Differential organ expression patterns of thyroid hormone receptor isoform genes in p,p'-DDE-treated adult male common frog, Rana temporaria

Gene expression and trace elements in Greenlandic ringed seals (Pusa hispida)

Marine top predators such as ringed seals biomagnify environmental contaminants; and with the increasing human activities in the Arctic, ringed seals are exposed to biologically significant concentrations of trace elements resulting in reproductive impairment, immunosuppression, and neurological damages. Little is known about the molecular effects of heavy metals on these vulnerable apex predators suffering from a rapidly changing Arctic with significant loss of sea-ice. In the present study, concentrations of cadmium (Cd), mercury (Hg) and selenium (Se) were measured in liver of sixteen Greenlandic ringed seals (nine adults and seven subadults) together with molecular biomarkers involved in bio-transformation, oxidative stress, endocrine disruption and immune activity in blood and blubber. The concentrations of trace elements increased in the following order: Hg > Se > Cd with levels of mercury and selenium being highest in adults. Aryl hydrocarbon receptor nuclear translocator (ARNT), peroxisome proliferator activated receptor alpha (PPARα, estrogen receptor alpha (ESR1), thyroid hormone receptor alpha (TRα) and interleukin - 2 (IL-2) mRNA transcript levels were highest in blubber, while heat shock protein 70 (HSP70) and interleukin - 10 (IL-10) were significantly higher in blood. There were no significant correlations between the concentrations of trace elements and mRNA transcript levels suggesting that stressors other than the trace elements investigated are responsible for the changes in gene expression levels. Since Hg seems to increase in Greenlandic ringed seals, there is a need to re-enforce health monitoring of this ringed seal population.

Microcystin-LR induced transgenerational effects of thyroid disruption in zebrafish offspring by endoplasmic reticulum stress-mediated thyroglobulin accumulation and apoptosis

MC-LR can interfere with thyroid function in fish, but the underlying mechanism is still unclear. Current study focuses to study the intergenerational inheritance of MC-LR-induced thyroid toxicity in zebrafish and in rat thyroid cells. In vivo experiments, adult female zebrafish (F0) were exposed to MC-LR (0, 5, and 25 μg/L) for 90 days and mated with male zebrafish without MC-LR exposure to generate F1 generation. F1 embryos were allowed to develop normally to 7 days post-fertilization (dpf) in clear water. In the F0 generation, MC-LR induced disturbance of the hypothalamic-pituitary-thyroid (HPT) axis, leading to a decrease in the production of thyroid hormones. Maternal MC-LR exposure also induced growth inhibition by altering thyroid hormones (THs) homeostasis and interfering with thyroid metabolism and development in F1 offspring. Mechanistically, MC-LR caused excessive accumulation of ROS and induced ER stress that further lead to activation of UPR in the F0 and F1 offspring of zebrafish. Interestingly, our findings suggested that MC-LR exposure hampered thyroglobulin turnover by triggering IRE1 and PERK pathway in zebrafish and FRTL-5 thyroid cells, thus disturbing the thyroid endocrine system and contributing to the thyroid toxicity from maternal to its F1 offspring of zebrafish. Particularly, inhibition of the IRE1 pathway by siRNA could alleviate thyroid development injury induced by MC-LR in FRTL-5 cells. In addition, MC-LR induced thyroid cell apoptosis by triggering ER stress. Taken together, our results demonstrated that maternal MC-LR exposure causes thyroid endocrine disruption by ER stress contributing to transgenerational effects in zebrafish offspring.

Agrochemicals disrupt multiple endocrine axes in amphibians

Concern over global amphibian declines and possible links to agrochemical use has led to research on the endocrine disrupting actions of agrochemicals, such as fertilizers, fungicides, insecticides, acaricides, herbicides, metals, and mixtures. Amphibians, like other species, have to partition resources for body maintenance, growth, and reproduction. Recent studies suggest that metabolic impairments induced by endocrine disrupting chemicals, and more particularly agrichemicals, may disrupt physiological constraints associated with these limited resources and could cause deleterious effects on growth and reproduction. Metabolic disruption has hardly been considered for amphibian species following agrichemical exposure. As for metamorphosis, the key thyroid hormone-dependent developmental phase for amphibians, it can either be advanced or delayed by agrichemicals with consequences for juvenile and adult health and survival. While numerous agrichemicals affect anuran sexual development, including sex reversal and intersex in several species, little is known about the mechanisms involved in dysregulation of the sex differentiation processes. Adult anurans display stereotypical male mating calls and female phonotaxis responses leading to successful amplexus and spawning. These are hormone-dependent behaviours at the foundation of reproductive success. Therefore, male vocalizations are highly ecologically-relevant and may be a non-invasive low-cost method for the assessment of endocrine disruption at the population level. While it is clear that agrochemicals disrupt multiple endocrine systems in frogs, very little has been uncovered regarding the molecular and cellular mechanisms at the basis of these actions. This is surprising, given the importance of the frog models to our deep understanding of developmental biology and thyroid hormone action to understand human health. Several agrochemicals were found to have multiple endocrine effects at once (e.g., targeting both the thyroid and gonadal axes); therefore, the assessment of agrochemicals that alter cross-talk between hormonal systems must be further addressed. Given the diversity of life-history traits in Anura, Caudata, and the Gymnophiona, it is essential that studies on endocrine disruption expand to include the lesser known taxa. Research under ecologically-relevant conditions will also be paramount. Closer collaboration between molecular and cellular endocrinologists and ecotoxicologists and ecologists is thus recommended.

Octylphenol influence growth and development of Rana chensinensis tadpoles via disrupting thyroid function

Octylphenol (OP), a class of endocrine disrupting chemicals (EDCs), could produce adverse effects on developmental process of animals. Thyroid hormone is one of the important hormones involved in animal development. To determine whether OP affect the growth and development of amphibian larvae via interfering the thyroid function, Rana chensinensis larvae at Gosner stage 29 were exposed to 10^-8, 10^-7 and 10^-6 mol/L OP in the present study. Results demonstrated that OP could decrease the body length and mass and retard the development of tadpoles. The histologic evaluation showed microscopic structures of thyroid gland were changed in 10^-7 and 10^-6 mol/L OP treated groups on day 40. The expression levels of Dio2, Dio3, TRα and TRβ mRNA in the liver, brain, skin and tail of tadpoles were detected by qRT-PCR, when treated with OP for 20, 30, 40 and 50 day, respectively. The results of qRT-PCR showed OP could affect the expressions of Dio2, Dio3, TRα and TRβ mRNA in the four tissues, and then influence the activity and function of THs, further affecting the growth and development of the tadpoles.

Semicarbazide-induced thyroid disruption in Japanese flounder (Paralichthys olivaceus) and its potential mechanisms

Semicarbazide (SMC) is a carcinogenic and genotoxic substance that has been found in aquatic systems. SMC may also cause thyroid follicular epithelial cell injury in rats, but the thyroid-disrupting properties of SMC and its potential mechanisms remain unclear. In this study, we exposed fertilized eggs of Japanese flounder (Paralichthys olivaceus) to 1, 10, 100, and 1000μg/L SMC for 55 d to assess the impact of SMC exposure on the thyroid system. The number of larvae in each metamorphic stage was counted, the concentrations of whole-body thyroid hormones (THs) 3,5,3'-triiodothyronine (T3) and thyroxine (T4) were measured, and the transcription levels of genes involved in the hypothalamic-pituitary-thyroid (HPT) axis and gamma-aminobutyric acid (GABA) synthesis were quantified. The results showed that 10μg/L SMC significantly increased whole-body T3 levels, and 100 and 1000μg/L SMC markedly enhanced whole-body T4 and T3 levels. Furthermore, 100μg/L SMC exposure shortened the time it took for flounder larvae to complete metamorphosis by 2 d as compared to the control group. Thus, this study demonstrated that SMC exerted thyroid-disrupting effects on Japanese flounder. SMC-mediated stimulation of TH levels was primarily related to transcriptional alterations of pituitary-derived thyroid stimulating hormone β-subunit (tshβ) and hepatic deiodinase (id). In the 10 and 100μg/L SMC exposure groups, the increased TH levels may have resulted from inhibition of TH metabolism caused by down-regulation of id3 mRNA expression, while at 1000μg/L SMC-exposed group, up-regulation of tshβ and id1 transcripts was expected to enhance the synthesis of T4 and the conversion of T4 to T3 and, consequently, result in higher T4 and T3 levels. In addition, 1000μg/L SMC-induced down-regulation in glutamic acid decarboxylase gad65 and gad67 transcription may have also contributed to the increased TH levels. The thyroid-disrupting effects of 10 and 100μg/L SMC indicated that environmentally relevant concentrations of SMC posed potential environmental risks to aquatic organisms. Overall, our results demonstrated for the first time that SMC exhibited thyroid-disrupting properties by affecting the HPT axis and GABA synthesis, providing theoretical support for environmental risk assessment.

Effects of ZnO nanoparticles on perfluorooctane sulfonate induced thyroid-disrupting on zebrafish larvae

Perfluorooctane sulfonate (PFOS) and ZnO nanoparticles (nano-ZnO) are widely distributed in the environment. However, the potential toxicity of co-exposure to PFOS and nano-ZnO remains to be fully elucidated. The test investigated the effects of co-exposure to PFOS and nano-ZnO on the hypothalamic-pituitary-thyroid (HPT) axis in zebrafish. Zebrafish embryos were exposed to a combination of PFOS (0.2, 0.4, 0.8mg/L) and nano-ZnO (50mg/L) from their early stages of life (0-14days). The whole-body content of TH and the expression of genes and proteins related to the HPT axis were analyzed. The co-exposure decreased the body length and increased the malformation rates compared with exposure to PFOS alone. Co-exposure also increased the triiodothyronine (T3) levels, whereas the thyroxine (T4) content remained unchanged. Compared with the exposure to PFOS alone, exposure to both PFOS (0.8mg/L) and nano-ZnO (50mg/L) significantly up-regulated the expression of corticotropin-releasing factor, sodium/iodidesymporter, iodothyronine deiodinases and thyroid receptors and significantly down-regulated the expression of thyroid-stimulating hormone, thyroglobulin (TG), transthyretin (TTR) and thyroid receptors. The protein expression levels of TG and TTR were also significantly down-regulated in the co-exposure groups. In addition, the expression of the thyroid peroxidase gene was unchanged in all groups. The results demonstrated that PFOS and nano-ZnO co-exposure could cause more serious thyroid-disrupting effects in zebrafish than exposure to PFOS alone. Our results also provide insight into the mechanism of disruption of the thyroid status by PFOS and nano-ZnO.

Microcystin-RR exposure results in growth impairment by disrupting thyroid endocrine in zebrafish larvae

Recent studies have shown that cyanobacteria-derived microcystins (MCs) have the potential to disrupt endocrine systems. However, the effects of microcystin-RR (MC-RR) and their underlying mechanisms are poorly resolved in fish. In this study, MC-RR exposure through submersion caused serious developmental toxicity, such as growth delay and depressed heart rates in zebrafish larvae. We also detected decreased levels of thyroid hormones (THs), suggesting that MC-RR-triggered thyroid endocrine disruption might contribute to the growth impairment observed in developing zebrafish. To further our understanding of mechanisms of MC-RR-induced endocrine toxicity, quantitative real-time PCR (QPCR) analysis was performed on hypothalamic-pituitary-thyroid (HPT) axis related genes, i.e., corticotropin-releasing factor (CRF), thyroid-stimulating hormone (TSH), sodium/iodide symporter (NIS), thyroglobulin (TG), thyroid receptors (TRα and TRβ) and iodothyronine deiodinases (Dio1 and Dio2), of developing zebrafish embryos exposed to 0, 0.3, 1.0 or 3.0mgL(-1) MC-RR until 96h post-fertilization. Our results showed that transcription pattern of HPT axis related genes were greatly changed by MC-RR exposure, except TG gene. Furthermore, western blot was used to validate the results of gene expression. The results showed protein synthesis of TG was not affected, while that of NIS was significantly up-regulated, which are in accordance with gene expression. The overall results indicated that exposure to MC-RR can induce developmental toxicity, which might be associated with thyroid endocrine disruption in developing zebrafish larvae.

Molecular characterization and developmental expression patterns of thyroid hormone receptors (TRs) and their responsiveness to TR agonist and antagonist in Rana nigromaculata

Considering some advantages of Rana nigromaculata as an experimental species, we propose that this species, like Xenopus laevis, could be used to assay thyroid hormone (TH) signaling disrupting actions. To validate the utilizability of R. nigromaculata, we investigated the responsiveness of R. nigromaculata to a TH receptor (TR) agonist (T3) and antagonist (amiodarone) by analyzing expression, based on characterizing TR cDNA and developmental expression patterns. With high levels of identity with the corresponding genes in X. laevis, both TRα and TRβ in R. nigromaculata exhibited roughly similar developmental expression patterns to those of X. laevis, in spite of some species-specific differences. Both TRα and TRβ expression had greater changes in the liver and intestine than in the tail and brain during metamorphosis. T3 exposure for 2days induced more dramatic increases of TRβ expression in stage 27 than in stage 34 tadpoles but not in stage 42 tadpoles, showing that the responsiveness of R. nigromaculata to TH decreased with development and disappeared at the onset of metamorphic climax. Corresponding to greater changes of TRβ expression in the liver and intestine than in the tail and brain during metamorphosis, the liver and intestine had higher responsiveness to exogenous T3 than the tail and brain. Amiodarone inhibited T3-induced TRβ expression. Our results show that R. nigromaculata can be used as a model species for assaying TH signaling disrupting actions by analyzing TRβ expression, and intestine tissues at stage 27 are ideal test materials due to high responsiveness and easy accessibility.

Crossover of the hypothalamic pituitary-adrenal/interrenal, -thyroid, and -gonadal axes in testicular development

Besides the well-known function of thyroid hormones (THs) for regulating metabolism, it has recently been discovered that THs are also involved in testicular development in mammalian and non-mammalian species. THs, in combination with follicle stimulating hormone, lead to androgen synthesis in Danio rerio, which results in the onset of spermatogenesis in the testis, potentially relating the hypothalamic-pituitary-thyroid (HPT) gland to the hypothalamic-pituitary-gonadal (HPG) axes. Furthermore, studies in non-mammalian species have suggested that by stimulating the thyroid-stimulating hormone (TSH), THs can be induced by corticotropin-releasing hormone. This suggests that the hypothalamic-pituitary-adrenal/interrenal gland (HPA) axis might influence the HPT axis. Additionally, it was shown that hormones pertaining to both HPT and HPA could also influence the HPG endocrine axis. For example, high levels of androgens were observed in the testis in Odonthestes bonariensis during a period of stress-induced sex-determination, which suggests that stress hormones influence the gonadal fate toward masculinization. Thus, this review highlights the hormonal interactions observed between the HPT, HPA, and HPG axes using a comparative approach in order to better understand how these endocrine systems could interact with each other to influence the development of testes.

Alteration of thyroid hormone levels and related gene expression in Chinese rare minnow larvae exposed to mercury chloride

Mercury is a prominent environmental contaminant that causes endocrine disorder to human and other organisms. But little is known about the response of the thyroid functions and hypothalamic-pituitary-thyroid (HPT) axis to mercury in teleosts and the few studies that are available have not yielded consistent results. In this study, expression profiles of corticotropin-releasing hormone (crh), thyroid stimulating hormone beta (tshβ), solute carrier family 5 (sodium iodide symporter) member 5 (slc5a5), thyroglobulin (tg), thyroid hormone receptor alpha (trα) and thyroid hormone receptor beta (trβ) genes were determined in whole-body of Chinese rare minnow (Gobiocypris rarus) larvae after exposure to different levels of Hg(2+) (0, 0.1 and 0.3 mg/l) for 4 days, as well as the thyroid hormones (THs) levels. Moreover, the 96-h lethal concentration of Hg(2+) on rare minnow larvae was determined as 0.32 mg/l. The results showed that crh, tg, trα and trβ mRNA levels were significantly up-regulated in the larvae, but the gene expression of tshβ and slc5a5 was not significantly changed in our study. Besides, the THs levels increased in the whole-body of fish, especially the thyroxine (T4) level. The above results indicated that Hg(2+) could alter some genes expression in the HPT axis which could be used as the potential biomarkers for evaluating the environmental Hg(2+)-induced stress in fish.

Effects of waterborne cadmium on thyroid hormone levels and related gene expression in Chinese rare minnow larvae

Cadmium is a heavy metal abundant in the environment that can induce endocrine disorder and toxicity in aquatic organisms at low levels. However, its effects on the thyroid system in fish are still unclear. In this study, the thyroid hormone (TH) levels and the expression profiles of genes related to hypothalamic- pituitary-thyroid (HPT) axis, including corticotropin-releasing hormone (crh), thyroid stimulating hormone beta (tshβ), solute carrier family 5 (sodium iodide symporter) member 5 (slc5a5), thyroglobulin (tg), thyroid hormone receptor alpha (trα) and thyroid hormone receptor beta (trβ), were determined in whole body of Chinese rare minnow (Gobiocypris rarus) larvae after exposure to different levels of Cd(2+) (0, 0.5 and 2.5mg/L) for 4days. And the 96-h lethal concentration of Cd(2+) on rare minnow larvae was determined as 2.59mg/L. The results showed that crh, slc5a5, tg and tshβ mRNA levels were significantly up-regulated in the larvae, but the gene expression of trα and trβ was down-regulated in a concentration-dependent manner. Besides, the THs levels decreased in the whole-body of fish, especially the thyroxine (T4) level. The above results indicated that Cd(2+) could alter gene expression in the HPT axis that might subsequently contribute to thyroid disruption.

Waterborne exposure to microcystin-LR alters thyroid hormone levels and gene transcription in the hypothalamic-pituitary-thyroid axis in zebrafish larvae

Microcystin-leucine-arginine (MCLR) is the most toxic and the most commonly encountered variant of microcystins (MCs) in aquatic environment, and it has the potential for disrupting thyroid hormone homeostasis, but the molecular mechanisms underlying this process have not yet been clarified. In the present study, we observed body growth retardation associated with decreased levels of thyroid hormones (THs) in zebrafish larvae, highlighting the interferences of MCLR with the growth of fish larvae. To further our understanding of mechanisms of MCLR-induced endocrine toxicity, quantitative real-time PCR analysis was performed on hypothalamic-pituitary-thyroid (HPT) axis related genes of developing zebrafish embryos exposed to 100, 300 and 500 μg L(-1) MCLR until 96 h post-fertilization. The expression of several genes in the HPT system, i.e., corticotropin-releasing factor (CRF), thyroid-stimulating hormone (TSH), sodium/iodide symporter (NIS), thyroglobulin (TG), thyroid receptors (TRα and TRβ) and iodothyronine deiodinases (Dio1 and Dio2) was examined using quantitatively real-time PCR. The gene expression levels of CRF, TSH, NIS and TG were significantly induced after exposure to 500 μg L(-1) MCLR. The transcription of TRs gene was down-regulated in a concentration-dependent manner. Up-regulation and down-regulation of Deio1 and Deio2 gene expression, respectively, were observed upon exposure to MCLR. The above results indicated that MCLR could alter gene expression in the HPT axis which might subsequently contribute to MCLR-induced thyroid disruption.

Regulation of thyroid hormone related genes mRNA expression by exogenous T₃ in larvae and adult Chinese rare minnow (Gobiocypris rarus)

In this study, the expression time and profiles of thyroid hormone receptor alpha (trα), type I and II deiodinase enzymes (d1 and d2), transthyretin (ttr), sodium iodide symporter (nis), and thyrotropin-releasing hormone receptor (trhr) genes in Chinese rare minnow (Gobiocypris rarus) were determined using real-time PCR. Meanwhile, the changes of these genes were investigated by exogenous T(3) (3.8 nM) in larvae and adult fish. The retardation of swim bladder development and growth inhibition were observed for larvae, and the transcription of trα, d1, d2, nis, and trhr was significantly down-regulated at the end of exposure (21 d). In adults, a down-regulation of trα, d1, nis, and trhr mRNA levels occurred at 7th or 14th day of exposure, but returned back to their normal levels similar to control at the end of exposure. The down-regulation of gene mRNA expression could serve as a compensatory mechanism for the activation of thyroid system.

Waterborne exposure to PFOS causes disruption of the hypothalamus-pituitary-thyroid axis in zebrafish larvae

Thyroid hormones (THs) play an important role in the normal development and physiological functions in fish. Environmental chemicals may adversely affect thyroid function by disturbing gene transcription. Perfluorooctane sulfonate (PFOS), a persistent compound, is widely distributed in the aquatic environment and wildlife. In the present study, we investigated whether PFOS could disrupt the hypothalamic-pituitary-thyroid (HPT) axis. Zebrafish embryos were exposed to various concentrations of PFOS (0, 100, 200 and 400 microgL(-1)) and gene expression patterns were examined 15d post-fertilization. The expression of several genes in the HPT system, i.e., corticotropin-releasing factor (CRF), thyroid-stimulating hormone (TSH), sodium/iodide symporter (NIS), thyroglobulin (TG), thyroid peroxidase (TPO), transthyretin (TTR), iodothyronine deiodinases (Dio1 and Dio2) and thyroid receptor (TRalpha and TRbeta), was quantitatively measured using real-time PCR. The gene expression levels of CRF and TSH were significantly up-regulated and down-regulated, respectively, upon exposure to 200 and 400 microg L(-1) PFOS. A significant increase in NIS and Dio1 gene expression was observed at 200 microg L(-1) PFOS exposure, while TG gene expression was down-regulated at 200 and 400 microg L(-1) PFOS exposure. TTR gene expression was down-regulated in a concentration-dependent manner. Up-regulation and down-regulation of TRalpha and TRbeta gene expression, respectively, was observed upon exposure to PFOS. The whole body thyroxine (T(4)) content remained unchanged, whereas triiodothyronine (T(3)) levels were significantly increased, which could directly reflect disrupted thyroid hormone status after PFOS exposure. The overall results indicated that PFOS exposure could alter gene expression in the HPT axis and that mechanisms of disruption of thyroid status by PFOS could occur at several steps in the synthesis, regulation, and action of thyroid hormones.

Effects of exposure to acetochlor on the expression of thyroid hormone related genes in larval and adult rare minnow (Gobiocypris rarus)

Acetochlor is a commonly used herbicide in agricultural environments, which can accelerate T(3)-induced metamorphosis in amphibians. However, effects of acetochlor on the thyroid system in fish are still unclear at environmentally relevant concentrations. In this study, expression profiles of thyroid hormone receptor alpha (tralpha), deiodinase (d1 and d2), malic enzyme (me), and sodium iodide symporter (nis) genes were determined in larval and adult rare minnow (Gobiocypris rarus) after exposure to different levels of acetochlor (20, 200, and 2000ng/l) for 21 days, respectively. Furthermore, plasma thyroid hormones (THs) levels in adults were evaluated simultaneously. The results showed that d1, d2, me, and nis mRNA levels were significantly down-regulated in the larvae. Similar expression changes of these genes in female brains were found. However, in adult livers, these gene expressions had no significantly effects, except the tralpha mRNA level significantly up-regulated. These results indicated that exposure to acetochlor could result in tissue-specific alternative expression of TH-related genes in adults. Moreover, the expression of d2 and me showed a positive correlation with plasma T(4) levels in female brains. Therefore, larval development and adult brain of rare minnow could be affected by acetochlor at environmentally relevant concentrations.

Changes of thyroid hormone levels and related gene expression in Chinese rare minnow (Gobiocypris rarus) during 3-amino-1,2,4-triazole exposure and recovery

Thyroid hormones (THs) play an important role in the development and metabolism of fish through their influences on genetic transcription and are targets for endocrine disruptive agents in the aquatic environment. Amitrole is a pesticide potentially interfering with thyroid hormone regulation. In this study, the rare minnow (Gobiocypris rarus) was exposed to different levels of 3-amino-1,2,4-triazole (amitrole) and allowed to recover in clean water. Plasma TH levels and the expression of TH-related genes, including transthyretin (ttr), deiodinases (d1 and d2), and the thyroid hormone receptor (tralpha) from the livers and brains were evaluated. After exposure, the plasma TH levels did not change. Histopathological observations showed that livers were degenerated at 10,000 ng/l and these damages could be recovered by the withdrawal of amitrole. However, the ttr, d1, and d2 mRNA levels in the livers of males were significantly up-regulated in all exposure groups (p<0.05). The ttr and d2 mRNA levels were significantly up-regulated at 10,000 ng/l and 10, 100, and 1000 ng/l in the livers of females, respectively (p<0.05). In the brains of males, a twofold increase of d2 mRNA levels at > or = 100 ng/l and a fivefold decrease of tralpha mRNA levels at > or = 10 ng/l were observed (p<0.05), whereas no significant differences were observed in the expression of d2 and tralpha in the brains of females. After a recovery period, the ttr, d1, and d2 mRNA levels in the livers of males returned to control levels, but the tralpha mRNA levels were irreversibly decreased at all treatments (p<0.05). In addition, the d2 mRNA levels in the livers of females were significantly induced at > or = 100 ng/l. Moreover, the d2 mRNA levels in the brains of males and females were up-regulated at 10,000 ng/l. These results indicated that amitrole exposure could result in alternations of ttr, d1, d2, and tralpha gene expression in different tissues of the rare minnow. The expression of these TH-related genes in males was more sensitive to amitrole than those of females. Recovery in clean water was associated with the selective regulation of TH-related gene transcription in the rare minnow. Therefore, these TH-related genes can serve as biomarkers to screen the effects of thyroid disruption chemicals in rare minnow.

Thyroid hormone-dependent gene expression as a biomarker of short-term 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) exposure in European common frog (Rana temporaria) tadpoles

The effects on thyroid hormone-dependent gene biomarker responses of the persistent organochlorine pesticide metabolite 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) were investigated after exposure of 4-week-old European common frog (Rana temporaria) (stage 36) tadpoles to two (0.001 and 0.01 ppm) DDE concentrations. Total body weight, total length, and tail length and width increased after 3-day exposure to DDE. Expression patterns of genes encoding for growth hormone, thyroid-stimulating hormone (TSHbeta) and thyroid hormone receptor (TRalpha and TRbeta) isoforms were evaluated in the head, body and tail regions using a validated real-time polymerase chain reaction (PCR) method. The mRNA expression of growth hormone in the body, and TSHbeta in the head showed significant DDE concentration-dependent decreases. While DDE caused variable effects on TRalpha mRNA steady-state, the expression of TRbeta was significantly decreased in the tail by DDE in a concentration-specific manner. The effect of DDE exposure on TRbeta mRNA expression showed a negative correlation with tail length and width during the exposure period. The unique pattern of a DDE-induced decrease of tail TRbeta expression probably reflects the significant role of this thyroid hormone receptor isoform in tail re-absorption and overall metamorphosis in anuran species. Therefore, the present study shows that the evaluation of thyroid hormone-dependent genes may represent quantitative biomarkers of acute exposure to organochlorine pesticides in anuran species during critical developmental periods such as metamorphosis. Given the widespread environmental levels of DDT and its metabolites, these pollutants will remain a subject of concern and their effects on anuran species should be studied in more detail.

Effects of short-term exposure to the DDT metabolite p,p'-DDE on bone tissue in male common frog (Rana temporaria)

Experimental studies as well as studies in free-ranging animals have shown that endocrine-disrupting chemicals (EDCs) impair bone tissue composition and strength. The aim of the present study was to expand our studies on bone tissue in a new group of animals by investigating whether bone tissue in frogs is an additional potential target of EDCs. Adult male European common frogs (Rana temporaria) were divided into 5 groups (n = 20) and injected (sc, single injection) with p,p'-DDE, a total dose of 0.01, 0.1, 1, or 10 mg of p,p'-DDE/kg body weight, respectively. A control group was treated with the vehicle (corn oil). Two weeks after injection the frogs were euthanized and samples taken. The diaphysis of the excised left femur was scanned using peripheral quantitative computed tomography (pQCT) and cortical variables, such as cortical bone mineral density (BMD), cortical cross-sectional area (CSA), and periosteal circumference, were determined. In addition, biomechanical three-point bending of the bones was conducted, with the load being applied to the same point as where the pQCT measurement was performed. The results from the pQCT measurements show that bone tissue in male frogs exposed to p,p'-DDE is negatively affected. A significant decrease in cortical BMD at the diaphysis was observed in frogs exposed to 1 mg p,p'-DDE. However, the biomechanical testing of the bones showed no significant differences between exposed and control group. Although this is the only study performed to date examining the possible relationships between EDCs and negative effects on frog bones, it supports both previous experimental findings in rodents and findings in free-ranging animals.

Toxicological housekeeping genes: do they really keep the house?

It is assumed that the expression of housekeeping genes is constant regardless of experimental conditions. In toxicology, this assumption has indeed become a misconception of reasonable concern, as these so-called housekeeping genes vary considerably across different experimental conditions and thereby lead to an erroneous interpretation of the expression profile of a target gene. Given that the choice of reference gene will ultimately influence statistical interpretation of toxicological data, it is essential to validate potential reference genes prior to their use, to establish their suitability for a specific experimental purpose. Therefore, the aim of this study is to quantitatively evaluate the most commonly used housekeeping genes in toxicology research for their suitability as reference endpoints, and thus provide toxicology researchers who have little experience in molecular biology but find themselves interested or involved with gene expression analysis with a summary of information necessary for re-evaluating their procedures. We show that the expression pattern of beta-actin, beta-tubulin, 18S ribosomal RNA (18S rRNA), and elongation factor-lalpha (EF-lalpha), representing commonly used housekeeping genes in toxicology, was modulated on the basis of random exposure condition and time, in both in vivo and in vitro test systems of Atlantic salmon (Salmo salar). Based on the data presented herein and several other reports by other researchers, there are very few biological justifications to refer to anything as a housekeeping gene in real-time PCR assays for toxicological research. However, given the absolute need for normalization genes to correct for sample-to-sample variations, the choice of internal control gene should be determined empirically on the basis of the individual exposure condition and by the individual researcher.