Molecular mechanisms of interleukin‐10‐mediated inhibition of NF‐κB activity: a role for p50

Nuclear factor kappa B (NF‐κB) is a transcription factor pivotal for the development of inflammation. A dysregulation of NF‐κB has been shown to play an important role in many chronic inflammatory diseases including rheumatoid arthritis, inflammatory bowel disease and psoriasis. Although classical NF‐κB, a heterodimer composed of the p50 and p65 subunits, has been well studied, little is known about gene regulation by other hetero‐ and homodimeric forms of NF‐κB. While p65 possesses a transactivation domain, p50 does not. Indeed, p50/p50 homodimers have been shown to inhibit transcriptional activity. We have recently shown that Interleukin‐10 exerts its anti‐inflammatory activity in part through the inhibition of NF‐κB by blocking IκB kinase activity and by inhibiting NF‐κB already found in the nucleus. Since the inhibition of nuclear NF‐κB could not be explained by an increase of nuclear IκB, we sought to further investigate the mechanisms involved in the inhibition of NF‐κB by IL‐10. We show here that IL‐10 selectively induced nuclear translocation and DNA‐binding of p50/p50 homodimers in human monocytic cells. TNF‐α treatment led to a strong translocation of p65 and p50, whereas pretreatment with IL‐10 followed by TNF‐α blocked p65 translocation but did not alter the strong translocation of p50. Furthermore, macrophages of p105/p50‐deficient mice exhibited a significantly decreased constitutive production of MIP‐2α and IL‐6 in comparison to wild type controls. Surprisingly, IL‐10 inhibited high constitutive levels of these cytokines in wt macrophages but not in p105/p50 deficient cells. Our findings suggest that the selective induction of nuclear translocation and DNA‐binding of the repressive p50/p50 homodimer is an important anti‐inflammatory mechanism utilized by IL‐10 to repress inflammatory gene transcription.