Mechanism of action of the antiproliferative natural product triptolide

Small molecules are widely used as tools to study and perturb biological systems. The ability of bioactive small molecules to quickly switch on or off the activity of proteins is not matched by any other known method. The main obstacle to a wider use of small molecules for studying biology is the difficulty of identifying compounds that can specifically alter the function of a given protein or protein family. The goal of my PhD thesis was to identify new potent and specific inhibitors of untargeted proteins that can be used as tools to study biology and serve as starting points for the development of new drugs down the road.;One approach for identification of inhibitors for untargeted protein families is to study potent bioactive natural products with unknown mechanism of action. I decided to focus on a natural product called triptolide. Triptolide has been shown to exhibit potent antiproliferative and immunosuppressive activities. Preclinical studies in animal models have revealed that triptolide is effective against cancer, collagen-induced arthritis, skin allograft rejection and bone marrow transplantation. An analog of this natural product called F60008 was shown to be active in patients with Acute Myelogenous Leukemia (AML) in Phase I clinical trials. Its molecular mechanism of action, however, has remained largely elusive to date.;I showed that triptolide works by covalently binding to XPB subunit of Transcription Factor IIH (TFIIH) and inhibiting its ATPase activity. This leads to inhibition of cell proliferation through selective impairment of Nucleotide Excision Repair and RNA polymerase II-dependent transcription initiation both of which require the activity of TFIIH.;Triptolide is the first known selective inhibitor of RNA polymerase II-dependent transcription initiation and the first known specific inhibitor of a member of helicase superfamily 2 that is not a nucleotide analog. Triptolide can serve as a basis for the design of specific inhibitors of XPB homologues that participate in transcription, translation, splicing, DNA repair and histone remodeling. And the knowledge of the mechanism of action of this natural product will help to advance the clinical testing of this potential chemotherapeutic agent.