Growth hormone (GH) and insulin-like growth factor-I (IGF-I) treatment of the GH-deficient dwarf rat: differential effects on IGF-I transcription start site expression in hepatic and extrahepatic tissues and lack of effect on type I IGF receptor mRNA expression

Effect of human milk formula with bovine colostrum supplementation on bone mineral density in infant cynomolgus macaques

Insulin-like growth factor 1 (IGF1) is a regulator of human growth during infancy and childhood, known to promote bone and muscle growth as well as lipid accumulation. This study aimed to investigate the effects of formula milk with or without IGF1 supplementation (in the form of pure IGF1 or bovine colostrum) on growth and body composition in infant cynomolgus macaques during the first 6 months of life. Three groups of infants were nursery-reared and received formula milk with or without IGF1 or bovine colostrum supplementation for 4 months, and a fourth group consisting of breast-fed infants was included for comparison (n=6 for each group). Ranked-based analysis of covariance was used to detect differences between adjusted means for sex. No differences in weight, height, fat mass, and fat-free mass could be detected between groups. However, bone mineral density (BMD) was significantly different between groups at the end of formula feeding. Infants that received bovine colostrum supplementation displayed higher mean BMD than infants of all other groups, with no differences between the latter three groups. In conclusion, our results suggest that supplementation with bovine colostrum can enhance BMD in formula-fed infants, an effect that apparently does not depend on IGF1. Bovine colostrum supplementation could be beneficial for long-term bone health in infants with suboptimal bone growth.

The Regulation of IGF-1 Gene Transcription and Splicing during Development and Aging

It is commonly known that the insulin-like growth factor-I gene contains six exons that can be differentially spliced to create multiple transcript variants. Further, there are two mutually exclusive leader exons each having multiple promoter sites that are variably used. The mature IGF-I protein derived from the multiplicity of transcripts does not differ suggesting a regulatory role for the various transcript isoforms. The variant forms possess different stabilities, binding partners, and activity indicating a pivotal role for the isoforms. Research has demonstrated differential expression of the IGF-I mRNA transcripts in response to steroids, growth hormone, and developmental cues. Many studies of different tissues have focused on assessing the presence, or putative action, of the transcript isoforms with little consideration of the transcriptional mechanisms that generate the variants or the translational use of the transcript isoforms. Control points for the latter include epigenetic regulation of splicing and promoter usage in response to development or injury, RNA binding proteins and microRNA effects on transcript stability, and preferential use of two leader exons by GH and other hormones. This review will detail the current knowledge of the mechanical, hormonal, and developmental stimuli regulating IGF-1 promoter usage and splicing machinery used to create the variants.

Maternal Hyperglycemia Disrupts Histone 3 Lysine 36 Trimethylation of the IGF-1 Gene

In utero environmental adaptation may predispose to lifelong morbidity. Organisms fine-tune gene expression to achieve environmental adaptation by epigenetic alterations of histone markers of gene accessibility. One example of epigenetics is how uteroplacental insufficiency-induced intrauterine growth restriction (IUGR), which predisposes to adult onset insulin resistance, decreases postnatal IGF-1 mRNA variants and the gene elongation mark histone 3 trimethylation of lysine 36 of the IGF-1 gene (H3Me3K36). Limitations in the study of epigenetics exist due to lack of a primary transgenic epigenetic model. Therefore we examined the epigenetic profile of insulin-like growth factor 1 (IGF-1) in a well-characterized rat model of maternal hyperglycemia to determine if the epigenetic profile of IGF-1 is conserved in disparate models of in utero adaptation. We hypothesized that maternal hyperglycemia would increase IGF-1 mRNA variants and H3Me3K36. However maternal hyperglycemia decreased hepatic IGF-1 mRNA variants and H3Me3K36. This finding is intriguing given that despite different prenatal insults and growth, both maternal hyperglycemia and IUGR predispose to adult onset insulin resistance. We speculate that H3Me3K36 of the IGF-1 gene is sensitive to the glucose level of the prenatal environment, with resultant alteration of IGF-1 mRNA expression and ultimately vulnerability to adult onset insulin resistance.

A growth stimulus is needed for IGF-1 to induce skeletal muscle hypertrophy in vivo

Here, we characterise new strains of normal and dystrophic (mdx) mice that overexpress Class 2 IGF-1 Ea in skeletal myofibres. We show that transgenic mice have increased muscle levels of IGF-1 (~13-26 fold) and show striking muscle hypertrophy (~24-56% increase in mass). Adult normal muscles were resistant to elevated IGF-1; they reached adult steady state and maintained the same mass from 3 to 12 months. By contrast, dystrophic muscles from mdx/IGF-1(C2:Ea) mice continued to increase in mass during adulthood. IGF-1 signalling was evident only in muscles that were growing as a result of normal postnatal development (23-day-old mice) or regenerating in response to endogenous necrosis (adult mdx mice). Increased phosphorylation of Akt at Ser473 was not evident in fasted normal adult transgenic muscles, but was 1.9-fold higher in fasted normal young transgenic muscles compared with age-matched wild-type controls and fourfold higher in fasted adult mdx/IGF-1(C2:Ea) compared with mdx muscles. Muscles of adult mdx/IGF-1(C2:Ea) mice showed higher p70S6K(Thr421/Ser424) phosphorylation and both young transgenic and adult mdx/IGF-1(C2:Ea) mice had higher phosphorylation of rpS6(Ser235/236). The level of mRNA encoding myogenin was increased in normal young (but not adult) transgenic muscles, indicating enhanced myogenic differentiation. These data demonstrate that elevated IGF-1 has a hypertrophic effect on skeletal muscle only in growth situations.

Endotoxin-induced growth hormone resistance in skeletal muscle

Inflammation-induced skeletal muscle wasting is a serious clinical problem and arises in part because of resistance to GH-stimulated IGF-I expression. Although it is established that in the liver, resistance develops because of impaired signaling through the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) transduction pathway, together with a more distal defect in STAT5 DNA-binding activity, the situation in skeletal muscle is unclear. Accordingly, we set out to characterize the mechanisms behind the skeletal muscle resistance to GH in rats with acute inflammation induced by endotoxin. Endotoxin caused significant declines in GH-stimulated STAT5a/b phosphorylation and IGF-I gene expression, and this occurred despite a lack of change in signaling protein levels or phosphorylation of JAK2. In whole muscle, GH-stimulated phospho-STAT5a/b levels were reduced by half, and in the nucleus, phospho-STAT5b levels were similarly reduced. Furthermore, the binding of phosphorylated STAT5b to DNA was reduced and to a similar extent to the reduction in nuclear phosphorylated STAT5b. Interestingly, GH-induced androgen receptor gene expression was also suppressed. Thus, it appears that skeletal muscle resistance to GH-stimulated IGF-I expression in acute endotoxemia arises from a defect in STAT5b signaling, with a proportionate reduction in STAT5b DNA binding. Finally, it appears that resistance to GH-induced androgen receptor expression also develops and, together with the attenuated GH-induced IGF-I expression, likely plays an important role in the muscle wasting that arises in endotoxin-induced inflammation.

Epigenetics: intrauterine growth retardation (IUGR) modifies the histone code along the rat hepatic IGF-1 gene

Intrauterine growth restriction (IUGR) decreases serum insulin growth factor-1 (IGF-1) levels. IGF-1 is an epigenetically regulated gene that has two promoters, alternative exon 5 splicing, and multiple termination sites. The regulation of gene expression involves the whole gene, as evidenced by the aforementioned IGF-1 paradigm. We hypothesized that IUGR in the rat would affect hepatic IGF-1 expression and alter the epigenetic characteristics of the IGF-1 gene along its length. IUGR was induced through a bilateral uterine artery ligation of the pregnant rat, a well-characterized model of IUGR. Pups from anesthesia and sham-operated dams were used as controls. Real-time RT-PCR and ELISA was used to measure expression at day of life (DOL) 0 and 21. Bisulfite sequencing and chromatin immunoprecipitation (ChIP) quantified IGF-1 epigenetic characteristics. A nontranscribed intergenic control was used for ChIP studies. IUGR decreased hepatic and serum IGF-1. Concurrently, IUGR modified epigenetic characteristics, particularly the histone code, along the length of the hepatic IGF-1 gene. Many changes persisted postnatally, and the postnatal effect of IUGR on the histone code was gender-specific. We conclude that IUGR modifies epigenetic characteristics of the rat hepatic IGF-1 gene along the length of the whole gene.

Autocrine IGF-1 action in adipocytes controls systemic IGF-1 concentrations and growth

OBJECTIVE

IGF-1 and the IGF-1 receptor (IGF-1R) have been implicated in the regulation of adipocyte differentiation and lipid accumulation in vitro.

RESEARCH DESIGN AND METHODS

To investigate the role of IGF-1 receptor in vivo, we have inactivated the Igf-1r gene in adipose tissue (IGF-1R(aP2Cre) mice) using conditional gene targeting strategies.

RESULTS

Conditional IGF-1R inactivation resulted in increased adipose tissue mass with a predominantly increased lipid accumulation in epigonadal fat pads. However, insulin-stimulated glucose uptake into adipocytes was unaffected by the deletion of the IGF-1R. Surprisingly, IGF-1R(aP2Cre) mice exhibited markedly increased somatic growth in the presence of elevated IGF-1 serum concentrations, and IGF-1 mRNA expression was significantly increased in liver and adipose tissue. IGF-1 stimulation of wild-type adipocytes significantly decreased IGF-1 mRNA expression, whereas the opposite effect was observed in IGF-1R-deficient adipocytes.

CONCLUSIONS

IGF-1R signaling in adipocytes does not appear to be crucial for the development and differentiation of adipose tissue in vivo, but we identified a negative IGF-1R-mediated feedback mechanism of IGF-1 on its own gene expression in adipocytes, indicating an unexpected role for adipose tissue IGF-1 signaling in the regulation of IGF-1 serum concentrations in control of somatic growth.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

Skeletal muscle myofibrillar mRNA expression in heart failure: relationship to local and circulating hormones

Chronic heart failure is characterized by changes in skeletal muscle that contribute to exercise intolerance and muscle weakness. To determine whether changes in the quantity and isoform distribution of key myofibrillar proteins are related to altered gene expression, we measured skeletal muscle myofibrillar mRNA abundance in nine heart failure patients (mean +/- SE; 63 +/- 3 yr) and nine controls (70 +/- 3 yr). In addition, we assessed the relationship of circulating levels of anabolic and catabolic hormones, as well as local expression of insulin-like growth factor (IGF)-I, to myofibrillar mRNA abundance. Heart failure patients were characterized by lower abundance of mRNA encoding the myosin heavy chain (MHC) I isoform (P < 0.01), whereas MHC IIa and MHC IIx mRNA did not differ between groups. Actin mRNA was also lower in heart failure patients compared with controls (P < 0.001). The expression of each MHC isoform transcript correlated with its respective protein product (MHC I: r = 0.656, P < 0.01; MHC IIa: r = 0.489, P < 0.05; MHC IIx: r = 0.505, P < 0.05; n = 18 for all). In addition to changes in myofibrillar transcripts, we found lower (P < 0.01) skeletal muscle IGF-1Ea mRNA content in heart failure patients. Myofibrillar mRNA levels were positively associated with skeletal muscle IGF-1Ea transcript levels (range of r values: 0.663-0.765; P values: <0.01 to <0.001) and modestly associated with circulating markers of immune activation (range of r values: -0.487 to -0.555; P values: <0.05 to <0.03). Our findings suggest that alterations in skeletal muscle MHC content and isoform distribution in heart failure may derive, in part, from changes in MHC gene expression. The relationships of myofibrillar mRNA content to both local and circulating hormones further suggest that alterations in the balance between anabolic and catabolic hormones in heart failure patients may influence skeletal muscle myofibrillar protein phenotype by altering gene expression.

Comparison of GH, IGF-I, and testosterone with mRNA of receptors and myostatin in skeletal muscle in older men

Growth hormone (GH), insulin-like growth factor I (IGF-I), and testosterone (T) are important mediators of muscle protein synthesis, and thus muscle mass, all of which decline with age. We hypothesized that circulating hormones would be related to the transcriptional levels of their respective receptors and that this expression would be negatively related to expression of the myostatin gene. We therefore determined content of mRNA transcripts (by RT-PCR) for GH receptor (GHR), IGF-I, androgen receptor (AR), and myostatin in skeletal muscle biopsy samples from 27 healthy men >65 yr of age. There were no significant relationships between age, lean body mass, or percent body fat and transcript levels of GHR, IGF-I, AR, or myostatin. Moreover, there were no significant correlations of serum GH, IGF-I, or T with their corresponding target mRNA levels (GHR, intramuscular IGF-I, or AR) in skeletal muscle. However, GHR was negatively correlated (r = -0.60, P = 0.001) with myostatin mRNA levels. The lack of apparent relationships of muscle transcripts with their respective ligands in healthy older adults suggests that age-related deficits in both GH and T may lead to an increase in myostatin expression and a disassociation in autocrine IGF-I effects on muscle protein synthesis, both of which could contribute to age-related sarcopenia.

Roles of growth hormone and insulin-like growth factor 1 in mouse postnatal growth

To examine the relationship between growth hormone (GH) and insulin-like growth factor 1 (IGF1) in controlling postnatal growth, we performed a comparative analysis of dwarfing phenotypes manifested in mouse mutants lacking GH receptor, IGF1, or both. This genetic study has provided conclusive evidence demonstrating that GH and IGF1 promote postnatal growth by both independent and common functions, as the growth retardation of double Ghr/Igf1 nullizygotes is more severe than that observed with either class of single mutant. In fact, the body weight of these double-mutant mice is only approximately 17% of normal and, in absolute magnitude ( approximately 5 g), only twice that of the smallest known mammal. Thus, the growth control pathway in which the components of the GH/IGF1 signaling systems participate constitutes the major determinant of body size. To complement this conclusion mainly based on extensive growth curve analyses, we also present details concerning the involvement of the GH/IGF1 axis in linear growth derived by a developmental study of long bone ossification in the mutants.

CrkII participation in the cellular effects of growth hormone and insulin-like growth factor-1. Phosphatidylinositol-3 kinase dependent and independent effects

We have examined the role of CrkII in the cellular response to both human growth hormone (hGH) and human insulin-like growth factor-1 (hIGF-1). We have demonstrated that overexpression of the adaptor molecule enhances both basal phosphatidylinositol 3-kinase (PI 3-kinase) activity and also dramatically enhances the ability of both hormones to stimulate PI 3-kinase activity in the cell. Many of the effects of CrkII overexpression on hGH- and hIGF-1-stimulated cellular function can then be attributed to CrkII enhancement of PI 3-kinase stimulation by these hormones. Thus, CrkII-enhanced PI 3-kinase activity is used to enhance actin filament reorganization in response to both hGH and hIGF-1, to enhance stress activated protein kinase (SAPK) activity in response to hGH, and to diminish STAT5-mediated transcription in response to hGH. It is apparent, however, that CrkII also regulates cellular function independent of its ability to stimulate PI 3-kinase activity. This is evidenced by the ability of CrkII, in a PI 3-kinase-independent manner, to diminish the activation of p44/42 mitogen-activated protein kinase in response to both hGH and hIGF-1 and to inhibit the activation of SAPK by hIGF-1. Therefore, despite the common use of CrkII to activate PI 3-kinase, CrkII also allows hGH or hIGF-1 to selectively switch the activation of SAPK. Thus, common utilization of CrkII by hGH and hIGF-1 allows the execution of common cellular effects of these hormones, concomitant with the retention of hormonal specificity.

IGF-I treatment increases motility and improves morphology of immature spermatozoa in the GH-deficient dwarf (dw/dw) rat

It has recently been shown that short-term growth hormone (GH) treatment can increase the motility of spermatozoa in the GH-deficient dw/dw rat. To examine whether the effects of GH on motility of immature spermatozoa are mediated by an increase in plasma concentrations of IGF-I, we treated GH-deficient dw/dw rats with 2 microg/g/day of IGF-I using osmotic minipumps. Body weight (saline 227+/-5 g, IGF-I 253+/-4 g) and IGF-I concentrations in blood plasma (saline 472+/-19.9 ng/ml, IGF-I 986+/-43.6 ng/ml) and seminal vesicle fluid (saline 30.9+/-1.7 ng/ml, IGF-I 47.9+/-2.9 ng/ml) were significantly increased with IGF-I treatment (P<0.001), similar to the observed responses to GH therapy in our earlier study. While epididymal fluid IGF-I concentrations were not changed, IGF-I treatment significantly increased the number of immature motile spermatozoa (saline 14.4+/-3.5%, IGF-I 28.3+/-4.1%, P<0.05) and the number of spermatozoa with normal morphology (control 65.7+/-3.3%, IGF-I 75+/-1.9%, P<0.05). These data suggest that increasing the circulating concentrations of IGF-I in the GH-deficient rat can improve the motility and morphology of immature spermatozoa and thus mimic, at least in part, the effects of GH.