Liver production of insulin-like growth factor-I (IGF-I) is a major point of control in the growth hormone (GH)/IGF axis, the endocrine system regulating body growth in fishes and other vertebrates. Pituitary GH stimulates hepatocyte production of IGF-I; however, in catabolic states, hepatocyte GH resistance results in decreases in liver IGF-I production. To investigate endocrine mechanisms leading to the development of hepatocyte GH resistance, we examined the regulation of IGF-I mRNA level by GH and metabolic hormones in primary culture of salmon hepatocytes. Cells were cultured in RPMI medium, and exposed to insulin (Ins, 10⁻⁶ M), glucagon (Glu, 10⁻⁶ M), triiodothyronine (T₃, 10⁻⁷ M), dexamethasone (Dex, 10⁻⁶ M) and glucagon-like peptide (GLP, 10⁻⁶ M), in the presence and absence of GH (5×10⁻⁹ M). GH always increased IGF-I mRNA. None of the other hormones tested alone affected IGF-I mRNA. However, Dex, Ins and Glu reduced the response to GH. The response to GH was inhibited by Dex at concentrations of 10⁻¹² M and above, by Ins at 10⁻⁹ M and above, and by Glu only at 10⁻⁶ M. Inhibition of GH response by glucocorticoids is found in other vertebrates. Salmon hepatocytes were very sensitive to Dex, suggesting that glucocorticoids may play an important role in salmon growth regulation even in unstressed conditions. Inhibition of GH response by Ins is the opposite of what is found in mammals and chickens, suggesting that the role of Ins in growth regulation may differ between fishes and tetrapods. To examine mechanisms for modulation of GH sensitivity, we measured hepatocyte GH receptor (GHR) mRNA levels. Ins inhibited and Dex stimulated GHR mRNA, suggesting that different mechanisms mediate the inhibition of GH response by these hormones. This study shows that glucocorticoids, Ins, and Glu induce GH resistance in cultured salmon hepatocytes.

Igf1 and Igf2 stimulate growth and development of vertebrates. Circulating Igfs are produced by the liver. In mammals, Igf1 mediates the postnatal growth-promoting effects of growth hormone (Gh), whereas Igf2 stimulates fetal and placental growth. Hepatic Igf2 production is not regulated by Gh in mammals. Little is known about the regulation of hepatic Igf2 production in nonmammalian vertebrates. We examined the regulation of igf2 mRNA level by metabolic hormones in primary cultured coho salmon hepatocytes. Gh, insulin, the glucocorticoid agonist dexamethasone (Dex), and glucagon increased igf2 mRNA levels, whereas triiodothyronine (T₃) decreased igf2 mRNA levels. Gh stimulated igf2 mRNA at physiological concentrations (0.25×10⁻⁹ M and above). Insulin strongly enhanced Gh stimulation of igf2 at low physiological concentrations (10⁻¹¹ M and above), and increased basal igf2 (10⁻⁸ M and above). Dex stimulated basal igf2 at concentrations comparable to those of stressed circulating cortisol (10⁻⁸ M and above). Glucagon stimulated basal and Gh-stimulated igf2 at supraphysiological concentrations (10⁻⁷ M and above), whereas T₃ suppressed basal and Gh-stimulated igf2 at the single concentration tested (10⁻⁷ M). These results show that igf2 mRNA level is highly regulated in salmon hepatocytes, suggesting that liver-derived Igf2 plays a significant role in salmon growth physiology. The synergistic regulation of igf2 by insulin and Gh in salmon hepatocytes is similar to the regulation of hepatic Igf1 production in mammals.

IGF-binding proteins (IGFBPs) modulate the effects of the IGFs, major stimulators of vertebrate growth and development. In mammals, IGFBP-1 inhibits the actions of IGF-I. Rapid increases in circulating IGFBP-1 occur during catabolic states. Insulin and glucocorticoids are the primary regulators of circulating IGFBP-1 in mammals. Insulin inhibits and glucocorticoids stimulate hepatocyte IGFBP-1 gene expression and production. A 22 kDa IGFBP in salmon blood also increases during catabolic states and has recently been identified as an IGFBP-1 homolog. We examined the hormonal regulation of salmon IGFBP-1 mRNA levels and protein secretion in primary cultured salmon hepatocytes. The glucocorticoid agonist dexamethasone progressively increased hepatocyte IGFBP-1 mRNA levels (eightfold) and medium IGFBP-1 immunoreactivity over concentrations comparable with stressed circulating cortisol levels (10⁻⁹–10⁻⁶ M). GH progressively reduced IGFBP-1 mRNA levels (0.3-fold) and medium IGFBP-1 immunoreactivity over physiological concentrations (5 × 10⁻¹¹–5 × 10⁻⁹ M). Unexpectedly, insulin slightly increased hepatocyte IGFBP-1 mRNA (1.4-fold) and did not change medium IGFBP-1 immunoreactivity over physiological concentrations and above (10⁻⁹–10⁻⁶ M). Triiodothyronine had no effect on hepatocyte IGFBP-1 mRNA, whereas glucagon increased IGFBP-1 mRNA (2.2-fold) at supraphysiological concentrations (10⁻⁶ M). This study suggests that the major inhibitory role of insulin in the regulation of liver IGFBP-1 production in mammals is not found in salmon. However, regulation of salmon liver IGFBP-1 production by other metabolic hormones is similar to what is found in mammals.