HYALURONAN FRAGMENTS ACTIVATE AN NF-KAPPA-B I-KAPPA-B-ALPHA AUTOREGULATORY LOOP IN MURINE MACROPHAGES/

Macrophages play an important role in the acute tissue inflammatory re sponse through the release of cytokines and growth factors in response to stimuli such as lipopolysaccharide (LPS). Macrophage inflammatory effector functions are also influenced by interactions with the extrac ellular matrix (ECM). Such macrophage-ECM interactions may be importan t in regulating chronic inflammatory responses. Recent evidence has su ggested that hyaluronan (HA), a glycosaminoglycan (GAG) component of E CM can induce inflammatory gene expression in murine macrophages. HA e xists in its native form as a large polymer, but is found as smaller f ragments under inflammatory conditions. The NF-kappa B/I-kappa B trans criptional regulatory system has been shown to be a critical component of the host inflammatory response. We examined the effects of high mo lecular weight HA and lower molecular weight HA fragments on NF-kappa B activation in mouse macrophages. Only the smaller HA fragments were found to activate NF-kappa B DNA binding activity. After HA stimulatio n, I-kappa B alpha mRNA was induced and I-kappa B alpha protein levels , which initially decreased, were restored. The induction of I-kappa B alpha expression was not observed for other GAGs. The time course of I-kappa B alpha protein regeneration in response to HA fragments was c onsistent with an autoregulatory mechanism. In support of this mechani sm, in vitro translated murine I-kappa B alpha inhibited HA fragment-i nduced NF-kappa B DNA binding activity. The NF-kappa B DNA binding com plex in HA-stimulated extracts was found to contain p50 and p65 subuni ts. Activation of the NF-kappa B/I-kappa B system in macrophages by EC M fragments may be an important mechanism for propagating the tissue i nflammatory response.