Native low-density lipoprotein uptake by Macrophage Colony-Stimulating Factor-differentiated human macrophages is mediated by macropinocytosis and micropinocytosis (134.12)

Abstract

Abstract Human macrophage colony-stimulating factor (M-CSF)-differentiated macrophages—the predominant macrophage phenotype in human atherosclerotic plaques—constitutively take up native low-density lipoprotein (LDL) by fluid-phase pinocytosis, but the pinocytotic pathways mediating LDL uptake by these macrophages have not been thoroughly examined. We identified the kinase inhibitor SU6656 and the Rho GTPase inhibitor toxin B as inhibitors of macrophage fluid-phase pinocytosis of LDL. Assessment of macropinocytosis by time-lapse microscopy revealed that both drugs almost completely inhibited macropinocytosis, although LDL uptake and cholesterol accumulation by macrophages was only partially inhibited (~40%) by these agents. We therefore investigated the role of micropinocytosis in mediating LDL uptake in macrophages and identified bafilomycin A1 as an additional partial inhibitor (~40%) of macrophage LDL uptake that targeted micropinocytosis. When macrophages were incubated with both bafilomycin A1 and SU6656, inhibition of LDL uptake was additive (reaching 80%), showing that these inhibitors target different pathways. Microscopic analysis of fluid-phase uptake pathways in these macrophages confirmed that LDL uptake occurs through both macropinocytosis and micropinocytosis. Our findings show that human M-CSF-differentiated macrophages take up native LDL by macropinocytosis and micropinocytosis, underscoring the importance of both pathways in mediating LDL uptake by these cells.