Although the classical chemotactic receptor for complement anaphylatoxin C5a has been associated with polymorphonuclear and mononuclear phagocytes, several recent studies have indicated that this receptor is expressed on nonmyeloid cells including human endothelial cells, vascular smooth muscle cells, bronchial and alveolar epithelial cells, hepatocytes, and in the human hepatoma cell line HepG2. In this study, we examined the possibility that other members of the chemotactic receptor family are expressed in HepG2 cells and human liver, and the possibility that such receptors mediate changes in acute phase gene expression in HepG2 cells. Using polymerase chain reaction (PCR) amplification of HepG2 mRNA with primers based on highly conserved regions of the chemotactic subgroup of the G protein-coupled receptor family, we identified a PCR fragment from the formyl-methionyl-leucyl-phenylalanine (FMLP) receptor, as well as one from the C5a receptor. Immunostaining with antipeptide antisera to FMLPR confirmed the presence of this receptor in HepG2 cells. Receptor binding studies showed specific saturable binding of a radioiodinated FMLP analogue to HepG2 cells (Kd -2.47 nM; R -6 x 103 plasma membrane receptors per cell). In situ hybridization analysis showed the presence of FMLPIk mRNA in parenchymal cells of the human liver in vivo. Both C5a and FMLP mediated concentrationand time-dependent changes in synthesis of acute phase proteins in HepG2 cells induding increases in complement C3, factor B, and αl-antichymotrypsin, as well as concomitant decreases in albumin and transferrin synthesis. The effects of CSa and FMLP on the synthesis of these acute phase proteins was evident at concentrations as low as 1 nM, and they were specifically blocked by antipeptide antisera for the corresponding receptor. In contrast to the effect of other mediators of hepatic acute phase gene regulation, such as interleukin 6, the effects of C5a and FMLP were reversed by increased concentrations well above the saturation point of the respective receptor. These results suggest that acute phase gene regulation by CSa and FMLP is desensitized at high concentrations, a property that is unique among the several known mechanisms for hepatic acute phase gene regulation.
ASJC Scopus subject areas
- Immunology and Allergy