Abstract 434: A novel extracellular chaperokine function of Hsp90 is essential for EphA2-driven cell migration in glioblastoma

Abstract

Abstract Glioblastoma multiforme (GBM) is among the most common and aggressive brain tumors with a strong propensity to invade and migrate into surrounding normal brain tissue. It is precisely this infiltrative and aggressive nature of GBM tumor cells that renders the disease largely incurable. The lethality of GBM is in large part due in part to the diffuse tumor boundary characterized by these infiltrating cells, which precludes effective cancer removal. Therefore, strategies designed to circumvent the infiltrative nature of these tumors is urgently needed. Heat shock protein (Hsp90) is a molecular chaperone for numerous intracellular proteins that support cancer growth. Interestingly, this protein has recently been shown to function extracellularly, as a chaperokine via transduction of autocrine signaling mediated by the multi-functional LRP1 (LDL receptor-related protein 1) receptor. This extracellular Hsp90 (eHsp90) LRP1 signaling pathway possesses pro-motility function within the context of a wound healing model. We therefore asked whether this signaling nodule might also contribute to the highly invasive properties associated with GBM tumor cells. We find that pharmacologic inhibition of eHsp90 potently inhibits GBM cell motility and suppresses the activation of several effector molecules known to mediate this process. In our search for upstream signaling partners, we turned our attention to EphA2, a known pro-motility receptor tyrosine kinase whose overexpression in GBM is prognostic for this disease. Surprisingly, we report that eHsp90 is necessary to sustain EphA2-dependent pro-motility function. EphA2 is a unique RTK in that it exhibits pro-motility functions in the absence of ligand due to its association with AKT, an event that induces subsequent AKT-dependent phosphorylation of EphA2. Inhibition of eHsp90 potently decreased AKT phosphorylation and inhibited the association between EphA2 and AKT. Pharmacologic targeting of eHsp90 also diminished src phosphorylation, a molecular event that is a prerequisite for AKT phosphorylation. Genetic suppression of eHsp90 signaling via downregulation its receptor LRP1 elicited similar inhibitory effects upon src and AKT, and abrogated association between EphA2 and LRP1. Taken together, our results support the notion that an eHsp90-LRP1 signaling axis sustains src activation, thereby promoting AKT phosphorylation and maintenance of the EphA2-AKT protein complex, the latter of which is essential for driving GBM motility and aggressiveness. Clinically, constitutive activation of both src and AKT are commonly observed in GBM tumor specimens. We now demonstrate that eHsp90 may be a critical upstream mediator essential to the activation of these signaling proteins, thereby providing the rationale for targeting eHsp90 in GBM. 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 434.