Lipoprotein-mediated inhibition of endothelial cell production of platelet-derived growth factor-like protein depends on free radical lipid peroxidation.

Cultured vascular endothelial cells produce several mitogens including a platelet-derived growth factor-like protein (PDGF-c). We previously reported that acetylated low density lipoprotein (acetyl-LDL) caused accumulation of cholesterol and specific inhibition of PDGF-c production by bovine aortic endothelial cells (Fox, P. L., and DiCorleto, P. E. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 4774-4778). We have now examined the role of cholesterol and other lipids on the inhibition of production of PDGF-c. Incubation of endothelial cells with free cholesterol/albumin complexes resulted in a large increase in cellular cholesterol content but did not inhibit PDGF-c production, demonstrating that cholesterol itself is not inhibitory. Involvement of lipid peroxides in the suppression of PDGF-c production was indicated by three observations. LDL modified in vitro by free radical lipid peroxidation quantitatively inhibited PDGF-c production. The inhibition was dependent on the level of LDL oxidation (as measured by thiobarbituric acid reactivity) and was specific since total protein synthesis was not affected. Inhibition of PDGF-c production by acetyl-LDL was also dependent on peroxidation. A lipid extract from oxidized LDL, but not from native LDL, specifically inhibited PDGF-c production. Chloroquine, monensin, and NH4Cl, inhibitors of lysosomal hydrolytic activity, did not prevent acetyl-LDL-mediated inhibition of PDGF-c production, indicating that cellular metabolism of the lipoprotein was not required for the inhibition. Furthermore, acetyl-LDL suppressed PDGF-c production by endothelial cells even in the presence of butylated hydroxytoluene, an inhibitor of lipid peroxidation, suggesting that cellular propagation of free radicals was not required for the inhibition. Finally, inhibition of PDGF-c production may be regulated at the post-transcriptional level since Northern blot analysis using a v-sis probe showed that the PDGF B-chain mRNA amounts were unaffected by oxidized or acetylated LDL. In summary, levels of an oxidized lipoprotein that have no effect on endothelial cell viability or protein synthetic rates can completely suppress production of a growth factor which may act as a paracrine mitogen in normal and pathological vascular processes.