Objective: Production of the serum amyloid A (SAA) proteins in the liver of patients with arthritis can be increased from approximately 1 microgram/ml to > 1000 micrograms/ml, while fibrinogen (Fg) can be increased from 2 to 9 mg/ml. The increases appear to be regulated by mediators similar to those found in inflamed joints, e.g., interleukins 1 and 6 (IL-1 and IL-6, respectively). The sensitivity and dose response of SAA and Fg synthesis by hepatoma cells to IL-1 and IL-6 was investigated to understand the relationship between the inflammatory cytokines produced in inflamed joints and the acute phase protein response in the liver of arthritis patients.
Methods: SAA and Fg mRNA and protein production in human Hep3B cells stimulated by human monocyte conditioned medium (CM) containing known amounts of IL-1 and IL-6, or stimulated by corresponding concentrations of recombinant IL-1 and IL-6 was analyzed by ELISA and Northern blot hybridization techniques.
Results: Increases in SAA mRNA and protein were dose dependent in the presence of IL-1 and IL-6 at concentrations ranging from 0.1 and 1 ng/ml, respectively, to 10 and 100 ng/ml, respectively. In the presence of IL-1 receptor antagonist (IL-1ra), there was a 75% decrease in SAA production and > 100% increase in Fg production by cells stimulated with CM.
Conclusion: Our results demonstrate that the thousand fold dynamic range associated with the acute phase SAA response requires IL-1 acting synergistically with cytokine(s) like IL-6. Optimum conditions for apoSAA production are suboptimal for Fg as indicated by the differential effects of IL-1ra.