Biomedical subjects

Richard E Peterson

Publications and source records attributed to Richard E Peterson.

At least 19 recordsLinked to original sources

PubMed2006

The aryl hydrocarbon receptor inhibits prostate carcinogenesis in TRAMP mice.

The aryl hydrocarbon receptor (AhR) is a transcription factor that mediates the inhibitory effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on prostate growth and also modulates normal prostate development. This is evidenced by AhR null mice (Ahr-/-) having smaller dorsolateral and anterior prostates, even though all prostate lobes remain histologically normal. To test the hypothesis that loss of the AhR increases the rate of prostate carcinogenesis, the incidence of macroscopic prostate tumors was determined in Ahr+/+, Ahr+/- and Ahr-/- C57BL/6J transgenic adenocarcinoma of the mouse prostate (TRAMP) mice at 35, 70, 105, 140, 175 and 210 days of age. From 140 days, prostate tumor incidence was greater in Ahr-/- (60%) and Ahr+/- (43%) mice than in Ahr+/+ mice (16%). Allele quantification did not indicate a loss of the wild-type Ahr allele in heterozygous TRAMP tumors, suggesting that tumor formation in these mice was not due to a loss of Ahr heterozygosity. Prostatic SV40 large T antigen mRNA expression and protein localization were comparable in TRAMP mice of each Ahr genotype. Prostates from all mice of each Ahr genotype were histologically indistinguishable, exhibiting diffuse epithelial hyperplasia by 105 days of age. mRNA expression and protein localization for molecular markers of neuroendocrine differentiation, including chromogranin A and neuropilin-1, were elevated in prostate tumors compared to tumor-free ventral prostates, regardless of Ahr genotype or age. Taken together, these results demonstrate that the Ahr inhibits prostate carcinogenesis in C57BL/6J TRAMP mice by interfering with neuroendocrine differentiation.

Animals↗

PubMed2006

Blocking expression of AHR2 and ARNT1 in zebrafish larvae protects against cardiac toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin.

The zebrafish (Danio rerio) has become an attractive vertebrate model for studying developmental processes, and is emerging as a model system for studying the mechanisms by which xenobiotic compounds perturb normal development. Embryos treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) shortly after fertilization exhibit a range of adverse effects on the heart: an early reduction in cardiac myocyte number, followed by a change in heart looping and morphology, with an apparent compaction of the ventricle and overall decrease in heart size. These changes are accompanied by impaired cardiac function including a decrease in cardiac output and eventually irreversible ventricular standstill. The mechanisms involved in mediating effects of TCDD on the heart remain unknown. However, it is widely accepted that aryl hydrocarbon receptor (AHR) activation mediates endpoints of TCDD toxicity in vertebrates. In zebrafish, there are multiple forms of AHR and AHR nuclear translocator protein (ARNT) raising the question about whether different endpoints of TCDD toxicity are mediated by different components of the AHR/ARNT pathway. To address this question we used morpholino oligonucleotide technology to specifically block the expression of zfAHR2, zfARNT1, zfARNT2, and zfCYP1A, and assessed the previously described effects of TCDD on heart morphology, size, and function in the developing morphants. We report that blocking zfAHR2 and zfARNT1 expression provided protection against the TCDD-mediated alteration in heart morphology, reduced cardiac myocyte number, decreased cardiac output and ventricular standstill in zebrafish larvae, while the zfarnt2 and zfcyp1a morpholinos did not block the TCDD-induced cardiac toxicity.

Animals↗

PubMed2006

The 2005 World Health Organization reevaluation of human and Mammalian toxic equivalency factors for dioxins and dioxin-like compounds.

In June 2005, a World Health Organization (WHO)-International Programme on Chemical Safety expert meeting was held in Geneva during which the toxic equivalency factors (TEFs) for dioxin-like compounds, including some polychlorinated biphenyls (PCBs), were reevaluated. For this reevaluation process, the refined TEF database recently published by Haws et al. (2006, Toxicol. Sci. 89, 4-30) was used as a starting point. Decisions about a TEF value were made based on a combination of unweighted relative effect potency (REP) distributions from this database, expert judgment, and point estimates. Previous TEFs were assigned in increments of 0.01, 0.05, 0.1, etc., but for this reevaluation, it was decided to use half order of magnitude increments on a logarithmic scale of 0.03, 0.1, 0.3, etc. Changes were decided by the expert panel for 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.3), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.03), octachlorodibenzo-p-dioxin and octachlorodibenzofuran (TEFs = 0.0003), 3,4,4',5-tetrachlorbiphenyl (PCB 81) (TEF = 0.0003), 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) (TEF = 0.03), and a single TEF value (0.00003) for all relevant mono-ortho-substituted PCBs. Additivity, an important prerequisite of the TEF concept was again confirmed by results from recent in vivo mixture studies. Some experimental evidence shows that non-dioxin-like aryl hydrocarbon receptor agonists/antagonists are able to impact the overall toxic potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, and this needs to be investigated further. Certain individual and groups of compounds were identified for possible future inclusion in the TEF concept, including 3,4,4'-TCB (PCB 37), polybrominated dibenzo-p-dioxins and dibenzofurans, mixed polyhalogenated dibenzo-p-dioxins and dibenzofurans, polyhalogenated naphthalenes, and polybrominated biphenyls. Concern was expressed about direct application of the TEF/total toxic equivalency (TEQ) approach to abiotic matrices, such as soil, sediment, etc., for direct application in human risk assessment. This is problematic as the present TEF scheme and TEQ methodology are primarily intended for estimating exposure and risks via oral ingestion (e.g., by dietary intake). A number of future approaches to determine alternative or additional TEFs were also identified. These included the use of a probabilistic methodology to determine TEFs that better describe the associated levels of uncertainty and "systemic" TEFs for blood and adipose tissue and TEQ for body burden.

Animals↗

PubMed2006

Aryl hydrocarbon receptor activation produces heart-specific transcriptional and toxic responses in developing zebrafish.

Proper regulation of the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, is required for normal vertebrate cardiovascular development. AHR hyperactivation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during zebrafish (Danio rerio) development results in altered heart morphology and function, culminating in death. To identify genes that may cause cardiac toxicity, we analyzed the transcriptional response to TCDD in zebrafish hearts. Zebrafish larvae were exposed to TCDD for 1 h at 72 h after fertilization (hpf), and the hearts were extracted for microarray analysis at 1, 2, 4, and 12 h after exposure (73, 74, 76, and 84 h postfertilization). The remaining body tissue was also collected at each time for comparison. TCDD rapidly induced expression in 42 genes within 1 to2hof exposure. These genes function in xenobiotic metabolism, proliferation, heart contractility, and pathways that regulate heart development. Furthermore, these expression changes preceded signs of cardiovascular toxicity, characterized by decreased stroke volume, peripheral blood flow, and a halt in heart growth. This identifies strong candidates for important AHR target genes. It is noteworthy that the TCDD-induced transcriptional response in the hearts of zebrafish larvae was substantially different from that induced in the rest of the body tissues. One of the biggest differences included a cluster of genes that were down-regulated 12 h after exposure in heart tissue, but not in the body samples. More than 70% of the transcripts in this heart-specific cluster promote cellular growth and proliferation. Thus, the developing heart stands out as being responsive to TCDD at both the level of toxicity and gene expression.

Animals↗

PubMed2006

Understanding dioxin developmental toxicity using the zebrafish model.

Zebrafish (Danio rerio) have advantages over mammals as an animal model for investigating developmental toxicity. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin, TCDD), a persistent global contaminant, is the most comprehensively studied developmental toxicant in zebrafish. The hallmark responses of TCDD developmental toxicity manifested in zebrafish larvae include edema, anemia, hemorrhage, and ischemia associated with arrested growth and development. Heart and vasculature development and function are severely impaired, and jaw malformations occur secondary to inhibited chondrogenesis. The swim bladder fails to inflate, and the switch from embryonic to adult erythropoiesis is blocked. This profile of developmental toxicity responses, commonly referred to as "blue sac syndrome" because the edematous yolk sac appears blue, is observed in the larval form of all freshwater fish species exposed to TCDD at the embryonic stage of development. Components of the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator (AHR/ARNT) signaling pathway in zebrafish have been identified and functionally characterized. Their role in mediating TCDD toxicity has been determined using morpholinos to specifically knockdown the translation of zfAHR1, zfAHR2, zfARNT1, and zfARNT2 mRNAs, respectively, and a line of zfARNT2 null mutant zebrafish has provided further insight. These studies have shown that zfAHR2 and zfARNT1 mediate TCDD developmental toxicity. In addition, the growing use of molecular and genomic tools for research on zebrafish have led to advances in our understanding of the mechanism of TCDD developmental toxicity at the molecular level, including the recent finding that toxicity is not mediated by increased cytochrome P4501A (zfCYP1A) expression.

Animals↗

PubMed2005

Identification of zebrafish ARNT1 homologs: 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity in the developing zebrafish requires ARNT1.

To use the zebrafish (Danio rerio) as a model to study 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) developmental toxicity, it is essential to know which proteins are involved in mediating toxicity. Previous work has identified zfAHR2 as the receptor that binds TCDD mediating downstream responses. Although zfARNT2b can form a functional heterodimer with zfAHR2 in vitro, zfarnt2 null mutants show no protection against endpoints of TCDD developmental toxicity, demonstrating that zfARNT2b cannot be the physiological dimerization partner for zfAHR2 mediating responses to TCDD in zebrafish embryos. The purpose of the current study was to identify an alternate dimerization partner(s) for zfAHR2 that may function to mediate TCDD developmental toxicity. By searching zebrafish genomic sequence and using the polymerase chain reaction-based rapid amplification of cDNA ends technique, three forms of cDNA that seem to be alternate mRNA splice variants of a zebrafish homolog of ARNT1 were detected. Analysis of the zfARNT1 proteins in vitro demonstrates that the two longest forms of zfARNT1, zfARNT1b and zfARNT1c, can form functional heterodimers with zfAHR2. However, the shortest form, zfARNT1a, seems to be nonfunctional with zfAHR2 in vitro. To determine whether a zfARNT1 protein functions with zfAHR2 in vivo, a morpholino targeted against the 5' end of zfARNT1 (zfarnt1-MO) was used. Injection of the zfarnt1-MO before TCDD treatment significantly decreases the induction of zfCYP1A mRNA and protein. In addition, zfarnt1 morphants show complete protection against TCDD-induced pericardial edema and show partial protection against reduced blood flow and craniofacial malformations caused by TCDD, demonstrating the role of zfARNT1 proteins in mediating these responses.

Amino Acid Sequence↗

PubMed2005

Altered prostatic epithelial proliferation and apoptosis, prostatic development, and serum testosterone in mice lacking cyclin-dependent kinase inhibitors.

Normal prostatic development and some prostatic diseases involve altered expression of the cell-cycle regulators p27 and p21 (also known as CDKN1B and CDKN1A, respectively). To determine the role of these proteins in the prostate, we examined prostatic phenotype and development in mice lacking p27 and/or p21. In p27-knockout (p27KO) mice, epithelial proliferation was increased 2- and 3.8-fold in the ventral and dorsolateral prostate, respectively, versus wild-type (WT) mice, although prostatic weights were not different. Epithelial apoptosis was increased in p27KO mice and may account for the lack of a concurrent increase in weight. Testosterone deficiency observed in this group was not the cause of this increase, because vehicle- and testosterone-treated p27KO mice had similar percentages of apoptotic cells. Also observed was a trend toward a decreased functional epithelial cytodifferentiation, indicating a potential role of p27 in this process. Conversely, dorsolateral prostate and seminal vesicle (SV) of p21-knockout (p21KO) mice, and all prostatic lobes and SV of p21/p27 double-knockout mice, weighed significantly less compared to the WT mice, and their epithelial proliferation was normal. Decreased testosterone concentrations may contribute to the decreased prostatic weights. However, other factors may be involved, because testosterone replacement only partially restored prostatic weights. We conclude that loss of p27 increases prostatic epithelial proliferation and alters differentiation but does not result in prostatic hyperplasia because of increased epithelial cell loss. The p21KO mice showed phenotypes distinctly different from those of p27KO mice, suggesting nonredundant roles of p21 and p27 in prostatic development. Loss of p27 or of both p21 and p27 results in serum testosterone deficiency, complicating analysis of the prostatic effects of these cell-cycle regulators.

Animals↗

PubMed2005

In utero and lactational 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure: effects on the prostate and its response to castration in senescent C57BL/6J mice.

In utero and lactational 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure inhibits ventral, dorsolateral, and anterior prostate development in C57BL/6 mice. To determine if prostatic abnormalities persist into senescence, mice born to dams given TCDD (5 mug/kg, po) or vehicle on gestation day 13 were examined at 100 and 510 days of age. Half the mice were castrated ten days prior to necropsy in order to assess androgen dependence, while the remaining mice were sham castrated. Effects of TCDD on the dorsolateral and anterior prostate of senescent sham-castrated mice were relatively subtle, whereas the ventral prostate was rudimentary or absent. Castration of vehicle-exposed mice caused far greater reductions in prostate lobe weights, epithelial cell height, and androgen-dependent gene expression (MP25 and probasin) in young mice than in senescent ones, while cell proliferation was decreased by castration in young mice and increased in senescence. Responses to castration were similar at 100 days of age in vehicle- and TCDD-exposed mice. At 510 days, however, TCDD-exposed mice were substantially more responsive to castration by most indices than vehicle-exposed mice. These results demonstrate that prostatic androgen dependence in mice declines substantially with age in several key ways, and that in utero and lactational TCDD exposure protects against this decline. Surprisingly, TCDD increased the incidence of cribriform structures in dorsolateral prostate ducts, from 2-3% in vehicle-exposed senescent mice to 16% in sham-castrated and to 7% in castrated senescent mice. Collectively, these results demonstrate that effects of in utero and lactational TCDD exposure on the prostate persist into senescence, and suggest that in utero and lactational TCDD exposure retards the aging process in the prostate. However, because cribriform structures are often considered to be associated with prostate carcinogenesis, these results also suggest that TCDD exposure early in development may increase susceptibility to prostate cancer.

Aging↗

PubMed2005

Zebrafish as a model vertebrate for investigating chemical toxicity.

Zebrafish (Danio rerio) has been a prominent model vertebrate in a variety of biological disciplines. Substantial information gathered from developmental and genetic research, together with near-completion of the zebrafish genome project, has placed zebrafish in an attractive position for use as a toxicological model. Although still in its infancy, there is a clear potential for zebrafish to provide valuable new insights into chemical toxicity, drug discovery, and human disease using recent advances in forward and reverse genetic techniques coupled with large-scale, high-throughput screening. Here we present an overview of the rapidly increasing use of zebrafish in toxicology. Advantages of the zebrafish both in identifying endpoints of toxicity and in elucidating mechanisms of toxicity are highlighted.

Animals↗

PubMed2005

Heart malformation is an early response to TCDD in embryonic zebrafish.

The zebrafish (Danio rerio) has become an attractive vertebrate model for studying developmental processes, and is emerging as a model system for studying the mechanisms by which toxic compounds perturb normal development. When exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) shortly after fertilization, zebrafish embryos exhibit pericardial edema and reduced blood flow by 72 h post fertilization (hpf). To better understand the progression of dioxin toxicity in zebrafish, we have examined the effects of TCDD on heart development. At 72 hpf, TCDD-treated embryos exhibited altered looping, with the atria positioned distinctly posterior to the ventricles, contrary to the looping of control hearts, where the two chambers lied side by side. Moreover, the ventricles in dioxin-exposed hearts became more compact, and the atria elongated in comparison to controls. These defects are not secondary to pericardial edema because they were observed when edema formation was suppressed with osmotic support. In addition to morphological changes, TCDD produced functional deficits in the developing hearts, including blood regurgitation and a striking ventricular standstill that became prevalent by 120 hpf. We also assessed the effect of TCDD on the heart size using stereological measurements, which demonstrated significant reduction in heart tissue volume at 72 hpf. Perhaps our most significant finding was a decrease in the total number of cardiomyocytes in TCDD-exposed embryos by 48 hpf, one day prior to observable effects on peripheral blood flow. We conclude that the developing heart is an important target for TCDD in zebrafish.

Abnormalities, Drug-Induced↗

PubMed2005

Opposing roles of two isoforms of the Prx1 homeobox gene in chondrogenesis.

The Prx1 homeobox gene is critical for cartilage and bone development as suggested by previous expression studies and demonstrated by gene targeting. However, neither approach assessed the individual roles of the two isoforms Prx1a and Prx1b. In this study, Western blot analysis demonstrates that, in the early stages of chondrogenesis, during mesenchymal condensation, only Prx1a is expressed. Higher level Prx1b expression is concomitant with the formation of a defined perichondrium. Prx1a overexpression in limb micro mass cultures results in an increase in the number of prechondrogenic condensations and cartilage nodules, whereas overexpression of Prx1b results in a decrease. Prx1a increases the percentage of proliferating cells in micro mass cultures and decreases apoptosis. The Prx1b isoform does not alter proliferation, but it does increase apoptosis, which is opposite of Prx1a. These results suggest that the Prx1a:Prx1b ratio and the alternative splicing mechanism that generates these two isoforms are critical in controlling chondrogenesis.

Animals↗

PubMed2005

Sonic hedgehog signaling regulates the expression of insulin-like growth factor binding protein-6 during fetal prostate development.

At the onset of ductal morphogenesis in the developing prostate, Shh expression condenses at evaginations of urogenital sinus epithelium and activates Gli transcription factors in the adjacent mesenchyme. Abrogation of Hedgehog signaling disrupts proper prostatic budding, ductal growth, and branching. We now show that Hedgehog signaling regulates the expression of insulin-like growth factor binding protein-6 (Igfbp-6) in the developing mouse prostate. Igfbp-6 is a secreted factor that specifically binds insulin-like growth factor-II (IGF-II), prevents its binding to the IGF-I receptor, and is thought to regulate the activity of IGF-II in growth and differentiation. Igfbp-6 is expressed in both the developing and adult prostate. In the urogenital sinus, Igfbp-6 mRNA colocalized with Ptc1 and Gli1 mRNA in the mesenchyme, while Igfbp-6 protein was found in both the mesenchymal and epithelial layers. Exogenous Shh peptide induced expression of Igfbp-6 in the developing prostate while the chemical inhibitor of Hedgehog signaling, cyclopamine, reduced its expression. These studies show that Igfbp-6 is an actual target of Shh signaling in the urogenital sinus and provide the first evidence for a linkage between the Hedgehog and IGF signaling pathways in prostate development.

Animals↗

PubMed2005

Zebrafish and cardiac toxicology.

Model systems are a mainstay in toxicological research. Zebrafish are rapidly becoming an important model organism for studying vertebrate development. The advantages of zebrafish: short reproductive cycle, production of numerous transparent, synchronously developing embryos, low cost, and standardization make zebrafish an attractive model for toxicologists as well. The use of these fish to study heart development has moved forward very rapidly, laying the groundwork for studying the effects of chemicals on cardiac development and function. Here we describe approaches that can be used to study cardiac toxicity in developing zebrafish, focusing on examples where zebrafish embryos have been especially useful in understanding the impact of specific toxicants on heart development and function.

Animals↗

PubMed2004

ARNT2 is not required for TCDD developmental toxicity in zebrafish.

ZfAHR2 has been identified as the receptor that is essential for mediating the developmental toxicity caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in zebrafish. One form of zfARNT2, zfARNT2b, forms a functional heterodimer with zfAHR2 that specifically recognizes XREs in gel shift experiments and induces XRE-driven transcription in COS-7 cells treated with TCDD. However, it has not been demonstrated that zfARNT2b acts as the physiological dimerization partner for zfAHR2 to mediate TCDD toxicity in developing zebrafish. An antisense morpholino targeted against zfARNT2 (zfarnt2-MO) along with a line of mutant zebrafish lacking expression of the zfarnt2 gene have been used to test the hypothesis that zfARNT2 mediates the developmental toxicity of TCDD. Injection of the zfarnt2-MO decreased expression of the zfARNT2 protein but did not provide any protection against the formation of pericardial edema at 72 hpf. In addition, in TCDD dose response studies the zfarnt2(-/-) embryos showed no protection against three endpoints of TCDD toxicity observed at 96 hpf: pericardial edema, reduced trunk blood flow, and shortened lower jaw. Finally, immunostaining results at 96 hpf demonstrate that the zfarnt2(-/-) embryos show a similar pattern of TCDD-induced zfCYP1A expression as WT embryos. These results demonstrate that zfARNT2 is not essential for mediating TCDD developmental toxicity in zebrafish and suggest that alternate dimerization partner(s) exist for zfAHR2 in vivo.

Abnormalities, Drug-Induced↗

PubMed2004

Pattern of male reproductive system effects after in utero and lactational 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in three differentially TCDD-sensitive rat lines.

Male reproductive effects induced by in utero and lactational exposure to TCDD were analyzed in three rat lines that are differently sensitive to TCDD. Rats from lines A, B, and C were selectively bred from TCDD-resistant Han/Wistar (Kuopio, H/W) and TCDD-sensitive Long-Evans (Turku/AB, L-E) rats and exhibited very different LD50 values for TCDD: >10,000, 830, and 40 microg/kg in males, respectively. The resistance in line A rats was linked to a mutated H/W-type aryl hydrocarbon receptor (Ahr(hw)) and in line B rats to a H/W-type unknown allele B (B(hw)). Line C rats had no resistance alleles. Influence of the resistance alleles on developmentally induced male reproductive effects of TCDD was studied by exposing pregnant females to TCDD (0.03, 0.1, 0.3, or 1 microg/kg) on gestation day (GD) 15. Male progeny were sacrificed on postnatal day (PND) 70. Next, the dams were given 1 microg/kg TCDD on GD 15 and male progeny were sacrificed on PND 14, 21, 28, 35, or 49. Serum testosterone concentration, male sex organ weights, and testicular and cauda epididymal sperm numbers were analyzed; the most sensitive end point was decreased sperm numbers. The dose of 1 microg/kg TCDD reduced daily sperm production by 9.3, 25, and 36%, and cauda epididymal sperm reserves by 18, 42, and 49% in rat lines A, B, and C when measured on PND 70, respectively. The most consistent and significant effect was decreased weight of prostate lobes. The growth of the male reproductive organs was not markedly affected by the resistance alleles Ahr(hw) and B(hw). In contrast, the effects on sperm parameters appeared to be slightly modified by the resistance alleles. Thus, the intraspecies genetic differences in C-terminal transactivation domain of AHR appear to modify the sensitivity to only certain dioxin-induced male reproductive effects.

Animals↗

PubMed2004

A constitutively active arylhydrocarbon receptor induces growth inhibition of jurkat T cells through changes in the expression of genes related to apoptosis and cell cycle arrest.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to suppress T cell-dependent immune reactions through the activation of the arylhydrocarbon receptor (AhR). Our previous findings suggest that TCDD inhibits the activation and subsequent expansion of T cells following antigen stimulation in mice, leading to a decreased level of T cell-derived cytokines involved in antibody production. In the present study, we investigated the effects of activated AhR on T cells by transiently expressing a constitutively active AhR (CA-AhR) mutant in AhR-null Jurkat T cells. In agreement with our previous findings, CA-AhR markedly inhibited the growth of Jurkat T cells. The inhibited cell growth was found to be concomitant with both an increase in the annexin V-positive apoptotic cells and the accumulation of cells in the G(1) phase. The growth inhibition was also shown to be mediated by both xenobiotic response element (XRE)-dependent and -independent mechanisms, because an A78D mutant of the CA-AhR, which lacks the ability of XRE-dependent transcription, partially inhibited the growth of Jurkat T cells. Furthermore, we demonstrated that CA-AhR induces expression changes in genes related to apoptosis and cell cycle arrest. These expression changes were shown to be solely mediated in an XRE-dependent manner, because the A78D mutant of the CA-AhR did not induce them. To summarize, these results suggest that AhR activation causes apoptosis and cell cycle arrest, especially through expression changes in genes related to apoptosis and cell cycle arrest by the XRE-dependent mechanism, leading to the inhibition of T cell growth.

Apoptosis↗

PubMed2004

Aryl hydrocarbon receptors in urogenital sinus mesenchyme mediate the inhibition of prostatic epithelial bud formation by 2,3,7,8-tetrachlorodibenzo-p-dioxin.

In utero exposure of male C57BL/6 mice to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) prevents prostatic epithelial buds from forming in the ventral region of the urogenital sinus (UGS) and reduces the number of buds that form in the dorsolateral region. This inhibition of budding is aryl hydrocarbon receptor (AHR) dependent and appears to be the primary cause of lobe-specific prostate abnormalities in TCDD-exposed mice. TCDD can inhibit prostatic epithelial bud formation by acting directly on the UGS in vitro, but whether it does so via AHR in UGS mesenchyme, epithelium, or both was unknown. To address this issue, UGS mesenchyme and epithelium from gestation day (GD) 15 wild-type C57BL/6J male mice were isolated, recombined, and cultured in vitro for 5 days with 10(-8) M 5alpha-dihydrotestosterone (DHT) and either 10(-9) M TCDD or vehicle. Prostatic epithelial buds were viewed by light microscopy after removal of mesenchyme. Effects depended greatly on which portions of the mesenchyme were used: TCDD had little if any effect when whole UGS epithelium (UGE) was recombined with ventral plus dorsolateral mesenchyme, tended to reduce bud numbers in recombinants made with UGE and dorsolateral mesenchyme, and severely reduced bud numbers in recombinants made with UGE and ventral mesenchyme (VM). [VM + UGE] recombinants prepared from wild-type and AHR knockout (Ahr(-/-)) mice were then cultured with DHT to determine the site of action of TCDD. AHR null mutation alone had no effect on budding. TCDD severely inhibited prostatic epithelial bud formation in recombinants that contained mesenchymal AHR, whereas bud formation was not inhibited by TCDD in recombinants lacking mesenchymal AHR, regardless of epithelial AHR status. These results demonstrate that UGS mesenchyme and not UGS epithelium is the site of action of TCDD. Therefore, the initial events responsible for abnormal UGS (and ultimately prostate) development occur within the UGS mesenchyme, and changes in gene expression in the UGS epithelium responsible for inhibited prostatic budding are secondary to the direct effects of TCDD on UGS mesenchyme.

Animals↗

PubMed2004

Evidence that inhibited prostatic epithelial bud formation in 2,3,7,8-tetrachlorodibenzo-p-dioxin-exposed C57BL/6J fetal mice is not due to interruption of androgen signaling in the urogenital sinus.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) inhibits the androgen-dependent processes by which the urogenital sinus (UGS) of fetal mice forms prostatic epithelial buds. This inhibition is mediated by aryl hydrocarbon receptors in UGS mesenchyme and causes prostate lobes to develop abnormally. Experiments were conducted to test the hypothesis that TCDD inhibits prostatic budding in C57BL/6J mice by inhibiting androgen signaling. In utero TCDD exposure sufficient to inhibit budding (5 microg/kg maternal dose on gestation day [GD] 13) had no effect on testicular testosterone content on GD 16 or 18. Nor did it inhibit the conversion of testosterone to 5alpha-dihydrotestosterone (DHT) by the UGS. Both hydroxyflutamide (OH-flutamide; a competitive androgen receptor antagonist) and TCDD inhibited prostatic epithelial budding by UGSs cultured in vitro with DHT. To determine if TCDD inhibits responsiveness to androgens, primary mesenchymal cells prepared from UGSs cultured for three days with DHT were transiently transfected with an androgen-responsive reporter plasmid (MMTV-luciferase). OH-flutamide prevented DHT from increasing luciferase activity in these cells but TCDD did not. The same results were obtained when the mesenchymal cells were isolated from UGSs cultured with both DHT and TCDD. The lack of effect of TCDD on androgen-dependent gene expression was not due to inability of transfected UGS mesenchymal cells to respond to TCDD, as shown by significant increases in luciferase activity after transfection with plasmids containing CYP1A1 and CYP1B1 promoters. Finally, while OH-flutamide prevented DHT from altering androgen receptor and 5alpha-reductase type II mRNA expression in UGS organ culture, TCDD had no such effects. Collectively, these results suggest that TCDD inhibits prostatic epithelial bud formation without impairing the androgen receptor signaling pathway.

Androgen Antagonists↗