Tissue IGF-I protein and mRNA responses to a single injection of somatotropin

Swine were divided into four groups of 11 animals at 40 kg body wt. Swine within a group were given a single porcine somatotropin (pST) injection (200 micrograms/kg) or buffer at 0800. Blood, liver (L), latissimus dorsi (LD), semitendinosus (STS), vastus lateralis (VL), dorsal subcutaneous (SQ), and perirenal (PR) adipose tissues were sampled at 0, 1, 2, 4, 8, 12, 16, and 24 h postinjection. Blood urea nitrogen was depressed by 16 h. Insulin was elevated by approximately 350% at 8 h. Lipogenic enzyme activities in adipose tissues were not affected by pST treatment. Insulin-like growth factor I (IGF-I) mRNA levels increased rapidly in SQ, PR, and L to a single pST administration, whereas they increased only slightly in VL. IGF-I mRNA concentrations in LD and STS were unaffected by pST treatment. IGF-I protein content of tissues changed little during the first 24 h postinjection. These data suggest that individual tissues differ in timing and degree of response to pST. Conflicting results reported after pST treatment could, in part, be due to tissue selection for sampling or sample timing.

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Regulation of porcine insulin-like growth factor I and growth hormone receptor mRNA expression by energy status

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.1994.266.5.e776 ◽

1994 ◽

Vol 266 (5) ◽

pp. E776-E785 ◽

Cited By ~ 5

Author(s):

P. A. Weller ◽

M. J. Dauncey ◽

P. C. Bates ◽

J. M. Brameld ◽

P. J. Buttery ◽

...

Keyword(s):

Growth Hormone ◽

Growth Factor ◽

Growth Rates ◽

Mrna Levels ◽

Energy Status ◽

Factor I ◽

Receptor Mrna ◽

Gh Receptor ◽

Igf I ◽

Class 1

Regulation of insulin-like growth factor I (IGF-I) and growth hormone (GH) receptor mRNA in liver and muscle by energy status was assessed in 2-mo-old pigs by altering thermoregulatory demand and energy intake over a 5-wk period to produce a range of plasma IGF-I concentrations from 3.5 +/- 0.7 to 28.9 +/- 6.2 nmol/l. These values were related directly to growth rates (0.06 +/- 0.02 to 0.44 +/- 0.01 kg/day) and total hepatic IGF-I mRNA levels. Increased growth rates were accompanied by an increase in hepatic class 1 and class 2 IGF-I mRNA levels and an increase in the ratio of class 2 to class 1 IGF-I mRNA in liver, suggesting a distinct role for class 2 expression in the endocrine growth response. High levels of class 1 transcripts and a virtual absence of class 2 transcripts characterized all muscle tissues examined, and there was no correlation with plasma IGF-I levels. This suggests that growth promotion in response to increased energy status is regulated via endocrine hepatic IGF-I rather than via a paracrine response. The levels of GH receptor mRNA were positively correlated with overall growth rate (P < 0.005) in liver and negatively correlated (P < 0.05) in muscle, indicating distinct tissue-specific effects of energy status.

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Insulin-like growth factor I stimulates degradation of an mRNA transcript encoding the 14 kDa ubiquitin-conjugating enzyme

Biochemical Journal ◽

10.1042/bj3190455 ◽

1996 ◽

Vol 319 (2) ◽

pp. 455-461 ◽

Cited By ~ 44

Author(s):

Simon S WING ◽

Nathalie BEDARD

Keyword(s):

Skeletal Muscle ◽

Growth Factor ◽

Binding Proteins ◽

Mrna Levels ◽

Insulin Like Growth Factor ◽

Mrna Transcript ◽

Factor I ◽

Igf I ◽

Ubiquitin Conjugating Enzyme ◽

Growth Factor I

Upon fasting, the ubiquitin-dependent proteolytic system is activated in skeletal muscle in parallel with the increases in rates of proteolysis. Levels of mRNA encoding the 14 kDa ubiquitin-conjugating enzyme (E214k), which can catalyse the first irreversible reaction in this pathway, rise and fall in parallel with the rates of proteolysis [Wing and Banville (1994) Am. J. Physiol. 267, E39-E48], indicating that the conjugation of ubiquitin to proteins is a regulated step. To characterize the mechanisms of this regulation, we have examined the effects of insulin, insulin-like growth factor I (IGF-I) and des(1–3) insulin-like growth factor I (DES-IGF-I), which does not bind IGF-binding proteins, on E214k mRNA levels in L6 myotubes. Insulin suppressed levels of E214k mRNA with an IC50 of 4×10-9 M, but had no effects on mRNAs encoding polyubiquitin and proteasome subunits C2 and C8, which, like E214k, also increase in skeletal muscle upon fasting. Reduction of E214k mRNA levels was more sensitive to IGF-I with an IC50 of approx. 5×10-10 M. During the incubation of these cells for 12 h there was significant secretion of IGF-I-binding proteins into the medium. DES-IGF-I, which has markedly reduced affinity for these binding proteins, was found to potently reduce E214k mRNA levels with an IC50 of 3×10-11 M. DES-IGF-I did not alter rates of transcription of the E214k gene, but enhanced the rate of degradation of the 1.2 kb mRNA transcript. The half-life of the 1.2 kb transcript was approximately one-third that of the 1.8 kb transcript and can explain the more marked regulation of this transcript observed previously. This indicates that the additional 3´ non-coding sequence in the 1.8 kb transcript confers stability. These observations suggest that IGF-I is an important regulator of E214k expression and demonstrate, for the first time, stimulation of degradation of a specific mRNA transcript by this hormone, while overall RNA accumulates.

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Altered skeletal pattern of gene expression in response to spaceflight and hindlimb elevation

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.1994.267.6.e822 ◽

1994 ◽

Vol 267 (6) ◽

pp. E822-E827 ◽

Cited By ~ 18

Author(s):

D. D. Bikle ◽

J. Harris ◽

B. P. Halloran ◽

E. Morey-Holton

Keyword(s):

Gene Expression ◽

Alkaline Phosphatase ◽

Bone Formation ◽

Bone Mineralization ◽

Mrna Levels ◽

Skeletal Unloading ◽

Compensatory Response ◽

Factor I ◽

Igf I ◽

Skeletal Pattern

Spaceflight leads to osteopenia, in part by inhibiting bone formation. Using an animal model (hindlimb elevation) that simulates the weightlessness of spaceflight, we and others showed a reversible inhibition of bone formation and bone mineralization. In this study, we have measured the mRNA levels of insulin-like growth factor I (IGF-I), IGF-I receptor (IGF-IR), alkaline phosphatase, and osteocalcin in the tibiae of rats flown aboard National Aeronautics and Space Administration Shuttle Flight STS-54 and compared the results with those obtained from their ground-based controls and from the bones of hindlimb-elevated animals. Spaceflight and hindlimb elevation transiently increase the mRNA levels for IGF-I, IGF-IR, and alkaline phosphatase but decrease the mRNA levels for osteocalcin. The changes in osteocalcin and alkaline phosphatase mRNA levels are consistent with a shift toward decreased maturation, whereas the rise in IGF-I and IGF-IR mRNA levels may indicate a compensatory response to the fall in bone formation. We conclude that skeletal unloading during spaceflight or hindlimb elevation resets the pattern of gene expression in the osteoblast, giving it a less mature profile.

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Inhibition by Interleukin-1β and Tumor Necrosis Factor-α of the Insulin-Like Growth Factor I Messenger Ribonucleic Acid Response to Growth Hormone in Rat Hepatocyte Primary Culture*

Endocrinology ◽

10.1210/endo.138.3.4966 ◽

1997 ◽

Vol 138 (3) ◽

pp. 1078-1084 ◽

Cited By ~ 80

Author(s):

Jean-Paul Thissen ◽

Josiane Verniers

Keyword(s):

Gene Expression ◽

Growth Factor ◽

Tumor Necrosis Factor ◽

Tumor Necrosis ◽

Mrna Levels ◽

Factor I ◽

Tnf Α ◽

Igf I ◽

Necrosis Factor ◽

I Gene

Abstract The cytokines are the putative mediators of the catabolic reaction that accompanies infection and trauma. Evidence suggests that their catabolic actions are indirect and potentially mediated through changes in hormonal axis such as the hypothalamo-pituitary-adrenal axis. Insulin-like growth factor I (IGF-I) is a GH-dependent growth factor that regulates the protein metabolism. To determine whether cytokines can directly inhibit the production of IGF-I by the liver, we investigated the regulation of IGF-I gene expression by interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α (10 ng/ml) in a model of rat primary cultured hepatocytes. Hepatocytes were isolated by liver collagenase perfusion and cultured on Matrigel 48 h before experiments. Each experiment was performed in at least three different animals. In the absence of GH, IL-1β and TNF-α did not affect the IGF-I messenger RNA (mRNA) basal levels, whereas IL-6 increased it by a factor of 2.5 after 24 h (P < 0.05). GH (500 ng/ml) alone stimulated the IGF-I gene expression markedly (5- to 10-fold increase) after 24 h (P < 0.001). IL-1β, and TNF-α to a lesser extent, dramatically inhibited the IGF-I mRNA response to GH (IL-1β: −82%, P < 0.001 and TNF-α: −47%, P < 0.01). The half-maximal inhibition of the IGF-I mRNA response to GH was observed for a concentration of IL-1β between 0.1 and 1 ng/ml. Moreover, IL-1β abolished the IL-6-induced IGF-I mRNA response. In contrast, IL-6 did not impair the IGF-I mRNA response to GH. To determine the potential role of the GH receptor (GHR) and the GH-binding protein (GHBP) in this GH resistance, we assessed the GHR and GHBP mRNAs response to these cytokines. GH alone did not affect the GHR/GHBP mRNA levels. IL-1β markedly decreased the GHR and GHBP mRNA levels (respectively, −68% and −60%, P < 0.05). Neither TNF-α nor IL-6 affected the GHR/GHBP gene expression. In conclusion, our results show that IL-1β, and TNF-α to a lesser extent, blunt the IGF-I mRNA response to GH. The resistance to GH induced by IL-1β might be mediated by a decrease of GH receptors, as suggested by the marked reduction of GHR mRNA. These findings suggest that decreased circulating IGF-I, in response to infection and trauma, may be caused by a direct effect of cytokines at the hepatocyte level.

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Identification of Androgen Response Elements in the Insulin-Like Growth Factor I Upstream Promoter

Endocrinology ◽

10.1210/en.2006-1653 ◽

2007 ◽

Vol 148 (6) ◽

pp. 2984-2993 ◽

Cited By ~ 63

Author(s):

Yong Wu ◽

Weidong Zhao ◽

Jingbo Zhao ◽

Jiangping Pan ◽

Qiaqia Wu ◽

...

Keyword(s):

Binding Sites ◽

Molecular Mechanisms ◽

Reporter Genes ◽

Mrna Levels ◽

Lncap Cells ◽

Factor I ◽

Upstream Promoter ◽

Igf I ◽

In Cis ◽

I Gene

Testosterone stimulates the expression of IGF-I in cells and tissues that include prostate, muscle and muscle satellite cells, and the uterus. Here, the molecular mechanisms of this effect of testosterone were explored. Testosterone increased IGF-I mRNA levels in HepG2 and LNCaP cells and stimulated the activity of reporter genes controlled by 1.6 kb of the upstream promoter of the human IGF-I gene. An androgen-responsive region that was located between −1320 and −1420 bases upstream of the first codon was identified by truncation studies. The androgen-responsive region was found to contain two sequences resembling known androgen receptor (AR)-binding sites from the Pem1 gene. Reporter genes incorporating these sequences were strongly stimulated by androgens. Each of the androgen-responsive elements (AREs) bound recombinant AR-DNA-binding domain in gel-shift experiments; binding was greatly enhanced by sequences flanking the apparent AR-binding half-sites. Testosterone induced recruitment of AR to sequences of genomic DNA containing these AREs. The two AREs were activated 5-fold more by AR than glucocorticoid receptor. Collectively, these findings indicate the presence of two AREs within the IGF-I upstream promoter that act in cis to activate IGF-I expression. These AREs seem likely to contribute to the up-regulation of the IGF-I gene in prostate tissues, HepG2 cells, and potentially other tissues.

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Regulation by Adrenocorticotropin (ACTH), Angiotensin II, Transforming Growth Factor-β, and Insulin-Like Growth Factor I of Bovine Adrenal Cell Steroidogenic Capacity and Expression of ACTH Receptor, Steroidogenic Acute Regulatory Protein, Cytochrome P450c17, and 3β-Hydroxysteroid Dehydrogenase*

Endocrinology ◽

10.1210/endo.141.5.7457 ◽

2000 ◽

Vol 141 (5) ◽

pp. 1599-1607 ◽

Cited By ~ 37

Author(s):

Christine Le Roy ◽

J. Yuan Li ◽

Douglas M. Stocco ◽

Dominique Langlois ◽

José M. Saez

Keyword(s):

Growth Factor ◽

Transforming Growth Factor ◽

Regulatory Protein ◽

Hydroxysteroid Dehydrogenase ◽

Steroidogenic Acute Regulatory Protein ◽

Mrna Levels ◽

Factor I ◽

Acth Receptor ◽

Igf I ◽

Steroidogenic Acute Regulatory

Abstract The purpose of this study was to evaluate the time-course effect of a 36-h treatment with ACTH (10−8m), transforming growth factor-β1 (TGFβ1; 10−10m), angiotensin II (AngII; 10−7m), and insulin-like growth factor I (IGF-I; 10−8m) on the steroidogenic capacity of bovine adrenocortical cells (BAC) and on messenger RNA (mRNA) levels of ACTH receptor, cytochrome P450c17, 3β-hydroxysteroid dehydrogenase (3βHSD), steroidogenic acute regulatory protein (StAR), and StAR protein. ACTH and IGF-I enhanced, in a time-dependent manner, the acute 2-h ACTH-induced cortisol production, whereas TGFβ1 and AngII markedly reduced it. ACTH, IGF-I, and AngII increased ACTH receptor mRNA, but the opposite was observed after TGFβ1 treatment. ACTH and IGF-I increased P450c17 and 3βHSD mRNAs, whereas AngII and TGFβ1 had the opposite effects. However, the effects of the four peptides on ACTH-induced cortisol production appeared before any significant alterations of the mRNA levels occurred. The most marked and rapid effect of the four peptides was on StAR mRNA. The stimulatory effect of ACTH was seen within 1.5 h, peaked at 4–6 h, and declined thereafter, but at the end of the 36-h pretreatment, the levels of StAR mRNA and protein were higher than those in control cells. IGF-I also enhanced StAR mRNA levels within 1.5 h, and these levels remained fairly constant. The effects of AngII on StAR mRNA expression were biphasic, with a peak within 1.5–3 h, followed by a rapid decline to almost undetectable levels of both mRNA and protein. TGFβ1 had no significant effect during the first 3 h, but thereafter StAR mRNA declined, and at the end of the experiment the StAR mRNA and protein were almost undetectable. Similar results were observed when cells were treated with ACTH plus TGFβ1. A 2-h acute ACTH stimulation at the end of the 36-h pretreatment caused a higher increase in StAR mRNA and protein in ACTH- or IGF-I-pretreated cells than in control cells, which, in turn, had higher levels than cells pretreated with TGFβ1, ACTH plus TGFβ1, or AngII. These results and the fact that the stimulatory (IGF-I) or inhibitory (AngII and TGFβ1) effects on ACTH-induced cortisol production were more pronounced than those on the ability of cells to transform pregnenolone into cortisol strongly suggest that regulation of StAR expression is one of the main factors, but not the only one, involved in the positive (IGF-I) or negative (TGFβ1 and AngII) regulation of BAC for ACTH steroidogenic responsiveness. A high correlation between steady state mRNA level and acute ACTH-induced cortisol production favors this conclusion.

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Differential regulation of the expression of fast-type sarcoplasmic-reticulum Ca2+-ATPase by thyroid hormone and insulin-like growth factor-I in the L6 muscle cell line

Biochemical Journal ◽

10.1042/bj3030467 ◽

1994 ◽

Vol 303 (2) ◽

pp. 467-474 ◽

Cited By ~ 9

Author(s):

M H M Thelen ◽

A Muller ◽

M J Zuidwijk ◽

G C van der Linden ◽

W S Simonides ◽

...

Keyword(s):

Transcriptional Regulation ◽

Growth Factor ◽

Thyroid Hormone ◽

Sarcoplasmic Reticulum ◽

Mrna Stability ◽

Half Life ◽

Mrna Levels ◽

Factor I ◽

Igf I ◽

Mrna Half Life

The aim of this study was to investigate the mechanism(s) underlying the thyroid-hormone (L-tri-iodothyronine, T3)-induced elevation of fast-type sarcoplasmic-reticulum Ca(2+)-ATPase (SERCA1) levels in L6 myotubes and the potentiating effect of insulin-like growth factor-I (IGF-I) [Muller, van Hardeveld, Simonides and van Rijn (1991) Biochem. J. 275, 35-40]. T3 increased the SERCA1 protein level (per microgram of DNA) by 160%. The concomitant increase in the SERCA1 mRNA level was somewhat higher (240%). IGF-I also increased SERCA1 protein (110%) and mRNA levels (50%), whereas IGF-I + T3 increased SERCA1 protein and mRNA levels by 410% and 380% respectively. These SERCA1 mRNA analyses show that the more-than-additive action of T3 and IGF-I on SERCA1 expression is, at least in part, pre-translational in nature. Further studies showed that the half-life of SERCA1 protein in L6 cells (17.5 h) was not altered by T3. In contrast, IGF-I prolonged the half-life of SERCA1 protein 1.5-1.9-fold, which may contribute to the disproportional increase in SERCA1 protein content compared with mRNA by IGF-I. Measurements of SERCA1 mRNA half-life (as determined by actinomycin D chase) showed no difference from the control values (15.5 h) in the presence of T3 or IGF-I alone. When T3 and IGF-I were both present, the SERCA1 mRNA half-life was prolonged 2-fold. No significant effects of T3 and IGF-I were observed on the half-life of total protein (37.4 h) and total RNA (37.0 h). The absence of an effect of T3 on SERCA1 protein and mRNA stability, when it was present alone, suggested transcriptional regulation, which was confirmed by nuclear run-on experiments, showing a 3-fold increase in transcription frequency of the SERCA1 gene by T3. We conclude that the synergistic stimulating effects of T3 and IGF-I on SERCA1 expression are the result of both transcriptional and post-transcriptional regulation. T3 acts primarily at the transcriptional level by increasing the transcription frequency of the SERCA1 gene, whereas IGF-I seems to act predominantly at post-transcriptional levels by enhancing SERCA1 protein and mRNA stability, the latter, however, only in the presence of T3.

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IGF-I measurement across blood, interstitial fluid, and muscle biocompartments following explosive, high-power exercise

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.00275.2012 ◽

2012 ◽

Vol 303 (10) ◽

pp. R1080-R1089 ◽

Cited By ~ 13

Author(s):

Bradley C. Nindl ◽

Maria L. Urso ◽

Joseph R. Pierce ◽

Dennis E. Scofield ◽

Brian R. Barnes ◽

...

Keyword(s):

Interstitial Fluid ◽

Vastus Lateralis ◽

Repetition Maximum ◽

Time Points ◽

Factor I ◽

Plyometric Exercise ◽

Igf I ◽

Mrna Content ◽

Exercise Induced ◽

Induced Changes

Insulin-like growth factor-I (IGF-I) resides across different biocompartments [blood, interstitial fluid (ISF), and muscle]. Whether circulating IGF-I responses to exercise reflect local events remains uncertain. We measured the IGF-I response to plyometric exercise across blood, ISF, and muscle biopsy from the vastus lateralis. Twenty volunteers (8 men, 12 women, 22 ± 1 yr) performed 10 sets of 10 plyometric jump repetitions at a 40% 1-repetition maximum. Blood, ISF, and muscle samples were taken pre- and postexercise. Circulating IGF-I increased postexercise: total IGF-I (preexercise = 546 ± 42, midexercise = 585 ± 43, postexercise = 597 ± 45, +30 = 557 ± 42, +60 = 536 ± 40, +120 = 567 ± 42 ng/ml; midexercise, postexercise, and +120 greater than preexercise, P < 0.05); Free IGF-I (preexercise = 0.83 ± 0.09, midexercise = 0.78 ± 0.10, postexercise = 0.79 ± 0.11, +30 = 0.93 ± 0.10, +60 = 0.88 ± 0.10, + 120 = 0.91 ± 0.11 ng/ml; +30 greater than all other preceding time points, P < 0.05). No exercise-induced changes were observed for ISF IGF-I (preexercise = 2.35 ± 0.29, postexercise = 2.46 ± 0.35 ng/ml). No changes were observed for skeletal muscle IGF-I protein, although IGF-I mRNA content increased ∼40% postexercise. The increase in circulating total and free IGF-I was not correlated with increases in ISF IGF-I or muscle IGF-I protein content. Our data indicate that exercise-induced increases in circulating IGF-I are not reflective of local IGF-I signaling.

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Effects of tri-iodothyronine and insulin-like growth factor-I (IGF-I) on alkaline phosphatase activity, [3H]thymidine incorporation and IGF-I receptor mRNA in cultured rat epiphyseal chondrocytes

Journal of Endocrinology ◽

10.1677/joe.0.1350115 ◽

1992 ◽

Vol 135 (1) ◽

pp. 115-123 ◽

Cited By ~ 42

Author(s):

C. Ohlsson ◽

A. Nilsson ◽

O. Isaksson ◽

J. Bentham ◽

A. Lindahl

Keyword(s):

Alkaline Phosphatase ◽

Growth Factor ◽

Thymidine Incorporation ◽

Control Culture ◽

Mrna Levels ◽

Factor I ◽

Receptor Mrna ◽

Alp Activity ◽

Igf I ◽

Treated Culture

ABSTRACT The effects of tri-iodothyronine (T3) and insulin-like growth factor-I (IGF-I) on [3H]thymidine incorporation, alkaline phosphatase (ALP) activity and IGF-I receptor mRNA levels were studied in rat epiphyseal chondrocytes cultured in monolayer. Chondrocytes from enzymatically digested rat tibia epiphyseal growth plates were seeded in monolayer culture and precultured for 7–14 days in Ham's F-12 medium supplemented with 10% (v/v) newborn calf serum and 1% (v/v) of a serum substitute. After preculture the medium was changed to Ham's F-12 medium containing 1% (v/v) serum from hypophysectomized rats, and the effects of T3 and/or IGF-I on DNA synthesis ([3H]thymidine incorporation), ALP activity (a late marker of differentiated epiphyseal chondrocytes) and IGF-I receptor mRNA levels were studied. ALP activity was increased by T3 in a dose-dependent manner with a maximal response at 10 μg T3/1 (678 ±86% compared with control culture). The increase in ALP activity was accompanied by a concomitant decrease in [3H]thymidine incorporation (52 ±14% compared with control culture). Human GH (hGH; 50 μg/l) and IGF-I (25 μg/l) had no stimulatory effect on ALP activity. However IGF-I (10 μg/l) exerted an inhibition on the T3 (10 μg/l)-induced increase in ALP activity (64 ± 9% compared with T3-treated culture). T3 (3 μg/l) inhibited the increase in [3H]thymidine incorporation caused by 25 μg IGF-I/l(51 ± 13% compared with IGF-I-treated culture). Furthermore, IGF-I receptor mRNA levels were increased by 10 μg T3/l (137 ±4·2% compared with control culture) while no effect of hGH (50 μg/l) or IGF-I (25 μg/l) was demonstrated. Both T3 and IGF-I were shown to interact with epiphyseal chondrocytes and both substances seemed to affect cell proliferation and maturation and therefore longitudinal bone growth. Furthermore, the results indicated that IGF-I is important for proliferation of the cells while T3 initiates the terminal differentiation of epiphyseal chondrocytes. Journal of Endocrinology (1992) 135, 115–123

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Methionine improves the performance and breast muscle growth of broilers with lower hatching weight by altering the expression of genes associated with the insulin-like growth factor-I signalling pathway

British Journal Of Nutrition ◽

10.1017/s0007114513002419 ◽

2013 ◽

Vol 111 (2) ◽

pp. 201-206 ◽

Cited By ~ 20

Author(s):

Chao Wen ◽

Ping Wu ◽

Yueping Chen ◽

Tian Wang ◽

Yanmin Zhou

Keyword(s):

Growth Factor ◽

Mrna Level ◽

Muscle Growth ◽

Breast Muscle ◽

Broiler Chicks ◽

Mrna Levels ◽

Insulin Like Growth Factor ◽

Factor I ◽

Igf I ◽

Growth Factor I

The present study aimed to investigate the responses of broilers with different hatching weights (HW) to dietary methionine (Met). A total of 192 1-d-old Arbor Acres broiler chicks with different HW (heavy: 48·3 (sem 0·1) g and light: 41·7 (sem 0·1) g) were allocated to a 2 (HW) × 2 (Met) factorial arrangement with six replicates of eight chicks. Control starter (1–21 d) and finisher (22–42 d) diets contained 0·50 and 0·43 % Met, respectively. Corresponding values for a high-Met treatment were 0·60 and 0·53 %. Light chicks had poorer (P< 0·05) growth performance and breast muscle weight and lower (P< 0·05) insulin-like growth factor-I (IGF-I) concentration and mRNA level in breast muscle than heavy chicks when both were fed the control diets. High-Met diets improved performance and promoted breast muscle growth and IGF-I concentration in light chicks (P< 0·05). Increased IGF-I and target of rapamycin (TOR) mRNA levels as well as decreased eIF4E-binding protein 1 (4EBP1), atrogin-1 and forkhead box O 4 (FOXO4) mRNA levels were induced by high-Met diets in light chicks (P< 0·05). In conclusion, the Met requirement of broilers might depend on their HW and Met levels used in the control diets in the present study were adequate for heavy chicks but inadequate for light chicks, resulting in poorer performance and breast muscle growth, which were improved by increasing dietary Met supply presumably through alterations in IGF-I synthesis and gene expression of the TOR/4EBP1 and FOXO4/atrogin-1 pathway.

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