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

Alexis C. McBrayer,Susan L. Bellis,Alencia V. Woodard-Grice,John K. Wakefield,Ya Zhuo

doi:10.1074/jbc.m800836200

Alexis C. McBrayer, Susan L. Bellis + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m800836200

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Abstract

Differentiation of monocytes into macrophages is accompanied by increased cell adhesiveness, due in part to the activation of alpha4beta1 integrins. Here we report that the sustained alpha4beta1 activation associated with macrophage differentiation results from expression of beta1 integrin subunits that lack alpha2-6-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 beta1 hyposialylation and enhanced alpha4beta1-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, beta1 hyposialylation, and alpha4beta1-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 beta1 subunit. Collectively, these results describe a novel mechanism for alpha4beta1 regulation and further suggest an unanticipated role for BACE1 in macrophage function.

Highlights

Associate with vascular cell adhesion molecule-1 (VCAM-1)4 on the surface of endothelial cells, an event crucial for monocyte transmigration [1,2,3,4]
Expression of Hyposialylated ␤1 Integrins Is Temporally Correlated with Cell Adhesion to VCAM-1—We previously determined that the onset of hyposialylated ␤1 integrin expression occurs ϳ7–9 h following the initiation of phorbol myristate acetate (PMA) treatment, and by 12–15 h, most of the wild-type integrin has been replaced by the hyposialylated glycoform [11]
To examine whether hyposialylated ␤1 integrin expression was temporally correlated with enhanced integrin activity, we established a time course for PMA-dependent U937 cell adhesion to VCAM-1

Summary

Introduction

Associate with vascular cell adhesion molecule-1 (VCAM-1)4 on the surface of endothelial cells, an event crucial for monocyte transmigration [1,2,3,4]. To verify that the delayed sustained phase of VCAM-1 binding was mediated by ␣4␤1 integrins, cells were treated overnight with PMA and subjected to adhesion assays in the presence of function-blocking antibodies. We observed that PMA did not significantly increase binding to denatured BSA, further suggesting that enhanced cell adhesiveness is because of integrin activation, rather than nonspecific mechanisms.

Results

Conclusion