Heparinlike Species and Inhibition of Proliferation in the Vessel Wall

Abstract

In the pathogenesis of arteriosclerosis, proliferation of smooth muscle cells (SMC) is a key early step. We have shown that heparin inhibits the growth of SMC both in vivo and in vitro. We have also demonstrated that cultured endothelium secretes a highly antiproliferative heparin species. This antiproliferative activity of heparin has the following characteristics: (1) only certain cell types are sensitive, e.g.. SMC, renal mesangial cells; (2) heparin is by far the most potent of the glycosaminoglycans; (3) anticoagulant and nonanticoagulant heparins are equally effective; and (4) the effect is reversible. The smallest inhibitory fragment of heparin is a pentasaccharide containing a critical 3-O-sulfate. An N-desulfated, re-N-acetylated heparin derivative loses anticoagulant but not antiproliferative activity. Heparin blocks SMC in late Gl, ruling out a direct interaction with platelet-derived growth factor. Other effects of heparin are (1) the induction of secretion of a 35-KDa protein into the medium and (2) the inhibition of nucleoside uptake. Because we have recently found that heparin inhibits the proliferation of rat cervical epithelial cells and that epidermal growth factor (EGF) overcomes this inhibition, we have investigated the effect of various growth factors in reversing the heparin inhibition of VSM. There are species differences in both growth responses to EGF and EGF reversal of heparin inhibition. With heparin-inhibited rat VSM, there is no reversal with EGF (at a dose as high as 50 ng/ml) or with fibroblast growth factor, platelet-derived growth factor, somatomedin C., insulin, and transferrin. With calf VSM, however, EGF (at a dose as low as 0.5 ng/ml) does overcome the heparin inhibition. Although EGF is a potent mitogen for calf VSM, in rat VSM little growth stimulatory effects are found. Interestingly, this correlates with EGF reversibility in the calf and lack of EGF reversibility in the rat. We conclude that EGF reversal of heparin inhibition may not be a universal characteristic of VSM growth regulation.