Inhibition of growth requires close proximity of syndecan-1 to the cell surface. (A) Effect of media preincubated with immobilized syndecan-1 on myeloma cell growth. ARH-77 cells were grown in media preincubated with syndecan-1 immobilized on sepharose beads or beads only (as control). Cell growth is expressed as a percentage of control. (B) Transwell assay. ARH-77 cells were grown in transwell inserts, physically separated from syndecan-1 (4 nmol/L) immobilized on sepharose beads, or beads only (as control) in the lower wells. Cells grown in the absence of a transwell but with exogenous, soluble syndecan-1 (4 nmol/L) serve as a positive control (Syn-1 no transwell). Cell growth is expressed as a percentage of control.