EPHA2 has been described to promote the growth of OVK-18 cells^[3]. Thaker et al., evaluated EPHA2 expression in OC cell lines by Western blot analysis. EG, 222, and SKOV3 OC cell lines overexpressed EPHA2, whereas A2780-PAR and HIO-180 exhibited low to absent EPHA2 expression ^[4]. Both OVCAR3 and SKOV3 cells demonstrated strong EPHA2 and ephrin-A1 mRNA expression, as detected through reverse transcription polymerase chain reaction (RT-PCR) and Western blot^[5]. Moreover, EPHA2 overexpression promoted cell–extracellular matrix attachment in A2780 cells, increased anchorage-independent cell growth in vitro, promoted tumorigenesis in an orthotopic mouse model of OC, and resulted in enhanced microvessel density (MVD)^[6].

EPHB2 demonstrated only small variations in RNA expression across OC cell lines. Promoter hypermethylation of EPHB2, EPHB3, and EPHB4 did not, however, seem to play an important role in ovarian tumors, as reported by Wu et al.^[7]. Davidson et al., conducted affymetrix U133A array analysis for angiogenic gene expression in multiple OC cell lines. The genes encoding ephrin-B2 and EPHB4 were upregulated in mutant TP53 cells; ephrin-B2 was upregulated in the A2780 line; and EPHB4 was overexpressed in the larger pool of mutant TP53 lines. In contrast, EPHB2 showed high expression levels in wild-type TP53 cell lines, while overexpression of ephrin-A3 was induced by hypoxia^[8]. EPHB4 was found to be highly expressed in OC cell lines and to regulate cell migration and invasion. Treatment of Hoc-7 cells with progesterone led to a dose-dependent reduction in EPHB4 expression, while inhibition of EPHB4 by specific siRNA or antisense oligonucleotides resulted in reduced viability and led to apoptosis as well as activation of the death receptor caspase pathway in this OC cell line. Interestingly, EPHB4 synthetic antisense oligodeoxynucleotides significantly inhibited tumor growth in mice-bearing human OC xenografts^[9]. Ma et al., transfected antisense EPHB4 and shRNA vectors into A2780 and SKOV3 cells. Co-transfection with both vectors could inhibit growth, induce apoptosis, and reduce invasive ability of OC cells, accompanied by downregulation of EPHB4 and the PI3K/Akt/mTOR pathway^[10].

Ephrin-A1 expression was shown to be induced in OC cells by the pleotropic transcription nuclear factor kappa B (NFκB) after stimulation with tumor necrosis factor alpha (TNF-α) and interleukin 1 beta (IL-1β)^[11]. Ephrin-A3 expression was upregulated by hypoxia and promoted endothelial cell migration and adhesion in OC cell lines^[8]. Unlike ephrin-A1, Jukonen et al., suggested that endogenous ephrin-A5 is an inefficient activator of EPHA2-pY588 signaling and receptor internalization in OC cells^[12].

Ephrin-B2 was found to be overexpressed in OC cell lines^[8].

Table 1 summarizes the role of the EPH/ephrin system in OC cell lines and human xenografts.

The upregulation of EPHA1 in OC tissues reveals its implication in disease onset and progression^[13]. EPHA1 expression has been reported to be increased by more than 10-fold in OC specimens through quantitative real-time (RT-PCR)^[14] and to predispose OC patients to adverse clinical outcomes^[15].

Similarly, EPHA2 has led to overexpression in OC tissues in a series of studies^{[3][4][5][14][16]}. High levels of EPHA2 protein expression have been associated with a higher tumor grade, advanced disease stage, high MVD, strong stromal reaction and epithelial matrix metalloproteinase (MMP)-9, epithelial MMP-2, and epithelial membrane-type 1 MMP expression, as well as shorter patient survival ^[4][5][16]. Specifically, EPHA2 processing, activated by membrane-type 1 MMP, was involved in malignant transformation of ovarian tumors^[17].

EPHA4 expression in OC patients correlated with early relapse and showed an adverse clinical association when expressed in both OC tumor cells and tumor-associated macrophages^[15].

On the contrary, loss of EPHA5 immunohistochemistry (IHC) expression in OC cases was shown to be associated with tumor grade, International Federation of Gynecology and Obstetrics (FIGO) stage, and poor patient outcome^[18].

Liu et al., employed a tissue microarray IHC analysis and detected high EPHA8 protein expression in 44.80% of OC tissues, but only 6.67–15% of normal or benign ovarian tissues. A high EPHA8 protein level correlated with older age at diagnosis, higher FIGO stage, positive lymph nodes (LNs), the presence of metastasis, positive ascitic fluid, and higher serum CA-125 levels^[19].

Loss of EPHB1 protein expression in OC tissue specimens was associated with higher tumor grade, the presence of metastasis, a high proliferative index (assessed by Ki67 expression), and significantly worse overall survival (OS)^[20]. OC patients older than 60 years of age exhibited higher EPHB2 expression and poorer survival compared with younger ones^[21]. On the other hand, Gao et al., performed IHC staining of EPHB3 and underlined that EPHB3 protein expression is significantly reduced in OC specimens, as well as negatively associated with histological grade and FIGO stage^[22]. EPHB4 expression was shown to be upregulated in OC and correlated with an adverse clinical outcome, advanced disease stage, the presence of ascites, poorer OS, and poorer response to chemotherapy^{[9][15][21][23]}.

The expression of EPHB6 was associated with grade and TNM stage, and negatively correlated with Ki67 expression levels in OC samples, while patients with low EPHB6 protein expression exhibited a poorer outcome^[24].

Elevated levels of ephrin-A1 expression led to a more aggressive tumor phenotype and were associated with poor survival in OC specimens^[21]. High ephrin-A5 expression correlated with a more aggressive subtype of OC, as well as with poor patient OS^[12][14][25].

High-grade OCs showed the greatest ephrin-B expression, with a strong correlation found between ephrin-B expression and MVD, disease recurrence, and a decrease in OS^[26]. Schaner et al., investigated the genes that are more highly expressed in ovarian than breast carcinomas and identified ephrin-B1 as one of the best discriminators more highly expressed in OC^[27]. Ephrin-B2 was overexpressed in OC tissue samples, significantly increased with clinical stages, and correlated with poor survival^[23]. Ephrin-B3 has been described to correlate with EPHB4 expression in ovarian tumor specimens^[28].

Table 2 summarizes the role of the EPH/ephrin system in OC tissue samples.