Estradiol-induced hepatic gene expression in the rainbow trout (Oncorhynchus mykiss).

• Main Author: MacKay, Mairi Elizabeth.
• Other Authors: Ph.D.
• Format: Text
• Language: English
• Published: Dalhousie University 2014
• Subjects: Chemistry; Biochemistry; Agriculture; Fisheries and Aquaculture; Canada; Atlantic salmon
• Online Access: http://hdl.handle.net/10222/55173

In order to gain a more complete understanding of the process of vitellogenesis in fish, we have studied the effects of estradiol (E2) on the levels of hepatic estrogen receptors (ER), and on the induction of ER mRNA and vitellogenin (Vg) mRNA and protein in the rainbow trout, Oncorhynchus mykiss. We have characterized high-affinity estrogen-building sites (K$\sb{\rm d}$ 1-3 nM) in cytosol and nuclear extracts from livers of juvenile trout. These sites are specific for estrogens and bind antiestrogens with high relative affinity. These features are common to estrogen receptors in other species and distinguish the hepatic estradiol-binding activity from the lower-affinity, less specific steroid-binding components of plasma. While examining the concentration and distribution of hepatic ER following E2-treatment, we noted a marked effect of environmental temperature on ER dynamics. In fish maintained at 15$\sp\circ$C, nuclear ER increased 10-fold within the first 24 hours after injection. Similar levels of nuclear ER accumulated in fish kept at 9$\sp\circ$C or 4$\sp\circ$C, but the response was slower. Levels of cytosol ER rose 4- to 5-fold after treatment, also with a delayed response at cooler temperatures. Up-regulation of ER mRNA was demonstrated as well, with increased amounts of message appearing rapidly after treatment at all temperatures. The enhanced expression of ER mRNA preceded the appearance of Vg mRNA and remained 3- to 10-fold higher than in untreated controls over the time course examined. However, even in animals receiving multiple hormone injections, elevated levels of hepatic estrogen-binding activity were not maintained and had decreased almost to pretreatment levels by 10 days. This was notably different from the pattern of ER induction seen previously in Atlantic salmon. The initial appearance and the amounts of Vg mRNA and protein induced by E2 were affected by acclimation temperature, as well as by hormone dosage, and generally paralleled the increases in nuclear ER. In chronically-treated fish maintained at 15$\sp\circ$C, accumulation of both Vg mRNA and serum Vg occurred in two phases, with a plateau of several days followed by a rapid increase between 7 and 10 days. The second phase coincided with pronounced increases in hepato-somatic index and total liver RNA, but at this time hepatic ER activity was no greater than in untreated fish. This is clearly different from the situation in Xenopus liver, where high concentrations of nuclear ER appear to be necessary for the maintenance of vitellogenesis. We suggest that factors other than the estrogen receptor may be induced in trout liver by high levels of E2 and may play a role in maintaining and amplifying the output of Vg following the initial ER-mediated response. Thesis (Ph.D.)--Dalhousie University (Canada), 1990.