Altered Glycosylation in Cancer: Sialic Acids and Sialyltransferases

Abnormal protein glycosylation, resulting in expression of altered carbohydrate determinants, is well associated with malignant transformation of the cell. One family of important molecules related to aberrant glycosylation is sialic acids (SAs) and their derivatives, which are ubiquitous at the terminal positions of the oligosaccharides of glycoproteins. Sialylation affects the half-lives of many circulating glycoproteins and plays roles in a variety of biologic processes such as cell-cell communication, cell-matrix interaction, adhesion, and protein targeting. The transfer of sialic acids from CMP-sialic acids to the acceptor carbohydrates is catalyzed by the sialyltransferase (ST) family, which includes 20 glycoprotein- and glycolipid-specific α2,3-, α2,6- and α2,8-linkage transferring enzymes described up to date. Cell surface SA levels are mainly correlated with the mRNA levels of ST genes. In human, STs are expressed in many tissues at different levels. Moreover, the level of ST expression is dramatically changed during cancer transformation and this alteration can be achieved transcriptionally through tissue-specific or cell type-specific promoters that lead to the production of mRNA species which diverge in the 5'-untranslated region. Evidence shows that altered ST expression have a significant correlation with oncogenesis, tumor progression, and lymph node metastases. Therefore, the functional roles of ST in cancer pathogenesis should be elucidated with the assistance of advanced molecular technology. In this paper, an overview of Sialylation changes in cancer is highlighted. Getting insights and understandings of altered glycosylation in cancers will offer a brand-new vision in modifying cancer behavior and treating these highly lethal diseases in the near future.