Triptolide (PG490, 97% pure) is a diterpenoid triepoxide with potent anti-inflammatory and immunosuppressive effects in transformed human bronchial epithelial cells and T cells (Qiu D, Zhao G, Aoki Y, Shi L, Uyei A, Nazarian S, Ng JC-H, and Kao PN.J Biol Chem 274: 13443–13450, 1999). Triptolide, with an IC₅₀ of ∼20–50 ng/ml, inhibits normal and transformed human bronchial epithelial cell expression of interleukin (IL)-6 and IL-8 stimulated by phorbol 12-myristate 13-acetate (PMA), tumor necrosis factor-α, or IL-1β. Nuclear runoff and luciferase reporter gene assays demonstrate that triptolide inhibits IL-8 transcription. Triptolide also inhibits the transcriptional activation, but not the DNA binding, of nuclear factor-κB. A cDNA array and clustering algorithm analysis reveals that triptolide inhibits expression of the PMA-induced genes tumor necrosis factor-α, IL-8, macrophage inflammatory protein-2α, intercellular adhesion molecule-1, integrin β₆, vascular endothelial growth factor, granulocyte-macrophage colony-stimulating factor, GATA-3, fra-1, and NF45. Triptolide also inhibits constitutively expressed cell cycle regulators and survival genes cyclins D1, B1, and A1, cdc-25, bcl-x, and c-jun. Thus anti-inflammatory, antiproliferative, and proapoptotic properties of triptolide are associated with inhibition of nuclear factor-κB signaling and inhibition of genes known to regulate cell cycle progression and survival.