Isolation of two interleukin-6 response element binding proteins from acute phase rat livers

Proteins binding at the IL-6 response element of the rat alpha 2 macroglobulin gene were purified by a combination of conventional chromatographic procedures and binding-site specific DNA affinity chromatography. The proteins were purified from the nuclei of rat livers, excised at the peak of an experimentally induced acute phase response. By this procedure three polypeptides of 92, 91 and 86 kD were enriched more than 6,000-fold. Partial proteolysis with lysyl endopeptidase and aminoacid sequence analysis of proteolytic peptides identified the 86 and 91 kD species as the transcription factor Stat3 and the 92 kD species as a Stat factor distinct from Stats 1 and 3. cDNA clones for Stats 1, 3 and this 92 kD factor were isolated from a cDNA library prepared with mRNA from acute phase rat livers. Parts of their DNA sequences were determined and the sequences of the purified peptides were found in these cDNA sequences. Thus, the identity of the factors as Stat3 and a Stat factor different from Stats 1 and 3 was confirmed. These results suggest, that APRF/Stat3 and p91/Stat1 are not the only factors mediating the effects of IL-6 on class 2 acute phase genes. The 92 kD Stat factor binding at the IL-6 RE probably also functions as a transcription factor in the cytokine-induced activation of the alpha 2M gene.