A histone methylation-MAPK signaling axis drives durable epithelial-mesenchymal transition in hypoxic pancreatic cancer

The tumor microenvironment in pancreatic ductal adenocarcinoma (PDAC) plays a key role in tumor progression and response to therapy. The dense PDAC stroma causes hypovascularity, which leads to hypoxia. Here, we showed that hypoxia drives long-lasting epithelial-mesenchymal transition (EMT) in PDAC primarily through a positive-feedback histone methylation-MAPK signaling axis. Transformed cells preferentially underwent EMT in hypoxic tumor regions in multiple model systems. Hypoxia drove a cell-autonomous EMT in PDAC cells which, unlike EMT in response to growth factors, could last for weeks. Furthermore, hypoxia reduced histone demethylase KDM2A activity, suppressed PP2 family phosphatase expression, and activated MAPKs to post-translationally stabilize histone methyltransferase NSD2, leading to an H3K36me2-dependent EMT in which hypoxia-inducible factors played only a supporting role. Hypoxia-driven EMT could be antagonized in vivo by combinations of MAPK inhibitors. Collectively, these results suggest hypoxia promotes durable EMT in PDAC by inducing a histone methylation-MAPK axis that can be effectively targeted with multi-drug therapies, providing a potential strategy for overcoming chemoresistance.

Divergent transcriptomic signatures from putative mesenchymal stimuli in glioblastoma cells

In glioblastoma, a mesenchymal phenotype is associated with especially poor patient outcomes. Various glioblastoma microenvironmental factors and therapeutic interventions are purported drivers of the mesenchymal transition, but the degree to which these cues promote the same mesenchymal transitions and the uniformity of those transitions, as defined by molecular subtyping systems, is unknown. Here, we investigate this question by analyzing publicly available patient data, surveying commonly measured transcripts for mesenchymal transitions in glioma-initiating cells (GIC), and performing next-generation RNA sequencing of GICs. Analysis of patient tumor data reveals that TGFβ, TNFα, and hypoxia signaling correlate with the mesenchymal subtype more than the proneural subtype. In cultured GICs, the microenvironment-relevant growth factors TGFβ and TNFα and the chemotherapeutic temozolomide promote expression of commonly measured mesenchymal transcripts. However, next-generation RNA sequencing reveals that growth factors and temozolomide broadly promote expression of both mesenchymal and proneural transcripts, in some cases with equal frequency. These results suggest that glioblastoma mesenchymal transitions do not occur as distinctly as in epithelial-derived cancers, at least as determined using common subtyping ontologies and measuring response to growth factors or chemotherapeutics. Further understanding of these issues may identify improved methods for pharmacologically targeting the mesenchymal phenotype in glioblastoma.

Understanding how transmembrane domains regulate interactions between human BST-2 and the SARS-CoV-2 accessory protein ORF7a

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID, replicates at intracellular membranes. Bone marrow stromal antigen 2 (BST-2; tetherin) is an antiviral response protein that inhibits transport of viral particles after budding within infected cells. RNA viruses such as SARS-CoV-2 use various strategies to disable BST-2, including use of transmembrane 'accessory' proteins that interfere with BST-2 oligomerization. ORF7a is a small, transmembrane protein present in SARS-CoV-2 shown previously to alter BST-2 glycosylation and function. In this study, we investigated the structural basis for BST-2 ORF7a interactions, with a particular focus on transmembrane and juxtamembrane interactions. Our results indicate that transmembrane domains play an important role in BST-2 ORF7a interactions and mutations to the transmembrane domain of BST-2 can alter these interactions, particularly single-nucleotide polymorphisms in BST-2 that result in mutations such as I28S. Using molecular dynamics simulations, we identified specific interfaces and interactions between BST-2 and ORF7a to develop a structural basis for the transmembrane interactions. Differences in glycosylation are observed for BST-2 transmembrane mutants interacting with ORF7a, consistent with the idea that transmembrane domains play a key role in their heterooligomerization. Overall, our results indicate that ORF7a transmembrane domain interactions play a key role along with extracellular and juxtamembrane domains in modulating BST-2 function.

An integrated mechanistic and data-driven computational model predicts cell responses to high- and low-affinity EGFR ligands

The biophysical properties of ligand binding heavily influence the ability of receptors to specify cell fates. Understanding the rules by which ligand binding kinetics impact cell phenotype is challenging, however, because of the coupled information transfers that occur from receptors to downstream signaling effectors and from effectors to phenotypes. Here, we address that issue by developing an integrated mechanistic and data-driven computational modeling platform to predict cell responses to different ligands for the epidermal growth factor receptor (EGFR). Experimental data for model training and validation were generated using MCF7 human breast cancer cells treated with the high- and low-affinity ligands epidermal growth factor (EGF) and epiregulin (EREG), respectively. The integrated model captures the unintuitive, concentration-dependent abilities of EGF and EREG to drive signals and phenotypes differently, even at similar levels of receptor occupancy. For example, the model correctly predicts the dominance of EREG over EGF in driving a cell differentiation phenotype through AKT signaling at intermediate and saturating ligand concentrations and the ability of EGF and EREG to drive a broadly concentration-sensitive migration phenotype through cooperative ERK and AKT signaling. Parameter sensitivity analysis identifies EGFR endocytosis, which is differentially regulated by EGF and EREG, as one of the most important determinants of the alternative phenotypes driven by different ligands. The integrated model provides a new platform to predict how phenotypes are controlled by the earliest biophysical rate processes in signal transduction and may eventually be leveraged to understand receptor signaling system performance depends on cell context.

One-sentence summary

Integrated kinetic and data-driven EGFR signaling model identifies the specific signaling mechanisms that dictate cell responses to EGFR activation by different ligands.

Single-cell RNA sequencing reveals microenvironment context-specific routes for epithelial-mesenchymal transition in pancreas cancer cells

In the PDAC tumor microenvironment, multiple factors initiate the epithelial-mesenchymal transition (EMT) that occurs heterogeneously among transformed ductal cells, but it is unclear if different drivers promote EMT through common or distinct signaling pathways. Here, we use single-cell RNA sequencing (scRNA-seq) to identify the transcriptional basis for EMT in pancreas cancer cells in response to hypoxia or EMT-inducing growth factors. Using clustering and gene set enrichment analysis, we find EMT gene expression patterns that are unique to the hypoxia or growth factor conditions or that are common between them. Among the inferences from the analysis, we find that the FAT1 cell adhesion protein is enriched in epithelial cells and suppresses EMT. Further, the receptor tyrosine kinase AXL is preferentially expressed in hypoxic mesenchymal cells in a manner correlating with YAP nuclear localization, which is suppressed by FAT1 expression. AXL inhibition prevents EMT in response to hypoxia but not growth factors. Relationships between FAT1 or AXL expression with EMT were confirmed through analysis of patient tumor scRNA-seq data. Further exploration of inferences from this unique dataset will reveal additional microenvironment context-specific signaling pathways for EMT that may represent novel drug targets for PDAC combination therapies.

Correction: A computational modeling approach for predicting multicell spheroid patterns based on signaling-induced differential adhesion

[This corrects the article DOI: 10.1371/journal.pcbi.1010701.].

Abstract C054: Hypoxia promotes a durable epithelial-mesenchymal transition in pancreas cancer through a histone methylation-MAPK signaling axis

Pancreatic ductal adenocarcinoma (PDAC) tumors are poorly vascularized and exhibit regions of hypoxia. Here, we demonstrate that this feature of the tumor microenvironment promotes epithelial-mesenchymal transition (EMT), which occurs early in PDAC and drives chemoresistance, and we identify the underlying signaling mechanism. Analysis of publicly-available human transcriptomics and proteomics demonstrated that PDAC cells or tumors enriched in mesenchymal markers were also enriched in markers of hypoxia or HIF activity. Furthermore, in lineage-traced autochthonous, orthotopic patient-derived xenograft, and orthotopic or subcutaneous implanted cell line models of PDAC, hypoxic tumor tissue regions were enriched for neoplastic cells that had undergone EMT. In cell culture experiments, PDAC cells from human and mouse tumors exhibited an ability to undergo EMT in response to 1% O2, with loss of membranous E-cadherin, increased vimentin protein expression, and transcriptional changes indicative of both hypoxia and EMT. Moreover, EMT in response to hypoxia was substantially more persistent than that observed in response to growth factors, and a hypoxia fate mapping system revealed that once-hypoxic cells could retain mesenchymal characteristics outside hypoxic tumor regions. To understand the mechanism for EMT in response to low oxygen tension, we constructed a multivariable linear regression model of the dependence of the hypoxic gene signature on different gene sets in PDAC ductal cells, which identified MAPK signaling as the most important feature. Consistent with the model inference, in both in vitro and in vivo settings, hypoxic cells showing evidence of EMT displayed elevated MAPK signaling. We further demonstrated that MAPK activation in hypoxia was potentiated by suppressed activity of a histone demethylase and concomitant loss of protein phosphatase expression, which reinforced the mechanism by stabilizing the expression of a histone methyltransferase. Thus, this study identifies a tumor microenvironment-initiated mechanism leading to EMT and nominates several potential drug targets whose antagonism may promote PDAC chemoresponse.

Citation Format: Brooke A. Brown, Paul J. Myers, Sara J. Adair, Jason R. Pitarresi, Shiv Sah Teli, Peppi Karppinen, Ben Z. Stanger, Todd W. Bauer, Matthew J. Lazzara. Hypoxia promotes a durable epithelial-mesenchymal transition in pancreas cancer through a histone methylation-MAPK signaling axis [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 C054.

A computational modeling approach for predicting multicell spheroid patterns based on signaling-induced differential adhesion

Physiological and pathological processes including embryogenesis and tumorigenesis rely on the ability of individual cells to work collectively to form multicell patterns. In these heterogeneous multicell systems, cell-cell signaling induces differential adhesion between cells that leads to tissue-level patterning. However, the sensitivity of pattern formation to changes in the strengths of signaling or cell adhesion processes is not well understood. Prior work has explored these issues using synthetically engineered heterogeneous multicell spheroid systems, in which cell subpopulations engage in bidirectional intercellular signaling to regulate the expression of different cadherins. While engineered cell systems provide excellent experimental tools to observe pattern formation in cell populations, computational models of these systems may be leveraged to explore more systematically how specific combinations of signaling and adhesion parameters can drive the emergence of unique patterns. We developed and validated two- and three-dimensional agent-based models (ABMs) of spheroid patterning for previously described cells engineered with a bidirectional signaling circuit that regulates N- and P-cadherin expression. Systematic exploration of model predictions, some of which were experimentally validated, revealed how cell seeding parameters, the order of signaling events, probabilities of induced cadherin expression, and homotypic adhesion strengths affect pattern formation. Unsupervised clustering was also used to map combinations of signaling and adhesion parameters to these unique spheroid patterns predicted by the ABM. Finally, we demonstrated how the model may be deployed to design new synthetic cell signaling circuits based on a desired final multicell pattern.

MECHANISTIC AND DATA-DRIVEN MODELS OF CELL SIGNALING: TOOLS FOR FUNDAMENTAL DISCOVERY AND RATIONAL DESIGN OF THERAPY

A full understanding of cell signaling processes requires knowledge of protein structure/function relationships, protein-protein interactions, and the abilities of pathways to control phenotypes. Computational models offer a valuable framework for integrating that knowledge to predict the effects of system perturbations and interventions in health and disease. Whereas mechanistic models are well suited for understanding the biophysical basis for signal transduction and principles of therapeutic design, data-driven models are particularly suited to distill complex signaling relationships among samples and between multivariate signaling changes and phenotypes. Both approaches have limitations and provide incomplete representations of signaling biology, but their careful implementation and integration can provide new understanding for how manipulating system variables impacts cellular decisions.

Breast Cancer Antiestrogen Resistance 3 (BCAR3) promotes tumor growth and progression in triple-negative breast cancer

Triple-Negative Breast Cancers (TNBCs) constitute roughly 10-20% of breast cancers and are associated with poor clinical outcomes. Previous work from our laboratory and others has determined that the cytoplasmic adaptor protein Breast Cancer Antiestrogen Resistance 3 (BCAR3) is an important promoter of cell motility and invasion of breast cancer cells. In this study, we use both in vivo and in vitro approaches to extend our understanding of BCAR3 function in TNBC. We show that BCAR3 is upregulated in ductal carcinoma in situ (DCIS) and invasive carcinomas compared to normal mammary tissue, and that survival of TNBC patients whose tumors contained elevated BCAR3 mRNA is reduced relative to individuals whose tumors had less BCAR3 mRNA. Using mouse orthotopic tumor models, we further show that BCAR3 is required for efficient TNBC tumor growth. Analysis of publicly available RNA expression databases revealed that MET receptor signaling is strongly correlated with BCAR3 mRNA expression. A functional role for BCAR3-MET coupling is supported by data showing that both proteins participate in a single pathway to control proliferation and migration of TNBC cells. Interestingly, the mechanism through which this functional interaction operates appears to differ in different genetic backgrounds of TNBC, stemming in one case from potential differences in the strength of downstream signaling by the MET receptor and in another from BCAR3-dependent activation of an autocrine loop involving the production of HGF mRNA. Together, these data open the possibility for new approaches to personalized therapy for individuals with TNBCs.

CX-5461 Treatment Leads to Cytosolic DNA-Mediated STING Activation in Ovarian Cancer

Epithelial ovarian cancer (EOC) is the deadliest of the gynecologic malignancies, with an overall survival rate of <30%. Recent research has suggested that targeting RNA polymerase I (POL I) with small-molecule inhibitors may be a viable therapeutic approach to combating EOC, even when chemoresistance is present. CX-5461 is one of the most promising POL I inhibitors currently being investigated, and previous reports have shown that CX-5461 treatment induces DNA damage response (DDR) through ATM/ATR kinase. Investigation into downstream effects of CX-5461 led us to uncovering a previously unreported phenotype. Treatment with CX-5461 induces a rapid accumulation of cytosolic DNA. This accumulation leads to transcriptional upregulation of 'STimulator of Interferon Genes' (STING) in the same time frame, phosphorylation of IRF3, and activation of type I interferon response both in vitro and in vivo. This activation is mediated and dependent on cyclic GMP-AMP synthase (cGAS). Here, we show THAT CX-5461 leads to an accumulation of cytosolic dsDNA and thereby activates the cGAS-STING-TBK1-IRF3 innate immune pathway, which induces type I IFN. CX-5461 treatment-mediated immune activation may be a powerful mechanism of action to exploit, leading to novel drug combinations with a chance of increasing immunotherapy efficacy, possibly with some cancer specificity limiting deleterious toxicities.

Phosphorylation of slit diaphragm proteins NEPHRIN and NEPH1 upon binding of HGF promotes podocyte repair

Phosphorylation (activation) and dephosphorylation (deactivation) of the slit diaphragm proteins NEPHRIN and NEPH1 are critical for maintaining the kidney epithelial podocyte actin cytoskeleton and, therefore, proper glomerular filtration. However, the mechanisms underlying these events remain largely unknown. Here we show that NEPHRIN and NEPH1 are novel receptor proteins for hepatocyte growth factor (HGF) and can be phosphorylated independently of the mesenchymal epithelial transition receptor in a ligand-dependent fashion through engagement of their extracellular domains by HGF. Furthermore, we demonstrate SH2 domain–containing protein tyrosine phosphatase-2–dependent dephosphorylation of these proteins. To establish HGF as a ligand, purified baculovirus-expressed NEPHRIN and NEPH1 recombinant proteins were used in surface plasma resonance binding experiments. We report high-affinity interactions of NEPHRIN and NEPH1 with HGF, although NEPHRIN binding was 20-fold higher than that of NEPH1. In addition, using molecular modeling we constructed peptides that were used to map specific HGF-binding regions in the extracellular domains of NEPHRIN and NEPH1. Finally, using an in vitro model of cultured podocytes and an ex vivo model of Drosophila nephrocytes, as well as chemically induced injury models, we demonstrated that HGF-induced phosphorylation of NEPHRIN and NEPH1 is centrally involved in podocyte repair. Taken together, this is the first study demonstrating a receptor-based function for NEPHRIN and NEPH1. This has important biological and clinical implications for the repair of injured podocytes and the maintenance of podocyte integrity.

Mathematical Model Predicts that Acceleration of Diabetic Wound Healing is Dependent on Spatial Distribution of VEGF-A mRNA (AZD8601)

Introduction

Pharmacologic approaches for promoting angiogenesis have been utilized to accelerate healing of chronic wounds in diabetic patients with varying degrees of success. We hypothesize that the distribution of proangiogenic drugs in the wound area critically impacts the rate of closure of diabetic wounds. To evaluate this hypothesis, we developed a mathematical model that predicts how spatial distribution of VEGF-A produced by delivery of a modified mRNA (AZD8601) accelerates diabetic wound healing.

Methods

We modified a previously published model of cutaneous wound healing based on coupled partial differential equations that describe the density of sprouting capillary tips, chemoattractant concentration, and density of blood vessels in a circular wound. Key model parameters identified by a sensitivity analysis were fit to data obtained from an in vivo wound healing study performed in the dorsum of diabetic mice, and a pharmacokinetic model was used to simulate mRNA and VEGF-A distribution following injections with AZD8601. Due to the limited availability of data regarding the spatial distribution of AZD8601 in the wound bed, we performed simulations with perturbations to the location of injections and diffusion coefficient of mRNA to understand the impact of these spatial parameters on wound healing.

Results

When simulating injections delivered at the wound border, the model predicted that injections delivered on day 0 were more effective in accelerating wound healing than injections delivered at later time points. When the location of the injection was varied throughout the wound space, the model predicted that healing could be accelerated by delivering injections a distance of 1–2 mm inside the wound bed when compared to injections delivered on the same day at the wound border. Perturbations to the diffusivity of mRNA predicted that restricting diffusion of mRNA delayed wound healing by creating an accumulation of VEGF-A at the wound border. Alternatively, a high mRNA diffusivity had no effect on wound healing compared to a simulation with vehicle injection due to the rapid loss of mRNA at the wound border to surrounding tissue.

Conclusions

These findings highlight the critical need to consider the location of drug delivery and diffusivity of the drug, parameters not typically explored in pre-clinical experiments, when designing and testing drugs for treating diabetic wounds.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-021-00678-9.

Glioblastoma Cell Resistance to EGFR and MET Inhibition Can Be Overcome via Blockade of FGFR-SPRY2 Bypass Signaling

SPRY2 is a purported tumor suppressor in certain cancers that promotes tumor growth and resistance to receptor tyrosine kinase inhibitors in glioblastoma. Here, we identify a SPRY2-dependent bypass signaling mechanism in glioblastoma that drives resistance to EGFR and MET inhibition. In glioblastoma cells treated with EGFR and MET inhibitors, SPRY2 expression is initially suppressed but eventually rebounds due to NF-κB pathway activation, resultant autocrine FGFR activation, and reactivation of ERK, which controls SPRY2 transcription. In cells where FGFR autocrine signaling does not occur and ERK does not reactivate, or in which ERK reactivates but SPRY2 cannot be expressed, EGFR and MET inhibitors are more effective at promoting death. The same mechanism also drives acquired resistance to EGFR and MET inhibition. Furthermore, tumor xenografts expressing an ERK-dependent bioluminescent reporter engineered for these studies reveal that this bypass resistance mechanism plays out in vivo but can be overcome through simultaneous FGFR inhibition.

Data-Driven Computational Modeling Identifies Determinants of Glioblastoma Response to SHP2 Inhibition

Oncogenic protein tyrosine phosphatases have long been viewed as drug targets of interest, and recently developed allosteric inhibitors of SH2 domain-containing phosphatase-2 (SHP2) have entered clinical trials. However, the ability of phosphatases to regulate many targets directly or indirectly and to both promote and antagonize oncogenic signaling may make the efficacy of phosphatase inhibition challenging to predict. Here we explore the consequences of antagonizing SHP2 in glioblastoma, a recalcitrant cancer where SHP2 has been proposed as a useful drug target. Measuring protein phosphorylation and expression in glioblastoma cells across 40 signaling pathway nodes in response to different drugs and for different oxygen tensions revealed that SHP2 antagonism has network-level, context-dependent signaling consequences that affect cell phenotypes (e.g., cell death) in unanticipated ways. To map specific signaling consequences of SHP2 antagonism to phenotypes of interest, a data-driven computational model was constructed based on the paired signaling and phenotype data. Model predictions aided in identifying three signaling processes with implications for treating glioblastoma with SHP2 inhibitors. These included PTEN-dependent DNA damage repair in response to SHP2 inhibition, AKT-mediated bypass resistance in response to chronic SHP2 inhibition, and SHP2 control of hypoxia-inducible factor expression through multiple MAPKs. Model-generated hypotheses were validated in multiple glioblastoma cell lines, in mouse tumor xenografts, and through analysis of The Cancer Genome Atlas data. Collectively, these results suggest that in glioblastoma, SHP2 inhibitors antagonize some signaling processes more effectively than existing kinase inhibitors but can also limit the efficacy of other drugs when used in combination. SIGNIFICANCE: These findings demonstrate that allosteric SHP2 inhibitors have multivariate and context-dependent effects in glioblastoma that may make them useful components of some combination therapies, but not others.

Abstract PO-055: Signaling regulation of epithelial-mesenchymal transition in the hypoxic tumor microenvironment of pancreas cancer

Epithelial-mesenchymal transition (EMT) is a normal developmental process by which epithelial cells lose their polarity and cell-cell adhesions to become more motile and invasive mesenchymal cells. EMT is associated with the earliest stages in metastasis and promotes chemoresistance in a variety of carcinomas, including pancreatic ductal adenocarcinoma (PDAC). While specific growth factors (e.g., transforming growth factor beta) are perhaps the most well studied inducers of EMT, the low oxygen tension (hypoxia) characteristic of the PDAC tumor microenvironment has also been reported to drive EMT. The signaling pathways that promote EMT in the hypoxic PDAC tumor microenvironment may therefore represent useful drug targets for combination therapy approaches as adjuvants to augment the efficacy of chemotherapy. However, hypoxia-driven EMT has not been well characterized in PDAC, nor have the relevant hypoxia-driven signaling pathways been identified. Our lab recently developed a statistical model of the multivariate signaling regulation of growth factor-induced EMT to identify that the ERK, JNK, and NF-kappaB pathways coordinate to drive EMT in PDAC cells. In the present study, we sought to determine the roles of those pathways in promoting EMT in PDAC cells in response to hypoxia. We created an in vitro model of hypoxia-driven EMT by culturing PDAC cells in 1% oxygen and characterizing dynamic changes in the expression of hypoxia inducible factors (HIF) 1 and 2 alpha, as well as the concomitant loss of E-cadherin protein and transcripts, increased abundance of vimentin-positive cells, and increased expression of the mesenchymal transcripts VIM, SNAI1, and SNAI2. Hypoxia-driven EMT was observed in human PDAC cell lines, patient-derived xenograft cell lines, and KPC murine cell lines. To probe for the activation of specific signaling pathways, we used a combination of immunoblotting and immunofluorescence microscopy. Automated image analysis of immunofluorescence images was used to understand the relevance of specific signaling pathways for explaining the heterogeneity of EMT within populations of cells. We further identified potential kinase targets for antagonizing EMT in the hypoxic microenvironment and confirmed their roles by pharmacological inhibition and/or siRNA-mediated knockdown. We also found that specific kinase pathways are involved in the stabilization of HIF proteins in hypoxia. The relevance of these pathways in driving EMT is being further validated through analyses of a patient-derived xenograft mouse model of PDAC and a HIF1A deletion/Kras mutant autochthonous mouse model of PDAC.

Citation Format: Brooke A. McGirr, Nicholas M. Seyler, Matthew J. Lazzara. Signaling regulation of epithelial-mesenchymal transition in the hypoxic tumor microenvironment of pancreas cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2020 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2020;80(22 Suppl):Abstract nr PO-055.

Calcium-stimulated disassembly of focal adhesions mediated by an ORP3/IQSec1 complex

Coordinated assembly and disassembly of integrin-mediated focal adhesions (FAs) is essential for cell migration. Many studies have shown that FA disassembly requires Ca²⁺ influx, however our understanding of this process remains incomplete. Here, we show that Ca²⁺ influx via STIM1/Orai1 calcium channels, which cluster near FAs, leads to activation of the GTPase Arf5 via the Ca²⁺-activated GEF IQSec1, and that both IQSec1 and Arf5 activation are essential for adhesion disassembly. We further show that IQSec1 forms a complex with the lipid transfer protein ORP3, and that Ca²⁺ influx triggers PKC-dependent translocation of this complex to ER/plasma membrane (PM) contact sites adjacent to FAs. In addition to allosterically activating IQSec1, ORP3 also extracts PI4P from the PM, in exchange for phosphatidylcholine. ORP3-mediated lipid exchange is also important for FA turnover. Together, these findings identify a new pathway that links calcium influx to FA turnover during cell migration.

Abstract C34: Signaling regulation of epithelial-mesenchymal transition in pancreas cancer cells cultured under hypoxic conditions

Pancreatic ductal adenocarcinoma (PDAC) tumors exhibit hypoxia due to high oxygen demand and oxygen diffusional limitations (1). Hypoxia has been reported as a driver of epithelial-mesenchymal transition (EMT) (2), which occurs early in pancreas cancer dissemination and promotes resistance to chemotherapy (3,4). Druggable signaling pathways that promote EMT in the hypoxic PDAC microenvironment may therefore represent useful targets for combination therapy. Our lab previously studied the roles of ERK and SHP2 in driving EMT in response to growth factors (5,6). More recently, our lab employed an unbiased data-driven modeling approach and found that the ERK, JNK, and NF-kappaB pathways coordinate to drive robust EMT in PDAC cells treated with combinations of growth factors (unpublished). In the present study, we sought to determine the roles of those pathways in promoting EMT in PDAC cells in response to a shift from normoxic to hypoxic conditions. The immortalized human PDAC cell line HPAF-II, which exhibits baseline epithelial characteristics, was used for these studies. In simple two-dimensional cell culture, a shift from 20% to 1% oxygen for ≤120 hr led to several indications of EMT, including decreased epithelial cell cluster circularity, reduced junctional E-cadherin expression and an increased fraction of vimentin-positive cells (determined by an automated image analysis pipeline), and increased expression of the mesenchymal transcripts VIM, SNAI1, and SNAI2 (determined by RT-qPCR). A hypoxic cellular response was confirmed by immunoblotting, which showed an early (4 hr) peak in HIF-1alpha expression and a relatively late (48 hr) peak in HIF-2alpha expression, as well as increased expression of the HIF target genes PGK1 and SLC2A1. In three-dimensional cultures of HPAF-II cell spheroids in a collagen I matrix, a shift to hypoxia promoted cell invasion into the matrix, a behavior also observed when HPAF-II cells were treated with exogenous growth factors that drive EMT. To probe for the activation of the ERK, JNK, and NF-kappaB pathways, we used a combination of immunoblotting and immunofluorescence microscopy. A shift from 20% to 1% oxygen increased the phosphorylation of ERK and promoted the nuclear localization of c-Jun, a substrate of JNK. Inhibition of MEK or JNK at the same time as the shift to 1% oxygen prevented the loss of junctional E-cadherin that otherwise occurred, with JNK inhibition having the greater effect. JNK inhibition also abrogated loss of CDH1 transcripts and the increase of VIM transcripts otherwise observed in response to hypoxia. Thus, activation of the ERK and JNK pathways in PDAC cells in response to a shift to low oxygen conditions promoted EMT. These pathways may therefore represent useful targets for antagonizing EMT in the hypoxic microenvironment characteristic of PDAC tumors.

References: 1. Bertout et al., Nat Rev Cancer 2008. 2. Marie-Egyptienne et al., Cancer Lett 2013. 3. Rhim et al., Cell 2012. 4. Zheng et al., Nature 2015. 5. Buonato et al., Cancer Res 2014. 6. Buonato et al., J Cell Sci 2015.

Citation Format: Brooke A. McGirr, Nicholas M. Seyler, Matthew J. Lazzara. Signaling regulation of epithelial-mesenchymal transition in pancreas cancer cells cultured under hypoxic conditions [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr C34.

Disrupting the transmembrane domain-mediated oligomerization of protein tyrosine phosphatase receptor J inhibits EGFR-driven cancer cell phenotypes

Receptor protein tyrosine phosphatases (RPTPs) play critical regulatory roles in mammalian signal transduction. However, the structural basis for the regulation of their catalytic activity is not fully understood, and RPTPs are generally not therapeutically targetable. This knowledge gap is partially due to the lack of known natural ligands or selective agonists of RPTPs. Contrary to what is known from structure-function studies of receptor tyrosine kinases (RTKs), RPTP activities have been reported to be suppressed by dimerization, which may prevent RPTPs from accessing their RTK substrates. We report here that homodimerization of protein tyrosine phosphatase receptor J (PTPRJ, also known as DEP-1) is regulated by specific transmembrane (TM) residues. We found that disrupting these interactions destabilizes homodimerization of full-length PTPRJ in cells, reduces the phosphorylation of the known PTPRJ substrate epidermal growth factor receptor (EGFR) and of other downstream signaling effectors, antagonizes EGFR-driven cell phenotypes, and promotes substrate access. We demonstrate these observations in human cancer cells using mutational studies and identified a peptide that binds to the PTPRJ TM domain and represents the first example of an allosteric agonist of RPTPs. The results of our study provide fundamental structural and functional insights into how PTPRJ activity is tuned by TM interactions in cells. Our findings also open up opportunities for developing peptide-based agents that could be used as tools to probe RPTPs' signaling mechanisms or to manage cancers driven by RTK signaling.

Effects of Proximal Tubule Shortening on Protein Excretion in a Lowe Syndrome Model

BACKGROUND

Lowe syndrome (LS) is an X-linked recessive disorder caused by mutations in OCRL, which encodes the enzyme OCRL. Symptoms of LS include proximal tubule (PT) dysfunction typically characterized by low molecular weight proteinuria, renal tubular acidosis (RTA), aminoaciduria, and hypercalciuria. How mutant OCRL causes these symptoms isn't clear.

METHODS

We examined the effect of deleting OCRL on endocytic traffic and cell division in newly created human PT CRISPR/Cas9 OCRL knockout cells, multiple PT cell lines treated with OCRL-targeting siRNA, and in orcl-mutant zebrafish.

RESULTS

OCRL-depleted human cells proliferated more slowly and about 10% of them were multinucleated compared with fewer than 2% of matched control cells. Heterologous expression of wild-type, but not phosphatase-deficient, OCRL prevented the accumulation of multinucleated cells after acute knockdown of OCRL but could not rescue the phenotype in stably edited knockout cell lines. Mathematic modeling confirmed that reduced PT length can account for the urinary excretion profile in LS. Both ocrl mutant zebrafish and zebrafish injected with ocrl morpholino showed truncated expression of megalin along the pronephric kidney, consistent with a shortened S1 segment.

CONCLUSIONS

Our data suggest a unifying model to explain how loss of OCRL results in tubular proteinuria as well as the other commonly observed renal manifestations of LS. We hypothesize that defective cell division during kidney development and/or repair compromises PT length and impairs kidney function in LS patients.

Localization dynamics of endogenous fluorescently labeled RAF1 in EGF-stimulated cells

Activation of the epidermal growth factor (EGF) receptor (EGFR) at the cell surface initiates signaling through the RAS-RAF-MAPK/ERK1/2 pathway and receptor endocytosis. Whether this signaling continues from endosomes remains unclear, because RAS is predominantly located on the plasma membrane, and the localization of endogenous RAF kinases, downstream effectors of RAS, is not defined. To examine RAF localization, we labeled endogenous RAF1 with mVenus using gene editing. From 10 to 15% of RAF1-mVenus (<2000 molecules/cell), which was initially entirely cytosolic, transiently translocated to the plasma membrane after EGF stimulation. Following an early burst of translocation, the membrane-associated RAF1-mVenus was undetectable by microscopy or subcellular fractionation, and this pool was estimated to be <200 molecules per cell. In contrast, persistent EGF-dependent translocation of RAF1-mVenus to the plasma membrane was driven by the RAF inhibitor sorafenib, which increases the affinity of Ras-GTP:RAF1 interactions. RAF1-mVenus was not found in EGFR-containing endosomes under any conditions. Computational modeling of RAF1 dynamics revealed that RAF1 membrane abundance is controlled most prominently by association and dissociation rates from RAS-GTP and by RAS-GTP concentration. The model further suggested that the relatively protracted activation of the RAF-MEK1/2-ERK1/2 module, in comparison with RAF1 membrane localization, may involve multiple rounds of cytosolic RAF1 rebinding to active RAS at the membrane.

An engineering design approach to systems biology

Measuring and modeling the integrated behavior of biomolecular-cellular networks is central to systems biology. Over several decades, systems biology has been shaped by quantitative biologists, physicists, mathematicians, and engineers in different ways. However, the basic and applied versions of systems biology are not typically distinguished, which blurs the separate aspirations of the field and its potential for real-world impact. Here, we articulate an engineering approach to systems biology, which applies educational philosophy, engineering design, and predictive models to solve contemporary problems in an age of biomedical Big Data. A concerted effort to train systems bioengineers will provide a versatile workforce capable of tackling the diverse challenges faced by the biotechnological and pharmaceutical sectors in a modern, information-dense economy.

Abstract 4559: Sprouty2 differentially regulates signaling and phenotypic responses of glioblastoma cells to DNA damaging agents and receptor kinase inhibitors

Key to improving glioblastoma patient survival is the exploration of the signaling mechanisms that regulate cellular response to approved and investigational therapeutics, including DNA damaging agents and targeted therapeutics such as EGFR and MET kinase inhibitors. Previous work from our group demonstrated that Sprouty2 (SPRY2) surprisingly acted more as a tumor promoter than tumor suppressor in glioblastoma cell lines and tumor xenografts, with SPRY2 knockdown reducing proliferation of human glioblastoma cells and antagonizing the growth of subcutaneous tumor xenografts in mice. We hypothesized here that these effects may result from a perturbation in the cell cycle and that such effects may augment cell cycle shifts induced by DNA damaging agents or kinase inhibitors. Flow cytometry measurements indicated shifts in DNA content distribution in glioblastoma cell lines consistent with cell cycle arrest in response to SPRY2 knockdown alone, and that these shifts were substantially augmented by carboplatin or to a lesser degree by a combination of EGFR and MET inhibitors, but not by temozolomide. Western blotting indicated that SPRY2 knockdown resulted in increased phosphorylation of Ataxia Telangiectasia Mutated (ATM), a kinase involved in the DNA damage response pathway, at a serine residue (1981) that regulates the dissociation of inactivate ATM homodimers into activated monomers. In response to carboplatin, but not the other therapeutics tested, a robust increase was observed in Checkpoint kinase 1 (CHK1) phosphorylation at serine 345, which has been reported to cause cell cycle arrest. This effect was further enhanced by SPRY2 knockdown, which may help explain why the largest cell cycle perturbations were observed in SPRY2-deficient cells treated with carboplatin. Increased phosphorylation of p38 was also observed in response to carboplatin or EGFR and MET inhibitors. This may have contributed to cell cycle arrest, but may also have cooperated with the inhibition of AKT phosphorylation that occurred only in response to EGFR and MET inhibition to drive cell death, an effect that was also augmented by SPRY2 knockdown. Overall, this work has identified a small network of SPRY2-regulated signaling processes that control phenotypic responses of glioblastoma cells to different clinically relevant therapeutics and provides motivation for further study of the role of SPRY2 in glioblastoma.

Citation Format: Nisha G. Sosale, Sally Shin, Matthew J. Lazzara. Sprouty2 differentially regulates signaling and phenotypic responses of glioblastoma cells to DNA damaging agents and receptor kinase inhibitors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4559.

Abstract 2111: Mitogen-activated protein kinase-driven Sprouty2 expression mediates resistance to receptor tyrosine kinase-targeted therapeutics in glioblastoma cells

The therapeutic efficacy of inhibitors of oncogenic kinases in glioblastoma multiforme (GBM) has been disappointing to date. In glioblastoma and other malignancies, resistance to targeted therapeutics can arise through dynamic rewiring of cell signaling pathways responsible for tumorigenesis and survival. Here, we identify one such signaling rewiring process in glioblastoma cells that promotes expression of Sprouty2 (SPRY2). SPRY2 expression is of interest because we recently identified SPRY2 as a driver of glioblastoma cell and tumor proliferation and resistance to kinase inhibitors. Specifically, we found that SPRY2 depletion reduced the ability of glioblastoma cells to form colonies in soft agar, to form subcutaneous tumors in mice, and to resist co-inhibition of the EGFR and MET receptor tyrosine kinases. In the present study we found that, in a panel of glioblastoma cell lines, SPRY2 expression was initially reduced in response to inhibition of EGFR and MET. However, at later time points (24-48 hr after receptor inhibition) resurgent SPRY2 expression was observed even as EGFR and MET phosphorylation remained suppressed. To identify regulators responsible for resurgent SPRY2 expression, we undertook a quantitative systems biology approach based on partial least squares regression (PLSR) modeling wherein the phosphorylation states of multiple signaling pathways known to be regulated by EGFR and MET were measured at six time points during the 48 hrs after treatment of cells with EGFR and MET inhibitors. The PLSR model identified late stage (48 hr) phosphorylation of extracellular signal-regulated kinase (ERK) as having a role in cellular resistance to the inhibitors distinct from ERK phosphorylation at all other times. Inspection of the experimental data revealed that phosphorylated ERK, which was initially suppressed in response to EGFR and MET inhibition, also displayed a resurgence that paralleled the trend in SPRY2 expression. This correlation between ERK phosphorylation and SPRY2 expression is consistent with the documented ability of ERK to regulate SPRY2 expression transcriptionally. The importance of resurgent ERK phosphorylation was confirmed in experiments showing that its suppression through the use of a MEK inhibitor prevented resurgent SPRY2 expression and augmented cell death in response to EGFR and MET inhibitors. We hypothesize that resurgent ERK phosphorylation and resultant SPRY2 expression occur due to the ability of glioblastoma cells to activate receptor tyrosine kinases other than EGFR and MET in response to inhibition of those receptors. Antibody microarray experiments have provided some initial clues about the identity of those receptors, and experiments are ongoing to validate those results.

Citation Format: Evan K. Day, Matthew J. Lazzara. Mitogen-activated protein kinase-driven Sprouty2 expression mediates resistance to receptor tyrosine kinase-targeted therapeutics in glioblastoma cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2111.

Cell signaling regulation by protein phosphorylation: a multivariate, heterogeneous, and context-dependent process

Proper spatiotemporal regulation of protein phosphorylation in cells and tissues is required for normal development and homeostasis, but aberrant protein phosphorylation regulation leads to various diseases. The study of signaling regulation by protein phosphorylation is complicated in part by the sheer scope of the kinome and phosphoproteome, dependence of signaling protein functionality on cellular localization, and the complex multivariate relationships that exist between protein phosphorylation dynamics and the cellular phenotypes they control. Additional complexities arise from the ability of microenvironmental factors to influence phosphorylation-dependent signaling and from the tendency for some signaling processes to occur heterogeneously among cells. These considerations should be taken into account when measuring cell signaling regulation by protein phosphorylation.

EGF augments TGFβ-induced epithelial-mesenchymal transition by promoting SHP2 binding to GAB1

In many epithelial cells, epidermal growth factor (EGF) augments the epithelial-mesenchymal transition (EMT) that occurs when cells are treated with transforming growth factor β (TGFβ). We demonstrate that this augmentation requires activation of SH2 domain-containing phosphatase-2 (SHP2; also known as PTPN11), a proto-oncogene. In lung and pancreatic cancer cell lines, reductions in E-cadherin expression, increases in vimentin expression and increases in cell scatter rates were larger when cells were treated with TGFβ and EGF versus TGFβ or EGF alone. SHP2 knockdown promoted epithelial characteristics basally and antagonized EMT in response to TGFβ alone or in combination with EGF. Whereas EGF promoted SHP2 binding to tyrosine phosphorylated GAB1, which promotes SHP2 activity, TGFβ did not induce SHP2 association with phosphotyrosine-containing proteins. Knockdown of endogenous SHP2 and reconstitution with an SHP2 mutant with impaired phosphotyrosine binding ability eliminated the EGF-mediated EMT augmentation that was otherwise restored with wild-type SHP2 reconstitution. These results demonstrate roles for basal and ligand-induced SHP2 activity in EMT and further motivate efforts to identify specific ways to inhibit SHP2, given the role of EMT in tumor dissemination and chemoresistance.

Sprouty2 Drives Drug Resistance and Proliferation in Glioblastoma

Glioblastoma multiforme (GBM) is notoriously resistant to therapy, and the development of a durable cure will require the identification of broadly relevant regulators of GBM cell tumorigenicity and survival. Here, we identify Sprouty2 (SPRY2), a known regulator of receptor tyrosine kinases (RTK), as one such regulator. SPRY2 knockdown reduced proliferation and anchorage-independent growth in GBM cells and slowed xenograft tumor growth in mice. SPRY2 knockdown also promoted cell death in response to coinhibition of the epidermal growth factor receptor (EGFR) and the c-MET receptor in GBM cells, an effect that involved regulation of the ability of the p38 mitogen-activated protein kinase (MAPK) to drive cell death in response to inhibitors. Analysis of data from clinical tumor specimens further demonstrated that SPRY2 protein is definitively expressed in GBM tissue, that SPRY2 expression is elevated in GBM tumors expressing EGFR variant III (EGFRvIII), and that elevated SPRY2 mRNA expression portends reduced GBM patient survival. Overall, these results identify SPRY2 and the pathways it regulates as novel candidate biomarkers and therapeutic targets in GBM.

IMPLICATIONS

SPRY2, counter to its roles in other cancer settings, promotes glioma cell and tumor growth and cellular resistance to targeted inhibitors of oncogenic RTKs, thus making SPRY2 and the cell signaling processes it regulates potential novel therapeutic targets in glioma.

Exocyst Sec10 protects renal tubule cells from injury by EGFR/MAPK activation and effects on endocytosis

Acute kidney injury is common and has a high mortality rate, and no effective treatment exists other than supportive care. Using cell culture models, we previously demonstrated that exocyst Sec10 overexpression reduced damage to renal tubule cells and speeded recovery and that the protective effect was mediated by higher basal levels of mitogen-activated protein kinase (MAPK) signaling. The exocyst, a highly-conserved eight-protein complex, is known for regulating protein trafficking. Here we show that the exocyst biochemically interacts with the epidermal growth factor receptor (EGFR), which is upstream of MAPK, and Sec10-overexpressing cells express greater levels of phosphorylated (active) ERK, the final step in the MAPK pathway, in response to EGF stimulation. EGFR endocytosis, which has been linked to activation of the MAPK pathway, increases in Sec10-overexpressing cells, and gefitinib, a specific EGFR inhibitor, and Dynasore, a dynamin inhibitor, both reduce EGFR endocytosis. In turn, inhibition of the MAPK pathway reduces ligand-mediated EGFR endocytosis, suggesting a potential feedback of elevated ERK activity on EGFR endocytosis. Gefitinib also decreases MAPK signaling in Sec10-overexpressing cells to levels seen in control cells and, demonstrating a causal role for EGFR, reverses the protective effect of Sec10 overexpression following cell injury in vitro. Finally, using an in vivo zebrafish model of acute kidney injury, morpholino-induced knockdown of sec10 increases renal tubule cell susceptibility to injury. Taken together, these results suggest that the exocyst, acting through EGFR, endocytosis, and the MAPK pathway is a candidate therapeutic target for acute kidney injury.

p53Ψ is a transcriptionally inactive p53 isoform able to reprogram cells toward a metastatic-like state

Although much is known about the underlying mechanisms of p53 activity and regulation, the factors that influence the diversity and duration of p53 responses are not well understood. Here we describe a unique mode of p53 regulation involving alternative splicing of the TP53 gene. We found that the use of an alternative 3' splice site in intron 6 generates a unique p53 isoform, dubbed p53Ψ. At the molecular level, p53Ψ is unable to bind to DNA and does not transactivate canonical p53 target genes. However, like certain p53 gain-of-function mutants, p53Ψ attenuates the expression of E-cadherin, induces expression of markers of the epithelial-mesenchymal transition, and enhances the motility and invasive capacity of cells through a unique mechanism involving the regulation of cyclophilin D activity, a component of the mitochondrial inner pore permeability. Hence, we propose that p53Ψ encodes a separation-of-function isoform that, although lacking canonical p53 tumor suppressor/transcriptional activities, is able to induce a prometastatic program in a transcriptionally independent manner.

Multivariate signaling regulation by SHP2 differentially controls proliferation and therapeutic response in glioma cells

Information from multiple signaling axes is integrated in the determination of cellular phenotypes. Here, we demonstrate this aspect of cellular decision making in glioblastoma multiforme (GBM) cells by investigating the multivariate signaling regulatory functions of the protein tyrosine phosphatase SHP2 (also known as PTPN11). Specifically, we demonstrate that the ability of SHP2 to simultaneously drive ERK1/2 and antagonize STAT3 pathway activities produces qualitatively different effects on the phenotypes of proliferation and resistance to EGFR and c-MET co-inhibition. Whereas the ERK1/2 and STAT3 pathways independently promote proliferation and resistance to EGFR and c-MET co-inhibition, SHP2-driven ERK1/2 activity is dominant in driving cellular proliferation and SHP2-mediated antagonism of STAT3 phosphorylation prevails in the promotion of GBM cell death in response to EGFR and c-MET co-inhibition. Interestingly, the extent of these SHP2 signaling regulatory functions is diminished in glioblastoma cells that express sufficiently high levels of the EGFR variant III (EGFRvIII) mutant, which is commonly expressed in GBM. In cells and tumors that express EGFRvIII, SHP2 also antagonizes the phosphorylation of EGFRvIII and c-MET and drives expression of HIF-1α and HIF-2α, adding complexity to the evolving understanding of the regulatory functions of SHP2 in GBM.

Differential parsing of EGFR endocytic flux among parallel internalization pathways in lung cancer cells with EGFR-activating mutations

Due to the existence of parallel pathways for receptor endocytosis and their complexities, a quantitative understanding of receptor endocytosis in normal and pathological settings requires computational analysis. Here, we develop a mechanistic model of epidermal growth factor receptor (EGFR) endocytosis to determine the relative contributions of three parallel pathways: clathrin-dependent internalization mediated by mitogen-inducible gene 6 (MIG6), an endogenous EGFR kinase inhibitor that links EGFR to endocytic proteins; clathrin-dependent internalization mediated by the ubiquitin ligase CBL, which can be sequestered by the regulatory protein Sprouty2; or alternative pathways that may be non-clathrin mediated. We applied the model to interpret our previous measurements of EGFR endocytosis in lung cancer cells. Interestingly, our results suggest that MIG6 is responsible for at least as much wild-type EGFR internalization as CBL, indicating that a significant fraction of internalizing EGFR may be incapable of driving signaling. Model results also suggest that MIG6's endocytic function is reduced for the kinase-activated and internalization-impaired EGFR mutants found in some lung cancers. Analysis of Sprouty2 knockdown data indicates that Sprouty2 regulates EGFR endocytosis primarily by controlling EGFR expression, rather than by sequestering CBL, and supports the notion that CBL-mediated internalization is impaired for EGFR mutants. We further demonstrate that differences in internalization between wild-type and mutant EGFR cannot explain differences in EGF-mediated EGFR degradation without concomitant changes in EGFR recycling, which we previously quantified. This work provides new quantitative insights into EGFR trafficking in lung cancer and provides a framework for studying parallel endocytosis pathways for other receptors.

ERK1/2 blockade prevents epithelial-mesenchymal transition in lung cancer cells and promotes their sensitivity to EGFR inhibition

Overcoming cellular mechanisms of de novo and acquired resistance to drug therapy remains a central challenge in the clinical management of many cancers, including non-small cell lung cancer (NSCLC). Although much work has linked the epithelial-mesenchymal transition (EMT) in cancer cells to the emergence of drug resistance, it is less clear where tractable routes may exist to reverse or inhibit EMT as a strategy for drug sensitization. Here, we demonstrate that extracellular signal-regulated kinase (ERK) 1/2 (mitogen-activated protein kinase 3/1, MAPK3/1) signaling plays a key role in directing the mesenchymal character of NSCLC cells and that blocking ERK signaling is sufficient to heighten therapeutic responses to EGF receptor (EGFR) inhibitors. MEK1/2 (MAPKK1/2) inhibition promoted an epithelial phenotype in NSCLC cells, preventing induction of EMT by exogenous TGF-β. Moreover, in cells exhibiting de novo or acquired resistance to the EGFR inhibitor gefitinib, MEK inhibition enhanced the sensitivity to gefitinib and slowed cell migration. These effects only occurred, however, if MEK was inhibited for a period sufficient to trigger changes in EMT marker expression. Consistent with these findings, changes in EMT phenotypes and markers were also induced by the expression of mutant KRAS in a MEK-dependent manner. Our results suggest that prolonged exposure to MEK or ERK inhibitors may not only restrain EMT but also overcome naïve or acquired resistance of NSCLC to EGFR-targeted therapy in the clinic.

Regulation of EGFR trafficking and cell signaling by Sprouty2 and MIG6 in lung cancer cells

The duration and specificity of epidermal growth factor receptor (EGFR) activation and signaling are determinants of cellular decision processes and are tightly regulated by receptor dephosphorylation, internalization and degradation. In addition, regulatory proteins that are upregulated or activated post-transcriptionally upon receptor activation may initiate feedback loops that play crucial roles in spatiotemporal regulation of signaling. We examined the roles of Sprouty2 (SPRY2) and mitogen-inducible gene 6 (MIG6), two feedback regulators of EGFR trafficking and signaling, in lung cancer cells with or without EGFR-activating mutations. These mutations are of interest because they confer unusual cellular sensitivity to EGFR inhibition through a mechanism involving an impairment of EGFR endocytosis. We found that the endocytosis of wild-type and mutant EGFR was promoted by SPRY2 knockdown and antagonized by MIG6 knockdown. SPRY2 knockdown also significantly reduced extracellular signal-regulated kinase (ERK) phosphorylation, EGFR expression, and EGFR recycling. In a cell line expressing mutant EGFR, this effect on ERK led to a marked increase in cell death response to EGFR inhibition. The effects of SPRY2 knockdown on EGFR endocytosis and recycling were primarily the result of the concomitant change in EGFR expression, but this was not true for the observed changes in ERK phosphorylation. Thus, our study demonstrates that SPRY2 and MIG6 are important regulators of wild-type and mutant EGFR trafficking and points to an EGFR expression-independent function of SPRY2 in the regulation of ERK activity that may impact cellular sensitivity to EGFR inhibitors, especially in the context of EGFR mutation.

Abstract 291: ERK1/2 pathway activation controls epithelial-mesenchymal transition in NSCLC cells

Understanding and overcoming the cellular mechanisms that underlie de novo and acquired therapy resistance remains a challenge in treating many cancers including non-small cell lung cancer (NSCLC). Despite the prevalence of epidermal growth factor receptor (EGFR) over-activation in NSCLC and other cancers, most patients are resistant to EGFR inhibitors. A number of studies have linked mesenchymal cellular phenotypes and epithelial-mesenchymal transition (EMT) with resistance to chemotherapeutics and EGFR inhibitors across cancer types. Here, we demonstrate in NSCLC cells that MAPK/ERK signaling plays a key role in determining mesenchymal cellular characteristics and response to the EGFR inhibitor gefitinib. In cultured NSCLC cell lines, pharmacological inhibition of MEK pushed cells toward a more epithelial phenotype and prevented EMT induction by exogenous transforming growth factor beta. In addition, chronic MEK inhibition synergistically enhanced cellular sensitivity to gefitinib in a reversible manner and slowed cell migration in wound healing assays in NSCLC cell lines with both de novo and acquired resistance to gefitinib. Importantly, our results demonstrate that the observed cell phenotypic changes occur on a time scale corresponding to that with which epithelial and mesenchymal marker expression change in response to ERK inhibition. This time scale is much larger than that for changes in ERK activity itself, reflecting the need for downstream ERK-dependent processes to affect phenotypic changes. Changes in the same cellular phenotypes and associated changes in epithelial and mesenchymal markers also occurred as a result of KRAS mutant expression in a MEK-dependent manner. Overall, this study draws a strong connection between ERK pathway activation and mesenchymal characteristics in NSCLC and suggests potential new opportunities for expanding the efficacy of currently available EGFR inhibitors.

Citation Format: Janine M. Buonato, Matthew J. Lazzara. ERK1/2 pathway activation controls epithelial-mesenchymal transition in NSCLC cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 291. doi:10.1158/1538-7445.AM2013-291

Computational analysis of the regulation of EGFR by protein tyrosine phosphatases

The tyrosine phosphorylated epidermal growth factor receptor (EGFR) initiates numerous cell signaling pathways. Although EGFR phosphorylation levels are ultimately determined by the balance of receptor kinase and protein tyrosine phosphatase (PTP) activities, the kinetics of EGFR dephosphorylation are not well understood. Previous models of EGFR signaling have generally neglected PTP activity or computed PTP activity by considering data that do not fully reveal the kinetics and compartmentalization of EGFR dephosphorylation. We developed a compartmentalized, mechanistic model to elucidate the kinetics of EGFR dephosphorylation and the coupling of this process to phosphorylation-dependent EGFR endocytosis. Model regression against data from HeLa cells for EGFR phosphorylation response to EGFR activation, PTP inhibition, and EGFR kinase inhibition led to the conclusion that EGFR dephosphorylation occurs at the plasma membrane and in the cell interior with a timescale that is smaller than that for ligand-mediated EGFR endocytosis. The model further predicted that sufficiently rapid dephosphorylation of EGFR at the plasma membrane could potentially impede EGFR endocytosis, consistent with recent experimental findings. Overall, our results suggest that PTPs regulate multiple receptor-level phenomena via their action at the plasma membrane and cell interior and point to new possibilities for targeting PTPs for modulation of EGFR dynamics.

Reactivation of ERK signaling causes resistance to EGFR kinase inhibitors

The clinical efficacy of epidermal growth factor receptor (EGFR) kinase inhibitors is limited by the development of drug resistance. The irreversible EGFR kinase inhibitor WZ4002 is effective against the most common mechanism of drug resistance mediated by the EGFR T790M mutation. Here, we show, in multiple complementary models, that resistance to WZ4002 develops through aberrant activation of extracellular signal-regulated kinase (ERK) signaling caused by either an amplification of mitogen-activated protein kinase 1 (MAPK1) or by downregulation of negative regulators of ERK signaling. Inhibition of MAP-ERK kinase (MEK) or ERK restores sensitivity to WZ4002 and prevents the emergence of drug resistance. We further identify MAPK1 amplification in an erlotinib-resistant EGFR-mutant non-small cell lung carcinoma patient. In addition, the WZ4002-resistant MAPK1-amplified cells also show an increase both in EGFR internalization and a decrease in sensitivity to cytotoxic chemotherapy. Our findings provide insights into mechanisms of drug resistance to EGFR kinase inhibitors and highlight rational combination therapies that should be evaluated in clinical trials.

Abstract 9: Imbalanced oncogenic signaling: Analogies between structurally distinct EGFR mutants of relevance to lung and brain cancers

The epidermal growth factor receptor (EGFR) is expressed at elevated levels in a large fraction of brain, lung, breast, prostate, and head-and-neck cancers. Efforts to treat such cancers with EGFR-targeted antibodies and kinase inhibitors have had limited success, suggesting that the level of EGFR expression in a tumor is not generally a good predictor of response to drugs targeting EGFR. In some cases, however, response to EGFR-targeted agents correlates with the presence of certain EGFR mutations. For example, in non-small cell lung cancer (NSCLC), the small percentage of patients harboring kinase-activating mutations of EGFR (mostly small in-frame deletion and point mutations) respond unusually well to EGFR kinase inhibitors such as gefitinib and erlotinib. In glioblastoma multiforme (GBM), the presence of the EGFRvIII mutation (a distinct deletion mutation which also promotes elevated EGFR activity) is also associated with increased response to EGFR inhibitors, at least in cells expressing PTEN. Settings where the elevated activity of a particular kinase promotes cellular responsiveness to drugs targeting the kinase have been identified as examples of the phenomenon of “oncogene addiction.” One of the prevailing models for EGFR oncogene addiction holds that EGFR mutations simultaneously lead to the increased activities of pro-survival and pro-apoptotic signaling pathways, with the latter persisting longer in response to EGFR inhibition and ultimately prevailing in determining cellular phenotypes. Here, we offer an alternative explanation for at least part of the altered cellular sensitivity to EGFR inhibition observed in the context of EGFR mutation in NSCLC and GBM. Specifically, we demonstrate that downstream of these structurally distinct EGFR mutants an important pathway involving the activation of ERK via the protein tyrosine phosphatase SHP2 is functionally impaired, and that this perturbs cellular responsiveness to gefitinib. Based upon immunoprecipitation, immunofluorescence, activity, and receptor trafficking studies, this mechanism appears to involve the sequestration of biochemically active SHP2 with endocytosis-impaired EGFR mutants at the plasma membrane in a manner which prevents the participation of SHP2 in the complete activation of ERK. Thus, our findings demonstrate that distinct EGFR-activating mutations may result in qualitatively similar perturbations to downstream signaling leading to the surprising impairment of receptor-mediated ERK activation. Given that elevated expression of these EGFR mutants generally promotes cellular response to EGFR inhibitors, this work identifies SHP2 and ERK as potentially critical nodes in the signaling network downstream of EGFR for co-inhibition in settings where EGFR inhibitors are not effective as single agents.

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 9. doi:1538-7445.AM2012-9

Impaired SHP2-mediated extracellular signal-regulated kinase activation contributes to gefitinib sensitivity of lung cancer cells with epidermal growth factor receptor-activating mutations

Most non-small cell lung cancers (NSCLC) display elevated expression of epidermal growth factor receptor (EGFR), but response to EGFR kinase inhibitors is predominantly limited to NSCLC harboring EGFR-activating mutations. These mutations are associated with increased activity of survival pathways, including phosphatidylinositol 3-kinase/AKT and signal transducer and activator of transcription 3/5. We report that EGFR-activating mutations also surprisingly lead to decreased ability to activate extracellular signal-regulated kinase (ERK) compared with wild-type EGFR. In NSCLC cells and mouse embryonic fibroblasts expressing mutant EGFR, this effect on ERK correlates with decreased EGFR internalization and reduced phosphorylation of SHP2, a tyrosine phosphatase required for the full activation of ERK. We further show that ERK activation levels affect cellular response to gefitinib. NSCLC cells with EGFR mutation display reduced gefitinib sensitivity when ERK activation is augmented by expression of constitutively active mutants of mitogen-activated protein kinase/ERK kinase (MEK). Conversely, in a NSCLC cell line expressing wild-type EGFR, gefitinib treatment along with or following MEK inhibition increases death response compared with treatment with gefitinib alone. Our results show that EGFR-activating mutations may promote some survival pathways but simultaneously impair others. This multivariate alteration of the network governing cellular response to gefitinib, which we term "oncogene imbalance," portends a potentially broader ability to treat gefitinib-resistant NSCLC.

Model of albumin reabsorption in the proximal tubule

Normally, the small amount of albumin which passes through the glomerular capillary wall is almost completely reabsorbed in the proximal tubule, via an endocytic mechanism, but the reabsorptive process can be overwhelmed if the filtered load of albumin is too large. To examine the factors that control the fractional reabsorption of albumin (f), we developed a mathematical model which assumes saturable endocytosis kinetics with a maximum reabsorptive capacity, V(max), and which includes the effects of flow and diffusion in the lumen. Limitations in albumin transport from the bulk tubule fluid to the endocytic sites at the bases of the microvilli had only a modest (8%) effect on the value of V(max) needed to fit micropuncture data on tubule albumin concentrations in rats. For moderate changes in filtered load, there was much greater sensitivity of f to SNGFR than to the albumin concentration of the filtrate (C(0)). A 50% increase in SNGFR was predicted to cause four- to fivefold increases in albumin excretion in rats or humans. For large increases in C(0), as might result from defects in glomerular sieving, there was a threshold at which the reabsorptive process became saturated and f fell sharply. That threshold corresponded to sieving coefficients of 10(-3) to 10(-2), the higher values occurring at reduced SNGFR. The predictions of the present model contrast with those of one proposed recently by Smithies (32), which does not include the effects of tubule flow rate.

Glomerular filtration of albumin: how small is the sieving coefficient?

A model was developed to describe how the concentration of albumin in proximal tubule fluid will vary with axial position, including the effects of luminal flow and water and albumin reabsorption. The results show that the high albumin sieving coefficients proposed recently cannot be reconciled with micropuncture data in rats.

Effects of concentration on the partitioning of macromolecule mixtures in agarose gels

To test the effects of solute concentration on the equilibrium partitioning of single macromolecules and macromolecule mixtures between bulk solutions and gels, the partition coefficient in agarose was measured for BSA and for four narrow fractions of Ficoll with Stokes radii of 30-59 A. Solutions of each test macromolecule were equilibrated with a known volume of gel, final liquid concentrations measured, and partition coefficients (gel concentration divided by bulk concentration) calculated by applying a material balance. The partition coefficient of each macromolecule was measured in 4 and 6% gels under dilute conditions and with BSA present at initial concentrations up to 13.5 g/dl. As expected, the partition coefficients decreased with increasing agarose concentration and with increasing macromolecular size. Moreover, increasing the BSA concentration increased the partition coefficient of BSA itself and that of all four Ficolls. This effect was most pronounced for the largest test solutes. Measurements at two ionic strengths confirmed that electrostatic interactions were negligible under the conditions used. The experimental results were compared with predictions from a previously developed excluded volume theory for the partitioning of mixtures of rigid, spheroidal macromolecules in fibrous media. Agarose was represented as a randomly oriented array of cylindrical fibers, BSA as a prolate spheroid, and Ficoll as a sphere. The quantitative agreement between the model predictions and the data was generally quite good, indicating that steric interactions among solute molecules and between solute molecules and gel fibers could explain the partitioning results. The theory is simple enough computationally to be applied to a variety of processes that are influenced by the equilibrium partitioning of macromolecules.

Protein partitioning driven by excluded-volume interactions in an aqueous nonionic micellar?gel system

The separation and purification of biomolecules in aqueous media driven by excluded-volume interactions is a well-established concept. In this article we propose a new separations method, based on excluded-volume principles, consisting of an aqueous micellar-gel system (AMGS). Specifically, an outer aqueous phase containing cylindrically shaped micelles of the nonionic surfactant n-decyl tetra (ethylene oxide) (C10E4) is physically separated from an inner aqueous phase defined by the interior volume of gel beads, from which the micelles are completely excluded because of their shape and size. In the AMGS, the concentration of the micelles outside the gel beads is sufficiently high that the volume excluded to a biomolecule in the solution external to the gel beads is much larger than that within the gel beads. Accordingly, when biomolecules are introduced into the AMGS, they partition preferentially into the gel-bead phase, according to their sizes, as a result of the greater effect of the excluded-volume interactions with the C10E4 micelles present in the aqueous phase outside the gel beads. The new AMGS is more versatile and adaptable than the conventional two-phase aqueous C10E4 micellar system because the micelle volume fraction is independent of the temperature and because the effects of entrainment are eliminated. After demonstrating the experimental feasibility of creating the new AMGS, the three proteins myoglobin, ovalbumin, and BSA-FITC, and the enzyme G6PD, were partitioned in the AMGS and their partitioning behavior was found to follow the experimental excluded-volume trends dictated by the interactions of the biomolecules with the C10E4 micelles. Specifically, the measured partition coefficients of the four biomolecules into the micellar phase were found to be less than unity and to decrease with increasing biomolecule size. A theoretical description of the partitioning behavior of the biomolecules in the new AMGS was formulated, based on excluded-volume considerations, and the predicted biomolecule partition coefficients were found to compare favorably with those measured for the four biomolecules studied.

Effects of plasma proteins on sieving of tracer macromolecules in glomerular basement membrane

It was found previously that the sieving coefficients of Ficoll and Ficoll sulfate across isolated glomerular basement membrane (GBM) were greatly elevated when BSA was present at physiological levels, and it was suggested that most of this increase might have been the result of steric interactions between BSA and the tracers (5). To test this hypothesis, we extended the theory for the sieving of macromolecular tracers to account for the presence of a second, abundant solute. Increasing the concentration of an abundant solute is predicted to increase the equilibrium partition coefficient of a tracer in a porous or fibrous membrane, thereby increasing the sieving coefficient. The magnitude of this partitioning effect depends on solute size and membrane structure. The osmotic reduction in filtrate velocity caused by an abundant, mostly retained solute will also tend to elevate the tracer sieving coefficient. The osmotic effect alone explained only about one-third of the observed increase in the sieving coefficients of Ficoll and Ficoll sulfate, whereas the effect of BSA on tracer partitioning was sufficient to account for the remainder. At physiological concentrations, predictions for tracer sieving in the presence of BSA were found to be insensitive to the assumed shape of the protein (sphere or prolate spheroid). For protein mixtures, the theoretical effect of 6 g/dl BSA on the partitioning of spherical tracers was indistinguishable from that of 3 g/dl BSA and 3 g/dl IgG. This suggests that for partitioning and sieving studies in vitro, a good experimental model for plasma is a BSA solution with a mass concentration matching that of total plasma protein. The effect of plasma proteins on tracer partitioning is expected to influence sieving not only in isolated GBM but also in intact glomerular capillaries in vivo.

Structural determinants of glomerular permeability

Recent progress in relating the functional properties of the glomerular capillary wall to its unique structure is reviewed. The fenestrated endothelium, glomerular basement membrane (GBM), and epithelial filtration slits form a series arrangement in which the flow diverges as it enters the GBM from the fenestrae and converges again at the filtration slits. A hydrodynamic model that combines morphometric findings with water flow data in isolated GBM has predicted overall hydraulic permeabilities that are consistent with measurements in vivo. The resistance of the GBM to water flow, which accounts for roughly half that of the capillary wall, is strongly dependent on the extent to which the GBM surfaces are blocked by cells. The spatial frequency of filtration slits is predicted to be a very important determinant of the overall hydraulic permeability, in keeping with observations in several glomerular diseases in humans. Whereas the hydraulic resistances of the cell layers and GBM are additive, the overall sieving coefficient for a macromolecule (its concentration in Bowman's space divided by that in plasma) is the product of the sieving coefficients for the individual layers. Models for macromolecule filtration reveal that the individual sieving coefficients are influenced by one another and by the filtrate velocity, requiring great care in extrapolating in vitro observations to the living animal. The size selectivity of the glomerular capillary has been shown to be determined largely by the cellular layers, rather than the GBM. Controversial findings concerning glomerular charge selectivity are reviewed, and it is concluded that there is good evidence for a role of charge in restricting the transmural movement of albumin. Also discussed is an effect of albumin that has received little attention, namely, its tendency to increase the sieving coefficients of test macromolecules via steric interactions. Among the unresolved issues are the specific contributions of the endothelial glycocalyx and epithelial slit diaphragm to the overall hydraulic resistance and macromolecule selectivity and the nanostructural basis for the observed permeability properties of the GBM.

Effects of Multisolute Steric Interactions on Membrane Partition Coefficients

A key parameter in membrane and chromatographic separations is the partition coefficient, the equilibrium ratio of the solute concentration in a porous or fibrous material to that in bulk solution. The theoretical effects of solute size on partition coefficients in straight pores or randomly oriented fiber matrices have been investigated previously for very dilute solutions, where solute-solute interactions are negligible, and also for more concentrated solutions consisting of spherical solutes of uniform size. For concentrated solutions it has been found that steric and other repulsive interactions among solutes increase the partition coefficient above the dilute limit. To extend the results for porous or fibrous media to include concentrated mixtures of solutes with different sizes or shapes, we used an excluded volume approach. In this formulation, which describes steric interactions only, partition coefficients were computed by summing all volumes excluded to a solute molecule by virtue of its finite size, the finite size of other solutes, and the presence of fixed obstacles (pore walls or fibers). For a mixture of two spherical solutes, the addition of any second solute at finite concentration increased the partition coefficient of the first solute. That increase was sensitive to the size of the second solute; for a given volume fraction of the second solute, the smaller its radius, the larger the effect. When the total volume fraction of solutes was fixed, an increase in the amount of a second, smaller solute increased the partition coefficient of the first solute, whereas an increase in the amount of a second, larger solute had the opposite effect. Results were obtained also for oblate or prolate spheroidal solutes and for fibrous media containing fibers of different radii. For constant total fiber volume fraction, an increase in the amount of a second, smaller fiber decreased the partition coefficient of a spherical solute, whereas an increase in the amount of a second, larger fiber had the opposite effect. Overall, the theory suggests that the introduction of heterogeneities, whether as mixtures of solute sizes or mixtures of fiber sizes, may cause partition coefficients to differ markedly from those of uniform systems. Copyright 2000 Academic Press.