Shed syndecan-1 binds to the extracellular matrix to promote invasion and angiogenesis. (A) Matrigel invasion chambers were incubated with medium conditioned by either HPSE-low or HPSE-high CAG myeloma cells or with medium from HPSE-high cells that was immunodepleted of syndecan-1 or treated with Hep III or with medium from cells expressing enzymatically inactive heparanase (M225). After 1 hour, the medium was removed, endothelial cells were seeded on the Matrigel, and after overnight incubation, cells that invaded through the Matrigel were counted. Data are mean ± SD from 3 independent experiments. *P < .05 vs HPSE-low. **P < .05 vs HPSE-high. (B) The level of syndecan-1 present in the conditioned medium before (■) and after (□) incubation with the Matrigel was determined by ELISA. Treatment of conditioned medium with Hep III abolished binding of syndecan-1 to the Matrigel.