Background: Epithelial to mesenchymal transition (EMT) correlates with increased metastatic potential and poor prognosis. Results: Secreted eHsp90 induces EMT, matrix metalloproteinase activity and cell motility. Conclusion: EMT inducing activity of eHsp90 provides a mechanistic basis for its tumorigenic and metastatic function. Significance: The requirement for eHsp90 in supporting tumorigenic events indicates that targeting eHsp90 may represent a therapeutic approach to improve prostate cancer patient survival.

BackgroundExtracellular Hsp90 protein (eHsp90) potentiates cancer cell motility and invasion through a poorly understood mechanism involving ligand mediated function with its cognate receptor LRP1. Glioblastoma multiforme (GBM) represents one of the most aggressive and lethal brain cancers. The receptor tyrosine kinase EphA2 is overexpressed in the majority of GBM specimens and is a critical mediator of GBM invasiveness through its AKT dependent activation of EphA2 at S897 (P-EphA2S897). We explored whether eHsp90 may confer invasive properties to GBM via regulation of EphA2 mediated signaling.Principal FindingsWe find that eHsp90 signaling is essential for sustaining AKT activation, P-EphA2S897, lamellipodia formation, and concomitant GBM cell motility and invasion. Furthermore, eHsp90 promotes the recruitment of LRP1 to EphA2 in an AKT dependent manner. A finding supported by biochemical methodology and the dual expression of LRP1 and P-EphA2S897 in primary and recurrent GBM tumor specimens. Moreover, hypoxia mediated facilitation of GBM motility and invasion is dependent upon eHsp90-LRP1 signaling. Hypoxia dramatically elevated surface expression of both eHsp90 and LRP1, concomitant with eHsp90 dependent activation of src, AKT, and EphA2.SignificanceWe herein demonstrate a novel crosstalk mechanism involving eHsp90-LRP1 dependent regulation of EphA2 function. We highlight a dual role for eHsp90 in transducing signaling via LRP1, and in facilitating LRP1 co-receptor function for EphA2. Taken together, our results demonstrate activation of the eHsp90-LRP1 signaling axis as an obligate step in the initiation and maintenance of AKT signaling and EphA2 activation, thereby implicating this pathway as an integral component contributing to the aggressive nature of GBM.

Activated ␣ 2 -macroglobulin (␣ 2 M*) signals predominantly through cell surface GRP78 (CS-GRP78) to promote proliferation and survival of cancer cells; however, the molecular mechanism remains obscure. c-MYC is an essential transcriptional regulator that controls cell proliferation. We hypothesize that ␣ 2 M*/CS-GRP78-evoked key signaling events are required for transcriptional activation of c-MYC target genes. Activation of CS-GRP78 by ␣ 2 M* requires ligation of the GRP78 primary amino acid sequence (Leu 98 -Leu 115 ). After stimulation with ␣ 2 M*, CS-GRP78 signaling activates 3-phosphoinositide-dependent protein kinase-1 (PDK1) to induce phosphorylation of PLK1, which in turn induces c-MYC transcription. We demonstrate that PLK1 binds directly to c-MYC and promotes its transcriptional activity by phosphorylating Ser 62 . Moreover, activated c-MYC is recruited to the E-boxes of target genes FOSL1 and ID2 by phosphorylating histone H3 at Ser 10 . In addition, targeting the carboxyl-terminal domain of CS-GRP78 with a mAb suppresses transcriptional activation of c-MYC target genes and impairs cell proliferation. This work demonstrates that ␣ 2 M*/CS-GRP78 acts as an upstream regulator of the PDK1/PLK1 signaling axis to modulate c-MYC transcription and its target genes, suggesting a therapeutic strategy for targeting c-MYC-associated malignant progression.␣ 2 -Macroglobulin (␣ 2 M) 2 is a plasma protein that interacts with and entraps virtually all proteinases, thereby blocking access to their substrates (1). In prostate cancer patients, ␣ 2 M is proteolytically activated (␣ 2 M*) and signals predominantly through interaction with cell surface GRP78 (K d ϳ 50 -100 pM), promoting proliferation and survival of cancer cells (2, 3). GRP78 is a stress-inducible, prosurvival, endoplasmic reticulum chaperone belonging to the HSP70 family. It is composed of an ATPase domain, a peptide binding domain, and a COOHterminal domain of unknown function (4 -6). Several different cell types, including proliferating endothelial cells and tumor cells, express GRP78 on their surface (7-15). GRP78 expression at the cell surface and its ligation by ␣ 2 M* are clearly implicated in the development of metastatic prostate cancer (2, 9, 16 -19). However, the mechanism by which ␣ 2 M*/cell surface GRP78 (CS-GRP78) signaling regulates gene transcription and their responses in cell proliferation is unknown.CS-GRP78 is a multifunctional receptor that forms complexes with phosphatidylinositol 3-kinase (PI3K) and enhances phosphatidylinositol 3,4,5-trisphosphate production, consistent with its novel role as a regulator of the PI3K/Akt signaling pathway. Thus it promotes cell proliferation, survival, metastasis, and chemoresistance (9, 20 -22). CS-GRP78, through its NH 2 -terminal domain, drives PI3K/Akt activity (2), whereas targeting the COOH-terminal domain with antibody promotes apoptotic signaling (21,23). Recently, we demonstrated in vivo that targeting the GRP78 COOH-terminal domain with monoclonal antibody C38 (C38 mAb) delays tumor gro...

Tumor vascularization is an essential modulator of early tumor growth, progression, and therapeutic outcome. Although antiangiogenic treatments appear promising, intrinsic and acquired tumor resistance contributes to treatment failure. Clinical inhibition of the molecular chaperone heat shock protein 90 (Hsp90) provides an opportunity to target multiple aspects of this signaling resiliency, which may elicit more robust and enduring tumor repression relative to effects elicited by specifically targeted agents. This review highlights several primary effectors of angiogenesis modulated by Hsp90 and describes the clinical challenges posed by the redundant circuitry of these pathways. The four main topics addressed include (1) Hsp90-mediated regulation of HIF/VEGF signaling, (2) chaperone-dependent regulation of HIF-independent VEGF-mediated angiogenesis, (3) Hsp90-dependent targeting of key proangiogenic receptor tyrosine kinases and modulation of drug resistance, and (4) consideration of factors such as tumor microenvironment that pose several challenges for the clinical efficacy of anti-angiogenic therapy and Hsp90-targeted strategies.