1. ¹Department of Dermatology and Venereology, Laboratory for Experimental Dermatology, Otto-von-Guericke-University Magdeburg, Magdeburg, Germany
2. ²Department of Apoptosis Regulation, German Cancer Research Center, Heidelberg, Germany
3. ³Laboratory for Experimental Oncology and Radiation, Academisch Medisch Centrum UVA, Amsterdam, Netherlands
4. ⁴Rudolf Virchow Center, DFG Research Center for Experimental Biomedicine University of Würzburg, Würzburg, Germany
5. ⁵Department of Dermatology, Venereology and Allergology, University of Würzburg, Würzburg, Germany
6. ⁶Medical Policlinic, University of Würzburg, Würzburg, Germany

Correspondence: Dr M Leverkus, Department of Dermatology and Venereology, Laboratory for Experimental Dermatology, Otto-von-Guericke-University Magdeburg, Leipziger Street 44, Magdeburg, Saxonia-Anhalt 39120, Germany. E-mail: leverkus@medizin.uni-magdeburg.de

⁷These authors contributed equally to this work.

Received 22 August 2007; Revised 2 November 2007; Accepted 8 November 2007; Published online 17 December 2007.

Abstract

Death ligands such as tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and certain forms of CD95L are attractive therapeutic options for metastatic melanoma. Since knowledge about the regulation of death receptor sensitivity in melanoma is sparse, we have analysed these signaling pathways in detail. The loss of CD95 or TRAIL-R1, but not of TRAIL-R2, surface expression correlated with apoptosis sensitivity in a panel of melanoma cell lines. In contrast, the expression of proteins of the apical apoptosis signaling cascade (FADD, initiator caspases-8 and cFLIP) did not predict apoptosis sensitivity. Since both TRAIL-R1 and -R2 transmit apoptotic signals, we asked whether cFLIP, highly expressed in several of the cell lines tested, is sufficient to maintain resistance to TRAIL-R2-mediated apoptosis. Downregulation of cFLIP in TRAIL-R2-positive, TRAIL-resistant IGR cells dramatically increased TRAIL sensitivity. Conversely ectopic expression of cFLIP in TRAIL-sensitive, TRAIL-R2-expressing RPM-EP melanoma cells inhibited TRAIL- and CD95L-mediated cell death. Thus, modulation of cFLIP is sufficient to sensitize TRAIL-R2-expressing cells for TRAIL. Taken together, albeit expressing all proteins necessary for death receptor-mediated apoptosis, TRAIL-R1 negative melanoma cells cannot undergo TRAIL- or CD95L-induced apoptosis due to expression of cFLIP. Hence, cFLIP represents an attractive therapeutic target for melanoma treatment, especially in combination with TRAIL receptor agonists.