Interleukin 6, the third mediator of acute‐phase reaction, modulates hepatic protein synthesis in human and mouse. Comparison with interleukin 1 β and tumor necrosis factor‐α

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Abstract
Interleukin 6 ( IL6 ) is the new definition of a group of cytokines previously named according to their biological activity, e.g. B cell stimulatory factor 2 (BSF‐2), hybridoma plasmocytoma‐growth factor (HGF), interferon‐β2 (IFN‐β2), hepatocyte stimulating factor (HSF). It has recently been suggested that IL6 may represent the major mediator of actue‐phase protein response whereas IL 1β and TNF‐α could play a minor role. We compared the effect of the three cytokine on hepatic protein synthesis by performing in vitro as well as in vivo experiments. Human hepatoma cells (PLC/PRFS) were exposed to each cytokine separately for 20 h, and the effect was then studied at the protein and RNA level. All three cytokines reduced albumin and increased C3 and ceruloplasmin biosynthesis. The cytokines induced the same effect at the RNA level indicating that the modulation was pretranslational. The effect of the cytokines was specific since actin gene expression was not changed; furthermore the effect was blocked by specific antibodies against the cytokines. The effect of the single cytokines was dose and time dependent, and quantitatively comparable. None of the cytokines was able to alter αl‐anti‐trypsin synthesis. In vivo experiments with mice showed that IL 1β and TNF‐a both induce serum amyloid A (SAA) mRNA in the mouse liver and increase factor B (Bf) gene expression. Human recombinant IL6 induced SAA gene expression and it also had a weak positive effect on Bf gene expression after i.p. injection. These data demonstrate that the three cytokines studied are quantitatively and qualitatively comparable, and that all three are probably involved in acute‐phase protein response.