cancers Review EphA2 and EGFR: Friends in Life, Partners in Crime. Can EphA2 Be a Predictive Biomarker of Response to Anti-EGFR Agents? Mario Cioce 1,* and Vito Michele Fazio 1,2,3,* 1 Laboratory of Molecular Medicine and Biotechnology, Department of Medicine, University Campus Bio-Medico of Rome, 00128 Rome, Italy 2 Laboratory of Oncology, Fondazione IRCCS Casa Sollievo della Sofferenza, 71013 San Giovanni Rotondo, Italy 3 Institute of Translational Pharmacology, National Research Council of Italy (CNR), 00133 Rome, Italy * Correspondence: [email protected] (M.C.); [email protected] (V.M.F.) Simple Summary: The Ephrin receptors and their ligands play important roles in organ formation and tissue repair, by orchestrating complex programs of cell adhesion and repulsion, however, this same system plays a role in cancer development In fact, EphA2 levels are higher in tumors vs normal tissue and further increased upon treatment, in vivo and in vitro. Changes in the molecular status of EphA2, of its subcellular localization, the absence of ligand and signals derived from the tumor context unleash the oncogenic role of EphA2 and its broad ability to promote resistance to radiotherapy, chemotherapy and targeted agents, including inhibitors of Epidermal-Growth-Factor- Receptor (EGFR). High levels of EphA2 may reduce response to cetuximab even in RAS wt CRC patients. In this work, we aim to review the current knowledge of the EphA2 function which is crucial for achieving a more effective therapeutic management of tumors resistant to EGFR inhibitors and to many other agents. Abstract: The Eph receptors represent the largest group among Receptor Tyrosine kinase (RTK) Citation: Cioce, M.; Fazio, V.M. families. The Eph/ephrin signaling axis plays center stage during development, and the deep EphA2 and EGFR: Friends in Life, perturbation of signaling consequent to its dysregulation in cancer reveals the multiplicity and Partners in Crime. Can EphA2 Be a complexity underlying its function. In the last decades, they have emerged as key players in solid Predictive Biomarker of Response to tumors, including colorectal cancer (CRC); however, what causes EphA2 to switch between tumor- Anti-EGFR Agents? Cancers 2021, 13, suppressive and tumor-promoting function is still an active theater of investigation. This review 700. https://doi.org/10.3390/ summarizes the recent advances in understanding EphA2 function in cancer, with detail on the cancers13040700 molecular determinants of the oncogene-tumor suppressor switch function of EphA2. We describe tumor context-specific examples of EphA2 signaling and the emerging role EphA2 plays in supporting Academic Editor: Roberta Alfieri Received: 10 January 2021 cancer—stem—cell-like populations and overcoming therapy-induced stress. In such a frame, we Accepted: 4 February 2021 detail the interaction of the EphA2 and EGFR pathway in solid tumors, including colorectal cancer. Published: 9 February 2021 We discuss the contribution of the EphA2 oncogenic signaling to the resistance to EGFR blocking agents, including cetuximab and TKIs. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: EphA2; EGFR; TKI; ephrins; CRC; CSCs; drug resistance; cetuximab; intra-tumor hetero- published maps and institutional affil- geneity; inter-tumor heterogeneity iations. 1. Introduction Copyright: © 2021 by the authors. 1.1. General Structure of Eph Receptors and Ephrin Ligands Licensee MDPI, Basel, Switzerland. The EphA2 receptor belongs to the Eph (erythropoietin-producing human hepatocel- This article is an open access article lular) superfamily, the largest among tyrosine kinase receptor families [1]. Eph receptors distributed under the terms and are classified into Eph-A and Eph-B subfamilies depending on their sequence homologies conditions of the Creative Commons and binding affinity for their cognate ephrin ligands. Although Eph receptors preferentially Attribution (CC BY) license (https:// bind ligands of the same class, cross-binding has been described [for a review, [2–4] (Figure creativecommons.org/licenses/by/ 1). All Eph receptors contain an extracellular region, with a conserved N-terminal globular 4.0/). Cancers 2021, 13, 700. https://doi.org/10.3390/cancers13040700 https://www.mdpi.com/journal/cancers Cancers 2021, 13, x 2 of 24 tors are classified into Eph-A and Eph-B subfamilies depending on their sequence homol- Cancers 2021, 13, 700 ogies and binding affinity for their cognate ephrin ligands. Although Eph receptors pref-2 of 23 erentially bind ligands of the same class, cross-binding has been described [for a review, [2–4] (Figure 1). All Eph receptors contain an extracellular region, with a conserved N- terminal globular ligand-binding domain (LBD), a cysteine-rich domain which comprises a Sushiligand-binding and an epidermal domain growth (LBD), factor a cysteine-rich (EGF)-like domain domain which and two comprises fibronectin a Sushi type and-III an epidermal growth factor (EGF)-like domain and two fibronectin type-III repeats (FN1 and repeats (FN1 and FN2). The intracellular region contains a juxtamembrane region (JM), a FN2). The intracellular region contains a juxtamembrane region (JM), a tyrosine kinase tyrosine kinase domain, a sterile alpha motif (SAM) domain, and a PDZ (Post-synaptic domain, a sterile alpha motif (SAM) domain, and a PDZ (Post-synaptic density protein-95, density protein-95, Drosophila disc large tumor suppressor (Dlg), Zona occludens-1) do- Drosophila disc large tumor suppressor (Dlg), Zona occludens-1) domain-binding motif main-binding motif that are responsible for the interaction with effector molecules. The that are responsible for the interaction with effector molecules. The receptor homo- and receptor homo- and hetero-oligomerization involves the extracellular domains (LBD and hetero-oligomerization involves the extracellular domains (LBD and cys-rich domain) [5,6]. cys-rich domain) [5,6]. The SAM domain is involved in receptor-receptor interactions, The SAM domain is involved in receptor-receptor interactions, possibly aiding homo- or possibly aiding homo- or hetero-oligomerization. The ectodomain and the intracellular hetero-oligomerization. The ectodomain and the intracellular domain are linked by a domain are linked by a transmembrane helix (TM) [for reviews, [2,3] (Figure 1). Ephrins transmembrane helix (TM) [for reviews, [2,3] (Figure1). Ephrins ligands are divided into ligands are divided into ephrin-A and ephrin-B subclasses [5,7]. Ephrin-A proteins (A1– ephrin-A and ephrin-B subclasses [5,7]. Ephrin-A proteins (A1–A6) are anchored to the A6)extracellular are anchored cell to membranethe extracellular via a glycosylcell membrane phosphatidylinositol via a glycosyl (GPI)phosphatidylinositol linkage that could (GPI)be linkage released that to activatecould beEphA released receptors to activate at distance EphA receptors [8]. Ephrin-B at distance members [8]. Ephrin (B1–B3)-B are memberstransmembrane (B1–B3) are proteins transmembrane containing protein a cytoplasmics containing domain a cytoplasmic with several domain conserved with tyrosinesev- eralresidues conserved and tyrosine a terminal residues PDZ-binding and a terminal motif allowing PDZ-binding the interaction motif allowing with proteins the interac- involved tionin with cytoskeleton proteins organizationinvolved in cytoskeleton and cell adhesion organization (Figure 1and). Thus, cell Eph-ephrinadhesion (Figure signaling 1). is Thus,transduced Eph-ephrin either signaling directly is (intransduced the case either of ephrin-Bs) directly or(in by the interaction case of ephrin with-Bs) intracellular or by interactionproteins with (like intracellular Fyn) or other proteins transmembrane (like Fyn) proteinsor other transmembrane (like the neurotrophin proteins receptor (like the p75) neurotrophin(as for ephrin-As receptor [9 ]p75) (Figure (as for1). ephrin-As [9] (Figure 1). FigureFigure 1. The 1. structureThe structure of Eph of receptors Eph receptors and their and ligands their ligands is shown. isshown. Eph receptors Eph receptors are consisting are consisting of an extracellular of an extracellular struc- turestructure consisting consisting of an ephrin of an ephrinbinding binding domain domain connected connected to two to fibronectin two fibronectin type- type-IIIIII repeats repeats by a bycysteine a cysteine-rich-rich EGF EGF-like-like motif.motif. The Thejuxtamembrane juxtamembrane region region connects connects the theextracellular extracellular portion portion of the of thereceptor receptor to the to theintracellular intracellular kinase kinase domain domain that that is linked to a sterile alpha motif (SAM) domain and PDZ-binding motif. Eph ligands (ephrin-A/B) are composed of a GPI- is linked to a sterile alpha motif (SAM) domain and PDZ-binding motif. Eph ligands (ephrin-A/B) are composed of a anchored receptor binding domain in the case of the ephrin-A type and a receptor-binding domain connected by a jux- GPI-anchored receptor binding domain in the case of the ephrin-A type and a receptor-binding domain connected by a tamembrane domain to a cytoplasmic domain and a PDZ interaction motif, in the case of ephrin-B. Eph-Ephrin signaling is transducedjuxtamembrane either domain directly to (in a cytoplasmicthe case of ephrin domain-Bs) and or a by PDZ interaction interaction with motif, Fyn in (as the has case been of ephrin-B.observed Eph-Ephrinwith ephrin signaling-As). Ligandis transduced binding likely either initiates directly clustering, (in the