Abstract 3115: Ephrin-dependent and -independent activities of the receptor tyrosine kinase EphB4

Abstract

Abstract Recent evidence has shown that the EphB4 receptor tyrosine kinase is upregulated in various types of cancer. However, it is unclear how the EphB4 receptor may contribute to oncogenesis because its activation by its preferred ligand, ephrin-B2, appears to be low in most cancer cell types. We have found that activation of EphB4 in breast cancer cells by stimulation with exogenous ephrin-B2 Fc ligand activates a tumor suppressor pathway that inhibits cell growth and migration. Our laboratory has also recently demonstrated an ephrin-independent activity of EphB4 in breast cancer cells, which may play a role in the tumors where the ligand is absent. By using RNA interference to downregulate EphB4 in MCF-7 and MDA-MB-435 cells, we found that EphB4 inhibits integrin-mediated substrate adhesion, spreading and migration, and reduces β1-integrin protein levels. Through these effects, EphB4 expression would be expected to inhibit tumor development and growth. Ephrin binding does not appear to be important for this EphB4-dependent regulation of β1-integrin levels. Here, we show that EphB4 knock down also dramatically increases β1-integrin protein levels in PC3 prostate cancer cells. Stimulation with a soluble form of the ligand, ephrin-B2 Fc, also reduces EphB4 expression. Therefore, experiments are in progress to determine if EphB4 downregulation by ligand also results in decreased β1-integrin protein levels, besides the inhibition of integrin-mediated cell adhesion induced by EphB4 ligand-dependent signaling. Experiments are also in progress to investigate the mechanism by which EphB4 regulates integrin levels, and whether it involves regulation of mRNA levels or protein stability. Interestingly, a recent report shows that the decreased expression of EphB4 in intestinal tumors from APCmin/+ mice that lack one EphB4 allele causes increased transcription of genes involved in cell attachment to the extracellular matrix and invasion (Dopeso et al., 2009, Cancer Res 69:7430). However, an increase in β1-integrin mRNA level was not detected in this model. Therefore, it will be interesting to investigate whether EphB4 regulates integrins differently in different cancer cell types, the mechanisms involved, and the effects on cell migration and invasion. 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 3115.