Li You

Androgen receptor expression in the testes and epididymides of prenatal and postnatal Sprague-Dawley rats.

The androgen receptor (AR) plays a critical role in sexual differentiation and in the virilization of the male reproductive system. A clear understanding of AR expression at the early stages of sexual development will help elucidate the sensitivity of perinatal animals to endocrine modulation by external agents, such as some environmental chemicals. Immunohistochemistry was used in this study to localize the AR in the differentiating testis and epididymis of Sprague-Dawley rats starting from gestation day 15 until postnatal day 21. Positive AR staining was found on gestation day 15 in the mesenchymal as well as in the epithelial cells in the mesonephros. Weak staining was also observed in a small number of interstitial cells in the primordial testis at this age. The fetal interstitial and peritubular myoid cells showed positive AR immunoreactivity early in development, but the Sertoli cells did not overtly express the receptors until postnatal day 5. The intensity of staining and number of AR-positive cells in the testis and epididymis increased over time. The epithelium in the mesonephros-derived tissues, including rete testis and epididymis, appeared to exhibit a higher capacity to express AR than the rest of the testicular tissue. The results demonstrate that AR expression in the primordial male reproductive system is highly specific to time and cell type and modify previous understanding on the timing of AR expression in the testicular tissue. Since AR-positive cells at various developmental stages may be potential sites of interaction with chemicals that adversely affect sexual differentiation, improved understanding of AR ontogeny will help in investigating the effects of AR-reactive agents, such as environmental antiandrogens, with respect to specific windows of sensitivity.

Genistein and methoxychlor modulate the activity of natural killer cells and the expression of phenotypic markers by thymocytes and splenocytes in F0 and F1 generations of Sprague–Dawley rats

The isoflavone genistein (GE) and methoxychlor (MXC) have been shown to be estrogenic in both in vitro and in vivo experimental systems. The objective of the present study was to evaluate the effects of GE and MXC on the immune system in adult and developing rats and the potential interaction between these compounds in their immunomodulatory actions. Timely pregnant Sprague-Dawley rats were exposed to GE (300 or 800 ppm), MXC (800 ppm), or their combinations in feed starting on day 1 of gestation. The offspring were exposed to these chemicals gestationally and lactationally. Immunological evaluation was performed on postnatal day 22. In F0 females, exposure to GE had no effect on the percentages of thymocyte subsets, but caused a significant decrease in the absolute thymus weight at the 800-ppm dose level. In the spleen, GE did not affect the activity of natural killer cells but induced changes in the percentages of splenic T lymphocyte subsets. Exposure to MXC produced no effect on the immune parameters examined except for a decrease in the percentage of CD4+CD8- thymocytes. Additionally, minimal interaction between GE and MXC was observed. In F(1) males, both GE and MXC decreased the percentage of CD4+CD8- thymocytes, but only GE increased spleen natural killer cell activity. MXC in combination with 300 ppm-GE, but not separately, produced significant decreases in the absolute weights of thymus and spleen. In F1 females, GE decreased the percentage of CD4+CD8- thymocytes, increased the percentage of CD4+CD8+ thymocytes, and decreased the activity of spleen natural killer cells. In contrast, MXC increased the percentages of spleen natural killer cells and CD8+ T cells. Overall, the results demonstrate that both GE and MXC can modulate the immune system with greater effects observed in developing rats. Moreover, male and female rats have differential responses to these compounds. A lack of interaction between these two estrogenic chemicals in modulating these immune parameters indicates that their effects on the immune system might involve other mechanisms in addition to the estrogen receptors.

Gene expression profiles in mammary gland of male rats treated with genistein and methoxychlor.

Humans and wildlife are frequently exposed to mixtures of natural and synthetic endocrine-active compounds. To understand the impact of dietary phytoestrogen on the susceptibility to synthetic chemicals in the environment, we studied the effects of a binary mixture consisting of the isoflavone genistein and the pesticide methoxychlor on the development of the mammary gland. Sprague-Dawley rats were exposed to genistein at 800ppm, methoxychlor at 800ppm, or their combination through dietary administration to dams during pregnancy and lactation and to the offspring directly after weaning. At post-natal day (PND) 90, offspring rats were killed and their inguinal mammary glands collected for gene expression analysis utilizing the Clontech Atlas Rat 1.2 cDNA array, which contains probes for 1176 genes. Treatment with both genistein and methoxychlor altered gene expression profiles of the mammary glands in male rats, and the effects were more prominent in the combination treatment than the single-compound groups. Specific gene changes suggested that treatments affected the stromal and epithelial compartments of the mammary, involving genes controlling growth factor signaling, apoptosis, and tissue remodeling. This study demonstrates that dietary phytoestrogens in combination with a synthetic endocrine-active chemical can cause unique effects in endocrine-responsive tissues and highlights the importance of studying the effects of chemical combinations on the multiple biological processes underlying toxicological responses.

Modification of endocrine active potential by mixtures

Wildlife and humans are exposed to a complex mixture of endocrine active chemicals. The activity of a specific chemical in any mixture can be modified through interactions with other components of the mixture. The toxic equivalency factor (TEF) approach for risk assessment was developed for chemicals such as halogenated aromatics that induce their effects through ligand-activated receptors. For persistent halogenated aromatic AhR agonists, this approach has some utility. However, the use of the TEF approach for endocrine active compounds is confounded by the unique tissue- and response-specific activities of these structurally diverse compounds. The term selective receptor modulator describes the ability of a natural or synthetic receptor ligand to manifest agonist activity in one tissue or for one response and antagonist activity in other tissues or for another response in the same tissue. Thus, it is possible for chemicals in a mixture to behave in an additive manner for one response and an antagonist manner for another response. A mechanisms-based hazard risk assessment of endocrine active chemical mixtures must account for these multiple variables.