Induction of hepatic 8-oxo-deoxyguanosine adducts by 2,3,7,8-tetrachlorodibenzo-p-dioxin in Sprague-Dawley rats is female-specific and estrogen-dependent.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a hepatocarcinogen that induces sex-specific hepatic neoplastic alterations in female, but not male, rats. It has been hypothesized that TCDD-induced alterations in estrogen metabolism lead to increased generation of reactive oxygen species. The resulting oxidative damage to DNA may contribute to TCDD-induced tumor promotion and hepatocarcinogenesis. This hypothesis is supported by previous observations of increased 8-oxo-deoxyguanosine (8-oxo-dG) adduct formation in the livers of intact, but not ovariectomized (OVX), rats following chronic exposure to TCDD. The aim of the current study was to more clearly define the roles of hormonal regulation, gender, dose-response, and exposure duration in TCDD induction of 8-oxo-dG adducts. Diethylnitrosamine (DEN)-initiated male and female (both intact and OVX) rats were exposed to TCDD in the presence or absence of 17 beta-estradiol. Following 30 weeks of exposure, hepatic 8-oxo-dG adduct levels were significantly higher in TCDD-treated intact female rats, and TCDD-treated OVX female rats receiving supplemental 17 beta-estradiol, when compared to respective corn oil vehicle controls. In DEN-initiated female rats exposed to a range of TCDD concentrations for 30 weeks, TCDD induced 8-oxo-dG adduct levels in a dose-dependent manner. However, 8-oxo-dG adduct levels were not altered in TCDD-treated male or OVX female rats following 30 weeks of exposure. In noninitiated female rats, the level of 8-oxo-dG adducts 4 days following a single dose of TCDD was not significantly different than in control rats. Additionally, 8-oxo-dG adduct formation was not affected by exposure to TCDD for 20 weeks in intact female rats. These data suggest that the induction of 8-oxo-dG adduct levels by TCDD is likely a response to chronic oxidative imbalance. These studies provide strong evidence that the induction of 8-oxo-dG by TCDD occurs via a chronic, sex-specific, estrogen-dependent mechanism.

Regulation of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced tumor promotion by 17 beta-estradiol in female Sprague--Dawley rats.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatocarcinogen in female but not in male rats. In an initiation-promotion model, ovariectomy inhibits TCDD-induced cell replication and reduces both TCDD-induced tumor formation and the promotion of enzyme-altered hepatocellular foci (AHF). The aim of this study was to determine the involvement of the ovarian hormone 17 beta-estradiol in the induction of cell proliferation and development of putative preneoplastic AHF following chronic exposure to TCDD. Diethylnitrosamine (DEN)-initiated ovariectomized (OVX) female rats were treated with TCDD for 20 or 30 weeks in the presence and absence of 17 beta-estradiol administered continuously by implanted 90-day release pellets. Following 20 weeks of treatment, cell proliferation in TCDD-treated rats was decreased regardless of ovarian hormones. Following 30 weeks of exposureTCDD, only significantly induced cell proliferation in OVX rats receiving supplemental 17 beta-estradiol. These data demonstrate that the the transitory mitoinhibition of cell replication by TCDD is not hormonally responsive, but that induction of cell replication at later time points is. TCDD exposure resulted in elevated AHF expressing gamma-glutamyltranspeptidase (GGT) in intact, but not OVX rats at both time points. TCDD also induced GGT-positive AHF in 17 beta-estradiol-supplemented OVX rats. TCDD induced AHF expressing the placental form of glutathione-S-transferase (PGST) in both intact and OVX rats regardless of 17 beta-estradiol exposure, indicating that the modulating effect of 17 beta-estradiol on AHF was specific to the GGT-positive phenotype.

Increased tumor necrosis factor-alpha production by peripheral blood leukocytes from TCDD-exposed rhesus monkeys.

Previous work has shown that exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is associated with a dose-dependent increase in the incidence and severity of endometriosis in the rhesus monkey. Studies also suggest that immune mechanisms participate in TCDD-mediated toxicity and the pathogenesis of endometriosis. Thirteen years after TCDD treatment was terminated, we characterized the phenotypic distribution of peripheral blood mononuclear cells (PBMC) from TCDD-exposed and -unexposed rhesus monkeys and determined the ability of these cells to produce cytokines and exert cytolytic activity against NK and T-cell-sensitive cell lines. We also determined whether elevated serum levels of TCDD, dioxin-like PHAH congeners, and triglycerides correlated with changes in PBMC phenotype or function. For all animals, TCDD exposure correlated with increased PBMC tumor necrosis factor-alpha (TNF-alpha) secretion in response to stimulation by T-cell mitogen and decreased cytolytic activity against NK-sensitive target cells. Furthermore, increased production of this cytokine by PHA-stimulated leukocytes was associated with elevated serum triglyceride levels. Leukocyte TNF-alpha secretion in response to viral antigen and PBMC production of interferon gamma (IFNgamma), IL-6, and IL-10 following exposure to mitogen or antigen were unaffected by previous TCDD treatment. Although TCDD exposure was not associated with changes in PBMC surface antigen expression, elevated serum concentrations of TCDD, 1,2,3,6,7,8-hexachlorodibenzofuran and 3,3',4,4',5-pentachlorobiphenyl correlated with increased numbers of CD3+/CD25- and CD3-/CD25+ leukocytes and enhanced secretion of TNF-alpha by mitogen-stimulated PBMC. These findings indicate that TCDD-exposed rhesus monkeys with endometriosis exhibit long-term alterations in systemic immunity associated with elevated serum levels of specific PHAH congeners.

Serum levels of TCDD and dioxin-like chemicals in Rhesus monkeys chronically exposed to dioxin: correlation of increased serum PCB levels with endometriosis.

Humans and animals are exposed daily to a complex mixture of polyhalogenated aromatic hydrocarbons (PHAHs). Previous work has shown that exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is associated with a dose-dependent increase in the incidence and severity of endometriosis in the rhesus monkey. Dioxin-like chemicals can also exert effects in combination with TCDD via the aryl hydrocarbon receptor. This study demonstrates that the serum levels of TCDD and specific dioxin-like PHAH congeners were increased in TCDD-treated animals with endometriosis 13 years after the TCDD exposure. Nine TCDD-exposed and 6 unexposed female rhesus monkeys were evaluated for serum content of relevant compounds and for endometriosis by surgical laparoscopy. Additional studies were done on 4 animals that died 7 to 11 years after exposure to TCDD and 4 lead-treated animals with no history of PHAH treatment. For TCDD-exposed and unexposed animals, TCDD exposure correlated with an increased serum TCDD concentration. Furthermore, TCDD exposure and an elevated serum TCDD concentration were associated with increased serum levels of triglycerides, 1,2,3,6,7,8-hexachlorodibenzofuran, 3,3',4,4'-tetrachlorobiphenyl (TCB) and 3,3'4,4',5-pentachlorobiphenyl (PnCB). Importantly, the animals with elevated serum levels of 3,3',4,4'-TCB, 3,3',4,4',5-PnCB and an increased total serum TEQ had a high prevalence of endometriosis, and the severity of disease correlated with the serum concentration of 3,3,',4,4'-TCB. Increased serum concentrations of coplanar PCBs were also present in lead-treated animals. Implications of these findings for human health and the prevalence of endometriosis in humans will be discussed.

Serum hormone levels in humans with low serum concentrations of 2,3,7,8-TCDD.

We measured current serum hormone and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) concentrations in 37 men who sprayed 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in the State of Victoria, Australia. TCDD levels were consistently significantly inversely related to prolactin levels in all analyses. In correlation analyses, TCDD levels were also inversely related to triiodothyronine (T3), thyroid-stimulating hormone (TSH), and testosterone levels, and positively associated with glucagon levels. The mean serum TCDD concentration in these sprayers was between 2.6 and 8.1 parts per trillion (ppt). Since such TCDD levels are commonly found in the general population in countries such as the US, the results could suggest that background levels of TCDD in the general population could have an effect on hormone levels. The findings are preliminary and need to be replicated in order to evaluate their full public health significance.

Induction of lung lesions in female rats following chronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin.

2,3,7,8,-Tetrachlorodibenzo-p-dioxin (TCDD) has been classified as a known human carcinogen, and epidemiologic studies identify the lung as one of the target organs. Few experimental studies have attempted to characterize pulmonary effects of TCDD exposure. In this study, we characterize the induction of lesions in the lung by chronic oral TCDD exposure in diethylnitrosamine (DEN)-initiated or noninitiated female Sprague-Dawley rats. Two or 18 weeks after initiation, rats were treated with TCDD continuously for 14, 30, or 60 weeks by biweekly oral gavage (1,750 ng TCDD/kg) at a dose equivalent to 125 ng/kg body weight per day (controls received corn oil). To assess the time dependence and reversibility of potential changes, some groups included withdrawal periods of 16 or 30 weeks after 30 weeks of TCDD treatment. TCDD treatment alone for 60 weeks caused significant increases in alveolar-bronchiolar (AB) metaplasia. TCDD treatment of DEN-initiated animals for 60 weeks resulted in a significant increase in bronchiolar epithelial hyperplasia. These increases were not observed in animals treated with TCDD for 30 weeks followed by corn oil for 30 weeks, indicating that the development of these lesions required continuous exposure to TCDD. AB hyperplasia increased in an age-dependent manner after DEN initiation but was unaffected by TCDD treatment. Expression of the aromatic hydrocarbon receptor (AHR) and induction of CYP1A1 was observed only in bronchiolar Clara and ciliated cells, indicating that the mechanism of induction of AB metaplasia may be mediated by the AHR. TCDD elimination half-life was monophasic in the lung, and serum and was estimated to be 39.7 days and 44.6 days, respectively, independent of age, tissue TCDD concentration, or body weight. This is the first report to identify the AB region as a target for TCDD-induced metaplastic and proliferative changes after chronic oral exposure.

Endotoxin (lipopolysaccharide)-induced nitric oxide production in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated Fischer rats: detection of nitrosyl hemoproteins by EPR spectroscopy.

Electron paramagnetic resonance (EPR) spectroscopy was used to study the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on endotoxin (lipopolysaccharide)-induced nitric oxide (NO) production in Fischer rats. We found that rats treated with 50 microg/kg TCDD had increased sensitivity to endotoxin, resulting in an approximately 2-fold increase in the level of NO production detected as nitrosylhemoglobin (HbNO) in venous blood. At lower concentrations (< or = 5 microg/kg), TCDD did not affect the endotoxin-induced NO production. The TNF-alpha serum concentration was found to parallel that of NO. TCDD alone did not induce the production of detectable HbNO or TNF-alpha. We found that TCDD induced a dose-dependent increase in the EPR signal intensity of (Fe(3+)) low-spin methemoprotein complexes found in the liver and kidney. These species with EPR resonance at g = 2.43, 2.26, and 1.92 are attributed to low-spin Fe(3+) in cytochromes P450 and P420. Our data confirm previous studies that have shown that TCDD induces a dose-dependent increase in the production of some cytochrome P450 enzymes. However, in rats that were subsequently challenged with endotoxin, a smaller increase in the EPR intensity of these species was observed. The decrease in the low-spin Fe(3+) cytochrome P450 EPR signal in endotoxin-challenged rats could be due to one or more of the following occurring: (1) cytochrome destruction, (2) reduction of the ferric to the ESR-silent ferrous oxidation state of cytochromes by nitric oxide, and/or (3) formation of ferrous nitrosyl cytochrome complexes that contribute, in part, to the characteristic five-coordinate nitrosyl hemoprotein triplet also observed in these tissues. Since low concentrations of endotoxin can leak from the gut lumen into the systemic circulation, this investigation explores the possibility that endotoxin interaction with TCDD may be, in part, responsible for the effects of TCDD observed in these tissues.

Ovarian tumors in rats induced by chronic 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a multispecies reproductive toxicant, and it has been recently classified by IARC as a known human carcinogen. Here, we report that TCDD promotes the development of ovarian tumors in an initiation-promotion model in female Sprague Dawley rats. Rats were initiated with diethylnitrosamine (DEN) or vehicle at 70 days of age. Starting 2 or 18 weeks after initiation, rats were exposed biweekly to TCDD at a daily average dose of 125 ng/kg/day for 14, 30, or 60 weeks continuously or for 30 weeks plus withdrawal periods of 16 or 30 weeks. Fifteen of 76 (20%) rats initiated with DEN and promoted with TCDD for various lengths of time developed ovarian sex cord-stromal tumors of Sertoli cell type, whereas no ovarian tumors developed in 86 rats used as vehicle controls or that received DEN alone or TCDD alone. The highest tumor incidence occurred in 6 of 14 rats (43%) after 60 weeks of continuous TCDD after DEN initiation. One of six rats developed a tumor by 30 weeks of exposure. Because most effects of TCDD can be attributed to its activation of the aryl hydrocarbon receptor (AhR), the presence and localization of AhR was determined in the rat ovary and in the ovarian tumors by reverse transcription-PCR, immunohistochemistry, and in situ hybridization. AhR was localized to oocytes, granulosa and thecal cells of growing follicles, surface epithelial cells, and epithelial cells lining single tubules in ovaries from adult control Sprague Dawley rats. Neoplastic cells in the ovarian tumors were also positive for both AhR message and protein. These results indicate that the ability of TCDD to cause ovarian tumors is dependent on initiation, length of promotion, and age of the animal when exposed and evaluated. The tumor type induced by TCDD in this experimental system is the same histological subtype as that reported from an early study of youngsters exposed during an industrial accident in Seveso, Italy.

Levels of seven urinary phthalate metabolites in a human reference population.

Using a novel and highly selective technique, we measured monoester metabolites of seven commonly used phthalates in urine samples from a reference population of 289 adult humans. This analytical approach allowed us to directly measure the individual phthalate metabolites responsible for the animal reproductive and developmental toxicity while avoiding contamination from the ubiquitous parent compounds. The monoesters with the highest urinary levels found were monoethyl phthalate (95th percentile, 3,750 ppb, 2,610 microg/g creatinine), monobutyl phthalate (95th percentile, 294 ppb, 162 microg/g creatinine), and monobenzyl phthalate (95th percentile, 137 ppb, 92 microg/g creatinine), reflecting exposure to diethyl phthalate, dibutyl phthalate, and benzyl butyl phthalate. Women of reproductive age (20-40 years) were found to have significantly higher levels of monobutyl phthalate, a reproductive and developmental toxicant in rodents, than other age/gender groups (p < 0.005). Current scientific and regulatory attention on phthalates has focused almost exclusively on health risks from exposure to only two phthalates, di-(2-ethylhexyl) phthalate and di-isononyl phthalate. Our findings strongly suggest that health-risk assessments for phthalate exposure in humans should include diethyl, dibutyl, and benzyl butyl phthalates.

Parting thoughts

It has been said that there is no stronger urge than the urge to edit someone else's writing. Upon my retirement from the National Institute of Environmental Health Sciences (NIEHS), and concurrently from my position as co-editor-in-chief of Environmental Health Perspectives, I find that perhaps the stronger urge is not to edit but rather to editorialize. Therefore, I would like to provide some parting thoughts, and with them hopefully some insights gained from my experiences, which have spanned the broad spectrum of environmental health, from basic science to public health policy to science communication. I am grateful to have had the opportunity to work on many different aspects of the environmental health continuum during my time at the NIEHS. In looking back on a career of 30 years in environmental health, including 28 years as co-editor-in-chief of this journal, I am struck by a kaleidoscope of thoughts, emotions, and nostalgia, and in juxtaposition to these remembrances, a new perspective on the future. As the field of environmental health continues to expand as both a scientific discipline and a global movement, I feel that there are some major components which it must enthusiastically encompass if the field is to continue to provide real answers to the most pressing environmental health issues of our day.

Hepatocarcinogenesis in female Sprague-Dawley rats following discontinuous treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin.

In this study, we investigated the time course of promotion of tumors and putatively preneoplastic altered hepatic foci in the livers of diethylnitrosamine (DEN)-initiated female Sprague-Dawley rats. These rats had been treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) under different dosing regimens, but we used the same administered biweekly dose of 1.75 microg/kg of body weight. Animals were treated continuously for up to 60 weeks, or continuously for 30 weeks, followed by cessation of treatment for up to 30 weeks. In addition, TCDD treatment in these groups was begun either 2 or 18 weeks after initiation with DEN. Liver tumors were only observed in animals after 60 weeks on the study and were increased by continuous TCDD treatment, relative to controls. The incidence of hepatocellular adenoma and carcinoma combined, in animals treated with TCDD for 30 weeks followed by no TCDD treatment for 30 weeks (17%), was lower than in animals receiving either TCDD (79%) or vehicle control (corn oil) alone (55%) for 60 weeks. The lower liver-tumor incidence after cessation of TCDD treatment paralleled time-dependent decreases in the volume fraction occupied by placental glutathione S-transferase-positive altered hepatic foci and the number of foci per unit volume, but not the mean focus volume that exhibited a time-dependent increase after cessation of TCDD treatment. Cessation of TCDD treatment led to reductions in liver TCDD levels, and these changes were reflected in a cessation of reduced body weight because of TCDD treatment. These data indicate that liver-tumor promotion by TCDD in female rats is dependent upon continuous exposure to TCDD, and that alterations in patterns of TCDD exposure can have significant effects on tumor incidence not reflected by standard measures of dioxin exposure.

Toxicity of chronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin in diethylnitrosamine-initiated ovariectomized rats implanted with subcutaneous 17 beta-estradiol pellets.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatocarcinogen in female but not in male rats. Several lines of evidence suggest a key role of ovarian hormones, presumably estrogen, in the mechanism of TCDD-induced hepatocarcinogenesis. The aim of this current study was to determine the toxicity of co-treatment with TCDD and 17 beta-estradiol and assess the efficacy of 90-day subcutaneous constant release 17 beta-estradiol pellets. Ovariectomized (OVX) female Sprague-Dawley rats were initiated with diethylnitrosamine (DEN) and treated with TCDD for 20 or 30 weeks in the presence and absence of 17 beta-estradiol. TCDD concentrations were equivalent in livers of TCDD-treated sham operated and OVX rats following 20 weeks of treatment. Following 30 weeks of TCDD treatment, liver TCDD concentrations were higher in OVX rats than in intact rats. TCDD concentrations in livers of TCDD-treated OVX rats receiving supplemental 17 beta-estradiol were similar to intact rats following either 20 or 30 weeks of treatment. Mean hepatic background TCDD concentrations in untreated rats were 2-fold higher in intact rats compared to OVX rats, regardless of 17-estradiol exposure following 20, but not 30 weeks of treatment. Serum indicators of hepatocellular and hepatobiliary toxicity indicated transient hepatotoxicity in TCDD-treated OVX rats receiving 17 beta-estradiol. Histopathological alterations indicated hepatotoxicity induced by exposure to TCDD following either 20 or 30 weeks of exposure. No excess hepatotoxicity was associated with 17 beta-estradiol-supplementation in TCDD-exposed OVX female Sprague-Dawley rats. Serum 17 beta-estradiol concentrations were not constant and resulted in supra-physiological levels that decreased over time, resulting in target physiological serum 17 beta-estradiol concentrations following several weeks of release. Treatment with 17 beta-estradiol resulted in uterine weights and total body weights comparable to sham-operated female rats. These data confirm the efficacy of supplemental subcutaneous 17 beta-estradiol pellets on the induction of estrogenic responses in TCDD-treated rats and indicate no increased hepatotoxicity associated with 17 beta-estradiol exposure in TCDD-treated rats.

Levels of methyleugenol in a subset of adults in the general U.S. population as determined by high resolution mass spectrometry.

We developed a sensitive and accurate analytical method for quantifying methyleugenol (ME) in human serum. Our method uses a simple solid-phase extraction followed by a highly specific analysis using isotope dilution gas chromatography-high resolution mass spectrometry. Our method is very accurate; its limit of detection is 3.1 pg/g and its average coefficient of variation is 14% over a 200-pg/g range. We applied this method to measure serum ME concentrations in adults in the general U.S. population. ME was detected in 98% of our samples, with a mean ME concentration of 24 pg/g (range < 3.1-390 pg/g). Lipid adjustment of the data did not alter the distribution. Bivariate and multivariate analyses using selected demographic variables showed only marginal relationships between race/ethnicity and sex/fasting status with serum ME concentrations. Although no demographic variable was a good predictor of ME exposure or dose, our data indicate prevalent exposure of U.S. adults to ME. Detailed pharmacokinetic studies are required to determine the relationship between ME intake and human serum ME concentrations.

Strategies for assessing the implications of malformed frogs for environmental health.

The recent increase in the incidence of deformities among natural frog populations has raised concern about the state of the environment and the possible impact of unidentified causative agents on the health of wildlife and human populations. An open workshop on Strategies for Assessing the Implications of Malformed Frogs for Environmental Health was convened on 4-5 December 1997 at the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina. The purpose of the workshop was to share information among a multidisciplinary group with scientific interest and responsibility for human and environmental health at the federal and state level. Discussions highlighted possible causes and recent findings directly related to frog deformities and provided insight into problems and strategies applicable to continuing investigation in several areas. Possible causes of the deformities were evaluated in terms of diagnostics performed on field amphibians, biologic mechanisms that can lead to the types of malformations observed, and parallel laboratory and field studies. Hydrogeochemistry must be more integrated into environmental toxicology because of the pivotal role of the aquatic environment and the importance of fates and transport relative to any potential exposure. There is no indication of whether there may be a human health factor associated with the deformities. However, the possibility that causal agents may be waterborne indicates a need to identify the relevant factors and establish the relationship between environmental and human health in terms of hazard assessment.

Gas-phase molecular structures of third row transition-metal hexafluorides WF6, ReF6, OsF6, IrF6, and PtF6. An electron-diffraction and ab initio study.

The molecular structures of WF6, ReF6, OsF6, IrF6, and PtF6 have been measured by electron diffraction from the gases, the last from both PtF6 itself and from a vapor assumed to consist of a mixture of O2 and PtF6 obtained by heating the salt O2PtF6. For models of Oh symmetry the bond lengths in the first three members of the series are essentially identical, but the Ir-F and Pt-F bonds are respectively about 0.01 and 0.02 A longer. Models of D4h symmetry were also tested for ReF6, OsF6, and IrF6 in which operation of the Jahn-Teller effect is thought possible. For these models the same trend was seen in the average bond-length values. The effect of three-atom multiple scattering was also investigated, and experimental estimates of the effects of vibrational averaging ("shrinkage") on the distances were obtained. Normal-coordinate analyses based on the observed wavenumbers yielded stretching force constants consistent with the usual inverse bond-length/force-constant relationship. Ab initio molecular orbital optimizations of the molecules constrained to Oh symmetry were carried out at several levels of theory and basis-set size. Less extensive optimizations of ReF6, OsF6, and IrF6 with D4h symmetry were also carried out. The best overall agreement with both the experimental values and the distance trend for Oh symmetry was obtained with the Hay-Wadt (n+1)VDZ basis on the metals and the aug-cc-pVTZ on the fluorines at the MP2 level, but these bases with B3P86 and B3PW91 density functional theory were nearly as good and with B3LYP only slightly worse. The D4h structures for ReF6, OsF6, and IrF6 with the cited bases at the B3P86 level were slightly more stable (respectively 0.8, 2.6, and 1.4 kcal/mol) with the axial bonds shorter by about 0.04 A in ReF6 and 0.07 A in OsF6, but about 0.05 A longer in IrF6. The significance of these values is uncertain. The experimental bond lengths (rg/A) with estimated 2sigma uncertainties for the models of Oh symmetry are W-F = 1.829(2), Re-F = 1.829(2), Os-F = 1.828(2), Ir-F = 1.839(2), and Pt-F = 1.852(2); the Pt-F value from the O2PtF6 sample was 1.851(2) A. Although the experimental data neither confirm nor refute the existence of the Jahn-Teller effect in ReF6, OsF6, and IrF6, they ensure that if present the distortion from Oh symmetry must be small.

Dose-dependent localization of TCDD in isolated centrilobular and periportal hepatocytes.

Dose-response relationships for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) suggest a differential sensitivity of liver cell types to the induction of cytochrome P450 gene expression, and that the induction of hepatic protein CYP1A2 causes sequestration of TCDD. In addition, immunolocalization of hepatic CYP1A1/1B1/1A2 proteins is not uniform after exposure to TCDD. The mechanism for the regio-specific induction of hepatic P450s by TCDD is unknown, but may involve the differential distribution of participants in the AhR-mediated pathway and/or regional P450 isozymes, as well as, non-uniform distribution/sequestration of TCDD. Therefore, this study examined the effects of TCDD in unfractionated, centrilobular and periportal hepatocytes isolated from female Sprague-Dawley rats acutely exposed (3 days) to a single oral dose of 0.01-10.0 microg [3H]TCDD/kg. A dose-dependent increase in concentration of TCDD was accompanied by a dose-dependent increase in CYP1A1, CYP1A2, and CYP1B1 mRNA expression and associated enzymes in all liver-cell populations. Centrilobular hepatocytes showed a 2.7- to 4.5-fold higher concentration of TCDD as compared to the periportal hepatocytes at doses up to 0.3 microg TCDD/kg. Centrilobular hepatocytes also exhibited an elevated MROD activity as compared to the periportal hepatocytes at doses up to 0.3 microg TCDD/kg. Furthermore, centrilobular hepatocytes showed an elevated concentration of induced CYP1A2 and CYP1B1 mRNA as compared to periportal hepatocytes within the 0.01- and 0.3-microg TCDD/kg-treatment groups. This is the first study to demonstrate that a dose-dependent difference in distribution of TCDD exists between centrilobular and periportal cells that might be related to regional differences in P450 induction.

Medicinal herbs in the United States: research needs.

Virtually all cultures have, throughout history, used a variety of plants or materials derived from plants for the prevention and treatment of disease. Evidence of the beneficial therapeutic effects of these medicinal herbs is seen in their continued use. Additionally, the development of modern chemistry permitted the isolation of chemicals from medicinal herbs that have served as drugs or starting materials for the synthesis of many important drugs used today. Many more modern drugs have been synthesized as a result of knowledge gained from studies of mechanisms of actions of chemicals first isolated from medicinal herbs. Thus, medicinal herbs have played a major role in the development of modern medicine and continue to be widely used in their original form. Whereas it is generally agreed that most medicinal herbs are safe under the conditions used, some are toxic and should be avoided even though they are readily available, and others have significant adverse side effects when misused. Also, little has been done to investigate potential adverse effects that may be associated with extended or high-dose use of medicinal herbs. Thus, concern has been expressed that the lack of quality control used in the preparation of medicinal herbs, plus their unregulated sale and uninformed use, pose potential adverse health effects for consumers. There is also concern regarding potential herb/herb or herb/drug interactions and possible untoward health effects of medicinal herbs in sensitive subpopulations such as the young and the elderly and certain genetically predisposed individuals. In this paper, we discuss these concerns at some length and make recommendations for additional research and education discussed in the recent International Workshop to Evaluate Research Needs on the Use and Safety of Medicinal Herbs.

Species, interindividual, and tissue specificity in endocrine signaling.

The activity of endocrine-active agents exhibits specificity at many levels. Differential responsiveness to these agents has been observed between different species and extends to interindividual differences within a species and between different tissues as well. In cases where they have been identified, the biologic and molecular mechanisms underlying this specificity are quite diverse. Determinants of species specificity include differences that exist in receptor binding, gene transcription, and cellular responses to endocrine-active compounds between species. Interindividual differences in responsiveness may be determined at the level of genetic polymorphisms in hormone-metabolizing enzymes, hormone receptors, and in those genes that are transactivated by these receptors, as well as during changing windows of susceptibility that occur as a function of age, such as prenatal and postmenopausal exposures. Extrinsic factors such as diet can also impact individual susceptibility to endocrine-active agents. Tissue-specific determinants of susceptibility are well documented, but little is known regarding the mechanisms underlying these different responses. Differences in the expression of accessory proteins for steroid hormone receptors and different patterns of receptor expression, estrogen receptor alpha and estrogen receptor beta; for example, may contribute to tissue specificity, as may differences in the pattern of expression of other genes such as hormone-metabolizing enzymes. The use of animal model systems and development of appropriate mathematical models has the potential to yield additional valuable information for elucidating the role of these determinants of specificity at low-dose exposures and for improved risk assessments for the adverse health effects of endocrine-active compounds.