Cellular responses to hematoporphyrin-induced photooxidative damage in Fanconi anemia, xeroderma pigmentosum and normal human fibroblasts

Identification of nucleolin and nucleophosmin as genotoxic stress-responsive RNA-binding proteins

Genotoxic stress (DNA damage) can elicit multiple responses in mammalian cells, including the activation of numerous cascades of signal transduction that result in the activation of cellular genes involved in growth control, DNA repair and apoptosis. In an earlier report, we have shown that DNA-damaging agents can also induce the RNA-binding activity of several specific proteins that favor a double stem-loop RNA structure. Here we report the purification and identification of nucleophosmin (NPM) and nucleolin as two genotoxic stress-responsive RNA-binding proteins. UV radiation induces the protein expression levels and RNA-binding activity of NPM while nucleolin RNA-binding activity increases after UV or ionizing radiation exposure. Moreover, we have identified 40 mRNA ligands that are potentially regulated by nucleolin, several of which are stress-responsive transcripts. In addition, our data indicate that activation of nucleolin RNA-binding activity by genotoxic stress is mediated by stress-activated protein kinase p38. Our findings suggest that activation of the RNA-binding properties of nucleolin and NPM is part of the cellular response to genotoxic stress.

Fanconi's anaemia cells have normal steady-state levels and repair of oxidative DNA base modifications sensitive to Fpg protein

Cells from Fanconi's anaemia (FA) patients are abnormally sensitive to oxygen. However, a distinct genetic defect in either the cellular defence against reactive oxygen species (ROS) or in their metabolic generation has not been identified to date. Recently, the gene for the human 8-hydroxyguanine (8-oxoG) glycosylase, which removes this oxidative base modification from the genome, has been localized on chromosome 3p25, i.e., in the same region as the FA complementation group D (FAD) gene. We therefore studied the removal of photosensitization-induced 8-oxoG residues from the DNA of FA cells, using Fpg protein, the bacterial 8-oxoG glycosylase, to quantify the lesions by alkaline elution. Similar repair kinetics (approx. 50% removal within 2 h) were observed in Epstein-Barr virus (EBV) immortalized lymphoid cells from FA complementation groups A, B, C and D and in control cells from normal donors, as well as in primary fetal lung fibroblasts not yet assigned to a specific complementation group. The susceptibility for the induction of oxidative DNA modifications by photosensitization was similar in all cells. In addition, the background (steady-state) levels of Fpg-sensitive oxidative DNA base modifications, which reflect the balance between generation and removal of the lesions, were similar in control and FA cells. It is concluded that both the generation and the overall removal of 8-oxoG residues in nuclear DNA is not impaired in FA cells.

Adduct formation, mutagenesis and nucleotide excision repair of DNA damage produced by reactive oxygen species and lipid peroxidation product

Reactive oxygen species are formed constantly in living organisms, as products of the normal metabolism, or as a result of many different environmental influences. Here we review the knowledge of formation of DNA damage, the mutations caused by reactive oxygen species and the role of the excision repair processes, that protect the organism from oxidative DNA damage. In particular, we have focused on recent studies that demonstrate the important role of nucleotide excision repair. We propose two major roles of nucleotide excision repair as 1) a backup when base excision repair of small oxidative lesions becomes saturated, and as 2) a primary repair pathway for DNA damage produced by lipid peroxidation products.

Cross-resistance to photofrin-mediated photodynamic therapy and UV light and recovery from photodynamic therapy damage in Rif-8A mouse fibrosarcoma cells measured using viral capacity

Photodynamic therapy (PDT) utilizes the localized delivery of light to activate a photosensitizing drug (such as Photofrin) which is selectively retained by the tumour tissues. The intrinsic in vitro sensitivity of tumour cells to PDT is thought to be an important determinant of clinical tumour response to PDT. In this work we show the feasibility of using a viral capacity assay for adenovirus (Ad) DNA synthesis as an indicator of cellular sensitivity to and recovery from Photofrin-mediated PDT. Rif-1 mouse fibrosarcoma cells and a PDT resistant derivative, Rif-8A, as well as Chinese hamster ovary (CHO) cells and CHO-MDR multi-drug resistant mutant cells were studied. Consistent with the clonogenic survival of these cells, the capacity of PDT-treated cells for Ad DNA synthesis was greater for Rif-8A compared to Rif-1 cells and for CHO-MDR compared to CHO-N cells. Delaying infection of the Rif cells from immediately after, to 6 hours after PDT, resulted in an increased capacity for Ad DNA synthesis, which was greater for Rif-8A compared to Rif-1 cells, suggesting that the increased resistance of Rif-8A cells to PDT results from an elevated recovery and/or repair of PDT damage. The capacity of UV-irradiated cells for Ad DNA synthesis was also greater for Rif-8A compared to Rif-1 cells indicating a cross-resistance of Rif-8A cells to UV. These results suggest some overlap in the types of cellular damage induced by UV and PDT and/or overlap in the pathways for the repair of UV and PDT damage in Rif cells.

Repair of ultraviolet B and singlet oxygen-induced DNA damage in xeroderma pigmentosum cells

Ultraviolet B (UVB) (290-320 nm) is capable of damaging the DNA molecule directly by generating predominantly pyrimidine dimers. UVA (320-400 nm) does not alter the DNA molecule directly. However, when it is absorbed by cellular photosensitizers, it can damage the DNA molecule indirectly, e.g., by mediation of singlet oxygen, generating predominantly 8-hydroxyguanine. These indirect effects have been implicated in the mutagenic, genotoxic, and carcinogenic effects of UVA. To study the processing of directly and indirectly UV-induced DNA damage in intact, DNA-repair-proficient and -deficient human cells, we used the replicating plasmid pRSVcat, either irradiated with up to 10 kJ/m2 UVB or treated with the photosensitizer methylene blue plus visible light (which generates singlet oxygen). These treated plasmids were introduced into lymphoblast lines from normal donors or from patients with xeroderma pigmentosum (XP) complementation groups A, C, D, E, and variant. DNA repair was assessed by measuring activity of reactivated chloramphenicol-acetyl-transferase enzyme, encoded by the plasmid's cat gene, in cell extracts after 3 d. As expected, the repair of UVB-induced DNA damage was reduced in all XP cell lines, and the degree varied with the complementation group. XP-A, -D, -E, and -variant cells were normally efficient in the repair of singlet oxygen-induced DNA damage. Only three of four XP-C cell lines showed a markedly reduced repair of these lesions. This indicates differential DNA-repair pathways for directly and indirectly UV-induced DNA damage in human cells and suggests that both may be affected in XP-C.

Processing of directly and indirectly ultraviolet-induced DNA damage in human cells

Mutations caused by ultraviolet (UV)-induced DNA damage represent the initial genetic changes in the tumorigenesis of UV-induced skin cancer. Different wavelengths of UV radiation cause different kinds of DNA damage and mutations. UVB (290-320 nm) generates pyrimidine dimers by direct excitation of the DNA molecule. UVA (320-400 nm) can damage the DNA only indirectly through a photosensitized reaction. This indirect action is mediated mainly by singlet oxygen, which generates purine base modifications, and has been implicated in the carcinogenic effects of UVA. In order to study the processing of directly and indirectly UV-induced DNA damage in human cells, we first treated the replicating plasmid pRSVcat with up to 10 kJ/m2 UVB or with the photosensitizer methylene blue plus visible light (which generates singlet oxygen) in vitro. Then, the damaged plasmid was transfected into normal or repair deficient xeroderma pigmentosum complementation group A (XP-A) cells. DNA repair was assessed by measuring activity of reactivated chloramphenicol acetyltransferase (CAT) enzyme, encoded by the plasmid's cat gene, in cell extracts after 3 days. While XP-A cells exhibited a significantly reduced repair of UVB-induced DNA damage, they showed a normal repair of singlet oxygen-induced DNA damage. This indicates a differential DNA repair pathway for directly and indirectly UV-induced DNA damage in human cells. Irradiation of the plasmid with UVA alone did not result in a genotoxic effect. Only in conjunction with a cell extract, which provides all candidate cellular photosensitizers, did we find a reduced CAT activity after transfection. This indicates that the genotoxicity of UVA is mediated by a cellular photosensitizer.