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Figure 1

Figure 1. Malignant cells release glycoproteins carrying disease-related carbohydrate epitopes into the interstitial space, where they can reach the circulation. From: Sweetening the pot: adding glycosylation to the biomarker discovery equation.

Cancer is associated with major changes in the glycan biosynthetic machinery, including (1) upregulation of fucosyltransferases (FucTs), sialyltransferases (SiaTs), and the MGAT5 gene product, which is involved in the elaboration of highly branched N-linked glycans. Disease-relevant proteinases such as MMPs and ADAM family members are also upregulated. Changes in the expression of glycosyltransferases result in altered glycan assembly, which occurs in the endoplasmic reticulum and Golgi (2). Accordingly, the glycoprotein products of tumor cells carry aberrant carbohydrate structures as compared to their normal counterparts. Typical changes include increased levels of fucose (red triangle), sialic acid (purple diamond), the addition of polylactosamine units (repeating sequences of galactose [yellow circle] and N-acetylglucosamine [blue square]), and higher-ordered branching of N-linked glycans. O-linked glycans are also affected in cancer, typically carrying incomplete or prematurely truncated structures relative to those found on normal cells. After secretion or proteolytic cleavage, glycosylated molecules and/or their cleavage products are released into the interstitial space (3), where they can enter the circulation (4). Since glycoproteins and mucins usually carry many carbohydrate chains, the signals they produce are highly amplified as compared to proteins, making them attractive candidates as biomarkers.

Penelope M. Drake, et al. Clin Chem. ;56(2):223-236.

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