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Laboratory of Grassland Science, University Farm, Graduate School of Bioagricultural Sciences, Nagoya University, Togo, Aichi 470-0151, Japan and * Laboratory of Animal Nutrition, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan
2To whom correspondence should be addressed. E-mail: kitak{at}agr.nagoya-u.ac.jp.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • insulin-like growth factor-I • insulin-like growth factor binding protein-2 • gene expression • nutrition • chickens
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). As in mammals, the growth rate of chickens, which varies widely under various nutritional conditions, is closely related to the plasma IGF-I concentration (2
–6
). It is well known that in the circulation and extracellular fluids, IGF-I is bound to specific and high affinity binding proteins (insulin-like growth factor binding proteins, IGFBP), six of which have been identified in mammalian species (7
). It has been reported that several species of IGFBP with different sizes exist in the plasma of chickens (8
–10
). So far, IGFBP-2 (11
) and IGFBP-5 (12
) have been isolated and characterized in chickens. A few studies examining the regulation of chicken IGFBP have been reported. A 30-kDa IGFBP in chicken plasma, which seems to be IGFBP-2, is nutritionally regulated (4
,13
). In these reports, the 30-kDa IGFBP in plasma was increased by food restriction and reduced by refeeding. In rats, hepatic IGFBP-2 mRNA gene expression has been shown to be increased by 2 d of food deprivation (14
), dietary protein limitation (15
,16
) and dietary energy restriction (17
). However, little information exists concerning the nutritional regulation of IGFBP-2 gene expression in various tissues of young chickens. Therefore, in the present study, we examined the time course changes in IGFBP-2 gene expression in various tissues of food-deprived/refed young chickens to elucidate which tissues contribute to circulating IGFBP-2 and to clarify the relationship between circulating IGFBP-2 and IGF-I activity. Furthermore, the influence of the quality and quantity of dietary protein on IGFBP-2 gene expression in several tissues of young chickens was also examined. |
MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Animals and experimental procedures.
In Experiment 1, single-comb 1-d-old White Leghorn male chicks (n = 200) were obtained from a local hatchery (Hattori Yokei, Nagoya, Japan). Chicks were fed a commercial chick mash diet [Pre-Chick, crude protein (CP) 210 g/kg, metabolizable energy (ME) 11.8 kJ/g; Marubeni Shiryo, Tokyo, Japan] until 2 wk of age. From 2 to 6 wk of age, chicks were given a different type of commercial chick mash diet (Chick-15, CP, 190 g/kg; ME, 11.8 kJ/g; Marubeni Shiryo). Chicks were housed in stainless steel cages in a temperature-controlled (29 ± 1°C) room with continuous illumination. At 6 wk of age, 35 birds of uniform body weight were selected and divided evenly into 7 experimental groups of 5 birds each. Thirty chicks in 6 experimental groups were food deprived for 2 d, and 5 chicks in another group were allowed free access to the diet Chick-15. After 2 d of food deprivation, 25 food-deprived chicks in 5 experimental groups were allowed free access to the diet. At 1, 2, 6 and 24 h after refeeding, chicks were anesthetized with diethylether and a blood sample was taken by heart puncture. After neck dislocation, brain, gizzard, small intestine, liver and kidney were removed, washed with physiological saline and blotted. Tissue samples were immediately frozen in liquid nitrogen and stored at -80°C until analyzed. At 48 h after refeeding, only blood samples were taken from the remaining 5 birds. Plasma samples were stored at -20°C until analyzed. Animal care was in compliance with applicable guidelines from the Nagoya University Policy on Animal Care and Use.
In Experiment 2, White Leghorn male chicks were obtained and maintained until 6 wk of age as described in Experiment 1. At 6 wk of age, 40 birds of uniform body weight were selected and divided evenly into 8 experimental groups of 5 birds each. Thirty-five chicks in 7 experimental groups were food deprived for 2 d, and 5 chicks in another group were allowed free access to a semipurified complete diet. After 2 d of food deprivation, 30 food-deprived chicks in 6 experimental groups were fed freely one of six semipurified experimental diets. The experimental diets were as follows: complete diet, protein-free diet, carbohydrate-free diet, fat-free diet, protein diet and carbohydrate diet. The composition of the experimental diets is shown in Table 1
. The remaining group of 5 food-deprived birds served as food-deprived (not refed) controls. Five fed chickens continued to receive free access to the complete diet. In Experiment 1, the decrease in gizzard and hepatic IGFBP-2 gene expression stimulated by food deprivation was observed 2–6 h after refeeding. Therefore, at 6 h after refeeding, livers were removed after neck dislocation. All samples were treated as described in Experiment 1.
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) and Scott et al. (20
), the dietary protein requirement for male Leghorn-type chicks is 18.0 and 21.5%, respectively. In the present study, therefore, 20% dietary protein was set as the requirement. The birds were allowed free access to one of the following 5 experimental diets: 1) isolated soybean protein diet (CP 20%) supplemented with essential amino acids (methionine, threonine and tryptophan); 2) isolated soybean protein diet (CP 20%) not supplemented with amino acids; 3) casein diet (CP 20%) supplemented with essential amino acids (arginine, methionine and glycine); 4) casein diet (CP 20%) not supplemented with amino acids; and 5) isolated soybean protein diet (CP 5%) supplemented with essential amino acids (methionine, threonine and tryptophan). The calculated ME of all diets was set at 12.6 kJ/g. The composition of the experimental diets is shown in Table 2
. On d 7 of consuming the experimental diets, blood, brain, gizzard, liver and kidney were removed as described in Experiment 1.
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Plasma concentrations of IGF-I (Experiments 1 and 3) and insulin (Experiment 1) were measured by RIA. The procedure was described in detail previously (21
). Polyclonal antiserum raised in rabbits against human IGF-I was generously given by Dr. P. C. Owens (Department of Obstetrics and Gynecology, Adelaide University, Adelaide, SA, Australia). Polyclonal antiserum from guinea pigs raised against chicken insulin was generously donated by Dr. J. P. McMurtry (USDA, Growth Biology Laboratory, Beltsville, MD).
Hepatic IGF-I mRNA.
To detect hepatic IGF-I mRNA, a ribonuclease protection assay was conducted according to the procedure described previously (4
) (Experiment 1). Total RNA was extracted from liver samples by using a commercial total RNA extraction reagent TRISOL Reagent (Life Technologies, Frederick, MD). Chicken IGF-I cDNA was generously donated by Dr. P. Rotwein (Washington University School of Medicine, St. Louis, MO) and subcloned into plasmid pSP73 (Bresatec, Adelaide, SA, Australia) for generating sense and antisense chicken IGF-I mRNA.
Tissue IGFBP-2 mRNA.
Total RNA was extracted from tissue samples by using a commercial total RNA extraction reagent TRISOL Reagent. Thereafter, to detect tissue IGFBP-2, Northern hybridization was conducted according to the procedure described by Sambrook et al. (22
). In this assay, 10 µg of RNA sample was used. Chicken IGFBP-2 cDNA (11
) was generously donated by Dr. T. J. Schoen (Laboratory of Retinal Cell and Molecular Biology, National Eye Institute, NIH, Bethesda, MD). The chicken IGFBP-2 cDNA fragment was isolated from plasmid vector pBluescript KS + by cutting with Not I/Sal I. In the present study, to certify that equal amounts of total RNA samples were loaded onto agarose gels for electrophoresis, chicken ribosomal protein S17 mRNA was measured. Chicken ribosomal protein S17 cDNA (23
) was generously given by Dr. B. Trueb (Laboratorium fur Biochemie I, ETH Zentrum, Zurich, Switzerland). Chicken ribosomal protein S17 cDNA was isolated from plasmid vector pUC19 by cutting with Eco RI. The 32P-labeled probes (specific activity 0.2 x 109 dpm/µg) were synthesized according to the protocol of a random primer DNA labeling kit (Takara Shuzo, Kyoto, Japan). The intensities of chicken IGFBP-2 and ribosomal protein S17 bands were measured using a bioimaging analysis system (BAS 2000, Fuji Photo Film,Tokyo,Japan).
Statistical analysis.
In all experiments, data were analyzed by one-way ANOVA to assess the significance of the effects of nutritional treatment. Then, Duncan’s multiple range test was performed to compare all means. In Experiment 3, two-way ANOVA was also performed to test main and interactive effects of protein source and amino acid supplementation of CP 20% diets. All statistical analyses were performed using the General Linear Model Procedures (GLM; SAS/STAT Version 6, SAS Institute, Cary, NC). Differences between means were considered to be significant at P < 0.05. Regression analysis was also performed using GLM.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Plasma insulin concentration was reduced by 2 d of food deprivation and increased after 2–6 h of refeeding (Table 3
). Food deprivation significantly decreased plasma IGF-I concentration to about half of that of fed controls. When chickens were refed a commercial diet, plasma IGF-I concentrations remained at a low level similar to unfed controls until sometime after 24 h of refeeding and had reached a level greater than that of fed controls by 48 h after refeeding. Food deprivation for 2 d halved hepatic IGF-I mRNA levels compared with fed controls. Hepatic IGF-I gene expression was stimulated by refeeding and increased gradually during the period of refeeding.
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Chicken IGFBP-2 mRNA was detected in the brain, gizzard, liver and kidney, but was barely detectable in the small intestine. In brain, IGFBP-2 mRNA was detected in fed chicks; it was significantly decreased by 2 d of food deprivation and did not increase during 24 h of refeeding (Fig. 1A
). IGFBP-2 gene expression was detected in the gizzard of fed chickens (Fig. 1
B) and was almost doubled by food deprivation. After 1 h of refeeding, gizzard IGFBP-2 gene expression remained at the level of food-deprived chickens but thereafter, the level of IGFBP-2 mRNA decreased to that of fed controls. In the liver of fed chicks, almost no IGFBP-2 mRNA was detected (Fig. 1
C). However, 2 d of food deprivation markedly increased hepatic IGFBP-2 gene expression, and this level remained even after 2 h of refeeding. After 6 h of refeeding, little IGFBP-2 mRNA could be detected in the liver. In the kidney, IGFBP-2 gene expression was detected but was not influenced by food deprivation and refeeding (Fig. 1
D).
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When chickens were food deprived for 2 d, hepatic IGFBP-2 mRNA levels increased to about twice that of fed controls (Fig. 2
). Refeeding all of the experimental diets significantly decreased IGFBP-2 gene expression to the level of fed controls within 6 h.
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A significant interaction between protein source and amino acid supplementation was observed for body weight gain (Table 4
). When young chickens were fed a 20% soybean protein diet but not the 20% casein diet, amino acid supplementation increased body weight gain. Birds fed a low, 5% soybean protein diet lost a small amount of body weight during the experiment. Food intake did not differ among the groups.
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Tissue IGFBP-2 mRNA (Experiment 3).
In brain, there was no interaction between protein source and amino acid supplementation on IGFBP-2 mRNA, but its level was lower in chickens fed the low protein diet than in all other groups (Table 5
). A significant interaction between protein source and amino acid supplementation was observed in gizzard IGFBP-2 mRNA level, and the level was greater in chickens fed the low protein diet than in those fed the 20% soybean protein, amino acid–supplemented diet. A lack of amino acids in the 20% soybean protein diet significantly increased IGFBP-2 mRNA in the gizzard to the level of the low protein group. Regardless of amino acid supplementation, feeding 20% casein diets increased gizzard IGFBP-2 mRNA to a level similar to that of chickens fed a low protein diet. In the kidney, neither quantity and quality of dietary protein affected IGFBP-2 mRNA level. Hepatic IGFBP-2 mRNA was not detected in any of the treatment groups.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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,25
). Like mammals, several findings suggest the important role of IGF-I for growth and metabolism in chickens (2
–6
). In the present study, plasma concentration of IGF-I varied widely with changes in nutritional conditions. As shown in Table 3
, 2
d of food deprivation significantly decreased both plasma IGF-I concentration and hepatic IGF-I mRNA level to about a half of that observed in fed controls. When chickens were administrated radiolabeled chicken IGF-I, the calculated half-life of IGF-I in plasma was only 54 min (9
). Therefore, plasma IGF-I concentration lowered by food deprivation may result from not only the suppression of hepatic IGF-I gene expression but also the rapid clearance of IGF-I from plasma. Lowered plasma IGF-I concentration caused by food deprivation remained low until at least 24 h after refeeding and had reached a level greater than that of fed controls by 48 h of refeeding, which is in agreement with the results of McMurtry (26
). However, hepatic IGF-I mRNA level decreased by 2 d of food deprivation had recovered by 2 h of refeeding. Taken together, these results suggest that food deprivation markedly reduces plasma IGF-I concentration and that the restoration of lowered plasma IGF-I concentration requires longer than 24 h after refeeding associated with the period for translating IGF-I mRNA to IGF-I peptide.
As shown in Table 4
, feeding a low protein diet markedly decreased plasma IGF-I concentration, consistent with other chicken studies (2
,4
,6
). Moreover, a significant interaction between protein source and amino acid supplementation was observed for body weight gain. In general, methionine (methionine + cystine) and arginine are the first limiting amino acids in isolated soybean protein and casein diets, respectively (Table 2)
. The methionine concentration in the isolated soybean protein diet without amino acid supplements corresponded to 63% of the recommended methionine requirement of 3.0 g/kg (19
). The arginine level in the casein diet without amino acid supplements corresponded to 81% of the recommended arginine requirement of 10.0 g/kg (19
). Except for methionine in the soybean protein diet and arginine in the casein diet, dietary essential amino acids met the recommended requirements. As shown in Table 4
, body weight gain and plasma IGF-I concentration of chickens fed a casein diet without amino acid supplements were higher than those of birds fed an isolated soybean protein diet without amino acid supplements. Moreover, body weight gain and plasma IGF-I concentration were related to one another. The regression equation between body weight gain and plasma IGF-I concentration was as follows: body weight gain (g/7 d) = -61 + 8.0 x plasma IGF-I concentration (µg/L) (r = 0.92, P < 0.0001). These results strongly suggest that in young chickens, body weight gain was influenced not only by dietary protein quantity but also by dietary protein quality, especially essential amino acid levels, and that body weight gain may be regulated in part by changes in plasma IGF-I concentration.
In the circulation and extracellular fluids, IGF-I is bound to IGFBP; in avian species, IGFBP-2 is thought to be the main IGFBP (9
,11
,27
). The objective of the present study was to examine the influence of nutrition on IGFBP-2 gene expression in various tissues of young chickens and to elucidate the relation of circulating IGFBP-2 to IGF-I in the plasma. In Experiment 3, feeding a low protein diet did not stimulate hepatic IGFBP-2 gene expression of young chickens. Leili and Scanes (28
) reported that one of the plasma IGFBP corresponding to IGFBP-2 did not increase when chickens were fed low protein diets. These results indicate that the stimulation of hepatic IGFBP-2 gene expression by dietary protein restriction is less than that due to food deprivation. As represented in Figures 1
c and 2, hepatic IGFBP-2 gene expression was increased by 2 d of food deprivation and decreased to the level of fed controls within 6 h of refeeding with any macronutrients. In rats, a similar response of hepatic IGFBP-2 gene expression to food deprivation followed by refeeding a complete diet was observed (14
,29
). Interestingly, the response of hepatic IGFBP-2 gene expression to food deprivation and refeeding was opposite to that of hepatic IGF-I gene expression (Table 3)
, which suggests two contrary functions of IGFBP-2 in plasma. Recently Hoeflich et al. (30
) generated transgenic mice in which expression of a mouse IGFBP-2 complementary DNA was controlled by the cytomegalovirus promoter and revealed that IGFBP-2 represented a negative regulator of postnatal growth in mice, potentially by reducing the bioavailability of IGF-I. Therefore, the first function of IGFBP-2 in plasma is the inhibitory activity, preventing IGF-I from associating with cell surface receptors and inhibiting IGF-I activity. Second, IGFBP-2 may act as stable reservoir of circulating free IGF-I, which is readily degraded by proteases, especially under conditions of malnutrition. Furthermore, McMurtry et al. (31
) showed that IGF-I associated with IGFBP had a longer half-life than free IGF-I. Therefore, the rapid increase in tissue IGFBP-2 gene expression by food deprivation (Fig. 1)
would provide a large amount of circulating IGFBP-2 to prevent the proteolysis of IGF-I by forming the IGF-IGFBP complex in plasma.
The gizzard is one of gastrointestinal organs specific to avians. Because the effects on gizzard IGFBP-2 gene expression due to changes in nutritional conditions have not been clarified so far, in the present study, the influence on IGFBP-2 gene expression in the gizzard was also investigated. As shown in Figure 1
b, the response of gizzard IGFBP-2 gene expression was similar to that in the liver. Previously, it was determined that one of the plasma IGFBP, which seemed to be IGFBP-2, detected by Western ligand blot was increased by both food restriction and protein limitation in chickens (4
,32
). Further, the response of circulating IGFBP-2 protein to malnutrition seemed to correspond to the alteration in gizzard and hepatic IGFBP-2 gene expression. Therefore, the mechanisms regulating plasma IGFBP-2 concentration may be attributed to the changes in IGFBP-2 gene expression in the liver and gizzard.
Several years ago, Schoen et al. (11
) characterized a genomic DNA for chicken IGFBP-2 and revealed that it contains an insulin response element in the 5' upstream region. Similar insulin response elements have been identified in both human IGFBP-1 and rat IGFBP-3 genes (33
,34
), whose expression is negatively regulated by insulin. As shown in Table 3
, plasma insulin concentration was reduced by 2 d of food deprivation and increased after 2–6 h of refeeding, which was the opposite of the changes in gizzard and hepatic IGFBP-2 gene expression. Recently, we demonstrated in vivo that gizzard and hepatic IGFBP-2 gene expression increased by 2 d of food deprivation was rapidly reduced by intravenous administration of bovine insulin in young chickens (35
). These results indicate that in these birds, the reduction in gizzard and hepatic IGFBP-2 gene expression in vivo stimulated by malnutrition may be regulated in part by the increase in plasma insulin concentration via an insulin-response element.
In the present study, the significant interaction between protein source and amino acid supplementation was observed in gizzard IGFBP-2 mRNA levels. In both the soybean protein and casein diet groups, the deficiency of essential amino acids stimulated an increase in gizzard IGFBP-2 gene expression to the level of the low protein diet group (Table 4)
. Although amino acid supplementation to the soybean protein diet significantly decreased gizzard IGFBP-2 mRNA level, a similar reduction was not observed in chickens fed a casein diet supplemented with amino acids. It has been reported that serum amino acid concentrations are considerably different in rats fed dietary casein or isolated soybean protein (36
). Furthermore, the evidence that several amino acids have the potency to regulate the expression of hepatic IGFBP-1 gene has been provided (37
–40
). These findings suggest that the different responses of gizzard IGFBP-2 gene expression to different dietary protein sources, casein and isolated soybean protein, may be due to the change in serum amino acid concentrations; this issue should be examined in the future.
In contrast to the liver and gizzard, as represented in Figure 1a
and Table 5
, IGFBP-2 mRNA was detected in the brain of well-fed young chickens and was reduced by food deprivation and feeding a low protein diet, which supports to the similar results observed in rats (15
,17
). These results suggest that brain IGFBP-2 gene expression may be decreased under conditions of malnutrition. Regardless of the considerable amount of IGFBP-2 in the brain, its specific function has not been clarified. Recently, Russo et al. (41
) reported that IGFBP-2 was abundant in rat olfactory bulb, a brain region undergoing postnatal differentiation and remodeling, and was bound to proteoglycans in cell membranes of the olfactory bulb. These findings suggest that cell-associated IGFBP-2 may have an important role in directing IGF-I to specific sites in the brain. Surprisingly, as shown in Figure 1
d, no alteration of IGFBP-2 gene expression was observed in the kidney of food-deprived and refed chickens. Bisbis et al. (42
) reported that insulin binding to renal membrane was increased by 2 d of food deprivation and decreased rapidly by refeeding, indicating that renal IGFBP-2 gene expression seems to be independent of the binding of insulin to its receptor.
In conclusion, in young chickens, gizzard and hepatic IGFBP-2 mRNA levels were increased by 2 d of food deprivation and decreased rapidly by refeeding. Feeding a low protein diet increased gizzard IGFBP-2 mRNA levels. The interactive effect of dietary protein source and amino acid supplementation was observed in gizzard IGFBP-2 mRNA levels. The supplementation of amino acids to a soybean protein diet, but not a casein diet, significantly reduced gizzard IGFBP-2 gene expression stimulated by amino acid deficiency. The response of IGFBP-2 gene expression to variations in nutritional status was rapid and different in several tissues of young chickens, which would help modulate the growth-promoting effect of circulating IGF-I by making the IGF-IGFBP complex.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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3 Present address: Laboratory of Animal Nutrition, Department of Animal Husbandry, Faculty of Agriculture, Syiah Kuala University, Darussalam, Banda Aceh 23111-Indonesia.
4 Present address: Cooperative Research Centre for Tissue Growth and Repair, PO Box 10065 Gouger Street, Adelaide, South Australia 5000, Australia.
5 Abbreviations used; CP, crude protein; IGF-I, insulin-like growth factor-I; IGFBP-2, insulin-like growth factor binding protein-2; ME, metabolizable energy.
Manuscript received 18 June 2001. Initial review completed 23 July 2001. Revision accepted 22 October 2001.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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