Structure-activity relationships for a large diverse set of natural, synthetic, and environmental estrogens.

Understanding structural requirements for a chemical to exhibit estrogen receptor (ER) binding has been important in various fields. This knowledge has been directly and indirectly applied to design drugs for human estrogen replacement therapy, and to identify estrogenic endocrine disruptors. This paper reports structure-activity relationships (SARs) based on a total of 230 chemicals, including both natural and xenoestrogens. Activities were generated using a validated ER competitive binding assay, which covers a 10(6)-fold range. This study is focused on identification of structural commonalities among diverse ER ligands. It provides an overall picture of how xenoestrogens structurally resemble endogenous 17beta-estradiol (E(2)) and the synthetic estrogen diethylstilbestrol (DES). On the basis of SAR analysis, five distinguishing criteria were found to be essential for xenoestrogen activity, using E(2) as a template: (1) H-bonding ability of the phenolic ring mimicking the 3-OH, (2) H-bond donor mimicking the17beta-OH and O-O distance between 3- and 17beta-OH, (3) precise steric hydrophobic centers mimicking steric 7alpha- and 11beta-substituents, (4) hydrophobicity, and (5) a ring structure. The 3-position H-bonding ability of phenols is a significant requirement for ER binding. This contributes as both a H-bond donor and acceptor, although predominantly as a donor. However, the 17beta-OH contributes as a H-bond donor only. The precise space (the size and orientation) of steric hydrophobic bulk groups is as important as a 17beta-OH. Where a direct comparison can be made, strong estrogens tend to be more hydrophobic. A rigid ring structure favors ER binding. The knowledge derived from this study is rationalized into a set of hierarchical rules that will be useful in guidance for identification of potential estrogens.

Does caloric restriction induce hormesis?

The question of whether caloric restriction (CR) is hormetic is addressed in terms of two common definitions of the term. In terms of the older definition, i.e., a growth-stimulatory effect when lower doses of a compound which resulted in growth inhibition at higher doses, CR is better characterized as a co-hormetic (i.e., a paradigm which at relatively "low doses," in combination with some stimulus, will evince increased growth (proliferation) and at higher "doses" will inhibit this increased proliferation) rather than a hormetic agent. Mechanisms such as cellular selection of cellular subpopulations, increases in receptor efficiency, and preservation of cellular proliferative potential can interact with agents and produce increased growth as long as the CR is not too severe. In terms of a broader definition, i.e., nonmonotonic dose-response behavior of a compound for any adverse response, CR appears to be hormetic, both as a result of body weight (BW) loss and other potential mechanisms. The impact of changes in BW, or frank CR, can be considered a component of every test for hormesis, and is thus capable for interaction with any other agent. The changes that BW loss (or CR) induce are so profound that any aspect of an agent's action - metabolism, pharmacokinetics, pharmacodynamics - can modulate the response of an organism to an agent. Similarly, other effects of a chemical that induce BW loss, e.g., physical activity or temperature dysregulation, can also induce dose-response curves that appear hormetic. The interaction of the hormetic agents of BW loss and CR can influence agent tests. Controlling these factors may make it possible to dissect the key components of a hormetic response. In addition, the effects of CR or BW loss appear to extrapolate well across species [Colman R, Kemnitz JW. Aging experiments using nonhuman primates. In: Yu BP (Ed), Methods in Aging Research. CRC Press, Boca Raton, FL, 1999, pp. 249-267]. Thus there is some reason to believe that these hormetic factors may be important for humans, and may already be a factor for tests of potentially adverse agents already conducted in humans.

The estrogen receptor relative binding affinities of 188 natural and xenochemicals: structural diversity of ligands.

We have utilized a validated (standardized) estrogen receptor (ER) competitive-binding assay to determine the ER affinity for a large, structurally diverse group of chemicals. Uteri from ovariectomized Sprague-Dawley rats were the ER source for the competitive-binding assay. Initially, test chemicals were screened at high concentrations to determine whether a chemical competed with [3H]-estradiol for the ER. Test chemicals that exhibited affinity for the ER in the first tier were subsequently assayed using a wide range of concentrations to characterize the binding curve and to determine each chemical's IC50 and relative binding affinity (RBA) values. Overall, we assayed 188 chemicals, covering a 1 x 10(6)-fold range of RBAs from several different chemical or use categories, including steroidal estrogens, synthetic estrogens, antiestrogens, other miscellaneous steroids, alkylphenols, diphenyl derivatives, organochlorines, pesticides, alkylhydroxybenzoate preservatives (parabens), phthalates, benzophenone compounds, and a number of other miscellaneous chemicals. Of the 188 chemicals tested, 100 bound to the ER while 88 were non-binders. Included in the 100 chemicals that bound to the ER were 4-benzyloxyphenol, 2,4-dihydroxybenzophenone, and 2,2'-methylenebis(4-chlorophenol), compounds that have not been shown previously to bind the ER. It was also evident that certain structural features, such as an overall ring structure, were important for ER binding. The current study provides the most structurally diverse ER RBA data set with the widest range of RBA values published to date.

Reduced levels of catalase activity potentiate MPP+-induced toxicity: comparison between MN9D cells and CHO cells.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been shown to be toxic by inducing oxygen free radicals in the mammalian nervous system, especially in the nigrostriatal dopaminergic system. The present study was designed to compare the toxic effects of MPP+, the active metabolite of MPTP, in MN9D neuronal cells that exhibit relatively low levels of catalase activity, as compared to CHO cells, which exhibit high levels of catalase activity. The survival of the MN9D cells in the presence of 250 microM MPP+ was less than 10%, whereas CHO cells exhibited 70% survival at the same concentration of MPP+. The ED50 values of MPP+ in MN9D and CHO cell lines were 60-600 microM, respectively. MN9D cells contain less catalase, an enzyme believed to be involved in the detoxification of free radicals compared to CHO cells. The catalase activity was 2 Units/mg protein in MN9D cells and 30 U/mg protein in CHO cells. The catalase activity in CHO cells increased with increasing MPP+ concentrations from 100-500 microM, however, it decreased at 1 mM MPP+. In contrast, catalase activity in MN9D remained the same at all MPP+ concentrations. When the CHO cells were pre-treated with 10-25 mM 3-aminotriazole (3-AT), which inhibits catalase activity, and exposed to MPP+ at various concentrations, they became susceptible to MPP+. It is evident from these data that the differential susceptibility to MPP+ in these two cell lines are due to differences in catalase activity. In addition, the inhibition of constituentive catalase activity in CHO cells by 3-AT treatment enhances their susceptibility. In conclusion, the study demonstrates that catalase activity represents an important defence mechanism in MPTP-induced toxicity.

Growth curves and survival characteristics of the animals used in the Biomarkers of Aging Program.

The collaborative Interagency Agreement between the National Center for Toxicological Research (NCTR) and the National Institute on Aging (NIA) was aimed at identifying and validating a panel of biomarkers of aging in rodents in order to rapidly test the efficacy and safety of interventions designed to slow aging. Another aim was to provide a basis for developing biomarkers of aging in humans, using the assumption that biomarkers that were useful across different genotypes and species were sensitive to fundamental processes that would extrapolate to humans. Caloric restriction (CR), the only intervention that consistently extends both mean and maximal life span in a variety of species, was used to provide a model with extended life span. C57BI/6NNia, DBA/2JNia, B6D2F1, and B6C3F1 mice and Brown Norway (BN/RijNia), Fischer (F344/NNia) and Fischer x Brown Norway hybrid (F344 x BN F1) rats were bred and maintained on study. NCTR generated data from over 60,000 individually housed animals of the seven different genotypes and both sexes, approximately half ad libitum (AL) fed, the remainder CR. Approximately half the animals were shipped to offsite NIA investigators internationally, with the majority of the remainder maintained at NCTR until they died. The collaboration supplied a choice of healthy, long-lived rodent models to investigators, while allowing for the development of some of the most definitive information on life span, food consumption, and growth characteristics in these genotypes under diverse feeding paradigms.

Cadmium-induced 8-hydroxydeoxyguanosine formation, DNA strand breaks and antioxidant enzyme activities in lymphoblastoid cells.

The effect of cadmium ion (Cd) and ascorbic acid (Asc) on the induction of oxidative DNA damage and on the activities of antioxidant enzymes were investigated in human lymphoblastoid cells (AHH-1 TK+/-). Cd at low concentrations of 5-35 microM induced the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) and caused nuclear DNA strand breaks. The formation both of 8-OHdG and of DNA strand breaks was dose-dependent at the low Cd concentration; both parameters were linearly correlated with each other (R = 0.932 and P = 0.0209). 8-OHdG formation by Cd plateaued at a Cd concentration of 50 microM. Asc also induced 8-OHdG formation, but it had no synergistic effect with Cd on the formation of 8-OHdG or DNA strand breaks. Cd at the concentration of 50 microM induced the nuclear activity of the antioxidant enzymes, catalase and superoxide dismutase (SOD). Furthermore, Cd caused a decrease in the concentration of reduced glutathione (GSH) and an increase in concentration of the oxidized form (GSSG). While Asc had no observable effect on SOD activity, it did increase nuclear catalase activity in cells. This effect on catalase was synergistic with that of Cd. The linear correlation between 8-OHdG and DNA strand breaks induced by Cd at the lower Cd concentrations (< or = 50 microM), suggested that the extent of formation of DNA strand breaks induced by Cd may be offset by their induction of the formation of 8-OHdG and antioxidant enzyme activities.

Actions and interactions of nickel and magnesium on the transformation response of transformed cells in culture.

Nickel (Ni) and magnesium (Mg) exert separate and interacting effects on cells: Ni is toxic while Mg enhances the transformation response of transformed cells and protects from heavy metal-induced toxicity. Transformed rat liver epithelial cells were used in the soft agar (SA) assay to measure the effect of Ni and/or Mg on the expression of anchorage independence. Cells were exposed to +/- Ni and +/- Mg in a single passage of growth medium (GM) prior to assay in SA. The cells were then treated with +/- Ni and +/- Mg in the SA resulting in a 4 x 4 treatment matrix yielding 16 Ni/Mg combinations. Nickel was expected to decrease the transformation frequency (TF) and did so in 6 of the 16 cases. Magnesium was expected to enhance the TF independently of Ni; Mg increased TF values in 7 of 16 cases. The Ni-Mg interaction occurred in 11 of 16 cases. In general, Mg and Ni effects were observed more in GM than is SA. It is not evident from this study why the Ni, Mg, and Ni-Mg effects are not observed universally, but it is evident that metal-metal interactions are not simply defined or analyzed in biological systems. A refined factorial design may be useful in further separating such interactions. From the point of view of the implementation of the SA assay, in which test substances are typically dose previous to the implementation of putting the exposed cells in SA, it is clear that assay results can be markedly altered by the presence of the test compound in the soft agar.

Damage to DNA by cadmium or nickel in the presence of ascorbate.

The interactive effects of the anti-oxidant ascorbate (Asc) and the metals cadmium (Cd, as CdCl2) or nickel (Ni, as NiCl2) on the in vitro formation of breaks in double-stranded deoxyribonucleic acid (d/s DNA) were determined. Concentrations of 50 microM Cd or 200 microM Ni were dosed for 4 hours in factorial combinations with 500 microM Asc in RPMI 1640 medium (7 percent bovine serum) in which AHH-1 TK+/- cells (a spontaneously transformed human B lymphoblastoid cell line by Gentest Corp.) were replicating. In combination with Asc, Cd caused significant d/s DNA breaks (p < 0.01, n = 5), while Cd in the absence of Asc produced only a slight (but not significantly different) amount of d/s DNA damage when compared to the cells with no Cd added. The Asc alone was not damaging. The Cd caused damage to the d/s DNA only when Asc was present. The percent of d/s DNA remaining following the respective treatments was: +Cd+Asc, 13 +/- 3; +Cd-Asc, 46 +/- 8; -Cd+Asc, 54 +/- 5; -Cd-Asc, 55 +/- 7. Conversely, the presence of Ni resulted in increased amounts (percent) of d/s DNA compared to control values: +Ni+Asc, 63 +/- 5; +Ni-Asc, 58 +/- 5; -Ni+Asc, 52 +/- 1; -Ni-Asc, 51 +/- 4, (p < 0.05, n = 3). The contrasting results between Cd and Ni in the presence of Asc may reside in the point of action; while Cd acts directly on DNA, Ni is reported to act on heterochromatin. Although Asc is a recognized anti-oxidant, its presence in the media mixture potentiated d/s DNA damage from the Cd. This may be caused by a Fenton-type reaction in which an antioxidant in the presence of metal generates hydroxyl radicals and consequently d/s DNA breaks. Oxidative reactions between metals, oxygen, and antioxidants such as Asc may represent an important mechanism of cell death, toxicity, and transformation.

Protective effect of magnesium on DNA strand breaks induced by nickel or cadmium.

Magnesium, an essential metal that is important in the normal functioning of DNA, has been shown to interact with some of the toxic heavy metals in respect to biochemical and molecular mechanisms and in altering the tumorigenic process. This study examined the influence of magnesium in combination with nickel and cadmium in respect to damage of the DNA molecule. The purpose of this study was to evaluate the influence of magnesium on the amelioration of the toxic metals nickel and cadmium in respect to sustaining DNA damage. Two types of lymphocytes were used, i.e., primary Fischer 344 rat splenocytes and AHH-1 TK+/-, a human B-lymphoblastoid cell line that has been spontaneously transformed. These cells were grown in either a magnesium-free or magnesium-supplemented RPMI 1640 medium that was specifically formulated for this study. A 2 x 2 factorial design was employed with magnesium and either nickel or cadmium serving as the two factors. The experimental groups were as follows: +Mg+Ni, +Mg-Ni, -Mg+Ni, -Mg-Ni, with cadmium alternating for the nickel in the subsequent studies. The nickel or cadmium was added at a concentration of 50 mumol/L. The presence of double-stranded DNA was determined in each of the respective treatment groups with the two types of cell lines. Based on the results of this study, nickel is not directly toxic to DNA, whereas cadmium produces damage directly on the DNA molecule.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of caloric restriction in animals on cellular function, oncogene expression, and DNA methylation in vitro.

While the life-extending and disease-modulating effects of caloric restriction (CR) are well documented in whole animal studies and in correlative experiments using cells taken from CR animals, very few studies have used cells in culture after their removal from the CR-fed animal. In using this in vivo-->in vitro approach we have attempted to examine the proposition that the effects of CR can be transferred to individual cells by analyzing the cellular functions of proliferation and transformation, the activation of oncogenes, and the methylation of DNA as a function only of diet. Pancreatic acinar cells excised from CR-fed Brown-Norway rats and placed in rich medium showed different responses compared to cells from ad libitum (AL)-fed controls. CR had the effect of slowing growth rate and protecting against spontaneous and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced transformation over 14 passages of cells in culture. At the molecular level, cells from the CR animals showed reduced c-Ha-ras oncogene expression and mutation as well as reduced mutation of the p53 suppressor gene. CR also increased genomic methylation of ras DNA. We conclude that the effects of CR treatment of the animal are transferred to individual cells and note that these responses (decreased proliferation and transformation; depressed oncogene expression and mutation and decreased suppressor gene mutation; and increased oncogene methylation) are cellular and molecular analogs of in vivo weight loss, life extension, and carcinogenesis modulation, which are hallmarks of CR in the whole animal. The fact that these responses are seen generations after the cells are removed from the CR-treated animal indicates that CR causes a permanent predisposition of pancreatic acinar cells to these modulated responses and shows the value of the in vivo-->in vitro protocol in studies that relate diet to cellular and molecular function.

An in vitro pancreas acinar cell model for testing the modulating effects of caloric restriction and ageing on cellular proliferation and transformation.

Pancreatic acinar cells were isolated for culture from a young (Y) and an old (O) Brown-Norway or Fischer 344 rat fed an ad libitum (AL) or calorically restricted (CR) diet. The cells were cultured and cellular growth rates were determined as a function of passage number. An overall increase in cellular growth rate and transformation frequency with age and/or AL diet relative to youth as well as a decrease with CR diet were concordant with reported responses in vivo. Transformation frequency was measured in Brown-Norway cells and followed the same pattern as the growth response: AL/O > AL/Y = CR/Y > CR/O. The cellular model is shown to fit the general multistage requirements of the carcinogenic process as well as general age and diet characteristics of pancreatic cancer. This pancreatic acinar cell age-diet approach may prove to be a valuable tool for determining mechanisms of exocrine pancreatic carcinogenesis as well as other disease states; it may also be of utility in in vitro gerontological nutritional and pharmacological studies since some of the age and diet determinants of biological effects appear to be segregable. Propensity of cells from an old and/or AL diet animal for faster growth and for cellular transformation are programmed into the cells by the time of their excision from the animal (as late as 14 months), indicating a heritable component in the model or a mechanism that is dependent upon elements that control gene expression.

Effect of magnesium on the growth and cell cycle of transformed and non-transformed epithelial rat liver cells in vitro.

The effects of magnesium (Mg) restriction on cell growth and the cell cycle were determined in transformed (TRL-8) and non-transformed (TRL-12-15) epithelial-like rat liver cells. Cells were cultured in RPMI 1640 medium in which the Mg concentration was reduced to 0.5, 0.1, and 0 x the concentration in the regular RPMI 1640 media (100mg/l). Cell growth in the transformed cells was not influenced by the Mg restriction as greatly as in the non-transformed cell line. Transit through the cell cycle also exhibited an independence of the Mg in the medium in the transformed cells. When transformed cells were grown for two generations in Mg-limited medium, the growth rate slowed to a rate similar to that demonstrated by the non-transformed cells. Analysis by flow cytometry showed that transit through the cell cycle was minimally slowed in Mg deficient transformed cells; however, transit through the G1 and S phases in the non-transformed cells was slowed. The TRL-8 cells in Mg-limited medium resulted in fewer nuclei in G1 with subsequent increases in the percentages of S-phase nuclei. The TRL 12-15 cells reacted oppositely with the number of G1 nuclei increased and the number of S-phase nuclei decreased. In respect to growth, these results show that epithelial cells respond in a similar manner to Mg-limitation as do fibroblast cells. The transformed cells exhibited a level of independence from Mg in respect to growth, reproduction, and cell-cycle kinetics.

Evaluation of the genotoxicity of gentian violet in bacterial and mammalian cell systems.

Previous studies indicate that gentian violet (GV), a triphenylmethane dye used in agriculture and human medicine, is a clastogen in vitro and a carcinogen in chronically exposed mice and rats. Data on its genotoxic activity, however, have been incomplete and partly contradictory. Mutagenesis and DNA damage experiments were conducted to re-evaluate the genotoxic potential of GV in both bacterial and mammalian cell systems. GV was mutagenic in Salmonella typhimurium tester strains TA97 and TA104, but there was little mutagenic activity detected in strains TA98 and TA100. A rat liver homogenate fraction (S9) tended to increase mutagenicity. The major microsomal metabolites of GV, pentamethylpararosaniline and N,N,N',N'-tetramethylpararosaniline were less mutagenic in TA97 and TA104, while N,N,N',N"-tetramethylpararosaniline was a weak mutagen in Salmonella. GV was not mutagenic in Chinese hamster ovary (CHO) cell strain CHO-K1-BH4, and was a questionable mutagen in CHO-AS52 cells. While GV produced DNA damage as measured by sedimentation of nucleotids derived from B6C3F1 mouse lymphocytes treated in vitro, no damage was found in lymphocytes isolated from mice dosed with GV. GV was also a weak producer of gene amplification in an SV40-transformed Chinese hamster cell line. The results indicate that GV is a point mutagen in bacteria; however, since similar exposure conditions produced weak mutagenic activity in mammalian cells, GV may be carcinogenic by virtue of its clastogenic activity.

Role of nitroreduction in the synergistic mutational response induced by mixtures of 1- and 3-nitrobenzo[a]pyrene in Salmonella typhimurium.

Previous studies showed that binary mixtures of the environmental pollutants 1- and 3-nitrobenzo[a]pyrene produced a synergistic mutational response in the Salmonella reversion assay. Since nitroreduction is believed to mediate the direct-acting mutagenicity of the individual compounds, we have examined the role of nitroreduction in the mutagenicity of mixtures of 1- and 3-nitrobenzo[a]pyrene in the Salmonella plate incorporation assay. While mixtures of 1- and 3-nitrobenzo[a]pyrene induced up to 183% more revertants in strain TA98 than produced by equivalent amounts of the individual compounds, in the nitroreductase-deficient strain TA98NR the same mixtures only induced up to 57% more revertants than the individual compounds. Analysis of mixtures of 1- and 3-nitrosobenzo[a]pyrene (the two-electron reduction products of 1- and 3-nitrobenzo[a]pyrene) for mutation induction in TA98 yielded no evidence of a synergistic effect between the compounds. The mutagenicity of the mixtures was dependent upon the amount of the more mutagenic component. Salmonella cultures were also incubated with mixtures of 1- and 3-nitrobenzo[a]pyrene, as well as with equivalent amounts of the individual compounds. In two experiments, nitroreductive ability, as measured by the amount of 1-nitropyrene metabolized to 1-aminopyrene in 1 hr, was increased 9 to 105% in cultures pretreated with the mixtures as compared with cultures pretreated with the individual compounds. The results of this study support the hypothesis that nitroreduction is a major factor in the synergistic mutational response induced by 1- and 3-nitrobenzo[a]pyrene in Salmonella typhimurium.

Hepatocyte-mediated mutagenicity of mononitrobenzo[a]pyrenes in Salmonella typhimurium strains.

The mononitro-substituted isomers of benzo[a]pyrene (B[a]P), 1-, 3- and 6-nitrobenzo[a]pyrene (NB[a]P), are environmental pollutants and are metabolized to mutagens in Salmonella by rat-liver homogenate postmitochondrial supernatant (S9) fractions. In this study, activation of these compounds to mutagens was investigated using the hepatocyte-mediated Salmonella mutagenicity assay. Hepatocytes from rats treated with Aroclor 1254 activated both 3-NB[a]P and 1-NB[a]P to mutagens, while 6-NB[a]P was not mutagenic. The positive mutagenicity responses were functions of both the chemical dose and the hepatocyte concentration. By using a nitroreductase-deficient strain (TA98NR) and a transesterificase-deficient strain (TA98/1,8-DNP6), it was verified that the direct-acting mutagenicities of 1- and 3-NB[a]P primarily were due to metabolic processes involving nitroreduction while the S9- and hepatocyte-mediated mutagenicity responses were also dependent on transesterification. When compared with the mutagenic responses produced with S9, the mutations induced by 1- and 3-NB[a]P in the presence of hepatocytes were relatively more dependent upon nitroreductase metabolism and less on transesterification. Thus, intact hepatocytes were capable of activating 1- and 3-NB[a]P to mutagenic metabolites and some of these metabolites appeared to be different from those produced by S9.

Mutagenicity of the mononitrobenzo[a]pyrenes in Chinese hamster ovary cells mediated by rat hepatocytes or liver S9.

Previous studies have shown that 1- and 3-nitrobenzo[a]pyrene (NBaP) were mutagenic in the Salmonella reversion assay without exogenous activation and that 1-, 3- and 6-NBaP were mutagenic in the presence of hepatocytes or liver homogenate (S9). In the present study, 1-, 3- and 6-NBaP were tested for mutagenicity in Chinese hamster ovary (CHO) cells under activation conditions similar to those used in the bacterial studies. None of the NBaPs was mutagenic without exogenous activation and none was mutagenically activated by hepatocytes from unpretreated rats or rats pretreated with Aroclor 1254 or 3-methylcholanthrene. Benzo-[a]pyrene (BaP), the parent compound, induced a strong mutagenic response under all hepatocyte mediation conditions. The NBaPs did produce positive mutagenic responses with S9 activation (3- = 1- greater than 6-NBaP), but these moderate responses were less than those of BaP. The difference between the bacterial and CHO results under the variety of activation conditions suggests the importance of the endogenous metabolism of the target cell as well as the source and the type of exogenous metabolic activation.