IL-1 beta maturation: evidence that mature cytokine formation can be induced specifically by nigericin

Mouse peritoneal macrophages stimulated with LPS produce large amounts of pro-IL-1 beta. When these cells were pulse-labeled with [35S]methionine, however, little labeled cytokine appeared in the medium after a chase, and that which was externalized was not processed to its mature biologically active form. In an effort to promote proteolytic maturation of IL-1 beta, macrophages were treated with agents that were expected to compromise their viability. The calcium ionophore A23187 and the detergent saponin caused complete release of nonprocessed 35-kDa pro-IL-1 beta and liberation into the extracellular medium of the cytoplasmic marker enzyme LDH and the lysosomal enzyme beta-N-acetylglucosaminidase. Hypotonic lysis resulted in the release of a 20-kDa IL-1 beta species that was distinct from the 17-kDa mature species. Importantly, incubation of the murine macrophages with the potassium/proton ionophore nigericin led to a quantitative conversion of pro-IL-1 beta to a 17-kDa species. The N-terminus of this nigericin-derived product possessed the amino acid sequence expected for mature biologically active IL-1 beta. Monensin, an ionophore similar to nigericin, did not induce release or proteolysis of IL-1 beta. Complete release of mature IL-1 beta required concentrations of nigericin in excess of 2 microM and a minimum of 10 min of treatment. Mature 17-kDa IL-1 beta was observed within the nigericin-treated cells before their lysis. Nigericin's effect was not limited to mouse peritoneal macrophages, inasmuch as the ionophore also induced release and proteolytic maturation of IL-1 beta produced by LPS-stimulated human peripheral blood monocytes. Treatment of macrophages with LPS and nigericin, therefore, results in a unique series of intracellular events that promote formation of mature 17-kDa IL-1 beta.