Inhibition of eukaryotic transcription by a natural product from Thunder God vine

Natural products have been a valuable source of novel anticancer drugs; they have also served as powerful molecular probes for different cellular processes. In contrast to cell proliferation, cell cycle regulation and a variety of signaling pathways, few potent small molecule inhibitors are known that target the fundamental process of transcription in eukaryotic cells. We have been studying the mechanism of action of triptolide, a natural product isolated from a traditional Chinese medicinal plant with potent antitumor activity. Using a top-down approach, we identified XPB, a DNA-dependent ATPase and a subunit of the general transcription factor TFIIH, as a molecular target for triptolide. We showed that triptolide binds covalently to XPB, blocking its DNA-dependent ATPase activity. In efforts to further elucidate the molecular interaction between triptolide and XPB, we recently identified the amino acid residue in XPB that is covalently modified by triptolide. We have also assessed the functional group in triptolide that is involved in the covalent modification of XPB. To assess the functional relevance of XBP as a molecular target of triptolide, we generated knock-in cells with a drug-resistant mutant of XPB and showed that it confers dramatic resistance to triptolide, strongly supporting XPB as the physiologically relevant target of triptolide. Together, these findings revealed for the first time a highly specific and potent small molecular inhibitor of the basal transcription machinery that has implications in the future development of inhibitors of transcription as anticancer drugs.