Inhibition of LDL Degradation in Cultured Human Fibroblasts Induced by

Cultured porcine aortic endothelial cells (EC) secrete a potent mitogen into their serum-free culture medium which stimulates DNA synthesis in quiescent human fibroblasts. Previous work has shown that endothelial cell-conditioned medium (ECCM) also stimulates parallel, concentration-dependent increases in low density lipoprotein (LDL) cell surface binding and internalization. LDL degradation, however, was inhibited in ECCM-treated cells. Studies were conducted to determine whether the EC-derived growth factor (EDGF) and the endothelial-derived inhibitor of LDL degradation (EDID) are separate and distinct components, and to further delineate some of the characteristics of EDID. Time-course studies showed that the mitogenic activity released by EC into ECCM increased steadily during the first 24 hours and came to a plateau at about 48 hours of conditioning, whereas inhibitory activity increased linearly in ECCM for at least 6 days. Mitogenic activity in ECCM was stable under the same conditions. Ultrafiltration experiments showed that EDGF activity has an apparent molecular weight in excess of 30,000; in contrast EDID activity was not retained by filters with a 2000 dalton retention limit. EDID activity also failed to bind t