Targeting oncogenic Eph receptor proteins with Hsp90 inhibitors

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Eph receptor tyrosine kinases (RTKs), with 16 distinct members, constitute the largest kinase family. The members of this superfamily regulate cellular boundaries, cell migration, and vascular morphogenesis. The Eph RTKs are highly promising targets in a wide variety of cancers, due to their selective overexpression and activation in tumors. Eph RTKs are subdivided into 2 families (EphA and EphB), based on their sequence homologies and ligand-binding specificities. EphA2 and several EphB family members have emerged as pivotal players in angiogenesis, invasion, and tumorigenic progression and are correlated with a highly invasive phenotype. Eph RTKs associate with their ligands, the ephrins, which are normally found on the cell surface of opposing cells. Cell contact promotes ligand binding, which triggers phosphorylation of the C-terminal kinase domain and transmittance of cellular signaling cascades. Ligand binding has a tumor-inhibitory role for EphA2, resulting receptor internalization and protein downregulation mediated by a proteasome-dependent pathway. In cancer cells, dysregulated cell-cell contact translates to diminished receptor-ligand assocation, resulting in overexpression of active receptor. Many oncogenic RTKs interact with heat shock protein 90 (Hsp90), a molecular chaperone involved in mediating the proper conformation and function of a multitude of signaling proteins. We report here that the Eph RTKs examined in this study, EphA2 and EphB1, form a complex with Hsp90 and co-chaperones. This association is mediated by the kinase domain of the receptor, as deletions of this domain abrogate both Hsp90 binding and sensitivity to drug. This is a significant finding, as Hsp90 inhibitors, such as the clinically-utilized 17-AAG, mediate the proteasome-dependent downregulation of Hsp90 substrates, or ‘client’ proteins. Moreover, we demonstrate that Hsp90 inhibiting agents efficiently diminish the levels of Eph receptor. These data demonstrate that Hsp90 plays an essential role in eph RTK function and may contribute to the anti-angiogenic and anti-migratory properties of 17-AAG. Furthermore, this finding suggests that Hsp90 inhibitors may be exploited as a novel means to downregulate the selective overexpression of these receptors in cancers.