Tumors frequently contain mutations in the ras genes, resulting in the constitutive activation of the Ras-activated signaling pathway. The activation of Ras is involved not only in tumor progression but also in the development of resistance of the tumor cells to platinum-based chemotherapeutic agents. To investigate the potential mechanisms underlying this resistance, we analyzed the effect of activated H-Ras on the expression of the nucleotide excision repair genes. Here we identified ERCC1, which is one of the key enzymes involved in nucleotide excision repair, as being markedly up-regulated by the activated H-Ras. From promoter analysis of ERCC1, an increase in the Ap1 transcriptional activity as a result of the expression of the oncogenic H-Ras was found to be crucial for this induction. In addition, ERCC1 small interfering RNA expression was shown to reduce the oncogenic H-Ras-mediated increase in the DNA repair activity as well as to suppress the oncogenic H-Ras-mediated resistance of the cells to platinum-containing chemotherapeutic agents. These results suggest that the oncogenic H-Ras-induced ERCC1, which activates the DNA repair capacity, may be involved in the protection of the cells against platinum-based anticancer agents.

Bcl-2 stimulates mutagenesis after the exposure of cells to DNA-damaging agents. However, the biological mechanisms of Bcl-2-mediated mutagenesis have remained largely obscure. Here we demonstrate that the Bcl-2-mediated suppression of hMSH2 expression results in a reduced cellular capacity to repair mismatches. The pathway linking Bcl-2 expression to the suppression of mismatch repair (MMR) activity involves the hypophosphorylation of pRb, and then the enhancement of the E2F-pRb complex. This is followed by a decrease in hMSH2 expression. MMR has a key role in protection against deleterious mutation accumulation and in maintaining genomic stability. Therefore, the decreased MMR activity by Bcl-2 may be an underlying mechanism for Bcl-2-promoted oncogenesis.

Cadmium is a well known human and animal carcinogen and is a ubiquitous contaminant in the environment. Although the carcinogenic mechanism of cadmium is a multifactorial process, oxidative DNA damage is believed to be of prime importance. In particular, cadmium suppresses the capacity of cells to repair oxidative DNA damage. In this study, cadmium treatment led to a significant increase in ␥-ray-induced 8-oxoguanine (8-oxoG) formation. Western blotting and semiquantitative reverse transcription-PCR revealed that cadmium treatment caused a decrease in the expression level of human OGG1 (8-oxoguanine-DNA glycosylase-1; hOGG1) in human fibroblast GM00637 and HeLa S3 cells. In addition, the cadmium-mediated decrease in hOGG1 transcription was the result of decreased binding of the transcription factor Sp1 to the hOGG1 promoter. Finally, we show that an increase in the functional hOGG1 expression level could inhibit the cadmium-mediated increase in ␥-ray-induced 8-oxoG accumulation as well as in ␥-radiation-induced mutation frequency at the HPRT (hypoxanthine-guanine phosphoribosyltransferase) gene locus. These results suggest that cadmium attenuates removal of ␥-ray-induced 8-oxoG adducts, which in turn increases the mutation frequency, and that this effect might, at least in part, result from suppression of hOGG1 transcription via inactivation of Sp1 as a result of cadmium treatment.

Human 8-oxoguanine DNA glycosylase (hOGG1) is the main defense enzyme against mutagenic effects of cellular 7,8-dihydro-8-oxoguanine. In this study, we investigated the biological role of hOGG1 in DNA damage -related apoptosis induced by hydrogen peroxide (H 2 O 2 ) -derived oxidative stress. The down-regulated expression of hOGG1 by its small interfering RNA prominently triggers the H 2 O 2 -induced apoptosis in human fibroblasts GM00637 and human lung carcinoma H1299 cells via the p53-mediated apoptotic pathway. However, the apoptotic responses were specifically inhibited by hOGG1 overexpression.

Metallothioneins (MT) play an important biological role in preventing oxidative damage to cells. We have previously demonstrated that the efficiency of the protective effect of MT-III against the DNA degradation from oxidative damage was much higher than that of MT-I/II. As an extension of the latter investigation, this study aimed to assess the ability of MT-III to suppress 8-oxoguanine (8-oxoG), which is one of the major base lesions formed after an oxidative attack to DNA and the mutant frequency of the HPRT gene in human fibroblast GM00637 cells upon exposure to ␥-rays. We found that human MT-III expression decreased the level of 8-oxoG and mutation frequency in the ␥-irradiated cells. Using an 8-oxoguanine DNA glycosylase (OGG1)-specific siRNAs, we also found that MT-III expression resulted in the suppression of the ␥-radiation-induced 8-oxoG accumulation and mutation in the OGG1-depleted cells. Moreover, the down-regulation of MT in human neuroblastoma SKNSH cells induced by MT-specific siRNA led to a significant increase in the 8-oxoG level, after exposure to ␥-irradiation. These results suggest that under the conditions of ␥-ray oxidative stress, MT-III prevents the ␥-radiation-induced 8-oxoG accumulation and mutation in normal and hOGG1-depleted cells, and this suppression might, at least in part, contribute to the anticarcinogenic and neuroprotective role of MT-III. The metallothioneins (MT)1 are a group of intracellular metal-binding proteins of low molecular mass (6 -7 kDa) that are widely distributed in a broad range of eukaryotic species from yeast to mammals (1, 2). In both mice and humans, there are four classes of quite similar MT proteins, MT-I to MT-IV. MT-I and MT-II are widely expressed in all tissues, whereas MT-III and MT-IV are expressed mainly in the central nervous system and the squamous epithelia, respectively (3-7). Whereas much is known about the chemical properties and genetic regulation of MT, the actual physiological role of MT is largely unknown. The first recognized function of MT is the detoxification of heavy metals such as cadmium and mercury (8, 9). Subsequently, a number of cellular functions have been proposed for MT, including regulating essential metal homeostasis (10, 11), contributing to the control of cellular proliferation and apoptosis (12, 13), and protecting against radiation and oxidative damage (14, 15). The role of MT in oxidative damage has been aggressively investigated, and the vast majority of studies show that MT is a potent antioxidant that protects against various oxidative damage from reactive oxygen species (ROS) in vitro and in vivo because of their multiple cysteines. In vitro up-regulation of MT has been correlated with resistance to cytotoxicity induced by various hydroxyl radical generators, and the rate constant of MT for a reaction with hydroxyl radicals is more than 100ϫ higher than that of glutathione (16 -21). In addition, MT is 50ϫ more effective in protecting DNA from hydroxyl radicals than glutathione on a molar basis (22). In vivo, the i...