Adaptation and resistance to chemicals in the environment is a critical part of the evolutionary process. As a result, a wide variety of defence systems that protect cells against chemical insult have evolved. Such chemical resistance mechanisms appear to play a central role in determining the sensitivity of human tumours to treatment with chemotherapeutic drugs. The glutathione S-transferases (GST) are important detoxification enzymes whose over-expression has been associated with drug-resistance. In order to evaluate this possibility we have expressed the human Alpha-class and Pi-class GST cDNAs that encode GST B1B1 and GST pi in the yeast Saccharomyces cerevisiae. The expression of GST B1B1 or GST pi resulted in a marked reduction in the cytotoxic effects of chlorambucil, a bifunctional alkylating agent, and an anthracycline, adriamycin. These data provide direct evidence that the over-expression of GST in cells can confer resistance to anticancer drugs.

Redox homeostasis involves factors that ensure proper function of cells. The excess reactive oxygen species (ROS) leads to oxidative stress and increased risk of oxidative damage to cellular components. In contrast, upon reductive stress, insufficient ROS abundance may result in faulty cell signalling. It may be expected that dietary antioxidants, depending on their standard reduction potentials (E°), will affect both scenarios. In our study, for the first time, we systematically tested the relationship among E°, chemical properties, and biological effects in HT29 cells for a series of structurally different catechins and a major endogenous antioxidant - glutathione (GSH), at both physiological and dietary concentrations. Among chemical antioxidant activity tests, the strongest correlation with E° was seen using a DPPH assay. The values of E° were also highly correlated with cellular antioxidant activity (CAA) values determined in HT29 cells. Our results indicated that physiological concentrations (1-10 µM) of tested catechins stabilized the redox status of cells, which was not exhibited at higher concentrations. This stabilization of redox homeostasis was mirrored by constant, dose and E° independent CAA values, uninhibited growth of HT29 cells, modulation of hydrogen peroxide-induced DNA damage, as well as effects at the genomic level, where either up-regulation of three redox-related genes (ALB, CCL5, and HSPA1A) out of 84 in the array (1 µM) or no effect (10 µM) was observed for catechins. Higher catechin concentrations (over 10 µM) increased CAA values in a dose- and E°-dependent manner, caused cell growth inhibition, but surprisingly did not protect HT29 cells against reactive oxygen species (ROS)-induced DNA fragmentation. In conclusion, dose-dependent effects of dietary antioxidants and biological functions potentially modulated by them may become deregulated upon exposure to excessive doses.

The role of DNA methylation and recently discovered hydroxymethylation in the function of the human epigenome is currently one of the hottest topics in the life sciences. Progress in this field of research has been further accelerated by the discovery that alterations in the methylome are not only associated with key functions of cells and organisms, such as development, differentiation and gene expression, but may underlie a number of human diseases, including cancer. This review describes both well established and more recent observations concerning alterations in the methylome, i.e. the global and local distribution of 5-methylcytosines, involved in its normal functions. Then, the changes in DNA methylation pattern seen in cancer cells are discussed in the context of their utilisation in cancer diagnostics and treatment. On this basis, comparisons are made between natural covalent DNA modification and that induced by genotoxic agents, chemical carcinogens and antitumour drugs as regards their impact on epigenetic mechanisms. The available data suggest that DNA damage by genotoxins can mimic epigenetic markers and in consequence disrupt the proper function of the epigenome. On the other hand, the same processes in cancer cells, e.g. DNA demethylation as a result of DNA methyltransferase blocking or the induction of DNA repair by DNA adducts, may restore the activity of hypermethylated anticancer genes. The observed multiple mechanisms by which genotoxic agents directly affect methylome function suggest that chemical carcinogens act primarily as epigenome disruptors, whereas mutations are secondary events that occur at later stages of cancer development when genome-protecting mechanisms have already been deregulated.