SPHINGOMYELINASE INDUCES AGGREGATION AND FUSION, BUT PHOSPHOLIPASE A(2) ONLY AGGREGATION, OF LOW-DENSITY-LIPOPROTEIN (LDL) PARTICLES - 2 DISTINCT MECHANISMS LEADING TO INCREASED BINDING STRENGTH OF LDL TO HUMAN AORTIC PROTEOGLYCANS

During atherogenesis, low density lipoprotein (LDL) particles bind to extracellular matrix proteoglycans in the arterial wall, become modifi ed, and appear as aggregated and fused particles, Sphingomyelinase (SM ase) and phospholipase A(2) (PLA(2)) have been found in the arterial w all, and, moreover, lesional LDL shows signs of hydrolysis of both sph ingomyelin and phosphatidylcholine, We have now studied the effects of these two lipolytic modifications on the aggregation and fusion of LD L particles by hydrolyzing the particles with Bacillus cereus SMase or bee venom PLA(2). In addition, the binding strengths of the modified LDL to human aortic proteoglycans (PG) were analyzed on an affinity co lumn, We found that SMase induced aggregation and fusion of LDL, but P LA(2) induced only aggregation of the particles, In addition, the SMas e-induced aggregation and fusion of LDL was promoted by pretreatment o f LDL with PLA(2). Determination of the binding strengths of the hydro lyzed LDL revealed that mere lipolysis of LDL without aggregation or f usion, either by SMase or PLA(2), did not affect the binding of the pa rticles to PG. Aggregation and fusion of lipolyzed LDL particles, howe ver, increased their strength of binding to PG, Active lysine residues in apolipoprotein B-100 (apoB-100) appear to be involved in the bindi ng of LDL to PG, and, in fact, quantitative C-13 NMR analysis revealed that, in the fused LDL particles, the number of active lysine residue s per apoB-100 moiety was increased. Moreover, aggregation and fusion of LDL increased the number of apoB-100 copies and, consequently, the number of active lysine residues per aggregate or fused particle. Our present findings therefore (i) show that treatment of LDL with SMase a nd PLA, generates modified LDL particles, which then bind to human aor tic PG with increased strength, and (ii) suggest that SMase- and PLA(2 )-induced aggregation and fusion of LDL are potential mechanisms leadi ng to focal retention of extracellular lipid in the arterial wall.