Stems Cells and the Pathways to Aging and Cancer

Determining the influence of telomere dysfunction and DNA damage on stem and progenitor cell aging: what markers can we use?

The decline in organ maintenance and function is one of the major problems limiting quality of life during aging. The accumulation of telomere dysfunction and DNA damage appears to be one of the underlying causes. Uncapping of chromosome ends in response to critical telomere shortening limits the proliferative capacity of human cells by activation of DNA damage checkpoints inducing senescence or apoptosis. Telomere shortening occurs in the vast majority of human tissues during aging and in chronic diseases that increase the rate of cell turnover. There is emerging evidence that telomere shortening can limit the maintenance and function of adult stem cells -- a cell type of utmost importance for organ maintenance and regeneration. In mouse models, telomere dysfunction leads to a depletion of adult stem cell compartments suggesting that stem cells are very sensitive to DNA damage. Both the rarity of stem and progenitor cells in adult organs and their removal in response to damage make it difficult to assess the impact of telomere dysfunction and DNA damage on stem and progenitor cell aging. Such approaches require the development of sensitive biomarkers recognizing low levels of telomere dysfunction and DNA damage in stem and progenitor cells. Here, we review experimental data on the prevalence of telomere dysfunction and DNA damage during aging and its possible impact on stem and progenitor cell aging.

Normal ovarian surface epithelial label-retaining cells exhibit stem/progenitor cell characteristics

Ovulation induces cyclic rupture and regenerative repair of the ovarian coelomic epithelium. This process of repeated disruption and repair accompanied by complex remodeling typifies a somatic stem/progenitor cell-mediated process. Using BrdU incorporation and doxycycline inducible histone2B-green fluorescent protein pulse-chase techniques, we identify a label-retaining cell population in the coelomic epithelium of the adult mouse ovary as candidate somatic stem/progenitor cells. The identified population exhibits quiescence with asymmetric label retention, functional response to estrous cycling in vivo by proliferation, enhanced growth characteristics by in vitro colony formation, and cytoprotective mechanisms by enrichment for the side population. Together, these characteristics identify the label-retaining cell population as a candidate for the putative somatic stem/progenitor cells of the coelomic epithelium of the mouse ovary.

Endocytosis and cancer: an 'insider' network with dangerous liaisons

From the signaling point of view, endocytosis has long been regarded as a major mechanism of attenuation, through the degradation of signaling receptors and, in some cases, of their ligands. This outlook has changed, over the past decade, as it has become clear that signaling persists in the endocytic route, and that intracellular endocytic stations (the 'signaling endosomes') actually contribute to the sorting of signals in space and time. Endocytosis-mediated recycling of receptors and of signaling molecules to specific regions of the plasma membrane is also coming into focus as a major mechanism in the execution of spatially restricted functions, such as cell motility. In addition, emerging evidence connects endocytosis as a whole, or individual endocytic proteins, to complex cellular programs, such as the control of the cell cycle, mitosis, apoptosis and cell fate determination. Thus, endocytosis seems to be deeply ingrained into the cell regulation blueprint and its subversion is predicted to play an important role in human diseases: first and foremost, cancer.

Failure of CDKN2A/B (INK4A/B-ARF)-mediated tumor suppression and resistance to targeted therapy in acute lymphoblastic leukemia induced by BCR-ABL

Deletions of the CDKN2A/B tumor suppressor locus and of the IKAROS and PAX5 genes that promote B-lineage development occur frequently in lymphoid, but not myeloid leukemias initiated by the BCR-ABL tyrosine kinase. Why is this the case, and how do these genetic lesions contribute to an aggressive disease that fails to durably respond to targeted kinase inhibitors?

HOX expression patterns identify a common signature for favorable AML

Deregulated HOX expression, by chromosomal translocations and myeloid-lymphoid leukemia (MLL) rearrangements, is causal in some types of leukemia. Using real-time reverse transcription-PCR, we examined the expression of 43 clustered HOX, polycomb, MLL and FLT3 genes in 119 newly diagnosed adult acute myeloid leukemias (AMLs) selected from all major cytogenetic groups. Downregulated HOX expression was a consistent feature of favorable AMLs and, among these cases, inv(16) cases had a distinct expression profile. Using a 17-gene predictor in 44 additional samples, we observed a 94.7% specificity for classifying favorable vs intermediate/unfavorable cytogenetic groups. Among other AMLs, HOX overexpression was associated with nucleophosmin (NPM) mutations and we also identified a phenotypically similar subset with wt-NPM. In many unfavorable and other intermediate cytogenetic AMLs, HOX levels resembled those in normal CD34+ cells, except that the homogeneity characteristic of normal samples was not present. We also observed that HOXA9 levels were significantly inversely correlated with survival and that BMI-1 was overexpressed in cases with 11q23 rearrangements, suggesting that p19(ARF) suppression may be involved in MLL-associated leukemia. These results underscore the close relationship between HOX expression patterns and certain forms of AML and emphasize the need to determine whether these differences play a role in the disease process.

Age-related upregulation of Drosophila caudal gene via NF-kappaB in the adult posterior midgut

The Drosophila midgut has emerged as a powerful model system for the investigation of fundamental cellular pathways relevant to intestinal stem cell biology. Understanding the age-related changes in the adult Drosophila midgut may provide insights into the molecular mechanisms that link aging to the modulation of adult stem cell population. The caudal-related homeobox genes encode intestine-specific transcription factors required for normal intestinal development and maintenance. Here, we demonstrate that caudal gene expression is upregulated in the adult posterior midgut in response to age and oxidative stress, and that overexpression of Caudal can stimulate cell proliferation in the adult posterior midgut. We further demonstrate that the age- and oxidative-stress-related upregulation of the caudal gene is mediated by the NF-kappaB binding site located in the 5'-flanking region of the caudal gene. Our results may contribute to an understanding of the mechanisms of age-related changes in the number and activity of intestinal stem cells and progenitors in the Drosophila adult midgut.

Antidepressants and Cdk inhibitors: releasing the brake on neurogenesis?

It is now clear that neurogenesis occurs in the brain of adult mammals. Many studies have attempted to establish relationships among neurogenesis, depression and the mechanism of action of antidepressant drugs. Therapeutic effects of antidepressants appear to be linked to increased neurogenesis in the hippocampus. Cdk inhibitors are expressed in multiple brain regions, presumably maintaining quiescence in differentiated neurons. Recently, the abundant expression of p21(Cip1) was found in neuroblasts and in newly developing neurons in the subgranular zone of the hippocampus, a region where adult neurogenesis occurs. Chronic treatment with the tricyclic antidepressant imipramine markedly decreased p21(Cip1) mRNA and protein levels and stimulated neurogenesis in this region. These results suggest that p21(Cip1) restrains neurogenesis in the hippocampus, and antidepressant-induced stimulation of neurogenesis might be a consequence of decreased p21(Cip1) expression, with the subsequent release of neuronal progenitor cells from the blockade of proliferation. These findings suggest the potential for new therapeutic strategies for the treatment of depression that target cell cycle proteins. However, there is a possibility that long-term stimulation of neurogenesis might exhaust the proliferation potentials of neuronal progenitors.

Aging and disease as modifiers of efficacy of cell therapy

Cell therapy is a promising option for treating ischemic diseases and heart failure. Adult stem and progenitor cells from various sources have experimentally been shown to augment the functional recovery after ischemia, and clinical trials have confirmed that autologous cell therapy using bone marrow-derived or circulating blood-derived progenitor cells is safe and provides beneficial effects. However, aging and risk factors for coronary artery disease affect the functional activity of the endogenous stem/progenitor cell pools, thereby at least partially limiting the therapeutic potential of the applied cells. In addition, age and disease affect the tissue environment, in which the cells are infused or injected. The present review article will summarize current evidence for cell impairment during aging and disease but also discuss novel approaches how to reverse the dysfunction of cells or to refresh the target tissue. Pretreatment of cells or the target tissue by small molecules, polymers, growth factors, or a combination thereof may provide useful approaches for enhancement of cell therapy for cardiovascular diseases.

Stem cells and aging in the hematopoietic system

The effector cells of the blood have limited lifetimes and must be replenished continuously throughout life from a small reserve of hematopoietic stem cells (HSCs) in the bone marrow. Although serial bone marrow transplantation experiments in mice suggest that the replicative potential of HSCs is finite, there is little evidence that replicative senescence causes depletion of the stem cell pool during the normal lifespan of either mouse or man. Studies conducted in murine genetic models defective in DNA repair, intracellular ROS management, and telomere maintenance indicate that all these pathways are critical to the longevity and stress response of the stem cell pool. With age, HSCs show an increased propensity to differentiate towards myeloid rather than lymphoid lineages, which may contribute to the decline in lymphopoiesis that attends aging. Challenges for the future include assessing the significance of 'lineage skewing' to immune dysfunction, and investigating the role of epigenetic dysregulation in HSC aging.

Replicative stress, stem cells and aging

DNA synthesis is a remarkably vulnerable phase in the cell cycle. In addition to introduction of errors during semi-conservative replication, the inherently labile structure of the replication fork, as well as numerous pitfalls encountered in the course of fork progression, make the normally stable double stranded molecule susceptible to collapse and recombination. As described in this issue, maintenance of genome integrity in the face of such events is essential to prevent the premature onset of age-related diseases. At the organismal level, the roles for such maintenance are numerous; however, the preservation of stem and progenitor cell pools may be particularly important as indicated by several genetically engineered mouse models. Stresses on stem and progenitor cell pools, in the form of telomere shortening (Terc(-/-)) or other genome maintenance failures (ATR(mKO), Ku86(-/-), LIG4(Y288C), XPD(R722W/R722W), etc.), have been shown to degrade tissue renewal capacity and accelerate the appearance of age-related phenotypes. In the case of telomere shortening, exhaustion of replicative potential appears to be at least partially dependent on the cell cycle regulatory component of the DNA damage response. Therefore, both the genome maintenance mechanisms that counter DNA damage and the cell cycle checkpoint responses to damage strongly influence the onset of age-related diseases and do so, at least in part, by affecting long-term stem and progenitor cell potential.