PARTICIPATION OF D-GALACTOSE-SPECIFIC RECEPTORS OF LIVER MACROPHAGES IN RECOGNITION OF FIBRONECTIN-OPSONIZED PARTICLES

The interaction of immobilized human or rat plasma fibronectin with isolated rat liver macrophages was studied in a model system using colloidal gold of 17-nm diameter (Au-17) as test particles. Plasma fibronectin (pFn)-coated gold particles were rapidly bound and endocytosed via the coated pit-coated vesicle pathway as demonstrated by photometry, and light and electron microscopy. The isolated macrophages bind 2.5 +/- 2 particles/10-mu-m of plasma membrane (incubation at 4-degrees), equalling a binding capacity of approximately 3.5 x 10(4) pFn-Au-17 particles per cell. Binding and uptake (at 37-degrees) was specifically inhibited by D-galactose-related carbohydrates, but not by D-Mannose, N-acetyl-D-glucosamine, nor by excess soluble pFn. Uptake was also inhibited by lactosylated bovine serum albumin at a concentration of 10(-6) M but not by bovine serum albumin. India ink uptake by the liver macrophages in the presence of fibronectin was also inhibited by D-galactose-related monosaccharides. The presence of terminal, nonreducing D-galactosyl groups on pFn could be demonstrated by agglutination experiments with the D-galactose-specific plant lectin, Ricinus communis agglutinin (RCA), which could also be used for isolation of pFn from rat plasma. The 29-kDa molecular mass D-galactose-specific receptor, known to be expressed on the liver macrophage membrane and recently shown to be a membrane-bound form of C-reactive protein, was found to bind the pFn-coated gold particles in dot blotting experiments. It was concluded that the D-galactose-specific macrophage receptor binds to terminal D-galactose-related units of immobilized pFn and participates in recognition of fibronectin-opsonized particles.