Vasculogenic Mimicry in Prostate Cancer: The Roles of EphA2 and PI3K

BACKGROUND Aggressive tumor cells can form perfusable networks that mimic normal vasculature and enhance tumor growth and metastasis. A number of molecular players have been implicated in such vasculogenic mimicry, among them the receptor tyrosine kinase EphA2, which is aberrantly expressed in aggressive tumors. Here we study the role and regulation of EphA2 in vasculogenic mimicry in prostate cancer where this phenomenon is still poorly understood. METHODS Vasculogenic mimicry was characterized by tubules whose cellular lining was negative for the endothelial cell marker CD34 but positive for periodic acid-Schiff staining, and/or contained red blood cells. Vasculogenic mimicry was assessed in 92 clinical samples of prostate cancer and analyzed in more detail in three prostate cancer cell lines kept in three-dimensional culture. Tissue samples and cell lines were also assessed for total and phosphorylated levels of EphA2 and its potential regulator, Phosphoinositide 3-Kinase (PI3K). In addition, the role of EphA2 in vasculogenic mimicry and in cell migration and invasion were investigated by manipulating the levels of EphA2 through specific siRNAs. Furthermore, the role of PI3K in vasculogenic mimicry and in regulating EphA2 was tested by application of an inhibitor, LY294002. RESULTS Immunohistochemistry of prostate cancers showed a significant correlation between vasculogenic mimicry and high expression levels of EphA2, high Gleason scores, advanced TNM stage, and the presence of lymph node and distant metastases. Likewise, two prostate cancer cell lines (PC3 and DU-145) formed vasculogenic networks on Matrigel and expressed high EphA2 levels, while one line (LNCaP) showed no vasculogenic networks and lower EphA2 levels. Specific silencing of EphA2 in PC3 and DU-145 cells decreased vasculogenic mimicry as well as cell migration and invasion. Furthermore, high expression levels of PI3K and EphA2 phosphorylation at Ser897 significantly correlated with the presence of vasculogenic mimicry and in vitro inhibition of PI3K by LY294002 disrupted vasculogenic mimicry, potentially through a reduction of EphA2 phosphorylation at Ser897. CONCLUSIONS The expression levels of PI3K and EphA2 are positively correlated with vasculogenic mimicry both in vivo and in vitro. Moreover, phosphorylation levels of EphA2 regulated by PI3K are also significantly associated with vasculogenic mimicry in vivo. Based on its functional implication in vasculogenic mimicry in vitro, EphA2 signaling may be a potential therapeutic target in advanced prostate cancer.