TAK1 mediates lipopolysaccharide-induced RANTES promoter activation in BV-2 microglial cells

Lipopolysaccharide (LPS), a part of the outer membrane of gram-negative bacteria activates the expression of the regulated upon activation, normal T cell expressed and secreted (RANTES), which plays an important role in the chemo-attraction of leukocytes during the inflammatory response. Recently, we found that LPS-induced RANTES production is mediated by the activation of NF-kappaB, but, the upstream regulatory mechanism involved in mediating this NF-kappaB activation was unclear. In this study, we investigated signal transducing molecules that mediate LPS-induced RANTES promoter activation and found the followings. First, LPS activates the RANTES gene promoter through NF-kappaB binding sites. Second, the expression of dominant negative mutants of TGF-beta-activated kinasel (TAK1) and NF-kappaB-inducing kinase (NIK), blocked the LPS-induced transcriptional activation of RANTES promoter. Moreover, the overexpression of TAK1 along with TAK1-binding protein 1 (TAB1), or NIK stimulated the transcriptional activation of RANTES in the absence of external stimuli. Third, we showed that endogenous TAK1 is phosphorylated by LPS stimulation, and that the association between TAK1 and tumour necrosis factor receptor-associated factor 6 (TRAF6) is constitutive and not induced by LPS treatment. These results indicate that NF-kappaB mediates LPS-triggered RANTES induction and that TAK1 as well as NIK, as NF-kappaB activators participates in LPS-triggered RANTES induction.