Abacavir, an anti-HIV-1 drug, targets TDP1-deficient adult T cell leukemia

REV3 promotes cellular tolerance to 5-fluorodeoxyuridine by activating translesion DNA synthesis and intra-S checkpoint

The drug floxuridine (5-fluorodeoxyuridine, FUdR) is an active metabolite of 5-Fluorouracil (5-FU). It converts to 5-fluorodeoxyuridine monophosphate (FdUMP) and 5-fluorodeoxyuridine triphosphate (FdUTP), which on incorporation into the genome inhibits DNA replication. Additionally, it inhibits thymidylate synthase, causing dTMP shortage while increasing dUMP availability, which induces uracil incorporation into the genome. However, the mechanisms underlying cellular tolerance to FUdR are yet to be fully elucidated. In this study, we explored the mechanisms underlying cellular resistance to FUdR by screening for FUdR hypersensitive mutants from a collection of DT40 mutants deficient in each genomic maintenance system. We identified REV3, which is involved in translesion DNA synthesis (TLS), to be a critical factor in FUdR tolerance. Replication using a FUdR-damaged template was attenuated in REV3-/- cells, indicating that the TLS function of REV3 is required to maintain replication on the FUdR-damaged template. Notably, FUdR-exposed REV3-/- cells exhibited defective cell cycle arrest in the early S phase, suggesting that REV3 is involved in intra-S checkpoint activation. Furthermore, REV3-/- cells showed defects in Chk1 phosphorylation, which is required for checkpoint activation, but the survival of FUdR-exposed REV3-/- cells was further reduced by the inhibition of Chk1 or ATR. These data indicate that REV3 mediates DNA checkpoint activation at least through Chk1 phosphorylation, but this signal acts in parallel with ATR-Chk1 DNA damage checkpoint pathway. Collectively, we reveal a previously unappreciated role of REV3 in FUdR tolerance.

A comparative study of different antiviral treatment protocols in HCV related cryoglobulinemic vasculitis

The treatment of HCV and its sequelae are used to be predominantly based on Interferon (IFN). However, this was associated with significant adverse events as a result of its immunostimulant capabilities. Since their introduction, the directly acting antiviral drugs (DAAs), have become the standard of care to treat of HCV and its complications including mixed cryoglobulinemic vasculitis (MCV). In spite of achieving sustained viral response (SVR), there appeared many reports describing unwelcome complications such as hepatocellular and hematological malignancies as well as relapses. Prolonged inflammation induced by a multitude of factors, can lead to DNA damage and affects BAFF and APRIL, which serve as markers of B-cell proliferation. We compared, head-to-head, three antiviral protocols for HCV-MCV treatment As regards the treatment response and relapse, levels of BAFF and APRIL among pegylated interferon α-based and free regimens (Sofosbuvir + Ribavirin; SOF-RIBA, Sofosbuvir + Daclatasvir; SOF-DACLA). Regarding clinical response HCV-MCV and SVR; no significant differences could be identified among the 3 different treatment protocols, and this was also independent form using IFN. We found no significant differences between IFN-based and free regimens DNA damage, markers of DNA repair, or levels of BAFF and APRIL. However, individualized drug-to-drug comparisons showed many differences. Those who were treated with IFN-based protocol showed decreased levels of DNA damage, while the other two IFN-free groups showed increased DNA damage, being the worst in SOF-DACLA group. There were increased levels of BAFF through follow-up periods in the 3 protocols being the best in SOF-DACLA group (decreased at 24 weeks). In SOF-RIBA, CGs relapsed significantly during the follow-up period. None of our patients who were treated with IFN-based protocol had significant clinico-laboratory relapse. Those who received IFN-free DAAs showed a statistically significant relapse of constitutional manifestations. Our findings suggest that IFN-based protocols are effective in treating HCV-MCV similar to IFN-free protocols. They showed lower levels of DNA damage and repair. We believe that our findings may offer an explanation for the process of lymphoproliferation, occurrence of malignancies, and relapses by shedding light on such possible mechanisms.

Dominant roles of BRCA1 in cellular tolerance to a chain-terminating nucleoside analog, alovudine

Alovudine is a chain-terminating nucleoside analog (CTNA) that is frequently used as an antiviral and anticancer agent. Generally, CTNAs inhibit DNA replication after their incorporation into nascent DNA during DNA synthesis by suppressing subsequent polymerization, which restricts the proliferation of viruses and cancer cells. Alovudine is a thymidine analog used as an antiviral drug. However, the mechanisms underlying the removal of alovudine and DNA damage tolerance pathways involved in cellular resistance to alovudine remain unclear. Here, we explored the DNA damage tolerance pathways responsible for cellular tolerance to alovudine and found that BRCA1-deficient cells exhibited the highest sensitivity to alovudine. Moreover, alovudine interfered with DNA replication in two distinct mechanisms: first: alovudine incorporated at the end of nascent DNA interfered with subsequent DNA synthesis; second: DNA replication stalled on the alovudine-incorporated template strand. Additionally, BRCA1 facilitated the removal of the incorporated alovudine from nascent DNA, and BRCA1-mediated homologous recombination (HR) contributed to the progressive replication on the alovudine-incorporated template. Thus, we have elucidated the previously unappreciated mechanism of alovudine-mediated inhibition of DNA replication and the role of BRCA1 in cellular tolerance to alovudine.

Repair of topoisomerase 1-induced DNA damage by tyrosyl-DNA phosphodiesterase 2 (TDP2) is dependent on its magnesium binding

Topoisomerases are enzymes that relax DNA supercoiling during replication and transcription. Camptothecin, a topoisomerase 1 (TOP1) inhibitor, and its analogs trap TOP1 at the 3'-end of DNA as a DNA-bound intermediate, resulting in DNA damage that can kill cells. Drugs with this mechanism of action are widely used to treat cancers. It has previously been shown that tyrosyl-DNA phosphodiesterase 1 (TDP1) repairs TOP1-induced DNA damage generated by camptothecin. In addition, tyrosyl-DNA phosphodiesterase 2 (TDP2) plays critical roles in repairing topoisomerase 2 (TOP2)-induced DNA damage at the 5'-end of DNA and in promoting the repair of TOP1-induced DNA damage in the absence of TDP1. However, the catalytic mechanism by which TDP2 processes TOP1-induced DNA damage has not been elucidated. In this study, we found that a similar catalytic mechanism underlies the repair of TOP1- and TOP2-induced DNA damage by TDP2, with Mg2+-TDP2 binding playing a role in both repair mechanisms. We show chain-terminating nucleoside analogs are incorporated into DNA at the 3'-end and abort DNA replication to kill cells. Furthermore, we found that Mg2+-TDP2 binding also contributes to the repair of incorporated chain-terminating nucleoside analogs. Overall, these findings reveal the role played by Mg2+-TDP2 binding in the repair of both 3'- and 5'-blocking DNA damage.

Zidovudine and Interferon Alfa based regimens for the treatment of adult T-cell leukemia/lymphoma (ATLL): a systematic review and meta-analysis

BACKGROUND

ATLL (Adult T-Cell Leukemia/Lymphoma) is an aggressive hematological malignancy. This T-cell non-Hodgkin lymphoma, caused by the human T-cell leukemia virus type 1 (HTLV-1), is challenging to treat. There is no known treatment for ATLL as of yet. However, it is recommended to use Zidovudine and Interferon Alfa-based regimens (AZT/IFN), chemotherapy, and stem cell transplant. This study aims to review the outcome of patients with different subtypes of ATLL treated with Zidovudine and Interferon Alfa-based regimens.

METHODS

A systematic search was carried out for articles evaluating outcomes of ATLL treatment by AZT/IFN agents on human subjects from January 1, 2004, until July 1, 2022. Researchers assessed all studies regarding the topic, followed by extracting the data. A random-effects model was used in the meta-analyses.

RESULTS

We obtained fifteen articles on the AZT/IFN treatment of 1101 ATLL patients. The response rate of the AZT/IFN regimen yielded an OR of 67% [95% CI: 0.50; 0.80], a CR of 33% [95% CI: 0.24; 0.44], and a PR of 31% [95% CI: 0.24; 0.39] among individuals who received this regimen at any point during their treatment. Our subgroup analyses' findings demonstrated that patients who received front-line and combined AZT/IFN therapy responded better than those who received AZT/IFN alone. It is significant to note that patients with indolent subtypes of disease had considerably higher response rates than individuals with aggressive disease.

CONCLUSION

IFN/AZT combined with chemotherapy regimens is an effective treatment for ATLL patients, and its use in the early stages of the disease may result in a greater response rate.

HTLV-1 bZIP factor impairs DNA mismatch repair system

Adult T-cell leukemia (ATL) is a peripheral T-cell malignancy caused by human T-cell leukemia virus type 1 (HTLV-1). Microsatellite instability (MSI) has been observed in ATL cells. Although MSI results from impaired mismatch repair (MMR) pathway, no null mutations in the genes encoding MMR factors are detectable in ATL cells. Thus, it is unclear whether or not impairment of MMR causes the MSI in ATL cells. HTLV-1 bZIP factor (HBZ) protein interacts with numerous host transcription factors and significantly contributes to disease pathogenesis and progression. Here we investigated the effect of HBZ on MMR in normal cells. The ectopic expression of HBZ in MMR-proficient cells induced MSI, and also suppressed the expression of several MMR factors. We then hypothesized that the HBZ compromises MMR by interfering with a transcription factor, nuclear respiratory factor 1 (NRF-1), and identified the consensus NRF-1 binding site at the promoter of the gene encoding MutS homologue 2 (MSH2), an essential MMR factor. The luciferase reporter assay revealed that NRF-1 overexpression enhanced MSH2 promoter activity, while co-expression of HBZ reversed this enhancement. These results supported the idea that HBZ suppresses the transcription of MSH2 by inhibiting NRF-1. Our data demonstrate that HBZ causes impaired MMR, and may imply a novel oncogenesis driven by HTLV-1.

TDP1 and TOP1 as targets in anticancer treatment of NSCLC: Activity and protein level in normal and tumor tissue from 150 NSCLC patients correlated to clinical data

OBJECTIVES

Topoisomerase 1 (TOP1) is a drug target used in anticancer treatment of various cancer types. The effect of the TOP1 drugs can be counteracted by the enzymatic activity of tyrosyl-DNA phosphodiesterase 1 (TDP1). Thus, to elucidate the relevance of combining TDP1 and TOP1 as drug targets for anticancer treatment in NSCLC, TDP1 and TOP1 was for the first time quantified in a large cohort of paired normal and tumor tissue from NSCLC patients, and data were correlated between the two enzymes and to clinical data.

MATERIALS AND METHODS

TDP1 and TOP1 activity and protein concentration were measured in paired normal and tumor tissue from 150 NSCLC patients using TDP1 and TOP1 specific biosensors and ELISA. TDP1 and TOP1 activity and protein concentration were correlated to clinical data.

RESULTS

TDP1 and TOP1 activity and protein concentration were significantly upregulated from normal to tumor tissue for the individual patients, but did not correlate to any of the clinical data. TDP1 and TOP1 activity were upregulated in 89.3% and 82.7% of the patients, respectively, and correlated in both normal and tumor tissue. The same tendency was observed for protein concentration with an upregulation of TDP1 and TOP1 in 73.0% and 84.4% of the patients, respectively. The activity and protein concentration correlated in normal and tumor tissue for both TDP1 and TOP1.

CONCLUSION

The upregulations of TDP1 and TOP1 from normal to tumor tissue combined with the observation that TDP1 and TOP1 did not correlate to any of the clinical data indicate that both proteins are important for development or maintenance of the tumor cells in NSCLC. Correlations between TDP1 and TOP1 indicate a biological dependency and potential co-regulation of the enzymes. These observations is encouraging in relation to using TOP1 and TDP1 as targets in anticancer treatment of NSCLC.

Current status of drug repositioning in hematology

INTRODUCTION

Drug repositioning (DR) is defined as determining new therapeutic applications for existing drugs. This approach is advantageous over de novo drug discovery in accelerating clinical development, in terms of lower costs, a shortened development period, a well-known action mechanism, a feasible dosage, and an acceptable safety profile.

AREAS COVERED

This work was aimed at reviewing agents with successful DR in hematology.

EXPERT OPINION

Thalidomide and plerixafor have been successfully repositioned for treating multiple myeloma and harvesting peripheral blood stem cells, respectively. The former was originally developed as a sedative and the latter as an anti-HIV drug. Currently, the feasibility of repositioning various agents is being explored (e.g. an anti-influenza virus drug oseltamivir for primary immune thrombocytopenia, an anti-HIV drug abacavir for adult T-cell leukemia, and a macrolide antibiotic clarithromycin for multiple myeloma). Furthermore, bosutinib for chronic myeloid leukemia or the antiplatelet drug cilostazol have been suggested to have clinical benefits for the management of amyotrophic lateral sclerosis and ischemic stroke, respectively. To promote DR, effective application of artificial intelligence or stem cell models, comprehensive database construction shared between academia and pharmaceutical companies, suitable handling of drug patents, and wide cooperation in the area of specialty are warranted.

Activation of PERK-ATF4-CHOP pathway as a novel therapeutic approach for efficient elimination of HTLV-1-infected cells

Patients with adult T-cell leukemia (ATL) exhibit a poor prognosis and overall survival rate when treated with standard chemotherapy, highlighting the continued requirement for the development of novel safe and effective therapies for human T-cell leukemia virus type 1 (HTLV-1)-related diseases. In this study, we demonstrated that MK-2048, a second-generation HIV-1 integrase (IN) inhibitor, potently and selectively kills HTLV-1-infected cells. Differential transcriptome profiling revealed significantly elevated levels of gene expression of the unfolded protein response (UPR) PKR-like ER kinase (PERK) signaling pathway in ATL cell lines following MK-2048 treatment. We also identified a significant downregulation in glucose regulated protein 78 (GRP78), a master regulator of the UPR in the CD4+CADM1+ HTLV-1-infected cell population of primary HTLV-1 carrier peripheral blood mononuclear cells (PBMCs) (n = 9), suggesting that HTLV-1-infected cells are hypersensitive to endoplasmic reticulum (ER) stress-mediated apoptosis. MK-2048 efficiently reduced proviral loads in primary HTLV-1 carrier PBMCs (n = 4), but had no effect on the total numbers of these cells, indicating that MK-2048 does not affect the proliferation of HTLV-1-uninfected PBMCs. MK-2048 specifically activated the ER stress-related proapoptotic gene, DNA damage-inducible transcript 3 protein (DDIT3), also known as C/EBP homologous protein (CHOP), in HTLV-1-infected but not uninfected cells of HTLV-1-carrier PBMCs. Our findings demonstrated that MK-2048 selectively induces HTLV-1-infected cell apoptosis via the activation of the UPR. This novel regulatory mechanism of the HIV IN inhibitor MK-2048 in HTLV-1-infected cells provides a promising prophylactic and therapeutic target for HTLV-1-related diseases including ATL.

Tyrosyl-DNA phosphodiesterase 2 (TDP2) repairs topoisomerase 1 DNA-protein crosslinks and 3'-blocking lesions in the absence of tyrosyl-DNA phosphodiesterase 1 (TDP1)

Topoisomerase I (TOP1) resolves DNA topology during replication and transcription. The enzyme forms an intermediate TOP1 cleavage complex (TOP1cc) through transient TOP1-DNA-protein crosslinks. Camptothecin is a frontline anticancer agent that freezes this reaction intermediate, leading to the generation of irreversible TOP1ccs that act as 3'-blocking lesions. It is widely accepted that TOP1cc is repaired via a two-step pathway involving proteasomal degradation of TOP1cc to the crosslinked peptide, followed by removal of the TOP1cc-derived peptide from DNA by tyrosyl-DNA phosphodiesterase 1 (TDP1). In the present study, we developed an assay system to estimate repair kinetics of TOP1cc separately in the first and second steps, using monoclonal antibodies against the TOP1 protein and the TOP1 catalytic site peptide-DNA complex, respectively. Although TDP1-deficient (TDP1^-/-) TK6 cells had normal kinetics of the first step, a delay in the kinetics of the second step was observed relative to that in wild-type cells. Tyrosyl-DNA phosphodiesterase 2 (TDP2) reportedly promotes the repair of TOP1-induced DNA damage in the absence of TDP1. The present assays additionally demonstrated that TDP2 promotes the second, but not the first, step of TOP1cc repair in the absence of TDP1. We also analyzed sensitivities of TK6 cells with deficiencies in TDP1 and/or TDP2 to agents that produce 3' -blocking lesions. These experiments showed that TDP1^-/-TDP2^-/- cells were more sensitive to the agents Azidothymidine (zidovudine), Cytarabine, Abacavir, Gemcitabine, and Trifluridine than TDP1^-/- or TDP2^-/- cells. Taken together, our findings confirm the roles of TDP2 in the repair of 3'-blocking lesions.

CtIP-BRCA1 complex and MRE11 maintain replication forks in the presence of chain terminating nucleoside analogs

Chain-terminating nucleoside analogs (CTNAs), which cannot be extended by DNA polymerases, are widely used as antivirals or anti-cancer agents, and can induce cell death. Processing of blocked DNA ends, like camptothecin-induced trapped-topoisomerase I, can be mediated by TDP1, BRCA1, CtIP and MRE11. Here, we investigated whether the CtIP-BRCA1 complex and MRE11 also contribute to cellular tolerance to CTNAs, including 2',3'-dideoxycytidine (ddC), cytarabine (ara-C) and zidovudine (Azidothymidine, AZT). We show that BRCA1-/-, CtIPS332A/-/- and nuclease-dead MRE11D20A/- mutants display increased sensitivity to CTNAs, accumulate more DNA damage (chromosomal breaks, γ-H2AX and neutral comets) when treated with CTNAs and exhibit significant delays in replication fork progression during exposure to CTNAs. Moreover, BRCA1-/-, CtIPS332A/-/- and nuclease-dead MRE11D20A/- mutants failed to resume DNA replication in response to CTNAs, whereas control and CtIP+/-/- cells experienced extensive recovery of DNA replication. In summary, we provide clear evidence that MRE11 and the collaborative action of BRCA1 and CtIP play a critical role in the nuclease-dependent removal of incorporated ddC from replicating genomic DNA. We propose that BRCA1-CTIP and MRE11 prepare nascent DNA ends, blocked from synthesis by CTNAs, for further repair.

Tyrosyl-DNA phosphodiesterases: rescuing the genome from the risks of relaxation

Tyrosyl–DNA Phosphodiesterases 1 (TDP1) and 2 (TDP2) are eukaryotic enzymes that clean-up after aberrant topoisomerase activity. While TDP1 hydrolyzes phosphotyrosyl peptides emanating from trapped topoisomerase I (Top I) from the 3′ DNA ends, topoisomerase 2 (Top II)-induced 5′-phosphotyrosyl residues are processed by TDP2. Even though the canonical functions of TDP1 and TDP2 are complementary, they exhibit little structural or sequence similarity. Homozygous mutations in genes encoding these enzymes lead to the development of severe neurodegenerative conditions due to the accumulation of transcription-dependent topoisomerase cleavage complexes underscoring the biological significance of these enzymes in the repair of topoisomerase–DNA lesions in the nervous system. TDP1 can promiscuously process several blocked 3′ ends generated by DNA damaging agents and nucleoside analogs in addition to hydrolyzing 3′-phosphotyrosyl residues. In addition, deficiency of these enzymes causes hypersensitivity to anti-tumor topoisomerase poisons. Thus, TDP1 and TDP2 are promising therapeutic targets and their inhibitors are expected to significantly synergize the effects of current anti-tumor therapies including topoisomerase poisons and other DNA damaging agents. This review covers the structural aspects, biology and regulation of these enzymes, along with ongoing developments in the process of discovering safe and effective TDP inhibitors.

Conflicting effects of poly(ADP-ribose) polymerase inhibitor on cell-mediated and virion-mediated HTLV-1 infection

Adult T-cell leukemia and human T-cell leukemia virus type 1 (HTLV-1) - associated myelopathy/tropical spastic paraparesis, which develop after HTLV-1 infection, are difficult to cure. In particular, the mode of HTLV-1 propagation is not well understood. Poly (ADP-ribose) polymerase-1 is reported to be a co-activator of HTLV-1 Tax protein; however, the effects of polyADP-ribosylation on infectivity of HTLV-1 have not been fully clarified. We studied the effects of a PARP inhibitor on two modes of HTLV-1 transmission: through cell adhesion between MT-2 cells (an HTLV-1-infected cell line) and uninfected cells and through virus particles produced by HTLV-1-producing c77 cells. Although the PARP inhibitor decreased HTLV-1 infection through cell adhesion, it increased HTLV-1 infection through virion production and caused apoptosis of HTLV-1-infected cells. Thus, careful consideration is required for clinical application of PARP inhibitors in HTLV-1 patients.

Mammalian Tyrosyl-DNA Phosphodiesterases in the Context of Mitochondrial DNA Repair

Mammalian mitochondria contain four topoisomerases encoded in the nuclear genome: TOP1MT, TOP2α, TOP2β, and TOP3α. They also contain the two known tyrosyl-DNA phosphodiesterases (TDPs): TDP1 and TDP2, including a specific TDP2^S isoform. Both TDP1 and TDP2 excise abortive topoisomerase cleavage complexes (TOPccs), yet their molecular structures and mechanisms are different. TDP1 is present across eukaryotes, from yeasts to humans and belongs to the phospholipase D family. It functions without a metal cofactor and has a broad activity range, as it also serves to cleanse blocking 3′-DNA ends bearing phosphoglycolate, deoxyribose phosphate, nucleoside, nucleoside analogs (zidovudine), abasic moieties, and with a lower efficiency, TOP2ccs. Found in higher vertebrates, TDP2 is absent in yeast where TDP1 appears to perform its functions. TDP2 belongs to the exonuclease/endonuclease/phosphodiesterase family and requires magnesium as a cofactor to excise TOP2ccs, and it also excises TOP1ccs, albeit with a lower efficiency. Here, we review TDP1 and TDP2 in the context of mitochondrial DNA repair and discuss potential new research areas centered on the mitochondrial TDPs.

Toxicity and genotoxicity induced by abacavir antiretroviral medication alone or in combination with zidovudine and/or lamivudine in Drosophila melanogaster

Abacavir (ABC), zidovudine (AZT), and lamivudine (3TC) are nucleoside analog reverse transcriptase inhibitors (NRTIs) widely used as combination-based antiretroviral therapy against human immunodeficiency virus. Despite effective viral suppression using NRTI combinations, genotoxic potential of NRTIs can be increased when administered in combination. This study investigated the toxic and genotoxic potential of ABC when administered alone or in combination with AZT and/or 3TC using the somatic mutation and recombination test in Drosophila melanogaster. This test simultaneously evaluated two events related to carcinogenic potential: mutation and somatic recombination. The results indicated that ABC was responsible for toxicity when administered alone or in combination with AZT and/or 3TC. In addition, all treatment combinations increased frequencies of mutation and somatic recombination. The combination of AZT/3TC showed the lowest genotoxic activity compared to all combinations with ABC. Therefore, our results indicated that ABC was responsible for a significant portion of genotoxic activity of these combinations. Somatic recombination was the main genetic event observed, ranging from 83.7% to 97.7%.

Probing the evolutionary conserved residues Y204, F259, S400 and W590 that shape the catalytic groove of human TDP1 for 3'- and 5'-phosphodiester-DNA bond cleavage

Tyrosyl-DNA phosphodiesterase 1 (TDP1) is an ubiquitous DNA repair enzyme present in yeast, plants and animals. It removes a broad range of blocking lesions at the ends of DNA breaks. The catalytic core of TDP1 consists in a pair of conserved histidine-lysine-asparagine (HKN) motifs. Analysis of the human TDP1 (hTDP1) crystal structure reveals potential involvement of additional residues that shape the substrate binding site. In this biochemical study, we analyzed four such conserved residues, tyrosine 204 (Y204), phenylalanine 259 (F259), serine 400 (S400) and tryptophan 590 (W590). We show that the F259 residue of hTDP1 is critical for both 3'- and 5'-phosphodiesterase catalysis. We propose that the double π-π interactions of the F259 residue with the -2 and -3 nucleobases serve to position the nucleopeptide substrate in phase with the active site histidines of hTDP1. Mutating Y204 of hTDP1 to phenylalanine (Y204F), as in fly and yeast TDP1 enzymes, had minor impact on TDP1 activity. In constrast, we find that S400 enhances 3'-processing activity while it suppresses 5'-processing activity, thereby promoting specificity for 3'-substrates. W590 is selectively important for 5'-processing. These results reveal the impact of conserved amino acid residues that participate in defining the DNA binding groove around the dual HKN catalytic core motif of TDP1, and their differential roles in facilitating the 3'- vs 5'-end processing activities of hTDP1.

The dominant role of proofreading exonuclease activity of replicative polymerase ε in cellular tolerance to cytarabine (Ara-C)

Chemotherapeutic nucleoside analogs, such as Ara-C, 5-Fluorouracil (5-FU) and Trifluridine (FTD), are frequently incorporated into DNA by the replicative DNA polymerases. However, it remains unclear how this incorporation kills cycling cells. There are two possibilities: Nucleoside analog triphosphates inhibit the replicative DNA polymerases, and/or nucleotide analogs mis-incorporated into genomic DNA interfere with the next round of DNA synthesis as replicative DNA polymerases recognize them as template DNA lesions, arresting synthesis. To address the first possibility, we selectively disrupted the proofreading exonuclease activity of DNA polymerase ε (Polε), the leading-strand replicative polymerase in avian DT40 and human TK6 cell lines. To address the second, we disrupted RAD18, a gene involved in translesion DNA synthesis, a mechanism that relieves stalled replication. Strikingly, POLE1^exo−/− cells, but not RAD18^−/− cells, were hypersensitive to Ara-C, while RAD18^−/− cells were hypersensitive to FTD. gH2AX focus formation following a pulse of Ara-C was immediate and did not progress into the next round of replication, while gH2AX focus formation following a pulse of 5-FU and FTD was delayed to the next round of replication. Biochemical studies indicate that human proofreading-deficient Polε-exo⁻ holoenzyme incorporates Ara-CTP, but subsequently extend from this base several times less efficiently than from intact nucleotides. Together our results suggest that Ara-C acts by blocking extension of the nascent DNA strand and is counteracted by the proofreading activity of Polε, while 5-FU and FTD are efficiently incorporated but act as replication fork blocks in the subsequent S phase, which is counteracted by translesion synthesis.

Alpha-terpineol affects synthesis and antitumor activity of triterpenoids fromAntrodia cinnamomeamycelia in solid-state culture

To enhance production ofAntrodia cinnamomeatriterpenoids (ACTs) from mycelia in solid-state culture, α-terpineol was added to the medium as an elicitor at an optimal concentration of 0.05 mL L⁻¹.

Dual inhibition of the mTORC1 and mTORC2 signaling pathways is a promising therapeutic target for adult T-cell leukemia

Adult T‐cell leukemia (ATL) has a poor prognosis as a result of severe immunosuppression and rapid tumor progression with resistance to conventional chemotherapy. Recent integrated‐genome analysis has revealed mutations in many genes involved in the T‐cell signaling pathway, suggesting that the aberration of this pathway is an important factor in ATL pathogenesis and ATL‐cell proliferation. We screened a siRNA library to examine signaling‐pathway functionality and found that the PI3K/Akt/mTOR pathway is critical to ATL‐cell proliferation. We therefore investigated the effect of mammalian target of rapamycin (mTOR) inhibitors, including the dual inhibitors PP242 and AZD8055 and the mTORC1 inhibitors rapamycin and everolimus, on human T‐cell leukemia virus type 1 (HTLV‐1)‐infected‐cell and ATL‐cell lines. Both dual inhibitors inhibited the proliferation of all tested cell lines by inducing G1‐phase cell‐cycle arrest and subsequent cell apoptosis, whereas the effects of the 2 mTORC1 inhibitors were limited, as they did not induce cell apoptosis. In the ATL‐cell lines and in the primary ATL samples, both dual inhibitors inhibited phosphorylation of AKT at serine‐473, a target of mTORC2, as well as that of S6K, whereas the mTORC1 inhibitors only inhibited mTORC1. Furthermore, AZD8055 more significantly inhibited the in vivo growth of the ATL‐cell xenografts than did everolimus. These results indicate that the PI3K/mTOR pathway is critical to ATL‐cell proliferation and might thus be a new therapeutic target in ATL.

HTLV-1 bZIP factor suppresses TDP1 expression through inhibition of NRF-1 in adult T-cell leukemia

Adult T-cell leukemia (ATL) is an aggressive T-cell malignancy caused by human T-cell leukemia virus type 1 (HTLV-1). We recently reported that abacavir, an anti-HIV-1 drug, potently and selectively kills ATL cells. This effect was attributed to the reduced expression of tyrosyl-DNA-phosphodiesterase 1 (TDP1), a DNA repair enzyme, in ATL cells. However, the molecular mechanism underlying the downregulation of TDP1 in ATL cells remains elusive. Here we identified the core promoter of the TDP1 gene, which contains a conserved nuclear respiratory factor 1 (NRF-1) binding site. Overexpression of NRF-1 increased TDP1-promoter activity, whereas the introduction of dominant-negative NRF-1 repressed such activity. Overexpression of NRF-1 also upregulated endogenous TDP-1 expression, while introduction of shNRF-1 suppressed TDP1 in Jurkat T cells, making them susceptible to abacavir. These results indicate that NRF-1 is a positive transcriptional regulator of TDP1-gene expression. Importantly, we revealed that HTLV-1 bZIP factor (HBZ) protein which is expressed in all ATL cases physically interacts with NRF-1 and inhibits the DNA-binding ability of NRF-1. Taken together, HBZ suppresses TDP1 expression by inhibiting NRF-1 function in ATL cells. The HBZ/NRF-1/TDP1 axis provides new therapeutic targets against ATL and might explain genomic instability leading to the pathogenesis of ATL.

TDP1 is Critical for the Repair of DNA Breaks Induced by Sapacitabine, a Nucleoside also Targeting ATM- and BRCA-Deficient Tumors

2'-C-cyano-2'-deoxy-1-β-d-arabino-pentofuranosylcytosine (CNDAC) is the active metabolite of the anticancer drug, sapacitabine. CNDAC is incorporated into the genome during DNA replication and subsequently undergoes β-elimination that generates single-strand breaks with abnormal 3'-ends. Because tyrosyl-DNA phosphodiesterase 1 (TDP1) selectively hydrolyzes nonphosphorylated 3'-blocking ends, we tested its role in the repair of CNDAC-induced DNA damage. We show that cells lacking TDP1 (avian TDP1^-/- DT40 cells and human TDP1 KO TSCER2 and HCT116 cells) exhibit marked hypersensitivity to CNDAC. We also identified BRCA1, FANCD2, and PCNA in the DNA repair pathways to CNDAC. Comparing CNDAC with the chemically related arabinosyl nucleoside analog, cytosine arabinoside (cytarabine, AraC) and the topoisomerase I inhibitor camptothecin (CPT), which both generate 3'-end blocking DNA lesions that are also repaired by TDP1, we found that inactivation of BRCA2 renders cells hypersensitive to CNDAC and CPT but not to AraC. By contrast, cells lacking PARP1 were only hypersensitive to CPT but not to CNDAC or AraC. Examination of TDP1 expression in the cancer cell line databases (CCLE, GDSC, NCI-60) and human cancers (TCGA) revealed a broad range of expression of TDP1, which was correlated with PARP1 expression, TDP1 gene copy number and promoter methylation. Thus, this study identifies the importance of TDP1 as a novel determinant of response to CNDAC across various cancer types (especially non-small cell lung cancers), and demonstrates the differential involvement of BRCA2, PARP1, and TDP1 in the cellular responses to CNDAC, AraC, and CPT. Mol Cancer Ther; 16(11); 2543-51. ©2017 AACR.

Repriming by PrimPol is critical for DNA replication restart downstream of lesions and chain-terminating nucleosides

PrimPol is a DNA damage tolerance enzyme possessing both translesion synthesis (TLS) and primase activities. To uncover its potential role in TLS-mediated IgVλ hypermutation and define its interplay with other TLS polymerases, PrimPol(-/-) and PrimPol(-/-)/Polη(-/-)/Polζ (-/-) gene knockouts were generated in avian cells. Loss of PrimPol had no significant impact on the rate of hypermutation or the mutation spectrum of IgVλ. However, PrimPol(-/-) cells were sensitive to methylmethane sulfonate, suggesting that it may bypass abasic sites at the IgVλ segment by repriming DNA synthesis downstream of these sites. PrimPol(-/-) cells were also sensitive to cisplatin and hydroxyurea, indicating that it assists in maintaining / restarting replication at a variety of lesions. To accurately measure the relative contribution of the TLS and primase activities, we examined DNA damage sensitivity in PrimPol(-/-) cells complemented with polymerase or primase-deficient PrimPol. Polymerase-defective, but not primase-deficient, PrimPol suppresses the hypersensitivity of PrimPol(-/-) cells. This indicates that its primase, rather than TLS activity, is pivotal for DNA damage tolerance. Loss of TLS polymerases, Polη and Polζ has an additive effect on the sensitivity of PrimPol(-/-) cells. Moreover, we found that PrimPol and Polη-Polζ redundantly prevented cell death and facilitated unperturbed cell cycle progression. PrimPol(-/-) cells also exhibited increased sensitivity to a wide variety of chain-terminating nucleoside analogs (CTNAs). PrimPol could perform close-coupled repriming downstream of CTNAs and oxidative damage in vitro. Together, these results indicate that PrimPol's repriming activity plays a central role in reinitiating replication downstream from CTNAs and other specific DNA lesions.

Chemical Incorporation of Chain-Terminating Nucleoside Analogs as 3'-Blocking DNA Damage and Their Removal by Human ERCC1-XPF Endonuclease

Nucleoside/nucleotide analogs that lack the 3′-hydroxy group are widely utilized for HIV therapy. These chain-terminating nucleoside analogs (CTNAs) block DNA synthesis after their incorporation into growing DNA, leading to the antiviral effects. However, they are also considered to be DNA damaging agents, and tyrosyl-DNA phosphodiesterase 1, a DNA repair enzyme, is reportedly able to remove such CTNA-modifications of DNA. Here, we have synthesized phosphoramidite building blocks of representative CTNAs, such as acyclovir, abacavir, carbovir, and lamivudine, and oligonucleotides with the 3′-CTNAs were successfully synthesized on solid supports. Using the chemically synthesized oligonucleotides, we investigated the excision of the 3′-CTNAs in DNA by the human excision repair cross complementing protein 1-xeroderma pigmentosum group F (ERCC1-XPF) endonuclease, which is one of the main components of the nucleotide excision repair pathway. A biochemical analysis demonstrated that the ERCC1-XPF endonuclease cleaved 2–7 nt upstream from the 3′-blocking CTNAs, and that DNA synthesis by the Klenow fragment was resumed after the removal of the CTNAs, suggesting that ERCC1-XPF participates in the repair of the CTNA-induced DNA damage.