two p53 homologs, p63 and p73, have been shown to play a role in cellular senescence and/or aging. Importantly, p53, p63, and p73 are necessary for the maintenance of adult stem cells. Therefore, understanding the dual role the p53 protein family in cancer and aging is critical to solve cancer and longevity in the future. In this chapter, we provide an overview on how p53, p63, p73, and their isoforms regulate cellular senescence and aging.

Establishment and characterization of two human breast carcinoma cell lines by spontaneous immortalization: Discordance between Estrogen, Progesterone and HER2/neu receptors of breast carcinoma tissues with derived cell lines

Background

Breast cancer is one of the most common cancers among women throughout the world. Therefore, established cell lines are widely used as in vitro experimental models in cancer research.

Methods

Two continuous human breast cell lines, designated MBC1 and MBC2, were successfully established and characterized from invasive ductal breast carcinoma tissues of Malaysian patients. MBC1 and MBC2 have been characterized in terms of morphology analysis, population doubling time, clonogenic formation, wound healing assay, invasion assay, cell cycle, DNA profiling, fluorescence immunocytochemistry, Western blotting and karyotyping.

Results

MBC1 and MBC2 exhibited adherent monolayer epithelial morphology at a passage number of 150. Receptor status of MBC1 and MBC2 show (ER⁺, PR⁺, HER2⁺) and (ER⁺, PR^-, HER2⁺), respectively. These results are in discordance with histopathological studies of the tumoral tissues, which were triple negative and (ER^-, PR^-, HER2⁺) for MBC1 and MBC2, respectively. Both cell lines were capable of growing in soft agar culture, which suggests their metastatic potential. The MBC1 and MBC2 metaphase spreads showed an abnormal karyotype, including hyperdiploidy and complex rearrangements with modes of 52–58 chromosomes per cell.

Conclusions

Loss or gain in secondary properties, deregulation and specific genetic changes possibly conferred receptor changes during the culturing of tumoral cells. Thus, we hypothesize that, among heterogenous tumoral cells, only a small minority of ER⁺/PR⁺/HER2⁺ and ER⁺/PR^-/HER2⁺ cells with lower energy metabolism might survive and adjust easily to in vitro conditions. These cell lines will pave the way for new perspectives in genetic and biological investigations, drug resistance and chemotherapy studies, and would serve as prototype models in Malaysian breast carcinogenesis investigations.

General RBP expression in human tissues as a function of age

Gene expression patterns vary dramatically in a tissue-specific and age-dependent manner. RNA-binding proteins that regulate mRNA turnover and/or translation (TTR-RBPs) critically affect the subsets of expressed proteins. Although many proteins implicated in age-related processes are encoded by mRNAs that are targets of TTR-RBPs, very little is known regarding the tissue- and age-dependent expression of TTR-RBPs in humans. Recent analysis of TTR-RBPs expression using human tissue microarray has provided us interesting insight into their possibly physiologic roles as a function of age. This analysis has also revealed striking discrepancies between the levels of TTR-RBPs in senescent human diploid fibroblasts (HDFs), widely used as an in vitro model of aging, and the levels of TTR-RBPs in tissues from individuals of advancing age. In this article, we will review our knowledge of human TTR-RBP expression in different tissues as a function of age.

Absence of replicative senescence in cultured cells from the short-lived killifish Nothobranchius furzeri

A major challenge in age research is the absence of short-lived vertebrate model organisms. The turquoise killifish Nothobranchius furzeri has the shortest known lifespan of a vertebrate that can be bred in captivity. The short lived GRZ strain only reaches a maximum age of 3-4 months, whereas other strains (MZM) reach 6-10 months. Most importantly, the short lifespan is associated with typical signs of ageing. To find out more about possible cellular factors that might contribute to the short lifespan and to the difference in lifespan between strains, we analyzed the expression of markers for cellular senescence. Expression of Tp53, Cdkn1a and Cdkn2a/b in skin revealed no change in the short-lived GRZ but increased expression of the cell cycle inhibitors Cdkn1a and Cdkn2a/b in the long-lived MZM strain with age. This suggests that expression of distinct cell cycle inhibitors reflects rather chronological than biological age in N. furzeri. To study the relationship of organismal life span and in vitro life span of cells, we established a primary cell culture model. For both strains we demonstrate here the absence of replicative senescence as analysed by morphology, expression of Cdkn1a and Cdkn2a/b, population doubling times and γH2AFX in long-term and short-term cultured cells. We reason this to be on account of sustained telomerase activity and maintained telomeric length. Hence, we propose that differences in maximum life span of different N. furzeri strains is not reflected by differences in proliferation speed or replicative potential of the respective cultured cells.

MicroRNAs as a novel cellular senescence regulator

Cellular senescence is a program activated in normal cells in response to various types of stresses and is manifested by permanent arrest of cell cycle. Cellular senescence is closely related to tumor suppression, and may contribute to the ageing of organisms. The complex senescence cell phenotype has many different mechanisms. Recent studies have provided important insights regarding the role played by miRNAs during cellular senescence as a novel molecular mechanism. In this article, we will review the latest advances in the identification and validation of senescence-regulatory miRNAs and the possible mechanisms.

Donor age-specific injury and immune responses

Utilization rates of organs from elderly donors have shown the highest proportional increase during the last decade. Clinical reports support the concept of transplanting older organs. However, the engraftment of such organs has been linked to accelerated immune responses based on ageing changes per se and a proinflammatory environment subsequent to compromised injury and repair mechanism. We analyzed the clinical consequences of transplanting older donor organs and present mechanistic aspects correlating age, injury repair and effects on host immunoresponsiveness.

Aging kidney: modern perspectives for an ‘old’ problem

The scientific community and health systems will have to address the increasing incidence of reduced kidney function because of progressive global aging. The aim of this review is to describe the morphological and functional alterations that characterize the aging kidney and to suggest not only pharmaceutical pathways but also lifestyle changes that could be beneficially targeted.

The aging effect of chemotherapy on cultured human mesenchymal stem cells

Various agents, including chemotherapeutic drugs, can induce cell senescence. However, the mechanisms involved in the aging pathway, particularly the stress that chemotherapy imposes on telomeres, are still undefined. To address these issues, human mesenchymal stem cells (MSCs) were assessed as target cells to investigate the initiation of the aging process by chemotherapy. The MSCs were obtained from bone marrow (BM) cells from normal adults and grown in the presence of platelet lysates. Cultured MSCs were identified for immunophenotype, and for growth and differentiation properties. The MSCs were exposed to 10 nM doxorubicin and 500 ng/mL etoposide, sublethal doses that induce DNA double-stranded breaks. Telomere length (TL) was assessed by flow-fluorescence in situ hybridization and Southern blotting. Initial TL shortening was detectable in MSCs at 5 days after drug exposure, with progressive reduction compared with untreated cells at 7, 14, 21, and 28 days in culture. After a single exposure, MSCs were unable to regain the lost telomere sequences for up to 28 days in culture. The ATM phosphorylation was documented early after drug exposure, while no telomerase activation was observed. Chemotherapy-induced TL shortening was associated with reduced clonogenic activity in vitro and accelerated adipose differentiation. Analogous behavior in the differentiation pattern was observed in naturally aged MSCs. These results indicate that cultured MSCs represent a useful cellular model to investigate novel drugs that may favor or, conversely, might prevent TL loss in human stem cells. The TL shortening is a permanent signature of previous chemotherapy-mediated DNA damage, and predicts impaired proliferative and differentiation potential.

Human umbilical cord stromal stem cell express CD10 and exert contractile properties

BACKGROUND

It has been demonstrated that human umbilical cord stromal stem cells (UCSSCs) are bio-equivalent to bone marrow mesenchymal stem cells. However, little is known about their tissue origin or in vivo functions, and data on their expansion properties are limited due to early senescence in the culture methods described to date.

METHODS

UC sections and cultured UCSSCs were analyzed with a panel of 12 antibodies. UCSSCs were grown in low-FCS containing medium at 5% or 21% oxygen and were assayed for their clonogenic properties, karyotype stability, expression of specific cellular markers, and multi-lineage potential. UCSSC contractile properties were evaluated by using collagen gel contraction assays under cytokine stimulus.

RESULTS

Immunohistochemistry studies showed that the UCSSCs were derived from the Wharton's jelly and not from the vascular smooth muscle sheath of the blood vessels. UCSSC growth properties were increased in a 5% oxygen atmosphere in comparison to normoxic culture conditions. In both culture conditions, UCSSCs were CD14-, CD34-, and CD45-negative while expressing high levels of CD73, CD90 and CD105 and maintaining their differentiation potentialities. UCSSCs expressed alpha smooth muscle actin and behaved as functional myofibroblasts when cellular contraction was challenged with appropriate stimuli.

CONCLUSIONS

UCSCs are mesenchymal stem cells that reside in the perivascular area of Wharton's jelly and are phenotypically and functionally related to myofibroblasts.

Sp100 as a potent tumor suppressor: accelerated senescence and rapid malignant transformation of human fibroblasts through modulation of an embryonic stem cell program

Identifying the functions of proteins, which associate with specific subnuclear structures, is critical to understanding eukaryotic nuclear dynamics. Sp100 is a prototypical protein of ND10/PML nuclear bodies, which colocalizes with Daxx and the proto-oncogenic PML. Sp100 isoforms contain SAND, PHD, Bromo, and HMG domains and are highly sumoylated, all characteristics suggestive of a role in chromatin-mediated gene regulation. A role for Sp100 in oncogenesis has not been defined previously. Using selective Sp100 isoform-knockdown approaches, we show that normal human diploid fibroblasts with reduced Sp100 levels rapidly senesce. Subsequently, small rapidly dividing Sp100 minus cells emerge from the senescing fibroblasts and are found to be highly tumorigenic in nude mice. The derivation of these tumorigenic cells from the parental fibroblasts is confirmed by microsatellite analysis. The small rapidly dividing Sp100 minus cells now also lack ND10/PML bodies, and exhibit genomic instability and p53 cytoplasmic sequestration. They have also activated MYC, RAS, and TERT pathways and express mesenchymal to epithelial transdifferentiation (MET) markers. Reintroduction of expression of only the Sp100A isoform is sufficient to maintain senescence and to inhibit emergence of the highly tumorigenic cells. Global transcriptome studies, quantitative PCR, and protein studies, as well as immunolocalization studies during the course of the transformation, reveal that a transient expression of stem cell markers precedes the malignant transformation. These results identify a role for Sp100 as a tumor suppressor in addition to its role in maintaining ND10/PML bodies and in the epigenetic regulation of gene expression.

Detailed analysis of DNA repair and senescence marker kinetics over the life span of a human fibroblast cell line

We examined phosphorylation of H2AX, a marker for DNA double-strand breaks over the life of a human fibroblast cell line. This marker was compared with a number of other cellular senescence and DNA repair endpoints. An increase in γH2AX foci number was observed after 24 hours of repair time following DNA damage over the course of fibroblast passaging. Progressive and relatively constant changes in growth retardation, doubling time, and telomere length were also observed. The fraction of cells expressing β-gal, a marker of cellular senescence, increased considerably around the 40th passage as did some other cell morphology endpoints. The detectable γH2AX foci at 24 hours after ionizing radiation were far fewer than the number detected at 1 hour across all passage numbers. We conclude that although residual DNA damage level increases with passage number, it is unlikely to be the result of less efficient DNA repair in the aged fibroblast since most DNA damage is repaired, even at late passages.

Cell senescence in the aging kidney

Pedigree genetics and environment modulate the biological process of aging. The permanent and irreversible growth arrest of cell senescence is a central paradigm of aging. Various pathophysiologic pressures such as oxidative stress and mitochondrial injury can also induce senescence. Senescent cells secrete altered levels of growth factors, show increased susceptibility to apoptosis, and associate with delayed repair and regeneration in the aging kidney. Here we discuss new progress in understanding renal aging, focusing on mechanisms of cell senescence and possible interventions to modulate age-related organ damage.

Knockout of Ku86 accelerates cellular senescence induced by high NaCl

NaCl induces DNA breaks, thus leading to cellular senescence. Here we showed that Ku86 deficiency accelerated the high NaCl-induced cellular senescence. We find that 1) high NaCl induces rapid cellular senescence in Ku86 deficient(xrs5) cells, 2) Ku86 deficiency shortens lifespan of C. elegans in high NaCl, and 3) cellular senescence is greatly accelerated in renal inner medullas of Ku86 (-/-) mice. Further, although water balance is known to be compromised in old mice, this occurs at much earlier age in Ku86(-/-) mice. When subjected to mild water restriction, 3 month old Ku86(-/-), but not Ku86(+/+),mice rapidly become dehydrated as evidenced by decrease in body weight, increased production of antidiuretic hormone,increased urine osmolality and decreased urine volume. The deficiency in water balance does not occur in Ku86(+/+)mice until they are much older (14 months). We conclude that Ku86 deficiency accelerates high NaCl(-) induced cellular senescence,particularly in the renal medulla where NaCl normally is high.

Cellular senescence: its role in tumor suppression and aging

In normal tissue, cell division is carefully regulated to maintain the correct proliferative balance. Abnormal cell division underlies many hypoproliferative and hyperproliferative disorders, including cancer, and a better understanding of the mechanisms involved could lead to new strategies for treatment and prevention. Cellular senescence, a state of irreversible growth arrest, was first described as a limit to the replicative life span of somatic cells after serial cultivation in vitro. Recently, however, it has also been shown to be triggered prematurely by potentially oncogenic stimuli such as oncogene expression, oxidative stress, and DNA damage in cell culture studies. These data suggest that cellular senescence is therefore acting as a tumor-protective fail-safe mechanism. However, the significance of cellular senescence has remained an issue of debate over the years, with the possibility that it might be a cell culture-related artifact. Recent reports on oncogene-induced senescence detected in premalignant tumors have provided evidence to validate its role as a physiological response to prevent oncogenesis in vivo. In this review, we discuss the mechanisms for cellular senescence and its roles in vivo.

Biological characterization of long-term cultured human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) have generated a great deal of interest in clinical applications. The reason is that they may have the plasticity needed to differentiate into multiple lineages and the ability to expand ex vivo. For the therapeutic applications of hMSCs to be of practical use, it is crucial to assess the efficacy and safety of hMSCs in long-term ex vivo expansion. In this study, we cultured hMSCs by population doubling (PD) 60, and investigated their growth, osteogenic and adipogenic differential abilities, change of surface markers, telomerase activity, telomere length, and gene expression related to tumorigenesis. An in vivo tumorigenesis assay was also carried out. In long-term expanded hMSCs, the cells became aged above PD 30 and their adipogenic and osteogenic differentiation potential decreased. Telomerase activity unchanged whereas telomere length decreased and karyotypes were not changed. Gene expressions related to tumorigenesis decreased in proportion as the PD of hMSCs increased. In vivo transplantation of long-term cultured hMSCs to nude mice did not result in tumor formation. These findings suggest that diverse tests for cellular therapy should be considered during the ex vivo culture of hMSCs, particularly when a prolonged and extended propagation period is required.

Endothelial progenitor cells dysfunction and senescence: contribution to oxidative stress

The identification of endothelial progenitor cells (EPCs) has led to a significant paradigm in the field of vascular biology and opened a door to the development of new therapeutic approaches. Based on the current evidence, it appears that EPCs may make both direct contribution to neovascularization and indirectly promote the angiogenic function of local endothelial cells via secretion of angiogenic factors. This concept of arterial wall repair mediated by bone marrow (BM)-derived EPCs provided an alternative to the local "response to injury hypothesis" for development of atherosclerotic inflammation. Increased oxidant stress has been proposed as a molecular mechanism for endothelial dysfunction, in part by reducing nitric oxide (NO) bioavailability. EPCs function may also be highly dependent on a well-controlled oxidant stress because EPCs NO bioavailability (which is highly sensitive to oxidant stress) is critical for their in vivo function. The critical question is whether oxidant damage directly leads to an impairment in EPCs function. It was revealed that activation of angiotensin II (Ang II) type 1 receptor stimulates nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase in the vascular endothelium and leads to production of reactive oxygen species. We observed that Ang II accelerates both BM- and peripheral blood (PB)-derived EPCs senescence by a gp91phox-mediated increase of oxidative stress, resulting in EPCs dysfunction. Consistently, both Ang II receptor 1 blockers (ARBs) and angiotensin converting enzyme (ACE) inhibitors have been reported to increase the number of EPCs in patients with cardiovascular disease. In this review, we describe current understanding of the contributions of oxidative stress in cardiovascular disease, focusing on the potential mechanisms of EPCs senescence.

Function of JunB in transient amplifying cell senescence and progression of human prostate cancer

PURPOSE

Replicative senescence in cells acts as a barrier against excessive proliferation and carcinogenesis. Transient amplifying cells (TAC) are a subset of basal cell populations within the prostate from which cancers are thought to originate; therefore, we focused on prostate TAC to investigate the molecular mechanisms by which the TAC may be able to evade senescence.

EXPERIMENTAL DESIGN

TAC clones were isolated from each zone within the whole prostate and analyzed in flow cytometry. Prostate cancer cells were transfected with junB small interfering RNA (siRNA) and examined by chorioallantoic membrane assay for cancer invasion. Immunohistochemical analysis was done in primary and metastatic prostate cancer specimens.

RESULTS

TAC populations showed increased expression of p53, p21, p16, and pRb, resulting in senescence. TAC clones with reduced p16 expression successfully bypassed this phase. We further found close correlation between the levels of junB and p16 expression. Repeated transfection of junB siRNA in prostatic TAC allowed the cells to escape senescence presumably through inactivation of p16/pRb. The chorioallantoic membrane invasion assay showed much lower in invasive cancer cells with high expression of junB; conversely, silencing of junB by transfection with junB siRNA promoted invasion. We also found that metastatic prostate cancers, as well as cancers with high Gleason scores, showed significantly low junB immunopositivity.

CONCLUSIONS

JunB is an essential upstream regulator of p16 and contributes to maintain cell senescence that blocks malignant transformation of TAC. JunB thus apparently plays an important role in controlling prostate carcinogenesis and may be a new target for cancer prevention and therapy.

Markers of cellular senescence in zero hour biopsies predict outcome in renal transplantation

Although chronological donor age is the most potent predictor of long-term outcome after renal transplantation, it does not incorporate individual differences of the aging-process itself. We therefore hypothesized that an estimate of biological organ age as derived from markers of cellular senescence in zero hour biopsies would be of higher predictive value. Telomere length and mRNA expression levels of the cell cycle inhibitors CDKN2A (p16INK4a) and CDKN1A (p21WAF1) were assessed in pre-implantation biopsies of 54 patients and the association of these and various other clinical parameters with serum creatinine after 1 year was determined. In a linear regression analysis, CDKN2A turned out to be the best single predictor followed by donor age and telomere length. A multiple linear regression analysis revealed that the combination of CDKN2A values and donor age yielded even higher predictive values for serum creatinine 1 year after transplantation. We conclude that the molecular aging marker CDKN2A in combination with chronological donor age predict renal allograft function after 1 year significantly better than chronological donor age alone.

The proto-oncogene LRF is under post-transcriptional control of MiR-20a: implications for senescence

MicroRNAs (miRNAs) are short 20–22 nucleotide RNA molecules that act as negative regulators of gene expression via translational repression: they have been shown to play a role in development, proliferation, stress response, and apoptosis. The transcriptional regulator LRF (Leukemia/lymphoma Related Factor) has been shown to prevent p19ARF transcription and consequently to inhibit senescence in mouse embryonic fibroblasts (MEF). Here we report, for the first time, that LRF is post-transcriptionally regulated by miR-20a. Using a gene reporter assay, direct interaction of miR-20a with the LRF 3′UTR is demonstrated. To validate the interaction miR-20a/3′UTR LRF miR-20a was over-expressed, either by transient transfection or retroviral infection, in wild type mouse embryo fibroblasts and in LRF-null MEF derived from LRF knock-out mice. We observed LRF decrease, p19ARF increase, inhibition of cell proliferation and induction of senescence. The comparison of miR-20a activity in wt and LRF-null MEF indicates that LRF is the main mediator of the miR-20a-induced senescence and that other targets are cooperating. As LRF down-regulation/p19ARF induction is always accompanied by E2F1 down-regulation and increase of p16, we propose that all these events act in synergy to accomplish miR-20a-induced senescence in MEF. Senescence has been recently revaluated as a tumor suppressor mechanism, alternative to apoptosis; from this point of view the discovery of new physiological “senescence inducer” appears to be promising as this molecule could be used as anticancer drug.

Melanomagenesis: overcoming the barrier of melanocyte senescence

Although melanoma ultimately progresses to a highly aggressive and metastatic disease that is typically resistant to currently available therapy, it often begins as a benign nevus consisting of a clonal population of hyperplastic melanocytes that cannot progress because they are locked in a state of cellular senescence. Once senescence is overcome, the nevus can exhibit dysplastic features and readily progress to more lethal stages. Recent advances have convincingly demonstrated that senescence represents a true barrier to the progression of many types of cancer, including melanoma. Thus, understanding the mechanism(s) by which melanoma evades senescence has become a priority in the melanoma research community. Senescence in most cells is regulated through some combination of activities within the RB and p53 pathways. However, differences discovered among various tumor types, some subtle and others quite profound, have revealed that senescence frequently operates in a context-dependent manner. Here we review what is known about melanocyte senescence, and how such knowledge may provide a much-needed edge in our struggles to contain or perhaps vanquish this often-fatal malignancy.

Hypertension induces somatic cellular senescence in rats and humans by induction of cell cycle inhibitor p16INK4a

There is increasing evidence for a role of somatic cellular senescence in physiological aging but also in injury and disease. Cell cycle inhibitor p16(INK4a) is the key mediator for stress and aberrant signaling induced senescence. Here we report that elevated blood pressure markedly induced p16(INK4a) expression in rat kidneys and hearts, as well as in human kidneys. In kidneys from deoxycorticosterone acetate-salt-treated rats, p16(INK4a) induction was found in tubular, glomerular, interstitial, and vascular cells and correlated with the typical histopathologic features of hypertensive target organ damage. p16(INK4a) expression also correlated with phospho-p38, a positive upstream regulator of p16(INK4a) expression. In left ventricles, increased p16(INK4a) expression was found in myocardium and cardiac arteries. Antihypertensive medication consistent of hydrochlorothiazide, hydralazine, and reserpine ameliorated the histopathologic changes and attenuated p16(INK4a) expression in kidneys of deoxycorticosterone acetate-salt-treated rats. Nonantihypertensive administration of spironolactone also reduced kidney damage and p16(INK4a) expression. p16(INK4a) induction was further observed in kidneys from hypertensive transgenic rats heterozygous for the mouse Ren-2 gene and was prevented by the angiotensin II type 1 receptor blocker losartan. In human kidney biopsies showing hypertensive nephrosclerosis, increased p16(INK4a) expression was found compared with age-matched normotensive control subjects. Thus, hypertension induces cellular senescence via p16(INK4a), possibly through p38, thereby contributing to hypertensive target organ damage. This detrimental effect can be overcome by different therapeutic drug strategies.

Evidence-based roads to the promotion of health in old age

The increase in life expectancy, along with the accompanying ongoing increase in the proportion and absolute numbers of nonagenarians and centenarians have set forth the curiosity regarding the question of the quality of health in very old age. Studies on that issue have pointed to the fact that the very old people are actually healthier than originally predicted on the basis of the earlier studies on aging. Current efforts are thus invested in elucidating the possible basis of health in the very old people, as well as better understanding of potential causes of frailty and common diseases in old age. This review recounts on the various aspects underlying evidence-based recommendations for healthy life in old age. We focus on the genetic and non-genetic bases of aging and longevity, and the various directions towards the promotion of health, both via avoiding, or eliminating risk factors and deleterious effects, as well as conducting healthy lifestyle - in terms of proper nutrition and physical exercise. Next, we touch upon preventive medicine, particularly as related to vaccination, with a note also on the need for a reasonable use of medications. In addition, we report about the developing area of regenerative medicine and its potential in relation to the prevention of damage and possible strategies towards tissue repair in cases of age-related degenerative processes.

Cellular senescence, cardiovascular risk, and CKD: a review of established and hypothetical interconnections

Cellular senescence is associated with shortened or damaged telomeres and is characterized by permanent exit from the cell cycle, morphological changes, and altered function. It develops after repeated cell divisions and also can be induced prematurely by stress conditions. The senescent phenotype, depending on cell type and atherosclerosis phase, seems to be a proatherosclerotic one: it promotes endothelial dysfunction and appears to be implicated in plaque destabilization, as well as in endothelial progenitor cell alteration. Many traditional and nontraditional cardiovascular disease risk factors induce senescence in a variety of vascular cells. Several of these factors, such as diabetes, hypertension, oxidative stress, and inflammation, are clustered in patients with chronic kidney disease. In a limited number of recent studies, stress-induced premature cellular senescence in this biologically aged population also was described. The hypothesis that premature cellular senescence might be considered an additional atherosclerosis-inducing factor in patients with chronic kidney disease is proposed.

Interrelation amongst differentiation, senescence and genetic instability in long-term cultures of fibroblasts exposed to different radiation qualities

BACKGROUND AND PURPOSE

The goal of the present study was to investigate aging and genetic instability in the progeny of human fibroblasts exposed to X-rays and carbon ions.

MATERIALS AND METHODS

Following irradiation, cells were regularly subcultured until senescence. At selected time-points BrdU-labelling index, expression of cell cycle related proteins, cell differentiation pattern and chromosome aberrations were assessed.

RESULTS

After exposure, an immediate cell cycle arrest occurred followed by a period of a few weeks where premature differentiation and senescence were observed. In all cultures cycling cells expressing low levels of cell cycle inhibiting proteins were present and finally dominated the populations. About 5months after exposure, the cellular and molecular changes attributed to differentiation and senescence reappeared and persisted. Concurrently, genetic instability was observed, but the aberration yields and types differed between repeated experiments. The descendants of cells exposed to carbon ions did not senesce earlier and displayed a similar rate of genetic instability as the X-ray progeny. For high doses an impaired cell cycle regulation and extended life span was observed, but finally cell proliferation ceased in all populations.

CONCLUSIONS

The descendants of irradiated fibroblasts undergo stepwise senescence and differentiation. Genetic instability is frequent and an extension of the life span may occur.

Senescent phenotypes of skin fibroblasts from patients with Tangier disease

Tangier disease (TD) is characterized by a deficiency of high density lipoprotein (HDL) in plasma and patients with TD have an increased risk for coronary artery disease (CAD). Recently, we reported that fibroblasts from TD exhibited large and flattened morphology, which is often observed in senescent cells. On the other hand, data have accumulated to show the relationship between cellular senescence and development of atherosclerotic CAD. The aim of the present study was to investigate whether TD fibroblasts exhibited cellular senescence. The proliferation of TD fibroblasts was gradually decreased at population doubling level (PDL) approximately 10 compared with control cells. TD cells practically ceased proliferation at PDL approximately 30. DNA synthesis was markedly decreased in TD fibroblasts. TD cells exhibited a higher positive rate for senescence-associated beta-galactosidase (SA-beta-gal), which is one of the biomarkers of cellular senescence in vitro. These data showed that TD cells reached cellular senescence at an earlier PDL compared with controls. Although, there was no difference in the telomere length of fibroblasts between TD and controls at the earlier passage (PDL 6), the telomere length of TD cells was shorter than that of controls at the late passage (PDL 25). Taken together, the current study demonstrates that the late-passaged TD fibroblasts showed senescent phenotype in vitro, which might be related to the increased cardiovascular manifestations in TD patients.

Manipulation of cell cycle progression can counteract the apparent loss of correction frequency following oligonucleotide-directed gene repair

Background

Single-stranded oligonucleotides (ssODN) are used routinely to direct specific base alterations within mammalian genomes that result in the restoration of a functional gene. Despite success with the technique, recent studies have revealed that following repair events, correction frequencies decrease as a function of time, possibly due to a sustained activation of damage response signals in corrected cells that lead to a selective stalling. In this study, we use thymidine to slow down the replication rate to enhance repair frequency and to maintain substantial levels of correction over time.

Results

First, we utilized thymidine to arrest cells in G1 and released the cells into S phase, at which point specific ssODNs direct the highest level of correction. Next, we devised a protocol in which cells are maintained in thymidine following the repair reaction, in which the replication is slowed in both corrected and non-corrected cells and the initial correction frequency is retained. We also present evidence that cells enter a senescence state upon prolonged treatment with thymidine but this passage can be avoided by removing thymidine at 48 hours.

Conclusion

Taken together, we believe that thymidine may be used in a therapeutic fashion to enable the maintenance of high levels of treated cells bearing repaired genes.

Mitogenic signalling and the p16INK4a–Rb pathway cooperate to enforce irreversible cellular senescence

The p16(INK4a) cyclin-dependent kinase inhibitor has a key role in establishing stable G1 cell-cycle arrest through activating the retinoblastoma (Rb) tumour suppressor protein pRb in cellular senescence. Here, we show that the p16(INK4a) /Rb-pathway also cooperates with mitogenic signals to induce elevated intracellular levels of reactive oxygen species (ROS), thereby activating protein kinase Cdelta (PKCdelta) in human senescent cells. Importantly, once activated by ROS, PKCdelta promotes further generation of ROS, thus establishing a positive feedback loop to sustain ROS-PKCdelta signalling. Sustained activation of ROS-PKCdelta signalling irreversibly blocks cytokinesis, at least partly through reducing the level of WARTS (also known as LATS1), a mitotic exit network (MEN) kinase required for cytokinesis, in human senescent cells. This irreversible cytokinetic block is likely to act as a second barrier to cellular immortalization ensuring stable cell-cycle arrest in human senescent cells. These results uncover an unexpected role for the p16(INK4a)-Rb pathway and provide a new insight into how senescent cell-cycle arrest is enforced in human cells.

Progressive loss of SIRT1 with cell cycle withdrawal

Sir2 is an NAD+-dependent deacetylase that regulates lifespan in yeast, worms and flies. The mammalian orthologs of Sir2 include SIRT1 in humans and mice. In this study, we analyzed the level of SIRT1 in human lung fibroblasts (IMR90) and mouse embryonic fibroblasts (MEFs) from mice with normal, accelerated, and delayed aging. SIRT1 protein, but not mRNA, decreased significantly with serial cell passage in both human and murine cells. Mouse SIRT1 decreased rapidly in prematurely senescent (p44 Tg) MEFs, remained high in MEFs with delayed senescence (Igf-1r-/-), and was inversely correlated with senescence-activated beta-galactosidase (SA-betaGal) activity. Reacquisition of mitotic capability following spontaneous immortalization of serially passaged wild-type MEFs restored the level of SIRT1 to that of early passage, highly proliferative MEFs. In mouse and human fibroblasts, we found a significant positive correlation between the levels of SIRT1 and proliferating cell nuclear antigen (PCNA), a DNA processing factor expressed during S-phase. In the animal, we found that SIRT1 decreased with age in tissues in which mitotic activity also declines, such as the thymus and testis, but not in tissues such as the brain in which there is little change in mitotic activity throughout life. Again, the decreases in SIRT1 were highly correlated with decreases in PCNA. Finally, loss of SIRT1 with age was accelerated in mice with accelerated aging but was not observed in long-lived growth hormone-receptor knockout mice. Thus, as mitotic activity ceases in mouse and human cells in the normal environment of the animal or in the culture dish, there is a concomitant decline in the level of SIRT1.

Ganoderiol F, a ganoderma triterpene, induces senescence in hepatoma HepG2 cells

Ganoderiol F (GolF), a tetracyclic triterpene, was isolated from Ganoderma amboinense and found to induce senescence of cancer cell lines. GolF induced growth arrest of cancer cell lines HepG2, Huh7 and K562, but exerted much less effect in hepatoma Hep3B cells and normal lung fibroblast MRC5 cells, and no effect on peripheral blood mononuclear cells. GolF treatment of the cancer cells, with the exception of Hep3B, resulted in prompt inhibition of DNA synthesis and arrest of cell progression cycle in G1 phase. Short-term exposure of HepG2 cells to GolF temporarily arrested progression of the cell cycle; cell growth was recovered if the drug was withdrawn from the medium after a 24-h exposure. After 18 days of continuous treatment of HepG2 cells with 30 muM GolF, over 50% of cells were found to be enlarged and flattened, and were beta-galactosidase positive phenotypes of senescent cells. GolF was found to inhibit activity of topoisomerases in vitro, which may contribute to the inhibition of cellular DNA synthesis. Activation of the mitogen-activated protein kinase EKR and up-regulation of cyclin-dependent kinase inhibitor p16 were found in early stages of GolF treatment and were presumed to cause cell-cycle arrest and trigger premature senescence of HepG2 cells. The growth-arrest and senescence induction capability on cancer cells suggest anticancer potential of GolF.

Reduced c-Myc signaling triggers telomere-independent senescence by regulating Bmi-1 and p16(INK4a)

Increased mitogenic signaling by positive effectors such as Ras or Myc can trigger senescence in normal cells, a response believed to function as a tumor-suppressive mechanism. We report here the existence of a checkpoint that monitors hypoproliferative signaling imbalances. Normal human fibroblasts with one copy of the c-myc gene inactivated by targeted homologous recombination switched with an increased frequency to a telomere-independent senescent state mediated by the cyclin-dependent kinase inhibitor p16(INK4a). p16(INK4a) expression was regulated by the Polycomb group repressor Bmi-1, which we show is a direct transcriptional target of c-Myc. The Myc-Bmi circuit provides a mechanism for the conversion of environmental inputs that converge on c-Myc into discrete cell-fate decisions coupled to cell-cycle recruitment. A mechanism for limiting the proliferation of damaged or otherwise physiologically compromised cells would be expected to have important consequences on the generation of replicatively senescent cells during organismal aging.

The human DEK proto-oncogene is a senescence inhibitor and an upregulated target of high-risk human papillomavirus E7

The human DEK proto-oncogene is a nucleic acid binding protein with suspected roles in human carcinogenesis, autoimmune disease, and viral infection. Intracellular DEK functions, however, are poorly understood. In papillomavirus-positive cervical cancer cells, downregulation of viral E6/E7 oncogene expression results in cellular senescence. We report here the specific repression of DEK message and protein levels in senescing human papillomavirus type 16- (HPV16-) and HPV18-positive cancer cell lines as well as in primary cells undergoing replicative senescence. Cervical cancer cell senescence was partially overcome by DEK overexpression, and DEK overexpression was sufficient for extending the life span of primary keratinocytes, supporting critical roles for this molecule as a senescence regulator. In order to determine whether DEK is a bona fide HPV oncogene target in primary cells, DEK expression was monitored in human keratinocytes transduced with HPV E6 and/or E7. The results identify high-risk HPV E7 as a positive DEK regulator, an activity that is not shared by low-risk HPV E7 protein. Experiments in mouse embryo fibroblasts recapitulated the observed E7-mediated DEK induction and demonstrated that both basal and E7-induced regulation of DEK expression are controlled by the retinoblastoma protein family. Taken together, our results suggest that DEK upregulation may be a common event in human carcinogenesis and may reflect its senescence inhibitory function.

Induction of p21WAF1/CIP1 by human synovial sarcoma-associated chimeric oncoprotein SYT-SSX1

Oncogenic protein provokes cell cycle arrest termed premature senescence. In this process Ras has been known to induce cyclin-dependent kinase inhibitor (CKI) p16(INK4A) in primary fibroblasts. Here, we present a novel finding that human chimeric oncoprotein SYT-SSX1 induces CKI p21(WAF1/CIP1) (p21) for suppression of cell growth. In human synovial sarcoma cell lines, the expression levels of p21 were high and the transcriptional activity of the p21 gene promoter was significantly elevated. The transient expression of SYT-SSX1-induced activation of the p21 gene promoter in human diploid fibroblasts. The N-terminus deletion form of SYT-SSX1, which failed to bind to hBRM one of the chromatin remodeling factors, preserved the p21 induction ability. This effect of SYT-SSX1 was similar in extent in both wild-type and p53-deficient HCT116 cell lines. Furthermore, the introduction of mutation in Sp1/Sp3 binding sites of the p21 gene promoter abolished the SYT-SSX1-induced transcriptional activity of its promoter. In SW13 cells, the stable expression of SYT-SSX1 suppressed cell growth in culture. These results suggest that SYT-SSX1 is able to induce p21 in a manner independent on hBRM and p53 but dependent on Sp1/Sp3.

Spontaneous human adult stem cell transformation

Human adult stem cells are being evaluated widely for various therapeutic approaches. Several recent clinical trials have reported their safety, showing them to be highly resistant to transformation. The clear similarities between stem cell and cancer stem cell genetic programs are nonetheless the basis of a recent proposal that some cancer stem cells could derive from human adult stem cells. Here we show that although they can be managed safely during the standard ex vivo expansion period (6-8 weeks), human mesenchymal stem cells can undergo spontaneous transformation following long-term in vitro culture (4-5 months). This is the first report of spontaneous transformation of human adult stem cells, supporting the hypothesis of cancer stem cell origin. Our findings indicate the importance of biosafety studies of mesenchymal stem cell biology to efficiently exploit their full clinical therapeutic potential.

Tumorigenesis: the adaptation of mammalian cells to sustained stress environment by epigenetic alterations and succeeding matched mutations

Recent studies indicate that during tumorigenic transformations, cells may generate mutations by themselves as a result of error-prone cell division with participation of error-prone polymerases and aberrant mitosis. These mechanisms may be activated in cells by continuing proliferative and survival signaling in a sustained stress environment (SSE). The paper hypothesizes that long-term exposure to this signaling epigenetically reprograms the genome of some cells and, in addition, leads to their senescence. The epigenetic reprogramming results in: (i) hypermethylation of tumor-suppressor genes involved in the onset of cell-cycle arrest, apoptosis and DNA repair; (ii) hypomethylation of proto-oncogenes associated with persistent proliferative activity; and (iii) the global demethylation of the genome and activation of DNA repeats. These epigenetic changes in the proliferating cells associate with their replicative senescence and allow the reprogrammed senescent cells to overcome the cell-cycle arrest and to activate error-prone replications. It is hypothesized that the generation of mutations in the error-prone replications of the epigenetically reprogrammed cells is not random. The mutations match epigenetic alterations in the cellular genome, namely gain of function mutations in the case of hypomethylation and loss of functions in the case of hypermethylation. In addition, continuing proliferation of the cells imposed by signaling in SSE speeds up the natural selection of the mutant cells favoring the survival of the cells with mutations that are beneficial in the environment. In this way, a stress-induced replication of the cells epigenetically reprograms their genome for quick adaptation to stressful environments providing an increased rate of mutations, epigenetic tags to beneficial mutations and quick selection process. In combination, these processes drive the origin of the transformed mammalian cells, cancer development and progression. Support from genomic, biochemical and medical studies for the proposed hypothesis, and its implementations are discussed.

The signals and pathways activating cellular senescence

Cellular senescence is a program activated by normal cells in response to various types of stress. These include telomere uncapping, DNA damage, oxidative stress, oncogene activity and others. Senescence can occur following a period of cellular proliferation or in a rapid manner in response to acute stress. Once cells have entered senescence, they cease to divide and undergo a series of dramatic morphologic and metabolic changes. Cellular senescence is thought to play an important role in tumor suppression and to contribute to organismal aging, but a detailed description of its physiologic occurrence in vivo is lacking. Recent studies have provided important insights regarding the manner by which different stresses and stimuli activate the signaling pathways leading to senescence. These studies reveal that a population of growing cells may suffer from a combination of different physiologic stresses acting simultaneously. The signaling pathways activated by these stresses are funneled to the p53 and Rb proteins, whose combined levels of activity determine whether cells enter senescence. Here we review recent advances in our understanding of the stimuli that trigger senescence, the molecular pathways activated by these stimuli, and the manner by which these signals determine the entry of a population of cells into senescence.

When cells get stressed: an integrative view of cellular senescence

Cells entering a state of senescence undergo a permanent cell cycle arrest, accompanied by a set of functional and morphological changes. Senescence of cells occurs following an extended period of proliferation in culture or in response to various physiologic stresses, yet little is known about the role this phenomenon plays in vivo. The study of senescence has focused largely on its hypothesized role as a barrier to extended cell division, governed by a division-counting mechanism in the form of telomere length. Here, we discuss the biological functions of cellular senescence and suggest that it should be viewed in terms of its role as a general cellular stress response program, rather than strictly as a barrier to unlimited cycles of cell growth and division. We also discuss the relative roles played by telomere shortening and telomere uncapping in the induction of senescence.

When cells get stressed: an integrative view of cellular senescence

Cells entering a state of senescence undergo a permanent cell cycle arrest, accompanied by a set of functional and morphological changes. Senescence of cells occurs following an extended period of proliferation in culture or in response to various physiologic stresses, yet little is known about the role this phenomenon plays in vivo. The study of senescence has focused largely on its hypothesized role as a barrier to extended cell division, governed by a division-counting mechanism in the form of telomere length. Here, we discuss the biological functions of cellular senescence and suggest that it should be viewed in terms of its role as a general cellular stress response program, rather than strictly as a barrier to unlimited cycles of cell growth and division. We also discuss the relative roles played by telomere shortening and telomere uncapping in the induction of senescence.

Genes of aging

According to developmental genetics theories, aging is a genetically programmed and controlled continuum of development and maturation. Being dynamic and malleable processes, development and aging are controlled not only by genes but also by environmental and epigenetic influences that predominate in the second half of life. Genetic mutations affect many phenotypes in flies, worms, rodents, and humans which share several diseases or their equivalents, including cancer, neurodegeneration, and infectious disorders as well as their susceptibility to them. Life span and stress resistance are closely linked. Oxidative stress actually constitutes a defined hypothesis of aging in that macromolecule oxidative damage accumulates with age and tends to be associated with life expectancy. DNA methylation, a force in the regulation of gene expression, is also one of the biomarkers of genetic damage. The mitotic clock of aging is marked, if not guided, by telomeres, essential genetic elements stabilizing natural chromosomic ends. The dream of humans to live longer, healthy lives is being tested by attempts to modify longevity in animal models, frequently by dietary manipulation. The quest continues to understand the mechanisms of healthy aging, one of the most compelling areas of research in the 21st century.