The effect of GH overexpression on GHR and IGF-I gene regulation in different genotypes of GH-transgenic zebrafish.

Most biological actions of growth hormone (GH) are mediated by the insulin-like growth factor I (IGF-I) that is produced after the interaction of the hormone with a specific cell surface receptor, the GH receptor (GHR). Even though the GH excess on fish metabolism is poorly known, several species have been genetically engineered for this hormone in order to improve growth for aquaculture. In some GH-transgenic fish growth has been dramatically increased, while in others high levels of transgene expression have shown inhibition of the growth response. In this study, we used for the first time different genotypes (hemizygous and homozygous) of a GH-transgenic zebrafish (Danio rerio) lineage as a model for studying the GH resistance induced by different GH transgene expression levels. The results obtained here demonstrated that homozygous fish did not grow as expected and have a lower condition factor, which implies a catabolic state. These findings are explained by a decreased IGF-I and GHR gene expression as a consequence of GH resistance. Together, our results demonstrated that homozygous GH-transgenic fish showed similar characteristics to the starvation-induced fish and could be an interesting model for studying the regulation of the GH/GHR/IGF-I axis in fish.