Heparin Causes the Accumulation of Heparan Sulfate in Cultures of Arterial Smooth Muscle Cells

Heparin infusion in an experimental animal model of arterial injury causes a significant increase in the proteoglycan (PG) component of the extracellular matrix in the injured arteries (Snow, A. D., Bolender, R. P., Wight, T. N., and Clowes, A. W. (1990)Am. J. Pathol.137, 313–330). The mechanisms responsible for this heparin-induced increase in arterial PGs are not understood. To address this question, we have examined the effect of heparin on PG synthesis and accumulation by aortic smooth muscle cells in culture. Heparin causes a dose-dependent increase in the accumulation of PGs while the greatest percentage change among the different types of PGs is a doubling of the amount of heparan sulfate (HS) accumulated in the cell layer. There is also a selective enrichment of HS in the trypsin-resistant component of the cell layer indicating a specific modification in intracellular PGs. This change is due to the accumulation of large HS glycosaminoglycans (GAGs) which elute atKav≃ 0.3 on Sepharose CL-6B (Mr≃ 50,000) under dissociative conditions and which are absent from the controls. These GAGs are located inside the cell as is indicated by their retention after heparitinase or trypsin treatment of the cell layer. Furthermore, the increased accumulation of labeled HS in steady-state heparin-treated cells above controls does not appear for several hours after the addition of [35S]sulfate, indicating that HS accumulation is due to decreased degradation of the HS and not new synthesis. This possibility is further supported by the fact that degradation of the intracellular large HS in trypsinized cells is delayed for 2 h by incubation with heparin. These results suggest that heparin may reduce the intracellular degradation of HS through competitive inhibition of endogenous heparitinase.