Proteolytic Shedding of ST6Gal-I by BACE1 Regulates the Glycosylation and Function of α4β1 Integrins

Abstract

Differentiation of monocytes into macrophages is accompanied by increased cell adhesiveness, due in part to the activation of ␣4␤1 integrins. Here we report that the sustained ␣4␤1 activation associated with macrophage differentiation results from expression of ␤1 integrin subunits that lack ␣2-6-linked sialic acids, a carbohydrate modification added by the ST6Gal-I sialyltransferase. During differentiation of U937 monocytic cells and primary human CD14؉ monocytes, ST6Gal-I is down-regulated, leading to ␤1 hyposialylation and enhanced ␣4␤1-dependent VCAM-1 binding. Importantly, ST6Gal-I down-regulation results from cleavage by the BACE1 secretase, which we show is dramatically up-regulated during macrophage differentiation. BACE1 up-regulation, ST6Gal-I shedding, ␤1 hyposialylation, and ␣4␤1-dependent VCAM-1 binding are all temporally correlated and share the same signaling mechanism (protein kinase C/Ras/ ERK). Preventing ST6Gal-I down-regulation (and therefore integrin hyposialylation), through BACE1 inhibition or ST6Gal-I constitutive overexpression, eliminates VCAM-1 binding. Similarly, preventing integrin hyposialylation inhibits a differentiation-induced increase in the expression of an activation-dependent conformational epitope on the ␤1 subunit. Collectively, these results describe a novel mechanism for ␣4␤1 regulation and further suggest an unanticipated role for BACE1 in macrophage function.

A Systematic Study of Site-specific GalNAc-type O-Glycosylation Modulating Proprotein Convertase Processing

“…Many Golgi glycosyltransferases are known to be shed as catalytically active enzymes by cleavage in the stem region, but little is known about the mechanism and only two examples have been characterized to our knowledge. Thus, shedding a soluble active form of ST6Gal-I has been shown to be performed by BACE-1 (40). PC processing has been shown to regulate secretion of the ␤3GlcNAc transferase Lunatic fringe (41).…”

Remodeling of Marrow Hematopoietic Stem and Progenitor Cells by Non-self ST6Gal-1 Sialyltransferase

“…The role of terminal glycosylation in cell-cell interactions and in signal transduction is well documented, particularly in immune cells, e.g. during selectin-mediated homing of leukocytes to lymph nodes and inflamed sites (23,24), in galectin-mediated apoptotic selection during lymphocyte maturation (25,26), and in altered integrin and death receptor signaling by ␣2,6-sialylation (27)(28)(29)(30). Lactosaminyl glycan-based epitopes and their selectin receptors are also important for the homing of stem cells to bone marrow niches (31,32), and recent indications suggest that differential glycan recognition may serve as a checkpoint between hematopoietic stem cell (HSC) quiescence and proliferation (33).…”

Sialylation of the Fas Death Receptor by ST6Gal-I Provides Protection against Fas-mediated Apoptosis in Colon Carcinoma Cells

“…Furthermore, we show that ␣2-6 sialylation of Fas inhibits Fas signaling through both 1) blocking the binding of FADD to the Fas cytoplasmic tail, which is the first step in DISC formation, and 2) inhibiting internalization of the stimulated Fas receptor, which is necessary for Fas apoptotic signaling (4). Sialylation can alter receptor function through several mechanisms, including conformational alteration (38), clustering (39), and differential internalization rate, depending on the specific receptor. Consistent with our work, the CD45 and PECAM receptors have been shown to be targets for ␣2-6 sialylation by ST6Gal-I, and this sialylation affects internalization in both cases (39 -41).…”