The proliferative effect of platelets and hyperlipidemic serum on stationary primary cultures

Outgrowths from explants of aortic media which have become stationary in the presence of a medium containing 10% normal serum have been studied. Homologous hyperlipidemic serum and especially its low density lipoprotein fraction has been shown to induce a second episode of proliferation in these cultures. Cell proliferation was evaluated by direct measurement of cell colony size and/or incorporation of [3H]thymidine visualized by autoradiography. The possibility has been investigated that the increase in arterial smooth muscle cell proliferation produced by hyperlipidemic serum might actually be due to a platelet factor present in that serum. Platelet-poor and platelet-rich sera were prepared from hyperlipidemic donors and added in a concentration of 5% to the culture medium. Both were equally effective in inducing proliferation; on the other hand, the addition of platelets from either hyperlipidemic or normolipidemic animals had no additive effect. The proliferation-stimulating effect of hyperlipidemic serum occurred even when the stationary cultures were maintained in a medium containing platelet-poor plasma serum for 2 weeks, prior to the addition of hyperlipidemic serum also derived from platelet-poor plasma. It is concluded that the proliferative effect of hyperlipidemic serum on stationary primary cultures does not depend on the presence of platelet-derived material. The implications of these observations on plaque formation are discussed.