Endothelial heparan sulfate is necessary but not sufficient for control ofvascular smooth muscle cell growth

The state of the endothelial cell (EC) determines the nature of its control of vascular smooth muscle cell (vSMC) biology. Conditioned medium from pos tconfluent ECs inhibits VSMC proliferation, whereas subconfluent conditione d medium from the same ECs has a stimulatory effect. We and others have ide ntified confluent endothelial cells' production of heparan sulfate proteogl ycans (HSPG) as critical to vSMC growth control. The question that arises i s whether the stimulation that is observed with subconfluent cells is from (1) aberrant HSPG production, (2) elaboration of noninhibitory species of H SPG, or (3) production of other factors, such as mitogens, which counteract the inhibitory HSPG to stimulate vSMCs. We studied the relative effects of conditioned medium produced by both subconfluent and postconfluent EC cult ures on vSMC growth. Conditioned medium was fractionated into nonproteoglyc an (non-PG) and proteoglycan (PG) components by anion-exchange chromatograp hy. The PG fractionation profile and the antiproliferative activity of the HSPGs isolated from both subconfluent and postconfluent EC-conditioned medi a were similar. However, the HSPG fraction alone could not approach the inh ibitory potential of unfractionated conditioned medium from postconfluent E C cultures. Non-PG proteins produced by the endothelial cultures had no eff ect on VSMC growth on their own. Yet, when they were mixed together with HS PG fractions, from either subconfluent or postconfluent EC cultures, the fu ll growth effects were returned. Non-PG protein fractions from postconfluen t cultures with HSPG fractions gave maximal inhibition of vSMC growth, wher eas non-PG protein fractions from subconfluent EC cultures with HSPG fracti ons produced the maximal stimulation. Thus, whereas the net stimulatory or inhibitory effect on VSMC growth of EC-conditioned medium is density depend ent, this effect does not result from a difference in the antiproliferative heparan sulfate component but rather from non-PG proteins that interact wi th the heparan sulfates. J. Cell. Physiol. 184:93-100, 2000. (C) 2000 Wiley -Liss, Inc.