Differential gene expression in normal and transformed human mammary epithelial cells in response to oxidative stress

Oxidative stress plays a key role in breast carcinogenesis. To investigate whether normal and malignant breast epithelial cells differ in their responses to oxidative stress, we examined the global gene expression profiles of three cell types, representing cancer progression from a normal to a malignant stage, under oxidative stress. Normal human mammary epithelial cells (HMECs), an immortalized cell line (HMLER-1), and a tumorigenic cell line (HMLER-5) were exposed to increased levels of reactive oxygen species (ROS) by treatment with glucose oxidase. Functional analysis of the metabolic pathways enriched with differentially expressed genes demonstrated that normal and malignant breast epithelial cells diverge substantially in their response to oxidative stress. Whereas normal cells exhibit the up-regulation of antioxidant mechanisms, cancer cells are unresponsive to the ROS insult. However, the gene expression response of normal HMECs under oxidative stress is comparable to that of the malignant cells under normal conditions, indicating that altered redox status is persistent in breast cancer cells, which makes them resistant to increased generation of ROS. We discuss some of the possible adaptation mechanisms of breast cancer cells under persistent oxidative stress that differentiate them from normal mammary epithelial cells as regards the response to acute oxidative stress.     

Cytotoxic and Genotoxic Consequences of Heat Stress Are Dependent on the Presence of Oxygen in Saccharomyces cerevisiae

Lethal heat stress generates oxidative stress in Saccharomyces cerevisiae, and anaerobic cells are several orders of magnitude more resistant than aerobic cells to a 50 degrees C heat shock. Here we characterize the oxidative effects of this heat stress. The thermoprotective effect in anaerobic cells was not due to expression of HSP104 or any other heat shock gene, raising the possibility that the toxicity of lethal heat shock is due mainly to oxidative stress. Aerobic but not anaerobic heat stress caused elevated frequencies of forward mutations and interchromosomal DNA recombination. Oxidative DNA repair glycosylase-deficient strains under aerobic conditions showed a powerful induction of forward mutation frequencies compared to wild-type cells, which was completely abolished under anaerobiosis. We also investigated potential causes for this oxygen-dependent heat shock-induced genetic instability. Levels of sulfhydryl groups, dominated mainly by the high levels of the antioxidant glutathione (reduced form) and levels of vitamin E, decreased after aerobic heat stress but not after anaerobic heat stress. Aerobic heat stress also led to an increase in mitochondrial membrane disruption of several hundredfold, which was 100-fold reduced under anaerobic conditions.     

Effect of hydrogen peroxide production and the Fenton reaction on membrane composition of Streptococcus pneumoniae

As part of its aerobic metabolism, Streptococcus pneumoniae generates high levels of H(2)O(2) by pyruvate oxidase (SpxB), which can be further reduced to yield the damaging hydroxyl radicals via the Fenton reaction. A universal conserved adaptation response observed among bacteria is the adjustment of the membrane fatty acids to various growth conditions. The aim of the present study was to reveal the effect of endogenous reactive oxygen species (ROS) formation on membrane composition of S. pneumoniae. Blocking carbon aerobic metabolism, by growing the bacteria at anaerobic conditions or by the truncation of the spxB gene, resulted in a significant enhancement in fatty acid unsaturation, mainly cis-vaccenic acid. Moreover, reducing the level of OH(.) by growing the bacteria at acidic pH, or in the presence of an OH(.) scavenger (salicylate), resulted in increased fatty acid unsaturation, similar to that obtained under anaerobic conditions. RT-PCR results demonstrated that this change does not originate from a change in mRNA expression level of the fatty acid synthase II genes. We suggest that endogenous ROS play an important regulatory role in membrane adaptation, allowing the survival of this anaerobic organism at aerobic environments of the host.