17-hydroxy-jolkinolide B inhibits signal transducers and activators of transcription 3 signaling by covalently cross-linking Janus kinases and induces apoptosis of human cancer cells.

Constitutive activation of the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway occurs frequently in cancer cells and contributes to oncogenesis. Among the members of STAT family, STAT3 plays a pivotal role in the development and progression of human tumors. The STAT3-mediated signaling pathway has been recognized as a promising anticancer target. Here, we show that 17-Hydroxy-jolkinolide B (HJB), a diterpenoid from the Chinese medicinal herb Euphorbia fischeriana Steud, strongly inhibits interleukin (IL)-6-induced as well as constitutive STAT3 activation. Furthermore, we show that HJB directly targets the JAK family kinases, JAK1, JAK2, and TYK2, by inducing dimerization of the JAKs via cross-linking. Addition of DTT or glutathione prevents the JAK cross-linking and blocks the inhibitory effects of HJB on IL-6-induced STAT3 activation, suggesting that HJB may react with cystein residues of JAKs to form covalent bonds that inactivate JAKs. Liquid chromatography/mass spectrometry analysis confirmed that each HJB reacted with two thiols. The effect of HJB on the JAK/STAT3 pathway is specific as HJB has no effect on platelet-derived growth factor, epidermal growth factor, or insulin-like growth factor I signaling pathways. Finally, we show that HJB inhibits growth and induces apoptosis of tumor cells, particularly those tumor cells with constitutively activated STAT3. We propose that the natural compound HJB is a promising anticancer drug candidate as a potent STAT3 signaling inhibitor.