R. Dickerson

2,3,7,8-tetrachlorodibenzo-p-dioxin inhibition of 17β-estradiol-induced increases in rat uterine epidermal growth factor receptor binding activity and gene expression

Abstract Treatment of immature female Sprague-Dawley rats with 17β-estradiol (5 μg/animal) resulted in an increase in uterine epidermal growth factor (EGF) receptor binding activity. Moreover, in a separate study it was also shown that 17β-estradiol increased steady-state levels of rat uterine EGF receptor mRNA as determined by Northern analysis. In contrast, 2,3,7,8-tetrachlorodibenzo- p -dioxin (TCDD) caused a dose-response decrease in constitutive rat uterine EGF receptor binding activity and this was paralleled by a decrease in steady-state levels of uterine EGF receptor mRNA. Cotreatment of the animals with both TCDD (16 nmol/kg) and 17β-estradiol (5 μg/rat) gave results which showed that TCDD significantly inhibited the estrogen-induced increases in rat uterine EGF receptor binding activity and EGF receptor mRNA levels. These results further extend the range of antiestrogenic properties of TCDD and suggest that the inhibition of growth factor expression may play a role in the growth-inhibiting properties of TCDD in estrogen-responsive tissues or cells.

Reduced Leydig cell volume and function in adult rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin without a significant effect on spermatogenesis

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to alter testicular function. However, its effect on the efficiency of spermatogenesis or on Leydig cell volume has not been determined in adult rats. In two replicas, adult male rats received a single intraperitoneal injection of TCDD at a rate of 0, 12.5, 25.0, or 50.0 micrograms/kg body weight. Rats were sacrificed 4 weeks after treatment. The cytosolic Ah receptor in the testis was estimated at 10.3 +/- 1.2 fmol/mg protein in these adult rats. The presence of the Ah receptor at this concentration in the testis reveals that the testis is a possible target organ for TCDD-induced responses. Left testes were homogenized and testicular spermatids were counted by phase contrast cytometry to determine daily sperm production. Right testes were vascularly perfused with glutaraldehyde, embedded in Epon 812, sectioned at 0.5 micron, stained with toluidine blue and evaluated by stereology for germ cells or Leydig cells. Body weight was reduced (P < 0.01) in a dose-dependent fashion. Testicular weight and daily sperm production per testis were not significantly reduced by TCDD. Androgen receptor concentrations in the testis and prostate were not affected. Weights of two androgen-sensitive organs (seminal vesicles and epididymis) were reduced (P < 0.01) in a dose-dependent fashion and serum concentrations of testosterone were reduced in a dose-dependent fashion in Replica 2. Due to low numbers of animals in Replica 1, the reduced Leydig cell volume was not significant after TCDD treatment; however, in Replica 2 there was a dose-dependent reduction (P < 0.01) in volume per testis of Leydig cell cytoplasm, nuclei, or total Leydig cell volume. Production of testosterone was sufficient to maintain spermatogenesis quantitatively; however, TCDD caused a dose-dependent reduction in Leydig cell function and Leydig cell volume per testis. This study showed for the first time that TCDD-induced androgen deficiency of male rats may be explained by the loss of total volume of Leydig cell cytoplasm. This study also further illustrates the reserve capacity of Leydig cell function to maintain spermatogenesis when the volume of these cells is significantly reduced.

The effect of 6-nitro-1,3,8-trichlorodibenzofuran as a partial estrogen in the female rat uterus

Administration of 6-nitro-1,3,8-trichlorodibenzofuran (6-NCDF) caused a dose- and time-dependent increase in uterine wet weight and cytosolic and nuclear estrogen receptor (ER) and progesterone receptor (PR) levels in immature female Sprague-Dawley rats. These estrogenic effects persisted for up to 96 or 144 hr after initial administration of 6-NCDF and could be observed at a dose as low as 2 mumol/kg. In contrast, 6-NCDF (25 mumol/kg) did not increase rat uterine peroxidase activity or epidermal growth factor (EGF) receptor binding activity. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), which exhibits a broad spectrum of antiestrogenic effects in the female rat uterus, inhibited the 17 beta-estradiol-induced increase in uterine wet weights, cytosolic and nuclear ER and PR levels, peroxidase activity, and EGF receptor binding activity. In contrast, 2,3,7,8-TCDD inhibited the uterotropic effects caused by 6-NCDF but did not affect the 6-NCDF-induced uterine ER and PR levels. 6-NCDF is a weak inducer of hepatic microsomal ethoxyresorufin O-deethylase activity and competitively binds to the aryl hydrocarbon (Ah) receptor but not the PR or ER. Thus both 6-NCDF and 2,3,7,8-TCDD, two ligands which bind to the Ah receptor, exhibit both partial estrogenic and antiestrogenic properties and serve as useful models for delineating the complex biochemical interactions between the ER and Ah receptor signal transduction pathways.

The Structure-Dependent Effects of Heptachlorodibenzofuran Isomers in Male C57BL/6 Mice: Immunotoxicity and onooxygenase Enzyme Induction

The dose-response effects of the 1,2,3,4,6,7,8-, 1,2,3,4,7,8,9-, 1,2,3,4,6,8,9-, and 1,2,3,4,6,7,9-heptachlorodibenzofurans (HpCDFs) on the splenic plaque-forming cell (PFC) response to sheep erythrocytes and on the induction of hepatic microsomal aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin-O-deethylase (EROD) activities were determined in male C57BL/6 mice. The ED50 values for the decrease in the PFCs/spleen, the number of PFCs/10(6) viable cells, and the induction of AHH and EROD activities were 1,2,3,4,6,7,8-HpCDF, 0.011, 0.018, 0.11, and 0.315 mumol/kg, respectively; 1,2,3,4,7,8,9-HpCDF, 0.012, 0.054, 0.70, and 0.20 mumol/kg, respectively; 1,2,3,4,6,7,9-HpCDF, 1.2, 1.3, greater than 43, and greater than 43 mumol/kg, respectively; 1,2,3,4,6,8,9-HpCDF, 1.5, 3.4, 22, and 22 mumol/kg, respectively. It was apparent from these studies that the 2,3,7,8-substituted HpCDF isomers (1,2,3,4,6,7,8- and 1,2,3,4,7,8,9-) were significantly more potent than the compounds which contained only three lateral C1 groups. These results were obtained using a multiple dosing regimen in which 10 separate doses of the HpCDF isomers were administered to the mice by intraperitoneal injection over a period of 12 days. However, when the mice were treated with a single dose of an HpCDF congener, which was equivalent to the total dose used in the multiple dose study, the responses were comparable. A comparison of the relative immunotoxic potencies of the 2,3,7,8-substituted HpCDFs and 2,3,7,8-tetrachlorodibenzo-p-dioxin showed that the latter compound was approximately 10 times more active than the HpCDFs.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on spermatogenesis and leydig cell volume in adult rats

Abstract Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters testicular function; however, its effect on daily sperm production and Leydig cell volume has not been determined in adult rats. Adult male Sprague-Dawley rats (300–350 g) received 0, 12.5, 25.0, or 50.0 μg/kg body weight doses of TCDD by a single intraperitoneal injection. The body weight was reduced (P 0.05) to have reduced Leydig cell volume after TCDD treatment, and rats in Exp. 2 revealed a dose-dependent reduction (P

Estrogen Receptor Signaling and Crosstalk with the Ah Receptor in Endometrial Cancer Cells

Common risk factors for the development of endometrial and breast cancer include early menarche, late menopause, null parity, and later age at first birth, indicating that “lifetime” exposure to estrogens increases the incidence of both tumors. In contrast, smoking protects against the development of endometrial cancer, whereas the role of smoking in breast cancer incidence is equivocal and may be dependent on the timing and duration of smoking. Constituents of cigarette smoke bind and activate the aryl hydrocarbon receptor (AhR), and research in this laboratory has focused on characterizing the inhibitory AhR-estrogen receptor (ER) a crosstalk in endometrial and breast cancer cell lines. Both Ishikawa and ECC1 endometrial cancer cells express the AhR and ERoc proteins by Western blot analysis. Moreover, AhR ligands such as benzo[a]pyrene (BaP) and/or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induce CYP1A1-dependent activity or reporter gene activity in cells transfected with constructs containing dioxin-responsive elements as promoters. Estrogen responsiveness was also confirmed in these cells, as evidenced by gene/reporter gene assay and the induction of cell proliferation by 17β-estradiol (E2). Inhibitory AhR-ERα crosstalk studies have shown that TCDD and/or BaP inhibit E2-induced growth of endometrial cancer cells and also block hormone-activated reporter gene/gene responses. Although there are several possible mechanisms for the interaction between AhR and ERα signaling pathways, the role of AhR-mediated downregulation of ERα will be discussed as one possible mechanism. In addition, selective AhR modulators have been developed for the treatment of breast and endometrial cancer and the potential use of these compounds alone or in combination with tamoxifen will be outlined.