Syndecan-1 Expression in Epithelial Cells Is Induced by Transforming Growth Factor β through a PKA-dependent Pathway*

Syndecans comprise a major family of cell surface heparan sulfate proteoglycans (HSPGs). Syndecans bind and modulate a wide variety of biological molecules through their heparan sulfate (HS) moiety. Although all syndecans contain the ligand binding HS chains, they likely perform specific functions in vivo because their temporal and spatial expression patterns are different. However, how syndecan expression is regulated has yet to be clearly defined. In this study, we examined how syndecan-1 expression is regulated in epithelial cells. Our results showed that among several bioactive agents tested, only forskolin and three isoforms of TGFβ (TGFβ1-TGFβ3) significantly induced syndecan-1, but not syndecan-4, expression on various epithelial cells. Steady-state syndecan-1 mRNA was not increased by TGFβ treatment and cycloheximide did not inhibit syndecan-1 induction by TGFβ, indicating that TGFβ induces syndecan-1 in a post-translational manner. However, TGFβ induction of syndecan-1 was inhibited by transient expression of a dominant-negative construct of protein kinase A (PKA) and by specific inhibitors of PKA. Further (i) syndecan-1 cytoplasmic domains were Ser-phosphorylated when cells were treated with TGFβ and this was inhibited by a PKA inhibitor, (ii) PKA was co-immunoprecipitated from cell lysates by anti-syndecan-1 antibodies, (iii) PKA phosphorylated recombinant syndecan-1 cytoplasmic domains in vitro, and (iv) expression of a syndecan-1 construct with its invariant Ser286 replaced with a Gly was not induced by TGFβ. Together, these findings define a regulatory mechanism where TGFβ signals through PKA to phosphorylate the syndecan-1 cytoplasmic domain and increases syndecan-1 expression on epithelial cells.

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