Modification of low density lipoproteins by polymorphonuclear cell elastase leads to enhanced uptake by human monocyte-derived macrophages via the low density lipoprotein receptor pathway

Abstract

In previous studies we reported that polymorphonuclear cell (PMN) elastase cleaves apoB-100 of human plasma low density lipoprotein (LDL) into seven or eight large Mr fragments (1, Polacek, D., R.E. Byrne, G.M. Fless, and A.M. Scanu. 1986. J. Biol. Chem. 261: 2057-2063). In the present studies we examined the interaction of native and elastase-digested LDL (ED-LDL) with primary cultures of human monocyte-derived macrophages (HMD-M). For this purpose LDL was digested with purified PMN elastase, re-isolated by ultracentrifugation at d 1.063 g/ml to remove the enzyme, and radiolabeled with 125I. At all LDL concentrations in the medium, the degradation of 125I-labeled ED-LDL was 1.5- to 2.5-fold greater than that of 125I-labeled native LDL, and for both lipoproteins species it was further enhanced by prior incubation of the cells in autologous lipoprotein-deficient serum (ALPDS). ED-LDL incubated with HMD-M in a medium containing [14C]oleate stimulated cholesteryl [14C]oleate formation 2- to 3-fold more than native LDL. In competitive degradation experiments, unlabeled ED-LDL did not inhibit the degradation of 125I-labeled acetylated LDL, whereas it caused a 90% inhibition of the degradation of 125I-labeled native LDL. At 4 degrees C, the binding of both 125I-labeled native and 125I-labeled ED-LDL was specific and of a high affinity. At saturation (Bmax), the binding of 125I-labeled ED-LDL was 2-fold higher (68 ng/mg cell protein) than that of 125I-labeled native LDL (31 ng/mg), with Kd values of 6.5 x 10(-8) M and 2.1 x 10(-8) M, respectively. A possible explanation of the binding data was provided by electrophoretic analyses suggesting that ED-LDL was twice the size of native LDL and thus potentially capable of delivering proportionately more cholesterol to the cells. Taken together, the results indicate that 1) digestion of LDL by purified PMN elastase results in a greater mass of ED-LDL (relative to native LDL) being degraded per unit time by HMD-M; 2) uptake of ED-LDL occurs via the LDL receptor; and 3) LDL digested by PMN elastase undergoes a physical change that may be responsible for its unique interactions with HMD-M. We speculate that if this process were to occur in vivo during an inflammatory process, macrophages could acquire excess cholesterol and be transformed into foam cells which are considered to be precursors of the atherosclerotic process.