In vitro incubation of low-density lipoproteins with inflammatory cells causes enhanced degradation by macrophages in culture

Abstract

Utilizing the polyvinyl sponge-implant model, we have reported in vivo modification of low-density lipoproteins (LDL) isolated from interstitial inflammatory fluid (IF) of the rabbit. Further studies on the metabolism of IF-LDL by resident mouse peritoneal macrophages (MPM), demonstrated enhanced uptake and degradation of these modified lipoproteins by scavenger mechanisms. Based upon these studies, we attempted to examine the mechanisms of these observed in vivo modifications in IF-LDL by in vitro incubation of plasma LDL with inflammatory fluid subfractions. Incubation of LDL with inflammatory cells at 37°C resulted in an increased anodal electrophoretic mobility and alteration in apolipoprotein (APO) composition. Subsequent incubation of this modified plasma LDL with MPM resulted in a significant increase in cell surface binding and an increase in the appearance of degradation products in the medium. The formation of lipid peroxides, measured as thiobarbituric acid-reacting substances (T Bars), increased with the time of LDL incubation with inflammatory cells. Conversely, incubation of LDL with cell-free, lipoprotein-deficient IF (LPDIF,d>1.210 g/ml) significantly inhibited LDL degradation by MPM. LPDIF did not alter the electrophoretic mobility of LDL or result in the appearance of T Bars in the medium. These results implicate peroxidative reactions associated with an inflammatory response as mediators of the in vivo modifications in IF-LDL which facilitates enhanced uptake via the scavenger receptor in MPM.