Differential signaling activation by insulin and insulin-like growth factors I and II upon binding to insulin receptor isoform A

Insulin receptor alternative splicing in breast and prostate cancer

Cancer etiology represents an intricate, multifactorial orchestration where metabolically associated insulin-like growth factors (IGFs) and insulin foster cellular proliferation and growth throughout tumorigenesis. The insulin receptor (IR) exhibits two splice variants arising from alternative mRNA processing, namely IR-A, and IR-B, with remarkable distribution and biological effects disparities. This insightful review elucidates the structural intricacies, widespread distribution, and functional significance of IR-A and IR-B. Additionally, it explores the regulatory mechanisms governing alternative splicing processes, intricate signal transduction pathways, and the intricate association linking IR-A and IR-B splicing variants to breast and prostate cancer tumorigenesis. Breast cancer and prostate cancer are the most common malignant tumors with the highest incidence rates among women and men, respectively. These findings provide a promising theoretical framework for advancing preventive strategies, diagnostic modalities, and therapeutic interventions targeting breast and prostate cancer.

Insulin Receptor Isoforms and Insulin Growth Factor-like Receptors: Implications in Cell Signaling, Carcinogenesis, and Chemoresistance

This comprehensive review thoroughly explores the intricate involvement of insulin receptor (IR) isoforms and insulin-like growth factor receptors (IGFRs) in the context of the insulin and insulin-like growth factor (IGF) signaling (IIS) pathway. This elaborate system encompasses ligands, receptors, and binding proteins, giving rise to a wide array of functions, including aspects such as carcinogenesis and chemoresistance. Detailed genetic analysis of IR and IGFR structures highlights their distinct isoforms, which arise from alternative splicing and exhibit diverse affinities for ligands. Notably, the overexpression of the IR-A isoform is linked to cancer stemness, tumor development, and resistance to targeted therapies. Similarly, elevated IGFR expression accelerates tumor progression and fosters chemoresistance. The review underscores the intricate interplay between IRs and IGFRs, contributing to resistance against anti-IGFR drugs. Consequently, the dual targeting of both receptors could present a more effective strategy for surmounting chemoresistance. To conclude, this review brings to light the pivotal roles played by IRs and IGFRs in cellular signaling, carcinogenesis, and therapy resistance. By precisely modulating these receptors and their complex signaling pathways, the potential emerges for developing enhanced anti-cancer interventions, ultimately leading to improved patient outcomes.

Comparative proteomic analysis of insulin receptor isoform A and B signaling

The insulin receptor (IR) gene undergoes differential splicing generating two IR isoforms, IR-A and IR-B. The roles of IR-A in cancer and of IR-B in metabolic regulation are well known but the molecular mechanisms responsible for their different biological effects are poorly understood. We aimed to identify different or similar protein substrates and signaling linked to each IR isoforms. We employed mouse fibroblasts lacking IGF1R gene and expressing exclusively either IR-A or IR-B. By proteomic analysis a total of 2530 proteins were identified and quantified. Proteins and pathways mostly associated with insulin-activated IR-A were involved in cancer, stemness and interferon signaling. Instead, proteins and pathways associated with insulin-stimulated IR-B-expressing cells were mostly involved in metabolic or tumor suppressive functions. These results show that IR-A and IR-B recruit partially different multiprotein complexes in response to insulin, suggesting partially different functions of IR isoforms in physiology and in disease.

Subventricular zone adult mouse neural stem cells require insulin receptor for self-renewal

The insulin receptor (INSR) is an evolutionarily conserved signaling protein that regulates development and cellular metabolism. INSR signaling promotes neurogenesis in Drosophila; however, a specific role for the INSR in maintaining adult neural stem cells (NSCs) in mammals has not been investigated. We show that conditionally deleting the Insr gene in adult mouse NSCs reduces subventricular zone NSCs by ∼70% accompanied by a corresponding increase in progenitors. Insr deletion also produced hyposmia caused by aberrant olfactory bulb neurogenesis. Interestingly, hippocampal neurogenesis and hippocampal-dependent behaviors were unperturbed. Highly aggressive proneural and mesenchymal glioblastomas had high INSR/insulin-like growth factor (IGF) pathway gene expression, and isolated glioma stem cells had an aberrantly high ratio of INSR:IGF type 1 receptor. Moreover, INSR knockdown inhibited GBM tumorsphere growth. Altogether, these data demonstrate that the INSR is essential for a subset of normal NSCs, as well as for brain tumor stem cell self-renewal.

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Insulin receptor (INSR) is essential for adult SVZ neural stem cell self-renewal

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INSR deletion causes hyposmia with increased olfactory bulb neurogenesis

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Hippocampal stem cells (and associated behaviors) do not require INSR

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Glioblastomas overexpress INSR pathway components required for tumorsphere growth

Novel Mechanisms of Tumor Promotion by the Insulin Receptor Isoform A in Triple-Negative Breast Cancer Cells

The insulin receptor isoform A (IR-A) plays an increasingly recognized role in fetal growth and tumor biology in response to circulating insulin and/or locally produced IGF2. This role seems not to be shared by the IR isoform B (IR-B). We aimed to dissect the specific impact of IR isoforms in modulating insulin signaling in triple negative breast cancer (TNBC) cells. We generated murine 4T1 TNBC cells deleted from the endogenous insulin receptor (INSR) gene and expressing comparable levels of either human IR-A or IR-B. We then measured IR isoform-specific in vitro and in vivo biological effects and transcriptome in response to insulin. Overall, the IR-A was more potent than the IR-B in mediating cell migration, invasion, and in vivo tumor growth. Transcriptome analysis showed that approximately 89% of insulin-stimulated transcripts depended solely on the expression of the specific isoform. Notably, in cells overexpressing IR-A, insulin strongly induced genes involved in tumor progression and immune evasion including chemokines and genes related to innate immunity. Conversely, in IR-B overexpressing cells, insulin predominantly induced the expression of genes primarily involved in the regulation of metabolic pathways and, to a lesser extent, tumor growth and angiogenesis.

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.

Isoform- and Paralog-Switching in IR-Signaling: When Diabetes Opens the Gates to Cancer

Insulin receptor (IR) and IR-related signaling defects have been shown to trigger insulin-resistance in insulin-dependent cells and ultimately to give rise to type 2 diabetes in mammalian organisms. IR expression is ubiquitous in mammalian tissues, and its over-expression is also a common finding in cancerous cells. This latter finding has been shown to associate with both a relative and absolute increase in IR isoform-A (IR-A) expression, missing 12 aa in its EC subunit corresponding to exon 11. Since IR-A is a high-affinity transducer of Insulin-like Growth Factor-II (IGF-II) signals, a growth factor is often secreted by cancer cells; such event offers a direct molecular link between IR-A/IR-B increased ratio in insulin resistance states (obesity and type 2 diabetes) and the malignant advantage provided by IGF-II to solid tumors. Nonetheless, recent findings on the biological role of isoforms for cellular signaling components suggest that the preferential expression of IR isoform-A may be part of a wider contextual isoform-expression switch in downstream regulatory factors, potentially enhancing IR-dependent oncogenic effects. The present review focuses on the role of isoform- and paralog-dependent variability in the IR and downstream cellular components playing a potential role in the modulation of the IR-A signaling related to the changes induced by insulin-resistance-linked conditions as well as to their relationship with the benign versus malignant transition in underlying solid tumors.

Unraveling the Resistance of IGF-Pathway Inhibition in Ewing Sarcoma

Simple Summary

The insulin-like growth factor-1 receptor (IGF1R) is a receptor commonly overexpressed and overactivated in a variety of cancers, including Ewing sarcoma, and promotes cell growth and survival. After promising results with targeting and inhibiting the receptor in vitro, multiple different IGF1R targeting compounds have been clinically tried but showed limited efficacy. Here we discuss several possible resistance mechanisms which could explain why IGF1R targeting fails in the clinic and discuss possible ways to overcome these resistances.

Abstract

Insulin-like growth factor-1 receptor (IGF1R) inhibitors are effective in preclinical studies, but so far, no convincing benefit in clinical studies has been observed, except in some rare cases of sustained response in Ewing sarcoma patients. The mechanism of resistance is unknown, but several hypotheses are proposed. In this review, multiple possible mechanisms of resistance to IGF-targeted therapies are discussed, including activated insulin signaling, pituitary-driven feedback loops through growth hormone (GH) secretion and autocrine loops. Additionally, the outcomes of clinical trials of IGF1-targeted therapies are discussed, as well as strategies to overcome the possible resistance mechanisms. In conclusion, lowering the plasma insulin levels or blocking its activity could provide an additional target in cancer therapy in combination with IGF1 inhibition. Furthermore, because Ewing sarcoma cells predominantly express the insulin receptor A (IRA) and healthy tissue insulin receptor B (IRB), it may be possible to synthesize a specific IRA inhibitor.

The IGF-II-Insulin Receptor Isoform-A Autocrine Signal in Cancer: Actionable Perspectives

Insulin receptor overexpression is a common event in human cancer. Its overexpression is associated with a relative increase in the expression of its isoform A (IR^A), a shorter variant lacking 11 aa in the extracellular domain, conferring high affinity for the binding of IGF-II along with added intracellular signaling specificity for this ligand. Since IGF-II is secreted by the vast majority of malignant solid cancers, where it establishes autocrine stimuli, the co-expression of IGF-II and IR^A in cancer provides specific advantages such as apoptosis escape, growth, and proliferation to those cancers bearing such a co-expression pattern. However, little is known about the exact role of this autocrine ligand–receptor system in sustaining cancer malignant features such as angiogenesis, invasion, and metastasis. The recent finding that the overexpression of angiogenic receptor kinase EphB4 along with VEGF-A is tightly dependent on the IGF-II/IR^A autocrine system independently of IGFIR provided new perspectives for all malignant IGF2omas (those aggressive solid cancers secreting IGF-II). The present review provides an updated view of the IGF system in cancer, focusing on the biology of the autocrine IGF-II/IR^A ligand–receptor axis and supporting its underscored role as a malignant-switch checkpoint target.

The Neglected Insulin: IGF-II, a Metabolic Regulator with Implications for Diabetes, Obesity, and Cancer

When originally discovered, one of the initial observations was that, when all of the insulin peptide was depleted from serum, the vast majority of the insulin activity remained and this was due to a single additional peptide, IGF-II. The IGF-II gene is adjacent to the insulin gene, which is a result of gene duplication, but has evolved to be considerably more complicated. It was one of the first genes recognised to be imprinted and expressed in a parent-of-origin specific manner. The gene codes for IGF-II mRNA, but, in addition, also codes for antisense RNA, long non-coding RNA, and several micro RNA. Recent evidence suggests that each of these have important independent roles in metabolic regulation. It has also become clear that an alternatively spliced form of the insulin receptor may be the principle IGF-II receptor. These recent discoveries have important implications for metabolic disorders and also for cancer, for which there is renewed acknowledgement of the importance of metabolic reprogramming.

Insulin Receptor Isoform A Modulates Metabolic Reprogramming of Breast Cancer Cells in Response to IGF2 and Insulin Stimulation

Previously published work has demonstrated that overexpression of the insulin receptor isoform A (IR-A) might play a role in cancer progression and metastasis. The IR has a predominant metabolic role in physiology, but the potential role of IR-A in cancer metabolic reprogramming is unknown. We aimed to characterize the metabolic impact of IR-A and its ligand insulin like growth factor 2 (IGF2) in human breast cancer (BC) cells. To establish autocrine IGF2 action, we generated human BC cells MCF7 overexpressing the human IGF2, while we focused on the metabolic effect of IR-A by stably infecting IGF1R-ablated MCF7 (MCF7^IGF1R-ve) cells with a human IR-A cDNA. We then evaluated the expression of key metabolism related molecules and measured real-time extracellular acidification rates and oxygen consumption rates using the Seahorse technology. MCF7/IGF2 cells showed increased proliferation and invasion associated with aerobic glycolysis and mitochondrial biogenesis and activity. In MCF7^IGF1R-ve/IR-A cells insulin and IGF2 stimulated similar metabolic changes and were equipotent in eliciting proliferative responses, while IGF2 more potently induced invasion. The combined treatment with the glycolysis inhibitor 2-deoxyglucose (2DG) and the mitochondrial inhibitor metformin blocked cell invasion and colony formation with additive effects. Overall, these results indicate that IGF2 and IR-A overexpression may contribute to BC metabolic reprogramming.

Insulin/IGF signaling and discoidin domain receptors: An emerging functional connection

The insulin/insulin-like growth factor system (IIGFs) plays a fundamental role in the regulation of prenatal and postnatal growth, metabolism and homeostasis. As a consequence, dysregulation of this axis is associated with growth disturbance, type 2 diabetes, chronic inflammation and tumor progression. A functional crosstalk between IIGFs and discoidin domain receptors (DDRs) has been recently discovered. DDRs are non-integrin collagen receptors that canonically undergo slow and long-lasting autophosphorylation after binding to fibrillar collagen. While both DDR1 and DDR2 functionally interact with IIGFs, the crosstalk with DDR1 is so far better characterized. Notably, the IIGFs-DDR1 crosstalk presents a feed-forward mechanism, which does not require collagen binding, thus identifying novel non-canonical action of DDR1. Further studies are needed to fully explore the role of this IIGFs-DDRs functional loop as potential target in the treatment of inflammatory and neoplastic disorders.

Insulin receptor in the brain: Mechanisms of activation and the role in the CNS pathology and treatment

While the insulin receptor (IR) was found in the CNS decades ago, the brain was long considered to be an insulin-insensitive organ. This view is currently revisited, given emerging evidence of critical roles of IR-mediated signaling in development, neuroprotection, metabolism, and plasticity in the brain. These diverse cellular and physiological IR activities are distinct from metabolic IR functions in peripheral tissues, thus highlighting region specificity of IR properties. This particularly concerns the fact that two IR isoforms, A and B, are predominantly expressed in either the brain or peripheral tissues, respectively, and neurons express exclusively IR-A. Intriguingly, in comparison with IR-B, IR-A displays high binding affinity and is also activated by low concentrations of insulin-like growth factor-2 (IGF-2), a regulator of neuronal plasticity, whose dysregulation is associated with neuropathologic processes. Deficiencies in IR activation, insulin availability, and downstream IR-related mechanisms may result in aberrant IR-mediated functions and, subsequently, a broad range of brain disorders, including neurodevelopmental syndromes, neoplasms, neurodegenerative conditions, and depression. Here, we discuss findings on the brain-specific features of IR-mediated signaling with focus on mechanisms of primary receptor activation and their roles in the neuropathology. We aimed to uncover the remaining gaps in current knowledge on IR physiology and highlight new therapies targeting IR, such as IR sensitizers.

Long-acting insulin analogs and cancer

AIMS

Hyperinsulinemia is a recognized risk factor for cancer and plays a major role for the increased cancer incidence in diabetic patients. Whether insulin analogs, and particularly long-acting analogs, worsen the pro-cancer effect of excess insulin is still controversial.

DATA SYNTHESIS

In this paper we summarize the biological bases for the potential detrimental effect of long-acting analogs on cancer cells and review the in vitro and in vivo evidence on this issue. Because of their different molecular structure relative to native insulin, insulin analogs may activate the insulin receptor (IR) and the post receptor pathways differently. Most, but not all, in vitro evidence indicate that long-acting analogs may have a stronger mitogenic potency than insulin on cancer cells. Notably insulin glargine, the most studied long-acting analog, also has a higher affinity for the insulin-like growth factor (IGF)-1 receptor, a potent growth mediator. In vitro observations, however, may not reflect what occurs in vivo when analogs are metabolized to derivatives with a different mitogenic activity. Clinical studies, mostly retrospective and predominantly concerning glargine, provide contrasting results. The only perspective trial found no cancer increase in patients treated with glargine. All these studies, however, have severe weaknesses because of the insufficient evaluation of important factors such as dose administered, length of exposure, patient follow-up duration and site-specific cancer investigation. Moreover, whether cancer promotion is a long-acting analog class characteristic or a specific effect of a single agent is not clear.

CONCLUSIONS

In conclusion the carcinogenic risk of long-acting analogs, and specifically glargine, can be neither confirmed nor excluded. A personalized and shared decision, considering all the individual risk factors (metabolic and non-metabolic), is the suggestion for the clinician.

Type I insulin-like growth factor receptor signaling in hematological malignancies

The insulin-like growth factor (IGF) signaling system plays key roles in the establishment and progression of different types of cancer. In agreement with this idea, substantial evidence has shown that the type I IGF receptor (IGF-IR) and its primary ligand IGF-I are important for maintaining the survival of malignant cells of hematopoietic origin. In this review, we discuss current understanding of the role of IGF-IR signaling in cancer with a focus on the hematological neoplasms. We also address the emergence of IGF-IR as a potential therapeutic target for the treatment of different types of cancer including plasma cell myeloma, leukemia, and lymphoma.

Insulin Receptor Isoforms in Physiology and Disease: An Updated View

The insulin receptor (IR) gene undergoes differential splicing that generates two IR isoforms, IR-A and IR-B. The physiological roles of IR isoforms are incompletely understood and appear to be determined by their different binding affinities for insulin-like growth factors (IGFs), particularly for IGF-2. Predominant roles of IR-A in prenatal growth and development and of IR-B in metabolic regulation are well established. However, emerging evidence indicates that the differential expression of IR isoforms may also help explain the diversification of insulin and IGF signaling and actions in various organs and tissues by involving not only different ligand-binding affinities but also different membrane partitioning and trafficking and possibly different abilities to interact with a variety of molecular partners. Of note, dysregulation of the IR-A/IR-B ratio is associated with insulin resistance, aging, and increased proliferative activity of normal and neoplastic tissues and appears to sustain detrimental effects. This review discusses novel information that has generated remarkable progress in our understanding of the physiology of IR isoforms and their role in disease. We also focus on novel IR ligands and modulators that should now be considered as an important strategy for better and safer treatment of diabetes and cancer and possibly other IR-related diseases.

IGF system targeted therapy: Therapeutic opportunities for ovarian cancer

The insulin-like growth factor (IGF) system comprises multiple growth factor receptors, including insulin-like growth factor 1 receptor (IGF-1R), insulin receptor (IR) -A and -B. These receptors are activated upon binding to their respective growth factor ligands, IGF-I, IGF-II and insulin, and play an important role in development, maintenance, progression, survival and chemotherapeutic response of ovarian cancer. In many pre-clinical studies anti-IGF-1R/IR targeted strategies proved effective in reducing growth of ovarian cancer models. In addition, anti-IGF-1R targeted strategies potentiated the efficacy of platinum based chemotherapy. Despite the vast amount of encouraging and promising pre-clinical data, anti-IGF-1R/IR targeted strategies lacked efficacy in the clinic. The question is whether targeting the IGF-1R/IR signaling pathway still holds therapeutic potential. In this review we address the complexity of the IGF-1R/IR signaling pathway, including receptor heterodimerization within and outside the IGF system and downstream signaling. Further, we discuss the implications of this complexity on current targeted strategies and indicate therapeutic opportunities for successful targeting of the IGF-1R/IR signaling pathway in ovarian cancer. Multiple-targeted approaches circumventing bidirectional receptor tyrosine kinase (RTK) compensation and prevention of system rewiring are expected to have more therapeutic potential.

Proinsulin Promotes Self-Renewal of a Hematopoietic Progenitor Cell Line In Vitro

The objective of this study was to assess the effects of exogenously expressed proinsulin on the biological characters of a hematopoietic stem cell line (HSC) and erythroid myeloid lymphoid (EML) cells and explore new strategies for cell therapy for type I diabetes. EML cells were transduced with lentivirus particles carrying the human proinsulin (proINS) gene. The positive transduced cells were selected based on green fluorescence protein (GFP) positivity and puromycin resistance. Overexpression of proINS was confirmed via real-time PCR and Western blotting. The functional activity of the human proINS secreted by EML cells was elucidated by analyzing the activation of insulin receptor and its downstream signaling. Pro-INS + EML cells were able to prime the phosphorylation of insulin receptor as well as induce the expression of downstream genes of insulin receptor. Furthermore, Wnt3a can significantly promote self-renewal of Pro-INS + EML cells. However, we did not observe significant changes in the proliferation and differentiation of INS + EML cells, compared to the control EML cells. Our results might be useful for developing a new therapy for diabetes mellitus.

IGF-I induces upregulation of DDR1 collagen receptor in breast cancer cells by suppressing MIR-199a-5p through the PI3K/AKT pathway

Discoidin Domain Receptor 1 (DDR1) is a collagen receptor tyrosine-kinase that contributes to epithelial-to-mesenchymal transition and enhances cancer progression. Our previous data indicate that, in breast cancer cells, DDR1 interacts with IGF-1R and positively modulates IGF-1R expression and biological responses, suggesting that the DDR1-IGF-IR cross-talk may play an important role in cancer.

In this study, we set out to evaluate whether IGF-I stimulation may affect DDR1 expression. Indeed, in breast cancer cells (MCF-7 and MDA-MB-231) IGF-I induced significant increase of DDR1 protein expression, in a time and dose dependent manner. However, we did not observe parallel changes in DDR1 mRNA. DDR1 upregulation required the activation of the PI3K/AKT pathway while the ERK1/2, the p70/mTOR and the PKC pathways were not involved. Moreover, we observed that DDR1 protein upregulation was induced by translational mechanisms involving miR-199a-5p suppression through PI3K/AKT activation. This effect was confirmed by both IGF-II produced by cancer-associated fibroblasts from human breast cancer and by stable transfection of breast cancer cells with a human IGF-II expression construct. Transfection with a constitutively active form of AKT was sufficient to decrease miR-199a-5p and upregulate DDR1. Accordingly, IGF-I-induced DDR1 upregulation was inhibited by transfection with pre-miR-199a-5p, which also impaired AKT activation and cell migration and proliferation in response to IGF-I.

These results demonstrate that, in breast cancer cells, a novel pathway involving AKT/miR-199a-5p/DDR1 plays a role in modulating IGFs biological responses. Therefore, this signaling pathway may represent an important target for breast cancers with over-activation of the IGF-IR axis.

Expression of insulin receptor (IR) A and B isoforms, IGF-IR, and IR/IGF-IR hybrid receptors in vascular smooth muscle cells and their role in cell migration in atherosclerosis

Background

Abnormal proliferation and migration of vascular smooth muscle cells (VSMCs) is a major contributor to the development of atherosclerotic process. In a previous work, we demonstrated that the insulin receptor isoform A (IRA) and its association with the insulin-like growth factor-I receptor (IGF-IR) confer a proliferative advantage to VSMCs. However, the role of IR and IGF-IR in VSMC migration remains poorly understood.

Methods

Wound healing assays were performed in VSMCs bearing IR (IRLoxP^+/+ VSMCs), or not (IR^−/− VSMCs), expressing IRA (IRA VSMCs) or expressing IRB (IRB VSMCs). To study the role of IR isoforms and IGF-IR in experimental atherosclerosis, we used ApoE^−/− mice at 8, 12, 18 and 24 weeks of age. Finally, we analyzed the mRNA expression of total IR, IRB isoform, IGF-IR and IGFs by qRT-PCR in the medial layer of human aortas.

Results

IGF-I strongly induced migration of the four cell lines through IGF-IR. In contrast, insulin and IGF-II only caused a significant increase of IRA VSMC migration which might be favored by the formation of IRA/IGF-IR receptors. Additionally, a specific IGF-IR inhibitor, picropodophyllin, completely abolished insulin- and IGF-II-induced migration in IRB, but not in IRA VSMCs. A significant increase of IRA and IGF-IR, and VSMC migration were observed in fibrous plaques from 24-week-old ApoE^−/− mice. Finally, we observed a marked increase of IGF-IR, IGF-I and IGF-II in media from fatty streaks as compared with both healthy aortas and fibrolipidic lesions, favoring the ability of medial VSMCs to migrate into the intima.

Conclusions

Our data suggest that overexpression of IGF-IR or IRA isoform, as homodimers or as part of IRA/IGF-IR hybrid receptors, confers a stronger migratory capability to VSMCs as might occur in early stages of atherosclerotic process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12933-016-0477-3) contains supplementary material, which is available to authorized users.

Insulin-like growth factor axis targeting in cancer and tumour angiogenesis - the missing link

Numerous molecular players in the process of tumour angiogenesis have been shown to offer potential for therapeutic targeting. Initially denoted to be involved in malignant transformation and tumour progression, the insulin-like growth factor (IGF) signalling axis has been subject to therapeutic interference, albeit with limited clinical success. More recently, IGFs and their receptors have received attention for their contribution to tumour angiogenesis, which offers novel therapeutic opportunities. Here we review the contribution of this signalling axis to tumour angiogenesis, the mechanisms of resistance to therapy and the interplay with other pro-angiogenic pathways, to offer insight in the renewed interest in the application of IGF axis targeting agents in anti-cancer combination therapies.

Cyclin G2 promotes cell cycle arrest in breast cancer cells responding to fulvestrant and metformin and correlates with patient survival

Definition of cell cycle control proteins that modify tumor cell resistance to estrogen (E2) signaling antagonists could inform clinical choice for estrogen receptor positive (ER+) breast cancer (BC) therapy. Cyclin G2 (CycG2) is upregulated during cell cycle arrest responses to cellular stresses and growth inhibitory signals and its gene, CCNG2, is directly repressed by E2-bound ER complexes. Our previous studies showed that blockade of HER2, PI3K and mTOR signaling upregulates CycG2 expression in HER2+ BC cells, and that CycG2 overexpression induces cell cycle arrest. Moreover, insulin and insulin-like growth factor-1 (IGF-1) receptor signaling strongly represses CycG2. Here we show that blockade of ER-signaling in MCF7 and T47D BC cell lines enhances the expression and nuclear localization of CycG2. Knockdown of CycG2 attenuated the cell cycle arrest response of E2-depleted and fulvestrant treated MCF7 cells. These muted responses were accompanied by sustained inhibitory phosphorylation of retinoblastoma (RB) protein, expression of cyclin D1, phospho-activation of ERK1/2 and MEK1/2 and expression of cRaf. Our work indicates that CycG2 can form complexes with CDK10, a CDK linked to modulation of RAF/MEK/MAPK signaling and tamoxifen resistance. We determined that metformin upregulates CycG2 and potentiates fulvestrant-induced CycG2 expression and cell cycle arrest. CycG2 knockdown blunts the enhanced anti-proliferative effect of metformin on fulvestrant treated cells. Meta-analysis of BC tumor microarrays indicates that CCNG2 expression is low in aggressive, poor-prognosis BC and that high CCNG2 expression correlates with longer periods of patient survival. Together these findings indicate that CycG2 contributes to signaling networks that limit BC.

Elucidating the Activation Mechanism of the Insulin-Family Proteins with Molecular Dynamics Simulations

The insulin-family proteins bind to their own receptors, but insulin-like growth factor II (IGF-II) can also bind to the A isoform of the insulin receptor (IR-A), activating unique and alternative signaling pathways from those of insulin. Although extensive studies of insulin have revealed that its activation is associated with the opening of the B chain-C terminal (BC-CT), the activation mechanism of the insulin-like growth factors (IGFs) still remains unknown. Here, we present the first comprehensive study of the insulin-family proteins comparing their activation process and mechanism using molecular dynamics simulations to reveal new insights into their specificity to the insulin receptor. We have found that all the proteins appear to exhibit similar stochastic dynamics in their conformational change to an active state. For the IGFs, our simulations show that activation involves two opening locations: the opening of the BC-CT section away from the core, similar to insulin; and the additional opening of the BC-CT section away from the C domain. Furthermore, we have found that these two openings occur simultaneously in IGF-I, but not in IGF-II, where they can occur independently. This suggests that the BC-CT section and the C domain behave as a unified domain in IGF-I, but as two independent domains in IGF-II during the activation process, implying that the IGFs undergo different activation mechanisms for receptor binding. The probabilities of the active and inactive states of the proteins suggest that IGF-II is hyperactive compared to IGF-I. The hinge residue and the hydrophobic interactions in the core are found to play a critical role in the stability and activity of IGFs. Overall, our simulations have elucidated the crucial differences and similarities in the activation mechanisms of the insulin-family proteins, providing new insights into the molecular mechanisms responsible for the observed differences between IGF-I and IGF-II in receptor binding.

Probing Receptor Specificity by Sampling the Conformational Space of the Insulin-like Growth Factor II C-domain

Insulin and insulin-like growth factors I and II are closely related protein hormones. Their distinct evolution has resulted in different yet overlapping biological functions with insulin becoming a key regulator of metabolism, whereas insulin-like growth factors (IGF)-I/II are major growth factors. Insulin and IGFs cross-bind with different affinities to closely related insulin receptor isoforms A and B (IR-A and IR-B) and insulin-like growth factor type I receptor (IGF-1R). Identification of structural determinants in IGFs and insulin that trigger their specific signaling pathways is of increasing importance in designing receptor-specific analogs with potential therapeutic applications. Here, we developed a straightforward protocol for production of recombinant IGF-II and prepared six IGF-II analogs with IGF-I-like mutations. All modified molecules exhibit significantly reduced affinity toward IR-A, particularly the analogs with a Pro-Gln insertion in the C-domain. Moreover, one of the analogs has enhanced binding affinity for IGF-1R due to a synergistic effect of the Pro-Gln insertion and S29N point mutation. Consequently, this analog has almost a 10-fold higher IGF-1R/IR-A binding specificity in comparison with native IGF-II. The established IGF-II purification protocol allowed for cost-effective isotope labeling required for a detailed NMR structural characterization of IGF-II analogs that revealed a link between the altered binding behavior of selected analogs and conformational rearrangement of their C-domains.

Novel method demonstrates differential ligand activation and phosphatase-mediated deactivation of insulin receptor tyrosine-specific phosphorylation

Insulin receptor signaling is a complex cascade leading to a multitude of intracellular functional responses. Three natural ligands, insulin, IGF1 and IGF2, are each capable of binding with different affinities to the insulin receptor, and result in variable biological responses. However, it is likely these affinity differences alone cannot completely explain the myriad of diverse cellular outcomes. Ligand binding initiates activation of a signaling cascade resulting in phosphorylation of the IR itself and other intracellular proteins. The direct catalytic activity along with the temporally coordinated assembly of signaling proteins is critical for insulin receptor signaling. We hypothesized that determining differential phosphorylation among individual tyrosine sites activated by ligand binding or dephosphorylation by phosphatases could provide valuable insight into insulin receptor signaling. Here, we present a sensitive, novel immunoassay adapted from Meso Scale Discovery technology to quantitatively measure changes in site-specific phosphorylation levels on endogenous insulin receptors from HuH7 cells. We identified insulin receptor phosphorylation patterns generated upon differential ligand activation and phosphatase-mediated deactivation. The data demonstrate that insulin, IGF1 and IGF2 elicit different insulin receptor phosphorylation kinetics and potencies that translate to downstream signaling. Furthermore, we show that insulin receptor deactivation, regulated by tyrosine phosphatases, occurs distinctively across specific tyrosine residues. In summary, we present a novel, quantitative and high-throughput assay that has uncovered differential ligand activation and site-specific deactivation of the insulin receptor. These results may help elucidate some of the insulin signaling mechanisms, discriminate ligand activity and contribute to a better understanding of insulin receptor signaling. We propose this methodology as a powerful approach to characterize agonists and antagonists of the insulin receptor and can be adapted to serve as a platform to evaluate ligands of alternate receptor systems.

Novel cross talk between IGF-IR and DDR1 regulates IGF-IR trafficking, signaling and biological responses

The insulin-like growth factor-I receptor (IGF-IR), plays a key role in regulating mammalian development and growth, and is frequently deregulated in cancer contributing to tumor initiation and progression. Discoidin domain receptor 1 (DDR1), a collagen receptor tyrosine-kinase, is as well frequently overexpressed in cancer and implicated in cancer progression. Thus, we investigated whether a functional cross-talk between the IGF-IR and DDR1 exists and plays any role in cancer progression.

Using human breast cancer cells we found that DDR1 constitutively associated with the IGF-IR. However, this interaction was enhanced by IGF-I stimulation, which promoted rapid DDR1 tyrosine-phosphorylation and co-internalization with the IGF-IR. Significantly, DDR1 was critical for IGF-IR endocytosis and trafficking into early endosomes, IGF-IR protein expression and IGF-I intracellular signaling and biological effects, including cell proliferation, migration and colony formation. These biological responses were inhibited by DDR1 silencing and enhanced by DDR1 overexpression.

Experiments in mouse fibroblasts co-transfected with the human IGF-IR and DDR1 gave similar results and indicated that, in the absence of IGF-IR, collagen-dependent phosphorylation of DDR1 is impaired.

These results demonstrate a critical role of DDR1 in the regulation of IGF-IR action, and identify DDR1 as a novel important target for breast cancers that overexpress IGF-IR.

Insulin receptor isoforms: an integrated view focused on gestational diabetes mellitus

The human insulin receptor (IR) exists in two isoforms that differ by the absence (IR-A) or the presence (IR-B) of a 12-amino acid segment encoded by exon 11. Both isoforms are functionally distinct regarding their binding affinities and intracellular signalling. However, the underlying mechanisms related to their cellular functions in several tissues are only partially understood. In this review, we summarize the current knowledge in this field regarding the alternative splicing of IR isoform, tissue-specific distribution and signalling both in physiology and disease, with an emphasis on the human placenta in gestational diabetes mellitus (GDM). Furthermore, we discuss the clinical relevance of IR isoforms highlighted by findings that show altered insulin signalling due to differential IR-A and IR-B expression in human placental endothelium in GDM pregnancies. Future research and clinical studies focused on the role of IR isoform signalling might provide novel therapeutic targets for treating GDM to improve the adverse maternal and neonatal outcomes.

Insulin receptor isoform A confers a higher proliferative capability to pancreatic beta cells enabling glucose availability and IGF-I signaling

The main compensatory response to insulin resistance is the pancreatic beta cell hyperplasia to account for increased insulin secretion. In fact, in a previous work we proposed a liver-pancreas endocrine axis with IGF-I (insulin-like growth factor type I) secreted by the liver acting on IRA insulin receptor in beta cells from iLIRKO mice (inducible Liver Insulin Receptor KnockOut) that showed a high IRA/IRB ratio. However, the role of insulin receptor isoforms in the IGF-I-induced beta cell proliferation as well as the underlying molecular mechanisms remain poorly understood. For this purpose, we have used four immortalized mouse beta cell lines: bearing IR (IRLoxP), lacking IR (IRKO), expressing exclusively IRA (IRA), or alternatively expressing IRB (IRB). Pancreatic beta cell proliferation studies showed that IRA cells are more sensitive than those expressing IRB to the mitogenic response induced by IGF-I, acting through the pathway IRA/IRS-1/2/αp85/Akt/mTORC1/p70S6K. More importantly, IRA beta cells, but not IRB, showed an increased glucose uptake as compared with IRLoxP cells, this effect being likely owing to an enhanced association between Glut-1 and Glut-2 with IRA. Overall, our results strongly suggest a prevalent role of IRA in glucose availability and IGF-I-induced beta cell proliferation mainly through mTORC1. These results could explain, at least partially, the role played by the liver-secreted IGF-I in the compensatory beta cell hyperplasia observed in response to severe hepatic insulin resistance in iLIRKO mice.

Ligand-Binding Affinity at the Insulin Receptor Isoform-A and Subsequent IR-A Tyrosine Phosphorylation Kinetics are Important Determinants of Mitogenic Biological Outcomes

The insulin receptor (IR) is a tyrosine kinase receptor that can mediate both metabolic and mitogenic biological actions. The IR isoform-A (IR-A) arises from alternative splicing of exon 11 and has different ligand binding and signaling properties compared to the IR isoform-B. The IR-A not only binds insulin but also insulin-like growth factor-II (IGF-II) with high affinity. IGF-II acting through the IR-A promotes cancer cell proliferation, survival, and migration by activating some unique signaling molecules compared to those activated by insulin. This observation led us to investigate whether the different IR-A signaling outcomes in response to IGF-II and insulin could be attributed to phosphorylation of a different subset of IR-A tyrosine residues or to the phosphorylation kinetics. We correlated IR-A phosphorylation to activation of molecules involved in mitogenic and metabolic signaling (MAPK and Akt) and receptor internalization rates (related to mitogenic signaling). We also extended this study to incorporate two ligands that are known to promote predominantly mitogenic [(His(4), Tyr(15), Thr(49), Ile(51)) IGF-I, qIGF-I] or metabolic (S597 peptide) biological actions, to see if common mechanisms can be used to define mitogenic or metabolic signaling through the IR-A. The threefold lower mitogenic action of IGF-II compared to insulin was associated with a decreased potency in activation of Y960, Y1146, Y1150, Y1151, Y1316, and Y1322, in MAPK phosphorylation and in IR-A internalization. With the poorly mitogenic S597 peptide, it was a decreased rate of tyrosine phosphorylation rather than potency that was associated with a low mitogenic potential. We conclude that both decreased affinity of IR-A binding and kinetics of IR-A phosphorylation can independently lead to a lower mitogenic activity. None of the studied parameters could account for the lower metabolic activity of qIGF-I.

Novel Aspects Concerning the Functional Cross-Talk between the Insulin/IGF-I System and Estrogen Signaling in Cancer Cells

The insulin/IGF system plays an important role in cancer progression. Accordingly, elevated levels of circulating insulin have been associated with an increased cancer risk as well as with aggressive and metastatic cancer phenotypes. Numerous studies have documented that estrogens cooperate with the insulin/IGF system in multiple pathophysiological conditions. The biological responses to estrogens are mainly mediated by the estrogen receptors (ER)α and ERβ, which act as transcription factors; however, several studies have recently demonstrated that a member of the G protein-coupled receptors, named GPR30/G-protein estrogen receptor (GPER), is also involved in the estrogen signaling in normal and malignant cells as well as in cancer-associated fibroblasts (CAFs). In this regard, novel mechanisms linking the action of estrogens through GPER with the insulin/IGF system have been recently demonstrated. This review recapitulates the relevant aspects of this functional cross-talk between the insulin/IGF and the estrogenic GPER transduction pathways, which occurs in various cell types and may account for cancer progression.

Targeting the insulin-like growth factor pathway in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the third leading cause of cancer-related deaths worldwide. Only 30%-40% of the patients with HCC are eligible for curative treatments, which include surgical resection as the first option, liver transplantation and percutaneous ablation. Unfortunately, there is a high frequency of tumor recurrence after surgical resection and most HCC seem resistant to conventional chemotherapy and radiotherapy. Sorafenib, a multi-tyrosine kinase inhibitor, is the only chemotherapeutic option for patients with advanced hepatocellular carcinoma. Patients treated with Sorafenib have a significant increase in overall survival of about three months. Therefore, there is an urgent need to develop alternative treatments. Due to its role in cell growth and development, the insulin-like growth factor system is commonly deregulated in many cancers. Indeed, the insulin-like growth factor (IGF) axis has recently emerged as a potential target for hepatocellular carcinoma treatment. To this aim, several inhibitors of the pathway have been developed such as monoclonal antibodies, small molecules, antisense oligonucleotides or small interfering RNAs. However recent studies suggest that, unlike most tumors, HCC development requires increased signaling through insulin growth factor II rather than insulin growth factor I. This may have great implications in the future treatment of HCC. This review summarizes the role of the IGF axis in liver carcinogenesis and the current status of the strategies designed to target the IGF-I signaling pathway for hepatocellular carcinoma treatment.

Inhibition of insulin receptor function by a human, allosteric monoclonal antibody: a potential new approach for the treatment of hyperinsulinemic hypoglycemia

Novel therapies are needed for the treatment of hypoglycemia resulting from both endogenous and exogenous hyperinsulinema. To provide a potential new treatment option, we identified XMetD, an allosteric monoclonal antibody to the insulin receptor (INSR) that was isolated from a human antibody phage display library. To selectively obtain antibodies directed at allosteric sites, panning of the phage display library was conducted using the insulin-INSR complex. Studies indicated that XMetD bound to the INSR with nanomolar affinity. Addition of insulin reduced the affinity of XMetD to the INSR by 3-fold, and XMetD reduced the affinity of the INSR for insulin 3-fold. In addition to inhibiting INSR binding, XMetD also inhibited insulin-induced INSR signaling by 20- to 100-fold. These signaling functions included INSR autophosphorylation, Akt activation and glucose transport. These data indicated that XMetD was an allosteric antagonist of the INSR because, in addition to inhibiting the INSR via modulation of binding affinity, it also inhibited the INSR via modulation of signaling efficacy. Intraperitoneal injection of XMetD at 10 mg/kg twice weekly into normal mice induced insulin resistance. When sustained-release insulin implants were placed into normal mice, they developed fasting hypoglycemia in the range of 50 mg/dl. This hypoglycemia was reversed by XMetD treatment. These studies demonstrate that allosteric monoclonal antibodies, such as XMetD, can antagonize INSR signaling both in vitro and in vivo. They also suggest that this class of allosteric monoclonal antibodies has the potential to treat hyperinsulinemic hypoglycemia resulting from conditions such as insulinoma, congenital hyperinsulinism and insulin overdose.

The emerging role of insulin and insulin-like growth factor signaling in cancer stem cells

Cancer cells frequently exploit the IGF signaling, a fundamental pathway mediating development, cell growth, and survival. As a consequence, several components of the IGF signaling are deregulated in cancer and sustain cancer progression. However, specific targeting of IGF-IR in humans has resulted efficacious only in small subsets of cancers, making researches wondering whether IGF system targeting is still worth pursuing in the clinical setting. Although no definite answer is yet available, it has become increasingly clear that other components of the IGF signaling pathway, such as IR-A, may substitute for the lack of IGF-IR, and induce cancer resistance and/or clonal selection. Moreover, accumulating evidence now indicates that IGF signaling is a central player in the induction/maintenance of epithelial mesenchymal transition (EMT) and cell stemness, two strictly related programs, which play a key role in metastatic spread and resistance to cancer treatments. Here we review the evidences indicating that IGF signaling enhances the expression of transcription factors implicated in the EMT program and has extensive cross-talk with specific pathways involved in cell pluripotency and stemness maintenance. In turn, EMT and cell stemness activate positive feed-back mechanisms causing up-regulation of various IGF signaling components. These findings may have novel translational implications.

Insulin-like growth factor-II (IGF-II) and IGF-II analogs with enhanced insulin receptor-a binding affinity promote neural stem cell expansion

The objective of this study was to employ genetically engineered IGF-II analogs to establish which receptor(s) mediate the stemness promoting actions of IGF-II on mouse subventricular zone neural precursors. Neural precursors from the subventricular zone were propagated in vitro in culture medium supplemented with IGF-II analogs. Cell growth and identity were analyzed using sphere generation and further analyzed by flow cytometry. F19A, an analog of IGF-II that does not bind the IGF-2R, stimulated an increase in the proportion of neural stem cells (NSCs) while decreasing the proportion of the later stage progenitors at a lower concentration than IGF-II. V43M, which binds to the IGF-2R with high affinity but which has low binding affinity to the IGF-1R and to the A isoform of the insulin receptor (IR-A) failed to promote NSC growth. The positive effects of F19A on NSC growth were unaltered by the addition of a functional blocking antibody to the IGF-1R. Altogether, these data lead to the conclusion that IGF-II promotes stemness of NSCs via the IR-A and not through activation of either the IGF-1R or the IGF-2R.

Ligand-mediated endocytosis and trafficking of the insulin-like growth factor receptor I and insulin receptor modulate receptor function

The insulin-like growth factor system and its two major receptors, the IGF receptor I (IGF-IR) and IR, plays a central role in a variety of physiological cellular processes including growth, differentiation, motility, and glucose homeostasis. The IGF-IR is also essential for tumorigenesis through its capacity to protect cancer cells from apoptosis. The IR is expressed in two isoforms: the IR isoform A (IR-A) and isoform B (IR-B). While the role of the IR-B in the regulation of metabolic effects has been known for several years, more recent evidence suggests that the IR, and in particular the IR-A, may be involved in the pathogenesis of cancer. Ligand-mediated endocytosis of tyrosine-kinases receptors plays a critical role in modulating the duration and intensity of receptors action but while the signaling pathways induced by the IGF-IR and IR are quite characterized, very little is still known about the mechanisms and proteins that regulate ligand-induced IGF-IR and IR endocytosis and trafficking. In addition, how these processes affect receptor downstream signaling has not been fully characterized. Here, we discuss the current understanding of the mechanisms and proteins regulating IGF-IR and IR endocytosis and sorting and their implications in modulating ligand-induced biological responses.

Tumors, IGF-2, and hypoglycemia: insights from the clinic, the laboratory, and the historical archive

Tumors of mesenchymal and epithelial origin produce IGF-2, which activates pathways in the tumors. In a minority of patients, the tumors (hepatomas, fibromas, and fibrosarcomas are the most common among many) release into the circulation enough IGF-2-related peptides to mimic the fasting hypoglycemia characteristic of patients with insulin-producing islet-cell tumors. Rarely, markedly elevated IGF-2 levels produce somatic changes suggestive of acromegaly. Typically, the elevated IGF-2 levels are associated with suppressed plasma levels of insulin, IGF-1, and GH. Complicating the pathophysiology are the IGF binding proteins (IGFBPs) that can bind IGF-2 and IGF-1, modifying hormone metabolism and action. IGFBP concentrations are often altered in the presence of these tumors. At the cellular level, the 3 hormone-related ligands, IGF-2, IGF-1, and insulin, all bind to 4 (or more) types of IGF-1 receptor (IGF-1R) and insulin receptor (IR). Each receptor has its own characteristic affinity for each ligand, a tyrosine kinase, and overlapping profiles of action in the target cells. The IGF-2R, in addition to binding mannose-6-phosphate-containing proteins, provides an IGF-2 degradation pathway. Recent evidence suggests IGF-2R involvement also in signal transduction. Surgery, the treatment of choice, can produce a cure. For patients not cured by surgery, multiple therapies exist, for the tumor and for hypoglycemia. Potential future therapeutic approaches are sketched. From 1910 to 1930, hypoglycemia, insulin, insulinomas, and non-islet-cell tumors were recognized. The latter third of the century witnessed the emergence of the immunoassay for insulin; the IGFs, their binding proteins, and assays to measure them; and receptors for the insulin-related peptides as well as the intracellular pathways beyond the receptor. In closing, we replace non-islet-cell tumor hypoglycemia, an outdated and misleading label, with IGF-2-oma, self-explanatory and consistent with names of other hormone-secreting tumors.

IGF-II promotes stemness of neural restricted precursors

Insulin-like growth factor (IGF)-I and IGF-II regulate brain development and growth through the IGF type 1 receptor (IGF-1R). Less appreciated is that IGF-II, but not IGF-I, activates a splice variant of the insulin receptor (IR) known as IR-A. We hypothesized that IGF-II exerts distinct effects from IGF-I on neural stem/progenitor cells (NSPs) via its interaction with IR-A. Immunofluorescence revealed high IGF-II in the medial region of the subventricular zone (SVZ) comprising the neural stem cell niche, with IGF-II mRNA predominant in the adjacent choroid plexus. The IGF-1R and the IR isoforms were differentially expressed with IR-A predominant in the medial SVZ, whereas the IGF-1R was more abundant laterally. Similarly, IR-A was more highly expressed by NSPs, whereas the IGF-1R was more highly expressed by lineage restricted cells. In vitro, IGF-II was more potent in promoting NSP expansion than either IGF-I or standard growth medium. Limiting dilution and differentiation assays revealed that IGF-II was superior to IGF-I in promoting stemness. In vivo, NSPs propagated in IGF-II migrated to and took up residence in periventricular niches while IGF-I-treated NSPs predominantly colonized white matter. Knockdown of IR or IGF-1R using shRNAs supported the conclusion that the IGF-1R promotes progenitor proliferation, whereas the IR is important for self-renewal. Q-PCR revealed that IGF-II increased Oct4, Sox1, and FABP7 mRNA levels in NSPs. Our data support the conclusion that IGF-II promotes the self-renewal of neural stem/progenitors via the IR. By contrast, IGF-1R functions as a mitogenic receptor to increase precursor abundance.

Diverse functions of IGF/insulin signaling in malignant and noncancerous prostate cells: proliferation in cancer cells and differentiation in noncancerous cells

The insulin-like growth factor (IGF) pathway represents one of the most studied molecular regulatory networks in oncology. Clinical trials investigating the therapeutic value of anti-IGF1 receptor (IGF1R) therapies in cancer, including prostate cancer, are ongoing. However, the multiple functions of the IGF network in the prostate are not entirely known. To elucidate the effects of IGF and insulin (INS) on prostate cells, we stimulated prostate cancer (PC3, DU145, LNCaP, DUCaP) and noncancerous prostate cells (EP156T, RWPE-1) and observed differing responses: whereas cancer cells responded to IGF and INS exposure by way of enhanced cell proliferation and glucose consumption, basal to luminal differentiation was induced in noncancerous cells. The same diverse responses were observed when the growth factor receptors IGF1R or INSR were overexpressed. Down-regulation of IGF1R or INSR isoform A (INSRA) also inhibited only proliferation of cancer cells. The proliferative response induced by the INSR in cancer cells was mediated solely by the INSRA. Moreover we observed that the receptors of the IGF network mutually influence their expression and exert redundant functions, thus underscoring the functional molecular network formed by IGF, INS, IGF1R, and INSR. Collectively we found that both IGF1R and INSRA have oncogenic effects in prostate cancer, but the IGF network also has important physiological functions in the noncancerous prostate. These data provide new insights into the biology of the IGF network in the prostate, thereby facilitating the design and interpretation of clinical studies investigating IGF1R targeting agents.

Identification of common and distinctive mechanisms of resistance to different anti-IGF-IR agents in Ewing's sarcoma

IGF system contributes significantly to many human malignancies. Targeting IGF-I receptor (IGF-IR) has been reported to be active against several tumors, but particular efficacy was observed only against a minority of Ewing's sarcoma patients. Identification of mechanisms of acquired resistance to anti-IGF-IR agents is mandatory to individualize their use in clinics and optimize cure costs. In this study, we compared gene expression profiles of cells made resistant with three different anti-IGF-IR drugs (human antibodies AVE1642, Figitumumab, or tyrosine kinase inhibitor NVP-AEW541) to highlight common and distinctive mechanisms of resistance. Among common mechanisms, we identified two molecular signatures that distinguish sensitive from resistant cells. Annotation analysis indicated some common altered pathways, such as insulin signaling, MAPK pathway, endocytosis, and modulation of some members of the interferon-induced transmembrane protein family. Among distinctive pathways/processes, resistance to human antibodies involves mainly genes regulating neural differentiation and angiogenesis, whereas resistance to NVP-AEW541 is mainly associated with alterations in genes concerning inflammation and antigen presentation. Evaluation of the common altered pathways indicated that resistant cells seem to maintain intact the IGF-IR internalization/degradation route of sensitive cells but constantly down-regulated its expression. In resistant cells, the loss of proliferative stimulus, normally sustained by IGF-I/IGF-IR autocrine loop in Ewing's sarcoma cells, is compensated by transcriptional up-regulation of IGF-II and insulin receptor-A; this signaling seems to favor the MAPK pathway over the v-akt murine thymoma viral oncogene homolog 1 pathway. Overall, complexity of IGF system requires analytical evaluation of its components to select those patients that may really benefit from this targeted therapy and support the idea of cotargeting IGF-IR and insulin receptor-A to increase the efficacy.

Insulin-like growth factor physiology: what we have learned from human studies

Although very similar to insulin and its receptor; the modus operandi of the insulin-like growth factors (IGFs) within the body is very different from that of the traditional peptide hormone. The IGF-binding proteins bind the IGFs with greater affinity than the cell surface receptors, enabling them to tightly control tissue activity. In addition to their role in fetal and childhood growth, IGFs play an important role in metabolic regulation. This article describes the basic underlying human physiology of IGFs, how this differs from that of experimental models, and why some information can only be learned from human clinical studies.

Amino acids potentiate insulin signaling in CHO-K1 at high glucose conditions

BACKGROUND AND AIMS

Amino acids reportedly increase the glucose uptake under high glucose conditions. However, there are controversies in the role of amino acids in diabetes mellitus. The present study explores the insulin signaling pathway involved in glucose uptake mediated by amino acids in CHO-K1 cells.

METHODS

CHO-K1 cells were exposed to normal (7 mM) and high glucose (17 and 27 mM) with 100 nM insulin in the presence and absence of amino acid mixtures (AAM) in varying concentration (5 and 20 mM) followed by the assays, insulin receptor tyrosine kinase (IRTK) and phosphatidylinositol 3 kinase (PI3K) by autoradiography, protein kinase B (Akt) and glucose transporter (GLUT4) by Western blot and glycogen synthase (GS) by HPLC.

RESULTS

The addition of 5 and 20 mM AAM significantly increased IRTK and PI3K activity (ANOVA p = 0.025, p = 0.003, respectively) with increasing glucose concentration. Addition of 5 mM AAM in the presence of normal glucose significantly increased the levels of phosphorylated Akt Ser473 (p = 0.02) with no significant change at high glucose. At 20 mM AAM there was a significant decrease in Akt phosphorylation (p = 0.035) that was increased by high glucose concentration. GLUT4 protein levels were increased with AAM (5 mM) along with increase in glycogen synthase activity at all glucose concentrations (p <0.05).

CONCLUSIONS

Amino acids as a mixture is beneficial in augmenting insulin signaling pathway via IRTK/PI3K/GLUT4 pathway along with activation of GS in CHO-K1 cells, thereby ensuring increased intracellular glucose availability.

Proinsulin binds with high affinity the insulin receptor isoform A and predominantly activates the mitogenic pathway

Proinsulin is generally regarded as an inactive prohormone because of its low metabolic activity. However, proinsulin appears to regulate embryo development in animal models. In this study, we evaluated whether proinsulin may differentially bind to and activate the two insulin receptor (IR) isoforms (IR-A and IR-B), because IR-A is a relatively low-specificity receptor that is prevalent in fetal and cancer cells and is able to mediate the growth effects of IGF-II. Mouse R(-) fibroblasts devoid of IGF-I receptor (IGF-IR) and stably transfected with cDNA encoding either human IR-A or IR-B (R(-) /IR-A and R(-) /IR-B cells) were used. Three human cancer cell lines were also studied. We found that proinsulin stimulated phosphorylation of IR-A with an EC(50) of 4.5 ± 0.6 nm and displaced [(125)I]insulin from IR-A with a similar EC(50). In contrast, proinsulin EC(50) values for stimulation of IR-B phosphorylation and for [(125)I]insulin displacement from IR-B were approximately 7-fold higher. Proinsulin did not bind or activate IGF-IR or IR/IGF-IR hybrids. Via IR-A, proinsulin activated the ERK/p70S6K pathway to a similar degree as insulin but elicited a weaker Akt response. Despite its low metabolic activity, proinsulin was almost equipotent as insulin in inducing cell proliferation and migration in cells expressing various IR-A levels. In conclusion, proinsulin is a selective IR-A ligand and may induce biological effects through this IR isoform.

Molecular basis of signaling specificity of insulin and IGF receptors: neglected corners and recent advances

Insulin and insulin-like growth factor (IGF) receptors utilize common phosphoinositide 3-kinase/Akt and Ras/extracellular signal-regulated kinase signaling pathways to mediate a broad spectrum of "metabolic" and "mitogenic" responses. Specificity of insulin and IGF action in vivo must in part reflect expression of receptors and responsive pathways in different tissues but it is widely assumed that it is also determined by the ligand binding and signaling mechanisms of the receptors. This review focuses on receptor-proximal events in insulin/IGF signaling and examines their contribution to specificity of downstream responses. Insulin and IGF receptors may differ subtly in the efficiency with which they recruit their major substrates (IRS-1 and IRS-2 and Shc) and this could influence effectiveness of signaling to "metabolic" and "mitogenic" responses. Other substrates (Grb2-associated binder, downstream of kinases, SH2Bs, Crk), scaffolds (RACK1, β-arrestins, cytohesins), and pathways (non-receptor tyrosine kinases, phosphoinositide kinases, reactive oxygen species) have been less widely studied. Some of these components appear to be specifically involved in "metabolic" or "mitogenic" signaling but it has not been shown that this reflects receptor-preferential interaction. Very few receptor-specific interactions have been characterized, and their roles in signaling are unclear. Signaling specificity might also be imparted by differences in intracellular trafficking or feedback regulation of receptors, but few studies have directly addressed this possibility. Although published data are not wholly conclusive, no evidence has yet emerged for signaling mechanisms that are specifically engaged by insulin receptors but not IGF receptors or vice versa, and there is only limited evidence for differential activation of signaling mechanisms that are common to both receptors. Cellular context, rather than intrinsic receptor activity, therefore appears to be the major determinant of whether responses to insulin and IGFs are perceived as "metabolic" or "mitogenic."

Insulin and insulin-like growth factor II differentially regulate endocytic sorting and stability of insulin receptor isoform A

The insulin receptor isoform A (IR-A) binds both insulin and insulin-like growth factor (IGF)-II, although the affinity for IGF-II is 3-10-fold lower than insulin depending on a cell and tissue context. Notably, in mouse embryonic fibroblasts lacking the IGF-IR and expressing solely the IR-A (R-/IR-A), IGF-II is a more potent mitogen than insulin. As receptor endocytosis and degradation provide spatial and temporal regulation of signaling events, we hypothesized that insulin and IGF-II could affect IR-A biological responses by differentially regulating IR-A trafficking. Using R-/IR-A cells, we discovered that insulin evoked significant IR-A internalization, a process modestly affected by IGF-II. However, the differential internalization was not due to IR-A ubiquitination. Notably, prolonged stimulation of R-/IR-A cells with insulin, but not with IGF-II, targeted the receptor to a degradative pathway. Similarly, the docking protein insulin receptor substrate 1 (IRS-1) was down-regulated after prolonged insulin but not IGF-II exposure. Similar results were also obtained in experiments using [NMeTyr(B26)]-insulin, an insulin analog with IR-A binding affinity similar to IGF-II. Finally, we discovered that IR-A was internalized through clathrin-dependent and -independent pathways, which differentially regulated the activation of downstream effectors. Collectively, our results suggest that a lower affinity of IGF-II for the IR-A promotes lower IR-A phosphorylation and activation of early downstream effectors vis à vis insulin but may protect IR-A and IRS-1 from down-regulation thereby evoking sustained and robust mitogenic stimuli.

Understanding the mechanism of insulin and insulin-like growth factor (IGF) receptor activation by IGF-II

Background

Insulin-like growth factor-II (IGF-II) promotes cell proliferation and survival and plays an important role in normal fetal development and placental function. IGF-II binds both the insulin-like growth factor receptor (IGF-1R) and insulin receptor isoform A (IR-A) with high affinity. Interestingly both IGF-II and the IR-A are often upregulated in cancer and IGF-II acts via both receptors to promote cancer proliferation. There is relatively little known about the mechanism of ligand induced activation of the insulin (IR) and IGF-1R. The recently solved IR structure reveals a folded over dimer with two potential ligand binding pockets arising from residues on each receptor half. Site-directed mutagenesis has mapped receptor residues important for ligand binding to two separate sites within the ligand binding pocket and we have recently shown that the IGFs have two separate binding surfaces which interact with the receptor sites 1 and 2.

Methodology/Principal Findings

In this study we describe a series of partial IGF-1R and IR agonists generated by mutating Glu12 of IGF-II. By comparing receptor binding affinities, abilities to induce negative cooperativity and potencies in receptor activation, we provide evidence that residue Glu12 bridges the two receptor halves leading to receptor activation.

Conclusions/Significance

This study provides novel insight into the mechanism of receptor binding and activation by IGF-II, which may be important for the future development of inhibitors of its action for the treatment of cancer.

Insulin receptor and cancer

The widespread epidemic of obesity and type 2 diabetes has raised concern for the impact of these disorders as risk factors for cancer and has renewed the interest for studies regarding the involvement of hyperinsulinemia and insulin receptor (IR) in cancer progression. Overexpression of IR in cancer cells may explain their increased sensitivity to hyperinsulinemia. Moreover, IR isoform A (IR-A) together with autocrine production of its ligand IGF2 is emerging as an important mechanism of normal and cancer stem cell expansion and is a feature of several malignancies. De novo activation of the IR-A/IGF2 autocrine loop also represents a mechanism of resistance to anticancer therapies. Increasing knowledge of the IR role in cancer has important implications for cancer prevention, which should include control of insulin resistance and hyperinsulinemia in the population and meticulous evaluation of new antidiabetic drugs for their metabolic:mitogenic ratio. We are now aware that several anticancer treatments may induce or worsen insulin resistance that may limit therapy efficacy. Future anticancer therapies need to target the IR-A pathway in order to inhibit the tumor promoting effect of IR without impairing the metabolic effect of insulin.

Research resource: New and diverse substrates for the insulin receptor isoform A revealed by quantitative proteomics after stimulation with IGF-II or insulin

The isoform A of the insulin receptor (IR) (IR-A) is a bifunctional receptor, because it binds both insulin and IGF-II. IR-A activation by IGF-II plays a role in development, but its physiological role in adults is unknown. IGF-II signaling through IR-A is deregulated in cancer and favors tumor progression. We hypothesized that IGF-II binding to the IR-A elicits a unique signaling pathway. In order to obtain an unbiased evaluation of IR-A substrates differentially involved after IGF-II and insulin stimulation, we performed quantitative proteomics of IR-A substrates recruited to tyrosine-phosphorylated protein complexes using stable isotope labeling with amino acids in cell culture in combination with antiphosphotyrosine antibody pull down and mass spectrometry. Using cells expressing only the human IR-A and lacking the IGF-I receptor, we identified 38 IR-A substrates. Only 10 were known IR mediators, whereas 28 substrates were not previously related to IR signaling. Eleven substrates were recruited by stimulation with both ligands: two equally recruited by IGF-II and insulin, three more strongly recruited by IGF-II, and six more strongly recruited by insulin. Moreover, 14 substrates were recruited solely by IGF-II and 13 solely by insulin stimulation. Interestingly, discoidin domain receptors, involved in cell migration and tumor metastasis, and ephrin receptor B4, involved in bidirectional signaling upon cell-cell contact, were predominantly activated by IGF-II. These findings indicate that IR-A activation by IGF-II elicits a unique signaling pathway that may play a distinct role in physiology and in disease.

Insulin receptor isoforms and insulin-like growth factor receptor in human follicular cell precursors from papillary thyroid cancer and normal thyroid

CONTEXT

Factors involved in the biology of normal and cancer stem/precursor cells from the thyroid are unknown. Thyroid cancer cells are responsive to insulin and IGF-I and IGF-II and often overexpress the insulin receptor (IR) and the IGF-I receptor (IGF-IR).

OBJECTIVE

We investigated the role of IR isoforms (IR-A and IR-B), IGF-IR, and their ligands in thyroid follicular cell precursors both normal and malignant.

DESIGN

We established cultures of follicular cell precursors as thyrospheres from three papillary thyroid cancers and the corresponding nonaffected tissues. The expression of IR, IGF-IR, and their ligands was evaluated by quantitative RT-PCR and, in one case, also by Western blot. The effects of insulin and IGFs on thyrosphere growth and self-renewal were evaluated.

RESULTS

Thyrospheres were characterized by the expression of stem cell markers and low/absent thyroid specific markers. Thyrospheres from normal tissue, but not from cancer tissue, could be induced to differentiate. Both IR isoforms, IGF-IR, IGF-I and IGF-II, were expressed at high levels in thyrospheres and markedly decreased in differentiating cells. IR-A was the predominant isoform in thyrospheres, especially from cancer, while IR-B was predominant in differentiating cells. Cancer thyrosphere growth was stimulated by insulin and IGFs.

CONCLUSIONS

Our data suggest that IR isoforms and IGF-IR play a role in the biology of follicular thyroid precursors. Cell differentiation is associated with marked changes in the expression of these receptors and cognate ligands. These data may provide insight for future differentiation therapies in thyroid cancer.