The complement-activating protease P100 is expressed by hepatocytes and is induced by IL-6 in vitro and during the acute phase reaction in vivo.

Ra reactive factor, a lectin present in the sera of a wide variety of vertebrates, is composed of mannan-binding proteins and a serine protease termed P100, which is known to activate complement. Using differential mRNA display technology to study the "activation"-dependent gene expression of hepatic stellate cells (HSC), we partially cloned a cDNA encoding the rat homolog of P100, which displayed 94% and 88% homology to mouse and human P100 cDNA, respectively. In the rat P100, specific transcripts 5.4, 4.0, and 3.3 kb in size were detected in major amounts in normal liver, but were absent or near the detection limit in other organs. Among the different liver cell populations studied during primary culture, P100-specific transcripts of 4.0 kb were prominent in HSC and present in hepatocytes and hepatoma cells, whereas Kupffer cells and sinusoidal endothelial cells were P100-negative. In addition to 4.0-kb mRNA, freshly isolated hepatocytes also contained transcripts of 5.4 and 3.3 kb, which were down-regulated during primary culture. In situ hybridization of normal liver tissue confirmed the in vitro data in that P100 was expressed by hepatocytes and nonparenchymal liver cells, which probably represent HSC. In vitro P100 steady-state mRNA levels of hepatocytes were stimulated by IL-6 and/or dexamethasone. During the acute phase reaction induced by turpentine injection, P100 steady-state mRNA levels were up-regulated in rat liver. The data demonstrate that: (a) the liver is the primary site for P100 expression in the rat; (b) HSC and hepatocytes appear to represent the cellular sources; and (c) P100 steady-state mRNA levels are up-regulated by the acute phase mediators IL-6 and dexamethasone in vitro and during the acute phase reaction in vivo, suggesting that P100 represents a novel, positive acute-phase gene in the rat.