Effect of heparin on vascular smooth muscle cells. I. Cell metabolism

Previous work from our laboratory has shown that heparin inhibits the proliferation of vascular smooth muscle cells in vivo and in vitro. The mechanism of action of this glycosaminoglycan is unknown. In this communication, we have examined the antiproliferative effect of heparin on smooth muscle and other cell types, and have investigated several aspects of heparin on smooth muscle cell metabolism. Smooth muscle and closely related cell types from several species, including human, were much more sensitive to heparin than any other cell type tested, including primary and established cell lines, normal and transformed cell pairs, fibroblasts, epithelial, and endothelial cells. Flow microfluorimetric analysis of cell cycle distribution indicated that heparin blocked either the G0----S transition or a very early S-phase event in smooth muscle cells. Heparin rapidly inhibited DNA and RNA synthesis, but did not affect the rate of protein synthesis. The decrease in nucleic acid synthesis could be accounted for by an inhibition of thymidine and uridine uptake. Interestingly, heparin did not block amino acid or glucose transport. Although no change in the overall rate of protein synthesis was observed in the presence of heparin, we noted at least two changes in the synthesis of specific proteins by smooth muscle cells: two 35,000-dalton proteins which appeared in the culture medium of heparin-treated cells, and the transient disappearance of a 48,000-dalton protein in the substrate attached material of smooth muscle cells exposed to heparin. The role of the observed changes in smooth muscle cell metabolism is yet to be determined, but they may provide valuable clues to the molecular mechanisms controlling the antiproliferative activity of heparin.