Acetyl-boswellic acids inhibit lipopolysaccharide-mediated TNF-alpha induction in monocytes by direct interaction with IkappaB kinases.

Expression of proinflammatory cytokines by monocytes is tightly regulated by transcription factors such as NF-kappaB. In this study, we show that, in LPS-stimulated human peripheral monocytes, the pentacyclic triterpenes acetyl-alpha-boswellic acid (AalphaBA) and acetyl-11-keto-beta-boswellic acid (AKbetaBA) down-regulate the TNF-alpha expression. AalphaBA and AKbetaBA inhibited NF-kappaB signaling both in LPS-stimulated monocytes as detected by EMSA, as well as in a NF-kappaB-dependent luciferase gene reporter assay. By contrast, the luciferase expression driven by the IFN-stimulated response element was unaffected, implying specificity of the inhibitory effect observed. Both AalphaBA and AKbetaBA did not affect binding of recombinant p50/p65 and p50/c-Rel dimers to DNA binding sites as analyzed by surface plasmon resonance. Instead, both pentacyclic triterpenes inhibited the LPS-induced degradation of IkappaBalpha, as well as phosphorylation of p65 at Ser(536) and its nuclear translocation. AalphaBA and AKbetaBA inhibited specifically the phosphorylation of recombinant IkappaBalpha and p65 by IkappaBalpha kinases (IKKs) immunoprecipitated from LPS-stimulated monocytes. In line with this, AalphaBA and AKbetaBA also bound to and inhibited the activities of active human recombinant GST-IKKalpha and His-IKKbeta. The LPS-triggered induction of TNF-alpha in monocytes is dependent on IKK activity, as confirmed by IKK-specific antisense oligodeoxynucleotides. Thus, via their direct inhibitory effects on IKK, AalphaBA and AKbetaBA convey inhibition of NF-kappaB and subsequent down-regulation of TNF-alpha expression in activated human monocytes. These findings provide a molecular basis for the anti-inflammatory properties ascribed to AalphaBA- and AKbetaBA-containing drugs and suggest acetyl-boswellic acids as tools for the development of novel therapeutic interventions.