Proteolytic Shedding of ST6Gal-I by BACE1 Regulates the Glycosylation and Function of α4β1 Integrins*
- Alencia V. Woodard-Grice‡1,
- Alexis C. McBrayer§,
- John K. Wakefield¶2,
- Ya Zhuo‡ and
- Susan L. Bellis‡3
- ‡Department of Physiology and Biophysics and §Department of Surgery, University of Alabama at Birmingham, Birmingham, Alabama 35294 and ¶Tranzyme Pharma, Birmingham, Alabama 35294
- 3 To whom correspondence should be addressed: Dept. of Physiology and Biophysics, Rm. 982A MCLM, 1918 University Blvd., Birmingham, AL 35294. Tel.: 205-934-3441; Fax: 205-975-9028; E-mail: bellis{at}physiology.uab.edu.
Abstract
Differentiation of monocytes into macrophages is accompanied by increased cell adhesiveness, due in part to the activation of α4β1 integrins. Here we report that the sustained α4β1 activation associated with macrophage differentiation results from expression of β1 integrin subunits that lack α2–6-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 β1 hyposialylation and enhanced α4β1-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, β1 hyposialylation, and α4β1-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 β1 subunit. Collectively, these results describe a novel mechanism for α4β1 regulation and further suggest an unanticipated role for BACE1 in macrophage function.
Footnotes
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↵4 The abbreviations used are: VCAM-1, vascular cell adhesion molecule-1; PMA, phorbol myristate acetate; ST6Gal-I, β-galactoside α2,6-sialyltransferase; FN, fibronectin; BACE1, β-site APP-cleaving enzyme 1; APP, amyloid precursor protein; ST6, U937 cells constitutively expressing ST6Gal-I; EV, empty vector U937 cells; SNA, S. nigra agglutinin; HRP, horseradish peroxidase; PKC, protein kinase C; ERK, extracellular signal-regulated kinase; Par, parental U937 cells; BSA, bovine serum albumin; MEK, mitogen-activated protein kinase/ERK kinase.
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↵* This work was supported, in whole or in part, by National Institutes of Health Grant R01CA84248. This work was also supported in part by grants from the American Heart Association and the Mizutani Foundation for Glycoscience (to S. L. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵1 Supported by a Ruth L. Kirschstein National Research Service Award predoctoral fellowship.
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↵2 Present address: Open Biosystems, Inc., Huntsville, AL.
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- Received January 31, 2008.
- Revision received July 21, 2008.
- The American Society for Biochemistry and Molecular Biology, Inc.
