Requirement of RIZ1 for cancer prevention by methyl-balanced diet

The typical Western diet is not balanced in methyl nutrients that regulate the level of the methyl donor S-adenosylmethionine (SAM) and its derivative metabolite S-adenosylhomocysteine (SAH), which in turn may control the activity of certain methyltransferases. Feeding rodents with amino acid defined and methyl-imbalanced diet decreases hepatic SAM and causes liver cancers. RIZ1 (PRDM2 or KMT8) is a tumor suppressor and functions in transcriptional repression by methylating histone H3 lysine 9. Here we show that a methyl-balanced diet conferred additional survival benefits compared to a tumor-inducing methyl-imbalanced diet only in mice with wild type RIZ1 but not in mice deficient in RIZ1. While absence of RIZ1 was tumorigenic in mice fed the balanced diet, its presence did not prevent tumor formation in mice fed the imbalanced diet. Unlike most of its related enzymes, RIZ1 was upregulated by methyl-balanced diet. Methyl-balanced diet did not fully repress oncogenes such as c-Jun in the absence of RIZ1. The data identify RIZ1 as a critical target of methyl-balanced diet in cancer prevention. The molecular understanding of dietary carcinogenesis may help people make informed choices on diet, which may greatly reduce the incidence of cancer

UV- irradiation induces oxidative damage to mitochondrial DNA primarily through hydrogen peroxide

Mitochondria are cell organella observed in eukaryotic cells and they play important roles in various basic biological phenomena such as apoptosis as well as production of energy via oxidative phosphorylation. Mitochondria have their own genome, mitochondrial DNA (mtDNA), in the matrix. In this decade, studies have found that mutations of mtDNA occur in various types of diseases and these mutations may result from oxidative damages by reactive oxygen species (ROS) generated in mitochondria. Furthermore, cytotoxic stress such as inadequate accumulation of ROS triggers pathways of apoptosis in mitochondria; mitochondria release cytochrome c into cytoplasma. Thus, mitochondria are a central regulatory element in stress-induced events. UV-irradiation activates the generation of ROS in cells, leading to consequent mutations in mtDNA. However, eukaryotes have the self-defense mechanisms against the adverse effects exerted by ROS. Manganese superoxide dismutase (MnSOD) and catalase (CAT) are thought to be involved in the mechanisms and to convert ROS to less toxic substances; MnSOD catalyses O2- to H2O2 and CAT converts H2O2 to water. We studied further roles of ROS in mtDNA damage by ROS in UV-irradiation. We introduced a cDNA of MnSOD and/or CAT into the human hepatoma cell line, SK-HEP1. Analyses using HPLC showed that UV-irradiation increased the accumulation of 8-hydroxy-2\u27-deoxyguanosine (8OHdG), a common marker for oxidatively damaged mtDNA, in these cells; a significantly increased level of 8OHdG was observed in all these cell lines. UVirradiation induced the accumulation of 8OHdG at higher level in MnSOD transfectants than in the control and CAT- or MnSOD/CAT- transfectants. Upon UV-irradiation, on the other hand, the accumulation of 8OHdG was reduced in cells overexpressing CAT when compared to that in the control cells. The accumulated levels of 8OHdG were in parallel with those of H2O2. Our results suggest that H2O2 play a key role in mtDNA damage in UV-irradatied cells. Our results also indicate that increased activity of MnSOD may cause a toxic effect on mtDNA by UV-irradiation, if the H2O2 cannot be converted.Apoptosis200

Cells overexpressing catalase are more sensitive to TNF : Role of superoxide anion in TNF -induced apoptosis

96th Annual Meetin

UV-irradiation induces oxidative damage to mitochondrial DNA primarily through hydrogen peroxide : Analysis of 8-oxodGuo by HPLC

Roles of reactive oxygen species (ROS) in damage to mitochondrial DNA (mtDNA) following ultraviolet (UV)-irradiation were investigated in the human hepatoma cell line SK-HEP-1. We altered the intracellular status of ROS by the overexpression of manganese superoxide dismutase (MnSOD) and/or catalase. Using HPLC, we analyzed 8-oxo-7,8-dihydro-2\u27-deoxyguanosine (8-oxodGuo), known as a marker of damage to DNA molecules. UV-irradiation resulted in the accumulation of 8-oxodGuo in these cells. The overexpression of MnSOD enhanced the accumulation of 8-oxodGuo by UV. The co-overexpression of catalase inhibited the accumulation of 8-oxodGuo by UV in MnSOD-transfectants. The overexpression of MnSOD reduced the colony forming capacity in SK-HEP-1 cells and the co-overexpression of catalase with MnSOD stimulated the capacity compared to control. UV-irradiation inhibited the colony forming capacity in these cells; no difference was observed among the capacities of control, MnSOD- and catalase-transfectants. However, the overexpression of MnSOD/catalase significantly rescued the reduction of colony forming capacity by UV-irradiation. Our results suggest that the accumulation of hydrogen peroxide plays a key role in the oxidative damage to mtDNA of UV-irradiated cells, and also that the overexpression of both MnSOD and catalase reduces the mtDNA damage and blocks the growth inhibition by UV. Our results also indicate that the increased activity of MnSOD may lead to a toxic effect on mtDNA by UV-irradiation