Telomere Dysfunction-induced Foci Research Articles

Late-life aerobic exercise reverses DNA damage and telomere dysfunction in non-atheroprone aortic regions with advanced age

Cellular senescence, a state of cell cycle arrest, contributes to arterial aging. The dominant factor driving cellular senescence is DNA damage. Moreover, telomeres, a repeated DNA sequence at the end of chromosomes, are especially vulnerable to DNA damage induced by DNA-damaging stimuli such as atheroprone shear stress. Although the adoption of aerobic exercise in later life is an effective strategy to mitigate arterial aging, its direct relationship with DNA damage and telomere dysfunction has yet to be elucidated. Additionally, the impact of late-life aerobic exercise, particularly in regions exposed to atheroprone and non-atheroprone shear stress, may not yield uniform benefits, requiring further exploration. We sought to investigate the effects of late-life aerobic exercise on DNA damage and telomere dysfunction in endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) in aortic regions exposed to distinct shear stress patterns such as the major aortic arch, minor aortic arch, and thoracic aorta. Fifteen male C57BL6 (22-23 mo) were given access to voluntary running wheel (VWR) for 4 weeks. They were classified as high-running (HR), moderate-running (MR), and low-running (LR) groups based on their habitual VWR distances (n=5 per group). Habitual VWR distance was greater in HR and MR compared to LR (75.9±11.2 and 21.3±1.6 vs. 5.8±2 m/day; P≤0.0003). Maximal treadmill running distance improved in HR, did not change in MR, and diminished in the LR (HR, MR vs. LR; P≤0.035) and was correlated to habitual VWR distance at 1-month post-intervention (R2=0.252, P=0.056). Immunofluorescence-fluorescent in situ hybridization was performed to detect the abundance of 53BP1 foci, a marker of DNA damage, and colocalization to telomeres, a measure of telomere-specific DNA damage known as telomere dysfunction-induced foci (TIF). The percentage of ECs containing 53BP1 foci and TIF was lower in the non-atheroprone aortic regions, i.e. major aortic arch and thoracic aorta, of HR compared to LR (P≤0.002), but there was no difference in the atheroprone, i.e. minor aortic arch (P≥0.899). A strong to moderate correlation was observed between 53BP1 foci and habitual VWR distance in major aortic arch and thoracic aorta (R2=0.584 and R2=0.54, P≤0.002) but only between VWR and TIF in the thoracic aorta (R2=0.356, P=0.019). The 53BP1 foci and TIF in VSMCs were comparable across groups in all aortic regions (P≥0.102). Likewise, Telomere length in ECs and VSMCs were similar across groups regardless of aortic region (P≥0.296). In conclusion, our findings suggest that increased level of habitual aerobic exercise in later life has a beneficial impact on DNA damage and telomere dysfunction in ECs, particularly in aortic regions exposed to extended periods of non-atheroprone shear stress. Notably, the volume of aerobic exercise was inversely associated with DNA damage and telomere dysfunction. These effects were independent of telomere length. The improvement in DNA damage and telomere dysfunction associated with habitual aerobic exercise were specific to ECs, as VSMCs remained unaffected in all aortic regions and results were independent of telomere length. Funded in part by awards from National Institutes of Health Awards R01 AG060395, R01 AG077751, R01 AG076748, T32 HL007576, T32 HL139451, Veteran's Affairs Merit Review Award I01 BX004492 and Nora Eccles Treadwell Foundation. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Effect of miR-181b Treatment on Senescent Human Aortic Smooth Muscle Cells

Large Artery Stiffness (LAS) is independently associated with increased cardiovascular morbidity and mortality and its development is multifactorial and has been associated with the onset of cellular senescence (CS). CS is defined as a stable cell cycle arrest, characterized by an altered cell phenotype with impaired function and detrimental secretory profile. Vascular smooth muscle cells (VSMCs) are present in two main phenotypes: the contractile phenotype seen in young populations, and the synthetic phenotype related to aging and senescent VSMCs. Micro-RNAs (miR) are non-coding RNAs that can alter protein expression. miR-181b has been demonstrated to undergo downregulation with aging with a close to 80% decrease, this correlates with an upregulation of markers of senescence. To test the hypothesis that miR-181b could modulate the senescent phenotype in VSMCs and promote a more contractile phenotype, a cellular model comprising young and old human aortic smooth muscle cells (HASMCs) was employed. These cells were transfected with either a miR-181b mimic or a scrambled miR control (SCR), resulting in an 11-fold increase in the young and a 3.3-fold increase in the old. Cell proliferation was measured using electric cell-substrate impedance sensing (ECIS) technology, where it was found to be lower in older HASMCs (1.05±0.001) compared to the young (1.1±0.001, p<0.05). When miR-181b was introduced, there was a significant increase in proliferation for both age groups by 0.03-fold compared to SCR (p<0.05). Old HASMCs demonstrated a decreased average ionomycin-induced contractile response (3.65%±0.46) when compared to the young (7.17%±0.60, p<0.05). Lipofectamine-mediated transfection of HASMCs with miR-181b resulted in an increased contractility response in the old HASMCs (5.46%±0.91) compared to scrambled control (1.41%±0.39, p<0.05), while there was no significant difference for the young cells. The percentage of Senescence-Associated-β-galactosidase (Saβ-gal) positive cells, a marker of senescence, was higher in old (39.74%±3.04) vs young (4.35%±0.72, p<0.05). The percentage of Saβ-gal positive cells tended to decrease in old VSMCs after miR-181b transfection (SCR 79.65%±4.38 vs Old181b 70.39±4.03, p=0.2.02), with no effect on young cells. Immunofluorescence in situ hybridization (IF-FISH) staining for the double-stranded DNA break marker (53BP1) was lower in the young (4.65%) compared to old HASMCs (32.69%). Likewise, the percentage of cells with one or more 53BP1 foci colocalization in telomeres, referred to as Telomere dysfunction-induced foci (TIF), was higher in old (11.54%) compared to young (2.33%) VSMCs (p=0.002). However, miR-181b transfection did not impact these markers regardless of age (p=0.94). Transfection of HASMCs with miR-181b did not impact senescent cell abundance, but improved contractility and cellular proliferation. Combined with our prior data demonstrating a senomorphic effect of miR-181b transfection to improve cellular stiffness and favorably modulate of senescence-associated secretory factors, our results demonstrate an enhancement in the phenotypic characteristics of the old HASMCs group, suggesting that miR-181b participates in the modulation of the senescent VSMCs phenotype. This translates to a potential improvement in HASMC health that could reduce the development of LAS. National Institutes of Health Awards: K08 AG070281 (ET), T32 HL139451 (DM), R01 AG048366 (LAL), R01 AG060395 (AJD), and Western Institute for Veterans Research (ET). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The RNA-binding motif protein 14 regulates telomere integrity at the interface of TERRA and telomeric R-loops.

Telomeric repeat-containing RNA (TERRA) and its formation of RNA:DNA hybrids (or TERRA R-loops), influence telomere maintenance, particularly in human cancer cells that use homologous recombination-mediated alternative lengthening of telomeres. Here, we report that the RNA-binding motif protein 14 (RBM14) is associated with telomeres in human cancer cells. RBM14 negatively regulates TERRA expression. It also binds to TERRA and inhibits it from forming TERRA R-loops at telomeres. RBM14 depletion has several effects, including elevated TERRA levels, telomeric R-loops, telomere dysfunction-induced DNA damage foci formation, particularly in the presence of DNA replication stress, pRPA32 accumulation at telomeresand telomere signal-free ends. Thus, RBM14 protects telomere integrity via modulating TERRA levels and its R-loop formation at telomeres.

ZBTB40 is a telomere-associated protein and protects telomeres in human ALT cells

Alternative lengthening of telomeres (ALTs) mechanism is activated in some somatic, germ cells, and human cancer cells. However, the key regulators and mechanisms of the ALT pathway remain elusive. Here we demonstrated that ZBTB40 is a novel telomere-associated protein and binds to telomeric dsDNA through its N-terminal BTB (BR-C, ttk and bab) or POZ (Pox virus and Zinc finger) domain in ALT cells. Notably, the knockout or knockdown of ZBTB40 resulted in the telomere dysfunction-induced foci and telomere lengthening in the ALT cells. The results also show that ZBTB40 is associated with ALT-associated promyelocytic leukemia nuclear bodies, and the loss of ZBTB40 induces the accumulation of the ALT-associated promyelocytic leukemia nuclear bodies in U2OS cells. Taken together, our results implicate that ZBTB40 is a key player of telomere protection and telomere lengthening regulation in human ALT cells.

Heat-Killed Staphylococcus aureus Induces Bone Mass Loss through Telomere Erosion

The mechanism of systemic osteoporosis caused by chronic infection is not completely clear, and there is a lack of reasonable interventions for this disease. In this study, heat-killed S. aureus (HKSA) was applied to simulate the inflammation caused by the typical clinical pathogen and to explore the mechanism of systemic bone loss caused by it. In this study, we found that the systemic application of HKSA caused bone loss in mice. Further exploration found that HKSA caused cellular senescence, telomere length shortening, and telomere dysfunction-induced foci (TIF) in limb bones. As a well-known telomerase activator, cycloastragenol (CAG) significantly alleviated HKSA-induced telomere erosion and bone loss. These results suggested that telomere erosion in bone marrow cells is a possible mechanism of HKSA-induced bone loss. CAG may protect against HKSA-induced bone loss by alleviating telomere erosion in bone marrow cells.

Natural Product Library Screens Identify Sanguinarine Chloride as a Potent Inhibitor of Telomerase Expression and Activity.

Reverse transcriptase hTERT is essential to telomerase function in stem cells, as well as in 85–90% of human cancers. Its high expression in stem cells or cancer cells has made telomerase/hTERT an attractive therapeutic target for anti-aging and anti-tumor applications. In this study, we screened a natural product library containing 800 compounds using an endogenous hTERT reporter. Eight candidates have been identified, in which sanguinarine chloride (SC) and brazilin (Braz) were selected due to their leading inhibition. SC could induce an acute and strong suppressive effect on the expression of hTERT and telomerase activity in multiple cancer cells, whereas Braz selectively inhibited telomerase in certain types of cancer cells. Remarkably, SC long-term treatment could cause telomere attrition and cell growth retardation, which lead to senescence features in cancer cells, such as the accumulation of senescence-associated β-galactosidase (SA-β-gal)-positive cells, the upregulation of p16/p21/p53 pathways and telomere dysfunction-induced foci (TIFs). Additionally, SC exhibited excellent capabilities of anti-tumorigenesis, both in vitro and in vivo. In the mechanism, the compound down-regulated several active transcription factors including p65, a subunit of NF-κB complex, and reintroducing p65 could alleviate its suppression of the hTERT/telomerase. Moreover, SC could directly bind hTERT and inhibit telomerase activity in vitro. In conclusion, we identified that SC not only down-regulates the hTERT gene’s expression, but also directly affects telomerase/hTERT. The dual function makes this compound an attractive drug candidate for anti-tumor therapy.

Enhanced radiosensitivity by 6-thio-dG via increasing telomere dysfunction and ataxia telangiectasia mutated inhibition in non-small cell lung cancer

ObjectiveTo investigate the radiosensitivity of 6-thio-dG and its underlying molecular mechanisms in non-small cell lung cancer (NSCLC). MethodsH1299 and A549 NSCLC cells were pretreated with 6-thio-dG for one week and then exposed to γ-irradiation. Cell proliferation and survival were quantified using clonogenic assays. DNA damage was assessed using immunofluorescence for γH2AX. Telomere dysfunction-induced foci analysis was performed by the co-localization of telomere signals (FISH) and γH2AX. Telomere fusion was defined as two telomere signals merged into one at the chromosome by immuno-FISH in metaphase spreads. Proteins related to the DNA damage response were detected using Western blot analysis. Apoptosis wasanalyzed using flow cytometry and Western blot. ResultsThe presence of 6-thio-dG increased the radio sensitivity of H1299 and A549 ​cells (P<0.05), but had no effect on the normal human lung fibroblast line, MRC5. 6-thio-dG pretreatment significantly reduced the clonogenic potential induced by γ-ray irradiation and aggravated genomic DNA and telomeric DNA damage (P<0.05). In addition, 6-thio-dG pretreatment effectively increased γ-ray irradiation induced telomere dysfunction (P<0.05), resulting in disruption of chromosome stability and inhibition of the ATM pathway, thereby impairing genomic DNA and telomeric DNA repair, which was closely associated with enhanced drug-mediated radiation-induced apoptosis. Conclusions6-thio-dG increases the radiosensitivity of NSCLC by inhibiting ATM and inducing telomere dysfunction, which can potentially be used as a strategy for radiotherapy for NSCLC.

Molecular Mechanisms of Alveolar Epithelial Stem Cell Senescence and Senescence-Associated Differentiation Disorders in Pulmonary Fibrosis.

Pulmonary senescence is accelerated by unresolved DNA damage response, underpinning susceptibility to pulmonary fibrosis. Recently it was reported that the SARS-Cov-2 viral infection induces acute pulmonary epithelial senescence followed by fibrosis, although the mechanism remains unclear. Here, we examine roles of alveolar epithelial stem cell senescence and senescence-associated differentiation disorders in pulmonary fibrosis, exploring the mechanisms mediating and preventing pulmonary fibrogenic crisis. Notably, the TGF-β signalling pathway mediates alveolar epithelial stem cell senescence by mechanisms involving suppression of the telomerase reverse transcriptase gene in pulmonary fibrosis. Alternatively, telomere uncapping caused by stress-induced telomeric shelterin protein TPP1 degradation mediates DNA damage response, pulmonary senescence and fibrosis. However, targeted intervention of cellular senescence disrupts pulmonary remodelling and fibrosis by clearing senescent cells using senolytics or preventing senescence using telomere dysfunction inhibitor (TELODIN). Studies indicate that the development of senescence-associated differentiation disorders is reprogrammable and reversible by inhibiting stem cell replicative senescence in pulmonary fibrosis, providing a framework for targeted intervention of the molecular mechanisms of alveolar stem cell senescence and pulmonary fibrosis. Abbreviations: DPS, developmental programmed senescence; IPF, idiopathic pulmonary fibrosis; OIS, oncogene-induced replicative senescence; SADD, senescence-associated differentiation disorder; SALI, senescence-associated low-grade inflammation; SIPS, stress-induced premature senescence; TERC, telomerase RNA component; TERT, telomerase reverse transcriptase; TIFs, telomere dysfunction-induced foci; TIS, therapy-induced senescence; VIS, virus-induced senescence.

Increase in lamin B1 promotes telomere instability by disrupting the shelterin complex in human cells.

Telomere maintenance is essential to preserve genomic stability and involves telomere-specific proteins, DNA replication and repair proteins. Lamins are key components of the nuclear envelope and play numerous roles, including maintenance of the nuclear integrity, regulation of transcription, and DNA replication. Elevated levels of lamin B1, one of the major lamins, have been observed in some human pathologies and several cancers. Yet, the effect of lamin B1 dysregulation on telomere maintenance remains unknown. Here, we unveil that lamin B1 overexpression drives telomere instability through the disruption of the shelterin complex. Indeed, lamin B1 dysregulation leads to an increase in telomere dysfunction-induced foci, telomeric fusions and telomere losses in human cells. Telomere aberrations were preceded by mislocalizations of TRF2 and its binding partner RAP1. Interestingly, we identified new interactions between lamin B1 and these shelterin proteins, which are strongly enhanced at the nuclear periphery upon lamin B1 overexpression. Importantly, chromosomal fusions induced by lamin B1 in excess were rescued by TRF2 overexpression. These data indicated that lamin B1 overexpression triggers telomere instability through a mislocalization of TRF2. Altogether our results point to lamin B1 as a new interacting partner of TRF2, that is involved in telomere stability.

Telomere uncapping as a possible mechanism for chemotherapy‐induced vascular toxicity

Cardiovascular toxicity is one of the detrimental side effects of chemotherapy that limits treatment. Specifically, chemotherapeutics such as doxorubicin (DOXO) can induce endothelial dysfunction, including reduced endothelial nitric oxide production, cellular senescence, and changes in genes that regulate telomeres, all of which may contribute to toxicity. Telomeres are DNA–protein structures that cap chromosome ends. When telomeres are damaged, they become uncapped and recognized as damaged DNA leading to cellular senescence. Importantly, telomere dysfunction is a biomarker of aging and is associated with numerous diseases including cardiovascular disease. To examine the effects of chemotherapy on vascular endothelial cells (ECs), we used two common chemotherapeutics, DOXO and etoposide (ETOP), both of which act as topoisomerase II inhibitors. Moreover, DOXO intercalates with DNA base pairs resulting in DNA strand breaks. However, whether chemotherapeutics induce telomere dysfunction associated with senescence and, therefore, promote endothelial dysfunction remains elusive. Here, we test the hypothesis that DOXO and ETOP induce endothelial cell telomere uncapping using doses of DOXO (0.1uM) and ETOP (10uM) shown by us and others to induce senescence. We examined telomere uncapping using colocalization of 53BP1, a double-stranded DNA (dsDNA) break marker, and telomeres, by performing immunofluorescence fluorescent in situ hybridization (IF-FISH) on human umbilical vein ECs. At confluence, ECs were treated with either DOXO or ETOP for 24 hours, and 24 and 48 hours later, ECs were collected for RNA isolation or were fixed for IF-FISH. To evaluate senescence, we examined mRNA expression of the senescence marker p21. p21 expression tended to be elevated after 24 hours of treatment (both p > 0.09) and was significantly elevated 48 hours after both DOXO and ETOP treatment (Fig. 1, p<0.001). These results corroborate prior findings that chemotherapeutics such as DOXO and ETOP induce senescence. Twenty-four hours post-treatment, the percentage of cells with ≥ 1 dsDNA break was elevated to 72.5 ± 6.3% in DOXO (n=175 cells) and 70.4 ± 13.6% in ETOP (n=97 cells) treated ECs compared to 13.2 ± 4.8% in the vehicle control group (n=310 cells, Fig. 2a, both p<0.0001). Moreover, the percentage of cells with ≥ 1 Telomere dysfunction-induced Foci (TIF) also increased to 46.6 ± 5.3% in DOXO treated ECs, and 52.2 ± 9.5% in ETOP treated ECs, compared to 7.6 ± 2.8% in the vehicle control group (Fig. 2b, p<0.0001). To determine if dsDNA breaks and TIFs were associated with DOXO and ETOP induced EC senescence, we performed a bivariate correlation analysis. For both treatments, the percentage of ECs with ≥ 1 dsDNA break and ≥ 1 TIF were correlated to p21 mRNA expression at 48 hours post treatment (DOXO: r=0.77 and r=0.78, p<0.01; ETOP: r=0.88 and r=0.91, all p<0.01). These data suggest that DOXO and ETOP may lead to senescence induced endothelial dysfunction via telomere uncapping, providing mechanistic insight into the vascular consequences of chemotherapy.

SIRT6 enhances telomerase activity to protect against DNA damage and senescence in hypertrophic ligamentum flavum cells from lumbar spinal stenosis patients.

Lumbar spinal stenosis (LSS) is a condition wherein patients exhibit age-related fibrosis, elastin-to-collagen ratio reductions, and ligamentum flavum hypertrophy. This study was designed to assess the relationship between SIRT6 and telomerase activity in hypertrophic ligamentum flavum (LFH) cells from LSS patients. We observed significant reductions in SIRT6, TPP1, and POT1 protein levels as well as increases in telomerase reverse transcriptase (TERT) levels and telomerase activity in LFH tissues relative to non- hypertrophic ligamentum flavum (LFN) tissues. When SIRT6 was overexpressed in these LFH cells, this was associated with significant increases in telomerase activity and a significant reduction in fibrosis-related protein expression. These effects were reversed, however, when telomerase activity was inactivated by hTERT knockdown in these same cells. SIRT6 overexpression was further found to reduce the frequency of senescence-associated β-galactosidase (SA-β-Gal)-positive LFH cells and to decrease p16, MMP3, and L1 mRNA levels and telomere dysfunction-induced foci (TIFs) in LFH cells. In contrast, hTERT knockdown-induced telomerase inactivation eliminated these SIRT6-dependent effects. Overall, our results indicate that SIRT6 functions as a key protective factor that prevents cellular senescence and telomere dysfunction in ligamentum flavum cells, with this effect being at least partially attributable to SIRT6-dependent telomerase activation.

Telomere Dysfunction in Oocytes and Embryos From Obese Mice.

Maternal obesity impairs oocyte quality and embryo development. However, the potential molecular pathways remain to be explored. In the present study, we examined the effects of obesity on telomere status in oocytes and embryos obtained from mice fed with high-fat diet (HFD). Of note, telomere shortening was observed in both oocytes and early embryos from obese mice, as evidenced by the reduced expression of telomerase reverse transcriptase and activity of telomerase. Moreover, quantitative analysis of telomere dysfunction-induced foci (TIFs) revealed that maternal obesity induces the defective telomeres in oocytes and embryos. Meanwhile, the high frequency of aneuploidy was detected in HFD oocytes and embryos as compared to controls, accompanying with the increased incidence of apoptotic blastocysts. In conclusion, these results indicate that telomere dysfunction might be a molecular pathway mediating the effects of maternal obesity on oocyte quality and embryo development.

A DNA Damage Response-Independent Mechanism for Telomere Shortening-Elicited Age-Related Pathologies

Telomere attrition is associated with telomeropathies and age-related pathologies. In telomeropathies, telomere uncapping induces a DNA damage response (DDR) that drives apoptosis or senescence. However, a defined mechanism by which telomere attrition contributes to other age-related pathologies has not been determined. Telomere integrity is maintained by shelterin, a six-protein complex. Rap1 is the only shelterin member that is not essential for telomere capping but engages non-telomeric DNA and regulates gene transcription. We hypothesized that non-telomeric Rap1 accumulation could contribute to age-related pathologies in a DDR-independent manner. To test this, we used CRISPR/Cas9 editing to generate a Rap1 mutant mouse model in which Rap1 at telomeres is prevented, leaving only non-telomeric Rap1. Indirect immunostaining showed no differences in telomere dysfunction-induced DDR foci in Rap1 mutant compared to wild-type primary fibroblasts. Cell fractionation/western blotting of fibroblasts from Rap1 mutants demonstrated decreased Rap1 expression and Rap1 re-localization off telomeres, which mimics the same alteration of Rap1 in human cells with telomere attrition. Rap1 mutant mice exhibited increased body weight and altered metabolic and immune-response transcripts in various tissues, indicating that altered transcription could account for some of the observed phenotypes related to telomere attrition. In conclusion, telomere shortening may facilitate non-telomeric Rap1, which alters gene transcription and drives metabolic and immune dysfunction in a DDR-independent manner.

MiR-490 suppresses telomere maintenance program and associated hallmarks in glioblastoma.

Glioblastoma (GBM) is the most aggressive cancer of central nervous system with worst patient outcome. Telomere maintenance is a crucial mechanism governing GBM initiation and progression making it an attractive target. microRNAs (miRNAs) have shown therapeutic potential in GBM. Earlier, we showed miR-490 is downregulated in GBM patients and plays a tumor suppressive role. Here, we show that miR-490 regulates telomere maintenance program in GBM by directly targeting Telomeric Repeat-binding Factor 2 (TERF2) of the shelterin complex, Tankyrase 2 (TNKS2) and Serine/Threonine-protein kinase, SMG1. Overexpression of miR-490 resulted in effects characteristic to hampered telomere maintenance via TERF2 inhibition. These include induction of telomere dysfunction-induced foci and global DNA damage (53BP1 foci), along with an increase in p-γH2AX levels. Further, it led to inhibition of telomere maintenance hallmarks via reduced stemness (SOX2 and SOX4 downregulation) and induction of senescence (H3K9me3 marks gain and SIRT1 downregulation). It also initiated downstream DNA damage response (DDR) leading to p53 pathway activation. Moreover, microarray data analysis highlighted an overlap between miR-490expression and REST-inhibition responses in GBM. Thus, miR-490-mediated targeting of telomere maintenance could be therapeutically important in GBM.

Establishing a density-based method to separate proliferating and senescent cells from bone marrow stromal cells.

To assist in the study of cellular aging, we established a new method of enriching physiologically aged bone marrow stromal cells (BMSCs) in animals of any age using a Percoll density gradient centrifugation technique. BMSCs from mice 2 months of age were isolated, and their cellular age determined (over 80% Scal-1+ CD29+ CD11b- CD45- CD105- and able to differentiate into osteoblasts, adipocytes, and chondrocytes). As proof –of principle, cells were aged in vitro and confirmed by low bromodeoxyuridine (BrdU) incorporation and senescence-associated β-galactosidase (SA-β-gal) staining. Proliferating cells were enriched in high-density gradient layers, and senescent cells were enriched in low-density gradient layers. We confirmed that over 80% of cells from the low-density gradient layer were senescent with SA-β-gal staining and telomere dysfunction-induced foci (TIF) assay. This density-based method, which can separate proliferating and senescent BMSCs, could be used to study mechanisms of physiologic cell aging and may have implications for the use of BMSCs in clinical transplant applications.

MiR-185 targets POT1 to induce telomere dysfunction and cellular senescence.

Protection of telomere 1 (POT1), the telomeric single-stranded DNA (ssDNA)-binding protein in the shelterin complex, has been implicated in the DNA damage response, tumorigenesis and aging. Telomere dysfunction induced by telomere deprotection could accelerate cellular senescence in primary human cells. While previous work demonstrated the biological mechanism of POT1 in aging and cancer, how POT1 is posttranscriptionally regulated remains largely unknown. To better understand the POT1 regulatory axis, we performed bioinformatic prediction, and selected candidates were further confirmed by dual-luciferase reporter assay. Collectively, our results revealed that miR-185 can significantly reduce POT1 mRNA and protein levels by directly targeting the POT1 3’-untranslated region (3’-UTR). Overexpression of miR-185 increased telomere dysfunction-induced foci (TIF) signals in both cancer cells and primary human fibroblasts. Elevated miR-185 led to telomere elongation in the telomerase-positive cell line HTC75, which was phenotypically consistent with POT1 knocking down. Moreover, miR-185 accelerated the replicative senescence process in primary human fibroblasts in a POT1-dependent manner. Interestingly, increased serum miR-185 could represent a potential aging-related biomarker. Taken together, our findings reveal miR-185 as a novel aging-related miRNA that targets POT1 and provide insight into the telomere and senescence regulatory network at both the intracellular and extracellular levels.

SMCHD1 promotes ATM-dependent DNA damage signaling and repair of uncapped telomeres.

Structural maintenance of chromosomes flexible hinge domain-containing protein 1 (SMCHD1) has been implicated in X-chromosome inactivation, imprinting, and DNA damage repair, and mutations in SMCHD1 can cause facioscapulohumeral muscular dystrophy. More recently, SMCHD1 has also been identified as a component of telomeric chromatin. Here, we report that SMCHD1 is required for DNA damage signaling and non-homologous end joining (NHEJ) at unprotected telomeres. Co-depletion of SMCHD1 and the shelterin subunit TRF2 reduced telomeric 3'-overhang removal in time-course experiments, as well as the number of chromosome end fusions. SMCHD1-deficient cells displayed reduced ATM S1981 phosphorylation and diminished formation of γH2AX foci and of 53BP1-containing telomere dysfunction-induced foci (TIFs), indicating defects in DNA damage checkpoint signaling. Removal of TPP1 and subsequent activation of ATR signaling rescued telomere fusion events in TRF2-depleted SMCHD1 knockout cells. Together, these data indicate that SMCHD1 depletion reduces telomere fusions in TRF2-depleted cells due to defects in ATM-dependent checkpoint signaling and that SMCHD1 mediates DNA damage response activation upstream of ATM phosphorylation at uncapped telomeres.

Targeted Reduction of Senescent Cell Burden Alleviates Focal Radiotherapy-Related Bone Loss.

Clinical radiotherapy treats life-threatening cancers, but the radiation often affects neighboring normal tissues including bone. Acute effects of ionizing radiation include oxidative stress, DNA damage, and cellular apoptosis. We show in this study that a large proportion of bone marrow cells, osteoblasts, and matrix-embedded osteocytes recover from these insults only to attain a senescent profile. Bone analyses of senescence-associated genes, senescence-associated beta-galactosidase (SA-β-gal) activity, and presence of telomere dysfunction-induced foci (TIF) at 1, 7, 14, 21, and 42 days post-focal radiation treatment (FRT) in C57BL/6 male mice confirmed the development of senescent cells and the senescence-associated secretory phenotype (SASP). Accumulation of senescent cells and SASP markers were correlated with a significant reduction in bone architecture at 42 days post-FRT. To test if senolytic drugs, which clear senescent cells, alleviate FRT-related bone damage, we administered the senolytic agents, dasatinib (D), quercetin (Q), fisetin (F), and a cocktail of D and Q (D+Q). We found moderate alleviation of radiation-induced bone damage with D and Q as stand-alone compounds, but no such improvement was seen with F. However, the senolytic cocktail of D+Q reduced senescent cell burden as assessed by TIF+ osteoblasts and osteocytes, markers of senescence (p16 Ink4a and p21), and key SASP factors, resulting in significant recovery in the bone architecture of radiated femurs. In summary, this study provides proof of concept that senescent cells play a role in radiotherapy-associated bone damage, and that reduction in senescent cell burden by senolytic agents is a potential therapeutic option for alleviating radiotherapy-related bone deterioration. © 2020 American Society for Bone and Mineral Research.

BIBR1532, a Selective Telomerase Inhibitor, Enhances Radiosensitivity of Non-Small Cell Lung Cancer Through Increasing Telomere Dysfunction and ATM/CHK1 Inhibition

Telomerase is reactivated in non-small cell lung cancer (NSCLC), and it increases cell resistance to irradiation through protecting damaged telomeres and enhancing DNA damage repair. We investigated the radiosensitizing effect of BIBR1532, a highly selective telomerase inhibitor, and its corresponding mechanism in NSCLC. Cell proliferation, telomerase activity, and telomere dysfunction-induced foci were measured with CCK-8 assay, real-time fluorescent quantitative polymerase chain reaction, and immunofluorescence. The effect of BIBR1532 on the response of NSCLC cells to radiation was analyzed using clonogenic survival and xenograft tumor assays. Cell death and cell senescence induced by BIBR1532 or ionizing radiation (IR), or both, were detected with western blotting, flow cytometry, and senescence-association β-galactosidase staining assay. We observed dose-dependent direct cytotoxicity of BIBR1532 at relatively high concentrations in NSCLC cells. Low concentrations of BIBR1532 did not appear toxic to NSCLC cells; however, they substantially increased the therapeutic efficacy of IR invitro by enhancing IR-induced apoptosis, senescence, and mitotic catastrophe. Moreover, in a mouse xenograft model, BIBR1532 treatment synergized with IR at nontoxic dose levels promoted the antitumor efficacy of IR without toxicity to hematologic and internal organs. Mechanistically, lower concentrations of BIBR1532 effectively inhibited telomerase activity and increased IR-induced telomere dysfunction, resulting in disruption of chromosomal stability and inhibition of the ATM/CHK1 (ataxia-telangiectasia-mutated/Checkpoint kinase 1) pathway, which impaired DNA damage repair. Our findings demonstrate that disturbances in telomerase function by nontoxic dose levels of BIBR1532 effectively enhance the radiosensitivity of NSCLC cells. This finding provides a rationale for the clinical assessment of BIBR1532 as a radiosensitizer.

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

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