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Developmental Endocrinology Section, Laboratory of Toxicology, Environmental Toxicology Program, Division of Intramural Research, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709 [R. R. N., E. P. B., W. N. J.], and Department of Pathology, Wake Forest University School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27157 [B. B.]
| ABSTRACT |
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| Introduction |
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2040 mg per day (46 mg/kg/day) of soy isoflavones, of which genistein makes up >65%, whereas adults consuming a moderate to large amount of soy in the diet are exposed to
1 mg/kg/day soy isoflavones (4)
. Earlier reports suggested beneficial effects of genistein exposure early in life, e.g., breast cancer prevention (5
, 6)
and improved cholesterol synthesis rates (7)
, but an increasing number of reports are now describing long-term deleterious effects of genistein (8)
and of another phytoestrogen, coumestrol (9
, 10) . One recent report showed an increase in carcinogen-induced mammary carcinogenesis after prenatal exposure of rats to genistein (11)
, and another associated a diet high in phytoestrogens with the development of insulin-dependent diabetes (12)
. Still another report highlighted the increased incidence of hypospadias in male offspring of vegetarian mothers (13)
. To further study the effects of phytoestrogen exposure early in life, we treated mice neonatally with genistein or an equivalent estrogenic dose of DES and determined the long-term carcinogenic potential. | Materials and Methods |
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Comparison of the Estrogenicity of Genistein and DES in Neonatal Pups.
Female pups were given daily s.c. injections of genistein or DES dissolved in corn oil or of corn oil alone (as control) on days 15 of neonatal life. A minimum of eight pups per compound was used. On the afternoon of day 5, mice were killed and body weights and uterine weights recorded. Data are expressed as percentage uterine weight increase over values in corn oil-treated control mice. A dose response for genistein has been previously reported in the immature mouse uterotropic bioassay (15)
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Tumor Induction.
Female pups were treated by daily s.c. injections on days 15 with genistein (50 mg/kg) or DES (0.001 mg/kg) dissolved in corn oil or with corn oil alone (as control). These doses are
100 µg/pup/day genistein and 0.002 µg/pup/day DES and are equal in estrogenic potency. Mice were weaned on day 21 and housed four per cage. Mice were killed by cervical dislocation at 18 months of age. Reproductive tract tissues were removed, fixed in 10% neutral buffered formalin, embedded in paraffin, and sectioned at 6 µm. Tissue sections were stained with H&E and evaluated by light microscopy. If a microscopic lesion was observed, additional serial sections were made to include the entire area of pathological change.
| Results |
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The range of uterine abnormalities in the uteri of mice exposed neonatally to genistein or DES is also shown in Table 2
. The incidence of CEH of the uterus was similar in both DES- and genistein-treatment groups [54% (7 of 13) and 47% (8 of 17), respectively]. A low incidence of CEH occurred in the control mice in this study [19% (3 of 16)]. Squamous metaplasia of the uterus occurred in 38% (5 of 13) of the DES and 64% (11 of 17) of the genistein groups. In addition to these uterine lesions, more severe pathologies occurred after neonatal exposure to both compounds. Atypical hyperplasia of the uterus occurred in 5% (1 of 17) of the genistein-treated mice. Furthermore, some animals had cellular alterations that progressed to uterine adenocarcinoma (Fig. 1, A and B)
. These tumors were usually well differentiated and characterized by irregularly shaped glands with little intervening stroma. Some lesions extended through the myometrium to the serosal surface of the uterus. Nuclear pleomorphism and mitotic figures were frequently observed. In genistein-treated mice, 35% (6 of 17) developed this tumor, and at the approximate equal estrogenic dose of DES, 31% (4 of 13) had uterine adenocarcinoma; there were no cases of uterine adenocarcinoma in any controls in this study.
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| Discussion |
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Of particular significance in this study is the incidence of uterine adenocarcinoma after neonatal exposure to genistein. This compound is readily available to many infants during the first year of life as a component of soy-based formulas. The amount of soy isoflavones found in some infant formulas, of which genistein and its conjugates account for >65%, approaches 40 mg/liter of formula (4)
. An infant consuming 1 liter of soy-based infant formula would ingest
27 mg of genistein per day. The amount of genistein used in our study (50 mg) is slightly higher than the amount consumed by infants, but it is certainly within one order of magnitude of the level of human exposure. We are currently evaluating the carcinogenic potential of lower doses of genistein as well as investigating the effects of genistein if exposure occurs through the diet instead of s.c. injection as described in this study.
In another report from our laboratory, lower doses of genistein caused alterations in the ovary such as multioocyte follicles (18) ; a dose-related increase in multioocyte follicles starting at a dose of 5 mg/kg/day was observed in 2-month-old mice. Previous work by Iguchi et al. (19) has shown that multioocyte follicles resulting from neonatal DES exposure are less fertile than single oocyte follicles. This data suggest that fertility could also be affected in mice exposed neonatally to genistein. Future studies in our laboratory will investigate fertility in genistein-treated mice as well as mechanisms involved in the formation of multioocyte follicles.
The findings of the present study raise concerns over the amount of phytoestrogens in soy-based infant formulas and other soy-based products that are fed to young children. Additional studies are needed to determine the potential effects in humans exposed to high quantities of phytoestrogens during critical stages of neonatal or early development. Ongoing National Toxicology Program (NTP) studies designed to address multigenerational effects of genistein,3 will help to determine the risks of developmental exposure to genistein and other endocrine-disrupting chemicals. The potential risks and benefits need to be thoroughly assessed to determine the appropriate balance of exposures of these chemicals during development and the permanent effects that may follow.
| FOOTNOTES |
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1 To whom requests for reprints should be addressed, at National Institute of Environmental Health Sciences, MD E4-02, P. O. Box 12233, Research Triangle Park, NC 27709. Phone: (919) 541-0738; Fax: (919) 541-4634; E-mail: newbold1{at}niehs.nih.gov ![]()
2 The abbreviations used are: DES, diethylstilbestrol; NIEHS, National Institute of Environmental Health Sciences; PPL, progressive proliferative lesion; CEH, cystic endometrial hyperplasia. ![]()
3 B. Delclos and R. R. Newbold. Dose range studies ofgenistein in the rat, submitted for publication. ![]()
Received 2/ 6/01. Accepted 4/ 3/01.
| REFERENCES |
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