Chen, Yin

Abstract Glycans decorate most proteins and lipids on the cell surface, and glycoproteins serve a critical role in cell-cell communication. The terminal modification of many glycans is a sialic acid. There are twenty different mammalian sialyltransferases, each with a unique specificity. Recently, we demonstrated that the sialyltransferase ST8Sia6, which adds a terminal alpha-2,8 disialic acid to a preexisting sialic acid on cell surface proteins, generates ligands for the inhibitory receptor Siglec-E. In patients, ST8Sia6 overexpression is associated with worse outcome. We have previously shown that ST8sia6 overexpression in a mouse tumor model accelerates tumor growth, dependent upon Siglec-E expression by immune cells. To understand the basis for ST8Sia6 function in tumors, proteomic analysis of murine tumor lines was performed to identify proteins modified by ST8Sia6. Here, we demonstrate that ST8Sia6 sialylates CD44. Siglecs vary significantly between species, with 5 non-conserved Siglecs in mice compared to 10 non-conserved Siglecs in humans. The closest ortholog to Siglec-E in mice is Siglec-7 in humans. We also demonstrate that ST8Sia6 sialylates CD44, generating ligands for Siglec-7. Thus, ST8sia6 has a conserved role in both mice and humans in modifying glycans on CD44 to generate ligands for Siglecs. As CD44 expression on tumors is associated with increased metastasis, we predict that ST8Sia6 may also have a role in this process which is under investigation. Supported by grants from 2T32AI007425-23 to DJF and NIH Glycobiology Common Fund supplement to R01CA243545-01A1 to VSS.

Abstract Glycans decorate almost all proteins and lipids on the cell surface, and they play a critical role in cell-cell interactions. Terminal sialic acid addition to glycans helps the immune system distinguish between “self” and “non-self.” The enzyme ST8sia6 adds alpha-2,8 disialic acids to cell surface receptors, and this sialic acid addition promotes binding to the inhibitory receptor Siglec-E. We have previously shown ST8sia6 overexpression in tumors accelerates tumor growth in a Siglec-E dependent manner. Proteomic analysis in murine tumor lines identified CD44 as a target for ST8sia6. CD44 expression on tumors is associated with metastasis and chemoresistance. Since humans and mice have different sialic acid structures, we examined whether ST8sia6 makes CD44 a ligand to the ortholog of Siglec-E in human cells, Siglec-7. When HEK293T cells overexpress ST8sia6 and CD44 together, Siglec-7 binding was significantly increased compared to ST8sia6 or CD44 overexpression alone. Thus, ST8sia6 has a conserved role from mouse to human in decorating CD44 with sialic acid. Our study proposes that ST8sia6 sialylation of CD44 could suppress immune responses and enables cancer progression in human. Supported by grant: 2T32AI007425-23 to DJF and R01 CA243545-01S1 to V.S.S.