ATHEROGENIC CONCENTRATIONS OF NATIVE LOW-DENSITY LIPOPROTEINS DOWN-REGULATE NITRIC-OXIDE-SYNTHASE MESSENGER-RNA AND PROTEIN-LEVELS IN ENDOTHELIAL-CELLS

The nitric oxide synthase family of proteins is the unique class of ma mmalian enzymes that metabolizes L-arginine to form nitric oxide (NO). The atherogenic action of low-density lipoproteins (LDL) may be media ted, in part, by its effects on endothelial-derived nitric oxide. To d etermine whether native LDL (nLDL), at atherogenic concentrations, are capable of modulating NO synthase expression, we treated human umbili cal vein endothelial cells with increasing concentrations of human nLD L (0-240 mg cholesterol/dl) for various time periods (2-48 h). Norther n and western blot analyses indicate that both endothelial NO-synthase mRNA and protein are down-regulated by atherogenic concentrations of nLDL (180 and 240 mg cholesterol/dl) after 48 h of incubation. Cyclohe ximide and actinomycin D experiments suggest that this down-regulation operates at a transcriptional level. Additionally, treatment of the c ells with high-density lipoproteins, at human physiological concentrat ions (45 mg cholesterol/dl), does not appear to alter the expression o f endothelial NO synthase which seems to indicate that nLDL affect the gene transcription rate by a specific and concentration-dependent mec hanism. These findings may have important implications because they pr ovide a novel mechanism by which hypercholesterolemia induces early ch anges on endothelial cells that could have pathophysiological signific ance in the atherosclerotic process.