Purification and characterization of the acid-labile subunit of rat serum insulin-like growth factor binding protein complex

Endocrine and cellular physiology and pathology of the insulin-like growth factor acid-labile subunit

The acid-labile subunit (ALS) of the insulin-like growth factor (IGF) binding protein (IGFBP) complex, encoded in humans by IGFALS, has a vital role in regulating the endocrine transport and bioavailability of IGF-1 and IGF-2. Accordingly, ALS has a considerable influence on postnatal growth and metabolism. ALS is a leucine-rich glycoprotein that forms high-affinity ternary complexes with IGFBP-3 or IGFBP-5 when they are occupied by either IGF-1 or IGF-2. These complexes constitute a stable reservoir of circulating IGFs, blocking the potentially hypoglycaemic activity of unbound IGFs. ALS is primarily synthesized by hepatocytes and its expression is lower in non-hepatic tissues. ALS synthesis is strongly induced by growth hormone and suppressed by IL-1β, thus potentially serving as a marker of growth hormone secretion and/or activity and of inflammation. IGFALS mutations in humans and Igfals deletion in mice cause modest growth retardation and pubertal delay, accompanied by decreased osteogenesis and enhanced adipogenesis. In hepatocellular carcinoma, IGFALS is described as a tumour suppressor; however, its contribution to other cancers is not well delineated. This Review addresses the endocrine physiology and pathology of ALS, discusses the latest cell and proteomic studies that suggest emerging cellular roles for ALS and outlines its involvement in other disease states.

Normal range for acid-labile subunit in paediatric patients in Spain and its association with age, sex, pubertal stage and other growth factors

INTRODUCTION

The acid-labile subunit (ALS) plays an important role in the endocrine effects of insulin-like growth factors (IGFs) on target tissues. Historically, it has attracted limited attention. The aim of our study was to describe the normal range of ALS in healthy children and its association with other growth factors.

PATIENTS AND METHODS

We designed a cross-sectional descriptive study. We collected data on age, height, body mass index, gestational age, anthropometry at birth and serum levels of ALS, IGF1 and IGFBP3 in healthy children aged 2-15 years with a normal height. The levels of ALS, IGF1 and IGFBP3 were measured by ELISA. We fitted GAMLSS normalization models to standardize the variables.

RESULTS

Samples were collected from 446 children. In prepubertal children, the levels of ALS, IGF1 and IGFBP3 were positively correlated in both sexes and with age (P < .01). We found significant differences in the levels of ALS, IGF1 and IGFBP3 and the IGF1/IGFBP3 molar ratio between the sexes and higher levels in pubertal boys (P < .01). We generated normal probability plots for each sex for each of the components of the ternary complex and for the IGF1/IGFBP3 and IGFBP3/ALS molar ratios. In addition, we extracted equations from the models for the calculation of z-scores for age and sex.

CONCLUSIONS

This study may contribute age- and sex-specific reference values for IGF1, IGFBP3 and ALS levels and IGF1/IGFBP3 and IGFBP3/ALS ratios in Spanish children and suggests an association between age, sex, and pubertal stage.

The role of acid-labile subunit (ALS) in the modulation of GH-IGF-I action

Acid-labile subunit (ALS) deficiency (ACLSD) constitutes the first monogenic defect involving a member of the Insulin-like Growth Factor (IGF) binding protein system. The lack of ALS completely disrupts the circulating IGF system. Autocrine/paracrine action of local produced IGF-I could explain the mild effect on growth. In the present work we have revised the more relevant clinical and biochemical consequences of complete ACLSD in 61 reported subjects from 31 families. Low birth weight and/or length, reduced head circumference, height between -2 and -3 SD, pubertal delay and insulin resistance are commonly observed. Partial ACLSD could be present in children initially labeled as idiopathic short stature, presenting low IGF-I levels, suggesting that one functional IGFALS allele is insufficient to stabilize ternary complexes. Dysfunction of the GH-IGF axis observed in ACLSD may eventually result in increased risk for type-2 diabetes and tumor progression. Consequently, long term surveillance is recommended in these patients.

Current Insights into the Role of the Growth Hormone-Insulin-Like Growth Factor System in Short Children Born Small for Gestational Age

BACKGROUND

The reason for the insufficient catch-up growth seen in 10% of children born small for gestational age (SGA) is poorly understood. Disturbances in the growth hormone (GH) - insulin-like growth factor (IGF) axis might underlie this failure to show sufficient catch-up growth.

CONCLUSION

This review summarizes insights gained in the molecular and (epi) genetic mechanisms of the GH-IGF axis in short children born SGA. The most notable anomalies of the IGF system are the lowered IGF-I levels in both cord blood and the placenta, and the increased expression of IGF-binding proteins (IGFBP)-1 and IGFBP-2, which inhibit IGF-I, in the placenta of SGA neonates. These observations suggest a decreased bioactivity of IGF-I in utero. IGF-I levels remain reduced in SGA children with short stature, as well as IGFBP-3 and acid-labile subunit levels. Proteolysis of IGFBP-3 appears to be increased.

Blocking transmembrane219 protein signaling inhibits autophagy and restores normal cell death

Autophagy plays a vital role in tumor therapy and survival of dormant tumor cells. Here we describe a novel function of a protein known as Transmembrane 219 (TM219) as an autophagy activator. TM219 is a small membrane protein expressed in all known human tissues except the thymus. We used biochemical approaches to identify calmodulin and calmodulin dependent protein kinase II as a part of TM219 protein complex. Then, we employed in vitro reconstitution system and fluorescence anisotropy to study the requirements of TM219 to bind calmodulin in vitro. We also used this system to study the effects of a synthetic peptide derived from the sequence of the short cytoplasmic tail of TM219 (SCTT) on calmodulin-TM219 receptor interactions. We conjugated SCTT peptide with a pH Low Insertion peptide (pHLIP) for optimal cellular delivery. We finally tested the effects of SCTT-pHLIP on triple negative human breast cancer cells in three dimension culture. Our data defined a novel function of TM219 protein and an efficient approach to inhibit it.

The insulin-like growth factor-1 system in the adult mammalian brain and its implications in central maternal adaptation

Our knowledge on the bioavailability and actions of insulin-like growth factor-1 (IGF-1) has markedly expanded in recent years as novel mechanisms were discovered on IGF binding proteins (IGFBPs) and their ability to release IGF-1. The new discoveries allowed a better understanding of the endogenous physiological actions of IGF-1 and also its applicability in therapeutics. The focus of the present review is to summarize novel findings on the neuronal, neuroendocrine and neuroplastic actions of IGF-1 in the adult brain. As most of the new regulatory mechanisms were described in the periphery, their implications on brain IGF system will also be covered. In addition, novel findings on the effects of IGF-1 on lactation and maternal behavior are described. Based on the enormous neuroplastic changes related to the peripartum period, IGF-1 has great but largely unexplored potential in maternal adaptation of the brain, which is highlighted in the present review.

Amyloid beta-mediated epigenetic alteration of insulin-like growth factor binding protein 3 controls cell survival in Alzheimer's disease

Swedish double mutation (KM670/671NL) of amyloid precursor protein (APP) is reported to increase toxic amyloid β (Aβ) production via aberrant cleavage at the β-secretase site and thereby cause early-onset Alzheimer's disease (AD). However, the underlying molecular mechanisms leading to AD pathogenesis remains largely unknown. Previously, our transcriptome sequence analyses revealed global expressional modifications of over 600 genes in APP-Swedish mutant-expressing H4 (H4-sw) cells compared to wild type H4 cells. Insulin-like growth factor binding protein 3 (IGFBP3) is one gene that showed significantly decreased mRNA expression in H4-sw cells. In this study, we investigated the functional role of IGFBP3 in AD pathogenesis and elucidated the mechanisms regulating its expression. We observed decreased IGFBP3 expression in the H4-sw cell line as well as the hippocampus of AD model transgenic mice. Treatment with exogenous IGFBP3 protein inhibited Aβ_1–42- induced cell death and caspase-3 activity, whereas siRNA-mediated suppression of IGFBP3 expression induced cell death and caspase-3 cleavage. In primary hippocampal neurons, administration of IGFBP3 protein blocked apoptotic cell death due to Aβ_1–42 toxicity. These data implicate a protective role for IGFBP3 against Aβ_1–42-mediated apoptosis. Next, we investigated the regulatory mechanisms of IGFBP3 expression in AD pathogenesis. We observed abnormal IGFBP3 hypermethylation within the promoter CpG island in H4-sw cells. Treatment with the DNA methyltransferase inhibitor 5-aza-2′-deoxycytidine restored IGFBP3 expression at both the mRNA and protein levels. Chronic exposure to Aβ_1–42 induced IGFBP3 hypermethylation at CpGs, particularly at loci −164 and −173, and subsequently suppressed IGFBP3 expression. Therefore, we demonstrate that expression of anti-apoptotic IGFBP3 is regulated by epigenetic DNA methylation, suggesting a mechanism that contributes to AD pathogenesis.

Pathological impact of SMN2 mis-splicing in adult SMA mice

Loss-of-function mutations in SMN1 cause spinal muscular atrophy (SMA), a leading genetic cause of infant mortality. The related SMN2 gene expresses suboptimal levels of functional SMN protein, due to a splicing defect. Many SMA patients reach adulthood, and there is also adult-onset (type IV) SMA. There is currently no animal model for adult-onset SMA, and the tissue-specific pathogenesis of post-developmental SMN deficiency remains elusive. Here, we use an antisense oligonucleotide (ASO) to exacerbate SMN2 mis-splicing. Intracerebroventricular ASO injection in adult SMN2-transgenic mice phenocopies key aspects of adult-onset SMA, including delayed-onset motor dysfunction and relevant histopathological features. SMN2 mis-splicing increases during late-stage disease, likely accelerating disease progression. Systemic ASO injection in adult mice causes peripheral SMN2 mis-splicing and affects prognosis, eliciting marked liver and heart pathologies, with decreased IGF1 levels. ASO dose–response and time-course studies suggest that only moderate SMN levels are required in the adult central nervous system, and treatment with a splicing-correcting ASO shows a broad therapeutic time window. We describe distinctive pathological features of adult-onset and early-onset SMA.

Growth hormone: does it have a therapeutic role in fracture healing?

BACKGROUND

The role of growth hormone (GH) in augmenting fracture healing has been postulated for over half a century. GH has been shown to play a role in bone metabolism and this can be mediated directly or indirectly through IGF-I.

OBJECTIVES

The use of GH was evaluated as a possible therapeutic agent in augmenting fracture healing.

METHOD

A literature search was undertaken on GH and its effect on bone fracture healing primarily using MEDLINE/OVID (1950 to January 2009). Key words and phrases including 'growth hormone', 'insulin like growth factor', 'insulin like growth factor binding protein', 'insulin like growth factor receptor', 'fracture repair', 'bone healing', 'bone fracture', 'bone metabolism', 'osteoblast' and 'osteoclast' were used in different combinations. Manual searches of the bibliography of key papers were also undertaken.

RESULTS

Current evidence suggests a positive role of GH on fracture healing as demonstrated by in vitro studies on osteoblasts, osteoclasts and the crosstalk between the two. Animal studies have demonstrated a number of factors influencing the effect of GH in vivo such as dose, timing and method of administration. Application of this knowledge in humans is limited but clearly demonstrates a positive effect on fracture healing. Concern has been raised in the past regarding the safety profile of the pharmacological use of GH when used in critically ill patients.

CONCLUSION

The optimal dose and method of administration is still to be determined, and the safety profile of this novel use of GH needs to be investigated prior to establishing its widespread use as a fracture-healing agent.

The effect of different patterns of growth hormone administration on the IGF axis and somatic and skeletal growth of the dwarf rat

Normal childhood growth is determined by ultradian and infradian variations in GH secretion, yet GH treatment of children with short stature is restricted to daily fixed doses. We have used GH-deficient dwarf rats to determine whether variable GH dose regimens promote growth more effectively than fixed doses. Animals were treated with saline or 4.2 mg of recombinant bovine GH given as 1) 700 microg/wk in 100 microg/day doses, 2) alternating weekly doses of 966 (138 microg/day) or 434 microg (62 microg/day), or 3) 700 microg/wk in randomized daily doses (5-250 microg/day). Body weight and length were measured weekly. Femur and tibia lengths and internal organ, fat pad, and muscle weights were recorded at the end of the study (6 wk); blood was collected for IGF axis measurements. GH promoted femur [F(3,60) = 14.67, P < 0.05], tibia [F(3,60) = 14.90, P < 0.05], muscle [F(3,60) = 10.37, P < 0.05], and organ growth [liver: F(3,60) = 9.30, P < 0.05; kidney: F(3,60) = 2.82, P < 0.05] and an increase in serum IGF-I [F(3,60) = 9.18, P < 0.05] and IGFBP-3 [F(3,60) = 6.70, P < 0.05] levels. IGF-I levels correlated with final weight (r = 0.45, P < 0.05) and length (r = 0.284, P < 0.05) in the whole cohort, but within each group, growth parameters correlated with serum IGF-I only in animals treated with random GH doses. The variable regimens promoted femur length (P < 0.05) and muscle (P < 0.05) and kidney (P < 0.05) weight more effectively than treatment with the fixed regimen. This study demonstrates that aspects of growth are improved following introduction of infradian variation to GH treatment in a GH-deficient model. The data suggest that varying the pattern of GH doses administered to children may enhance growth performance without increasing the overall GH dose.

The acid-labile subunit is required for full effects of exogenous growth hormone on growth and carbohydrate metabolism

Normal postnatal growth is dependent in part on overlapping actions of GH and IGF-I. These actions reflect GH stimulation of IGF-I production in liver and extrahepatic tissues, representing respectively the endocrine and autocrine/paracrine arms of the IGF system. Recent experiments in genetically modified mice show that each source of IGF-I can compensate for absence of the other but do not resolve their relative role in postnatal growth. In an effort to address this issue, we studied the GH responsiveness of mice harboring a null mutation of the acid-labile subunit (ALS). Null ALS mice have a substantial reduction in endocrine IGF-I but, unlike other models of plasma IGF-I deficiency, have no obvious additional endocrine defects. Wild type and null ALS mice of both sexes received daily sc injections of saline or recombinant bovine GH between d 35 and 63 of postnatal age. The GH-stimulated body weight gain of null ALS mice was reduced by more than 30% relative to wild type mice, irrespective of sex. Reductions in GH responsiveness were also seen for kidney and linear growth. Absence of ALS eliminated the ability of GH to increase plasma IGF-I despite intact GH-dependent stimulation of IGF-I expression in liver, adipose tissue, and skeletal muscle. GH treatment was also less efficient in antagonizing insulin action in null ALS mice. Overall, these results suggest that the GH effects mediated by endocrine IGF-I depends on ALS, and accordingly null ALS mice are less responsive to exogenous GH therapy.

Postnatal nutrition alters body composition in adult offspring exposed to maternal protein restriction

The insulin-like growth factor (IGF) system is altered with intra-uterine growth retardation and in adult metabolic disease. The aim of the present study was to observe effects of continued protein restriction on the IGF-I system and body composition in offspring of mothers fed a low-protein (LP) diet. Offspring from Wistar dams fed either a 20 % (CON) or 8 % (LP) protein diet during gestation and lactation were studied at birth, 10 d, weaning and at 12 weeks after maintenance on either the 8 % (lp) or 20 % (con) protein diet from weaning. LP offspring had reduced weaning weights (P < 0·05) and reduced serum insulin (P < 0·005). Serum IGF-I (P < 0·001) and acid-labile subunit (ALS) (P < 0·0001) were reduced at 10 and 21 d. Hepatic expression of IGF-I (P < 0·05) and ALS (P < 0·005) were reduced at 10 and 21 d. IGF binding protein (IGFBP)-1 hepatic expression was elevated at 10 d (P < 0·001) but not at 21 d. Adult LP-con offspring had reduced body weight (P < 0·05), lean (P < 0·0001) and bone (P < 0·0001) but not fat (P = 0·6) mass with no persistent effects on IGF-I, ALS and IGFBP-1.Postnatal lp feeding reduced lean mass (P < 0·0001) and bone mass (P < 0·0001) in CON and LP animals. Percentage fat (LP P = 0·04; CON P = 0·6) and IGFBP-1 (LP P = 0·01; CON P = 0·2) were increased in LP-lp but not CON-lp offspring. This suggests that postnatal nutrition is important in the effects of maternal protein restriction on adult body composition and that IGFBP-1 may be involved.

Age-dependent regulation of the acid-labile subunit in response to fasting-refeeding in rats

The GH-dependent, hepatocyte-derived acid-labile subunit (ALS) regulates IGF release from the serum by forming ternary complexes containing IGF binding protein (IGFBP)-3 or IGFBP-5. Malnutrition suppresses ALS and IGF-I expression in a development-dependent manner. Our aim was to investigate whether the effect of feeding following fasting was similarly age dependent. We fasted juvenile and adult rats for 48 h and then refed them, collecting serum and liver tissue at 8, 24, and 48 h. These were compared with rats before fasting (0 h controls) and animals fed throughout the study (free-fed controls). During fasting, serum ALS fell to 25 +/- 5.3% of 0 h controls in juveniles but only 56 +/- 6% in adults. Within 24 h of refeeding, ALS in juveniles had returned to 0 h control levels, and by 48 h to free-fed levels, whereas there was no significant refeeding response in adults during this period. Circulating IGF-I and IGFBP-5 showed similar age-dependent responses to refeeding, rising significantly faster in juveniles. IGFBP-3 did not show this response. Furthermore, hepatic ALS and IGF-I mRNA showed no age-differential response to fasting and refeeding, suggesting posttranscriptional regulation. Neither regulation of hepatic GH receptor nor ALS clearance rates could explain the age-dependent effect. We hypothesize that development-dependent regulation of ALS and IGF-I during refeeding may involve a posttranscriptional hepatic response that is not GH dependent.

Regulation of the acid-labile subunit in sustained endotoxemia

The effect of sustained endotoxemia on expression of the acid-labile subunit (ALS) in relation to hepatic markers of altered GH and insulin sensitivity was examined. Juvenile rats were injected with endotoxin twice daily for 48 h, causing reduced food intake and attenuated growth. In pair-fed controls, food restriction caused marked suppression of ALS gene expression and circulating levels within 12 h, and endotoxemia augmented this effect. This acute effect of endotoxin corresponded temporally with transient induction of suppressor of cytokine signaling (SOCS)-3, cytokine-inducible SH2-containing protein (CIS), phosphoenolpyruvate carboxykinase (PEPCK), and insulin-like growth factor-binding protein (IGFBP)-1 and suppression of GH receptor (GHR). During the subsequent 36 h of sustained endotoxin treatment, expression of ALS recovered to, and then rose above, that of their pair-fed controls. This effect was paralleled by other ternary complex components. The inductive effect of sustained endotoxemia relative to pair-fed controls could not be explained by differences in expression of GHR, SOCS-3, or CIS but coincided with normalized PEPCK and IGFBP-1 levels, suggesting better hepatic insulin sensitivity in these animals. These data may indicate that, in sustained endotoxemia, ALS levels are regulated through modulation of hepatic insulin sensitivity.

Responses of insulin-like growth factor (IGF)-I and IGF-binding proteins to nutritional status in peroxisome proliferator-activated receptor-alpha knockout mice

Peroxisome proliferator-activated receptor alpha (PPARalpha) plays a central role in glucose and lipid homeostasis. Mice lacking PPARalpha(-/-) have a sexually dimorphic phenotype. We have characterized the IGF system in wild type and PPARalpha-/- mice. In normal mice fasting IGF-I and the IGFBP-3 ternary complex were 2-fold higher in males than in females. PPARalpha influenced the IGF/IGFBP response to feeding, particularly in males. Compared to wild type, male PPARalpha-/- mice had 40% lower total fasting IGF-I concentrations, decreased ALS and less IGFBP-3 ternary complex formation, but within 4 h of refeeding there was an increase in IGF-I and IGFBP-3 ternary complex to values similar to controls. Circulating IGFBP protease activity was induced in male PPARalpha-/- mice during refeeding. IGFBP-1 and insulin concentrations were higher in males than females, and were increased by PPARalpha knockout, suggesting significant hepatic insulin resistance. We speculate that gender differences in the IGF system contribute to the PPARalpha-/- phenotype.

Acid-labile subunit regulation during the early stages of liver regeneration: implications for glucoregulation

The initiation of liver regeneration is regulated by endogenously produced growth factors and cytokines and is accompanied by suppression of growth hormone (GH) binding to hepatocytes. We have demonstrated some of these factors, particularly GH, which modulate acid-labile subunit (ALS) expression in vitro. Consequently, we investigated ALS hepatic mRNA and serum levels in rats for 24 h after partial hepatectomy (PHx). There was a significant suppression of ALS gene expression (approximately 50%, P < 0.005) and serum levels (approximately 30%, P < 0.02) by 12 h in PHx rats relative to controls. Relative to intact animals, hepatic mRNA and serum levels of ALS were suppressed by approximately 60% at 24 h. Similarly, hepatic GH receptor mRNA levels were significantly reduced in PHx animals. Moreover, hepatocytes isolated from PHx animals were less responsive to GH than those from controls. Overall, our results demonstrate that suppression of ALS gene expression and serum levels during liver regeneration relates to lowered hepatic GH sensitivity. Suppressed circulating ALS may alter insulin-like growth factor bioavailability and constitute a mechanism to maintain relatively normal glucoregulation after loss of liver mass.

Inactivation of the acid labile subunit gene in mice results in mild retardation of postnatal growth despite profound disruptions in the circulating insulin-like growth factor system

Insulin-like growth factors (IGFs) I and II are important regulators of cell proliferation and differentiation. After birth, plasma IGFs, representing mostly liver-derived IGFs, circulate in ternary complexes of 150 kDa consisting of one molecule each of IGF, IGF-binding protein (IGFBP) 3, and an acid labile subunit (ALS). Onset of ALS synthesis after birth is the primary factor driving the formation of ternary complexes. Capture of IGFs by ALS is thought to allow the development of a plasma reservoir without negative effects such as hypoglycemia and cell proliferation. To evaluate the importance of ALS and ternary complexes, we have created mice in which the ALS gene has been inactivated. The mutation was inherited in a Mendelian manner, without any effects on survival rates and birth weights. A growth deficit was observed in null mice after 3 weeks of life and reached 13% by 10 weeks. This modest phenotype was observed despite reductions of 62 and 88% in the concentrations of plasma IGF-I and IGFBP-3, respectively. Increased turnover accounted for these reductions because indices of synthesis in liver and kidney were not decreased. Surprisingly, absence of ALS did not affect glucose and insulin homeostasis. Therefore, ALS is required for postnatal accumulation of IGF-I and IGFBP-3 but, consistent with findings supporting a predominant role for locally produced IGF-I, is not critical for growth. This model should be useful to determine whether presence of ALS is needed for other actions of liver-derived IGF-I and for maintenance of homeostasis in presence of high circulating levels of IGF-II.

Conservation of a growth hormone-responsive promoter element in the human and mouse acid-labile subunit genes

During extrauterine life, insulin-like growth factors (IGFs) circulate in a ternary serum complex with one IGF-binding protein-3 (IGFBP-3) or IGFBP-5 protein and with a single acid-labile subunit (ALS). GH increases levels of this ternary complex; in mice, this effect is achieved in part by the ability of GH to stimulate mouse ALS (mALS) transcription through an interferon-gamma-activated sequence-like element (GLE) in the mALS promoter. To begin studying how GH regulates human ALS (hALS) gene expression, we cloned the hALS gene and found that it spans approximately 3.3 kb of DNA at chromosomal region 16p13.3. The hALS gene has two exons separated by a 1235-bp intron, which is found at the identical site in rat and mouse ALS genes. Sequence analysis reveals that the hALS 5'-flanking sequence is homologous to the mALS promoter, and that the GH-responsive GLE in the mALS promoter is conserved in both sequence and location in the hALS gene. The region spanning from -755 to -4 bp 5' to the hALS ATG translation start codon directs expression of a luciferase reporter gene in primary rat hepatocytes, and GH increases reporter expression in the presence of the native, but not a mutant, GLE in the hALS promoter. These data suggest that GH stimulates hALS and mALS gene expression by a similar mechanism, which involves at least in part a conserved GLE in the ALS promoter.

Anabolic effects of insulin-like growth factor in combination with insulin-like growth factor binding protein-3 in severely burned adults

BACKGROUND

The purpose of this study was to determine the anabolic effects of recombinant human insulin-like growth factor-I (rhIGF-1) complexed with its principal binding protein IGF-1 binding protein-3 (IGFBP-3) in severely burned adults.

METHODS

Ten burned adults were studied consecutively after receiving saline (pretreatment), then rhIGF-1/IGFBP-3 (treatment) for 5 days. Doses were 1, 2, and 4 mg/kg per day. Glucose, electrolytes, hormones, and leg muscle protein metabolism were determined. Nine other studies were performed on similarly injured adults at comparable times to the treatment studies to control for time effects.

RESULTS

Serum IGF-1 and IGFBP-3 levels increased with all doses, but no incremental increases were found. Leg protein balance improved with rhIGF-1/IGFBP-3, which was associated with an increase in muscle protein fractional synthetic rate. These effects were independent of time. All patients were euglycemic without electrolyte imbalances.

CONCLUSION

Net protein synthesis in the isolated leg of severely burned adults improved with rhIGF-1/IGFBP-3 without development of glucose abnormalities.

Changes in the circulating IGF system during short-term fasting and refeeding in rats

There is little information on free insulin-like growth factor I (IGF-I) and its regulatory proteins during fasting and refeeding. Therefore, we examined rats during fasting (0, 1, 2, and 3 days) and refeeding (3, 6, and 12 h and 1, 2, 3, and 7 days) (n = 6-9). Serum was analyzed for insulin, C-peptide, growth hormone (GH), free and total IGF-I, IGF-binding protein (IGFBP)-1 and -3, and the acid-labile subunit (ALS). Additionally, liver mRNA for IGF-I, IGFBP-1, and ALS was determined. Fasting reduced serum levels of GH, free and total IGF-I, IGFBP-3, and ALS, whereas IGFBP-1 was increased (P < 0.0001). Refeeding normalized IGFBP-1 at 3 h and GH at 12 h. Free IGF-I changed in parallel with total IGF-I, ALS, and IGFBP-3, being normalized at 48 h of refeeding. IGFBP-1 (peptide and mRNA) correlated inversely with insulin and C-peptide (P < 0.001). The correlation between peptide and mRNA was relatively strong for IGFBP-1 (r(2) = 0.36; P < 0.0001), moderate for IGF-I (r(2) = 0.18; P < 0.0005), and insignificant for ALS. In conclusion, insulin appears to regulate IGFBP-1 in fasted and refed rats. However, the normal inverse relationship between free IGF-I and IGFBP-1 was absent, and free IGF-I changed in parallel with total IGF-I and thus ALS and IGFBP-3. Finally, the regulation of the hepatic synthesis of IGF-I, IGFBP-1, and ALS seems to differ substantially.

Parenteral amino acid intake alters the anabolic actions of insulin-like growth factor I in rats

The anabolic properties of insulin-like growth factor (IGF) I are attenuated by oral diets that are low in protein. However, it is not known whether parenteral nutrition (PN) providing a low amino acid (AA) input will influence IGF-I action. With the use of a rat model, this study examined the interaction between AA input (1.27 and 0.62 g N. kg body wt(-1). 24 h(-1), AA and 1/2AA groups, respectively) and recombinant human IGF-I (rhIGF-I, 2.5 mg. kg body wt(-1). 24 h(-1)) infusion on the composition of the carcass and organs and on plasma insulin, IGF-I, IGF-binding protein 1 (IGFBP-1), and acid-labile subunit (ALS) concentrations. Carcass protein deposition only occurred in the AA groups (P < 0.003) and was not influenced by administration of rhIGF-I. However, visceral protein loss persisted in the AA group but was prevented by rhIGF-I infusion. The changes in water content of the carcass and the organs were generally in the expected proportion of normal lean tissue. The accumulation of lipid that follows the infusion of the AA-deficient PN was prevented by rhIGF-I infusion, which may indicate an improved energy utilization. Neither serum insulin nor ALS concentrations were influenced by the level of AA infusion but were reduced by rhIGF-I administration. However, plasma IGF-I levels were elevated by higher AA infusion and by IGF-I administration. Also, IGFBP-1 concentrations were reduced by the higher AA infusion and increased with rhIGF-I administration. Interestingly, there was a significant interaction effect between both of these influences. It is concluded that free IGF-I concentration, which may be regulated by IGFBP-1 through a direct effect of AAs on the liver, may have an important role in regulating anabolism in visceral and possibly skeletal tissue during PN.

Serum free insulin-like growth factor-I is dose-dependently decreased by methylprednisolone and related to body weight changes in rats

Glucocorticoids usually inhibit growth despite a paradoxical increase in total IGF-I. To investigate the effect of methylprednisolone on free IGF-I, rats were treated with for 3 days (0, 1, 2, 4, and 6 mg/kg per day). A dose-dependent decrease in ultrafiltrated serum free IGF-I was observed, being lowest after 6 mg/kg (P < 0.001 all groups vs controls). Total IGF-I was increased in the groups receiving 2 mg/kg (P < 0.05). Weight change in the 24 h prior to blood sampling was positively correlated with free IGF-I (R = 0.46, P = 0.0002), but not with total IGF-I. Immunoassayable IGFBP-1 was decreased in rats given 4 mg/kg (P = 0.001), whereas there was no change in IGFBP-3 or acid-labile subunit. We propose that in rats the glucocorticoid-induced weight loss may in part be due to suppression of circulating free IGF-I.

Developmental changes in serum levels of free and total insulin-like growth factor I (IGF-I), IGF-binding protein-1 and -3, and the acid-labile subunit in rats

We have recently described a competitive binding assay for rat insulin-like growth factor-binding protein-3 (IGFBP-3) based on the ability of IGFBP-3 to form a ternary complex with the acid-labile subunit (ALS) in the presence of IGF-I. Using this assay we studied groups of male (n = 6) and female rats (n = 6) at 20, 30, 40, 50, 60, 80, and 130 days of age. Nonfasting serum levels of IGFBP-3 were compared with those of total (extractable) IGF-I (tIGF-I) and ALS as well as IGFBP-3 determined by ligand blotting. Additionally, we studied the relationship between ultrafiltered free IGF-I (fIGF-I) and immunoassayable IGFBP-1. IGFBP-3 was dependent on age only (P < 0.0001), but tended to be higher in males than in females (P = 0.06); between 20-130 days levels increased from 6.5 +/- 1.7 to 73.6 +/- 7.2 nmol/liter in males and from 5.4 +/- 1.6 to 51.3 +/- 8.0 nmol/liter in females. IGFBP-3 correlated positively with tIGF-I (r = 0.90; P < 0.0001), ALS (r = 0.92; P < 0.0001), and IGFBP-3, as determined by ligand blotting (r = 0.88; P < 0.0001). The molar ratio of IGFBP-3 to tIGF-I increased from 0.23 +/- 0.04 to 0.76 +/- 0.04 (P < 0.0001) without any sex dependence. An age- and sex-dependent decrease in IGFBP-1 was observed (P < 0.0001), from 10.9 +/- 2.5 to 1.2 +/- 0.2 nmol/liter in females and from 8.9 +/- 0.7 to 0.2 +/- 0.04 nmol/liter in males. Free IGF-I (fIGF-I) increased with age (from 0.7 +/- 0.2 to 7.1 +/- 0.5 nmol/liter; P < 0.0001), and levels were inversely correlated with IGFBP-1 (r = -0.80; P < 0.0001). In young rats, IGFBP-1 circulated in a 10-fold molar excess over the level of fIGF-I, whereas in older rats, fIGF-I exceeded IGFBP-1 by an average of 9-fold in females and by up to almost 60-fold in males. We conclude that in rats 1) IGFBP-3 and fIGF-I are strongly age dependent; 2) IGFBP-3 correlates positively with ALS and tIGF-I; and 3) fIGF-I and IGFBP-1 are inversely correlated. This is in accordance with clinical findings. However, in humans the adult level of fIGF-I rarely exceeds 0.3 nmol/liter, and IGFBP-1 usually circulates in excess of fIGF-I. Thus, our results also imply species differences in the IGF systems of humans and rats.

Molecular regulation of insulin-like growth factor-I and its principal binding protein, IGFBP-3

The insulin-like growth factors (IGFs) have diverse anabolic cellular functions, and structure similar to that of proinsulin. The distribution of IGFs and their receptors in a wide variety of organs and tissues enables the IGFs to exert endocrine, paracrine, and autocrine effects on cell proliferation and differentiation, caloric storage, and skeletal elongation. IGF-I exhibits particular metabolic responsiveness, and circulating IGF-I originates predominantly in the liver. Hepatic IGF-I production is controlled at the level of gene transcription, and transcripts are initiated largely in exon 1. Hepatic IGF-I gene transcription is reduced in conditions of protein malnutrition and diabetes mellitus, and our laboratory has used in vitro transcription to study mechanisms related to diabetes. We find that the presence of sequences downstream from the major transcription initiation sites in exon 1 is necessary for the diabetes-induced decrease in IGF-I transcription. Six nuclear factor binding sites have been identified within the exon 1 downstream region, and footprint sites III and V appear to be necessary for metabolic regulation; region V probes exhibit a decrease in nuclear factor binding with hepatic nuclear extracts from diabetic animals. IGFs in biological fluids are associated with IGF binding proteins, and IGFs circulate as a 150-kDa complex that consists of an IGF, an IGFBP-3, and an acid-labile subunit. Circulating IGFBP-3 originates mainly in hepatic nonparenchymal cells, where IGF-I increases IGFBP-3 mRNA stability, but insulin increases IGFBP-3 gene transcription. Regulation of IGFBP-3 gene transcription by insulin appears to be mediated by an insulin-responsive element, which recognizes insulin-responsive nuclear factors in both gel mobility shift assays and southwestern blots. Studies of mechanisms underlying the modulation of IGF-I and IGFBP-3 gene transcription, and identification of critical nuclear proteins involved, should lead to new understanding of the role and regulation of these important growth factors in diabetes mellitus and other metabolic disorders.

Interleukin-1 beta suppresses growth hormone-induced acid-labile subunit mRNA levels and secretion in primary hepatocytes

Cytokines are thought to mediate the catabolic states induced by infection and trauma. Recent evidence suggests that the cytokine interleukin-1 beta (IL-1 beta) directly inhibits the anabolic insulin-like growth factor (IGF)-I: growth hormone (GH) axis. The biological activity of circulating IGF is regulated by the hepatocyte derived, GH-dependent acid-labile subunit (ALS) of the 140-kDa IGF binding protein (IGFBP) complex. ALS buffers the growth and metabolic effects of the insulin-like growth factors by sequestering them in a ternary complex with IGFBP-3. To determine whether IL-1 beta has a direct effect on hepatic ALS production, we have examined its effect on ALS mRNA levels and secretion in hepatocytes under GH-induced and basal conditions. In the presence of GH (30 ng/mL) half-maximal reduction of ALS mRNA levels and secretion was induced by between 0.3-3 ng/mL rhIL-1 beta (P < 0.05). However, under basal conditions IL-1 beta had no significant effect on ALS mRNA levels, and only a slight suppression of secretion. Our study suggests that IL-1 beta regulates ALS gene expression and secretion in a way that is dependent, in part, on interaction with the GH signalling pathway.

Binding sites and binding properties of binary and ternary complexes of insulin-like growth factor-II (IGF-II), IGF-binding protein-3, and acid-labile subunit

We have examined regions of rat IGF-binding protein-3 (IGFBP-3) important for complex formations using two kinds of deletion mutants, three kinds of chimera molecules between rat IGFBP-3 and rat IGFBP-2, and a synthetic peptide (41 residues, Glu52-Ala92) derived from rat IGFBP-3. Solid-phase binding assays using 96-well microtiter plates were designed to quantitate the relative binding affinities. It was found that not only the IGFBP-3 derivatives with the amino-terminal, cysteine-rich domain (N domain) but also the synthetic peptide maintained affinity for IGF-II. Ternary complex formation was observed with full-length IGFBP-3 and chimera IGFBP, the carboxyl-terminal cysteine-rich domain (C domain) of which was derived from IGFBP-3, unlike the mutants lacking the C domain and the chimera IGFBPs, the C domain of which was derived from IGFBP-2. These results were confirmed by affinity cross-linking experiments. Furthermore, the IGFBP-3 derivatives that possessed the C domain of IGFBP-3 bound to the acid-labile subunit, even in the absence of IGFs. Finally, we observed sites in IGF-II important for the ternary complex formation using various IGF-II mutants. These IGF-II mutants, which contained a substitution of Tyr27 for Leu, had extremely reduced activity. These results strongly suggest that: 1) the N domain, containing at least Glu52-Ala92, of rat IGFBP-3 is important for binding to IGF-II; 2) the C domain of IGFBP-3 is essential for binding to the acid-labile subunit both in the presence and absence of IGF-II; and 3) Tyr27 of IGF-II is important for the ternary complex formation.

Organization and chromosomal localization of the gene encoding the mouse acid labile subunit of the insulin-like growth factor binding complex

After birth, most of insulin-like growth factor I and II (IGFs) circulate as a ternary complex formed by the association of IGF binding protein 3-IGF complexes with a serum protein called acid-labile subunit (ALS). ALS retains the IGF binding protein-3-IGF complexes in the vascular compartment and extends the t1/2 of IGFs in the circulation. Synthesis of ALS occurs mainly in liver after birth and is stimulated by growth hormone. To study the basis for this regulation, we cloned and characterized the mouse ALS gene. Comparison of genomic and cDNA sequences indicated that the gene is composed of two exons separated by a 1126-bp intron. Exon 1 encodes the first 5 amino acids of the signal peptide and contributes the first nucleotide of codon 6. Exon 2 contributes the last 2 nt of codon 6 and encodes the remaining 17 amino acids of the signal peptide as well as the 580 amino acids of the mature protein. The polyadenylylation signal, ATTAAA, is located 241 bp from the termination codon. The cDNA and genomic DNA diverge 16 bp downstream from this signal. Transcription initiation was mapped to 11 sites over a 140-bp TATA-less region. The DNA fragment extending from nt -805 to -11 (ATG, +1) directed basal and growth hormone-regulated expression of a luciferase reporter plasmid in the rat liver cell line H4-II-E. Finally, the ALS gene was mapped to mouse chromosome 17 by fluorescence in situ hybridization.

Developmental differences in the IGF-I system response to severe and chronic calorie malnutrition

Recent studies in children suggest that there are age-related differences in the insulin-like growth factor I (IGF-I) response to malnutrition. To extend this observation, immature 4-wk-old male rats were fasted for 3 days, fed ad libitum (control), or fed 60 or 40% of control calories (restricted) and compared with 8-wk-old young adults. Over the 3-wk study period, serum total IGF-I levels of the older rats were stable despite reduced insulin levels, whereas IGF-I increased 2.2-fold in the younger controls. With the 40% diet, younger and older rats changed body weight +1 and -1 body wt/day, respectively (P < 0.0001). The restricted younger animals reduced serum IGF-I IGF binding protein-3, acid-labile subunit, and growth hormone binding protein levels significantly more than the restricted older animals. Fasting decreased most of these parameters by 40%, serum insulin by approximately 80%, and body weight by 9%, regardless of age. We conclude that the suppression of the IGF-I system in response to chronic undernutrition, but not acute fasting, is greater in maturing than young adult rats.