Abstract
The organochlorine pesticide methoxychlor (MXC) is an environmental estrogen known to stimulate the expression of the egg-yolk protein, vitellogenin (Vtg) in fish species. To begin to understand the underlying mechanisms for how MXC exerts its deleterious effects on the endocrine system, male largemouth bass (Micropterus salmoides) were treated with 2.5, 10, or 25mg/kg MXC and compared to fish pair-treated with 1mg/kg 17 beta-estradiol (E2), and vehicle control. Fish were sacrificed 24, 48, or 72 h following treatment. The liver and testes were then assayed for changes in expression of the three bass estrogen receptors (ERs alpha, beta a, and beta b) in tissues, as well as Vtg and cytochrome P450 (CYP) 3A isoform 68 in the liver and steroidogenic acute regulatory protein (StAR) in the testes. In the liver, significant increases in gene expression were seen for each of the genes measured by 24 h and each returned to the level of the vehicle by 72 h. Total testosterone 6 beta-hydroxylase activity, reflective of CYP3A activity, was also increased by 24h for all of the exposures. In the testes, ER alpha was unaffected by any treatment, ERbetaa was up-regulated only by MXC, peaking at 24h for the 2.5 and 10mg/kg MXC and at 48 h for the 25mg/kg MXC treatment. By 72 h, the MXC effects had disappeared, while E2 significantly decreased the expression of ER beta a mRNA. ER beta b expression in the testes was stimulated by all concentrations of MXC by 24 h and the effect remained up to 72 h, whereas E2 had no effect. Finally, StAR expression was also found to be decreased by E2 and all MXC treatments. However, the effect on StAR expression by E2 occurred within 24h, while the effect by all concentrations of MXC was not seen until 72 h after treatment. The stimulatory effects of E2 and 25mg/kg MXC on the expression of the ERs in the liver were opposite to the responses seen in the testes, suggesting an inverted relationship between these two tissue types. These results provide a possible mechanism showing that alterations in reproductive signaling in male fish by xenoestrogens not only increase Vtg expression in the liver, but may also decrease reproductive success by muting some of the estrogen signals required for sperm production.