Glycan structures decorate a wide variety of cell types—dotting the surface of blood cells, linked to lipid moieties on neurons, and studding the membrane of tumor cells. These carbohydrate structures create a cellular language that is translated by binding partners, eliciting specific downstream effects. And for some cancers, the effects can be devastating.
Consider carbohydrate chains that contain sialic acid, a negatively charged sugar that frequently terminates the carbohydrate moieties of glycoproteins and glycolipids. Sialic acid can be linked to sub-terminal sugars through an alpha-2-6 bond to N-acetylgalactosamine (GalNAc) or N-acetylglucosamine (GlcNAc), an alpha-2,3 or alpha-2,6 bond to galactose (Gal) or through an alpha-2-8-bond to another sialic acid, forming polysialic acids.  Because of its terminal position and negative charge, sialic acid influences molecular and cellular interactions in a manner referred to as “sialosignaling”. These interactions have been postulated to activate self-amplifying signaling pathways that drive cancer progression. 1
To understand the role of sialic acid in cell signaling and disease, it’s necessary to study the enzymes that direct the addition of sialic acids to acceptors. Sialyltransferases (STases) catalyze the transfer of sialic acid from CMP-sialic acid to an acceptor carbohydrate chain. Like many other glycosyltransferases (the general term for enzymes that catalyze any sugar transfer from a donor to an acceptor), they are often integral membrane proteins, and may be situated near other glycosyltransferases working in “assembly line” fashion.
In humans, up-regulation of STase activity has been observed in plasma of cancer patients., An increase in activity of specific STases is correlated with invasiveness and progression for many different cancers. For example, high expression of alpha2,3-sialyltransferase type I (ST3GalI) is associated with advanced stage epithelial ovarian cancer, which has the highest mortality rate among all gynecologic cancers. Over-expression of ST6-GalI is correlated with the aggressiveness of prostate cancer: ST6-GalI regulates the proliferation, growth, migration, and invasion of prostate cancer tumors, as well as similar processes in hepatocellular carcinoma, ovarian cancer, and breast cancer.  In thyroid cancer, follicular thyroid carcinoma invasiveness is mediated by ST6GalNAcII through the PI3K/Akt/NF-κB signaling pathway.  Gastric carcinoma cells with increased expression of ST3GAL4, which synthesizes the Sialyl Lewis X structure, have an invasive phenotype and show c-Met activation. And Sialyl Lewis X antigens are also linked to post-operative recurrence of non-small cell lung cancer and reduced survival.
Given the wide range of cancers that are affected by sialic acid-containing structures, how might these effects occur? In the case of Sialyl-Le(a) or Sialyl-Le(x) structures, it seems that the presence of sialic acid directly promotes the metastatic process through binding to selectins (E- or P-selectin), which enable endothelial interaction at other sites in the body.  Another group of sialylated antigens, the Thomsen-Friedenreich (TF)-related antigens, may exert their impact on invasiveness of bladder cancer, colon cancer, breast cancer, and other forms of cancer through interaction with Siglec 15 lectin, which is expressed by macrophages.1 Distinct mechanisms could regulate the involvement of glycoproteins modified by polysialic acid (PSA). Long chains of PSA are correlated with malignancy in cancers of neuroectodermal origin, which could be mediated by E-cadherin and/or by increased numbers of cell-substratum focal adhesions.1
These are but a few examples of the many ways in which sialic acid-containing structures and STase activity are involved in seemingly disparate aspects of tumorigenesis. Given that hypersialylation of tumor cell surface proteins is a key determinant of over a dozen different cancers, the race to identify and characterize is becoming competitive, with compelling rationale for a variety of possible strategies. And while STase inhibitor drug discovery is an emerging goal, challenges remain—particularly with regard to identifying candidate inhibitors that are specific, selective, cell-permeable, and amenable to synthesis.
Implementation of sensitive assays that can be used for high-throughput screening (HTS) for discovery of STase inhibitors will undoubtedly be crucial. The Transcreener® AMP2/GMP2 Assay is capable of detecting STase activity through sensitive, robust fluorescence signal detection of CMP released from CMP-sialic acid. The broad applicability of this approach holds significant advantage for HTS, which will require a sensitive yet flexible format to identify diverse candidate inhibitors.
– Robyn M. Perrin, PhD
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 Wen KC, Sung PL, Hsieh SL, Chou YT, Lee OK, Wu CW, Wang PH 2017. α2,3-sialyltransferase type I regulates migration and peritoneal dissemination of ovarian cancer cells. Oncotarget. 8(17):29013-29027
 Wei A, Fan B, Zhao Y, Zhang H, Wang L, Yu X, Yuan Q, Yang D, Wang S. 2016. ST6Gal-I overexpression facilitates prostate cancer progression via the PI3K/Akt/GSK-3β/β-catenin signaling pathway. Oncotarget. 7(40):65374-65388.
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 Yu CJ, Shih JY, Lee YC, Shun CT, Yuan A, Yang PC. 2005. Sialyl Lewis antigens: association with MUC5AC protein and correlation with post-operative recurrence of non-small cell lung cancer. Lung Cancer. 47(1):59-67.
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