Aberrant DNA topoisomerase II activity, radioresistance and inherited susceptibility to cancer

Response of DNA repair enzymes in murine fibrosarcoma, lymphosarcoma and ascites cells following gamma irradiation

The response of different tumours to radiation varies. This variation has been attributed to, among others, varying DNA repair capabilities The response of three tumour lines, differing in their sensitivities to radiation, namely, murine fibrosarcoma, lymphosarcoma and ascites, was studied by following the activities of enzymes known to be involved in DNA repair. The activities of poly (ADP-ribose) polymerase (PARP), DNA polymerase b and DNA ligase in fibrosarcoma, lymphosarcoma and ascites recorded varying degrees of increase following gamma irradiation (2 Gy). The increase was more pronounced in fibrosarcoma, which recorded a maximum 2 h after irradiation for b polymerase, and at 4 h for ligase and PARP, thereafter declining to near normal levels after 24 h. In contrast, the activity of DNA Topoisomerase I declined, corresponding to an increase in the PARP activity. The maximum increase in the activity of beta polymerase, ligase and PARP from lymphosarcoma and ascites was observed 2 h after irradiation with a corresponding decrease in Topoisomerase I activity. Search for the target enzymes and proteins for modification by PARP in gamma -irradiated fibrosarcoma tumour cells revealed that nuclei, and not chromatin, were preferentially modified by PARP. Among the nuclear proteins, histones were found to be ribosylated. The enzyme topoisomerase was ribosylated by PARP in vitro, and this modification was found to inhibit topoisomerase activity. We speculate that a possible role of PARP is to coordinate the activities of other enzymes in DNA repair by selectively inhibiting certain enzymes by the ribosylation process.

MEK/ERK pathway protects ionizing radiation-induced loss of mitochondrial membrane potential and cell death in lymphocytic leukemia cells

MEK/ERK-mediated signals have recently been found to inhibit Fas-mediated cell death through inhibition of caspase-8 activity. It remains unknown whether MEK/ERK-mediated signals affect ionizing radiation (IR)-induced cell death. Here we demonstrate that MEK/ERK-mediated signals selectively inhibit IR-induced loss of mitochondrial membrane potential (DeltaPsi(m)) and subsequent cell death. In Jurkat cells, TPA strongly activated ERK and inhibited the IR-induced caspase-8/Bid cleavage and the loss of DeltaPsi(m). The inhibitory effect of TPA was mostly abrogated by pretreatment of a specific MEK inhibitor PD98059, indicating that the effect depends upon MEK/ERK-mediated signals. Moreover, BAF-B03 transfectants expressing IL-2 receptor (IL-2R) beta(c) chain lacking the acidic region, which is responsible for MEK/ERK-mediated signals, revealed higher sensitivity to IR than the transfectants expressing wild-type IL-2R. Interestingly, the signals could neither protect the DeltaPsi(m) loss nor cell death in UV-irradiated cells. These data imply that the anti-apoptotic effect of MEK/ERK-mediated signals appears to selectively inhibit the IR-induced cell death through protection of the DeltaPsi(m) loss. Our data enlighten an anti-apoptotic function of MEK/ERK pathway against IR-induced apoptosis, thereby implying its contribution to radioresistance.

Sister chromatid exchanges and DNA topoisomerase II inhibitors: effect of low concentrations of etoposide (VP-16) in ataxia telangiectasia lymphoblastoid cell lines

The correlation between etoposide (VP-16) cytotoxicity and the induction of sister chromatid exchanges (SCEs) suggested that the promotion of DNA recombination events may be crucial for the activity of antitopoisomerase drugs. To further evaluate this hypothesis, we investigated the correlation between VP-16 induction of SCEs, chromosomal aberrations and cell cycle alterations in lymphoblastoid cell lines derived from patients affected by ataxia telangiectasia (AT), whose cells are known as hypersensitive to the cytotoxic and clastogenic activity of DNA topoisomerase II inhibitors. Our present study has shown that AT homozygous and heterozygous cell lines exposed to low VP-16 concentrations, although hypersensitive to the induction of chromosomal aberrations, exhibit an induction of SCEs comparable to that found in normal cell lines. Moreover, while the clastogenic effect of the drug was directly correlated to the reduction of the mitotic index, the enhancement of SCE frequencies, obtained over the same range of VP-16 concentrations, was not paralleled by a modification of proliferation index. Thus, these results suggest that etoposide retains in AT cells a strong clastogenic and cytostatic activity which is independent from DNA recombination events and which may be important for the induction of cell death by this kind of drug.

Effect of v-rasH on sensitivity of NCI-H82 human small cell lung cancer cells to cisplatin, etoposide, and camptothecin

Expression of v-ras(H) in NCI-H82 human small cell lung cancer (SCLC) cells results in a line (NCI-H82ras(H)) with a non-small cell phenotype (Mabry et al., Proc Natl Acad Sci USA 85: 6523-6527, 1988). This v-ras(H) -associated phenotypic change is prevented by treatment with trans-retinoic acid (tRA) (Kalemkarian et al., Cell Growth Differ 5: 55-60, 1994). The present studies were performed to examine changes in drug sensitivity that accompanied these phenotypic changes. v-ras(H) expression was associated with increased metallothionein-IIa (MT-IIa) mRNA and decreased levels of nonprotein sulfhydryls in NCI-H82ras(H) cells compared with -H82 cells. These changes were accompanied by the development of CdCl2 resistance without any change in cisplatin sensitivity. In contrast, growth of parental NCI-H82 cells in 1 microM tRa resulted in increased MT-IIa mRNA without any change in nonprotein sulfhydryls. In these cells, a 3.3-fold increase in cisplatin IC50 was observed. Examination of the action of topoisomerase (topo) poisons revealed that NCI-H82 and -H82ras(H) cells had indistinguishable levels of topo II polypeptides and indistinguishable sensitivities to etoposide, an agent that is often combined with cisplatin clinically. On the other hand, v-ras(H) expression was accompanied by a 2-fold increase in topo I activity and a 1.7-fold decrease in IC50 for the topo I-directed agent camptothecin. These changes resulted in 30-fold lower survival of NCI-H82ras(H) cells compared with -H82 cells at camptothecin concentrations as low as 10 nM. In summary, these studies demonstrate that chronic tRA treatment is accompanied by decreased cisplatin sensitivity in NCI-H82 human SCLC cells. In contrast, v-ras(H) expression is not associated with any change in cisplatin or etoposide sensitivity, but is accompanied by increased camptothecin sensitivity.

Abnormal pattern of post-gamma-ray DNA replication in radioresistant fibroblast strains from affected members of a cancer-prone family with Li-Fraumeni syndrome

Non-malignant dermal fibroblast strains, cultured from affected members of a Li-Fraumeni syndrome (LFS) family with diverse neoplasms associated with radiation exposure, display a unique increased resistance to the lethal effects of gamma-radiation. In the studies reported here, this radioresistance (RR) trait has been found to correlate strongly with an abnormal pattern of post-gamma-ray DNA replicative synthesis, as monitored by radiolabelled thymidine incorporation and S-phase cell autoradiography. In particular, the time interval between the gamma-ray-induced shutdown of DNA synthesis and its subsequent recovery was greater in all four RR strains examined and the post-recovery replication rate was much higher and was maintained longer than in normal and spousal controls. Alkaline sucrose sedimentation profiles of pulse-labelled cellular DNA indicated that the unusual pattern of DNA replication in irradiated RR strains may be ascribed to anomalies in both replicon initiation and DNA chain elongation processes. Moreover, the RR strain which had previously displayed the highest post-gamma-ray clonogenic survival was found to harbour a somatic (codon 234) mutation (presumably acquired during culture in vitro) in the same conserved region of the p53 tumour-suppressor gene as the germline (codon 245) mutation in the remaining three RR strains from other family members, thus coupling the RR phenotype and abnormal post-gamma-ray DNA synthesis pattern with faulty p53 expression. Significantly, these two aberrant radioresponse end points, along with documented anomalies in c-myc and c-raf-1 proto-oncogenes, are unprecedented among other LFS families carrying p53 germline mutations. We thus speculate that this peculiar cancer-prone family may possess in its germ line a second, as yet unidentified, genetic defect in addition to the p53 mutation.

Topoisomerase expression in cancer cell lines and clinical samples

Topoisomerase II activity has been previously associated with chemosensitivity to cytotoxic agents in cell lines made resistant to drugs in vitro. Examination of unselected cancer cell lines, however, shows a relatively poor correlation between topoisomerase II content and intrinsic chemoresistance. Studies of topoisomerase II expression in clinical materials from human tumor biopsies also demonstrate a poor relationship with the response of the cancers to induction chemotherapy. A major problem with assessing topoisomerase II activity in clinical materials is the marked heterogeneity of the enzyme among the cells and the associated high proportion of tumor cells which are not traversing the cell cycle. While the activation of oncogenes may disregulate topoisomerase II expression in some experimental systems, there is currently no evidence that enzyme activity is disconnected from cell cycling in clinical cancer specimens. Novel techniques of topoisomerase II measurement may permit more accurate correlation of enzyme activity with clinical chemosensitivity.

Immunological evidence for co-expression of cluster-5 and cluster-5A small cell lung cancer antigen on a single molecule

We have investigated immunologically the molecular association of the cell-surface sialoglycoprotein antigens cluster-5 (CL-5) and cluster-5A (CL-5A), known to be co-expressed in human small-cell lung cancer (SCLC). CL-5 antigen is exclusively defined by IgM antibodies as represented by MAb LAM8, whereas CL-5A antigen is exclusively defined by IgG antibodies as represented by MAbs SWA20 and SEN31. Because of the unavailability of purified antigens, the question of the molecular relationship between these antigens was addressed by immunological studies. We generated an anti-anti-idiotypic MAb of the IgG isotype using a CL-5-antigen-mimicking anti-idiotype defined by rat MAb Ly8-229 as an immunogen to circumvent the avidity problems observed with the IgM MAb LAM8 in binding-competition experiments. In addition, we developed a heterologous double antibody sandwich assay able to identify circulating CL-5/5A antigens in pre-treatment sera of patients with SCLC. The results of both types of immunological studies demonstrated the expression of CL-5 and CL-5A antigens on a single molecule, both in cellular assays and in assays detecting antigens shed into the serum of patients with SCLC.

Hypersensitivity of lymphoblastoid lines derived from ataxia telangiectasia patients to the induction of chromosomal aberrations by etoposide (VP-16)

Mammalian DNA topoisomerase II represents the cellular target of many antitumor drugs, such as epipodophyllotoxin VP-16 (etoposide). The mechanism by which VP-16 exerts its cytotoxic and antineoplastic actions has not yet been firmly established, although the unique correlation between sensitivity to ionizing radiation and to topoisomerase II inhibitors suggest the involvement of DNA double-strand breaks. In the present study we analyzed the chromosomal sensitivity of lymphoblastoid cell lines derived from ataxia telangiectasia (AT) patients to low concentrations of the drug. Our results indicate that AT derived cells are hypersensitive to the clastogenic activity of VP-16 either when the drug is present for the whole duration of the cell cycle or specifically in the G2 phase, confirming that the induction of DNA double strand breaks, to which AT cells seem typically sensitive, could have an important role in the biological activity of VP-16.

Anti-tumor activity of a blocked ricin immunotoxin with specificity against the cluster-5A antigen associated with human small-cell lung cancer

The monoclonal antibody (MAb SEN31, a mouse IgG1 which recognizes the cluster-5a antigen on small-cell lung cancer (SCLC) cells, was used to prepare a selective and potent blocked ricin immunotoxin. In a series of experiments in vitro and in a SCLC xenograft model in nude mice, the tumor localization potential of the radiolabeled antibody SEN31 and the anti-tumor activity of the immunotoxin SEN31-bR, the non-specific binding activity of which had been greatly reduced by blocking of the galactose binding domains of the B-chain, was determined. Radiolabeling of SEN31 was performed by linking a 67Ga-labeled desferrioxamine moiety to the oligosaccharide side chains of the antibody in order to preserve the specific cell-binding activity. 67Ga-SEN31 bound to the antigenic sites on cells of the SW2 SCLC cell line, with a dissociation constant of 3.5 nM and, when injected i.v., selectively localized at the site of s.c.-growing SW2 tumor xenografts in nude mice, with a tumor-to-blood ratio of 3.5. The immunotoxin SEN31-bR was potently and selectively active against SCLC cell lines both of classic and of variant morphologies. At a concentration of 300 pM the immunotoxin selectively eliminated 4.5 logs of clonogenic tumor cells. In nude mice, SEN31-bR was cleared from the blood with biphasic kinetics following i.v. injection and maintained a stable serum level during continuous i.p. infusion. The growth of s.c. SW2 solid-tumor xenografts was delayed following a single i.v. injection or a continuous i.p. infusion, each at a non-toxic dose.

The prognostic value of early CA125 serum assay in epithelial ovarian carcinoma

We examined the prognostic value of early serum CA125 assay in 58 patients with advanced epithelial ovarian cancer together with residual disease, age, tumour grade, performance status, and the presence of ascites or adhesions at primary surgery. CA125 was a highly significant predictor of both progression free and overall survival after the first cycle and throughout primary chemotherapy. After the first cycle, CA125 was by far the most significant predictor of progression free survival (P < 0.0005). At this time, CA125 was a highly significant predictor of survival (P < 0.005), but did not add to performance status (P < 0.001) in multivariate analysis. We were able to identify three statistically-distinct prognostic groups. Patients in the upper quartile, with CA125 levels greater than 450 U ml-1, had a very poor median survival of 7 months. Patients in the lower quartile, with CA125 levels less than 55 U ml-1 had a good median survival of 23 months. Those in the two interquartile groups, who had CA125 levels ranging from 58-221 U ml-1 and 228-434 U ml-1, had relatively intermediate median survival times of 16 months and 15 months respectively. Although CA125 levels provided significant prognostic information, in the majority of patients CA125 merely confirmed overall clinical impression.

p53 mutations increase resistance to ionizing radiation

Mouse and human tumors of diverse origin frequently have somatically acquired mutations or rearrangements of the p53 gene, or they have lost one or both copies of the gene. Although wild-type p53 protein is believed to function as a tumor-suppressor gene, it is as yet unclear how p53 mutations lead to neoplastic development. Wild-type p53 has been postulated to play a role in DNA repair, suggesting that expression of mutant forms of p53 might alter cellular resistance to the DNA damage caused by gamma radiation. Moreover, p53 is thought to function as a cell cycle checkpoint after irradiation, also suggesting that mutant p53 might change the cellular proliferative response to radiation. We have used transgenic mice expressing one of two mutant alleles of p53 to test this prediction. Our results show that expression of both mutant variants of the mouse p53 gene significantly increases the cellular resistance of a variety of hematopoietic cell lineages to gamma radiation. These observations provide direct evidence that p53 mutations affect the cellular response to DNA damage, either by increasing DNA repair processes or, possibly, by increasing cellular tolerance to DNA damage. The association of p53 mutations with increased radioresistance suggests possible mechanisms through which alterations in the p53 gene might lead to oncogenic transformation.

Tumor-antigen-specific humoral immune response of animals to anti-idiotypic antibodies and comparative serological analysis of patients with small-cell lung carcinoma

We have previously developed 3 monoclonal anti-idiotypic antibodies (Ab2) of LOU rat origin directed against the binding site of the murine monoclonal IgM LAM8, which recognizes the small-cell lung carcinoma (SCLC)-associated sialoglycoprotein antigen sGP 90-135. The aim of this study was to compare the efficiencies of these 3 Ab2, designated LY8-229, LX8-531 and LX8-632, to induce antigen-specific immunity in different animal species without prior exposure of the recipients to the nominal antigen, and thereby possibly select an Ab2 candidate for active immunotherapy against SCLC in patients. The feasibility of this approach was further evaluated by a serological analysis of patients with SCLC compared with healthy individuals, in whom the spontaneous antibody reactivities against SCLC cell lines and Ab2 were tested. LY8-229 was shown to be the most effective Ab2 in inducing antigen-specific antibodies in BALB/c mice, CBA/J/Zur mice and one NZW rabbit. Furthermore, LY8-229 was the only Ab2 against which significantly elevated idiotype-specific antibody reactivities existed in sera of patients with SCLC. These reactivities correlated positively with binding to antigen-positive tumor cells. Our findings suggest that LY8-229 represents in its reactivity pattern the nominal SCLC antigen in humans also, and therefore may be of diagnostic and possibly therapeutic relevance for patients with SCLC.