br Disclaimer Statement br Introduction The erythropoietin p
Introduction The erythropoietin-producing hepatoma (Eph) receptors represent the largest class of receptor tyrosine kinases (RTKs). They are type I transmembrane proteins that interact with their membrane-bound ligands the ephrins and facilitate cell-to-cell contacts resulting in bidirectional intracellular signaling [1,2]. Ephrin stimulation causes receptor dimerization and autophosphorylation of the Eph receptor on the juxtamembrane regions and cytoplasmic tails driving the subsequent recruitment of down-stream signaling molecules . These include SH2 and SH3 adapter proteins, Src family kinases, PI3K, MAP kinases, small GTPases, guanine nucleotide exchange factors, and phosphatases, each of which contribute to the complex cell repulsion and adhesion pathways that modulate cell shape, motility and attachment . Based on sequence homologies and their binding affinities and characteristics, the Eph receptors are grouped into subclasses namely, Eph A and Eph B receptors, although there is significant redundancy and cross talk between subclasses [, , , ]. Furthermore, while the members of the ephrin-A subclass (ephrin-A1 to -A5) are tethered to the membrane via a glycosylphosphatidylinositol (GPI) anchor, the members of the ephrin-B subclass (ephrin-B1 to -B3) are transmembrane proteins and have a highly conserved cytoplasmic domain . The EphB subfamily, the largest of receptor tyrosine kinases, comprises of six members (EphB1 to 6). Five of them are catalytically competent, while the more divergent EphB6 is catalytically dead . Unlike many of the other members in this family that exhibit promiscuity , the EphB4 receptor has distinctive specificity for a single ligand, ephrinB2; however, EphB4 also binds to ephrinB1 and ephrinB3 albeit weakly . Physiologically, EphB4 plays important roles in vascular remodeling of primitive capillary networks into distinct 77 9 sale and veins [12,13]. On the other hand, EphB4 is also known to regulate vascularization in malignant tumours , and is up-regulated in numerous cancer types including upper aerodigestive cancers (reviewed, [8,15,16]). Nonetheless, since Eph-ephrin interactions have been shown to either promote or inhibit tumour growth [8,17], the role of these receptors in cancer remains a matter for debate. However, crosstalk between elevated Eph receptors and other oncogenes, such as the ErbB family of receptor tyrosine kinases is thought to result in enhanced cell proliferation and tumorigenesis, presumably independent of ephrin stimulation (reviewed, ) (Fig. 1). Here, we review the function of EphB4, its potential as a therapeutic target, the developments to date on small molecule inhibitors, and EphB4 immunotherapy in upper aerodigestive malignancies.
EphB4 as a therapeutic target in aerodigestive cancers
Pharmacologically targeting EphB4
Biomarker potential Comparatively speaking, very little is known about the potential of EphB4 as a biomarker, especially in upper aerodigestive malignancies. A recent study by Yu et al.  who evaluated cell-free RNA content in the peripheral blood as a potential biomarker for detecting CTCs in NSCLC patients, found that the expression of EphB4 in PBMCs correlated with histopathological type. A survival analysis showed that the patients with enhanced expression of EphB4 along 3 other mRNAs in PBMCs had poorer disease-free survival and overall survival than those without (all P < 0.0001) suggesting that alteration of cell-free RNA content in peripheral blood might have clinical ramifications in the diagnosis and treatment of NSCLC patients. Similarly, Yavrouian et al.  examined the expression of EphB4 and EphrinB2 in tumour tissue and surrounding normal tissue in patients with head and neck squamous cell carcinoma (HNSCC) to evaluate its association with overall patient survival. Overall survival in patients was then compared with EphB4 and EphrinB2 expression. The authors found that EphB4 overexpression was greater than that of EphrinB2 compared with normal tissue. There was a statistically significant decrease in overall survival among patients with elevated EphB4 and EphrinB2 expression (P < 0.001) suggesting that EphB4 and EphrinB2 overexpression is associated with poor overall survival in patients with HNSCC.