Denbinobin, a 1,4-phenanthrenequinone firstly isolated from the stems of Dendrobium moniliforme (Shi-Hu in Chinese medicine), has been reported to exhibit anti-tumoral and anti-inflammatory activities through mechanism(s) not yet fully understood. Because of the critical role of the transcription factor NF-kappaB and of ROS-induced activation of stress regulated kinases in tumorigenesis, we have investigated the effect of denbinobin on these pathways. We found that denbinobin is a potent inhibitor of TNFalpha and PMA-induced NF-kappaB activation, and that it can block the phosphorylation and degradation of IkappaBalpha by inhibiting TAK1 activity, an event lying upstream of IKK activation. Moreover, treatment with denbinobin not only elicited apoptotic signalling, including mitochondrial membrane dysfunction, activation of caspases and cleavage of poly(ADP-ribose) polymerase, but also induced intracellular reactive oxygen species (ROS) generation and sustained activation of the mitogen-activated kinases (MAPKs) ERK1+2, p38 and JNK 1+2. The apoptotic effects of denbinobin could be prevented by pre-treatment with the intracellular ROS scavenger N-acetyl-l-cysteine, but not by pharmacological inhibition of MAPKs, suggesting that intracellular ROS generation underlies denbinobin-induced apoptosis, and that this effect takes place in an MAPKs-independent pathway. To define the structural elements critical for these activities, a series of phenanthrenequinones with different substituents in the phenanthrene- and/or in the quinone ring were prepared and assayed for NF-kappaB inhibition and ROS production. In this way, the major structure-activity relationships and the structural elements critical for the activity of denbinobin could be established.