Quantitative assessment of bleomycin-induced poly(ADP-ribosyl)ation in human lymphocytes by immunofluorescence and image analysis

Inhibition of homologous recombination by treatment with BVDU (brivudin) or by RAD51 silencing increases chromosomal damage induced by bleomycin in mismatch repair-deficient tumour cells

Mismatch repair (MMR) has been shown to control homologous recombination (HR) by aborting strand exchange between divergent sequences. We previously demonstrated that MMR-deficient tumour cells are more resistant to chromosomal damage induced by bleomycin (BLM) during the G(2) phase, likely due to the lack of the MMR inhibitory effect on HR. Aim of this study was to investigate whether inhibition of HR by the nucleoside analogue BVDU [(E)-5(2-bromovinyl)-2'-deoxyuridine, brivudin], or silencing of genes involved in HR function, might affect sensitivity of MMR-deficient tumour cells to DNA damage induced by BLM in G(2). The results indicated that BVDU increased chromatid damage and DNA double strand breaks induced by BLM only in MMR-deficient MT-1, HL-60R, HCT116 cells, which are more resistant to BLM with respect to MMR-proficient TK-6, HL-60S and HCT116/3-6 lines. Silencing of RAD51, a key component of HR, increased sensitivity of MMR-deficient HCT-15 cells to BLM clastogenicity; in this case combined treatment with BVDU had no additional effect. Similarly, treatment with BVDU did not affect BLM clastogenicity in CAPAN-1 cells, characterized by a defective HR due to BRCA2 mutations. Conversely, BVDU increased chromatid breaks induced by BLM in HCT-15 cells transiently silenced for DNA-PK catalytic subunit, which plays a key role in non-homologous end joining. The BVDU-mediated increase of chromatid breaks in MMR-deficient cells did not depend on its previously reported inhibitory effect on poly(ADP-ribose) polymerase (PARP). In fact, it was observed also in cells stably silenced for PARP-1, which is responsible for most of cellular PARP activity. These data support the suggestion that the higher sensitivity of MMR-proficient versus MMR-deficient cells to BLM-induced chromatid breaks in the G(2) phase is a consequence of the inhibition of HR by MMR. In MMR-deficient cells, BVDU attenuates the repair of BLM-induced DSBs and this is likely to occur via inhibition of HR.

Activation of deoxycytidine kinase by deoxyadenosine: implications in deoxyadenosine-mediated cytotoxicity

The inborn deficiency of adenosine deaminase is characterised by accumulation of excess amounts of cytotoxic deoxyadenine nucleotides in lymphocytes. Formation of dATP requires phosphorylation of deoxyadenosine by deoxycytidine kinase (dCK), the main nucleoside salvage enzyme in lymphoid cells. Activation of dCK by a number of genotoxic agents including 2-chlorodeoxyadenosine, a deamination-resistant deoxyadenosine analogue, was found previously. Here, we show that deoxyadenosine itself is also a potent activator of dCK if its deamination was prevented by the adenosine deaminase inhibitor deoxycoformycin. In contrast, deoxycytidine was found to prevent stimulation of dCK by various drugs. The activated form of dCK was more resistant to tryptic digestion, indicating that dCK undergoes a substrate-independent conformational change upon activation. Elevated dCK activities were accompanied by decreased pyrimidine nucleotide levels whereas cytotoxic dATP pools were selectively enhanced. dCK activity was found to be downregulated by growth factor and MAP kinase signalling, providing a potential tool to slow the rate of dATP accumulation in adenosine deaminase deficiency.

Central role for the Werner syndrome protein/poly(ADP-ribose) polymerase 1 complex in the poly(ADP-ribosyl)ation pathway after DNA damage

A defect in the Werner syndrome protein (WRN) leads to the premature aging disease Werner syndrome (WS). Hallmark features of cells derived from WS patients include genomic instability and hypersensitivity to certain DNA-damaging agents. WRN contains a highly conserved region, the RecQ conserved domain, that plays a central role in protein interactions. We searched for proteins that bound to this region, and the most prominent direct interaction was with poly(ADP-ribose) polymerase 1 (PARP-1), a nuclear enzyme that protects the genome by responding to DNA damage and facilitating DNA repair. In pursuit of a functional interaction between WRN and PARP-1, we found that WS cells are deficient in the poly(ADP-ribosyl)ation pathway after they are treated with the DNA-damaging agents H2O2 and methyl methanesulfonate. After cellular stress, PARP-1 itself becomes activated, but the poly(ADP-ribosyl)ation of other cellular proteins is severely impaired in WS cells. Overexpression of the PARP-1 binding domain of WRN strongly inhibits the poly(ADP-ribosyl)ation activity in H2O2-treated control cell lines. These results indicate that the WRN/PARP-1 complex plays a key role in the cellular response to oxidative stress and alkylating agents, suggesting a role for these proteins in the base excision DNA repair pathway.

Microarray as a model for quantitative visualization chemistry

For visualization of proteins or nucleic acids, direct and indirect in situ fluorescence and absorption methods (immunohistochemistry and cytochemistry) have existed for many years. The authors describe a new experimental approach using microarray as a model to quantitatively compare both visualization methods. The spots obtained with the microarray robot had a progressive twofold decrease in concentrations and are used as objects with known amounts of DNA. Subsequent hybridization resulted in a direct fluorescence (DF) label or in hapten for indirect fluorescence (IF) and absorption. The results show that the image of the object in the IF method is larger than that in the DF method because of an edge effect, with stronger staining at the circumference. This leads to a higher plateau level and an 8- to 10-fold reduction in the detection threshold for IF compared with DF. These features are especially useful for one-color DNA-related microarray analysis, such as single nucleotide polymorphism, loss of heterozygosity, and mutation analysis, provided that the spots are not designed directly adjacent to each other, so that the edge effect is taken into account. The slope of the linear range for the IF method is much steeper than for the DF method, pointing to a narrow dynamic range in immunohistochemistry. It is noteworthy that the detection limit for absorption images after indirect immunoenzyme visualization is lower than for the DF images. The indirect immunohistochemistry semiquantitative absorption signal was at least similar compared with the DF fluorescence. In conclusion, an explanation for the difficulties experienced in quantitative immunohistochemistry is provided, and the data emphasize that in general, for daily pathology, semiquantitative patterns should suffice. Indirect labeling of DNA has useful characteristics for application in microarray analyses because of the large signal enhancement.

Induction of poly(ADP-ribosyl)ation and DNA damage in human peripheral lymphocytes after treatment with (-)-epigallocatechin-gallate

With regard to a future use of tea polyphenols in intervention trials with individuals at high cancer risk, the effects of the tea ingredient (-)-epigallocatechin gallate (EGCG) on poly(ADP-ribose) (PAR) levels and on DNA damage were investigated in human lymphocytes. A dose- and time-dependent elevation of both PAR formation as assessed by quantitative immunofluorescence analysis and DNA damage as assessed by the comet assay were observed after treatment with EGCG at 20, 40 and 80 microM for 10-240 min. Maximum levels of PAR formation and of DNA damage were observed after 10 min at all concentrations tested. Increased PAR levels were still detectable by 240 min in the 40 and 80 microM groups. At the lowest concentration, which is near the physiological peak values found after tea ingestion, PAR formation was not correlated with DNA damage. Here, EGCG led to pronounced PAR levels, whereas the comet assay was almost negative. In contrast, such marked differences in time course and extent of both genotoxicity and PAR formation following EGCG treatment were not detected after gamma-irradiation. Our results suggest that the known chemopreventive effects of EGCG, the main constituent of tea, may be partly attributed to an induction of PAR formation.

Reduced poly(ADP-ribosyl)ation in lymphocytes of laryngeal cancer patients: results of a case-control study

Poly(ADP-ribose) polymerase (PARP), a nuclear enzyme that is catalytically activated by DNA strand breaks, plays a complex role in DNA repair. Using NAD(+) as a precursor, it catalyzes the formation of ADP-ribose polymers, which are attached to various proteins. Defects in DNA repair pathways have been associated with increased risks for cancer in humans. We investigated whether differences in the activity of PARP are associated with the risk for laryngeal cancer. In a case-control study on genetic, lifestyle and occupational risk factors for laryngeal cancer, PARP activity was assessed as DNA damage-induced poly(ADP-ribose) formation in human peripheral blood lymphocytes by quantitative immunofluorescence analysis. Polymer formation was determined as the cellular response to bleomycin, a well-known inducer of DNA strand breaks, in lymphocytes from 69 laryngeal cancer patients and 125 healthy controls. The frequency of bleomycin-induced polymer formation, measured as mean pixel intensity, was significantly lower in cases (74.6, SE = 3.7) than in controls (94.5, SE = 3.5) and not influenced by smoking, age or sex. There was no significant difference between cases (59.1, SE = 5.2) and controls (50.5, SE = 3.7) in basal polymer formation (in cells not treated with bleomycin). When the highest tertile of polymer formation was used as the reference, the odds ratio for the lowest tertile of bleomycin-induced polymer formation was 3.79 (95% confidence interval 1.37-10.47, p = 0.01). Peripheral blood lymphocytes from laryngeal cancer patients thus showed significantly less bleomycin-induced poly(ADP-ribose) formation. Our results suggest that a reduced capacity of somatic cells to synthesize poly(ADP-ribose) might be associated with an increased risk for laryngeal cancer. The underlying mechanism remains to be investigated.