Brazilein Impedes IKK Activation by Disrupting RIPK1 Polyubiquitination, Increasing Apoptotic Susceptibility in Cells with Constitutively Active NF-κB

Ubiquitination of RIPK1 serves as a critical regulatory switch in determining the outcome of prosurvival NF-κB signaling by linking the TNFR1 signaling complex to upstream IKK activation. Therefore, identifying bioactive compounds that modulate RIPK1 ubiquitination has emerged as a promising strategy to enhance the therapeutic efficacy of TNF, particularly in cancers with constitutively active NF-κB signaling. In our previous <i>in vitro</i> phytochemical study, we demonstrated that brazilin, isolated from <i>Caesalpinia sappan</i> L., inhibits the catalytic activity of the IKK complex during TNF-mediated NF-κB activation without affecting RIPK1 ubiquitination at high concentrations (~50 μM), raising concerns about off-target effects. In this study, we now report that brazilein, an oxidized derivative of brazilin, acts as a potent inhibitor of RIPK1-dependent NF-κB activation upon TNFR1 engagement. Our findings reveal that brazilein markedly suppresses upstream IKK signaling events, including TNFR1-associated RIPK1 polyubiquitination and its interaction with IKKβ. In contrast, brazilein does not affect NIK/IKKα-mediated non-canonical NF-κB activation induced by LIGHT, indicating its specificity for the canonical NF-κB pathway. Moreover, brazilein not only sensitizes cells to TNF-induced apoptosis but also induces apoptosis in A20-deficient and oncogenically transformed cells with constitutive NF-κB activity. Taken together, these results suggest a novel mechanism by which brazilein exerts anti-IKK activity through inhibition of RIPK1 ubiquitination, highlighting its potential as a candidate for NF-κB-targeted cancer therapy.