EphA2 and EGFR: Friends in Life, Partners in Crime. Can EphA2 Be a Predictive Biomarker of Response to Anti-EGFR Agents?

Abstract

The Eph receptors represent the largest group among Receptor Tyrosine kinase (RTK) families. The Eph/ephrin signaling axis plays center stage during development, and the deep perturbation of signaling consequent to its dysregulation in cancer reveals the multiplicity and complexity underlying its function. In the last decades, they have emerged as key players in solid tumors, including colorectal cancer (CRC); however, what causes EphA2 to switch between tumor-suppressive and tumor-promoting function is still an active theater of investigation. This review summarizes the recent advances in understanding EphA2 function in cancer, with detail on the 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 cancer-stem-cell-like populations and overcoming therapy-induced stress. In such a frame, we detail the interaction of the EphA2 and EGFR pathway in solid tumors, including colorectal cancer. We discuss the contribution of the EphA2 oncogenic signaling to the resistance to EGFR blocking agents, including cetuximab and TKIs.

Keywords: CRC; CSCs; EGFR; EphA2; TKI; cetuximab; drug resistance; ephrins; inter-tumor heterogeneity; intra-tumor heterogeneity.

Figure 1

The structure of Eph receptors and their ligands is shown. Eph receptors are consisting of an extracellular structure consisting of an ephrin binding domain connected to two fibronectin type-III repeats by a cysteine-rich EGF-like motif. The juxtamembrane region connects the extracellular portion of the receptor to the intracellular kinase domain that is linked to a sterile alpha motif (SAM) domain and PDZ-binding motif. Eph ligands (ephrin-A/B) are composed of a GPI-anchored receptor binding domain in the case of the ephrin-A type and a receptor-binding domain connected by a juxtamembrane domain to a cytoplasmic domain and a PDZ interaction motif, in the case of ephrin-B. Eph-Ephrin signaling is transduced either directly (in the case of ephrin-Bs) or by interaction with Fyn (as has been observed with ephrin-As). Ligand binding likely initiates clustering, aided by receptor-receptor interactions mediated by the SAM domain and by the PDZ (Post-synaptic density protein-95, Drosophila disc large tumor suppressor (Dlg), Zona occludens-1)-domain-binding motif. The formed complexes mediate bi-directional signaling called ephrin “reverse” and Eph “forward” signaling.

Figure 2

EphA2 signaling in normal (A) and cancer (B) cells. (A) In untransformed cells, EphA2 is engaged by its ligands, mainly EphrinA1 and highly tyrosine-phosphorylated. This mediates cell adhesion/repulsion through activation of Rac/Rho GTPAses and RASGap. Ligand binding also mediates inhibition of MAPK and AKT. Upon ligand binding, the EphA2 is targeted to endosomes in a CBL-mediated process. (B) In cancer cells, unliganded and overexpressed EphA2 is mainly phosphorylated in ser897 by PI3k/AKT, ERK/RSK, PKA, and PKC, in response to oncogenic stimuli The Akt-mTORC1, Raf-MEK-ERK, and Pyk2-Src-ERK signaling pathways were identified as the downstream signaling of the EphA2 non-canonical pathway. S897-phosphorylated EphA2 recruits Ephexin4 that in turn acts on RhoG to promote cell migration and anoikis resistance (this latter effect through a RhoG-AKT pathway). Further, FAK-integrin mediates cell adhesion and migration and may promote CSC features, including drug resistance (please also see Figure 3). The phospho-tyrosine content of EphA2 is also reduced by the LMW-PTPase, frequently overexpressed in cancer. The pro-tumorigenic contribution of EphA2 may thus derive from ligand independency, overexpression, reduced phospho-tyrosine content, and increased serine/threonine phosphorylation. Additionally, ligand-stimulated EphA2 negatively modulates the recycling of EGFR, by inhibiting AKT/PIKfyve, thus reducing the amount of available EGFR on the plasma membrane and migration. On the other hand, such feedback is attenuated in transformed cells, where EGFR levels in the plasma membrane are increased and this correlates with ligand independency of EphA2 and activation of motile responses to EGF.

Figure 3

Main mechanism(s) of EphA2 mediated resistance to therapy. (A). EphA2-mediated activation of PI3K /AKT and MAPK mediated resistance to the EGFR blocking antibody cetuximab (and, possibly, panitumumab). (B–D) EphA2-driven activation of RSK/MTORC1/S6K1/BAD and FAK signaling mediated the resistance to anti-EGFR TKI. (E). Paclitaxel activated the Cell Adhesion Mediated-Drug Resistance (CM-DR) (mediated by mutEphB6-EphA2 interaction), through JNK/CDH11/RhoA/FAK signaling. (F). Platinum-based therapies activated EphA2-mediated EMT driven by wnt-mediated activation of Snail, Slug, and Twist. Physical and functional interaction of EphA2 and YAP has been shown to mediate platinum resistance as well. Please note that while representing independent findings, it is likely that, especially for the anti-EGFR agents (A–D), the described mechanisms may coexist, given the similarity of action between these TKIs. Please also note that for the sake of clarity, in this scheme the tumor-tissue specificity has not been considered. The description of paclitaxel- and cisplatin-EphA2 driven response has been introduced here since those therapies can be used with EGFR TKI or anti-EGFR mabs in combination settings or further lines of treatment. Thin arrows indicate direct signaling, bold arrows indicate a more complex and rather indirect involvement in the indicated biological process.