Enhancement of XPG mRNA expression by human interferon-beta in Cockayne syndrome cells

Molecular regulation of UV-induced DNA repair

Ultraviolet (UV) radiation from sunlight is a major etiologic factor for skin cancer, the most prevalent cancer in the United States, as well as premature skin aging. In particular, UVB radiation causes formation of specific DNA damage photoproducts between pyrimidine bases. These DNA damage photoproducts are repaired by a process called nucleotide excision repair, also known as UV-induced DNA repair. When left unrepaired, UVB-induced DNA damage leads to accumulation of mutations, predisposing people to carcinogenesis as well as to premature aging. Genetic loss of nucleotide excision repair leads to severe disorders, namely, xeroderma pigmentosum (XP), trichothiodystrophy (TTD) and Cockayne syndrome (CS), which are associated with predisposition to skin carcinogenesis at a young age as well as developmental and neurological conditions. Regulation of nucleotide excision repair is an attractive avenue to preventing or reversing these detrimental consequences of impaired nucleotide excision repair. Here, we review recent studies on molecular mechanisms regulating nucleotide excision repair by extracellular cues and intracellular signaling pathways, with a special focus on the molecular regulation of individual repair factors.

Transcriptional regulation of human DNA repair genes following genotoxic stress: trigger mechanisms, inducible responses and genotoxic adaptation

DNA repair is the first barrier in the defense against genotoxic stress. In recent years, mechanisms that recognize DNA damage and activate DNA repair functions through transcriptional upregulation and post-translational modification were the focus of intensive research. Most DNA repair pathways are complex, involving many proteins working in discrete consecutive steps. Therefore, their balanced expression is important for avoiding erroneous repair that might result from excessive base removal and DNA cleavage. Amelioration of DNA repair requires both a fine-tuned system of lesion recognition and transcription factors that regulate repair genes in a balanced way. Transcriptional upregulation of DNA repair genes by genotoxic stress is counteracted by DNA damage that blocks transcription. Therefore, induction of DNA repair resulting in an adaptive response is only visible through a narrow window of dose. Here, we review transcriptional regulation of DNA repair genes in normal and cancer cells and describe mechanisms of promoter activation following genotoxic exposures through environmental carcinogens and anticancer drugs. The data available to date indicate that 25 DNA repair genes are subject to regulation following genotoxic stress in rodent and human cells, but for only a few of them, the data are solid as to the mechanism, homeostatic regulation and involvement in an adaptive response to genotoxic stress.

Involvement of heat shock protein 27 in the susceptibility of KT human breast cancer cells to UVC and interferon lethality

Revealing the key molecules regulating the stress-response pathways in human cells is an intriguing problem. Chaperones, such as glucose-regulated protein 78 (GRP78) and heat shock protein 27 (HSP27), are important molecules for protecting the viability of human cells; however, it remains to be further clarified whether the molecules differentially modulate cellular responses to various types of stressors, such as DNA-damaging ultraviolet ray C (principally 254-nm wavelength, UVC) and cytocidal cytokine interferons. In the present study, the human breast cancer cell lines KT and MCF-7 were examined for GRP78 and HSP27 expression following exposure to UVC and human interferon-β (HuIFN-β). The KT cells demonstrated a higher sensitivity to both UVC and HuIFN-β lethality than MCF-7 cells. The cellular expression levels of GRP78 in KT cells, assessed by western blot analysis, were approximately 2-fold higher than that in MCF-7 cells, while the expression of HSP27 in the KT cells was 20% of the expression in the MCF-7 cells. Decreased resistance to UVC lethality was observed in GRP78 siRNA-transfected KT cells. In addition, HSP27 cDNA transfection of KT cells resulted in an increased resistance to UVC lethality. The cDNA-transfected KT cells showed an increased viability against HuIFN-β, compared with that of empty vector-transfected cells. By contrast, KT cells pretreated with HuIFN-β and irradiated with UVC demonstrated an increased resistance to UVC lethality, in association with increased levels of HSP27 expression. Thus, HSP27 may control the survival response pathways to both UVC and HuIFN-β in the human cells examined.

Involvement of LEU13 in interferon-induced refractoriness of human RSa cells to cell killing by X rays

Culture of human cells with human interferon alpha and beta (IFNA and IFNB) results in increased resistance of the cells to cell killing by X rays. To identify candidate genes responsible for the IFN-induced X-ray resistance, we searched for genes whose expression levels are increased in human RSa cells treated with IFNA, using an mRNA differential display method and Northern blotting analysis. RSa cells, which showed increased survival (assayed by colony formation) after X irradiation when they were treated with IFNA prior to irradiation, showed increased expression levels of LEU13 (IFITM1) mRNA after IFNA treatment alone. In contrast, IF(r) and F-IF(r) cells, both of which are derived from RSa cells, showed increased X-ray resistance and high constitutive LEU13 mRNA expression levels compared to the parental RSa cells. Furthermore, the IFNA-induced resistance of RSa cells to killing by X rays was suppressed by antisense oligonucleotides for LEU13 mRNA. LEU13, a leukocyte surface protein, was previously reported to mediate the actions of IFN such as inhibition of cell proliferation. The present results suggest a novel role of LEU13 different from that in the inhibition of cell proliferation, involved in IFNA-induced refractoriness of RSa cells to X rays.

Studies on p53 and Bax protein expression in Cockayne syndrome cells after UV irradiation and interferon-beta treatment

Human interferon (HuIFN) has a protective effect against ultraviolet (UV)-induced killing of Cockayne syndrome (CS) and xeroderma pigmentosum (XP) cells. Irradiation with ultraviolet (UV) resulted in nuclear accumulation of p53 in normal human fibroblast cells, and this accumulation was suppressed by treatment with HuIFN-beta. On the other hand, a large amount of p53 was found in both nuclear and cytoplasmic fractions of one SV40-transformed XP and two SV40-transformed CS cell strains irrespective of UV irradiation. Treatment with HuIFN-beta reduced the level of pro-apoptotic Bax protein without suppression of nuclear accumulation of p53 in the CS cells but not in the XP cells. These findings suggest that there are different mechanisms of UV-refractoriness caused by HuIFN-beta in UV-sensitive CS and XP cells.

Mutagenicity of bisphenol A and its suppression by interferon-alpha in human RSa cells

Bisphenol A is used as a monomer in the production of polycarbonate plastic products. The widespread use of bisphenol A has raised concerns about its effects in humans. Since there is little information on the mutagenic potential of the chemical, the mutagenicity of bisphenol A was tested using human RSa cells, which has been utilized for identification of novel mutagens. In genomic DNA from cells treated with bisphenol A at concentrations ranging from 1x10(-7) to 1x10(-5)M, base substitution mutations at K-ras codon 12 were detected using PCR and differential dot-blot hybridization with mutant probes. Mutations were also detected using the method of peptide nucleic acid (PNA)-mediated PCR clamping. The latter method enabled us to detect the mutation in bisphenol A-treated cells at a dose (1x10(-8)M) equivalent to that typically found in the environment. Induction of ouabain-resistant (Oua(R)) phenotypic mutation was also found in cells treated with 1x10(-7) and 1x10(-5)M of bisphenol A. The induction of K-ras codon 12 mutations and Oua(R) mutations was suppressed by pretreating RSa cells with human interferon (HuIFN)-alpha prior to bisphenol A treatment. The cells treated with bisphenol A at the concentration of 1x10(-6)M elicited unscheduled DNA synthesis (UDS). These findings suggested that bisphenol A has mutagenicity in RSa cells as well as mutagens that have been tested in these cells, and furthermore, that a combination of the PNA-mediated PCR clamping method with the human RSa cell line may be used as an assay system for screening the mutagenic chemicals at very low doses.

Radiation inducible DNA repair processes in eukaryotes

Eukaryotic cells respond to radiation-induced damage in DNA and other cellular components by turning on cascades of regulatory events which constitute a complex network of pathways of cell cycle checkpoints, DNA repair and damage tolerance mechanisms, recombination and delayed cell death (apoptosis). By virtue of the high homology in structure and function of yeast and mammalian proteins several DNA repair pathways that may be upregulated in response to radiation, and some of their regulatory factors involved in sensing of damage, signal transduction by protein kinase cascades and transcription have been identified. In yeast, genes for DNA synthesis and replicative damage bypass, for base and nucleotide excision repair, in particular global genome repair, and for crucial steps in DNA double strand break repair by homologous recombination show enhanced expression in response to radiation. In mammalian cells, the identification of homologous genes and upregulated homologous DNA repair pathways makes fast progress. It is, however, evident that the regulatory network is considerably more complex than in yeast. The improved understanding on the molecular level of the radiation-inducible cellular responses to radiation is of high public interest. Especially, the response to very low doses may have relevance for the risk estimation for ionising radiation and, possibly as well, ultraviolet light (UV-B), and for the design of suitable dose fractionation schemes for radiotherapy.