ACETYLATED LDL ENDOCYTOSIS BY THE HUMAN MONOCYTIC MONO MAC 6SR CELLS IS NOT MEDIATED BY THE MACROPHAGE TYPE-I AND TYPE-II SCAVENGER RECEPTORS

We recently reported that the human monocytic Mono Mac 6sr cell line c onstitutively takes up and degrades acetylated (acLDL) and oxidized LD L through receptor-specific pathways. The present studies were underta ken to further characterize the acLDL binding site on a functional and molecular basis. The degradation of acLDL increased during differenti ation of Mono Mac 6sr cells with lipopolysaccharide (10 ng/mL, 72 hour s) and low concentrations of phorbol 12-myristate 13-acetate (PMA; 0.1 to 1.0 ng/mL, 72 hours). Higher doses of PMA (5 or 10 ng/mL). however , decreased acLDL degradation. Scatchard plots of acLDL binding in unt reated and LPS-differentiated Mono Mac 6sr cells were nonlinear and su ggested the presence of more than one binding site. Although the ligan d specificity of the acLDL receptor in Mono Mac 6sr cells resembles th at of the macrophage type I and type II scavenger receptors, we did no t detect mRNA of either receptor type in untreated or differentiated M ono Mac 6sr cells by means of Northern blotting and reverse transcript ion polymerase chain reaction. Furthermore, ligand blotting with I-125 -acLDL failed to detect the 220-kD types I and II scavenger receptor p rotein. Thus, Mono Mac 6sr cells express an acLDL receptor that is dis tinct from the type I and type II scavenger receptor found in human mo nocyte-derived macrophages but that, like the latter, is induced durin g monocytic differentiation.