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Fc Gamma Receptors in the Hepatic Sinusoid
© Braathen et al; licensee BioMed Central Ltd 2004
Published: 14 January 2004
Receptors that recognize the Fc-portion of antibodies mediate endocytosis of antibody-containing complexes (ICs) [1, 2]. The liver is the main organ for uptake of IC from circulation. Phagocytosis of antibody-coated particles takes place in Kupffer cells (KC) whereas endocytosis of soluble IC takes place both in KC and in liver endothelial cells (LEC) [3, 4]. ICs have, in comparison with ligands of the mannose-receptor or the scavenger receptors, been shown to be delayed in the endocytic pathway en route to lysosomes [4, 5] suggesting that the ICs and the Fc gamma Rs may use a unique pathway to lysosomes. Whereas the receptors of mannose and scavenger receptors are recycled constitutively between endosomes and plasma membrane, with or without bound ligand , the traffic of the Fc gamma R may be controlled by the ligand. Polyvalent ligands (ICs) which bring about receptor cross-linking may direct the receptor-ligand-complex from a recycling route to late endosomes and eventually to lysosomes. The latter pathway will lead to receptor-downregulation . Few studies have been done to determine whether hepatic Fc gamma Rs are subject to downregulation by their specific ligands. If these receptors were lost from KC and/or LEC after uptake of immune complexes a fatal result would be that the liver were left defenseless until new receptors were synthesized. The purpose of the present investigation was to determine whether uptake of polyvalent ligands would bring about a reduction in the number of Fc gamma Rs in rat KC and LEC.
Animals and reagents
Male Wistar rats, weighing 200–300 g, were obtained from M–llegaard and Bomholt (Ry, Denmark). Eagle minimal essential medium, glutamine and penicillin/streptomycin were obtained from Bio Whittaker, INC (Walkersville, MD). Na125I was purchased from Isopharma (Kjeller, Norway).
Preparation and culture of cells
Isolated liver cells and pure cultures of KC and LEC were prepared as described earlier . For all experiments cells were grown on Petri dishes (6 cm in diameter). The mouse peritoneal macrophage cell line J774-A1 was cultured in monolayers in 6-cm dishes with approximately 1.5 – 107 cells in each dish. DMEM containing 10% fetal calf serum, 1% pen/strep (penicillin 10,000 U/ml, streptomycin 10,000 ug/ml) and 1% L-glutamine was used.
Radiolabeling and preparation of aggregated IgG
Human IgG was diluted with PBS and iodinated with 40 MBq Na 125I in a iodogen coated glass tube in a final volume of 100 microliters. The radioiodination was carried out for 30 min at room temperature and arrested by passage over a Sephadex G-25 PD-10 column. Aggregated human IgG was (AgIgG) prepared by incubating the labeled IgG (1.3 mg/ml) for 45 min at 63 degrees C.
Antibodies against Fc gamma Rs-receptors
Monoclonal antibodies, mouse anti-rat CD32 and rat anti mouse CD32/16 were from Research Diagnostics Inc, NJ, USA).
Uptake and degradation of 125IAgIgG in KC, LEC and J774 cells
Antibodies against Fc gamma Rs inhibit binding of 125I-Ag IgG to LEC, KC and J774 cells
Down-regulation of Fc gamma Rs in LEC, KC and J774 cells
Fc gamma Rs, following cross-linking by multivalent ligands, may be redirected from a recycling to a lysosomal route . The preliminary data presented in this report suggest that uptake of AgIgG leads to partial downregulation of the Fc gamma Rs in KC and J774 cells but apparently not in LEC. The latter result was unexpected as it suggests that the Fc gamma Rs in these cells are able to release the polyvalent ligand intracellularly and recycle to the plasma membrane. Ligands internalized via the Fc gamma Rs in LEC are degraded efficiently in the lysosomes of these cells [3, 4], and it may be calculated that the number of ligand molecules internalized during 4 hours in the LEC is higher than the number of surface receptors. It should be noted though, that an unknown pool of Fc gamma Rs may be present inside the cells from which receptors may be recruited and replace those internalized during the experiment. The mechanisms whereby endocytic receptors are routed to lysosomes or redirected from a recycling to a lysosomal route have been enigmatic. Recent studies have revealed that conjugation of proteins with ubiquitin is a signal for lysosomal sorting , and it has been shown that Fc gamma RIIA on leucocytes can be ubiquitylated and that endocytosis (but not phagocytosis) of aggregated IgG seems to require ubiquitylation of the receptor . The different effects of receptor crosslinking in KC and LEC could conceivably be due to differences in receptor ubiquitylation and/or the expression of ubiquitin-binding proteins in the two types of cells in the liver sinusoid.
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