Structure, expression, and regulation of the IGF-I gene

The Role of Insulin-like Growth Factor-1 (IGF-1) in the Control of Neuroendocrine Regulation of Growth

In mammals, the neuroendocrine system, which includes the communication between the hypothalamus and the pituitary, plays a major role in controlling body growth and cellular metabolism. GH produced from the pituitary somatotroph is considered the master regulator of somatic development and involved, directly and indirectly, in carbohydrate and lipid metabolism via complex, yet well-defined, signaling pathways. GH production from the pituitary gland is primarily regulated by the counter-regulatory effects of the hypothalamic GHRH and SST hormones. The role of IGF-1 feedback regulation in GH production has been demonstrated by pharmacologic interventions and in genetically modified mouse models. In the present review, we discuss the role of IGF-1 in the regulation of the GH-axis as it controls somatic growth and metabolic homeostasis. We present genetically modified mouse models that maintain the integrity of the GH/GHRH-axis with the single exception of IGF-1 receptor (IGF-1R) deficiency in the hypothalamic GHRH neurons and somatotroph that reveals a novel mechanism controlling adipose tissues physiology and energy expenditure.

Immunohistochemical expression of insulin-like growth factor-1Ec in primary endometrial carcinoma: Association with PTEN, p53 and survivin expression

Chronic hyperinsulinemia due to insulin resistance and elevated levels of insulin-like growth factor (IGF)-1 and IGF-2 are suggestive of a significantly higher risk of endometrial carcinoma. There is a wealth of evidence showing differential expression of IGF-1 isoforms in various types of cancer. In the present study, 99 archived endometrial carcinoma tissue sections were retrospectively assessed by immunohistochemistry for IGF-1Ec isoform expression. Expression of IGF-1Ec was also assessed in nine cases of non-neoplastic endometrial tissue adjacent to the tumor, in 30 cases with normal endometrium and in 30 cases with endometrial hyperplasia. Furthermore, the association between IGF-1Ec and the concurrent expression of phosphatase and tensin homologue deleted on chromosome 10 (PTEN), p53 or survivin was assessed, as well as their combined expression in association with clinicopathological variables. In endometrial carcinoma, IGF-1Ec expression was high in non-endometrioid carcinoma (serous papillary or clear cell carcinoma) compared with that in endometrioid adenocarcinoma. IGF-1Ec expression was also high in the presence of tumoral necrosis. Furthermore, there was a significant correlation between the histological differentiation and the sum of staining intensity and the number of IGF-1Ec immunopositive cells in endometrial carcinoma. There was a moderate negative correlation between co-expression of IGF-1Ec and PTEN, for both the number of immunopositive cells (P=0.006, ρ=−0.343) and the sum of staining (scores and intensity; P=0.006, ρ=−0.343). Furthermore, there was a positive correlation between the sum of staining (scores and intensity) and co-expression of IGF-1Ec and survivin (P=0.043, ρ=0.225). However, there was no association between concomitant expression of IGF-1Ec and p53. These results emphasized the importance of IGF-1Ec expression during development of non-estrogen dependent endometrial adenocarcinoma. IGF-1Ec and PTEN may function opposingly during endometrial carcinogenesis. By contrast, IGF-1Ec and survivin may share common molecular pathways and may promote, in parallel, tumoral development.

Plausible Links Between Metabolic Networks, Stem Cells, and Longevity

Aging is an inevitable consequence of life, and all multicellular organisms undergo a decline in tissue and organ functions as they age. Several well-known risk factors, such as obesity, diabetes, and lack of physical activity that lead to the cardiovascular system, decline and impede the function of vital organs, ultimately limit overall life span. Over recent years, aging research has experienced an unparalleled growth, particularly with the discovery and recognition of genetic pathways and biochemical processes that control to some extent the rate of aging.In this chapter, we focus on several aspects of stem cell biology and aging, beginning with major cellular hallmarks of aging, endocrine regulation of aging and its impact on stem cell compartment, and mechanisms of increased longevity. We then discuss the role of epigenetic modifications associated with aging and provide an overview on a most recent search of antiaging modalities.

Microsatellite polymorphism in the P1 promoter region of the IGF‑1 gene is associated with endometrial cancer

Endometrial carcinoma (EC) is the most common type of gynecological malignancy. Studies have demonstrated that the insulin growth factor (IGF) pathway is implicated in the development of endometrial tumors and that the serum levels of IGF-1 are affected by estrogen. Most EC cells with high microsatellite instability (MSI-H) accumulate mutations at a microsatellite sequence in the IGF-1 gene. The present study investigated the CA repeat polymorphism in the P1 promoter region of the IGF-1 gene among Caucasian females with endometrial hyperplasia, EC and healthy control subjects, whose blood serum and surgical tissue specimens were analyzed. Differences or correlations between the analyzed parameters [serum levels of IGF-1 and IGF binding protein (IGFBP)-1 and IGFBP-3 as well as estrogens among the polymorphisms] were verified using the χ², Mann-Whitney U, Kruskal-Wallis or Spearman's rank correlation tests. A PCR amplification and DNA sequencing analysis was used for identification of (CA)_n repeats in the P1 region of IGF-1. ELISA was used to determine the blood serum levels of IGF-1, IGFBP-1, IGFBP-3 and estrogens. Furthermore, IGF-1 was assessed in endometrial tissues by immunohistochemical analysis. The present study indicated no statistically significant differences between serum levels of IGF-1, IGFBP-1, IGFBP-3 and estrone, estriol and estradiol in the control and study groups. A significant correlation was identified between the IGF-1 levels and estrone levels in the MSI-H polymorphism (r=−0.41, P=0.012) as well as a highly negative correlation between IGF-1 levels and the estradiol levels in the MSI-H polymorphism (r=−0.6, P=0.002). Genotypes without the 19 CA allele were predominantly found in EC. Furthermore, statistical analysis indicated that the number of IGF-1-expressing cells was significantly elevated in MSI-H type 18-20 (P= 0.0072), MSI-L type 19-20 (P=0.025) and microsatellite-stable MSS type 19-19 (P=0.024) compared with those in the MSI-H 20-20 genotype. The present study suggested that it is rather likely that the polymorphisms in the IGF-1 promoter are associated with EC in Caucasian females with regard to its development. In the present study, polymorphisms of the IGF-1 promoter may have been introduced during the genesis of EC and contributed to it by leading to aberrant expression of IGF-1.

Thyroxine modifies the effects of growth hormone in Ames dwarf mice

Ames dwarf (df/df) mice lack growth hormone (GH), thyroid stimulating hormone and prolactin. Treatment of juvenile df/df mice with GH alone stimulates somatic growth, reduces insulin sensitivity and shortens lifespan. Early‐life treatment with thyroxine (T4) alone produces modest growth stimulation but does not affect longevity. In this study, we examined the effects of treatment of juvenile Ames dwarf mice with a combination of GH + T4 and compared them to the effects of GH alone. Treatment of female and male dwarfs with GH + T4 between the ages of 2 and 8 weeks rescued somatic growth yet did not reduce lifespan to match normal controls, thus contrasting with the previously reported effects of GH alone. While the male dwarf GH + T4 treatment group had no significant effect on lifespan, the female dwarfs undergoing treatment showed a decrease in maximal longevity. Expression of genes related to GH and insulin signaling in the skeletal muscle and white adipose tissue (WAT) of female dwarfs was differentially affected by treatment with GH + T4 vs. GH alone. Differences in the effects of GH + T4 vs. GH alone on insulin target tissues may contribute to the differential effects of these treatments on longevity.

Finite mixture clustering of human tissues with different levels of IGF-1 splice variants mRNA transcripts

BACKGROUND

This study addresses a recurrent biological problem, that is to define a formal clustering structure for a set of tissues on the basis of the relative abundance of multiple alternatively spliced isoforms mRNAs generated by the same gene. To this aim, we have used a model-based clustering approach, based on a finite mixture of multivariate Gaussian densities. However, given we had more technical replicates from the same tissue for each quantitative measurement, we also employed a finite mixture of linear mixed models, with tissue-specific random effects.

RESULTS

A panel of human tissues was analysed through quantitative real-time PCR methods, to quantify the relative amount of mRNA encoding different IGF-1 alternative splicing variants. After an appropriate, preliminary, equalization of the quantitative data, we provided an estimate of the distribution of the observed concentrations for the different IGF-1 mRNA splice variants in the cohort of tissues by employing suitable kernel density estimators. We observed that the analysed IGF-1 mRNA splice variants were characterized by multimodal distributions, which could be interpreted as describing the presence of several sub-population, i.e. potential tissue clusters. In this context, a formal clustering approach based on a finite mixture model (FMM) with Gaussian components is proposed. Due to the presence of potential dependence between the technical replicates (originated by repeated quantitative measurements of the same mRNA splice isoform in the same tissue) we have also employed the finite mixture of linear mixed models (FMLMM), which allowed to take into account this kind of within-tissue dependence.

CONCLUSIONS

The FMM and the FMLMM provided a convenient yet formal setting for a model-based clustering of the human tissues in sub-populations, characterized by homogeneous values of concentrations of the mRNAs for one or multiple IGF-1 alternative splicing isoforms. The proposed approaches can be applied to any cohort of tissues expressing several alternatively spliced mRNAs generated by the same gene, and can overcome the limitations of clustering methods based on simple comparisons between splice isoform expression levels.

Clinical assays for quantitation of insulin-like-growth-factor-1 (IGF1)

Insulin-like growth factor 1 (IGF1), a 70 amino acid peptide hormone is the principal mediator of effects of growth hormone (GH). Since GH secretion is pulsatile in nature and is affected by many factors including sleep, feeding and exercise it is not a reliable marker for diagnosis of GH related disorders. On the other hand, IGF1 levels does not undergo short-term fluctuations in the manner that GH does making it the preferred IGF1 biomarker for the diagnosis of growth related disorders. There are several immunoassays available for IGF1 determination. Since majority (>90%) of IGF1 circulates as a ternary complex bound to its principal carrier/binding protein, IGF binding protein 3 (IGFBP3) and acid labile subunit (ALS), the assay methodology used to quantitate IGF1 has to dissociate IGF1 from IGFBPs prior to quantitation. IGFBPs are known to be a source of interference in immunoassays and many techniques have been employed to circumvent this issue. Immunoassays rely on antibody specificity towards IGF1 and differential cross reactivity towards IGFBPs. Mass spectrometry (MS) has also been employed for quantitation of IGF1. Liquid chromatography tandem mass spectrometry (LC-MS/MS) assays for IGF1 rely on generating tryptic peptides followed by selective reaction monitoring (SRM) while LC high resolution accurate-mass mass spectrometry (LC-HRAMS) approaches for intact IGF1 rely on mass accuracy for reliable, robust and accurate quantitation. This review article will focus on the clinical assays available and the clinical utility of quantitative assessment of IGF1. IGF1 quantitation using diverse assay platforms including immunoassay, LC-MS/MS and LC-HRAMS are discussed in detail.

Analysis of cytosine-adenine repeats in P1 promoter region of IGF-1 gene in peripheral blood cells and cervical tissue samples of females with cervical intraepithelial lesions and squamous cervical cancer

High oncogenic risk human papillomaviruses (HPVs) are closely associated with cancer of the cervix. However, HPV infection alone may not be sufficient to cause cervical cancer, and other factors or cofactors may have a cumulative effect on the risk of progression from cervical HPV infection to cancer. The present study investigates the cytosine-adenine (CA) repeat polymorphism in the P1 promoter region of the insulin-like growth factor-1 (IGF-1) gene among cervical precancerous and cancer patients and healthy control females. The association between these polymorphisms, tissue and blood serum levels of IGF-1, and cervical cancer risk and progression is evaluated. The material for analysis consisted of blood cells and postoperative tissues from patients diagnosed with low-grade squamous intraepithelial lesions (L-SILs), high-grade squamous intraepithelial lesions (H-SILs) and invasive cervical cancer (ICC). A polymerase chain reaction amplification and the sequencing of DNA were used for the identification of (CA)_n repeats in the IGF-1 P1 region and detection of HPV DNA. The blood serum concentration of IGF was determined by enzyme-linked immunosorbent assay. The identification of the IGF-1 protein in the cervical tissues was performed by immunohistochemical analysis. The range of the length of the CA repeats in the study DNA was 11 to 21. However, the most common allele length and genotype in the control and study patients from serum and tissues was 19 CA repeats and a homozygous genotype of CA19/19. Statistically significant differences in the concentration of IGF-1 in the blood serum were observed between H-SILs and controls, only (p=0.047). However, the concentration of IGF-1 in the group of females with CA19/19, CA19<19 and CA19>19 was significantly higher in the group of patients with H-SIL (P=0.041) and ICC (P=0.048) in comparison with the control group. An association was detected between CA repeat length <19 and/or >19, IGF concentration in blood serum and tissues and the development of cervical cancer.

Optimizing IGF-I for skeletal muscle therapeutics

It is virtually undisputed that IGF-I promotes cell growth and survival. However, the presence of several IGF-I isoforms, vast numbers of intracellular signaling components, and multiple receptors results in a complex and highly regulated system by which IGF-I actions are mediated. IGF-I has long been recognized as one of the critical factors for coordinating muscle growth, enhancing muscle repair, and increasing muscle mass and strength. How to optimize this panoply of pathways to drive anabolic processes in muscle as opposed to aberrant growth in other tissues is an area that deserves focus. This review will address how advances in the bioavailability, potency, and tissue response of IGF-I can provide new potential directions for skeletal muscle therapeutics.

IGF expression in HPV-related and HPV-unrelated human cancer cells

The human Igf-1 gene not only produces insulin‑like growth factor-I (IGF-I), but also different carboxy‑terminal extensions, known as E peptides, through alternative splicing. We and others have shown that human Eb peptide (hEb) derived from Igf-1 has intrinsic biological activity and is localized to nuclei of transfected cells. Since hEb actions can complement the activity of IGF-I itself, the aim of the present study was to compare IGF-I isoforms at the endogenous protein and transcript level in cancer cell lines, including HeLa, U2OS, HepG2 and K562 cells. Quantitative real-time PCR (qRT‑PCR) using Igf-1 isoform specific primers was performed to determine expression patterns, using β-actin as a reference gene. The overall relative Igf-1 transcript level was different across the cell lines, with ~80-fold higher expression in K562 (130.2±31.2) than in U2OS cells (1.7±1.1). The relative copy number of Igf-1b was the highest in HepG2 (69.9±28.6) and K562 cells (28.3±6.7), whereas the relative copy numbers of Igf-1a and Igf-1c were significantly higher in K562 cells compared to all other cell lines. Immunoblotting using total cell lysates, cytoplasmic and nuclear fractions were carried out to determine the level and distribution of IGF-I proteins. K562 cells exhibited the highest level of hEb in total cell lysates and nuclear fractions and no cell lines displayed hEb in the cytoplasmic fractions. In contrast, IGF-IA was the highest in HeLa cells and was enriched only in the cytoplasmic fraction. Since relatively low IGF-1A transcript level but relatively high pro‑IGF-1A protein level is plausible, we hypothesized that these transcripts could be processed with higher efficiency and/or the protein product may be stabilized by viral HPV oncogenes in HeLa cells. We assert that while it is important to analyze Igf-1 transcript level, it may be more relevant to determine the IGF isoforms at the protein level.

Insulin-like growth factor 1 physiology: lessons from mouse models

Insulin-like growth factor 1 (IGF-1) is a pleiotropic polypeptide. Its expression is tightly regulated and it plays significant roles during early development, maturation, and adulthood. This article discusses the roles of IGF-1 in determination of body size, skeletal acquisition, muscle growth, carbohydrate metabolism, and longevity, as learned from mouse models.

Expression of various insulin-like growth factor-1 mRNA isoforms in colorectal cancer

Aim of the study

Several epidemiological studies have attempted to demonstrate a relationship between increased serum level of insulin-like growth factor 1 (IGF-1) and an augmented risk of developing colorectal cancers (CRC).

The human IGF-1 gene is composed of 6 exons and demonstrated expression of 6 different splice variants (isoforms) of mRNA (IA, IB, IC, IIA, IIB and IIC).

The aim of the study was to evaluate the expression of different isoforms of IGF-1 mRNA in CRC and normal colon tissue.

Material and methods

13 paired tissue specimens (colorectal tumor and non-tumor tissues) were analyzed using both quantitative polymerase chain reaction (PCR) and immunocytochemistry methods (IHC). The expression of classes I and II and variants A, B, C of IGF-1 mRNA were measured.

Results

In CRC higher amounts of IGF-1 class II mRNA than class I mRNA were detected. Among A, B, C isoforms, A variant of IGF-1 mRNA prevailed. The amounts of IGF-1 class I and class II mRNAs and of IGF-1 variant B mRNA were lowered in CRC as compared to the control. In CRC significant correlations were detected between reciprocal expression of class I and class II as well as between I and II isoforms and A, B and C.

Conclusions

Expression of IGF-1 mRNA isoforms differs between normal and CRC tissues. Even if all isoforms of IGF-1 mRNA manifested correlations with each other in tissues of CRC, expression of all transcripts (except that of isoform A) was significantly decreased as compared to the control.

Genetic correlates of early accelerated infant growth associated with juvenile-onset type 1 diabetes

OBJECTIVE

We previously showed that accelerated growth predisposing to development of childhood-onset type 1 diabetes (T1D) is restricted to the first year after birth. We assessed whether this phenomenon of increased early growth is associated with variants of two genes, insulin-like growth factor-1 (IGF1) and insulin variable number of tandem repeats (INS-VNTR), whose products are components of the growth axis.

PATIENTS AND METHODS

Patients and their siblings were genotyped for the INS-VNTR and for an IGF1 microsatellite. We tested for difference in first year growth, i.e., increased weight standard deviation score (SDS), a reliable measure of especially first year growth, between carriers and non-carriers of these gene variants, using a repeated measurement and regression analysis.

RESULTS

In patients, growth did not differ between carriers and non-carriers of the INS-VNTR*III allele, while carriership of this allele in siblings was positively associated with increased first year growth. In both patients and siblings, non-carriership of the IGF1*194 allele was positively associated with growth. Birth size was not associated with either variant.

CONCLUSIONS/DISCUSSION

Non-carriership of the IGF1*194 allele was positively associated with accelerated first year growth in both patients and siblings, independent of disease. This IGF1 variant may therefore contribute to increased first year growth, but cannot explain the association of first year growth with diabetes. An effect on growth of the INS-VNTR was detected in healthy siblings, but not in patients, suggesting that disease supersedes a growth effect of INS-VNTR.

Expression of insulin-like growth factor 1 isoforms in the rabbit oculomotor system

OBJECTIVE

The insulin-like growth factor-1 (IGF-1) gene encodes two isoforms, IGF-1Ea and IGF-1Eb. Both isoforms can regulate skeletal muscle growth and strength. It has been suggested that IGF-Eb may be more potent in promoting skeletal muscle hypertrophy. Precise contractile force regulation is particularly important in the oculomotor system. However, expression of these isoforms in mammalian extraocular muscles (EOMs) is unknown. Here, we examined their expression in rabbit EOMs and the innervating nerve, two potential sources for myogenic growth factors, and compared isoform expression between EOMs and limb skeletal muscles.

DESIGN

Expression of IGF-1 isoforms was quantified by real-time RT-PCR in adult rabbit EOMs, trochlear and ophthalmic nerves, and compared with expression in rabbit limb skeletal muscles. The presence of mature IGF-1 peptide in the muscles was further examined by Western blot.

RESULTS

Both IGF-1Ea and IGF-1Eb were expressed in the EOM and the trochlear nerve. Both isoforms were expressed at significantly higher levels (9-fold) in EOM than in limb skeletal muscle. Transcripts of IGF-1 isoforms, of IGF-1 receptor and of IGF binding proteins showed a gradient distribution along the EOM from proximal to distal. The mature IGF-1 protein showed the same gradient distribution in the EOM.

CONCLUSIONS

Expression of relatively abundant amounts of both IGF-1 splicing isoforms in EOMs, and at a significantly higher level than in limb skeletal muscle, underscores the potential relevance of these myogenic growth factors in EOM plasticity and force regulation.

Cross-talk between the insulin-like growth factor (IGF) axis and membrane integrins to regulate cell physiology

The biology of cross-talk between activated growth factor receptors and cell-surface integrins is an area which has attracted much interest in recent years (Schwartz and Ginsberg, 2002). This review discusses the relationship between the insulin-like growth factor (IGF) axis and cell-surface integrin receptors in the regulation of various aspects of cell physiology. Key to these interactions are signals transmitted between integrins and the IGF-I receptor (IGF-IR) when either or both are bound to their cognate ligands and we will review the current state of knowledge in this area. The IGF axis comprises many molecular components and we will also discuss the potential role of these species in cross-talk with the integrin receptor. With respect to integrin ligands, we will mainly focus on the well-characterized interactions of the two extracellular matrix (ECM) glycoproteins fibronectin (FN) and vitronectin (VN) with cell-surface ligands, and, how this affects activity through the IGF axis. However, we will also highlight the importance of other integrin activation mechanisms and their impact on IGF activity.

The insulin-like growth factor (IGF)-I E-peptides are required for isoform-specific gene expression and muscle hypertrophy after local IGF-I production

Insulin-like growth factor I (IGF-I) coordinates proliferation and differentiation in a wide variety of cell types. The igf1 gene not only produces IGF-I, but also generates multiple carboxy-terminal extensions, the E-peptides, through alternative splicing leading to different isoforms. It is not known if the IGF-I isoforms share a common pathway for their actions, or if there are specific actions of each protein. Viral administration of murine IGF-IA, IGF-IB, and mature IGF, which lacked an E-peptide extension, was utilized to identify IGF-I isoform-specific responsive genes in muscles of young growing mice. Microarray analysis revealed responses that were driven by increased IGF-I regardless of the presence of E-peptide, such as Bcl-XL. In contrast, distinct expression patterns were observed after viral delivery of IGF-IA or IGF-IB, which included matrix metalloproteinase 13 (MMP13). Expression of Bcl-XL was prevented when viral administration of the IGF-I isoforms was performed into muscles of MKR mice, which lack functional IGF-I receptors on the muscle fibers. However, MMP13 expression persisted under the same conditions after viral injection of IGF-IB. At 4 mo after viral delivery, expression of IGF-IA or IGF-IB promoted muscle hypertrophy, but viral delivery of mature IGF-I failed to increase muscle mass. These studies provide evidence that local production of IGF-I requires the E-peptides to drive hypertrophy in growing muscle and that both common and unique pathways exist for the IGF-I isoforms to promote biological effects.

The insulin-like growth factor (IGF)-I E-peptides modulate cell entry of the mature IGF-I protein

Insulin-like growth factor (IGF)-I is a critical protein for cell development and growth. Alternative splicing of the igf1 gene gives rise to multiple isoforms. In rodents, proIGF-IA and proIGF-IB have different carboxy-terminal extensions called the E-peptides (EA and EB) and upon further posttranslational processing, produce the identical mature IGF-I protein. Rodent EB has been reported to have mitogenic and motogenic effects independent of IGF-I. However, effects of EA or EB on mature IGF-I, or whether proIGF-IA and proIGF-IB have different properties, have not been addressed. To determine whether the presence of EA or EB affected the distribution and stability of mature IGF-I protein, transient transfections of cDNAs encoding murine IGF-IA, IGF-IB, and mature IGF-I were performed in C2C12 cells, a skeletal muscle cell line. IGF-I secretion was measured by enzyme-linked immunosorbent assay of the media, and did not differ between expression of proIGF-IA, proIGF-IB, or mature IGF-I expression. Next, epitope-tagged constructs were transfected to determine cellular distribution of IGF-I, EA, and EB in the cells throughout the culture. IGF-I was detected in significantly fewer nontransfected cells in cultures transfected with mature IGF-I compared with transfection of proIGF-IA or proIGF-IB. These results demonstrate that EA and EB are not required for IGF-I secretion but that they increase cell entry of IGF-I from the media. This study provides evidence that the EA and EB may modulate IGF-I in addition to having independent activity.

Insulin-like growth factor-I for the treatment of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that affects both upper and lower motorneurons (MN) resulting in weakness, paralysis and subsequent death. Insulin-like growth factor-I (IGF-I) is a potent neurotrophic factor that has neuroprotective properties in the central and peripheral nervous systems. Due to the efficacy of IGF-I in the treatment of other diseases and its ability to promote neuronal survival, IGF-I is being extensively studied in ALS therapeutic trials. This review covers in vitro and in vivo studies examining the efficacy of IGF-I in ALS model systems and also addresses the mechanisms by which IGF-I asserts its effects in these models, the status of the IGF-I system in ALS patients, results of clinical trials, and the need for the development of better delivery mechanisms to maximize IGF-I efficacy. The knowledge obtained from these studies suggests that IGF-I has the potential to be a safe and efficacious therapy for ALS.

Serum complexes of insulin-like growth factor-1 modulate skeletal integrity and carbohydrate metabolism

Serum insulin-like growth factor (IGF) -1 is secreted mainly by the liver and circulates bound to IGF-binding proteins (IGFBPs), either as binary complexes or ternary complexes with IGFBP-3 or IGFBP-5 and an acid-labile subunit (ALS). The purpose of this study was to genetically dissect the role of IGF-1 circulatory complexes in somatic growth, skeletal integrity, and metabolism. Phenotypic comparisons of controls and four mouse lines with genetic IGF-1 deficits-liver-specific IGF-1 deficiency (LID), ALS knockout (ALSKO), IGFBP-3 (BP3) knockout, and a triply deficient LID/ALSKO/BP3 line-produced several novel findings. 1) All deficient strains had decreased serum IGF-1 levels, but this neither predicted growth potential or skeletal integrity nor defined growth hormone secretion or metabolic abnormalities. 2) IGF-1 deficiency affected development of both cortical and trabecular bone differently, effects apparently dependent on the presence of different circulating IGF-1 complexes. 3) IGFBP-3 deficiency resulted in increased linear growth. In summary, each IGF-1 complex constituent appears to play a distinct role in determining skeletal phenotype, with different effects on cortical and trabecular bone compartments.

Overexpression of mIGF-1 in keratinocytes improves wound healing and accelerates hair follicle formation and cycling in mice

Insulin-like growth factor 1 (IGF-1) is an important regulator of growth, survival, and differentiation in many tissues. It is produced in several isoforms that differ in their N-terminal signal peptide and C-terminal extension peptide. The locally acting isoform of IGF-1 (mIGF-1) was previously shown to enhance the regeneration of both muscle and heart. In this study, we tested the therapeutic potential of mIGF-1 in the skin by generating a transgenic mouse model in which mIGF-1 expression is driven by the keratin 14 promoter. IGF-1 levels were unchanged in the sera of hemizygous K14/mIGF-1 transgenic animals whose growth was unaffected. A skin analysis of young animals revealed normal architecture and thickness as well as proper expression of differentiation and proliferation markers. No malignant tumors were formed. Normal homeostasis of the putative stem cell compartment was also maintained. Healing of full-thickness excisional wounds was accelerated because of increased proliferation and migration of keratinocytes, whereas inflammation, granulation tissue formation, and scarring were not obviously affected. In addition, mIGF-1 promoted late hair follicle morphogenesis and cycling. To our knowledge, this is the first work to characterize the simultaneous, stimulatory effect of IGF-1 delivery to keratinocytes on two types of regeneration processes within a single mouse model. Our analysis supports the use of mIGF-1 for skin and hair regeneration and describes a potential cell type-restricted action.

Running on empty: how p53 controls INS/IGF signaling and affects life span

In higher organisms dependent on the regenerative ability of tissue stem cells to maintain tissue integrity throughout adulthood, the failure of stem cells to replace worn out, dead, or damaged cells is seen as one mechanism that limits life span. In these organisms, tumor suppressors such as p53 are central participants in the control of longevity because they regulate stem cell proliferation. Several recent reports have identified p53 as a longevity gene in organisms such as Caenorhabditis elegans and Drosophila melanogaster, which lack proliferative stem cells in all but the germline and have relatively short life spans. This has forced us to reevaluate the role of p53 in the control of life span. We discuss how p53 might regulate longevity in both long- and short-lived species by controlling the activity of insulin-like molecules that operate in proliferating and non-proliferating compartments of adult somatic tissues. We also discuss the hierarchical structure of life span regulation where loss of p53 has life span extending effects. Finally, we suggest a molecular mechanism by which p53 might facilitate the response to severe nutrient deprivation that allows metabolically active cells to survive periods of starvation. Paradoxically, loss of p53 function in these cells would compromise life span.

Neuroprotective effects of short peptides derived from the Insulin-like growth factor 1

Insulin-like growth factor I (IGF-1) is a peptide synthesized in response to growth hormone stimulation. While most of the circulating IGF-1 comes from the liver, it can also be produced in other tissues and both its expression and processing undergo tissue-specific regulation. The predominant form, IGF-1Ea is a circulating factor while two others, IGF-1Eb and IGF-1Ec (MGF), are mostly expressed in different tissues or in response to various stimuli and show some preferences with respect to the signal transduction pathways they activate. In skeletal muscle specific forms of IGF-1 play a role in development and growth and in addition to these physiological roles IGF-1 functions in the damaged muscle. IGF-1 is also important for the developing and adult brain and can reduce neuronal death caused by different types of injuries. Like many other peptide hormones IGF-1 originates from a precursor pro-hormone that undergoes extensive post-translational modifications. Processing liberates the mature peptide, which acts via the specific IGF-1 receptor but additional short peptides can arise from both N- and C-termini of various IGF-1 isoforms. These derivatives function as autonomous biologically active peptides and extremely potent neuroprotective agents. Their biological effects are independent of the activation of the IGF-1 receptor. Unfortunately, little is known about their mechanism(s) of action. Likewise, the existence of the endogenous production and wider biological effects of these short peptides are uncertain. However, considering the difference in the modes of action it might be possible to dissociate the unwanted and potentially dangerous mitogenic activity of the full-length IGF-1 exerted via its receptor from the neuroprotective effects of short derivatives mediated through different pathways. Such small molecules show good penetration through the blood brain barrier, can be inexpensively manufactured and modified to increase their stability. Therefore, they are good candidates for development into a neuroprotective therapeutic modality.

Enhancing repair of the mammalian heart

The injured mammalian heart is particularly susceptible to tissue deterioration, scarring, and loss of contractile function in response to trauma or sustained disease. We tested the ability of a locally acting insulin-like growth factor-1 isoform (mIGF-1) to recover heart functionality, expressing the transgene in the mouse myocardium to exclude endocrine effects on other tissues. supplemental mIGF-1 expression did not perturb normal cardiac growth and physiology. Restoration of cardiac function in post-infarct mIGF-1 transgenic mice was facilitated by modulation of the inflammatory response and increased antiapoptotic signaling. mIGF-1 ventricular tissue exhibited increased proliferative activity several weeks after injury. The canonical signaling pathway involving Akt, mTOR, and p70S6 kinase was not induced in mIGF-1 hearts, which instead activated alternate PDK1 and SGK1 signaling intermediates. The robust response achieved with the mIGF-1 isoform provides a mechanistic basis for clinically feasible therapeutic strategies for improving the outcome of heart disease.

Effect of angiotensin II with follicle cells and insulin-like growth factor-I or insulin on bovine oocyte maturation and embryo development

The objective was to evaluate the effects of angiotensin II (Ang II), insulin-like growth factor-I (IGF-I) and insulin on the nuclear and cytoplasmic maturation of bovine oocytes in the presence of follicular cells. Cumulus-oocyte complexes (COCs) were cultured for 22h in the presence of follicular cells (control with cells) and Ang II, IGF-I or insulin (treatments), or in the absence of follicular cells (control without cells). Using these five groups, Experiment 1 was conducted with and without the addition of gonadotrophins. Only oocytes in the Ang II group resumed meiosis at rates (88.2+/-1.8% and 90.7+/-4.3% for oocytes cultured in the absence or presence of LH/FSH, respectively) similar to those observed for oocytes cultured in the absence of follicular cells (89.7+/-0.3% and 92.6+/-2.6%; P<0.01). In Experiment 2, the effect of Ang II alone and in combination with IGF-I or insulin on oocyte maturation for 7h (germinal vesicle breakdown), 12h (metaphase I) and 22h (metaphase II) was evaluated in a design similar to that of the first experiment. Ang II plus IGF-I or insulin induced the resumption of meiosis, irrespective of the presence of gonadotrophins (P<0.01). Experiment 3 used groups similar Experiment 2 to determine the rate of subsequent embryo development, using fetal calf serum (FCS) in the culture medium. The COCs were cultured in maturation medium for 1h (1+23h), 12h (12+12h) or 24h in the presence of follicular cells and the respective treatments and for the remaining period in the absence of follicular cells to complete 24h. In Experiment 4, BSA was used in lieu of serum in the maturation medium in a 12+12h maturation system. Oocytes matured using the 12+12h system with BSA or FCS in the presence of Ang II+IGF-I had higher rates of blastocyst formation than the other treatments (P<0.05). In conclusion, Ang II reversed the inhibitory effect of follicular cells on nuclear maturation of bovine oocytes, irrespective of the presence of gonadotrophins, IGF-I and insulin. However, oocyte cytoplasmatic maturation (i.e., subsequent embryo development), was higher when Ang II and IGF-I were present in the maturation medium containing follicular cells cultured for 12+12h.

The neuroprotective effects of a locally acting IGF-1 isoform

In the last decade, dramatic progress has been made in elucidating the molecular defects underlying a number of neuromuscular diseases. With the characterization of mutations responsible for muscle and nerve dysfunction in several inherited pathologies, and the identification of novel signaling pathways, in which subtle alterations can lead to significant defects in tissue metabolism, the field is poised to devise successful strategies for treatment of this debilitating and often fatal group of human ailments. Yet progress in therapeutic application has been slow despite our newly gained knowledge of basic biology. Hence, where direct therapeutic approaches to address the primary diseases are still sub-optimal, it may be more effective to focus on strategies for improving neuromuscular function. Among potential candidates, insulin-like growth factor (IGF-1) has been involved in several anabolic pathways in both skeletal muscle and the nervous system and it is a promising candidate to attenuate neuromuscular diseases. In this review, we will discuss the role of IGF-1 isoforms in neuromuscular diseases and the contribution of muscle-produced IGF-1 (mIGF-1) to motor neuron survival and activity.

Viral expression of insulin-like growth factor-I isoforms promotes different responses in skeletal muscle

Insulin-like growth factor I (IGF-I) is a critical protein for skeletal muscle development and regeneration. Its ability to promote skeletal muscle hypertrophy has been demonstrated by several methods. Alternative splicing of the Igf-1 gene does not affect the mature IGF-I protein but does produce different E peptide extensions, which have been reported to modify the potency of IGF-I. Viral-mediated delivery of murine IGF-IA and IGF-IB into skeletal muscle of 2-wk-old and 6-mo-old mice was utilized to compare the effects of the isoforms on muscle mass. In young mice, tissue content of IGF-I protein was significantly higher in rAAV-treated muscles than control muscles at 1, 2, and 4 mo postinjection. Viral injection of IGF-IB produced two- to sevenfold more IGF-I than rAAVIGF-IA. Hypertrophy was observed 2 and 4 mo postinjection, where both rAAVIGF-IA and rAAVIGF-IB were equally effective in increasing muscle mass. These results suggest that there is a threshold of IGF-I production necessary to promote muscle hypertrophy in young growing animals regardless of isoform. In 6-mo-old animals, only rAAVIGF-IA produced significant increases in muscle size, even though increased IGF-I content was observed after injection of both isoforms. Therefore, the ability for IGF-IB to promote muscle hypertrophy is only effective in growing animals, suggesting that the bioavailability of this isoform or its receptor affinity diminishes with age.

The effect of recombinant human growth hormone and resistance training on IGF-I mRNA expression in the muscles of elderly men

The expression of two isoforms of insulin-like growth factor-I (IGF-I): mechano growth factor (MGF) and IGF-IEa were studied in muscle in response to growth hormone (GH) administration with and without resistance training in healthy elderly men. A third isoform, IGF-IEb was also investigated in response to resistance training only. The subjects (age 74 +/- 1 years, mean +/- S.E.M) were assigned to either resistance training with placebo, resistance training combined with GH administration or GH administration alone. Real-time quantitative RT-PCR was used to determine mRNA levels in biopsies from the vastus lateralis muscle at baseline, after 5 and 12 weeks in the three groups. GH administration did not change MGF mRNA at 5 weeks, but significantly increased IGF-IEa mRNA (237%). After 12 weeks, MGF mRNA was significantly increased (80%) compared to baseline. Five weeks of resistance training significantly increased the mRNA expression of MGF (163%), IGF-IEa (68%) and IGF-IEb (75%). No further changes were observed after 12 weeks. However, after 5 weeks of training combined with GH treatment, MGF mRNA increased significantly (456%) and IGF-IEa mRNA by (167%). No further significant changes were noted at 12 weeks. The data suggest that when mechanical loading in the form of resistance training is combined with GH, MGF mRNA levels are enhanced. This may reflect an overall up-regulation of transcription of the IGF-I gene prior to splicing.

Transcriptional regulation of IGF-I expression in skeletal muscle

The present study investigated the role of transcription in the regulation of insulin-like growth factor (IGF)-I expression in skeletal muscle. RT-PCR was used to determine endogenous expression of IGF-I pre-mRNA and mRNA in control (Con) and functionally overloaded (FO) rat plantaris. The transcriptional activities of five different-length IGF-I promoter fragments controlling transcription of a firefly luciferase (FLuc) reporter gene were tested in vitro by transfection of myoblasts or in vivo during FO by direct gene transfer into the plantaris. Increased endogenous IGF-I gene transcription during 7 days of plantaris FO was evidenced by an approximately 140-160% increase (P < 0.0001) in IGF-I pre-mRNA (a transcriptional marker). IGF-I mRNA expression also increased by approximately 90% (P < 0.0001), and it was correlated (R = 0.93; P < 0.0001) with the pre-mRNA increases. The three longest IGF-I exon 1 promoters induced reporter gene expression in proliferating C2C12 and L6E9 myoblasts. In differentiated L6E9 myotubes, promoter activity increased approximately two- to threefold over myoblasts. Overexpression of calcineurin and MyoD increased the activity of the -852/+192 promoter in C2C12 myotubes by approximately 5- and approximately 18-fold, respectively. However, FO did not induce these exogenous promoter fragments. Nevertheless, the present findings are consistent with the hypothesis that the IGF-I gene is transcriptionally regulated during muscle hypertrophy in vivo as evidenced by the induction of the endogenous IGF-I pre-mRNA during plantaris FO. The exon 1 promoter region of the IGF-I gene is sufficient to direct inducible expression in vitro; however, an in vivo response to FO may require elements outside the -852/+346 region of the exon 1 IGF-I promoter or features inherent to the endogenous IGF-I gene.

Gene therapy for cardiac cachexia?

The prevention or attenuation of disease-related skeletal muscle degeneration has been a common goal in the treatment of cardiac cachexia. Cell-based therapies are complicated by insufficient numbers of autologous myoblasts and by ineffective incorporation into host muscle. Pharmacological administration of growth hormone in a variety of clinical conditions characterized by an increase in catabolic rate have been associated with increases in mortality and morbidity, resulting in a decrease in the clinical use of growth hormone and its downstream effector, insulin-like growth factor-1 and a decline in general research into anabolic treatment strategies. In mouse models, however, the selective expression of a muscle-specific transgene encoding a locally acting IGF-1 isoform induces muscle hypertrophy, prevents age- or disease-related atrophy, by increasing stem cell recruitment to injured or degenerating tissue. This gene-based approach avoids hypertrophic effects on distal organs such as the heart, and eliminates risk of possible neoplasms induced by inappropriate high expression levels of circulating IGF-1. The potential therapeutic role of locally expressed IGF-1 is discussed in the context of current strategies for the attenuation of cardiac cachexia.

Regulation of insulin-like growth factor I (IGF-I) gene expression in brain of transgenic mice expressing an IGF-I-luciferase fusion gene

Insulin-like growth factor I (IGF-I) plays an important role in the development and function of the central nervous system (CNS). Little is known, however, about the factors and mechanisms involved in regulation of CNS IGF-I gene expression. To facilitate our goal to define mechanisms of IGF-I gene regulation in the CNS, we generated several lines of transgenic (Tg) mice that express firefly luciferase (LUC) under control of a 11.3-kb fragment from the 5' region of the rat IGF-I gene. Consistent with expression of the native IGF-I gene in murine brain, expression of the transgene predominated in neurons and astrocytes and used promoter 1, the major IGF-I promoter in the CNS and in most tissues. Transgene messenger RNA and protein expression rapidly increased after birth and peaked at postnatal (P) day 4 in all brain regions studied. LUC activities in all regions then gradually decreased to 0.5-4% of their peak values at P31, except for the olfactory bulb, which maintained about one third of its maximal activity. Compared with littermate controls, administration of dexamethasone decreased LUC activity and transgenic IGF-I messenger RNA abundance, whereas GH significantly increased the expression of the transgene. Addition of GH to cultured fetal brain cells from Tg mice for 12 h also increased LUC activity in a dose-dependent manner (77-388%). These results show that this IGF-I promoter transgene is expressed in a fashion similar to the endogenous IGF-I gene, and thus indicates that the transgene contains cis-elements essential for developmental, GH, and glucocorticoid regulation of IGF-I gene expression in the CNS. These Tg mice should serve as an useful model to study mechanisms of IGF-I gene regulation in the brain.