Triptolide (1) is a structurally unique diterpene triepoxide isolated from a traditional Chinese medicinal plant with anti-inflammatory, immunosuppressive, contraceptive and antitumor activities. Its molecular mechanism of action, however, has remained largely elusive to date. We report that triptolide covalently binds to human XPB/ERCC3, a subunit of the transcription factor TFIIH, and inhibits its DNA-dependent ATPase activity, which leads to the inhibition of RNA Polymerase II mediated transcription and likely nucleotide excision repair. The identification of XPB as the target of triptolide accounts for the majority of the known biological activities of triptolide. These findings also suggest that triptolide can serve as a novel molecular probe for studying transcription and, potentially, as a new type of anticancer agents through inhibition of the ATPase activity of XPB.
A decline in electron transport chain (ETC) activity is associated with many human diseases. Although diminished mitochondrial ATP production is recognized as a source of pathology, the contribution of the associated reduction in the ratio of the amount of oxidized nicotinamide adenine dinucleotide (NAD+) to that of its reduced form (NADH) is less clear. We used a water-forming NADH oxidase from L. brevis (LbNOX) as a genetic tool for inducing a compartment-specific increase of the NAD+/NADH ratio in human cells. We used LbNOX to demonstrate the dependence of key metabolic fluxes, gluconeogenesis, and signaling on the cytosolic or mitochondrial NAD+/NADH ratios. Expression of LbNOX in the cytosol or mitochondria ameliorated proliferative and metabolic defects caused by an impaired ETC. The results underscore the role of reductive stress in mitochondrial pathogenesis and demonstrate the utility of targeted LbNOX for direct, compartment-specific manipulation of redox state.
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