The insulin-like growth factor 2 gene in mammals: Organizational complexity within a conserved locus

The role of imprinting genes' loss of imprints in cancers and their clinical implications

Genomic imprinting plays an important role in the growth and development of mammals. When the original imprint status of these genes is lost, known as loss of imprinting (LOI), it may affect growth, neurocognitive development, metabolism, and even tumor susceptibility. The LOI of imprint genes has gradually been found not only as an early event in tumorigenesis, but also to be involved in progression. More than 120 imprinted genes had been identified in humans. In this review, we summarized the most studied LOI of two gene clusters and 13 single genes in cancers. We focused on the roles they played, that is, as growth suppressors and anti-apoptosis agents, sustaining proliferative signaling or inducing angiogenesis; the molecular pathways they regulated; and especially their clinical significance. It is notable that 12 combined forms of multi-genes' LOI, 3 of which have already been used as diagnostic models, achieved good sensitivity, specificity, and accuracy. In addition, the methods used for LOI detection in existing research are classified into detection of biallelic expression (BAE), differentially methylated regions (DMRs), methylation, and single-nucleotide polymorphisms (SNPs). These all indicated that the detection of imprinting genes' LOI has potential clinical significance in cancer diagnosis, treatment, and prognosis.

Unveiling the connection: Long-chain non-coding RNAs and critical signaling pathways in breast cancer

Breast cancer is a complex and diverse condition that disrupts multiple signaling pathways essential for cell proliferation, survival, and differentiation. Recently, the significant involvement of long-chain non-coding RNAs (lncRNAs) in controlling key signaling pathways associated with breast cancer development has been discovered. This review aims to explore the interaction between lncRNAs and various pathways, including the AKT/PI3K/mTOR, Wnt/β-catenin, Notch, DNA damage response, TGF-β, Hedgehog, and NF-κB signaling pathways, to gain a comprehensive understanding of their roles in breast cancer. The AKT/PI3K/mTOR pathway regulates cell growth, survival, and metabolic function. Recent data suggests that specific lncRNAs can influence the functioning of this pathway, acting as either oncogenes or tumor suppressors. Dysregulation of this pathway is commonly observed in breast cancer cases. Moreover, breast cancer development has been associated with other pathways such as Wnt/β-catenin, Notch, TGF-β, Hedgehog, and NF-κB. Emerging studies have identified lncRNAs that modulate breast cancer's growth, progression, and metastasis by interacting with these pathways. To advance the development of innovative diagnostic tools and targeted treatment options, it is crucial to comprehend the intricate relationship between lncRNAs and vital signaling pathways in breast cancer. By fully harnessing the therapeutic potential of lncRNAs, there is a possibility of developing more effective and personalized therapy choices for breast cancer patients. Further investigation is necessary to comprehensively understand the role of lncRNAs within breast cancer signaling pathways and fully exploit their therapeutic potential.

Human IGF2 Gene Epigenetic and Transcriptional Regulation: At the Core of Developmental Growth and Tumorigenic Behavior

Regulation of the human IGF2 gene displays multiple layers of control, which secures a genetically and epigenetically predetermined gene expression pattern throughout embryonal growth and postnatal life. These predominantly nuclear regulatory mechanisms converge on the function of the IGF2-H19 gene cluster on Chromosome 11 and ultimately affect IGF2 gene expression. Deregulation of such control checkpoints leads to the enhancement of IGF2 gene transcription and/or transcript stabilization, ultimately leading to IGF-II peptide overproduction. This type of anomaly is responsible for the effects observed in terms of both abnormal fetal growth and increased cell proliferation, typically observed in pediatric overgrowth syndromes and cancer. We performed a review of relevant experimental work on the mechanisms affecting the human IGF2 gene at the epigenetic, transcriptional and transcript regulatory levels. The result of our work, indeed, provides a wider and diversified scenario for IGF2 gene activation than previously envisioned by shedding new light on its extended regulation. Overall, we focused on the functional integration between the epigenetic and genetic machinery driving its overexpression in overgrowth syndromes and malignancy, independently of the underlying presence of loss of imprinting (LOI). The molecular landscape provided at last strengthens the role of IGF2 in cancer initiation, progression and malignant phenotype maintenance. Finally, this review suggests potential actionable targets for IGF2 gene- and regulatory protein target-degradation therapies.

Insulin‐like growth factor‐2 is a promising candidate for the treatment and prevention of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia. Current AD treatments slow the rate of cognitive decline, but do not restore lost function. One reason for the low efficacy of current treatments is that they fail to target neurotrophic processes, which are thought to be essential for functional recovery. Bolstering neurotrophic processes may also be a viable strategy for preventative treatment, since structural losses are thought to underlie cognitive decline in AD. The challenge of identifying presymptomatic patients who might benefit from preventative treatment means that any such treatment must meet a high standard of safety and tolerability. The neurotrophic peptide insulin-like growth factor-2 (IGF2) is a promising candidate for both treating and preventing AD-induced cognitive decline. Brain IGF2 expression declines in AD patients. In rodent models of AD, exogenous IGF2 modulates multiple aspects of AD pathology, resulting in (1) improved cognitive function; (2) stimulation of neurogenesis and synaptogenesis; and, (3) neuroprotection against cholinergic dysfunction and beta amyloid-induced neurotoxicity. Preclinical evidence suggests that IGF2 is likely to be safe and tolerable at therapeutic doses. In the preventative treatment context, the intranasal route of administration is likely to be the preferred method for achieving the therapeutic effect without risking adverse side effects. For patients already experiencing AD dementia, routes of administration that deliver IGF2 directly access the CNS may be necessary. Finally, we discuss several strategies for improving the translational validity of animal models used to study the therapeutic potential of IGF2.

Genetic ablation of the preptin-coding portion of Igf2 impairs pancreatic function in female mice

Preptin is a 34-amino acid peptide derived from the E-peptide of pro-insulin-like growth factor 2 and is co-secreted with insulin from β-cells. Little is understood about the effects of endogenous preptin on whole body glucose metabolism. We developed a novel mouse model in which the preptin portion of Igf2 was genetically ablated in all tissues, hereafter referred to as preptin knockout (KO), and tested the hypothesis that the removal of preptin will lead to a decreased insulin response to a metabolic challenge. Preptin KO and wild-type (WT) mice underwent weekly fasting blood glucose measurements, intraperitoneal insulin tolerance tests (ITT) at 9, 29, and 44 wk of age, and an oral glucose tolerance test (GTT) at 45 wk of age. Preptin KO mice of both sexes had similar Igf2 exon 2-3 mRNA expression in the liver and kidney compared with WT mice, but Igf2 exon 3-4 (preptin) expression was not detectable. Western blot analysis of neonatal serum indicated that processing of pro-IGF2 translated from the KO allele may be altered. Preptin KO mice had similar body weight, body composition, β-cell area, and fasted glucose concentrations compared with WT mice in both sexes up to 47 wk of age. Female KO mice had a diminished ability to mount an insulin response following glucose stimulation in vivo. This effect was absent in male KO mice. Although preptin is not essential for glucose homeostasis, when combined with previous in vitro and ex vivo findings, these data show that preptin positively impacts β-cell function.NEW & NOTEWORTHY This is the first study to describe a model in which the preptin-coding portion of the Igf2 gene has been genetically ablated in mice. The mice do not show reduced size at birth associated with Igf2 knockout suggesting that IGF2 functionality is maintained, yet we demonstrate a change in the processing of mature Igf2. Female knockout mice have diminished glucose-stimulated insulin secretion, whereas the insulin response in males is not different to wild type.

Insulin-like Growth Factor-2 (IGF-2) in Fibrosis

The insulin family consists of insulin, insulin-like growth factor 1 (IGF-1), insulin-like growth factor 2 (IGF-2), their receptors (IR, IGF-1R and IGF-2R), and their binding proteins. All three ligands are involved in cell proliferation, apoptosis, protein synthesis and metabolism due to their homologous sequences and structural similarities. Insulin-like growth factor 2, a member of the insulin family, plays an important role in embryonic development, metabolic disorders, and tumorigenesis by combining with three receptors with different degrees of affinity. The main pathological feature of various fibrotic diseases is the excessive deposition of extracellular matrix (ECM) after tissue and organ damage, which eventually results in organic dysfunction because scar formation replaces tissue parenchyma. As a mitogenic factor, IGF-2 is overexpressed in many fibrotic diseases. It can promote the proliferation of fibroblasts significantly, as well as the production of ECM in a time- and dose-dependent manner. This review aims to describe the expression changes and fibrosis-promoting effects of IGF-2 in the skin, oral cavity, heart, lung, liver, and kidney fibrotic tissues.

IGF2: Development, Genetic and Epigenetic Abnormalities

In the 30 years since the first report of parental imprinting in insulin-like growth factor 2 (Igf2) knockout mouse models, we have learnt much about the structure of this protein, its role and regulation. Indeed, many animal and human studies involving innovative techniques have shed light on the complex regulation of IGF2 expression. The physiological roles of IGF-II have also been documented, revealing pleiotropic tissue-specific and developmental-stage-dependent action. Furthermore, in recent years, animal studies have highlighted important interspecies differences in IGF-II function, gene expression and regulation. The identification of human disorders due to impaired IGF2 gene expression has also helped to elucidate the major role of IGF-II in growth and in tumor proliferation. The Silver-Russell and Beckwith-Wiedemann syndromes are the most representative imprinted disorders, as they constitute both phenotypic and molecular mirrors of IGF2-linked abnormalities. The characterization of patients with either epigenetic or genetic defects altering IGF2 expression has confirmed the central role of IGF-II in human growth regulation, particularly before birth, and its effects on broader body functions, such as metabolism or tumor susceptibility. Given the long-term health impact of these rare disorders, it is important to understand the consequences of IGF2 defects in these patients.

The Insulin-like Growth Factor Signalling Pathway in Cardiac Development and Regeneration

Insulin and Insulin-like growth factors (IGFs) perform key roles during embryonic development, regulating processes of cell proliferation and survival. The IGF signalling pathway comprises two IGFs (IGF1, IGF2), two IGF receptors (IGFR1, IGFR2), and six IGF binding proteins (IGFBPs) that regulate IGF transport and availability. The IGF signalling pathway is essential for cardiac development. IGF2 is the primary mitogen inducing ventricular cardiomyocyte proliferation and morphogenesis of the compact myocardial wall. Conditional deletion of the Igf1r and the insulin receptor (Insr) genes in the myocardium results in decreased cardiomyocyte proliferation and ventricular wall hypoplasia. The significance of the IGF signalling pathway during embryonic development has led to consider it as a candidate for adult cardiac repair and regeneration. In fact, paracrine IGF2 plays a key role in the transient regenerative ability of the newborn mouse heart. We aimed to review the current knowledge about the role played by the IGF signalling pathway during cardiac development and also the clinical potential of recapitulating this developmental axis in regeneration of the adult heart.

IGF2 is a potential factor in RAI-refractory differentiated thyroid cancer

Differentiated thyroid cancer (DTC) is the most frequent endocrine tumor with a good prognosis after primary treatment in most cases. By contrast, 30–40% of patients with metastatic DTC are unresponsive to ¹³¹I radioactive iodide (RAI) treatment due to tumor dedifferentiation. Currently, underlying molecular mechanisms of dedifferentiation remain elusive and predictive biomarkers are lacking. Therefore, the present study aimed to identify molecular biomarkers in primary tumors associated with RAI refractoriness. A retrospective cohort was gathered consisting of RAI-sensitive patients with DTC and RAI-refractory patients with poorly DTC. In all patients, extensive intratumoral mutation profiling, gene fusions analysis, telomerase reverse transcriptase (TERT) promoter mutation analysis and formalin-fixed paraffin-embedded-compatible RNA sequencing were performed. Genetic analyses revealed an increased mutational load in RAI-refractory DTC, including mutations in AKT1, PTEN, TP53 and TERT promoter. Transcriptomic analyses revealed profound differential expression of insulin-like growth factor 2 (IGF2), with up to 100-fold higher expression in RAI-refractory DTC compared with in RAI-sensitive DTC cases. ELISA revealed significant lower IGF2 plasma concentrations after surgery and subsequent ¹³¹I RAI therapy in patients with DTC compared with pretreatment baseline. Overall, the current findings suggested that the tumor-promoting growth factor IGF2 may have a potential role in acquiring RAI refractoriness.

Per1/Per2-Igf2 axis-mediated circadian regulation of myogenic differentiation

Circadian rhythms regulate cell proliferation and differentiation, but circadian control of tissue regeneration remains elusive at the molecular level. Here, we show that proper myoblast differentiation and muscle regeneration are regulated by the circadian master regulators Per1 and Per2. Depletion of Per1 or Per2 suppressed myoblast differentiation in vitro and muscle regeneration in vivo, demonstrating their nonredundant functions. Both Per1 and Per2 were required for the activation of Igf2, an autocrine promoter of myoblast differentiation, accompanied by Per-dependent recruitment of RNA polymerase II, dynamic histone modifications at the Igf2 promoter and enhancer, and the promoter–enhancer interaction. This circadian epigenetic priming created a preferred time window for initiating myoblast differentiation. Consistently, muscle regeneration was faster if initiated at night, when Per1, Per2, and Igf2 were highly expressed compared with morning. This study reveals the circadian timing as a significant factor for effective muscle cell differentiation and regeneration.

Insulin-like growth factors: Ligands, binding proteins, and receptors

BACKGROUND

The insulin-like growth factor family of ligands (IGF-I, IGF-II, and insulin), receptors (IGF-IR, M6P/IGF-IIR, and insulin receptor [IR]), and IGF-binding proteins (IGFBP-1-6) play critical roles in normal human physiology and disease states.

SCOPE OF REVIEW

Insulin and insulin receptors are the focus of other chapters in this series and will therefore not be discussed further. Here we review the basic components of the IGF system, their role in normal physiology and in critical pathology's. While this review concentrates on the role of IGFs in human physiology, animal models have been essential in providing understanding of the IGF system, and its regulation, and are briefly described.

MAJOR CONCLUSIONS

IGF-I has effects via the circulation and locally within tissues to regulate cellular growth, differentiation, and survival, thereby controlling overall body growth. IGF-II levels are highest prenatally when it has important effects on growth. In adults, IGF-II plays important tissue-specific roles, including the maintenance of stem cell populations. Although the IGF-IR is closely related to the IR it has distinct physiological roles both on the cell surface and in the nucleus. The M6P/IGF-IIR, in contrast, is distinct and acts as a scavenger by mediating internalization and degradation of IGF-II. The IGFBPs bind IGF-I and IGF-II in the circulation to prolong their half-lives and modulate tissue access, thereby controlling IGF function. IGFBPs also have IGF ligand-independent cell effects.

Promoter-specific expression of the imprinted IGF2 gene in bovine oocytes and pre-implantation embryos

The bovine IGF2 locus is a genomic region with alternative transcripts controlled by five promoters (P0, P1, P2, P3 and P4). As transcriptional regulation can affect messenger RNA (mRNA) stability and translation, and thus, subsequent biological effects, this study evaluated the bovine IGF2 promoter-specific expression patterns in oocytes and pre-implantation embryos produced in vitro by our standard IVP procedures. Immature and matured oocytes, and pre-implantation embryos at the 1-, 2-, 4-, 8- and 16-cell, and at early morula, compact morula, blastocyst and expanded blastocyst stages were collected in three pools of five structures per stage, in four replicates. Total RNA was extracted and subjected to RT-qPCR, using four sets of IGF2 promoter-specific primers covering transcripts driven by promoters P0/P1, P2, P3 and P4, with fragments sequenced for confirmation. Expression of P2- and P4-derived transcripts showed an initial peak between immature (P4) or matured (P2/P4) oocytes and 2-cell embryos, gradually falling until embryo genome activation (EGA), rising again at compaction and cavitation. P0/P1-derived transcripts were identified after EGA, during compaction, whereas P3 activity was not detected at any stage. Our findings suggest that P0/P1 and P2 likely have secondary roles during early stages, whereas P3 may be more relevant later in development. P4 seems to be the main pathway for bovine IGF2 expression during oocyte maturation and embryo development and, therefore, the main target to influence IVP in modulation of embryo growth and in studies in developmental biology.

Insulin-Like Growth Factor 2 As a Possible Neuroprotective Agent and Memory Enhancer-Its Comparative Expression, Processing and Signaling in Mammalian CNS

A number of studies performed on rodents suggest that insulin-like growth factor 2 (IGF-2) or its analogs may possibly be used for treating some conditions like Alzheimer’s disease, Huntington’s disease, autistic spectrum disorders or aging-related cognitive impairment. Still, for translational research a comparative knowledge about the function of IGF-2 and related molecules in model organisms (rats and mice) and humans is necessary. There is a number of important differences in IGF-2 signaling between species. In the present review we emphasize species-specific patterns of IGF-2 expression in rodents, humans and some other mammals, using, among other sources, publicly available transcriptomic data. We provide a detailed description of Igf2 mRNA expression regulation and pre-pro-IGF-2 protein processing in different species. We also summarize the function of IGF-binding proteins. We describe three different receptors able to bind IGF-2 and discuss the role of IGF-2 signaling in learning and memory, as well as in neuroprotection. We hope that comprehensive understanding of similarities and differences in IGF-2 signaling between model organisms and humans will be useful for development of more effective medicines targeting IGF-2 receptors.

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.

Insulin-Like Growth Factor 2 (IGF2) Signaling in Colorectal Cancer-From Basic Research to Potential Clinical Applications

Colorectal cancer (CRC) is one of the most common cancers in men and women worldwide as well as is the leading cause of death in the western world. Almost a third of the patients has or will develop liver metastases. While genetic as well as epigenetic mechanisms are important in CRC pathogenesis, the basis of the most cases of cancer is unknown. High spatial and inter-patient variability of the molecular alterations qualifies this cancer in the group of highly heterogeneous tumors, which makes it harder to elucidate the mechanisms underlying CRC progression. Determination of highly sensitive and specific early diagnosis markers and understanding the cellular and molecular mechanism(s) of cancer progression are still a challenge of the current era in oncology of solid tumors. One of the accepted risk factors for CRC development is overexpression of insulin-like growth factor 2 (IGF2), a 7.5-kDa peptide produced by liver and many other tissues. IGF2 is the first gene discovered to be parentally imprinted. Loss of imprinting (LOI) or aberrant imprinting of IGF2 could lead to IGF2 overexpression, increased cell proliferation, and CRC development. IGF2 as a mitogen is associated with increased risk of developing colorectal neoplasia. Higher serum IGF2 concentration as well as its tissue overexpression in CRC compared to control are associated with metastasis. IGF2 protein was one of the three candidates for a selective marker of CRC progression and staging. Recent research indicates dysregulation of different micro- and long non-coding RNAs (miRNAs and lncRNAs, respectively) embedded within the IGF2 gene in CRC carcinogenesis, with some of them indicated as potential diagnostic and prognostic CRC biomarkers. This review systematises the knowledge on the role of genetic and epigenetic instabilities of IGF2 gene, free (active form of IGF2) and IGF-binding protein (IGFBP) bound (inactive form), paracrine/autocrine secretion of IGF2, as well as mechanisms of inducing dysplasia in vitro and tumorigenicity in vivo. We have tried to answer which molecular changes of the IGF2 gene and its regulatory mechanisms have the most significance in initiation, progression (including liver metastasis), prognosis, and potential anti-IGF2 therapy in CRC patients.