Dietary cadmium inhibits spontaneous hepatocarcinogenesis in C3H/HeN mice and hepatitis in A/J mice, but not in C57BL/6 mice

Metal-mediated Epigenetic Regulation of Gene Expression

Epigenetics is the study of heritable changes in gene expression that occur without alterations in the DNA sequence. Several studies have shown that environmental chemicals can alter epigenetic modifications, including histone modifications and DNA methylation. Environmental chemicals may show toxic effects via epigenetic mechanism-regulated changes in gene expression. Previously, we reported that zinc treatment rapidly decreased Lys(4)-trimethylated and Lys(9)-acetylated histone H3 in the metallothionein (MT) promoter, and also decreased total histone H3. The chromatin structure in the MT promoter may be locally disrupted by zinc-induced nucleosome removal. We also showed that chromium (VI) inhibited MT gene transcription by modifying the transcription potential of the co-activator p300. MT is a small cysteine-rich protein that is active in zinc homeostasis, cadmium detoxification, and protection against reactive oxygen species. Epigenetic changes might influence the cytoprotective function of the MT gene. In this review, I briefly summarize the results of previous studies and discuss the mechanisms and toxicological significance of metal-mediated epigenetic modifications.

Suppression of alkylating agent induced cell transformation and gastric ulceration by low-dose alkylating agent pretreatment

Exposure to mild stress by chemicals and radiation causes DNA damage and leads to acquired stress resistance. Although the linear no-threshold (LNT) model of safety assessment assumes risk from any dose, evidence from radiological research demonstrates a conflicting hormetic phenomenon known as the hormesis effect. However, the mechanisms underlying radiation hormesis have not yet been clarified, and little is known about the effects of low doses of chemical carcinogens. We analyzed the efficacy of pretreatment with low doses of the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on the subsequent induction of cell transformation and gastric ulceration by high-dose MNNG. We used an in vitro Balb/3T3 A31-1-1 cell transformation test and monitored the formation of gastric ulcers in 5-week-old male ICR mice that were administered MNNG in drinking water. The treatment concentrations of MNNG were determined by the cell survival rate and past reports. For low-dose in vitro and in vivo experiments, MNNG was used at 0.028 μM, and 2.8 μg/mL, respectively. The frequency of cell transformation induced by 10 μm MNNG was decreased by low-dose MNNG pretreatment to levels similar to that of spontaneous transformation. In addition, reactive oxygen species (ROS) and mutation frequencies induced by 10 μm MNNG were decreased by low-dose MNNG pretreatment. Importantly, low-dose MNNG pretreatment had no effect on cell proliferation. In vivo studies showed that the number of gastric ulcers induced by 1 mg/mL MNNG decreased after low-dose MNNG pretreatment. These data indicate that low-dose pretreatment with carcinogens may play a beneficial role in the prevention of chemical toxicity under specified conditions.

Selenium and cadmium levels and ratios in prostates, livers, and kidneys of nonsmokers and smokers

Cadmium (Cd) from cigarette smoke, environmental, and nutritional sources accumulates in the human prostate where it interacts with selenium (Se) in a manner suggesting the formation of a 1:1 Cd-Se-protein complex. At low Cd exposures and adequate Se status, this interaction may be beneficial as it results in the detoxification of Cd. At higher exposures, Cd may weaken or abolish the anticarcinogenic effects of Se and increase prostate cancer risk. In some older men and especially in smokers, Cd levels in prostates reach levels in stoichiometric excess over Se, which may be the reason why heavy smokers are at higher risk of developing lethal forms of prostate cancer. In the liver and the kidneys, Cd accumulates as well, but in these organs, Cd is efficiently sequestered through metallothionein formation. In the prostate, this mode of detoxification is not available or less efficient, causing Cd to interact with Se and to increase prostate cancer risk.

Nitric oxide as a target of complementary and alternative medicines to prevent and treat inflammation and cancer

Nitric oxide (NO) and associated reactive nitrogen species (RNS) are involved in many physiological functions. There has been an ongoing debate to whether RNS can inhibit or perpetuate chronic inflammation and associated carcinogenesis. Although the final outcome depends on the genetic make-up of its target, the surrounding microenvironment, the activity and localization of nitric oxide synthase (NOS) isoforms, and overall levels of NO/RNS, evidence is accumulating that in general, RNS drive inflammation and cancers associated with inflammation. To this end, many complementary and alternative medicines (CAMs) that work in chemoprevention associated with chronic inflammation, are inhibitors of excessive NO observed in inflammatory conditions. Here, we review recent literature outlining a role of NO/RNS in chronic inflammation and cancer, and point toward NO as one of several targets for the success of CAMs in treating chronic inflammation and cancer associated with this inflammation.

Murine strain differences in 8-hydroxy-deoxyguanosine formation in hepatic DNA induced by oxidized lard and dietary oils

Difference of 8-hydroxy-deoxyguanosine (8-OH-dG) formation in liver DNA in C3H/HeN and in C57BL/6 mice--fed oxidized lard and dietary oils (soybean and sardine)--was investigated. The blank levels of 8-OH-dG were higher in C3H/HeN mice (highly sensitive to liver tumorigenesis) than in C57BL/6 mice (resistant strain). The level of 8-OH-dG increased much more in C3H/HeN mice than in the C57BL/6 mice fed by oxidized lard and dietary oil treatment. Feeding oxidized lard and dietary oils increased 8-oxo-guanine DNA glycosylase I (OGG1) and mRNA 8-oxo-dGTPase in C57BL/6 mice. On the other hand, no appreciable change of mRNA in the C3H/HeN mice was observed. The formation differences of 8-OH-dG from the two murine strains fed with oxidized lard and dietary oils may be associated with the different mRNA levels in the DNA repair enzymes because the mRNA levels in the DNA repair enzymes were much lower in C3H/HeN mice than in C57BL/6 mice.

Metallothionein-I/II double knockout mice are hypersensitive to lead-induced kidney carcinogenesis: role of inclusion body formation

Lead is an environmental nephrotoxicant and probable human carcinogen. Elucidating factors predisposing populations to lead toxicity is an important public health issue. Recently, we found that metallothionein-I/-II double knockout (metallothionein-null) mice that are unable to produce the major forms of metallothionein do not produce lead inclusion bodies, which are thought to mitigate lead toxicity, and were sensitive to the subchronic toxic effects of lead exposure (10 weeks), showing modestly diminished renal function and nephromegaly compared with wild-type (WT) mice. It is unclear how this knockout might impact lead carcinogenesis. Thus, the effects of lead(II) acetate were tested in groups (n = 25) of male metallothionein-null and WT mice receiving drinking water with 0, 1,000, 2,000, or 4,000 parts per million lead for up to 104 weeks. Renal proliferative lesions (adenoma and cystic tubular atypical hyperplasia) were much more common and more severe in lead-exposed metallothionein-null mice than in WT mice. A metastatic renal cell carcinoma also occurred in a lead-treated metallothionein-null mouse, whereas none occurred in WT mice. Lead-induced renal proliferative lesions showed marked overexpression of cyclin D1, a common feature of human renal tumors. Renal lead-containing nuclear inclusion bodies were frequently observed in WT mice but did not form in metallothionein-null mice. Metallothionein was often found associated with the outer portion of these inclusion bodies. Thus, the metallothionein-null mice cannot form renal inclusion bodies, even after protracted lead exposure, and this increases the carcinogenic potential of lead. Poor production of metallothionein may predispose human populations to lead carcinogenicity.

Low-Dose Cadmium Exposure Reduces Human Prostate Cell Transformation in Culture and Up-Regulates Metallothionein and MT-1G mRNA

Chronic low-level exposure to environmental toxins, including cadmium (Cd), is a growing problem in the industrialized world. One promising strategy for protection from these toxins is the use of low-dose exposure of environmental chemicals to induce cell tolerance and recovery, a phenomenon known as "protective hormesis". Hormetic [low-dose stimulatory] effects occur in a variety of systems and with a number of chemicals. Cd is a potent carcinogen in rodents and has also been linked to human lung and prostate cancers. In the present study, we have evaluated the protective effects of low and ultra-low dose, long-term Cd exposure in the normal human prostate cells, RWPE-1. Cells were exposed to low and ultra-low doses (0, 0 (S(-36)), 10(-6), 10(-7), 10(-18), 10(-21), 10(-32), or 10(-36)M) of Cd for 20 weeks followed by treatment with 10(-5)M Cd for another 8 weeks. Continuous exposure of RWPE-1 cells to 10(-5)M Cd results in malignant transformation. However, cells pretreated with low and ultra-low doses of Cd had delayed transformation compared with controls. In addition, the number of transformed cell mounds was lower in pretreated cells indicating that low and ultra-low dose exposure had protective effects against high-dose Cd induced carcinogenesis. The expression of metallothionein (MT), the primary Cd detoxification protein, was induced by low-dose exposure to Cd and maintained during the 20 weeks. In addition, MT-1G mRNA was up-regulated 2- to 3-fold by low-dose and ultralow-dose Cd exposures and may be the mechanism of protective hormesis in this model. MT-1G mRNA might also serve as a biological indicator of very low-dose environmental Cd exposure.