Acetyl-boswellic acids inhibit lipopolysaccharide-mediated TNF-α induction in monocytes by direct interaction with IκB kinases

Expression of proinflammatory cytokines by monocytes is tightly regulated by transcription factors such as NF-κB. In this study, we show that, in LPS-stimulated human peripheral monocytes, the pentacyclic triterpenes acetyl-α-boswellic acid (AαBA) and acetyl-11-keto-β-boswellic acid (AKβBA) down-regulate the TNF-α expression. AαBA and AKβBA inhibited NF-κB signaling both in LPS-stimulated monocytes as detected by EMSA, as well as in a NF-κB-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 AαBA and AKβBA 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 IκBα, as well as phosphorylation of p65 at Ser ⁵³⁶ and its nuclear translocation. AαBA and AKβBA inhibited specifically the phosphorylation of recombinant IκBα and p65 by IκBα kineses (IKKs) immunoprecipitated from LPS-stimulated monocytes. In line with this, AαBA and AKβBA also bound to and inhibited the activities of active human recombinant GST-IKKα and His-IKKβ. The LPS-triggered induction of TNF-α in monocytes is dependent on IKK activity, as confirmed by IKK-specific antisense oligodeoxynucleotides. Thus, via their direct inhibitory effects on IKK, AαBA and AKβBA convey inhibition of NF-κB and subsequent down-regulation of TNF-α expression in activated human monocytes. These findings provide a molecular basis for the anti-inflammatory properties ascribed to AαBA- and AKβBA-containing drugs and suggest acetyl-boswellic acids as tools for the development of novel therapeutic interventions.