Cell-surface heparan sulfate. Mechanisms of proteoglycan-cell association

We have shown previously that exogenous heparan sulfate and heparin reversibly bind to isolated hepatocytes in a ligand-receptor type of interaction (Kjellen, L., Oldberg, A., Rubin, K., and Hook, M. (1977) Biochem. Biophys. Res. Commun. 74, 126-133). In the present investigation, the mode of association between heparan sulfate proteoglycans and hepatocytes has been studied. The binding of isolated heparan sulfate proteoglycans to cells is demonstrated, and is shown to require the presence of intact polysaccharide chains. Endogenous heparan sulfate proteoglycans were displaced from the cells on addition of heparin, indicating that the polysaccharide receptor participates also in the association of endogenous heparan sulfate proteoglycans with the cell surface. However, about one-third of the cell-surface heparan sulfate was not displaced by repeated incubations with heparin, but was released by mild trypsin treatment. The amount of heparan sulfate released by trypsin was independent of previous heparin treatment, indicating that the cell association of the heparan sulfate in this pool does not involve the polysaccharide-receptor interaction. The specificity of the heparan sulfate-receptor interaction was investigated in three types of experiments, including (a) binding of [³H]heparan sulfate to cells, (b) inhibition of [³H]heparan sulfate-cell binding by unlabeled polysaccharides, and (c) displacement of endogenous heparan sulfate from cells by added polysaccharides. The results suggest that heparin-like polysaccharides of high sulfate content are preferred as ligands and that the ability of heparin to displace endogenous heparan sulfate increases with increasing molecular weight. An apparent association constant of 1 x 10⁷M^-1 was determined for the binding of heparan sulfate chains to the cell-surface receptors. The number of available receptors per cell was calculated to be 4 x 10⁶.