IGF-1 receptor activity in the Golgi of migratory cancer cells depends on adhesion-dependent phosphorylation of Tyr1250 and Tyr1251

Brefeldin A and M-COPA block the export of RTKs from the endoplasmic reticulum via simultaneous inactivation of ARF1, ARF4, and ARF5

Normal receptor tyrosine kinases (RTKs) need to reach the plasma membrane (PM) for ligand-induced activation, whereas its cancer-causing mutants can be activated before reaching the PM in organelles, such as the Golgi/trans-Golgi network (TGN). Inhibitors of protein export from the endoplasmic reticulum (ER), such as brefeldin A (BFA) and 2-methylcoprophilinamide (M-COPA), can suppress the activation of mutant RTKs in cancer cells, suggesting that RTK mutants cannot initiate signaling in the ER. BFA and M-COPA block the function of ADP-ribosylation factors (ARFs) that play a crucial role in ER-Golgi protein trafficking. However, among ARF family proteins, the specific ARFs inhibited by BFA or M-COPA, that is, the ARFs involved in RTKs transport from the ER, remain unclear. In this study, we showed that M-COPA blocked the export of not only KIT but also PDGFRA/EGFR/MET RTKs from the ER. ER-retained RTKs could not fully transduce anti-apoptotic signals, thereby leading to cancer cell apoptosis. Moreover, a single knockdown of ARF1, ARF3, ARF4, ARF5, or ARF6 could not block ER export of RTKs, indicating that BFA/M-COPA treatment cannot be mimicked by the knockdown of only one ARF member. Interestingly, simultaneous transfection of ARF1, ARF4, and ARF5 siRNAs mirrored the effect of BFA/M-COPA treatment. Consistent with these results, in vitro pulldown assays showed that BFA/M-COPA blocked the function of ARF1, ARF4, and ARF5. Taken together, these results suggest that BFA/M-COPA targets at least ARF1, ARF4, and ARF5; in other words, RTKs require the simultaneous activation of ARF1, ARF4, and ARF5 for their ER export.

Autocrine IGF-II-Associated Cancers: From a Rare Paraneoplastic Event to a Hallmark in Malignancy

The paraneoplastic syndrome referred in the literature as non-islet-cell tumor hypoglycemia (NICTH) and extra-pancreatic tumor hypoglycemia (EPTH) was first reported almost a century ago, and the role of cancer-secreted IGF-II in causing this blood glucose-lowering condition has been widely established. The landscape emerging in the last few decades, based on molecular and cellular findings, supports a broader role for IGF-II in cancer biology beyond its involvement in the paraneoplastic syndrome. In particular, a few key findings are constantly observed during tumorigenesis, (a) a relative and absolute increase in fetal insulin receptor isoform (IRA) content, with (b) an increase in IGF-II high-molecular weight cancer-variants (big-IGF-II), and (c) a stage-progressive increase in the IGF-II autocrine signal in the cancer cell, mostly during the transition from benign to malignant growth. An increasing and still under-exploited combinatorial pattern of the IGF-II signal in cancer is shaping up in the literature with respect to its transducing receptorial system and effector intracellular network. Interestingly, while surgical and clinical reports have traditionally restricted IGF-II secretion to a small number of solid malignancies displaying paraneoplastic hypoglycemia, a retrospective literature analysis, along with publicly available expression data from patient-derived cancer cell lines conveyed in the present perspective, clearly suggests that IGF-II expression in cancer is a much more common event, especially in overt malignancy. These findings strengthen the view that (1) IGF-II expression/secretion in solid tumor-derived cancer cell lines and tissues is a broader and more common event compared to the reported IGF-II association to paraneoplastic hypoglycemia, and (2) IGF-II associates to the commonly observed autocrine loops in cancer cells while IGF-I cancer-promoting effects may be linked to its paracrine effects in the tumor microenvironment. Based on these evidence-centered considerations, making the autocrine IGF-II loop a hallmark for malignant cancer growth, we here propose the functional name of IGF-II secreting tumors (IGF-IIsT) to overcome the view that IGF-II secretion and pro-tumorigenic actions affect only a clinical sub-group of rare tumors with associated hypoglycemic symptoms. The proposed scenario provides an updated logical frame towards biologically sound therapeutic strategies and personalized therapeutic interventions for currently unaccounted IGF-II-producing cancers.

IGF-1R targeting in cancer - does sub-cellular localization matter?

The insulin-like growth factor receptor (IGF-1R) was among the most intensively pursued kinase targets in oncology. However, even after a slew of small-molecule and antibody therapeutics reached clinical trials for a range of solid tumors, the initial promise remains unfulfilled. Mechanisms of resistance to, and toxicities resulting from, IGF-1R-targeted drugs are well-catalogued, and there is general appreciation of the fact that a lack of biomarker-based patient stratification was a limitation of previous clinical trials. But no next-generation therapeutic strategies have yet successfully exploited this understanding in the clinic.Currently there is emerging interest in re-visiting IGF-1R targeted therapeutics in combination-treatment protocols with predictive biomarker-driven patient-stratification. One such biomarker that emerged from early clinical trials is the sub-cellular localization of IGF-1R. After providing some background on IGF-1R, its drugging history, and the trials that led to the termination of drug development for this target, we look more deeply into the correlation between sub-cellular localization of IGF-1R and susceptibility to various classes of IGF-1R - targeted agents.

Golgi retention and oncogenic KIT signaling via PLCγ2-PKD2-PI4KIIIβ activation in gastrointestinal stromal tumor cells

Most gastrointestinal stromal tumors (GISTs) develop due to gain-of-function mutations in the tyrosine kinase gene, KIT. We recently showed that mutant KIT mislocalizes to the Golgi area and initiates uncontrolled signaling. However, the molecular mechanisms underlying its Golgi retention remain unknown. Here, we show that protein kinase D2 (PKD2) is activated by the mutant, which causes Golgi retention of KIT. In PKD2-inhibited cells, KIT migrates from the Golgi region to lysosomes and subsequently undergoes degradation. Importantly, delocalized KIT cannot trigger downstream activation. In the Golgi/trans-Golgi network (TGN), KIT activates the PKD2-phosphatidylinositol 4-kinase IIIβ (PKD2-PI4KIIIβ) pathway through phospholipase Cγ2 (PLCγ2) to generate a PI4P-rich membrane domain, where the AP1-GGA1 complex is aberrantly recruited. Disruption of any factors in this cascade results in the release of KIT from the Golgi/TGN. Our findings show the molecular mechanisms underlying KIT mislocalization and provide evidence for a strategy for inhibition of oncogenic signaling.

EHD1-dependent traffic of IGF-1 receptor to the cell surface is essential for Ewing sarcoma tumorigenesis and metastasis

Overexpression of the EPS15 Homology Domain containing 1 (EHD1) protein has been linked to tumorigenesis but whether its core function as a regulator of intracellular traffic of cell surface receptors plays a role in oncogenesis remains unknown. We establish that EHD1 is overexpressed in Ewing sarcoma (EWS), with high EHD1 mRNA expression specifying shorter patient survival. ShRNA-knockdown and CRISPR-knockout with mouse Ehd1 rescue established a requirement of EHD1 for tumorigenesis and metastasis. RTK antibody arrays identified IGF-1R as a target of EHD1 regulation in EWS. Mechanistically, we demonstrate a requirement of EHD1 for endocytic recycling and Golgi to plasma membrane traffic of IGF-1R to maintain its surface expression and downstream signaling. Conversely, EHD1 overexpression-dependent exaggerated oncogenic traits require IGF-1R expression and kinase activity. Our findings define the RTK traffic regulation as a proximal mechanism of EHD1 overexpression-dependent oncogenesis that impinges on IGF-1R in EWS, supporting the potential of IGF-1R and EHD1 co-targeting.

EHD1-dependent traffic of IGF-1 receptor to the cell surface is essential for Ewing sarcoma tumorigenesis and metastasis

Overexpression of EPS15 Homology Domain containing 1 (EHD1) has been linked to tumorigenesis but whether its core function as a regulator of intracellular traffic of cell surface receptors plays a role in oncogenesis remains unknown. We establish that EHD1 is overexpressed in Ewing sarcoma (EWS), with high EHD mRNA expression specifying shorter patient survival. ShRNA and CRISPR-knockout with mouse Ehd1 rescue established a requirement of EHD1 for tumorigenesis and metastasis. RTK antibody arrays identified the IGF-1R as a target of EHD1 regulation in EWS. Mechanistically, we demonstrate a requirement of EHD1 for endocytic recycling and Golgi to plasma membrane traffic of IGF-1R to maintain its surface expression and downstream signaling. Conversely, EHD1 overexpression-dependent exaggerated oncogenic traits require IGF-1R expression and kinase activity. Our findings define the RTK traffic regulation as a proximal mechanism of EHD1 overexpression-dependent oncogenesis that impinges on IGF-1R in EWS, supporting the potential of IGF-1R and EHD1 co-targeting.

Emerging Role of IGF-1 in Prostate Cancer: A Promising Biomarker and Therapeutic Target

Prostate cancer (PCa) is a highly heterogeneous disease driven by gene alterations and microenvironmental influences. Not only enhanced serum IGF-1 but also the activation of IGF-1R and its downstream signaling components has been increasingly recognized to have a vital driving role in the development of PCa. A better understanding of IGF-1/IGF-1R activity and regulation has therefore emerged as an important subject of PCa research. IGF-1/IGF-1R signaling affects diverse biological processes in cancer cells, including promoting survival and renewal, inducing migration and spread, and promoting resistance to radiation and castration. Consequently, inhibitory reagents targeting IGF-1/IGF-1R have been developed to limit cancer development. Multiple agents targeting IGF-1/IGF-1R signaling have shown effects against tumor growth in tumor xenograft models, but further verification of their effectiveness in PCa patients in clinical trials is still needed. Combining androgen deprivation therapy or cytotoxic chemotherapeutics with IGF-1R antagonists based on reliable predictive biomarkers and developing and applying novel agents may provide more desirable outcomes. This review will summarize the contribution of IGF-1 signaling to the development of PCa and highlight the relevance of this signaling axis in potential strategies for cancer therapy.

A Self-Assembling Probe for Imaging the States of Golgi Apparatus in Live Single Cells

Despite the enormous progress in genomics and proteomics, it is still challenging to assess the states of organelles in living cells with high spatiotemporal resolution. Based on our recent finding of enzyme-instructed self-assembly of a thiophosphopeptide that targets the Golgi Apparatus (GA) instantly, we use the thiophosphopeptide, which is enzymatically responsive and redox active, as an integrative probe for revealing the state of the GA of live cells at the single cell level. By imaging the probe in the GA of live cells over time, our results show that the accumulation of the probe at the GA depends on cell types. By comparison to a conventional Golgi probe, this self-assembling probe accumulates at the GA much faster and are sensitive to the expression of alkaline phosphatases. In addition, subtle changes of the fluorophore results in slightly different GA responses. This work illustrates a novel class of active molecular probes that combine enzyme-instructed self-assembly and redox reaction for high-resolution imaging of the states of subcellular organelles over a large area and extended times.

Enzyme-Responsive Peptide Thioesters for Targeting Golgi Apparatus

The Golgi apparatus (GA) is the hub of intracellular trafficking, but selectively targeting GA remains a challenge. We show an unconventional types of peptide thioesters, consisting of an aminoethyl thioester and acting as substrates of thioesterases, for instantly targeting the GA of cells. The peptide thioesters, above or below their critical micelle concentrations, enter cells mainly via caveolin-mediated endocytosis or macropinocytosis, respectively. After being hydrolyzed by GA-associated thioesterases, the resulting thiopeptides form dimers and accumulate in the GA. After saturating the GA, the thiopeptides are enriched in the endoplasmic reticulum (ER). Their buildup in ER and GA disrupts protein trafficking, thus leading to cell death via multiple pathways. The peptide thioesters target the GA of a wide variety of cells, including human, murine, and Drosophila cells. Changing d-diphenylalanine to l-diphenylalanine in the peptide maintains the GA-targeting ability. In addition, targeting GA redirects protein (e.g., NRAS) distribution. This work illustrates a thioesterase-responsive and redox-active molecular platform for targeting the GA and controlling cell fates.

Enzymatic Noncovalent Synthesis for Targeting Subcellular Organelles

Enzymatic noncovalent synthesis (ENS) exploits enzymatic reactions to produce spatially organized higher-order supramolecular assemblies that modulate cellular processes. While ENS is a general mechanism to create higher-order assemblies of proteins for diverse cellular functions, the exploration of ENS of other bioactive molecules, such as peptides or small organic molecules, is rather limited. Since ENS generates non-diffusive supramolecular assemblies locally, it provides a unique approach to targeting subcellular organelles. In this Review, we highlight the recent progress of the application of ENS of peptide assemblies for targeting subcellular organelles. After a brief introduction of the concept of ENS, we introduce the case of generating artificial filaments by ENS in cell cytosol, then discuss the use of ENS for targeting endoplasmic reticulum, mitochondria, Golgi apparatus, and lysosomes, and finally we describe the targeting of nucleus by ENS. We hope to illustrate the promise of ENS, as a localized molecular process in an open system, for understanding diseases, controlling cell behaviors, and developing new therapeutics.

Type 1 Insulin-Like Growth Factor Receptor Nuclear Localization in High-Grade Glioma Cells Enhances Motility, Metabolism, and In Vivo Tumorigenesis

Gliomas are the most frequent solid tumors in children. Among these, high-grade gliomas are less common in children than in adults, though they are similar in their aggressive clinical behavior. In adults, glioblastoma is the most lethal tumor of the central nervous system. Insulin-like growth factor 1 receptor (IGF1R) plays an important role in cancer biology, and its nuclear localization has been described as an adverse prognostic factor in different tumors. Previously, we have demonstrated that, in pediatric gliomas, IGF1R nuclear localization is significantly associated with high-grade tumors, worst clinical outcome, and increased risk of death. Herein we explore the role of IGF1R intracellular localization by comparing two glioblastoma cell lines that differ only in their IGF1R capacity to translocate to the nucleus. In vitro, IGF1R nuclear localization enhances glioblastoma cell motility and metabolism without affecting their proliferation. In vivo, IGF1R has the capacity to translocate to the nucleus and allows not only a higher proliferation rate and the earlier development of tumors but also renders the cells sensitive to OSI906 therapy. With this work, we provide evidence supporting the implications of the presence of IGF1R in the nucleus of glioma cells and a potential therapeutic opportunity for patients harboring gliomas with IGF1R nuclear localization.

Revising Endosomal Trafficking under Insulin Receptor Activation

The endocytosis of ligand-bound receptors and their eventual recycling to the plasma membrane (PM) are processes that have an influence on signalling activity and therefore on many cell functions, including migration and proliferation. Like other tyrosine kinase receptors (TKR), the insulin receptor (INSR) has been shown to be endocytosed by clathrin-dependent and -independent mechanisms. Once at the early endosome (EE), the sorting of the receptor, either to the late endosome (LE) for degradation or back to the PM through slow or fast recycling pathways, will determine the intensity and duration of insulin effects. Both the endocytic and the endosomic pathways are regulated by many proteins, the Arf and Rab families of small GTPases being some of the most relevant. Here, we argue for a specific role for the slow recycling route, whilst we review the main molecular mechanisms involved in INSR endocytosis, sorting and recycling, as well as their possible role in cell functions.

Enzymatic Assemblies of Thiophosphopeptides Instantly Target Golgi Apparatus and Selectively Kill Cancer Cells*

Changing an oxygen atom of the phosphoester bond in phosphopeptides by a sulfur atom enables instantly targeting Golgi apparatus (GA) and selectively killing cancer cells by enzymatic self-assembly. Specifically, conjugating cysteamine S-phosphate to the C-terminal of a self-assembling peptide generates a thiophosphopeptide. Being a substrate of alkaline phosphatase (ALP), the thiophosphopeptide undergoes rapid ALP-catalyzed dephosphorylation to form a thiopeptide that self-assembles. The thiophosphopeptide enters cells via caveolin-mediated endocytosis and macropinocytosis and instantly accumulates in GA because of dephosphorylation and formation of disulfide bonds in Golgi by themselves and with Golgi proteins. Moreover, the thiophosphopeptide potently and selectively inhibits cancer cells (HeLa) with the IC50 (about 3 μM), which is an order of magnitude more potent than that of the parent phosphopeptide.

The Role of Nuclear Insulin and IGF1 Receptors in Metabolism and Cancer

Insulin (InsR) and insulin-like growth factor-1 (IGF1R) receptors mediate the metabolic and growth-promoting actions of insulin and IGF1/IGF2, respectively. Evidence accumulated in recent years indicates that, in addition to their typical cell-surface localization pattern and ligand-activated mechanism of action, InsR and IGF1R are present in the cell nucleus of both normal and transformed cells. Nuclear translocation seems to involve interaction with a small, ubiquitin-like modifier protein (SUMO-1), although this modification is not always a prerequisite. Nuclear InsR and IGF1R exhibit a number of biological activities that classically fit within the definition of transcription factors. These nuclear activities include, among others, sequence-specific DNA binding and transcriptional control. Of particular interest, nuclear IGF1R was capable of binding and stimulating its cognate gene promoter. The physiological relevance of this autoregulatory mechanism needs to be further investigated. In addition to its nuclear localization, studies have identified IGF1R in the Golgi apparatus, and this particular distribution correlated with a migratory phenotype. In summary, the newly described roles of InsR and IGF1R as gene regulators, in concert with their atypical pattern of subcellular distribution, add a further layer of complexity to traditional models of cell signaling. Furthermore, and in view of the emerging role of IGF1R as a potential therapeutic target, a better understanding of the mechanisms responsible for nuclear IGF1R transport and identification of IGF1R interactors might help optimize target directed therapies in oncology.

The tale of a tail: The secret behind IGF-1R's oncogenic power

The C-terminal tail of insulin-like growth factor 1 receptor (IGF-1R) has long been appreciated to drive much of this receptor's oncogenic power. In this issue of Science Signaling, Rieger et al. have shown that Tyr¹²⁵⁰ and Tyr¹²⁵¹ of IGF-1R are autophosphorylated in a cell adhesion-dependent manner, uncovering a previously unknown plasma membrane-Golgi trafficking route for IGF-1R in migratory cells, an integral part of the malignant phenotype.

Controlled Signaling-Insulin-Like Growth Factor Receptor Endocytosis and Presence at Intracellular Compartments

Ligand-induced activation of the IGF-1 receptor triggers plasma-membrane-derived signal transduction but also triggers receptor endocytosis, which was previously thought to limit signaling. However, it is becoming ever more clear that IGF-1R endocytosis and trafficking to specific subcellular locations can define specific signaling responses that are important for key biological processes in normal cells and cancer cells. In different cell types, specific cell adhesion receptors and associated proteins can regulate IGF-1R endocytosis and trafficking. Once internalized, the IGF-1R may be recycled, degraded or translocated to the intracellular membrane compartments of the Golgi apparatus or the nucleus. The IGF-1R is present in the Golgi apparatus of migratory cancer cells where its signaling contributes to aggressive cancer behaviors including cell migration. The IGF-1R is also found in the nucleus of certain cancer cells where it can regulate gene expression. Nuclear IGF-1R is associated with poor clinical outcomes. IGF-1R signaling has also been shown to support mitochondrial biogenesis and function, and IGF-1R inhibition causes mitochondrial dysfunction. How IGF-1R intracellular trafficking and compartmentalized signaling is controlled is still unknown. This is an important area for further study, particularly in cancer.