Telomere Dysfunction and Senescence in Stem Cell and Tissues Aging

Amyloid β and Tumorigenesis: Amyloid β-Induced Telomere Dysfunction in Tumor Cells

Stem cells are believed to be closely associated with tissue degeneration during aging. Studies of human genetic diseases and gene-targeted animal models have provided evidence that functional decline of telomeres and deregulation of cell cycle checkpoints contribute to the aging process of tissue stem cells. Telomere dysfunction can induce DNA damage response via key cell cycle checkpoints, leading to cellular senescence or apoptosis depending on the tissue type and developmental stage of a specific stem cell compartment. Telomerase mutation and telomere shortening have been observed in a variety of hematological disorders, such as dyskeratosis congenital, aplastic anemia, myelodysplastic syndromes and leukemia, in which the hematopoietic stem cells (HSC) are a major target during the pathogenesis. Moreover, telomere dysfunction is able to induce both cell-intrinsic checkpoints and environmental factors limiting the self-renewal capacity and differentiation potential of HSCs. Crucial components in the cascade of DNA damage response, including ataxia telangiectasia mutated, CHK2, p53, p21 and p16/p19(ARF), play important roles in HSC maintenance and self-renewal in the scenarios of both sufficient telomere reserve and dysfunctional telomere. Therefore, a further understanding of the molecular mechanisms underlying HSC aging may help identity new therapeutic targets for stem cell-based regenerative medicine.

Effects of hydrogen peroxide, doxorubicin and ultraviolet irradiation on senescence of human dental pulp stem cells

Telomere Shortening Promotes Chromosomal Instability and Predicts Malignant Clonal Evolution in Aplastic Anemia.

<div>Abstract<p><b>Purpose:</b> BRAF is frequently activated by gene fusion or point mutation in pilocytic astrocytoma, the most common pediatric brain tumor. We investigated the functional effect of constitutive BRAF activation in normal human neural stem and progenitor cells to determine its role in tumor induction in the brain.</p><p><b>Experimental Design:</b> The constitutively active BRAF<sup>V600E</sup> allele was introduced into human neurospheres, and its effects on MAPK (mitogen-activated protein kinase) signaling, proliferation, soft agarose colony formation, stem cell phenotype, and induction of cellular senescence were assayed. Immunohistochemistry was used to examine p16<sup>INK4a</sup> levels in pilocytic astrocytoma.</p><p><b>Results:</b> BRAF<sup>V600E</sup> expression initially strongly promoted colony formation but did not lead to significantly increased proliferation. BRAF<sup>V600E</sup>-expressing cells subsequently stopped proliferating and induced markers of oncogene-induced senescence including acidic β-galactosidase, PAI-1, and p16<sup>INK4a</sup> whereas controls did not. Onset of senescence was associated with decreased expression of neural stem cell markers including SOX2. Primary pilocytic astrocytoma cultures also showed induction of acidic β-galactosidase activity. Immunohistochemical examination of 66 pilocytic astrocytomas revealed p16<sup>INK4a</sup> immunoreactivity in the majority of cases, but patients with tumors negative for p16<sup>INK4a</sup> had significantly shorter overall survival.</p><p><b>Conclusions:</b> BRAF activation in human neural stem and progenitor cells initially promotes clonogenic growth in soft agarose, suggesting partial cellular transformation, but oncogene-induced senescence subsequently limits proliferation. Induction of senescence by BRAF may help explain the low-grade pathobiology of pilocytic astrocytoma, whereas worse clinical outcomes associated with tumors lacking p16<sup>INK4a</sup> expression could reflect failure to induce senescence or an escape from oncogene-induced senescence. <i>Clin Cancer Res; 17(11); 3590–9. ©2011 AACR</i>.</p></div>

Cellular senescence prevents the proliferation of cells at risk for neoplastic transformation. Nonetheless, the senescence response is thought to be antagonistically pleiotropic and thus contribute to aging phenotypes, including, ironically, late life cancers. The cancer-promoting activity of senescent cells is likely due to secreted molecules, the identity of which remains largely unknown. Here, we have shown that senescent fibroblasts, much more than presenescent fibroblasts, stimulate tumor vascularization in mice. Weakly malignant epithelial cells co-injected with senescent fibroblasts had larger and greater numbers of blood vessels compared with controls. Accordingly, increased vascular endothelial growth factor (VEGF) expression was a frequent characteristic of senescent human and mouse fibroblasts in culture. Importantly, conditioned medium from senescent fibroblasts, more than medium from presenescent cells, stimulates cultured human umbilical vein endothelial cells to invade a basement membrane, a hallmark of angiogenesis. Increased VEGF expression was specific to the senescent phenotype and increased whether senescence was induced by replicative exhaustion, overexpression of p16(Ink4a), or overexpression of oncogenic RAS. The senescence-dependent increase in VEGF production was accompanied by very little increase in hypoxic-inducible (transcription) factor 1 alpha protein levels, and hypoxia further induced VEGF in senescent cells. This result suggests the rise in VEGF expression at senescence is not a hypoxic response. Our findings may in part explain why senescent cells stimulate tumorigenesis in vivo and support the idea that senescent cells may facilitate age-associated cancer development by secreting factors that promote malignant progression.

Cellular senescence in dental pulp stem cells

Chapter Eight - Telomeres in Lung Diseases

From cells to organisms: can we learn about aging from cells in culture?

BackgroundChronic kidney disease (CKD) is highly prevalent worldwide, and its global burden is substantial and growing. CKD displays a number of features of accelerated senescence. Tubular cell senescence is a common biological process that contributes to CKD progression. Tubulointerstitial inflammation is a driver of tubular cell senescence and a common characteristic of CKD. However, the mechanism by which the interstitial inflammation drives tubular cell senescence remains unclear. This paper aims to explore the role of exosomal miRNAs derived from macrophages in the development of tubular cell senescence.MethodsAmong the identified inflammation-related miRNAs, miR-155 is considered to be one of the most important miRNAs involved in the inflammatory response. Macrophages, the primary immune cells that mediate inflammatory processes, contain a high abundance of miR-155 in their released exosomes. We assessed the potential role of miR-155 in tubular cell senescence and renal fibrosis. We subjected miR-155−/− mice and wild-type controls, as well as tubular epithelial cells (TECs), to angiotensin II (AngII)-induced kidney injury. We assessed kidney function and injury using standard techniques. TECs were evaluated for cell senescence and telomere dysfunction in vivo and in vitro. Telomeres were measured by the fluorescence in situ hybridization.ResultsCompared with normal controls, miR-155 was up-regulated in proximal renal tubule cells in CKD patients and mouse models of CKD. Moreover, the expression of miR-155 was positively correlated with the extent of renal fibrosis, eGFR decline and p16INK4A expression. The overexpression of miR-155 exacerbated tubular senescence, evidenced by increased detection of p16INK4A/p21expression and senescence-associated β-galactosidase activity. Notably, miR-155 knockout attenuates renal fibrosis and tubule cell senescence in vivo. Interestingly, once released, macrophages-derived exosomal miR-155 was internalized by TECs, leading to telomere shortening and dysfunction through targeting TRF1. A dual-luciferase reporter assay confirmed that TRF1 was the direct target of miR-155. Thus, our study clearly demonstrates that exosomal miR-155 may mediate communication between macrophages and TECs, subsequently inducing telomere dysfunction and senescence in TECs.ConclusionsOur work suggests a new mechanism by which macrophage exosomes are involved in the development of tubule senescence and renal fibrosis, in part by delivering miR-155 to target TRF1 to promote telomere dysfunction. Our study may provide novel strategies for the treatment of AngII-induced kidney injury.

There is growing evidence that the accumulation of DNA damage and telomere dysfunction contributes to declines in stem cell function and tissue maintenance during aging and in response to chronic disease. Checkpoint responses that mediate the decline in stem cell function in response to DNA damage remain yet to be delineated. Here we conducted a functional genomic in vivo screen to identify genes impairing the function of hematopoietic stem cells in the context of chronic DNA damage induced by telomere dysfunction. Screening a shRNA library of 1000 cancer related genes revealed that shRNA mediated knock down of the transcription factor Batf was most potent to rescue the function of telomere dysfunctional stem cells. Specifically, Batf knockdown was 500fold enriched in telomere dysfunctional HSCs compared to wildtype HSCs when transplanted into lethally irradiated recipients. Our study shows that Batf is strongly upregulated in HSCs in response to telomere dysfunction or gamma-irradiation induced DNA damage and during mouse aging. Upregulation of Batf in HSCs in response to DNA damage was G-CSF/Stat3 dependent. shRNA-mediated knockdown of Batf rescued the repopulation capacity of telomere dysfunctional stem cells in transplanted recipients. This rescue in HSC function was associated with impaired induction of lymphocyte differentiation markers, cell cycle inhibitors (p53, p21, p16), and apoptosis in telomere dysfunctional or gamma-irradiated HSCs. Irradiation experiments revealed that lymphoid competent HSCs (CD150lowCD34-KSL) were most sensitive to DNA damage induced stem cell depletion, which was rescued by shRNA mediated knockdown of Batf. Together, this study provides the first experimental evidence that DNA damage leads to induction of a Batf-dependent differentiation checkpoint limiting the repopulation capacity of HSCs and the self-renewal of lymphoid

Telomeres in cancer

Telomerase gene therapy rescues telomere length, bone marrow aplasia, and survival in mice with aplastic anemia

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

Localization of αII Spectrin to Telomeres After DNA Interstrand Cross-Link Damage Is Deficient In Fanconi Anemia Group A Cells, Which May Lead to Telomere Dysfunction.