Fecal microbial metabolism of polyphenols and its effects on human gut microbiota

We investigated the biotransformation of four common dietary polyphenols, rutin, quercetin, chlorogenic acid and caffeic acid, in an in vitro mixed culture model of human intestinal microbiota, to determine effects on human gut bacteria. All four compounds were biotransformed rapidly, disappearing from the medium within 0.5 h and later replaced by known phenolic acid breakdown products, at concentrations up to hundreds of micromolar, much higher than in no-polyphenol control experiments. Quantitative PCR was used to measure effects of the polyphenols on the balance between the major groups of intestinal bacteria that are known to influence gut health, i.e., Bifidobacterium spp., Bacteroidetes, and Firmicutes. Fermentation of polyphenols stimulated proliferation of bifidobacteria and decreased the ratio of Firmicutes to Bacteroidetes, relative to controls. Polyphenols also stimulated short chain fatty acid production by the bacteria. Pure bifidobacterial cultures were treated separately with either fermented media isolated from the incubations, the pure test polyphenols, or the biotransformation products detected in the fermentations. Growth stimulation was observed only with fermented polyphenol media and the pure biotransformation products. It appears that dietary polyphenols may have the ability to modify the gut microbial balance, but this effect is indirect, i.e., it is mediated by biotransformation products, rather than the original plant compounds.

Phenotypic changes associated with RNA interference silencing of chalcone synthase in apple (Malus × domestica)

We have identified in apple (Malus × domestica) three chalcone synthase (CHS) genes. In order to understand the functional redundancy of this gene family RNA interference knockout lines were generated where all three of these genes were down-regulated. These lines had no detectable anthocyanins and radically reduced concentrations of dihydrochalcones and flavonoids. Surprisingly, down-regulation of CHS also led to major changes in plant development, resulting in plants with shortened internode lengths, smaller leaves and a greatly reduced growth rate. Microscopic analysis revealed that these phenotypic changes extended down to the cellular level, with CHS-silenced lines showing aberrant cellular organisation in the leaves. Fruit collected from one CHS-silenced line was smaller than the 'Royal Gala' controls, lacked flavonoids in the skin and flesh and also had changes in cell morphology. Auxin transport experiments showed increased rates of auxin transport in a CHS-silenced line compared with the 'Royal Gala' control. As flavonoids are well known to be key modulators of auxin transport, we hypothesise that the removal of almost all flavonoids from the plant by CHS silencing creates a vastly altered environment for auxin transport to occur and results in the observed changes in growth and development.

Apple skin patterning is associated with differential expression of MYB10

BACKGROUND

Some apple (Malus × domestica Borkh.) varieties have attractive striping patterns, a quality attribute that is important for determining apple fruit market acceptance. Most apple cultivars (e.g. 'Royal Gala') produce fruit with a defined fruit pigment pattern, but in the case of 'Honeycrisp' apple, trees can produce fruits of two different kinds: striped and blushed. The causes of this phenomenon are unknown.

RESULTS

Here we show that striped areas of 'Honeycrisp' and 'Royal Gala' are due to sectorial increases in anthocyanin concentration. Transcript levels of the major biosynthetic genes and MYB10, a transcription factor that upregulates apple anthocyanin production, correlated with increased anthocyanin concentration in stripes. However, nucleotide changes in the promoter and coding sequence of MYB10 do not correlate with skin pattern in 'Honeycrisp' and other cultivars differing in peel pigmentation patterns. A survey of methylation levels throughout the coding region of MYB10 and a 2.5 Kb region 5' of the ATG translation start site indicated that an area 900 bp long, starting 1400 bp upstream of the translation start site, is highly methylated. Cytosine methylation was present in all three contexts, with higher methylation levels observed for CHH and CHG (where H is A, C or T) than for CG. Comparisons of methylation levels of the MYB10 promoter in 'Honeycrisp' red and green stripes indicated that they correlate with peel phenotypes, with an enrichment of methylation observed in green stripes.

CONCLUSIONS

Differences in anthocyanin levels between red and green stripes can be explained by differential transcript accumulation of MYB10. Different levels of MYB10 transcript in red versus green stripes are inversely associated with methylation levels in the promoter region. Although observed methylation differences are modest, trends are consistent across years and differences are statistically significant. Methylation may be associated with the presence of a TRIM retrotransposon within the promoter region, but the presence of the TRIM element alone cannot explain the phenotypic variability observed in 'Honeycrisp'. We suggest that methylation in the MYB10 promoter is more variable in 'Honeycrisp' than in 'Royal Gala', leading to more variable color patterns in the peel of this cultivar.

Preparative enzymatic synthesis of glucuronides of zearalenone and five of its metabolites

The resorcylic acid lactones zearalenone ( 1), alpha-zearalenol ( 2), beta-zearalenol ( 3), alpha-zearalanol (zeranol) ( 4), beta-zearalanol (taleranol) ( 5), and zearalanone ( 6) were converted to their glucuronides on a preparative scale in good yields. Reactions were conducted with bovine uridine 5'-diphosphoglucuronyl transferase (UDPGT) as catalyst and uridine 5'-diphosphoglucuronic acid (UDPGA) as cofactor. The glucuronides were isolated by column chromatography and characterized by NMR spectroscopy and mass spectrometry. Although the principal products were 4- O-glucuronides (i.e., linkage through a phenolic hydroxyl), significant quantities of the 6'- O-glucuronides (i.e., linkage through the aliphatic hydroxyl) of alcohols 2, 4, and 5 were also isolated. In the case of 3, the 2- O-glucuronide was isolated as the minor product. Overall isolated yields of glucuronides, performed on a 20-50 mg scale, were typically ca. 80% based on the resorcylic acid lactone starting material. LC-UV-MS (2) analysis of purified specimens revealed MS (2) fragmentations useful for defining the point of attachment of the glucuronide moiety to the zearalenone nucleus.

The potential influence of fruit polyphenols on colonic microflora and human gut health

The effect of common dietary polyphenols on growth of human gut bacteria and their adhesion to enterocytes was investigated. The influence on the growth of a probiotic (Lactobacillus rhamnosus), a commensal (Escherichia coli) and two pathogenic bacteria (Staphylococcus aureus, Salmonella typhimurium) was determined, together with effects on adhesion of pathogenic and probiotic bacteria to cultured Caco-2 cells. All polyphenols, except rutin, were found to affect the viability of representative gut flora in vitro, at doses likely to be present in the gastrointestinal tract, but to differing degrees. Naringenin and quercetin were the most active with the lowest minimum inhibitory concentrations for all the four bacteria tested. The remaining polyphenols had the most marked effect on the Gram positive enteropathogen S. aureus. Naringenin and phloridzin were the most effective inhibitors of S. typhimurium adherence to Caco-2 enterocytes while phloridzin and rutin enhanced the adherence of the probiotic L. rhamnosus. Polyphenols appear to have potential to alter gut microecology and, by affecting the total number of beneficial microflora in the gut, may confer positive gut health benefits.

Polyphenolic phytochemicals--just antioxidants or much more?

Polyphenolic phytochemicals are ubiquitous in plants, in which they function in various protective roles. A 'recommended' human diet contains significant quantities of polyphenolics, as they have long been assumed to be 'antioxidants' that scavenge excessive, damaging, free radicals arising from normal metabolic processes. There is recent evidence that polyphenolics also have 'indirect' antioxidant effects through induction of endogenous protective enzymes. There is also increasing evidence for many potential benefits through polyphenolic-mediated regulation of cellular processes such as inflammation. Inductive or signalling effects may occur at concentrations much lower than required for effective radical scavenging. Over the last 2-3 years, there have been many exciting new developments in the elucidation of the in vivo mechanisms of the health benefits of polyphenolics. We summarise the current knowledge of the intake, bio-availability and metabolism of polyphenolics, their antioxidant effects, regulatory effects on signalling pathways, neuro-protective effects and regulatory effects on energy metabolism and gut health.

Comparison of enzymically glucuronidated flavonoids with flavonoid aglycones in an in vitro cellular model of oxidative stress protection

This study modeled, in vitro, the potential effect of conjugative (phase II) metabolism on the cytoprotective capacity of fruit flavonoids against oxidative stress. Flavonoid aglycones were compared with their corresponding isomeric mixtures of glucuronides for their ability to enhance the survival of cultured human Jurkat T and neuroblastoma cells stressed with hydrogen peroxide. Various polyphenolic compounds were tested as substrates in vitro for an ovine liver glucuronyl transferase preparation. Flavonoids and their glycoside derivatives were found to be good substrates, whereas phenolic acids were either poor or nonsubstrates. Five common flavonoids were glucuronidated to prepare mixtures for bioassay testing. Glucuronidation generally weakened the cytoprotective capacities of flavonoids (in the presence of H(2)O(2)), but some compounds were weakened much more than others. The concentration that halved cell death was well below 0.5 microM for most flavonoids tested, but glucuronidation increased median effective concentration values to a range of 1-16 microM. This compares with the generally accepted physiological range (0.1-10 microM) for circulating dietary polyphenolics detected in the body. Therefore, some flavonoids may retain a reduced cytoprotective capacity in vitro, after glucuronidation, whereas others may be effectively inactivated.

Comparison of the relative recovery of polyphenolics in two fruit extracts from a model of degradation during digestion and metabolism

To simulate the effects of digestion and metabolism on the survival of different polyphenolic compounds, extracts of blueberry and apple were deglycosylated by acid hydrolysis, followed by enzymic glucuronidation under neutral conditions, yielding approximately 5% overall recovery of polyphenolics. The major polyphenolics before and after the treatment were compared, to estimate which species are likely to be present in the intestinal lumen, undegraded and available for absorption, after consumption of the fruit. Whereas blueberry extract consisted predominantly of anthocyanins, epicatechin and caffeoyl quinate esters, the major components of the treated extract were quercetin glucuronides and (unglucuronidated) caffeoyl quinates, with only traces of anthocyanidin derivatives. In apple extract, compositional changes were less marked, but caffeoyl quinates, procyanidins and quercetin were enriched at the expense of caffeic acid, epicatechin and catechin. Hydrophobic compounds like phloretin and quercetin were extensively glucuronidated, whereas caffeic acid and caffeoyl quinate were not. These results suggest that the major polyphenolic components of a fruit are not necessarily the most important contributors to any health benefits because the polyphenolic composition in the intestinal lumen and consequently, in the circulation, may be considerably different.

Red colouration in apple fruit is due to the activity of the MYB transcription factor, MdMYB10

Anthocyanin concentration is an important determinant of the colour of many fruits. In apple (Malus x domestica), centuries of breeding have produced numerous varieties in which levels of anthocyanin pigment vary widely and change in response to environmental and developmental stimuli. The apple fruit cortex is usually colourless, although germplasm does exist where the cortex is highly pigmented due to the accumulation of either anthocyanins or carotenoids. From studies in a diverse array of plant species, it is apparent that anthocyanin biosynthesis is controlled at the level of transcription. Here we report the transcript levels of the anthocyanin biosynthetic genes in a red-fleshed apple compared with a white-fleshed cultivar. We also describe an apple MYB transcription factor, MdMYB10, that is similar in sequence to known anthocyanin regulators in other species. We further show that this transcription factor can induce anthocyanin accumulation in both heterologous and homologous systems, generating pigmented patches in transient assays in tobacco leaves and highly pigmented apple plants following stable transformation with constitutively expressed MdMYB10. Efficient induction of anthocyanin biosynthesis in transient assays by MdMYB10 was dependent on the co-expression of two distinct bHLH proteins from apple, MdbHLH3 and MdbHLH33. The strong correlation between the expression of MdMYB10 and apple anthocyanin levels during fruit development suggests that this transcription factor is responsible for controlling anthocyanin biosynthesis in apple fruit; in the red-fleshed cultivar and in the skin of other varieties, there is an induction of MdMYB10 expression concurrent with colour formation during development. Characterization of MdMYB10 has implications for the development of new varieties through classical breeding or a biotechnological approach.

Methylated polyphenols are poor “chemical” antioxidants but can still effectively protect cells from hydrogen peroxide-induced cytotoxicity

Several polyphenolic compounds, including flavonoids and phenolic acids, were compared with their per-methylated forms in both chemical and cell-based assays for antioxidant capacity. Methylation largely eliminated "chemical" antioxidant capacity, according to ferric reducing antioxidant power and oxygen radical absorbance capacity assays. Methylation, however, only moderately reduced protection of human Jurkat cells in culture, from hydrogen peroxide-mediated cytotoxicity, at physiologically relevant concentrations. Neither methylated nor un-methylated compounds were detectably metabolized by the cells. It appears that the protective mechanism of polyphenolic antioxidants against high concentrations of hydrogen peroxide in human cells may be largely unrelated to chemical antioxidant capacity.

Structural analysis of carrageenans from the red alga, Callophyllis hombroniana Mont. Kütz (Kallymeniaceae, Rhodophyta)

The use of a range of modern analytical techniques has facilitated the structural characterisation of the polysaccharide from the New Zealand endemic red alga, Callophyllis hombroniana. The native polysaccharide contains a number of structural units with the largest proportion consisting of 3-linked beta-D-galactopyranosyl 2-sulfate units, alternating with 4-linked 3,6-anhydro-alpha-D-galactopyranosyl 2-sulfate units, that is, theta-carrageenan (36 mol%). C. hombroniana is the first red seaweed reported to naturally contain such a large proportion of theta-carrageenan.

Comparative effects of dieldrin on hepatic ploidy, cell proliferation, and apoptosis in rodent liver

Dieldrin-induced hepatocarcinogenesis, which is seen only in the mouse, apparently occurs through a nongenotoxic mechanism. Previous studies have demonstrated that dieldrin induces hepatic DNA synthesis in mouse, but not rat liver. A number of nongenotoxic hepatocarcinogens have been shown to increase hepatocyte nuclear ploidy following acute and subchronic treatment in rodents, suggesting that an induction of hepatocyte DNA synthesis may occur without a concomitant increase in cell division. The current study examined the effects of dieldrin on changes in hepatocyte DNA synthesis, mitosis, apoptosis, and ploidy in mouse liver (the sensitive strain and target tissue for dieldrin-induced carcinogenicity) and the rat liver (an insensitive species). Male F344 rats and B6C3F1 mice were treated with 0, 1, 3, or 10 mg dieldrin/kg diet and were sampled after 7, 14, 28, or 90 d on diet. Liver from mice fed 10 mg dieldrin/kg diet exhibited significantly increased DNA synthesis and mitosis at 14, 28, or 90 d on diet. In rats, no increase in DNA synthesis or mitotic index was observed. The apoptotic index in liver of mice and rats did not change over the 90-d study period. Exposure of mice to only the highest dose of dieldrin produced a significant increase in octaploid (8N) hepatocytes and a decrease in diploid (2N) hepatocytes, which were restricted primarily to centrilobular hepatocytes, with the periportal region showing little or no change from control. No changes in hepatocyte nuclear ploidy were observed in the rat. This study demonstrates that exposure to high concentrations of dieldrin is accompanied by increased nuclear ploidy and mitosis in mouse, but not rat, liver. It is proposed that the observed increase in nuclear ploidy in the mouse may reflect an adaptive response to dieldrin exposure.

Enzymatic Preparation of Lysergol β-D-Glucuronide

Enzymatic synthesis of lysergol β-D-glucuronide (2) using ovine microsomal uridine 5'-diphosphoglucuronyl transferase and uridine 5'-diphosphoglucuronic acid (Na3-salt) is demonstrated.

Cancer dose-response modeling of epidemiological data on worker exposures to aldrin and dieldrin

The paper applies classical statistical principles to yield new tools for risk assessment and makes new use of epidemiological data for human risk assessment. An extensive clinical and epidemiological study of workers engaged in the manufacturing and formulation of aldrin and dieldrin provides occupational hygiene and biological monitoring data on individual exposures over the years of employment and provides unusually accurate measures of individual lifetime average daily doses. In the cancer dose-response modeling, each worker is treated as a separate experimental unit with his own unique dose. Maximum likelihood estimates of added cancer risk are calculated for multistage, multistage-Weibull, and proportional hazards models. Distributional characterizations of added cancer risk are based on bootstrap and relative likelihood techniques. The cancer mortality data on these male workers suggest that low-dose exposures to aldrin and dieldrin do not significantly increase human cancer risk and may even decrease the human hazard rate for all types of cancer combined at low doses (e.g., 1 microgram/kg/day). The apparent hormetic effect in the best fitting dose-response models for this data set is statistically significant. The decrease in cancer risk at low doses of aldrin and dieldrin is in sharp contrast to the U.S. Environmental Protection Agency's upper bound on cancer potency based on mouse liver tumors. The EPA's upper bound implies that lifetime average daily doses of 0.0000625 and 0.00625 microgram/kg body weight/day would correspond to increased cancer risks of 0.000001 and 0.0001, respectively. However, the best estimate from the Pernis epidemiological data is that there is no increase in cancer risk in these workers at these doses or even at doses as large as 2 micrograms/kg/day.

Monograph: reassessment of human cancer risk of aldrin/dieldrin

In 1987, the US Environmental Protection Agency (EPA) classified aldrin and dieldrin as category B2 carcinogens, i.e. probable human carcinogens, based largely on the increase in liver tumors in mice fed either organochlorine insecticide. At that date, the relevant epidemiology was deemed inadequate to influence the cancer risk assessment. More time has now elapsed since early exposures of manufacturing workers to aldrin/dieldrin; therefore, updated epidemiological data possess more power to detect exposure-related differences in cancer risk and mortality. Also, recent experimental studies provide a plausible mode of action to explain the mouse specificity of dieldrin-induced hepatocarcinogenesis and call into question the relevance of this activity to human cancer risk. This monograph places this new information within the historic and current perspectives of human cancer risk assessment, including EPA's 1996 Proposed Guidelines for Carcinogen Risk Assessment. Updated epidemiological studies of manufacturing workers in which lifetime exposures to aldrin/dieldrin have been quantified do not indicate increased mortality or cancer risk. In fact, at the middle range of exposures, there is evidence of a decrease in both mortality from all causes and cancer. Recent experimental studies indicate that dieldrin-induced hepatocarcinogenesis in mice occurs through a nongenotoxic mode of action, in which the slow oxidative metabolism of dieldrin is accompanied by an increased production of reactive oxygen species, depletion of hepatic antioxidant defenses (particularly alpha-tocopherol), and peroxidation of liver lipids. Dieldrin-induced oxidative stress or its sequelae apparently result in modulation of gene expression that favors expansion of initiated mouse, but not rat, liver cells; thus, dieldrin acts as a nongenotoxic promoter/accelerator of background liver tumorigenesis in the mouse. Within the framework of EPA's Proposed Guidelines for Carcinogen Risk Assessment, it is proposed that the most appropriate cancer risk descriptor for aldrin/dieldrin, relating to the mouse liver tumor response, is 'not likely a human carcinogen', a descriptor consistent with the example of phenobarbital cited by EPA.

Role of oxidative stress in the selective toxicity of dieldrin in the mouse liver

Dieldrin, an organochlorine insecticide, induces hepatic tumors in mice but not in rats. Although the mechanism(s) responsible for this species specificity is not fully understood, accumulating evidence indicates that oxidative stress may be involved. This study examined the association of dieldrin-induced hepatic DNA synthesis with the modulation of biomarkers of oxidative damage to lipids (malondialdehyde [MDA]) and DNA (8-hydroxy-2-deoxyguanosine [oh8dG]), in male B6C3F1 mice and F344 rats fed dieldrin (0.1, 1.0, or 10 mg/kg diet) for 7, 14, 28, and 90 days. The nonenzymatic components of the antioxidant defense system (ascorbic acid, glutathione, and alpha-tocopherol) were also examined. Increased urinary MDA was observed in mice fed 0.1, 1.0, or 10 mg dieldrin/kg diet for 7, 14, 28, and 90 days; while increased hepatic MDA was seen only after 7 days in mice fed 0.1, 1.0, or 10 mg dieldrin/kg diet and after 14 days in mice fed 10 mg/kg diet. In rats, dieldrin had no effect on either hepatic MDA or urine MDA levels after 7, 14, and 28 days of treatment. A dose-dependent increase in urinary MDA was observed in rats at the 90-day sampling time. The only significant elevation in urinary or hepatic oh8dG content was limited to urinary oh8dG in mice fed 10 mg/kg dieldrin diet for 14 days. Dietary dieldrin produced sustained decreases in hepatic and serum alpha-tocopherol and sustained elevations in hepatic ascorbic acid in both mice and rats. Rats, however, possessed a three- to four-fold higher content of endogenous or basal (control) hepatic alpha-tocopherol; and, even when fed 10 mg dieldrin/kg diet, the levels of hepatic alpha-tocopherol were maintained at higher levels than those of mice fed control diet. In both rats and mice fed dieldrin, transient (14 and 28 days on diet) elevations in hepatic glutathione were observed. These data support the hypothesis that the species specificity of dieldrin-induced hepatotoxicity may be related to dieldrin's ability to induce oxidative stress in the liver of mice, but not in rats. Only in mice fed dieldrin was a temporal association of increases in hepatic MDA content and hepatic DNA synthesis seen, suggesting that oxidative damage (shown by increased lipid peroxidation) may be involved in early events in dieldrin-induced hepatocarcinogenesis. Rats may be protected from dieldrin-induced oxidative stress by a more effective antioxidant defense system, characterized by higher basal levels of hepatic alpha-tocopherol and ascorbic acid than that seen in the mouse.

Some implications for quantitative risk assessment if hormesis exists

The existence of hormesis should impact quantitative risk assessment in at least seven fundamental ways. (1) The dose-response models for bioassay and epidemiological data should have greater flexibility to fit the observed shape of the dose-response data and no longer be forced to always be linearly increasing at low doses. (2) Experimental designs should be altered to provide greater opportunity to identify the hormetic component of a dose-response relationship. (3) Rather than a lifetime average daily dose or its analog for shorter time periods, dose scales or metrics should be used that reflect the age or time dependence of the dose level. (4) Low-dose risk characterization should include the likelihood of beneficial effects and the likelihood that a dose level has reasonable certainty of no appreciable adverse health effects. (5) Exposure assessments should make greater efforts to characterize the distribution of actual doses from exposure rather than just upper bounds. (6) Uncertainty characterizations should be expanded to include both upper and lower bounds, and there should be an increased explicit use of expert judgement and weight-of-evidence based distributional analyses reflecting more of the available relevant dose-response information and alternative risk characterizations. (7) Risk should be characterized in terms of the net effect of a dose on health rather than a dose's effect on a single factor affecting health - for example, risk would be better expressed in terms of mortality from all causes combined rather than a specific type of fatal disease.

Vitamin E modulation of dieldrin-induced hepatic focal lesion growth in mice

The effect of vitamin E on dieldrin-induced hepatic focal lesion growth in male B6C3F1 mice previously treated with diethylnitrosamine (DEN) was investigated. After hepatic focal lesions were formed, mice were placed into one of the following treatment groups: Group 1, 50 mg vitamin E/kg diet (control NIH-07 diet); Group 2, 10 mg dieldrin/kg NIH-07 diet; Group 3, 10 mg dieldrin and 450 mg vitamin E/kg NIH-07 diet; and Group 4, 450 mg vitamin E/kg NIH-07 diet. Mice were killed and necropsied after 30 and 60 d of dietary treatment. The effect of treatment on lesion growth was examined by measuring the number of focal lesions per liver and the relative hepatic focal lesion volume. In addition, the possible cellular mechanism of focal hepatocyte growth was investigated by examining both focal DNA synthesis and apoptosis. Dieldrin treatment alone (Group 2) increased the focal lesion volume, focal lesion number, and focal lesion labeling index. Supplementation with vitamin E (Group 3) blocked this effect. Vitamin E supplementation to the diet alone (Group 4) also enhanced focal lesion growth and increased the number of lesions per liver, the relative focal volume, and the labeling index in hepatic focal lesions. Interestingly, vitamin E supplementation inhibited apoptosis in normal liver but did not produce an observable decrease in apoptosis in hepatic focal lesions. The present study showed that dieldrin (Group 2) or vitamin E supplementation alone (Group 4) promoted the growth of hepatic focal lesions in mice. However, when vitamin E is supplemented to dieldrin-fed mice (Group 3), there is an inhibition of hepatic focal lesion growth.

Modeling to incorporate defense mechanisms into the estimation of dose responses

Several adverse health effects (including cancer and noncancer effects) may be the result of an imbalance between exogenous and endogenous invading substances and defense mechanisms. In these cases the probability of an adverse effect depends on how much the exposure to a substance increases or decreases the number of defenders or their efficiency as well as increasing or decreasing the number of invaders. Rather than using a dose scale such as parts per million or milligram/kilogram/day in these cases, dose-response models can directly incorporate the impact of defense mechanisms by using a dose scale that corresponds to the number of invaders that break through the defenders and become free to do their damage. The number of breakthroughs at a specific age, the cumulative number of breakthroughs by a specific age, or the cumulative number of breakthroughs in a window of time would usually be the appropriate age-dependent dose. Although a lifetime average daily dose level can be used as a surrogate for an age-dependent dose in simplistic dose-response models, the age-dependent dose itself can be used in more biologically based models that include time, reflect the key role of feedback mechanisms, and treat the human body as an age-dependent dynamic system responding to internal and external stimuli and not as a system at equilibrium. Some illustrative biologic examples of defense mechanisms and invader-defender interactions are presented. Several numerical examples are given in which the dose incorporates the age-dependent effects of a substance on the number of invaders, the number of defenders, and/or the defenders' efficiencies.

The role of oxidative stress in chemical carcinogenesis

Oxidative stress results when the balance between the production of reactive oxygen species (ROS) overrides the antioxidant capability of the target cell; oxidative damage from the interaction of reactive oxygen with critical cellular macromolecules may occur. ROS may interact with and modify cellular protein, lipid, and DNA, which results in altered target cell function. The accumulation of oxidative damage has been implicated in both acute and chronic cell injury including possible participation in the formation of cancer. Acute oxidative injury may produce selective cell death and a compensatory increase in cell proliferation. This stimulus may result in the formation of newly initiated preneoplastic cells and/or enhance the selective clonal expansion of latent initiated preneoplastic cells. Similarly, sublethal acute oxidative injury may produce unrepaired DNA damage and result in the formation of new mutations and, potentially, new initiated cells. In contrast, sustained chronic oxidative injury may lead to a nonlethal modification of normal cellular growth control mechanisms. Cellular oxidative stress can modify intercellular communication, protein kinase activity, membrane structure and function, and gene expression, and result in modulation of cell growth. We examined the role of oxidative stress as a possible mechanism by which nongenotoxic carcinogens may function. In studies with the selective mouse liver carcinogen dieldrin, a species-specific and dose-dependent decrease in liver antioxidant concentrations with a concomitant increase in ROS formation and oxidative damage was seen. This increase in oxidative stress correlated with an increase in hepatocyte DNA synthesis. Antioxidant supplementation prevented the dieldrin-induced cellular changes. Our findings suggest that the effect of nongenotoxic carcinogens (if they function through oxidative mechanisms) may be amplified in rodents but not in primates because of rodents' greater sensitivity to ROS. These results and findings reported by others support a potential role for oxidative-induced injury in the cancer process specifically during the promotion stage.

Role of oxidative stress in the mechanism of dieldrin's hepatotoxicity

The production of reactive oxygen species (ROS) by toxic chemicals has been implicated in acute and chronic disease states, including cancer. This increase in cellular ROS can lead to a state of oxidative stress. Many compounds selectively induce hepatic tumors in mice but not rats. The mechanism for the induction of hepatic cancer by these compounds and the observed species selectivity of this effect are not known but may be related to the induction of oxidative stress. Dieldrin is one such compound and is used in the present study to characterize the relationship between oxidative stress and the observed selective hepatotoxicity of dieldrin in mice. It was found that dieldrin induced oxidative stress in the mouse but not the rat, and the observed oxidative stress correlated with the induction of DNA S-phase synthesis. This evidence suggests that the induction of oxidative stress may be a mechanism by which dieldrin and other mouse specific compounds selectively induce their hepatic toxic effects in mice.

Reversibility of promoter induced hepatic focal lesion growth in mice

The effect of cessation of phenobarbital and dieldren treatment on hepatic focal lesion growth in male B6C3F1 mice was investigated. Following induction of lesions by diethylnitrosamine, mice were placed on control NIH-07 diet (control diet) or NIH-07 diet containing either dieldrin (10.0 mg/kg diet) or phenobarbital (500 mg/kg diet). Mice were sacrificed after 30 and 60 days of dietary treatment. Two additional groups of mice were fed either the dieldren- or phenobarbital-containing diet for 30 days followed by feeding of NIH-07-only diet for an additional 30 days. The effect of treatment and removal of dieldrin or phenobarbital on lesion growth was examined by measuring both the number of focal lesions per liver and the relative volume of focal lesions. In addition, the rate of cell proliferation and programmed cell death in focal lesion growth was investigated by examining DNA synthesis and apoptosis in the focal lesions. Dietary dieldrin or phenobarbital increased the number of focal lesions and the focal lesion volume. In both dieldrin- and phenobarbital-treated mice, an increased number of eosinophilic lesions were seen. The focal lesion volume was increased in both eosinophilic and basophilic lesions. Dieldrin and phenobarbital treatment also increased the DNA synthetic labeling index in both eosinophilic and basophilic lesions. Removal of dieldrin or phenobarbital from the diet after 30 days of promoter treatment decreased the total number and volume of hepatic focal lesions. The labeling index of the focal lesions was also decreased in these mice. At the terminal sacrifice, the percentage of apoptotic cells in focal lesions was higher in mice fed dieldrin- or phenobarbital-containing diets for the entire 60 days than in mice returned to control diet for the last 30 days. Eosinophilic lesions were more dependent on the presence of a promoting stimulus than the basophilic lesions. These data indicate that induction and maintenance of the growth of some preneoplastic lesions in the mouse may be dependent upon continuous tumor promoter treatment.

Selective dieldrin promotion of hepatic focal lesions in mice

Chronic exposure to a number of chlorinated pesticides, including dieldrin, results in an increased incidence and/or multiplicity of hepatocellular neoplasia in mice, with no such effect in similarly treated rats. One possible explanation of this observed selective carcinogenicity is species-specific hepatic tumor promotion. In the present study we examined the dose-response effect of dieldrin (at several doses) on focal lesion growth (tumor promotion), hepatocyte apoptosis and DNA synthesis in rat and mouse liver. Preneoplastic focal hepatic lesions were produced by diethylnitrosamine (DEN). After the lesions developed, mice and rats were placed into one of the following dose groups: control (NIH-07 diet) or 0.1, 1.0 or 10.0 mg dieldrin/kg diet. Increased focal lesion volume, number of foci per liver and focal DNA synthetic labeling index were observed in 10 mg dieldrin/kg diet-treated mice, but not in similarly treated rats. Dieldrin at dietary concentrations of 0.1 and 1.0 mg/kg diet produced an increase in the number of preneoplastic lesions (0.1 mg/kg diet at 7 days only) and focal volume (0.1 mg/kg diet at 7 and 30 days, 1.0 mg/kg diet at 30 days), but these concentrations did not increase focal DNA labeling index. At dietary concentrations of 0.1, 1.0 and 10 mg dieldrin/kg diet no significant change in lesion percent volume, number of preneoplastic lesions per liver or preneoplastic lesion DNA labeling index was seen in treated rats compared with control rats. Apoptosis, a form of programed cell death, was not decreased in foci by any concentration of dieldrin in either rats or mice. Thus our results suggest that dieldrin may function as a mouse-specific tumor promoter through increased lesion DNA synthesis.

Dose dependence of phenobarbital promotion of preneoplastic hepatic lesions in F344 rats and B6C3F1 mice: effects on DNA synthesis and apoptosis

Phenobarbital (PB), a non-genotoxic hepatocarcinogen in rodents, has been studied extensively but its mechanism of carcinogenic action is unclear. PB appears to function as a tumor promoter by selectively inducing the growth of preneoplastic hepatocytes. In the present study, the comparative effects of PB at tumor-promoting and non-promoting doses were examined in male B6C3F1 mice and male F344 rats. In addition, the mechanism by which PB produced the selective induction of preneoplastic cell growth (increased DNA synthesis/cell proliferation and/or decreased apoptosis) was investigated. Preneoplastic focal lesions were produced using diethylnitrosamine (DEN). After the lesions were histologically apparent, mice and rats were fed PB (10, 100, or 500 mg/kg NIH-07 diet) or control diet and sampled after 7, 30 and 60 days of treatment In both mice and rats, 100 and 500 mg PB/kg increased the number and the relative volume of focal lesions. In rats and mice, 10 mg PB/kg did not enhance focal lesion growth. The preneoplastic lesions that clonally expanded due to phenobarbital treatment were predominantly eosinophilic in appearance. In addition, DNA synthesis in focal hepatocytes was significantly increased in the 100 and 500 mg PB/kg diet. In PB-treated mice and rats, there also was a significant decrease in the rates of apoptosis in focal hepatocytes. Therefore, our data showed that PB at doses of 100 and 500 mg/kg diet promoted focal hepatic lesion growth both by increasing DNA synthesis and cell proliferation and by decreasing the rate of apoptosis.

Subchronic effects of dieldrin and phenobarbital on hepatic DNA synthesis in mice and rats

Dieldrin, an organochlorine pesticide, has been shown to be hepatocarcinogenic in mice but not rats. Phenobarbital, in contrast, induces hepatic tumors in both mice and rats. Previous studies have shown that acute dietary exposure of rats or mice to either dieldrin or phenobarbital produces several liver changes, including centrilobular hypertrophy, induction of hepatic cytochrome P450, and increased liver weight. The present study examined the subchronic effect of dieldrin (0.1, 1.0, 3.0, 10.0 mg dieldrin/kg diet) and phenobarbital (10, 50, 100, 500 mg phenobarbital/kg diet) on the induction of hepatic DNA synthesis and hepatocyte lethality in male B6C3F1 mice and male F344 rats. Eight-week-old animals were treated as above and evaluated for hepatic DNA synthesis after 7, 14, 21, 28, and 90 days of continual treatment to dieldrin or phenobarbital. Maximal induction of hepatic DNA synthesis in mice was seen at the 14-, 21-, and 28-day sampling times. In rats, no significant increase in hepatic DNA synthesis or hepatocyte lethality was observed at any dose of dieldrin investigated. Phenobarbital produced a significant increase in hepatic DNA synthesis in both rat and mouse liver following 7 days of treatment. The induction of DNA synthesis in rat liver was transient, with the labeling index returning to control levels by 14 days of treatment. In contrast, mice treated with phenobarbital showed a significant increase in hepatic DNA synthesis throughout the treatment. In both mice and rats, dieldrin and phenobarbital induced hepatic DNA synthesis selectively in the centrilobular region of the hepatic lobule. The lack of an increase in serum enzymes indicative of hepatic damage and the absence of liver histopathology in mice or rats fed dieldrin or phenobarbital indicate that the induction of DNA synthesis was not mediated by a cytolethal, compensatory hyperplastic response, suggesting a mitogenic mechanism. Therefore, the species-specific induction of hepatic DNA synthesis by either dieldrin or phenobarbital correlated with the previously observed species-specific induction of hepatic cancer by these two compounds.

Oxidative stress in nongenotoxic carcinogenesis

The induction of oxidative stress in the target tissue has been proposed as a possible mechanism of action for nongenotoxic carcinogens. A variety of nongenotoxic hepatocarcinogens including peroxisome proliferators, organochlorines, barbiturates, and metals have been shown to produce an increase in reactive oxygen species (ROS) in the liver. Our group has examined the induction of oxidative stress by the organochlorine mouse hepatic carcinogen, dieldrin. Using a salicylate spin trap assay, dieldrin was found to produce mouse liver-specific increases in ROS in cultured hepatocytes. Increased amounts of hepatic 8-hydroxy-2'-deoxyguanosine and malondialdehyde (MDA) and decreased levels of cellular antioxidants were also seen in cultured mouse hepatocytes following dieldrin treatment. In subchronically dieldrin-treated mice and rats, hepatic vitamin E (Vit E) was decreased correlated with dieldrin dose. While Vit E levels were decreased in both rats and mice, the normal lower levels of Vit E in the mouse resulted in a subsequent oxidative stress, evidenced by an increase in MDA formation in the mouse liver. Dieldrin also produced a dose-dependent increase in DNA synthesis in the mouse (not the rat) following subchronic treatment. These effects seen in both cells in culture and in vivo were species specific, organ specific, and dose dependent which directly correlated with the observed pattern of cancer induction for dieldrin in rodents (mouse liver-specific). These findings support a possible role for the induction of oxidative stress in nongenotoxic hepatic carcinogenesis possibly through modulation of gene expression.

Challenges to default assumptions stimulate comprehensive realism as a new tier in quantitative cancer risk assessment

The current practice in carcinogen risk assessment of using a linearized multistage model and assuming low-dose linearity is based on several false premises. In many cases linearity at low doses would not be expected based on the interaction between the multiple components in the carcinogenic process. The two-stage growth models, involving multiple mutations and cell birth and death rates, provide one means of exploring these interactions. In addition, if carcinogenesis is considered to be the imbalance between invading substances and defense mechanisms, then the cancer probability depends on how much the substance increases or decrease the number of defenders or their efficiency as well as increasing or decreasing the number of invaders. Challenges to low-dose linearity and other default assumptions have stimulated the development of new risk assessment methodologies as have the need for more realistic estimates of risk, better uncertainty characterization, and greater utilization of cost-benefit analyses, and other tools for risk management decision making. "Comprehensive realism" is an emerging quantitative weight-of-evidence risk assessment methodology which is designed to reflect all of the relevant and available information and the current state of knowledge about the health risks associated with a substance.

Effect of dietary antioxidants on dieldrin-induced hepatotoxicity in mice

An increasing number of reports suggest that oxidative stress plays a role in the toxicity of various xenobiotics, including organochlorine pesticides and drugs such as phenobarbital. Antioxidants appear to be protective against the damage induced by an acute dose of endrin, supporting the theory of a role for reactive oxygen in the toxicity of this class of compounds. The current study examined the effects of the dietary administration of vitamin C (400 mg/kg diet) or vitamin E (200 mg DL-alpha-tocopherol acetate/kg diet) on hepatotoxicity induced by subchronic (7 or 28 days) feeding of dieldrin (1, 3 and 10 mg/kg diet) to male B6C3F1 mice. Hepatoxicity induced by feeding of dieldrin for 28 days was evidenced by liver enlargement, hypertrophy of centrolobular hepatocytes, induction of hepatic ethoxyresorufin O-deethylase activity, and increased DNA synthesis in hepatocytes, particularly in centrolobular hepatocytes. Neither vitamin inhibited the dose-dependent increase in liver/body weight ratios, hypertrophy of centrolobular hepatocytes, or induction of hepatic ethoxyresorufin O-deethylase. Vitamin E, however, inhibited hepatic DNA synthesis at all dietary intakes of dieldrin, while vitamin C was inhibitory at 1 and 3, but stimulatory at 10 mg dieldrin per kg diet. The major changes in DNA labeling occurred in the centrolobular zones, but were not consistently inhibited by vitamins C or E. The ability of antioxidant vitamins to inhibit dieldrin-induced hepatic DNA synthesis suggests oxidative stress is involved in the toxicity of this compound; however, the inability of these vitamins to prevent all hepatotoxic changes indicates other factors are also involved.

Another flaw in the linearized multistage model upper bounds on human cancer potency

The statistical methodology used by the EPA and other federal agencies to characterize human cancer potencies based on animal experiments greatly exaggerates the estimates and bounds for human cancer risks in some instances. The current methodology incorporates simplified assumptions and approximations that fail to properly assess the impact of quantitative differences in human and experimental animal background transition rates from stage to stage in the multistage carcinogenic process. Because the majority of tumorigenic responses in rats and mice occur in organs with a high background tumor incidence and, hence, high background stage transition rates, the current simplified methodology often significantly overstates human risk. This newly recognized flaw in conjunction with the several previously recognized flaws in the current characterizations of human cancer potencies argues strongly for a change away from the current characterizations based solely on a default screening methodology to a more comprehensive, biologically based risk assessment methodology.