ANDROGEN INHIBITION OF VITELLOGENIN GENE-EXPRESSION IN TILAPIA (OREOCHROMIS-NILOTICUS)

Treatment of mature female tilapia (Oreochromis niloticus) with high l evels of androgen (17 alpha-methyltestosterone, 17 alpha MT) results i n a pronounced decline in plasma vitellogenin levels as determined by gel electrophoresis. Total RNA extracted from livers of treated fish a nd vehicle-injected controls was analyzed by Northern and slot blot hy bridization using an oligonucleotide complementary to a sequence in th e 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 (E(2)) treatment induced the 6.5-kb mRNA in mature male tilapia. The antiestrogen, tam oxifen, strongly decreased vitellogenin mRNA levels in mature females. Radioimmunoassay of serum from control and 17 alpha MT-treated female tilapia showed a marked reduction in serum E(2) levels, from 11.4 +/- 2.6 ng/ml in controls to 2.2 +/- 0.13 ng/ml in treated fish. Tamoxife n, however, resulted in increased serum E(2) levels, probably by block ing E(2) negative feedback. The serum E(2)-lowering effect of 17 alpha MT suggests an inhibitory site of action on gonadotropin production a t the hypothalamic-pituitary axis, possibly through an androgen recept or or through an estrogen receptor after local aromatization of 17 alp ha MT. (C) 1996 Academic Press, Inc.