Comprehensive approaches to understand regulatory functions of cancer-associated glycosphingolipids

Maliganant transformation of cells results in the expression of cancer-associated antigens, and many of them are often glycosphingolipids. We have isolated cDNAs of glycosyltransferase genes involved in the synthesis of those cancer-associated glycolipids, and have analyzed their roles in the malignant properties of cancer cells. Generally, monosialyl glycolipids tended to suppress cancer phenotypes, while disialyl compounds enhanced the malignant properties. To investigate mechanisms by which these glycosphingolipids regulate cell signals and phenotypes, we have approached effects of glycosphingolipids by identifying molecules interacting with them in the vicinity of the membrane. Those molecules were classified into two groups with cis-acting mode and trans-acting mode. To analyze cis-acting molecules, we used an EMARS-MS (enzyme-mediated activation of radical sources-mass spectrometry) approach (Kotani N and Honke K) by labeling neighboring proteins within 200 nm around target glycolipids. Among identified molecules around ganglioside GD3 on a melanoma cell line, neogenin-1 is the most intriguing one because of its restricted localization in lipid rafts in GD3-positive cells, and its functions as a transcription factor via cleavage with ・-secretase. PDGF receptor ・ was also identified as one of GD3-associating molecules on mouse gliomas. It formed a molecular complex with GD3 and Yes, leading to enhanced cell invasion. As for trans-acting molecules, sialic acid-recognizing lectins (Siglecs) such as siglec-7 and siglec-9 interact with sialyl compounds on the cell surface, leading to modulation of innate immunity towards cancer cells. Thus, we propose here that gangliosides regulate cancer phenotypes by modulation of signaling at lipid rafts, and by regulation of microenvironments. Regulation of inflammatory reaction by sialylation has been also analyzed with focus on IgG.