ROLE OF MACROPHAGE GLYCOSAMINOGLYCANS IN THE CELLULAR CATABOLISM OF OXIDIZED LDL BY MACROPHAGES

Macrophage binding sites for oxidized LDL (Ox-LDL) include class A sca venger receptors (SR-As), the CD-36 molecule. and an additional but hi therto unidentified binding site. Because cell-surface glycosaminoglyc ans (GAGs) were previously shown to be involved in the cellular uptake of native LDL and lipoprotein(a), several strategies to assess the pa rticipation of heparan sulfate (HS) and chondroitin sulfate (CS) in ma crophage catabolism of Ox-LDL were used. First, incubation of J-774 A. 1 macrophage-like cells with either heparinase or chondroitinase, or w ith both enzymes together, reduced the binding, uptake, and degradatio n of I-125-Ox-LDL by 20% to 45%, in comparison with control nontreated cells, while catabolism of I-125-labeled acetylated LDL (Ac-LDL) and native LDL were unaffected. Second, the proteoglycan (PG) cellular con tent was increased by cell enrichment with exogenous GAGs or by using human monocyte-derived macrophages from two patients with Sanfilippo m ucopolysaccharidosis, which are characterized by cellular HS accumulat ion. In these macrophages, cellular uptake of I-125-Ox-LDL increased, while catabolism of I-125-Ac-LDL and native LDL were unaffected. Exper iments using conditioned media from control, heparinase-digested, or c hondroitinase-digested macrophages indicated that neither secreted GAG s nor released digestion products played any role in Ox-LDL catabolism . To evaluate potential interactions between cell-surface GAGs and kno wn receptors for Ox-LDL. we used excess unlabeled Ac-LDL to block SR-A s or anti-CD-36 antibodies to block CD-36, and then examined the catab olism of I-125-Ox-LDL by GAG-enriched or -depleted macrophages. Both e xcess unlabeled Ac-LDL and anti-CD-36 antibodies reduced I-125-Ox-LDL catabolism, but only excess unlabeled Ac-LDL completely abolished the increase in I-125-Ox-LDL catabolism on GAG enrichment of the cells, in dicating a cooperation between exogenous GAGs and cell-surface SR-As i n the catabolism of OX-LDL. Moreover, the addition of GAGases to macro phages that were preincubated with anti-CD-36 antibodies and excess Ac -LDL further reduced macrophage degradation of Ox-LDL in comparison wi th cells that were pretreated only with anti-CD-36 antibodies and Ac-L DL, indicating a more complex role for endogenous GAGs. Overall, these studies demonstrate a substantial contribution of macrophage-associat ed GAGs in the catabolism of Ox-LDL, which is mediated in part by a co operation between GAGs and cell-surface SR-As.