Reverse pharmacology, also called the "bedside to bench" approach, that deals with new uses for a well known molecular entity has been used extensively in
cancer drug development to identify novel compounds and delineate their mechanisms of action. Here, we show that
nimbolide, a
triterpenoid isolated from Azadirachta indica, enhanced the apoptosis induced by inflammatory
cytokines and chemotherapeutic agents in
tumor cells. This
limonoid abrogated the expression of
proteins associated with cell survival (Bcl-2, Bcl-xL, IAP-1, and IAP-2), proliferation (
cyclin D1), invasion (MMP-9), and angiogenesis (
VEGF), all regulated by nuclear factor (NF)-κB.
Nimbolide inhibited the activation of NF-κB induced by
carcinogens and inflammatory stimuli. Constitutively active NF-κB found in most
tumor cells was also inhibited. We found that suppression of NF-κB activation by
nimbolide was caused by inhibition of IκB
kinase (IKK), which led to suppression of IκBα phosphorylation and degradation, nuclear translocation,
DNA binding, and gene transcription.
Reducing agent reversed the action of the
limonoid, suggesting the involvement of a
cysteine residue. Replacement of Cys(179) of IKK-β with
alanine abolished the effect of
nimbolide, suggesting that Cys(179) plays a critical role in inhibiting the NF-κB activation. Overall, our results indicate that
nimbolide can sensitize
tumor cells to chemotherapeutic agents through interaction with IKK, leading to inhibition of NF-κB-regulated
proteins.