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Methoxychlor As a Model for Environmental Estrogens Critical Reviews in Toxicology ISSN: 1040-8444 (Print) 1547-6898 (Online) Journal homepage: http://www.tandfonline.com/loi/itxc20 Methoxychlor as a Model for Environmental Estrogens Audrey M. Cummings To cite this article: Audrey M. Cummings (1997) Methoxychlor as a Model for Environmental Estrogens, Critical Reviews in Toxicology, 27:4, 367-379 To link to this article: http://dx.doi.org/10.3109/10408449709089899 Published online: 25 Sep 2008. Submit your article to this journal Article views: 76 View related articles Citing articles: 8 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=itxc20 Download by: [University of Lethbridge] Date: 26 September 2015, At: 00:46 Critical Reviews in Toxicology, 27(4):367-379 ( 1997) Methoxychlor as a Model for Environmental Estrogens Audrey M. Cummings * Endocrinology Branch, Reproductive Toxicology Division, NHEERL, USEPA, Research Triangle Park, NC * Address all correspondence to: Audrey M. Cummings MD-72, NHEERL, USEPA, Research Triangle Park, NC 277 1 1 ABSTRACT: Estrogens can have a variety of physiological effects, especially on the reproductive system. Chemicals with estrogenic activity that are present in the environment may thus be considered potentially hazardous to development and/or reproduction. Methoxychlor is one such chemical, a chlorinated hydrocarbon pesticide with proestrogenic activity. Metabolism of the chemical either in vivo or using liver microsomes produces 2,2-bis(p-hydroxyphenyl)- 1,l,l -trichloroethane (HPTE), the active estrogenic form, and the delineation of this mechanism is reviewed herein. When administered in vivo, methoxychlor has adverse effects on fertility, early pregnancy, and in utero development in females as well as adverse effects on adult males such as altered social behavior following prenatal exposure to methoxychlor. Effects of methoxychlor on the female have been studied extensively, whereas reports on the chemical's effects on males are less common. From the studies reviewed here, the reproductive toxicity of methoxychlor is evident, but the significance of this toxicity with respect to human health remains to be determined. KEY WORDS: methoxychlor, proestrogen, HPTE, estrogenic pesticide. 1. INTRODUCTION mone binds to its receptor, a conformational change results in active or transformed receptors. The ac- Any exogenously administered estrogen can tive, dimer form binds to the genome at an accep- have toxic effects, depending on the dose applied. tor site or steroid response element (SRE), thus For example, estrogen administered during early activating or inhibiting the transcription of spe- pregnancy can block implantation or terminate cific messenger WAS.Estrogen-induced mRNAs pregnancy.lg2This hormone can also accelerate em- are subsequently processed and then translated by bryo transport through the reproductive tract that, in ribosomes in the cytoplasm to produce a new itself, can impair early pregnan~y.~Effects of ex- protein, such as an enzyme. This new protein thus cess estrogen on the decidualization of the uterus, alters the function of the cell and, ultimately, the a growth and differentiation of the endometrium hormone target tissue.6, Downloaded by [University of Lethbridge] at 00:46 26 September 2015 that is necessary in the rat and human for implanta- The uterus is a primary target organ for estro- tion, can include a reduction or obliteration of decid- gens. Estrogen is uterotrophic. The administration of ual tissue formati~n.~?~However, the same hormone estrogen to immature or ovariectomized rats will that has all of these effects is necessary for preg- result in the rapid growth of uterine tissue, imbibi- nancy and decidualization when present at the tion of fluid, and increase in uterine weight by 6 h.7 proper levels and in proportion with progester- Estradiol can promote the activity of a number of one. uterine enzymes, including omithine decarboxy- The mechanism by which estrogen exerts many lase,8 creatine kinase?JO uterine peroxidase,I1J2 of its effects is well known (Figure 1). The circulat- glucose-6-phosphate dehydr~genase,'~and others. ing hormone binds to a receptor molecule in the The uterotrophic response and these specific en- cytoplasm or nucleus of target cells (those cells of zyme activities have been used as markers for the organs containing receptors). When the steroid hor- action of environmental estrogens in animals. 1040-8444/97/$.50 0 1997 by CRC Press LLC 367 Cell Membrane Nuciear Membrane RWA UAA FIGURE 1. Molecular pathway for steroid hormone action. S, steroid hormone, NHP, non-histone protein. (Reproduced with permission.6) Downloaded by [University of Lethbridge] at 00:46 26 September 2015 Estrogens have significant effects following of 10 to 100 mgkg; the chemical also produced perinatal exposure, specifically in brain and precocious vaginal opening, persistent vaginal reproductive tract development. Perinatal admin- estrus, and anovulation.22The polychlorinated istration of estrogen can induce a male pattern of biphenyl (PCB) Aroclor 1221 produced uterine brain differentiati~n.’~The synthetic estrogen di- growth in weanling rats, and when administered ethylstilbestrol (DES), administered to rodents to pups on days 2 and 3 of life it induced preco- perinatally, can reduce fertility and produce struc- cious puberty and persistent vaginal estrus, which tural abnormalities of the oviduct, uterus, cervix, are hallmarks of an estrogen.23Using the 18-h and ~agina.’~J~DES also alters sex differentia- glycogen response of the immature rat uterus, tion of the rodent central nervous system, where, Bitman and evaluated a number of PCB for example, the sexually dimorphic nucleus in compounds. PCB’s numbers 43, 44, 45, and 46 the preoptic area (SDN-POA) of the rat hypo- (corresponding to Aroclors 1221,1232,1242,and thalamus is considerably larger in male rats than 1248) were shown to be estrogenically active. 4- in females.I4 Octylphenol is a wastewater treatment plant ef- DDT was one of the first estrogenic pesticides fluent chemical that is a breakdown product of found in the environment to be banned. Original- detergents. It blocks early pregnancy in rats (un- ly, Burlington and LindemanI7demonstrated that published data) and has been shown to be estro- DDT inhibited testes growth and secondary genic.25 sexual characteristics in cockerels, suggesting Methoxychlor (MXC) is a pesticide that was that it was estrogenic. Later it was reported that developed to replace DDT. Considerable inter- 2,2’-bis-(pp-hydroxyphenyl)-1, 1 , 1 -trichloroethane est has been paid to MXC due to the fact that it appeared to have estrogenic activity, while DDT continues to be used, is found in the environ- did not.’* When o,p’ DDT (l,l,l-trichloro-2(p- ment, and is a proestrogen. chlorophenyl)-2-(o- chloropheny1)ethane) was compared with p,p’-DDT by the measure of in- creased uterine and oviductal weight, water con- II. GENERAL CHARACTERISTICS tent, glycogen and RNA in rats, chickens, and OF METHOXYCHLOR quail, o,p‘ but not p,p’-DDT was found to be estrogenically active. l9 Methoxychlor has a chlorinated, double ring Further work with DDT analogues have structure (Figure 2). Technical-grade MXC con- shown that 500 pg of o,p’-DDT injected daily tains 50 to 88% of the pure chemical, and the into young rats advances vaginal opening and remainder is isomers and reaction products. It increases ovarian and uterine weights.20In ma- was first synthesized in 1893, and commercial ture ovariectomized rats, o,p’-DDT treatment production in the U.S. began in 1946. MXC is with 10 mg/d resulted in an increase in uterine approved for use as an insecticide on 87 agricul- weight, a thickened vacuolated uterine epithe- tural crops and on beef cattle, dairy cattle, goats, lium, and the induction of cornified vaginal sheep, and swine.26Tradenames include Maralate, Downloaded by [University of Lethbridge] at 00:46 26 September 2015 smears with reduced serum luteinizing hormone Marlate, and Metox, and the chemical name is (LH).20 Two homologues of DDT, p,p‘-DDD 2,2-di(p-methoxy phenyl) 1, 1,l-trichloroethane. (2,2-bis(p-chlorophenyl)-1,l -dichloroethane) and A report published in 198327stated that MXC p,p’-DDE (2,2-bis(p-chlorophenyl)-1,l -dichloro- was found in 1.2% of composite samples of food ethylene) were not estrogenic in these studies, in the U.S., representing an intake of 0.1 to 0.8 pg while one of the major metabolites of o,p’-DDT, MXC/d for 16- to 19-year old males. When admin- p,p’-DDA (2,2-bis(p-chlorophenyl)acetic acid) istered orally to mice, 98.3% of the MXC is elimi- was found to exhibit estrogenic activity. nated by 24 h.28Both mice and insects metabolize Chlordecone (Kepone) is another pesticide methoxychlor to 2-(p-hydroxyphenyl)-2-(p- with estrogenic characteristics that is no longer methoxypheny1)-l,l,l-trichloroethane and 2,2-bis- used due to its toxicity.21Kepone has been shown (p-hydroxypheny1)- l,l,1-trichloroethane via to produce uterine growth in weanling rats at doses O-demethylation.28When rats were fed methoxy- 369 CI-c -CI I CI CHEMICAL FORMULA: FIGURE 2. Structure of methoxychlor: 1,1,1-trichloro-2,2-bis(prnethoxyphenyl)ethane. (By permission, Ref. 6.) chlor at 25 mgkg in the diet, the chemical was not positive and negative, the National Cancer Insti- detected in body fat at 18 weekszyRats receiving tute concluded that methoxychlor is not an animal MXC in the diet for 2 years at 25, 200, or 1600 ~arcinogen.~~ mgkg did store the chemical in the fat, and MXC was also found in the kidney, liver, and brain.’, The first publication of a chronic oral study 111. CHEMISTRY OF METHOXYCHLOR was published in 1950.’’ The LD,, in rats at 72 h AND ITS MECHANISMS was found to be 5 g/kg for the technical-grade MXC.
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