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.

The tumor suppressor p53 is an important regulator of intracellular reactive oxygen species (ROS) levels, although downstream mediators of p53 remain to be elucidated. Here, we show that p53 and its downstream targets, p53-inducible ribonucleotide reductase (p53R2) and p53-inducible gene 3 (PIG3), physically and functionally interact with catalase for efficient regulation of intracellular ROS, depending on stress intensity. Under physiological conditions, the antioxidant functions of p53 are mediated by p53R2, which maintains increased catalase activity and thereby protects against endogenous ROS. After genotoxic stress, high levels of p53 and PIG3 cooperate to inhibit catalase activity, leading to a shift in the oxidant/antioxidant balance toward an oxidative status, which could augment apoptotic cell death. These results highlight the essential role of catalase in p53-mediated ROS regulation and suggest that the p53/p53R2-catalase and p53/PIG3-catalase pathways are critically involved in intracellular ROS regulation under physiological conditions and during the response to DNA damage, respectively. Reactive oxygen species (ROS) are generated as products or by-products in cells and function as signaling molecules and cellular toxicants.1,2 Therefore, a series of antioxidant mechanisms maintain and protect intracellular redox homeostasis.2-4 A shift in the balance between oxidants and antioxidants toward oxidation causes DNA mutations, protein oxidation, and lipid peroxidation, which eventually lead to loss of molecular function 1,2,5 and contribute to the pathogenesis of human diseases, including aging and cancer. 3The p53 protein has been proposed as a critical regulator of intracellular ROS levels. Upon activation following DNA damage, p53 can activate several genes that result in increased ROS generation, which contributes to the induction of apoptosis in cells with unrepaired DNA damage.6-10 The induction of apoptosis is central to the tumor-suppressive activity of p53. 11,12 By using the serial analysis of gene expression technique to evaluate the patterns of gene expression following p53 expression, a series of p53-inducible genes (PIG genes) have been identified that are predicted to encode proteins that could generate ROS. 8 Of particular interest is p53-inducible gene 3 (PIG3), which shares sequence similarity with NADPH-quinine oxidoreductase, and is induced by p53 before the onset of apoptosis and contributes to ROS generation.8 Thus, PIG3 is believed to be one of the major factors involved in p53-induced apoptosis through ROS generation. This was the first clear connection between p53 and ROS generation, but the molecular mechanisms of PIG3-induced ROS generation have not yet been elucidated. Under physiological condition, basal levels of p53 can also upregulate antioxidant genes that function to lower ROS levels, and this antioxidant function of p53 is important in preventing oxidative stress-induced DNA damage and tumor development under low-stress conditions.13-21 Thus, p53 has opposing roles in ...

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.

Aberrant expression of apurinic-apyrimidinic endonuclease-1 (APEX1) has been reported in numerous human solid tumors and is positively correlated with cancer progression; however, the role of APEX1 in tumor progression is poorly defined. Here, we show that APEX1 contributes to aggressive colon cancer behavior and functions as an upstream activator in the Jagged1/Notch signaling pathway. APEX1 overexpression or knockdown in human colon cancer cell lines induced profound changes in malignant properties such as cell proliferation, anchorage-independent growth, migration, invasion, and angiogenesis in vitro and in tumor formation and metastasis in mouse xenograft models. These oncogenic effects of APEX1 were mediated by the upregulation of Jagged1, a major Notch ligand. Furthermore, APEX1 expression was associated with Jagged1 in various colon cancer cell lines and in tissues from colon cancer patients. This finding identifies APEX1 as a positive regulator of Jagged1/Notch activity and suggests that it is a potential therapeutic target in colon cancers that exhibit high levels of Jagged1/Notch signaling.

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