Effects of postnatal exposure to a mixture of polychlorinated biphenyls, p,p′-dichlorodiphenyltrichloroethane, and p-p′-dichlorodiphenyldichloroethene in prepubertal and adult female Sprague-Dawley rats

The postnatal period is a critical phase of development and a time during which humans are exposed to higher levels of persistent organic pollutants (POPs), than during subsequent periods of life. There is a paucity of information describing effects of postnatal exposure to environmentally relevant mixtures of POPs, such as polychlorinated biphenyls (PCBs), p, p'-dichlorodiphenyltrichloroethane (DDT), and p, p'-dichlorodiphenyldichloroethene (DDE). To provide data useful for the risk assessment of postnatal exposure to POPs, mixtures containing 19 PCBs, DDT, and DDE were prepared according to their concentrations previously measured in the milk of Canadian women, and dose-response effects were tested on the proliferation of MCF7-E3 cells in vitro, and in vivo experiments. Female neonates were exposed by gavage at postnatal days (PNDs) 1, 5, 10, 15, and 20 with dosages equivalent to 10, 100, and 1000 times the estimated human exposure level over the first 24 days of life. The MCF7-E3 cells showed a 227% increase in the AlamarBlue proliferation index, suggesting estrogen-like properties of the mixture, but this was not confirmed in vivo, given the absence of uterotrophic effects at PND21. An increase (511%) in hepatic ethoxyresorufin-o-deethylase activity at the dose 100 x was the most sensitive endpoint among those measured at PND21 ( organ weight, mammary gland and ovarian morphometry, hepatic enzyme inductions, serum thyroxine and pituitary hormones). In liver samples from older female rats ( previously involved in a mammary tumor study [Desaulniers et al., Toxicol. Sci. 75: 468 - 480, 2001]), hepatic metabolism of C-14-estradiol-17 beta (E2) at PND55 to PND62 was significantly higher in the 1000 x compared to the control group, but hepatic detoxification enzyme activities had already returned to control values. The production of hepatic 2-hydroxy-E2 decreased, whereas that of estrone increased with age. In conclusion, the smallest dose of the mixture to induce significant effects was 100 x, and mixture-induced changes in the hepatic metabolism of estrogens might be a sensitive indicator of persistent effects.