RESUMEN
Accurate exposure information for cosmetic products and ingredients is needed in order to conduct safety assessments. Essential information includes both the amount of cosmetic product applied, and the frequency of use. To obtain current data, a study to assess consumer use practices was undertaken. The study included three widely used cosmetic product types: lipstick, body lotion, and face cream. Three hundred and sixty women, ages 19-65 years, who regularly use the products of interest, were recruited at ten different geographical locations within the US. The number of recruits was chosen to ensure a minimum of 300 completes per product type. Subjects were provided with prototype test products, and kept diaries and recorded detailed daily usage information over a two week period. Products were weighed at the start and completion of the study in order to determine the total amount of product used. Statistical analysis of the data was conducted to derive summary distribution of use patterns. The mean and median usage per application, respectively, for the three products was: face cream, 1.22 g and 0.84 g; lipstick, 10 mg and 5 mg; and body lotion, 4.42 g and 3.45 g. The mean and median usage per day for the three products was: face cream, 2.05 g and 1.53 g; lipstick, 24 mg and 13 mg; and body lotion, 8.70 g and 7.63 g. The mean number of applications per day for face cream and lipstick was 1.77 and 2.35, respectively. For body lotion, the mean number of applications per day was dependent on body area, and was 2.12, 1.52, 1.11, 0.95, 0.43, 0.26, and 0.40 for hands, arms, legs, feet, neck and throat, back, and other body areas, respectively. The effect of product preference on use practices was also investigated. This study provides current cosmetic exposure information for commonly used products which will be useful for risk assessment purposes.
Asunto(s)
Seguridad de Productos para el Consumidor , Cosméticos/administración & dosificación , Cosméticos/toxicidad , Administración Tópica , Adolescente , Adulto , Distribución por Edad , Anciano , Femenino , Humanos , Persona de Mediana Edad , Registros , Medición de Riesgo , Absorción Cutánea , Estados UnidosRESUMEN
Various proteins contain EGF-like domains that are not ligands for the EGF receptor. In the present study a cognate polypeptide for residues 361-406 of the mouse EGF precursor was synthesized by the solid-phase method. The product was renatured under oxidative conditions since it probably has an EGF-like array of three cystine disulfide bonds in its native state. HPLC analysis of the renaturation reaction revealed formation of a peak material with no apparent free-SH groups. Accordingly, the HPLC retention time of this product was readily increased by treatment (reduction of disulfides) with dithiothreitol. The renatured 46-mer (PEGF-1) did not displace 125I-EGF bound to rat liver membranes and 125I-PEGF-1 did not exhibit specific binding to membrane preparations from the mouse liver, mammary gland, or kidney, with or without Ca2+ in the binding medium. Although PEGF-1 contains a putative Ca2+ binding motif, specific binding of this cation by the polypeptide could not be demonstrated by electromobility shiff or incubation with 45Ca2+. Immunoassay of PEGF-1 and EGF in fractions obtained following gel filtration of mouse urine revealed multiple peaks of PEGF-1 immunoreactivity with the major peaks eluting at an Mr > 30 kDa. In contrast, virtually all the EGF immunoreactivity eluted at a volume similar to that of 125I-EGF. These data suggest that selective cleavage of the PEGF-1 domain from the precursor does not occur with the proclivity known for that of EGF. Instead, the PEGF-1 probably functions coordinately with other EGF-like domains while tethered to the precursor backbone. Finally, localization of PEGF-1 immunoreactivity occurred only in cell populations of the mouse previously demonstrated as sites for EGF/EGF precursor, which suggests that PEGF-1 is exclusively a domain of the EGF precursor.
Asunto(s)
Factor de Crecimiento Epidérmico/metabolismo , Precursores de Proteínas/metabolismo , Secuencia de Aminoácidos , Animales , Sitios de Unión , Radioisótopos de Calcio , Factor de Crecimiento Epidérmico/síntesis química , Factor de Crecimiento Epidérmico/orina , Receptores ErbB/metabolismo , Proteínas de la Membrana/metabolismo , Ratones , Datos de Secuencia Molecular , Fragmentos de Péptidos/síntesis química , Fragmentos de Péptidos/metabolismo , Fragmentos de Péptidos/orina , Precursores de Proteínas/síntesis química , Renaturación de Proteína , Estructura Terciaria de Proteína , RatasRESUMEN
Past studies have shown that epidermal growth factor (EGF) is able to mimic the uterotropic effects of estrogen in the rodent. These studies have suggested a "cross-talk" model in which EGF receptor (EGF-R) signaling results in activation of nuclear estrogen receptor (ER) and its target genes in an estrogen-independent manner. Furthermore, in vitro studies have indicated the requirement for ER in this mechanism. To verify the requirement for ER in an in vivo system, EGF effects were studied in the uteri of ER knockout (ERKO) mice, which lack functional ER. The EGF-R levels, autophosphorylation, and c-fos induction were observed at equivalent levels in both genotypes indicating that removal of ER did not disrupt the EGF responses. Induction of DNA synthesis and the progesterone receptor gene in the uterus were measured after EGF treatment of both ERKO and wild-type animals. Wild-type mice showed increases of 4.3-fold in DNA synthesis, as well as an increase in PR mRNA after EGF treatment. However, these responses were absent in ERKO mice, confirming that the estrogen-like effects of EGF in the mouse uterus do indeed require the ER. These data conclusively demonstrate the coupling of EGF and ER signaling pathways in the rodent reproductive tract.
Asunto(s)
Factor de Crecimiento Epidérmico/farmacología , Receptores de Estrógenos/fisiología , Transducción de Señal , Alelos , Animales , Secuencia de Bases , Electroforesis en Gel de Poliacrilamida , Femenino , Ratones , Ratones Noqueados , Datos de Secuencia Molecular , Fosforilación , Proteínas Proto-Oncogénicas c-fos/genética , Proteínas Proto-Oncogénicas c-fos/metabolismo , Receptores de Estrógenos/genética , Útero/metabolismoRESUMEN
A physiological dosimetric model of the disposition of TCDD in the rat (Kohn et al., Toxicol. Appl. Pharmacol. 120, 138-154, 1993) was extended to include effects of dioxin on serum concentrations of thyroid hormones in the rat. The extended model included distribution of blood among major vessels and tissue capillary beds and resorption of TCDD released into the gut lumen from the liver by cell lysis consequent to cytotoxicity. TCDD metabolism was represented by Hill kinetics. Parameter values were estimated by fitting time-course data for a single oral subcutaneous injection of TCDD and dose-response data for biweekly oral dosing. The extended model included new compartments for the thyroid and thyroxine-sensitive tissues (e.g., pituitary, kidney, and brown fat), secretion and tissue uptake of thyroid hormones, binding of 3,5,3'-triiodothyronine (T3) and 3,5,3',5'-tetraiodothyronine (thyroxine, T4) to proteins in blood and tissues, deiodination of iodothyronines, and glucuronidation of T4 by the hepatic UDP-glucuronosyltransferase (UGT) activity induced by TCDD. Secretion of thyroid hormones was modeled as regulated by thyrotropin (TSH), whose secretion was modeled as regulated by the hypothalamic factors thyrotropin releasing hormone and somatostatin. Release of the hypothalamic factors was modeled as under feedback control by the blood T4 level. Induction of UGT was modeled as stimulated by the Ah receptor-TCDD complex. The extended model fit the observed dose-response of P450 isozymes and Ah and estrogen receptors following repeated oral doses with comparable accuracy as the earlier model. The fit to liver and fat TCDD levels following single and repeated oral and subcutaneous doses was improved over the earlier model. The revised model's predicted liver TCDD concentrations at very low doses were verified experimentally. The model reproduced the responses observed for blood T3, T4, and TSH after 31 weeks of biweekly oral dosing of rats with TCDD. The model also predicted responses of UGT mRNA and UGT enzymatic activity comparable to those observed in TCDD-treated rats in experiments whose data were not used in constructing the model. Calculated increases in blood TSH levels are consistent with prolonged stimulation of the thyroid and may represent an early stage in the induction of thyroid tumors identified in previous two-year bioassays. Thus, increases in UGT activity may be useful as a biomarker for tumorigenic changes in hormone levels subsequent to TCDD exposure.
Asunto(s)
Dibenzodioxinas Policloradas/toxicidad , Glándula Tiroides/efectos de los fármacos , Tiroxina/sangre , Triyodotironina/sangre , Tejido Adiposo/metabolismo , Administración Oral , Animales , Sistema Enzimático del Citocromo P-450/metabolismo , Relación Dosis-Respuesta a Droga , Glucuronosiltransferasa/metabolismo , Inyecciones Subcutáneas , Isoenzimas , Hígado/efectos de los fármacos , Hígado/enzimología , Hígado/metabolismo , Modelos Biológicos , Dibenzodioxinas Policloradas/administración & dosificación , Dibenzodioxinas Policloradas/sangre , Dibenzodioxinas Policloradas/farmacocinética , ARN Mensajero/metabolismo , Ratas , Receptores Colinérgicos/efectos de los fármacos , Receptores Colinérgicos/metabolismo , Receptores de Estrógenos/efectos de los fármacos , Receptores de Estrógenos/metabolismo , Glándula Tiroides/enzimología , Glándula Tiroides/metabolismo , Hormona Liberadora de Tirotropina/sangre , Tiroxina/efectos de los fármacos , Distribución Tisular , Triyodotironina/efectos de los fármacosRESUMEN
Epidermal growth factor (EGF) receptor has been implied as having a role in certain actions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). After a single dose of TCDD, the receptor has been shown to be downregulated in several tissues including the liver. Two rat substrains, the Han/Wistar (Kuopio; H/W) rat and the Long-Evans (Turku AB; L-E) rat exhibit over a 1000-fold difference in their sensitivity to the lethal effect of TCDD. This large sensitivity difference was utilized in the current study to investigate whether or not a correlation exists between TCDD lethality and biochemical endpoints related to the hepatic EGF receptor. In the TCDD-sensitive L-E strain both the B(max) of the EGF receptor and the receptor protein as measured by Western blots, decreased dose and time dependently. Ten days after a lethal dose of TCDD (50 µg/kg), the downregulation was 80%. In the resistant H/W strain, two non-lethal doses were used (50 and 500 µg/kg), since the lethal dose is not known. These doses caused a downregulation already at 4 days after dosing, but no further decrease by day 10. The activity of phosphoenolpyruvate carboxykinase (PEPCK, the main gluconeogenetic enzyme in the liver and a proposed target of TCDD) decreased in H/W rats at least to the same extent as in L-E rats at both 4 and 10 days. It is concluded that EGF receptor downregulation is different in the two rat strains studied, despite the fact that a classical Ah receptor-regulated response (CYP1A1 induction) is similar. The results demonstrate that downregulation of the EGF receptor by TCDD is strain-dependent as well as dose- and time-dependent.
RESUMEN
Numerous xenobiotics regulate cellular functions by altering transcription of target genes. Use of sensitive and specific biomarkers based on gene transcript levels may help clarify the shape of the dose-response curve in the low-dose region associated with human exposures to environmental concentrations of chemicals. We have quantified gene transcription induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin in both animal models and humans with the use of Northern analysis and PCR-based methods. In addition, we describe a rapid and sensitive in vitro assay that we have used to screen chemicals and biological samples for their ability to alter gene transcription. Whereas some of the endpoints in our studies such as cytochrome P-450 1A1 are predictive indicators of exposure and dose, other gene responses such as growth factors are more complex and represent a critical event, progression, or adaptation to a pathological alteration. In conclusion, measurement of toxicant-induced gene transcription will contribute to the usefulness of biomarkers in addressing issues of human health and environmentally induced disease.
Asunto(s)
Biomarcadores , Regulación Enzimológica de la Expresión Génica/efectos de los fármacos , Dibenzodioxinas Policloradas/efectos adversos , Transcripción Genética/efectos de los fármacos , Animales , Northern Blotting , Citocromo P-450 CYP1A1 , Sistema Enzimático del Citocromo P-450/metabolismo , Modelos Animales de Enfermedad , Relación Dosis-Respuesta a Droga , Exposición a Riesgos Ambientales/efectos adversos , Salud Ambiental , Receptores ErbB/efectos de los fármacos , Receptores ErbB/genética , Regulación Enzimológica de la Expresión Génica/genética , Humanos , Oxidorreductasas/metabolismo , Reacción en Cadena de la Polimerasa , Transcripción Genética/genéticaAsunto(s)
Carcinógenos/toxicidad , Dibenzodioxinas Policloradas/toxicidad , Receptores de Hidrocarburo de Aril/fisiología , Animales , Sistema Enzimático del Citocromo P-450/efectos de los fármacos , Relación Dosis-Respuesta a Droga , Receptores ErbB/efectos de los fármacos , Femenino , Humanos , Hígado/efectos de los fármacos , Ratas , Medición de Riesgo , Estados Unidos , United States Environmental Protection AgencyRESUMEN
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent tumor promoter in two-stage models of hepatocarcinogenesis. This study focuses on the persistence or reversibility of TCDD-mediated changes in livers after 30 weeks of treatment and cessation of treatment. Diethylnitrosamine (DEN) initiated animals (175 mg/kg) were promoted bi-weekly with TCDD at a dose equivalent to 125 ng/kg/day for 30 weeks without or with a following waiting period of 32 weeks before necropsy. 2,3,7,8-Tetrachlorodibenzo-p-dioxin liver concentration decreased 300-fold above background. Induction of CYP1A1 dependent enzyme activity decreased according to TCDD tissue levels. In contrast, cell proliferation, as measured by BrdU-labeling index, was still 2.8-fold increased over controls in the TCDD group with waiting period compared to a 4-fold increase over controls at the end of the 30 week dosing period. Enzyme altered hepatic foci expressing the placental form of glutathione S-transferase decreased in number but the remaining foci were significantly increased in size and the percent of liver occupied by foci was higher at the end of the waiting period as compared to livers at the end of the dosing period. Liver tumor incidence at the end of the waiting period was 71% (5 of 7 animals) and the livers showed an increase in bile duct lesions with only mild toxicity. There was pronounced bile duct proliferation in DEN/TCDD treated animals after the waiting period with intense expression of TGF alpha in bile duct epithelial cells at detected by immunohistochemical methods. In comparison, at the end of the 30 week dosing period the livers showed more severe toxicity and only mild bile duct proliferation. Also, one small hepatocellular adenoma was observed. It is concluded that as opposed to CYP1A1 induction the more complex biological responses, cell proliferation and selective growth of certain preneoplastic foci, are persistent after prolonged TCDD treatment within the experimental framework of our study.
Asunto(s)
Carcinógenos/toxicidad , Neoplasias Hepáticas Experimentales/inducido químicamente , Hígado/efectos de los fármacos , Dibenzodioxinas Policloradas/toxicidad , Animales , División Celular/efectos de los fármacos , Dietilnitrosamina , Femenino , Glutatión Transferasa/metabolismo , Hígado/enzimología , Hígado/patología , Dibenzodioxinas Policloradas/metabolismo , Ratas , Ratas Sprague-DawleyRESUMEN
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a multisite carcinogen. Although the hepatocarcinogenic actions of TCDD have received the most attention, it has been demonstrated in several rodent carcinogenicity bioassays that TCDD causes a dose-related increase in thyroid follicular cell adenomas and carcinomas. The purpose of the present experiment was to investigate the dose-response relationship for thyroid function alterations in female Sprague-Dawley rats following chronic treatment with TCDD. TCDD was administered via oral gavage biweekly for 30 weeks at average daily equivalent doses of 0.1-125 ng/kg/day, thereby more than encompassing the dose range historically used in previous TCDD rodent bioassays. The endpoints examined include serum levels of thyroxine (T4), triiodothyronine (T3), and thyroid-stimulating hormone (TSH). In addition, the induction of the dioxin-responsive genes UDP-glucuronosyltransferase-1 (UGT1) and cytochrome P450 1A1 (CYP1A1) in liver were measured using reverse-transcriptase-polymerase chain reaction (RT-PCR). In agreement with previous hypotheses, TCDD appears to alter thyroid function via a secondary mechanism, namely increased excretion of T4-glucuronide resulting from TCDD induction of UGT1. The observed follicular cell hyperplasia and hypertrophy are consistent with the observed elevated TSH levels and may represent the early stages in the progression of thyroid carcinogenesis. Therefore, TCDD induces alterations in thyroid hormone function, probably as a result of chronic perturbations of liver-pituitary-thyroid axis.
Asunto(s)
Dibenzodioxinas Policloradas/toxicidad , Hormonas Tiroideas/biosíntesis , Tirotropina/biosíntesis , Animales , Antitiroideos/metabolismo , Secuencia de Bases , Sistema Enzimático del Citocromo P-450/genética , Inducción Enzimática/genética , Femenino , Regulación de la Expresión Génica/genética , Glucuronosiltransferasa/genética , Datos de Secuencia Molecular , Dibenzodioxinas Policloradas/metabolismo , Reacción en Cadena de la Polimerasa , ARN Mensajero/metabolismo , Radioinmunoensayo , Ratas , Ratas Sprague-Dawley , Glándula Tiroides/efectos de los fármacos , Glándula Tiroides/patología , Tiroxina/antagonistas & inhibidores , Tiroxina/biosíntesis , Triyodotironina/antagonistas & inhibidores , Triyodotironina/biosíntesisRESUMEN
Administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent tumor promoter, to rats resulted in a dose-dependent decrease in hepatic plasma membrane epidermal growth factor receptor (EGFR). The present study is the first to quantify and compare alterations in hepatic EGFR levels in female Sprague-Dawley rats 7 days after a single oral gavage dose of TCDD (0, 1, 5, 25, and 50 micrograms/kg) using three different techniques: (1) equilibrium receptor binding, (2) EGF induced receptor autophosphorylation, and (3) Western blot detection with a rabbit anti-rat EGFR polyclonal antibody. All three methods similarly demonstrated that the level of hepatic EGFR is significantly decreased at a dose of TCDD as low as 1 micrograms/kg. We showed that the immunoblot technique is a sensitive and quantitative alternative to radioligand binding assays. It is concluded that TCDD decreased total EGFR protein and maximum binding capacity without altering ligand binding affinity (Kd). The results demonstrated that ligand-induced autophosphorylation capacity and basal phosphotyrosine residues of plasma membrane EGFR were both decreased parallel with the decrease in EGFR protein, suggesting no TCDD-related alteration in the inherent functional ability of the receptor to undergo activation. Furthermore, it was found that the dose-response curve for EGFR protein level determined by Western blot analysis was similar for both male and female Sprague-Dawley rats.
Asunto(s)
Receptores ErbB/efectos de los fármacos , Receptores ErbB/metabolismo , Hígado/efectos de los fármacos , Dibenzodioxinas Policloradas/toxicidad , Animales , Ácidos y Sales Biliares/análisis , Western Blotting , Sistema Enzimático del Citocromo P-450/sangre , Sistema Enzimático del Citocromo P-450/metabolismo , Inducción Enzimática , Enzimas/sangre , Receptores ErbB/inmunología , Femenino , Hígado/anatomía & histología , Hígado/ultraestructura , Masculino , Tamaño de los Órganos , Fosforilación/efectos de los fármacos , Unión Proteica/efectos de los fármacos , Ratas , Ratas Sprague-Dawley , Factores SexualesRESUMEN
Ethinyl estradiol (EE) is a strong promoter of hepatocarcinogenesis in female rats. A common effect shared by many non-genotoxic hepatic promoters is the stimulation of hyperplastic growth. However, studies by others with several hepatic promoters have shown that following an initial, transient increase in liver growth, continued exposure causes an inhibition in basal and/or induced growth. We have shown that EE also causes an initial, transient increase in hepatocyte proliferation (Carcinogenesis, 7, 2007-20014, 1986). The objective of the investigation reported here was to determine whether chronic EE treatment also becomes inhibitory to basal and/or induced liver growth. Female Lewis rats were treated with EE at 2.5 and 5.0 micrograms/rat/day using time-release tablets. Seven days prior to sacrifice, the rats were implanted with osmotic minipumps containing bromodeoxyuridine (BrdU) to allow the cumulative labeling of replicating hepatocytes. At sacrifice, liver tissue was fixed, sectioned and the percent labeled hepatocyte nuclei determined by immunohistochemistry. The results of these experiments revealed that, as expected, during the first 7 days of treatment, EE increased hepatocyte proliferation. However, after 28 and 42 days of EE treatment, the basal level of liver growth was dramatically inhibited. Thus, hepatocyte nuclear labeling indices, compared to controls, were reduced by 72 and 88% after 28 and 42 days of treatment respectively. An analysis of 125I-labeled EGF binding to isolated liver membranes revealed that EGF receptor levels decreased during the initial period of growth, but had returned to control levels by day 21 when replication had ceased. In another experiment, rats were treated with EE at 5.0 micrograms/rat/day for 21 days; control rats received placebo time-release tablets. At the end of that time, the rats were surgically partially hepatectomized and the level of subsequent regenerative DNA synthesis was determined using [3H]thymidine administered 2 h prior to sacrifice 24, 48, 72 and 96 h later. The results showed that EE caused an inhibition of regenerative growth. While at each time point the level of DNA synthesis in EE-treated rats was not significantly less than in corresponding controls, statistical analysis indicated that overall, EE caused a significant reduction in liver growth. The results of these studies demonstrate that chronic EE treatment leads to the appearance of a mitosuppressed state in the liver which is characterized by reduced cell turnover and decreased growth responsiveness.(ABSTRACT TRUNCATED AT 400 WORDS)
Asunto(s)
Etinilestradiol/farmacología , Hígado/citología , Hígado/efectos de los fármacos , Animales , División Celular/efectos de los fármacos , Depresión Química , Receptores ErbB/metabolismo , Femenino , Hepatectomía , Hígado/fisiología , Regeneración Hepática/efectos de los fármacos , Mitosis/efectos de los fármacos , Ratas , Ratas Endogámicas Lew , Estimulación Química , Factores de TiempoAsunto(s)
Neoplasias Hepáticas Experimentales/inducido químicamente , Dibenzodioxinas Policloradas/toxicidad , Receptores de Hidrocarburo de Aril/fisiología , Animales , Citocromo P-450 CYP1A2 , Sistema Enzimático del Citocromo P-450/biosíntesis , Sistema Enzimático del Citocromo P-450/genética , Relación Dosis-Respuesta a Droga , Inducción Enzimática/efectos de los fármacos , Receptores ErbB/biosíntesis , Receptores ErbB/genética , Femenino , Glucuronosiltransferasa/biosíntesis , Glucuronosiltransferasa/genética , Modelos Biológicos , Oxidorreductasas/biosíntesis , Oxidorreductasas/genética , Ratas , Receptores de Hidrocarburo de Aril/efectos de los fármacos , Medición de Riesgo , Transducción de Señal , Tirotropina/metabolismoRESUMEN
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent liver tumor promoter in rats, with females being more sensitive than males. The epidermal growth factor receptor (EGFR) pathway has been implicated in altered cell growth patterns induced by tumor promoters. We investigated hepatic EGFR levels in a two-stage initiation promotion model. The TCDD doses were chosen to encompass the dose range administered in a previous chronic bioassay currently used to determine the cancer potency commonly used for human health risk assessments. TCDD was administered biweekly by oral gavage to female Sprague-Dawley rats for 30 weeks following initiation by a single dose of diethylnitrosamine (DEN). TCDD-mediated decreased EGF receptor levels were demonstrated in intact but not ovariectomized animals, consistent with previous tumor data. Likewise, previous studies have shown that TCDD induces cell proliferation in intact rats but not ovariectomized rats. We report a significant dose-dependent decrease in plasma membrane EGF receptor maximum binding capacity in both initiated and non-initiated intact rats at TCDD doses equivalent to 3.5, 10.7, 35.7 and 125 ng/kg/day. There was a significant correlation between EGF receptor effects and liver TCDD concentration. The decrease in plasma membrane EGFR determined by equilibrium binding was confirmed quantitatively by EGF stimulation of EGFR autophosphorylation as well as qualitatively by immunohistochemical detection in control and treated rats. These results demonstrate that the observed down modulation of the EGFR by TCDD is ovarian-dependent and is a sensitive effect induced at dose levels associated with TCDD hepatocarcinogenicity in rodent bioassays.
Asunto(s)
Receptores ErbB/metabolismo , Hígado/metabolismo , Dibenzodioxinas Policloradas/farmacología , Análisis de Varianza , Animales , División Celular/efectos de los fármacos , Membrana Celular/metabolismo , Dietilnitrosamina , Relación Dosis-Respuesta a Droga , Regulación hacia Abajo/efectos de los fármacos , Receptores ErbB/análisis , Receptores ErbB/efectos de los fármacos , Femenino , Hígado/química , Hígado/citología , Neoplasias Hepáticas Experimentales/inducido químicamente , Ovariectomía , Fosforilación , Dibenzodioxinas Policloradas/toxicidad , Ratas , Ratas Sprague-DawleyAsunto(s)
Dibenzodioxinas Policloradas/toxicidad , Animales , Bioensayo , Pruebas de Carcinogenicidad , Cricetinae , Femenino , Humanos , Masculino , Ratones , Ratas , Factores de RiesgoRESUMEN
Improved methods for estimating the shape of the response curve for effects of exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are needed in order to evaluate possible adverse health effects of TCDD. A mathematical model has been constructed to describe TCDD-mediated alterations in hepatic proteins in the rat. In this model it was assumed that TCDD mediates increases in the liver concentration of transforming growth factor-alpha (TGF-alpha) by a mechanism which requires the aryl hydrocarbon (Ah) receptor. TGF-alpha subsequently binds to the epidermal growth factor (EGF) receptor, a process which is known to cause internalization of this receptor in hepatocytes. This action is thought to be an early event in the generation of a mitogenic signal. Because TCDD decreases binding of EGF in the livers of intact female rats but not in ovariectomized rats, this effect was further assumed to be dependent on estrogen action. The model postulates Ah receptor-dependent effects on the concentration of cytochrome P450 1A2 (CYP1A2), which is involved in the metabolism of estradiol, and on the concentration of the estrogen receptor. The model also incorporates information on induction of cytochrome P450 1A1 (CYP1A1) by TCDD. The biochemical response curves for all these proteins were hyperbolic (Hill exponents in the equations for their expression were found to be 1), indicating a proportional relationship between target tissue dose and protein concentration at low administered doses of TCDD. The model successfully reproduced the observed tissue distribution of TCDD, the concentrations of CYP1A1 and CYP1A2, and the effects of TCDD on the Ah, estrogen, and EGF receptors over a wide dose range.
Asunto(s)
Expresión Génica/efectos de los fármacos , Hígado/efectos de los fármacos , Modelos Biológicos , Dibenzodioxinas Policloradas/toxicidad , Animales , División Celular/efectos de los fármacos , Sistema Enzimático del Citocromo P-450/metabolismo , Relación Dosis-Respuesta a Droga , Receptores ErbB/efectos de los fármacos , Estrógenos/metabolismo , Femenino , Dibenzodioxinas Policloradas/farmacocinética , Ratas , Distribución TisularRESUMEN
There is renewed controversy regarding safe exposure levels for dioxin. At the heart of this controversy is the hypothesis that toxic effects of dioxin are receptor-mediated and therefore a "threshold" should exist below which no toxic effects can occur. Our research focuses on the ability of dioxin to alter protein levels in rodent livers. Established effects of exposure to dioxin are the induction of cytochrome P450-1A1 and P450-1A2 and a reduction in the maximal binding of the epidermal growth factor receptor in rat livers. An initiation-promotion protocol was used to study the effects of dioxin in female Sprague-Dawley rats. Animals were administered a single initiating dose of diethylnitrosamine followed by 16 biweekly gavage doses of 2,3,7,8-TCDD. Steady-state pharmacodynamic models were fit to these data assuming a combination of Hill kinetics and Michaelis-Menten kinetics. Two classes of models were developed which postulate two different mechanisms for the constitutive expression and TCDD-induced alterations in the levels of these proteins. The results are consistent with models which follow proportionate response in the low-dose region (no threshold) and with models which allow for a low-dose threshold. In all cases studied, the best fitting model exhibited no "threshold" for the effects of TCDD on the modulation of these proteins. The finding is consistent with the knowledge that for some receptor-mediated responses, there is a proportional relationship between receptor occupancy and biological response, even at low ligand concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)