Abstract
Glioblastomas are very difficult tumors to treat because they are highly invasive and disseminate within the normal brain, resulting in newly growing tumors. We have identified netrin-1 as a molecule that promotes glioblastoma invasiveness. As evidence, netrin-1 stimulates glioblastoma cell invasion directly through Matrigel-coated transwells, promotes tumor cell sprouting and enhances metastasis to lymph nodes in vivo. Furthermore, netrin-1 regulates angiogenesis as shown in specific angiogenesis assays such as enhanced capillary endothelial cells (EC) sprouting and by increased EC infiltration into Matrigel plugs in vivo, as does VEGF-A. This netrin-1 signaling pathway in glioblastoma cells includes activation of RhoA and cyclic AMP response element-binding protein (CREB). A novel finding is that netrin-1-induced glioblastoma invasiveness and angiogenesis are mediated by activated cathepsin B (CatB), a cysteine protease that translocates to the cell surface as an active enzyme and co-localizes with cell surface annexin A2 (ANXA2). The specific CatB inhibitor CA-074Me inhibits netrin-1-induced cell invasion, sprouting, and Matrigel plug angiogenesis. Silencing of CREB suppresses netrin-1-induced glioblastoma cell invasion, sprouting, and CatB expression. It is concluded that netrin-1 plays an important dual role in glioblastoma progression by promoting both glioblastoma cell invasiveness and angiogenesis in a RhoA-, CREB-, and CatB-dependent manner. Targeting netrin-1 pathways may be a promising strategy for brain cancer therapy.