Regulation of Syndecan-4 Phosphorylation in Vivo*
- From the Angiogenesis Research Center, Cardiovascular Division, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215
Abstract
Recent studies suggest that some of the heparan sulfate-carrying proteoglycans may directly participate in signaling via their cytoplasmic tail. The present investigation addresses the potential involvement of syndecan-4, a widely expressed transmembrane proteoglycan, in this process. We found that the cytoplasmic tail of syndecan-4 is phosphorylated on a single serine residue (Ser183) in growth-arrested NIH 3T3 fibroblasts, with a stoichiometry of 0.3 mol Pi/mol syndecan-4. Treatment of the cells with a protein kinase C (PKC)-activating phorbol ester lead to a 2.5-fold increase in Ser183 phosphorylation. This increase was inhibited by a generic PKC inhibitor but not by an inhibitor specific to the calcium-dependent conventional PKCs, suggesting that the cytoplasmic tail of syndecan-4 is phosphorylated by a calcium-independent novel PKC isozyme. Application of 10–30 ng/ml basic fibroblast growth factor (bFGF) produced a 2–3-fold reduction in the phosphorylation of syndecan-4. Because treatment with the phosphatase inhibitor calyculin prevented the bFGF-induced decrease in syndecan-4 phosphorylation, the effect of bFGF appears to be mediated by a protein serine/threonine phosphatase type 1 or 2A. We conclude that the cytoplasmic tail of syndecan-4 is subject to in vivo phosphorylation on Ser183, which is regulated by the activities of a novel PKC isozyme and a bFGF-dependent serine/threonine phosphatase.
Footnotes
-
↵* This work was supported by National Institutes of Health Grant HL-53793 (to M. S.), National Institutes of Health Training Grant HL-07374 (to A. H.) and American Heart Association Scientist Development Grant 9730282N (to A. H.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
-
↵‡ To whom correspondence should be addressed: Cardiovascular Div., RW453, Beth Israel Deaconess Medical Center, 330 Brookline Ave., Boston, MA 02215. Tel.: 617-667-5364; Fax: 617-975-5201; E-mail:msimons{at}bidmc.harvard.edu.
-
↵1 The abbreviations used are: bFGF, basic fibroblast growth factor; DMEM, Dulbecco’s modified Eagle’s medium; GAG, glycosaminoglycan; PAGE, polyacrylamide gel electrophoresis; PBS, phosphate-buffered saline; PMA, phorbol 12-myristate 13-acetate; PKC, protein kinase C; cPKC, conventional PKC; nPKC, novel PKC; PVDF, polyvinylidene fluoride.
-
- Received November 21, 1997.
- Revision received February 16, 1998.
- The American Society for Biochemistry and Molecular Biology, Inc.
