Effects of X-irradiation and sodium butyrate on cell-cycle traverse on normal and radiosensitive lymphoblastoid cells

ATM protein kinase: the linchpin of cellular defenses to stress

ATM is the most significant molecule involved in monitoring the genomic integrity of the cell. Any damage done to DNA relentlessly challenges the cellular machinery involved in recognition, processing and repair of these insults. ATM kinase is activated early to detect and signal lesions in DNA, arrest the cell cycle, establish DNA repair signaling and faithfully restore the damaged chromatin. ATM activation plays an important role as a barrier to tumorigenesis, metabolic syndrome and neurodegeneration. Therefore, studies of ATM-dependent DNA damage signaling pathways hold promise for treatment of a variety of debilitating diseases through the development of new therapeutics capable of modulating cellular responses to stress. In this review, we have tried to untangle the complex web of ATM signaling pathways with the purpose of pinpointing multiple roles of ATM underlying the complex phenotypes observed in AT patients.

Simultaneous Measurement of Cell Cycle Phase Position and Ionizing Radiation-induced DNA Strand Breakage in Single Human Tumour Cells Using Laser Scanning Confocal Imaging

Techniques for the assessment of DNA damage and repair in individual cells are pertinent to several areas of research, in particular the study of the heterogeneity of tumour cell populations in response to anticancer agents. We describe an adaptation of an in situ alkaline denaturation assay performed on individual nuclei of lysed cells, termed nucleoids, trapped within an agarose film. A novel aspect of the technique described in the application of confocal laser scanning fluorescence microscopy for the measurement of nucleoid relaxation in response to DNA damage. The volumes of spherical nucleoids and their relative DNA contents were determined by ethidium bromide staining and the analysis of confocal sections through the equatorial planes of the nucleoids. Mean nucleoid volume increased as a linear function of X-ray dose (0.5-8 Gy) administered to intact cells prior to lysis. We provide evidence of heterogeneity, in asynchronous cultures, in the DNA unfolding/unwinding characteristics of cells irrespective of cell cycle age. Bivariate plots of relative DNA content versus nucleoid volume allowed the direct assessment of cellular repair capacity with respect to cell cycle position.

Histone deacetylase 1 gene expression and sensitization of multidrug-resistant neuroblastoma cell lines to cytotoxic agents by depsipeptide

BACKGROUND

Genes that are overexpressed in multidrug-resistant neuroblastomas relative to drug-sensitive neuroblastomas may provide targets for modulating drug resistance.

METHODS

We used microarrays to compare the gene expression profile of two drug-sensitive neuroblastoma cell lines with that of three multidrug-resistant neuroblastoma cell lines. RNA expression of selected overexpressed genes was quantified in 17 neuroblastoma cell lines by reverse transcription-polymerase chain reaction (RT-PCR). Small-interfering RNAs (siRNAs) were used for silencing gene expression. Cytotoxicity of melphalan, carboplatin, etoposide, and vincristine and cytotoxic synergy (expressed as combination index calculated by CalcuSyn software, where combination index < 1 indicates synergy and > 1 indicates antagonism) were measured in cell lines with a fluorescence-based assay of cell viability. All statistical tests were two-sided.

RESULTS

A total of 94 genes were overexpressed in the multidrug-resistant cell lines relative to the drug-sensitive cell lines. Nine genes were selected for RT-PCR analysis, of which four displayed higher mRNA expression in the multidrug-resistant lines than in the drug-sensitive lines: histone deacetylase 1 (HDAC1; 2.3-fold difference, 95% confidence interval [CI] = 1.0-fold to 3.5-fold, P = .025), nuclear transport factor 2-like export factor (4.2-fold difference, 95% CI = 1.7-fold to 7.6-fold, P = .0018), heat shock 27-kDa protein 1 (2.5-fold difference, 95% CI = 1.0-fold to 87.7-fold, P = .028), and TAF12 RNA polymerase II, TATA box-binding protein-associated factor, 20 kDa (2.2-fold, 95% CI = 0.9-fold to 6.0-fold, P = .051). siRNA knockdown of HDAC1 gene expression sensitized CHLA-136 neuroblastoma cells to etoposide up to fivefold relative to the parental cell line or scrambled siRNA-transfected cells (P<.001). Cytotoxicity of the histone deacetylase inhibitor depsipeptide was tested in combination with melphalan, carboplatin, etoposide, or vincristine in five multidrug-resistant neuroblastoma cell lines, and synergistic cytotoxicity was demonstrated at a 90% cell kill of treated cells (combination index < 0.8) in all cell lines.

CONCLUSION

High HDAC1 mRNA expression was associated with multidrug resistance in neuroblastoma cell lines, and inhibition of HDAC1 expression or activity enhanced the cytotoxicity of chemotherapeutic drugs in multidrug-resistant neuroblastoma cell lines. Thus, HDAC1 is a potential therapeutic target in multidrug-resistant neuroblastoma.

DNA damaging and cell cycle effects of the topoisomerase I poison camptothecin in irradiated human cells

This study addressed the potential radiosensitizing and DNA-damaging actions of the DNA topoisomerase I poison camptothecin (CPT) on SV40 transformed normal (MRC5CVI) and ataxia-telangiectasia (AT5BIVA) fibroblast cell lines. In both cell lines CPT induced a dose-dependent delay of cells in S phase, followed by a dose-dependent trapping in G2/M phase. Acute X-irradiation produced patterns of G2/M arrest and S-phase delay similar to those observed for CPT in the MRC5CVI cell line, but no S phase delay was observed in the AT5BIVA cell line consistent with the ataxiatelangiectasia phenotype of this cell line. X-irradiation of CPT-treated cells resulted in additive prolongation of S phase delay in MRC5CVI cultures and additive effects for cell killing in both cell lines. The potential for topoisomerase I-DNA cross-linking by CPT was not altered by 24h pretreatment with CPT, or by acute X-irradiation. Hypersensitivity of AT5BIVA to CPT was not attributable to elevated levels of complex trapping. These findings suggest that in a rapidly proliferating human tumour there is unlikely to be synergistic therapeutic gain when the two agents are used concurrently, and that previously reported radiosensitization by CPT is restricted to G0 phase cells.

A new human plasma cell line, Karpas 620, with translocations involving chromosomes 1, 11 and 14

We report here the establishment of a new cell line, Karpas 620 (K620), from the peripheral blood of an elderly woman with an IgG-kappa plasma cell leukaemia (PCL). The line has the same hypotetraploid karyotype as the fresh cells from the patient. The cultured cells have the ultrastructural appearance of plasma cells with abundant rough endoplasmic reticulum (RER) and secrete kappa light chain. They are positive for surface antigens HLA DR, and WR17 (CD 37) and negative for CD1, CD3, CD4 and CD8. Using high resolution (HR) cytogenetic analysis it has been possible to identify all the marker chromosomes including several rearrangements commonly seen in malignancies of B cell lineage. These are a 14q+ marker with a typical 'Burkitt' morphology der(14)(pter----q32.3::8q24.1----qter) but with no reciprocal 8q-, and three translocations involving chromosome 11 at q13 with partners other than chromosome 14, namely 1q32.1, 8q24.22 and 13q14.3. An earlier report of molecular studies on the DNA of K620 has shown a rearrangement near the region on 11q13 designated BCL-1 (Rabbitts et al. 1988). This is the first report of a rearrangement in the region of 11q13 in a cell line originating from a case of plasma cell leukaemia.

Enhanced sensitivity to camptothecin in ataxia-telangiectasia cells and its relationship with the expression of DNA topoisomerase I

The antitumour drug camptothecin (CPT) can trap covalently bound topoisomerase I-DNA intermediates as complexes which conceal single-strand scissions. In an attempt to evaluate the cytotoxic potential of these lesions in human cells we have measured: (1) cell cycle delay and cell killing by CPT in primary and transformed fibroblasts, and in lymphoblastoid lines derived from normal, X-ray sensitive ataxia-telangiectasia (A-T) and xeroderma pigmentosum (XP) donors; (2) the properties of sublines obtained by high-dose selection in CPT: (3) levels of drug-induced DNA strand scission in intact cells; (4) the cellular availability of extractable topoisomerase I. The drug induced a marked cell cycle block in G2 phase, the magnitude of the block being closely related to cell kill. XP group A cells showed normal sensitivity to CPT, whereas A-T derived cells were consistently hypersensitive (3-5 fold) in a manner which could not be related to a primary deficiency in topoisomerase I activity, abnormal capacity for complex formation or anomalies in the intracellular generation of DNA strand breaks. A CPT-resistant A-T subline had reduced topoisomerase I activity but retained the characteristic of hypersensitivity to X-radiation. The subline lost resistance upon in vitro passage with evidence that resistance was initially an unstable feature of a subpopulation of cells. The findings have implications for the role of topoisomerase I in the in vitro phenotype of A-T cells, and the contribution made by topoisomerase I-dependent damage to the cytotoxic action of CPT.

Colcemid inhibits growth during early G1 in normal but not in tumorigenic lymphocytes

Mitogenically stimulated human and mouse lymphocytes enter the cell cycle (G0, G1A, G1B, S, G2+M) via a newly recognized subphase, G1'. This subphase precedes G1A and is distinct from G0. The G1' subphase is absent in immortalized and tumorigenic lymphoblastoid cell lines (LCLs) by cytofluorimetric criteria. Furthermore, colcemid inhibits transition through the G0/G1' as well as G2 phases in mitogen-stimulated lymphocytes and in LCLs. Tumorigenic LCLs are not sensitive to growth inhibition by colcemid during early G1. These observations suggest that a progressive series of changes have occurred during G0/G1' which lead to deregulation of growth control.