The involvement of growth hormone in growth regulation, energy homeostasis and immune function in the gilthead sea bream (Sparus aurata):: a short review

The aim of this mini-review is to provide a comprehensive survey of the physiological role of growth hormone (GH) in the Mediterranean sea bream (Sparus aurata). For this purpose, sea bream GH is now available as a recombinant and bioactive protein in a practically unlimited amount. In juvenile and adult fish, the liver is the most important target for the direct action of GH. Nevertheless, it must be noted that in sea bream larvae the greater concentration of GH-binding sites occurs in the head region, which also exhibits a fast growth. This finding suggests that GH exerts at this early stage of development a direct action on growing tissues rather than a systemic one mediated by hepatic IGF-I. However, the GH-liver axis is later a sensitive marker of growth performance, and seasonal changes in circulating GH levels and hepatic GH-binding are well characterized. The effect of age, water temperature and feeding regimes has also been studied. As a characteristic feature, fasting and malnourished fish show a decrease in hepatic GH-binding and circulating IGF-I, which increases pituitary GH release due to a lack of negative feedback inhibition. Interestingly, the up-regulation of plasma GH levels has also been described in fish fed to visual satiety. This, together with a decreased feed conversion, is more evident in fish fed high energy diets, and it has been suggested that this metabolic derangement is an adaptive response to protect adipose tissue and perhaps other organs and tissues from the excessive lipid deposition, when abundant energy is available. Experimental evidence also indicates that GH is able to exert a direct effect on sea bream erythroid and immunocytes. It is now recognized that GH receptors are present in erythroid, lymphoid and myeloid cells, and both GH and IGF-I have a mitogenic effect. GH also acts as a phagocytic-activating factor, and the presence of GH transcripts in the head kidney has been demonstrated by RT-PCR. All this provides suitable evidence for a pleiotropic and crucial role of GH in a lower vertebrate species such as sea bream.