Oral exposure to genistin, the glycosylated form of genistein, during neonatal life adversely affects the female reproductive system

Abstract

Background: Developmental exposure to environmental estrogens is associated with adverse consequences later in life. Exposure to genistin (GIN), the glycosylated form of the phytoestrogen genistein (GEN) found in soy products, is of concern because approximately 20% of U.S. infants are fed soy formula. High circulating levels of GEN have been measured in the serum of these infants, indicating that GIN is readily absorbed, hydrolyzed, and circulated.

Objectives: We investigated whether orally administered GIN is estrogenic in neonatal mice and whether it causes adverse effects on the developing female reproductive tract.

Methods: Female CD-1 mice were treated on postnatal days 1-5 with oral GIN (6.25, 12.5, 25, or 37.5 mg/kg/day; GEN-equivalent doses), oral GEN (25, 37.5, or 75 mg/kg/day), or subcutaneous GEN (12.5, 20, or 25 mg/kg/day). Estrogenic activity was measured on day 5 by determining uterine wet weight gain and induction of the estrogen-responsive gene lactoferrin. Vaginal opening, estrous cyclicity, fertility, and morphologic alterations in the ovary/reproductive tract were examined.

Results: Oral GIN elicited an estrogenic response in the neonatal uterus, whereas the response to oral GEN was much weaker. Oral GIN altered ovarian differentiation (i.e., multioocyte follicles), delayed vaginal opening, caused abnormal estrous cycles, decreased fertility, and delayed parturition.

Conclusions: Our results support the idea that the dose of the physiologically active compound reaching the target tissue, rather than the administered dose or route, is most important in modeling chemical exposures. This is particularly true with young animals in which phase II metabolism capacity is underdeveloped relative to adults.

Keywords: development; diethylstilbestrol; endocrine disruptors; environmental estrogen; isoflavone; ovary.

Figure 1

Chemical structures of GIN (oral exposure) and GEN (sc exposure). MW, molecular weight.

Figure 2

Effects of neonatal treatment with GEN and GIN on female mice on PND5. C, corn oil control. (A) Uterotropic bioassay after sc or oral GEN or oral GIN; values shown are mean ± SE uterine wet weight for each treatment group (n = 8 mice per group). (B) Real-time RT-PCR for LF shown as mean ± SE of LF expression normalized to cyclophilin × 10,000. *p < 0.05, compared with controls, Dunnett’s test.

Figure 3

Serum levels of GEN (mean ± SE) after oral exposure to GIN 37.5 mg/kg on PNDs 1–5. Serum was collected at 0.5, 1, 2, 4, 8, 24, and 48 hr after the last treatment (n = 4–6 samples per group).

Figure 4

Fertility assessment of mice treated neonatally with oral GIN. C, corn oil control. (A) Percentage of vaginal plug–positive mice delivering live pups by age (2, 4, and 6 months). (B) Number of live pups per litter (mean ± SE) by age (2, 4, and 6 months). (C) Percentage of mice delivering live pups for all ages combined (mean ± SE). *p < 0.05, compared with controls by Fisher’s exact test. **p < 0.05, compared with controls by Mann-Whitney test.

Figure 5

Timing of delivery of live pups. C, corn oil control. (A) Percentage of vaginal plug–positive mice delivering on time for each group by age (2, 4, and 6 months). (B) Percentage of mice per treatment group by age (2, 4, and 6 months) that delivered in the late afternoon or 1–2 days late, or that delivered no live pups. *p < 0.05 compared with controls by Fisher’s exact test.