Fathead Minnow Vitellogenin: Complementary Dna Sequence and Messenger Rna and Protein Expression After 17-Estradiol Treatment
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Environmental Toxicology and Chemistry, Vol. 19, No. 4, pp. 972±981, 2000 Printed in the USA 0730-7268/00 $9.00 1 .00 FATHEAD MINNOW VITELLOGENIN: COMPLEMENTARY DNA SEQUENCE AND MESSENGER RNA AND PROTEIN EXPRESSION AFTER 17b-ESTRADIOL TREATMENT JOSEPH J. KORTE,*² MICHAEL D. KAHL,² KATHLEEN M. JENSEN,² MUMTAZ S. PASHA,² LOUISE G. PARKS,³ GERALD A. LEBLANC,³ and GERALD T. A NKLEY² ²U.S. Environmental Protection Agency, Mid-Continent Ecology Division, 6201 Congdon Boulevard, Duluth, Minnesota 55804 ³North Carolina State University, Department of Toxicology, Box 7633, Raleigh, North Carolina 27695, USA (Received 9 April 1999; Accepted 9 August 1999) AbstractÐInduction of vitellogenin (VTG) in oviparous animals has been proposed as a sensitive indicator of environmental contaminants that activate the estrogen receptor. In the present study, a sensitive ribonuclease protection assay (RPA) for VTG messenger RNA (mRNA) was developed for the fathead minnow (Pimephales promelas), a species proposed for routine endocrine- disrupting chemical (EDC) screening. The utility of this method was compared with an enzyme-linked immunosorbent assay (ELISA) speci®c for fathead minnow VTG protein. Assessment of the two methods included kinetic characterization of the plasma VTG protein and hepatic VTG mRNA levels in male fathead minnows following intraperitoneal injections of 17b-estradiol (E2) at two dose levels (0.5, 5.0 mg/kg). Initial plasma E2 concentrations were elevated in a dose-dependent manner but returned to normal levels within 2 d. Liver VTG mRNA was detected within 4 h, reached a maximum around 48 h, and returned to normal levels in about 6 d. Plasma VTG protein was detectable within 16 h of treatment, reached maximum levels at about 72 h, and remained near these maximum levels for at least 18 d. While the RPA was about 1,000 times more sensitive than the ELISA, the ELISA appears superior for routine screening tests. The ELISA method is relatively simple to perform and, because males lack a clearance mechanism for VTG, the protein remains at relatively high concentrations in the plasma for an extended period of time. As part of the development of the RPA, the complementary DNA (cDNA) sequence for fathead minnow VTG was determined and the deduced amino acid sequence compared with VTG sequences for other ®sh species. KeywordsÐVitellogenin Pimephales promelas 17b-Estradiol mRNA cDNA INTRODUCTION for screening EDCs is international in scope, with Organization Concern for possible adverse effects of endocrine-disrupt- for Economic Cooperation and Development member coun- ing chemicals (EDCs) on humans and wildlife prompted the tries working on methods development and test harmonization U.S. Environmental Protection Agency to identify research on with this species [8]. this topic as one of six high-priority issues [1]. Coincident There are a number of existing assay protocols with the with this, the U.S. Environmental Protection Agency received fathead minnow, including partial- and full-life cycle tests [9± a legislated mandate to develop and implement a screening 12], that would capture adverse effects on estrogen, androgen, program for detecting potential EDCs [2,3]. To meet this man- and/or thyroid function. However, the endpoints typically as- date, a number of expert workshops were initiated to help sessed in these tests (e.g., growth rate, developmental anom- identify potential screening methods [4±6], and a multistake- alies, reproductive success) are also re¯ective of potential im- holder advisory group was formed to provide recommenda- pacts associated with a variety of other toxicity mechanisms. tions to the U.S. Environmental Protection Agency for devel- Hence, in developing fathead minnow screening methods spe- oping and implementing an EDC screening program. This ci®c for EDCs, there has been an emphasis on assessment of group, the Endocrine Disruptor Screening and Testing Advi- biological endpoints that are directly related to mechanisms sory Committee, developed a list of screening assays suitable of concern. For example, the protocol recommended for tier for tier 1 (initial) testing for chemicals that could cause toxicity 1 screening incorporates assessment of plasma vitellogenin through disruption of reproductive and developmental pro- (VTG) concentrations in test animals [5,7]. Under normal con- cesses controlled by estrogen, androgen, and thyroid hormones ditions, the production of VTG, a precursor to egg yolk protein, [7]. The tier 1 battery recommended by the Endocrine Dis- is induced in the liver of oviparous females by the interaction ruptor Screening and Testing Advisory Committee includes of estrogens with the estrogen receptor (ER) [13,14]. Males three mammalian tests, one amphibian assay (with Xenopus also possess the gene for VTG, but plasma concentrations of laevis), and a test with the fathead minnow (Pimephales pro- the protein typically remain small, presumably due to low melas). Although the most immediate application is in the levels of endogenous estrogens [15,16]. However, exposure of United States, interest in using the fathead minnow as a model males to exogenous estrogens/estrogen mimics can result in a marked induction of plasma VTG, which tends to remain el- * To whom correspondence may be addressed evated because of lack of mechanisms to clear the protein[17]. ([email protected]). This has been successfully exploited as a biomarker of ex- This manuscript has been reviewed in accordance with of®cial posure of ®sh to ER agonists in the environment [e.g., 18± U.S. Environmental Protection Agency (U.S. EPA) procedures; how- ever, the content does not re¯ect U.S. EPA policy. Mention of trade 23]. Further, a number of studies have demonstrated a corre- names does not imply endorsement by the U.S. EPA or the U.S. lation between VTG induction and adverse reproductive effects government. both in male and female ®sh [24±28]. 972 Expression of fathead minnow vitellogenin mRNA and protein Environ. Toxicol. Chem. 19, 2000 973 There are a number of techniques available for measuring compared with E2 standards. Extraction ef®ciency, as deter- VTG. An effective, but relatively nonspeci®c, measure of VTG mined by recovery of tritiated steroids extracted from plasma, can be made through assaying alkaline-labile protein-bound was 85 to 90% (data not shown). phosphorus in ®sh plasma [17,27,29]. More speci®c measure- ment techniques for VTG include radioimmunoassays and en- Plasma vitellogenin measurement zyme-linked immunosorbent assays (ELISA); these approach- Plasma VTG protein was measured using a competitive es have been used successfully for measuring fathead minnow ELISA with an antibody directed against fathead minnow VTG VTG [8,16,28,30]. It also is possible to assess events asso- [30]. Brie¯y, puri®ed fathead minnow VTG was serially di- ciated with VTG induction further upstream through mea- luted to obtain standard concentrations between 6 and 1,500 surement of VTG mRNA [31,32]; however, this approach has ng/ml. These standards and dilutions of plasma between 1:325 not been commonly utilized, in part, because of lack of a (controls and samples with little VTG expected) and 1:675,000 standard technique for measuring VTG mRNA. The purpose (treated samples with high VTG expected) were combined with of the present study, therefore, was to develop and validate a an equal volume of a 1:40,000 dilution of primary antibody ribonuclease protection assay (RPA) for measuring VTG and incubated for1hat378C. Two hundred-microliter aliquots mRNA in the fathead minnow. As part of this study, we as- were removed in duplicate, added to 96-well microtiter plates sessed the kinetics of mRNA versus protein induction in male that had been coated with fathead minnow VTG, and incubated ®sh treated with 17b-estradiol (E2). In conjunction with de- for1hat378C. After washing the plates, a 1:40,000 or 1: velopment of the RPA, we also were able to completely se- 80,000 dilution of goat antirabbit peroxidase-conjugated sec- quence the fathead minnow VTG complementary DNA ondary antibody (Bio-Rad, Hercules, CA, USA) was added (cDNA) and compare the deduced amino acid sequence to and incubated for1hat378C. After washing the plates again, published sequences for other ®sh species. the peroxidase substrate was added and allowed to react for 5 to 10 min. The reaction was stopped with 100 mlof1M MATERIALS AND METHODS phosphoric acid and the response measured using a 450-nm Treatment of ®sh ®lter in a plate reader (Model 3550, Bio-Rad). Some samples were reassayed at new dilutions because the values obtained To characterize induction of VTG mRNA and protein, adult were outside the reliable range of the standards. (seven±nine-month-old) male fathead minnows from on-site cultures were anesthetized via water exposure to tricaine meth- Preparation of probe and target RNA for the ribonuclease anesulfonate (100 mg/L) buffered with NaHCO3 (200 mg/L) protection assay and given intraperitoneal (ip) injections of E2 in 10% (v/v) Detailed presentation of the principles and procedures in- ethanol in corn oil. The ®sh were injected at 10 l/g of body m volved in the development and use of the RPA for measure- weight to achieve E2 doses of 5.0 (high) and 0.5 mg/kg (low). ment of speci®c mRNA sequences can be found in the manual Control ®sh received injections of the ethanol-corn oil only. for the Ambion RPA IITM kit (Austin, TX, USA). The ®rst step After injection, the ®sh were held in 10-L tanks receiving 150 in development of the RPA was to obtain a probe that had the ml/min of sand-®ltered Lake Superior water at 25 C. The pho- 8 complementary sequence for the VTG mRNA. To achieve this, toperiod was 16:8 light:dark, and ®sh were fed frozen brine we isolated total RNA from a pool of livers from seven fathead shrimp (San Francisco Bay Brand, San Francisco, CA, USA) minnows 48 h after an ip injection of E2 at 5 mg/kg (see daily.