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Induction of Three Vitellogenins by 17beta-Estradiol with Concurrent Inhibition of the Growth Hormone-Insulin-Like Growth Factor 1 Axis in a Euryhaline Teleost, the Tilapia (Oreochromis mossambicus)¹

Hawaii Institute of Marine Biology,³ University of Hawaii, Kaneohe, Hawaii 96744 Department of Zoology,⁴ North Carolina State University, Raleigh, North Carolina 27695-7617 Hokkaido National Fisheries Research Institute,⁵ Kushiro, Hokkaido 085-0802, Japan Graduate School of Fisheries Sciences,⁶ Hokkaido University, Hakodate, Hokkaido 041-8611, Japan

ABSTRACT

The objective of the present study was to utilize the male Mozambique tilapia (Oreochromis mossambicus) as a model for examining the molecular mechanisms that mediate the physiological transition between somatic and gonadal growth in female teleost fish, and in vertebrates in general. Partial cDNAs that encode multiple forms of vitellogenin (Vtg), which is the major precursor of yolk proteins, were cloned from estrogen-treated males and utilized to develop real-time quantitative RT-PCR assays, which were supplemented by an assay for Vtg immunoreactivity in the plasma. Alignment analyses of the amino acid sequences deduced from the vtg cDNAs revealed three distinct tilapia Vtgs, which were categorized as Aa-, Ab-, and C-type Vtgs. A single injection of male tilapias with 17beta-estradiol (E₂) at 5 µg/g body weight significantly increased the plasma E₂ and hepatic levels of all three vtg transcripts within 1 day. Plasma E₂ levels declined after 3 days, whereas the plasma Vtg immunoreactivity and hepatic levels of the three vtg transcripts continued to increase. Hepatic expression of the estrogen receptor (esr) 1 gene, but not the esr2 gene, also increased markedly 1 day after E₂ injection and remained elevated for 5 days. While plasma growth hormone (Gh) levels were unaffected, hepatic expression of transcripts that encoded the Gh receptor and insulin-like growth factor 1 (Igf1) was suppressed by E₂, as were the plasma Igf1 levels. These results clearly suggest a distinct negative interplay between the growth and reproductive axes at the molecular level of key hepatic regulatory pathways involved in the control of energy utilization by gonadal and somatic growth processes.

estradiol, estrogen receptor, growth hormone, insulin-like growth factor, vitellogenin

¹Supported in part by an EPA STAR fellowship (FP91642801) to L.K.D., and by grants from the National Science Foundation (IBN 04-17250, IOB 05-17769, OISE 04-36347), US Department of Agriculture (2005-35206-15285), and the Pauley Foundation Summer Program 2004 and 2005 to E.G.G.

Correspondence: ²E. Gordon Grau, Hawaii Institute of Marine Biology, University of Hawaii, PO Box 1346, Kaneohe, Hawaii 96744. FAX: 808 956 3014; e-mail: grau{at}hawaii.edu