Cucurbitacin B suppresses the transactivation activity of RelA/p65

Cucurbitacin B, a natural triterpenoid is well-known for its strong anticancer activity, and recent studies showed that the compound inhibits JAK/STAT3 pathway. In this study, we demonstrate for the first time that cucurbitacin B is also a potent inhibitor of NF-κB activation. Our results showed that cucurbitacin B inhibited TNF-α-induced expression of NF-κB reporter gene and NF-κB target genes in a dose-dependent manner, however, it did not prevent either stimuli-induced degradation of IκBα or nuclear translocation and DNA-binding activity of NF-κB. On the other hand, cucurbitacin B dose-dependently suppressed not only NF-κB activation induced by overexpression of RelA/p65 but also transactivation activity of RelA/p65 subunit of NF-κB. Consistently, treatment of HeLa cells with the compound significantly suppressed TNF-α-induced activation of Akt and phosphorylation of Ser536 in RelA/p65, which is required for transactivation activity. Consequently, cucurbitacin B inhibited TNF-α-induced expression of NF-κB-dependent anti-apoptotic proteins such as c-IAP1, c-IAP2, XIAP, TRAF1, and TRAF2 and sensitized TNF-α-induced cell death. Taken together, our results demonstrated that cucurbitacin B could be served as a valuable candidate for the intervention of NF-κB-dependent pathological condition such as cancer.     

A distinct role of neutrophil lactoferrin in RelA/p65 phosphorylation on Ser536 by recruiting TNF receptor-associated factors to IkappaB kinase signaling complex

The activation of NF-kappaB by neutrophil lactoferrin (Lf) is regulated via the IkappaB kinase (IKK) signaling cascade, resulting in the sequential phosphorylation and degradation of IkappaB. In this study, we observed that Lf protein augmented p65 phosphorylation at the Ser(536), but not the Ser(276) residue, and stimulated the translocation of p65 into the nucleus. Lf was also shown to enhance the association between p65 and CREB-binding protein/p300 in vivo. To elucidate the mechanism by which Lf triggers these signaling pathways, we attempted to delineate the roles of the upstream components of the IKK complex, using their dominant-negative mutants and IKKalpha(-/-) and IKKbeta(-/-) mouse embryonic cells. We demonstrated that both IKKalpha and IKKbeta as well as NF-kappaB-inducing kinase are indispensable for Lf-induced p65 phosphorylation. However, MAPK kinase kinase 1 is not essentially required for this activation. We also observed that Lf-induced p65 phosphorylation was either partially or completely abrogated as the result of treatment with the mutant forms of TNFR-associated factor (TRAF) 2, TRAF5, or TRAF6. Moreover, we demonstrated that Lf directly interacted with TRAF5. Expression of the dominant-negative mutant of TRAF5 or its small interfering RNA almost completely abrogated the Lf-induced p65 phosphorylation. These results suggest that signaling pathways, including TRAFs/NF-kappaB-inducing kinase/IKKs, may be involved in the regulation of Lf-induced p65 activation, thereby resulting in the activation of members of the NF-kappaB family.