Metabolic and mitogenic effects of IGF-II in rainbow trout (Oncorhynchus mykiss) myocytes in culture and the role of IGF-II in the PI3K/Akt and MAPK signalling pathways

Primary cultures of rainbow trout skeletal muscle cells were used to examine the role of insulin-like growth factor II (IGF-II) in fish muscle metabolism and growth, and to compare its main signal transduction pathways with those of IGF-I. IGF-II stimulated 2-deoxy-d-glucose (2-DG) uptake in trout myocytes at concentrations of between 5 and 100 nM, with similar maximal effects and temporal pattern to IGF-I (100 nM). The results of incubation with inhibitors (Wortmannin and CKB) indicated that IGF-II stimulates glucose uptake through the same mechanisms as IGF-I. In addition, IGF-II stimulated myoblast DNA synthesis (measured by thymidine incorporation) at relatively low concentrations (0.1-10 nM), with the maximum increase at 1 nM (167 ± 17% with respect to control values). The cells were immunoreactive against ERK 1/2 MAPK and Akt/PKB, components of the two main signal transduction pathways for the IGF-I receptor. IGF-II stimulated the phosphorylation of the protein MAPK, especially at the proliferation stage (increases of up to 125.7 ± 16.9% and 125.3 ± 3.3% with respect to control in IGF-II- and IGF-I-treated cells, respectively). In contrast, the effects of both IGFs on the activation of the PI3K/Akt pathway were stronger in fully differentiated myocytes and in early-formed fibres (up to 359 ± 18.5% in IGF-II-treated cells with respect to control). These results indicate that IGF-II has both mitogenic and metabolic effects in trout muscle cells, which are equivalent to those found in response to IGF-I. Both IGFs exert these effects though the same signalling pathways (MAPK and PI3K/Akt).