Cell‐free‐synthesized interleukin‐6 (BSF‐2/IFN‐β2) exhibits hepatocyte‐stimulating activity

Secretory products of cultured human blood monocytes contain a hepatocyte‐stimulating factor which is able to induce the acute‐phase proteins α2‐macroglobulin and fibrinogen in rat liver cells. Total RNA was isolated from unstimulated and lipopolysaccharide‐stimulated human monocytes and translated in a reticulocyte lysate. The capability of the cell‐free synthesized proteins to induce the acute‐phase proteins α2‐macroglobulin and fibrinogen was assayed in rat hepatocyte primary cultures and in the rat hepatoma cell line Fao. The products translated from the mRNA of lipopolysaccharide‐stimulated human monocytes induced mRNAs for α2‐macro‐globulin and fibrinogen and therefore contain hepatocyte‐stimulating factor. The translation products of unstimulated monocytes had no effect.A cDNA containing the coding sequence for interleukin‐6 (B‐cell stimulatory factor 2, interferon‐β2/26‐kDa protein, interleukin HP1) derived from human T‐cells cloned into the transcription vector pGEM4 was transcribed in vitro. Translation of the isolated RNA in a reticulocyte lysate led to the synthesis of a protein of about 25 kDa. This cell‐free synthesized interleukin‐6 exhibited hepatocyte‐stimulating activity measured by the induction of β‐fibrinogen mRNA in Fao cells. Using an antibody against interleukin‐6, two proteins of 22 kDa and 23 kDa were immunoprecipitated from the culture medium of lipopolysaccharide‐stimulated human monocytes. These two proteins were not synthesized by unstimulated monocytes.When total RNA from unstimulated human monocytes and lipopolysaccharide‐stimulated human monocytes and lymphocytes was subjected to Northern analysis and hybridized with the interleukin‐6 cDNA, a strong hybridization signal corresponding to an RNA of about 1300 bases was detected only in the RNA from lipopolysaccharide‐stimulated human monocytes, indicating that human monocytes express the interleukin‐6 gene after stimulation. The data presented in this paper strongly suggest that hepatocyte‐stimulating factor from human monocytes and interleukin‐6 from T‐cells are identical.