Increased Perfluorooctanesulfonate (PFOS) Toxicity and Accumulation Is Associated with Perturbed Prostaglandin Metabolism and Increased Organic Anion Transport Protein (OATP) Expression

Perfluorooctanesulfonate (PFOS) is a widespread environmental pollutant with a long half-life and clearly negative outcomes on metabolic diseases such as fatty liver disease and diabetes. Male and female Cyp2b-null and humanized CYP2B6-transgenic (hCYP2B6-Tg) mice were treated with 0, 1, or 10 mg/kg/day PFOS for 21 days, and surprisingly it was found that PFOS was retained at greater concentrations in the serum and liver of hCYP2B6-Tg mice than those of Cyp2b-null mice, with greater differences in the females. Thus, Cyp2b-null and hCYP2B6-Tg mice provide new models for investigating individual mechanisms for PFOS bioaccumulation and toxicity. Overt toxicity was greater in hCYP2B6-Tg mice (especially females) as measured by mortality; however, steatosis occurred more readily in Cyp2b-null mice despite the lower PFOS liver concentrations. Targeted lipidomics and transcriptomics from PFOS-treated Cyp2b-null and hCYP2B6-Tg mouse livers were performed and compared to PFOS retention and serum markers of toxicity using PCA. Several oxylipins, including prostaglandins, thromboxanes, and docosahexaenoic acid metabolites, are associated or inversely associated with PFOS toxicity. Both lipidomics and transcriptomics indicate PFOS toxicity is associated with PPAR activity in all models. GO terms associated with reduced steatosis were sexually dimorphic with lipid metabolism and transport increased in females and circadian rhythm associated genes increased in males. However, we cannot rule out that steatosis was initially protective from PFOS toxicity. Moreover, several transporters are associated with increased retention, probably due to increased uptake. The strongest associations are the organic anion transport proteins (Oatp1a4-6) genes and a long-chain fatty acid transport protein (fatp1), enriched in female hCYP2B6-Tg mice. PFOS uptake was also reduced in cultured murine hepatocytes by OATP inhibitors. The role of OATP1A6 and FATP1 in PFOS transport has not been tested. In summary, Cyp2b-null and hCYP2B6-Tg mice provided unique models for estimating the importance of novel mechanisms in PFOS retention and toxicity.

Detoxification Cytochrome P450s (CYPs) in Families 1-3 Produce Functional Oxylipins from Polyunsaturated Fatty Acids

This manuscript reviews the CYP-mediated production of oxylipins and the current known function of these diverse set of oxylipins with emphasis on the detoxification CYPs in families 1-3. Our knowledge of oxylipin function has greatly increased over the past 3-7 years with new theories on stability and function. This includes a significant amount of new information on oxylipins produced from linoleic acid (LA) and the omega-3 PUFA-derived oxylipins such as α-linolenic acid (ALA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA). However, there is still a lack of knowledge regarding the primary CYP responsible for producing specific oxylipins, and a lack of mechanistic insight for some clinical associations between outcomes and oxylipin levels. In addition, the role of CYPs in the production of oxylipins as signaling molecules for obesity, energy utilization, and development have increased greatly with potential interactions between diet, endocrinology, and pharmacology/toxicology due to nuclear receptor mediated CYP induction, CYP inhibition, and receptor interactions/crosstalk. The potential for diet-diet and diet-drug/chemical interactions is high given that these promiscuous CYPs metabolize a plethora of different endogenous and exogenous chemicals.

Human CYP2B6 produces oxylipins from polyunsaturated fatty acids and reduces diet-induced obesity

Multiple factors in addition to over consumption lead to obesity and non-alcoholic fatty liver disease (NAFLD) in the United States and worldwide. CYP2B6 is the only human detoxification CYP whose loss is associated with obesity, and Cyp2b-null mice show greater diet-induced obesity with increased steatosis than wildtype mice. However, a putative mechanism has not been determined. LC-MS/MS revealed that CYP2B6 metabolizes PUFAs, with a preference for metabolism of ALA to 9-HOTrE and to a lesser extent 13-HOTrE with a preference for metabolism of PUFAs at the 9- and 13-positions. To further study the role of CYP2B6 in vivo, humanized-CYP2B6-transgenic (hCYP2B6-Tg) and Cyp2b-null mice were fed a 60% high-fat diet for 16 weeks. Compared to Cyp2b-null mice, hCYP2B6-Tg mice showed reduced weight gain and metabolic disease as measured by glucose tolerance tests, however hCYP2B6-Tg male mice showed increased liver triglycerides. Serum and liver oxylipin metabolite concentrations increased in male hCYP2B6-Tg mice, while only serum oxylipins increased in female hCYP2B6-Tg mice with the greatest increases in LA oxylipins metabolized at the 9 and 13-positions. Several of these oxylipins, specifically 9-HODE, 9-HOTrE, and 13-oxoODE, are PPAR agonists. RNA-seq data also demonstrated sexually dimorphic changes in gene expression related to nuclear receptor signaling, especially CAR > PPAR with qPCR suggesting PPARγ signaling is more likely than PPARα signaling in male mice. Overall, our data indicates that CYP2B6 is an anti-obesity enzyme, but probably to a lesser extent than murine Cyp2b's. Therefore, the inhibition of CYP2B6 by xenobiotics or dietary fats can exacerbate obesity and metabolic disease potentially through disrupted PUFA metabolism and the production of key lipid metabolites.

The reproductive effects of the cancer chemotherapy agent, Carmofur, on Daphnia magna are mediated by its metabolite, 5-Fluorouracil

Carmofur is an antineoplastic agent that inhibits ceramidase, a key enzyme in the sphingolipid pathway. Previous research suggests carmofur represses reproductive maturity in Daphnia magna. The purpose of this experiment was to confirm carmofur's effects on fecundity and reproductive maturity over two generations. A chronic toxicity test found reproductive maturity was delayed from 9 to 19 days by 0.80 μM carmofur with a 99.7% drop in reproduction, probably caused by delayed ovarian development. Second generation effects were even greater with 0% reproductive success at 0.40 μM. To our surprise, carmofur was not measured in the media by HPLC 24 h after exposure. Previous research indicated that carmofur is unstable in water and hydrolyzed into 5-fluorouracil (5-FU). Therefore, the chronic toxicity study was repeated with 5-FU and similar effects on reproductive maturity were observed at similar concentrations despite very different acute toxicities (48 h carmofur LC50 = 1.93 μM; 5-FU LC50 = 207 μM). 5-FU delayed reproductive maturity from 9 to 21 days with a 71.12% drop in reproduction at 0.80 μM and greater effects in the 2^nd generation similar to carmofur. 5-FU was found stable in aquatic media and HPLC confirmed 5-FU was hydrolyzed from carmofur within 24 h. In conclusion, carmofur and 5-FU reduce fecundity because they delay reproductive maturity and ovarian development in Daphnia magna. We conclude that the reproductive effects observed after carmofur treatment are primarily mediated by its breakdown product, 5-FU. This further underscores the importance of measuring chemical concentrations and evaluating chemical metabolism and decomposition when determining toxicity, especially of chemotherapeutic agents.Clinical trials registration Not applicable.

Increased toxicity and retention of perflourooctane sulfonate (PFOS) in humanized CYP2B6-Transgenic mice compared to Cyp2b-null mice is relieved by a high-fat diet (HFD)

PFOS is a persistent, fluorosurfactant used in multiple products. Murine Cyp2b's are induced by PFOS and high-fat diets (HFD) and therefore we hypothesized that human CYP2B6 may alleviate PFOS-induced steatosis. Cyp2b-null and hCYP2B6-Tg mice were treated with 0, 1, or 10 mg/kg/day PFOS by oral gavage for 21-days while provided a chow diet (ND) or HFD. Similar to murine Cyp2b10, CYP2B6 is inducible by PFOS. Furthermore, three ND-fed hCYP2B6-Tg females treated with 10 mg/kg/day PFOS died during the exposure period; neither Cyp2b-null nor HFD-fed mice died. hCYP2B6-Tg mice retained more PFOS in serum and liver than Cyp2b-null mice presumably causing the observed toxicity. In contrast, serum PFOS retention was reduced in the HFD-fed hCYP2B6-Tg mice; the opposite trend observed in HFD-fed Cyp2b-null mice. Hepatotoxicity biomarkers, ALT and ALP, were higher in PFOS-treated mice and repressed by a HFD. However, PFOS combined with a HFD exacerbated steatosis in all mice, especially in the hCYP2B6-Tg mice with significant disruption of key lipid metabolism genes such as Srebp1, Pparg, and Hmgcr. In conclusion, CYP2B6 is induced by PFOS but does not alleviate PFOS toxicity presumably due to increased retention. CYP2B6 protects from PFOS-mediated steatosis in ND-fed mice, but increases steatosis when co-treated with a HFD.

Evaluation of oral citrulline administration as a mitigation strategy for fescue toxicosis in sheep

Gestating ewes consuming ergot alkaloids, from endophyte-infected (E+) tall fescue seed, suffer from intrauterine growth restriction and produce smaller lambs. Arginine (Arg) supplementation has been shown to increase birth weight and oral citrulline (Cit) administration is reported to increase arginine concentrations. Two experiments were conducted to: 1) evaluate if oral supplementation with Cit or water, to ewes consuming E+ fescue seed, increases lamb birth weight and 2) determine the effectiveness of Cit and citrulline:malate as an oral drench and elevating circulating levels of Cit to determine levels and dose frequency. In experiment 1, gestating Suffolk ewes (n = 10) were assigned to one of two treatments [oral drench of citrulline-malate 2:1 (CITM; 81 mg/kg/d of citrulline) or water (TOX)] to start on d 86 of gestation and continued until parturition. Ewes on CITM treatment had decreased (P < 0.05) plasma Arg and Cit concentrations during gestation. At birth, lambs from CITM ewes had reduced (P < 0.05) crude fat and total fat but did not differ (P > 0.05) in birth weight from lambs born to TOX ewes. In experiment 2, nonpregnant Suffolk ewes (n = 3) were assigned to either oral citrulline (CIT; 81 mg/kg/d), citrulline-malate 2:1 (CITM; 81 mg/kg/d of citrulline), or water (CON) drench in a Latin Square design for a treatment period of 4 d with a washout period of 3 d. On d 4, blood samples were collected at 0, 0.5, 1, 2, 3, 4, 6, 8, 10, 12, and 18 h post drench. Oral drenching of CIT and CITM increased (P < 0.0001) Cit concentrations within 2 h and levels remained elevated for 6 h. Apparent half-life of elimination for CIT and CITM were 8.484 and 10.392 h, respectively. Our results show that lamb birth weight was not altered with a single oral drench of citrulline-malate; however, lamb body composition was altered. The level and frequency of citrulline dosing may need to be greater in order to observe consistent elevation of Cit/Arg concentrations to determine its effectiveness in mitigating fescue toxicosis.

20th Pollutant Responses in Marine Organisms (PRIMO 20): Global issues and fundamental mechanisms caused by pollutant stress in marine and freshwater organisms

The 20th Pollutant Responses in Marine Organisms (PRIMO 20) conference provided a forum for scientists from around the world to communicate novel toxicological research findings specifically focused on aquatic organisms, by combining applied and basic research at the intersection of environmental and mechanistic toxicology. The work highlighted in this special issue of Aquatic Toxicology, a special issue of Marine Environmental Research, and presented through posters and presentations, encompass important and emerging topics in freshwater and marine toxicology. This includes multiple types of emerging contaminants including microplastics and UV filtering chemicals. Other studies aimed to further our understanding of the effects of endocrine disrupting chemicals, pharmaceuticals, and personal care products. Further research presented in this virtual issue examined the interactive effects of chemicals and pathogens, while the final set of manuscripts demonstrates continuing efforts to combine traditional biomonitoring, data from -omic technologies, and modeling for use in risk assessment and management. An additional goal of PRIMO meetings is to address the link between environmental and human health. Several articles in this issue of Aquatic Toxicology describe the appropriateness of using aquatic organisms as models for human health, while the keynote speakers, as described in the editorial below, presented research that highlighted bioaccumulation of contaminants such as PFOS and mercury from fish to marine mammals and coastal human populations such as the Gullah/GeeChee near Charleston, South Carolina, USA.

Gender differences in diet-induced steatotic disease in Cyp2b-null mice

Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease; however, progression to nonalcoholic steatohepatitis (NASH) is associated with most adverse outcomes. CYP2B metabolizes multiple xeno- and endobiotics, and male Cyp2b-null mice are diet-induced obese (DIO) with increased NAFLD. However, the DIO study was not performed long enough to assess progression to NASH. Therefore, to assess the role of Cyp2b in fatty liver disease progression from NAFLD to NASH, we treated wildtype (WT) and Cyp2b-null mice with a normal diet (ND) or choline-deficient, L-amino acid-defined high fat diet (CDAHFD) for 8 weeks and determined metabolic and molecular changes. CDAHFD-fed WT female mice gained more weight and had greater liver and white adipose tissue mass than their Cyp2b-null counterparts; males experienced diet-induced weight loss regardless of genotype. Serum biomarkers of liver injury increased in both CDAHFD-fed female and male mice; however CDAHFD-fed Cyp2b-null females exhibited significantly lower serum ALT, AST, and ASP concentrations compared to WT mice, indicating Cyp2b-null females were protected from liver injury. In both genders, hierarchical clustering of RNA-seq data demonstrates several gene ontologies responded differently in CDAHFD-fed Cyp2b-null mice compared to WT mice (lipid metabolism > fibrosis > inflammation). Oil Red O staining and direct triglycerides measurements confirmed that CDAHFD-fed Cyp2b-null females were protected from NAFLD. CDAHFD-fed Cyp2b-null mice showed equivocal changes in fibrosis with transcriptomic and serum markers suggesting less inflammation due to glucocorticoid-mediated repression of immune responses. In contrast to females, CDAHFD-fed Cyp2b-null males had higher triglyceride levels. Results indicate that female Cyp2b-null mice are protected from NAFLD while male Cyp2b-null mice are more susceptible to NAFLD, with few significant changes in NASH development. This study confirms that increased NAFLD development does not necessarily lead to progressive NASH. Furthermore, it indicates a role for Cyp2b in fatty liver disease that differs based on gender.

Chemical Activation of the Constitutive Androstane Receptor Leads to Activation of Oxidant-Induced Nrf2

Exposure to environmentally relevant chemicals that activate the xenobiotic receptors aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and peroxisome proliferator-activated receptor alpha (PPARα) in rodent test systems often leads to increases in oxidative stress (OS) that contributes to liver cancer induction. We hypothesized that activation of the oxidant-induced transcription factor Nrf2 could be used as a surrogate endpoint for increases in OS. We examined the relationships between activation of xenobiotic receptors and Nrf2 using previously characterized gene expression biomarkers that accurately predict modulation. Using a correlation approach (Running Fisher Test), the biomarkers were compared with microarray profiles in a mouse liver gene expression compendium. Out of the 163 chemicals examined, 47% from 53 studies activated Nrf2. We found consistent coupling between CAR and Nrf2 activation. Out of the 41 chemicals from 32 studies that activated CAR, 90% also activated Nrf2. CAR was activated earlier and at lower doses than Nrf2, indicating CAR activation preceded Nrf2 activation. Nrf2 activation by 2 CAR activators was abolished in CAR-null mice. We hypothesized that Nrf2 is activated by reactive oxygen species from the increased activity of enzymes encoded by Cyp2b family members. However, Nrf2 was similarly activated in the livers of both TCPOBOP-treated wild-type and Cyp2b9/10/13-null mice. This study provides evidence that Nrf2 activation (1) often occurs after exposure to xenobiotic chemicals, (2) is tightly linked to activation of CAR, and (3) does not require induction of 3 Cyp2b genes secondary to CAR activation.

Phase 0 of the Xenobiotic Response: Nuclear Receptors and Other Transcription Factors as a First Step in Protection from Xenobiotics

This mini-review examines the crucial importance of transcription factors as a first line of defense in the detoxication of xenobiotics. Key transcription factors that recognize xenobiotics or xenobiotic-induced stress such as reactive oxygen species (ROS), include AhR, PXR, CAR, MTF, Nrf2, NF-κB, and AP-1. These transcription factors constitute a significant portion of the pathways induced by toxicants as they regulate phase I-III detoxication enzymes and transporters as well as other protective proteins such as heat shock proteins, chaperones, and anti-oxidants. Because they are often the first line of defense and induce phase I-III metabolism, could these transcription factors be considered the phase 0 of xenobiotic response?

Cyp2b-null male mice are susceptible to diet-induced obesity and perturbations in lipid homeostasis

Obesity is an endemic problem in the United States and elsewhere, and data indicate that in addition to overconsumption, exposure to specific chemicals enhances obesity. CYP2B metabolizes multiple endo- and xenobiotics, and recent data suggests that repression of Cyp2b activity increases dyslipidemia and age-onset obesity, especially in males. To investigate the role played by Cyp2b in lipid homeostasis and obesity, we treated wildtype and Cyp2b-null mice with a normal (ND) or 60% high-fat diet (HFD) for 10 weeks and determined metabolic and molecular changes. Male HFD-fed Cyp2b-null mice weigh 15% more than HFD-fed wildtype mice, primarily due to an increase in white adipose tissue (WAT); however, Cyp2b-null female mice did not demonstrate greater body mass or WAT. Serum parameters indicate increased ketosis, leptin and cholesterol in HFD-fed Cyp2b-null male mice compared to HFD-fed wildtype mice. Liver triglycerides and liver:serum triglyceride ratios were higher than their similarly treated wildtype counterparts in Cyp2b-null male mice, indicating a role for Cyp2b in fatty acid metabolism regardless of diet. Furthermore, RNAseq demonstrates that hepatic gene expression in ND-fed Cyp2b-null male mice is similar to HFD-fed WT male mice, suggestive of fatty liver disease progression and a role for Cyp2b in lipid homeostasis. Females did not show as demonstrative changes in liver health, and significantly fewer changes in gene expression, as well as gene expression associated with liver disease. Overall our data indicates that the repression or inhibition of CYP2B may exacerbate metabolic disorders and cause obesity by perturbing fatty acid metabolism, especially in males.

Cyp2b-Knockdown Mice Poorly Metabolize Corn Oil and Are Age-Dependent Obese

We previously made a RNAi-based cytochrome P450 2b (Cyp2b)-knockdown (Cyp2b-KD) mouse to determine the in vivo role of the Cyp2b subfamily in xenobiotic detoxification. Further studies reported here indicate a role for Cyp2b in unsaturated fatty-acid (UFA) metabolism and in turn obesity. Mice were treated intraperitoneally (i.p.) with 100 μL corn oil as a carrier or the potent Cyp2b-inducer 3,3',5,5'-Tetrachloro-1,4-bis(pyridyloxy)benzene (TCPOBOP (TC)) dissolved in corn oil. Surprisingly, female Cyp2b-KD mice but not male mice showed increased liver lipid accumulation. Male Cyp2b-KD mice had higher serum triacylglycerols, cholesterol, very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) than wildtype (WT) mice; females had higher cholesterol, LDL, and HDL. Thus, Cyp2b-KD mice are unable to clear a high bolus dose of corn oil, potentially because the Cyp2b-KD mice were unable to metabolize the UFA in the corn oil. Therefore, WT and Cyp2b-KD mice were housed for 35 weeks and necropsies performed to test whether Cyp2b-KD mice develop age onset obesity. Cyp2b-KD mice exhibited a significant increase in body weight caused by an increase in white adipose tissue deposition relative to WT mice. Serum cholesterol, triacylglycerol, LDL, and VLDL were significantly greater in 35-week-old Cyp2b-KD males compared to WT males; only serum triacylglycerol and LDL were higher in females. In conclusion, changes in Cyp2b expression led to perturbation in lipid metabolism and depuration in Cyp2b-KD mice. This suggests that Cyp2b is more than a detoxification enzyme, but also involved in the metabolism of UFA, as Cyp2b-KD mice have increased the body weight, fat deposition, and serum lipids.

High fat diet induced obesity is mitigated in Cyp3a-null female mice

Recent studies indicate a role for the constitutive androstane receptor (CAR), pregnane X-receptor (PXR), and hepatic xenobiotic detoxifying CYPs in fatty liver disease or obesity. Therefore, we examined whether Cyp3a-null mice show increased obesity and fatty liver disease following 8-weeks of exposure to a 60% high-fat diet (HFD). Surprisingly, HFD-fed Cyp3a-null females fed a HFD gained 50% less weight than wild-type (WT; B6) females fed a HFD. In contrast, Cyp3a-null males gained more weight than WT males, primarily during the first few weeks of HFD-treatment. Cyp3a-null females also recovered faster than WT females from a glucose tolerance test; males showed no difference in glucose tolerance between the groups. Serum concentrations of the anti-obesity hormone, adiponectin are 60% higher and β-hydroxybutyrate levels are nearly 50% lower in Cyp3a-null females than WT females, in agreement with reduced weight gain, faster glucose response, and reduced ketogenesis. In contrast, Cyp3a-null males have higher liver triglyceride concentrations and lipidomic analysis indicates an increase in phosphatidylinositol, phosphatidylserine and sphingomyelin. None of these changes were observed in females. Last, Pxr, Cyp2b, and IL-6 expression increased in Cyp3a-null females following HFD-treatment. Cyp2b and Fatp1 increased, while Pxr, Cpt1a, Srebp1 and Fasn decreased in Cyp3a-null males following a HFD, indicating compensatory biochemical responses in male (and to a lesser extent) female mice fed a HFD. In conclusion, lack of Cyp3a has a positive effect on acclimation to a HFD in females as it improves weight gain, glucose response and ketosis.

RNA sequencing indicates that atrazine induces multiple detoxification genes in Daphnia magna and this is a potential source of its mixture interactions with other chemicals

Atrazine is an herbicide with several known toxicologically relevant effects, including interactions with other chemicals. Atrazine increases the toxicity of several organophosphates and has been shown to reduce the toxicity of triclosan to D. magna in a concentration dependent manner. Atrazine is a potent activator in vitro of the xenobiotic-sensing nuclear receptor, HR96, related to vertebrate constitutive androstane receptor (CAR) and pregnane X-receptor (PXR). RNA sequencing (RNAseq) was performed to determine if atrazine is inducing phase I-III detoxification enzymes in vivo, and estimate its potential for mixture interactions. RNAseq analysis demonstrates induction of glutathione S-transferases (GSTs), cytochrome P450s (CYPs), glucosyltransferases (UDPGTs), and xenobiotic transporters, of which several are verified by qPCR. Pathway analysis demonstrates changes in drug, glutathione, and sphingolipid metabolism, indicative of HR96 activation. Based on our RNAseq data, we hypothesized as to which environmentally relevant chemicals may show altered toxicity with co-exposure to atrazine. Acute toxicity tests were performed to determine individual LC₅₀ and Hillslope values as were toxicity tests with binary mixtures containing atrazine. The observed mixture toxicity was compared with modeled mixture toxicity using the Computational Approach to the Toxicity Assessment of Mixtures (CATAM) to assess whether atrazine is exerting antagonism, additivity, or synergistic toxicity in accordance with our hypothesis. Atrazine-triclosan mixtures showed decreased toxicity as expected; atrazine-parathion, atrazine-endosulfan, and to a lesser extent atrazine-p-nonylphenol mixtures showed increased toxicity. In summary, exposure to atrazine activates HR96, and induces phase I-III detoxification genes that are likely responsible for mixture interactions.

Annotation of the Nuclear Receptors in an Estuarine Fish species, Fundulus heteroclitus

The nuclear receptors (NRs) are ligand-dependent transcription factors that respond to various internal as well as external cues such as nutrients, pheromones, and steroid hormones that play crucial roles in regulation and maintenance of homeostasis and orchestrating the physiological and stress responses of an organism. We annotated the Fundulus heteroclitus (mummichog; Atlantic killifish) nuclear receptors. Mummichog are a non-migratory, estuarine fish with a limited home range often used in environmental research as a field model for studying ecological and evolutionary responses to variable environmental conditions such as salinity, oxygen, temperature, pH, and toxic compounds because of their hardiness. F. heteroclitus have at least 74 NRs spanning all seven gene subfamilies. F. heteroclitus is unique in that no RXRα member was found within the genome. Interestingly, some of the NRs are highly conserved between species, while others show a higher degree of divergence such as PXR, SF1, and ARα. Fundulus like other fish species show expansion of the RAR (NR1B), Rev-erb (NR1D), ROR (NR1F), COUPTF (NR2F), ERR (NR3B), RXR (NR2B), and to a lesser extent the NGF (NR4A), and NR3C steroid receptors (GR/AR). Of particular interest is the co-expansion of opposing NRs, Reverb-ROR, and RAR/RXR-COUPTF.

Exchange of polar lipids from adults to neonates in Daphnia magna: Perturbations in sphingomyelin allocation by dietary lipids and environmental toxicants

Because xenosensing nuclear receptors are also lipid sensors that regulate lipid allocation, we hypothesized that toxicant-induced modulation of HR96 activity would alter lipid profiles and the balance between adult survival and neonate production following exposure in Daphnia magna. Adult daphnids were exposed to unsaturated fatty acid- and toxicant- activators or inhibitors of HR96 and later starved to test whether chemical exposure altered allocation toward survival or reproduction. The HR96 activators, linoleic acid and atrazine, decreased reproduction as expected with concomitant changes in the expression of HR96 regulated genes such as magro. The HR96 inhibitors, docosahexaenoic acid (DHA) and triclosan, increased reproduction or neonate starvation survival, respectively. However, pre-exposure to triclosan increased in neonate survival at the expense of reproductive maturation. Lipidomic analysis revealed that sphingomyelins (SM) are predominantly found in neonates and therefore we propose are important in development. DHA and triclosan increased neonatal SM, consistent with HR96’s regulation of Niemann-Pick genes. While DHA altered expression of magro, Niemann-Pick 1b, mannosidase, and other HR96-regulated genes as expected, triclosan primarily perturbed sphingomyelinase and mannosidase expression indicating different but potentially overlapping mechanisms for perturbing SM. Overall, SM appears to be a key lipid in Daphnia maturation and further support was provided by carmofur, which inhibits sphingomyelin/ceramide metabolism and in turn severely represses Daphnia maturation and initial brood production. In conclusion, toxicants can perturb lipid allocation and in turn impair development and reproduction.

Compensatory changes in CYP expression in three different toxicology mouse models: CAR-null, Cyp3a-null, and Cyp2b9/10/13-null mice

Targeted mutant models are common in mechanistic toxicology experiments investigating the absorption, metabolism, distribution, or elimination (ADME) of chemicals from individuals. Key models include those for xenosensing transcription factors and cytochrome P450s (CYP). Here we investigated changes in transcript levels, protein expression, and steroid hydroxylation of several xenobiotic detoxifying CYPs in constitutive androstane receptor (CAR)-null and two CYP-null mouse models that have subfamily members regulated by CAR; the Cyp3a-null and a newly described Cyp2b9/10/13-null mouse model. Compensatory changes in CYP expression that occur in these models may also occur in polymorphic humans, or may complicate interpretation of ADME studies performed using these models. The loss of CAR causes significant changes in several CYPs probably due to loss of CAR-mediated constitutive regulation of these CYPs. Expression and activity changes include significant repression of Cyp2a and Cyp2b members with corresponding drops in 6α- and 16β-testosterone hydroxylase activity. Further, the ratio of 6α-/15α-hydroxylase activity, a biomarker of sexual dimorphism in the liver, indicates masculinization of female CAR-null mice, suggesting a role for CAR in the regulation of sexually dimorphic liver CYP profiles. The loss of Cyp3a causes fewer changes than CAR. Nevertheless, there are compensatory changes including gender-specific increases in Cyp2a and Cyp2b. Cyp2a and Cyp2b were down-regulated in CAR-null mice, suggesting activation of CAR and potentially PXR following loss of the Cyp3a members. However, the loss of Cyp2b causes few changes in hepatic CYP transcript levels and almost no significant compensatory changes in protein expression or activity with the possible exception of 6α-hydroxylase activity. This lack of a compensatory response in the Cyp2b9/10/13-null mice is probably due to low CYP2B hepatic expression, especially in male mice. Overall, compensatory and regulatory CYP changes followed the order CAR-null > Cyp3a-null > Cyp2b-null mice.

Perturbations in polar lipids, starvation survival and reproduction following exposure to unsaturated fatty acids or environmental toxicants in Daphnia magna

Acclimating to toxicant stress is energy expensive. In laboratory toxicology tests dietary conditions are ideal, but not in natural environments where nutrient resources vary in quality and quantity. We compared the effects of additional lipid resources, docosahexaenoic acid (n-3; DHA) or linoleic acid (n-6; LA), or the effects of the toxicants, atrazine or triclosan on post-treatment starvation survival, reproduction, and lipid profiles. Chemical exposure prior to starvation had chemical-specific effects as DHA showed moderately beneficial effects on starvation survival and all of the other chemicals showed adverse effects on either survival or reproduction. Surprisingly, pre-exposure to triclosan inhibits adult maturation and in turn completely blocks reproduction during the starvation phase. The two HR96 activators tested, atrazine and LA adversely reduce post-reproduction survival 70% during starvation and in turn show poor fecundity. DHA and LA show distinctly different lipid profiles as DHA primarily increases the percentage of large (>37 carbon) phosphatidylcholine (PC) species and LA primarily increases the percentage of smaller (<37 carbon) PC species. The toxicants atrazine and triclosan moderately perturb a large number of different phospholipids including several phosphatidylethanolamine species. Some of these polar lipid species may be biomarkers for diets rich in specific fatty acids or toxicant classes. Overall our data demonstrates that toxicants can perturb lipid utilization and storage in daphnids in a chemical specific manner, and different chemicals can produce distinct polar lipid profiles. In summary, biological effects caused by fatty acids and toxicants are associated with changes in the production and use of lipids.

The HR96 activator, atrazine, reduces sensitivity of D. magna to triclosan and DHA

HR96 is a CAR/PXR/VDR ortholog in invertebrates, and a promiscuous endo- and xenobiotic nuclear receptor involved in acclimation to toxicants. Daphnia HR96 is activated by chemicals such as atrazine and linoleic acid (LA) (n-6 fatty acid), and inhibited by triclosan and docosahexaenoic acid (DHA) (n-3 fatty acid). We hypothesized that inhibitors of HR96 may block the protective responses of HR96 based on previously performed luciferase assays. Therefore, we performed acute toxicity tests with two-chemical mixtures containing a HR96 inhibitor (DHA or triclosan) and a HR96 activator (LA or atrazine). Surprisingly, results demonstrate that triclosan and DHA are less toxic when co-treated with 20-80 μM atrazine. Atrazine provides concentration-dependent protection as lower concentrations have no effect and higher concentrations cause toxicity. LA, a weaker HR96 activator, did not provide protection from triclosan or DHA. Atrazine's protective effects are presumably due to its ability to activate HR96 or other toxicologically relevant transcription factors and induce protective enzymes. Atrazine did not significantly induce glucosyltransferase, a crucial enzyme in triclosan detoxification. However, atrazine did increase antioxidant activities, crucial pathways in triclosan's toxicity, as measured through GST activity and the TROLOX equivalence assay. The increase in antioxidant capacity is consistent with atrazine providing protection from a wide range of toxicants that induce ROS, including triclosan and unsaturated fatty acids predisposed to lipid peroxidation.

The HR96 activator, atrazine, reduces sensitivity of D. magna to triclosan and DHA

HR96 is a CAR/PXR/VDR ortholog in invertebrates, and a promiscuous endo- and xenobiotic nuclear receptor involved in acclimation to toxicants. Daphnia HR96 is activated by chemicals such as atrazine and linoleic acid (LA) (n-6 fatty acid), and inhibited by triclosan and docosahexaenoic acid (DHA) (n-3 fatty acid). We hypothesized that inhibitors of HR96 may block the protective responses of HR96 based on previously performed luciferase assays. Therefore, we performed acute toxicity tests with two-chemical mixtures containing a HR96 inhibitor (DHA or triclosan) and a HR96 activator (LA or atrazine). Surprisingly, results demonstrate that triclosan and DHA are less toxic when co-treated with 20-80 μM atrazine. Atrazine provides concentration-dependent protection as lower concentrations have no effect and higher concentrations cause toxicity. LA, a weaker HR96 activator, did not provide protection from triclosan or DHA. Atrazine's protective effects are presumably due to its ability to activate HR96 or other toxicologically relevant transcription factors and induce protective enzymes. Atrazine did not significantly induce glucosyltransferase, a crucial enzyme in triclosan detoxification. However, atrazine did increase antioxidant activities, crucial pathways in triclosan's toxicity, as measured through GST activity and the TROLOX equivalence assay. The increase in antioxidant capacity is consistent with atrazine providing protection from a wide range of toxicants that induce ROS, including triclosan and unsaturated fatty acids predisposed to lipid peroxidation.

Arachidonic acid enhances reproduction in Daphnia magna and mitigates changes in sex ratios induced by pyriproxyfen

Arachidonic acid is 1 of only 2 unsaturated fatty acids retained in the ovaries of crustaceans and an inhibitor of HR97g, a nuclear receptor expressed in adult ovaries. The authors hypothesized that, as a key fatty acid, arachidonic acid may be associated with reproduction and potentially environmental sex determination in Daphnia. Reproduction assays with arachidonic acid indicate that it alters female:male sex ratios by increasing female production. This reproductive effect only occurred during a restricted Pseudokirchneriella subcapitata diet. Next, the authors tested whether enriching a poorer algal diet (Chlorella vulgaris) with arachidonic acid enhances overall reproduction and sex ratios. Arachidonic acid enrichment of a C. vulgaris diet also enhances fecundity at 1.0 µM and 4.0 µM by 30% to 40% in the presence and absence of pyriproxyfen. This indicates that arachidonic acid is crucial in reproduction regardless of environmental sex determination. Furthermore, the data indicate that P. subcapitata may provide a threshold concentration of arachidonic acid needed for reproduction. Diet-switch experiments from P. subcapitata to C. vulgaris mitigate some, but not all, of arachidonic acid's effects when compared with a C. vulgaris-only diet, suggesting that some arachidonic acid provided by P. subcapitata is retained. In summary, arachidonic acid supplementation increases reproduction and represses pyriproxyfen-induced environmental sex determination in D. magna in restricted diets. A diet rich in arachidonic acid may provide protection from some reproductive toxicants such as the juvenile hormone agonist pyriproxyfen. Environ Toxicol Chem 2015;34:527-535. © 2014 SETAC.

Annotation of the Daphnia magna nuclear receptors: comparison to Daphnia pulex

Most nuclear receptors (NRs) are ligand-dependent transcription factors crucial in homeostatic physiological responses or environmental responses. We annotated the Daphnia magna NRs and compared them to Daphnia pulex and other species, primarily through phylogenetic analysis. Daphnia species contain 26 NRs spanning all seven gene subfamilies. Thirteen of the 26 receptors found in Daphnia species phylogenetically segregate into the NR1 subfamily, primarily involved in energy metabolism and resource allocation. Some of the Daphnia NRs, such as RXR, HR96, and E75 show strong conservation between D. magna and D. pulex. Other receptors, such as EcRb, THRL-11 and RARL-10 have diverged considerably and therefore may show different functions in the two species. Curiously, there is an inverse association between the number of NR splice variants and conservation of the LBD. Overall, D. pulex and D. magna possess the same NRs; however not all of the NRs demonstrate high conservation indicating the potential for a divergence of function.

The HR97 (NR1L) group of nuclear receptors: a new group of nuclear receptors discovered in Daphnia species

The recently sequenced Daphnia pulex genome revealed the NR1L nuclear receptor group consisting of three novel receptors. Phylogenetic studies show that this group is related to the NR1I group (CAR/PXR/VDR) and the NR1J group (HR96), and were subsequently named HR97a/b/g. Each of the HR97 paralogs from Daphnia magna, a commonly used crustacean in toxicity testing, was cloned, sequenced, and partially characterized. Phylogenetic analysis indicates that the HR97 receptors are present in primitive arthropods such as the chelicerates but lost in insects. qPCR and immunohistochemistry demonstrate that each of the receptors is expressed near or at reproductive maturity, and that HR97g, the most ancient of the HR97 receptors, is primarily expressed in the gastrointestinal tract, mandibular region, and ovaries, consistent with a role in reproduction. Transactivation assays using an HR97a/b/g-GAL4 chimera indicate that unlike Daphnia HR96 that is promiscuous with respect to ligand recognition, the HR97 receptors do not respond to many of the ligands that activate CAR/PXR/HR96 nuclear receptors. Only three putative ligands of HR97 receptors were identified in this study: pyriproxyfen, methyl farnesoate, and arachidonic acid. Only arachidonic acid, which acts as an inverse agonist, alters HR97g activity at concentrations that would be considered within physiologically relevant ranges. Overall, this study demonstrates that, although closely related to the promiscuous receptors in the NR1I and NR1J groups, the HR97 receptors are mostly likely not multi-xenobiotic sensors, but rather may perform physiological functions, potentially in reproduction, unique to crustaceans and other non-insect arthropod groups.

The time- and age-dependent effects of the juvenile hormone analog pesticide, pyriproxyfen on Daphnia magna reproduction

Pyriproxyfen is an insecticidal juvenile hormone analog that perturbs insect and tick development. Pyriproxyfen also alters parthenogenic reproduction in non-target cladoceran species as it induces male production that can lead to a decrease in fecundity, a reduction in population density, and subsequent ecological effects. In this study, we investigate the impacts of pyriproxyfen on Daphnia magna reproduction using a series of male production screening assays. These assays demonstrate that pyriproxyfen increases male production in a concentration-dependent fashion with an EC50 of 156pM (50.24ngL(-1)); a concentration considered environmentally relevant. Furthermore, pyriproxyfen decreases overall fecundity at all ages tested (7, 14, 21-d old female parthenogenic daphnids). Juvenile (3-d old) and reproductively mature (10-d old) female daphnids were also exposed to 155pM pyriproxyfen for 2-12d and reproduction measured for 16d to compare the effects of short-term and prolonged exposures, and determine the potential for recovery. Results indicate that longer pyriproxyfen exposures (8-12d) extend male production and decrease reproduction; however, daphnids exposed for only 2-4d recover and produce a relatively normal abundance of neonates. In addition, juvenile daphnids are also very sensitive to pyriproxyfen, but the primary effect on juvenile daphnids is reduced reproduction and protracted development not male production. Taken together, continued use of pyriproxyfen around water bodies needs due caution because of its potential adverse effects with significant developmental delays and male production compounded by prolonged exposure.

Daphnia HR96 is a promiscuous xenobiotic and endobiotic nuclear receptor

Daphnia pulex is the first crustacean to have its genome sequenced. The genome project provides new insight and data into how an aquatic crustacean may respond to environmental stressors, including toxicants. We cloned Daphnia pulex HR96 (DappuHR96), a nuclear receptor orthologous to the CAR/PXR/VDR group of nuclear receptors. In Drosophila melanogaster, (hormone receptor 96) HR96 responds to phenobarbital exposure and has been hypothesized as a toxicant receptor. Therefore, we set up a transactivation assay to test whether DappuHR96 is a promiscuous receptor activated by xenobiotics and endobiotics similar to the constitutive androstane receptor (CAR) and the pregnane X-receptor (PXR). Transactivation assays performed with a GAL4-HR96 chimera demonstrate that HR96 is a promiscuous toxicant receptor activated by a diverse set of chemicals such as pesticides, hormones, and fatty acids. Several environmental toxicants activate HR96 including estradiol, pyriproxyfen, chlorpyrifos, atrazine, and methane arsonate. We also observed repression of HR96 activity by chemicals such as triclosan, androstanol, and fluoxetine. Nearly 50% of the chemicals tested activated or inhibited HR96. Interestingly, unsaturated fatty acids were common activators or inhibitors of HR96 activity, indicating a link between diet and toxicant response. The omega-6 and omega-9 unsaturated fatty acids linoleic and oleic acid activated HR96, but the omega-3 unsaturated fatty acids alpha-linolenic acid and docosahexaenoic acid inhibited HR96, suggesting that these two distinct sets of lipids perform opposing roles in Daphnia physiology. This also provides a putative mechanism by which the ratio of dietary unsaturated fats may affect the ability of an organism to respond to a toxic insult. In summary, HR96 is a promiscuous nuclear receptor activated by numerous endo- and xenobiotics.

Lentiviral-mediated RNAi knockdown yields a novel mouse model for studying Cyp2b function

There are few in vivo knockout models available to study the function of Cyp2 members involved in the metabolism of endogenous and exogenous chemicals. These models may help provide insight into the cytochrome P450s (CYPs) responsible for the detoxification and activation of drugs, environmental toxicants, and endobiotics. The aim of this work is to produce a potent Cyp2b-knockdown (KD) mouse for subsequent study of Cyp2b function. We made a quintuple Cyp2b-KD mouse using lentiviral-promoted short hairpin RNA (shRNA) homologous to all five murine Cyp2b subfamily members (Cyp2b9, 2b10, 2b13, 2b19, and 2b23). The Cyp2b-KD mice are viable, fertile, and without obvious gross abnormalities except for an increase in liver weight. Expression of the three hepatic Cyp2b members, 2b9, 2b10, and 2b13, is significantly repressed as demonstrated by quantitative real-time PCR and Western blotting. The constitutive androstane receptor activator, 1,4-Bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP), was used to determine if shRNA-mediated Cyp2b10 repression could be outcompeted by Cyp2b10 induction. TCPOBOP-treated Cyp2b-KD mice show 80-90% less Cyp2b protein expression than TCPOBOP-treated wild-type (WT) mice, demonstrating that Cyp induction does not outcompete the repressive function of the shRNA. Untreated and TCPOBOP-treated Cyp2b-KD mice are poor metabolizers of parathion compared with WT mice. Furthermore, Cyp2b-KD mice are sensitive to parathion, an organophosphate insecticide primarily metabolized by Cyp2b enzymes, when compared with WT mice. In summary, we designed an shRNA construct that repressed the expression and activity of multiple Cyp2b enzymes. We foresee that this novel Cyp2b-KD mouse model will significantly improve our understanding of the role of Cyp2b enzymes in chemical sensitivity and drug metabolism.

Selective imaging and killing of cancer cells with protein-activated near-infrared fluorescing nanoparticles

We present a general approach for the selective imaging and killing of cancer cells using protein-activated near-infrared emitting and cytotoxic oxygen generating nanoparticles. Poly(propargyl acrylate) (PA) particles were surface modified through the copper-catalyzed azide/alkyne cycloaddition of azide-terminated indocyanine green (azICG), a near-infrared emitter, and poly(ethylene glycol) (azPEG) chains of various molecular weights. The placement of azICG onto the surface of the particles allowed for the chromophores to complex with bovine serum albumin when dispersed in PBS that resulted in an enhancement of the dye emission. In addition, the inclusion of azPEG with the chromophores onto the particle surface resulted in a synergistic ninefold enhancement of the fluorescence intensity, with azPEGs of increasing molecular weight amplifying the response. Human liver carcinoma cells (HepG2) overexpress albumin proteins and could be employed to activate the fluorescence of the nanoparticles. Preliminary PDT studies with HepG2 cells combined with the modified particles indicated that a minor exposure of 780 nm radiation resulted in a statistically significant reduction in cell growth.

Dose-responsive gene expression changes in juvenile and adult mummichogs (Fundulus heteroclitus) after arsenic exposure

The present study investigated arsenic's effects on mummichogs (Fundulus heteroclitus), while also examining what role that gender or exposure age might play. Adult male and female mummichogs were exposed to 172 ppb, 575 ppb, or 1720 ppb arsenic as sodium arsenite for 10 days immediately prior to spawning. No differences were noted in the number or viability of eggs between the groups, but there was a significant increase in deformities in 1720 ppb arsenic exposure group. Total RNA from adult livers or 6-week old juveniles was used to probe custom macroarrays for changes in gene expression. In females, 3% of the genes were commonly differentially expressed in the 172 and 575 ppb exposure groups compared to controls. In the males, between 1.1 and 3% of the differentially expressed genes were in common between the exposure groups. Several genes, including apolipoprotein and serum amyloid precursor were commonly expressed in either a dose-responsive manner or were dose-specific, but consistent across genders. These patterns of regulation were confirmed by QPCR. These findings will provide us with a better understanding of the effects of dose, gender, and exposure age on the response to arsenic.

Constitutive androstane receptor -null mice are sensitive to the toxic effects of parathion: association with reduced cytochrome p450-mediated parathion metabolism [corrected]

Constitutive androstane receptor [corrected](CAR) is activated by several chemicals and in turn regulates multiple detoxification genes. Our research demonstrates that parathion is one of the most potent, environmentally relevant CAR activators with an EC(50) of 1.43 microM. Therefore, animal studies were conducted to determine whether CAR was activated by parathion in vivo. Surprisingly, CAR-null mice, but not wild-type (WT) mice, showed significant parathion-induced toxicity. However, parathion did not induce Cyp2b expression, suggesting that parathion is not a CAR activator in vivo, presumably because of its short half-life. CAR expression is also associated with the expression of several drug-metabolizing cytochromes P450 (P450). CAR-null mice demonstrate lower expression of Cyp2b9, Cyp2b10, Cyp2c29, and Cyp3a11 primarily, but not exclusively in males. Therefore, we incubated microsomes from untreated WT and CAR-null mice with parathion in the presence of esterase inhibitors to determine whether CAR-null mice show perturbed P450-mediated parathion metabolism compared with that in WT mice. The metabolism of parathion to paraoxon and p-nitrophenol (PNP) was reduced in CAR-null mice with male CAR-null mice showing reduced production of both paraoxon and PNP, and female CAR-null mice showing reduced production of only PNP. Overall, the data indicate that CAR-null mice metabolize parathion slower than WT mice. These results provide a potential mechanism for increased sensitivity of individuals with lower CAR activity such as newborns to parathion and potentially other chemicals due to decreased metabolic capacity.

Interactions of the crustacean nuclear receptors HR3 and E75 in the regulation of gene transcription

Endocrine signal transduction occurs through cascades that involve the action of both ligand-dependent and ligand-independent nuclear receptors. In insects, two such nuclear receptors are HR3 and E75 that interact to transduce signals initiated by ecdysteroids. We have cloned these nuclear receptors from the crustacean Daphnia pulex to assess their function as regulators of gene transcription in this ecologically and economically important group of organisms. Both nuclear receptors from D. pulex (DappuHR3 (group NR1F) and DappuE75 (group NR1D)) exhibit a high degree of sequence similarity to other NR1F and NR1D group members that is indicative of monomeric binding to the RORE (retinoid orphan receptor element). DappuE75 possesses key amino acid residues required for heme binding to the ligand-binding domain. Next, we developed a gene transcription reporter assay containing a luciferase reporter gene driven by the RORE. DappuHR3, but not DappuE75, activated transcription of the luciferase gene in this system. Co-transfection experiments revealed that DappuE75 suppressed DappuHR3-dependent luciferase transcription in a dose-dependent manner. Electrophoretic mobility shift assays confirmed that DappuHR3 bound to the RORE. However, we found no evidence that DappuE75 similarly bound to the response element. These experiments further demonstrated that DappuE75 prevented DappuHR3 from binding to the response element. In conclusion, DappuHR3 functions as a transcriptional activator of genes regulated by the RORE and DappuE75 is a negative regulator of this activity. DappuE75 does not suppress the action of DappuHR3 by occupying the response element but presumably interacts directly with the DappuHR3 protein. Taken together with the previous demonstration that daphnid HR3 is highly induced by 20-hydroxyecdysone, these results support the premise that HR3 is a major component of ecdysteroid signaling in some crustaceans and is under the negative regulatory control of E75.

Integrating omic technologies into aquatic ecological risk assessment and environmental monitoring: hurdles, achievements, and future outlook

BACKGROUND

In this commentary we present the findings from an international consortium on fish toxicogenomics sponsored by the U.K. Natural Environment Research Council (Fish Toxicogenomics-Moving into Regulation and Monitoring, held 21-23 April 2008 at the Pacific Environmental Science Centre, Vancouver, BC, Canada).

OBJECTIVES

The consortium from government agencies, academia, and industry addressed three topics: progress in ecotoxicogenomics, regulatory perspectives on roadblocks for practical implementation of toxicogenomics into risk assessment, and dealing with variability in data sets.

DISCUSSION

Participants noted that examples of successful application of omic technologies have been identified, but critical studies are needed to relate molecular changes to ecological adverse outcome. Participants made recommendations for the management of technical and biological variation. They also stressed the need for enhanced interdisciplinary training and communication as well as considerable investment into the generation and curation of appropriate reference omic data.

CONCLUSIONS

The participants concluded that, although there are hurdles to pass on the road to regulatory acceptance, omics technologies are already useful for elucidating modes of action of toxicants and can contribute to the risk assessment process as part of a weight-of-evidence approach.

Annotation, phylogenetics, and expression of the nuclear receptors in Daphnia pulex

BACKGROUND

The nuclear receptor superfamily currently consists of seven gene subfamilies that encompass over 80 distinct receptor proteins. These transcription factors typically share a common five-domain structure with a highly conserved DNA-binding domain. Some nuclear receptors are ubiquitous among the metazoans, while others are unique to specific phylogenetic groups. Crustaceans represent the second largest group of arthropods with insects being the largest. However, relative to insects, little is known about the nuclear receptors of crustaceans. The aim of this study was to identify putative nuclear receptors from the first assembled genome of a crustacean Daphnia pulex http://wFleaBase.org. Nuclear receptor expression was evaluated and receptors were subjected to phylogenetic analyses to gain insight into evolution and function.

RESULTS

Twenty-five putative nuclear receptors were identified in D. pulex based on the presence of a conserved DNA-binding domain. All of the nuclear receptor protein sequences contain a highly homologous DNA-binding domain and a less conserved ligand-binding domain with the exception of the NR0A group. These receptors lack a ligand-binding domain. Phylogenetic analysis revealed the presence of all seven receptor subfamilies. The D. pulex genome contains several nuclear receptors that have vertebrate orthologs. However, several nuclear receptor members that are represented in vertebrates are absent from D. pulex. Notable absences include receptors of the 1C group (peroxisome proliferators-activated receptors), the 3A group (estrogen receptor), and the 3C group (androgen, progestogen, mineralcorticoid, and glucocorticoid receptors). The D. pulex genome also contains nuclear receptor orthologs that are present in insects and nematodes but not vertebrates, including putative nuclear receptors within the NR0A group. A novel group of receptors, designated HR97, was identified in D. pulex that groups with the HR96/CeNHR8/48/DAF12 clade, but forms its own sub-clade. Gene products were detected in adult female D. pulex for 21 of the 25 receptors.

CONCLUSION

Nuclear receptors are ancient proteins with highly conserved DNA-binding domains. The DNA-binding domains of the nuclear receptors of D. pulex contain the same degree of conservation that is typically found within nuclear receptors of other species. Most of the receptors identified in D. pulex have orthologs within the vertebrate and invertebrate lineages examined with the exception of the novel HR97 group and the Dappu-HR10 and potentially the Dappu-HR11 receptors found in D. pulex. These groups of receptors may harbour functions that are intrinsic to crustacean physiology.

The cytochrome P450 (CYP) gene superfamily in Daphnia pulex

BACKGROUND

Cytochrome P450s (CYPs) in animals fall into two categories: those that synthesize or metabolize endogenous molecules and those that interact with exogenous chemicals from the diet or the environment. The latter form a critical component of detoxification systems.

RESULTS

Data mining and manual curation of the Daphnia pulex genome identified 75 functional CYP genes, and three CYP pseudogenes. These CYPs belong to 4 clans, 13 families, and 19 subfamilies. The CYP 2, 3, 4, and mitochondrial clans are the same four clans found in other sequenced protostome genomes. Comparison of the CYPs from D. pulex to the CYPs from insects, vertebrates and sea anemone (Nematostella vectensis) show that the CYP2 clan, and to a lesser degree, the CYP4 clan has expanded in Daphnia pulex, whereas the CYP3 clan has expanded in insects. However, the expansion of the Daphnia CYP2 clan is not as great as the expansion observed in deuterostomes and the nematode C. elegans. Mapping of CYP tandem repeat regions demonstrated the unusual expansion of the CYP370 family of the CYP2 clan. The CYP370s are similar to the CYP15s and CYP303s that occur as solo genes in insects, but the CYP370s constitute approximately 20% of all the CYP genes in Daphnia pulex. Lastly, our phylogenetic comparisons provide new insights into the potential origins of otherwise mysterious CYPs such as CYP46 and CYP19 (aromatase).

CONCLUSION

Overall, the cladoceran, D. pulex has a wide range of CYPs with the same clans as insects and nematodes, but with distinct changes in the size and composition of each clan.

A concentration addition model for the activation of the constitutive androstane receptor by xenobiotic mixtures

The effects of contaminants are typically studied in individual exposures; however, environmental exposures are rarely from a single contaminant. Therefore, the study of chemical mixtures is important in determining the effects of xenobiotics. The constitutive androstane receptor (CAR) responds to endobiotics and xenobiotics, and in turn induces detoxification enzymes involved in their elimination. First, we compared several androgens as inverse agonists, including androgens allegedly used by Bay Area Laboratory Co-operative to enhance athletic performance. CAR inverse agonists ranked in order of potency were dihydroandrosterone (DHA) > tetrahydrogestrinone (THG) > androstanol > norbolethone. Therefore, we used DHA as an inverse agonist during transactivation assays. Next, we examined the effects of several pesticides, plasticizers, steroids, and bile acids on CAR activation. Our data demonstrates that several pesticides and plasticizers, including diethylhexylphthalate, nonylphenol, cypermethrin, and chlorpyrifos activate CAR. Both full and partial CAR activators were discovered, and EC(50) values and Hillslopes were determined for use in the concentration addition models. Concentration addition models with and without restraint values to account for partial activators were developed. Measured results from transactivation assays with a mixture of two to five chemicals indicate that the concentration addition model without restraints correctly predicts activity unless all of the chemicals in the mixture are partial activators, and then restraint values be considered. Overall, our data indicates that it is important to consider that we are exposed to a milieu of chemicals, and the efficacy of each individual chemical is not the sole factor in determining CAR's activity in mixture modeling.

Fundulus as the premier teleost model in environmental biology: opportunities for new insights using genomics

A strong foundation of basic and applied research documents that the estuarine fish Fundulus heteroclitus and related species are unique laboratory and field models for understanding how individuals and populations interact with their environment. In this paper we summarize an extensive body of work examining the adaptive responses of Fundulus species to environmental conditions, and describe how this research has contributed importantly to our understanding of physiology, gene regulation, toxicology, and ecological and evolutionary genetics of teleosts and other vertebrates. These explorations have reached a critical juncture at which advancement is hindered by the lack of genomic resources for these species. We suggest that a more complete genomics toolbox for F. heteroclitus and related species will permit researchers to exploit the power of this model organism to rapidly advance our understanding of fundamental biological and pathological mechanisms among vertebrates, as well as ecological strategies and evolutionary processes common to all living organisms.

COX-2 overexpression as a biomarker of early cervical carcinogenesis: a pilot study

BACKGROUND

Recent studies have demonstrated that cyclooxygenase-2 (COX-2) expression is up-regulated in a number of cancers. Selective inhibition of COX-2 offers a potential pharmacological strategy for cancer prevention. The COX-2 isoform is induced in response to inflammatory factors and is expressed in premalignant lesions, including cervical tissues. Few studies have investigated COX-2 expression as a biomarker for early cervical carcinogenesis. In this preliminary study, we assessed the variability of COX-2 overexpression in cervical premalignant lesions.

METHODS

Fifty-two patients were recruited and consented. Paired abnormal and control (normal) cervical biopsies were obtained and evaluated for high-risk human papillomavirus (HPV), inflammation, histopathological diagnosis, and COX-2 protein concentration by ELISA. Paired Student's t-test and general linear regression models were used to compare mean COX-2 protein concentrations among biopsy samples and selected risk variables.

RESULTS

Forty-seven of fifty-two paired biopsies were evaluated. COX-2 protein concentrations were 4.9-fold greater in abnormal biopsies (CIN 1 and CIN 2) than normal biopsies. COX-2 was also significantly increased in inflammation-positive biopsies. No significant association was found between COX-2 levels and HPV high-risk positivity, age, parity, STI history, or hormonal contraceptive use, but the sample size was small.

CONCLUSIONS

These results suggest that COX-2 induction begins in the premalignant phase of cervical carcinogenesis and is correlated with inflammation. A trial using a much larger number of specimens will allow further development of our understanding of COX-2 as a biomarker for use in chemoprevention trials.

The environmental estrogen, nonylphenol, activates the constitutive androstane receptor

Nonylphenol (NP) and its parent compounds, the nonylphenol ethoxylates are some of the most prevalent chemicals found in U.S. waterways. NP is also resistant to biodegradation and is a known environmental estrogen, which makes NP a chemical of concern. Our data show that NP also activates the constitutive androstane receptor (CAR), an orphan nuclear receptor important in the induction of detoxification enzymes, including the P450s. Transactivation assays demonstrate that NP increases murine CAR (mCAR) transcriptional activity, and NP treatment can overcome the inhibitory effects of the inverse agonist, androstanol, on mCAR activation. Treatment of wild-type (CAR +/+) mice with NP at 50 or 75 mg/kg/day increases Cyp2b protein expression in a dose-dependent manner as demonstrated by Western blotting, and was confirmed by quantitative reverse transcription-PCR of Cyp2b10 transcript levels. CAR-null (CAR -/-) mice show no increased expression of Cyp2b following NP treatment, indicating that CAR is required for NP-mediated Cyp2b induction. In addition, NP increases the translocation of CAR into the nucleus, which is the key step in the commencement of CAR's transcriptional activity. NP also induced CYP2B6 in primary human hepatocytes, and increased Cyp2b10 messenger RNA and protein expression in humanized CAR mice, indicating that NP is an activator of human CAR as well. In conclusion, NP is a CAR activator, and this was demonstrated in vitro with transactivation assays and in vivo with transgenic CAR mouse models.

The environmental estrogen, nonylphenol, activates the constitutive androstane receptor

Nonylphenol (NP) and its parent compounds, the nonylphenol ethoxylates are some of the most prevalent chemicals found in U.S. waterways. NP is also resistant to biodegradation and is a known environmental estrogen, which makes NP a chemical of concern. Our data show that NP also activates the constitutive androstane receptor (CAR), an orphan nuclear receptor important in the induction of detoxification enzymes, including the P450s. Transactivation assays demonstrate that NP increases murine CAR (mCAR) transcriptional activity, and NP treatment can overcome the inhibitory effects of the inverse agonist, androstanol, on mCAR activation. Treatment of wild-type (CAR +/+) mice with NP at 50 or 75 mg/kg/day increases Cyp2b protein expression in a dose-dependent manner as demonstrated by Western blotting, and was confirmed by quantitative reverse transcription-PCR of Cyp2b10 transcript levels. CAR-null (CAR -/-) mice show no increased expression of Cyp2b following NP treatment, indicating that CAR is required for NP-mediated Cyp2b induction. In addition, NP increases the translocation of CAR into the nucleus, which is the key step in the commencement of CAR's transcriptional activity. NP also induced CYP2B6 in primary human hepatocytes, and increased Cyp2b10 messenger RNA and protein expression in humanized CAR mice, indicating that NP is an activator of human CAR as well. In conclusion, NP is a CAR activator, and this was demonstrated in vitro with transactivation assays and in vivo with transgenic CAR mouse models.

Using mummichog (Fundulus heteroclitus) arrays to monitor the effectiveness of remediation at a superfund site in Charleston, South Carolina, U.S.A

We previously developed a cDNA array for mummichogs (Fundulus heteroclitus), an estuarine minnow, that is targeted for identifying differentially expressed genes from exposure to polycyclic aromatic hydrocarbons and several metals, including chromium. A chromium-contaminated Superfund site at Shipyard Creek in Charleston, South Carolina, USA, is undergoing remediation, providing us a unique opportunity to study the utility of arrays for monitoring the effectiveness of site remediation. Mummichogs were captured in Shipyard Creek in Charleston prior to remediation (2000) and after remediation began (2003 and 2005). Simultaneously, mummichogs were collected from a reference site at the Winyah Bay National Estuarine Research Reserve (NERR) in Georgetown, South Carolina, USA. The hepatic gene expression pattern of fish captured at Shipyard Creek in 2000 showed wide differences from the fish captured at NERR in 2000. Interestingly, as remediation progressed the gene expression pattern of mummichogs captured at Shipyard Creek became increasingly similar to those captured at NERR. The arrays acted as multidimensional biomarkers as the number of differentially expressed genes dropped from 22 in 2000 to four in 2003, and the magnitude of differential expression dropped from 3.2-fold in 2000 to no gene demonstrating a difference over 1.5-fold in 2003. Furthermore, the arrays indicated changes in the bioavailability of chromium caused by hydraulic dredging in the summer of 2005. This research is, to our knowledge, the first report using arrays as biomarkers for a weight-of-evidence hazard assessment and demonstrates that arrays can be used as multidimensional biomarkers to monitor site mitigation because the gene expression profile is associated with chromium bioavailability and body burden.

Hexavalent chromium reduces larval growth and alters gene expression in mummichog (Fundulus heteroclitus)

Hexavalent chromium [Cr(VI)] is a common bioavailable metal ion that causes oxidative stress, DNA adducts, and perturbs gene expression. Changes in gene expression are useful biomarkers of toxicant exposure that provide information about an organism's health, adaptability, and toxicant-specific effects. Therefore, we developed a cDNA array for the estuarine sentinel species mummichog (Fundulus heteroclitus). Mummichog larvae were exposed to concentrations ranging from 0 to 24 mg/L (462 microM) of Cr(VI) for 30 d, and growth was measured to determine the no-observable-effect concentration (1.5 mg/L) and the lowest-observable-effect concentration (3 mg/L). Body burdens from Cr(VI)-exposed fish showed a dose-dependent increase and were inversely correlated to body weight. Mummichog larvae exposed to Cr(VI) differentially expressed 16 genes in a dose-dependent manner, including GLUT-2, L-FABP, ATPase synthase 8, type II keratin, TBT-binding protein, and complement component C3-2. Many of these genes are involved in energy metabolism or growth, which is consistent with the reduced growth observed. In subsequent experiments, adults were exposed to Cr(VI) for 7 d at 0, 1.5, or 3 mg/L, because adult mummichog are used in monitoring Superfund sites. Hexavalent chromium altered the expression of 10 genes in adult liver, including HGFA, H-FABP, and complement component C3-2. Many of these genes also are involved in energy metabolism. The mummichog arrays provide a potential mechanism for the effects of Cr(VI) on growth. We anticipate using these arrays and the data they provide to monitor effects at polluted sites, to assess the bioavailability of chromium at these sites, and to investigate the efficacy of remediation in chromium-polluted estuaries.

Alterations in hepatic gene expression by trivalent chromium in Fundulus heteroclitus

Cr(III) is the dominant toxicant at some Superfund sites within the United States and therefore we are interested in its effects. Cr(III)s mechanisms are not well studied or understood because of its low bioavailability. We have attempted to characterize the effects of Cr(III) on gene expression in Fundulus heteroclitus (mummichog) liver. The NOEC and LOEC were determined at 32 and 64 mg/L, respectively, by measuring growth and mortality after exposing juveniles for 30 days. Secondary adult male exposures were performed at 32 mg/L, livers excised, and RNA extracted. Arrays were probed with cDNA from untreated or Cr(III)-exposed adult fish and gene expression was quantified. Cr(III) at 32 mg/L altered the expression of five genes, including GSTtau, GSTalpha, and ALDH4. Ultimately, we anticipate using this gene expression information to ascertain whether Cr(III) is bioavailable at potentially adverse concentrations in contaminated sites.

Gender-specific induction of cytochrome P450s in nonylphenol-treated FVB/NJ mice

Nonylphenol (NP) is a breakdown product of nonylphenol ethoxylates, which are used in a variety of industrial, agricultural, household cleaning, and beauty products. NP is one of the most commonly found toxicants in the United States and Europe and is considered a toxicant of concern because of its long half-life. NP is an environmental estrogen that also activates the pregnane X-receptor (PXR) and in turn induces P450s. No study to date has examined the gender-specific effects of NP on hepatic P450 expression. We provided NP at 0, 50 or 75 mg/kg/day for 7 days to male and female FVB/NJ mice and compared their P450 expression profiles. Q-PCR was performed on hepatic cDNA using primers to several CYP isoforms regulated by PXR or its relative, the constitutive androstane receptor (CAR). In female mice, NP induced Cyp2b10 and Cyp2b13, and downregulated the female-specific P450s, Cyp3a41 and Cyp3a44. In contrast, male mice treated with NP showed increased expression of Cyp2a4, Cyp2b9, and Cyp2b10. Western blots confirmed induction of Cyp2b subfamily members in both males and females. Consistent with the Q-PCR data, Western blots showed dose-dependent downregulation of Cyp3a only in females and induction of Cyp2a only in males. The overall increase in female-predominant P450s in males (Cyp2a4, 2b9) and the decrease in female-predominant P450s in females (Cyp3a41, 3a44) suggest that NP is in part feminizing the P450 profile in males and masculinizing the P450 profile in females. Testosterone hydroxylation was also altered in a gender-specific manner, as testosterone 16alpha-hydroxylase activity was only induced in NP-treated males. In contrast, NP-treated females demonstrated a greater propensity for metabolizing zoxazolamine probably due to greater Cyp2b induction in females. In conclusion, NP causes gender-specific P450 induction and therefore exposure to NP may cause distinct pharmacological and toxicological effects in males compared to females.

Physiological changes and differential gene expression in mummichogs (Fundulus heteroclitus) exposed to arsenic

Arsenic has been detected as a contaminant in water bodies around the world. Although a number of studies have shown toxicity to adult fish, little is known about its effects on the offspring. However, human epidemiological studies have shown that arsenic increases the number of stillbirths and prematurely born infants. We examined changes in the morphology and gene expression in juvenile mummichogs (Fundulus heteroclitus) whose parents were exposed to 230 ppb arsenic for 10 days immediately prior to spawning. The hatchlings of exposed fish had a 2.8-fold increased incidence of curved or stunted tails. Total RNA from 6-week-old hatchlings, reared in clean water, was used to construct a cDNA subtractive hybridization library. Using this library, we found 13 genes whose expression was altered in the hatchlings as a result of arsenic exposure. We confirmed differential expression by real-time PCR and found significant up-regulation of myosin light chain 2 (4.2-fold), type II keratin (1.5-fold), tropomyosin (3.1-fold) and parvalbumin (3.5-fold) in the hatchlings whose parents were exposed to arsenic. These genes are important during embryogenesis and their differential expression may be linked to the morphological changes observed in the hatchlings.

CAR and PXR: xenosensors of endocrine disrupters?

The pregnane X-receptor (PXR) and the constitutive androstane receptor (CAR) are orphan nuclear receptors activated by a variety of ligands. Currently it remains uncertain whether these receptors have a high-affinity ligand or instead function as more generalized steroid/xenobiotic sensors. Both receptors are important regulators of several steroid and xenobiotic detoxification enzymes and transporters (phases I-III) in the liver and intestine and thus are important regulators of adaptation to chemical stress. The detoxification proteins induced are responsible for the metabolism, deactivation and transport of bile acids, thyroid and steroid hormones, numerous environmental chemicals, and several drugs. PXR and CAR received their names because of steroid ligands that activate and inhibit their transcriptional activity, respectively. Interestingly, some steroids and steroid mimics activate one or both receptors, including several endocrine disrupting chemicals. Environmental estrogens, such as the pesticides methoxychlor, endosulfan, dieldrin, DDT, and the plasticizer nonylphenol activate either PXR or both PXR and CAR. Because PXR and CAR are activated by numerous steroids and endocrine disrupters, it appears that these receptors protect the integrity of the endocrine system. They recognize an increase in steroid-like chemicals and, in turn, induce detoxification. Furthermore, PXR and CAR induce enzymes, such as the CYP2B and CYP3A family members, responsible for the metabolism of steroid and thyroid hormones and this may alter their normal physiological function. This review summarizes the available data on the activity of endocrine disrupters and endocrine active chemicals on PXR and CAR, examines the role of PXR and CAR in protection from these chemicals, and evaluates potential adverse physiological consequences of PXR and CAR activation.

Effects of nonylphenol on hepatic testosterone metabolism and the expression of acute phase proteins in winter flounder (Pleuronectes americanus): comparison to the effects of Saint John's Wort

4-Nonylphenol (4-NP), a major by-product of alkylphenol ethoxylates, is used in several industries and as a consequence is quite common in rivers, estuaries and other aquatic environments that receive sewage discharges or are near offshore oil platforms. 4-NP is an environmental estrogen that also binds human and rodent Pregnane X-receptor (PXR), the orphan nuclear receptor that controls the expression of several detoxication genes in mammals, including several CYP3A and CYP2B family members. These P450s preferentially hydroxylate testosterone in the 6beta- and 16beta-positions, respectively. In this study, the effects of 4-NP on testosterone metabolism and hepatic CYP3A induction were compared to the effects of St. John's Wort (SJW), a well established mammalian PXR agonist, in winter flounder. Male winter flounder (Pleuronectes americanus) were injected with 100 mg/kg/day 4-NP or 500 mg/kg/day SJW or both (S and N) every 24 h. Forty-eight hours after the initial injections, flounder were euthanized. Western blots and testosterone 6beta-hydroxylation indicated that CYP3A was increased 50% by 4-NP, but was not affected by SJW. Testosterone 16beta-hydroxylase activity was also significantly increased in flounder treated with 4-NP (2.8 x), but not with SJW. This is not consistent with our hypothesis that both SJW and 4-NP would induce CYP3A. Subtractive hybridization was performed between control and 4-NP treated hepatic mRNA samples to isolate differentially expressed genes. Subtractive hybridization indicated that several acute phase proteins were altered by 4-NP. Quantitative real-time PCR (Q-PCR) confirmed 4-NP altered the expression of complement components C8b, cathepsin L, C-type lectin domain, FK506 binding protein 2 precursor (FKBP2) and an EST (expressed sequence tag). SJW and 4-NP treated flounder demonstrated similar induction profiles for the EST, cathepsin L and FKBP2, suggesting that SJW was at a sufficient dose to alter gene expression but not induce P450s. In conclusion, testosterone hydroxylase activity and Western blots indicate that SJW did not activate detoxication pathways in a similar manner to 4-NP.

The contribution of hepatic steroid metabolism to serum estradiol and estriol concentrations in nonylphenol treated MMTVneu mice and its potential effects on breast cancer incidence and latency

The two major pathways for the metabolism of estradiol-17beta (E2) are the 2- and 16-hydroxylase pathways. Research has suggested that the increased production of the estrogenically active 16-hydroxy products such as estriol (E3) may be involved in increased susceptibility to breast cancer. 4-Nonylphenol (4-NP) is an environmental estrogen that also can activate the pregnane-X receptor (PXR) and induce P-450 enzymes responsible for the production of E3. It is hypothesized that 4-NP may act in part as an environmental estrogen by increasing E3 production. Based on its affinity for the estrogen receptor (ER) alone, 4-NP may be more potent than predicted at increasing mammary cancer incidence in the MMTVneu mouse. Female mice were treated per os for 7 days at 0, 25, 50 or 75 mg kg(-1) day(-1) 4-NP to investigate the effects of 4-NP on hepatic estrogen metabolism after an acute treatment. 4-Nonylphenol increased the hepatic formation of E3 in a dose-dependent manner. However, serum E3 concentrations were only increased at 25 mg kg(-1) day(-1) presumably due to direct inhibition of E3 formation by 4-NP. MMTVneu mice were then treated for 32 weeks at 0, 30 or 45 mg kg(-1) day(-1) 4-NP to determine its effects on mammary cancer formation and estrogen metabolism. 4-Nonylphenol increased mammary cancer formation in the MMTVneu mice at 45 mg kg(-1) day(-1) but not at 30 mg kg(-1) day(-1). Mice treated with an equipotent dose of E2, 10 microg kg(-1) day(-1), based on the relative binding affinities of nonylphenol and estradiol for ER alpha, did not develop mammary cancer. This suggests that nonylphenol is more potent than predicted based on its affinity for the estrogen receptor. However, no changes in serum E3 concentrations or hepatic E3 production were measured after the chronic treatment. Changes in E3 formation were correlated with increased CYP2B levels after the 7 day 4-NP treatment, and repression of CYP2B and CYP3A after 32 weeks of 4-NP treatment. Microarray analysis and Q-PCR of liver mRNA from the mice treated for 32 weeks demonstrated a decrease in RXR alpha, the heterodimeric partner of the PXR, which may in part explain the repressed transcription of the P450s measured. In conclusion, 4-NP treatment for 32 weeks increased mammary cancer formation at a dose of 45 mg kg(-1) day(-1). However, chronic treatment with 4-NP did not increase hepatic E3 formation or serum E3 concentrations. The transient induction by 4-NP of hepatic E3 formation and serum concentrations is most likely not involved in the increased incidence of mammary cancer in MMTVneu mice since E3 serum concentrations were only increased at 25 mg kg(-1) day(-1), a dose that was not sufficient to induce mammary tumor formation. Nevertheless, the induced hepatic E3 production in the acute exposures to 4-NP was indicative of an increase in mammary cancer incidence after the chronic exposure.