Two growth hormone receptors in Nile tilapia (Oreochromis niloticus): Molecular characterization, tissue distribution and expression profiles in the gonad during the reproductive cycle

Growth hormone (GH) has many important physiological roles in the control of growth, metabolism and reproduction, which is mediated by growth hormone receptor (GHR). In this study, two cDNAs encoding GHR were isolated from the liver of Nile tilapia (Oreochromis niloticus). The two cDNAs, one consisting of 1908 bp and the other of 1728 bp, encoding for putative 635- and 575-amino acid GHR (designated ntGHR1 and ntGHR2, respectively), shared 34.4% identity in nucleotide sequence and 29.6% in deduced amino acid sequence. Northern blot analysis indicated a single 6.0-kb transcript of ntGHR1 and a single 4.0-kb transcript of ntGHR2 in the liver. Real-time RT-PCR analysis showed that both ntGHR1 and ntGHR2 mRNAs were presented in all tissues tested and expressed extremely highly in the liver. In most tissues, ntGHR2 expressed significantly higher than ntGHR1. Analysis of the ntGHRs expression profiles in the gonad during reproductive cycle indicated that the mRNA levels of ntGHRs in ovary were significantly higher at sexual matured stage while those in testis were significantly higher at sexual recrudescent stage, suggesting that GH/IGF-I axis might be involved in reproduction under a regulatory mechanism of GHR gene expression.