Mechanisms of Hormone Action

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)

Abstract
The objective of this study was to utilize the male Mozambique tilapia (Oreochromis mossambicus) as a model for examining molecular mechanisms mediating the physiological transition between somatic and gonadal growth in female teleost fish and in vertebrates in general. Partial cDNAs encoding multiple forms of vitellogenin (Vtg), the major precursor to 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 amino acid sequences deduced from the vtg cDNAs revealed three distinct tilapia Vtgs, which were categorized as an Aa-, Ab-, and C-type Vtg. A single injection of male tilapia with estradiol-17beta (E₂) at 5 µg/g body weight significantly increased plasma E₂ and hepatic levels of all three vtg transcripts within one day. Plasma E₂ levels declined after 3 days, whereas 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 one day after E₂ injection and remained elevated for 5 days. While plasma growth hormone (Gh) levels were unaffected, hepatic expression of transcripts encoding the Gh receptor and insulin-like growth factor 1 (Igf1) was suppressed by E₂, as were 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.

Key words: Estradiol • Growth hormone • Insulin-like growth factor • Vitellogenin • Tilapia

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