Cell cycle effects of the DNA topoisomerase inhibitors camptothecin and m-AMSA in lymphoblastoid cell lines from patients with Fanconi anemia

Nucleolin inhibits G4 oligonucleotide unwinding by Werner helicase

BACKGROUND

The Werner protein (WRNp), a member of the RecQ helicase family, is strongly associated with the nucleolus, as is nucleolin (NCL), an important nucleolar constituent protein. Both WRNp and NCL respond to the effects of DNA damaging agents. Therefore, we have investigated if these nuclear proteins interact and if this interaction has a possible functional significance in DNA damage repair.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that WRNp interacts with the RNA-binding protein, NCL, based on immunoprecipitation, immunofluorescent co-localization in live and fixed cells, and direct binding of purified WRNp to nucleolin. We also map the binding region to the C-terminal domains of both proteins. Furthermore, treatment of U2OS cells with 15 µM of the Topoisomerase I inhibitor, camptothecin, causes the dissociation of the nucleolin-Werner complex in the nucleolus, followed by partial re-association in the nucleoplasm. Other DNA damaging agents, such as hydroxyurea, Mitomycin C, and aphidicolin do not have these effects. Nucleolin or its C-terminal fragment affected the helicase, but not the exonuclease activity of WRNp, by inhibiting WRN unwinding of G4 tetraplex DNA structures, as seen in activity assays and electrophoretic mobility shift assays (EMSA).

CONCLUSIONS/SIGNIFICANCE

These data suggest that nucleolin may regulate G4 DNA unwinding by WRNp, possibly in response to certain DNA damaging agents. We postulate that the NCL-WRNp complex may contain an inactive form of WRNp, which is released from the nucleolus upon DNA damage. Then, when required, WRNp is released from inhibition and can participate in the DNA repair processes.

Is Fanconi anemia caused by a defect in the processing of DNA damage?

Fanconi anemia (FA) is an autosomal genetic disease characterized by a complex array of developmental disorders, a high predisposition to bone marrow failure and to acute myelogenous leukemia. The chromosomal instability and the hypersensitivity to DNA cross-linking agents led to its classification with the DNA repair disorders. This review aimed at establishing whether it is still appropriate to consider 1/approximately FA within a DNA repair framework taking into account the recently discovered genetic heterogeneity characteristics of the defect (eight complementation groups). We discuss the possibility that the FA proteins interact to form a complex which may control different functions, including the processing of specific DNA lesions. Such a complex may act as a sensor to initiate protective systems as well as transcription of specific genes specifying, among others proteins, growth factors. Such steps may be organized as a linear cascade or more likely under the form of a web network.

Cell cycle traverse in AHH-1 tk +/- human lymphoblastoid cells exposed to the chromosomal mutagen, m-amsa

AHH-1 tk +/- cells were exposed to the chemotherapeutic agent, m-amsa, both in complete medium and in medium without serum, subcultured in complete medium, and the effect on the traverse of the cell cycle determined by flow cytometric analysis of bromodeoxyuridine (BrdUrd)-labeled DNA. After exposure to m-amsa (day 0), the percentage of S-phase cells increased significantly (P < 0.0017) with increasing concentration. Cells also accumulated in G2/M as evidenced by the significant (P < 0.0026), concentration-dependent increase in the percentage of cells detected within this phase. Serum deprivation during exposure resulted in significantly (P = 0.024) more cells in S-phase than in cultures exposed to m-amsa in complete medium. After three days in culture, a significant (P = 0.0001) accumulation of cells in G2/M was present; the percentage of cells in G2/M did not differ significantly (P = 0.148) in cultures exposed to m-amsa in complete medium or in serum-free medium. However, a significant (P < 0.001) loss of S-phase cells was found in cultures exposed without serum. At day 7, no significant concentration effects were detected (GO/G1, P = 0.6026; S-phase, P = 0.9773; G2/M, P = 0.8401). These results demonstrate that exposure to m-amsa perturbs the traverse of the cell cycle, initially by inhibiting the completion of S-phase and followed by an accumulation of cells in G2/M. In addition, exposure to m-amsa under conditions of serum deprivation results in an increased percentage of cells in the initial S-phase after exposure, the loss of S-phase cells from the culture after three days, and the appearance of subdiploid peak, consistent with cells undergoing apoptosis.

Toxicity of camptothecin to Chinese hamster cells containing 5-hydroxymethyl-2'-deoxyuridine in their DNA

5-Hydroxymethyl-2'-deoxyuridine (hmdUrd) is incorporated into the DNA of V79 Chinese hamster cells as an analogue of thymidine. Incorporated residues are then recognized and excised by hmUra-DNA glycosylase (hmUDG). The removal of large numbers of hmUra residues and subsequent strand breakage is cytotoxic, as has been demonstrated by our finding that a mutant cell line, which is deficient in this enzyme, is resistant to hmdUrd (Boorstein et al., 1992a). In order to determine whether topoisomerase I plays a role in hmUDG initiated base excision repair, V79 cells and repair deficient V79mut1 cells were exposed to combinations of hmdUrd and the topoisomerase I inhibitors camptothecin (CPT), CPT-11, and beta-lapachone. Treatment of V79 cells with hmdUrd followed by non-toxic concentrations of camptothecin or CPT-11 showed significant enhancement of the baseline cytotoxicity of the hmdUrd alone. In contrast, camptothecin and CPT-11 had no effect in combination with hmdUrd in the V79mut1 cells. Non-toxic concentrations of beta-lapachone, which inhibits topoisomerase I by a different mechanism than camptothecin and CPT-11, produced no synergistic toxicity in V79 cells. Neither camptothecin nor CPT-11 inhibited removal of hmdUrd from hmdUrd treated cells, nor did they affect hmdUrd-induced poly(ADP-ribose) synthesis. Camptothecin did not alter the cell cycle distribution of either hmdUrd treated cells or untreated cells at concentrations sufficient to cause synergistic toxicity with hmdUrd. Results from our study indicate that the utility of topoisomerase I inhibitors may be enhanced by sensitizing cells with hmdUrd initiated repair activity which arrests cells in S-phase and produces DNA lesions that are further converted into lethal damage by camptothecin.

Comparison of the effects of DNA topoisomerase inhibitors on lymphoblasts from normal and Fanconi anemia donors

DNA topoisomerases modify supercoiled DNA through concerted breaking and rejoining of the DNA strands and consequently play a key role in DNA biosynthesis and processing. It has been suggested that topoisomerases may facilitate access to damaged sites of excision repair enzymes due to their property to relax supercoiled DNA. We show here that treatment with nalidixic acid and novobiocin, which affects topoisomerase II activity among other targets, impairs the incision of 8-methoxypsoralen photoinduced DNA interstrand cross-links in normal human fibroblasts. Since cells derived from Fanconi anemia (FA) demonstrate hypersensitivity to DNA cross-linking agents associated with a reduced repair efficiency of cross-links, we compared the effects of different topoisomerase I and II inhibitors on FA and normal lymphoblasts. No differences were found in growth inhibition or induction of chromosome aberrations between FA and normal cells. The specificity of inhibitors is questionable and even if topoisomerases are indeed inhibited alternative pathways may be involved. However, our observations provisionally suggested that topoisomerases activities are normal in FA cells.

Determinants of cellular sensitivity to topoisomerase-targeting antitumor drugs

It is now clear that topoisomerase activity level is an important determinant of sensitivity to topo drugs. The regulation of topoisomerases is no doubt complex and multifaceted and is probably accomplished through redundancy at many control levels. The mechanism(s) of altered topo I expression in certain tumor types is unknown, but may be related to the central importance of topoisomerases in proliferating cell functions (transcription, replication, etc.), and the aberrant and chronic activation of these functions as a result of specific tumorigenic alterations. Small differences in sensitivity to chemotherapy can have a dramatic effect on cure rates, and therefore subtle cell type-specific differences may be important determinants of drug sensitivity. Whether abnormal topoisomerase quantity and specific activity are associated with resistance or sensitivity to topoisomerase-targeted chemotherapy in the clinic is now being studied. Determinants downstream of cleavable complex formation that affect the sensitivity of tumor versus normal cells to topo drugs in particular and DNA-damaging agents in general are little known. The goal of enhancing selective tumor cell killing relative to the normal cells that are dose limiting may be achieved either by overcoming tumor cell resistance or by protecting normal cells. Both of these strategies will become more feasible as specific molecular differences between tumor and normal cells are being rapidly identified and new combination therapies that take advantage of these differences are being designed and tested.

DNA topoisomerases and the DNA lesion in human genetic instability syndromes

Genetically determined chromosomal instability entails, among other sequelae, a condition of elevated cancer risk. Patients with the autosomal recessive disorder Fanconi's anemia (FA) often develop leukemias of the monocytic lineage together with pancytopenia, whereas the Bloom's syndrome (BS) mutation confers an early and elevated incidence of neoplasia of no particular type. Cultured cells from FA patients show spontaneously elevated rates of chromosome aberrations and a hypersensitivity to DNA cross-linking agents. Cytogenetic evaluation of cells from BS patients revealed elevated rates of sister chromatid exchanges, which were sensitive to the bromodeoxyuridine (BrdU) concentration used in the assay. Such a BrdU sensitivity was also found in cultured cells from healthy subjects exposed to the intracellular superoxide generator paraquat or to bleomycin. Skin fibroblasts from FA and BS patients show poor growth, which in the case of FA could be mitigated by lowering the oxygen concentration to 5%. Lymphoblastoid B-cell lines derived from peripheral blood samples from FA and BS patients show elevated numbers of cells arrested in the G2 phase of the cell cycle. This phenomenon could also be provoked by exposing cell lines from healthy subjects to compounds interfering with the function of DNA topoisomerase I (camptothecin) or II (m-AMSA). To test for a putative deficiency of either DNA topoisomerase, B-cell cultures from FA and BS patients were compared with cell cultures from healthy subjects regarding their sensitivity towards camptothecin and m-AMSA. No difference in sensitivity to these agents was found in patient vs. control cell lines, thus ruling out a deficiency in DNA topoisomerase I or II as the prime defect in these conditions of elevated cancer risk. The similarity between the cell cycle kinetic patterns found in untreated FA cell lines and in normal cell lines exposed to camptothecin or m-AMSA suggest that the DNA lesion in FA, presumably being caused by an oxygen-related mechanism, may interfere with DNA topoisomerase function.