CD95 in cancer: tool or target? Ana Martin-Villalba, Enric Llorens-Bobadilla, and Damian Wollny Molecular Neurobiology, German Cancer Research Center (DKFZ), 69120 Heidelberg, Germany The role of CD95 (Fas/Apo1) in cancer has been a matter of debate for over 30 years. First discovered as an apo- ptosis-inducing molecule, CD95 soon emerged as a po- tential anticancer therapy. Yet accumulating evidence indicates a profound role for CD95 in alternative non- apoptotic signaling pathways that increase tumorigen- esis. This fact challenges the initial clinical idea of using CD95 as a ‘tumor killer’ while setting the stage for clinical studies targeting the nonapoptotic signaling branch of CD95. This review summarizes the findings surrounding manipulation of the CD95 pathway for cancer therapy, considering how one receptor can both promote and prevent cell growth. The complexity of killing a tumor Cancer defines a state of uncontrolled proliferation of cells that escape the homeostatic mechanisms controlling tissue growth. Proliferating tumor cells uncouple themselves from control mechanisms by modifying, and thereby using, the surrounding stroma and infiltrating immune cells. The extent of environmental manipulation ultimately deter- mines tumor progression. Ideally, anticancer therapies should target both tumor cells and their microenviron- ment. Thus, the design of effective therapies requires a comprehensive knowledge of the biology of the target in cellular systems, preclinical animal tumor models, and clinical trials. The case of CD95 is a paradigmatic example that illustrates how using isolated 2D cellular systems might be too reductionistic. Only recent studies using more sophisticated in vivo models have discovered additional, nonapoptotic functions of CD95. These studies dissect the contribution of multiple cellular players within the physi- ological environment and the differential response of tu- mor cells in isolation or embedded within tissue. In this article, we critically review the knowledge of CD95 func- tions gained over many years of research that has provided the foundation for recent clinical trials examining the inhibition of CD95 for the treatment of cancer. Lessons from cellular systems In 1989, the finding that injection of the antibody APO-1 was sufficient to rapidly and specifically eliminate tumor cells paved the way for the cloning and discovery of the receptor CD95 (Fas/Apo1), a prototypic trigger of the ‘ex- trinsic’ apoptotic pathway [1–3]. In the apoptotic para- digm, CD95 activation leads to the clustering of receptor molecules, stabilizing an open conformation of the intra- cellular death domain (DD) and allowing the recruitment of FADD (Fas-associated protein with death domain) (Box 1) [4]. Procaspase-8 is then recruited to the receptor complex through the death effector domain (DED) of FADD. However, when the number of procaspase molecules exceeds the number of FADD at the receptor, a caspase-activating chain can cluster and, together with FLIP (FLICE-like inhibitory protein), form the death-inducing signaling complex (DISC) [5,6]. The caspase chain efficiently activates caspase-8, ultimately leading to apoptotic cell death [7]. In addition to inducing apoptosis, the DISC complex has also been shown to regulate another type of cell death, namely, necrosis. Programmed necrosis is induced through the receptor-interacting protein 1 (RIP1) and RIP3 kinases, which themselves are negatively regulated by FADD and caspase-8 (Figure 1) [8–10]. However, when trying to generalize these findings to cancer, it was found that most tumor cells – although they rarely lose CD95 expression – appear to be refractory to CD95-induced cell death. This fact shifted the focus of research towards identifying mechanisms that would ren- der these cells resistant to apoptosis, disregarding the possibility of alternative pathways emanating from CD95. Seminal work by Marcus Peter’s laboratory using cells from breast, ovary, skin, lung, and kidney tumors showed that not only is apoptotic signaling blocked but also CD95 Opinion Box 1. Glossary Lessons from cellular systems Death domain (DD): protein–protein interaction domain characteristic of the death receptor subfamily. Fas-associated protein with death domain (FADD): adaptor protein binding death domain of CD95 through its own death domain. Death effector domain (DED): N-terminal protein–protein interaction domain of FADD required for caspase recruitment. Src-family kinases (SFK): receptor tyrosine kinase binding adaptor protein. What we learned from animal models Conditional mutation: genetic modification manifested exclusively under specific conditions (i.e., tissue-specific gene deletion). Lymphoproliferative disease: heterogeneous expansion of lymphoid cells occurring as a result of immune dysfunction. Lymphadenopathy: enlargement of lymph nodes. Splenomegaly: enlargement of the spleen. Glomerulonephritis: non-bacterial induced inflammation of the kidney glomeruli. Pulmonary fibrosis: formation of excess fibrous tissue in the lung. Fatal wasting syndrome: chronic loss of weight commonly found in AIDS. Adaptive cardiac hypertrophy: increase in heart muscle mass to compensate for pressure overload. Implications for therapy Phase I/II clinical trial: set of clinical tests to examine if an experimental medication or treatment is safe (Phase I) and effective (Phase II). Randomized clinical trial: trial with random allocation of participants to alternative treatment groups. Progression-free survival: staying free of disease progression for a number of patients within a certain time frame. 1471-4914/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/ j.molmed.2013.03.002 Corresponding author: Martin-Villalba, A. (a.martin-villalba@dkfz.de). Keywords: CD95; FAS; apoptosis; tumorigenesis; clinical studies; cancer therapy. Trends in Molecular Medicine, June 2013, Vol. 19, No. 6 329