Regulation of growth hormone (GH) receptor (GHR1 and GHR2) mRNA level by GH and metabolic hormones in primary cultured tilapia hepatocytes.

Growth hormone (GH) regulates essential physiological functions in teleost fishes, including growth, metabolism, and osmoregulation. Recent studies have identified two clades of putative receptors for GH (GHR1 clade and GHR2 clade) in fishes, both of which are highly expressed in the liver. Moreover, the liver is an important target for the anabolic effects of GH via endocrine IGFs, and liver sensitivity to GH is modulated by metabolic hormones. We investigated the effects of GH, insulin, glucagon, cortisol and triiodothyronine on GHR1 and GHR2 mRNA levels in primary cultured tilapia hepatocytes. Physiological concentrations of GH strongly stimulated GHR2 mRNA level (0.5-50×10(-9) M), but did not affect GHR1 mRNA level. Insulin suppressed stimulation of GHR2 mRNA level by GH (10(-8)-10(-6) M). Insulin increased basal GHR1 mRNA level (10(-8)-10(-6) M). Cortisol increased basal GHR2 mRNA level (10(-7)-10(-6) M), but did not consistently affect GH-stimulated GHR2 mRNA level. Cortisol increased basal GHR1 mRNA level (10(-9)-10(-6) M). Glucagon suppressed GH-stimulated GHR2 mRNA level and increased basal GHR1 mRNA level at a supraphysiological concentration (10(-6) M). A single injection of GH (5 µg/g) increased liver GHR2 mRNA level, and insulin injection (5 µg/g) decreased both basal and GH-stimulated GHR2 mRNA levels after 6 h. In contrast, insulin and GH injection had little effect on liver GHR1 mRNA level. This study shows that GHR1 and GHR2 gene expression are differentially regulated by physiological levels of GH and insulin in tilapia primary hepatocytes.

Metabolic hormones regulate basal and growth hormone-dependent igf2 mRNA level in primary cultured coho salmon hepatocytes: effects of insulin, glucagon, dexamethasone, and triiodothyronine.

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 (T(3)) decreased igf2 mRNA levels. Gh stimulated igf2 mRNA at physiological concentrations (0.25x10(-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 T(3) 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.

Prolactin receptor, growth hormone receptor, and putative somatolactin receptor in Mozambique tilapia: tissue specific expression and differential regulation by salinity and fasting.

In fish, pituitary growth hormone family peptide hormones (growth hormone, GH; prolactin, PRL; somatolactin, SL) regulate essential physiological functions including osmoregulation, growth, and metabolism. Teleost GH family hormones have both differential and overlapping effects, which are mediated by plasma membrane receptors. A PRL receptor (PRLR) and two putative GH receptors (GHR1 and GHR2) have been identified in several teleost species. Recent phylogenetic analyses and binding studies suggest that GHR1 is a receptor for SL. However, no studies have compared the tissue distribution and physiological regulation of all three receptors. We sequenced GHR2 from the liver of the Mozambique tilapia (Oreochromis mossambicus), developed quantitative real-time PCR assays for the three receptors, and assessed their tissue distribution and regulation by salinity and fasting. PRLR was highly expressed in the gill, kidney, and intestine, consistent with the osmoregulatory functions of PRL. PRLR expression was very low in the liver. GHR2 was most highly expressed in the muscle, followed by heart, testis, and liver, consistent with this being a GH receptor with functions in growth and metabolism. GHR1 was most highly expressed in fat, liver, and muscle, suggesting a metabolic function. GHR1 expression was also high in skin, consistent with a function of SL in chromatophore regulation. These findings support the hypothesis that GHR1 is a receptor for SL. In a comparison of freshwater (FW)- and seawater (SW)-adapted tilapia, plasma PRL was strongly elevated in FW, whereas plasma GH was slightly elevated in SW. PRLR expression was reduced in the gill in SW, consistent with PRL's function in freshwater adaptation. GHR2 was elevated in the kidney in FW, and correlated negatively with plasma GH, whereas GHR1 was elevated in the gill in SW. Plasma IGF-I, but not GH, was reduced by 4 weeks of fasting. Transcript levels of GHR1 and GHR2 were elevated by fasting in the muscle. However, liver levels of GHR1 and GHR2 transcripts, and liver and muscle levels of IGF-I transcripts were unaffected by fasting. These results clearly indicate tissue specific expression and differential physiological regulation of GH family receptors in the tilapia.

Metabolic hormones regulate insulin-like growth factor binding protein-1 mRNA levels in primary cultured salmon hepatocytes; lack of inhibition by insulin.

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 x 10(-11)-5 x 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.

Metabolic hormones modulate the effect of growth hormone (GH) on insulin-like growth factor-I (IGF-I) mRNA level in primary culture of salmon hepatocytes.

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 x 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.

Time course of the GH/IGF axis response to fasting and increased ration in chinook salmon (Oncorhynchus tshawytscha).

Body growth in vertebrates is chiefly regulated by the GH/IGF axis. Pituitary growth hormone (GH) stimulates liver insulin-like growth factor-I (IGF-I) production. During fasting, plasma IGF-I levels decline due to the development of liver GH resistance, while GH levels generally increase. In mammals, decreased insulin during fasting is thought to cause liver GH resistance. However, the sequence of events in the GH/IGF axis response to fasting is not well characterized, especially in non-mammalian vertebrates. We assessed the time course of the GH/IGF axis response to fasting and increased ration in chinook salmon. Fish were placed on Fasting, Increased, or Control rations, and sampled daily for 4 days and at more widely spaced intervals through 29 days. Plasma IGF-I, GH, insulin, and 41 kDa IGF binding protein (putative salmon IGFBP-3), and liver IGF-I gene expression were measured. Control and Increased ration fish did not differ strongly. Plasma IGF-I and 41 kDa IGFBP were significantly lower in Fasted versus Control fish from day 4 onward, and liver IGF-I gene expression was significantly lower from day 6 onward. Liver IGF-I gene expression and plasma IGF-I levels were correlated. Plasma insulin was lower in Fasted fish from day 6 onward. There was a trend toward increased GH in Fasted fish on days 1-2, and GH was significantly increased Fasted fish from day 3 onward. Fasted GH first increased (days 1-3) to a plateau of 10-20 ng/ml (days 4-12) and then increased dramatically (days 15-29), suggesting that the GH response to fasting had three phases. The early increase in GH, followed by the decrease in plasma IGF-I after 4 days, suggests that GH resistance developed within 4 days.

A quantitative real-time RT-PCR assay for salmon IGF-I mRNA, and its application in the study of GH regulation of IGF-I gene expression in primary culture of salmon hepatocytes.

The hormone insulin-like growth factor-I (IGF-I) regulates vertebrate growth. The liver produces most circulating IGF-I, under the control of pituitary growth hormone (GH) and nutritional status. To study the regulation of liver IGF-I production in salmon, we established a primary hepatocyte culture system and developed a TaqMan quantitative real-time RT-PCR assay for salmon IGF-I gene expression. A portion of the coho salmon acidic ribosomal phosphoprotein P0 (ARP) cDNA was sequenced for use as a reference gene. A systematic bias across the 96 well PCR plate was discovered in an initial IGF-I assay, which was corrected when the assay was redesigned. IGF-I mRNA levels measured with the validated assay correlated well with levels measured with an RNase protection assay, and were highest in liver compared with other tissues. We examined the time course of hepatocyte IGF-I gene expression over 48 h in culture, the response to a range of GH concentrations in hepatocytes from fed and fasted fish, and potential effects of variation in IGF-I in the medium. IGF-I gene expression decreased over time in culture in hepatocytes in plain medium, and in cells treated with 5 nM GH with or without a combination of metabolic hormones (1 microM insulin, 100 nM triiodothyronine, and 0.1 nM dexamethasone). GH stimulated IGF-I gene expression at all time points. In cells treated with GH plus metabolic hormones, IGF-I gene expression was intermediate between the controls and GH alone. Increasing concentrations of GH resulted in biphasic IGF-I gene expression response curves in cells from fed and fasted fish, with the threshold for stimulation from 0.5 to 2.5 nM GH, maximal response from 5 to 50 nM, and a reduced response at 500 nM. Medium IGF-I (5 nM) did not affect basal or GH stimulated IGF-I gene expression. This study shows that primary hepatocyte culture and the TaqMan IGF-I assay can be used to study the regulation of hepatic IGF-I gene expression in salmon, and provides the first evidence of a biphasic response to GH concentration in fish hepatocyte culture.

Effects of ration on somatotropic hormones and growth in coho salmon.

We examined the response of growth hormone (GH), total plasma insulin-like growth-factor I (IGF-I), and growth rate to a change in ration in coho salmon. Tanks of individually tagged fish were placed on high, medium, or low ration, and sampled every 2 weeks for 8 weeks to create a range of growth rates. Some fish received non-lethal blood draws, while others were sampled terminally. Plasma IGF-I levels were higher in high ration fish than in low ration fish from 4 weeks after the beginning of experimental diets to the end of the experiment. GH levels were low and similar in all fish after changing rations, except for the fish in the low ration group at week 2. IGF-I was strongly correlated with specific growth rate in weight in terminally sampled fish after 4 weeks. GH did not correlate with growth rate or IGF-I levels. Growth parameters (length, weight, specific growth rates in weight and length, and condition factor) responded to ration. Serial sampling reduced growth rates and hematocrit, but did not change hormone levels. This study shows that IGF-I responds to changed rations within 2-4 weeks in salmonids.