Inhibition of IκB kinase activity by acetyl-boswellic acids promotes apoptosis in androgen-independent PC-3 prostate cancer cells in vitro and in vivo

Signaling through NF-κB has been implicated in the malignant phenotype as well as the chemoresistance of various cancers. Here we show that the natural compounds acetyl-β-boswellic acid and acetyl-11-keto-β- boswellic acid (AKβBA) inhibit proliferation and elicit cell death in chemoresistant androgen-independent PC-3 prostate cancer cells in vitro and in vivo. Induction of apoptosis was demonstrated in cultured PC-3 cells by several parameters including mitochondrial cytochrome c release and DNA fragmentation. At the molecular level these compounds inhibit constitutively activated NF-κB signaling by intercepting the IκB kinase (IKK) activity; signaling through the interferon-stimulated response element remained unaffected, suggesting specificity for IKK inhibition. The impaired phosphorylation of p65 and the reduced nuclear translocation of NF-κB proteins were associated with down-regulation of the constitutively overexpressed and NF-κB-dependent antiapoptotic proteins Bcl-2 and Bcl-x_L. In addition, expression of cyclin D1, a crucial cell cycle regulator, was reduced as well. Down-regulation of IKK by antisense oligodeoxynucleotides confirmed the essential role of IKK inhibition for the proliferation of the PC-3 cells. Both compounds tested were active in vivo, yet AKβBA proved to be far superior. Indeed, topical application of water-soluble AKβBA-γ-cyclodextrin on PC-3 tumors xenografted onto chick chorioallantoic membranes induced concentration-dependent inhibition of proliferation as well as apoptosis. Similarly, in nude mice carrying PC-3 tumors, systemic application of AKβBA-γ-cyclodextrin inhibited tumor growth and triggered apoptosis in the absence of detectable systemic toxicity. Thus, AKβBA and related compounds acting on IKK might provide a novel approach for the treatment of chemoresistant human tumors such as androgen-independent human prostate cancers.