Tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) exhibits potent antitumour activity upon systemic administration in mice without showing the deleterious side effects observed with other apoptosisinducing members of the TNF family such as TNF and CD95L. TRAIL may, thus, have great potential in the treatment of human cancer. However, about 60% of tumour cell lines are not sensitive to TRAIL. To evaluate the mechanisms of tumour resistance to TRAIL, we investigated hepatocellular carcinoma (HCC) cell lines that exhibit differential sensitivity to TRAIL. Pretreatment with chemotherapeutic drugs, for example, 5-fluorouracil (5-FU), rendered the TRAIL-resistant HCC cell lines sensitive to TRAIL-induced apoptosis. Analysis of the TRAIL death-inducing signalling complex (DISC) revealed upregulation of TRAIL-R2. Caspase-8 recruitment to and its activation at the DISC were substantially increased after 5-FU sensitisation, while FADD recruitment remained essentially unchanged. 5-FU pretreatment downregulated cellular FLICE-inhibitory protein (cFLIP) and specific cFLIP downregulation by small interfering RNA was sufficient to sensitise TRAIL-resistant HCC cell lines for TRAIL-induced apoptosis. Thus, a potential mechanism for TRAIL sensitisation by 5-FU is the increased effectiveness of caspase-8 recruitment to and activation at the DISC facilitated by the downregulation of cFLIP and the consequent shift in the ratio of caspase-8 to cFLIP at the DISC.

Apoptosis of activated peripheral T cells during the termination phase of an immune response is critical to maintain T-cell homeostasis. Activated T cells can be removed by two mechanisms: activation-induced cell death (AICD) and death by neglect. AICD is triggered by death receptors, whereas death by neglect is induced by cytokine withdrawal. CD95 (APO-1/Fas) belongs to the subfamily of death receptors and plays a major role in AICD. In this review, we focus on the molecular mechanisms of AICD, in particular those involving the CD95 system. Moreover, we discuss the relative contribution of AICD and death by neglect to terminate a T-cell immune response. In order to become fully activated, T cells require a second signal provided by antigen-presenting cells. We discuss how these costimulatory signals counteract pro-apoptotic signals and, finally, which signals might protect T cells from death to generate a pool of memory T cells.

IntroductionInduction of apoptosis in cancer cells is one of the strategies of anticancer therapy. Apoptotic cell death can be induced through the extrinsic or the intrinsic signaling pathways that are ultimately coupled to the activation of effector caspases. 1,2 The extrinsic pathway involves ligation of death receptors followed by the formation of the death-inducing-signaling-complex (DISC) and activation of, for example, pro-caspase-8. Caspase-8 activates caspase-3, which cleaves target proteins leading to apoptosis. Intrinsic death stimuli directly or indirectly activate the mitochondrial pathway by inducing release of cytochrome c and formation of a cytosolic multiprotein complex, the apoptosome, composed of Apaf-1 and pro-caspase-9. Caspase-9 is activated at the apoptosome and, in turn, activates pro-caspase-3. This death pathway is largely controlled by the proapoptotic (eg, Bax, Bad, Bid, and Bak) and antiapoptotic (eg Bcl-2 and Bcl-x L ) Bcl-2 family proteins. 1,2 Caspase-8 may also induce cleavage of Bid, which induces the translocation of the proapoptotic Bcl-2 family proteins Bax and/or Bak to the mitochondrial membrane. 3 Many stimuli, such as growth factor deprivation, ionizing radiation, and reactive oxygen species (ROS), may trigger the intrinsic death pathway. Recently, Ca 2ϩ signals have been implicated to play an important role in regulation of cell death and survival. 1 Certain apoptotic stimuli induce release of Ca 2ϩ from stores in the endoplasmic reticulum (ER), which causes Ca 2ϩ overload of the mitochondria leading to the release of cytochrome c as part of a stress response. ROS, such as . O 2 Ϫ and its reduced product H 2 O 2 , have been considered as cytotoxic byproducts of cellular metabolism. Recent evidence indicates that H 2 O 2 can also serve as a signaling molecule to modulate various physiologic functions, including mobilization of intracellular Ca 2ϩ through activation of phospholipase C␥1 (PLC␥1), a key enzyme involved in Ca 2ϩ signaling. [4][5][6] Over the past decades, much effort has been invested into the search for agents that can differentially induce apoptotic death in cancer cells. In recent years, traditional Chinese herbal remedies have gradually gained considerable attention as a new source of anticancer drugs. Although their curative mechanisms are still largely unknown, some of the drugs have been used to treat cancer. 7 Extracts of the radix of the traditional Chinese herb Huang-Qin (Scutellaria baicalensis Georgi) are among the most popular herbal remedies used in China and several oriental countries for clinical treatment of hyperlipemia, atherosclerosis, hypertension, dysentery, common cold, and inflammatory diseases, such as atopic dermatitis. Huang-Qin extracts show low toxicity in different animals (The grand dictionary of Chinese herbs, 1977). The active components of Huang-Qin are confirmed to be flavonoids. Wogonin (5,7-dihydroxy-8-methoxyflavone) is one of the major bioactive flavonoids of Scutellaria baicalensis Georgi, which has been shown to have an...