Epigenetic silencing associated with aberrant methylation of promoter region CpG islands is one mechanism leading to loss of tumor suppressor function in human cancer. Profiling of CpG island methylation indicates that some genes are more frequently methylated than others, and that each tumor type is associated with a unique set of methylated genes. However, little is known about why certain genes succumb to this aberrant event. To address this question, we used Restriction Landmark Genome Scanning to analyze the susceptibility of 1,749 unselected CpG islands to de novo methylation driven by overexpression of DNA cytosine-5-methyltransferase 1 (DNMT1). We found that although the overall incidence of CpG island methylation was increased in cells overexpressing DNMT1, not all loci were equally affected. The majority of CpG islands (69.9%) were resistant to de novo methylation, regardless of DNMT1 overexpression. In contrast, we identified a subset of methylation-prone CpG islands (3.8%) that were consistently hypermethylated in multiple DNMT1 overexpressing clones. Methylation-prone and methylation-resistant CpG islands were not significantly different with respect to size, C؉G content, CpG frequency, chromosomal location, or promoter association. We used DNA pattern recognition and supervised learning techniques to derive a classification function based on the frequency of seven novel sequence patterns that was capable of discriminating methylation-prone from methylation-resistant CpG islands with 82% accuracy. The data indicate that CpG islands differ in their intrinsic susceptibility to de novo methylation, and suggest that the propensity for a CpG island to become aberrantly methylated can be predicted based on its sequence context.

Glutathione (GSH) is an important cellular antioxidant and has a critical role in maintaining the balance of cellular redox. In this study, we investigated the GSH biosynthesis genes involved in the elevation of endogenous GSH levels using an irradiation system with an irradiation dose rate of 1.78 mGy/h, which was about 40,000 times less than the dose rates used in other studies. The results showed that GSH levels were significantly increased in the low-dose (0.02 and 0.2 Gy) irradiated group compared to those in the non-irradiated group, but enzymatic antioxidants such as superoxide dismutase and catalase were not induced at any doses tested. The elevation in GSH was accompanied by elevated expression of glutamate-cysteine ligase modifier subunit, but no changes were observed in the expression of glutamate-cysteine ligase catalytic subunit and thioredoxin in de novo GSH synthesis. In the case of genes involved in the GSH regeneration cycle, the expression of glutathione reductase was not changed after irradiation, whereas glutathione peroxidase was only increased in the 0.2 Gy irradiated group. Collectively, our results suggest that the de novo pathway, rather than the regeneration cycle, may be mainly switched on in response to stimulation with long-term low-dose radiation in the spleen.

The objective of this study was to propose a feasibility of a cellular imaging assay as an alternative to the conventional cytotoxicity assay, such as MTS assay, for apoptosis monitoring. As an apoptosis monitoring parameter, affinity interaction between phosphatidylserine (PS) and annexin V was chosen. First, the specific binding affinity between annexin V and PS in phospholipid bilayers consisting of various molar (0-15%) composition of PS was measured using a surface plasmon resonance biosensor. As PS composition increased, the binding level of annexin V increased proportionally. Second, various concentrations (0.1-10 μM) of staurosporine were used as to induce apoptosis and introduced to MCF-7 breast carcinoma cells. The cellular fluorescence images from annexin V-FITC conjugate were obtained by confocal microscopy, and their fluorescence intensities were quantified by image scanning. Dose-apoptosis (or cell death) relationships were very similar to those from MTS and FACS assays. In summary, our cellular imaging method could serve as a quicker and simpler alternative to MTS (end point assay) and FACS (flow cytometry) to screen potential apoptosis inducers.