Cell-surface heparan sulfate and heparan-sulfate/chondroitin-sulfate hybrid proteoglycans of mouse mammary epithelial cells

Abstract

The hydrophobic cell-surface proteoglycans of mouse mammary epithelial cells were purified by gel filtration, ion-exchange chromatography, and liposome incorporation. The size of the proteoglycans appeared to be directly proportional to the size of their heparan-sulfate chains, larger proteoglycans yielding larger chains. The chondroitin sulfate chains, in contrast, showed no size heterogeneity. Digestion of 125I-labeled proteoglycans with heparitin-sulfate lyase and chondroitin ABC lyase yielded core proteins of approximately 93 kDa, approximately 85 kDa and approximately 38 kDa. Comparison with single enzyme digestions identified the 93-kDa and 85-kDa cores as components of hybrid proteoglycans that carried both heparan-sulfate and chondroitin-sulfate chains. Immunoblotting indicated that the 93-kDa and 85-kDa cores shared the epitope defined by monoclonal antibody 281-2. The 38-kDa core, in contrast, carried only heparan-sulfate chains and lacked the 281-2 epitope. Preparations enriched in heparan sulfate or in heparan-sulfate/chondroitin-sulfate hybrid proteoglycans were obtained by N-desulfation and ion-exchange chromatography. Hybrid proteoglycans accounting for the bulk of the chondroitin-sulfate and nearly half of the heparan-sulfate residues of the proteoglycans showed a similar polydispersity of heparan-sulfate chain sizes as found in proteoglycans that carried only, or predominantly, heparan-sulfate chains. These hybrids contained heparan-sulfate and chondroitin-sulfate chains in similar molar amounts. Analysis of 125I-labeled proteoglycans suggested that typical hybrid proteoglycans were composed of a 85-kDa core protein that carries a single chondroitin-sulfate chain and a single heparan-sulfate chain of variable length. A minority of hybrids seemed characterized by the variant, but possibly structurally related, 93-kDa core protein. The other half of the hydrophobic proteoglycans were composed of the 38-kDa core and carried only heparan-sulfate chains. The significance of the co-existence of hybrid and heparan-sulfate proteoglycans at the cell surface and possible relationships between the proteoglycans need to be further clarified.