Turnover of heparan sulfate proteoglycans from substratum adhesion sites of murine fibroblasts.

Substratum adhesion sites from murine Balb/c SVT2 fibroblasts are enriched in heparan sulfate proteoglycans which have been implicated in mediating adhesion of these cells to a fibronectin-adsorbed tissue culture substratum. Most of the heparan sulfate isolated from newly formed adhesion sites is found covalently attached to protein as proteoglycan while a significant portion of heparan sulfate from older sites has been identified as a single-chain species. This observation suggests that there may be catabolism of the heparan sulfate proteoglycan during the "maturation" of these adhesion sites at the cell's undersurface. Zwittergent 3-12 selectively extracts the single-chain class of heparan sulfate from either newly formed or "mature" adhesion sites while leaving the proteoglycan firmly bound in these sites. In an effort to further characterize the metabolism of these proteoglycans, substratum adhesion sites were isolated at various times after the cells had been pulse-radiolabeled using radioactive sulfate and subsequently chased. Greater than 80% of the sulfate-radiolabeled material is lost from the substratum-attached material within 24-48 h. Characterization of both the Zwittergent-soluble and -resistant heparan sulfate indicated that there was an initial accumulation followed by a rapid loss of a portion of the radiolabeled heparan sulfate as the single-chain Zwittergent-soluble class. However, most of the heparan sulfate proteoglycan was lost from the adhesion sites following approximately a 4-h time lag during the chase period without going through a smaller molecular weight intermediate. The turnover properties of the heparan sulfate proteoglycan in the EGTA-detachable cells were different from those in the substratum-attached fraction of the cell. The significance of these two different mechanisms of turnover of heparan sulfate proteoglycan in adhesion sites is discussed in relation to the role of this proteoglycan in mediating adhesion processes.