TRIGGERING OF THE HUMAN INTERLEUKIN-6 GENE BY INTERFERON-GAMMA AND TUMOR-NECROSIS-FACTOR-ALPHA IN MONOCYTIC CELLS INVOLVES COOPERATION BETWEEN INTERFERON REGULATORY FACTOR-I, NF-KAPPA-B, AND SP1 TRANSCRIPTION FACTORS

We investigated the molecular basis of the synergistic induction by it erferon-gamma (IFN-gamma)/tumor necrosis factor-alpha (TNF-alpha) of h uman interleukin-6 (IL-6) gene in THP-1 monocytic cells, and compared it with the basis of this induction by lipopolysaccharide (LPS), Funct ional studies with IL-6 promoter demonstrated that three regions are t he targets of the IFN-gamma and/or TNF-gamma action, whereas only one of these regions seemed to be implicated in LPS activation, The three regions concerned are: 1) a region between -73 and -36, which is the m inimal element inducible by LPS or TNF-alpha; 2) an element located be tween -181 and -73, which appeared to regulate the response to IFN-gam ma and TNF-alpha negatively; and 3) a distal element upstream of -224, which was inducible by IFN-gamma alone. LPS signaling was found to in volve MF kappa B activation by the p50/p65 heterodimers. Synergis tic induction of the IL-6 gene by IFN-gamma and TNF-alpha, in monocytic ce lls, involved cooperation between the IRF-1 and NF kappa B p65 homodim ers with concomitant removal of the negative effect of the retinoblast oma control element present in the IL-6 promoter. This removal occurre d by activation of the constitutive Sp1 factor, whose increased bindin g activity and phosphorylation were mediated by IFN-gamma.