Internalization of inactive EGF receptor into endosomes and the subsequent activation of endosome-associated EGF receptors. Epidermal growth factor

Two-Pulse Endosomal Stimulation of Receptor Tyrosine Kinases Induces Cell Proliferation

Signals transduced from ligand-activated receptor tyrosine kinases (RTKs) lead to a diverse array of biological outcomes, such as cell proliferation. Strict regulation of RTK activity is therefore necessary to prevent aberrancies in cell signaling that can lead to diseases such as cancer. RTKs are activated at the plasma membrane (PM) upon ligand binding. Contrary to the initial belief, RTK activity does not terminate immediately following endocytosis, instead RTKs remain active while being trafficked in endosomes. Here we describe a two-pulse endosomal stimulation approach which can specifically activate endosome-accumulated EGFR and drive cell proliferation.

Spatio-temporal regulation of EGFR signaling by the Eps15 homology domain-containing protein 3 (EHD3)

The epidermal growth factor (EGF) receptor EGFR is a major receptor tyrosine kinase whose role in gliomagenesis is well established. We have recently identified EHD3 [Eps15 homology (EH) domain-containing protein 3], an endocytic trafficking regulatory protein, as a putative brain tumor suppressor. Here, we investigate the underlying mechanisms, by establishing a novel mechanistic and functional connection between EHD3 and the EGFR signaling pathway. We show that, in response to stimulation with the EGF ligand, EHD3 accelerates the rate of EGFR degradation by dramatically increasing its ubiquitination. As part of this process, EHD3 also regulates EGFR endosomal trafficking by diverting it away from the recycling route into the degradative pathway. Moreover, we found that upon EGF activation, rather than affecting the total MAPK and AKT downstream signaling, EHD3 decreases endosome-based signaling of these two pathways, thus suggesting the contribution of EHD3 in the spatial regulation of EGFR signaling. This function explains the higher sensitivity of EHD3-expressing cells to the growth-inhibitory effects of EGF. In summary, this is the first report supporting a mechanism of EHD3-mediated tumor suppression that involves the attenuation of endosomal signaling of the EGFR oncogene.

Beclin1 antagonizes LAPTM4B-mediated EGFR overactivation in gastric cancer cells

Beclin1 is an essential autophagy regulator and a haploinsufficient tumor-suppressor. Reduced Berclin1 expression has been associated with many types of human malignancies including gastric cancer. However, the mechanism of how Beclin1 represses tumorigenesis of gastric cancer remains elusive. In recent proteomics study, we found that Beclin1 is associated with Lysosome-associated transmembrane protein 4β (LAPTM4B). LAPTM4B plays an important role in promoting the growth and proliferation of tumor cells, it is overexpressed in a variety of solid tumors and serves as a biomarker for tumor therapy. Further analysis showed that Beclin1 interacts with both the N- and C-termini of LAPTM4B and this interaction is independent of Vps34 complex. We demonstrated that Beclin1 competes with Epidermal growth factor receptor (EGFR) for LAPTM4B binding and Beclin1 can repress the LAPTM4B mediated EGFR activation and gastric cancer cell growth. Taken together, our study proposes a role of Beclin1 in repressing gastric cancer through disrupting the oncogenic promoting function of LAPTM4B.

ErbB Receptors and Cancer

The ErbB receptor family, also known as the EGF receptor family or type I receptor family, includes the epidermal growth factor (EGF) receptor (EGFR) or ErbB1/Her1, ErbB2/Her2, ErbB3/Her3, and ErbB4/Her4. Among all RTKs, EGFR was the first RTK identified and the first one linked to cancer. Thus, EGFR has also been the most intensively studied among all RTKs. ErbB receptors are activated after homodimerization or heterodimerization. The ErbB family is unique among the various groups of receptor tyrosine kinases (RTKs) in that ErbB3 has impaired kinase activity, while ErbB2 does not have a direct ligand. Therefore, heterodimerization is an important mechanism that allows the activation of all ErbB receptors in response to ligand stimulation. The activated ErbB receptors bind to many signaling proteins and stimulate the activation of many signaling pathways. The specificity and potency of intracellular signaling pathways are determined by positive and negative regulators, the specific composition of activating ligand(s), receptor dimer components, and the diverse range of proteins that associate with the tyrosine phosphorylated C-terminal domain of the ErbB receptors. ErbB receptors are overexpressed or mutated in many cancers, especially in breast cancer, ovarian cancer, and non-small cell lung cancer. The overexpression and overactivation of ErbB receptors are correlated with poor prognosis, drug resistance, cancer metastasis, and lower survival rate. ErbB receptors, especially EGFR and ErbB2 have been the primary choices as targets for developing cancer therapies.

Insulin Signalling: The Inside Story

Insulin signalling begins with binding to its cell surface insulin receptor (IR), which is a tyrosine kinase. The insulin receptor kinase (IRK) is subsequently autophosphorylated and activated to tyrosine phosphorylate key cellular substrates that are essential for entraining the insulin response. Although IRK activation begins at the cell surface, it is maintained and augmented following internalization into the endosomal system (ENS). The peroxovanadium compounds (pVs) were discovered to activate the IRK in the absence of insulin and lead to a full insulin response. Thus, IRK activation is both necessary and sufficient for insulin signalling. Furthermore, this could be shown to occur with activation of only the endosomal IRK. The mechanism of pV action was shown to be the inhibition of IRK-associated phosphotyrosine phosphatases (PTPs). Our studies showed that the duration and intensity of insulin signalling are modulated within ENS by the recruitment of cellular substrates to ENS; intra-endosomal acidification, which promotes dissociation of insulin from the IRK; an endosomal acidic insulinase, which degrades intra-endosomal insulin; and IRK-associated PTPs, which dephosphorylate and, hence, deactivate the IRK. Therefore, the internalization of IRKs is central to insulin signalling and its regulation.

Vascular Endothelial Cell Growth Factor A Acts via Platelet-Derived Growth Factor Receptor α To Promote Viability of Cells Enduring Hypoxia

Vascular endothelial cell growth factor A (VEGF) is a biologically and therapeutically important growth factor because it promotes angiogenesis in response to hypoxia, which underlies a wide variety of both physiological and pathological settings. We report here that both VEGF receptor 2 (VEGFR2)-positive and -negative cells depended on VEGF to endure hypoxia. VEGF enhanced the viability of platelet-derived growth factor receptor α (PDGFRα)-positive and VEGFR2-negative cells by enabling indirect activation of PDGFRα, thereby reducing the level of p53. We conclude that the breadth of VEGF's influence extends beyond VEGFR-positive cells and propose a plausible mechanistic explanation of this phenomenon.

Epidermal growth factor-stimulated Akt phosphorylation requires clathrin or ErbB2 but not receptor endocytosis

Upon ligand binding, the epidermal growth factor receptor (EGFR) activates signaling and undergoes endocytosis. EGFR signaling leading to Akt activation is impaired by perturbation of clathrin but not by inhibition of internalization through perturbation of dynamin. Clathrin may thus directly regulate receptor signaling at the cell surface.

Epidermal growth factor (EGF) binding to its receptor (EGFR) activates several signaling intermediates, including Akt, leading to control of cell survival and metabolism. Concomitantly, ligand-bound EGFR is incorporated into clathrin-coated pits—membrane structures containing clathrin and other proteins—eventually leading to receptor internalization. Whether clathrin might regulate EGFR signaling at the plasma membrane before vesicle scission is poorly understood. We compared the effect of clathrin perturbation (preventing formation of, or receptor recruitment to, clathrin structures) to that of dynamin2 (allowing formation of clathrin structures but preventing EGFR internalization) under conditions in which EGFR endocytosis is clathrin dependent. Clathrin perturbation by siRNA gene silencing, with the clathrin inhibitor pitstop2, or knocksideways silencing inhibited EGF-simulated Gab1 and Akt phosphorylation in ARPE-19 cells. In contrast, perturbation of dynamin2 with inhibitors or by siRNA gene silencing did not affect EGF-stimulated Gab1 or Akt phosphorylation. EGF stimulation enriched Gab1 and phospho-Gab1 within clathrin structures. ARPE-19 cells have low ErbB2 expression, and overexpression and knockdown experiments revealed that robust ErbB2 expression bypassed the requirement for clathrin for EGF-stimulated Akt phosphorylation. Thus clathrin scaffolds may represent unique plasma membrane signaling microdomains required for signaling by certain receptors, a function that can be separated from vesicle formation.

Endosomal signaling and oncogenesis

The endosomal system provides a route whereby nutrients, viruses, and receptors are internalized. During the course of endocytosis, activated receptors can accumulate within endosomal structures and certain signal-transducing molecules can be recruited to endosomal membranes. In the context of signaling and cancer, they provide platforms within the cell from which signals can be potentiated or attenuated. Regulation of the duration of receptor signaling is a pivotal means of refining growth responses in cells. In cancers, this is often considered in terms of mutations that affect receptor tyrosine kinases and maintain them in hyperactivated states of dimerization and/or phosphorylation. However, disruption to the regulatory control exerted by the assembly of protein complexes within the endosomal network can also contribute to disease among which oncogenesis is characterized in part by dysregulated growth, enhanced cell survival, and changes in the expression of markers of differentiation. In this chapter, we will discuss the role of proteins that regulate in endocytosis as tumor suppressors or oncogenes and how changing the fate of internalized receptors and concomitant endosomal signaling can contribute to cancer.

Targeting C4-demethylating genes in the cholesterol pathway sensitizes cancer cells to EGF receptor inhibitors via increased EGF receptor degradation

Persistent signaling by the oncogenic EGF receptor (EGFR) is a major source of cancer resistance to EGFR targeting. We established that inactivation of 2 sterol biosynthesis pathway genes, SC4MOL (sterol C4-methyl oxidase-like) and its partner, NSDHL (NADP-dependent steroid dehydrogenase-like), sensitized tumor cells to EGFR inhibitors. Bioinformatics modeling of interactions for the sterol pathway genes in eukaryotes allowed us to hypothesize and then extensively validate an unexpected role for SC4MOL and NSDHL in controlling the signaling, vesicular trafficking, and degradation of EGFR and its dimerization partners, ERBB2 and ERBB3. Metabolic block upstream of SC4MOL with ketoconazole or CYP51A1 siRNA rescued cancer cell viability and EGFR degradation. Inactivation of SC4MOL markedly sensitized A431 xenografts to cetuximab, a therapeutic anti-EGFR antibody. Analysis of Nsdhl-deficient Bpa(1H/+) mice confirmed dramatic and selective loss of internalized platelet-derived growth factor receptor in fibroblasts, and reduced activation of EGFR and its effectors in regions of skin lacking NSDHL.

SIGNIFICANCE

This work identifies a critical role for SC4MOL and NSDHL in the regulation of EGFR signaling and endocytic trafficking and suggests novel strategies to increase the potency of EGFR antagonists in tumors.

Targeting C4-demethylating genes in the cholesterol pathway sensitizes cancer cells to EGF receptor inhibitors via increased EGF receptor degradation

Persistent signaling by the oncogenic EGF receptor (EGFR) is a major source of cancer resistance to EGFR targeting. We established that inactivation of 2 sterol biosynthesis pathway genes, SC4MOL (sterol C4-methyl oxidase-like) and its partner, NSDHL (NADP-dependent steroid dehydrogenase-like), sensitized tumor cells to EGFR inhibitors. Bioinformatics modeling of interactions for the sterol pathway genes in eukaryotes allowed us to hypothesize and then extensively validate an unexpected role for SC4MOL and NSDHL in controlling the signaling, vesicular trafficking, and degradation of EGFR and its dimerization partners, ERBB2 and ERBB3. Metabolic block upstream of SC4MOL with ketoconazole or CYP51A1 siRNA rescued cancer cell viability and EGFR degradation. Inactivation of SC4MOL markedly sensitized A431 xenografts to cetuximab, a therapeutic anti-EGFR antibody. Analysis of Nsdhl-deficient Bpa(1H/+) mice confirmed dramatic and selective loss of internalized platelet-derived growth factor receptor in fibroblasts, and reduced activation of EGFR and its effectors in regions of skin lacking NSDHL.

SIGNIFICANCE

This work identifies a critical role for SC4MOL and NSDHL in the regulation of EGFR signaling and endocytic trafficking and suggests novel strategies to increase the potency of EGFR antagonists in tumors.

Differential regulation of transcription factors by location-specific EGF receptor signaling via a spatio-temporal interplay of ERK activation

It is well established that EGFR signals from both the plasma membrane (PM) and endosome (EN). However, very little is known about whether and how the EGFR signals at the PM and EN to differentially regulate various signaling pathways and the physiological outcomes. In this communication, we established a system that allowed the specific activations of EGFR at different cell locations: PM and EN. PM activation of EGFR is achieved by activation of endocytosis-deficient mutant EGFR1010LL/AA stably expressed in CHO cells (CHO-LL/AA cell). EN activation of EGFR is achieved by activating the wild type EGFR stably expressed in CHO cells (CHO-EGFR cell) after its internalization into EN with a previously reported protocol. We showed that both EGFR activations at PM and EN activated ERK to a similar level, but differentially stimulated transcriptional factors c-jun and c-fos. We further showed that EGFR activations at PM and EN resulted in differential spatio-temporal dynamics of phosphorylated ERK which caused the differential activation of two downstream substrates ELK1 and RSK. Finally we showed that EGFR signaling from PM and EN led to different physiological outcomes. CHO-LL/AA cells that only generate PM EGFR signals have a larger cell size and slower proliferation rate than CHO-EGFR cells. We conclude that location-specific EGFR activation differentially regulates cell functions through a spatio-temporal interplay of ERK activation.

Nonmuscle myosin II is required for internalization of the epidermal growth factor receptor and modulation of downstream signaling

Ligand-induced internalization of the epidermal growth factor receptor (EGFR) is an important process for regulating signal transduction, cellular dynamics, and cell-cell communication. Here, we demonstrate that nonmuscle myosin II (NM II) is required for the internalization of the EGFR and to trigger the EGFR-dependent activation of ERK and AKT. The EGFR was identified as a protein that interacts with NM II by co-immunoprecipitation and mass spectrometry analysis. This interaction requires both the regulatory light chain 20 (RLC20) of NM II and the kinase domain of the EGFR. Two paralogs of NM II, NM II-A, and NM II-B can act to internalize the EGFR, depending on the cell type and paralog content of the cell line. Loss (siRNA) or inhibition (25 μm blebbistatin) of NM II attenuates the internalization of the EGFR and impairs EGFR-dependent activation of ERK and AKT. Both internalization of the EGFR and downstream signaling to ERK and AKT can be partially restored in siRNA-treated cells by introduction of wild type (WT) GFP-NM II, but cannot be restored by motor mutant NM II. Taken together, these results suggest that NM II plays a role in the internalization of the EGFR and EGFR-mediated signaling pathways.

Scaffold proteins and immune-cell signalling

Over the past 20 years great progress has been made in defining most of the key signalling pathways that functionally regulate immune cells. Recently, it has become clear that scaffold proteins have a crucial role in regulating many of these signalling cascades. By binding two or more components of a signalling pathway, scaffold proteins can help to localize signalling molecules to a specific part of the cell or to enhance the efficacy of a signalling pathway. Scaffold proteins can also affect the thresholds and the dynamics of signalling reactions by coordinating positive and negative feedback signals. In this Review, we focus on recent progress in the understanding of the function of scaffold proteins in immune cells.

Endosomal trafficking of Src tyrosine kinase

Endosomal trafficking is an essential cellular process involved in the transport of proteins such as integrins, hormone receptors, growth factor receptors, receptor tyrosine kinases, and lipids (e.g. sphingomyelin). Regulation of this process is highly complex and involves Arf GAPs, SNAREs, Rab proteins, Rho GTPases and the actin cytoskeleton. In this article, we focus on the intracellular targeting of the Src family of non-receptor tyrosine kinases (nRTKs), and the role of endosomes in the delivery of nRTKs to the plasma membrane. Furthermore, we discuss the role of the actin cytoskeleton in this process and consider how endosome-regulated intracellular trafficking affects cell signalling.

The complexity of targeting EGFR signalling in cancer: from expression to turnover

The epidermal growth factor receptor (ErbB1 or EGFR) has been found to be altered in a variety of human cancers. A number of agents targeting these receptors, including specific antibodies directed against the ligand-binding domain of the receptor and small molecules that inhibit kinase activity are either in clinical trials or are already approved for clinical treatment. However, identifying patients that are likely to respond to such treatments has been challenging. As a consequence, it still remains important to identify additional alterations of the tumor cell that contribute to the response to EGFR-targeted agents. While EGFR-mediated signalling pathways have been well established, there is still a rather limited understanding of how intracellular protein-protein interactions, ubiquitination, endocytosis and subsequent degradation of EGFR contribute to the determination of sensitivity to EGFR targeting agents and are emerging areas of investigation. This review primarily focuses on the basic signal transduction pathways mediated through activated membrane bound and/or endosomal EGFR and emphasizes the need to co-target additional proteins that function either upstream or downstream of EGFR to improve cancer therapy.

Compartmentalization of growth factor receptor signalling

Spatial and temporal separation of signal transduction pathways often determines the specificity in cellular responses. Recent advances have improved our understanding of how growth factor signalling is influenced by the formation of molecular complexes (signalosomes) in distinct cellular compartments. There has also been new insight into the mechanisms that determine the signalling competence of these complexes and their role in receptor endocytosis, retrograde trafficking in neurons and restricted protein biosynthesis, and many examples have been found where signalosome deregulation leads to disease.

Both proteasomes and lysosomes degrade the activated erythropoietin receptor

Activation of the erythropoietin receptor (EpoR) after Epo binding is very transient because of the rapid activation of strong down-regulation mechanisms that quickly decrease Epo sensitivity of the cells. Among these down-regulation mechanisms, receptor internalization and degradation are probably the most efficient. Here, we show that the Epo receptor was rapidly ubiquitinated after ligand stimulation and that the C-terminal part of the Epo receptor was degraded by the proteasomes. Both ubiquitination and receptor degradation by the proteasomes occurred at the cell surface and required Janus kinase 2 (Jak2) activation. Moreover, Epo-EpoR complexes were rapidly internalized and targeted to the lysosomes for degradation. Neither Jak2 nor proteasome activities were required for internalization. In contrast, Jak2 activation was necessary for lysosome targeting of the Epo-EpoR complexes. Blocking Jak2 with the tyrphostin AG490 led to some recycling of internalized Epo-Epo receptor complexes to the cell surface. Thus, activated Epo receptors appear to be quickly degraded after ubiquitination by 2 proteolytic systems that proceed successively: the proteasomes remove part of the intracellular domain at the cell surface, and the lysosomes degrade the remaining part of the receptor-hormone complex. The efficiency of these processes probably explains the short duration of intracellular signaling activated by Epo.

Sorting nexin 16 regulates EGF receptor trafficking by phosphatidylinositol-3-phosphate interaction with the Phox domain

Sorting nexins (SNXs) containing the Phox (PX) domain are implicated in the regulation of membrane trafficking and sorting processes of epithelial growth factor receptor (EGFR). In this study, we investigated whether SNX16 regulates EGF-induced cell signaling by regulating EGFR trafficking. SNX16 is localized in early and recycling endosomes via its PX domain. Mutation of the PX domain disrupted the association between SNX16 and phosphatidylinositol 3-phosphate [PtdIns(3)P]. Treatment with wortmannin, a PtdIns 3-kinase inhibitor, abolished the endosomal localization of SNX16, suggesting that the intracellular localization of SNX16 is regulated by PtdIns 3-kinase activity. SNX16 was found to associate with EGFR after stimulation with EGF in COS-7 cells. Moreover, overexpression of SNX16 increased the rate of EGF-induced EGFR degradation and inhibited the EGF-induced up-regulation of ERK and serum response element (SRE). In addition, mutation in the PX domain significantly blocked the inhibitory effect of SNX16 on EGF-induced activation of ERK and SRE. From these results, we suggest that SNX16 directs the sorting of EGFR to the endosomal compartment and thus regulates EGF-induced cell signaling.

Internal and external autocrine VEGF/KDR loops regulate survival of subsets of acute leukemia through distinct signaling pathways

Besides being expressed on endothelial cells, vascular endothelial growth factor receptors (VEGFRs) are also functional on subsets of leukemias, resulting in autocrine loops that sustain leukemia migration and proliferation. While recent evidence suggests that VEGF supports hematopoietic stem cell survival via an internal loop, the molecular mechanisms whereby autocrine stimulation of VEGFR-2 (KDR) promotes leukemia growth are not well understood. Here we show on acute myeloid primary leukemias and cell lines that VEGF/KDR autocrine loops operate both internally and externally. First, we demonstrate that KDR is constitutively phosphorylated and located at the nucleus of VEGF-producing leukemias. Treatment with anti-VEGF antibody, which acts externally, blocked KDR nuclear translocation and inhibited nuclear factor κ B (NF-κB; p65 and c-rel) activation. In contrast, a KDR-specific intracellular inhibitor failed to block KDR nuclear translocation, but inhibited the constitutive activation of mitogen activated protein kinase (MAPK)/Erk and the phosphatidylinositol 3-kinase/AKT pathways. Notably, treatment with the anti-VEGF antibody alone had little effect on cell survival, while the internal inhibitor induced leukemia apoptosis, and the 2 drugs produced synergistic effects, together and with chemotherapy, reducing cell survival to a larger extent than either agent alone. Our results demonstrate that internal and external VEGF/KDR autocrine loops regulate leukemia survival via different mechanisms, and suggest that blocking both may have therapeutic potential.

Stimulation of cell proliferation by endosomal epidermal growth factor receptor as revealed through two distinct phases of signaling

Strong evidence indicates that endosome-localized epidermal growth factor receptor (EGFR) plays an important role in cell signaling. However, elimination of endosomal signaling does not attenuate EGF-induced physiological outcomes, arguing against physiological relevance. Recently we established a system to specifically activate endosome-associated EGFR in the absence of any plasma membrane activation of EGFR and showed that endosomal EGFR signaling is sufficient to support cell survival. However, this pure endosomal signaling of EGFR does not stimulate cell proliferation, because EGFR only remained activated for less than 2 h following its stimulation at endosomes, while DNA synthesis generally requires growth factor exposure for 8 h or more. Here we report that the prolonged requirement for EGF to stimulate epithelial cell proliferation can be substituted for with two short pulses of EGF. By combining the two short pulses of EGF stimulation with our previously established method to generate endosomal EGFR signaling, we are able to generate two pulses of endosomal EGFR signaling. In this way, we demonstrated that two pulses of endosomal EGFR signaling are sufficient to stimulate cell proliferation. The first pulse of EGFR signaling induces exit from quiescence into G(1) phase and appears to render cells responsive to subsequent mitogenic stimulus. This second pulse, required several hours later, drives cells through the restriction point of late G(1) and into S phase. We further showed that the two pulses of endosomal EGFR signaling engaged cell cycle machinery the same way as the two pulses of standard EGFR signaling. Moreover, two pulses of endosomal EGFR signaling stimulated downstream signaling cascades in a similar way to the two pulses of standard EGFR activation. The data therefore demonstrate that signals transduced from internalized EGFR, with or without a contribution from the plasma membrane, fully satisfy the physiological requirements for S-phase entry.

Ras pathway signaling on endomembranes

Until recently, the plasma membrane has been considered to be a unique platform from which emanate the signaling events regulating or regulated by Ras and its close relatives. For the past few years, the role of endosomes derived from the plasma membrane as platforms for Ras/mitogen-activated protein kinase signaling has been appreciated. More recently, the cytoplasmic face of the Golgi apparatus and endoplasmic reticulum have been shown to host Ras signaling. The biological implications of compartmentalized signaling are only beginning to emerge.