The cathepsin B inhibitor z-FA.fmk inhibits cytokine production in macrophages stimulated by lipopolysaccharide

Cathepsin B has previously been shown to proteolytically activate the proinflammatory caspase-11 in vitro. Here we show that Cathepsin B is not involved in activation of caspase-11 induced by lipopolysaccharide (LPS) and subsequent maturation of interleukin (IL)-1beta in macrophages. Nevertheless, we found that the Cathepsin B Inhibitor benzyloxycarbonyl-Phe-Ala-fluoromethylketone (z-FA.fmk) prevents LPS-induced production of IL-1alpha, IL-1beta, and tumor necrosis factor at the transcriptional level. The latter was not because of Cathepsin B inhibition, but was mediated by inhibition of the transactivation potential of the nuclear factor kappaB (NF-kappaB). z-FA.fmk did not prevent LPS-induced activation of p38 mitogen-activated protein kinase, which was shown to be involved in NF-kappaB transactivation in response to LPS. These results suggest that the previously described therapeutic effect of z-FA.fmk in the treatment of rheumatoid arthritis might not only result from inhibition of Cathepsin B but also implicates an important contribution from the inhibition of NF-kappaB-dependent gene expression.