TMIC-26. PRECLINICAL MODEL SYSTEMS OF GLIOBLASTOMA REVEAL MICROENVIRONMENTAL PROGRAMS AND DEPENDENCIES IN PATIENT TUMORS

Patient-derived model systems serve as a platform for translational research representing the heterogeneity of human cancers, and their success in recapitulating disease-driving genomic alterations is well-documented. While recent studies have demonstrated genomic and functional divergence in patient-derived models with passaging, the need for accurate preclinical models remains. Glioblastoma (GBM) is the most common and aggressive primary brain tumor, and thus far preclinical models have failed to consistently replicate the responses found in patients. We therefore aimed to evaluate the multi-omic fidelity of low-passage GBM model systems across in vitro and in vivo environments and to elucidate the molecular features in which they differ. To this end we established a biobank of glioma direct-from-patient orthotopic xenograft (GliomaPDOX) models and primary gliomasphere cultures (GSCs) and performed whole-exome and RNA sequencing of over 40 purified patient tumors and their matched GliomaPDOXs and GSCs to facilitate paired comparisons across a gradient of full tumor microenvironment (TME) presence. We observed global genomic and transcriptomic fidelity in both systems, but specific programmatic gene expression differences associated with cell-cell interactions in the brain TME, glial cell identity, and in vitro GSC-forming ability. GSCs and GSC-forming ability are strongly associated with an astrocytic gene expression signature, while more stem-like and oligodendrocytic patient tumors including IDH- and H3F3A-mutant GBMs more successfully engraft in GliomaPDOXs. This result implicates the brain TME as a support system for these more stem/oligo-like tumors. Transcription factor network analysis identified regulators of the NOTCH and MYC pathways as strongly enriched in this subgroup of patient tumors and their derivative xenografts, and provides potential targets for therapeutic intervention in near future experiments. Collectively, these findings underline the critical role of the TME in defining GBM cell state, reveal the heterogeneity of TME dependence across patient tumors, and link this dependency to therapeutically actionable molecular features.

Stress-induced EGF receptor signaling through STAT3 and tumor progression in triple-negative breast cancer

Elevated STAT3 activity is a hallmark of many epithelial carcinomas particularly in breast cancers where it is known to contribute to tumor progression through a variety of context-dependent biological responses. However, its role downstream of stress-exposed EGF receptors (EGFR) that are transactivated in endosomes independent of exogenous ligand has not been studied. This review discusses how STAT3 signaling induced by therapeutic stress in EGFR-driven triple-negative breast cancers (TNBC) might override normal epithelial homeostatic mechanisms and provide a survival advantage for tumor cells before they leave the primary tumor and spread to distant sites. Despite continued improvements in breast cancer treatment strategies, TNBC is still associated with poor prognosis and high risk of distant recurrence and death. Understanding EGFR-STAT3 signaling mechanisms regulating the earliest steps of tumor progression is a key to discovery of new targeted therapies against TNBC.

Mutual cross-talk between fibronectin integrins and the EGF receptor: Molecular basis and biological significance

Extension of the plasma membrane is one of the first steps in cell migration. Understanding how cells “choose” between various types of membrane protrusion enhances our knowledge of both normal and cancer cell physiology. The EGF receptor is a paradigm for understanding how transmembrane receptor tyrosine kinases regulate intracellular signaling following ligand stimulation. Evidence from the past decade indicates that EGF receptors also form macromolecular complexes with integrin receptors leading to EGF receptor transactivation during cell adhesion. However, relatively little is known about how these complexes form and impact cell migration. Our recent work characterized a molecular complex between EGF receptor and β3 integrin which recognizes RGD motifs in extracellular matrix proteins. Complex formation requires a dileucine motif (679-LL) in the intracellular juxtamembrane region of the EGF receptor that also controls whether or not the receptor undergoes Src kinase-dependent phosphorylation at Tyr-845. In contrast to wild-type receptors, mutant EGF receptors defective for Tyr-845 phosphorylation form complexes with β1 integrin that also binds RGD motifs. In addition, we have discovered that EGF receptor antagonizes small GTPase RhoA by mediating membrane recruitment of its regulatory GAP p190RhoGAP. In this addendum we discuss a potential new role for Src-dependent EGF receptor transactivation in integrin/EGF receptor complex formation. We also discuss how our study fits with previous observations linking p190RhoGAP to RhoA-dependent cytoskeletal rearrangements involved in cell migration, and provide new data that the EGF receptor is compartmentalized to relatively immature zyxin-poor focal adhesions which are the likely site of p190RhoGAP signaling.

STAT3 and epithelial-mesenchymal transitions in carcinomas

Cellular programs coupled to cycles of epithelial–mesenchymal transitions (EMTs) play critical roles during embryogenesis, as well as during tissue development, remodeling, and repair. Research over the last decade has established the importance of an ever-expanding list of master EMT transcription factors, whose activity is regulated by STAT3 and function to stimulate the rapid transition of cells between epithelial and mesenchymal phenotypes. Importantly, inappropriate reactivation of embryonic EMT programs in carcinoma cells underlies their metastasis to distant organ sites, as well as their acquisition of stem cell-like and chemoresistant phenotypes operant in eliciting disease recurrence. Thus, targeted inactivation of master EMT transcription factors may offer new inroads to alleviate metastatic disease. Here we review the molecular, cellular, and microenvironmental factors that contribute to the pathophysiological activities of STAT3 during its regulation of EMT programs in human carcinomas.

Epithelial to mesenchymal transition promotes breast cancer progression via a fibronectin-dependent STAT3 signaling pathway

We previously established that overexpression of the EGF receptor (EGFR) is sufficient to induce tumor formation by otherwise nontransformed mammary epithelial cells, and that the initiation of epithelial-mesenchymal transition (EMT) is capable of increasing the invasion and metastasis of these cells. Using this breast cancer (BC) model, we find that in addition to EGF, adhesion to fibronectin (FN) activates signal transducer and activator of transcription 3 (STAT3) through EGFR-dependent and -independent mechanisms. Importantly, EMT facilitated a signaling switch from SRC-dependent EGFR:STAT3 signaling in pre-EMT cells to EGFR-independent FN:JAK2:STAT3 signaling in their post-EMT counterparts, thereby sensitizing these cells to JAK2 inhibition. Accordingly, human metastatic BC cells that failed to activate STAT3 downstream of EGFR did display robust STAT3 activity upon adhesion to FN. Furthermore, FN enhanced outgrowth in three-dimensional organotypic cultures via a mechanism that is dependent upon β1 integrin, Janus kinase 2 (JAK2), and STAT3 but not EGFR. Collectively, our data demonstrate that matrix-initiated signaling is sufficient to drive STAT3 activation, a reaction that is facilitated by EMT during BC metastatic progression.

Abstract A17: Regulation of EGFR: Stat3 signaling by Mig6 is required for metastatic outgrowth in triple-negative breast cancer

Stat3 is an oncogene that enriches for tumor initiating breast cancer cells. Stat3 activity is driven by numerous signaling inputs, including those initiated by EGFR. Herein we shown that Stat3 is critically involved in EGFR-driven tumorigenesis as EGFR molecules harboring a mutation in the juxtamembrane domain that specifically prevents activation of Stat3 fail to transform mammary epithelial cells. Using this model of EGFR transformation, we previously established that the induction of epithelial-mesenchymal transition (EMT) enhances pulmonary tumor formation. Surprisingly, EMT drastically diminishes the capacity of EGFR to signal to Stat3. Indeed, using the MCF-10A progression series, we show that constitutive activation of Stat3 is consistent with the acquisition of a mesenchymal phenotype but independent of EGFR activity. As a compensatory mechanism, we find that EMT increases the expression and ability of fibronectin to directly activate Stat3 in an EGFR-independent, Jak2-dependent manor. Indeed, human triple-negative breast cancer (TNBC) cells that fail to activate Stat3 in response to EGF robustly activate Stat3 upon fibronectin adhesion. Consistent with these results, TNBC outgrowth in pulmonary 3D-organotypic cultures is enhanced by the addition of fibronectin, a process that is absolutely dependent on β1-integrin, Jak2 and Stat3, but independent of EGFR. Underscoring the importance of this switch from EGFR-mediated to fibronectin-mediated Stat3 activation in metastatic TNBC cells, we demonstrate that shRNA-mediated diminution of the EGFR inhibitory molecule MIG6 results in increased apoptosis and consequently, decreased pulmonary tumor formation by TNBC cells. Our data clearly demonstrate that matrix-initiated signaling is sufficient to drive Stat3 activation, a reaction that likely contributes to the resistance of metastatic TNBCs to molecular therapies, particularly those that target EGFR.

Citation Format: Mike K. Wendt, Nikolas Balanis, Barbara Schiemann, Cathleen Carlin, William Schiemann. Regulation of EGFR: Stat3 signaling by Mig6 is required for metastatic outgrowth in triple-negative breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr A17.

Abstract A24: Fibronectin-mediated activation of Stat3 facilitates the resistance of triple negative breast cancer to EGFR-targeted therapies

Breast cancer (BC) can be subdivided into at least five genetically distinct subtypes. Those classified as triple-negative BC (TNBC) are the most aggressive and do not respond to existing molecularly targeted therapies due to their lack of estrogen receptor and Her2. In the context of TNBC, expression of EGFR within the primary tumor is strongly linked to decreased patient survival, pointing to EGFR as an effective target in treating TNBC. However, EGFR-targeted therapies do not offer clinical benefit to TNBC patients. The mechanisms that underlie this disconnect between the diagnostic and therapeutic values of EGFR in TNBC are unknown. Signal-transducer and activator of transcription-3 (Stat3) is an established oncogene and the importance of constitutive Stat3 activity has recently been described in the maintenance of a tumor-initiating stem-like phenotype in BC. We establish herein that overexpression of WT-EGFR is capable of transforming normal mammary epithelial cells (MECs) through a Stat3-mediated process. Using this overexpression model, we show that activation of Stat3 can be initiated through EGF-induced signaling and via fibronectin (FN)-mediated transactivation of EGFR. Following the induction of epithelial-mesenchymal transition (EMT) by TGF-β, orthotopic mammary tumors formed by EGFR-transformed MECs progress to a TNBC phenotype and undergo postsurgical recurrence and pulmonary metastasis. This induction of metastasis is associated with a potent inhibition of EGF-induced activation of Stat3 signaling and a corresponding increase is FN-driven Stat3. Using the human TNBC cells lines MDA-MB-231 and MDA-MB-468 we show that unlike EGF-induced activation of Stat3, kinase inhibitors of EGFR (AG1478) or Src (PP2) activity do not block FN-mediated Stat3 activity. Consistent with these data and clinical findings we demonstrate that following the induction of EMT, even cells originally transformed by EGFR fail to respond to erlotinib therapy during the outgrowth of recurrent and metastatic lesions. Concomitant with the diminution of EGFR-mediated Stat3 signaling we used microarray and RT-PCR analyses to show that the IIIc isoform of FGFR1 is potently and stably upregulated following the induction of EMT. Along these lines, we have previously established that a gain in metastatic outgrowth potential is associated with the loss of EGFR expression in the D2-HAN model of metastatic pulmonary outgrowth. In contrast to EGFR, mesenchymal and metastatic D2.A1 cells express robust levels of FGFR1-IIIc. Importantly, FN was still capable of activating Stat3 in these EGFR-null, metastatic TNBC cells. To examine the role of FN transactivation of FGFR:Stat3 signaling in mediating the metastatic outgrowth of TNBC we used an organotypic 3-D culture model that accurately recapitulates the pulmonary microenvironment. Addition of FN to this system potently induces the outgrowth of D2.A1 cells, a process known to require β1 integrin. Genetic depletion of FGFR1 using an shRNA approach or pharmacological inhibition FGFR (PD17304) or Stat3 (Stattic) ablated the ability of FN to induce 3-D outgrowth of D2.A1 cells. Taken together, our studies indicate that EMT empowers TNBC cells with a secondary tumor-initiating phenotype by converting Stat3 activation from a ligand-dependent event to a constitutive event that occurs through FN:β1 integrin-mediated transactivation of FGFR-IIIc within the metastatic microenvironment. Specific targeting of FGFR-IIIc activity and/or its transactivation by FN may hold the key to preventing the outgrowth of metastatic TNBCs that are resistant to EGFR-targeted therapies.

β3 integrin-EGF receptor cross-talk activates p190RhoGAP in mouse mammary gland epithelial cells

Active RhoA localizes to plasma membrane, where it stimulates formation of focal adhesions and stress fibers. RhoA activity is inhibited by p190RhoGAP following integrin-mediated cell attachment to allow sampling of new adhesive environments. p190RhoGAP is itself activated by Src-dependent tyrosine phosphorylation, which facilitates complex formation with p120RasGAP. This complex then translocates to the cell surface, where p190RhoGAP down-regulates RhoA. Here we demonstrate that the epidermal growth factor receptor (EGFR) cooperates with β3 integrin to regulate p190RhoGAP activity in mouse mammary gland epithelial cells. Adhesion to fibronectin stimulates tyrosine phosphorylation of the EGFR in the absence of receptor ligands. Use of a dominant inhibitory EGFR mutant demonstrates that fibronectin-activated EGFR recruits p120RasGAP to the cell periphery. Expression of an inactive β3 integrin subunit abolishes p190RhoGAP tyrosine phosphorylation, demonstrating a mechanistic link between β3 integrin-activated Src and EGFR regulation of the RhoA inhibitor. The β3 integrin/EGFR pathway also has a positive role in formation of filopodia. Together our data suggest that EGFR constitutes an important intrinsic migratory cue since fibronectin is a key component of the microenvironment in normal mammary gland development and breast cancer. Our data also suggest that EGFR expressed at high levels has a role in eliciting cell shape changes associated with epithelial-to-mesenchymal transition.