Regulation of Syndecan-4 Phosphorylation in Vivo

• Published in: The Journal of biological chemistry Vol. 273; no. 18; pp. 10914 - 10918
• Main Authors: Horowitz, Arie, Simons, Michael
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.05.1998; American Society for Biochemistry and Molecular Biology
• Subjects: 3T3 Cells; Alkaloids; Amino Acid Sequence; Animals; Benzophenanthridines; Cell Membrane - metabolism; Cytoplasm - metabolism; Enzyme Inhibitors - pharmacology; Fibroblast Growth Factor 2 - metabolism; Membrane Glycoproteins - metabolism; Mice; Molecular Sequence Data; Phenanthridines - pharmacology; Phosphorylation; Protein Kinase C - antagonists & inhibitors; Protein Kinase C - metabolism; Proteoglycans - metabolism; Syndecan-4
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.273.18.10914

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.