ON THE PATHOGENESIS OF ATHEROSCLEROSIS - ENZYMATIC TRANSFORMATION OF HUMAN LOW-DENSITY-LIPOPROTEIN TO AN ATHEROGENIC MOIETY

Combined treatment with trypsin, cholesterol esterase, and neuraminida se transforms LDL, but not HDL or VLDL, to particles with properties a kin to those of lipid extracted from atherosclerotic lesions. Single o r double enzyme modifications, or treatment with phospholipase C, or s imple vortexing are ineffective. Triple enzyme treatment disrupts the ordered and uniform structure of LDL particles, and gives rise to the formation of inhomogeneous lipid droplets 10-200 nm in diameter with a pronounced net negative charge, but lacking significant amounts of ox idized lipid. Enzymatically modified LDL (E-LDL), but not oxidatively modified LDL (ox-LDL), is endowed with potent complement-activating ca pacity. As previously found for lipid isolated from atherosclerotic le sions, complement activation occurs to completion via the alternative pathway and is independent of antibody. E-LDL is rapidly taken up by h uman macrophages to an extent exceeding the uptake of acetylated LDL ( ac-LDL) or oxidatively modified LDL. After 16 h, cholesteryl oleate es ter formation induced by E-LDL (50 mu g/ml cholesterol) was in the ran ge of 6-10 nmol/mg protein compared with 3-6 nmol/mg induced by an equ ivalent amount of acetylated LDL. At this concentration, E-LDL was ess entially devoid of direct cytotoxic effects. Competition experiments i ndicated that uptake of E-LDL was mediated in part by ox-LDL receptor( s). Thus, similar to 90% of I-125-ox-LDL degradation was inhibited by a 20-fold excess of unlabeled E-LDL. Uptake of I-125-LDL was not inhib ited by E-LDL. We hypothesize that extracellular enzymatic modificatio n may represent an important step linking subendothelial deposition of LDL to the initiation of atherosclerosis.