DNA repair in human promyelocytic cell line, HL-60

Hematopoietic myeloid cell differentiation diminishes nucleotide excision repair

Myeloid cell differentiation is the process by which stem cells develop into mature monocytes or granulocytes. This process is achieved by the sequential activation of variety of genes. Disruption of this process can result in immunodeficiency, bone marrow failure syndrome, or leukemia. Acute promyelocytic leukemia (APL) is characterized by the t(15;17) translocation and can be treated by a combination of all-trans retinoic acid (ATRA) and anthracycline. This treatment can induce leukemic cell differentiation, leading to extremely high remission rates. XAB2, a molecule involved in nucleotide excision repair (NER), is downregulated during granulocyte differentiation and shows reduced expression in NB4 APL-derived cells in vitro. Differentiation of APL by ATRA treatment reduced XAB2 expression levels in vivo. These observations suggest that cellular differentiation is associated with reduced NER activity and provides new insights into combined differentiation induction. NB4 cells were more susceptible than the immature myeloid leukemic cell lines, Kasumi-3 and Kasumi-1, to the DNA interstrand crosslinking agent cisplatin.

DNA repair in differentiated cells: some new answers to old questions

Terminally differentiated cells need never replicate their genomes and may therefore dispense with the daunting task of maintaining several repair systems to constantly scan their entire complement of DNA. Obviously, transcribed genes need to be repaired, so that cells can carry out their specialized functions, but dedicated mechanisms such as transcription-coupled repair and differentiation-associated repair can ensure the maintenance of those transcriptionally active domains. Many groups have studied DNA repair in differentiated cells, often with divergent results, possibly because there are distinct classes of differentiated cells, with unique properties. Thus neurons ought to last for a lifetime, whereas myocytes are backed by precursor cells, while white blood cells like macrophages are constantly being replaced. More importantly, different DNA repair systems can vary in their response to cellular differentiation, possibly depending on whether they can be coupled to transcription. Nucleotide excision repair (NER) is probably the most versatile DNA repair system and is coupled to transcription. NER was shown to be attenuated by differentiation in several cell types, including neurons. The attenuation occurs only at the global genome level, with transcribed genes still being efficiently repaired. We have determined that this attenuation results from the lack of ubiquitination of a NER factor, most likely owing to differences in phosphorylation of the ubiquitin-activating enzyme E1. Because there is only one E1 in human cells, it is likely that other metabolic pathways are similarly affected, depending on whether they rely on an E2 enzyme which is sensitive to the state of E1 phosphorylation.

Nucleotide excision repair phenotype of human acute myeloid leukemia cell lines at various stages of differentiation

In previous studies it was shown that nucleotide excision repair (NER) is strongly attenuated at the global genome level in terminally differentiated neuron-like cells. NER was measured in several human acute myeloid leukemia cell lines, before and after differentiation into macrophage-like cells. Repair of cisplatin intrastrand GTG crosslinks in differentiated cells was strongly attenuated. There were also some variations between repair levels in naïve cells, but these were not correlated with the degree of differentiation. By contrast, the proficient repair of UV-induced (6-4)pyrimidine-pyrimidone photoproducts [(6-4)PPs] was not affected by differentiation. Although cyclobutane pyrimidine dimers (CPDs) were poorly repaired at the global genome level in all cell lines, differentiated or not, they were very efficiently removed from the transcribed strand of an active gene, indicating that transcription-coupled repair (TCR) is proficient in each cell line. CPDs were also removed from the non-transcribed strand of an active gene better than at the overall global genome level. This relatively efficient repair of the non-transcribed strand of active genes, when compared with global genomic repair (GGR), has been described previously in neuron-like cells and termed differentiation-associated repair (DAR). Here we show that it also can occur in actively growing cells that display poor GGR.

Molecular mechanisms of individual radiosensitivity studied in normal diploid human fibroblasts

The molecular mechanisms of individual radiosensitivity were studied in normal diploid human fibroblasts. For fibroblasts irradiated with X-rays in G1-phase the individual radiosensitivity was shown to be correlated with the extent of double-strand break (dsb) repair. The number of residual dsbs (including both non- and mis-rejoined dsbs) varied between 2 and 5% of the initial number induced and was low for resistant and high for sensitive strains. In the G1-phase dsbs are considered to be mostly repaired via the non-homologous end-joining pathway (NHEJ). However, so far none of the parameters tested for this pathway was found to be correlated with the number of residual dsbs. The parameters tested were mRNA expression, protein level and localisation and activity of the DNA-PK, which is the central complex of NHEJ. The dsb-repair capacity is also not regulated by the differentiation status, which varies substantially among fibroblast strains, whereas there is some indication that dsb repair might depend on the chromatin structure, with more efficient repair in cells with condensed DNA. Residual dsbs are converted into lethal chromosome aberrations finally leading to the loss of clonogenic activity, when cells pass through mitosis. Beside this so-called mitotic death, X-irradiated human fibroblasts are also inactivated via the TP53-dependent permanent G1-arrest, while apoptosis appears to be not important. On average, mitotic death and G1-arrest are equally effective, but there is a broad variation from one strain to the other, with a negative correlation between these two pathways. Fibroblast strains exhibiting only a moderate G1-arrest showed a high number of lethal aberrations and vice versa. This result points to a common regulator of both G1-arrest and dsb repair, which is presently under investigation.

Formation and repair of DNA double-strand breaks in gamma-irradiated K562 cells undergoing erythroid differentiation

Cellular differentiation is accompanied by gross changes in nuclear organization, metabolic pathways and gene expression characteristics. To investigate, whether the response to radiation damage is altered during cellular differentiation, we studied the formation and repair of DNA double-strand breaks in gamma-irradiated K562 erythroleukemia cells induced to differentiate by exposure to butyric acid. We applied an assay based on pulsed-field gel electrophoresis and Southern hybridization to measure break induction in several genomic restriction fragments. Pulsed-field gel electrophoresis of (14)C-labelled unrestricted DNA was used to study the rejoining of gamma-radiation-induced breaks in the whole genome. Total rejoining and joining of correct break ends in specific genomic regions was monitored by hybridization analysis of blots of unrestricted and restriction digested DNA with single-copy probes. The yields of gamma-ray-induced DNA double-strand breaks were found to decrease with differentiation by about 20%. Correct rejoining of radiation-induced breaks, as measured by the reconstitution of broken restriction fragments, was unaltered in differentiating cells compared to actively proliferating precursor cells. Total rejoining, however, appeared to be retarded in differentiating cells. The results suggest that in spite of the fundamental changes accompanying differentiation, the cellular damage response pathways are not essentially affected throughout erythroid differentiation.

Effects of PARP inhibition on drug and Fas-induced apoptosis in leukaemic cells

Poly (ADP-ribose) polymerase (PARP) is activated following binding to DNA strand breaks and is cleaved in cells undergoing apoptosis. Work predominantly in murine systems has suggested that inhibitors of PARP might potentiate the effects of chemotherapeutic agents and be used as adjuncts to cancer therapy. Therefore, we studied the role of PARP in drug-induced apoptosis in HL-60, myeloid leukaemia cells and found that pre-treatment with 3-aminobenzamide (3AB) or 6(5H)-phenanthridinone, inhibitors of PARP, resulted in resistance to, rather than potentiation of apoptotic death induced by DNA-damaging agents, idarubicin, etoposide and fludarabine, as determined by flow cytometry, following propidium iodide staining. 3AB treated CEM/VLB100, mdr-expressing human lymphoblastic leukaemia cells were also found to be more resistant to idarubicin compared to cells treated with idarubicin alone, however, apoptosis was not reduced in parental CCRF-CEM cells under the same conditions. Similar results were obtained using agents with primary modes of action which do not involve DNA damage, vinblastine and a fas-ligating antibody (CH11). The precise role of PARP has yet to be defined but might involve effects on cell cycle progression. We conclude that PARP activation appears to be involved in apoptosis in certain leukaemic cell lines and that these effects are independent of lineage or p-glycoprotein. Constitutive failure to activate PARP might be responsible for conferring resistance to apoptosis.

Proliferation kinetics and PCNA expression of HL-60 cells following ionizing irradiation and granulocytic differentiation

The human promyelocytic leukaemia cell line, HL-60, was investigated with regard to proliferation and terminal differentiation following irradiation. The cells were X-irradiated and induced with 1.25% dimethyl sulfoxide (DMSO) towards the granulocytic lineage. Proliferation was measured via cell growth, clonogenicity and the bromodeoxyuridine/DNA incorporation assay. Immunohistochemical detection of proliferating cell nuclear antigen (PCNA) expression was used to discriminate cycling from non-cycling cells. The differentiation obtained was proved by testing for the immune function of the respiratory burst (NBT reduction test). The HL-60 cells studied revealed a high radiosensitivity (D0 = 0.63 Gy). After induction with DMSO, declines in cell growth, clonogenicity and PCNA positivity of the cells indicated a decrease in proliferation and an increase in differentiation. Starting on day 2 in culture, irradiation after seeding with 1 Gy accelerated the loss of the PCNA expression in induced cells (46% v. 3% PCNA-negative control cells on day 3). Induced cells gained the capability of exerting the respiratory burst, which was found to be dose-dependent radiosensitive (42%, and 12% NBT-positive cells after 1 and 2 Gy, respectively, v. 53% NBT-positive control cells on day 8). Subpopulations in the cell line were evident in all parameters investigated. We discuss the HL-60 cell, not only as a model comparable to human progenitor cells, but also as a suitable tool in radiobiological research with regard to proliferation and differentiation following ionizing irradiation.

Apoptosis induced by anticancer drugs

Most of the cytotoxic anticancer drugs in current use have been shown to induce apoptosis in susceptible cells. The fact that disparate agents, which interact with different targets, induce cell death with some common features (endonucleolytic cleavage of DNA, changes in chromatin condensation) suggests that cytotoxicity is determined by the ability of the cell to engage this so-called 'programmed' cell death. The mechanism of the coupling of a stimulus (drug-target interaction) to a response (cell death) is not known, but modulation of this coupling may affect the outcome of drug treatment. This review surveys the recent evidence which supports the idea that the drug-target interaction per se is not the sole determinant of cellular sensitivity of cytotoxic drugs. Studies of the signals which might engage apoptosis, the genes which modulate it and the biochemical process of drug-induced apoptosis itself are described, where possible, for glucocorticoids, topoisomerase inhibitors, alkylating agents, antimetabolites and antihormones. It is suggested that identification of the gene products which couple the stimulus to the response, and so determine intrinsic cellular sensitivity (and resistance), will be important targets for new types of drugs. These might then allow responses to occur in the major cancers of man, which are chemoresistant.

Diadenosine tetraphosphate (Ap4A) levels in HL-60 cells during differentiation into granulocytes and monocytes

1. Diadenosine tetraphosphate (Ap4A) levels were determined in HL-60 cells differentiating into granulocytes or monocytes after treatment for 0-7 days with retinoic acid (RA) or 4-beta-phorbol-12-myristate-13-acetate (PMA) respectively. 2. The levels increased significantly compared to untreated control cells within 2 days and then declined again. 3. In RA treated cells the levels finally decreased far below those of untreated HL-60 cells and became equal to those found in human granulocytes. 4. PMA treatment had no effect on Ap4A levels in human granulocytes. 5. A possible interaction between Ap4A and ADP-ribosyl transferase is discussed.

ADP-ribosylation is involved in the integration of foreign DNA into the mammalian cell genome

The most commonly used DNA transfection method, which employs the calcium phosphate co-precipitation of the donor DNA, involves several discrete steps (1,2). These include the uptake of the donor DNA by the recipient cells, the transport of the DNA to the nucleus, transient expression prior to integration into the host cell genome, concatenation and integration of the transfected DNA into the host cell genome and finally the stable expression of the integrated genes (2,3). Both the concatenation and the integration of the donor DNA into the host genome involve the formation and ligation of DNA strand-breaks. In the present study we demonstrate that the nuclear enzyme, adenosine diphosphoribosyl transferase (ADPRT, E.C. 2.4.2.30), which is dependent on the presence of DNA strand breaks for its activity (4,5) and necessary for the efficient ligation of DNA strand-breaks in eukaryotic cells (4,6), is required for the integration of donor DNA into the host genome. However, ADPRT activity does not influence the uptake of DNA into the cell, its episomal maintenance or replication, nor its expression either before or after integration into the host genome. These observations strongly suggest the involvement of ADPRT activity in eukaryotic DNA recombination events.

Transient formation of DNA strand breaks during the induced differentiation of a human promyelocytic leukaemic cell line, HL-60

During the induced differentiation of the human promyelocytic leukaemic cell line, HL-60, along the myelocytic lineage, DNA strand-breaks are formed. These breaks which are formed in the face of a proficient DNA repair mechanism, are only transiently maintained and subsequently become religated. The ligation of these breaks requires the activity of the nuclear adenosine diphosphoribosyl transferase (ADPRT). Inhibition of nuclear ADPRT, an enzyme totally dependent on the presence of DNA strand-breaks for its activity and required for efficient DNA repair in eukaryotic cells, blocks the religation of these breaks but not their formation. The inhibition of DNA strand ligation in the differentiating HL-60 cells results in loss of viability and cell death.