DNA damage: a sensible mediator of the differentiation decision in hematopoietic stem cells and in leukemia

Stress-induced modification of Escherichia coli tRNA generates 5-methylcytidine in the variable loop

There has been recent interest in trying to understand the connection between transfer RNA (tRNA) posttranscriptional modifications and changes in-cellular environmental conditions. Here, we report on the identification of the modified nucleoside 5-methylcytidine (m5C) in Escherichia coli tRNAs. This modification was determined to be present at position 49 of tRNA Tyr-QUA-II. Moreover, m5C levels in this tRNA are significantly elevated under high reactive oxygen specieis (ROS) conditions in E. coli cells. We identified the known ribosomal RNA methyltransferase rsmF as the enzyme responsible for m5C synthesis in tRNA and enzyme transcript levels are responsive to elevated levels of ROS in the cell. We further find that changes in m5C levels in this tRNA are not specific to Fenton-like reaction conditions elevating ROS, but heat shock can also induce increased modification of tRNA Tyr-QUA-II. Altogether, this work illustrates how cells adapt to changing environmental conditions through variations in tRNA modification profiles.

DNA-PK inhibitor peposertib enhances p53-dependent cytotoxicity of DNA double-strand break inducing therapy in acute leukemia

Peposertib (M3814) is a potent and selective DNA-PK inhibitor in early clinical development. It effectively blocks non-homologous end-joining repair of DNA double-strand breaks (DSB) and strongly potentiates the antitumor effect of ionizing radiation (IR) and topoisomerase II inhibitors. By suppressing DNA-PK catalytic activity in the presence of DNA DSB, M3814 potentiates ATM/p53 signaling leading to enhanced p53-dependent antitumor activity in tumor cells. Here, we investigated the therapeutic potential of M3814 in combination with DSB-inducing agents in leukemia cells and a patient-derived tumor. We show that in the presence of IR or topoisomerase II inhibitors, M3814 boosts the ATM/p53 response in acute leukemia cells leading to the elevation of p53 protein levels as well as its transcriptional activity. M3814 synergistically sensitized p53 wild-type, but not p53-deficient, AML cells to killing by DSB-inducing agents via p53-dependent apoptosis involving both intrinsic and extrinsic effector pathways. The antileukemic effect was further potentiated by enhancing daunorubicin-induced myeloid cell differentiation. Further, combined with the fixed-ratio liposomal formulation of daunorubicin and cytarabine, CPX-351, M3814 enhanced the efficacy against leukemia cells in vitro and in vivo without increasing hematopoietic toxicity, suggesting that DNA-PK inhibition could offer a novel clinical strategy for harnessing the anticancer potential of p53 in AML therapy.

How Hematopoietic Stem Cells Respond to Irradiation: Similarities and Differences between Low and High Doses of Ionizing Radiations

In this review, we will specifically address the newest insights on the effect of low doses of ionizing radiations on the hematopoietic stem cells, which are prone to long-term deleterious effects. Impact of high doses of irradiation on hematopoietic cells has been widely studied over the years, in line with the risk of accidental or terrorist exposure to irradiation and with a particular attention to the sensitivity of the hematopoietic system. Recently, more studies have focused on lower doses of irradiation on different tissues, due to the increasing exposure caused by medical imaging, radiotherapy or plane travelling for instance. Hence, we will delineate similarities and discrepancies in HSC response to high and low doses of irradiation from these studies.

Prion protein deficiency impairs hematopoietic stem cell determination and sensitizes myeloid progenitors to irradiation

Highly conserved among species and expressed in various types of cells, numerous roles have been attributed to the cellular prion protein (PrPC). In hematopoiesis, PrPC regulates hematopoietic stem cell self-renewal but the mechanisms involved in this regulation are unknown. Here we show that PrPC regulates hematopoietic stem cell number during aging and their determination towards myeloid progenitors. Furthermore, PrPC protects myeloid progenitors against the cytotoxic effects of total body irradiation. This radioprotective effect was associated with increased cellular prion mRNA level and with stimulation of the DNA repair activity of the Apurinic/pyrimidinic endonuclease 1, a key enzyme of the base excision repair pathway. Altogether, these results show a previously unappreciated role of PrPC in adult hematopoiesis, and indicate that PrPC-mediated stimulation of BER activity might protect hematopoietic progenitors from the cytotoxic effects of total body irradiation.

Carcinogenesis and Reactive Oxygen Species Signaling: Interaction of the NADPH Oxidase NOX1-5 and Superoxide Dismutase 1-3 Signal Transduction Pathways

SIGNIFICANCE

Reduction/oxidation (redox) balance could be defined as an even distribution of reduction and oxidation complementary processes and their reaction end products. There is a consensus that aberrant levels of reactive oxygen species (ROS), commonly observed in cancer, stimulate primary cell immortalization and progression of carcinogenesis. However, the mechanism how different ROS regulate redox balance is not completely understood. Recent Advances: In the current review, we have summarized the main signaling cascades inducing NADPH oxidase NOX1-5 and superoxide dismutase (SOD) 1-3 expression and their connection to cell proliferation, immortalization, transformation, and CD34+ cell differentiation in thyroid, colon, lung, breast, and hematological cancers.

CRITICAL ISSUES

Interestingly, many of the signaling pathways activating redox enzymes or mediating the effect of ROS are common, such as pathways initiated from G protein-coupled receptors and tyrosine kinase receptors involving protein kinase A, phospholipase C, calcium, and small GTPase signaling molecules.

FUTURE DIRECTIONS

The clarification of interaction of signal transduction pathways could explain how cells regulate redox balance and may even provide means to inhibit the accumulation of harmful levels of ROS in human pathologies.

The relationship of mammal survivorship and body mass modeled by metabolic and vitality theories

A model describes the relationship between mammal body mass and survivorship by combining replicative senescence theory postulating a cellular basis of aging, metabolic theory relating metabolism to body mass, and vitality theory relating survival to vitality loss and extrinsic mortality. In the combined framework, intrinsic mortality results from replicative senescence of the hematopoietic stem cells and extrinsic mortality results from environmental challenges. Because the model expresses the intrinsic and extrinsic rates with different powers of body mass, across the spectrum of mammals, survivorship changes from Type I to Type II curve shapes with decreasing body mass. Fitting the model to body mass and maximum lifespan data of 494 nonvolant mammals yields allometric relationships of body mass to the vitality parameters, from which full survivorship profiles were generated from body mass alone. Because maximum lifespan data is predominantly derived from captive populations, the generated survivorship curves were dominated by intrinsic mortality. Comparison of the mass-derived and observed survivorship curves provides insights into how specific populations deviate from the aggregate of populations observed under captivity.

Lowering Etoposide Doses Shifts Cell Demise From Caspase-Dependent to Differentiation and Caspase-3-Independent Apoptosis via DNA Damage Response, Inducing AML Culture Extinction

Cytotoxic chemotherapy, still the most widely adopted anticancer treatment, aims at eliminating cancer cells inducing apoptosis with DNA damaging agents, exploiting the differential replication rate of cancer vs. normal cells; efficiency is evaluated in terms of extent of induced apoptosis, which depends on the individual cell sensitivity to a given drug, and on the dose. In this in vitro study, we report that the concentration of etoposide, a topoisomerase II poison widely used in clinics, determines both the kinetics of cell death, and the type of apoptosis induced. We observed that on a set of myeloid leukemia cell lines, etoposide at high (50 uM) dose promoted a rapid caspase-3-mediated apoptosis, whereas at low (0.5 uM) dose, it induced morphological and functional granulocytic differentiation and caspase-2-dependent, but caspase-3-independent, cell death, displaying features consistent with apoptosis. Both differentiation and caspase-2- (but not 3)-mediated apoptosis were contrasted by caffeine, a well-known inhibitor of the cellular DNA damage response (DDR), which maintained cell viability and cycling, indicating that the effects of low etoposide dose are not the immediate consequence of damage, but the result of a signaling pathway. DDR may be thus the mediator responsible for translating a mere dosage-effect into different signal transduction pathways, highlighting a strategic action in regulating timing and mode of cell death according to the severity of induced damage. The evidence of different molecular pathways induced by high vs. low drug doses may possibly contribute to explain the different effects of cytotoxic vs. metronomic therapy, the latter achieving durable clinical responses by treating cancer patients with stable, low doses of otherwise canonical cytotoxic drugs; intriguingly caspase-3, a major promoter of wounded tissue regeneration, is also a key factor of post-therapy cancer repopulation. All this suggests that cancer control in response to cytotoxic drugs arises from complex reprogramming mechanisms in tumor tissue, recently described as anakoinosis.

Genoprotective Effect of New Triazine Derivatives in Endosulfan Mediated Toxicity, an In vivo and In vitro Study

Background:

Recently, we reported synthesis and neuroprotective activity of some new 1,2,4-triazine derivatives against H2O2 and β-amyloid toxicity in two neurotic cell lines, SHSY5Y and PC12.

Methods:

The promising results obtained prompted us to further study on these potent neuroprotective agents. In the current study, in vivo anti-inflammatory effect and also genoprotective activity of these compounds in endosulfan-mediated toxicity were investigated. Compounds RT and SMO exhibited high anti-inflammatory effect at 3 and 4 hours after injection in 20 mg/kg, and were even more effective than Indomethacin (20 mg/kg).

Results:

Interestingly, compound SMO in 200 &#181;M was the best compound in reducing micronuclei significantly (P value <0.0001) in lymphocytes treated with endosulfan compared to control group.

Conclusion:

Herein, we report SMO as a genoprotective agent and a new drug candidate for endosulfan mediated toxicity.

Oxidative stress in chronic lymphocytic leukemia: still a matter of debate

There is a large body of evidence showing a strong correlation between carcinogenesis of several types of human tumors, including chronic lymphocytic leukemia (CLL), and oxidative stress (OS). The mechanisms by which OS may promote cancer pathogenesis have not been completely deciphered yet and, in CLL, as in other neoplasms, whether OS is a primary cause or simply a downstream effect of the disease is still an open question. It has been demonstrated that, in CLL, OS concomitantly results from increased reactive oxygen species (ROS) production, mainly ascribable to CLL cells mitochondrial activity, and impaired antioxidant defenses. Interestingly, OS evaluation in CLL patients, at diagnosis, seems to have a prognostic significance, thus getting new insights in the biological comprehension of the disease with potential therapeutic implications.

Low Oxidative Stress-Mediated Proliferation Via JNK-FOXO3a-Catalase Signaling in Transplanted Adult Stem Cells Promotes Wound Tissue Regeneration

Aims: Stem cells exposed to pathological levels of reactive oxygen species (ROS) at wound sites fail to regenerate tissue. The molecular mechanism underlying differential levels of ROS-mediated regulation of stem cells remains elusive. This study elucidates the mechanistic role of catalase at 10 μM H₂O₂-induced proliferation of mouse bone marrow stromal (BMSC) and hematopoietic (HSPC) stem/progenitor cells. Results: BMSCs and HSPCs depicted an increased growth rate and colony formation, in the presence of 10 μM but not 100 μM concentration of H₂O₂, an effect that was perturbed by Vit. C. Mechanistically, JNK activation-FOXO3a nuclear translocation and binding of FOXO3a to catalase promoter at 10 μM H₂O₂ led to an increased expression and activity of anti-oxidant gene, catalase. This was followed by an increased proliferative phenotype via the AKT-dependent pathway that was perturbed in the presence of catalase-inhibitor, 3-aminotriazole due to an increased ROS-mediated inactivation of AKT. Preclinically, 10 μM H₂O₂-mediated preconditioning of BMSCs/HSPCs transplantation accelerated wound closure, enhanced catalase expression, and decreased ROS levels at the wound site. Transplantation of male donor cells into female recipient mice or GFP-labeled BMSCs or HSPCs depicted an increased engraftment and proliferation in preconditioned cell transplanted groups as compared with the wound control. Wound healing occurred via keratinocyte generation and vascularization in preconditioned BMSCs, whereas only neo-vascularization occurred in the preconditioned HSPCs transplanted groups. Innovation and Conclusion: Our study suggests a distinct role of catalase that protects BMSCs and HSPCs from low ROS and promotes proliferation. Transplantation of preconditioned stem cells enhanced wound tissue regeneration with a better antioxidant defense mechanism-as a therapeutic approach in stem cell transplantation-mediated tissue regeneration. Antioxid. Redox Signal. 28, 1047-1065.

Senescent human hematopoietic progenitors show elevated expression of transposable elements and inflammatory genes

Genomic transposable elements (TEs) constitute the majority of the genome. Expression of TEs is known to activate the double-stranded RNA recognition pathway ("viral mimicry"), leading to the activation of interferon-stimulated genes, inflammation, and immune-mediated cell death. Recently, we showed that the expression of TEs is suppressed along with immune pathways in leukemic stem cells (LSCs) in acute myeloid leukemia, suggesting a potential mechanism for immune escape of LSCs. This indicated that, during oncogenesis, where there is escape from senescence, expression of TEs is suppressed. Senescence is known to activate the interferon response and inflammatory cytokines, known as the senescence-associated secretory phenotype (SASP). We characterized the transcriptome of senescent and active human hematopoietic stem and progenitor cells (HSPCs) in vivo and showed co-occurrence of overexpression of TEs, SASP genes, and gene pathways of inflammation in senescence. The percentage of circulating senescent HSPCs (s-HSPCs) did not increase with age, indicating active clearance. Induction of senescence in human HSPCs in vitro showed increased expression of TE and SASP genes. SASP is known to mediate clearance of senescent cells and active clearance of senescent cells has been shown to increase organismal fitness. We speculate that the expression of TEs in s-HSPCs could contribute to orderly clearance of the cells via activation of immune pathways, warranting further mechanistic studies. This is the first study to characterize the transcriptome of human s-HSPCs in vivo, revealing activated expression of TEs and inflammatory genes.

Arsenic trioxide inhibits glioma cell growth through induction of telomerase displacement and telomere dysfunction

Glioblastomas are resistant to many kinds of treatment, including chemotherapy, radiation and other adjuvant therapies. As₂O₃ reportedly induces ROS generation in cells, suggesting it may be able to induce telomerase suppression and telomere dysfunction in glioblastoma cells. We show here that As₂O₃ induces ROS generation as well as telomerase phosphorylation in U87, U251, SHG4 and C6 glioma cells. It also induces translocation of telomerase from the nucleus to the cytoplasm, thereby decreasing total telomerase activity. These effects of As₂O₃ trigger an extensive DNA damage response at the telomere, which includes up-regulation of ATM, ATR, 53BP1, γ-H₂AX and Mer11, in parallel with telomere fusion and 3′-overhang degradation. This ultimately results in induction of p53- and p21-mediated cell apoptosis, G2/M cell cycle arrest and cellular senescence. These results provide new insight into the antitumor effects of As₂O₃ and can perhaps contribute to solving the problem of glioblastoma treatment resistance.

Interstitial telomeric repeats-associated DNA breaks

During a cell's lifespan, DNA break formation is a common event, associated with many processes, from replication to apoptosis. Most of DNA breaks are readily repaired, but some are meant to persist in time, such as the chromosome ends, protected by telomeres. Besides them, eukaryotic genomes comprise shorter stretches of interstitial telomeric repeats. We assumed that the latter may also be associated with the formation of DNA breaks meant to persist in time. In zebrafish and mouse embryos, cells containing numerous breakage foci were identified. These breaks were not associated with apoptosis or replication, nor did they seem to activate DNA damage response machinery. Unlike short-living, accidental sparse breaks, the ones we found seem to be closely associated, forming discrete break foci. A PCR-based method was developed, allowing specific amplification of DNA regions located between inverted telomeric repeats associated with breaks. The cloning and sequencing of such DNA fragments were found to denote some specificity in their distribution for different tissue types and development stages.

The DNA damage response pathway in normal hematopoiesis and malignancies

In mammalian cells, the DNA damage response (DDR) prevents the replication and propagation of DNA errors to the next generation, thus maintaining genomic stability. At the heart of the DDR are the related signaling kinases ATM, ATR, and DNA-PK, which regulate DNA repair and associated events such as cell cycle checkpoints, chromatin remodeling, transcription, and ultimately apoptosis. Several findings highlight the occurrence of DDR in hemopoietic stem cells (HSCs), and persistence of DNA lesions in these cells promotes their functional decline and accumulation of leukemogenic mutations. Besides favoring tumor formation and progression, molecular defects that directly or indirectly inactivate certain DDR pathways can provide a therapeutic opportunity, since a reduced ability to repair DNA lesions renders hemopoietic malignancies vulnerable to genotoxic drugs acting also through synthetic lethal interactions. Here, we discuss the essential role of DDR in HSC maintenance and protection against leukemogenesis, and how acquired DDR dysfunctions or pharmacological agents that block this pathway can be effectively exploited for the treatment of various hematopoietic malignancies.

Slow release of etoposide from dextran conjugation shifts etoposide activity from cytotoxicity to differentiation: A promising tool for dosage control in anticancer metronomic therapy

Drug conjugation, improving drug stability, solubility and body permanence, allows achieving impressive results in tumor control. Here, we show that conjugation may provide a straightforward method to administer drugs by the emerging anticancer metronomic approach, presently consisting of low, repeated doses of cytotoxic drugs used in traditional chemotherapy, thus reducing toxicity without reducing efficiency; however, low dose maintenance in tumor sites is difficult. We show that conjugating the antitumor drug etoposide to dextran via pH-sensitive bond produces slow releasing, apoptosis-proficient conjugates rapidly internalized into acidic lysosomes; importantly, release of active etoposide requires cell internalization and acidic pH. Conjugation, without impairing etoposide-induced complete elimination of tumor cells, shifted the mode of apoptosis from cytotoxicity- to differentiation-related; interestingly, high conjugate doses acted as low doses of free etoposide, thus mimicking the effect of metronomic therapy. This indicates slow release as a promising novel strategy for stabilizing low drug levels in metronomic regimens.

DNA-damage response in hematopoietic stem cells: an evolutionary trade-off between blood regeneration and leukemia suppression

Self-renewing and multipotent hematopoietic stem cells (HSCs) maintain lifelong hematopoiesis. Their enormous regenerative potential coupled with lifetime persistence in the body, in contrast with the Progenitors, demand tight control of HSCs genome stability. Indeed, failure to accurately repair DNA damage in HSCs is associated with bone marrow failure and accelerated leukemogenesis. Recent observations exposed remarkable differences in several DNA-damage response (DDR) aspects between HSCs and Progenitors, especially in their DNA-repair capacities and susceptibility to apoptosis. Human HSCs in comparison with Progenitors exhibit delayed DNA double-strand break rejoining, persistent DDR signaling activation, higher sensitivity to the cytotoxic effects of ionizing radiation and attenuated expression of DNA-repair genes. Importantly, the distinct DDR of HSCs was also documented in mouse models. Nevertheless, physiological significance and the molecular basis of the HSCs-specific DDR features are only partially understood. Taking radiation-induced DDR as a paradigm, this review will focus on the current advances in understanding the role of cell-intrinsic DDR regulators and the cellular microenvironment in balancing stemness with genome stability. Pre-leukemia HSCs and clonal hematopoiesis evolvement will be discussed as an evolutionary compromise between the need for lifelong blood regeneration and DDR. Uniquely for this review, we outline the differences in HSCs-related DDR as highlighted by various experimental systems and attempt to provide their critical analysis.

The immune receptor Trem1 cooperates with diminished DNA damage response to induce preleukemic stem cell expansion

Fanconi anemia (FA) is an inherited bone marrow failure syndrome with extremely high risk of leukemic transformation. Here we investigate the relationship between DNA damage response (DDR) and leukemogenesis using the Fanca knockout mouse model. We found that chronic exposure of the Fanca^-/- hematopoietic stem cells to DNA crosslinking agent mitomycin C in vivo leads to diminished DDR, and the emergence/expansion of pre-leukemia stem cells (pre-LSCs). Surprisingly, although genetic correction of Fanca deficiency in the pre-LSCs restores DDR and reduces genomic instability, but fails to prevent pre-LSC expansion or delay leukemia development in irradiated recipients. Furthermore, we identified transcription program underlying dysregulated DDR and cell migration, myeloid proliferation, and immune response in the Fanca^-/- pre-LSCs. Forced expression of the downregulated DNA repair genes, Rad51c or Trp53i13, in the Fanca^-/- pre-LSCs partially rescues DDR but has no effect on leukemia, whereas shRNA knockdown of the upregulated immune receptor genes Trem1 or Pilrb improves leukemia-related survival, but not DDR or genomic instability. Furthermore, Trem1 cooperates with diminished DDR in vivo to promote Fanca^-/- pre-LSC expansion and leukemia development. Our study implicates diminishing DDR as a root cause of FA leukemogenesis, which subsequently collaborates with other signaling pathways for leukemogenic transformation.

Pluripotent Stem Cells and DNA Damage Response to Ionizing Radiations

Pluripotent stem cells (PSCs) hold great promise in regenerative medicine, disease modeling, functional genomics, toxicological studies and cell-based therapeutics due to their unique characteristics of self-renewal and pluripotency. Novel methods for generation of pluripotent stem cells and their differentiation to the specialized cell types such as neuronal cells, myocardial cells, hepatocytes and beta cells of the pancreas and many other cells of the body are constantly being refined. Pluripotent stem cell derived differentiated cells, including neuronal cells or cardiac cells, are ideal for stem cell transplantation as autologous or allogeneic cells from healthy donors due to their minimal risk of rejection. Radiation-induced DNA damage, ultraviolet light, genotoxic stress and other intrinsic and extrinsic factors triggers a series of biochemical reactions known as DNA damage response. To maintain genomic stability and avoid transmission of mutations into progenitors cells, stem cells have robust DNA damage response signaling, a contrast to somatic cells. Stem cell transplantation may protect against radiation-induced late effects. In particular, this review focuses on differential DNA damage response between stem cells and derived differentiated cells and the possible pathways that determine such differences.

Terminal differentiation induction as DNA damage response in hematopoietic stem cells by GADD45A

Hematopoietic stem cells (HSCs) sustain lifelong blood cell regeneration by balancing their ability for self-renewal with their ability to differentiate into all blood cell types. To prevent organ exhaustion and malignant transformation, long-lived HSCs, in particular, must be protected from exogenous and endogenous stress, which cause severe DNA damage. When DNA is damaged, distinct DNA repair mechanisms and cell fate controls occur in adult HSCs compared with committed cells. Growth arrest and DNA damage-inducible 45 alpha (GADD45A) is known to coordinate a variety of cellular stress responses, indicating the molecule is an important stress mediator. So far, the function of GADD45A in hematopoietic stem and progenitor cells is controversial and appears highly dependent on the cell type and stress stimulus. Recent studies have analyzed its role in cell fate decision control of prospectively isolated HSCs and have revealed unexpected functions of GADD45A, as discussed here. The upregulation of GADD45A by DNA damage-causing conditions results in enhanced HSC differentiation, probably to efficiently eliminate aberrant HSCs from the system. These findings, in concert with a few studies on other stem cell systems, have led us to propose DNA damage-induced differentiation as a novel DNA damage response mechanism in stem cells that circumvents the fatal consequences of cumulative DNA damage in the stem cell compartment.

DNA Damage Response in Hematopoietic Stem Cell Ageing

Maintenance of tissue-specific stem cells is vital for organ homeostasis and organismal longevity. Hematopoietic stem cells (HSCs) are the most primitive cell type in the hematopoietic system. They divide asymmetrically and give rise to daughter cells with HSC identity (self-renewal) and progenitor progenies (differentiation), which further proliferate and differentiate into full hematopoietic lineages. Mammalian ageing process is accompanied with abnormalities in the HSC self-renewal and differentiation. Transcriptional changes and epigenetic modulations have been implicated as the key regulators in HSC ageing process. The DNA damage response (DDR) in the cells involves an orchestrated signaling pathway, consisting of cell cycle regulation, cell death and senescence, transcriptional regulation, as well as chromatin remodeling. Recent studies employing DNA repair-deficient mouse models indicate that DDR could intrinsically and extrinsically regulate HSC maintenance and play important roles in tissue homeostasis of the hematopoietic system. In this review, we summarize the current understanding of how the DDR determines the HSC fates and finally contributes to organismal ageing.