Abstract 3139: EphB4-EphrinB2 receptor-ligand are downstream effectors and novel targets of PTEN deficient prostate cancer
Authors: Li Grace X. , Ma Binyun , Krasnoperov Valery G. , Siddiqi Imran , Sali Akash , Liang Gangning , Gill Inderbir S. , Pinski Jacek K. , Quinn David I. , Sadeghi Sarmad , Gill Parkash S. 
Published: August 2019

Abstract
Background EphB4 is upregulated in prostate cancer in over half the cases, and correlates with stage and survival. EprhinB2, the ligand for EphB4 has not been well studied. Functional studies show EphB4 provides a survival signal through the PI3K/PTEN/Akt/pS6 pathway. A therapeutic agent, soluble EphB4-albumin fusion protein (sEphB4) which blocks bidirectional signaling is currently in clinical development. We thus wished to conduct detailed investigation into the role of EphB4 and EphrinB2 in genetic mouse model of prostate cancer, in tumor initiation and progression, by knockout of EphB4 and by pharmacological agent (sEphB4).
Methods We studied the expression of Ephrinb2 in 180 samples of human prostate cancers and normal prostates using highly characterized, specific EphrinB2 mAb. We next used genetically engineered mouse model of prostate cancer condition deletion of PTEN in prostate epithelium, crossed with transgenic mouse line expressing luciferase. To test the role of EphB4-EphrinB2, we generated Floxed allele of Ephb4 to conditionally delete in prostate epithelium. PTENf/f, EphB4f/f and luciferase were crossed and imaged over time. Secondly, PTENf/f mice were treated with sEphB4 in prevention, established tumor and castration resistant prostate cancer (CRPC) cohorts. Prostate cell lines C4-2B, PC3 and 22RV1 were used to analyze the role of EphB4 in regulation of PI3K pathway and androgen receptor (AR).
Results EphrinB2 is expressed in 50% of human prostate cancers, but not the normal tissues. PTEN null mouse prostate tumors had elevated EphB4 and EphrinB2, but not other family members. Conditional deletion of EphB4 in the context of PTEN deletion in prostate epithelium abolished tumor formation and caused inhibition of the PI3K pathway. Treatment with sEphB4 precluded tumor development and induced tumor regression in both early prostate cancer and CRPC mice. PI3K/AKT/pS6 activity nearly complete declined in the treatment group. Mechanistic studies using EphB4 knock down showed downregulation of PI3K alpha, beta, not gamma and delta. Most surprisingly, AR levels were markedly reduced in sEphB4 treated tumors. Further studies in vitro showed decline in AR with EphB4 knock down, which could be rescued with ectopic expression of PI3K beta, but not other PI3K isoforms. These changes were recapitulated in a patient with late stage CRPC treated in a single patient IND for sEphB4.
Conclusions EphB4-EphrinB2, a receptor-ligand pair are expressed in prostate cancer and induced by loss of PTEN or activation of PI3K pathway. Genetic and pharmacological intervention validates the potential role as a downstream effectors through downregulation of PI3K signaling and AR. Drugs that can reduce AR levels and PI3Kβ inhibitors are potential candidates for prostate cancer therapy and sEphB4 holds therapeutic potential in prostate cancer.
Citation Format: Grace X. Li, Binyun Ma, Valery G. Krasnoperov, Imran Siddiqi, Akash Sali, Gangning Liang, Inderbir S. Gill, Jacek K. Pinski, David I. Quinn, Sarmad Sadeghi, Parkash S. Gill. EphB4-EphrinB2 receptor-ligand are downstream effectors and novel targets of PTEN deficient prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3139.