2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) reverses hyperglycemia in a type II diabetes mellitus rat model by a mechanism unrelated to PPAR gamma.

It has been asserted that exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increases the risk for diabetes mellitus in humans, observable as hyperglycemia resulting from insulin resistance. There is no animal model for the induction of diabetes by TCDD. On the contrary, TCDD has been shown to increase insulin sensitivity in rats. Therefore, a diabetic rat model was used to study the effects of TCDD on preexisting diabetes. Type II diabetes was induced in male rats by a high-fat diet and streptozotocin. After manifestation of the disease, these rats received loading dose rates (LDRs) of 3.2, 6.4, and 12.8 microg/kg of TCDD p.o., followed by weekly maintenance dose rates. Rats fed a high-fat diet and not dosed with streptozotocin nor with TCDD served as nondiabetic controls. By day 2, serum-glucose levels in diabetic rats treated with the high LDR of 12.8 microg/kg TCDD were already significantly reduced. By day 8, serum-glucose levels had decreased to control levels and were maintained for the duration of the study (32 days). Thus, TCDD effectively counteracted hyperglycemia in this diabetic rat model. In healthy animals, TCDD induced PPAR gamma transcription and activity in a different dose range than that observed for the hypoglycemic effect.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD) alter body weight by decreasing insulin-like growth factor I (IGF-I) signaling.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) affects glycemia due to reduced gluconeogenesis; when combined with a reduction in feed intake, this culminates in decreased body weight. We investigated the effects of steady-state levels of TCDD (loading dose rates of 0.0125, 0.05, 0.2, 0.8, and 3.2 microg/kg) or approximately isoeffective dose rates of 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD) (loading dose rates of 0.3125, 1.25, 5, 20, and 80 microg/kg) on body weight, phosphoenolpyruvate carboxykinase (PEPCK) mRNA expression and activity, and circulating concentrations of insulin, glucose, and insulin-like growth factor-I (IGF-I), and expression of hepatic phosphorylated AMP kinase-alpha (p-AMPK) protein in female Sprague-Dawley rats (approximately 250 gm) at 2, 4, 8, 16, 32, 64, and 128 days after commencement of treatment. At the 0.05 and 1.25 microg/kg loading dose rates of TCDD and HxCDD, respectively, there was a slight increase in body weight as compared to controls, whereas at the 3.2 and 80 microg/kg loading dose rates of TCDD and HxCDD, respectively, body weight of the rats was significantly decreased. TCDD and HxCDD also inhibited PEPCK activity in a dose-dependent fashion, as demonstrated by reductions in PEPCK mRNA and protein. Serum IGF-I levels of rats treated initially with 3.2 microg/kg TCDD or 80 microg/kg HxCDD started to decline at day 4 and decreased to about 40% of levels seen in controls after day 16, remaining low for the duration of the study. Eight days after initial dosing, hepatic p-AMPK protein was increased in a dose-dependent manner with higher doses of TCDD and HxCDD. There was no effect with any dose of TCDD or HxCDD on circulating insulin or glucose levels. In conclusion, doses of TCDD or HxCDD that began to inhibit body weight in female rats also started to inhibit PEPCK, inhibited IGF-I, while at the same time inducing p-AMPK.

Validation of Haber's Rule (dose x time = constant) in rats and mice for monochloroacetic acid and 2,3,7,8-tetrachlorodibenzo-p-dioxin under conditions of kinetic steady state.

Haber's Rule and associated time to coma after monochloroacetic acid (MCA) exposure in male Sprague-Dawley (SD) rats and time to death after 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in female Sprague-Dawley rats and male A/J mice were investigated at isoeffective or nearly isoeffective doses. Animals exposed to MCA received either single bolus intravenous (iv) doses or a loading dose rate via the iv route followed by a maintenance dose rate through subcutaneously implanted osmotic mini pumps. For TCDD, rats received a loading dose rate via bolus oral gavage followed by maintenance dose rates through iv injection every fourth day until death. Mice received both loading and maintenance (once a week) dose rates via oral gavage. Different dosing regimens were employed to demonstrate that the key to Haber's Rule lies not in the route of administration but in conducting experiments under conditions of kinetic steady state. Single doses of MCA produced inconsistent time responses but a reasonably constant c x t product (7657+/-391 mg/kg x min) which was not anticipated although it should have been expected because MCA's elimination half-life (2 h) is twice as long as its time to coma ( approximately 1h). Generation of kinetic steady state by infusion of MCA after iv injection of a loading dose rate resulted in a consistently decreasing time response with increasing dose which diminished the variability in the c x t (dose x time)=k relationship (8032+/-136 mg/kg x min). Both acute and chronic toxicity of TCDD under conditions of kinetic steady state yielded consistent time responses with inverse proportionality between dose and time leading to robust c x t=k products in both rats (1060+/-82 microg/kg x day) and mice (80+/-2 mg/kg x day).

Chronic toxicity and carcinogenicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin displays a distinct dose/time toxicity threshold (c x t = k) and a life-prolonging subthreshold effect.

Chronic toxicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) including its carcinogenicity was studied in female Sprague-Dawley rats in lifetime experiments. Six single dose and three multiple dose rate experiments were conducted with a single dose corn oil control group and a multiple dose rate corn oil control group, respectively. The lowest dose (1.0 mg/kg) of HpCDD and multiple dose rates of corn oil (4.0 ml/kg every other week) both prolonged the life of rats by about 2 months over that of single dose corn oil controls. Higher doses resulted in a predictable shortening of the life of rats after single dose administrations as well as after multiple dose rate administrations. The c x t = k paradigm previously validated for acute toxicity [Toxicol. Sci. 49 (1999) 102] was confirmed for chronic toxicity including carcinogenicity of HpCDD. The c x t = k product was independent of dosing regimen. Anemia and squamous cell carcinoma of the lungs were the earliest and most prevalent endpoints of toxicity. A dose of 2.1 mg/kg and 3.1 mg/kg of HpCDD caused 16.6% and 73.3% lung cancer, respectively. Liver cancer had a low prevalence and was a very late effect occurring only at doses lethal acutely for most rats in the three highest dosage groups. There was no correlation in the dose-dependence of non-malignant hepatic lesions and liver cancer.

Hormesis and risk assessment.

It is postulated in this paper that at low doses all chemicals have hormetic/hormoligotic (beneficial) effects in living organisms. It has been known since Paracelsus that at high doses all chemicals are toxic. The combination of low and high dose effects can be empirically described by a beta-curve or an inverted beta-curve. A mathematical method is suggested to determine the maximum of the beta-curve or the minimum of the inverted beta-curve, yielding a point estimate for risk assessment.

Kinetics of monochloroacetic acid at subtoxic and toxic doses in rats after single oral and dermal administrations.

Rats were administered a single oral (10 [subtoxic] or 225 [toxic, LD20] mg/kg) or dermal (125 mg/kg, LD20) dose of 14C-monochloroacetic acid (MCA) and the time-course (0.25, 0.75, 2, 4, 8, 16, and 32 h postadministration) of radioactivity determined in plasma, tissues, and excreta. At the subtoxic oral dose, concentration of 14C-MCA peaked at 0.1% of dose by 2 h. Most tissue profiles of MCA paralleled that of plasma with few exceptions. At the toxic oral dose, tissue concentrations remained initially below those seen after the subtoxic dose, because stomach retained most of the toxic dose for up to 8 h. Peak plasma concentration was reached within 0.25 h without an apparent subsequent uptake phase. Most of the dermal dose rapidly penetrated into the skin (>95% within 0.25 h) and remained sequestered there and released slowly. Concentration in plasma peaked at 0.36% of dose by 0.75 h and remained constant for up to 4 h. Peak tissue concentrations were reached between 2 and 4 h. Within 0.75 h, 9% of the dermally absorbed dose was metabolized by liver and eliminated through bile, all of which was subsequently reabsorbed. Two percent of MCA appeared in colon by 0.75 h, apparently as a result of direct transport through GI-wall in retrograde movement. About 70-80% of radioactivity recovered from the small intestine of orally dosed rats was parent compound. Fecal elimination was negligible ( 400 and < 450) and 175 (LD50 145) mg/kg after oral and dermal exposure, respectively.

Estradiol enhances and estriol inhibits the expression of CYP1A1 induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin in a mouse ovarian cancer cell line.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a ubiquitous pollutant and promoter of carcinogenesis. This study investigated the interaction between TCDD and different estrogens in a cancer cell line (ID8) derived from mouse ovarian epithelium. TCDD-induced ethoxyresorufin-O-deethylase (EROD) activity and cytochrome P4501A1 (CYP1A1) expression in a dose- and time-dependent manner. Estrogen receptor (ER) alpha mRNAs were constitutively expressed, but ER beta and progesterone receptor (PR) mRNAs were not expressed. Induction of EROD by TCDD was completely inhibited by a alpha-naphthoflavone and phenanthroline, two aryl hydrocarbon receptor (AhR) antagonists. Progesterone and gonadotropins (FSH and LH) had no effect on the induction of EROD by TCDD. Congeners of 17beta-estradiol (E2) increased the induction of EROD activity by TCDD dose-dependently in the relative potency order: estrone (El)>E2> or = 4-hydroxyestradiol (4OHE2)> or = 2-hydroxyestradiol (2OHE2). In contrast, estriol (E3) decreased EROD activity induced by TCDD. E2 increased TCDD-induced CYP1A1 protein and mRNA whereas E3 decreased both the protein and mRNA. E2 did not alter luciferase activity induced by TCDD in cells transfected with a luciferase reporter containing dioxin response elements (DRE) or a CYP1A1 promoter. In contrast, E3 dose-dependently decreased the luciferase activity. A pure anti-estrogen (ICI 182780) inhibited the interaction between E2 and TCDD but did not block E3's effect on EROD activity. These results indicate that E2 may affect TCDD-induced CYP1A1 expression by a mechanism different from E3 in ID8 cells. It appears that the potentiation of E2 in the induction of CYP1A1 by TCDD occurs by a mechanism involving ER alpha since a specific ER antagonist blocked the potentiation. The inhibitory effect of E3 may be due to a rapid direct effect on EROD and a later suppression of CYP1A1 expression.

Tissue-specific effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the activity of 5'-deiodinases I and II in rats.

Thyroid hormones play a complex role in the toxicity of polychlorinated dibenzo-p-dioxins and furans and related compounds. We investigated the toxicological significance of 5'-deiodinases I and II (5'-DI and 5'-DII) in the altered thyroid hormone status of TCDD-treated rats. Time courses and dose responses were determined for serum thyroxine (T4) and triiodothyronine (T3) levels, for 5'-DI activity in thyroid gland, liver and kidney, and for 5'-DII activity in brown adipose tissue (BAT). TCDD-treatment resulted in prompt and dose-dependent decrease in circulating T4 followed by a decrease in liver 5'-DI activity 1-2 days later and an apparent increase in BAT 5'-DII activity. Changes in liver 5'-DI and BAT 5'-DII activity were secondary to decreased T4 levels. Thyroid and kidney 5'-DI activities as well as circulating T3 levels were not affected. The results suggest that altered 5'-DI or 5'-DII activities do not significantly influence the circulating levels of T4 or T3 in TCDD-treated rats.

Alteration in ovarian gene expression in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin: reduction of cyclooxygenase-2 in the blockage of ovulation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a reproductive toxicant and endocrine disrupter that is known to block ovulation. This study was designed to investigate alterations in relevant ovarian genes that may be involved in the blockage of ovulation by TCDD in immature intact rats primed with equine chorionic gonadotropin (eCG). In this ovulation model, rats were given either 32 microg/kg TCDD or corn oil by gavage on 25 days of age. The next day, eCG (5 IU) was injected subcutaneously (s.c.) to stimulate follicular development. Ovulation occurs 72 h after administration of eCG in controls of this model. TCDD blocked ovulation at the expected time and also reduced both ovarian and body weights. At 72 h after eCG (the morning after expected ovulation), TCDD did not alter significantly serum concentrations of progesterone (P4) and androstenedione (A4). However, estradiol (E2) was significantly higher at 72 h after eCG in TCDD-treated rats when compared with controls. Western blots revealed that ovarian CYP1A1 was induced by TCDD. In addition, the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) were down- and up-regulated by TCDD, respectively, indicating that AhR-mediated signal transduction was altered in the ovary. Ovarian estrogen receptor (ER)alpha, ER beta and progesterone receptor (PR) were not altered significantly by TCDD, but ovarian glucocorticoid receptor (GR) was increased at 24h after TCDD and decreased at 72 h after eCG when compared with controls. TCDD induced the early appearance of ovarian plasminogen activator inhibitor type-1 (PAI-1), plasminogen activator inhibitor type-2 (PAI-2), urokinase plasminogen activator (uPA), and tissue plasminogen activator (tPA) at 24h after dosing when compared with controls. On the morning after ovulation (72 h after eCG), no significant differences between control and TCDD-treated rats were observed except that TCDD had still increased tPA and decreased PAI-2 when compared with controls. Interestingly, ovarian COX-2 was induced on the morning after ovulation (72 h after eCG) in controls, but was greatly inhibited in TCDD-treated rats at that time. On the other hand, COX-1 was constitutively expressed throughout the ovulatory period and remained unaffected by TCDD. Immunolocalization of COX-2 in the ovary revealed that TCDD inhibited COX-2 expression in the granulosa cell layer when assessed in the morning of expected ovulation. In conclusion, AhR signaling is activated in the ovary by TCDD and inhibition of COX-2 appeared to be a critical step in the TCDD blockage of ovulation because blockage or reduction of COX-2 expression is well known to be associated with failure of ovulation.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on serum inhibin concentrations and inhibin immunostaining during follicular development in female Sprague-Dawley rats.

Intact and hypophysectomized immature rats were pretreated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 0 or 32 microg/kg p.o.) and sacrificed throughout synchronized follicular development (0, 12, 24, 48, and 72 h after equine chorionic gonadotropin, eCG). TCDD administration to intact rats resulted in a premature elevation of serum FSH and LH by 12 h post-eCG. In intact rats pretreated with TCDD, the intensity of ovarian immunoreactivity for inhibin and the number of ovarian follicles staining for inhibin in midsaggital ovarian sections were decreased at the time of eCG administration (24 h post-TCDD) in comparison to controls. However, this decreased ovarian staining for inhibin was not associated with alterations in serum inhibin concentrations. Serum inhibin was suppressed in TCDD-treated rats when compared to intact controls only at 24 h post-eCG. Hypophysectomized animals exhibited no effect of TCDD on serum inhibin at any timepoint but did have decreased estradiol concentrations during follicular development. In summary, TCDD reduced serum concentrations of inhibin after the premature increases in FSH and LH suggesting that inhibin is not important in the initial elevation of FSH following exposure to TCDD.

From more to less than Haber's law.

We show how reformulating a first-order response theory with respect to both toxicant concentration and exposure time, in terms of relative instead of absolute increments, leads to a generalized version of Haber's law and to a combined sigmoid model for concentration and time response, derived on analytical rather than empirical grounds. More general presuppositions of Haber's law (without assuming sigmoid response) are briefly discussed. The relative intensities of time response and concentration response are measured by a single dimensionless parameter, characterizing each quadruple {toxicant, route, species, adverse effect}. Cross-species and cross-toxicant variability of this parameter is discussed and illustrated by new findings on algae and Daphniae exposed to selected inorganic and organic pollutants.

Endocrine disruption: fact or urban legend?

Endocrine disruptors (EDs) are substances that cause adverse health effects via endocrine-mediated mechanisms in an intact organism or its progeny or (sub) populations. Purported EDCs in personal care products include 4-MBC (UV filter) or parabens that showed oestrogenic activity in screening tests, although regulatory toxicity studies showed no adverse effects on reproductive endpoints. Hormonal potency is the key issue of the safety of EDCs. Oestrogen-based drugs, e.g. the contraceptive pill or the synthetic oestrogen DES, possess potencies up to 7 orders of magnitude higher than those of PCP ingredients; yet, in utero exposure to these drugs did not adversely affect fertility or sexual organ development of offspring unless exposed to extreme doses. Additive effects of EDs are unlikely due to the multitude of mechanisms how substances may produce a hormone-like activity; even after uptake of different substances with a similar mode of action, the possibility of additive effects is reduced by different absorption, metabolism and kinetics. This is supported by a number of studies on mixtures of chemical EDCs. Overall, despite of 20 years of research a human health risk from exposure to low concentrations of exogenous chemical substances with weak hormone-like activities remains an unproven and unlikely hypothesis.

Relationship between aryl hydrocarbon receptor-affinity and the induction of EROD activity by 2,3,7,8-tetrachlorinated phenothiazine and derivatives.

Reported herein are semi-empirical calculations of the molecular geometry of TCDD, TCPT, TCPT-sulfoxide (TCPT-O), TCPT-sulfone (TCPT-O(2)), N-methyl-TCPT (Me-TCPT), N-methyl-TCPT-sulfoxide (Me-TCPT-O), and N-methyl-TCPT-sulfone (Me-TCPT-O(2)), the characterization of their AhR binding affinity in rat hepatic cytosol, and their ability to induce EROD activity in a rat hepatoma cell line in vitro. Semi-empirical calculations yielded detailed information about the stereochemistry and the preferred conformation of each of these compounds. These results in combination with observations reported in this paper were used to determine structure-activity relationships. In vitro displacement of (3)H-TCDD was measured by increasing concentrations of the respective ligands. This assay revealed a strong binding affinity of TCPT to the AhR with a K(i) value of 1.08 nM. TCDD had a K(i) value of 0.54 nM. The affinity of TCPT derivatives for the AhR decreased with increasing degree of oxidation. Moreover, N-methylation further lowered the affinity, so that the N-methyl sulfone derivative of TCPT displayed the highest K(i) at approximately 1200 nM (=460.4 ng/ml). A corresponding trend was observed regarding the potency of TCPT and derivatives to induce EROD activity in vitro. However, the potencies were considerably lower than that of TCDD. Enzyme induction was measured in a rat hepatoma cell line H4IIEC/T3 by quantification of ethoxyresorufin-O-deethylase (EROD) activity. Induction was measured at 12, 24, 48 and 72 h to determine time dependence. Sulfoxidated and N-methylated phenothiazines displayed a lower potency than their respective parent compounds. TCPT and all derivatives induced enzyme activity at an efficacy similar to TCDD at all time points measured. The reported findings clearly separate the induction of EROD activity by TCPT and derivatives from their binding affinities to the AhR. In contrast, a direct correlation between the two is generally assumed in drug development, leading to - in our view - unwarranted termination of drug candidates. Therefore, a lack of such a correlation for TCPT and derivatives in fact supports their further development as possible drug leads.

From dioxin to drug lead--the development of 2,3,7,8-tetrachlorophenothiazine.

Polychlorinated dibenzo-p-dioxins are persistent environmental pollutants. The most potent congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), causes a wasting syndrome and is a potent carcinogen and immunosuppressant in the rat at high doses. However, low doses cause opposite effects to some of those observed at higher doses, resulting in chemoprevention, stimulation of the immune system, and longevity in experimental animals. The new TCDD analogue, 2,3,7,8-tetrachlorophenothiazine (TCPT), was developed to take advantage of the low-dose effects of dioxins that have potential application as therapeutics. Its development marked a deviation from the traditional scope of phenothiazine drug design by deriving biological effects from aryl substituents. TCPT was synthesized in three steps. The key ring-closing step was performed utilizing a Buchwald-Hartwig amination to provide TCPT in 37% yield. Its potency to induce CYP1A1 activity over 24 h was 370 times lower than that of TCDD in vitro. The elimination half-life of the parent compound in serum was 5.4 h in the rat and 2.7 h in the guinea pig, compared to 11 and 30 days, respectively, for TCDD. These initial findings clearly differentiate TCPT from TCDD and provide the basis for further studies of its potential as a drug lead.

Scientific foundations of hormesis. Part 2. Maturation, strengths, limitations, and possible applications in toxicology, pharmacology, and epidemiology.

The notion of hormesis has undergone numerous modifications in the course of the 20th century. Because of its unfortunate association with homeopathy, hormesis did not gain acceptance among biomedical professionals. The lack of a plausible mechanism for its occurrence may have contributed much to the rejection of this concept. This treatise outlines the conceptual struggle for an understanding of the widespread occurrence of low dose effects that appear to be opposite to those caused by high doses as also seen in hormesis. An incomplete conceptualization of time as a fundamental variable of effects (in addition to dose) is identified as one of the major reasons why hermetic responses were not observed more frequently than was reported by Calabrese and Baldwin. The definition of hormesis as an (over)compensation response to an inhibitory signal lacks a designation for (over)compensation responses to stimulatory signals in the other direction. Hormoligosis, which was coined by Luckey for all low-dose stimulatory responses of toxins, is suggested as a suitable term for generalizing the latter types of effects. Both types of effects are recognized as originating in a homeostatic overcompensation response that optimizes the ability of an organism to meet challenges beyond the limits of normal (unexercised) adaptation. Thus, repeated biochemical/physiologic/immunological, etc. exercises like physical exercise make an organism more fit and hence both hormetic and hormoligotic effects will have life-prolonging consequences. A more complete generalization was developed by linking hormesis/hormoligosis with the vast literature on Selye's general adaptation syndrome to stress. According to this broader view, stress is just one type of homeostatic exercise making organisms more fit for future biochemical/physiological/immunological, etc.challenges. Therefore, both hormesis and hormoligosis are manifestations of two nonmutational evolutionary principles--homeostasis and optimization.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces plasminogen activator inhibitor-1 through an aryl hydrocarbon receptor-mediated pathway in mouse hepatoma cell lines.

2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD), a ubiquitous environmental pollutant, elicits a variety of toxicities and is a well-known carcinogen. TCDD alters the expression of many genes including CYP1A1/2, CYP1B1, glutathione S-transferase Ya, aldehyde-3-dehydrogenase, NAD(P)H:quinone oxidoreductase, transforming growth factor (TGF)-alpha and TGF-beta. The present study was aimed at characterization of TCDD to induce plasminogen activator inhibitor-1 (PAI-1) in mouse hepatoma cell lines. A Hepa1c1c7 wild-type cell [H1(wt)], an aryl hydrocarbon receptor (AhR)-deficient mutant [H1(AhR(-))] and an AhR nuclear translocator (Arnt)-deficient mutant [H1(Arnt(-))] were used for this study. TCDD induced PAI-1 in H1(wt) cells, but not in H1(AhR(-)) and H1(Arnt(-)) mutants, indicating a functional role of the AhR-Arnt complex in this effect. Cycloheximide (CHX) treatment resulted in increased PAI-1 mRNA induction, indicating that this response to TCDD is a direct effect on transcription and not a secondary effect mediated by other TCDD-induced proteins. Transfection with PAI-1 promoter led to increased PAI-1 promoter activity in H1(wt) cells treated with TCDD, but no such effect occurred in H1(AhR(-)) or H1(Arnt(-)) cells, implying involvement of the AhR and Arnt. In addition, alpha-naphthoflavone and phenanthroline, two AhR antagonists, each blocked the enhancing effect of TCDD on PAI-1 promoter-coupled luciferase activity in H1(wt) cells. PAI-1 promoter deletion analysis indicated that TCDD-induced PAI-1 transcription was distinctly different from TGF-beta-dependent PAI-1 transcription, particularly in the region between -161 to +73. In summary, TCDD induced the PAI-1 gene directly via an AhR- and Arnt-dependent mechanism, which was distinctly different from TGF-beta-driven PAI-1 transcription.

Derivation of an occupational exposure limit (OEL) for n-propyl bromide using an improved methodology.

n-Propyl bromide is an industrial solvent with increasing production volume due to its use as a replacement for fluorohydrocarbons. Therefore, the number of occupationally exposed workers is growing accordingly. This manuscript presents a thorough evaluation of available animal and human data to derive an occupational exposure limit (OEL) for n-propyl bromide. In addition, structure activity relationship within the homologous series of methyl, ethyl, and n-propyl bromide and an identical spectrum of effects caused by similar doses of 2-propyl bromide are used to increase the confidence of the analysis. The structure activity relationship was entirely consistent for acute and subchronic (neurologic, reproductive, and hematopoietic) toxicities and for mutagenic potency in that CH3Br was more toxic than CH3CH2Br, which in turn was more toxic than CH3CH2CH2Br in every case in all species studied, including humans. Animals appeared to be similarly susceptible as, or slightly more susceptible than, humans to n-propyl bromide's toxicity. An OEL (60-90 ppm) was derived from a limited human study and supported by an across-the-toxic-spectrum comparison of animal and human data for both n-propyl and 2-propyl bromide. A carcinogenic classification was not deemed necessary at the recommended OEL based on very low mutagenic potency and the consistent structure activity relationship across the homologous series of these alkyl bromides.

Endocrine disruption by indole-3-carbinol and tamoxifen: blockage of ovulation.

Immature Sprague-Dawley rats received daily doses of indole-3-carbinol (I3C, 0-1.5 g/kg/day), 3,3'-diindolymethane (DIM, 0-400 mg/kg/day), tamoxifen (TAM, 0-0.5 mg/kg/day), or vehicle to determine if their antiestrogenic effects occur by the same mechanism and whether I3C's action is mediated by DIM. Follicular development was induced on day 24 of age by equine chorionic gonadotropin (eCG, 5 IU) 1 day after the initial dose. In a hormone replacement study, human chorionic gonadotropin (hCG, 10 IU sc, 48 h post-eCG) was used to mimic a normal preovulatoy luteinizing hormone (LH) surge following treatment with either I3C or TAM. Blood and ovaries were collected throughout follicular development and the number of ova shed was measured on the morning following expected ovulation (72 h post-eCG). I3C but not TAM reduced body weight gain at higher doses after 4 days of dosing. Ovarian weight gain and ovulation were inhibited by both I3C and TAM in a dose-dependent fashion. During the preovulatory period, both I3C and TAM blocked normal LH and follicle-stimulating hormone (FSH) surges and suppressed serum progesterone (P(4)) profoundly without changing circulating levels of estrogen (E(2)). At the time of expected ovulation, serum E(2) was increased in rats receiving I3C or tamoxifen, whereas serum P(4) was dose-dependently decreased. DIM exerted no significant effects on any of the endpoints studied, even at the highest dose, indicating that the antiestrogenic effects of I3C are not mediated by this metabolite of I3C. hCG successfully restored ovarian weight gain and ovulation in TAM-treated rats. However, hCG only partially reversed the blockage of ovulation by I3C, although ovarian weight gain was restored to normal. In summary, both I3C and TAM block ovulation by altering preovulatory concentrations of LH and FSH, but I3C appears to exert its effect(s) by (a) different mechanism(s) of action. I3C seems to act at both the ovarian and hypothalamic levels by mechanisms similar to those seen in TCDD-treated rats, whereas TAM appears to act only on the hypothalamic-pituitary axis as an anti-estrogen.