5.1.3. Regulation of the expression of insulin-like growth factors in fish muscle by insulin-like growth factors and nutrients
Previously, was exposed the discrepancy regarding whether or not the GH is a potent regulator of igf1 expression in fish muscle. A number of recent studies have demonstrated that igf1 expression can be strongly regulated by nutrients and the same IGFs (Fig. 3). In 9 day old myogenic cells of Atlantic salmon, amino acids alone are sufficient to stimulate the expression of igf1 and igf2 (Bower and Johnston, 2010a). Co-incubation of IGF2 and amino acids maintain the enhanced expression of both molecules (Bower and Johnston, 2010a). Co-incubation of IGF1 and amino acids not only maintains the increase, but also has a synergistic effect by upregulating igf1 expression almost 20-fold (Bower and Johnston, 2010a). These results clearly indicate a positive feedback mechanism for igf1 transcription. In 4 day old myogenic cells of gilthead sea bream, treatments with IGF2 alone and of IGF2 with the GH increase the expression of igf1 after 18 h of treatment (Jiménez-Amilburu et al., 2013). Co-incubation with IGF1 and the GH increases the expression of both igf1 and igf2 after 6 h of treatment (Jiménez- Amilburu et al., 2013). Altogether, these results show that the classic GH-activated JAK2/STAT5 signaling pathway is not the only responsible for the expression of igf1 in fish muscle, but that other signaling pathways (e.g. amino acids and IGFs) are strongly regulating
this process. Considering that it seems that the GH is not a strong regulator for the transcription of igf1 in fish muscle, and the fact that at least three teleost species show higher muscle contents of tGHR1s than of flGHR1 (Fig. 1), it is plausible to hypothesize that the signaling pathways activated by IGFs and amino acids could be the main pathways responsible for controlling the transcription of igf1 rather than GH signaling. This is particularly important for fish, which have a higher consumption of dietary proteins in comparison with mammals. Hence, it appears that IGFs and amino acids-derived nutrients are of greater importance in regulating igf1 expression in fish than in mammals. However, more studies using other fish species are needed to establish a clear view of which signaling pathways are the most important for IGF1 production
in fish muscle and whether there are large species differences or not.