Regulation of inflammatory signaling by the ST6Gal-I sialyltransferase

The ST6Gal-I sialyltransferase, an enzyme that adds α2-6-linked sialic acids to N-glycosylated proteins, regulates multiple immunological processes. However, the contribution of receptor sialylation to inflammatory signaling has been under-investigated. In the current study, we uncovered a role for ST6Gal-I in promoting sustained signaling through two prominent inflammatory pathways, NFκB and JAK/STAT. Using the U937 monocytic cell model, we determined that knockdown (KD) of ST6Gal-I expression had no effect on the rapid activation of NFκB by TNF (≤ 30 min), whereas long-term TNF-induced NFκB activation (2–6 hr) was diminished in ST6Gal-I-KD cells. These data align with prior work in epithelial cells showing that α2–6 sialylation of TNFR1 prolongs TNF-dependent NFκB activation. Similar to TNF, long-term, but not short-term, LPS-induced activation of NFκB was suppressed by ST6Gal-I KD. ST6Gal-I KD cells also exhibited reduced long-term IRF3 and STAT3 activation by LPS. Given that ST6Gal-I activity modulated LPS-dependent signaling, we conducted pull-down assays using SNA (a lectin specific for α2–6 sialic acids) to show that the LPS receptor, TLR4, is a substrate for sialylation by ST6Gal-I. We next assessed signaling by IFNγ, IL-6 and GM-CSF, and found that ST6Gal-I-KD had a limited effect on STAT activation induced by these cytokines. To corroborate these findings, signaling was monitored in bone marrow derived macrophages (BMDMs) from mice with myeloid-specific deletion of ST6Gal-I (LysMCre/ST6Gal-Ifl/fl). In agreement with data from U937 cells, BMDMs with ST6Gal-I knockout displayed reduced long-term activation of NFκB by both TNF and LPS, and diminished long-term LPS-dependent STAT3 activation. However, STAT activation induced by IFNγ, IL-6 and GM-CSF was comparable in wild-type and ST6Gal-I knockout BMDMs. These results implicate ST6Gal-I-mediated receptor sialylation in prolonging the activity of select signaling cascades including TNF/NFκB, LPS/NFκB, and LPS/STAT3, providing new insights into ST6Gal-I’s role in modulating the inflammatory phenotype of monocytic cells.

Glycosyltransferase ST6Gal-I promotes the epithelial to mesenchymal transition in pancreatic cancer cells

ST6Gal-I, an enzyme upregulated in numerous malignancies, adds α2-6-linked sialic acids to select membrane receptors, thereby modulating receptor signaling and cell phenotype. In this study, we investigated ST6Gal-I’s role in epithelial to mesenchymal transition (EMT) using the Suit2 pancreatic cancer cell line, which has low endogenous ST6Gal-I and limited metastatic potential, along with two metastatic Suit2-derived subclones, S2-013 and S2-LM7AA, which have upregulated ST6Gal-I. RNA-Seq results suggested that the metastatic subclones had greater activation of EMT-related gene networks than parental Suit2 cells, and forced overexpression (OE) of ST6Gal-I in the Suit2 line was sufficient to activate EMT pathways. Accordingly, we evaluated expression of EMT markers and cell invasiveness (a key phenotypic feature of EMT) in Suit2 cells with or without ST6Gal-I OE, as well as S2-013 and S2-LM7AA cells with or without ST6Gal-I knockdown (KD). Cells with high ST6Gal-I expression displayed enrichment in mesenchymal markers (N-cadherin, slug, snail, fibronectin) and cell invasiveness, relative to ST6Gal-I-low cells. Contrarily, epithelial markers (E-cadherin, occludin) were suppressed in ST6Gal-I-high cells. To gain mechanistic insight into ST6Gal-I’s role in EMT, we examined the activity of EGFR, a known EMT driver. ST6Gal-I-high cells had greater α2-6 sialylation and activation of EGFR than ST6Gal-I-low cells. The EGFR inhibitor, erlotinib, neutralized ST6Gal-I-dependent differences in EGFR activation, mesenchymal marker expression and invasiveness in Suit2 and S2-LM7AA, but not S2-013, lines. Collectively, these results advance our understanding of ST6Gal-I’s tumor-promoting function by highlighting a role for ST6Gal-I in EMT, which may be mediated, at least in part, by α2-6-sialylated EGFR.

Modulation of glycosyltransferase ST6Gal-I in gastric cancer-derived organoids disrupts homeostatic epithelial cell turnover

Programmed cell death promotes homeostatic cell turnover in the epithelium but is dysregulated in cancer. The glycosyltransferase ST6Gal-I is known to block homeostatic apoptosis through α2,6-linked sialylation of the death receptor TNFR1 in many cell types. However, its role has not been investigated in gastric epithelial cells or gastric tumorigenesis. We determined that human gastric antral epithelium rarely expressed ST6Gal-I, but the number of ST6Gal-I–expressing epithelial cells increased significantly with advancing premalignancy leading to cancer. The mRNA expression levels of ST6GAL-I and SOX9 in human gastric epithelial cells correlated positively with one another through the premalignancy cascade, indicating that increased epithelial cell expression of ST6Gal-I is associated with premalignant progression. To determine the functional impact of increased ST6Gal-I, we generated human gastric antral organoids from epithelial stem cells and differentiated epithelial monolayers from gastric organoids. Gastric epithelial stem cells strongly expressed ST6Gal-I, suggesting a novel biomarker of stemness. In contrast, organoid-derived epithelial monolayers expressed markedly reduced ST6Gal-I and underwent TNF-induced, caspase-mediated apoptosis, consistent with homeostasis. Conversely, epithelial monolayers generated from gastric cancer stem cells retained high levels of ST6Gal-I and resisted TNF-induced apoptosis, supporting prolonged survival. Protection from TNF-induced apoptosis depended on ST6Gal-I overexpression, because forced ST6Gal-I overexpression in normal gastric stem cell–differentiated monolayers inhibited TNF-induced apoptosis, and cleavage of α2,6-linked sialic acids from gastric cancer organoid-derived monolayers restored susceptibility to TNF-induced apoptosis. These findings implicate up-regulated ST6Gal-I expression in blocking homeostatic epithelial cell apoptosis in gastric cancer pathogenesis, suggesting a mechanism for prolonged epithelioid tumor cell survival.

Unliganded and CMP-Neu5Ac bound structures of human α-2,6-sialyltransferase ST6Gal I at high resolution

ABSTRACTSialic acid residues found as terminal monosaccharides in various types of glycan chains in cell surface glycoproteins and glycolipids have been identified as important contributors of cell-cell interactions in normal vs. abnormal cellular behavior and are pivotal in diseases such as cancers. In vertebrates, sialic acids are attached to glycan chains by a conserved subset of sialyltransferases with different enzymatic and substrate specificities. ST6Gal I is a sialyltransferase using activated CMP-sialic acids as donor substrates to catalyze the formation of a α2,6-glycosidic bond between the sialic acid residue and the acceptor disaccharide LacNAc. Understanding sialyltransferases at the molecular and structural level shed light into the function. We present here two human ST6Gal I structures, which show for the first time the enzyme in the unliganded state and with the full donor substrate CMP-Neu5Ac bound. Comparison of these structures reveal flexibility of the catalytic loop, since in the unliganded structure Tyr354 adopts a conformation seen also as an alternate conformation in the substrate bound structure. CMP-Neu5Ac is bound with the side chain at C-5 of the sugar residue directed towards empty space at the surface of the protein. Furthermore, the exact binding mode of the sialic acid moiety of the substrate directly involves sialylmotifs L, S and III and positions the sialylmotif VS in the immediate vicinity.PROTEIN DATA BANK ACCESSION CODESAtomic coordinates and structure factors of the human wild-type unliganded and CMP-Neu5Ac bound ST6Gal I have been deposited with the PDB with accession codes 6QVS and 6QVT, respectively.