ABSTRACT: The ST6Gal-I sialyltransferase, an enzyme that adds ?2-6-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, NF?B 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 NF?B by TNF (? 30 min), whereas long-term TNF-induced NF?B activation (2-6 hr) was diminished in ST6Gal-I-KD cells. These data align with prior work in epithelial cells showing that ?2-6 sialylation of TNFR1 prolongs TNF-dependent NF?B activation. Similar to TNF, long-term, but not short-term, LPS-induced activation of NF?B 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 ?2-6 sialic acids) to show that the LPS receptor, TLR4, is a substrate for sialylation by ST6Gal-I. We next assessed signaling by IFN?, 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 NF?B by both TNF and LPS, and diminished long-term LPS-dependent STAT3 activation. However, STAT activation induced by IFN?, 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/NF?B, LPS/NF?B, and LPS/STAT3, providing new insights into ST6Gal-I's role in modulating the inflammatory phenotype of monocytic cells.
