N-FORMYLPEPTIDE AND COMPLEMENT C5A RECEPTORS ARE EXPRESSED IN LIVER-CELLS AND MEDIATE HEPATIC ACUTE-PHASE GENE-REGULATION

Although the classical chemotactic receptor for complement anaphylatox in C5a has been associated with polymorphonuclear and mononuclear phag ocytes, several recent studies have indicated that this receptor is ex pressed on nonmyeloid cells including human endothelial cells, vascula r smooth muscle cells, bronchial and alveolar epithelial cells, hepato cytes, and in the human hepatoma cell line HepG2. In this study, we ex amined the possibility that other members of the chemotactic receptor family are expressed in HepG2 cells and human liver, and the possibili ty 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 chem otactic subgroup of the G protein-coupled receptor family, we identifi ed 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 bin ding of a radioiodinated FMLP analogue to HepG2 cells (K-d similar to 2.47 nM; R similar to 6 x 10(3) plasma membrane receptors per cell). I n situ hybridization analysis showed the presence of FMLPR mRNA in par enchymal cells of the human liver in vivo. Both C5a and FMLP mediated concentration and time-dependent changes in synthesis of acute phase p roteins in HepG2 cells including increases in complement C3, factor B, and alpha 1-antichymotrypsin, as well as concomitant decreases in alb umin and transferrin synthesis. The effects of C5a and FMLP on the syn thesis of these acute phase proteins was evident at concentrations as low as 1 nM, and they were specifically blocked by antipeptide antiser a for the corresponding receptor. In contrast to the effect of other m ediators of hepatic acute phase gene regulation, such as interleukin 6 , the effects of C5a and FMLP were reversed by increased concentration s well above the saturation point of the respective receptor. These re sults suggest that acute phase gene regulation by C5a and FMLP is dese nsitized at high concentrations, a property that is unique among the s everal known mechanisms for hepatic acute phase gene regulation.