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

The candidate tumor suppressor KILLER/DR5 is a DNA damageinducible p53-regulated death receptor for the tumor necrosis factorrelated apoptosis-inducing ligand (TRAIL), a promising agent for cancer therapy. The majority of studies on KILLER/DR5 have been focused on its role in TRAIL-induced apoptosis. However, its contribution to the inhibition of tumor growth and its role as a determinant of chemosensitivity are poorly understood. In the present study, we have generated stable human colon cancer cell lines, in which the function of KILLER/DR5 was ablated using inducible RNA interference. Inducible silencing of KILLER/DR5 in vivo by exposure of mice to doxycycline led to accelerated growth of bioluminescent tumor xenografts and conferred resistance to the chemotherapeutic agent 5-fluorouracil. Our results suggest that KILLER/DR5 may be a critical determinant for tumorigenicity and chemosensitivity.

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.

From the ‡ Molecular Biology of Selenium Section, Laboratory of Cancer Prevention, Center for Cancer Research, NCI, National Institutes of Health, Bethesda, Maryland 20892, §Neurobiology of Selenium, Neuroscience Research Center, Institute forExperimental Endocrinology, Charite Universita tsmedizin Berlin, 10117 Berlin, Germany, ¶ Science Applications International Corporation, Frederick Cancer Research and Development Center, Frederick, Maryland 21702, the School of Biological Sciences and Institute of Molecular Biology and Genetics, Seoul National University, Seoul 151-742, Korea, and the **Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588

Tumor necrosis factor–related apoptosis–inducing ligand (TRAIL) has attracted interest as an anticancer treatment, when used in conjunction with standard chemotherapy. We investigated the mechanistic basis for combining low-dose TRAIL with microtubule-targeting agents that invoke the mitotic checkpoint. Treatment of T98G and HCT116 cells with nocodazole alone resulted in a robust mitotic block with initially little cell death; low levels of cell death were also seen with TRAIL alone at 10ng/mL final concentration. In contrast, the addition of low-dose TRAIL to nocodazole was associated with maximally increased caspase-3, caspase-8, and caspase-9 activation, which efficiently abrogated the mitotic delay and markedly increased cell death. In contrast, the abrogation of mitotic checkpoint and increased cell death were blocked by inhibitors of caspase-8 and caspase-9 or pan-caspase inhibitor. The addition of TRAIL to either nocodazole or paclitaxel (Taxol) reduced levels of the mitotic checkpoint proteins BubR1 and Bub1. BubR1 mutated for the caspase cleavage sites, but not wild-type BubR1, was resistant to cleavage induced by TRAIL added to nocodazole, and partially blocked the checkpoint abrogation. These resultssuggest that adding a relatively low concentration of TRAIL to antimicrotubule agents markedly increases complete caspase activation. This in turn accentuates degradation of spindle checkpoint proteins such as BubR1 and Bub1, contributes to abrogation of the mitotic checkpoint, and induces cancer cell death. These results suggest that TRAIL may increase the anticancer efficacy of microtubule-targeting drugs. [Cancer Res 2008;68 (9):3440–9]