Effects of cytokine antagonists on the hepatic acute-phase response

Endotoxin-induced hepatic acute-phase protein synthesis has been thought to be primarily regulated through cytokines such as interleukin-1 (IL-1) and interleukin-6 (IL-6). Previously, it was found that a 23-kDa murine acute-phase protein, the lipopolysaccharide (LPS)-induced protein (LIP), was synthesized following treatment of hepatocytes in vitro with LPS. Since this protein was also induced by IL-1 and IL-6, the present studies were undertaken to determine if the effect of endotoxin was mediated through these cytokines. Primary cultures of murine hepatocytes were treated with LPS, IL-1, IL-6, or an LPS-stimulated macrophage supernatant in the presence or absence of the IL-1 receptor antagonist (IL-1 RA) and/or an anti-IL-6 antibody. The cells were then radiolabeled with [35S]methionine. LIP was detected by electrophoresis and autoradiography of the secreted proteins. In vitro, IL-1 RA completely inhibited the stimulation of LIP synthesis elicited by IL-1 and the macrophage supernatant, but did not affect LPS-stimulated synthesis of this protein. The anti-IL-6 antibody inhibited IL-6-triggered synthesis of LIP, but had no effect on LPS-stimulated synthesis. Hepatocytes isolated from mice treated in vivo with both IL-1 RA and LPS synthesized LIP to the same degree as hepatocytes isolated from mice treated with LPS alone. LPS-stimulated synthesis of LIP in vitro does not require IL-1 or IL-6 as an obligatory intermediate. These results are consistent with the hypothesis that endotoxin can directly stimulate hepatocyte acute-phase protein synthesis in the absence of cytokines.