Androgen Inhibition of Vitellogenin Gene Expression in Tilapia (Oreochromis niloticus)

Treatment of mature female tilapia (Oreochromis niloticus) with high levels of androgen (17α-methyltestosterone, 17αMT) results in a pronounced decline in plasma vitellogenin levels as determined by gel electrophoresis. Total RNA extracted from livers of treated fish and vehicle-injected controls was analyzed by Northern and slot blot hybridization using an oligonucleotide complementary to a sequence in the 3′ end of tilapia vitellogenin mRNA. The probe revealed an mRNA of 6.5 kb in liver from the control mature female fish which was decreased by 85% by androgen treatment. As expected, estradiol (E2) treatment induced the 6.5-kb mRNA in mature male tilapia. The antiestrogen, tamoxifen, strongly decreased vitellogenin mRNA levels in mature females. Radioimmunoassay of serum from control and 17αMT-treated female tilapia showed a marked reduction in serum E2levels, from 11.4 ± 2.6 ng/ml in controls to 2.2 ± 0.13 ng/ml in treated fish. Tamoxifen, however, resulted in increased serum E2levels, probably by blocking E2negative feedback. The serum E2-lowering effect of 17αMT suggests an inhibitory site of action on gonadotropin production at the hypothalamic–pituitary axis, possibly through an androgen receptor or through an estrogen receptor after local aromatization of 17αMT.