Variation in the Insulin-Like Growth Factor 1 Gene in Primates

Evaluating the Safety and Efficacy of Capromorelin in Rhesus Macaques (Macaca mulatta)

Nonhuman primates used in biomedical research may experience clinically significant weight loss for a variety of reasons. Episodes of anorexia (complete loss of appetite) or hyporexia (decreased appetite) can result in significant weight loss, potentially altering animal welfare and scientific studies. The FDA has approved several appetite stimulants for use in domestic species, but currently none are approved for use in NHP. Treatment of inappetence and weight loss in NHP often relies on the extralabel use of these compounds. Capromorelin is a ghrelin receptor agonist. As a growth hormone secretagogue, capromorelin increases appetite, leading to weight gain. Studies in several species have shown a positive correlation between capromorelin administration and weight gain; in 2017, an oral solution of capromorelin received FDA approval for use in dogs. We tested this solution in healthy adult rhesus macaques (n = 3 males and 3 females) for its effects on body weight and insulin like growth factor-1 (IGF-1). A control group (n = 2 males and 2 females) was used for comparison. Treated macaques received a 3mg/kg oral dose daily for 7 d. Clinical signs were observed daily. Weights were collected before, during and at the end of treatment. Blood was drawn before, during and after treatment for measurement of IGF-1 levels and standard hematology and biochemistry parameters. Baseline-adjusted mean body weights and IGF-1 levels were significantly higher in treated as compared with control monkeys after 7 d of beginning treatment (body weight of 10.5±0.1kg (mean ± SEM) and 10.1±0.1kg, respectively; IGF-1 of 758±43ng/mL and 639±22ng/mL, respectively). Capromorelin administration was not associated with appreciable changes in hematologic and biochemical values in treated macaques. These findings suggest that capromorelin may be useful for treating inappetence and weight loss in NHP, and based on blood analysis, a 7-d course of treatment does not appear to cause acute toxicity.

Insulin-like Growth Factor 1 and Prolactin Levels in Chimpanzees (Pan troglodytes) Across the Lifespan

As human and chimpanzee genomes show high homology for IGF1 and PRL, we analyzed the sera of 367 healthy chimpanzees obtained during routine physical examinations in a single colony and measured chimpanzee insulin-like growth factor (IGF)-1 and prolactin (PRL) levels across the lifespan using standard human immunoassays. Assuming chimpanzee IGF-1 levels peak during puberty as in humans, we randomly defined puberty as the age at which most IGF-1 levels were equal to or above the 90^th percentile for each sex (males, ages ≥7.00 but <9.20 years; females, ≥5.00 but <8.00 years). IGF-1 levels steadily increased at a similar rate in juvenile males and females and peaked in puberty, strongly correlating with age, then slowly decreased faster in adult males than in adult females. As a group, males had a higher mean IGF-1 level than did females, but comparison by age category showed similar mean IGF-1 levels in males and females. PRL levels increased with age in females more than in males and levels were twice as high in females than in males. One pubertal male reported to have short stature had lower IGF-1 and weight compared with other males in the age group, confirming suspected growth hormone deficiency; a second male of normal height but low IGF-1 may have had delayed puberty. Overall, results show that differences in IGF-1 levels over the lifespan in this cohort of chimpanzees largely mimic those seen in humans, while patterns of PRL changes are less similar.

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.

Regulation of gene expression by growth hormone

Growth hormone (GH) plays a pivotal role in many physiological processes in humans, and in other mammalian and non-mammalian vertebrate species, through actions on somatic growth, tissue development and repair, and intermediary metabolism. This review will focus on mechanisms of GH actions on gene expression, primarily from the perspective of the genes that encode proteins stimulated by GH to regulate somatic growth, especially insulin-like growth factor 1 (IGF-I), but also others that are induced or repressed by GH. Topics to be discussed will include a brief overview of GH-mediated signal transduction pathways and how these cascades alter the functions of responsive transcription factors, with a specific focus on STAT5B, a key member of the signal transducers and activators of transcription family, characterization of essential GH-regulated genes, and elucidation of mechanisms of their regulation from biochemical, genetic, and genomic perspectives.

Characterization of a novel alternatively-spliced 5' exon in the human insulin-like growth factor I (IGF-I) gene, expressed in liver and some cancers

In mammals, the large IGF-I gene comprises 6 exons, which are subject to alternative splicing. All transcripts contain exons 3 and 4, encoding mature IGF-I, but the other exons are included in various combinations, giving at least 6 possible mature mRNAs. At the 5' end, exons 1 and 2 are spliced alternatively to exon 3, giving different leader/signal sequences. It is shown in this study that in human an additional exon (designated exon 0) is present, upstream of exon 1. This can be spliced directly to exon 3 or, less frequently, into exon 1. Exon 0 is utilized in liver, in about 24% of IGF-I transcripts, to a minor extent in prostate and endometrium (<1% of transcripts), but not in any of 29 other normal human tissues examined. The exon 0 sequence includes an in-frame ATG/AUG, potentially providing a translation start point giving an IGF-I precursor with a very long signal peptide. However, this ATG is very close to the 5' end, and may not be included in all transcripts; an in-frame ATG in exon 3 could provide an alternative start point. Utilization of exon 0 was detected in other apes, and to a small extent in Old World monkeys, but not in New World monkeys, prosimians or various non-primate mammals. Exon 0 was not expressed in most human tumours, but was utilized in many prostate tumours, at levels much greater than seen in normal prostate, and in liver tumours, at a lower level than in normal liver.

Characterizing the complexity of Australian marsupial insulin-like growth factor 1 genes

Insulin-like growth factor 1 (IGF1) actions are essential for somatic growth and tissue repair. IGF1 gene regulation is controlled by many inputs, with growth hormone playing a major role. In most mammals, the 6-exon IGF1/Igf1 gene produces multiple transcripts via independent activity of its promoters plus alternative RNA splicing and differential polyadenylation. Here, by analyzing public genomic and RNA-sequencing repositories, I have characterized three Australian marsupial IGF1 genes. Koala, Tasmanian devil, and wallaby IGF1 are more complicated than other mammals, as they contain up to 11 exons, and encode multiple mRNAs and predicted protein precursors, including potentially novel isoforms. Moreover, just two of multiple growth hormone-stimulated transcriptional enhancers found in other IGF1/Igf1 loci are detected in these species. These observations define Australian marsupial IGF1 genes and demonstrate that comprehensive interrogation of genomic and RNA-sequencing resources is an effective strategy for characterizing genes and gene expression in otherwise experimentally intractable organisms.

Gene Mapping by RNA-sequencing: A Direct Way to Characterize Genes and Gene Expression through Targeted Queries of Large Public Databases

Recent advances in genomics present new opportunities for enhancing knowledge about gene regulation and function across a wide spectrum of organisms and species. Understanding and evaluating this information at the individual gene level is challenging, and not only requires extracting, collating and interpreting data from public genetic repositories, but also recognizing that much of the information has been developed through implementation of computationally based exon-calling algorithms, and thus may be inaccurate. Moreover, as these data usually have not been validated experimentally, results also may be incomplete and incorrect. This has created a quality-control problem for scientists who want to use individual gene-specific information in their research. Here, I describe a simple experimental strategy that takes advantage of the large amounts of untapped primary experimental data for characterizing gene expression that have been deposited in the Sequence Read Archive of the National Center for Biotechnology Information. The approach consists of a readily adaptable pipeline that may be used to confirm exons, to define 5' and 3' un-translated regions and the beginnings and ends of individual genes, and to quantify alternative RNA splicing. The series of experimental strategies described offers effective replacements for older molecular biological methods, and can rapidly and reproducibly resolve major gene mapping problems.

Quantifying promoter-specific Insulin-like Growth Factor 1 gene expression by interrogating public databases

The actions of insulin-like growth factor 1 (IGF1), a small, secreted protein, are essential for normal somatic growth in children and are important for tissue regeneration and repair in adults. Similar functions are conserved in other mammalian species. IGF1 gene regulation is complicated in mammals, with transcription being controlled by different hormonal, nutritional, and tissue-specific inputs. Quantifying IGF1 gene expression in different organs and tissues also has been difficult because of the variable contributions of its two promoters and because of the lack of standard platforms for analysis. Here, I have taken advantage of the wealth of information found in publicly accessible RNA-sequencing libraries to measure steady-state levels of IGF1 mRNAs from human and macaque, species chosen because they are not readily tractable experimental organisms, yet retain similar IGF1 gene organization. Results demonstrate that IGF1 transcripts are highly expressed in fat and liver in both species, and are induced during human adipocyte differentiation. IGF1 mRNAs also are increased in macaque skeletal muscle after selected dietary manipulations. In the organs and tissues examined, IGF1 promoter 1 appears to be far more active than promoter 2. Collectively, these observations show that interrogating large-scale public genomic resources is an effective strategy for quantifying gene expression across different tissues and species.

Insulinlike Growth Factor 1 Gene Variation in Vertebrates

IGF1-a small, single-chain, secreted peptide in mammals-is essential for normal somatic growth and is involved in a variety of other physiological and pathophysiological processes. IGF1 expression appears to be controlled by several different signaling mechanisms in mammals, with GH playing a key role by activating an inducible transcriptional pathway via the Jak2 protein kinase and the Stat5b transcription factor. Here, to understand aspects of Igf1 gene regulation over a substantially longer timeline than is discernible in mammals, Igf1 genes have been examined in 21 different nonmammalian vertebrates representing five different classes and ranging over ∼500 million years of evolutionary history. Parts of vertebrate Igf1 genes resemble components found in mammals. Conserved exons encoding the mature IGF1 protein are detected in all 21 species studied and are separated by a large intron, as seen in mammals; the single promoter contains putative regulatory elements that are similar to those functionally mapped in human IGF1 promoter 1. In contrast, GH-activated Stat5b-binding enhancers found in mammalian IGF1 loci are completely absent, there is no homolog of promoter 2 or exon 2 in any nonmammalian vertebrate, and different types of "extra" exons not present in mammals are found in birds, reptiles, and teleosts. These data collectively define properties of Igf1 genes and IGF1 proteins that were likely present in the earliest vertebrates and support the contention that common structural and regulatory features in Igf1 genes have a long evolutionary history.

Diversification of the insulin-like growth factor 1 gene in mammals

Insulin-like growth factor 1 (IGF1), a small, secreted peptide growth factor, is involved in a variety of physiological and patho-physiological processes, including somatic growth, tissue repair, and metabolism of carbohydrates, proteins, and lipids. IGF1 gene expression appears to be controlled by several different signaling cascades in the few species in which it has been evaluated, with growth hormone playing a major role by activating a pathway involving the Stat5b transcription factor. Here, genes encoding IGF1 have been evaluated in 25 different mammalian species representing 15 different orders and ranging over ~180 million years of evolutionary diversification. Parts of the IGF1 gene have been fairly well conserved. Like rat Igf1 and human IGF1, 21 of 23 other genes are composed of 6 exons and 5 introns, and all 23 also contain recognizable tandem promoters, each with a unique leader exon. Exon and intron lengths are similar in most species, and DNA sequence conservation is moderately high in orthologous exons and proximal promoter regions. In contrast, putative growth hormone-activated Stat5b-binding enhancers found in analogous locations in rodent Igf1 and in human IGF1 loci, have undergone substantial variation in other mammals, and a processed retro-transposed IGF1 pseudogene is found in the sloth locus, but not in other mammalian genomes. Taken together, the fairly high level of organizational and nucleotide sequence similarity in the IGF1 gene among these 25 species supports the contention that some common regulatory pathways had existed prior to the beginning of mammalian speciation.