Mini-review Engineering death receptor ligands for cancer therapy Harald Wajant a,⇑ , Jeannette Gerspach b , Klaus Pfizenmaier b a Division of Molecular Internal Medicine, Department of Internal Medicine II, University Hospital Würzburg, Würzburg, Germany b Institute of Cell Biology and Immunology, University of Stuttgart, Stuttgart, Germany article info Article history: Available online xxxx Keywords: Apoptosis CD95L Death ligands TNF TRAIL abstract CD95, TNFR1, TRAILR1 and TRAILR2 belong to a subgroup of TNF receptors which is char- acterized by a conserved cell death-inducing protein domain that connects these receptors to the apoptotic machinery of the cell. Activation of death receptors in malignant cells attracts increasing attention as a principle to fight cancer. Besides agonistic antibodies the major way to stimulate death receptors is the use of their naturally occurring ‘‘death ligands’’ CD95L, TNF and TRAIL. However, dependent from the concept followed to develop a death ligand-based therapy various limiting aspects have to be taken into consideration on the way to a ‘‘bedside’’ usable drug. Problems arise in particular from the cell associated transmembrane nature of the death ligands, the poor serum half life of the soluble frag- ments derived from the transmembrane ligands, the ubiquitous expression of the death receptors and the existence of additional non-death receptors of the death ligands. Here, we summarize strategies how these limitations can be overcome by genetic engineering. Ó 2011 Published by Elsevier Ireland Ltd. 1. Introduction The ligands and receptors of the tumor necrosis factor (TNF) family fulfill a variety of functions in the immune system, but have also been implicated in developmental processes and in the control of tissue homeostasis. With respect to the activation of intracellular signaling path- ways three subgroups of TNF receptors can be defined ([1] and Fig. 1). Firstly, TNF receptors that stimulate intra- cellular signaling pathways by recruitment of members of the TNF-receptor associated factor (TRAF) family of adapter proteins. Secondly, TNF receptors that have a conserved protein protein interaction domain in their cytoplasmic tail, called the death domain, which enables signaling by homotypic interaction with death domain containing- adapter proteins. As most of these receptors can trigger apoptosis by virtue of their death domain, they have been named death receptors. Especially, CD95 (Fas), tumor necrosis factor (TNF) receptor-1 (TNFR1), TNF-related apoptosis inducing ligand (TRAIL) receptor-1 (TRAILR1; DR4) and TRAILR2 (DR5) have been identified as potent inducers of apoptosis in a variety of cell types in vitro and in vivo. The ligands of these death receptors, CD95L, TRAIL and TNF, are accordingly often designated as death ligands. Thirdly, soluble and membrane-bound decoy receptors which compete with receptors of the two other subgroups for ligand binding. There is evidence that transformed cells are particularly sensitive for death receptor-induced apoptosis. So, there are currently considerable efforts to exploit death ligands in tumor therapy [2]. With respect to the development of a death ligand-based therapeutic concept, two principle strategies can be pursued. First, treatment with recombi- nant ligand proteins and secondly gene therapy with cells engineered to express death ligands. Dependent from the concrete strategy adopted to create a death ligand-based therapy, one or more of the following aspects has to be ta- ken into consideration on the way to the bedside.  The ligands of the TNF family, including all death ligands, are initially expressed as transmembrane proteins. From these transmembrane molecules soluble 0304-3835/$ - see front matter Ó 2011 Published by Elsevier Ireland Ltd. doi:10.1016/j.canlet.2010.12.019 ⇑ Corresponding author. Address: Division of Molecular Internal Med- icine, Department of Internal Medicine II, University Hospital Würzburg, Röntgenring 11, 97070 Würzburg, Germany. Tel.: +49 931 201 71010; fax: +49 931 201 71070. E-mail address: harald.wajant@mail.uni-wuerzburg.de (H. Wajant). Cancer Letters xxx (2011) xxx–xxx Contents lists available at ScienceDirect Cancer Letters journal homepage: www.elsevier.com/locate/canlet Please cite this article in press as: H. Wajant et al., Engineering death receptor ligands for cancer therapy, Cancer Lett. (2011), doi:10.1016/ j.canlet.2010.12.019