Duplication of growth hormone receptor (GHR) in fish genome: gene organization and transcriptional regulation of GHR type I and II in gilthead sea bream (Sparus aurata).

Nucleotide sequences encoding for functional growth hormone receptors (GHR) are now available in salmonids (coho and masu salmon) and other fish orders. Several authors have hypothesized a divergent evolution of salmonid GHRs, but the recent finding that a trout cDNA is related to non-salmonid (GHR type I) rather than to salmonid GHRs (GHR type II) points out a possible duplication of actively transcribed GHR genes. To address this issue, we search by RT-PCR for GHR type II in trout, gilthead sea bream, European sea bass, and turbot. Both in trout and gilthead sea bream, a cDNA sequence with all the characteristic features of masu and coho salmon GHRs (GHR type II) was found. Similarly, in European sea bass, a cDNA encoding for the intracellular domain of GHR type II was reported. No positive results were found in turbot, but searches in genome databases of fugu and zebrafish identified DNA sequences with a significant similarity to fish GHR type I and II, which are more related each other than to GHRs of tetrapods. Gene organization is, however, highly conserved through the evolution of vertebrates, and eight exons homologous to exons 2 and 4-10 of mammals were found in fish GHRs. Transcriptional regulation of GHR type I and II was also addressed by means of real-time PCR assays in gilthead sea bream. In liver and adipose tissue, GHR type I was the most abundant transcript, but the expression of GHR type I and II was practically equal in skeletal muscle. Both in liver and adipose tissue, the two GHRs were down-regulated by fasting. In skeletal muscle, the expression of GHR type I remained unaltered in fasted fish, whereas a 2- to 3-fold increase was found for GHR type II. All this provides suitable evidence for a tissue-specific regulation of fish GHRs, but further research is needed to determine whether these duplicated genes are evolved in a new or redundant fashion.