IL-6 regulates induction of C-reactive protein gene expression by activating STAT3 isoforms*

C-reactive protein (CRP) is synthesized in hepatocytes. The serum concentration of CRP increases dramatically during the acute phase response. In human hepatoma Hep3B cells, maximal CRP expression occurs in cells treated with the combination of IL-6 and IL-10. IL-6 induces transcription of the CRP gene and IL-10 synergistically enhances the effects of IL-6. We investigated the role of IL-6-activated transcription factor STAT3, also known as STAT3 alpha, in inducing CRP expression since we identified four consensus STAT3-binding sites centered at positions 72, 108, 134 and 164 on the CRP promoter. It has been shown previously that STAT3 binds to the site at 108 and induces CRP expression. We found that STAT3 also bound to the other three sites, and several STAT3-containing complexes were formed at each site, suggesting the presence of STAT3 isoforms and additional transcription factors in the complexes. Mutation of the STAT3 sites at 108, 134 or 164 resulted in decreased CRP expression in response to IL-6 and IL-10 treatment, although the synergy between IL-6 and IL 10 was not affected by the mutations. The STAT3 site at 72 could not be investigated employing mutagenesis. We also found that IL-6 activated two isoforms of STAT3 in Hep3B cells: STAT3 alpha which contains both a DNA binding domain and a transactivation domain and STAT30 which contains only the DNA-binding domain. Taken together, these findings raise the possibility that IL-6 not only induces CRP expression but also regulates the induction of CRP expression by activating STAT3 isoforms and by utilizing all four STAT3 sites.

Automated summary

C-reactive protein (CRP) is synthesized in hepatocytes and its serum concentration increases during the acute phase response. The combination of IL-6 and IL-10 leads to maximal CRP expression in human hepatoma Hep3B cells. The transcription factor STAT3, particularly its alpha isoform, plays a crucial role in inducing CRP expression by binding to the consensus STAT3-binding sites on the CRP promoter. IL-6 activates multiple isoforms of STAT3, including STAT3 alpha and STAT30, which regulate the induction of CRP expression through all four STAT3 sites.