Astrocytic tumor is the most prevalent primary brain tumor. However, the role of cell surface carbohydrates in astrocytic tumor invasion is not known. In a previous study, we showed that polysialic acid facilitates astrocytic tumor invasion and thereby tumor progression. Here, we examined the role of HNK-1 glycan in astrocytic tumor invasion. A Kaplan-Meier analysis of 45 patients revealed that higher HNK-1 expression levels were positively associated with increased survival of patients. To determine the role of HNK-1 glycan, we transfected C6 glioma cells, which lack HNK-1 glycan expression, with 1,3-glucuronyltransferase-P cDNA, generating HNK-1-positive cells. When these cells were injected into the mouse brain, the resultant tumors were 60% smaller than tumors emerging from injection of the mock-transfected HNK-1-negative C6 cells. HNK-1-positive C6 cells also grew more slowly than mocktransfected C6 cells in anchorage-dependent and anchorage-independent assays. C6-HNK-1 cells migrated well after treatment of anti-1 integrin antibody, whereas the same treatment inhibited cell migration of mock-transfected C6 cells. Similarly, ␣-dystroglycan containing HNK-1 glycan is different from those containing the laminin-binding glycans, supporting the above conclusion that C6-HNK-1 cells migrate independently from 1-integrin-mediated signaling. Moreover, HNK-1-positive cells exhibited attenuated activation of ERK 1/2 compared with mock-transfected C6 cells, whereas focal adhesion kinase activation was equivalent in both cell types. Overall, these results indicate that HNK-1 glycan functions as a tumor suppressor.HNK-1 glycan is a glucuronylated and sulfated carbohydrate expressed in a cell type-specific manner (1). HNK-1 glycan was originally characterized as an epitope recognized by a monoclonal antibody raised against human natural killer cells (2). The structure of HNK-1 glycan-positive glycolipid was identified as SO 3 3glucuronic acid 133Gal134GlcNAc133 Gal13 4Glc13ceramide (3). HNK-1 glycan SO 3 33GlcA133Gal is unique and is expressed on both glycolipids (4, 5), N-glycans, and O-glycans (1, 6 -8). HNK-1 glycans have been found on several cell adhesion molecules, including the neural cell adhesion molecule (NCAM), 3 myelin-associated glycoprotein, contactin, P0, GluR2, RPTP, and CD24 (9 -14). In humans and rats, HNK-1 glycan is found on migrating neural crest cells, cerebellar neurons, Schwann cells, and myelinated motor neurons. Neurite outgrowth of mouse motor neurons is facilitated by an HNK-1 glycolipid substratum. Similarly, neurite outgrowth is inhibited by HNK-1 glycan but not by non-sulfated HNK-1 precursor glycan (15, 16). These results indicate that sulfated HNK-1 glycan plays a critical role in cell-cell interaction and in cell migration.HNK-1 glycan is sequentially synthesized by two enzymes. The 1,3-glucuronyltransferase 1 (GlcAT-P, B3gat1) add 1,3-linked glucuronic acid (GlcA) to galactose (17), followed by addition of 3-sulfate to GlcA by the HNK-1 sulfotransferase (HNK-1ST, Chst10) (18...