5-Bromodeoxyuridine induces senescence-like phenomena in mammalian cells regardless of cell type or species

Regulatory Network Inference of Induced Senescent Midbrain Cell Types Reveals Cell Type-Specific Senescence-Associated Transcriptional Regulators

Cellular senescence of brain cell types has become an increasingly important perspective for both aging and neurodegeneration, specifically in the context of Parkinson's Disease (PD). The characterization of classical hallmarks of senescence is a widely debated topic, whereby the context in which a senescence phenotype is being investigated, such as the cell type, the inducing stressor, and/or the model system, is an extremely important aspect to consider when defining a senescent cell. Here, we describe a cell type-specific profile of senescence through the investigation of various canonical senescence markers in five human midbrain cell lines using chronic 5-Bromodeoxyuridine (BrdU) treatment as a model of DNA damage-induced senescence. We used principal component analysis (PCA) and subsequent regulatory network inference to define both unique and common senescence profiles in the cell types investigated, as well as revealed senescence-associated transcriptional regulators (SATRs). Functional characterization of one of the identified regulators, transcription factor AP4 (TFAP4), further highlights the cell type-specificity of the expression of the various senescence hallmarks. Our data indicates that SATRs modulate cell type-specific profiles of induced senescence in key midbrain cell types that play an important role in the context of aging and PD.

Faulty Gap Filling in Nucleotide Excision Repair Leads to Double-Strand Break Formation in Senescent Cells

The change of repair efficiency of UV-induced pyrimidine dimers due to aging was examined in replicatively senesced fibroblasts. The fibroblasts with repeated passages showed the characteristics of cellular senescence, including irreversible cell cycle arrest, elevated β-galactosidase activity, and senescence-associated secretory phenotype. The incision efficiency of oligonucleotide containing UV lesions was similar regardless of cell doubling levels, but the gap filling process was impaired in replicatively senescent cells. The releases of xeroderma pigmentosum group G, proliferating cell nuclear antigen, and replication protein A from damaged sites were delayed, which might have disturbed the DNA polymerase progression. The persistent single-stranded DNA was likely converted to double-strand breaks, leading to ataxia telangiectasia-mutated phosphorylation and 53BP1 foci formation. Phosphorylated histone H2AX (γ-H2AX) induction mainly occurred in G1 phase in senescent cells, not in S phase such as in normal cells, indicating that replication stress-independent double-strand breaks might be formed. MRE11 having nuclease activity accumulated to damaged sites at early time point after UV irradiation but not released in senescent cells. The pharmacological studies using specific inhibitors for the nuclease activity suggested that MRE11 contributed to the enlargement of single-stranded DNA gap, facilitating the double-strand break formation.

In Vitro Investigation of Therapy-Induced Senescence and Senescence Escape in Breast Cancer Cells Using Novel Flow Cytometry-Based Methods

Although cellular senescence was originally defined as an irreversible form of cell cycle arrest, in therapy-induced senescence models, the emergence of proliferative senescence-escaped cancer cells has been reported by several groups, challenging the definition of senescence. Indeed, senescence-escaped cancer cells may contribute to resistance to cancer treatment. Here, to study senescence escape and isolate senescence-escaped cells, we developed novel flow cytometry-based methods using the proliferation marker Ki-67 and CellTrace CFSE live-staining. We investigated the role of a novel senescence marker (DPP4/CD26) and a senolytic drug (azithromycin) on the senescence-escaping ability of MCF-7 and MDA-MB-231 breast cancer cells. Our results show that the expression of DPP4/CD26 is significantly increased in both senescent MCF-7 and MDA-MB-231 cells. While not essential for senescence induction, DPP4/CD26 contributed to promoting senescence escape in MCF-7 cells but not in MDA-MB-231 cells. Our results also confirmed the potential senolytic effect of azithromycin in senescent cancer cells. Importantly, the combination of azithromycin and a DPP4 inhibitor (sitagliptin) demonstrated a synergistic effect in senescent MCF-7 cells and reduced the number of senescence-escaped cells. Although further research is needed, our results and novel methods could contribute to the investigation of the mechanisms of senescence escape and the identification of potential therapeutic targets. Indeed, DPP4/CD26 could be a promising marker and a novel target to potentially decrease senescence escape in cancer.

Upregulated expression of lamin B receptor increases cell proliferation and suppresses genomic instability: implications for cellular immortalization

Mammalian somatic cells undergo terminal proliferation arrest after a limited number of cell divisions, a phenomenon termed cellular senescence. However, cells acquire the ability to proliferate infinitely (cellular immortalization) through multiple genetic alterations. Inactivation of tumor suppressor genes such as p53, RB and p16 is important for cellular immortalization, although additional molecular alterations are required for cellular immortalization to occur. Here, we aimed to gain insights into these molecular alterations. Given that cellular immortalization is the escape of cells from cellular senescence, genes that regulate cellular senescence are likely to be involved in cellular immortalization. Because senescent cells show altered heterochromatin organization, we investigated the implications of lamin A/C, lamin B1 and lamin B receptor (LBR), which regulate heterochromatin organization, in cellular immortalization. We employed human immortalized cell lines, KMST-6 and SUSM-1, and found that expression of LBR was upregulated upon cellular immortalization and downregulated upon cellular senescence. In addition, knockdown of LBR induced cellular senescence with altered chromatin configuration. Additionally, enforced expression of LBR increased cell proliferation likely through suppression of genome instability in human primary fibroblasts that expressed the simian virus 40 large T antigen (TAg), which inactivates p53 and RB. Furthermore, expression of TAg or knockdown of p53 led to upregulated LBR expression. These observations suggested that expression of LBR might be upregulated to suppress genome instability in TAg-expressing cells, and, consequently, its upregulated expression assisted the proliferation of TAg-expressing cells (i.e. p53/RB-defective cells). Our findings suggest a crucial role for LBR in the process of cellular immortalization.

5-Bromo-2'-deoxyuridine labeling: historical perspectives, factors influencing the detection, toxicity, and its implications in the neurogenesis

The halopyrimidine 5-bromo-2'-deoxyuridine (BrdU) is an exogenous marker of DNA synthesis. Since the introduction of monoclonal antibodies against BrdU, an increasing number of methodologies have been used for the immunodetection of this synthesized bromine-tagged base analogue into replicating DNA. BrdU labeling is widely used for identifying neuron precursors and following their fate during the embryonic, perinatal, and adult neurogenesis in a variety of vertebrate species including birds, reptiles, and mammals. Due to BrdU toxicity, its incorporation into replicating DNA presents adverse consequences on the generation, survival, and settled patterns of cells. This may lead to false results and misinterpretation in the identification of proliferative neuroblasts. In this review, I will indicate the detrimental effects of this nucleoside during the development of the central nervous system, as well as the reliability of BrdU labeling to detect proliferating neuroblasts. Moreover, it will show factors influencing BrdU immunodetection and the contribution of this nucleoside to the study of prenatal, perinatal, and adult neurogenesis. Human adult neurogenesis will also be discussed. It is my hope that this review serves as a reference for those researchers who focused on detecting cells that are in the synthetic phase of the cell cycle.

Nucleosome disruption by 5-bromodeoxyuridine leads to senescence

5-Bromodeoxyuridine (BrdU) exposure leads to senescence, but the mechanistic details remain elusive. In this issue, En A et al. unveil a role of the HBR domain in histone H2B as a potential mediator of the effects of BrdU both in yeast and in human cells. Comment on: https://doi.org/10.1111/febs.16584.

The key role of a basic domain of histone H2B N-terminal tail in the action of 5-bromodeoxyuridine to induce cellular senescence

5-Bromodeoxyuridine (BrdU), a thymidine analogue, is an interesting reagent that modulates various biological phenomena. BrdU, upon incorporation into DNA, causes destabilized nucleosome positioning which leads to changes in heterochromatin organization and gene expression in cells. We have previously shown that BrdU effectively induces cellular senescence, a phenomenon of irreversible growth arrest in mammalian cells. Identification of the mechanism of action of BrdU would provide a novel insight into the molecular mechanisms of cellular senescence. Here, we showed that a basic domain in the histone H2B N-terminal tail, termed the HBR (histone H2B repression) domain, is involved in the action of BrdU. Notably, deletion of the HBR domain causes destabilized nucleosome positioning and derepression of gene expression, as does BrdU. We also showed that the genes up-regulated by BrdU significantly overlapped with those by deletion of the HBR domain, the result of which suggested that BrdU and deletion of the HBR domain act in a similar way. Furthermore, we showed that decreased HBR domain function induced cellular senescence or facilitated the induction of cellular senescence. These findings indicated that the HBR domain is crucially involved in the action of BrdU, and also suggested that disordered nucleosome organization may be involved in the induction of cellular senescence.

Synergism of BCL-2 family inhibitors facilitates selective elimination of senescent cells

Accumulation of senescent cells in tissues with advancing age participates in the pathogenesis of several human age-associated diseases. Specific senescent secretome, the resistance of senescent cells to apoptotic stimuli, and lack of immune system response contribute to the accumulation of senescent cells and their adverse effects in tissues. Inhibition of antiapoptotic machinery, augmented in senescent cells, by BCL-2 protein family inhibitors represents a promising approach to eliminate senescent cells from tissues. This study aimed to explore synergistic and selective senolytic effects of anti-apoptotic BCL-2 family targeting compounds, particularly BH3 mimetics.

Using human non-transformed cells RPE-1, BJ, and MRC-5 brought to ionizing radiation-, oncogene-, drug-induced and replicative senescence, we found synergy in combining MCL-1 selective inhibitors with other BH3 mimetics. In an attempt to uncover the mechanism of such synergy, we revealed that the surviving subpopulation of cells resistant to individually applied ABT-737/ABT-263, MIK665, ABT-199, and S63845 BCL-2 family inhibitors showed elevated MCL-1 compared to untreated control cells indicating the presence of a subset of cells expressing high MCL-1 levels and, therefore, resistant to BCL-2 inhibitors within the original population of senescent cells.

Overall, we found that combining BCL-2 inhibitors can be beneficial for eliminating senescent cells, thereby enabling use of lower, potentially less toxic, doses of drugs compared to monotherapy, thereby overcoming the resistance of the subpopulation of senescent cells to monotherapy.

Peroxiredoxin 6 protects irradiated cells from oxidative stress and shapes their senescence-associated cytokine landscape

Cellular senescence is a complex stress response defined as an essentially irreversible cell cycle arrest mediated by the inhibition of cell cycle-specific cyclin dependent kinases. The imbalance in redox homeostasis and oxidative stress have been repeatedly observed as one of the hallmarks of the senescent phenotype. However, a large-scale study investigating protein oxidation and redox signaling in senescent cells in vitro has been lacking. Here we applied a proteome-wide analysis using SILAC-iodoTMT workflow to quantitatively estimate the level of protein sulfhydryl oxidation and proteome level changes in ionizing radiation-induced senescence (IRIS) in hTERT-RPE-1 cells. We observed that senescent cells mobilized the antioxidant system to buffer the increased oxidation stress. Among the antioxidant proteins with increased relative abundance in IRIS, a unique 1-Cys peroxiredoxin family member, peroxiredoxin 6 (PRDX6), was identified as an important contributor to protection against oxidative stress. PRDX6 silencing increased ROS production in senescent cells, decreased their resistance to oxidative stress-induced cell death, and impaired their viability. Subsequent SILAC-iodoTMT and secretome analysis after PRDX6 silencing showed the downregulation of PRDX6 in IRIS affected protein secretory pathways, decreased expression of extracellular matrix proteins, and led to unexpected attenuation of senescence-associated secretory phenotype (SASP). The latter was exemplified by decreased secretion of pro-inflammatory cytokine IL-6 which was also confirmed after treatment with an inhibitor of PRDX6 iPLA2 activity, MJ33. In conclusion, by combining different methodological approaches we discovered a novel role of PRDX6 in senescent cell viability and SASP development. Our results suggest PRDX6 could have a potential as a drug target for senolytic or senomodulatory therapy.

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SILAC-iodoTMT is a powerful tool to quantify redox imbalance in IRIS.

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Senescence in hTERT-RPE-1 cells is not accompanied by bulk cysteine oxidation.

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Antioxidant proteins are upregulated in senescent hTERT-RPE-1 cells.

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PRDX6 silencing affects redox homeostasis and viability of senescent cells.

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PRDX6 silencing alters secretome of senescent RPE-1 cells and suppresses IL-6.

Recent advances in nucleotide analogue-based techniques for tracking dividing stem cells: An overview

Detection of thymidine analogues after their incorporation into replicating DNA represents a powerful tool for the study of cellular DNA synthesis, progression through the cell cycle, cell proliferation kinetics, chronology of cell division, and cell fate determination. Recent advances in the concurrent detection of multiple such analogues offer new avenues for the investigation of unknown features of these vital cellular processes. Combined with quantitative analysis, temporal discrimination of multiple labels enables elucidation of various aspects of stem cell life cycle in situ, such as division modes, differentiation, maintenance, and elimination. Data obtained from such experiments are critically important for creating descriptive models of tissue histogenesis and renewal in embryonic development and adult life. Despite the wide use of thymidine analogues in stem cell research, there are a number of caveats to consider for obtaining valid and reliable labeling results when marking replicating DNA with nucleotide analogues. Therefore, in this review, we describe critical points regarding dosage, delivery, and detection of nucleotide analogues in the context of single and multiple labeling, outline labeling schemes based on pulse-chase, cumulative and multilabel marking of replicating DNA for revealing stem cell proliferative behaviors, and determining cell cycle parameters, and discuss preconditions and pitfalls in conducting such experiments. The information presented in our review is important for rational design of experiments on tracking dividing stem cells by marking replicating DNA with thymidine analogues.

The ERK-p38MAPK-STAT3 Signalling Axis Regulates iNOS Expression and Salmonella Infection in Senescent Cells

The cellular changes occurring due to senescence like proliferation arrest, increase in free radical levels, and secretion of pro-inflammatory cytokines have been well studied, but its associated alteration in intracellular signalling networks has been scarcely explored. In this study, we examine the roles of three major kinases viz. p38 MAPK, ERK, and STAT3 in regulating iNOS expression and thereby the levels of the free radical Nitric oxide in senescent cells. Our study revealed that these kinases could differentially regulate iNOS in senescent cells compared to non-senescent cells. Further, we tested the physiological relevance of these alterations with Salmonella infection assays and established an inter-regulatory network between these kinases unique to infected senescent cells. Overall, our findings show how key signalling networks may be rewired in senescent cells rendering them phenotypically different.

Inhibiting microglia proliferation after spinal cord injury improves recovery in mice and nonhuman primates

No curative treatment is available for any deficits induced by spinal cord injury (SCI). Following injury, microglia undergo highly diverse activation processes, including proliferation, and play a critical role on functional recovery.

In a translational objective, we investigated whether a transient pharmacological reduction of microglia proliferation after injury is beneficial for functional recovery after SCI in mice and nonhuman primates.

Methods: The colony stimulating factor-1 receptor (CSF1R) regulates proliferation, differentiation, and survival of microglia. We orally administrated GW2580, a CSF1R inhibitor that inhibits microglia proliferation. In mice and nonhuman primates, we then analyzed treatment outcomes on locomotor function and spinal cord pathology. Finally, we used cell-specific transcriptomic analysis to uncover GW2580-induced molecular changes in microglia.

Results: First, transient post-injury GW2580 administration in mice improves motor function recovery, promotes tissue preservation and/or reorganization (identified by coherent anti-stokes Raman scattering microscopy), and modulates glial reactivity.

Second, post-injury GW2580-treatment in nonhuman primates reduces microglia proliferation, improves motor function recovery, and promotes tissue protection.

Finally, GW2580-treatment in mice induced down-regulation of proliferation-associated transcripts and inflammatory associated genes in microglia that may account for reduced neuroinflammation and improved functional recovery following SCI.

Conclusion: Thus, a transient oral GW2580 treatment post-injury may provide a promising therapeutic strategy for SCI patients and may also be extended to other central nervous system disorders displaying microglia activation.

Incorporation of 5-Bromo-2'-deoxyuridine into DNA and Proliferative Behavior of Cerebellar Neuroblasts: All That Glitters Is Not Gold

The synthetic halogenated pyrimidine analog, 5-bromo-2'-deoxyuridine (BrdU), is a marker of DNA synthesis. This exogenous nucleoside has generated important insights into the cellular mechanisms of the central nervous system development in a variety of animals including insects, birds, and mammals. Despite this, the detrimental effects of the incorporation of BrdU into DNA on proliferation and viability of different types of cells has been frequently neglected. This review will summarize and present the effects of a pulse of BrdU, at doses ranging from 25 to 300 µg/g, or repeated injections. The latter, following the method of the progressively delayed labeling comprehensive procedure. The prenatal and perinatal development of the cerebellum are studied. These current data have implications for the interpretation of the results obtained by this marker as an index of the generation, migration, and settled pattern of neurons in the developing central nervous system. Caution should be exercised when interpreting the results obtained using BrdU. This is particularly important when high or repeated doses of this agent are injected. I hope that this review sheds light on the effects of this toxic maker. It may be used as a reference for toxicologists and neurobiologists given the broad use of 5-bromo-2'-deoxyuridine to label dividing cells.

Reciprocal Changes in miRNA Expression with Pigmentation and Decreased Proliferation Induced in Mouse B16F1 Melanoma Cells by L-Tyrosine and 5-Bromo-2'-Deoxyuridine

Background: Many microRNAs have been identified as critical mediators in the progression of melanoma through its regulation of genes involved in different cellular processes such as melanogenesis, cell cycle control, and senescence. However, microRNAs’ concurrent participation in syngeneic mouse B16F1 melanoma cells simultaneously induced decreased proliferation and differential pigmentation by exposure to 5-Brd-2′-dU (5’Bromo-2-deoxyuridine) and L-Tyr (L-Tyrosine) respectively, is poorly understood. Aim: To evaluate changes in the expression of microRNAs and identify which miRNAs in-network may contribute to the functional bases of phenotypes of differential pigmentation and reduction of proliferation in B16F1 melanoma cells exposed to 5-Brd-2′-dU and L-Tyr. Methods: Small RNAseq evaluation of the expression profiles of miRNAs in B16F1 melanoma cells exposed to 5-Brd-2′-dU (2.5 μg/mL) and L-Tyr (5 mM), as well as the expression by qRT-PCR of some molecular targets related to melanogenesis, cell cycle, and senescence. By bioinformatic analysis, we constructed network models of regulation and co-expression of microRNAs. Results: We confirmed that stimulation or repression of melanogenesis with L-Tyr or 5-Brd-2′-dU, respectively, generated changes in melanin concentration, reduction in proliferation, and changes in expression of microRNAs 470-3p, 470-5p, 30d-5p, 129-5p, 148b-3p, 27b-3p, and 211-5p, which presented patterns of coordinated and reciprocal co-expression, related to changes in melanogenesis through their putative targets Mitf, Tyr and Tyrp1, and control of cell cycle and senescence: Cyclin D1, Cdk2, Cdk4, p21, and p27. Conclusions: These findings provide insights into the molecular biology of melanoma of the way miRNAs are coordinated and reciprocal expression that may operate in a network as molecular bases for understanding changes in pigmentation and decreased proliferation induced in B16F1 melanoma cells exposed to L-Tyr and 5-Brd-2′-dU.

Administration of 5-bromo-2'-deoxyuridine interferes with neuroblast proliferation and promotes apoptotic cell death in the rat cerebellar neuroepithelium

The current study was conducted to assess whether a single administration of 5-bromo-2'-deoxyuridine (BrdU) interferes with cell proliferation and leads to the activation of apoptotic cellular events in the prenatal cerebellum. BrdU effects across a wide range of doses (25-300 μg/g b.w.) were analyzed using immunohistochemical and ultrastructural procedures. The pregnant rats were injected with BrdU at embryonic day 13, and their fetuses were sacrificed from 5 to 35 hr after exposure. The quantification of several parameters such as the density of mitotic figures, and BrdU and proliferating cell nuclear antigen (PCNA)-reactive cells showed that, in comparison with the saline injected rats, the administration of BrdU impairs the proliferative behavior of neuroepithelial cells. The above-mentioned parameters were significantly reduced in rats injected with 100 μg/g b.w. of BrdU. The reduction was more evident using 200 μg/g b.w. The most severe effects were found with 300 μg/g b.w. of BrdU. The present findings also revealed that high doses of BrdU lead to the activation of apoptotic cellular events as evidenced by both terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay and immunohistochemistry for active caspase-3. In comparison with saline rats, many apoptotic cells were found in rats injected with 100 μg/g b.w. of BrdU. The number of dying cells increased with 200 μg/g b.w. The most important number of apoptotic cells were observed in animals injected with 300 μg/g b.w. of BrdU. Ultrastructural studies confirmed the presence of neuroblasts at different stages of apoptosis.

Cellular Senescence-Inducing Small Molecules for Cancer Treatment

Recently, the chemotherapeutic drug-induced cellular senescence has been considered a promising anti-cancer approach. The drug-induced senescence, which shows both similar and different hallmarks from replicative and oncogene-induced senescence, was regarded as a key determinant of tumor response to chemotherapy in vitro and in vivo. To date, an amount of effective chemotherapeutic drugs that can evoke senescence in cancer cells have been reported. The targets of these drugs differ substantially, including senescence signaling pathways, DNA replication process, DNA damage pathways, epigenetic modifications, microtubule polymerization, senescence-associated secretory phenotype (SASP), and so on. By summarizing senescence-inducing small molecule drugs together with their specific traits and corresponding mechanisms, this review is devoted to inform scientists to develop novel therapeutic strategies against cancer through inducing senescence.

The role of lamin B receptor in the regulation of senescence-associated secretory phenotype (SASP)

Cellular senescence is a phenomenon of irreversible growth arrest of mammalian somatic cells. Senescent cells increase the production of secretory proteins such as inflammatory cytokines, a phenomenon termed senescence-associated secretory phenotype (SASP). SASP is known to have profound effects on organismal health and aging; however, the molecular mechanisms of SASP are not precisely understood. In our previous studies, we have shown that senescent cells show decreased function of lamin B receptor (LBR), a nuclear membrane protein that regulates heterochromatin organization. Here we examined the implication of LBR in the regulation of SASP because senescent cells show altered heterochromatin organization, which would affect gene expression. We found that knock-down of LBR up-regulated the expression of the SASP factors such as IL-6, IL-8, and MMP1 in HeLa cells, even though cellular senescence was not induced by LBR knock-down. Conversely, enforced expression of LBR suppressed their up-regulated expression in senescent cells induced by excess thymidine. Further, our gene expression profile analysis also showed that many secretory proteins were up-regulated by LBR knock-down. We then analyzed the regulatory mechanisms of the expression of SASP factors by LBR, and found that the promoters of these SASP factors associated with LBR in normally growing cells, but dissociated from it in senescent cells. Additionally, we found that enforced expression of LBR decreased the generation of cytoplasmic DNA, which could be involved in SASP, in senescent cells. These findings suggested that LBR would play crucial roles in the regulation of SASP.

Extract of Plumbago zeylanica enhances the growth of hair follicle dermal papilla cells with down-regulation of 5α-reductase type II

BACKGROUND

Cellular senescence, a phenomenon of irreversible growth arrest of mammalian cells, is involved in various age-related phenomena in organisms. Hair follicle dermal papilla (DP) cells play important roles in the regulation of hair growth and loss.

AIMS

We examined the implication of cellular senescence of DP cells in androgenetic alopecia (AGA), the most common form of male hair loss, and searched for the compounds that have a beneficial effect on the prevention of AGA.

PATIENTS/METHODS

Expression of the 5α-reductase type II (SRD5A2) gene, which plays a key role in the development of AGA, was examined by quantitative RT-PCR and Western blotting analysis in DP cells. Besides, DP cells were cultured with the extracts of herbs used in traditional Ayurvedic medicine to search for the compounds that have a beneficial effect on the growth of DP cells.

RESULTS

We found that expression of the SRD5A2 was up-regulated in senescent DP cells. We also found that the herbal extract of Plumbago zeylanica (root) enhanced the growth of DP cells and down-regulated the expression of SRD5A2 in DP cells. Further, plumbagin, an ingredient of P zeylanica, would be responsible for the above effects of P zeylanica.

CONCLUSION

These results suggested the possibility that senescent DP cells may have a role in the development of AGA through up-regulating SRD5A2 expression, and the P zeylanica extract and plumbagin may suppress its development through enhancing the growth of DP cells and down-regulating SRD5A2 expression in DP cells.

Lamin B receptor plays a key role in cellular senescence induced by inhibition of the proteasome

Cellular senescence is a terminal growth arrest phenomenon in mammalian cells. Coordinated regulation of protein synthesis and degradation is required to maintain protein homeostasis in cells; however, senescent cells exhibit decreased activity of the proteasome, a major cellular proteolytic machinery, with an accumulation of proteins. Indeed, we showed that MG132, a proteasome inhibitor, induced cellular senescence through an accumulation of proteins in human cells. We then investigated the mechanisms of cellular senescence induced by protein accumulation by treating cells with MG132. We found that lamin B receptor (LBR), a nuclear membrane protein that regulates heterochromatin organization, was mislocalized and down-regulated in cells on treatment with MG132. Importantly, enforced expression of LBR suppressed cellular senescence induced by MG132. We also showed that LBR was involved in the regulation of chromatin organization in senescent cells, and that endoplasmic reticulum stress and autophagy were likely to be involved in the mislocalization and down-regulation of LBR. These findings indicate that decreased LBR function was responsible for the induction of cellular senescence by MG132, and thus suggest that protein accumulation caused by inhibition of the proteasome induced cellular senescence probably through chromatin dysregulation in human cells.

Induction, regulation and roles of neural adhesion molecule L1CAM in cellular senescence

Aging involves tissue accumulation of senescent cells (SC) whose elimination through senolytic approaches may evoke organismal rejuvenation. SC also contribute to aging-associated pathologies including cancer, hence it is imperative to better identify and target SC. Here, we aimed to identify new cell-surface proteins differentially expressed on human SC. Besides previously reported proteins enriched on SC, we identified 78 proteins enriched and 73 proteins underrepresented in replicatively senescent BJ fibroblasts, including L1CAM, whose expression is normally restricted to the neural system and kidneys. L1CAM was: 1) induced in premature forms of cellular senescence triggered chemically and by gamma-radiation, but not in Ras-induced senescence; 2) induced upon inhibition of cyclin-dependent kinases by p16^INK4a; 3) induced by TGFbeta and suppressed by RAS/MAPK(Erk) signaling (the latter explaining the lack of L1CAM induction in RAS-induced senescence); and 4) induced upon downregulation of growth-associated gene ANT2, growth in low-glucose medium or inhibition of the mevalonate pathway. These data indicate that L1CAM is controlled by a number of cell growth- and metabolism-related pathways during SC development. Functionally, SC with enhanced surface L1CAM showed increased adhesion to extracellular matrix and migrated faster. Our results provide mechanistic insights into senescence of human cells, with implications for future senolytic strategies.

Lamin B receptor (LBR) is involved in the induction of cellular senescence in human cells

Cellular senescence is a phenomenon of irreversible growth arrest in mammalian somatic cells in culture. Various stresses induce cellular senescence and indeed, we have found that excess thymidine effectively induces cellular senescence in human cells. Further, many reports indicate the implication of chromatin proteins in cellular senescence. Here we analysed the role of lamin B receptor (LBR), a nuclear envelope protein that regulates heterochromatin organization, in cellular senescence induced by excess thymidine. We then found that the LBR protein was down-regulated and showed aberrant localization in cells upon induction of cellular senescence by excess thymidine. Additionally, we also found that knock-down of LBR facilitated the induction of cellular senescence by excess thymidine in cancerous HeLa cells, and importantly, it induced cellular senescence in normal human diploid fibroblast TIG-7 cells. These results suggested that decreased LBR function is involved in the induction of cellular senescence in human cells.

Azithromycin and Roxithromycin define a new family of "senolytic" drugs that target senescent human fibroblasts

Here, we employed a "senolytic" assay system as a screening tool, with the goal of identifying and repurposing FDA-approved antibiotics, for the targeting of the senescent cell population. Briefly, we used two established human fibroblast cell lines (MRC-5 and/or BJ) as model systems to induce senescence, via chronic treatment with a DNA-damaging agent, namely BrdU (at a concentration of 100 μM for 8 days). Cell viability was then monitored by using the SRB assay, to measure protein content. As a consequence of this streamlined screening strategy, we identified Azithromycin and Roxithromycin as two novel clinically-approved senolytic drugs. However, Erythromycin - the very closely-related parent compound - did not show any senolytic activity, highlighting the dramatic specificity of these interactions. Interestingly, we also show that Azithromycin treatment of human fibroblasts was indeed sufficient to strongly induce both aerobic glycolysis and autophagy. However, the effects of Azithromycin on mitochondrial oxygen consumption rates (OCR) were bi-phasic, showing inhibitory activity at 50 μM and stimulatory activity at 100 μM. These autophagic/metabolic changes induced by Azithromycin could mechanistically explain its senolytic activity. We also independently validated our findings using the xCELLigence real-time assay system, which measures electrical impedance. Using this approach, we see that Azithromycin preferentially targets senescent cells, removing approximately 97% of them with great efficiency. This represents a near 25-fold reduction in senescent cells. Finally, we also discuss our current results in the context of previous clinical findings that specifically document the anti-inflammatory activity of Azithromycin in patients with cystic fibrosis - a genetic lung disorder that results in protein mis-folding mutations that cause protein aggregation.

A new method for neonatal rat ventricular myocyte purification using superparamagnetic iron oxide particles

BACKGROUND

Neonatal rat ventricular myocytes (NRVMs) have proven to be an ideal research model for cardiac disease. However, the current methods to purify NRVMs have a limitation to obtain high purity. The purpose of this study was to develop a NRVM purification method by using superparamagnetic iron oxide particles (SIOP).

METHODS

NRVMs were purified by using SIOP (SIOP group). The differential attachment with or without bromodeoxyuridine (BrdU) treatment served as control and BrdU groups, respectively. The Percoll gradient (Percoll) and magnetic-activated cell sorting (MACS) methods were performed to compare the purity and viability of NRVMs with SIOP method.

RESULTS

The SIOP group enriched NRVMs up to 93.9 ± 2.0% purity determined by flow cytometry (FCM) and 95.6 ± 1.3% by immunofluorescence count (IF). In contrast, the control group gave purities of 71.9 ± 2.9% (by FCM) and 66.8 ± 8.9% (by IF), and the BrdU group obtained 82.0 ± 1.3% (by FCM) and 83.1 ± 2.4% (by IF). The purity of SIOP-isolated NRVMs was not different from that of Percoll and MACS groups. However, the cardiomyocytes separated by these methods, except SIOP protocol, were mixed with intrinsic cardiac adrenergic cells. NRVMs purified by SIOP shaped the similar three-dimensional morphology, with no difference in cell yield, viability and cytosolic Ca²⁺ homeostasis at 24 h after isolation compared with NRVMs in other groups. Furthermore, SIOP-purified NRVMs retained the responses to phenylephrine and lipopolysaccharide challenge.

CONCLUSION

We first reported an efficient and novel method to purify NRVMs using SIOP, which may help accelerate innovative research in the field of cardiomyocyte biology.

GNG11 (G-protein subunit γ 11) suppresses cell growth with induction of reactive oxygen species and abnormal nuclear morphology in human SUSM-1 cells

Enforced expression of GNG11, G-protein subunit γ 11, induces cellular senescence in normal human diploid fibroblasts. We here examined the effect of the expression of GNG11 on the growth of immortalized human cell lines, and found that it suppressed the growth of SUSM-1 cells, but not of HeLa cells. We then compared these two cell lines to understand the molecular basis for the action of GNG11. We found that expression of GNG11 induced the generation of reactive oxygen species (ROS) and abnormal nuclear morphology in SUSM-1 cells but not in HeLa cells. Increased ROS generation by GNG11 would likely be caused by the down-regulation of the antioxidant enzymes in SUSM-1 cells. We also found that SUSM-1 cells, even under normal culture conditions, showed higher levels of ROS and higher incidence of abnormal nuclear morphology than HeLa cells, and that abnormal nuclear morphology was relevant to the increased ROS generation in SUSM-1 cells. Thus, SUSM-1 and HeLa cells showed differences in the regulation of ROS and nuclear morphology, which might account for their different responses to the expression of GNG11. Thus, SUSM-1 cells may provide a unique system to study the regulatory relationship between ROS generation, nuclear morphology, and G-protein signaling.

Small molecule compounds that induce cellular senescence

To date, dozens of stress‐induced cellular senescence phenotypes have been reported. These cellular senescence states may differ substantially from each other, as well as from replicative senescence through the presence of specific senescence features. Here, we attempted to catalog virtually all of the cellular senescence‐like states that can be induced by low molecular weight compounds. We summarized biological markers, molecular pathways involved in senescence establishment, and specific traits of cellular senescence states induced by more than fifty small molecule compounds.

Combinatorial effects of continuous protein synthesis, ERK-signaling, and reactive oxygen species on induction of cellular senescence

Mammalian cells, when treated with sub-lethal doses of genotoxic stresses, slow down DNA synthesis but continue protein synthesis. Thus, these cells show an accumulation of proteins and undergo unbalanced growth. In the previous studies, we have shown that HeLa cells treated with excess thymidine or camptothecin undergo unbalanced growth, and prolonged unbalanced growth causes induction of cellular senescence, which is suppressed by restriction of protein synthesis or inhibition of ERK-signaling. In this study, we found that restriction of protein synthesis, inhibition of ERK-signaling, and elimination of reactive oxygen species showed a combinatorial effect on suppression of cellular senescence induced by excess thymidine or camptothecin. Of these, restriction of protein synthesis most effectively suppressed cellular senescence. Importantly, a similar combinatorial effect was observed in replicative senescence in normal human diploid fibroblasts. Our findings suggested that various stresses were cumulatively involved in cellular senescence, and suppression of cellular senescence was improved by combining the treatments that reduce the stresses.

Aberrant localization of lamin B receptor (LBR) in cellular senescence in human cells

5-Bromodeoxyuridine (BrdU), a thymidine analogue, induces cellular senescence in mammalian cells. BrdU induces cellular senescence probably through the regulation of chromatin because BrdU destabilizes or disrupts nucleosome positioning and decondenses heterochromatin. Since heterochromatin is tethered to the nuclear periphery through the interaction with the nuclear envelope proteins, we examined the localization of the several nuclear envelope proteins such as lamins, lamin-interacting proteins, nuclear pore complex proteins, and nuclear transport proteins in senescent cells. We have shown here that lamin B receptor (LBR) showed a change in localization in both BrdU-induced and replicative senescent cells.

Restriction of protein synthesis abolishes senescence features at cellular and organismal levels

Cellular senescence or its equivalence is induced by treatment of cells with an appropriate inducer of senescence in various cell types. Mild restriction of cytoplasmic protein synthesis prevented induction of all aspects of cellular senescence in normal and tumor-derived human cells. It allowed the cells to continuously grow with no sign of senescent features in the presence of various inducers. It also delayed replicative senescence in normal human fibroblasts. Moreover, it allowed for growth of the cells that had entered a senescent state. When adult worms of the nematode C. elegans were grown under protein-restricted conditions, their average and maximal lifespans were significantly extended. These results suggest that accumulation of cytoplasmic proteins due to imbalance in macromolecule synthesis is a fundamental cause of cellular senescence.

High concentrations of NaCl induce cell swelling leading to senescence in human cells

Cell swelling and retardation in DNA replication are always observed in senescent cells. When DNA replication is slowed down with RNA and protein syntheses unchanged in proliferating cells, it causes a phenomenon known as unbalanced growth. The purpose of this study is to assess the role of cell swelling in unbalanced growth in terms of senescence and investigate the mechanism underlying this phenomenon. We tried to induce cell swelling with minimum damage to cells in this study. We perturbed the osmoregulatory functions to induce cell swelling under hypotonic and hypertonic conditions in normal human fibroblasts. Addition of excess NaCl was found to induce significant cell and nuclear swelling in dose- and time-dependent manners. Excess NaCl immediately retarded DNA replication, accumulated cells at G1 phase of the cell cycle, and eventually deprived division potential of the cells. Such cells showed typical senescent cell shape followed by expression of the typical senescence-associated genes. Excess NaCl also activated ERK1/2, p38, and JNK of the mitogen activated protein kinase family. Addition of U0126, an inhibitor of ERK1/2, prevented appearance of senescent features induced by excess NaCl. These results suggest that hypertonic conditions induce cell swelling due to unbalanced growth, thereby leading to cellular senescence.

Disruptive chemicals, senescence and immortality

Carcinogenesis is thought to be a multistep process, with clonal evolution playing a central role in the process. Clonal evolution involves the repeated 'selection and succession' of rare variant cells that acquire a growth advantage over the remaining cell population through the acquisition of 'driver mutations' enabling a selective advantage in a particular micro-environment. Clonal selection is the driving force behind tumorigenesis and possesses three basic requirements: (i) effective competitive proliferation of the variant clone when compared with its neighboring cells, (ii) acquisition of an indefinite capacity for self-renewal, and (iii) establishment of sufficiently high levels of genetic and epigenetic variability to permit the emergence of rare variants. However, several questions regarding the process of clonal evolution remain. Which cellular processes initiate carcinogenesis in the first place? To what extent are environmental carcinogens responsible for the initiation of clonal evolution? What are the roles of genotoxic and non-genotoxic carcinogens in carcinogenesis? What are the underlying mechanisms responsible for chemical carcinogen-induced cellular immortality? Here, we explore the possible mechanisms of cellular immortalization, the contribution of immortalization to tumorigenesis and the mechanisms by which chemical carcinogens may contribute to these processes.

Repeated exposure to 5-bromo-2'-deoxyuridine causes decreased proliferation and low-grade inflammation in the lungs of mice

Incorporation of 5-bromo-2'-deoxyuridine (BrdU) into proliferating cells has been used to label dividing cells in many tissues. Although BrdU has been shown to be genotoxic, teratogenic and mutagenic, such adverse effects have largely been ignored by researchers. We determined whether long-term BrdU exposure causes any histopathological changes in the lungs of mice. Eight-week-old male C57/BL6J mice were administered BrdU by intraperitoneal injection on 3 consecutive days of each week for 14 weeks. While no obvious structural changes such as tissue damage, fibrosis, emphysema, airway remodeling, vascular thickening or tumorigenesis were noted, a moderate degree of macrophage infiltration was observed in the airways and lung parenchyma in the lungs of the mice exposed repeatedly to BrdU (BrdU-exposed mice). The proliferative activities of the airway and alveolar epithelial and mesenchymal cells were reduced in the BrdU-exposed mice, although the numbers of these cells in the lungs were maintained. Double immunofluorescence study of the lungs of the BrdU-exposed mice showed overexpression of IL-6 in the airway epithelial and alveolar wall cells, some of which were also double-positive for BrdU. These results indicate that long-term exposure to BrdU inhibits cell proliferation and induces low-grade inflammation in the lungs of mice. Our findings underscore the need for caution in the interpretation of studies that involve long-term exposure to BrdU.

Temporally distinct roles of ATM and ROS in genotoxic-stress-dependent induction and maintenance of cellular senescence

Cells exposed to genotoxic stress induce cellular senescence through a DNA damage response (DDR) pathway regulated by ATM kinase and reactive oxygen species (ROS). Here, we show that the regulatory roles for ATM kinase and ROS differ during induction and maintenance of cellular senescence. Cells treated with different genotoxic agents were analyzed using specific pathway markers and inhibitors to determine that ATM kinase activation is directly proportional to the dose of the genotoxic stress and that senescence initiation is not dependent on ROS or the p53 status of cells. Cells in which ROS was quenched still activated ATM and initiated the DDR when insulted, and progressed normally to senescence. By contrast, maintenance of a viable senescent state required the presence of ROS as well as activated ATM. Inhibition or removal of either of the components caused cell death in senescent cells, through a deregulated ATM–ROS axis. Overall, our work demonstrates existence of an intricate temporal hierarchy between genotoxic stress, DDR and ROS in cellular senescence. Our model reports the existence of different stages of cellular senescence with distinct regulatory networks.

Molecular basis for premature senescence induced by surfactants in normal human cells

Sublethal doses of surfactants as exemplified by NP-40 clearly induce premature senescence in normal human cells. To understand molecular basis for this phenomenon, we tried to suppress it with use of various inhibitors. An inhibitor of p38 of the MAPK family almost completely suppressed growth arrest and morphological changes induced by surfactants; however, other inhibitors tested had no effect. Oleic acid, a weak inducer of premature senescence, was found to suppress the effect of NP-40. Fluorescein-labeled oleic acid rapidly bound to the cell surface, and this binding was clearly blocked by pre-treatment with surfactants, suggesting that surfactants and oleic acid compete for binding to the cell surface. Moderate concentrations of cycloheximide, an inhibitor of protein synthesis, also suppressed the senescent features induced by NP-40. These results suggest that surfactants activate p38 signaling pathway by binding to the cell surface, and induce cellular senescence.

Regulation of transcription termination by glucosylated hydroxymethyluracil, base J, in Leishmania major and Trypanosoma brucei

Base J, β-d-glucosyl-hydroxymethyluracil, is an epigenetic modification of thymine in the nuclear DNA of flagellated protozoa of the order Kinetoplastida. J is enriched at sites involved in RNA polymerase (RNAP) II initiation and termination. Reduction of J in Leishmania tarentolae via growth in BrdU resulted in cell death and indicated a role of J in the regulation of RNAP II termination. To further explore J function in RNAP II termination among kinetoplastids and avoid indirect effects associated with BrdU toxicity and genetic deletions, we inhibited J synthesis in Leishmania major and Trypanosoma brucei using DMOG. Reduction of J in L. major resulted in genome-wide defects in transcription termination at the end of polycistronic gene clusters and the generation of antisense RNAs, without cell death. In contrast, loss of J in T. brucei did not lead to genome-wide termination defects; however, the loss of J at specific sites within polycistronic gene clusters led to altered transcription termination and increased expression of downstream genes. Thus, J regulation of RNAP II transcription termination genome-wide is restricted to Leishmania spp., while in T. brucei it regulates termination and gene expression at specific sites within polycistronic gene clusters.

5-Bromodeoxyuridine Increases Transient Expression of Ectopic Genes in Human Cells

Ectopic genes transferred to cells are temporally expressed, although this phenomenon has not yet been well characterized. We found that 5-bromodeoxyuridine dramatically increased transient expression of ectopic genes in human cells. This effect was elicited by adding 5-bromodeoxyuridine prior to or after transfection. No promoter specificity was observed. Real time PCR analysis showed an approximately 2-fold increase in mRNA levels. Since 5-bromodeoxyuridine decondenses heterochromatin and changes the nuclear envelope, these changes might affect transcriptional and post-transcriptional events in the gene expression of plasmids.

Proteasome Inhibitors Induce Changes in Chromatin Structure Characteristic of Senescent Human Fibroblasts

Inhibitors of proteasome induced premature senescence in normal human fibroblasts. Besides morphological alteration and expression of senescence marker genes, these cells manifested senescence-associated heterochromatic foci under staining of the nuclei with DAPI similar to normally senescent cells. These results suggest that declining ability in protein degradation may be involved in the formation of heterochromatic foci in senescent fibroblasts.

Targeting of DNA Damage Signaling Pathway Induced Senescence and Reduced Migration of Cancer cells

The heat shock 70 family protein, mortalin, has pancytoplasmic distribution pattern in normal and perinuclear in cancer human cells. Cancer cells when induced to senesce by either chemicals or stress showed shift in mortalin staining pattern from perinuclear to pancytoplasmic type. Using such shift in mortalin staining as a reporter, we screened human shRNA library and identified nine senescence-inducing siRNA candidates. An independent Comparative Genomic Hybridization analysis of 35 breast cancer cell lines revealed that five (NBS1, BRCA1, TIN2, MRE11A, and KPNA2) of the nine genes located on chromosome regions identified as the gain of locus in more than 80% cell lines. By gene-specific PCR, these five genes were found to be frequently amplified in cancer cell lines. Bioinformatics revealed that the identified targets were connected to MRN (MRE11-RAD50-NBS1) complex, the DNA damage-sensing complex. We demonstrate that the identified shRNAs triggered DNA damage response and induced the expression of tumor suppressor protein p16(INK4A) causing growth arrest of cancer cells. Furthermore, cells showed decreased migration, mediated by decrease in matrix metalloproteases. Taken together, we demonstrate that the MRN complex is a potential target of cancer cell proliferation and migration, and staining pattern of mortalin could serve as an assay to identify senescence-inducing/anticancer reagents.

Tumor dormancy, oncogene addiction, cellular senescence, and self-renewal programs

Cancers are frequently addicted to initiating oncogenes that elicit aberrant cellular proliferation, self-renewal, and apoptosis. Restoration of oncogenes to normal physiologic regulation can elicit dramatic reversal of the neoplastic phenotype, including reduced proliferation and increased apoptosis of tumor cells (Science 297(5578):63-64, 2002). In some cases, oncogene inactivation is associated with compete elimination of a tumor. However, in other cases, oncogene inactivation induces a conversion of tumor cells to a dormant state that is associated with cellular differentiation and/or loss of the ability to self-replicate. Importantly, this dormant state is reversible, with tumor cells regaining the ability to self-renew upon oncogene reactivation. Thus, understanding the mechanism of oncogene inactivation-induced dormancy may be crucial for predicting therapeutic outcome of targeted therapy. One important mechanistic insight into tumor dormancy is that oncogene addiction might involve regulation of a decision between self-renewal and cellular senescence. Recent evidence suggests that this decision is regulated by multiple mechanisms that include tumor cell-intrinsic, cell-autonomous mechanisms and host-dependent, tumor cell-non-autonomous programs (Mol Cell 4(2):199-207, 1999; Science 297(5578):102-104, 2002; Nature 431(7012):1112-1117, 2004; Proc Natl Acad Sci U S A 104(32):13028-13033, 2007). In particular, the tumor microenvironment, which is known to be critical during tumor initiation (Cancer Cell 7(5):411-423, 2005; J Clin Invest 121(6):2436-2446, 2011), prevention (Nature 410(6832):1107-1111, 2001), and progression (Cytokine Growth Factor Rev 21(1):3-10, 2010), also appears to dictate when oncogene inactivation elicits the permanent loss of self-renewal through induction of cellular senescence (Nat Rev Clin Oncol 8(3):151-160, 2011; Science 313(5795):1960-1964, 2006; N Engl J Med 351(21):2159-21569, 2004). Thus, oncogene addiction may be best modeled as a consequence of the interplay amongst cell-autonomous and host-dependent programs that define when a therapy will result in tumor dormancy.

ERK1/2 mediates unbalanced growth leading to senescence induced by excess thymidine in human cells

Excess thymidine induces unbalanced growth by delaying DNA replication and subsequently induces senescence in every human cell type. Our previous studies with use of inhibitors suggested that ERK1/2 has a major role in these processes. Here we directly assessed the roles of ERK1 and ERK2 in unbalanced growth induced by excess thymidine. Knockdown of ERK2 and ERK1 by vector-based RNA interference prevented loss of colony forming ability and appearance of senescence markers induced by excess thymidine in HeLa and TIG-7 cells, respectively. Such cells continued growing in the presence of excess thymidine. Double knockdown of ERK1 and ERK2 did not improve the effects of single knockdowns of ERK1 and ERK2 in either cell types. These results demonstrate that ERK1 or ERK2 has a major role in manifestation of unbalanced growth in human cells.

Accumulation of annexin A5 at the nuclear envelope is a biomarker of cellular aging

Cellular senescence is a permanent cell cycle arrest induced by short telomeres or oncogenic stress in vitro and in vivo. Because no single of the established biomarkers can reliably identify senescent cells, the application of new ones may aid the diagnosis of aged cells. Here we show that annexin A5 accumulates at the nuclear envelope during replicative and drug-induced cellular senescence in primary human fibroblasts. This new cellular aging phenotype that we have termed SA-ANX5 (senescence-associated accumulation at the nuclear envelope of annexin A5) is as efficient and quantitative as the well-established senescence-associated β-galactosidase activity assay and p21 immunoreactivity. SA-ANX5 is also observed in aged human skin where is exclusively detected in DNA damage foci-positive/Ki-67-negative cells. We also observed that depletion of annexin A5 by siRNA in human fibroblasts accelerates premature senescence through the p38MAP kinase pathway. These observations establish SA-ANX5 as a new biomarker for cellular aging and implicate a functional role for annexin A5 in cellular senescence.

Neural stem cells exposed to BrdU lose their global DNA methylation and undergo astrocytic differentiation

Bromodeoxyuridine (5-bromo-2′-deoxyuridine, BrdU) is a halogenated nucleotide of low toxicity commonly used to monitor DNA replication. It is considered a valuable tool for in vitro and in vivo studies, including the detection of the small population of neural stem cells (NSC) in the mammalian brain. Here, we show that NSC grown in self-renewing conditions in vitro, when exposed to BrdU, lose the expression of stem cell markers like Nestin, Sox2 and Pax6 and undergo glial differentiation, strongly up-regulating the astrocytic marker GFAP. The onset of GFAP expression in BrdU exposed NSC was paralleled by a reduced expression of key DNA methyltransferases (DNMT) and a rapid loss of global DNA CpG methylation, as we determined by our specially developed analytic assay. Remarkably, a known DNA demethylating compound, 5-aza-2′-deoxycytidine (Decitabine), had similar effect on demethylation and differentiation of NSC. Since our key findings apply also to NSC derived from murine forebrain, our observations strongly suggest more caution in BrdU uses in stem cells research. We also propose that BrdU and its related substances may also open new opportunities for differentiation therapy in oncology.

Ethynyldeoxyuridine (EdU) suppresses in vitro population expansion and in vivo tumor progression of human glioblastoma cells

Thymidine analogs (TAs) are synthetic nucleosides that incorporate into newly synthesized DNA. Halogenated pyrimidines (HPs), such as bromodeoxyuridine (BrdU), are a class of TAs that can be detected with antibodies and are commonly used for birthdating individual cells and for assessing the proliferative index of cell populations. It is well established that HPs can act as radiosensitizers when incorporated into DNA chains, but they are generally believed not to impair normal cell function in the absence of secondary stressors. However, we and others have shown that HP incorporation leads to a sustained suppression of cell cycle progression in mammalian cells, resulting in cellular senescence in somatic cells. In addition, we have shown that HP incorporation results in delayed tumor progression in a syngeneic rat model of glioma. Here we examine ethynyldeoxyuridine (EdU), a newly developed and alkylated TA, for its anti-cancer activity, both in vitro and in vivo. We show that EdU, like HPs, leads to a severe reduction in the proliferation rate of normal and transformed cells in vitro. Unlike HPs, however, EdU incorporation also causes DNA damage resulting in the death of a substantial subset of treated cells. When administered over an extended time as a monotherapy to mice bearing subcutaneous xenografts of human glioblastoma multiforme tumors, EdU significantly reduces tumor volume and increases survival without apparent significant toxicity. These results, combined with the fact that EdU readily crosses the blood-brain barrier, support the continued investigation of EdU as a potential therapy for malignant brain tumors.

The dark side of BrdU in neural stem cell biology: detrimental effects on cell cycle, differentiation and survival

5-Bromo-2'-deoxyuridin (BrdU) is frequently used in anaylsis of neural stem cell biology, in particular to label and to fate-map dividing cells. However, up to now, only a few studies have addressed the question as to whether BrdU labeling per se affects the cells to be investigated. Here, we focused on the potential impact of BrdU on neurosphere cultures derived from the adult rat brain and on proliferation of progenitors in vivo. In vitro, neurospheres were pulsed for 48 h with BrdU, and cell proliferation, cell cycle, differentiation, survival and adhesion properties were subsequently analyzed. BrdU inhibited the expansion of neural progenitors as assessed by MTS assay and increased the fraction of cells in the G0/G1-phase of the cell cycle. Moreover, BrdU increased cell death and dose-dependently induced adherence of NPCs. Cell adherence was accompanied by a reduced amount of active matrix-metalloproteinase-2 (MMP-2). Furthermore, BrdU repressed neuronal and oligodendroglial differentiation, whereas astroglial fate was not affected. In contrast to the in vitro situation, BrdU apparently did not influence endogenous proliferation of NPCs or neurogenesis in concentrations that are typically used for labeling of neural progenitors in vivo. Our results reveal so far uncharacterized effects of BrdU on adult NPCs. We conclude that, because of its ubiquitous use in stem cell biology, any potential effect of BrdU of NPCs has to be scrutinized prior to interpretation of data.

N-terminal short fragment of TUP1 confers resistance to 5-bromodeoxyuridine in the yeast Saccharomyces cerevisiae

Small molecules that exhibit biological activity have contributed to the understanding of the molecular mechanisms of various biological phenomena. 5-Bromodeoxyuridine (BrdU) is a thymidine analogue that modulates various biological phenomena such as cellular differentiation and cellular senescence in cultured mammalian cells. Although BrdU is thought to function through changing chromatin structure and gene expression, its precise molecular mechanisms are not understood. To study the molecular mechanism for the action of BrdU, we have employed the yeast Saccharomyces cerevisiae as a model system, and screened multi-copy suppressor genes that confer resistance to BrdU. Our genetic screen has revealed that expression of the N-terminal short fragment of TUP1, and also disruption of HDA1 or HOS1, histone deacetylases that interact with TUP1, conferred resistance to BrdU. These results suggest the implication of the chromatin proteins in the function of BrdU, and would provide novel clues to answer the old question of how BrdU modulates various biological phenomena.