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 E₂ levels, 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 E₂ levels, probably by blocking E₂ negative 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.