Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) exhibits specific tumoricidal activity and is under development for cancer therapy. Mismatch-repair-deficient colonic tumors evade TRAIL-induced apoptosis through mutational inactivation of Bax, but chemotherapeutics including Camptosar (CPT-11) restore TRAIL sensitivity. However, the signaling pathways in restoring TRAIL sensitivity remain to be elucidated. Here, we imaged p53 transcriptional activity in Bax-1-carcinomas by using bioluminescence, in vivo, and find that p53 is required for sensitization to TRAIL by CPT-11. Small interfering RNAs directed at proapoptotic p53 targets reveal TRAIL receptor KILLER DR5 contributes significantly to TRAIL sensitization, whereas Bak plays a minor role. Caspase 8 inhibition protects both CPT-11 pretreated wild-type and Bax-1-HCT116 cells from TRAIL-induced apoptosis, whereas caspase 9 inhibition only rescued the wild-type HCT116 cells from death induced by TRAIL. The results suggest a conversion in the apoptoticmechanism in HCT116 colon carcinoma from a type II pathway involving Bax and the mitochondria to a type I pathway involving efficient extrinsic pathway caspase activation. In contrast to Bax-1-cells, Bak-deficient human cancers undergo apoptosis in response to TRAIL or CPT-11, implying that these proteins have nonoverlapping functions. Our studies elucidate a mechanism for restoration of TRAIL sensitivity in MMR-deficient Bax-1- human cancers through p53-dependent activation of KILLER DR5 and reconstitution of a type I death pathway. Efforts to identify agents that up-regulate DR5 may be useful in cancer therapies restoring TRAIL sensitivity.

Tumor necrosis factor-related apoptosis-inducing ligand or Apo 2 ligand (TRAIL/Apo2L) is a member of the tumor necrosis factor (TNF) family of ligands capable of initiating apoptosis through engagement of its death receptors. TRAIL selectively induces apoptosis of a variety of tumor cells and transformed cells, but not most normal cells, and therefore has garnered intense interest as a promising agent for cancer therapy. TRA IL is expressed on different cells of the immune system and plays a role in both T-cell-and natural killer cell-mediated tumor surveillance and suppression of suppressing tumor metastasis. Som e mismatch-repair-deficient tumors evade TRAIL-induced apoptosis and acquire TRAIL resistance through different mechanisms. Deat hreceptors, members of the TNF receptor family, signal apoptosis independently of the p53 tumor-suppressor gene. TRAIL treatment in combination with chemo-or radiotherapy enhances TRAIL sensitivity or reverses TRAIL resistance by regulating the downstream effectors. Efforts to identify agents that activate death receptors or block specific effectors may improve therapeutic design. In this review, we summarize recent insights into the apoptosis-signaling pathways stimulated by TRAIL, present our current understanding of the physiological role of this ligand and the potential of its application for cancer therapy and prevention.

Mice homozygous for an allele encoding the selenocysteine (Sec) tRNA[Ser]Sec gene (Trsp) flanked by loxP sites were generated. Cre recombinase-dependent removal of Trsp in these mice was lethal to embryos. To investigate the role of Trsp in mouse mammary epithelium, we deleted this gene by using transgenic mice carrying the Cre recombinase gene under control of the mouse mammary tumor virus (MMTV) long terminal repeat or the whey acidic protein promoter. While both promoters target Cre gene expression to mammary epithelium, MMTV-Cre is also expressed in spleen and skin. Sec tRNA[Ser]Sec amounts were reduced by more than 70% in mammary tissue with either transgene, while in skin and spleen, levels were reduced only with MMTV-Cre. The selenoprotein population was selectively affected with MMTV-Cre in breast and skin but not in the control tissue, kidney. Moreover, within affected tissues, expression of specific selenoproteins was regulated differently and often in a contrasting manner, with levels of Sep15 and the glutathione peroxidases GPx1 and GPx4 being substantially reduced. Expression of the tumor suppressor genes BRCA1 and p53 was also altered in a contrasting manner in MMTV-Cre mice, suggesting greater susceptibility to cancer and/or increased cell apoptosis. Thus, the conditional Trsp knockout mouse allows tissue-specific manipulation of Sec tRNA and selenoprotein expression, suggesting that this approach will provide a useful tool for studying the role of selenoproteins in health.

During tumor development a switch to glycolytic metabolism known as the Warburg effect may provide cancer cells with a survival advantage and may also provide a therapeutic opportunity. A number of signals contribute to aerobic glycolysis including those mediated by HIF‑1, c‑Myc, Akt and Hexokinase. Recent studies have implicated the p53 tumor suppressor as a negative regulator of this switch. Using inducible p53 gene silencing in bioluminescent tumor xenografts we initially observed qualitatively similar levels of FDG uptake by PET small animal imaging in wild‑type p53‑expressing tumor xenografts and p53 gene‑silenced xenografts. We further evaluated glucose uptake using FDG‑PET/CT fusion imaging of green and red fluorescently-labeled wild‑type and p53-null human colon tumor xenografts. Our results demonstrate that the wild‑type p53‑expressing tumor xenografts exhibit high levels of glucose uptake, similar to those observed in p53‑null tumor xenografts, by quantitative PET imaging indicative of the glycolytic switch. Thus p53 function is not sufficient to suppress glucose uptake in cells and tumors that could theoretically support aerobic glycolysis. Introduction

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)is a member of the TNF superfamily and has been shown to induce apoptosis in cancer cells but not normal cells. TRAIL triggers apoptosis through binding to its receptors DR4 and KILLER/DR5. Chemo or radiotherapy induces apoptosis through activation of p53 in response to cellular damage, whereas TRAIL induces apoptosis independent of p53. Mutations or deletions of p53 occurred in more than half of human tumors confer resistance to chemo-radiotherapy. Treatment of TRAILresistant tumors with agents targeting death receptors, intrinsic Bcl-2 family members, inhibitor of apoptosis proteins or PI3K/Akt pathway restores the sensitivity to TRAIL-induced apoptosis. Combination of rhTRAIL or the agonist antibody for TRAIL receptor with conventional chemotherapeutic agents results in enhanced efficacy in preventing tumor progression and metastasis. Therefore, the rational design of TRAIL-based therapy combining with other modality that either synergizes to apoptosis induction or overcomes the resistance represents a challenging strategy to achieve the systemic tumor targeting and augment the antitumor activity of cancer therapeutics.