Differential expression of the C5a receptor on the main cell types of rat liver as demonstrated with a novel monoclonal antibody and by C5a anaphylatoxin-induced Ca2+ release

The C5-anaphylatoxin (C5a) is a protein of 74 (human) or 77 (rat) amino aci d residues, respectively, which is generated by limited proteolysis upon ac tivation of the fifth component of complement. Its generation may be induce d by both the classical and alternative pathways. C5a has been shown to ind irectly increase glucose output from hepatocytes (HC) in perfused rat liver by inducing prostanoid release from Kupffer cells (KC) and hepatic stellat e cells (HSC). A direct action of C5a on hepatocytes would require their ex pression of the specific C5a receptor (C5aR). In former studies using quant itative reverse transcription polymerase chain reaction (RT-PCR) it was sho wn that HC lack this receptor in contrast to KC, HSC and, probably, sinusoi dal endothelial cells (SEC), all of which contained mRNA for the C5aR in de creasing amounts. Using a novel monoclonal antibody (mAb R63) against the r at receptor, expression of the rat receptor on the four cell types was inve stigated by FAGS analysis, immunohistochemistry, and immunocytochemistry. T he data obtained were confirmed by functional studies in which the Ca2+ res ponse after stimulation of the isolated cells with recombinant rat C5a (nC5 a), the ligand for the receptor was recorded. The FAGS and the immunocytoch emical data presented here clearly indicate that rat HC do not express the C5aR, whereas KC have the highest expression level followed by HSC. SEC exp ressed the receptor only weakly. In line with these findings, a strong Ca2 response was observed after stimulation of KC and HSC, and a weak one with SEC. However, no signal was obtained upon stimulation of HC. The results o f this study support the indirect stimulation of glucose output from HC via prostanoid release from nonparenchymal liver cells and contradict the form erly proposed hypothesis of a direct action of C5 anaphylatoxin on hepatocy tes.