Tyrosine phosphorylation of syndecan-1 and -4 cytoplasmic domains in adherent B82 fibroblasts

The syndecans, a family of cell surface proteoglycans, have highly conserve d cytoplasmic domains that bind proteins containing PDZ domains and co-loca lize with the actin cytoskeleton. The syndecan cytoplasmic domains contain four conserved tyrosine residues, two of which are located within favorable sequences for phosphorylation. Endogenous tyrosine phosphorylation of synd ecans-1 and -4 is detected in adherent B82 fibroblasts. Approximately 1.5% of total syndecan is endogenously phosphorylated, while most, if not all, c ell surface syndecan is phosphorylated following treatment with the tyrosin e phosphatase inhibitor pervanadate. Syndecan phosphorylation is also detec ted in Raji-S1 and NMuMG cells, but only following treatment with vanadate or pervanadate, suggesting that endogenous phosphorylation is maintained in an "off" state in these cells. Endogenous syndecan phosphorylation in B82 cells is rapidly blocked by genistein (IC50 < 10 mu M) confirming the prese nce of a constitutively active kinase and a corresponding tyrosine phosphat ase. Phosphorylation is also inhibited by herbimycin A (IC50 < 1.0 mu M) an d staurosporine (IC50 < 1.0 nM), suggesting a role for Src family kinases i n regulating syndecan phosphorylation. Together, these data suggest an impo rtant role for tyrosine phosphorylation of the syndecan cytoplasmic domains in regulating downstream signaling events in response to cell adhesion and /or growth factor activity.