Insulin-like Growth Factor-1 Isoforms Research Articles

Construction of Plasmid DNA Expressing Two Isoforms of Insulin-Like Growth Factor-1 and Its Effects on Skeletal Muscle Injury Models.

Insulin-like growth factor-1 (IGF-1) plays a significant role in the development of various organs, and several studies have suggested that IGF-1 isoforms, IGF-1 Ea and IGF-1 Ec, are expressed in skeletal muscle to control its growth. In this study, we designed a novel nucleotide sequence, IGF-1-X10, consisting of IGF-1 exons and introns to simultaneously express both IGF-1 Ea and IGF-1 Ec. When transfected into human cells, the expression of both isoforms was observed at the transcript and protein levels. In an animal study, intramuscular injection of plasmid DNA comprising IGF-1-X10 induced the expression of IGF-1 Ea and IGF-1 Ec, leading to the production of functional IGF-1 protein. Finally, the efficacy of this plasmid DNA was tested in a cardiotoxin (CTX)-mediated muscle injury model and age-related muscle atrophy model. We found that IGF-1-X10 increased the muscle mass and controlled several key factors involved in the muscle atrophy program in both models. Taken together, these data suggest that IGF-1-X10 may be utilized in the form of gene therapy for the treatment of various muscle diseases related to IGF-1 deficiency.

Application of the Athlete Biological Passport Approach to the Detection of Growth Hormone Doping.

Because of its anabolic and lipolytic properties, growth hormone (GH) use is prohibited in sport. Two methods based on population-derived decision limits are currently used to detect human GH (hGH) abuse: the hGH Biomarkers Test and the Isoforms Differential Immunoassay. We tested the hypothesis that longitudinal profiling of hGH biomarkers through application of the Athlete Biological Passport (ABP) has the potential to flag hGH abuse. Insulin-like growth factor 1 (IGF-1) and procollagen III peptide (P-III-NP) distributions were obtained from 7 years of anti-doping data in elite athletes (n = 11 455) and applied as priors to analyze individual profiles from an hGH administration study in recreational athletes (n = 35). An open-label, randomized, single-site, placebo-controlled administration study was carried out with individuals randomly assigned to 4 arms: placebo, or 3 different doses of recombinant hGH. Serum samples were analyzed for IGF-1, P-III-NP, and hGH isoforms and the performance of a longitudinal, ABP-based approach was evaluated. An ABP-based approach set at a 99% specificity level flagged 20/27 individuals receiving hGH treatment, including 17/27 individuals after cessation of the treatment. ABP sensitivity ranged from 12.5% to 71.4% across the hGH concentrations tested following 7 days of treatment, peaking at 57.1% to 100% after 21 days of treatment, and was maintained between 37.5% and 71.4% for the low and high dose groups 1 week after cessation of treatment. These findings demonstrate that longitudinal profiling of hGH biomarkers can provide suitable performance characteristics for use in anti-doping programs.

Role of Alternatively Spliced Messenger RNA (mRNA) Isoforms of the Insulin-Like Growth Factor 1 (IGF1) in Selected Human Tumors.

Insulin-like growth factor 1 (IGF1) is a key regulator of tissue growth and development that is also implicated in the initiation and progression of various cancers. The human IGF1 gene contains six exons and five long introns, the transcription of which is controlled by two promoters (P1 and P2). Alternate promoter usage, as well as alternative splicing (AS) of IGF1, results in the expression of six various variants (isoforms) of mRNA, i.e., IA, IB, IC, IIA, IIB, and IIC. A mature 70-kDa IGF1 protein is coded only by exons 3 and 4, while exons 5 and 6 are alternatively spliced code for the three C-terminal E peptides: Ea (exon 6), Eb (exon 5), and Ec (fragments of exons 5 and 6). The most abundant of those transcripts is IGF1Ea, followed by IGF1Eb and IGF1Ec (also known as mechano-growth factor, MGF). The presence of different IGF1 transcripts suggests tissue-specific auto- and/or paracrine action, as well as separate regulation of both of these gene promoters. In physiology, the role of different IGF1 mRNA isoforms and pro-peptides is best recognized in skeletal muscle tissue. Their functions include the development and regeneration of muscles, as well as maintenance of proper muscle mass. In turn, in nervous tissue, a neuroprotective function of short peptides, produced as a result of IGF1 expression and characterized by significant blood-brain barrier penetrance, has been described and could be a potential therapeutic target. When it comes to the regulation of carcinogenesis, the potential biological role of different var iants of IGF1 mRNAs and pro-peptides is also intensively studied. This review highlights the role of IGF1 isoform expression (mRNAs, proteins) in physiology and different types of human tumors (e.g., breast cancer, cervical cancer, colorectal cancer, osteosarcoma, prostate and thyroid cancers), as well as mechanisms of IGF1 spliced variants involvement in tumor biology.

CRISPR-Cas9-induced IGF1 gene activation as a tool for enhancing muscle differentiation via multiple isoform expression.

Muscle wasting, or muscle atrophy, can occur with age, injury, and disease; it affects the quality of life and complicates treatment. Insulin-like growth factor 1 (IGF1) is a key positive regulator of muscle mass. The IGF1/Igf1 gene encodes multiple protein isoforms that differ in tissue expression, potency, and function, particularly in cellular proliferation and differentiation, as well as in systemic versus localized signaling. Genome engineering is a novel strategy for increasing gene expression and has the potential to recapitulate the diverse biology seen in IGF1 signaling through the overexpression of multiple IGF1 isoforms. Using a CRISPR-Cas9 gene activation approach, we showed that the expression of multiple IGF1 or Igf1 mRNA variants can be increased in human and mouse skeletal muscle myoblast cell lines using a single-guide RNA (sgRNA). We found increased IGF1 protein levels in the cell culture media and increased cellular phosphorylation of AKT1, the main effector of IGF1 signaling. We also showed that the expression of Class 1 or Class 2 mRNA variants can be selectively increased by changing the sgRNA target location. The expression of multiple IGF1 or Igf1 mRNA transcript variants in human and mouse skeletal muscle myoblasts promoted myotube differentiation and prevented dexamethasone-induced atrophy in myotubes in vitro. Our findings suggest that this novel approach for enhancing IGF1 signaling has potential therapeutic applications for treating skeletal muscle atrophy.

Overexpression of Mechano-Growth Factor Modulates Inflammatory Cytokine Expression and Macrophage Resolution in Skeletal Muscle Injury.

In muscle regeneration, infiltrating myeloid cells, such as macrophages mediate muscle inflammation by releasing key soluble factors. One such factor, insulin-like growth factor 1 (IGF-1), suppresses inflammatory cytokine expression and mediates macrophage polarization to anti-inflammatory phenotype during muscle injury. Previously the IGF-1Ea isoform was shown to be anti-inflammatory. Another isoform of IGF-1, mechano-growth factor (MGF), is structurally and functionally distinct from IGF-1Ea, but its role in muscle inflammation has not yet been characterized. In this study, we hypothesized that MGF expression in muscle injury modulates muscle inflammation. We first investigated changes of transcription and expression of MGF in response to skeletal muscle injury induced by cardiotoxin (CTX) in vivo. At 1–2 days post-injury, Mgf expression was significantly upregulated and positively correlated with that of inflammatory cytokines. Immunostaining revealed that infiltration of neutrophils and macrophages coincided with Mgf upregulation. Furthermore, infiltrating neutrophils and macrophages expressed Mgf, suggesting their contribution to MGF upregulation in muscle injury. Macrophages seem to be the predominant source of MGF in muscle injury, whereas neutrophil depletion did not affect muscle Mgf expression. Given the association of MGF and macrophages, we then studied whether MGF could affect macrophage infiltration and polarization. To test this, we overexpressed MGF in CTX-injured muscles and evaluated inflammatory marker expression, macrophage populations, and muscle regeneration outcomes. MGF overexpression delayed the resolution of macrophages, particularly the pro-inflammatory phenotype. This coincided with upregulation of inflammatory markers. Annexin V-based flow cytometry revealed that MGF overexpression likely delays macrophage resolution by limiting macrophage apoptosis. Although MGF overexpression did not obviously affect muscle regeneration outcomes, the findings are novel and provide insights on the physiological roles of MGF in muscle regeneration.

The COOH-terminus of the IGF-1Ec Isoform Enhances the Proliferation and Migration of Human MCF-7 Breast Cancer Cells.

The insulin-like growth factor-1 (IGF-1) signaling is well implicated in cancer biology, however the potential roles of the distinct IGF-1 isoforms in human malignancies are largely unknown. Recently, the carboxyl-terminal of the IGF-1Ec variant (hEc; 24aa) has been associated with osteosarcoma and prostate cancer. Herein, we investigated the potential role of hEc in breast cancer. Synthetic hEc peptide was administrated to MCF-7 and MDA-MB-231 breast cancer cells. In addition MCF-7 cells were engineered to overexpress hEc. The proliferation and migratory capacities in response to hEc were analyzed using MTT, trypan blue and wound healing/scratch assays, while the activation of the ERK/AKT signaling pathways were investigated using phospho western blotting. We found that exogenous administration of hEc stimulated the proliferation of estrogen-responsive MCF-7, but not that of hormone-resistant MDA-MB-231 cells. In addition, MCF-7 cells stably-overexpressing hEc acquired an increased proliferation rate and migratory capacity, as well as, enhanced ERK1/2 phosphorylation, compared to mock and wild-type cells. hEc stimulates the proliferation and migration of MCF-7 breast cancer cells and enhances the intracellular ERK1/2 signaling.

Insulin-like growth factor-1 isoforms in human ovary. Preliminary report on the expression of the IGF-1 gene in PCOS patients and healthy controls.

Insulin-like growth factor 1 (IGF-1), produced and secreted locally may affect the mechanisms of folliculogenesis and cause ovarian dysfunction, characteristic of PCOS. The expression of the IGF-1 gene gives rise to three different isoforms of the original molecule. Until now, the role of IGF-1 isoforms has been documented in the repair processes of damaged muscle fibers, cardomyocytes, hepatocytes, and neurons. The literature offers no reports on the presence and role of IGF-1 isoforms in the ovary The aim of the study was to assess the IGF-1A, B and C isoforms at the level of IGF-1 gene transcription in the ovaries of PCOS women and healthy controls. Serum samples and ovarian tissues from PCOS women, treated and non-treated with metformin (PCOS M(+); n = 12 and PCOS M(-), n = 37, respectively), and controls (n = 21) were obtained. The expression of mRNA species of IGF-1 in the ovaries was determined by quantitative RT-PCR. The presence of transcripts of three types of IGF-1 isoforms was observed in healthy controls and PCOS patients, regardless of metformin treatment. Total expression of all isoforms was higher in the M(-)(Me-26640) group as compared to the M(+)(Me- 13470) group, as well as controls (Me-17030)-(not significant, p = 0.061). Similar results for IGF-1A were obtained in all groups. The relative expression of IGF-1A was lower in the M(-)(86.02%) group and differed statistically from controls (91.38%) (p = 0.011). We detected the presence of mRNA for three IGF-1 isoforms in human ovary. To the best of our knowledge, this has been the first report on the presence of mRNA for three IGF-1 isoforms in human ovary. We found differences in the relative expression of IGF-1A isoforms between the investigated groups.

Platelet-rich plasma and skeletal muscle healing: a molecular analysis of the early phases of the regeneration process in an experimental animal model.

Platelet-rich plasma (PRP) has received increasing interest in applied medicine, being widely used in clinical practice with the aim of stimulating tissue healing. Despite the reported clinical success, there is still a lack of knowledge when considering the biological mechanisms at the base of the activity of PRP during the process of muscle healing. The aim of the present study was to verify whether the local delivery of PRP modulates specific molecular events involved in the early stages of the muscle regeneration process. The right flexor sublimis muscle of anesthetized Wistar rats was mechanically injured and either treated with PRP or received no treatment. At day 2 and 5 after surgery, the animals were sacrificed and the muscle samples evaluated at molecular levels. PRP treatment increased significantly the mRNA level of the pro-inflammatory cytokines IL-1β, and TGF-β1. This phenomenon induced an increased expression at mRNA and/or protein levels of several myogenic regulatory factors such as MyoD1, Myf5 and Pax7, as well as the muscular isoform of insulin-like growth factor1 (IGF-1Eb). No effect was detected with respect to VEGF-A expression. In addition, PRP application modulated the expression of miR-133a together with its known target serum response factor (SRF); increased the phosphorylation of αB-cristallin, with a significant improvement in several apoptotic parameters (NF-κB-p65 and caspase 3), indexes of augmented cell survival. The results of the present study indicates that the effect of PRP in skeletal muscle injury repair is due both to the modulation of the molecular mediators of the inflammatory and myogenic pathways, and to the control of secondary pathways such as those regulated by myomiRNAs and heat shock proteins, which contribute to proper and effective tissue regeneration.

Biological activity of the e domain of the IGF-1Ec as addressed by synthetic peptides.

Insulin-like growth factor-1 (IGF-1) is a multipotent growth factor involved in the growth, development and regulation of homeostasis in a tissue-specific manner. Alternative splicing, multiple transcription initiation sites and different polyadelynation signals give rise to diverse mRNA isoforms, such as IGF-1Ea, IGF-1Eb and IGF-1Ec transcripts. There is increasing interest in the expression of the IGF-1 isoforms and their potential distinct biological role. IGF-1Ec results from alternative splicing of exons 4-5-6 and its expression is upregulated in various conditions and pathologies. Recent studies have shown that IGF-1Ec is preferentially increased after injury in skeletal muscle during post-infarctal myocardium remodelling and in cancer tissues and cell lines. A synthetic analogue corresponding to the last 24 aa of the E domain of the IGF-1Ec isoform has been used to elucidate its potential biological role. The aim of the present review is to describe and discuss the putative bioactivity of the E domain of the IGF-1Ec isoform.

Anabolic effects of interval exercise training on skeletal muscle of patients with chronic heart failure

Purpose: Heart failure (HF) is associated with exercise intolerance and significant skeletal muscle abnormalities. Insulin-like growth factor 1 (IGF-1) is a significant regulator of skeletal muscle mass that promotes muscle hypertrophy. IGF-1 expression is reduced in skeletal muscle of HF patients. Three different IGF-1 isoforms are expressed in humans, namely IGF-1Ea, IGF-1Eb and IGF-1Ec, showing a differential expression pattern in skeletal muscle. Aim of this study was to evaluate the anabolic effect of high intensity interval exercise training program on skeletal muscle and fiber-type distribution of HF patients. Methods: This randomized controlled 2-group study consisted of 9 stable heart failure patients (males, age: 56±13 yrs, BMI: 27±4 kg/m2) who participated in a high intensity interval training rehabilitation program [with (n=5) or without (n=4) the addition of strength training] of 36 sessions, 3 times per week. All patients performed a symptom-limited cardiopulmonary exercise test before and after the completion of the program. At the same time they underwent percutaneous needle biopsies of the vastus lateralis muscle. Pathology measurements included the skeletal muscle fiber type distribution and cross sectional area (CSA) that were determined by adenosine triphosphatase staining. The mRNA expression of IGF-1Ea, IGF-1Eb, IGF-IEc and IGF-1R was investigated by quantitative real-time PCR. Results: After 3 months of exercise training all patients significantly improved their exercise capacity. A significant increase of the CSA of type I fibers from 4519±1071 to 4856±1076 μm2 (p=0.001) and of type II fibers from 4420±1057 to 4925±1049 μm2 (p=0.001) was found and there was a significant reshift to aerobic fibers from 45±9% to 53±7% (p=0.006). An increased expression of the IGF-1Ea, IGF-1Eb and IGF-1Ec transcripts was found following the exercise training compared with the pre-exercise expression levels [1.8±0.9 (p=0.02), 1.7±0.8 (p=0.03) and 2.1±1.4 (p=0.04) fold changes, respectively]. IGF-1R expression levels showed no changes. The percentage increase of CSA found to be correlated with the increase of IGF-1Eb (r=0.72, p=0.03). No significant changes between the two types of exercise were found. Conclusions: The results indicate that high intensity interval exercise training can improve aerobic capacity and may have an anabolic effect by increasing the cross sectional area of muscle fibers in HF patients, possibly via IGF-1 isoform-specific actions. There is also evidence that the pathologic shif to anaerobic fiber phenotype present in HF patients may be reversed by this intervention.

The E-domain region of mechano-growth factor inhibits cellular apoptosis and preserves cardiac function during myocardial infarction

Insulin-like growth factor-1 (IGF-1) isoforms are expressed via alternative splicing. Expression of the minor isoform IGF-1Eb [also known as mechano-growth factor (MGF)] is responsive to cell stress. Since IGF-1 isoforms differ in their E-domain regions, we are interested in determining the biological function of the MGF E-domain. To do so, a synthetic peptide analog was used to gain mechanistic insight into the actions of the E-domain. Treatment of H9c2 cells indicated a rapid cellular uptake mechanism that did not involve IGF-1 receptor activation but resulted in a nuclear localization. Peptide treatment inhibited the intrinsic apoptotic pathway in H9c2 cells subjected to cell stress with sorbitol by preventing the collapse of the mitochondrial membrane potential and inhibition of caspase-3 activation. Therefore, we administered the peptide at the time of myocardial infarction (MI) in mice. At 2 weeks post-MI cardiac function, gene expression and cell death were assayed. A significant decline in both systolic and diastolic function was evident in untreated mice based on PV loop analysis. Delivery of the E-peptide ameliorated the decline in function and resulted in significant preservation of cardiac contractility. Associated with these changes were an inhibition of pathologic hypertrophy and significantly fewer apoptotic nuclei in the viable myocardium of E-peptide-treated mice post-MI. We conclude that administration of the MGF E-domain peptide may provide a means of modulating local tissue IGF-1 autocrine/paracrine actions to preserve cardiac function, prevent cell death, and pathologic remodeling in the heart.Electronic supplementary materialThe online version of this article (doi:10.1007/s11010-013-1689-4) contains supplementary material, which is available to authorized users.

Rates of molecular evolution vary in vertebrates for insulin-like growth factor-1 (IGF-1), a pleiotropic locus that regulates life history traits

Insulin-like growth factor-1 (IGF-1) is a member of the vertebrate insulin/insulin-like growth factor/relaxin gene family necessary for growth, reproduction, and survival at both the cellular and organismal level. Its sequence, protein structure, and function have been characterized in mammals, birds, and fish; however, a notable gap in our current knowledge of the function of IGF-1 and its molecular evolution is information in ectothermic reptiles. To address this disparity, we sequenced the coding region of IGF-1 in 11 reptile species—one crocodilian, three turtles, three lizards, and four snakes. Complete sequencing of the full mRNA transcript of a snake revealed the Ea-isoform, the predominant isoform of IGF-1 also reported in other vertebrate groups. A gene tree of the IGF-1 protein-coding region that incorporated sequences from diverse vertebrate groups showed similarity to the species phylogeny, with the exception of the placement of Testudines as sister group to Aves, due to their high nucleotide sequence similarity. In contrast, long-branch lengths indicate more rapid divergence in IGF-1 among lizards and snakes. Additionally, lepidosaurs (i.e., lizards and snakes) had higher rates of non-synonymous:synonymous substitutions (dN/dS) relative to archosaurs (i.e., birds and crocodilians) and turtles. Tests for positive selection on specific codons within branches and evaluation of the changes in the amino acid properties, suggested positive selection in lepidosaurs on the C domain of IGF-1, which is involved in binding affinity to the IGF-1 receptor. Predicted structural changes suggest that major alterations in protein structure and function may have occurred in reptiles. These data propose new insights into the molecular co-evolution of IGF-1 and its receptors, and ultimately the evolution of IGF-1’s role in regulating life-history traits across vertebrates.

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

ObjectiveThe 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. DesignExpression 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. ResultsBoth 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. ConclusionsExpression 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.

Mechano Growth Factor E peptide (MGF-E), derived from an isoform of IGF-1, activates human muscle progenitor cells and induces an increase in their fusion potential at different ages

Loss of muscle mass and strength is a major problem during aging and the expression of Mechano Growth Factor (MGF), a member of the IGF-1 (insulin-like Growth Factor 1) super family, has been shown to be both exercise and age dependent. MGF, also called IGF-1Ec, has a unique E domain with a 49 bp insert in humans (52 bp in rodents; IGF-1Eb), which results in a reading frame shift during the IGF-1 gene splicing to produce a distinct mature isoform. We have studied the effects of the MGF-24aa-E peptide on proliferation and differentiation of primary human muscle cell cultures isolated from healthy subjects of different ages. We found that MGF-E peptide significantly increases the proliferative life span and delays senescence of satellite cells isolated from neonatal and young adult but not from old adult muscle, hypertrophy associated with a significant decrease in the percentage of reserve cells was observed in all cultures. It is concluded that the MGF-24aa-E peptide alone has a marked ability to enhance satellite cell activation, proliferation and fusion for muscle repair and maintenance and could provide a new strategy to combat age related sarcopenia without the oncogenic side effects observed for IGF1.

Increased IGF-1 in muscle modulates the phenotype of severe SMA mice

Spinal muscular atrophy (SMA) is an inherited motor neuron disease caused by the mutation of the survival motor neuron 1 (SMN1) gene and deficiency of the SMN protein. Severe SMA mice have abnormal motor function and small, immature myofibers early in development suggesting that SMN protein deficiency results in retarded muscle growth. Insulin-like growth factor 1 (IGF-1) stimulates myoblast proliferation, induces myogenic differentiation and generates myocyte hypertrophy in vitro and in vivo. We hypothesized that increased expression of IGF-1 specifically in skeletal muscle would attenuate disease features of SMAΔ7 mice. SMAΔ7 mice overexpressing a local isoform of IGF-1 (mIGF-1) in muscle showed enlarged myofibers and a 40% increase in median survival compared with mIGF-1-negative SMA littermates (median survival = 14 versus 10 days, respectively, log-rank P = 0.025). Surprisingly, this was not associated with a significant improvement in motor behavior. Treatment of both mIGF-1(NEG) and mIGF-1(POS) SMA mice with the histone deacetylase inhibitor, trichostatin A (TSA), resulted in a further extension of survival and improved motor behavior, but the combination of mIGF-1 and TSA treatment was not synergistic. These results show that increased mIGF-1 expression restricted to muscle can modulate the phenotype of SMA mice indicating that therapeutics targeted to muscle alone should not be discounted as potential disease-modifying therapies in SMA. IGF-1 may warrant further investigation in mild SMA animal models and perhaps SMA patients.

Class 2 IGF-1 isoforms are dispensable for viability, growth and maintenance of IGF-1 serum levels

Insulin-like growth factor 1 (IGF-1) is a pleiotropic factor involved in growth, cell survival and cellular differentiation. It exerts its functions through endocrine, paracrine or autocrine mechanisms. Circulating IGF-1 is essential for normal fetal and postnatal growth, although the published phenotypes of IGF-1 null animals have been only partially penetrant, presumably due to mixed genetic backgrounds. Molecular dissection of IGF-1 action is complicated by the existence of at least nine different IGF-1 isoforms, generated in both humans and rodents by usage of alternate promoters, differential splicing and different post-translational modifications. Several lines of evidence suggest that the Class 2 IGF-1 isoform is specifically destined for circulation, supporting an endocrine role of IGF-1 in normal growth processes. Using Cre/LoxP conditional gene targeting of exon 2 of the IGF-1 gene, we have generated a Class 2 IGF-1 knockout mouse line in a pure C57/Bl6 genetic background, where the specific removal of exon 2 ablated Class 2 IGF-1 isoform. Class 2 IGF-1 knockout mice exhibited normal development and postnatal growth patterns and had normal IGF-1 circulating levels, due to compensatory upregulation of Class 1 transcripts. In contrast, progeny of a total IGF-1 knockout line lacking exon 3 in the same genetic background were predictably smaller, displayed dramatically reduced IGF-1 receptor phosphorylation and all died perinatally, apparently due to respiratory failure. These results confirm that Class 2 signal peptide is not necessary for systemic circulation of IGF-1, revealing an internal compensation system for maintaining IGF-1 serum concentrations. We also uncover a vital requirement of IGF-1 for perinatal viability, previously obscured by modifiers in heterogeneous genetic backgrounds.

Local IGF-1 isoform protects cardiomyocytes from hypertrophic and oxidative stresses via SirT1 activity

Oxidative and hypertrophic stresses contribute to the pathogenesis of heart failure. Insulin-like growth factor-1 (IGF-1) is a peptide hormone with a complex post-transcriptional regulation, generating distinct isoforms. Locally acting IGF-1 isoform (mIGF-1) helps the heart to recover from toxic injury and from infarct. In the murine heart, moderate overexpression of the NAD(+)-dependent deacetylase SirT1 was reported to mitigate oxidative stress. SirT1 is known to promote lifespan extension and to protect from metabolic challenges. Circulating IGF-1 and SirT1 play antagonizing biological roles and share molecular targets in the heart, in turn affecting cardiomyocyte physiology. However, how different IGF-1 isoforms may impact SirT1 and affect cardiomyocyte function is unknown. Here we show that locally acting mIGF-1 increases SirT1 expression/activity, whereas circulating IGF-1 isoform does not affect it, in cultured HL-1 and neonatal cardiomyocytes. mIGF-1-induced SirT1 activity exerts protection against angiotensin II (Ang II)-triggered hypertrophy and against paraquat (PQ) and Ang II-induced oxidative stress. Conversely, circulating IGF-1 triggered itself oxidative stress and cardiomyocyte hypertrophy. Interestingly, potent cardio-protective genes (adiponectin, UCP-1 and MT-2) were increased specifically in mIGF-1-overexpressing cardiomyocytes, in a SirT1-dependent fashion. Thus, mIGF-1 protects cardiomyocytes from oxidative and hypertrophic stresses via SirT1 activity, and may represent a promising cardiac therapeutic.

Overexpression of IGF-1 in Muscle Attenuates Disease in a Mouse Model of Spinal and Bulbar Muscular Atrophy

Expansion of a polyglutamine tract in the androgen receptor (AR) causes spinal and bulbar muscular atrophy (SBMA). We previously showed that Akt-mediated phosphorylation of AR reduces ligand binding and attenuates the mutant AR toxicity. Here, we show that in culture insulin-like growth factor 1 (IGF-1) reduces AR aggregation and increases AR clearance via the ubiquitin-proteasome system through phosphorylation of AR by Akt. In vivo, SBMA transgenic mice overexpressing a muscle-specific isoform of IGF-1 selectively in skeletal muscle show evidence of increased Akt activation and AR phosphorylation and decreased AR aggregation. Augmentation of IGF-1/Akt signaling rescues behavioral and histopathological abnormalities, extends the life span, and reduces both muscle and spinal cord pathology of SBMA mice. This study establishes IGF-1/Akt-mediated inactivation of mutant AR as a strategy to counteract disease in vivo and demonstrates that skeletal muscle is a viable target tissue for therapeutic intervention in SBMA.

Interactions of muscle regulatory factors and isoforms of insulin growth factor‐1 in the muscle regenerative process

Interactions of muscle regulatory factors and isoforms of insulin growth factor‐1 in the muscle regenerative process

Counteracting muscle wasting in aging and neuromuscular diseases: the critical role of IGF-1

Most muscle pathologies are characterized by the progressive loss of muscle tissue due to chronic degeneration combined with the inability of regeneration machinery to replace the damaged muscle. These pathological changes, known as muscle wasting, can be attributed to the activation of several proteolytic systems, such as calpain, ubiquitin-proteasome and caspases, and to the alteration in muscle growth factors. Among them, insulin-like growth factor-1 (IGF-1) has been implicated in the control of skeletal muscle growth, differentiation, survival, and regeneration and has been considered a promising therapeutic agent in staving off the advance of muscle weakness. Here we review the molecular basis of muscle wasting associated with diseases, such as sarcopenia, muscular dystrophy and Amyotrophic Lateral Sclerosis, and discuss the potential therapeutic role of local IGF-1 isoforms in muscle aging and diseases.