Scavenger receptor-mediated recognition of maleyl bovine plasma albumin and the demaleylated protein in human monocyte macrophages.

Abstract

Maleyl bovine plasma albumin competed on an equimolar basis with malondialdehyde low density lipoprotein (LDL) in suppressing the lysosomal hydrolysis of 251-labeled malondialdehyde LDL mediated by the scavenger receptor of human monocyte macrophages. Maleyl bovine plasma albumin, in which 94% of the amino groups were modified, exhibited an anodic mobility in agarose electrophoresis 1.7 times that of the native protein. Incubation of maleyl bovine plasma albumin at pH 3.5 regenerated the free amino groups and restored the protein to the same electrophoretic mobility as native albumin. The demaleylated protein suppressed 75% of the hydrolysis of 1251-labeled malondialdehyde LDL and >80% of 1251-labeled maleyl bovine plasma albumin. The ability of the demaleylated protein to compete was abolished after treatment with guanidine hydrochloride. Although ligands recognized by the scavenger receptor typically are anionic, we propose that addition of new negative charge achieved by maleylation, rather than directly forming the receptor binding site(s), induces conformational changes in albumin as a prerequisite to expression of the recognition domain(s). The altered conformation of the modified protein apparently persists after removal of the maleyl groups. We conclude that the primary sequence of albumin, rather than addition of new negative charge, provides the recognition determinant(s) essential for interaction of maleyl bovine plasma albumin with the scavenger receptor.The scavenger receptor of macrophages binds a diverse array of polyanionic macromolecules, including modified human low density lipoprotein (LDL), maleyl-albumin (the half-cystyl derivative of bovine plasma albumin that has been modified by maleic anhydride), polyinosinic acid, and fucoidin (see review in ref. tyl-LDL by murine peritoneal macrophages (3, 12). In particular, maleyl human serum albumin was an effective competitive inhibitor, whereas acetyl albumin was not. Since maleylation results in the addition of net negative charge to peptidyl lysines, and acetylation acts to neutralize the positive charge of lysyl residues, it was postulated that the addition of new negative charges by maleylation was necessary to convert albumin into a molecule bound by the scavenger receptor (3).We have recently compared the ability of several lysinespecific reagents to convert LDL into a ligand bound by the scavenger receptor of human monocyte macrophages (13,14). Based on these studies, we have proposed that modification of LDL to produce a molecule recognized by the scavenger receptor effects a conformational transition of the apoB polypeptides, and that modification of the charge of specific critical peptidyl lysines results in expression of the receptor binding site(s) formed by the primary amino acid sequence of the LDL protein.These results prompted us to examine whether the negative charge provided by maleylation of the lysyl residues of bovine plasma albumin was essential for interaction of albumin with the scavenger receptor of human monocyte ...

Scavenger Receptors and Resistance to Inhaled Allergens

“…In vitro assays confirmed that the absence of receptors did not affect AM binding of OVA, as doubledeficient AMs bound Alexa-OVA to nearly the same extent as did control AMs (data not shown). These findings are consistent with previous reports indicating that, unlike chemically modified albumin, native albumin binding to MΦs is not mediated through SRAs (17)(18)(19), and also indicate that nebulization does not per se denature the allergenic proteins sufficiently to create SRA-binding domains.…”

“…AMs can efficiently bind and internalize unopsonized particles and bacteria through SRAs, leading to the clearance of inhaled matter from the airways and the reduction of the resulting inflammation (7-9), and are known to similarly bind modified proteins (17)(18)(19). To determine if SRAs could reduce allergic inflammation by simply 'scavenging' aeroallergen with a resulting decrease in allergen dose, we measured their ability to internalize inhaled allergens using FITC-OVA.…”

shibiremutations reveal distinct dynamin-independent and-dependent endocytic pathways in primary cultures ofDrosophilahemocytes

“…Probes for endocytosis and immunodetection mBSA was prepared as described earlier (Haberland and Fogelman, 1985). Fluorescent conjugates were made according to the instructions provided by the manufacturer with minor modifications.…”

Disruption of T Cell Tolerance to Self-Immunoglobulin Causes Polyclonal B Cell Stimulation Followed by Inactivation of Responding Autoreactive T Cells

“…In addition to maleylated proteins, SRs bind to many other ligands (31)(32)(33)40), and we have shown previously that SR-like binding capabilities are exhibited by all professional APCs (35), although classical SRs are known to be present only on cells of the monocytic lineage (31,33,34). To examine the role of macrophages in presentation of self-Ags to generate autoimmune responses, we used CGN to deplete macrophages of their functional capability at the time of immunization.…”

“…For this purpose, we have used maleylated self-Ags. Protein maleylation confers on them the ability to bind to scavenger receptors (SRs) 3 (31)(32)(33). Although classical SRs are restricted to cells of the monocytic lineage such as macrophages and dendritic cells (DCs) (33,34), other receptor molecules with the ability to bind SR ligands have also been shown to be present on B cells (34,35).…”