Featured Articles

Tumor suppression: Glycosyltransferases loom LARGE

Binding of glycosylated -DG to ligands has antimigratory activity. PNAS (2009) 106, 12109-12114 doi:10.1073/pnas.0904515106

Alpha-dystroglycan (-DG) is a highly glycosylated cell adhesion molecule that mediates epithelial cell interactions with the basement membrane. Major basement membrane proteins, including laminin, bind unique but uncharacterized -DG glycans. Deficient O-mannosyl glycosylation of -DG causes muscular dystrophy, and -DG defects are also associated with tumor progression. Although glycosyltransferase genes including LARGE (named for its huge genomic span) are thus implicated in muscular dystrophy, the molecular cause for -DG defects found in carcinomas, and their link to disease progression, are unknown.

Now, Fukuda and colleagues have identified a critical role for 3-N-acetylglucosaminyltransferase 1 (3GnT1), in collaboration with LARGE, in the synthesis of the laminin-binding glycans on -DG. Reporting in Proceedings of the National Academy of Sciences of the USA, the authors show that 3GnT1-dependent -DG glycans suppress tumor formation, and they propose a model for the anti-tumor activity whereby binding of glycosylated -DG to the basement membrane balances integrin-mediated pro-invasive signals.

A decrease in -DG has been reported in several carcinomas, based on the binding of antibodies to -DG laminin-binding glycans. In this study, the authors used an antibody to -DG core protein to distinguish that actually the laminin-binding glycans, and not the -DG protein, are reduced in various migratory and invasive carcinoma cell lines. Prostate cancer cells with high expression of the full/complete a-DG glycans (PC3-H cells) produced smaller tumors in mice, were associated with decreased metastasis and were much less invasive in a Boyden chamber assay in comparison to the cells expressing lower levels of mature glycans (PC3-L cells), despite equivalent -DG protein expression. The authors also observed that the PC3-L cells had undetectable 3GnT1 expression, in contrast to PC3-H and parental PC3 cells. Transfected into PC3-L cells, 3GnT1 restored the cell surface laminin-binding -DG glycans and reduced the invasive potential and tumor-forming ability of the cells. Blocking laminin binding to -DG glycans increased PC3-H invasiveness, without affecting the performance of PC3-L cells.

To probe the relative contributions of 3GnT1 and LARGE to the expression of laminin-binding -DG glycans, the authors used siRNA, overexpression and co-immunoprecipitation studies. LARGE was also able to increase levels of -DG laminin-binding glycans, but only in the presence of 3GnT1, and the two glycosyltransferases formed a complex in cells.

The exact nature of the laminin-binding glycans is still unclear, but they are located on -mannosyl oligosaccharides. Their involvement in tumor suppression is related to integrin signaling: downregulation of LARGE, 3GnT1 or -DG increased the phosphorylation of signaling molecules ERK and AKT, indicating that functionally glycosylated -DG attenuates downstream integrin-mediated signals. A fragment of laminin that binds to -DG glycans similarly attenuated migration of cell lines expressing the glycans and decreased phosphorylation of ERK in the cells.

Combining the results of this study, the authors propose a model wherein laminin binding to functionally glycosylated -DG induces signals that counteract the downstream signaling cascades initiated by integrin binding to extracellular matrix ligands. This maintains a balanced, non-malignant phenotype that is disrupted when 3GnT1 is downregulated. With the loss of the laminin-binding -DG glycans, pro-invasive integrin signaling proceeds unchecked. Further work is needed to fully characterize the tumor-suppressing glycans, and to determine whether overexpression of 3GnT1 can control tumor growth in vivo.

Author: Emma Leah