Intersectin - many facets of a scaffold protein

Intersectin (ITSN) is a multi-domain scaffold protein with a diverse array of functions including regulation of endocytosis, vesicle transport, and activation of various signal transduction pathways. There are two ITSN genes located on chromosomes 21 and 2 encoding for proteins ITSN1 and ITSN2, respectively. Each ITSN gene encodes two major isoforms, ITSN-Long (ITSN-L) and ITSN-Short (ITSN-S), due to alternative splicing. ITSN1 and 2, collectively referred to as ITSN, are implicated in many physiological and pathological processes, such as neuronal maintenance, actin cytoskeletal rearrangement, and tumor progression. ITSN is mis-regulated in many tumors, such as breast, lung, neuroblastomas, and gliomas. Altered expression of ITSN is also found in several neurodegenerative diseases, such as Down Syndrome and Alzheimer's disease. This review summarizes recent studies on ITSN and provides an overview of the function of this important family of scaffold proteins in various biological processes.

Probing RAS Function Using Monobody and NanoBiT Technologies

Missense mutations in the RAS family of oncogenes (HRAS, KRAS, and NRAS) are present in approximately 20% of human cancers, making RAS a valuable therapeutic target (Prior et al., Cancer Res 80:2969-2974, 2020). Although decades of research efforts to develop therapeutic inhibitors of RAS were unsuccessful, there has been success in recent years with the entrance of FDA-approved KRASG12C-specific inhibitors to the clinic (Skoulidis et al., N Engl J Med 384:2371-2381, 2021; Jänne et al., N Engl J Med 387:120-131, 2022). Additionally, KRASG12D-specific inhibitors are presently undergoing clinical trials (Wang et al., J Med Chem 65:3123-3133, 2022). The advent of these allele specific inhibitors has disproved the previous notion that RAS is undruggable. Despite these advancements in RAS-targeted therapeutics, several RAS mutants that frequently arise in cancers remain without tractable drugs. Thus, it is critical to further understand the function and biology of RAS in cells and to develop tools to identify novel therapeutic vulnerabilities for development of anti-RAS therapeutics. To do this, we have exploited the use of monobody (Mb) technology to develop specific protein-based inhibitors of selected RAS isoforms and mutants (Spencer-Smith et al., Nat Chem Biol 13:62-68, 2017; Khan et al., Cell Rep 38:110322, 2022; Wallon et al., Proc Natl Acad Sci USA 119:e2204481119, 2022; Khan et al., Small GTPases 13:114-127, 2021; Khan et al., Oncogene 38:2984-2993, 2019). Herein, we describe our combined use of Mbs and NanoLuc Binary Technology (NanoBiT) to analyze RAS protein-protein interactions and to screen for RAS-binding small molecules in live-cell, high-throughput assays.

Abstract B057: KRAS mutant-specific protein interactions reveal mechanisms in pancreatic cancer tumorigenesis and metabolic regulation

Pancreatic ductal adenocarcinoma (PDAC) is the third most deadly human cancer in the US with a five-year survival rate of 11%. KRAS is mutated in over 95% of PDAC patients and is a key driver of tumorigenesis. Despite the promise of targeted inhibition of the RAF-MEK-ERK MAPK signaling pathway, arguably the most critical KRAS-mediated signaling pathway, clinical trials targeting MEK/ERK signaling as a single-agent therapy have been unsuccessful, indicating the role of additional KRAS-specific signaling pathways. The most frequent KRAS mutations in PDAC are KRAS G12D (40%), KRAS G12V (33%) and KRAS G12R (17%). However, the KRAS G12R mutation is rare in lung and colorectal cancers (<1%), suggesting the presence of KRAS mutant-specific signaling, which remains poorly understood. While mutagenic processes may drive the observed mutation frequency data, many studies have demonstrated that mutant KRAS protein signaling drives the overall observed mutational frequencies. In agreement with this observation, the KRAS Q61L mutant is predicted to occur in PDAC but is rarely detected in the patient population. Therein, we hypothesize that mutation-specific signaling promotes tumorigenesis and that determination of the KRAS mutant-specific interactomes that promote pancreatic tumorigenesis in KRAS G12R yet hinder oncogenic fitness in KRAS Q61L will provide insight into the development of KRAS mutation-selective therapies in PDAC. Thus, we used doxycycline-inducible KRAS constructs combined with BioID proximity labeling to determine the mutant-selective interactomes of four KRAS mutant proteins in an isogenic immortalized pancreatic cell line. While we detected significant overlap in effector signaling, numerous mutant-selective differences were detected, including pathways regulating endocytosis and autophagy. Interestingly, the PDAC tumor microenvironment has been shown to have limited nutrient availability, which promotes macropinocytosis, the nonselective uptake of proteins and molecules from extracellular spaces, and autophagy, a mode of cellular recycling, to promote tumor proliferation. To replicate this environment in cell culture, we utilized a minimal glucose medium supplemented with albumin, a large protein that is absorbed via macropinocytosis. We show that this altered cell culture medium preferentially drives increased macropinocytosis and resistance to MEK MAPK and autophagy inhibition in KRAS G12R-mutant PDAC. Furthermore, while KRAS G12R PDAC cell lines continue to proliferate in the absence of glucose, many KRAS G12D mutant PDAC cell lines fail to sustain proliferation. To determine alternative potential therapeutic vulnerabilities, we have performed an RNA sequencing screen in high and low glucose medium, which has exposed an increase in receptor tyrosine kinase signaling and a reprogramming of metabolic processes in the tricarboxylic acid cycle. These studies provide a rationale for the limited success of MEK/ERK therapies in the clinic and we propose novel treatment strategies for KRAS G12R PDAC patients with elevated macropinocytosis.

Citation Format: Guy Aaron Hobbs, Rachel Burge, Amanda Linke, Kamala Sundararaj, John P. O'Bryan. KRAS mutant-specific protein interactions reveal mechanisms in pancreatic cancer tumorigenesis and metabolic regulation [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B057.

Identification of the nucleotide-free state as a therapeutic vulnerability for inhibition of selected oncogenic RAS mutants

RAS guanosine triphosphatases (GTPases) are mutated in nearly 20% of human tumors, making them an attractive therapeutic target. Following our discovery that nucleotide-free RAS (apo RAS) regulates cell signaling, we selectively target this state as an approach to inhibit RAS function. Here, we describe the R15 monobody that exclusively binds the apo state of all three RAS isoforms in vitro, regardless of the mutation status, and captures RAS in the apo state in cells. R15 inhibits the signaling and transforming activity of a subset of RAS mutants with elevated intrinsic nucleotide exchange rates (i.e., fast exchange mutants). Intracellular expression of R15 reduces the tumor-forming capacity of cancer cell lines driven by select RAS mutants and KRAS(G12D)-mutant patient-derived xenografts (PDXs). Thus, our approach establishes an opportunity to selectively inhibit a subset of RAS mutants by targeting the apo state with drug-like molecules.

Khan et al. develop a high-affinity monobody to nucleotide-free RAS that, when expressed intracellularly, inhibits oncogenic RAS-mediated signaling and tumorigenesis. This study reveals the feasibility of targeting the nucleotide-free state to inhibit tumors driven by oncogenic RAS mutants that possess elevated nucleotide exchange activity.

Neuroblastoma differentiation in vivo excludes cranial tumors

Neuroblastoma (NB), the most common cancer in the first year of life, presents almost exclusively in the trunk. To understand why an early-onset cancer would have such a specific localization, we xenotransplanted human NB cells into discrete neural crest (NC) streams in zebrafish embryos. Here, we demonstrate that human NB cells remain in an undifferentiated, tumorigenic state when comigrating posteriorly with NC cells but, upon comigration into the head, differentiate into neurons and exhibit decreased survival. Furthermore, we demonstrate that this in vivo differentiation requires retinoic acid and brain-derived neurotrophic factor signaling from the microenvironment, as well as cell-autonomous intersectin-1-dependent phosphoinositide 3-kinase-mediated signaling, likely via Akt kinase activation. Our findings suggest a microenvironment-driven explanation for NB's trunk-biased localization and highlight the potential for induced differentiation to promote NB resolution in vivo.

Targeting the "undruggable" RAS with biologics

RAS proteins represent critical drivers of tumor development and thus are the focus of intense efforts to pharmacologically inhibit these proteins in human cancer. Although recent success has been attained in developing clinically efficacious inhibitors to KRASG12C, there remains a critical need for developing approaches to inhibit additional mutant RAS proteins. A number of anti-RAS biologics have been developed which reveal novel and potentially therapeutically targetable vulnerabilities in oncogenic RAS. This review will discuss the growing field of anti-RAS biologics and potential development of these reagents into new anti-RAS therapies.

The P-type ATPase transporter ATP7A promotes angiogenesis by limiting autophagic degradation of VEGFR2

VEGFR2 (KDR/Flk1) signaling in endothelial cells (ECs) plays a central role in angiogenesis. The P-type ATPase transporter ATP7A regulates copper homeostasis, and its role in VEGFR2 signaling and angiogenesis is entirely unknown. Here, we describe the unexpected crosstalk between the Copper transporter ATP7A, autophagy, and VEGFR2 degradation. The functional significance of this Copper transporter was demonstrated by the finding that inducible EC-specific ATP7A deficient mice or ATP7A-dysfunctional ATP7Amut mice showed impaired post-ischemic neovascularization. In ECs, loss of ATP7A inhibited VEGF-induced VEGFR2 signaling and angiogenic responses, in part by promoting ligand-induced VEGFR2 protein degradation. Mechanistically, VEGF stimulated ATP7A translocation from the trans-Golgi network to the plasma membrane where it bound to VEGFR2, which prevented autophagy-mediated lysosomal VEGFR2 degradation by inhibiting autophagic cargo/adapter p62/SQSTM1 binding to ubiquitinated VEGFR2. Enhanced autophagy flux due to ATP7A dysfunction in vivo was confirmed by autophagy reporter CAG-ATP7Amut -RFP-EGFP-LC3 transgenic mice. In summary, our study uncovers a novel function of ATP7A to limit autophagy-mediated degradation of VEGFR2, thereby promoting VEGFR2 signaling and angiogenesis, which restores perfusion recovery and neovascularization. Thus, endothelial ATP7A is identified as a potential therapeutic target for treatment of ischemic cardiovascular diseases.

Selective and noncovalent targeting of RAS mutants for inhibition and degradation

Activating mutants of RAS are commonly found in human cancers, but to date selective targeting of RAS in the clinic has been limited to KRAS(G12C) through covalent inhibitors. Here, we report a monobody, termed 12VC1, that recognizes the active state of both KRAS(G12V) and KRAS(G12C) up to 400-times more tightly than wild-type KRAS. The crystal structures reveal that 12VC1 recognizes the mutations through a shallow pocket, and 12VC1 competes against RAS-effector interaction. When expressed intracellularly, 12VC1 potently inhibits ERK activation and the proliferation of RAS-driven cancer cell lines in vitro and in mouse xenograft models. 12VC1 fused to VHL selectively degrades the KRAS mutants and provides more extended suppression of mutant RAS activity than inhibition by 12VC1 alone. These results demonstrate the feasibility of selective targeting and degradation of KRAS mutants in the active state with noncovalent reagents and provide a starting point for designing noncovalent therapeutics against oncogenic RAS mutants.

Probing RAS Function with Monobodies

RAS is frequently mutated in human cancers with nearly 20% of all cancers harboring mutations in one of three RAS isoforms (KRAS, HRAS, or NRAS). Furthermore, RAS proteins are critical oncogenic drivers of tumorigenesis. As such, RAS has been a prime focus for development of targeted cancer therapeutics. Although RAS is viewed by many as undruggable, the recent development of allele-specific covalent inhibitors to KRAS(G12C) has provided significant hope for the eventual pharmacological inhibition of RAS (Ostrem et al., Nature 503(7477):548-551, 2013; Patricelli et al., Cancer Discov 6(3):316-329, 2016; Janes et al., Cell 172(3):578-589.e17, 2018; Canon et al., Nature 575(7781):217-223, 2019; Hallin et al., Cancer Discov 10(1):54-71, 2020). Indeed, these (G12C)-specific inhibitors have elicited promising responses in early phase clinical trials (Canon et al., Nature 575(7781):217-223, 2019; Hallin et al., Cancer Discov 10(1):54-71, 2020). Despite this success in pharmacologically targeting KRAS(G12C), the remaining RAS mutants lack readily tractable chemistries for development of covalent inhibitors. Thus, alternative approaches are needed to develop broadly efficacious RAS inhibitors. We have utilized Monobody (Mb) technology to identify vulnerabilities in RAS that can potentially be exploited for development of novel RAS inhibitors. Here, we describe the methods used to isolate RAS-specific Mbs and to define their inhibitory activity.

Caveolin-1 stabilizes ATP7A, a copper transporter for extracellular SOD, in vascular tissue to maintain endothelial function

Caveolin-1 (Cav-1) is a scaffolding protein and a major component of caveolae/lipid rafts. Previous reports have shown that endothelial dysfunction in Cav-1-deficient (Cav-1-/-) mice is mediated by elevated oxidative stress through endothelial nitric oxide synthase (eNOS) uncoupling and increased NADPH oxidase. Oxidant stress is the net balance of oxidant generation and scavenging, and the role of Cav-1 as a regulator of antioxidant enzymes in vascular tissue is poorly understood. Extracellular SOD (SOD3) is a copper (Cu)-containing enzyme that is secreted from vascular smooth muscle cells/fibroblasts and subsequently binds to the endothelial cells surface, where it scavenges extracellular [Formula: see text] and preserves endothelial function. SOD3 activity is dependent on Cu, supplied by the Cu transporter ATP7A, but whether Cav-1 regulates the ATP7A-SOD3 axis and its role in oxidative stress-mediated vascular dysfunction has not been studied. Here we show that the activity of SOD3, but not SOD1, was significantly decreased in Cav-1-/- vessels, which was rescued by re-expression of Cav-1 or Cu supplementation. Loss of Cav-1 reduced ATP7A protein, but not mRNA, and this was mediated by ubiquitination of ATP7A and proteasomal degradation. ATP7A bound to Cav-1 and was colocalized with SOD3 in caveolae/lipid rafts or perinucleus in vascular tissues or cells. Impaired endothelium-dependent vasorelaxation in Cav-1-/- mice was rescued by gene transfer of SOD3 or by ATP7A-overexpressing transgenic mice. These data reveal an unexpected role of Cav-1 in stabilizing ATP7A protein expression by preventing its ubiquitination and proteasomal degradation, thereby increasing SOD3 activity, which in turn protects against vascular oxidative stress-mediated endothelial dysfunction.

Biology, pathology, and therapeutic targeting of RAS

RAS was identified as a human oncogene in the early 1980s and subsequently found to be mutated in nearly 30% of all human cancers. More importantly, RAS plays a central role in driving tumor development and maintenance. Despite decades of effort, there remain no FDA approved drugs that directly inhibit RAS. The prevalence of RAS mutations in cancer and the lack of effective anti-RAS therapies stem from RAS' core role in growth factor signaling, unique structural features, and biochemistry. However, recent advances have brought promising new drugs to clinical trials and shone a ray of hope in the field. Here, we will exposit the details of RAS biology that illustrate its key role in cell signaling and shed light on the difficulties in therapeutically targeting RAS. Furthermore, past and current efforts to develop RAS inhibitors will be discussed in depth.

Abstract B01: Inhibition of RAS signaling and tumorigenesis through targeting vulnerabilities in RAS biochemistry

RAS GTPases are important mediators of oncogenesis in humans. However, pharmacologic inhibition of RAS has proved challenging. We have taken a novel approach to discover vulnerabilities in RAS that can be exploited to inhibit RAS signaling and tumorigenesis. Monobodies are single-domain synthetic binding proteins that achieve levels of affinity and selectivity similar to antibodies but are insensitive to the redox potential of their environment. We have developed a panel of monobodies that target distinct vulnerabilities in RAS. We recently described the activity of the NS1 monobody at inhibiting RAS signaling. NS1 binds to the α4-α5 allosteric lobe of RAS to prevent RAS dimerization and nanoclustering. When introduced into cells as a genetically encoded reagent, NS1 inhibits RAS signaling and oncogenic transformation in vitro through blocking the ability of RAS to self-associate and stimulate the dimerization and activation of RAF. Using a chemically regulated NS1 expression system, we demonstrate that targeting the α4-α5 dimerization interface with NS1 inhibits KRAS-driven tumors in vivo. In addition to NS1, we will discuss our results with monobodies targeting additional aspects of RAS biochemistry. Our results establish the importance of RAS dimerization through the α4-α5 region in mediating RAS signaling and oncogenic transformation of cells both in vitro and in vivo and reveal additional vulnerabilities in RAS that may be targeted to inhibit RAS-driven tumors.

Citation Format: Imran Khan, Russell Spencer-Smith, Kevin Teng, Akiko Koide, Shohei Koide, John P. O'Bryan. Inhibition of RAS signaling and tumorigenesis through targeting vulnerabilities in RAS biochemistry [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr B01.

Therapeutic targeting of RAS: New hope for drugging the "undruggable"

RAS is the most frequently mutated oncogene in cancer and a critical driver of oncogenesis. Therapeutic targeting of RAS has been a goal of cancer research for more than 30 years due to its essential role in tumor formation and maintenance. Yet the quest to inhibit this challenging foe has been elusive. Although once considered "undruggable", the struggle to directly inhibit RAS has seen recent success with the development of pharmacological agents that specifically target the KRAS(G12C) mutant protein, which include the first direct RAS inhibitor to gain entry to clinical trials. However, the limited applicability of these inhibitors to G12C-mutant tumors demands further efforts to identify more broadly efficacious RAS inhibitors. Understanding allosteric influences on RAS may open new avenues to inhibit RAS. Here, we provide a brief overview of RAS biology and biochemistry, discuss the allosteric regulation of RAS, and summarize the various approaches to develop RAS inhibitors.

Targeting the α4-α5 dimerization interface of K-RAS inhibits tumor formation in vivo

RAS genes are the most commonly mutated oncogenes in human cancers. Despite tremendous efforts over the past several decades, however, RAS-specific inhibitors remain elusive. Thus, targeting RAS remains a highly sought after goal of cancer research. Previously, we reported a new approach to inhibit RAS-dependent signaling and transformation in vitro through targeting the α4-α5 dimerization interface with a novel RAS-specific monobody, termed NS1. Expression of NS1 inhibits oncogenic K-RAS and H-RAS signaling and transformation in vitro. Here, we evaluated the efficacy of targeting RAS dimerization as an approach to inhibit tumor formation in vivo. Using a doxycycline (DOX) regulated NS1 expression system, we demonstrate that DOX-induced NS1 inhibited oncogenic K-RAS driven tumor growth in vivo. Furthermore, we observed context-specific effects of NS1 on RAS-mediated signaling in 2D vs 3D growth conditions. Finally, our results highlight the potential therapeutic efficacy of targeting the α4-α5 dimerization interface as an approach to inhibit RAS-driven tumors in vivo.

Pharmacological targeting of RAS: Recent success with direct inhibitors

RAS has long been viewed as undruggable due to its lack of deep pockets for binding of small molecule inhibitors. However, recent successes in the development of direct RAS inhibitors suggest that the goal of pharmacological inhibition of RAS in patients may soon be realized. This review will discuss the role of RAS in cancer, the approaches used to develop direct RAS inhibitors, and highlight recent successes in the development of novel RAS inhibitory compounds that target different aspects of RAS biochemistry. In particular, this review will discuss the different properties of RAS that have been targeted by various inhibitors including membrane localization, the different activation states of RAS, effector binding, and nucleotide exchange. In addition, this review will highlight the recent success with mutation-specific inhibitors that exploit the unique biochemistry of the RAS(G12C) mutant. Although this mutation in KRAS accounts for 11% of all KRAS mutations in cancer, it is the most prominent KRAS mutant in lung cancer suggesting that G12C-specific inhibitors may provide a new approach for treating the subset of lung cancer patients harboring this mutant allele. Finally, this review will discuss the involvement of dimerization in RAS function and highlight new approaches to inhibit RAS by specifically interfering with RAS:RAS interaction.

Akt2 (Protein Kinase B Beta) Stabilizes ATP7A, a Copper Transporter for Extracellular Superoxide Dismutase, in Vascular Smooth Muscle: Novel Mechanism to Limit Endothelial Dysfunction in Type 2 Diabetes Mellitus

OBJECTIVE

Copper transporter ATP7A (copper-transporting/ATPase) is required for full activation of SOD3 (extracellular superoxide dismutase), which is secreted from vascular smooth muscle cells (VSMCs) and anchors to endothelial cell surface to preserve endothelial function by scavenging extracellular superoxide. We reported that ATP7A protein expression and SOD3 activity are decreased in insulin-deficient type 1 diabetes mellitus vessels, thereby, inducing superoxide-mediated endothelial dysfunction, which are rescued by insulin treatment. However, it is unknown regarding the mechanism by which insulin increases ATP7A expression in VSMCs and whether ATP7A downregulation is observed in T2DM (type2 diabetes mellitus) mice and human in which insulin-Akt (protein kinase B) pathway is selectively impaired.

APPROACH AND RESULTS

Here we show that ATP7A protein is markedly downregulated in vessels isolated from T2DM patients, as well as those from high-fat diet-induced or db/db T2DM mice. Akt2 (protein kinase B beta) activated by insulin promotes ATP7A stabilization via preventing ubiquitination/degradation as well as translocation to plasma membrane in VSMCs, which contributes to activation of SOD3 that protects against T2DM-induced endothelial dysfunction. Downregulation of ATP7A in T2DM vessels is restored by constitutive active Akt or PTP1B^-/- (protein-tyrosine phosphatase 1B-deficient) T2DM mice, which enhance insulin-Akt signaling. Immunoprecipitation, in vitro kinase assay, and mass spectrometry analysis reveal that insulin stimulates Akt2 binding to ATP7A to induce phosphorylation at Ser1424/1463/1466. Furthermore, SOD3 activity is reduced in Akt2^-/- vessels or VSMCs, which is rescued by ATP7A overexpression.

CONCLUSION

Akt2 plays a critical role in ATP7A protein stabilization and translocation to plasma membrane in VSMCs, which contributes to full activation of vascular SOD3 that protects against endothelial dysfunction in T2DM.

Targeting the α4-α5 interface of RAS results in multiple levels of inhibition

Generation of RAS-targeted therapeutics has long been considered a "holy grail" in cancer research. However, a lack of binding pockets on the surface of RAS and its picomolar affinity for guanine nucleotides have made isolation of inhibitors particularly challenging. We recently described a monobody, termed NS1, that blocks RAS signaling and oncogenic transformation. NS1 binds to the α4-β6-α5 interface of H-RAS and K-RAS thus preventing RAS dimerization and nanoclustering, which in turn prevents RAS-stimulated dimerization and activation of RAF. Interestingly, NS1 reduces interaction of oncogenic K-RAS, but not H-RAS, with RAF and reduces K-RAS plasma membrane localization. Here, we show that these isoform specific effects of NS1 on RAS:RAF are due to the distinct hypervariable regions of RAS isoforms. NS1 inhibited wild type RAS function by reducing RAS GTP levels. These findings reveal that NS1 disrupts RAS signaling through a mechanism that is more complex than simply inhibiting RAS dimerization and nanoclustering.

Direct inhibition of RAS: Quest for the Holy Grail?

RAS GTPases (H-, K-, and N-RAS) are the most frequently mutated oncoprotein family in human cancer. However, the relatively smooth surface architecture of RAS and its picomolar affinity for nucleotide have given rise to the assumption that RAS is an "undruggable" target. Recent advancements in drug screening, molecular modeling, and a greater understanding of RAS function have led to a resurgence in efforts to pharmacologically target this challenging foe. This review focuses on the state of the art of RAS inhibition, the approaches taken to achieve this goal, and the challenges of translating these discoveries into viable therapeutics.

Inhibition of RAS function through targeting an allosteric regulatory site

RAS GTPases are important mediators of oncogenesis in humans. However, pharmacological inhibition of RAS has proved challenging. Here we describe a functionally critical region, located outside the effector lobe of RAS, that can be targeted for inhibition. We developed NS1, a synthetic binding protein (monobody) that bound with high affinity to both GTP- and GDP-bound states of H-RAS and K-RAS but not N-RAS. NS1 potently inhibited growth factor signaling and oncogenic H-RAS- and K-RAS-mediated signaling and transformation but did not block oncogenic N-RAS, BRAF or MEK1. NS1 bound the α4-β6-α5 region of RAS, which disrupted RAS dimerization and nanoclustering and led to blocking of CRAF-BRAF heterodimerization and activation. These results establish the importance of the α4-β6-α5 interface in RAS-mediated signaling and define a previously unrecognized site in RAS for inhibiting RAS function.

Intersectin scaffold proteins and their role in cell signaling and endocytosis

Intersectins (ITSNs) are a family of multi-domain proteins involved in regulation of diverse cellular pathways. These scaffold proteins are well known for regulating endocytosis but also play important roles in cell signaling pathways including kinase regulation and Ras activation. ITSNs participate in several human cancers, such as neuroblastomas and glioblastomas, while their downregulation is associated with lung injury. Alterations in ITSN expression have been found in neurodegenerative diseases such as Down Syndrome and Alzheimer's disease. Binding proteins for ITSNs include endocytic regulatory factors, cytoskeleton related proteins (i.e. actin or dynamin), signaling proteins as well as herpes virus proteins. This review will summarize recent studies on ITSNs, highlighting the importance of these scaffold proteins in the aforementioned processes.

Protein Kinase A (PKA) Phosphorylation of Shp2 Protein Inhibits Its Phosphatase Activity and Modulates Ligand Specificity

Pathological cardiac hypertrophy (an increase in cardiac mass resulting from stress-induced cardiac myocyte growth) is a major factor underlying heart failure. Src homology 2 domain-containing phosphatase (Shp2) is critical for cardiac function because mutations resulting in loss of Shp2 catalytic activity are associated with congenital cardiac defects and hypertrophy. We identified a novel mechanism of Shp2 inhibition that may promote cardiac hypertrophy. We demonstrate that Shp2 is a component of the protein kinase A anchoring protein (AKAP)-Lbc complex. AKAP-Lbc facilitates PKA phosphorylation of Shp2, which inhibits Shp2 phosphatase activity. We identified two key amino acids in Shp2 that are phosphorylated by PKA. Thr-73 contributes a helix cap to helix αB within the N-terminal SH2 domain of Shp2, whereas Ser-189 occupies an equivalent position within the C-terminal SH2 domain. Utilizing double mutant PKA phosphodeficient (T73A/S189A) and phosphomimetic (T73D/S189D) constructs, in vitro binding assays, and phosphatase activity assays, we demonstrate that phosphorylation of these residues disrupts Shp2 interaction with tyrosine-phosphorylated ligands and inhibits its protein-tyrosine phosphatase activity. Overall, our data indicate that AKAP-Lbc integrates PKA and Shp2 signaling in the heart and that AKAP-Lbc-associated Shp2 activity is reduced in hypertrophic hearts in response to chronic β-adrenergic stimulation and PKA activation. Therefore, although induction of cardiac hypertrophy is a multifaceted process, inhibition of Shp2 activity through AKAP-Lbc-anchored PKA is a previously unrecognized mechanism that may promote this compensatory response.

UCR1C is a novel activator of phosphodiesterase 4 (PDE4) long isoforms and attenuates cardiomyocyte hypertrophy

Hypertrophy increases the risk of heart failure and arrhythmia. Prevention or reversal of the maladaptive hypertrophic phenotype has thus been proposed to treat heart failure. Chronic β-adrenergic receptor (β-AR) stimulation induces cardiomyocyte hypertrophy by elevating 3',5'-cyclic adenosine monophosphate (cAMP) levels and activating downstream effectors such protein kinase A (PKA). Conversely, hydrolysis of cAMP by phosphodiesterases (PDEs) spatiotemporally restricts cAMP signaling. Here, we demonstrate that PDE4, but not PDE3, is critical in regulating cardiomyocyte hypertrophy, and may represent a potential target for preventing maladaptive hypertrophy. We identify a sequence within the upstream conserved region 1 of PDE4D, termed UCR1C, as a novel activator of PDE4 long isoforms. UCR1C activates PDE4 in complex with A-kinase anchoring protein (AKAP)-Lbc resulting in decreased PKA signaling facilitated by AKAP-Lbc. Expression of UCR1C in cardiomyocytes inhibits hypertrophy in response to chronic β-AR stimulation. This effect is partially due to inhibition of nuclear PKA activity, which decreases phosphorylation of the transcription factor cAMP response element-binding protein (CREB). In conclusion, PDE4 activation by UCR1C attenuates cardiomyocyte hypertrophy by specifically inhibiting nuclear PKA activity.

Phosphatidylinositol 3-kinase, class 2 beta (PI3KC2β) isoform contributes to neuroblastoma tumorigenesis

Phosphatidylinositol 3-kinases (PI3Ks) play important roles in human tumorigenesis. Activation of the PI3K target AKT is frequent in neuroblastoma (NB) and correlates with poor prognosis. PI3K pan-inhibitors reduce NB tumor formation but present severe toxicity, which limits their therapeutic potential. Therefore, defining the importance of specific PI3K isoforms may aid in developing more effective therapeutic strategies. We previously demonstrated that PI3K Class IIβ (PI3KC2β) and its regulator intersectin 1 (ITSN1) are highly expressed in primary NB tumors and cell lines. Silencing ITSN1 dramatically reduced the tumorigenic potential of NB cells. Interestingly, overexpression of PI3KC2β rescued the anchorage-independent growth of ITSN1-silenced cells suggesting that PI3KC2β mediates ITSN1's function in NB cells. To address the importance of PI3KC2β in NBs, we generated PI3KC2β-silenced lines and examined their biologic activity. Herein, we demonstrate that PI3KC2β-silencing inhibits early stages of NB tumorigenic growth. We also show that loss of endogenous PI3KC2β or ITSN1 reduces AKT activation but does not impact ERK-MAPK activation. These data reveal a novel role for PI3KC2β in human NB tumorigenesis.

Abstract A17: Differential role of Ras isoforms in regulating PI3KC2β: Novel role for nucleotide-free H-Ras in cell signaling

Ras proteins, like all GTPases, cycle through an active GTP-bound state and an inactive GDP-bound state that arises through the intrinsic GTPase activity of the protein. Activation of Ras requires the activity of GEFs which promote the release of GDP leading to the subsequent binding of GTP due to the relatively higher levels of GTP in cells. Given the high affinity of nucleotide-free Ras (nf-Ras) for nucleotides in vitro, it has been proposed that nf-Ras is a transient intermediate in vivo. However, we have discovered a potentially new role for nf-H-Ras in the regulation of PI3KC2β (1). Previously, we discovered that the scaffold protein intersectin 1 (ITSN1) interacts with and activates H-Ras on endocytic vesicles (2). In addition, ITSN1 binds and activates PI3KC2β on a similar population of vesicles (3). PI3KC2β, like Class I PI3Ks, contains a Ras binding domain (RBD) although PI3KC2β does not interact with RasGTP or RasGDP (4). Our recent studies, however, indicate that nf-H-Ras binds the PI3KC2β RBD and inhibits PI3KC2β activity in vitro (1). Furthermore, the complex of nf-H-Ras with PI3KC2β RBD was not disrupted by addition of 1mM GTP or GDP suggesting that this complex may exist in the cellular milieu of high guanine nucleotides (1). Consistent with this in vitro data, PI3KC2β preferentially interacts with H-Ras dominant-negative (Ras17N)>H-Ras WT>activated H-Ras (Ras61L or 12V) in cells (1). To determine whether these results can be extended to other GTPases, we have examined the interaction of PI3KC2β with K-Ras and various Ras-related GTPases (see poster by RS Smith et. al.). Interestingly, PI3KC2β preferentially interacts with K-Ras12V>WT> dominant-negative (K-Ras15A). Furthermore, K-Ras localizes PI3KC2β to the plasma membrane whereas H-Ras localizes PI3KC2β to the perinuclear region. Thus, different Ras GTPases interact with PI3KC2β in distinct manners. We will present data on the contrasting role of K-Ras and H-Ras on regulation of PI3KC2β. Our findings suggest that PI3KC2β stabilizes nf-H-Ras and that interaction of H-Ras and PI3KC2β mutually inhibit one another. However, the role of K-Ras in regulation of PI3KC2β appears distinct. These findings have important implications for Ras-dependent tumorigenesis.

References:

1. Wong, K. A., A. Russo, X. Wang, Y.-J. Chen, A. Lavie, and O. B. J. P. 2012. A new dimension to Ras function: a novel role for nucleotide-free Ras in Class II phosphatidylinositol 3-kinase beta (PI3K-C2β) regulation. PLoS ONE 7:e45360.

2. Mohney, R. P., M. Das, T. G. Bivona, R. Hanes, A. G. Adams, M. R. Philips, and J. P. O'Bryan. 2003. Intersectin activates Ras but stimulates transcription through an independent pathway involving JNK. J Biol Chem 278:47038-45.

3. Das, M., E. Scappini, N. P. Martin, K. A. Wong, S. Dunn, Y. J. Chen, S. L. Miller, J. Domin, and J. P. O'Bryan. 2007. Regulation of neuron survival through an intersectin-phosphoinositide 3'-kinase C2beta-AKT pathway. Mol Cell Biol 27:7906-17.

4. Arcaro, A., S. Volinia, M. J. Zvelebil, R. Stein, S. J. Watton, M. J. Layton, I. Gout, K. Ahmadi, J. Downward, and M. D. Waterfield. 1998. Human phosphoinositide 3-kinase C2beta, the role of calcium and the C2 domain in enzyme activity. J Biol Chem 273:33082-90.

Citation Format: Katy A. Wong, Russell Spencer Smith, Angela Russo, Arnon Lavie, John P. O'Bryan. Differential role of Ras isoforms in regulating PI3KC2β: Novel role for nucleotide-free H-Ras in cell signaling. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr A17. doi: 10.1158/1557-3125.RASONC14-A17

Abstract B57: Distinct interactions of Ras superfamily members with common protein targets

Ras superfamily members act as molecular switches to many key signaling pathways, including proliferation cytoskeletal integrity, differentiation and cellular cargo transport. The Ras subfamily, consisting of KRas, NRas and HRas are the most widely studied Ras-family members due to the high instance of Ras activating mutation in human cancers. Rab subfamily members are important regulators of endocytosis, consisting of Rab5 (early endosome), Rab7 (recycling endosome) and Rab11 (late endosome). We have discovered that common binding partners exist between these Ras subfamilies, resulting in distinct interactions and regulatory effects. Two such commonalities are scaffold protein ITSN1 and lipid kinase PI3KC2β. There is evidence that the learning disabilities associated with Down syndrome may be somewhat attributed to deregulated vesicular transport in the brain. DS patients also have a 10-20 fold increased risk of developing acute myeloid leukemia (AML). ITSN1, a regulator of endocytosis and activator of the oncogenic AKT pathway (1), is overexpressed in Down syndrome patients. PI3KC2β is a member of the PI3K family of lipid kinases, which phosphorylate phosphatidylinositol leading to AKT activation. Treatment of neuroblastoma and AML cell lines with PI3KC2β-specific inhibitors greatly decreases proliferation (2) and silencing PI3KC2β in neuroblastoma cell lines results in a 50% reduction in tumor volume in xenograft models. Our previous work demonstrated that nucleotide-free HRas preferentially binds to PI3KC2β, inhibiting its activity (see poster by JP O'Bryan; 3). The addition of ITSN1 disrupts this interaction, presumably restoring the active state of both proteins. In contrast, constitutively activated Rab5 and KRas preferentially interact with PI3KC2β. Interestingly KRas sequesters PI3KC2β to the plasma membrane. The current dogma states that Ras proteins must be GTP-loaded and associated with the plasma membrane to activate their classic effectors suggesting that KRas would be active in this capacity and may therefore positively regulate PI3KC2β activity. Here we present evidence of three Ras superfamily members interacting in distinct ways with the same target protein and provide support for their related yet contrasting roles in disease states.

Citation Format: Russell Spencer Smith, Katy A. Wong, Angela Russo, John P. O'Bryan. Distinct interactions of Ras superfamily members with common protein targets. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr B57. doi: 10.1158/1557-3125.RASONC14-B57

Receptor tyrosine kinase ubiquitylation involves the dynamic regulation of Cbl-Spry2 by intersectin 1 and the Shp2 tyrosine phosphatase

Ubiquitylation of receptor tyrosine kinases (RTKs) regulates their trafficking and lysosomal degradation. The multidomain scaffolding protein intersectin 1 (ITSN1) is an important regulator of this process. ITSN1 stimulates ubiquitylation of the epidermal growth factor receptor (EGFR) through enhancing the activity of the Cbl E3 ubiquitin ligase. However, the precise mechanism through which ITSN1 enhances Cbl activity is unclear. Here, we demonstrate that ITSN1 interacts with and recruits the Shp2 tyrosine phosphatase to Spry2 to enhance its dephosphorylation, thereby disrupting the inhibitory effect of Spry2 on Cbl and enhancing EGFR ubiquitylation. In contrast, expression of a catalytically inactive Shp2 mutant reversed the effect of ITSN1 on Spry2 dephosphorylation and decreased Cbl-mediated EGFR ubiquitylation. In addition, disruption of ITSN1 binding to Spry2 through point mutation of the Pro-rich ITSN1 binding site in Spry2 resulted in decreased Shp2-Spry2 interaction and enhanced Spry2 tyrosine phosphorylation. This study demonstrates that ITSN1 enhances Cbl activity, in part, by modulating the interaction of Cbl with Spry2 through recruitment of Shp2 phosphatase to the Cbl-Spry2 complex. These findings reveal a new level of complexity in the regulation of RTKs by Cbl through ITSN1 binding with Shp2 and Spry2.

A new dimension to Ras function: a novel role for nucleotide-free Ras in Class II phosphatidylinositol 3-kinase beta (PI3KC2β) regulation

The intersectin 1 (ITSN1) scaffold stimulates Ras activation on endocytic vesicles without activating classic Ras effectors. The identification of Class II phosphatidylinositol 3-kinase beta, PI3KC2β, as an ITSN1 target on vesicles and the presence of a Ras binding domain (RBD) in PI3KC2β suggests a role for Ras in PI3KC2β activation. Here, we demonstrate that nucleotide-free Ras negatively regulates PI3KC2β activity. PI3KC2β preferentially interacts in vivo with dominant-negative (DN) Ras, which possesses a low affinity for nucleotides. PI3KC2β interaction with DN Ras is disrupted by switch 1 domain mutations in Ras as well as RBD mutations in PI3KC2β. Using purified proteins, we demonstrate that the PI3KC2β-RBD directly binds nucleotide-free Ras in vitro and that this interaction is not disrupted by nucleotide addition. Finally, nucleotide-free Ras but not GTP-loaded Ras inhibits PI3KC2β lipid kinase activity in vitro. Our findings indicate that PI3KC2β interacts with and is regulated by nucleotide-free Ras. These data suggest a novel role for nucleotide-free Ras in cell signaling in which PI3KC2β stabilizes nucleotide-free Ras and that interaction of Ras and PI3KC2β mutually inhibit one another.

Intersectin (ITSN) family of scaffolds function as molecular hubs in protein interaction networks

Members of the intersectin (ITSN) family of scaffold proteins consist of multiple modular domains, each with distinct ligand preferences. Although ITSNs were initially implicated in the regulation of endocytosis, subsequent studies have revealed a more complex role for these scaffold proteins in regulation of additional biochemical pathways. In this study, we performed a high throughput yeast two-hybrid screen to identify additional pathways regulated by these scaffolds. Although several known ITSN binding partners were identified, we isolated more than 100 new targets for the two mammalian ITSN proteins, ITSN1 and ITSN2. We present the characterization of several of these new targets which implicate ITSNs in the regulation of the Rab and Arf GTPase pathways as well as regulation of the disrupted in schizophrenia 1 (DISC1) interactome. In addition, we demonstrate that ITSN proteins form homomeric and heteromeric complexes with each other revealing an added level of complexity in the function of these evolutionarily conserved scaffolds.

Abstract 3249: Intersectin 1 is required for neuroblastoma tumorigenesis

Neuroblastoma (NB) is a malignant tumor deriving from the neural crest and is responsible for 13% of cancer-related deaths in children. Phosphoinositide 3-kinases (PI3Ks) play a central role in NB tumorigenesis. PI3Ks activate AKT, which regulates survival and stabilizes the MYCN oncogene. Both MYCN and AKT are markers for poor prognosis in NBs. Inhibiting PI3Ks with pan-inhibitors blocks malignant progression in vivo; however, these inhibitors have high toxicity that limits their use in NB treatment. Though there are three classes of PI3Ks, only Class I isoforms have been implicated in NB tumorigenesis. We have previously found that the scaffold protein intersectin 1 (ITSN1) interacts with PI3KC2α and regulates its activation as well as AKT activation in neuronal cell lines. In addition, ITSN1 overexpression is transforming in rodent fibroblasts. Thus, we examined the role of the ITSN1-PI3KC2α pathway in human NB tumorigenesis. ITSN1 and PI3KC2α are highly expressed in primary human neuroblastoma tumors of varying stage and MYCN amplification status. In addition, both ITSN1 and PI3KC2α are expressed in NB tumor cell lines derived from high grade NB tumors. Silencing ITSN1 dramatically inhibited the anchorage-independent growth of NB tumor cells in vitro and tumor formation in xenograft assays in vivo independent of MYCN status. Conversely, heterologous overexpression of ITSN1 increased the formation of soft agar colonies. These results suggest that ITSN1 functions as an oncogene in NB cells. Furthermore, PI3KC2α overexpression restores anchorage-independent growth of ITSN1-depleted NB cells demonstrating that PI3KC2α mediates ITSN1's tumorigenic properties. This study reveals a new pathway, i.e., ITSN1-PI3KC2α, involved in regulating the pathogenesis of NBs and suggests that this pathway may represent a new target for therapeutic intervention in the treatment of NB patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3249. doi:1538-7445.AM2012-3249

Cardiac-specific expression of the tetracycline transactivator confers increased heart function and survival following ischemia reperfusion injury

Mice expressing the tetracycline transactivator (tTA) transcription factor driven by the rat α-myosin heavy chain promoter (α-MHC-tTA) are widely used to dissect the molecular mechanisms involved in cardiac development and disease. However, these α-MHC-tTA mice exhibit a gain-of-function phenotype consisting of robust protection against ischemia/reperfusion injury in both in vitro and in vivo models in the absence of associated cardiac hypertrophy or remodeling. Cardiac function, as assessed by echocardiography, did not differ between α-MHC-tTA and control animals, and there were no noticeable differences observed between the two groups in HW/TL ratio or LV end-diastolic and end-systolic dimensions. Protection against ischemia/reperfusion injury was assessed using isolated perfused hearts where α-MHC-tTA mice had robust protection against ischemia/reperfusion injury which was not blocked by pharmacological inhibition of PI3Ks with LY294002. Furthermore, α-MHC-tTA mice subjected to coronary artery ligation exhibited significantly reduced infarct size compared to control animals. Our findings reveal that α-MHC-tTA transgenic mice exhibit a gain-of-function phenotype consisting of robust protection against ischemia/reperfusion injury similar to cardiac pre- and post-conditioning effects. However, in contrast to classical pre- and post-conditioning, the α-MHC-tTA phenotype is not inhibited by the classic preconditioning inhibitor LY294002 suggesting involvement of a non-PI3K-AKT signaling pathway in this phenotype. Thus, further study of the α-MHC-tTA model may reveal novel molecular targets for therapeutic intervention during ischemic injury.

Cooperative role of caveolin-1 and C-terminal Src kinase binding protein in C-terminal Src kinase-mediated negative regulation of c-Src

In the present study, we assessed the cooperative roles of C-terminal Src kinase (Csk) binding protein (Cbp) and Caveolin-1 (Cav-1) in the mechanism of Src family tyrosine kinase (SFK) inhibition by Csk. SFKs are inactivated by phosphorylation of their C-terminal tyrosine by Csk. Whereas SFKs are membrane-associated, Csk is a cytoplasmic protein and therefore requires membrane adaptors such as Cbp or Cav-1 for recruitment to the plasma membrane to mediate SFK inhibition. To determine the specific role of Cav-1 and Cbp in SFK inhibition, we measured c-Src activity in the absence of each membrane adaptor. It is noteworthy that in lungs and fibroblasts from Cav-1(-/-) mice, we observed increased expression of Cbp compared with wild-type (WT) controls. However, both c-Src activity and Csk localization at the membrane were similar between Cav-1(-/-) fibroblasts and WT cells. Likewise, Cbp depletion by small interfering RNA (siRNA) treatment of WT cells had no effect on basal c-Src activity, but it increased the phosphorylation state of Cav-1. Immunoprecipitation then confirmed increased association of Csk with phosphomimicking Cav-1. Knockdown of Cbp by siRNA in Cav-1(-/-) cells revealed increased basal c-Src activity, and re-expression of WT Cav-1 in the same cells reduced basal c-Src activity. Taken together, these results indicate that Cav-1 and Cbp cooperatively regulate c-Src activity by recruiting Csk to the membrane where it phosphorylates c-Src inhibitory tyrosine 529. Furthermore, when either Cav-1 or Cbp expression is reduced or absent, there is a compensatory increase in the phosphorylation state or expression level of the other membrane-associated Csk adaptor to maintain SFK inhibition.

Bimolecular fluorescence complementation

Defining the subcellular distribution of signaling complexes is imperative to understanding the output from that complex. Conventional methods such as immunoprecipitation do not provide information on the spatial localization of complexes. In contrast, BiFC monitors the interaction and subcellular compartmentalization of protein complexes. In this method, a fluororescent protein is split into amino- and carboxy-terminal non-fluorescent fragments which are then fused to two proteins of interest. Interaction of the proteins results in reconstitution of the fluorophore (Figure 1). A limitation of BiFC is that once the fragmented fluorophore is reconstituted the complex is irreversible. This limitation is advantageous in detecting transient or weak interactions, but precludes a kinetic analysis of complex dynamics. An additional caveat is that the reconstituted flourophore requires 30min to mature and fluoresce, again precluding the observation of real time interactions. BiFC is a specific example of the protein fragment complementation assay (PCA) which employs reporter proteins such as green fluorescent protein variants (BiFC), dihydrofolate reductase, b-lactamase, and luciferase to measure protein:protein interactions. Alternative methods to study protein:protein interactions in cells include fluorescence co-localization and Förster resonance energy transfer (FRET). For co-localization, two proteins are individually tagged either directly with a fluorophore or by indirect immunofluorescence. However, this approach leads to high background of non-interacting proteins making it difficult to interpret co-localization data. In addition, due to the limits of resolution of confocal microscopy, two proteins may appear co-localized without necessarily interacting. With BiFC, fluorescence is only observed when the two proteins of interest interact. FRET is another excellent method for studying protein:protein interactions, but can be technically challenging. FRET experiments require the donor and acceptor to be of similar brightness and stoichiometry in the cell. In addition, one must account for bleed through of the donor into the acceptor channel and vice versa. Unlike FRET, BiFC has little background fluorescence, little post processing of image data, does not require high overexpression, and can detect weak or transient interactions. Bioluminescence resonance energy transfer (BRET) is a method similar to FRET except the donor is an enzyme (e.g. luciferase) that catalyzes a substrate to become bioluminescent thereby exciting an acceptor. BRET lacks the technical problems of bleed through and high background fluorescence but lacks the ability to provide spatial information due to the lack of substrate localization to specific compartments. Overall, BiFC is an excellent method for visualizing subcellular localization of protein complexes to gain insight into compartmentalized signaling.

Intersecting pathways in cell biology

The endocytic pathway is involved in activation and inhibition of cellular signaling. Thus, defining the regulatory mechanisms that link endocytosis and cellular signaling is of interest. An emerging link between these processes is a family of proteins called intersectins (ITSNs). These multidomain proteins serve as scaffolds in the assembly of endocytic vesicles and also regulate components of various signaling pathways, including kinases, guanosine triphosphatases, and ubiquitin ligases. This review summarizes research on the role of ITSNs in regulating both endocytic and signal transduction pathways, discusses the link between ITSNs and human disease, and highlights future directions in the study of ITSNs.

The ubiquitin-interacting motif protein, S5a, is ubiquitinated by all types of ubiquitin ligases by a mechanism different from typical substrate recognition

S5a/Rpn10 is a ubiquitin (Ub)-binding protein that is a subunit of the 26S proteasome but also exists free in the cytosol. It binds poly-Ub chains through its two Ub-interacting motifs (UIMs). We discovered that, unlike typical substrates of Ub ligases (E3s), S5a can be ubiquitinated by all E3s tested including multimeric and monomeric Ring finger E3s (MuRF1, Siah2, Parkin, APC, and SCF(betaTRCP1)), the U-box E3, CHIP, and HECT domain E3s (E6AP and Nedd4) when assayed with UbcH5 or related Ub-conjugating enzymes. However, the E2s, UbcH1 and UbcH13/Uev1a, which function by distinct mechanisms, do not support S5a ubiquitination. Thus, S5a can be used for assay of probably all E3s with UbcH5. Ubiquitination of S5a results from its binding to Ub chains on the E3 (after self-ubiquitination) or on the substrate, as a mutant lacking the UIM domain was not ubiquitinated. Furthermore, if the S5a UIM domains were fused to GST, the protein was rapidly ubiquitinated by MuRF1 and CHIP. In addition, polyubiquitination (but not monoubiquitination) of MuRF1 allowed S5a to bind to MuRF1 and accelerated S5a ubiquitination. This tendency of S5a to associate with the growing Ub chain can explain how S5a, unlike typical substrates, which are recognized by certain E3s through specific motifs, is ubiquitinated by all E3s tested and is rapidly degraded in vivo.

Regulation of neuron survival through an intersectin-phosphoinositide 3'-kinase C2beta-AKT pathway

While endocytosis attenuates signals from plasma membrane receptors, recent studies suggest that endocytosis also serves as a platform for the compartmentalized activation of cellular signaling pathways. Intersectin (ITSN) is a multidomain scaffolding protein that regulates endocytosis and has the potential to regulate various biochemical pathways through its multiple, modular domains. To address the biological importance of ITSN in regulating cellular signaling pathways versus in endocytosis, we have stably silenced ITSN expression in neuronal cells by using short hairpin RNAs. Decreasing ITSN expression dramatically increased apoptosis in both neuroblastoma cells and primary cortical neurons. Surprisingly, the loss of ITSN did not lead to major defects in the endocytic pathway. Yeast two-hybrid analysis identified class II phosphoinositide 3'-kinase C2beta (PI3K-C2beta) as an ITSN binding protein, suggesting that ITSN may regulate a PI3K-C2beta-AKT survival pathway. ITSN associated with PI3K-C2beta on a subset of endomembrane vesicles and enhanced both basal and growth factor-stimulated PI3K-C2beta activity, resulting in AKT activation. The use of pharmacological inhibitors, dominant negatives, and rescue experiments revealed that PI3K-C2beta and AKT were epistatic to ITSN. This study represents the first demonstration that ITSN, independent of its role in endocytosis, regulates a critical cellular signaling pathway necessary for cell survival.

Intersectin enhances huntingtin aggregation and neurodegeneration through activation of c-Jun-NH2-terminal kinase

Huntingon's disease is a progressive neurodegenerative disease arising from expansion of a polyglutamine (polyQ) tract in the protein huntingtin (Htt) resulting in aggregation of mutant Htt into nuclear and/or cytosolic inclusions in neurons. Mutant Htt affects multiple processes including protein degradation, transcription, signal transduction, fast axonal transport and endocytosis [reviewed in Ross, C.A. and Poirier, M.A. (2005) Opinion: what is the role of protein aggregation in neurodegeneration? Nat. Rev. Mol. Cell. Biol., 6, 891-898]. Here, we report that the endocytic and signal transduction scaffold intersectin (ITSN) increased aggregate formation by mutant Htt through activation of the c-Jun-NH(2)-terminal kinase (JNK)-MAPK pathway. Conversely, silencing ITSN or inhibiting JNK attenuated aggregate formation. Using a Drosophila model for polyQ repeat disease, we observed that ITSN enhanced polyQ-mediated neurotoxicity. A reciprocal relationship was observed between ITSN and Htt. While ITSN enhanced Htt aggregation and toxicity, Htt, in turn, inhibited the cooperativity between ITSN and the epidermal growth factor receptor signal transduction pathway. Finally, we observed that ITSN overexpression enhanced aggregation of polyQ-expanded androgen receptor (AR) as well as wild-type versions of both Htt and AR suggesting a broader involvement of ITSN in neurodegenerative diseases through destabilization of polyQ-containing proteins.

Intersectin regulates epidermal growth factor receptor endocytosis, ubiquitylation, and signaling

Receptor tyrosine kinases (RTKs) are critical for normal cell growth, differentiation, and development, but they contribute to various pathological conditions when disrupted. Activation of RTKs stimulates a plethora of pathways, including the ubiquitylation and endocytosis of the receptor itself. Although endocytosis terminates RTK signaling, it has emerged as a requisite step in RTK activation of signaling pathways. We have discovered that the endocytic scaffolding protein intersectin (ITSN) cooperated with epidermal growth factor receptor (EGFR) in the regulation of cell growth and signaling. However, a biochemical link between ITSN and EGFR was not defined. In this study, we demonstrate that ITSN is a scaffold for the E3 ubiquitin ligase Cbl. ITSN forms a complex with Cbl in vivo mediated by the Src homology (SH) 3 domains binding to the Pro-rich COOH terminus of Cbl. This interaction stimulates the ubiquitylation and degradation of the activated EGFR. Furthermore, silencing ITSN by RNA interference attenuated EGFR internalization as well as activation of the extracellular signal-regulated kinasemitogen-activated protein kinase pathway, thereby demonstrating the importance of ITSN in EGFR function. Given the cooperativity between ITSN and additional RTKs, these results point to an important evolutionarily conserved, regulatory role for ITSN in RTK function that is necessary for both signaling from receptors as well as the ultimate termination of receptor signaling.

The U-box ligase carboxyl-terminus of Hsc 70-interacting protein ubiquitylates Epsin

Epsin is an endocytic adaptor protein involved in the regulation of clathrin-dependent endocytosis. We and others have demonstrated that Epsin is ubiquitylated in cells and requires its ubiquitin interacting motifs (UIMs) for this modification. To further elucidate the mechanism of Epsin ubiquitylation, we initiated studies to identify the E3 ligase(s) that modifies Epsin. In this study, we discovered that the U-box ubiquitin ligase carboxyl-terminus of Hsc70 interacting protein (CHIP) ubiquitylated Epsin. Using an in vitro ubiquitylation assay, we demonstrate that CHIP specifically ubiquitylated Epsin in a UIM-dependent manner. Furthermore, overexpression of CHIP in cells increased Epsin ubiquitylation also in a UIM-dependent manner. Together, these data provide evidence that CHIP functions to ubiquitylate the endocytic protein Epsin.

Analysis of the Role of Ubiquitin-interacting Motifs in Ubiquitin Binding and Ubiquitylation

The ubiquitin-interacting motif (UIM) is a short peptide motif with the dual function of binding ubiquitin and promoting ubiquitylation. This motif is conserved throughout eukaryotes and is present in numerous proteins involved in a wide variety of cellular processes including endocytosis, protein trafficking, and signal transduction. We previously reported that the UIMs of epsin were both necessary and sufficient for its ubiquitylation. In this study, we found that many, but not all, UIM-containing proteins were ubiquitylated. When expressed as chimeric fusion proteins, most UIMs promoted ubiquitylation of the chimera. In contrast to previous studies, we found that UIMs do not exclusively promote monoubiquitylation but rather a mixture of mono-, multi-, and polyubiquitylation. However, UIM-dependent polyubiquitylation does not lead to degradation of the modified protein. UIMs also bind polyubiquitin chains of varying lengths and to different degrees, and this activity is required for UIM-dependent ubiquitylation. Mutational analysis of the UIM revealed specific amino acids that are important for both polyubiquitin binding and ubiquitin conjugation. Finally we provide evidence that UIM-dependent ubiquitylation inhibits the interaction of UIM-containing proteins with other ubiquitylated cellular proteins. Our results suggest a new model for the ubiquitylation of UIM-containing proteins.

N-type Ca2+ Channels as Scaffold Proteins in the Assembly of Signaling Molecules for GABAB Receptor Effects

An emerging concept in signal transduction is the organization of neuronal receptors and channels into microdomains in which signaling proteins are brought together to regulate functional responses. With the multiplicity of potential protein-protein interactions arises the need for the regulation and timing of these interactions. We have identified N-type Ca(2+) channel-signaling molecule complexes formed at different times upon activation of gamma-aminobutyric acid, type B, receptors. The first type of interaction involves pre-association of signaling proteins such as Src kinase with the Ca(2+) channel, because it is rapidly activated by the receptors and regulates the magnitude of the inhibition of the Ca(2+) channel. The second type of interaction involves signaling molecules that are recruited to the channel by receptor activation and control the rate of the channel response. Recruitment of members of the Ras pathway has two effects as follows: 1) modulation of the rate of onset of the gamma-aminobutyric acid-mediated inhibition of Ca(2+) current, and 2) activation of MAP kinase. Our results suggest that the Ca(2+) channel alpha(1) subunit functions as a dynamic scaffold allowing assembly of intracellular signaling components that alter channel activity and route signals to the MAP kinase pathway.

The Ews/Fli-1 fusion gene switches the differentiation program of neuroblastomas to Ewing sarcoma/peripheral primitive neuroectodermal tumors

Neuroblastoma (NB) and the Ewing sarcoma (ES)/peripheral primitive neuroectodermal tumor (PNET) family are pediatric cancers derived from neural crest cells. Although NBs display features of the sympathetic nervous system, ES/PNETs express markers consistent with parasympathetic differentiation. To examine the control of these differentiation markers, we generated NB x ES/PNET somatic cell hybrids. NB-specific markers were suppressed in the hybrids, whereas ES/PNET-specific markers were unaffected. These results suggested that the Ews/Fli-1 fusion gene, resulting from a translocation unique to ES/PNETs, might account for the loss of NB-specific markers. To test this hypothesis, we generated two different NB cell lines that stably expressed the Ews/Fli-1 gene. We observed that heterologous expression of the Ews/Fli-1 protein led to the suppression of NB-specific markers and de novo expression of ES/PNET markers. To determine the extent of changes in differentiation, we used the Affymetrix GeneChip Array system to observe global transcriptional changes of genes. This analysis revealed that the gene expression pattern of the Ews/Fli-1-expressing NB cells resembled that observed in pooled ES/PNET cell lines and differed significantly from the NB parental cells. Therefore, we propose that Ews/Fli-1 contributes to the etiology of ES/PNET by subverting the differentiation program of its neural crest precursor cell to a less differentiated and more proliferative state.

Intersectin activates Ras but stimulates transcription through an independent pathway involving JNK

Intersectin (ITSN) is a molecular scaffold involved in regulating endocytosis and mitogenic signaling. We previously demonstrated that ITSN transformed rodent fibroblasts, accelerated hormone-induced maturation of Xenopus oocytes, and activated the Elk-1 transcription factor through an MEK- and Erk-independent mechanism. We now demonstrate that ITSN complexes with the Ras guanine nucleotide exchange factor Sos1 leading to increased RasGTP levels. Using fluorescence resonant energy transfer analysis, we demonstrate that ITSN complexes with Ras in living cells leading to Ras activation on intracellular vesicles. These vesicles contain epidermal growth factor receptor but are distinct from transferrin-positive vesicles. However, Ras is not required for ITSN stimulation of transcription. Rather, we demonstrate that ITSN signals through JNK to activate Elk-1. Although ITSN activation of Elk-1 was Ras-independent, ITSN cooperates with Ras to synergistically activate JNK. These findings indicate that ITSN activates multiple intracellular signaling pathways and suggest that this adaptor protein may coordinately regulate the activity of these pathways in vivo.

The ubiquitin-interacting motifs target the endocytic adaptor protein epsin for ubiquitination

The covalent attachment of ubiquitin to proteins is an evolutionarily conserved signal for rapid protein degradation. However, additional cellular functions for ubiquitination are now emerging, including regulation of protein trafficking and endocytosis. For example, recent genetic studies suggested a role for ubiquitination in regulating epsin, a modular endocytic adaptor protein that functions in the assembly of clathrin-coated vesicles; however, biochemical evidence for this notion has been lacking. Epsin consists of an epsin NH(2)-terminal homology (ENTH) domain that promotes the interaction with phospholipids, several AP2 binding sites, two clathrin binding sequences, and several Eps15 homology (EH) domain binding motifs. Interestingly, epsin also possesses several recently described ubiquitin-interacting motifs (UIMs) that have been postulated to bind ubiquitin. Here, we demonstrate that epsin is predominantly monoubiquitinated and resistant to proteasomal degradation. The UIMs are necessary for epsin ubiquitination but are not the site of ubiquitination. Finally, we demonstrate that the isolated UIMs from both epsin and an unrelated monoubiquitinated protein, Eps15, are sufficient to promote ubiquitination of a chimeric glutathione-S-transferase (GST)-UIM fusion protein. Thus, our data suggest that UIMs may serve as a general signal for ubiquitination.

Rac and Rho mediate opposing hormonal regulation of the ether-a-go-go-related potassium channel

BACKGROUND

Previous studies of ion channel regulation by G proteins have focused on the larger, heterotrimeric GTPases, which are activated by heptahelical membrane receptors. In contrast, studies of the Rho family of smaller, monomeric, Ras-related GTPases, which are activated by cytoplasmic guanine nucleotide exchange factors, have focused on their role in cytoskeletal regulation.

RESULTS

Here we demonstrate novel functions for the Rho family GTPases Rac and Rho in the opposing hormonal regulation of voltage-activated, ether-a-go-go-related potassium channels (ERG) in a rat pituitary cell line, GH(4)C(1). The hypothalamic neuropeptide, thyrotropin-releasing hormone (TRH) inhibits ERG channel activity through a PKC-independent process that is blocked by RhoA(19N) and the Clostridium botulinum C3 toxin, which inhibit Rho signaling. The constitutively active, GTPase-deficient mutant of RhoA(63L) rapidly inhibits the channels when the protein is dialysed directly into the cell through the patch pipette, and inhibition persists when the protein is overexpressed. In contrast, GTPase-deficient Rac1(61L) stimulates ERG channel activity. The thyroid hormone triiodothyronine (T3), which antagonizes TRH action in the pituitary, also stimulates ERG channel activity through a rapid process that is blocked by Rac1(17N) and wortmannin but not by RhoA(19N).

CONCLUSIONS

Rho stimulation by G(13)-coupled receptors and Rac stimulation by nuclear hormones through PI3-kinase may be general mechanisms for regulating ion channel activity in many cell types. Disruption of these novel signaling cascades is predicted to contribute to several specific human neurological diseases, including epilepsy and deafness.

Mitogenesis and endocytosis: What's at the INTERSECTIoN?

Endocytosis is a regulated physiological process by which cell surface proteins are internalized along with extracellular factors such as nutrients, pathogens, peptides, toxins, etc. The process begins with the invagination of small regions of the plasma membrane which ultimately form intracellullar vesicles. These internalized vesicles may shuttle back to the plasma membrane to recycle the membrane components or they may be targeted for degradation. One role for endocytosis is in the attenuation of receptor signaling. For example, desensitization of activated membrane bound receptors such as G-protein coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs) occurs, in part, through endocytosis of the activated receptor. However, accumulating evidence suggests that endocytosis also mediates intracellular signaling. In this review, we discuss the experimental data that implicate endocytosis as a critical component in cellular signal transduction, both in the initiation of a signal as well as in the termination of a signal. Furthermore, we focus our attention on a recently described adaptor protein, intersectin (ITSN), which provides a link to both the endocytic and the mitogenic machinery of a cell. Thus, ITSN functions at a crossroad in the biochemical regulation of cell function.

The Nf2 tumor suppressor, merlin, functions in Rac-dependent signaling

Mutations in the neurofibromatosis type II (NF2) tumor suppressor predispose humans and mice to tumor development. The study of Nf2+/- mice has demonstrated an additional effect of Nf2 loss on tumor metastasis. The NF2-encoded protein, merlin, belongs to the ERM (ezrin, radixin, and moesin) family of cytoskeleton:membrane linkers. However, the molecular basis for the tumor- and metastasis- suppressing activity of merlin is unknown. We have now placed merlin in a signaling pathway downstream of the small GTPase Rac. Expression of activated Rac induces phosphorylation and decreased association of merlin with the cytoskeleton. Furthermore, merlin overexpression inhibits Rac-induced signaling in a phosphorylation-dependent manner. Finally, Nf2-/- cells exhibit characteristics of cells expressing activated alleles of Rac. These studies provide insight into the normal cellular function of merlin and how Nf2 mutation contributes to tumor initiation and progression.