Methylglyoxal-modified arginine residues — a signal for receptor-mediated endocytosis and degradation of proteins by monocytic THP-1 cells

Abstract

Non-enzymatic glycosylation or glycation of proteins to form advanced glycation endproducts (AGE) has been proposed as a process which provides a signal for the degradation of proteins. Despite this, the AGE which act a recognition factor for receptor-mediated endocytosis and degradation of glycated proteins by monocytes and macrophages has not been identified. Methylglyoxal, a reactive α-oxoaldehyde and physiological metabolite, reacted irreversibly with arginine residues in proteins to form N δ -(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine and N δ -(5-methyl-4-imidazolon-2-yl)ornithine residues. Human serum albumin minimally-modified with methylglyoxal (MG min-HSA) was bound by cell surface receptors of human monocytic THP-1 cells in vitro at 4°C: the binding constant K d value was 377±35 nM and the number of receptors per cell was 5.9±0.2×10 5 ( n=12). N α -Acetyl- N δ -(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine displaced MG min-HSA from THP-1 cells, suggesting that the N δ -(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine residue was the receptor recognition factor. At 37°C, MG min-HSA was internalised by THP-1 cells and degraded. Similar binding and degradation of human serum albumin modified by glucose-derived AGE was found but only when highly modified. MG min-HSA, therefore, is the first example of a protein minimally-modified by AGE-like compounds that binds specifically to monocyte receptors. The irreversible modification of proteins by methylglyoxal is a potent signal for the degradation of proteins by monocytic cells in which the arginine derivative, N δ -(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine, is the receptor recognition factor. This factor is not present in glucose-modified proteins.