Abstract 540: Role of EphB4-EphrinB2 as novel target in Her2 positive breast cancer

Background: Her2 amplification drives tumor growth in 20% of breast cancers. Her2 targeted therapies have improved survival, but eventual failure occurs and additional targeted therapies to overcome resistance are needed. EphB4 and EphrinB2 receptor kinase -ligand pair is induced in Her2 positive tumors and is a novel target.

Methods: Her2 positive tumor cell lines and Her2 transgenic mouse lines were studied for expression of EphB4-EphrinB2. The role of EphB4-EphrinB2 was examined by knock down with shRNA. In vivo tumor xenograft studies with Her2 positive cell lines resistant or sensitive to Her2 targeted therapy, and Her2 transgenic mouse were treated with soluble decoy receptor (soluble EphB4-Albumin fusion protein). Tumor response and activation status of Her2 were examined.

Results: EphB4 and EphrinB2 are induced in Her2 positive tumor cell lines with further induction in trastuzumab (Herceptin) resistant tumor lines. Knock down of EphB4 with shRNA reduced the cell viability by nearly 80% compared to control shRNA. In vivo studies with sEphB4-Albumin also reduced the tumor growth by 70-80% accompanied by the reduction in pHer2 and pEGFR. No effect was seen on pHer3. Combination with lapatinib and sEphB4-Albumin was additive. sEphB4-Albumin inhibited tumor growth in Her2 resistant tumor cell lines by 56%.

Conclusion: EphB4 and EphrinB2 are induced by Herceptin sensitive and resistant Her2 positive tumors. EphB4 provides survival advantage to Her2 positive trastuzumab sensitive and resistant tumors. sEphB4-Albumin inhibits tumor growth as single agent in both trastuzumab sensitive and resistant tumors. sEphB4-Albumin is currently in the clinical development and thus worthy of investigation in Her2 positive cancers.

Citation Format: A. Duarte, J. Quaresma, A. Trindade, P. Tripathy, X. Liu, V. Krasnoperov, R. Liu, P.S. Gill. Role of EphB4-EphrinB2 as novel target in Her2 positive breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 540. doi:10.1158/1538-7445.AM2013-540

Abstract 335: Therapeutic targeting of EphB4 with a novel monoclonal antibody

Objective: EphB4 is a transmembrane receptor tyrosine kinase that plays an important role in neural plasticity and angiogenesis. Its biological and clinical relevance in ovarian cancer is the focus of this study.

Methods: Benign ovarian (ML5 and ML10) and ovarian cancer (Hey, SKOV3ip1, A2780-par, A2780-cp20, and IGROV-af1) cell lines, 7 normal ovarian and 85 invasive ovarian cancer samples were evaluated for EphB4 expression by Western blot or immunohistochemistry. Univariate and multivariate analyses were performed for clinical correlation. EphB4 blockade was accomplished using a novel EphB4-targeted monoclonal antibody (EphB4-131). Systemic delivery of EphB4-131 antibody was utilized in vivo to target EphB4 in orthotopic mouse models of ovarian cancer.

Results: By Western blot analysis, all five malignant cell lines overexpressed EphB4, whereas the two benign cell lines had low EphB4 expression. All of the normal ovarian tissues had low or absent EphB4 expression. Among the invasive ovarian carcinomas (mean age of patients was 59.4 years), 73 (86%) had some level of EphB4 expression and overexpression (moderate or strong) was noted in 58% of all samples. EphB4 overexpression significantly correlated with advanced stage (III or IV; p<0.001) and presence of ascites (p<0.001). In multivariate analysis, EphB4 overexpression (p=0.04) and stage (p=0.03) were independent predictors of poor survival. EphB4-131 antibody led to decreased EphB4 expression in A2780-cp20 and IGROV-af1 cell lines, lasting for over 5 days. Compared to controls, EphB4-131 antibody alone significantly decreased tumor growth in A2780-cp20 (83%, p<0.01) and IGROV-af1 (80%, p<0.001) orthotopic ovarian cancer models. Combination therapy with EphB4-131 antibody and docetaxel resulted in the greatest tumor reduction in both A2780-cp20 and IGROV-af1 cell lines (94-98% reduction versus controls; p<0.01).

Conclusions: EphB4 overexpression is predictive of poor clinical outcome in ovarian cancer patients and is associated with aggressive features. Therapeutic targeting of EphB4 with the novel antibody EphB4-131 is an attractive strategy for further development.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 335.

Communication between ephrinB2 and EphB4 within the osteoblast lineage

Members of the ephrin and Eph family are local mediators of cell function through largely contact-dependent processes in development and in maturity. Production of ephrinB2 mRNA and protein are increased by PTH and PTHrP in osteoblasts. Both a synthetic peptide antagonist of ephrinB2/EphB4 receptor interaction and recombinant soluble extracellular domain of EphB4 (sEphB4), which is an antagonist of both forward and reverse EphB4 signaling, were able to inhibit mineralization and the expression of several osteoblast genes involved late in osteoblast differentiation. The findings are consistent with ephrinB2/EphB4 signaling within the osteoblast lineage having a paracrine role in osteoblast differentiation, in addition to the proposed role of osteoclast-derived ephrinB2 in coupling of bone formation to resorption. This local regulation might contribute to control of osteoblast differentiation and bone formation at remodeling sites, and perhaps also in modeling.

The receptor tyrosine kinase EphB4 is overexpressed in ovarian cancer, provides survival signals and predicts poor outcome

EphB4 is a member of the largest family of transmembrane receptor tyrosine kinases and plays critical roles in axonal pathfinding and blood vessel maturation. We wanted to determine the biological role of EphB4 in ovarian cancer. We studied the expression of EphB4 in seven normal ovarian specimens and 85 invasive ovarian carcinomas by immunohistochemistry. EphB4 expression was largely absent in normal ovarian surface epithelium, but was expressed in 86% of ovarian cancers. EphB4 expression was significantly associated with advanced stage of disease and the presence of ascites. Overexpression of EphB4 predicted poor survival in both univariate and multivariate analyses. We also studied the biological significance of EphB4 expression in ovarian tumour cells lines in vitro and in vivo. All five malignant ovarian tumour cell lines tested expressed higher levels of EphB4 compared with the two benign cell lines. Treatment of malignant, but not benign, ovarian tumour cell lines with progesterone, but not oestrogen, led to a 90% reduction in EphB4 levels that was associated with 50% reduction in cell survival. Inhibition of EphB4 expression by specific siRNA or antisense oligonucleotides significantly inhibited tumour cell viability by inducing apoptosis via activation of caspase-8, and also inhibited tumour cell invasion and migration. Furthermore, EphB4 antisense significantly inhibited growth of ovarian tumour xenografts and tumour microvasculature in vivo. Inhibition of EphB4 may hence have prognostic and therapeutic utility in ovarian carcinoma.

EphB4 receptor tyrosine kinase is expressed in bladder cancer and provides signals for cell survival

We sought to evaluate the biological function of the receptor tyrosine kinase EphB4 in bladder cancer. All of the nine bladder cancer cell lines examined express EphB4 and the receptor could be phosphorylated following stimulation with its cognate ligand, EphrinB2. Out of the 15 fresh bladder cancer specimens examined, 14 expressed EphB4 with a mean sevenfold higher level of expression compared to adjacent normal urothelium. EphB4 expression was regulated by several mechanisms: EPHB4 gene locus was amplified in 27% tumor specimens and 33% cell lines studied; inhibition of EGFR signaling downregulated EphB4 levels; and forced expression of wild-type p53 reduced EphB4 expression. EphB4 knockdown using specific siRNA and antisense oligodeoxynucleotides molecules led to a profound inhibition in cell viability associated with apoptosis via activation of caspase-8 pathway and downregulation of antiapoptotic factor, bcl-xl. Furthermore, EphB4 knockdown significantly inhibited tumor cell migration and invasion. EphB4 knockdown in an in vivo murine tumor xenograft model led to a nearly 80% reduction in tumor volume associated with reduced tumor proliferation, increased apoptosis and reduced tumor microvasculature. EphB4 is thus a potential candidate as a predictor of disease outcome in bladder cancer and as target for novel therapy.

A novel ubiquitously expressed alpha-latrotoxin receptor is a member of the CIRL family of G-protein-coupled receptors

Poisoning with alpha-latrotoxin, a neurotoxic protein from black widow spider venom, results in a robust increase of spontaneous synaptic transmission and subsequent degeneration of affected nerve terminals. The neurotoxic action of alpha-latrotoxin involves extracellular binding to its high affinity receptors as a first step. One of these proteins, CIRL, is a neuronal G-protein-coupled receptor implicated in the regulation of secretion. We now demonstrate that CIRL has two close homologs with a similar domain structure and high degree of overall identity. These novel receptors, which we propose to name CIRL-2 and CIRL-3, together with CIRL (CIRL-1) belong to a recently identified subfamily of large orphan receptors with structural features typical of both G-protein-coupled receptors and cell adhesion proteins. Northern blotting experiments indicate that CIRL-2 is expressed ubiquitously with highest concentrations found in placenta, kidney, spleen, ovary, heart, and lung, whereas CIRL-3 is expressed predominantly in brain similarly to CIRL-1. It appears that CIRL-2 can also bind alpha-latrotoxin, although its affinity to the toxin is about 14 times less than that of CIRL-1. When overexpressed in chromaffin cells, CIRL-2 increases their sensitivity to alpha-latrotoxin stimulation but also inhibits Ca2+-regulated secretion. Thus, CIRL-2 is a functionally competent receptor of alpha-latrotoxin. Our findings suggest that although the nervous system is the primary target of low doses of alpha-latrotoxin, cells of other tissues are also susceptible to the toxic effects of alpha-latrotoxin because of the presence of CIRL-2, a low affinity receptor of the toxin.

Structural requirements for alpha-latrotoxin binding and alpha-latrotoxin-stimulated secretion. A study with calcium-independent receptor of alpha-latrotoxin (CIRL) deletion mutants

Stimulation of neurotransmitter release by alpha-latrotoxin requires its binding to the calcium-independent receptor of alpha-latrotoxin (CIRL), an orphan neuronal G protein-coupled receptor. CIRL consists of two noncovalently bound subunits, p85, a heptahelical integral membrane protein, and p120, a large extracellular polypeptide with domains homologous to lectin, olfactomedin, mucin, the secretin receptor family, and a novel structural motif common for large orphan G protein-coupled receptors. The analysis of CIRL deletion mutants indicates that the high affinity alpha-latrotoxin-binding site is located within residues 467-891, which comprise the first transmembrane segment of p85 and the C-terminal half of p120. The N-terminal lectin, olfactomedin, and mucin domains of p120 are not required for the interaction with alpha-latrotoxin. Soluble p120 and all its fragments, which include the 467-770 residues, bind alpha-latrotoxin with low affinity suggesting the importance of membrane-embedded p85 for the stabilization of the complex of the toxin with p120. Two COOH-terminal deletion mutants of CIRL, one with the truncated cytoplasmic domain and the other with only one transmembrane segment left of seven, supported both alpha-latrotoxin-induced calcium uptake in HEK293 cells and alpha-latrotoxin-stimulated secretion when expressed in chromaffin cells, although with a different dose dependence than wild-type CIRL and its N-terminal deletion mutant. Thus the signaling domains of CIRL are not critically important for the stimulation of exocytosis in intact chromaffin cells by alpha-latrotoxin.

Neurexin I alpha is a major alpha-latrotoxin receptor that cooperates in alpha-latrotoxin action

alpha-Latrotoxin is a potent neurotoxin from black widow spider venom that binds to presynaptic receptors and causes massive neurotransmitter release. A surprising finding was the biochemical description of two distinct cell surface proteins that bind alpha-latrotoxin with nanomolar affinities; Neurexin I alpha binds alpha-latrotoxin in a Ca(2+)-dependent manner, and CIRL/latrophilin binds in a Ca(2+)-independent manner. We have now generated and analyzed mice that lack neurexin I alpha to test its importance in alpha-latrotoxin action. alpha-Latrotoxin binding to brain membranes from mutant mice was decreased by almost 50% compared with wild type membranes; the decrease was almost entirely due to a loss of Ca(2+)-dependent alpha-latrotoxin binding sites. In cultured hippocampal neurons, alpha-latrotoxin was still capable of activating neurotransmission in the absence of neurexin I alpha. Direct measurements of [3H]glutamate release from synaptosomes, however, showed a major decrease in the amount of release triggered by alpha-latrotoxin in the presence of Ca2+. Thus neurexin I alpha is not essential for alpha-latrotoxin action but contributes to alpha-latrotoxin action when Ca2+ is present. Viewed as a whole, our results show that mice contain two distinct types of alpha-latrotoxin receptors with similar affinities and abundance but different properties and functions. The action of alpha-latrotoxin may therefore be mediated by independent parallel pathways, of which the CIRL/latrophilin pathway is sufficient for neurotransmitter release, whereas the neurexin I alpha pathway contributes to the Ca(2+)-dependent action of alpha-latrotoxin.

Structure and evolution of neurexophilin

Using affinity chromatography on immobilized alpha-latrotoxin, we have purified a novel 29 kDa protein, neurexophilin, in a complex with neurexin l alpha. Cloning revealed that rat and bovine neurexophilins are composed of N-terminal signal peptides, nonconserved N-terminal domains (20% identity over 80 residues), and highly homologous C-terminal sequences (85% identity over 169 residues). Analysis of genomic clones from mice identified two distinct neurexophilin genes, one of which is more homologous to rat neurexophilin and the other to bovine neurexophilin. The first neurexophilin gene is expressed abundantly in adult rat and mouse brain, whereas no mRNA corresponding to the second gene was detected in rodents despite its abundant expression in bovine brain, suggesting that rodents and cattle primarily express distinct neurexophilin genes. RNA blots and in situ hybridizations revealed that neurexophilin is expressed in adult rat brain at high levels only in a scattered subpopulation of neurons that probably represent inhibitory interneurons; by contrast, neurexins are expressed in all neurons. Neurexophilin contains a signal sequence and is N-glycosylated at multiple sites, suggesting that it is secreted and binds to the extracellular domain of neurexin l alpha. This hypothesis was confirmed by binding recombinant neurexophilin to the extracellular domains of neurexin l alpha. Together our data suggest that neurexophilin constitutes a secreted glycoprotein that is synthesized in a subclass of neurons and may be a ligand for neurexins.

High affinity binding of alpha-latrotoxin to recombinant neurexin I alpha

alpha-Latrotoxin is a potent neurotoxin from black widow spider venom that stimulates neurotransmitter release. alpha-Latrotoxin is thought to act by binding to a high affinity receptor on presynaptic nerve terminals. In previous studies, high affinity alpha-latrotoxin binding proteins were isolated and demonstrated to contain neurexin I alpha as a major component. Neurexin I alpha is a cell surface protein that exists in multiple differentially spliced isoforms and belongs to a large family of neuron-specific proteins. Using a series of neurexin I-IgG fusion proteins, we now show that recombinant neurexin I alpha binds alpha-latrotoxin directly with high affinity (Kd approximately 4 nM). Binding of alpha-latrotoxin to recombinant neurexin I alpha is dependent on Ca2+ (EC50 approximately 30 microM). Our data suggest that neurexin I alpha is a Ca(2+)-dependent high affinity receptor for alpha-latrotoxin.