CELL-SURFACE EXPRESSION OF MOUSE MACROSIALIN AND HUMAN CD68 AND THEIRROLE AS MACROPHAGE RECEPTORS FOR OXIDIZED LOW-DENSITY-LIPOPROTEIN

We have previously identified a 94- to 97-kDa oxidized low density lip oprotein (LDL)-binding protein in mouse macrophages as macrosialin (MS ), a member of the lamp family. Earlier immunostaining studies have sh own that MS and its human homolog, CD68, are predominantly intracellul ar proteins, However, using sensitive techniques such as flow cytometr y (FAGS) and cell-surface-specific biotinylation, we now show that the re is significant surface expression of these proteins, FAGS analysis of intact cells using mAb FA/11 showed small but definite surface expr ession of MS in resident mouse peritoneal macrophages but this,vas gre atly enhanced with thioglycollate elicitation, Biotinylation of intact cells and detergent-solubilized cell preparations followed by immunop recipitation revealed 10-15% of the total MS content of elicited macro phages on the plasma membrane, Similar results were obtained with untr eated RAW 264.7 cells, FAGS analysis of intact THP-1 monocytic cells s howed minimal surface expression of CD68 on unactivated cells (4% of t otal cell content). Stimulation with phorbol 12-myristate W-acetate in creased both surface and total CD68 expression considerably, Furthermo re, the specific binding at 4 degrees C and uptake at 37 degrees C of I-125-labeled oxidized LDL by activated THP-1 cells was inhibited by 3 0-50% by CD68 mAbs KP-1 and EBM-11. Thus, although the sill-face expre ssion of MS/CD68 at steady-state represents only a small percentage of their total cellular content, these proteins can play a significant r ole in oxidized LDL uptake by activated macrophages in vitro and could contribute to foam cell formation in atherosclerotic lesions.